In the search for improvements in the environmental credentials of the organic coatings industry, several different aspects are currently being addressed, including the reduction of volatile organic compounds, development of UV curable coatings and the reduction of potentially hazardous components in the coatings formulations. The aim of this work was to develop the water based primers for coil coating applications as a substitution for chromate conversion coatings. The performance and durability of three novel water based systems applied on alkali cleaned HDGS has been investigated; XPS and ToF-SIMS have been employed to analyse the failure interfaces generated by various test methods designed to assess the performance and durability of each of the systems. In depth analysis has been carried out using ULAM for sample preparation, and XPS and ToF-SIMS were employed to analyse the buried metal/ primer interface, in order to investigate the interface chemistry. A method of post processing data, obtained by surface analysis techniques, was invented to gain a clear understanding of the possible chemical interactions occurred at the metal/primer interface. Adsorption studies were also carried out to encode the interfacial chemistry. The method was used to understand the role of adhesion promoter in the primer formulation, particularly the manner in which it interacts with the substrate and the topcoat and the way it improves the performance and durability; also to determine the role of anti-corrosive additives, especially the manner in which it prevents the corrosion and therefore its contribution toward the performance and durability. The distribution of these components across the primer layer was also investigated considering the possible opportunities of such components enhancing the primer interaction at either interfaces. The finding has been used for re-adjustment of some of process parameters in coil coating systems and specifically water based primers.
This study investigates the formation of a chromate conversion coating at Al–Cu–Fe–Mn intermetallic sites of an Al2219 alloy and the corrosion initiation at these sites in a 3.5% NaCl solution, using SEM, AES and EDX. Changes in the surface chemistry were monitored after progressive exposures to the solution up to 42 h. The coating was found to be thinner and more defective on the intermetallic. Initially, Al is dissolved and Al(OH)3 deposited on and around the intermetallic. After 42 h of exposure, Al(OH)3, Fe and Mn oxides and small particles of elemental Cu are deposited as corrosion products.
To investigate the role of intermetallic particles in the localised corrosion of AA7075-T6, three particles were monitored over 16 hours immersion in 3.5 wt.% KCl solution. These were examined using Auger electron spectroscopy, energy dispersive x-ray spectroscopy, scanning Kelvin probe force microscopy and focused ion beam-scanning electron microscopy. Despite similar Volta potential measurements, the corrosion microchemistry varied significantly with composition. A Al7Cu2Fe intermetallic resulted in trenching while a (Al,Cu)6(Fe,Cu) intermetallic showed crevice corrosion and sub-surface intergranular corrosion and a Al12Fe3Si intermetallic appeared to be galvanically inactive but showed crevice formation at the matrix interface and sub-surface intergranular corrosion.
Beryllium is an important metal in the nuclear industry for which there are no suitable replacements. It undergoes localised corrosion at the site of heterogeneities in the metal surface. Corrosion pits are associated with a range of second phase particles. To investigate the role of these particles in corrosion, a safe experimental protocol was established using an aluminium alloy as a corrosion material analogue. The 7075-T6 alloy had not previously been investigated using the experimental methodology used in this thesis. This work led to the development of the experimental methodology and safe working practices for handling beryllium. The range and composition of the second phase particles present in S-65 beryllium billet were identified using a combination of SEM, AES, EDX and WDX. Following the identification of a range of particles with various compositions, including the AlFeBe4 precipitate which has been previously associated with corrosion, the location of the particles were marked to enable their repeated study. Attention was focused on the microchemistry in the vicinity of second phase particles, as a function of immersion time in pH 7, 0.1 M NaCl solution. The corrosion process associated with different particles was followed by repeatedly relocating the particles to perform analysis by means of SEM, AES and EDX. The use of traditional chlorinated vapour degreasing solvents on beryllium was investigated and compared to two modern commercially available cleaning solutions designed as drop-in replacements. This work expanded the range of solvents suitable for cleaning beryllium and validated the conclusions from previous thermodynamic modelling. Additionally, a new experimental methodology has been developed which enables the acquisition of chemical state information from the surface of micron scale features. This was applied to sub-micron copper and iron particles, as well as a copper intermetallic
Secondary ion mass spectrometry (SIMS) is a technique that has evolved to be one of the most powerful techniques for the analysis of organic samples. Modern instruments are capable of obtaining three-dimensional information with high spatial resolution of a material with information as rich as a full mass spectrum at every voxel of the 3D structure, thus generating very large and complex datasets. Multivariate analysis (MVA) methods are used within the SIMS community, however, the absence of MVA in the software packages of instrument manufacturers together with constant increase in data and data analysis complexity demands practical data analysis solutions that are accessible to scientists of diverse backgrounds. This thesis aims to expand the applicability of three major MVA methods to complex SIMS datasets: Principal component analysis (PCA), non-negative matrix factorisation (NMF) and k-means clustering. This is achieved by establishing and validating existing and novel methodologies for the processing of large and complex datasets. Furthermore, it presents the development of a software that encompasses these methodologies and provide accessible and flexible analysis and data visualisation tools. Finally, it presents the application of the software to a series of experiments carried out at The Surface Analysis Laboratory of the University of Surrey in which data processing enabled deeper interpretation of the results and helped to achieve insights towards scientific and industrial problem solving.
Space photovoltaics is dominated by multi-junction (III-V) technology. However, emerging applications will require solar arrays with; high specific power (kW/kg), flexibility in stowage and deployment and a significantly lower cost than the current III-V technology offers. This research demonstrates direct deposition of thin film CdTe onto the radiation-hard cover glass that is normally laminated to any solar cell deployed in space. Four CdTe samples, with 9 defined contact device areas of 0.25 cm2, were irradiated with protons of 0.5 MeV energy and varying fluences. At the lowest fluence, 1×1012 cm-2, the relative efficiency of the solar cells was 95%. Increasing the proton fluence to 1×1013 cm-2 and then 1×1014 cm-2 decreased the solar cell efficiency to 82% and 4% respectively. At the fluence of 1×1013 cm-2, carrier concentration was reduced by an order of magnitude. Solar Cell Capacitance Simulator (SCAPS) modelling obtained a good fit from a reduction in shallow acceptor concentration with no change in the deep trap defect concentration. The more highly irradiated devices resulted in a buried junction characteristic of the external quantum efficiency, indicating further deterioration of the acceptor doping. This is explained by compensation from interstitial H+ formed by the proton absorption. An anneal of the 1×1014 cm-2 fluence devices gave an efficiency increase from 4% to 73% of the pre-irradiated levels, indicating that the compensation was reversible. CdTe with its rapid recovery through annealing, demonstrates a radiation hardness to protons that is far superior to conventional multi-junction III-V solar cells.
Lamb DA, Irvine SJC, Clayton AJ, Kartopu G, Barrioz V, Hodgson SD, Baker MA, Grilli R, Hall J, Underwood CI, Kimber R (2016)Characterization of MOCVD Thin-Film CdTe Photovoltaics on Space-Qualified Cover Glass, In: IEEE Journal of Photovoltaics6(2)pp. 557-561
This paper details the AM0 conversion efficiency of a metal-organic chemical vapor phase deposition thin-film cadmium telluride (CdTe) solar cell deposited onto a cerium-doped cover glass (100 μm). An AM0 best cell conversion efficiency of 12.4% (0.25-cm2 contact area) is reported. An AM0 mean efficiency of 12.1% over eight cells demonstrated good spatial uniformity. Excellent adhesion of the cell structure to the cover glass was observed with an adhesive strength of 38 MPa being measured before cohesive failure of the test adhesive. The device structure on cover glass was also subject to severe thermal shock cycling of +80 °C to -196 °C, showing no signs of delamination and no deterioration of the photovoltaic (PV) performance.
Lamb DA, Irvine SJC, Clayton AJ, Barrioz V, Kartopu G, Baker MA, Underwood CI, Grilli R, Kimber R, Hall J (2015)Lightweight and low-cost thin film photovoltaics for large area extra-terrestrial applications, In: IET Renewable Power Generation9(5)pp. 420-423
This work describes progress towards achieving a flexible, high specific power and low-cost photovoltaic (PV) for emerging large area space applications. The study reports the highest conversion efficiency of 15.3% AM1.5G for a CdTe device on ultra-thin cerium-doped cover glass, the standard protective material for extra-terrestrial PVs. The deposition technique used for all of the semiconductor layers comprising the device structure was atmospheric pressure metal organic chemical vapour deposition. Improvements to the device structure over those previously reported led to a Voc of 788 mV and a relatively low series resistance of 3.3 Ω·cm2. These were largely achieved by the introduction of a post-growth air anneal and a refinement of the front contact bus bars, respectively. The aluminium-doped zinc oxide transparent conductive oxide, being the first layer applied to the cover glass, was subject to thermal shock cycling +80 to (-) 196°C to test the adhesion under the extreme conditions likely to be encountered for space application. Scotch Tape testing and sheet resistance measurements before and after the thermal shock testing demonstrated that the aluminium-doped zinc oxide remained well adhered to the cover glass and its electrical performance unchanged.
Space photovoltaics is dominated by multi‐junction (III‐V) technology. However, emerging applications will require solar arrays with high specific power (kW/kg), flexibility in stowage and deployment, and a significantly lower cost than the current III‐V technology offers. This research demonstrates direct deposition of thin film CdTe onto the radiation‐hard cover glass that is normally laminated to any solar cell deployed in space. Four CdTe samples, with 9 defined contact device areas of 0.25 cm2, were irradiated with protons of 0.5‐MeV energy and varying fluences. At the lowest fluence, 1 × 1012 cm−2, the relative efficiency of the solar cells was 95%. Increasing the proton fluence to 1 × 1013 cm−2 and then 1 × 1014 cm−2 decreased the solar cell efficiency to 82% and 4%, respectively. At the fluence of 1 × 1013 cm−2, carrier concentration was reduced by an order of magnitude. Solar Cell Capacitance Simulator (SCAPS) modelling obtained a good fit from a reduction in shallow acceptor concentration with no change in the deep trap defect concentration. The more highly irradiated devices resulted in a buried junction characteristic of the external quantum efficiency, indicating further deterioration of the acceptor doping. This is explained by compensation from interstitial H+ formed by the proton absorption. An anneal of the 1 × 1014 cm−2 fluence devices gave an efficiency increase from 4% to 73% of the pre‐irradiated levels, indicating that the compensation was reversible. CdTe with its rapid recovery through annealing demonstrates a radiation hardness to protons that is far superior to conventional multijunction III‐V solar cells.
Wire and arc additive manufacture (WAAM) is a process in which an arc is used to melt and deposit beads of feedstock wire layer by layer onto a substrate and then subsequent layers to create near-net shape components. WAAM has the potential to reduce costs and material waste, as well as shorten lead-time, compared to conventional manufacturing methods. WAAM is not currently commercially viable, however, and one of the largest barriers is lack of reproducibility. This work investigates the influence of wire batch on reproducibility of the WAAM of aluminium. Particular attention is given to the external appearance and porosity of aluminium alloy 2319 (AA2319) components. Components were manufactured using 1.2 mm diameter AA2319 welding wire from different wire batches and manufacturers, and different process parameters. The surface deposit that resulted from the process on some components, which was a visible indicator of variability, was characterised using a qualitative coverage scale and five complementary techniques: scanning electron microscopy, X-ray photoelectron spectroscopy, X-ray diffraction, plasma focussed ion beam and Raman spectroscopy. Porosity of the components was also measured. Batch-to-batch variation of wire was analysed, in particular, the surface finish, diameter and composition. Surface deposit composition and coverage did not depend on wire batch. The surface deposit contained predominantly metal and metal oxide nanospheres, microspatter and carbon-rich microparticles. For AA2319 components, the metal was aluminium and for AA5183, the metals were magnesium and aluminium. There was a relationship between surface deposit coverage and wire feed speed (WFS): the higher the WFS, the greater percentage of surface deposit coverage. The results support the suggestion that surface deposit formation is caused by condensation of vapour and microspatter from feedstock wire. There was no statistically meaningful relationship between surface deposit coverage and porosity and surface deposit formation does not appear to influence commercial viability of WAAM. Wire batch was the dominant influence on porosity. Porosity ranged from 0.02 % to 6.0 % depending on wire batch used. There was no correlation between process parameters and porosity for the range of parameters used. There is evidence to suggest that internal voids and diameter of the wire affected porosity; wire with internal voids resulted in high porosity and wire with smaller diameter resulted in high porosity. These features can affect hydrogen content to the weld pool, arc stability, weld pool conduction and cooling rates, all of which influence porosity formation. The commercial viability of WAAM can be improved through control of feedstock material. This work showed that porosity could be reproducibly reduced using wire manufactured with a new manufacturing process that included diamond dies and a new cleaning process. Wire manufactured from the same ingot of material as other batches using the new process resulted in much lower porosity compared to the old process: 0.2 % compared to 1.2 %. Results suggest that changes to the wire manufacturing process may have produced wire with no internal voids and a larger diameter.
Ni/Al2O3 and Ni/CaO-MgO-Al2O3 catalysts were investigated for the biogas dry reforming reaction using CH4/CO2 mixtures with minimal dilution. Stability tests were conducted between 600 and 800 oC and TGA/DTG, Raman, STEM-HAADF, HR-TEM, XPS techniques were used to characterize the spent samples. Graphitized carbon allotrope structures, carbon nanotubes (CNTs) and amorphous carbon were formed on all samples. Metallic Ni0 was recorded for all (XPS), whereas a strong peak corresponding to Ni2O3/NiAl2O4 was observed for the Ni/Al2O3 sample (650–750°C). Stability tests confirmed that the Ni/CaO-MgO-Al2O3 catalyst deactivates at a more gradual rate and is more active and selective in comparison to the Ni/Al2O3 for all temperatures. The Ni/CaO-MgO-Al2O3 exhibits good durability in terms of conversion and selectivity, whereas the Ni/Al2O3 gradually loses its activity in CH4 and CO2 conversion, with a concomitant decrease of the H2 and CO yield. It can be concluded that doping Al2O3 with CaO-MgO enhances catalytic performance by: (a) maintaining the Ni0 phase during the reaction, due to higher dispersion and stronger active phase-support interactions, (b) leading to a less graphitic and more defective type of deposited carbon, and (c) facilitating the deposited carbon gasification due to the enhanced CO2 adsorption on its increased surface basic sites.
Siakavelas G.I, Charisiou N.D, AlKhoori S, AlKhoori A.A, Sebastian V, Hinder S.J, Baker M.A, Yentekakis I.V, Polychronopoulou K, Goula M.A (2021)Highly selective and stable nickel catalysts supported on ceria promoted with Sm2O3, Pr2O3 and MgO for the CO2 methanation reaction, In: Applied Catalysis. B, Environmental282
•Microwave assisted sol gel method produces selective CO2 methanation Ni catalysts.•The incorporation of Sm3+ and Pr3+ into the CeO2 lattice generates basic positions.•Sm3+ and Pr3+ oxygen vacancies suppress the agglomeration of Ni sites.•Presence of Mg2+ increases basicity and prevents Ni sintering during reaction.•Ni on Pr-Ce highly active, selective and stable for CO2 methanation reaction.
The present work reports on the investigation of the catalytic performance for the methanation of CO2 over Ni catalysts based on CeO2, and for the first time, of Ni catalysts supported on binary CeO2-based oxides, namely, Sm2O3-CeO2, Pr2O3-CeO2 and MgO-CeO2. The supports were obtained using the microwave assisted sol-gel method under reflux, while the catalysts were prepared by the wet impregnation method. For the investigation of the morphological, textural, structural and other intrinsic properties of the catalytic materials a variety of characterization techniques were used, i.e., Raman spectroscopy, XRD, N2 physisorption-desorption, CO2-TPD, H2-TPR, H2-TPD, XPS and TEM. Carbon deposition and sintering were investigated using TEM. It was shown that the addition of Sm3+ or Pr3+, incorporated into the lattice of CeO2, generated oxygen vacancies, but the Ni/Pr-Ce catalyst was found to possess more surface oxygen vacancies (e.g. Ce4+-Ov-Pr3+ entities). Moreover, modification of CeO2 using Sm3+ or Pr3+ restricted the agglomeration of nickel active sites and led to the genesis of Lewis basic positions. These characteristics improved the hydrogenation reaction at lower temperature. On the other hand, the addition of Mg2+ resulted at strong metal support interactions reinforcing the resistance of the Ni/Mg-Ce catalyst against sintering. Furthermore, the addition of Sm3+, Pr3+ and Mg2+ cations increased the overall basicity and the moderate adsorption sites and led to the formation of smaller Ni nano particles; these physico-chemical properties enhanced the CO2 methanation reaction. Finally, the activity experiments (WGHSV = 25,000 mL g−1 h−1, H2/CO2 = 4:1, T =350 °C) showed that at lower reaction temperature the Ni/Pr-Ce had the highest catalytic performance in terms of CO2 conversion (54.5%) and CH4 yield (54.5%) and selectivity (100%). The TOF values were found to follow the order Ni/Pr-Ce >> Ni/Mg-Ce > Ni/Sm-Ce > Ni/Ce.
Metals in the nuclear industry are often required to endure significant periods of storage or inactivity during their lifespan. A level of confidence is therefore required that they will retain the useful structural and functional properties in a given set of conditions for a given period of time. Corrosion is the primary means by which this is compromised for these materials. Studies regarding fundamental corrosion mechanisms can provide data to feed into overarching corrosion models that can predict behaviour with multiple variables (e.g. material grade, temperature, time, contamination type and amount, etc.). Firstly, early stage oxidation of a dilute depleted uranium-molybdenum alloy was analysed in situ under UHV conditions by AES and XPS. At the equivalent of less than 300 ns at 1 atm O2, U-5 wt. % Mo oxidises to form stoichiometric UO2. No molybdenum oxidation is observed. After an oxygen dose of approximately 39 L, the oxide layer approached a limiting thickness of approximately 2.4 nm. The oxidation kinetics followed a logarithmic rate law, with the best fit to the experimental data for the oxide thickness, d, being given by d = 1.26 log(0.12t + 0.56). Changes in oxygen KLL and 1s peak positions associated with transformation from chemisorbed oxygen to metal oxide were observed at similar oxygen doses of 2.3 and 2.6 L O2 by AES and XPS respectively, which opens up the possibility of using well characterised XPS chemical information to inform Auger peak shifts. A novel peak library tool was created with the primary purpose of rapid and accurate identification of Auger peaks. SKPFM was used in combination with optical microscopy and SEM/EDX to assess the range of second phase particle types (size, composition and practical nobility) in an S-65 beryllium sample and evaluate the effect of carbon contamination on corrosion of this material. Carbon deposited by thermal evaporation and pencil marks gave varying thicknesses and distributions of carbon on the surface, each with strong cathodic character with respect to the matrix, but causing no observable increase in corrosion. Pitting corrosion was shown to be associated with pre-existing second phase particles, especially in regions where pencil marking had disturbed the native oxide layer. An in situ technique for assessing the relative electrochemical behaviour of individual second phase particle types was sought to build on current knowledge of their chemical behaviour. Combinations of amperometric and potentiometric SECM methods were developed on model galvanic couple samples, before showing in situ cathodic behaviour of copper and iron containing second phase particles in a beryllium analogue sample, 7075 aluminium alloy (AA7075-T6), in agreement with the well characterised corrosion mechanism for these particle types. Preliminary results were obtained for S-65 beryllium, highlighting the capability of the method to resolve larger scale anodic features.
Failure of current standard pipeline materials in chloride processing environments has a high environmental and economic impact. Tantalum (Ta) is resistant to attack in HCl environments due to its passive oxide layer. Solid and lined Ta components are, however, prohibitively expensive for large-scale deployment. Ta chemical vapour deposition (CVD) coatings on standard carbon steel pipeline materials offer an economic solution with superior properties but have been demonstrated to exhibit a dual phase nature with a metastable, hard, and brittle beta (β) phase, and a ductile alpha (α) phase, and also detrimental interfacial characteristics. This project describes the deposition of bulk α-Ta CVD coatings on standard 0.3 – 0.35 wt.%Cmax pipelines and components. The potential for tailoring of CVD processes such coatings were investigated using a hybrid statistical design of experiment approach. Deposition rate, phase fraction and Knoop hardness of coatings was measured in response to varying deposition temperature, reagent gas flows, and sample position and finish. Statistical analysis identified deposition temperature, hydrogen (H2) reagent and argon (Ar) carrier gas flows as dominant parameters. Optimal levels for the desired properties were obtained and then applied to newly installed large-scale equipment. Ta coatings on higher carbon content steels demonstrated reduced interfacial defects and a reduction in interdiffusion zone thickness. Optical micrographs using an alternate oxalic acid based etch and EBSD highlight the distribution of the deposited α/β-Ta phase within coatings. β-Ta shows preferential positioning at the surface and in the bulk, and α-Ta at the interface. XRD diffraction data is used to calculate approximate coating phase fractions that ranged from 5 – 96%. Heat treatments identified a β→α transition temperature of >950°C, correlated through exothermic peak detection in DSC analysis. Coating adhesion improved with the reduction of interfacial defects and the application of the heat treatment to minimise β-Ta content. Three-point bend testing investigated the performance of single/dual Ta coatings 20-80µm thick on A105 steel with varying post-deposition heat treatments. Single layer coatings, and those with singular heat treatments, suffered delamination on test. Samples with dual Ta coatings that had a heat treatment after both deposition processes had no visible or acoustic sign of failure after bending through 90° demonstrating ductility of the coating. Coating integrity was proved through HCl immersion testing with no measurable mass loss detected. Initial Ta coated mild steel had a corrosion rate of 11.4 MPY (0.26 mm/yr). Using the preferred two-stage deposition and heat treatment process C-ring corrosion testing of stressed A105 steel samples resulted in a ‘nil’ corrosion rate of 0.06 MPY (0.0014 mm/yr) equivalent to that of solid Ta.
Maximising X-ray flux into a focussed spot during analysis by X-ray photoelectron spectroscopy (XPS) is often beneficial. Excitation X-rays for the analysis are usually generated by directing an electron beam at a thin aluminium film. The limiting factor for X-ray flux is potential damage to the thin film by heat from the electron beam. XPS anodes are therefore water cooled. Some commercial instruments also employ a diamond heat spreader beneath the anode film. Diamond has the highest thermal conductivity of any known material, and a cost-effective source is polycrystalline material grown by chemical vapour deposition (CVD). CVD diamond has been shown to have a coefficient of thermal conduction (CTC) that is inhomogeneous and anisotropic. The CTC varies with depth beneath the growth face of the diamond and is greater perpendicular to the face than parallel to it. The CTC can vary by a factor of 5 or more. This is caused by the microstructure of the CVD diamond. This thesis explores the effect of this CTC variation on the performance of the material as a heat spreader in XPS anodes. Detailed finite element analysis of an XPS anode is undertaken, which for the first time incorporates diamond CTC that is temperature dependent, inhomogeneous and anisotropic, and heat delivery into the aluminium film that is distributed over a realistic depth. CVD diamond samples from different suppliers were characterised by white light interferometry (WLI), atomic force microscopy (AFM), scanning electron microscopy (SEM), laser scanning confocal microscopy (LSCM) and Raman microscopy. A test facility was commissioned to perform destructive testing to compare (i) anodes with and without a diamond heat spreader (ii) CVD diamond from different manufacturers and (iii) to compare the relative performance of the nucleation and growth faces. This testing was not completed. The nucleation and growth faces were easily distinguishable by WLI. Surface voids were examined in detail by SEM and appeared to result from polishing damage, incomplete growth or local contamination. LSCM revealed apparent internal voids in some samples. The volume fraction of internal voids is small, so heat transport is unlikely to be affected, but they might be an indicator of lower quality CVD diamond. The material with the most voids was the material with the most non-diamond peaks in the Raman spectrum. Surface voids could cause hot spots if the aluminium film does not have good contact with the underlying diamond. LSCM also identified a highly reflective distinct layer within some samples that could potentially impede heat transport within the diamond. The FEA results indicate that a CVD diamond heat spreader reduces the aluminium temperature from 613 °C to 301 °C with a 20 W load. Having the anode material applied to the nucleation face instead of the growth face gave a temperature of 575 °C. Attempts to improve cooling by making the anode tip wall thinner, or by having the water directly cool the diamond actually caused a small increase in the temperature. The maximum temperature attained is highly sensitive to the aluminium film thickness, the aluminium CTC and the diamond CTC. 65% of the heat was removed by the cooling water from the walls of the anode rather than the tip, regardless of whether a heat spreader is employed. Physical testing was hampered by repeated equipment failures. The performance of the first electron gun suffered due to poor component quality and poor design choices. Even so, an electron beam was extracted from the prototype and moderately focussed onto a phosphor screen. It proved impossible to apply more than 7.5 kV to the electron gun without causing a flashover. A new electron gun design is under way. The FEA results have important implications for anode design. A diamond heat spreader is shown to be effective, but the choice of diamond face is critical. As most of the cooling occurs at the walls of the anode, the return route of the cooling water must fully exploit the area available by passing over as much wall area as possible. A narrow conduit for the effluent could miss important cooling opportunities. Attempts to bring the cooling fluid closer to the hot spot by thinning the tip impedes heat flow to the area where most heat is removed and leads to a temperature increase.
Silicon carbide (SiC) monofilaments are high strength, continuous ceramic fibres produced through chemical vapour deposition (CVD) and used to reinforce metal matrix composites. Such composites have excellent mechanical properties. However, they are expensive to manufacture and the monofilaments must be highly reproducible to ensure reliability of the resulting composite. TISICS Ltd are the sole producers of the material outside of the United States of America and have recently developed two new monofilaments, SM3256 (140 µm diameter) and SM3240 (100 µm diameter) with enhanced mechanical properties and reduced cost of production. These monofilaments and composite panels have been evaluated through tensile testing. They have been found to be highly reproducible over three years of production with the monofilaments possessing an average tensile strength of 4.0±0.2 GPa with a Weibull modulus of 50±10. Recent advances in plasma focussed ion beam (PFIB) milling techniques and scanning transmission electron microscopy (STEM) have been exploited to produce specimens revealing the interior of the monofilaments with unparalleled detail and precision. Raman spectroscopy and Auger spectroscopy have been used to characterise the microstructure and composition of the monofilaments and inform their development. The process for depositing a protective coating on the monofilaments has been improved, resulting in a 17% decrease in the total cost of CVD feedstock chemicals required. Previously unobserved nanoscale voids in the tungsten filament substrate have been identified as a critical process variable potentially responsible for the narrow strength distribution of the monofilaments. Analysis of the monofilament microstructures has indicated the potential for increasing the production speed of SM3256. Experimental trials have resulted in up to 75% faster production however a resulting decrease in performance demonstrates that further work is necessary. This research has resulted in significant cost reductions and has improved the economic viability of the monofilaments. The demonstration of reproducibility of the material properties has contributed to ongoing qualification for their use in aerospace components. The potential for further fundamental improvements to the process has been identified.
Kostoglou Nikolaos, Liao Chi-Wei, Wang Cheng-Yu, Kondo Junko N., Tampaxis Christos, Steriotis Theodore, Giannakopoulos Konstantinos, Kontos Athanassios G., Hinder Steven, Baker Mark, Bousser Etienne, Matthews Allan, Rebholz Claus, Mitterer Christian (2021)Effect of Pt nanoparticle decoration on the H2 storage performance of plasma-derived nanoporous graphene, In: CARBON171pp. 294-305
PERGAMON-ELSEVIER SCIENCE LTD
A nanoporous and large surface area (∼800 m2/g) graphene-based material was produced by plasma treatment of natural flake graphite and was subsequently surface decorated with platinum (Pt) nano-sized particles via thermal reduction of a Pt precursor (chloroplatinic acid). The carbon-metal nanocomposite showed a ∼2 wt% loading of well-dispersed Pt nanoparticles (<2 nm) across its porous graphene surface, while neither a significant surface chemistry alteration nor a pore structure degradation was observed due to the Pt decoration procedure. The presence of Pt seems to slightly promote the hydrogen sorption behavior at room temperature with respect to the pure graphene, thus implying the rise of “weak” chemisorption phenomena, including a potential hydrogen “spillover” effect. The findings of this experimental study provide insights for the development of novel graphene-based nanocomposites for hydrogen storage applications at ambient conditions.
Zhang Chunyang, Bhoyate Sanket, Zhao Chen, Kahol Pawan, Kostoglou Nikolaos, Mitterer Christian, Hinder Steven, Baker Mark, Constantinides Georgios, Polychronopoulou Kyriaki, Rebholz Claus, Gupta Ram (2019)Electrodeposited Nanostructured CoFe2O4 for Overall Water Splitting and Supercapacitor Applications, In: Catalysts9(2)
To contribute to solving global energy problems, a multifunctional CoFe2O4 spinel was synthesized and used as a catalyst for overall water splitting and as an electrode material for supercapacitors. The ultra-fast one-step electrodeposition of CoFe2O4 over conducting substrates provides an economic pathway to high-performance energy devices. Electrodeposited CoFe2O4 on Ni-foam showed a low overpotential of 270 mV and a Tafel slope of 31 mV/dec. The results indicated a higher conductivity for electrodeposited compared with dip-coated CoFe2O4 with enhanced device performance. Moreover, bending and chronoamperometry studies suggest excellent durability of the catalytic electrode for long-term use. The energy storage behavior of CoFe2O4 showed high specific capacitance of 768 F/g at a current density of 0.5 A/g and maintained about 80% retention after 10,000 cycles. These results demonstrate the competitiveness and multifunctional applicability of the CoFe2O4 spinel to be used for energy generation and storage devices.
In this study, a critical comparison between two low metal (Ni) loading catalysts is presented, namely Ni/Al2O3 and Ni/AlCeO3 for the glycerol steam reforming (GSR) reaction. The surface and bulk properties of the catalysts were evaluated using a plethora of techniques, such as N2 adsorption/desorption, ICP-AES, XRD, XPS, SEM/EDX, TEM, CO2-TPD, NH3-TPD, H2-TPR. Carbon deposited on the catalysts surfaces was probed using TPO, SEM and TEM. It is demonstrated that Ce-modification of Al2O3 induces an increase of the surface basicity and Ni dispersion. These features lead to a higher conversion of glycerol to gaseous products (60% to 80%), particularly H2 and CO2, enhancement of WGS reaction and a higher resistance to coke deposition. Allyl alcohol was found to be the main liquid product for the Ni/AlCeO3 catalyst, the production of which ceases over 700 oC. It is also highly significant that the Ni/AlCeO3 catalyst demonstrated stable values for H2 yield (2.9-2.3) and selectivity (89-81%), in addition to CO2 (75-67%) and CO (23-29%) selectivity during a (20h) long time-on-stream study. Following the reaction, SEM/EDX and TEM analysis showed heavy coke deposition over the Ni/Al2O3 catalyst, whereas for the Ni/AlCeO3 catalyst TPO studies showed the formation of more defective coke, the latter being more easily oxidized
AlKetbi M., Polychronopoulou K., Zedan Abdallah. F., Sebastián V., Baker Mark A., AlKhoori A., Jaoude M.A., Alnuaimi O., Hinder Steve S., Tharalekshmy Anjana, AlJaber Amina S. (2018)Tuning the activity of Cu-containing rare earth oxide catalysts for CO oxidation reaction: Cooling while heating paradigm in microwave-assisted synthesis, In: Materials Research Bulletin108pp. 142-150
(Ce-La-xCu)O2 catalysts with low (3 at.%) and high (10 at.%) Cu content were prepared by conventional microwave (MW) and enhanced microwave methods where air cooling (AC), while heating, was applied. The catalysts were tested for the CO oxidation reaction in the 25–500 °C range using 4%CO/20%O2/He feed gas. Varying spectroscopic, microscopic and catalytic studies were used to probe the effect of synthesis on the nanostructure and the CO oxidation performance. It was found that the synthesis method adopted impacts on the extent of the Cu doping into the (Ce-La)O2 fluorite lattice, hence leading to one and two phases system in the case of catalyst prepared through enhanced (AC) and conventional (MW) microwave methods, respectively. Furthermore, only Ce4+ species were found on the surface of the (Ce-La-10Cu)O2 catalysts synthesized using MW and AC (XPS studies), whereas oxygen vacant sites which are associated with Ce3+ ions were indicated in the sub-surface/bulk (Raman studies). Ultimately, the catalysts with the low and high Cu loading, prepared under the AC-promoted microwave method, presented a superior performance against CO oxidation, exhibiting an overall improvement of the catalytic activity by 16% and 32%, respectively.
Photocatalytic experiments on the pharmaceutical pollutant carbamazepine (CBZ) were conducted using sol-gel nitrogen-doped TiO-coated glass slides under a solar simulator. CBZ was stable to photodegradation under direct solar irradiation. No CBZ sorption to the catalyst surface was observed, as further confirmed by surface characterization using X-ray photoelectron spectroscopic analysis of N-doped TiO surfaces. When exposing the catalyst surface to natural organic matter (NOM), an excess amount of carbon was detected relative to controls, which is consistent with NOM remaining on the catalyst surface. The catalyst surface charge was negative at pH values from 4 to 10 and decreased with increasing pH, correlated with enhanced CBZ removal with increasing medium pH in the range of 5-9. A dissolved organic carbon concentration of 5mg/L resulted in ∼20% reduction in CBZ removal, probably due to competitive inhibition of the photocatalytic degradation of CBZ. At alkalinity values corresponding to CaCO addition at 100mg/L, an over 40% decrease in CBZ removal was observed. A 35% reduction in CBZ occurred in the presence of surface water compared to complete suppression of the photocatalytic process in wastewater effluent. © 2012 Elsevier B.V.
In the study presented herein, the catalytic activity and stability of a Ni catalyst supported on Y2O3–ZrO2 was examined for the first time in the glycerol steam reforming reaction and compared with a Ni/ZrO2. The addition of Y2O3 stabilized the ZrO2 tetragonal phase, increased the O2 storage capacity of the support and the medium strength acid sites of the catalyst, and although the Ni/Zr catalyst had a higher concentration of basic sites, the Ni/YZr presented more stable monodentate carbonates. Moreover, the Ni/YZr had substantially higher Ni surface concentration and smaller Ni particles. These properties influence the gaseous products’ distribution by increasing the H2 yield and selectivity and preventing the transformation of CO2 to CO, by inhibiting the reverse water gas shift (RWGS) reaction from taking place. For both catalysts the main liquid products identified were allyl alcohol, acetaldehyde, acetone, acrolein, acetic acid and acetol; these were subsequently quantified. The time-on-stream experiments showed that the Ni/YZr was more stable during reaction and had a higher H2 yield after 20 h (2.17 in comparison to 1.50 mol H2/mol C3H8O3, for the Ni/Zr). Extensive investigation of the carbon deposits showed that although lower amounts of coke were deposited on the Ni/Zr catalyst, these structures were more graphitic in nature and had fewer defects, which means they were harder to oxidize. Moreover, transmission electron microscopy (TEM) analysis showed that sintering of Ni nanoparticles during the reaction was significant for the Ni/Zr catalyst, as the mean particle diameter increased from an initial value of 48.2 to 67.9 nm, while it was almost absent on the Ni/YZr catalyst (the mean particle diameter increased from 42.1 to 47.4 nm).
Papageridis K.N, Charisiou N.D, Douvartzides S.L, Sebastian V., Hinder S.J, Baker M.A, AlKhoori S., Goula M.A, Polychronopoulou K. Effect of operating parameters on the selective catalytic deoxygenation of palm oil to produce renewable diesel over Ni supported on Al2O3, ZrO2 and SiO2 catalysts, In: Fuel Processing Technology
The present work investigated the production of Green Diesel through the deoxygenation of palm oil over Ni catalysts supported on γ-Αl2O3, ZrO2 and SiO2 for a continuous flow fixed bed reactor. A comprehensive experimental study was carried out in order to examine the effects of temperature, pressure, LHSV and H2/oil feed ratio on catalytic activity during short (6 h) and long (20 h) time-on-stream experiments. The catalysts were prepared through the wet impregnation method (8 wt.% Ni) and were extensively characterized by N2 adsorption/desorption, XRD, NH3-TPD, CO2-TPD, H2-TPD, H2-TPR, XPS, TEM/HR-TEM and Raman. The characterization of the materials prior to reaction revealed that although relatively small Ni nanoparticles were achieved for all catalysts (4.3 ± 1.6 nm, 6.1 ± 1.8 nm and 6.0 ± 1.8 nm for the Ni/Al2O3, Ni/ZrO2 and Ni/SiO2 catalysts, respectively), NiO was better dispersed on the Ni/ZrO2 catalyst, while the opposite was true for the Ni/SiO2 sample. In the case of Ni/Al2O3, part of Ni could not participate in the reaction due to its entrapment in the NiAl2O4 spinel phase. Regarding performance, although an increase in H2 pressure led to increases in paraffin conversion, the increase of temperature was beneficial only up to a critical value which differed for each catalytic system under consideration (375 oC, 300 oC and 350 oC for the Ni/Al2O3, Ni/ZrO2 and Ni/SiO2 catalysts, respectively). All catalysts favored the deCO2 and deCO deoxygenation paths much more extensively than HDO, irrespective of testing conditions. Time-on-stream experiments showed that all catalysts deactivated after about 6 h, which was attributed to the sintering of the Ni particles and/or their covering by a thin graphitic carbon shell.
Advanced hybrid joints, which incorporate a specially designed array of macro-scale pins that provide mechanical interlocking reinforcement, have been developed in order to address the challenges associated with joining fibre reinforced composites to metals. In the present work, important joint characteristics including strength, mechanical fatigue, damage tolerance and durability have been studied and discussed. The results indicate that with advanced hybrid joints it is possible to achieve the benefits of the respective bonded and bolted systems but with virtually zero net weight gain, or conceivably a weight reduction as the increased performance of the hybrid scheme could facilitate smaller joints. The authors also present initial results from a comprehensive manufacturing and scalability trial, and demonstrate that low-cost, large-scale manufacture of hybrid joints is now feasible.
Ultra-thin CdTe:As/Cd1-xZnxS photovoltaic solar cells with an absorber thickness of 0.5 µm were deposited by metal-organic chemical vapour deposition on indium tin oxide coated boro-aluminosilicate substrates. The Zn precursor concentration was varied to compensate for Zn leaching effects after CdCl2 activation treatment. Analysis of the solar cell composition and structure by X-ray photoelectron spectroscopy depth profiling and X-ray diffraction showed that higher concentrations of Zn in the Cd1-xZnxS window layer resulted in suppression of S diffusion across the CdTe/Cd1-xZnxS interface after CdCl2 activation treatment. Excessive Zn content in the Cd1-xZnxS alloy preserved the spectral response in the blue region of the solar spectrum, but increased series resistance for the solar cells. A modest increase in the Zn content of the Cd1-xZnxS alloy together with a post-deposition air anneal resulted in an improved blue response and an enhanced open circuit voltage and fill factor. This device yielded a mean efficiency of 8.3% over 8 cells (0.25 cm2 cell area) and best cell efficiency of 8.8%.
PET web samples have been treated by magnetically enhanced glow discharges powered using either medium frequency pulse direct current (p-DC) or low frequency high power pulse (HIPIMS) sources. The plasma pre-treatment processes were carried out in an Ar–O2 atmosphere using either Cu or Ti sputter targets. XPS, AFM and sessile drop water contact angle measurements have been employed to examine changes in surface chemistry and morphology for different pre-treatment process parameters. Deposition of metal oxide onto the PET surface is observed as a result of the sputter magnetron-based glow discharge web treatment. Using the Cu target, both the p-DC and HIPIMS processes result in the formation of a thin CuO layer (with a thickness between 1 and 11 nm) being deposited onto the PET surface. Employing the Ti target, both p-DC and HIPIMS processes give rise to a much lower concentration of Ti (< 5 at.%), in the form of TiO2 on the PET treated surface. The TiO2 is probably distributed as an island-like distribution covering the PET surface. Presence of Cu and Ti oxide constituents on the treated PET is beneficial in aiding the adhesion but alone (i.e. without oxygen plasma activation) is not enough to provide very high levels of hydrophilicity as is clear from sessile drop water contact angle measurements on aged samples. Exposure to the plasma treatments leads to a small amount of roughening of the substrate surface, but the average surface roughness in all cases is below 2.5 nm. The PET structure at the interface with a coating is mostly or wholly preserved. The oxygen plasma treatment, metal oxide deposition and surface roughening resulting from the HIPIMS and p-DC treatments will promote adhesion to any subsequent thin film that is deposited immediately following the plasma treatment.
Gravani S, Polychronopoulou K, Stolojan V, Cui Q, Gibson PN, Hinder SJ, Gu Z, Doumanidis CC, Baker MA, Rebholz C (2010)Growth and characterization of ceria thin films and Ce-doped gamma-Al2O3 nanowires using sol-gel techniques, In: NANOTECHNOLOGY21(46)465606
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γ-Al2O3 is a well known catalyst support. The addition of Ce to γ-Al2O3 is known to beneficially retard the phase transformation of γ-Al2O3 to α-Al2O3 and stabilize the γ-pore structure. In this work, Ce-doped γ-Al2O3 nanowires have been prepared by a novel method employing an anodic aluminium oxide (AAO) template in a 0.01 M cerium nitrate solution, assisted by urea hydrolysis. Calcination at 500 °C for 6 h resulted in the crystallization of the Ce-doped AlOOH gel to form Ce-doped γ-Al2O3 nanowires. Ce3 + ions within the nanowires were present at a concentration of < 1 at.%. On the template surface, a nanocrystalline CeO2 thin film was deposited with a cubic fluorite structure and a crystallite size of 6–7 nm. Characterization of the nanowires and thin films was performed using scanning electron microscopy, transmission electron microscopy, electron energy loss spectroscopy, x-ray photoelectron spectroscopy and x-ray diffraction. The nanowire formation mechanism and urea hydrolysis kinetics are discussed in terms of the pH evolution during the reaction. The Ce-doped γ-Al2O3 nanowires are likely to find useful applications in catalysis and this novel method can be exploited further for doping alumina nanowires with other rare earth elements.
© 2014 American Vacuum Society.A standard 30 nm thick Ta2O5 oxide layer grown on Ta was examined by XPS after Arn+ cluster ion bombardment at ion energies of 4 keV, 5 keV and 6 keV, with a cluster size of 1000 atoms. The reduction of Ta2O5, resulting from the preferential sputtering of oxygen after ion beam bombardment at different energies has been investigated. Survey spectra, C 1s, Ta 4f and O 1s spectra are presented for each profile at three stages: native surface, after reaching the steady-state oxide composition, and from the underlying metal substrate. It is necessary to reach a voltage of 6 keV to obtain a good sputter rate. The use of the cluster source seems to be promising to reduce the preferential sputtering phenomenon.
This paper reports on the structure and mechanical properties of ~ 2 μm thick nanocomposite (nc-) Ti(N,C)/amorphous diamond like carbon (a-C:H) coatings deposited on 100Cr6 steel substrates, using low temperature (~ 200 °C) DC reactive magnetron sputtering. The carbon content was varied with acetylene partial pressure in order to obtain single layer coatings with different a-C:H carbon phase fractions. The nanocrystalline Ti(N,C) phase is approximately stoichiometric for all coatings and the a-C:H phase fraction increases from 31 to 47 at.% as the coatings stoichiometry changed from TiC1.34 N0.51 to TiC2.48 N0.48, respectively. TiC1.34 N0.51 coatings showed the highest nanoindentation hardness (H) of ~ 14 GPa and a modulus (Er) of ~ 144 GPa; H reduced to < 6 GPa and Er to < 70 GPa for TiC2.48 N0.48 coatings. nc-Ti(N,C)/a-C:H coatings are promising candidates for applications where better matching of the modulus between a relatively low modulus substrate, hard loading support layer and low modulus-high H/E ratio top layer is required.
TiAlBN coatings have been deposited by electron beam (EB) evaporation from a single TiAlBN material source onto AISI 316 stainless steel substrates at a temperature of 450 °C and substrate bias of − 100 V. The stoichiometry and nanostructure have been studied by X-ray photoelectron spectroscopy, X-ray diffraction and transmission electron microscopy. The hardness and elastic modulus were determined by nanoindentation. Five coatings have been deposited, three from hot-pressed TiAlBN material and two from hot isostatically pressed (HIPped) material. The coatings deposited from the hot-pressed material exhibited a nanocomposite nc-(Ti,Al)N/a-BN/a-(Ti,Al)B2 structure, the relative phase fraction being consistent with that predicted by the equilibrium Ti–B–N phase diagram. Nanoindentation hardness values were in the range of 22 to 32 GPa. Using the HIPped material, coating (Ti,Al)B0.29N0.46 was found to have a phase composition of 72–79 mol.% nc-(Ti,Al)(N,B)1 − x+ 21–28 mol.% amorphous titanium boride and a hardness of 32 GPa. The second coating, (Ti,Al)B0.66N0.25, was X-ray amorphous with a nitride+boride multiphase composition and a hardness of 26 GPa. The nanostructure and structure–property relationships of all coatings are discussed in detail. Comparisons are made between the single-EB coatings deposited in this work and previously deposited twin-EB coatings. Twin-EB deposition gives rise to lower adatom mobilities, leading to (111) (Ti,Al)N preferential orientation, smaller grain sizes, less dense coatings and lower hardnesses.
Tsotsos C, Polychronopoulou K, Demas NG, Constantinides G, Gravani S, Böbel K, Baker MA, Polycarpou AA, Rebholz C (2010)Mechanical and high pressure tribological properties of nanocrystalline Ti(N,C) and amorphous C:H nanocomposite coatings, In: Diamond and Related Materials19(7-9)pp. 960-963
This paper reports on the mechanical and high pressure tribological properties of nanocrystalline (nc-) Ti(N,C)/amorphous (a-) C:H deposited, using low temperature (∼ 200 °C) DC reactive magnetron sputtering. The mechanical properties are affected by the nc-Ti(N,C)/a-C:H phase fraction ratio. For increasing C contents (from 31 to 47 at.%) an increase of the a-C:H phase content and a degradation of the nanocrystalline phase occurs leading to a reduction in nanoindentation hardness (H) values (from 15 to 9 GPa) and reduced modulus (Er) values (from 150 to 80 GPa). A strong correlation between H/E ratio and wear performance was exhibited by the coatings. The synthesized coatings survived up to 100 m sliding distance when tested using pin-on-disc sliding configuration at > 4.5 GPa contact pressures and the measured friction coefficient values were similar for all films (μ ∼ 0.21–0.25).
© 2014 American Vacuum Society.A standard 30 nm thick Ta2O5 oxide layer grown on Ta was examined by XPS after Ar+ ion bombardment at ion energies of 200 eV, 500 eV, and 3 keV. The reduction of Ta2O5, resulting from the preferential sputtering of oxygen after ion beam bombardment at different energies has been investigated. Survey spectra, C 1s, Ta 4f and O 1s spectra are presented for each profile at three stages: native surface, after reaching the steady-state oxide composition, and from the underlying metal substrate. Reducing the Ar+ energy from 3 keV to 200 eV makes no substantial difference in the degree of Ta2O5 reduction observed following ion bombardment.
Copyright © 2014 John Wiley & Sons, Ltd. Cerium-based conversion coatings are being investigated as alternatives to chromating treatments for the corrosion protection of aluminium and its alloys because of the environmentally unfriendly nature of the chromating process. This study investigates the surface film composition, structure and corrosion performance following a two-step surface treatment for an AA2024-T3 clad aluminium alloy. The two-step treatment comprised of an initial cerium conversion process involving immersion in an aqueous solution containing Ce3+ ions at 75°C followed by immersion in a propylene glycol solution at 75°C. The coating surface morphology, composition and structure have been studied using SEM, XPS, Auger spectroscopy and Fourier transform infrared spectroscopy, while corrosion resistance was evaluated using electrochemical impedance spectroscopy. The coating formed by the two-step treatment is an interconnecting fibrous (pseudo) boehmite layer with the incorporation of Ce3+ in the film. This two step treatment coating exhibits high impedance compared with the coatings formed through exposure to just stage 1 or stage 2 of the two-step treatment and shows good potential for improved corrosion protection.
Surface-sensitive techniques have been employed to characterise a model polymer substrate surface, poly(ethylene terephthalate) (PET), after a reactive sputter pre-treatment using magnetically enhanced Cu or Ti sputter targets in a mixed Ar-O glow discharge plasma. The plasmas are produced using either medium-frequency pulsed direct current (p-DC) or low-frequency high power impulse (HIPIMS) sources. X-ray photoelectron spectroscopy (XPS), time-of-flight secondary ion mass spectrometry (ToF-SIMS) and sessile drop water contact angles were employed to investigate changes in PET surface chemistry and properties following surface modification using different p-DC and HIPIMS process parameters. The XPS results indicate that the chemical composition of plasma-treated PET surfaces (p-DC or HIPIMS) depends strongly on the processing parameters employed such as sputter target material, magnetic array type and power supply technology. XPS results demonstrate that the sputter target material employed is of primary importance as it dictates the quantity of metal deposited/implanted into the PET surface. XPS results show that the use of a Cu target resulted in ∼ 31-35 at.% of Cu incorporated into the PET surface (as CuO), while the use of a Ti target resulted in only 1-4 at. % incorporation (as TiO ). The SIMS spectra and XPS depth profiles of Cu-treated PET indicate that the CuO has formed a discrete film at the surface, offering predominant or total coverage of the underlying PET. However, for Ti-treated PET, both PET and Ti SIMS peaks are observed, and the XPS C1s peak shape is characteristic of PET, indicating that Ti has not formed a discrete film, but instead TiO species have been incorporated, probably as an island-like distribution into the surface of the PET. The formation of CuO and TiO on the PET surface leads to a reduction in the contact angle compared to native PET. Hence, both p-DC and HIPIMS reactive plasma pre-treatments result in a more hydrophilic surface, promoting adhesion and offering a flexible means to introduce a wide range of surface chemistries and properties to polymeric surfaces. Copyright © 2012 John Wiley & Sons, Ltd. Copyright © 2012 John Wiley & Sons, Ltd.
Indium tin oxide (ITO) thin films with a specific resistivity of 3.5 × 10− 4 Ω cm and average visible light transmission (VLT) of 90% have been reactively sputtered onto A4 Polyethylene terephthalate (PET), glass and silicon substrates using a remote plasma sputtering system. This system offers independent control of the plasma density and the target power enabling the effect of the plasma on ITO properties to be studied. Characterization of ITO on glass and silicon has shown that increasing the plasma density gives rise to a decrease in the specific resistivity and an increase in the optical band gap of the ITO films. Samples deposited at plasma powers of 1.5 kW, 2.0 kW and 2.5 kW and optimized oxygen flow rates exhibited specific resistivity values of 3.8 × 10− 4 Ω cm, 3.7 × 10− 4 Ω cm and 3.5 × 10− 4 Ω cm and optical gaps of 3.48 eV, 3.51 eV and 3.78 eV respectively. The increase in plasma density also influenced the crystalline texture and the VLT increased from 70 to 95%, indicating that more oxygen is being incorporated into the growing film. It has been shown that the remote plasma sputter technique can be used in an in-line process to produce uniform ITO coatings on PET with specific resistivities of between 3.5 × 10− 4 and 4.5 × 10− 4 Ω cm and optical transmission of greater than 85% over substrate widths of up to 30 cm.
CNTs can have the ability to act as compliant small-scale springs or as shock resistance micro-contactors. This work investigates the performance of vertically-aligned CNTs (VA-CNTs) as micro-contactors in electromechanical testing applications for testing at wafer-level chip-scale-packaging (WLCSP) and wafer-level-packaging (WLP). Fabricated on ohmic substrates, 500-μm-tall CNT-metal composite contact structures are electromechanically characterized. The probe design and architecture are scalable, allowing for the assembly of thousands of probes in short manufacturing times, with easy pitch control. We discuss the effects of the metallization morphology and thickness on the compliance and electromechanical response of the metal-CNT composite contacts. Pd-metallized CNT contactors show up to 25 μm of compliance, with contact resistance as low as 460 mΩ (3.6 kΩ/μm) and network resistivity of 1.8 × 10−5 Ω cm, after 2500 touchdowns, with 50 μm of over-travel; they form reproducible and repeatable contacts, with less than 5% contact resistance degradation. Failure mechanisms are studied in-situ and after cyclic testing and show that, for top-cap-and-side metallized contacts, the CNT-metal shell provides stiffness to the probe structure in the elastic region, whilst reducing the contact resistance. The stable low resistance achieved, the high repeatability and endurance of the manufactured probes make CNT micro-contacts a viable candidate for WLP and WLCSP testing.
X-ray photoelectron spectroscopy (XPS) was carried out to analyse a commercially available pentanedioic acid powder. XPS spectra were obtained using incident monochromatic Al Ka radiation at 1486.6 eV. A survey spectrum together with O 1s and C 1s core level spectra are presented.
This is the second of two volumes focusing on the principal analytical techniques forcharacterizing metal alloys, semiconductors, polymers, and ceramics.
Recent interest in the fields of human motion monitoring, electronic skin, and human–machine interface technology demands strain sensors with high stretchability/compressibility (ε > 50%), high sensitivity (or gauge factor (GF > 100)), and long-lasting electromechanical compliance. However, current metal- and semiconductor-based strain sensors have very low (ε < 5%) stretchability or low sensitivity (GF < 2), typically sacrificing the stretchability for high sensitivity. Composite elastomer sensors are a solution where the challenge is to improve the sensitivity to GF > 100. We propose a simple, low-cost fabrication of mechanically compliant, physically robust metallic carbon nanotube (CNT)-polydimethylsiloxane (PDMS) strain sensors. The process allows the alignment of CNTs within the PDMS elastomer, permitting directional sensing. Aligning CNTs horizontally (HA-CNTs) on the substrate before embedding in the PDMS reduces the number of CNT junctions and introduces scale-like features on the CNT film perpendicular to the tensile strain direction, resulting in improved sensitivity compared to vertically-aligned CNT-(VA-CNT)-PDMS strain sensors under tension. The CNT alignment and the scale-like features modulate the electron conduction pathway, affecting the electrical sensitivity. Resulting GF values are 594 at 15% and 65 at 50% strains for HA-CNT-PDMS and 326 at 25% and 52 at 50% strains for VA-CNT-PDMS sensors. Under compression, VA-CNT-PDMS sensors show more sensitivity to small-scale deformation than HA-CNT-PDMS sensors due to the CNT orientation and the continuous morphology of the film, demonstrating that the sensing ability can be improved by aligning the CNTs in certain directions. Furthermore, mechanical robustness and electromechanical durability are tested for over 6000 cycles up to 50% tensile and compressive strains, with good frequency responses with negligible hysteresis. Finally, both types of sensors are shown to detect small-scale human motions, successfully distinguishing various human motions with reaction and recovery times of as low as 130 ms and 0.5 s, respectively.
Charisiou N.D., Siakavelas G., Tzounis L., Sebastian V., Monzon A., Baker M.A., Hinder S.J., Polychronopoulou K., Yentekakis I.V., Goula M.A. An in depth investigation of deactivation through carbon formation during the biogas dry reforming reaction for Ni supported on modified with CeO2 and La2O3 zirconia catalysts, In: International Journal of Hydrogen Energy43(41)pp. 18955-18976
The dry reforming of biogas on a Ni catalyst supported on three commercially available materials (ZrO2, La2O3-ZrO2 and CeO2-ZrO2), has been investigated, paying particular attention to carbon deposition. The DRM efficiency of the catalysts was studied in the temperature range of 500-800oC at three distinct space velocities, and their time-on-stream stability at four temperatures (550, 650, 750 and 800oC) was determined for 10 or 50 h operation. The morphological, textural and other physicochemical characteristics of fresh and spent catalysts together with the amount and type of carbon deposited were examined by a number of techniques including BET-BJH method, CO2 and NH3-TPD, XPS, SEM, TEM, STEM-HAADF, Raman spectroscopy, and TGA/DTG. The impact of the La2O3 and CeO2 modifiers on the DRM performance and time-on-stream stability of the Ni/ZrO2 catalyst was found to be very beneficial: up to 20 and 30% enhancement in CH4 and CO2 conversions respectively, accompanied with a CO-enriched syngas product, while the 50 h time-on-stream catalytic performance deterioration of ~30-35% on Ni/ZrO2 was limited to less than ~15-20% on the La2O3 and CeO2 modified samples. Their influence on the amount and type of carbon formed was substantial: it was revealed that faster oxidation of the deposited carbon at elevated temperatures occurs on the modified catalysts. Correlations between the La2O3 and CeO2-induced modifications on the surface characteristics and physicochemical properties of the catalyst with their concomitant support-mediated effects on the overall DRM performance and carbon deposition were revealed.
In the present study, Ni/Ce-Sm-xCu (x = 5, 7, 10 at.%) catalysts were prepared using microwave radiation coupled with sol-gel and followed by wetness impregnation method for the Ni incorporation. Highly dispersed nanocrystallites of CuO and NiO on the Ce-Sm-Cu support were found. Increase of Cu content seems to facilitate the reducibility of the catalyst according to the H₂ temperature-programmed reduction (H₂-TPR). All the catalysts had a variety of weak, medium and strong acid/basic sites that regulate the reaction products. All the catalysts had very high XC3H8O3 for the entire temperature (400–750 °C) range; from ≈84% at 400 °C to ≈94% at 750 °C. Ni/Ce-Sm-10Cu catalyst showed the lowest XC3H8O3-gas implying the Cu content has a detrimental effect on performance, especially between 450–650 °C. In terms of H₂ selectivity (SH2) and H₂ yield (YH2), both appeared to vary in the following order: Ni/Ce-Sm-10Cu ˃ Ni/Ce-Sm-7Cu ˃ Ni/Ce-Sm-5Cu, demonstrating the high impact of Cu content. Following stability tests, all the catalysts accumulated high amounts of carbon, following the order Ni/Ce-Sm-5Cu ˂ Ni/Ce-Sm-7Cu ˂ Ni/Ce-Sm-10Cu (52, 65 and 79 wt.%, respectively) based on the thermogravimetric analysis (TGA) studies. Raman studies showed that the incorporation of Cu in the support matrix controls the extent of carbon graphitization deposited during the reaction at hand.
X-ray photoelectron spectroscopy (XPS) was carried out to analyse a commercially available propanedioic acid (malonic acid) powder. XPS spectra were obtained using incident monochromatic Al Ka radiation at 1486.6 eV. A survey spectrum together with O 1s and C 1s core level spectra are presented. The presence of characteristic carbon and oxygen photoelectrons peaks allows the use these results as a reference for dicarboxylic acids.
X-ray photoelectron spectroscopy (XPS) was carried out to analyse a commercially available butanenedioic acid (succinic acid) powder. XPS spectra were obtained using incident monochromatic Al Ka radiation at 1486.6 eV. A survey spectrum together with O 1s and C 1s core level spectra are presented. The presence of characteristic carbon and oxygen photoelectrons peaks allows the use these results as a reference for dicarboxylic acids.
Most current analysis of nano-indentation test data assumes the sample to behave as an isotropic, homogeneous body. In practice, engineering materials such as structural steels, titanium alloys and high strength aluminium alloys are multi-phase metals with microstructural length scales that can be the same order of magnitude as the maximum achievable nano-indentation depth. This heterogeneity results in considerable scatter in the indentation load-displacement traces and complicates inverse analysis of this data. To address this problem, an improved and optimised inverse analysis procedure to estimate bulk tensile properties of heterogeneous materials using a new ‘multi-objective’ function has been developed which considers nano-indentation data obtained from several indentation sites. The technique was applied to S355 structural steel bulk samples as well as an autogenously electron beam welded sample where there is a local variation of material properties. Using the new inverse analysis approach on the S355 bulk material resulted in an error within 3% of the experimental yield strength and strain hardening exponent data, which compares to an approximate 9% error in the yield strength and an 8% error in the strain hardening exponent using a more conventional approach to the inverse analysis method. Applying the new method to indentation data from different regions of an S355 steel weld and using this data as an input into an FE model of the cross-weld, tensile data from the FE model resulted matching the experimentally measured properties to within 5%, confirming the efficacy of the new inverse analysis approach.
CeO2 and CexSm1-xO2 nanoparticle mixed oxides have been synthesized by microwave assisted sol-gel (MW sol-gel) and conventional sol-gel (C sol-gel) synthesis carried out at 60oC (typical sol-gel) and 100oC (approaching the MW temperature). Different characterization techniques, namely, XRD, BET, Raman, SEM, FTIR, TEM, XPS, H2-TPR, CO2-TPD, and XPS have been employed to understand the process-structure-properties relationship of the catalysts. The CO oxidation performance has been determined both in the absence and in the presence of H2 in the feed gas stream. Microwave heating yields a more thermally stable precursor material, which preserves 75% of its mass up to 600oC, attributable to the different chemical nature of the precursor, compared to the typical sol-gel material with the same composition. Varying the synthesis method has no profound effect on the surface area of the materials, which is in the range 4-35m2/g. Conventional sol-gel synthesis performed at 60 and 100oC yields CeO2 particles with a crystallite size of 29 nm and 24 nm compared to 21-27 nm for MW sol-gel synthesis (at different power values). The MW sol-gel CexSm1-xO2 catalysts exhibit a smaller crystallite size (12-18 nm). The pure ceria nanoparticles were shown to have a stoichiometry of approximately CeO1.95. The presence of Ce3+ and Sm3+ in the mixed oxide particles facilitates the presence of oxygen vacant sites, confirmed by Raman. Oxygen mobile species have been traced using H2-TPR studies and a compressive lattice strain in the 0.45-1.9% range of the cubic CexSm1-xO2 lattice were found to be strongly correlated with the CO oxidation performance in the presence and absence of H2 in the oxidation feed stream. MW sol-gel synthesis led to more active CeO2 and Ce0.5Sm0.5O2 catalysts, demonstrated by T50 (temperature where 50% CO conversion is achieved), being reduced by 131 oC and 47 oC, respectively, compared to typical sol-gel catalysts. Conventional synthesis performed at 100oC leads to a CeO2 catalyst of initially higher activity at a certain temperature window (220-420oC), though with a slower increase of XCO with temperature compared to the MW one. MW sol-gel synthesized Ce0.8Sm0.2O2 exhibited a high performance (~90%) for CO oxidation over a period of more than 20 h in stream. In addition the effect of reaction temperature and contact time (W/F) on the activity of the CeO2-based materials for CO oxidation kinetics were investigated. The activation energy of the reaction was found to be in the 36-43 kJ/mole range depending on the catalyst composition.
The glycerol steam reforming (GSR) reaction for H2 production was studied comparing the performance of Ni supported on ZrO2 and SiO2-ZrO2 catalysts. The surface and bulk properties were determined by ICP, BET, XRD, TPD, TPR, TPO, XPS, SEM and STEM-HAADF. It was suggested that the addition of SiO2 stabilizes the ZrO2 monoclinic structure, restricts the sintering of nickel particles and strengthens the interaction between Ni2+ species and support. It also removes the weak acidic sites and increases the amount of the strong acidic sites, whereas it decreases the amount of the basic sites. Furthermore, it influences the gaseous products’ distribution by increasing H2 yield and not favouring the transformation of CO2 in CO. Thus, a high H2/CO ratio can be achieved accompanying by negligible value for CO/CO2. From the liquid products quantitative analysis, it was suggested that acetone and acetaldehyde were the main products for the Ni/Zr catalyst, for 750oC, whereas for the Ni/SiZr catalyst allyl alcohol was the only liquid product for the same temperature. It was also concluded that the Ni/SiZr sample seems to be more resistant to deactivation however, for both catalysts a substantial amount of carbon exists on the catalytic surface in the shape of carbon nanotubes and amorphous carbon.
Protective coatings have been deposited on electrogalvanized steel by immersion in solutions containing 2-Butyne-1.4-diol propoxylate (CHO), cerium nitrate, sodium nitrate and sodium sulphate for different immersion periods. The surface morphology and chemical composition of the coatings formed on the electrogalvanized steel were studied using field emission gun scanning electron microscopy, X-ray photoelectron spectroscopy and Fourier Transform Infrared Spectroscopy. The corrosion resistance of the electrogalvanized steel prior to and after surface treatment was investigated by electrochemical impedance spectroscopy in 0.1 mol L NaCl solution. The results were compared to the performance of a chromate conversion coating in the same solution. The coatings formed on the electrogalvanized steel surface showed the presence of a mixed organic/inorganic layer containing CeO and CeO which improved the corrosion resistance of the substrate and showed a superior corrosion resistance to that provided by a chromate conversion coating.
Rix Michael, Baker Mark, Whiting Mark, Durman Ray P, Shatwell Robert A (2017)An Improved Silicon Carbide Monofilament for the Reinforcement of Metal Matrix Composites, In: Meyers M, Benavides HAC, Brühl SP, Colorado HA, Dalgaard E, Elias CN, Figueiredo RB, Mangalaraja RV, Garcia-Rincon O, Kawasaki M, Langdon TG, Marroquin MCG, da Cunha Rocha A, Schoenung JM, Costa e Silva A, Wells M, Yang W (eds.), Proceedings of the 3rd Pan American Materials Congresspp. 317-324
As part of ongoing research in the UK, TISICS have developed an improved 140 µm carbon coated silicon carbide monofilament for the reinforcement of metal matrix composites. The monofilament is fabricated in a single reactor using a high speed chemical vapor deposition process at a rate of 8 m/min (26 ft/min). Statistical analysis of monofilament properties over two years of production has demonstrated excellent reproducibility of the process. The monofilaments have an average tensile strength of 4.0 ± 0.2 GPa with a Weibull modulus of 50 ± 10. Composites incorporating the monofilaments show similar low variability in yield and tensile strength with the latter exhibiting a mean value above 90% of the maximum theoretical strength predicted by the rule of mixtures. By varying the volume fraction and orientation of the monofilament reinforcement, composite properties can be tailored to fit design requirements. Examples are given of demonstrator components made for the European aerospace sector.
The performance of single crystal CdZnTe radiation detectors is dependent on both the bulk and the surface properties of the material. After single crystal fabrication and mechanical polishing, modification of the surface to remove damage and reduce the surface leakage current is generally achieved through chemical etching followed by a passivation treatment. In this work, CdZnTe single crystals have been chemically etched using a bromine in methanol (BM) treatment. The BM concentrations employed were 0.2 and 2.0 (v/v) % and exposure times varied between 5 and 120 s. Angle resolved XPS and sputter depth profiling has been employed to characterize the surfaces for the different exposure conditions. A Te rich surface layer was formed for all exposures and the layer thickness was found to be independent of exposure time. The enriched Te layer thickness was accurately determined by calibrating the sputter rate against a CdTe layer of known thickness. For BM concentrations of 0.2 (v/v) % and 2 (v/v) %, the Te layer thickness was determined to be 1.3 ± 0.2 and 1.8 ± 0.2 nm, respectively. The BM etched surfaces have subsequently been passivated in a 30 wt.% HO solution employing exposure time of 15 s. The oxide layer thickness has been calculated using two standard XPS methodologies, based on the Beer-Lambert expression. The TeO thickness calculated from ARXPS data are slightly higher than the thickness obtained by the simplified Beer-Lambert expression. For BM exposures of 30-120 s followed by a passivation treatment of 30 wt. % HO solution employing an exposure time 15 s, the ARXPS method gave an average TeO thickness value of 1.20 nm and the simplified Beer-Lambert expression gave an average thickness value of 0.99 nm. © 2012 Elsevier B.V. All rights reserved.
With the progression towards higher aspect ratios and finer topographical dimensions in many micro- and nano-systems, it is of technological importance to be able to conformally deposit thin films onto such structures. Sputtering techniques have been developed to provide such conformal coverage through a combination of coating re-sputtering and ionised physical vapour deposition (IPVD), the latter by use of a secondary plasma source or a pulsed high target power (HiPIMS). This paper reports on the use of an alternate remote plasma sputtering technique in which a high density (>1013 cm-3) magnetised plasma is used for sputter deposition, and additionally is shown to provide IPVD and a re-sputtering capability. From the substrate I-V characteristics and optical emission spectroscopy (OES) data, it is shown that remote plasma sputtering is an inherently continuous IPVD process (without the need of a secondary discharge). Through the reactive deposition of Al2O3 onto complex structures, scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX) results demonstrate that applying a negative substrate bias during film growth can result in re-sputtering of deposited material and film growth on surfaces obscured from the initial sputter flux. Using 5:1 (height:width) aspect ratio trenches, the substrate bias was set to 0, -245 and -334 V. At 0 V substrate bias, the alumina coating is predominantly deposited on the horizontal surfaces; at -344 V, it is predominantly deposited onto the side walls and at -245 V a more uniform layer thickness is obtained over the trench. The process was optimised further by alternating the substrate bias between -222 and -267 V, with a 50 % residence time at each voltage, yielding a more uniform conformal coverage of the 5:1 aspect ratio structures over large areas.
Polychronopoulou K, Baker MA, Rebholz C, Neidhardt J, O'Sullivan M, Reiter AE, Kanakis K, Leyland A, Matthews A, Mitterer C (2009)The nanostructure, wear and corrosion performance of arc-evaporated CrBxNy nanocomposite coatings, In: SURFACE & COATINGS TECHNOLOGY204(3)pp. 246-255
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A thermodynamic study of the adsorption of an epoxy acrylate resin used for UV-cured coatings on two different anticorrosion pretreatments on aluminium alloys relevant to aerospace industry has been undertaken. Aluminium alloy Al2219 specimens, treated with an inorganic chromate based conversion coating (Alodine 1200S) and an organic titanium based conversion coating (Nabutan STI/310), were immersed in solutions of different concentrations of the resin and adsorption isotherms were determined by assessing the uptake of the adsorbate, as a function of solution concentration, by time-of-flight secondary ion mass spectrometry (ToF-SIMS). The results show different behaviour for the two substrates, which can be attributed to the organic component of the titanium based coating. In the case of the inorganic conversion coating a clear plateau is achieved at relatively low concentrations and at a lower level of adsorption than for the hybrid coating. The data for both the coatings conform well to the Langmuir model, the organic coating, as well as showing a higher level of adsorption of the resin, also presents oscillatory behaviour at low concentration, which is shown to be complementary to the behaviour of the reactive diluent included with the epoxy acrylate to aid processing. A discussion of this competitive adsorption of the epoxy resin and the diluent on the different substrates is presented, based on considerations of the chemistry of the systems under investigation.
Duarte DD, Bell SJ, Lipp J, Schneider A, Seller P, Veale MC, Wilson MD, Baker MA, Sellin PJ, Kachkanov V, Sawhney KJS (2013)Edge effects in a small pixel CdTe for X-ray imaging, In: JOURNAL OF INSTRUMENTATION8ARTN Ppp. ?-?
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Large area detectors capable of operating with high detection efficiency at energies above 30 keV are required in many contemporary X-ray imaging applications. The properties of high Z compound semiconductors, such as CdTe, make them ideally suitable to these applications. The STFC Rutherford Appleton Laboratory has developed a small pixel CdTe detector with 80×80 pixels on a 250 µm pitch. Historically, these detectors have included a 200 µm wide guard band around the pixelated anode to reduce the effect of defects in the crystal edge. The latest version of the detector ASIC is capable of four-side butting that allows the tiling of N×N flat panel arrays. To limit the dead space between modules to the width of one pixel, edgeless detector geometries have been developed where the active volume of the detector extends to the physical edge of the crystal. The spectroscopic performance of an edgeless CdTe detector bump bonded to the HEXITEC ASIC was tested with sealed radiation sources and compared with a monochromatic X-ray micro-beam mapping measurements made at the Diamond Light Source, U.K. The average energy resolution at 59.54 keV of bulk and edge pixels was 1.23 keV and 1.58 keV, respectively. 87% of the edge pixels present fully spectroscopic performance demonstrating that edgeless CdTe detectors are a promising technology for the production of large panel radiation detectors for X-ray imaging
Bell SJ, Schneider A, Seller P, Veale MC, Wilson MD, Baker MA, Perumal V, Sellin PJ, Chen H, Marthandam P (2013)A multi-technique characterization of electroless gold contacts on single crystal CdZnTe radiation detectors, In: Journal of Physics D: Applied Physics46(45)
Cadmium zinc telluride (CdZnTe) is now established as a popular choice of sensor for the detection of γ-rays and hard x-rays, leading to its adoption in security, medical and scientific applications. There are still many technical challenges involving the deposition of high-quality, uniform metal contacts on CdZnTe. A detailed understanding of the interface between the bulk CdZnTe and the metal contacts is required for improvements to be made. To understand these complex interfaces, a range of complementary materials characterization techniques have been employed, including x-ray photoelectron spectroscopy depth profiling, focused ion beam cross section imaging and energy dispersive x-ray spectroscopy. In this paper a number of Redlen CdZnTe detectors with asymmetric anode/cathode contacts have been investigated. The structures of the contacts were imaged and their compositions identified. It was found that the two stage electroless indium/electroless gold deposition process on 'polished only' surfaces formed a complex heterojunction on the cathode, incorporating compounds of gold, gold-tellurium, tellurium oxide (of varying stoichiometry) and cadmium chloride up to depths of several 100 nm. Trace amounts of indium were found, in the form of an indium-gold compound, or possibly indium oxide. At the surface of the CdZnTe bulk, a thin Cd depleted layer was observed. The anode heterojunction, formed by a single stage electroless gold deposition, was thinner and exhibited a simpler structure of gold and tellurium oxide. The differing (asymmetric) nature of the anode/cathode contacts gave rise to asymmetric current-voltage (I-V) behaviour and spectroscopy. © 2013 IOP Publishing Ltd.
Cadmium zinc telluride (CdZnTe) is a leading sensor material for spectroscopic X/-ray imaging in the fields of homeland security, medical imaging, industrial analysis and astrophysics. The metal-semiconductor interface formed during contact deposition is of fundamental importance to the spectroscopic performance of the detector and is primarily determined by the deposition method. A multi-technique analysis of the metalsemiconductor interface formed by sputter and electroless deposition of gold onto (111) aligned CdZnTe is presented. Focused ion beam (FIB) cross section imaging, X-ray photoelectron spectroscopy (XPS) depth profiling and current-voltage (IV) analysis have been applied to determine the structural, chemical and electronic properties of the gold contacts. In a novel approach, principal component analysis has been employed on the XPS depth profiles to extract detailed chemical state information from different depths within the profile. It was found that electroless deposition forms a complicated, graded interface comprised of tellurium oxide, gold/gold telluride particulates, and cadmium chloride. This compared with a sharp transition from surface gold to bulk CdZnTe observed for the interface formed by sputter deposition. The electronic (IV) response for the detector with electroless deposited contacts was symmetric, but was asymmetric for the detector with sputtered gold contacts. This is due to the electroless deposition degrading the difference between the Cd- and Te-faces of the CdZnTe (111) crystal, whilst these differences are maintained for the sputter deposited gold contacts. This work represents an important step in the optimisation of the metal-semiconductor interface which currently is a limiting factor in the development of high resolution CdZnTe detectors.
Ultrafast pulsed laser ablation has been investigated as a technique to machine CdWO4 single crystal scintillator and segment it into small blocks with the aim of fabricating a 2D high energy X-ray imaging array. Cadmium tungstate (CdWO4) is a brittle transparent scintillator used for the detection of high energy X-rays and γ-rays. A 6 W Yb:KGW Pharos-SP pulsed laser of wavelength 1028 nm was used with a tuneable pulse duration of 10 ps to 190 fs, repetition rate of up to 600 kHz and pulse energies of up to 1 mJ was employed. The effect of varying the pulse duration, pulse energy, pulse overlap and scan pattern on the laser induced damage to the crystals was investigated. A pulse duration of ≥500 fs was found to induce substantial cracking in the material. The laser induced damage was minimised using the following operating parameters: a pulse duration of 190 fs, fluence of 15.3 J cm−2 and employing a serpentine scan pattern with a normalised pulse overlap of 0.8. The surface of the ablated surfaces was studied using scanning electron microscopy, energy dispersive X-ray spectroscopy, atomic force microscopy and X-ray photoelectron spectroscopy. Ablation products were found to contain cadmium tungstate together with different cadmium and tungsten oxides. These laser ablation products could be removed using an ammonium hydroxide treatment.
N-doped TiO2 thin films have been deposited by reactive RF magnetron sputtering at different total gas pressures and varying O 2/N2 gas flow rates at 300 C. The thin film nanostructure has been studied by scanning electron microscopy, X-ray diffraction and X-ray photoelectron spectroscopy (XPS). Increasing the deposition pressure leads to reduced crystallinity of the thin films and a higher N2 flow rate was required to incorporate N into the growing film. This is attributed to the lower energy ion bombardment of the surface and N adatom chemical reactivity being reduced at higher total gas pressures. Ar+ ion sputtering of the deposited N-doped TiO2 thin films has enabled a detailed XPS investigation of the surface and bulk N species to be performed. Adsorbed N species have been identified on all the deposited thin film surfaces, with the most prevalent adsorbed N species occurring at a binding energy of approximately 400 eV, shown to originate from atmospheric contamination, most probably N containing organic species. The bulk N content varies between 0.6 and 6.0 at.% and N is located predominantly at substitutional sites in the TiO2. The presence of interstitial N, in the form of NO species, has been identified by XPS in some thin films deposited at higher deposition pressures. Hence, varying the total gas pressure may provide a route for tailoring the location of N in the bulk structure. At higher N contents (> 3 at.%), TiN is found as a secondary phase within the bulk structure and the presence of TiN leads to a sharp reduction in the band gap. Post-deposition annealing of low N containing films results in an N-doped TiO2 single phase anatase structure. © 2013 Elsevier B.V.
© 2015 Elsevier B.V. Membrane filtration is employed for water treatment and wastewater reclamation purposes, but membranes alone are unable to remove pollutant molecules and certain pathogens. Photocatalytically active N-doped TiO2 coatings have been deposited by sol-gel onto 200 nm pore size alumina membranes for water treatment applications using two different methods, via pipette droplets or spiral bar applicator. The uncoated and coated membranes were characterised by X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), X-ray diffraction (XRD) and energy dispersive X-ray spectrometry (EDX). Both coatings showed the presence of N-doped anatase, with a surface coverage between 84 and 92%, and nitrogen concentration (predominantly interstitial) of 0.9 at.%. The spiral bar applicator deposited coatings exhibit a thicker mud-cracked surface layer with limited penetration of the porous membrane, whilst the pipette deposited coatings have mostly penetrated into the bulk of the membrane and a thinner layer is present at the surface. The photocatalytic activity (PCA), measured through the degradation of carbamazepine (CBZ), under irradiation of a solar simulator was 58.6% for the pipette coating and 63.3% for the spiral bar coating. These photocatalytically active N-doped sol-gel coated membranes offer strong potential in forming the fundamental basis of a sunlight based water treatment system.
Hinder Steven, Polycarpou AA, Doumanidis CC, Boebel K, Polychronopoulou K, Rebholz C, Baker Mark, Theodorou L, Demas NG (2008)Nanostructure, mechanical and tribological properties of reactive magnetron sputtered TiCx coatings, In: DIAMOND AND RELATED MATERIALS17(12)pp. 2054-2061
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X-ray sources are used for both scientific instrumentation and inspection applications. In X-ray photoelectron spectroscopy (XPS), aluminum Kα X-rays are generated through electron beam irradiation of a copper-based X-ray anode incorporating a thin surface layer of aluminum. The maximum power operation of the X-ray anode is limited by the relatively low melting point of the aluminum. Hence, optimization of the materials and design of the X-ray anode to transfer heat away from the aluminum thin film is key to maximizing performance. Finite element analysis has been employed to model the heat transfer of a water-cooled copper-based X-ray anode with and without the use of a CVD (chemical vapour deposited) diamond heat spreader. The modeling approach was to construct a representative baseline model, and then to vary different parameters systematically, solving for a steady state thermal condition, and observing the effect of on the maximum temperature attained. The model indicates that a CVD diamond heat spreader (with isotropic thermal properties) brazed into the copper body reduces the maximum temperature in the 4 μm aluminum layer from 613 °C to 301 °C. Introducing realistic anisotropy in the TC (thermal conductivity) of the CVD diamond has no significant effect on heat transfer if the aluminum film is on the CVD diamond growth face (with the highest TC). However, if the aluminum layer is on the CVD diamond nucleation face (with the lowest TC), the maximum temperature is 575 °C. Implications for anode design are discussed.
Fully spectroscopic x/γ-ray imaging is now possible thanks to advances in the growth of wide-bandgap semiconductors. One of the most promising materials is cadmium zinc telluride (CdZnTe or CZT), which has been demonstrated in homeland security, medical imaging, astrophysics and industrial analysis applications. These applications have demanding energy and spatial resolution requirements that are not always met by the metal contacts deposited on the CdZnTe. To improve the contacts, the interface formed between metal and semiconductor during contact deposition must be better understood. Gold has a work function closely matching that of high resistivity CdZnTe and is a popular choice of contact metal. Gold contacts are often formed by electroless deposition however this forms a complex interface. The prior CdZnTe surface preparation, such as mechanical or chemo-mechanical polishing, and electroless deposition parameters, such as gold chloride solution temperature, play important roles in the formation of the interface and are the subject of the presented work. Techniques such as focused ion beam (FIB) cross section imaging, transmission electron microscopy (TEM), energy dispersive x-ray spectroscopy (EDS), x-ray photoelectron spectroscopy (XPS) and current − voltage (I–V) analysis have been used to characterize the interface. It has been found that the electroless reaction depends on the surface preparation and for chemo-mechanically polished (1 1 1) CdZnTe, it also depends on the A/B face identity. Where the deposition occurred at elevated temperature, the deposited contacts were found to produce a greater leakage current and suffered from increased subsurface voiding due to the formation of cadmium chloride.
Commercial α-Al2O3 photocatalytic membranes with a pore size of 200 and 800-nm were coated with N-doped TiO2 photocatalytic film using a sol-gel technique for concurrent bottom-up filtration and photocatalytic oxidation. X-ray diffraction confirmed that the deposited N-doped TiO2 films are in the form of anatase with 78-84% coverage of the membrane surface. The concentration of N found by X-ray photoelectron spectroscopy was in the range of 0.3-0.9 atomic percentage. Membrane permeability after coating decreased by 50% and 12% for the 200- and 800-nm membrane substrates, respectively. The impact of operational parameters on the photocatalytic activity (PCA) of the N-doped TiO2-coated membranes was examined in a laboratory flow cell based on degradation of the model micropollutant carbamazepine, using a solar simulator as the light source. The significant gap in degradation rate between flow through the membrane and flow on the surface of the membrane was attributed both to the hydraulic effect and in-pore PCA. N-doped TiO2-coated membranes showed enhanced activity for UV wavelengths, in addition to activity under visible light. Experiments of PCA under varying flow rates concluded that the process is in the mass-transfer control regime. Carbamazepine removal rate increased with temperature, despite the decrease in dissolved oxygen concentration.
Linear saturated dicarboxylic acids are a class of organic chemical compounds with two carboxyl functional groups (-COOH) at the extremities of their aliphatic chains. This class of organic acids can be represented by the general molecular formula HOOC-(CH2)n-COOH. The most common values for n with their respective acid names are present in Table I. The general chemical behavior and reactivity of these compounds are similar to monocarboxylic acids, and they are all widely used in the production of copolymers, such as polyamides and polyesters (Refs. 1–3). The easy conversion of carboxyl groups to esters has industrial importance since many esters are used as taste and odor enhancers. Carboxylic acids are also used as catalysts, replacing ecologically unfavorable organic halides (Ref. 4). Over the last three decades, interest in such acids has increased, specifically regarding their application to improve the corrosion resistance of metallic substrates such as zinc, copper, iron, and aluminum (Refs. 5–12). Research has also shown that carboxylic acids can be used as additives for the electro synthesis of polymeric protective coatings. Such coatings promote passivation of different metallic substrates, allowing the oxidation of the carboxylic acid monomers without concomitant reactions (Refs. 10–13). More recently, carboxylic acids have been used to generate hydrophobic surfaces on various metallic substrates (Fe, Al, Cu, Mg, Zn, Ti, etc.) forming self-assembled layers by adsorption, via carboxyl groups, to the positively charged metal surfaces (Refs. 14–18).
Zhao Chen, Zhang Chunyang, Bhoyate Sanket, Kahol Pawan, Kostoglou Nikolaos, Mitterer Christian, Hinder Steven, Baker Mark, Constantinides Georgios, Polychronopoulou Kyriaki, Rebholz Claus, Gupta Ram Nanostructured Fe-Ni Sulfide: A Multifunctional Material for Energy Generation and Storage, In: Catalysts
In recent years, gas cluster ion beams (GCIB) have become the cutting edge of ion beam technology to sputter etch organic materials in surface analysis. However, little is currently known on the ability of argon cluster ions (Arn+) to etch metal oxides and other technologically important inorganic compounds and no depth profiles have previously been reported. In this work, XPS depth profiles through a certified (European standard BCR-261T) 30 nm thick Ta2O5 layer grown on Ta foil using monatomic Ar+ and Ar1000+ cluster ions have been performed at different incident energies. The preferential sputtering of oxygen induced using 6 keV Ar1000+ ions is lower relative to 3 keV and 500 eV Ar+ ions. The depth profiling etch rate and depth resolution is substantially better for the monatomic beam compared to the cluster beam. Ar+ ions exhibit a steady state O/Ta ratio through the bulk oxide but Ar1000+ ions show a gradual decrease in the O/Ta ratio as a function of depth. Higher residual O concentrations are observed on the Ta bulk metal for the Ar1000+ profiles compared to the Ar+ profiles.
Duarte DD, Lipp JD, Schneider A, Seller P, Veale MC, Wilson MD, Baker MA, Sellin PJ (2016)Simulation of active-edge pixelated CdTe radiation detectors, In: NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION A-ACCELERATORS SPECTROMETERS DETECTORS AND ASSOCIATED EQUIPMENT806pp. 139-145
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The edge surfaces of single crystal CdTe play an important role in the electronic properties and performance of this material as an X-ray and γ-ray radiation detector. Edge effects have previously been reported to reduce the spectroscopic performance of the edge pixels in pixelated CdTe radiation detectors without guard bands. A novel Technology Computer Aided Design (TCAD) model based on experimental data has been developed to investigate these effects. The results presented in this paper show how localized low resistivity surfaces modify the internal electric field of CdTe creating potential wells. These result in a reduction of charge collection efficiency of the edge pixels, which compares well with experimental data.
Charisiou ND, Papageridis KN, Siakavelas G, Tzounis L, Kousi K, Baker Mark, Hinder Steven, Cabeza VS, Polychronopoulou K, Goula MA (2017)Glycerol Steam Reforming for Hydrogen Production over Nickel Supported on Alumina, Zirconia and Silica Catalysts, In: Topics in Catalysis60(15-16)pp. 1226-1250
The aim of the work was to investigate the influence of support on the catalytic performance of Ni catalysts for the glycerol steam reforming reaction. Nickel catalysts (8 wt%) supported on Al2O3, ZrO2, SiO2 were prepared by the wet impregnation technique. The catalysts’ surface and bulk properties, at their calcined, reduced and used forms, were determined by ICP, BET, XRD, NH3-TPD, CO2-TPD, TPR, XPS, TEM, TPO, Raman, SEM techniques. The Ni/Si sample, even if it was less active for T <600 °C, produces more gaseous products and reveals higher H2 yield for the whole temperature range. Ni/Zr and Ni/Si catalysts facilitate the WGS reaction, producing a gas mixture with a high H2/CO molar ratio. Ni/Si after stability tests exhibits highest values for total (70%) and gaseous products (45%) glycerol conversion, YH2 (2.5), SH2 (80%), SCO2 (65%), H2/CO molar ratio (6.0) and lowest values for SCO (31%), SCH4 (3.1%), CO/CO2 molar ratio (0.48) among all samples. The contribution of the graphitized carbon formed on the catalysts follows the trend Ni/Si (I D /I G = 1.34) < Ni/Zr (I D /I G = 1.08) < Ni/Al (I D /I G = 0.88) and indicates that the fraction of different carbon types depends on the catalyst’s support nature. It is suggested that the type of carbon is rather more important than the amount of carbon deposited in determining stability. It is confirmed that the nature of the support affects mainly the catalytic performance of the active phase and that Ni/SiO2 can be considered as a promising catalyst for the glycerol steam reforming reaction.
AlKhoori Ayesha A., Polychronopoulou Kyriaki, Belabbes Abderrezak, Jaoude Maguy Abi, Vega Lourdes F., Sebastian Victor, Hinder Steven, Baker Mark A., Zedan Abdallah F. (2020)Cu, Sm co-doping effect on the CO oxidation activity of CeO2. A combined experimental and density functional study, In: Applied Surface Science521146305
The co-doping effect of a rare earth (RE) metal and a transition metal (TM) on ceria oxidation catalysis through the evaluation of samarium-copper co-doped catalysts with Ce-Sm-xCu-O (x: 0–20 at.%, Ce/Sm = 1) nominal compositions, is discussed. The CO oxidation reaction was used as a prototype reaction due to its pivotal role in the fuel cell technology. Ce-Sm-20Cu-O catalyst presented a 64% increase in the CO oxidation activity compared to that of pristine ceria. Diffraction and Raman studies proved that the Cu, Sm co-doping induces many defects related to the dopants (Sm, Cu) and the oxygen vacant sites, while the presence of hybrid CuO/Ce-Sm(Cu)-O fluorite/SmO8 (cubic metastable) phases is the most representative scenario of this oxide microstructure. A size polydispersity of CuO phases was achieved by introducing air cooling during the microwave heating. Cu, Sm atoms were uniformly doped in CeO2 structure according to the HAADF-STEM studies. These results are in agreement with EDS analysis, where Cu, Sm and Ce are located in all the analyzed areas without any preferential distribution. The XPS studies demonstrated the co-presence of Cu2+/Cu1+ and Ce4+/Ce3+ redox couples in agreement with the Bader charge analysis from the ab initio calculations, the latter influencing greatly the oxidation activity of the catalysts. Density functional theory (DFT) calculations shed light on the oxide surface and the underlying mechanism governing the oxidation catalysis taking place. In particular, Cu2+ and Sm3+ dopants were found to be located in the nearest neighbor (NN) sites of oxygen vacancies. Different oxygen vacancies configurations were studied (single vs. double, surface vs. subsurface), where the single vacancies are more stable on the surface, whereas the double vacancies configurations are more stable on the subsurface. Regarding the Ce3+ location, in the presence of single and double oxygen vacancy, the Ce3+ ions prefer to be located in the 1st NN/2nd NN and 2nd NN of the first Ce layer, relative to the oxygen vacancy, respectively. The total Density of States (DOS) analysis of the co-doped systems revealed that the dopants induced new surface states inside the ceria band gap, which can accommodate the unpaired electrons of the vacant oxygen sites. These electronic modifications justify the much lower energy of oxygen vacancy formation (Evf) in both cases, the Sm-doped, and Cu, Sm -doped CeO2 (1 1 1) geometries. Specifically, the Evf lowering upon doping was found to be almost two times larger for the Cu adjacent oxygen vacancies (Cu2+-□) compared to the Sm ones (Sm3+-□), consistent with the CO adsorption trend as the Cu-Sm-CeO2 (1 1 1) system is energetically more favorable than the Sm-CeO2 (1 1 1) and pure CeO2 (1 1 1) surfaces.
Recent improvements in the growth of wide-bandgap semiconductors, such as cadmium zinc telluride (CdZnTe or CZT), has enabled spectroscopic X/γ-ray imaging detectors to be developed. These detectors have applications covering homeland security, industrial analysis, space science and medical imaging. At the Rutherford Appleton Laboratory (RAL) a promising range of spectroscopic, position sensitive, small-pixel Cd(Zn)Te detectors have been developed. The challenge now is to improve the quality of metal contacts on CdZnTe in order to meet the demanding energy and spatial resolution requirements of these applications. The choice of metal deposition method and fabrication process are of fundamental importance. Presented is a comparison of two CdZnTe detectors with contacts formed by sputter and electroless deposition. The detectors were fabricated with a 74 × 74 array of 200 μm pixels on a 250 μm pitch and bump-bonded to the HEXITEC ASIC. The X/γ-ray emissions from an 241Am source were measured to form energy spectra for comparison. It was found that the detector with contacts formed by electroless deposition produced the best uniformity and energy resolution; the best pixel produced a FWHM of 560 eV at 59.54 keV and 50% of pixels produced a FWHM better than 1.7 keV . This compared with a FWHM of 1.5 keV for the best pixel and 50% of pixels better than 4.4 keV for the detector with sputtered contacts.
Phosphating of sintered NdFeB magnets has been studied by immersion in a solution of 0.15 M NaH 2PO 4 acidified to pH 3.7 under polarization. Cyclic polarization experiments indicated that phosphating could be assisted by polarization, with the current density decreasing as the number of polarization cycles increased. Auger electron spectroscopy (AES) and Energy Dispersive X-ray Analysis (EDX) of magnets exposed to the phosphating treatment confirmed the formation of the phosphate layer over both main phases of the specimen, namely, the magnetic (Φ) and the Nd-rich phase. Electrochemical impedance spectroscopy (EIS) measurements performed on treated and untreated magnets immersed in synthetic saliva showed the phosphate conversion layer to improve the corrosion resistance and provided evidence of its porous nature. The phosphating procedure adopted in the present investigation is a promising surface treatment for improving the corrosion resistance of sintered NdFeB magnets.
Capacitive deionization (CDI) is an emerging desalination technology that still needs further development to enhance its performance for practical implementation. Herein, we present a hybrid CDI approach, which integrates the electrical double-layer (EDL) with the sodium-ion battery concept to improve the separation of sodium and chloride ions from saline water. The hybrid CDI cell is achieved by using hydrothermally-grown and uniformly dispersed prawn-like α-MnO₂/graphene (α-MnO₂/G) nanocomposite as anode material, and graphene at the cathode. In this paper, the effect of MnO₂ morphology on the electrode electrochemical performance and its effect on capacitive deionization performance have been fully investigated. In this configuration, the Na⁺ ions are inserted by the electrochemical reaction at the α-MnO₂/G electrode, whereas Cl⁻ ions are captured by the graphene-based electrode. The morphological dependent electrochemical properties of the obtained nanocomposites were studied deeply through CV and EIS analysis. The established hybrid CID cell provides an electrical capacitance as high as 375 F g⁻¹ at 10 mV s⁻¹, cation-selectivity, good electrical stability and low internal resistance. The hybrid CDI device also shows a stable and reversible salt insertion/de-insertion capacity up to 29.5 mg g⁻¹ at 1.2 V. These results demonstrate the suitability of prawn-like α-MnO₂/G nanocomposite to produce high-performance hybrid CDI cells.
Ce1-xSmxO2(x=0, 0.2, 0.5 and 0.8) nanofibers (NFs) were synthesized by coupling sol-gel with electrospinning and using poly-vinyl pyrrolidone (PVP) as the polymer medium, in an ethanol/water mixture. Control over the fabrication conditions was achieved through analysis of the most key synthetic factors, which include: (i) the applied field strength; (ii) the solution feed rate and (iii) the PVP content in the electrospinning solution. The optimum microstructural fiber morphology (high quality beeds-free fibers) was achieved using the following electrospinning parameters: an applied voltage of 18.5 kV, a 7 ml/hr of solution feed rate and a 12% (w/w) of PVP composition. Morphological features of the resulting fibers were examined by scanning electron microscopy (SEM). The average fiber diameter was typically found to be in the range of 200-1100 nm and 50-300 nm, before and after calcination at 500 oC, respectively. X-ray diffraction (XRD) results showed that the fluorite cubic structure was preserved for the entire Ce1-xSmxO2 compositional range studied, while elemental analysis using EELS and X-ray photoelectron spectroscopy (XPS) confirmed the purity of the bulk and surface composition of the fibers. Selected area electron diffraction (SAED) and high resolution transmission electron microscopy (HRTEM) proved that the NFs are highly crystalline. The thermal stability of the composite (polymer/inorganic nitrate salts) NFs was further investigated in an inert atmosphere (N2) using thermogravimetric analysis (TGA), which allowed the transformation process of the NFs from composite to oxide to be monitored. The reducibility of the metal oxide NFs (mobility of oxygen species in the fluorite cubic lattice) as well as their thermal stability in successive oxidation-reduction cycles was evaluated using temperature-programmed reduction in a H2 atmosphere (H2-TPR). Acidic-basic features of the NFs and powder surfaces were studied through temperature programmed desorption (TPD) using NH3 and CO2 as probe molecules, where weak, medium and strong acid sites were successfully traced with profound differences depending on the morphology. The NFs’ potential performance towards NH3 oxidation was also evaluated. Two types of basic sites, hydroxyl groups and surface lattice oxygen are present on the NFs, as probed by CO2 adsorption. Pyridine adsorption followed by infrared spectroscopy (Py-FT-IR) studies unveiled the more profound Lewis acid presence in Ce0.5Sm0.5O2 NFs compared to bulk powder Ce0.5Sm0.5O2.
The photocatalytic degradation of the model pollutant carbamazepine (CBZ) was investigated under simulated solar irradiation with an N-doped TiO2-coated Al2O3 photocatalytic membrane, using different water types. The photocatalytic membrane combines photocatalysis and membrane filtration in a single step. The impact of each individual constituent such as acidity, alkalinity, dissolved organic matter (DOM), divalent cations (Mg2+ and Ca2+), and Cl
A comparative study of the GSR performance for Ni/CaO-MgO-Al2O3 and Ni/Al2O3 catalysts is reported. Catalysts were synthesized applying the wet impregnation method at a constant metal loading (8 wt %). Synthesized samples were characterized by N2 adsorption/desorption, ICP, BET, XRD, NH3-TPD, CO2-TPD, H2-TPR, XPS, TEM, STEM-HAADF and EDS. The carbon deposited on their surface under reaction conditions was characterized by TPO, Raman and TEM. It was proven that the use of CaO-MgO as alumina modifiers leads to smaller nickel species crystallite size, increased basicity and surface amount of Ni0 phase. Thus, it increases the conversion to gaseous products favoring H2 and CO2 production to the detriment of CO formation, by enhancing the water gas-shift (WGS) reaction. No liquid products were produced by the Ni/modAl catalyst over 550 °C, whereas time on stream results confirmed that deactivation can be prevented, as apart from decreasing the amount of coke deposition the nature of carbon was altered towards less graphitic and more defective structures.
Nanograins of Ce-La-xCu-O oxides, of 16 nm2 area size, are tested as materials towards the CO oxidation . Preservation of the cubic lattice structure following La3+ and Cu2+ metal cations doping is confirmed based on the powder X-ray diffraction and Raman studies. From XPS, the presence of mixed Ce3+/Ce4+ and Cu2+/Cu1+ oxidation states was confirmed, which was more profound in the low Cu-content Ce-La-xCu-O catalysts. Cu increases the concentration of oxygen vacant sites in the doped-CeO2 according to the Raman intensity ratio IOv/IF2g of 1.58 and 1.78 with the increase in copper content from 7 to 20 at.% as compared to the lower value of 0.44 for the Ce-La. The mobility of the surface and bulk lattice oxygen is further investigated using 16O/18O isotopic exchange (TIIE), and is found to be Cu at.% dependent. For the case of Ce-La-20Cu, the participation of the lattice oxygen (OL) in the reaction mechanism has been demonstrated using transient experiments. Accordingly, the specific rate (μmol CO m-2s-1) of the CO oxidation reaction is found to be higher for the Ce-La-20Cu and Ce-La-7Cu catalysts, corroborating thus the presence of more mobile/labile oxygen species in those ternary catalysts as opposed to the other lower copper compositions.
Ni/Al2O3 and Ni/La2O-Al2O3 catalysts were investigated for the biogas reforming reaction using CH4/CO2 mixtures with minimal dilution. Stability tests at various reaction temperatures were conducted and TGA/DTG, Raman, STEM-HAADF, HR-TEM, XPS techniques were used to characterize the spent samples. Graphitized carbon allotrope structures, carbon nanotubes (CNTs) and amorphous carbon were formed on all samples. Metallic Ni0 was recorded for all (XPS), whereas a strong peak corresponding to Ni2O3/NiAl2O4, was observed for the Ni/Al sample (650–750°C). Stability tests confirm that the Ni/LaAl catalyst deactivates at a more gradual rate and is more active and selective in comparison to the Ni/Al for all temperatures. The Ni/LaAl exhibits good durability in terms of conversion and selectivity, whereas the Ni/Al gradually loses its activity in CH4 and CO2 conversion, with a concomitant decrease of the H2 and CO yield. It can be concluded that doping Al2O3 with La2O3 stabilizes the catalyst by (a) maintaining the Ni0 phase during the reaction, due to higher dispersion and stronger active phase-support interactions, (b) leading to a less graphitic and more defective type of deposited carbon and (c) facilitating the deposited carbon gasification due to the enhanced CO2 adsorption on its increased surface basic sites.
Zequine Camila, Bhoyate Sanket, Siam Khamis, Kahol Pawan K., Kostoglou Nikolaos, Mitterer Christian, Hinder Steven, Baker Mark, Constantinides Georgios, Rebholz Claus, Gupta Gautam, Li Xianglin, Gupta Ram K. (2018)Needle grass array of nanostructured nickel cobalt sulfide electrode for clean energy generation, In: Surface and Coatings Technology354pp. 306-312
Significant efforts have been focused on the search of earth-abundant elements to solve growing energy issues and to provide bifunctional behavior for both hydrogen and oxygen evolution reaction. Mixed transition metals could provide promising synergistic electrochemical properties and serve as bi-catalyst for overall water splitting process. In this study, a needle grass array of nanostructured nickel cobalt sulfide (NiCo2S4) was synthesized using a hydrothermal process. The synthesized NiCo2S4 electrodes showed promising electrocatalytic activity with a low overpotential of 148 mV and 293 mV for hydrogen and oxygen evolution reactions, respectively. The electrolyzer cell consisting of two NiCo2S4 electrodes displayed excellent performance with high electrochemical stability and low overall cell potential of 1.61 V to achieve a current density of 10 mA/cm2. Our study suggests that mixed transition metal chalcogenides such as NiCo2S4 could be used as efficient and stable electrocatalyst for overall water splitting process.
Pixelated Cd(Zn)Te radiation detectors are a promising technology for X-ray imaging applications but their areas are limited to 5 cm2. Active-edge sensors, without guard bands,are explored and characterised in this work to produce a large panel pixelated Cd(Zn)Te detector built of tiled modules with minimal gaps between them. The characterisation of an active-edge sensor fabricated using present processing technologies showed that 87 % of all edge pixels had excellent spectroscopic characteristics for X-ray imaging. However non-uniformities in the charge collection were observed in 34 % of the pixels. These were attributed to regions with poor charge collection efficiency up to 200 µm from the edge due to a low electric field strength near the edge that was caused by the high edge surface leakage currents. New techniques for the processing of the crystal edges were investigated with the aim of improving the sensitivity of the detectors up to the edge of the crystal. The leakage current was significantly reduced when the diced edges of CdTe sensors were lapped with a 3 µm alumina slurry followed by a polish with a 0.3 µm alumina slurry. This resulted in 60 % of edge pixels with excellent characteristics for X-ray imaging. The remaining 40 % presented poor spectroscopy due to damaged pixels, as a consequence of the difficulties in handling the 1 mm thick crystal whilst manually processing the edge surfaces. The polished and diced surfaces were illuminated edge-on, between the cathode and the anode, for the first time ever in CdTe. Poor detection and charge collection efficiency were observed within 12 µm from the polished edge surface and 80 µm from the diced edge surface. This was attributed to a high density of electron traps at the crystal edge due to dicing and processing that originated multiple trapping and de-trapping of charge carriers. This work concludes that active-edge CdTe radiation detectors are a promising technology for the production of a large Cd(Zn)Te radiation detector for X-ray imaging. The nonuniformities seen in the edge pixels are related to the high edge surface leakage currents due to the introduction of trap states during dicing. These are reduced by edge processing which creates active-edge pixels sensitive to radiation within 12 µm from the edge surface.
Pulsed laser ablation has been investigated as a method for the creation of thick segmented scintillator arrays for high-energy X-ray radiography. Thick scintillators are needed to improve the X-ray absorption at high energies, while segmentation is required for spatial resolution. Monte-Carlo simulations predicted that reflections at the inter-segment walls were the greatest source of loss of scintillation photons. As a result of this, fine pitched arrays would be inefficient as the number of reflections would be significantly higher than in large pitch arrays. Nanosecond and femtosecond pulsed laser ablation was investigated as a method to segment cadmium tungstate (CdWO_4). The effect of laser parameters on the ablation mechanisms, laser induced material changes and debris produced were investigated using optical and electron microscopy, energy dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy for both types of lasers. It was determined that nanosecond ablation was unsuitable due to the large amount of cracking and a heat affected zone created during the ablation process. Femtosecond pulsed laser ablation was found to induce less damage. The optimised laser parameters for a 1028 nm laser was found to be a pulse energy of 54 μJ corresponding to a fluence of 5.3 J cm^-2 a pulse duration of 190 fs, a repetition rate of 78.3 kHz and a laser scan speed of 707 mm s^-1 achieving a normalised pulse overlap of 0.8. A serpentine scan pattern was found to minimise damage caused by anisotropic thermal expansion. Femtosecond pulsed ablation was also found to create a layer of tungsten and cadmium sub-oxides on the surface of the crystals. The CdWO_4 could be cleaned by immersing the CdWO_4 in ammonium hydroxide at 45°C for 15 minutes. However, XPS indicated that the ammonium hydroxide formed a thin layer of CdCO_3 and Cd(OH)_2 on the surface. Prototype arrays were shown to be able to resolve features as small as 0.5 mm using keV energy X-rays. The most efficient prototype showed low detective quantum efficiency of 0.08±0.01 at 0 lp/mm using a tube voltage of 160 kVp.
Inverse analysis of nanoindentation data has attracted increasing interest in industry due to its ability to estimate the bulk tensile properties of materials and potentially offers an alternative technique to conventional characterisation methods. Inverse analysis of nanoindentation data is particularly valuable in applications where conventional techniques are not suitable due to either the scale of characterisation (very small regions) or because the testing is expensive and time consuming. Despite using best practices to minimise sources of error in the experimental data, given the scale of the indentations, the heterogeneity of material microstructure can create significant variability in the data, ultimately affecting the reliability of the inverse analysis solution. This thesis proposes and discusses pragmatic approaches to mitigate the effects of material heterogeneity on the accuracy of the inverse problem solution as well as of nanoindentation data in general. The work has involved finite element analysis modelling, nanoindentation and tensile testing. One mitigation approach consisted in the implementation and verification of a new ‘multi-objective’ function inverse analysis methodology where the bias of selecting only one experimental nanoindentation curve as representative of the homogenised response of the material is overcome. The new approach uses all the experimental curves generated from a grid of nanoindentations and employs a weighted averaging procedure. This methodology was applied to S355 steel samples through recording nanoindentation and tensile test data. Despite the variation present in the experimental nanoindentation load-depth curves, this being in the order of 13%, the ‘multi-objective’ function approach was found to estimate the tensile parameters with an error margin as low as 3-6% compared to an error margin of 9-20% for the conventional method. A framework of activities was also undertaken to monitor the variation of the measured nanoindentation properties (e.g. hardness) as function of the indentation depth, in relation to the average grain size of the material. Commercial purity aluminium 1050 samples (with varying average grain sizes) and S355 steel were employed as test materials. These results in addition to those from other materials were used to construct a look-up plot of the hardness COV values as function of the normalised nanoindentation depths (normalised with respect to the average grain diameter). The plot is based on upper and lower bound curves and intends to provide guidance on the selection of the nanoindentation testing parameters to minimise the variability of the indentation response.
Semiconductor materials have a vast range of applications varying from basic electronic products to astronomy and their semiconducting properties can be altered through the growth of binary or ternary compound materials. CdZnTe and CdTe are prominent materials in radiation detector and photovoltaic solar cell applications. For radiation detectors, in the fabrication process, surface preparation (chemical polishing and passivation) and contact deposition are key to the detector performance. This thesis investigates the effect of these two processing steps on CdZnTe detectors through varying the passivation procedures and gold contact configurations. The surface composition, layer thickness and non-uniformity resulting from the passivation treatments have been investigated using X-ray Photoelectron Spectroscopy (XPS), Scanning Transmission Electron Microscopy (STEM), Energy Dispersive X-ray spectroscopy (EDX) and other materials characterisation techniques. The device electrical and spectroscopic responses were measured using the I-V characteristics and alpha spectroscopy respectively. Passivation using 30 % H2O2 and 5% NaClO treatments develops a very thin oxide layer of up to ~2 nm, while NH4F/H2O2 and KOH+NH4F/H2O2 treatments yield oxide layers of varying thickness (30 – 142 nm) and metal oxides comprising of Te2/Te3, CdO and ZnO. Devices were fabricated in metal-semiconductor (MS) and metal-insulator-semiconductor (MIS) configurations. The MIS configuration improves the mobility-lifetime product, partial charge collection and leakage current of a CdZnTe device. The MIS device barrier heights were calculated to be 0.83 ± 0.02 eV and 0.86 ± 0.02 eV for very thin and thick oxide layers respectively. For ultra-thin (0.5 µm CdTe layer in) CdTe/CdZnS solar cells, XPS and X-ray diffraction (XRD) were employed to study the effect of varying the CdCl2 processing step. Increasing the degree of CdCl2 activation and annealing treatment was found to increase sulphur diffusion into the CdTe layer (up to a concentration of ~ 2 at.%). Cell performance measurements showed that the increase in S concentration is directly related to the open-circuit voltage (Voc), and increasing the degree of CdCl2 treatment gives higher Voc values.
Spectroscopic X-ray imaging is an emerging technology with applications in the fields of medical imaging, security, science and industrial analysis. The wide bandgap semiconductor cadmium zinc telluride (CdZnTe) is the leading sensor material for this technology. Refinements in the growth process have delivered consistent improvements in both yield and quality of single crystal CdZnTe. The fabrication of metal contacts required for electronic readout is now considered to be the limiting factor for small pixel CdZnTe detector performance. This includes preparation of the CdZnTe surface, deposition of the metal contacts and pixellation. The current work is concerned with understanding and improving the detector fabrication process. A range of complementary characterisation techniques have been used to analyse the metal-semiconductor interface formed by electroless and sputter deposition of gold contacts onto CdZnTe. The characterisation included focused ion beam (FIB) cross section imaging, chemical analysis with X-ray photoelectron spectroscopy (XPS) and electronic analysis with current-voltage (IV) measurements. The electroless deposition was found to produce a complicated interface consisting of a surface gold layer on top of a mixed interface of gold, tellurium oxide and cadmium chloride. The effective barrier height of this contact was measured to be 0.78 ± 0.04 eV under positive bias and 0.83 ± 0.02 eV under negative bias. The interface of sputter deposited gold contacts was simple in comparison, with a sharp interface between the gold and CdZnTe and a barrier height of 0.64 ± 0.02 eV under positive bias and 0.78 ± 0.02 eV under negative bias. An optimised detector fabrication process has been developed. This process involves forming gold contacts on mechanically polished-only CdZnTe by electroless deposition. Pixellation is achieved with a positive photoresist prior to gold deposition. A small pixel (250 μm pitch) CdZnTe detector fabricated in this manner and bonded to the HEXITEC ASIC has produced a single pixel FWHM energy resolution at 59.54 keV of 560 eV, with a modal FWHM resolution of 1.5 keV across the full 74 × 74 pixel array.
Gas cluster ion beam instruments represent the cutting edge in polymer depth profiling technology. This is because they can be used to analyse polymer samples while preserving the delicate chemical structure often lost when more conventional ion beams are used. In this thesis thorough investigations are conducted into the various advantages and disadvantages of using both Ar+ and Ar+n ions to analyse organic, inorganic and multi-layer samples by XPS and ToF-SIMS depth profiling. Analytical techniques including XPS, ToF-SIMS, AFM and laser interferometry were utilised to measure chemical and physical changes induced in samples by the two Ar ion species both during depth profiling and following crater formation. Significant observations were made in Ar+n depth profiling of inorganic metal oxide samples of effects not previously reported in the literature as well as the detection of the damage layer induced in polymer samples by cluster ions. This includes a reduction in levels of preferential sputtering in Ar+n compared to Ar+ depth profiles. Ar+n depth profiles of Ta2O5 on Ta and SiO2 on PET are also shown to lack the so-called “steady-state” region observed in Ar+ depth profiles. Evidence of O diffusion from the Ta2O5 layer to the underlying Ta metal is also reported in depth profiles acquired using Ar+n ions but not Ar+ ions. A method for the measurement of the rapid ejection of organic material at an inorganic/organic multi-layer interface is demonstrated showing a clear variation between the material ejection between Ar+ and Ar+n ion exposure. The observations discussed have serious implications for the current understanding of Ar+n bombardment of inorganic sample materials some of which have been published in the literature.
The development of flexible lightweight OLED devices requires oxygen/moisture barrier layer thin films with water vapour transmission rates (WVTR) of < 10-6 g/m2/day. This thesis reports on single and multilayer architecture barrier layers (mostly based on SiO2, Al2O3 and TiO2) deposited onto glass, Si and polymeric substrates using remote plasma sputtering. The reactive sputtering depositions were performed on Plasma Quest S500 based sputter systems and the morphology, nanostructure and composition of the coatings have been examined using SEM, EDX, STEM, XPS, XRD and AFM. The WVTR has been determined using industry standard techniques (e.g. MOCON) but, for rapid screening of the deposited layers, an in-house permeation test was also developed. SEM, XRD and STEM results showed that the coatings exhibited a dense, amorphous structure with no evidence of columnar growth. However, all of the single and multilayer coatings exhibited relatively poor WVTRs of > 1 x 10-1 g/m2/day at 38 °C and 85 % RH. Further characterisation indicated that the barrier films were failing due to the presence of substrate asperities and airborne particulates. Different mechanisms were investigated in an attempt to reduce the density of film defects including incorporation of a getter layer, modification of growth kinetics, plasma treatment and polymer planarising, but none were successful in lowering the WVTR. Review of this issue indicated that the achievement of good barrier layers was likely to be problematic in commercial practice due to the cost implications of adequately reducing particulate density and the need to cover deliberately non-planar surfaces and fabricated 3D structures. Conformal coverage would therefore be required to bury surface structures and to mitigate particulate issues. Studies of the remote plasma system showed that it both inherently delivered an ionised physical vapour deposition (IPVD) process and was compatible with bias re-sputtering of substrates. Accordingly, a process using RF substrate bias to conformally coat surfaces was developed to encapsulate surface particulates and seal associated permeation paths. An order of magnitude improvement in WVTR (6.7 x 10-2 g/m2/day) was measured for initial Al2O3 coatings deposited with substrate bias. The development of substrate bias to enhance conformal coverage provides significant new commercial benefit. Furthermore, conformal coverage of 5:1 aspect ratio structures have been demonstrated by alternating the substrate bias between -222 V and -267 V, with a 50 % dwell time at each voltage. Further development and optimisation of the substrate bias technique is required to fully explore the potential for further improving barrier properties and conformal coverage of high aspect ratio and other 3D structures.
Methylene diphenyl diisocyanate (MDI) is the most widely employed diisocyanate for the production of polyurethanes (PUs). This family of polymers is used for many applications including adhesives and coatings where an important characteristic is good adhesion. Considering how widely PUs are employed, the importance of MDI production in the world becomes evident. The phosgenation step, in the MDI production process, ultimately leads to fouling of the climbing lm evaporators (CFEs) employed, resulting in the loss of their thermal e- ciency. This work is concerned with the interactions between MDI (and related compounds) with metal substrates such as steels, which are employed in the CFEs as well as substrates for adhesion. For this study, surface analysis techniques, in particular X-ray photoelectron spectroscopy (XPS) and time of ight secondary ion mass spectrometry (ToF-SIMS), have been employed. Both substrates (316L and duplex steels) and adsorbates (MDI, polymeric MDI, methylene diphenyl amine and amine hydrochloride) have been characterised. The interaction between phenyl ring electrons (present in MDI and related compounds) and metal have been studied by observing the XPS - shake-up satellite at high spectral resolution. The interface between MDI and related compounds was then investigated. After the investigation of the model samples, plant facsimile samples were produced and analysed. Finally, actual plant samples were characterised. Proof was found of interaction between the phenyl ring electrons and the silicon substrate, as well as the formation of covalent bonds at the interface between MDI and steel as a result of the reaction between isocyanate and metal oxides and hydroxides, present on the surface of the steel. The facsimile samples showed the same types of interactions observed in the model samples and it was also found that the corrosion of the metal strongly influences the adhesion and fouling mechanisms. The samples from the plant showed similarities with the model and facsimile samples, proving that they provide a good basis for understanding the real world scenario.
This thesis contains work on three topics, the commissioning of an integrated surface science instrument, the ageing of LiH and the surface reactivity of U. The integrated system contains a variety of surface science techniques which have been used extensively in the investigations presented here. These techniques are: scanning electron microscopy (SEM), Auger electron spectroscopy (AES), x-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), secondary ion mass spectrometry (SIMS), temperature programmed decomposition (TPD), low energy electron diffraction (LEED) and molecular beam scattering (MBS). LiH ageing has been split into two regimes, wet and dry ageing. Wet ageing explicitly requires the presence of an external source of H2O, while dry ageing does not. This work has primarily looked at dry ageing, in which reactions are thermally driven, on both bulk and thin film LiH. Thin films of lithium were deposited in-situ on Ni(100) and were converted to LiH through exposure to H2 at 1mbar. Four reactions were found to occur in the dry ageing process. The rate-limiting mechanisms were found to be different in thin films compared to bulk. The rate-limiting mechanism for the decomposition of LiOH was found to be the rate of nucleation in bulk samples, whereas it was the rate of growth of nuclei in thin films. For the solid state reaction (LiH + LiOH -> Li2O + H2), the same mechanism was found to be rate limiting in both cases (i.e. 3D diffusion). However, the activation energy was determined to be significantly higher in a thin film, thought to be a result of a decrease in defect concentration and the number of grain boundaries in the thin film. The Li2O layer formed through the solid state reaction exhibited a high defect concentration and poor crystallinity, attributed to the large lattice mismatch between itself and LiH. This was shown to impact on its subsequent reactivity. On exposure of a LiH thin film to H2O, a chemisorbed H2O state was observed. High purity polycrystalline uranium metal sample was prepared in-situ by ambient temperature Ar+ sputtering. Annealing of the clean uranium sample caused segregation of UO2-x and UOxCy to the surface, similar to that observed in UO2 samples. H2 and H2O have been shown to dissociatively adsorb onto a uranium metal surface, with the latter causing partial surface oxidation. O2 exposure caused irreversible surface oxidation of uranium metal, which could be described by a precursor state model whereby adsorption directly onto the surface is more probable than adsorption mediated by a physisorbed second layer. A high purity UO2 surface was formed by heating the uranium metal in an O2 atmosphere. The interaction of D2O with UO2 at sub-ambient temperatures exhibited the formation of transient ice multilayers. The dynamics of D2 on the oxide surface suggested the occurrence of trap-desorption, rather than rotationally inelastic scattering.