Professor Robert Dorey

There is a great demand from patients requiring skin repair, as a result of poorly healed acute wounds or chronic wounds. These patients are at high risk of constant inflammation that often leads to life-threatening infections. Therefore, there is an urgent need for new materials that could rapidly stimulate the healing process and simultaneously prevent infections. Phosphate-based coacervates (PC) have been the subject of increased interest due to their great potential in tissue regeneration and as controlled delivery systems. Being bioresorbable, they dissolve over time and simultaneously release therapeutic species in a continuous manner. Of particular interest is the controlled release of metallic antibacterial ions (e.g. Ag+), a promising alternative to conventional treatments based on antibiotics, often associated with antibacterial resistance (AMR). This study investigates a series of PC gels containing a range of concentrations of the antibacterial ion Ag+ (0.1, 0.3 and 0.75 mol%). Dissolution tests have demonstrated controlled release of Ag+ over time, resulting in a significant bacterial reduction (up to 7 log), against both non-AMR and AMR strains of both Gram-positive and Gram-negative bacteria (Staphylococcus aureus, Enterococcus faecalis, Escherichia coli and Pseudomonas aeruginosa). Dissolution tests have also shown controlled release of phosphates, Ca2+, Na+ and Ag+ with most of the release occurring in the first 24 h. Biocompatibility studies, assessed using dissolution products in contact with human keratinocyte cells (HaCaT) and bacterial strains, have shown a significant increase in cell viability (p ≤ 0.001) when gels are dissolved in cell medium compared to the control. These results suggest that gel-like silver doped PCs are promising multifunctional materials for smart wound dressings, being capable of simultaneously inhibit pathogenic bacteria and maintain good cell viability.

A unified theoretical model applicable to different types of Triboelectric Nanogenerators (TENGs) is presented based on Maxwell’s equations, which fully explains the working principles of a majority of TENG types. This new model utilizes the distance-dependent electric field (DDEF) concept to derive a universal theoretical platform for all vertical charge polarization TENG types which overcomes the inaccuracies of the classical theoretical models as well as the limitations of the existing electric field-based model. The theoretical results show excellent agreement with experimental TENGs for all working modes, providing an improved capability of predicting the influence of different device parameters on the output behaviour. Finally, the output performances of different TENG types are compared. This work, for the first time, presents a unified framework of analytical equations for different TENG working modes, leading to an in-depth understanding of their working principles, which in turn enables more precise design and construction of efficient energy harvesters.

In this paper, electrohydrodynamic atomization combined with a polymeric micromoulding technique was used to form PZT single element devices using a PZT sol-gel slurry without an etching process. The PZT single element device was initially designed to work as a piezoelectric ultrasonic transducer consisting of a circular or a square of various sizes, which was produced and used to evaluate the process. The resulting PZT device had a homogenous microstructure. It was observed that the relative permittivity of the circular and square single element devices was especially high at small size due to the fringe effect. The results show that the radius and width of the PZT single circular and square element devices with a thickness of 15μm should be bigger than 400μm in order to reduce the fringe effect. © (2009) Trans Tech Publications, Switzerland.

Polymer composite materials are widely used in marine applications where an understanding of the long-term performance is essential for economic, safety and durability requirements. Although moisture absorption in composites has been studied for many years, the relationship between the mechanisms of moisture absorption and consequent changes in material behaviour has received much less attention. Understanding degradation is necessary for developing lifetime assessments. In this work, long-term exposure of a unidirectional carbon fibre epoxy composite material in water has been investigated in relation to mechanical property changes and moisture uptake. Using diffusion modelling and microscopy of the fracture surfaces, it is possible to correlate water absorption to experimental data showing how the position of water in the material causes changes to flexural properties (modulus decreased by 14% and strength by 20%) and in the glass transition temperature, T g reduced by 18%. Flexural modulus has been shown to be most affected by water interaction with the fibre interface; T g by water interaction within the resin; and flexural strength equally by water interaction with both fibre interface and resin.

The microstructural and stress evolution of thick (25 μm) alumina films on dense alumina substrates sintered at temperatures from 1300 °C to 1600 °C has been investigated. In this study the constraint on sintering was monitored in the absence of significant differences in thermal expansion between the film and the substrate. For comparison purposes unconstrained alumina pellets sintered at 1300 °C-1600 °C were also examined. Overall, the constrained alumina densified less than the free alumina, as expected, although at intermediate temperatures densification rates were comparable. Sintering in the direction perpendicular to the substrate was enhanced with respect to that parallel to the substrate as a means of stress relaxation. Using fluorescence spectroscopy the residual stresses of the films parallel to the substrates were measured; residual tensile stresses as high as 450±40 MPa were exhibited by the films. The considerable stress development resulted in cracking and delamination of the film from the substrate, subsequently film constraint was reduced and densification was not impeded. © 2014 The Authors.

Thick PZT films (1 - 20 m) have been prepared using a composite sol gel technique whereby PZT powder and a PZT producing sol are formed into a slurry and spin coated onto silicon wafers. The maximum relative permittivity obtained was approximately 80% of that exhibited by bulk PZT of comparable composition. However, the d33, f and e31, f[1] piezoelectric coefficients were shown to be significantly lower than that of bulk PZT. It has been proposed that the measured value of d33, f is affected appreciably by particle-particle rotation and substrate clamping leading to reduced poling efficiency which may also greatly reduce the value of e 31, f observed. Samples with high levels of porosity have been shown to exhibit a reduced value of d33. This was attributed to 31 and 51 mode piezoelectrically generated charges caused by the bending and shearing of particle-particle bridges. The effect of substrate clamping, on d 33, f and poling, has been studied by monitoring the changes in position and intensity of the (200)/(002) X-ray diffraction (XRD) peaks of composite films. The presence of the substrate was found to introduce tensile stresses parallel to the film plane which distorted the unit cell. Subsequent permanent polarisation following poling was found to be reduced due to the presence of these stresses. The discrepancies between the values of d 33 measured on thick films and bulk ceramics were highlighted as being of particular importance if thick film materials are to be modelled for device applications. Thick film piezoelectric coefficients (i.e. those of the combined film-substrate structure) should not be used in place of material piezoelectric coefficients when attempting to model the behaviour of devices. Such actions would inevitably lead to erroneous results. © 2002 Taylor & Francis.

This paper reports the preparation of dense and substrate-free PZT thick films. Electrohydrodynamic jet deposition and sol infiltration were utilized to produce dense PZT thick film, then wet chemical etching was employed to successfully remove the silicon substrate. Subsequently, a pure PZT thick film having a thickness of 14 µm without substrate was produced. The piezoresponse force microscopy technique was used to examine the piezoelectric constant (d33, f), it was found that the d33 was increased from 71 pm V−1 to 140 pm V−1, having a double increase. It was also observed that the remnant polarization (Pr) and relative permittivity (εr) of PZT film were distinctly improved after the removal of silicon substrate. The experimental result shows that the substrate clamping had great effects on the electrical properties of PZT films and its effect value was evaluated. In addition, the systematic theoretical analysis of the substrate clamping on film was deeply studied. The theoretical analysis agrees well with the experiment results, which can be used to estimate the effect value caused by the substrate clamping.

This paper presents the design and fabrication of a precision designed CSP receiver tube coating machine for research purposes, designed to deposit and examine the properties of novel anti-reflection (AR) coatings possessing a thickness in the nanometre range. The manufacturing process chain and in line thickness control technique are also described.

This paper details the prototyping of a novel three axial micro probe based on utilisation of piezoelectric sensors and actuators for true three dimensional metrology and measurements at micro- and nanometre scale. Computational mechanics is used first to model and simulate the performance of the conceptual design of the micro-probe. Piezoelectric analysis is conducted to understand performance of three different materials silicon, glassy carbon, and nickel - and the effect of load parameters (amplitude, frequency, phase angle) on the magnitude of vibrations. Simulations are also used to compare several design options for layout of the lead zirconium titanate (PZT) sensors and to identify the most feasible from fabrication point of view design. The material options for the realisation of the device have been also tested. Direct laser machining was selected as the primary means of production. It is found that a Yb MaPA based fiber laser was capable of providing the necessary precision on glassy carbon (GC), although machining trials on Si and Ni were less successful due to residual thermal effects.To provide the active and sensing elements on the flexures of the probe, PZT thick films are developed and deposited at low temperatures (

An actuator can be defined as a mechanical device that creates a physical movement within a system. While this definition can encompass many different devices, the focus of this chapter is on printed films that are able to impart an actuation action by virtue of being composed of an active material (piezoelectric, magnetostrictive and shape memory alloy) that deforms mechanically when subjected to an external stimulus. For film-based actuators, actuation is most commonly achieved by coupling the active material with an inactive support structure that induces a bending moment when the active material is made to contract or expand parallel to the film plane. The approaches used to integrate thick active films with a variety of substrates are examined, along with the limitations and microstructural effects that arise as a consequence of co-processing materials. © 2012 Woodhead Publishing Limited. All rights reserved.

The operational stability of organic–inorganic halide perovskite based solar cells is a challenge for widespread commercial adoption. The mobility of ionic species is a key contributor to perovskite instability since ion migration can lead to unfavourable changes in the crystal lattice and ultimately destabilisation of the perovskite phase. Here we study the nanoscale early-stage degradation of mixed-halide mixed-cation perovskite films under operation-like conditions using electrical scanning probe microscopy to investigate the formation of surface nanograin defects. We identify the nanograins as lead iodide and study their formation in ambient and inert environments with various optical, thermal, and electrical stress conditions in order to elucidate the different underlying degradation mechanisms. We find that the intrinsic instability is related to the polycrystalline morphology, where electrical bias stress leads to the build-up of charge at grain boundaries and lateral space charge gradients that destabilise the local perovskite lattice facilitating escape of the organic cation. This mechanism is accelerated by enhanced ionic mobility under optical excitation. Our findings highlight the importance of inhibiting the formation of local charge imbalance, either through compositions preventing ionic redistribution or local grain boundary passivation, in order to extend operational stability in perovskite photovoltaics.

Ferroelectric switching in bulk materials, at modest electric fields, is a relatively fast process, occurring on time scales of microseconds and less. A secondary retarded switching phenomenon also occurs on time scales of seconds and has previously been attributed to defect induced elevated energy barriers between polarisation states. As ferroelectric switching is a thermally activated process the barrier heights are also affected by temperature which is not constant in ferroelectric materials due to the electrocaloric effect. Here an additional EC induced retardation mechanism is proposed whereby EC induced temperature changes repeatedly temporarily prevent further FE switching during cooling cycles.

Lead zirconate titanate (PZT) thick films, a few tens of micrometres thick, are of technological interest for integration with microsystems to create micro electromechanical systems (MEMS) with high sensitivity and power output. This paper examines the challenges faced in integrating thick film PZT with other materials to create functional micro devices. Thermal, chemical and mechanical challenges associated with integration will be examined and potential solutions explored. © (2009) Trans Tech Publications, Switzerland.

The intermittency and discontinuous nature of power generation in Triboelectric Nanogenerators (TENGs) are arguably their most significant drawback, despite the promise demonstrated in low-power electronics. Herein, we introduce a novel technology to overcome this issue, in which, built-in systematic phase shifting of multiple poles is used to design a pseudo direct-current TENG. Unlike previous attempts of constructing near direct-current TENGs that base on the segmentation of electrodes of a sliding mode TENG, this technology introduces a new method that depends on planned excitation of constituent TENG units at different time intervals to obtain the necessary phase shifts, achieved by their structural design that contains an asymmetric spatial arrangement. Therefore, the direct current generation for TENG, which was previously limited to the sliding mode TENG units, are expanded to contact-mode TENGs. The technology allows for continuous and smooth operation of the driven loads and paves the way for a new dawn in energy scavenging from mechanical sources. We use the distance-dependent electric field (DDEF) platform to design the systematic phase shifting technology, which is experimentally demonstrated via a free-standing mode TENG (FSTENG) based design, to power a number of prototype devices. The resultant power output of the TENG indicates a crest factor close to 1.1 at relatively low frequencies, the best reported values for TENGs with contact-mode basic units, to date. This work provides a highly awaited solution to overcome the intermittency and sporadic nature of TENG outputs, thus, promoting the field towards powering next generation autonomous and mobile electronics.

Using thick and thin films instead of bulk functional materials presents tremendous advantages in the field of flexible electronics and component miniaturization. Here, a low-cost method to grow and release large-area, microscale thickness, freestanding, functional, ceramic foils is reported. It uses evaporation of sodium chloride to silicon wafer substrates as sacrificial layers, upon which functional lead titanate zirconate ceramic films are grown at 710 °C maximum temperature to validate the method. The freestanding, functional foils are then released by dissolution of the sacrificial sodium chloride in water and have the potential to be integrated into low-thermal stability printed circuits and flexible substrates. The optimization of the sodium chloride layer surface quality and bonding strength with the underlying wafer is achieved thanks to pre-annealing treatment.

The use of manufacturing methods commonly used for polymer matrix composites (PMCs) in the production of ceramic matrix composites (CMCs), as opposed to more traditional ceramic manufacturing methods, has the potential to reduce the cost of components. This work focuses on three typical PMC manufacturing methods and assesses their suitability for the production of an oxide-oxide porous matrix ceramic composite, starting from a commercially available pre-impregnated Nextel 610®/aluminium oxide material. While all the techniques can be used to produce CMCs, results showed that compared with vacuum bagging and warm pressing, autoclave processing produced the best outcome. It resulted in the most uniform thickness laminates and the lowest macro-porosity, as well as the highest flexural strength. 

Although many lead zirconate titanate (PZT) based MEMS have been demonstrated, the thermal incompatibility problems associated with in-situ fabrication of PZT films on the device substrate remain a major challenge. Process temperatures of 600-700 °C are common for PZT on silicon, however these temperatures can degrade silicon microelectronics and metal interconnects. By depositing the film on a separate fabrication substrate, such as sapphire, and then using a pulsed UV laser to aid its transfer to the device substrate the problems of thermal incompatibility are avoided [1]. In order to gain a better understanding of the effects of the laser radiation on the interfacial region of the film originally adjacent to the sapphire substrate, we are examining the merits of dual-beam SEM focussed-ion- beam etching (FIBSEM, Nova 200 Nanolab, FEI UK Ltd). Microstructural information from SEM, together with preliminary results using FIBSEM are presented.

Even in nonexcitable cells, the membrane potential Vm is fundamental to cell function, with roles from ion channel regulation, development, to cancer metastasis. Vm arises from transmembrane ion concentration gradients; standard models assume homogeneous extracellular and intracellular ion concentrations, and that Vm only exists across the cell membrane and has no significance beyond it. Using red blood cells, we show that this is incorrect, or at least incomplete; Vm is detectable in the extracellular ion concentration beyond the cell surface, and that modulating Vm produces quantifiable and consistent changes in extracellular potential. Evidence strongly suggests this is due to capacitive coupling between Vm and the electrical double layer, rather than molecular transporters. We show that modulating Vm changing the extracellular ion composition mimics the behaviour of voltage-activated ion channel in non-excitable channels. We also observe Vm-synchronised circadian rhythms in extracellular potential, with significant implications for cell-cell interactions and cardiovascular disease.

Ultrasonication is widely used to exfoliate two dimensional (2D) van der Waals layered materials such as graphene. Its fundamental mechanism, inertial cavitation, is poorly understood and often ignored in ultrasonication strategies resulting in low exfoliation rates, low material yields and wide flake size distributions, making the graphene dispersions produced by ultrasonication less economically viable. Here we report that few-layer graphene yields of up to 18% in three hours can be achieved by optimising inertial cavitation dose during ultrasonication. We demonstrate that inertial cavitation preferentially exfoliates larger flakes and that the graphene exfoliation rate and flake dimensions are strongly correlated with, and therefore can be controlled by, inertial cavitation dose. Furthermore, inertial cavitation is shown to preferentially exfoliate larger graphene flakes which causes the exfoliation rate to decrease as a function of sonication time. This study demonstrates that measurement and control of inertial cavitation is critical in optimising the high yield sonication-assisted aqueous liquid phase exfoliation of size-selected nanomaterials. Future development of this method should lead to the development of high volume flow cell production of 2D van der Waals layered nanomaterials.

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.

Silver nanowires are one of the prominent candidates for the replacement of the incumbent indium tin oxide in thin and flexible electronics applications. Their main drawback is their inferior electrical robustness. Here, the mechanism of the short duration direct current induced failure in large networks is investigated by current stress tests and by examining the morphology of failures. It is found that the failures are due to the heating of the film and they initiate at the nanowire junctions, indicating that the main failure mechanism is based on the Joule heating of the junctions. This failure mechanism is different than what has been seen in literature for single nanowires and sparse networks. In addition, finite element heating simulations are performed to support the findings. Finally, we suggest ways of improving these films, in order to make them more suitable for device applications.

Structural heath monitoring of engineering structures is of growing interest due to increased complexity of such structures and the ability to schedule maintenance when it is needed thus preventing unnecessary work or preventing failure. One such method for monitoring the structural health of large scale structures is through the detection of Acoustic Emissions (AE). A novel thick film Acoustic Emission sensor is presented. Piezoelectric thick film AE sensors were fabricated by creating and pattering lead zirconate titanate (PZT) thick films using a powder/sol composite ink deposition technique in conjunction with mechanical patterning of the subsequent films. The resultant AE sensors exhibit a response comparable to commercially available AE sensors. Comparative results between the thick film and commercial sensors will be reviewed and discussed.

The paper presents the development of a novel suspended membrane resistive gas sensor on a ceramic substrate. The sensor is designed and simulated to be fabricated by combining laser milling techniques, conductive ceramic technology, thin film technology, and semiconductor metal oxides. Trenches are created in the alumina substrate in order to define the geometry of the heater using laser processing of the substrate. The heater is completed by filling the trenches with conductive ceramic paste and then baking to remove the solvent from the paste. The next step consists of polishing the surface to obtain a surface roughness small enough for thin film technology. A dielectric (SiO 2 or ceramic) material is then deposited, acting as hot plate and also as electrical isolation between the heater and sensing electrode. The sensing electrode consists of an interdigitated resistor made of Au or Pt with thickness in the range of 2000 -3000 Å. The gas sensitive layer (SnO 2) is deposited by screen printing or spinning. When heated it react with gas molecules and changes its resistivity, thereby acting as a sensor. The final step involves releasing the sensor, enabling it to be suspended on four bridges, to minimise the dissipation of the heat in the substrate. © 2005 IEEE.

Particulate matter is ubiquitous in the environment, however industrial processes have increased the amount released into the air. Here, the authors demonstrate the initial development of a novel sensor capable of detecting airborne particulate matter in real time. Interdigitated microelectrodes (IDT) were printed on a silicon wafer substrate and exposed to Arizona Road Dust in a wind tunnel for periods of 2, 5, and 10 minutes with an air sample mass loading of 9.07mg/m3 at a velocity of 1.7m/s. Impedance measurements were taken every 30 seconds during exposure. The average loading efficiency was calculated to be 31%. Impedance measurements were recorded from the IDT samples showing that the impedance decreased in real time over the 10 minute exposure. The observed capturing coefficient was possibly due to surface-particle interaction phenomena, such as particle bounce, impaction and re-entrainment. IDTs have previously been used to detect nanoparticles within aqueous environments however this is the first report of such electrodes being used to successfully detect airborne particles.

Micromachining techniques, in combination with low temperature ceramic composite sol gel processing, have been used to fabricate annular array thickness-mode piezoelectric micro ultrasonic transducers (Tm-pMUT). The processing techniques of low temperature (720°C) composite sol gel ceramic (sol + ceramic powder) deposition and wet etching will be described and device architectures demonstrated. Using these techniques, high quality PZT materials with near bulk permittivity have been obtained. The Tm-pMUT device resonated in the range of 50-100MHz with a kt of between 0.3 and 0.4 depending on processing conditions. Examples of devices will be presented along with results of electrical and resonance measurements.

Energy harvesting devices based on piezoelectric, pyroelectric, and thermoelectric materials offer an attractive solution for batteryless and wireless sensor nodes for a range of sensor applications. Current devices are typically fabricated using semimanual approaches leading to higher costs and reduced yields as well as significant material wastage. Powder-based thick film devices have been shown to be capable of harvesting milliwatt levels of power while the associated printing technologies offer commercially attractive fabrication solutions. This paper provides a review of examples of recent piezoelectric, pyroelectric, and thermoelectric powder-based thick film energy harvester devices and outlines potential fabrication techniques, ink compositions, and ways to reduce processing temperatures that can be used to create integrated thick film energy harvesting devices. The key to the creation of such devices is the management of thermal budgets and processing environments to ensure the functional properties of the thick films are maximized.

The MEMS (Micro Electro-Mechanical Systems) market returned to growth in 2010. The total MEMS market is worth about $6.5 billion, up more than 11 percent from last year and nearly as high as its historic peak in 2007. MEMS devices are used across sectors as diverse as automotive, aerospace, medical, industrial process control, instrumentation and telecommunications forming the nerve center of products including airbag crash sensors, pressure sensors, biosensors and ink jet printer heads. Part of the MEMS cluster within the Micro & Nano Technologies Series, this book covers the fabrication techniques and applications of thick film piezoelectric micro electromechanical systems (MEMS). It includes examples of applications where the piezoelectric thick films have been used, illustrating how the fabrication process relates to the properties and performance of the resulting device. Other topics include: top-down and bottom-up fabrication of thick film MEMS, integration of thick films with other materials, effect of microstructure on properties, device performance, etc.

In this work, low temperature deposition of ceramics, in combination with micromachining techniques have been used to fabricate a kerfed, annular-array, high-frequency, micro ultrasonic transducer (with seven elements). This transducer was based on PZT thick film and operated in thickness mode. The 27 μm thick PZT film was fabricated using a low temperature (720 °C) composite sol-gel ceramic (sol + ceramic powder) deposition technique. Chemical wet etching was used to pattern the PZT thick film to produce the annular array ultrasonic transducer with a kerf of 90 μm between rings. A 67 MHz parallel resonant frequency in air was obtained. Pulse-echo responses were measured in water, showing that this device was able to operate in water medium. The resonance frequency and pulse-echo response have shown the frequency response presented additional resonance mode, which were due to the lateral modes induced by the small width-to-height ratios, especially for peripheral rings. A hybrid finite-difference (FD) and pseudospectral time-domain (PSTD) method (FD-PSTD) was used to simulate the acoustic field characteristics of two types of annular devices. One has no physical separation of the rings while the other has 90 μm kerf between each ring. The results show that the kerfed annular-array device has higher sensitivity than the kerfless one.

© 2015 Elsevier Ltd. This paper reports the use of a printing technique, called electrohydrodynamic jet printing, for producing PZT thick film micro-scale structures without additional material removing processes. The PZT powder was ball-milled and the effect of milling time on the particle size was examined. This ball-milling process can significantly reduce the PZT particle size and help to prepare stable composite slurry suitable for the E-Jet printing. The PZT micro-scale structures with different features were produced. The PZT lines with different widths and separations were fabricated through the control of the E-Jet printing parameters. The widths of the PZT lines were varied from 80μm to 200μm and the separations were changed from 5μm to 200μm. In addition, PZT walled structures were obtained by multi-layer E-Jet printing. The E-Jet printed PZT thick films exhibited a relative permittivity (er) of ~233 and a piezoelectric constant (d33, f) of ~66pCN-1.

High frequency bending mode membranes are fabricated using a 1μm PZT thick film deposited by sol-gel. Finite Element Analysis (FEA) is used to tailor the membrane radius to a resonant frequency in the 5-10MHz frequency range. Using a radius of 16 and 24μm, devices are produced as individual cells as well as 3x3 and 5x5 arrays. The arrays are designed so that the bottom electrode is common to all cells whereas the top electrode is only common to a column of 5 or 3 cells depending on the type of arrays. Within a column each cell was separated by a wall of silicon (having the length of the substrate of silicon) in order to sustain the array, reduce parasitic vibrations, hence cross-talk. A 16μm radius membrane shows a resonant frequency of 9MHz and a coupling coefficient of 11%. Suspended cells are also investigated to increase frequency further. Two arms-suspended cells resonate at 8.1MHz for a 24μm radius membrane. © 2006 IEEE.

High quality NaCo2O4 thermoelectrics are challenging to process due to the volatile nature of Na, the slow densification kinetics, and degradation of NaCo2O4 above 900–950 °C leading to the formation of Na-poor second phases. Fine grained sol-gel derived powders have been used to enhance the densification kinetics while pre-treatment of the NaCo2O4 powder with NaOH, to provide a Na rich environment, has been shown to mitigate Na loss at elevated temperatures. While insufficient to compensate for Na loss at processing temperatures of 1000 °C and above, at lower temperatures it is able to enhance densification and facilitate the formation of complex crystal structures yielding low thermal conductivity (0.66 Wm−1K−1) coupled with high electrical conductivity (3.8 × 103 Sm−1) and a Seebeck coefficient of 34.9. The resultant room temperature power factor and ZT were 6.19 × 10−6 Wm−1K−2 and 0.0026, respectively.

Under the broad umbrella of chemical solution depositions (CSD), synthesis of thick films (>1 μm) using a combination of sol and particles (consisting of particles >100 nm size) has first been reviewed. Here the sol is used to both enhance the sintering and performance of conventional powder films as well as being integral to the formation of true powder-sol composite films where the sol forms in integral part of the deposited ink. Advantages and limitation of these composite sol-gel processing techniques are considered and deposition routes explored. The subsequent sections are devoted to outline the novel concept of composite thin film synthesis using molecular precursors. Based on the authors' own experience and existing literature, the perspective, potential and possibilities of the electro-ceramic thin films synthesized using sub 100 nm particulate precursor sols has been outlined.