Browsing by Author "Wilson, Stephen A."
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Item Open Access Characterisation of PZT thin film micro-actuators using a silicon micro-force sensor(Elsevier, 2007-01-08) Duval, Fabrice F. C.; Wilson, Stephen A.; Ensell, Graham; Evanno, Nicolas M. P.; Cain, Markys G.; Whatmore, Roger W.This paper reports on the measurements of displacement and blocking force of piezoelectric micro-cantilevers. The free displacement was studied using a surface profiler and a laser vibrometer. The experimental data were compared with an analytical model which showed that the PZT thin film has a Young's modulus of 110 GPa and a piezoelectric coefficient d31,f of 30 pC/N. The blocking force was investigated by means of a micro-machined silicon force sensor based on the silicon piezoresistive effect. The generated force was detected by measuring a change in voltage within a piezoresistors bridge. The sensor was calibrated using a commercial nano-indenter as a force and displacement standard. Application of the method showed that a 700 μm long micro-cantilever showed a maximum displacement of 800 nm and a blocking force of 0.1 mN at an actuation voltage of 5 V, within experimental error of the theoretical predictions based on the known piezoelectric and elastic properties of the PZT film.Item Open Access Chemical synthesis of nanoparticles and electrohydrodynamic manipulation of nanoparticulate suspensions(Cranfield University, 2009-09) Libor, Zsuzsanna; Wilson, Stephen A.; Zhang, QiThis research concerns the study of nanoparticle synthesis methods and the influence that nanoparticles can have on the physical and electrical properties of non-polar fluids. In this study it is demonstrated that a very small volume fraction of nanoparticles can have a very large effect on the macroscopic properties of fluids. Characterization of nanoparticles and nanofluids has led to the creation of new techniques for controlled deployment of nanoparticles within larger structures. A new dielectrophoretic technique can be used for (1) uniformly sized droplet generation and manipulation with controllable droplet size, (2) oil-in-water emulsion creation in unique way, (3) recycling nanoparticles from nanoparticulate suspensions and (4) creation of core-shell structures. Different types, sizes and morphologies of nanoparticles have been made successfully by chemical synthesis and new synthesis routes have been created. A new wet chemical route has been devised to synthesise nickel nanoparticles with controllable size and different morphology including new shapes such as micro-flower and nano-stars. PZT nanoparticles have been synthesized hydrothermally with controllable size and new morphologies created such as nearly spherical nanoparticles and pellets. A novel wet chemical synthesis method was developed to produce core-shell structures of Ni- and also Fe3O4 - coated SiO2, BT, and PZT particles. The characterization of non-polar fluid-based nanofluids included a sedimentation study, studies of the fluidic properties (viscosity and surface tension) and electrical properties such as DC conductivity and dielectric permittivity. The results clearly show that the macroscopic properties of base fluids (silicone oil and perfluorinated oil) were changed even with a very low concentration (< 0.6 vol %) of nanoparticles added. The properties of nanofluids are found to depend on the properties of the base fluid and also on the properties of the dispersed nanoparticles. Importantly, the properties are demonstrated to depend on the fluid-particle interaction.Item Open Access Electric-field structuring of piezoelectric composite materials(Cranfield University, 1999-09) Wilson, Stephen A.; Whatmore, Roger W.Piezoelectric composite materials, consisting of a ferroelectric ceramic in an electrically-inactive polymer matrix, have been shown to greatly outperform single phase materials for certain applications. A new assembly technique, which electrically controls the spatial distribution of the ceramic within the polymer, promises to enhance the sensitivity of 0-3 type piezoelectric composites. The materials so-produced have a quasi 1-3 structure and it is intended that they will exhibit some of the advantages of 1- 3 piezoelectric composites, whilst retaining the simplicity of 0-3 manufacturing. The electric field structuring technique exploits the electrokinetic phenomenon of dielectrophoresis, which is responsible for the electrorheological effect. When a suspension of ceramic particles in an insulating fluid is exposed to a moderate AC electric field, the particles polarize and as a result they exhibit a mutually attractive force. Under suitable conditions the particles assemble into `pearl-chains', `fibrils' or columns, oriented parallel to the applied field. If the fluid is a resin pre-polymer, this can then be cured and the newly formed structures frozen into place to form a composite material with anisotropic properties. The key process parameters are explored and the implications of employing this method to produce technologically useful materials are discussed. It is demonstrated, for the first time, that dielectrophoresis can be used to induce anisotropic dielectric and piezoelectric properties in 55%vol. fraction ceramic / polymer composites. A model composite system of pure lead titanate in an epoxy resin is considered in basic detail. A method of producing a lead zirconate titanate (PZT) powder with a narrow particle size distribution, by flux growth, has been shown to be effective. New concepts in multiphase composites are introduced, whereby chains are formed within the confines of a second immiscible fluid or where particles of two different materials are mixed in a suspension, each material having its own `polarization signature'.Item Open Access Electrostatic manipulation of piezoelectric fibres using a sharp probe electrode in a dielectric liquid: Analysis of the electrohydrodynamic phenomena(Cranfield University, 2007-05) Capria, Ennio; Wilson, Stephen A.Micro-assembly techniques have been identified as a major technology ‘pillar’ that will underpin further advancements in integrated micro-and nano-systems. In practice, there is a generic requirement for component parts that are often fragile, or that have been prepared by mutually incompatible processes, to be brought together to make a complete working system. This thesis discusses an electrostatic positioning technique for micro-scale elements that could form the basis of an industrial process. A highly non-uniform field generated between a needle-like upper electrode and a bottom flat electrode can be used to electrostatically capture, displace, and relocate elements into a predefined spatial configuration. The very intense field at the needle tip can facilitate the collection of the material at a precise point. However charge injection and local dielectric breakdown must also be considered as they can induce instability near the tip, and consequently interfere with any picking up action. The principal physical phenomena and potential benefits are analysed and discussed, considering three different configurations to achieve the pick and place operation for a micro-fibre in the needle-plane configuration. The first two are operated on an isolated single fibre lying on a flat bottom electrode, applying respectively a DC or an AC voltage. The third case is that of a group of fibres, and it exploits a dielectrophoretic chain structuring effect to assist in the micro-manipulation technique. Experimentation has focussed on the importance of the charge transfer mechanisms, leading to a model which provides good agreement with the observed behaviour. Moreover, an analysis of the forces exerted on the fibres showed that they arise not only from a polarisation effect, but that there is also an electrophoretic contribution. The viability of the proposed technique has been demonstrated using lead zirconate titanate (PZT rods and carbon fibres).Item Open Access Enhanced DC conductivity of low volume-fraction nano-particulate suspensions in silicone and perfluorinated oils.(Iop Publishing Ltd, 2009-03-01T00:00:00Z) Wilson, Stephen A.; Libor, Zsuzsanna; Skordos, Alexandros A.; Zhang, QiThe contrasting DC conductivities of several different types of nanoparticles (nickel, barium titanate and magnetite) suspended in both silicone and perfluorinated oils have been measured and compared. Enhanced DC conductivity through interaction between the particles and the fluid has been demonstrated even at quite moderate fields and different types of nanoparticles have been shown to exhibit different behavioural trends. Whilst the DC enhancement is partly related to the concentration (or spatial arrangement) of the particles as expected, there is clear evidence that energy-activated (electric-field activated) processes also play a major role. It can be said that effective medium theories based solely on the electrical properties and volume fractions of the component materials have limited applicability when assessing the DC conductivity of these nanoparticle-fluid combinations at low volume fractions.Item Open Access Flextensional ultrasonic motor using the contour mode of a square piezoelectric plate.(IEEE Institute of Electrical and Electronics, 2004-08-01T00:00:00Z) Leinvuo, Joni T.; Wilson, Stephen A.; Whatmore, Roger W.This paper presents the design, fabrication, and characterization of a new type of standing wave piezoelectric ultrasonic motor. The motor uses a metallic flextensional amplifier, or cymbal, to convert the contour mode vibrations of a square piezoelectric ceramic plate into flexural oscillations, which are further converted to produce rotary actuation by means of an elastic-fin friction drive. The motor operates on a single-phase electrical supply. A beryllium copper rotor design with three-fin configuration was adopted, and the geometry was varied to include different material thicknesses, fin lengths, and inclinations. The best stall torque and no load speed for a 25-mm square motor were 0.72 Nmm and 895 r/minute, respectively. The behavior of the stator structure was analyzed by ANSYS finite element software using harmonic and modal analyses. The vibration mode estimated by finite element modeling (FEM) was confirmed by laser Doppler vibration measurements.Item Open Access Flextensional ultrasonic piezoelectric micro-motor.(IEEE Institute of Electrical and Electronics, 2006-12-01T00:00:00Z) Leinvuo, Joni T.; Wilson, Stephen A.; Whatmore, Roger W.; Cain, Markys G.This paper presents the experimental design, construction, and operational characteristics of a new type of standing wave piezoelectric ultrasonic micro-motor. The motor uses a composite stator, consisting of a metallic flextensional mode converter, or "cymbal," bonded to a 2-mm-square piezoelectric plate. The cymbal converts contour-mode vibrations of the plate into oscillations in the cymbal, perpendicular to the stator plane. These are further converted into rotational movement in a rotor pressed against the cymbal by means of an elastic-fin friction drive to produce the required rotary actuation. The motor operates on a single-phase electrical supply, and direct control of the output speed and torque can be achieved by adjusting the amplitude and frequency of the supply voltage. Noncontact optical techniques were used to assess the performance of the developed micro-motor. The operational characteristics were developed from the acceleration and deceleration characteristics. No-load output speed (11 rev s-1) and stall torque (27 nNm) were derived using high-speed imaging and image analysis. Maximum efficiency was 0.6%Item Open Access The impact of structural changes on the actuation of polypyrroles(Cranfield University, 2012-08) Melling, Daniel; Jager, Ir. Edwin W. H.; Skordos, Alexandros A.; Wilson, Stephen A.A new non-contact method for characterizing the time-dependent mechanical performance of electro-active polymer films has been developed and is described in detail. We first illustrate our new measuring technique by investigating the impact of film thickness on the actuation performance of polypyrrole. Our method is simple to perform and serves as a valuable tool for studying the long term stability and operational failure of the films, the effects of synthesis conditions and for the optimization of actuator performance. We have used our technique to investigate the impact that structural changes, such as crosslinking, have on the actuation of polypyrroles. An understanding of this relationship is necessary if forms of polypyrrole are to be produced that are capable of greater movement, operating speeds, in service lifetimes and force generation. In order to do this we have developed a logical synthetic strategy (blocking approach) which allows us to change the degree of crosslinking in electro-synthesised polypyrrole. Using our blocking approach we have been able to show the impact that structural changes make on the actuation performance of polypyrroles. We have shown that it is possible to monitor crosslinking and branching changes in polypyrroles indirectly using the irreversible expansion of these films. Our measurements are a form of “dynamic swelling study” and are analogous with solvent swelling studies used in the polymer industry for monitoring cross-linking changes. The irreversible expansion of polypyrrole films has been used to investigate the effects that polymerization potential has upon the levels of cross-linking and branching. We go on to identify the optimal conditions for producing the maximum expansion, strain and strain rate for PPy(DBS). In addition, we have used instrumented indentation as a secondary method for monitoring crosslinking changes. This has provided information that is consistent with those revealed by changes in the irreversible expansion of the polymer. Finally we present an in-depth theoretical discussion of how elemental analysis could be used as a more direct way to quantitatively determine the levels of crosslinking within polypyrroles. This work represents the first study of its kind aimed at understanding the impact that crosslinking and branching has upon the actuation performance of polypyrroles. As a result we are closer to being able to synthesize polypyrroles with improved actuator properties such as greater strains and strain rates.Item Open Access A new flextensional piezoelectric ultrasonic motor - design, fabrication and characterisation(Elsevier, 2007-01-08) Leinvuo, Joni T.; Wilson, Stephen A.; Whatmore, Roger W.; Cain, Markys G.This paper presents the techniques used for the characterisation of a new type of standing-wave piezoelectric ultrasonic motor. The motor uses a metallic flextensional amplifier, or “cymbal”, to convert the radial mode vibrations of a piezoelectric ceramic disc into flexural oscillations, which are further converted to produce rotary actuation by means of an elastic fin friction drive. The motor operates on a single-phase electrical supply. A beryllium copper rotor design with three-fin configuration was adopted. The best stall torque, no load speed, transient time and efficiency for a 25 mm motor were 2 N mm, 680 rpm, 2 ms and 4.8%, respectively. The operational characteristics of the motor were evaluated by using two methods: one based on the pulley–brake principle and one on high-speed imaging. The results obtained from using these two techniques are contrasted and compared.Item Open Access Rheological properties of magnetic and electro-active nanoparticles in non-polar liquids(Springer Science Business Media, 2011-08-31T00:00:00Z) Libor, Zsuzsanna; Wilson, Stephen A.; Zhang, QiThe rheological properties of two non-polar liquids [silicone oil or perfluorinated oil (FC70)] containing various types of particles, barium titanate, nickel and iron oxide, were investigated as functions of solid loading, particle size and shear rate. All the particles were synthesised in- house. The viscosities of either silicone oil or FC70 containing different solid loadings (10, 20 and 30 g/L) were measured over the shear rate range of 0.10-10 s(-1). All the nanofluids showed shear-thinning behaviour within this range and the viscosities increased with the increase of concentrations of nanoparticle and with the decrease of particle size. The highest increase of viscosity was found to be caused by nickel particles in silicone oil due to the formation of Ni network.Item Open Access Structure modification of 0–3 piezoelectric ceramic/polymer composites through dielectrophoresis(Institute of Physics, 2005-01-21) Wilson, Stephen A.; Maistros, George M.; Whatmore, Roger W.Anisotropic material properties can be induced in ceramic/polymer composites by applying an alternating electric field of moderate strength during processing. Under suitable conditions, particles of a ceramic filler material that are randomly dispersed in a liquid polymer or pre-polymer can be polarized and they then exhibit a collective response to localized gradients in the electric field. Typically, the particles experience a mutually attractive force which causes them to form 'pearl-chains' or columnar structures spanning the gap between electrodes. If the fluid is solidified, for example by curing the polymer resin, then the newly formed structures can be fixed in place to produce a composite with directional electrical and mechanical properties. Direct visual observations were made for low volume fraction dispersions of pure lead titanate in an epoxy pre-polymer under the influence of an electric field. The observed interaction was correlated with low-field dielectric measurements and existing theory to identify optimum assembly conditions. The dielectric properties of the fluid are predominant and the formation of chain-like structures is found to be both field strength and field frequency dependent. The dielectric permittivities of a range of structurally modified composites were measured and compared with existing theoretical models of di-phasic materials.Item Open Access Thick-film piezoelectric bimorph actuators for MEMS devices(Cranfield University, 2011-08) Owens, Sam; Wilson, Stephen A.Active ow control can be used to improve the aerodynamic e ciency of passenger aircraft, road tra c and wind turbines amongst other applications. This work describes the fabrication of an ultra-compact microvalve that has been designed as part of an active aerodynamic ow control system that generates airjets of a scale and velocity that have been shown to have desirable e ects on the macroscopic air ow. The design of the microvalve is based on criteria speci ed by the requirements of active ow control and the piezoelectric bimorph actuator which opens and closes the valve outlet. Cont/d.Item Open Access Ultra-precision grinding of piezoelectric ceramic thick films for fabrication of pre-stressed bimorph microactuators and development of new numerical models to assess induced thermal stress in multilayer structure(Cranfield University, 2005-09) Jourdain, R. P.; Wilson, Stephen A.The research programme aims to develop a novel 2.5 dimensional fabrication process for multilayer piezoelectric microactuators. Ultra-precision grinding was used in conjunction with standard micro-fabrication techniques and adhesive bonding technology to create a powerful bimorph structure. The multilayer structure forms the heart of a microvalve device and it comprises piezoelectric ceramic layers joined to a metal shim at relatively low temperature (200° Celsius). I contrast with existing thick deposition techniques this new fabrication technique does not adversely affect the electro-active properties of the ceramic material and it has the further advantage that it is not substrate sensitive making it compatible with the wide range of materials available for micro-scale device construction. The objectives of this project were twofold: > To investigate the mechanism of the material removal process for piezoelectric ceramics using a ultra-precision grinding machine tool on four different piezoelectric materials. The research work investigated both planarization and surface integrity. > To develop different models using the definite element analysis technique to analyse and assess the thermally induced stress due to the bonding process. Direct and initial stress loading conditions were analysed and applied to a multilayer structure. This work ends with a fully characterized symmetric structure for bimorph cantilevers. Grinding is shown to be a suitable machining technique to prepare the surface of piezoelectric ceramic discs. The required surface quality is fully achieved for the target microsystems application. Surface flatness and roughness have been scientifically investigated through a experimental plan. The infinite element analysis reveals some valuable results relating to stress intensity and explores the effect of changes due to thermal expansion coefficient mismatch and the influence of adhesive bond thickness in a multilayer structure. Use of initial stress loading allows simulation of complex processes of fabrication leading to optimization of the structure. h this way a improved fabrication process has been established which avoids any profile deformation. Finally the performance of the bimorph cantilevers is predicted through analytical and numerical modelling and a correlation is established.Item Open Access Ultra-precision grinding of PZT ceramics--Surface integrity control and tooling design(Elsevier, 2009-10) Arai, S.; Wilson, Stephen A.; Corbett, John; Whatmore, Roger W.A comprehensive statistical analysis of the factors controlling surface quality and form in ultra-precision grinding of polycrystalline lead zirconate titanate (PZT) ceramics has been conducted. The work focuses on practical grinding conditions and it includes an assessment of the interactions that exist between the method of material removal and the machine design. In the first phase of experimentation, defects including porosity and the fractural damage induced in the subsurface area were investigated. Machining trials were then conducted which were used to highlight the significant technical factors or combinations of technical factors that influence surface roughness, surface flatness and textural damage. A model for the systematic material removal mechanism which suggests that a relatively large depth of cut and ‘soft contact’ can be used to achieve improved surface integrity is proposed. In order to verify the suggested model, a series of design modifications to the tooling structure were made and the nature of the contact at the material removal interface was studied. Dramatic improvements in surface quality were achieved by incorporating a compliant polymer layer into the vacuum chuck used to hold the ceramics during grinding.