Thermal control is essential to guarantee the optimal performance of most advanced electronic devices or systems. In space, orbital satellites face the issues of high thermal gradients, heating, and different thermal loads mediated by differential illumination from the Sun. Todaýs state-of-the-art thermal control systems provide protection; however, they are bulky and restrict the mass and power budgets for payloads. Here, we develop a lightweight optical superlattice nanobarrier structure to provide a smart thermal control solution. The structure consists of a moisture and outgassing physical barrier (MOB) coupled with atomic oxygen (AO)-UV protection functionality. The nanobarrier exhibits transmission and reflection of light by controlling the optical gap of individual layers to enable high infrared emissivity and variable solar absorptivity (minimum Δα = 0.30) across other wavelengths. The multifunctional coating can be applied to heat-sensitive substrates by means of a bespoke room-temperature process. We demonstrate enhanced stability, energy-harvesting capability, and power savings by facilitating the radiation cooling and facility for active self-reconfiguration in orbit. In this way, the reduction of the operating temperature from ∼120 to ∼60 °C on space-qualified and nonmechanically controlled composite structures is also demonstrated.
(EN)This invention relates to a radiator 10 and to a method of making a radiator. In particular, this invention relates to a radiator 10 having thin-film 5 coatings that serve to increase the thermal emissivity of the entire structure. A radiator 10 is provided comprising a substrate 12, an amorphous carbon layer 16 and the metallic carbide forming layer 14 interposed between the substrate 12 and amorphous carbon layer 16. In addition, a method of making a radiator is provided comprising the steps of forming the metallic carbide-forming layer on 10 a substrate and forming an amorphous carbon layer on the metallic carbide- forming layer.
Some of the key challenges in the applications of graphene and carbon nanotubes are associated with their poor attachment to the substrate and poisoning of the catalyst by environmental contamination prior to the growth phase. Here we report a ‘protected catalyst’ technique which not only overcomes these challenges but also provides a new material production route compatible with many applications of carbon nanomaterials. The breakthrough technique involves capping the catalyst with a protective layer of a suitable material (examples include TiN, Cr, Ta) which protects the catalyst from environmental contaminants such as oxidation, etchant attack, etc., whilst maintaining carbon supply to the catalyst for the CVD growth of desired nanomaterial. A thin Fe catalyst layer remained protected due to the capping layer in the CF4 based reactive-ion-etching of SiO2. We show that the carbon nanostructures grown using this technique exhibit significantly improved adhesion to the substrate in sonication bath tests. We demonstrate the fabrication of 3D structures and CNT based vias in a buried catalyst arrangement using the protected catalyst technique. The technique also allows better control over various growth parameters such as number of graphene layers, growth rate, morphology, and structural quality.
The synthesis of high-quality nanomaterials depends on the efficiency of the catalyst and the growth temperature. To produce high-quality material, high-growth temperatures (often up to 1000 °C) are regularly required and this can limit possible applications, especially where temperature sensitive substrates or tight thermal budgets are present. In this study, we show that high-quality catalyzed nanomaterial growth at low substrate temperatures is possible by efficient coupling of energy directly into the catalyst particles by an optical method. We demonstrate that using this photothermal-based chemical vapor deposition method that rapid growth (under 4 min, which includes catalyst pretreatment time) of high-density carbon nanotubes can be grown at substrate temperatures as low as 415 °C with proper catalyst heat treatment. The growth process results in nanotubes that are high quality, as judged by a range of structural, Raman, and electrical characterization techniques, and are compatible with the requirements for interconnect technology.
We report an in situ imaging method and use it to reveal the mechanism for the formation of extended size sheets of graphene (carpets) in few-layer graphene using the solid-state process, taking place via a layer-by-layer growth mechanism, which can result in a stack of separate individual layers of graphene. This mechanism is revealed by an imaging method that allows the use of conventional (unmodified) scanning electron microscopy to image graphene growth in situ and in real time. With this dynamic imaging, we reveal for the first time the dynamics of flake nucleation and growth and show the dynamics of flake coalescence to form extended size polycrystalline graphene carpets, allowing one to deduce a growth model. This growth method produces graphene flakes with Raman spectral characteristics that closely resemble those from exfoliated flakes obtained using the “Scotch-tape” method. The material is highly electronically intrinsic, with I 2D/I G ratios as high as 5. The kinetics of electronic interconnectivity between flakes during blanket formation is imaged dynamically using a doping level contrast in an electron microscope in real time. Furthermore, the observations reveal that it is possible to maximize the time between the formation of each individual blanket, up to several minutes, thus facilitating the production of multiple decoupled graphene layers of precise thickness. This allows one to control the number of layers produced even when using catalysts of high activity and high-carbon solubility such as Fe.
With the realization of larger and more complex space installations, an increase in the surface area exposed to atomic oxygen (AO) and ultraviolet (UV) effects is expected, making structural integrity of space structures essential for future development. In a low Earth orbit (LEO), the effects of AO and UV degradation can have devastating consequences for polymer and composite structures in satellites and space installations. Composite materials such as carbon fiber-reinforced polymer (CFRP) or polymer materials such as polyetherimide and polystyrene are widely used in satellite construction for various applications including structural components, thermal insulation, and importantly radio frequency (RF) assemblies. In this paper, we present a multilayered material protection solution, a multilayered protection barrier, that mitigates the effects of AO and UV without disrupting the functional performance of tested assemblies. This multilayered protection barrier deposited via a custom-built plasma-enhanced chemical vapor deposition (PECVD) system is designed so as to deposit all necessary layers without breaking vacuum to maximize the adhesion to the surface of the substrate and to ensure no pinhole erosion is present. In the multilayer solution, a moisture and outgassing barrier (MOB) is coupled with an AO and UV capping layer to provide complete protection.
We have investigated the room temperature long channel field effect characteristics of a single graphene layer transistor incorporating a poly-4-vinyl-phenol (PVP) organic insulating layer, as an alternative to conventional oxide gate dielectric materials. High purity copper foils were used in the chemical vapour growth of the graphene layer and visible Raman analysis confirmed the presence of a high quality mono-layer carbon film. Using a channel length of 50 μm, a field effect hole mobility of 37 cm2/Vs was calculated, which demonstrates the possibility of an all carbon graphene based large area transistor with carrier mobilities above those found in conventional long channel all organic electronic transistors.
The ability to engineer a thin two-dimensional surface for light trapping across an ultra-broad spectral range is central for an increasing number of applications including energy, optoelectronics, and spectroscopy. Although broadband light trapping has been obtained in tall structures of carbon nanotubes with millimeter-tall dimensions, obtaining such broadband light–trapping behavior from nanometer-scale absorbers remains elusive. We report a method for trapping the optical field coincident with few-layer decoupled graphene using field localization within a disordered distribution of subwavelength-sized nanotexturing metal particles. We show that the combination of the broadband light–coupling effect from the disordered nanotexture combined with the natural thinness and remarkably high and wavelength-independent absorption of graphene results in an ultrathin (15 nm thin) yet ultra-broadband blackbody absorber, featuring 99% absorption spanning from the mid-infrared to the ultraviolet. We demonstrate the utility of our approach to produce the blackbody absorber on delicate opto-microelectromechanical infrared emitters, using a low-temperature, noncontact fabrication method, which is also large-area compatible. This development may pave a way to new fabrication methodologies for optical devices requiring light management at the nanoscale.
Despite the "darker than black" association attributed to carbon nanotube forests, here is shown that it is also possible to grow these structures, over heat-sensitive substrates, featuring highly transmissive characteristics from the UV to infrared wavelengths, for forest heights as high as 20 μm. The optical transmission is interpreted in terms of light propagation along channels that are self-generated by localized bundling of tubes, acting as waveguides. A good correlation is shown between the distribution of diameter sizes of these sub-wavelength voids and the transmission spectrum of the forests. For the shorter visible and near-UV wavelengths, this model shows that light propagates by channeling along individual vertical voids in the forests, which elucidates the origin for the widely-reported near-zero reflectance values observed in forests. For the longer infrared wavelengths, the mode spreads over many nanotubes and voids, and propagates along a "homogeneous effective medium". The strong absorption of the forest at the shorter wavelengths is correlated in terms of the stronger attenuation inside a waveguide cavity, according to the λ attenuation dependency of standard waveguide theory. The realization of this material can lead to novel avenues in new optoelectronic device design, where the carbon nanotube forests can be used as highly conducting "scaffolds" for optically active materials, whilst also allowing light to penetrate to significant depths into the structure, in excess of 20 μm, enabling optical functionality. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
Vertically aligned carbon nanotubes (VACNTs) present an exciting avenue for nanoelectronics due to their predetermined orientation and exceptional transport capabilities along the tube length, with the potential to be employed in a variety of optoelectronic applications. However, growth of VACNTs using conventional chemical vapor deposition (CVD) methods requires elevated temperatures (>720 °C) and therefore, the suitability of commonly used transparent conductive oxide (TCO) glasses, such as fluorine‐doped tin oxide (FTO) and indium‐tin oxide (ITO), as the substrates for nanotube growth are limited by their temperature‐sensitive nature. Here, the successful growth of multi‐walled VACNTs directly onto commonly used TCO glasses, FTO and ITO, using the photo‐thermal chemical vapor deposition (PTCVD) growth method is reported. The benefit of reflection, within the infrared region, of the TCO substrate and the effect of surface roughness on the growth of VACNTs is investigated. The application of VACNTs on ITO in inverted planar perovskite solar cells is investigated, which shows superior charge transfer, larger grain sizes in the perovskite film, and a champion device efficiency approaching 16%. Vertically aligned carbon nanotubes are grown directly onto temperature‐sensitive transparent conductive oxide glass; the morphology, quality and electrical properties are analyzed and used to fabricate optimized patterned carbon nanotube forest films which are used in perovskite solar cells to improve charge extraction resulting in a champion efficiency approaching 16%.
We report a method for the growth of carbon nanotubes on carbon fibre using a low temperature growth technique which is infused using a standard industrial process, to create a fuzzy fibre composite with enhanced electrical characteristics. Conductivity tests reveal improvements of 510% in the out-of-plane and 330% in the in-plane direction for the nanocomposite compared to the reference composite. Further analysis of current-voltage (I-V) curves confirm a transformation in the electron transport mechanism from charge - hopping in the conventional material, to an Ohmic diffusive mechanism for the carbon nanotube modified composite. Single fibre tensile tests reveal a tensile performance decrease of only 9.7% after subjecting it to our low temperature carbon nanotube growth process, which is significantly smaller than previous reports. Our low-temperature growth process uses substrate water-cooling to maintain the bulk of the fibre material at lower temperatures, whilst the catalyst on the surface of the carbon fibre is at optimally higher temperatures required for carbon nanotube growth. The process is large-area production compatible with bulk-manufacturing of carbon fibre polymer composites. © 2014 Elsevier Ltd. All rights reserved.
We report prediction of selected physical properties (e.g. glass transition temperature, moduli and thermal degradation temperature) using molecular dynamics simulations for a difunctional epoxy monomer (the diglycidyl ether of bisphenol A) when cured with p-3,30 -dimethylcyclohexylamine to form a dielectric polymer suitable for microelectronic applications. Plots of density versus temperature show decreases in density within the same temperature range as experimental values for the thermal degradation and other thermal events determined using e.g. dynamic mechanical thermal analysis. Empirical characterisation data for a commercial example of the same polymer are presented to validate the network constructed. Extremely close agreement with empirical data is obtained: the simulated value for the glass transition temperature for the 60 C cured epoxy resin (simulated conversion a = 0.70; experimentally determined a = 0.67 using Raman spectroscopy) is ca. 70–85 C, in line with the experimental temperature range of 60–105 C (peak maximum 85 C). The simulation is also able to mimic the change in processing temperature: the simulated value for the glass transition temperature for the 130 C cured epoxy resin (simulated a = 0.81; experimentally determined a = 0.73 using Raman and a = 0.85 using DSC) is ca. 105–130 C, in line with the experimental temperature range of 110–155 C (peak maximum 128 C). This offers the possibility of optimising the processing parameters in silico to achieve the best final properties, reducing labour- and material-intensive empirical testing. 2013
Carbon nanomaterials offer a number of possibilities for sensing platforms. The ability to chemically functionalize the surfaces of the nano-carbon, using hybrid or nano-composite structures, can further enhance the material properties. Complementary to the addition of any requisite chemical or biochemical functionality, such enhancements can take the form of improved electrical, optical or morphological properties which improve the transduction capabilities of the carbon nano-material, or facilitate detection of the transduced signal, for example by improving charge transfer to detection electronics. Here we review the methods of producing hybrid and nano-composite carbon structures for sensing systems, highlighting the advantages of the functionalization in each case and benchmark their performance against existing carbon-only devices. Finally, we detail some of the recent applications of hybrid and nano-composite carbon technologies in a wide variety of sensor technologies.
W.T. Young, S.R.P. Silva, J.V. Anguita, J.M. Shannon, K.P. Homewood, B.J. Sealy
Carbon fibre reinforced polymers (CFRP) were introduced to the aerospace, automobile and civil engineering industries for their high strength and low weight. A key feature of CFRP is the polymer sizing - a coating applied to the surface of the carbon fibres to assist handling, improve the interfacial adhesion between fibre and polymer matrix and allow this matrix to wet-out the carbon fibres. In this paper, we introduce an alternative material to the polymer sizing, namely carbon nanotubes (CNTs) on the carbon fibres, which in addition imparts electrical and thermal functionality. High quality CNTs are grown at a high density as a result of a 35 nm aluminium interlayer which has previously been shown to minimise diffusion of the catalyst in the carbon fibre substrate. A CNT modified-CFRP show 300%, 450% and 230% improvements in the electrical conductivity on the ‘surface’, ‘through-thickness’ and ‘volume’ directions, respectively. Furthermore, through-thickness thermal conductivity calculations reveal a 107% increase. These improvements suggest the potential of a direct replacement for lightning strike solutions and to enhance the efficiency of current de-icing solutions employed in the aerospace industry.
A new model which comprehensively explains the working principles of contact-mode Triboelectric Nanogenerators (TENGs) based on Maxwell’s equations is presented. Unlike previous models which are restricted to known simple geometries and derived using the parallel plate capacitor model, this model is generic and can be modified to a wide range of geometries and surface topographies. We introduce the concept of a distance-dependent electric field, a factor not taken in to account in previous models, to calculate the current, voltage, charge, and power output under different experimental conditions. The versatality of the model is demonstrated for non-planar geometry consisting of a covex-conave surface. The theoretical results show excellent agreement with experimental TENGs. Our model provides a complete understanding of the working principles of TENGs, and accurately predicts the output trends, which enables the design of more efficient TENG structures.
In this paper we present preliminary work on group IV photonic waveguides that may be suitable for mid-infrared wavelengths. Fabrication and experimental results for two waveguide structures are given.
© 2015.A methylnadic anhydride-cured diglycidylether of bisphenol A, is prepared and characterised and a mono-epoxy POSS reagent added (0.5-4wt-%) to produce a series of nanocomposites. Two reaction mechanisms are observed involving esterification at lower temperatures (60-180°C) and etherification at temperatures above 180°C. Using the Ozawa and Kissinger methods, the activation energy for the first reaction was found to be 87-90kJ/mol and 122-124kJ/mol for the second reaction. Incorporation of POSS into the epoxy-anhydride network increases the Tg and cross-link density, indicating a more rigid network, but the values do not follow a trend based solely on POSS content. The char yield increases with POSS content with very little change in the degradation temperature. Incorporation of POSS (1wt-%) can reduce the moisture uptake in the cured resin by ~25% at 75% relative humidity. This is accompanied by a lower impact on glass transition temperature: the Tg is reduced by 10K at saturation, compared with 31K for the unmodified epoxy.
We report prediction of selected physical properties (e.g. glass transition temperature, moduli and thermal degradation temperature) using molecular dynamics simulations for a difunctional epoxy monomer (the diglycidyl ether of bisphenol A) when cured with p-3,30 -dimethylcyclohexylamine to form a dielectric polymer suitable for microelectronic applications. Plots of density versus temperature show decreases in density within the same temperature range as experimental values for the thermal degradation and other thermal events determined using e.g. dynamic mechanical thermal analysis. Empirical characterisation data for a commercial example of the same polymer are presented to validate the network constructed. Extremely close agreement with empirical data is obtained: the simulated value for the glass transition temperature for the 60 C cured epoxy resin (simulated conversion a = 0.70; experimentally determined a = 0.67 using Raman spectroscopy) is ca. 70–85 C, in line with the experimental temperature range of 60–105 C (peak maximum 85 C). The simulation is also able to mimic the change in processing temperature: the simulated value for the glass transition temperature for the 130 C cured epoxy resin (simulated a = 0.81; experimentally determined a = 0.73 using Raman and a = 0.85 using DSC) is ca. 105–130 C, in line with the experimental temperature range of 110–155 C (peak maximum 128 C). This offers the possibility of optimising the processing parameters in silico to achieve the best final properties, reducing labour- and material-intensive empirical testing.
Octopus-like carbon nanofibres with leg diameters as small as 9 nm are reported, with a high yield over large areas, using a unique photo-thermal chemical vapour deposition system. The branched nature of these nanostructures leads to geometries ideal for increasing the surface area of contacts for many electronic and electrochemical devices. The manufacture of these structures involves a combination of a polyacrylonitrile/polysiloxane film covering the surface of cupronickel catalysts, supported on silicon. Acetylene is used as the carbon feedstock. High-resolution electron microscopy revealed a relationship between the geometry of the nanoparticles and the catalytic growth process, which can be tuned to maximise geometries (and therefore the surface area) and was obtained with a catalyst size of 125 nm. The technique proposed for growing these carbon octopi nanostructures is ideal to facilitate a new in situ transfer film process to place high-density carbon structures on secondary surfaces to produce high capacitance all-carbon contacts.
The quest to develop materials that enables the manufacture of dimensionally ultra-stable structures for critical-dimension components in spacecraft, has led to much research and evolution of carbon-fibre reinforced polymer materials (CFRP) over many decades. This has resulted in structural designs that feature a near-zero coefficient of thermal expansion. However, the dimensional instabilities that result from moisture ingression and release remains the fundamental vulnerability of the matrix, which restricts many such applications. Here, we address this challenge by developing a space-qualifiable physical surface barrier that blends within the mechanical properties of the composite, thus becoming part of the composite itself. The resulting enhanced composite features mechanical integrity and strength that is superior to the underlying composite, whilst remaining impervious to moisture and outgassing. We demonstrate production capability on a model-sized component for Sentinel-5 mission and demonstrate such capability for future European Space Agency (ESA) and National Aeronautics and Space Administration (NASA) programs such as Copernicus Extension, Earth Explorer and Science Cosmic Visions.
For carbon nanotubes (CNTs) to be exploited in electronic applications, the growth of high quality material on conductive substrates at low temperatures (
The growth of graphene on Ni using a photo-thermal chemical vapor deposition (PT-CVD) technique is reported. The non-thermal equilibrium nature of PT-CVD process resulted in a much shorter duration in both heating up and cooling down stages, thus allowing for a reduction in the overall growth time. Despite the reduced time for synthesis compared to standard thermal chemical vapor deposition (T-CVD), there was no decrease in the quality of the graphene film produced. Furthermore, the graphene formation under PT-CVD is much less sensitive to cooling rate than that observed for T-CVD process. Growth on Ni also allows for the alleviation of hydrogen blister damage that is commonly encountered during growth on Cu substrates and a lower processing temperature. To characterize the film’s electrical and optical properties, we further report the use of pristine PT-CVD grown graphene as the transparent electrode material in an organic photovoltaic device (OPV) with poly(3-hexyl)thiophene (P3HT)/phenyl-C61-butyric acid methyl ester (PCBM) as the active layer where the power conversion efficiency of the OPV cell is found to be comparable to that reported using pristine graphene prepared by conventional CVD.