CERES :: Browsing by Author "Dorey, Robert A."

Browsing by Author "Dorey, Robert A."

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Open Access
Apparent Reduction in the Value of the d 33 Piezoelectric Coefficient in PZT Thick Films.
(Taylor & Francis, 2002-01-01T00:00:00Z) Dorey, Robert A.; Whatmore, Roger W.
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 d 33, f and e 31, f [1] piezoelectric coefficients were shown to be significantly lower than that of bulk PZT. It has been proposed that the measured value of d 33, 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 d 33 . 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.
Open Access
Design and development of micro-electromechanical acoustic emission sensors
(Cranfield University, 2012-01) Pickwell, A. J.; Dorey, Robert A.; Mba, David
Non-Destructive Testing (NDT) is a vital technique in modern engi- neering, enabling the monitoring of the structural health of a com- ponent and therefore enabling the prediction of component failure. Once the structural health of a component is known, timely main- tenance can be carried out to prevent component failure which may have resulted in costly downtime or injury. One NDT technique which has been of increasing interest over recent years is Acoustic Emission (AE) monitoring. AE monitoring technology has been successfully combined with preventative maintenance, saving millions of pounds worldwide. Cont/d.
Open Access
Direct writing of lead zirconate titanate piezoelectric structures by electrohydrodynamic atomisation.
(Springer Science Business Media, 2007-12-01T00:00:00Z) Rocks, Sophie A.; Wang, D.; Sun, D.; Jayasinghe, S. N.; Edirisinghe, M. J.; Dorey, Robert A.
Direct writing, consisting of the directed deposition of individual droplets of ceramic suspension using ink jet printing can be used to produce piezoelectric structures with feature sizes in excess of 100 μm. This work presents an alternative direct writing technique, consisting of directed individual droplets produced by electrohydrodynamic atomisation (EHDA), where feature sizes an order of magnitude smaller can be achieved. This technique opens up the possibility of using direct writing technology to produce integrated MEMS devices. Low toxicity lead zirconate titantate (PZT) sol has been used in conjunction with the EHDA process to produce isolated features and lines with dimensions as small as 20 μm. These features, when fired, form the perovskite PZT crystal structure. Using an X–Y stage to move the sample, it is possible to create a variety of structu
Open Access
Effect of constrained sintering on the piezoelectric properties of PZT thick films
(Cranfield University, 2012-02) Tillman, Mark; Dorey, Robert A.
This thesis concerns the processing of thick lead zirconate titanate (PZT) films integrated with rigid substrates. The aim was to better understand the evolution of the microstructure, stress, and electrical properties of the films under constrained sintering conditions. This is an important process to understand because of the degrading effects that constrained sintering has on PZT films, which are employed vastly in commercial applications. It is hypothesised that the better understandings can lead to the development of PZT films that exhibit superior dielectric and piezoelectric properties than those in current production. The shrinkage of PZT films was examined in order to better understand the ways in which the rigidity of the substrate affects densification during sintering. This was done by processing isolated regions of PZT film on silicon substrates. These were sintered using a halogen bulb which exhibited a spot at a temperature of 725°C and with a ramp rate of less than 10 seconds. In this way the sintered regions could be ‘frozen’ mid sintering. The shrinkage of the films was determined at various sintering times. It was found that film shrinkage had finished within 2 minutes of sintering. The evolution of the constrained films during sintering was then examined as a function of the microstructure, stress and electrical property development. It was found that the grain sizes and electrical properties increased within 2 minutes of sintering. However, at longer sintering times there was a degradation of the films. Furthermore, tensile stresses developed during sintering which had degrading effects. This work was expanded upon by motioning the PZT films in a single line scan through the sintering spot to sinter larger areas of the film in one motion. This resulted in a high control over the sintering times, which was vital as the highest electrical properties were found at short sintering times. Next it was examined if the electrical properties could be further increased by applying a compressive stress. It was found that the dielectric properties increased as a result of increased domain wall vibrations. However, there was a decrease in domain reorientation during poling as a result of the effect of the compressive stress, thus the piezoelectric properties reduced. The evolution of PZT films under constrained sintering was better understood as a result of these studies, and led to the development of a sintering method in which the dielectric and piezoelectric properties were increased.
Open Access
Effect of sodium rich pretreatments and processing conditions on microstructure and property evolution of sodium cobalt oxide thermoelectric materials
(Cranfield University, 2014-06) Jakubczyk, Ewa; Dorey, Robert A.; Sansom, Christopher L.
Global environmental and sustainability issues have led to a growth in interest in oxide based thermoelectric materials. Sodium cobalt oxide, which presents low toxicity, is one of the most promising p-type thermoelectric materials for high temperature power generation applications. However, reproducibility and ease of manufacture limits its common use. NaCo2O4 bulk ceramic materials were prepared from powders synthesized using a solid state reaction (SSR) and sol gel (SG) method. The effect of time and temperature of treatment were investigated in order to determine their influence on microstructure and physical properties. The effects of three different Na-enriching pretreatments were evaluated with respect to microstructural evolution and their impact on thermoelectric and electric behaviour of the materials. Such modifications were found to be a critical factor affecting the microstructure of the bulk ceramic materials. The Na-rich pretreatments were found to improve density by up to 15%, increase electrical conductivity and help to compensate for Na loss at high sintering temperatures. The thermoelectric figure of merit ZT was found to increase for Na-rich pretreatment samples due to increases in Seebeck coefficient and low thermal conductivity. The highest value of ZT was found to be for the infiltration pretreatment where the value of 0.025 was observed at 350K. Na rich pretreatments, when compared with unpretreatment samples, reduces thermal conductivity by up to 35%, electrical resistivity by up to 67%, increases Seebeck coefficient by up to 23% and as a consequence increases ZT for ball milling preatreatment by 28%, for mixing preatreatment by 71% and for infiltrating by 250%. A range of films were also produced using a spin coating technique, with thicknesses ranging from 200 nm, for single sol gel layers, up to ~ 32μm for 4 (ink + 2 sol layers) structures. Several factors such as: process conditions, substrates, surfactant and base components used, were investigated in order to improve the quality of films. Process conditions were found to be a critical factor affecting the quality of films. The use of sol infiltration of each layer and a higher preheated temperature were found to reduce surface roughness by up to 23%. The films showed good electrical resistivity ranging from 260 to 500 μΩcm. The lowest value of electrical resistivity was found to be for films annealed at 700ºC.
Open Access
Electrohydrodynamic jet printing of PZT thick film micro-scale structures
(Elsevier, 2015-06-08) Wang, D.; Zhu, X.; Liang, J-S; Ren, T.; Zha, W.; Dong, W.; Rocks, Sophie A.; Dorey, Robert A.; Xu, Z.; Wang, X.
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 80 μm to 200 μm and the separations were changed from 5 μm to 200 μm. In addition, PZT walled structures were obtained by multi-layer E-Jet printing. The E-Jet printed PZT thick films exhibited a relative permittivity (ɛr) of ∼233 and a piezoelectric constant (d33, f) of ∼66 pC N−1.
Open Access
Enhancing thermoelectric properties of NaCo2O4 ceramics through Na pre-treatment induced nano-decoration
(Elsevier, 2019-02-19) Jakubczyk, E. M.; Mapp, A.; Chung, C. C.; Sansom, Christopher L.; Jones, J. L.; Dorey, Robert A.
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.
Open Access
Extending the functionality and efficiency of energy storage tanks in solar power plants
(2011-09-23T00:00:00Z) Sansom, Christopher L.; Dorey, Robert A.; Jones, Paul M.; Peterson, J.; Jakubczyk, Ewa
Improvements to the efficiency of Solar Power Plants are a key objective as the technology matures. Oneopportunity yet to be explored involves energy harvesting from hot components located within the powerplant, utilizing waste heat. We describe two approaches to energy harvesting in this context. These are basedfirstly on TEC device technology, where we describe the use of both commercially available components andour work to develop more efficient TEC devices based on nanostructured oxides. Secondly, we describe analternative thermomagnetic approach based on nanoparticle ferrofluids for thermal scavenging and theconversion of heat to usable electrical energy. For both approaches we present concept designs for theharvesting of waste heat from thermal energy storage (TES) tanks, in order to demonstrate the potential of thetechnology.
Open Access
Formation of PZT thick film single elements using EDHA deposition
(Transtec Publications; 1999, 2011-09-29) Wang, D. Z.; Dorey, Robert A.
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 wasproduced 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 effec
Open Access
The impact of Zero-valent Iron Nanoparticles upon Soil Microbial Communities is Context Dependent
(Ecomed Publishers, 2013-02-01T00:00:00Z) Pawlett, Mark; Ritz, Karl; Dorey, Robert A.; Rocks, Sophie A.; Ramsden, Jeremy J.; Harris, Jim A.
Purpose Nanosized zero valent iron (nZVI) is an effective land remediation tool, but there remains little information regarding its impact upon and interactions with the soil microbial community. Methods nZVI stabilised with sodium carboxymethyl cellulose was applied to soils of three contrasting textures and organic matter contents to determine impacts on soil microbial biomass, phenotypic (phospholipid fatty acid - PLFA), and functional (multiple substrate induced respiration - MSIR) profiles. Results The nZVI significantly reduced microbial biomass by 29% but onlywhere soil was amended with 5% straw. Effects of nZVI on MSIR profiles were only evident in the clay soils, and were independent of organic matter content. PLFA profiling indicated that the soil microbial community structure in sandy soils were apparently the most, and clay soils the least, vulnerable to nZVI suggesting a protective effect imparted by clays. Evidence of nZVI bactericidal effects on Gram negative bacteria and a potential reduction of Arbuscular Mycorrhizal fungi are presented. Conclusion Data implies that the impact of nZVI on soil microbial communities is dependent on organic matter content and soil mineral type. Thereby evaluations of nZVI toxicity on soil microbial communities should consider context. The reduction of AM fungi following nZVI application may have implications for land remediation.
Open Access
Innovative method to produce large-area freestanding functional ceramic foils
(Elsevier, 2018-03-31) Leighton, Glenn J. T.; Jones, Paul M.; Lonne, Quentin; Dorey, Robert A.; Giuliano, Fabien
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.
Open Access
An investigation of the photocatalytic properties of lithium niobate and barium titanate
(Cranfield University, 2012) Stock, Matt; Dunn, S. C.; Dorey, Robert A.
Efficiency of particulate semiconductors for driving photocatalytic reactions is impractically low due to the recombination of excited carriers and intermediate species at the interface. In the literature it has been demonstrated internal depolarisation fields in ferroelectric materials separate electron and hole carriers, this gives rise to spatially distinct reduction and oxidation processes. It is hypothesised this property can supress the rate of back reactions and carrier recombination to improve photocatalytic efficiency. In this thesis the properties of ferroelectric particulates for driving photocatalytic reactions are investigated. Lithium niobate and barium titanate powders were suspended in aqueous solutions of acid black 1 or rhodamine b dye. Adsorption studies compared retention of dye in the double layer by the different powders. Under UV or simulated solar illumination barium titanate or lithium niobate powders photocatalytically decolourised the dye solutions. Powders of lithium niobate powder doped with magnesium or iron showed altered reaction rates and structural selectivity of decolourisation reactions. Photochemical deposition of silver nanoparticles at the surface of the barium titanate or lithium niobate powders increased the rate of photocatalytic decolourisation of rhodamine b solutions under UV or simulated solar illumination. Photochemical reduction of carbon dioxide to form formic acid and formaldehyde over lithium niobate powder was studied under UV illumination. Solid-liquid phase reactions were carried out using aqueous suspensions of powder bubbled with carbon dioxide gas. Solid-gas phase reactions were investigated using a purpose built reaction vessels loaded with carbon dioxide gas and water vapour. Under solid-gas phase conditions the rate of formation of products over lithium niobate powder was greater than over titanium dioxide powder.
Open Access
Investigation of ZnO nanrod solar cells with layer-by-layer deposited CdTe quantum dot absorbers
(Cranfield University, 2011-08-03) Briscoe, Joe; Dunn, S. C.; Dorey, Robert A.
Innovation in solar cell design is required to reduce cost and compete with traditional power generation. Current innovative solar technologies include nanostructured dye-sensitised solar cells and polymer solar cells, which both contain organic materials with limited lifetime. This project aims to combine the advantages of ZnO nanorods and quantum dot (QD) absorbers in an all-inorganic solar cell, using the layer-by-layer (LbL) process to increase light absorption in the cell. Cont/d.
Open Access
Modelling, simulation and optimisation of a piezoelectric energy harvester
(Elsevier, 2014-10-31) Farnsworth, Michael; Tiwari, Ashutosh; Dorey, Robert A.
The power generation efficiency of piezoelectric energy harvesters is dependent on the coupling of their resonant frequency with that of the source vibration. The mechanical design of the energy harvester plays an important role in defining the resonant frequency characteristics of the system and therefore in order to maximize power density it is important for a designer to be able to model, simulate and optimise designs to match new target applications. This paper investigates a strategy for the application of soft computing techniques from the field of evolutionary computation towards the design optimisation of piezoelectric energy harvesters that exhibit the targeted resonant frequency response chosen by the designer. The advantages of such evolutionary techniques are their ability to overcome challenges such as multi-modal and discontinuous search spaces which afflict more traditional gradient-based methods. A single case study is demonstrated in this paper, with the coupling of a multi-objective evolutionary algorithm NSGA-II to a multiphysics simulator COMSOL. Experimental results show successful implementation of the schema with all 5 experimental tests producing optimal piezoelectric energy harvester designs that matched the desired frequency response of 250 Hz.
Open Access
Molten hydroxide synthesis as an alternative to molten salt sythesis for producing K0.5Na0.5NbO3 lead free ceramics
(Springer Science Business Media, 2012-02-01T00:00:00Z) Lusiola, Tony; Bortolani, Francesca; Zhang, Qi; Dorey, Robert A.
Lead-free piezoelectric materials have grown in importance through increased environmental concern and subsequent EU and worldwide legislation, with the aspiration to reduce the use of Pb-based materials in all sectors. Integration of the next generation of lead-free piezoelectric materials with substrates to form functional micro devices has received less attention. Low temperature synthesis methods for K0.5Na0.5NbO3 powder were developed to overcome the issue of poor purity of the final product during high temperature sintering. Molten hydroxide synthesis (MHS), derived from molten salt synthesis (MSS), has been developed to overcome a Na ion preference in the molten salt synthesis reaction that leads to NaNbO3 production instead of K0.5Na0.5NbO3 when stoichiometric amounts of precursors are used. MHS makes use of a KOH molten reaction aid in place of the NaCl/KCl molten salt mix of the MSS. In a two stage reaction K rich intermediate niobates are produced and subsequent reactions with Na species produce KNN.
Open Access
Nanostructured thin films and coatings
(Hindawi Publishing Corporation, 2008-12-31T00:00:00Z) Xiao, P.; Dorey, Robert A.
Open Access
New Advances in Forming Functional Ceramics for Micro Devices
(2006-10-10T00:00:00Z) Dorey, Robert A.; Rocks, Sophie A.; Dauchy, Florent; Navarro, A.
Micro electromechanical systems (MEMS) are finding uses in an increasing number of diverse applications. Currently the fabrication techniques used to produce such MEMS devices are primarily based on 2-D processing of thin films. The challenges faced by producing more complex structures (e.g. high aspect ratio, spans, and multi-material structures) require the development of new processing techniques. Potential solutions to these challenges based on low temperature processing of functional ceramics, selective chemical patterning, and micro-moulding are presented to show that it is possible to create complex functional ceramic structures which incorporate non-ceramic conducting and support structures. The capabilities of both techniques are compared and the relative advantages of each explored.
Open Access
Oxide Thermoelectric Energy Harvesting Materials
(Cranfield University, 2014-11) James, Ashley; Dorey, Robert A.
Conventional thermoelectric materials found in many thermoelectric devices have unfavourable properties; they often suffer instability at high temperatures and contain toxic metals which pose a hazard to the environment. Oxide thermoelectric materials are stable, less toxic and could eventually replace conventional materials. The thermoelectric performance of oxide materials currently do not match conventional materials however, there is potential for improvement through doping and altering the microstructure and chemistry through modification of the processing conditions. This project aims to examine the doping and processing conditions and the effect this has upon the thermoelectric behaviour of oxide based thermoelectric materials. Zinc oxide (ZnO) has been investigated as an oxide thermoelectric material and doping of ZnO with aluminium (Al) and antimony (Sb) by mixed oxide synthesis was investigated. Al2O3 and Sb2O3 were used as aluminium (Al) and antimony (Sb) dopant sources for ZnO, which were reacted with ZnO at temperatures of 1000°C-1300°C. Al was found to incorporate effectively into the ZnO system and was shown to produce n-type behaviour. The Sb doped ZnO material was also found to display n-type behaviour which is intriguing as Sb is considered a p-type dopant in the ZnO system; at low levels <1.0at.%, Sb incorporates onto the Zn site rather than the O site as expected, which leads to n-type behaviour. The addition of Sb dopant leads to the formation of secondary phase of Zn7Sb2O12, which appears to increase the Seebeck coefficient by an energy filtering effect with higher levels of dopant leading to higher levels of secondary phase. Grain size and porosity also play a significant role in both the Al and Sb doped systems with small grains and higher levels of porosity leading to higher values of Seebeck coefficient up to -100µV.K-1 for Al (0.5at.%) and - 115µV.K-1 for Sb (0.8at.%). The ZT figure of merits were found to be highest for materials sintered at 1300°C with values of 6×10-5 and 2×10-10 for Al and Sb doped ZnO respectively, these values are low compared to literature values, which are in the region of 0.01. This is due to high electrical resistivities of the synthesised samples, which is linked to porosity. A better understanding of the effects that microstructure plays on thermoelectric behaviour has been developed and procedures to isolate the contributions from grain size, and degree of dopant incorporation to the thermoelectric properties have been conducted.
Open Access
Patterned crack-free PZT thick films for micro-electromechanical system applications
(Springer Science Business Media, 2007-05-01T00:00:00Z) Dauchy, Florent; Dorey, Robert A.
The fabrication and structuring of multilayer-thick film piezoelectric (PZT-lead zirconate titanate) structures, using composite sol-gel techniques and wet etching is described. The composite sol-gel technique involves producing a PZT powder/sol composite slurry which when spun down, yields films a few micrometres thick. Repeated layering and infiltration has been used to produce PZT films between 10 and 40 μm thick. Due to the low firing temperature (<720°C), it has also been possible to produce PZT films with embedded thin (ca. 100 nm thick) metal electrodes. The PZT thick films have also been structured using a wet etching technique. Examples of features and cavities with lateral dimensions in the order of tens of micrometres are presented. The ability to fabricate and structure thick functional films with embedded metal electrode structures offers the possibility to create novel micro-device structures suitable for use in micro-electromechanical systems (MEMS
Open Access
Pyroelectric ceramics and thin films for applications in uncooled infra-red sensor arrays
(Royal Swedish Academy of Sciences, 2007-11-22T00:00:00Z) Whatmore, Roger W.; Zhang, Qi; Shaw, Christopher P.; Dorey, Robert A.; Alcock, Jeffrey R.
Pyroelectric infra-red detector arrays provide an attractive solution to the problem of collecting spatial information in the far IR. They are only sensitive to changes in the IR flux and are well suited to sensing movements of people. The applications of low cost arrays with limited ( a few hundred) elements for people sensing and radiometry will be illustrated. The performances of uncooled pyroelectric arrays are ultimately driven by the materials used. For this reason, continuous improvements in materials technology and figures-of-merit (FoM) are important. The performance of dense, bulk pyroelectric ceramics has not increased for several years, but nevertheless it is possible to obtain significant improvements in performance through the use of tape-cast, functionally gradient materials (FGMs) in which controlled porosity is used to control the permittivity and heat capacity of the material. A model for the performance of such a material will be presented, and compared with experimentally determined properties of FGMs based on a modified-PZT pyroelectric ceramic. The use of ferroelectric thin films is offering considerable potential for low cost and high performance. It will be shown that the introduction of controlled amounts of porosity can have a significant positive effect on the relevant pyroelectric FoM.