Papers by Author: Chris R. Bowen

Abstract: The paper examines the dynamic properties of bistable cross-shaped laminate plates for broadband energy harvesting applications by converting mechanical vibration energy into the electrical power output. Bistable laminates plates coupled to piezoelectric transducers were excited by application of harmonic excitations and exhibited a range of vibration patterns. The vibration patterns included single-well oscillations and snap-through vibrations of both periodic and chaotic character; such vibration patterns led to a different power output. Classical spectral analysis of measured voltage, displacement and velocity time histories indicated the presence of a variety of nonlinear and chaotic phenomena. As a result, an analysis of the measured displacement and voltage time histories was carried out with the use of the Recurrence Plots and the Recurrence Quantification Analysis methods. The Recurrence Plots method was used for detection of qualitative changes in the dynamic behaviour of the non-linear harvesting system. In order to facilitate interpretation of piezoelectric voltage and laminate displacement, a detailed analysis using Recurrence Plots, Recurrence Quantification Analysis was employed.

Abstract: This paper describes the study of lime/metakaolin pastes using impedance spectroscopy during the first three weeks after mixing. Changes in the composition, structure and morphology were obtained using scanning electron microscopy, energy dispersive analysis of X-rays, differential scanning calorimetry, thermal gravimetric analysis and mercury intrusion porosimetry. The impedance data was studied with reference to changes in ac conductivity, relative permittivity and bulk resistance. Electrical behaviour was found consistent with structural and compositional changes. This study highlights the importance of impedance spectroscopy as a method for the non destructive monitoring and evaluation of lime based hydraulic binders.

Abstract: This paper characterises the properties of unsymmetric bistable carbon fibre composites actuated using a single piezoelectric macro fibre actuator. A piezoelectric–laminate combination under two-axis constraint was evaluated in terms of its load–deflection characteristics, snap-through force and stiffness. The load–deflection characteristics indicate that snap-through force and laminate stiffness increase as a function of piezoelectric actuator drive-voltage. Both snap-through force and laminate stiffness are significantly increased under two-axis constraint, as compared to single-axis constraint. Snap-through force and stiffness are shown to be affected by localised curvature variation caused by laminate ageing.

Abstract: This paper reports experimental and modelling results on the manufacture and properties of hydroxyapatite / BaTiO3 ceramic composites and studies their electromechanical properties with ferroelectric ceramic volume fractions, mFC ³ 0.7. In these composites the bio-active properties of hydroxyapatite are combined with the electromechanical properties of a perovskite-type ferroelectric BaTiO3 ceramic in an attempt to create a novel polarised bone-substitute material. Experimental results of the volume fraction dependences of the effective piezoelectric coefficients * 31 d (mFC), * 33 d (mFC) and dielectric permittivity e *s 33 (mFC) of stress free samples are analysed within the framework of a modified model of a porous piezo-active material that is described in terms of 1–3 (one-dimensional rods in a continuous matrix) and 2–2 connectivity (laminates). The role of several structural elements and physical factors in forming the electromechanical properties of the composites is discussed. It is shown that performance of these materials typical properties are 5 pC / N < | * 31 d |< 45 pC / N, 20 pC / N < * 33 d < 100 pC / N and 400 < e *s 33 / 0 e < 1300.

Abstract: Titanium oxide (TiO2) nanopowders can be reproducibly formed by hydroxylation of titanium organic complexes. The crystallisation to anatase and rutile can be controlled by systematic calcination and a complex range of properties optimized for specific applications. Characterisation of the powders has been undertaken using advanced physical techniques. The morphology of the TiO2 powders is determined by solution concentration and precipitation phenomena, particularly temperature and stirring regime. However the fine powders have limitations in terms of processing flexibility particularly when nanostructured and organised features are desired, due to their fine particle structure and inability to be sintered without undergoing complete phase change. Anodising titanium metal can overcome these difficulties and under appropriate conditions semi-ordered nanotubes of TiO2 have been prepared. These can be heat treated to develop the phase of choice, anatase or rutile. A mechanism for the formation of the nanotubes has been proposed which is based on the linkage of pores developed in the anodized oxidation product. The pores are driven to into alignment by the applied potential and link up to form the tubular structures. A degree of control of the tube size and wall thickness is shown possible by control of applied voltage. The nanotubes have been investigated using SEM, TEM, XRD and Raman spectroscopy to elucidate the structure and postulate the formation mechanism.

Abstract: This paper studies the effects of porosity, sintering atmosphere and composition on the polarisation behaviour of calcium phosphate (CaP) ceramics. Hydroxyapatite (HA) and α- tricalcium phosphate (α-TCP) materials were prepared as both dense discs and as porous samples with interconnecting porosity. Materials were sintered in either air or water vapour at 1300°C and poled at a variety of temperatures, applied electric fields and time periods to optimise the polarisation properties of the materials. Thermally Stimulated Depolarisation Current (TSDC) measurements were used to investigate the degree of polarisation of the HA and α-TCP ceramics. A high depolarisation current was observed for both the porous HA and α-TCP ceramics. Sintering in water vapour also increased the depolarisation current due to reduced dehydration reactions, which result in a greater concentration of hydroxyl ions.

Abstract: The demand in the medical industry for load bearing materials is ever increasing. The techniques currently used for the manufacture of such materials are not optimized in terms of porosity and mechanical strength. This study adopts a microstructural shape design approach to the production of open porous materials, which utilizes spatial periodicity as a simple way to generate the models. A set of triply periodic surfaces expressed via trigonometric functions in the implicit form are presented. A geometric description of the topology of the microstructure is necessary when macroscopic properties such as mechanical strength, stiffness and isotropy are required to be optimised for a given value of volume fraction. A distinction between the families of structures produced is made on the basis of topology. The models generated have been used successfully to manufacture both a range of structures with different volume fractions of pores and samples of functional gradient material using rapid prototyping.

Abstract: Hydroxyapatite (HA) and barium titanate (BT) powders were mixed and sintered to form hydroxyapatite – barium titanate (HABT) ceramics. These materials were then poled and their piezoelectric properties were measured. The microstructure of unpoled samples was examined using scanning electron microscopy (SEM).The piezoelectric constants (d33 and d31) of the ceramics were found to be dependent on the proportion of BT in the ceramic In materials containing less than 70% BT, no piezoelectric effect was found. Above this value, the piezoelectric constant increased with the addition of BT up to a value of 108pCN-1 for pure BT. Values of d33 for ceramics containing more than 80% BT are above values previously shown to have a positive influence on bone growth in vivo. SEM analysis indicated that the grain size within the materials decreased as the proportion of BT in the material was reduced. Examination of the microstructure of the ceramics indicated the presence of electrical domains in the 100% BT and 95% BT ceramics. Domains were not visible below 95% BT. The reduction in grain size may influence the reduction in piezoelectric activity within the materials but cannot be considered to be the only cause.

Abstract: This paper describes the effective electromechanical properties of hydroxyapatite-BaTiO3 0-3 composites as a potential bone substitute material. The permittivity, d3j and g3j piezoelectric coefficients are calculated for composites at a range of compositions and connectivities to understand the relationships between composite composition and piezoelectric and dielectric properties. The properties of the composites can be tailored by adjusting the volume fraction and aspect ratio of the BaTiO3 particles and the shape and level of porosity in the hydroxyapatite matrix.

Abstract: Unsymmetrical carbon fibre/epoxy composites with bonded piezoelectric fibre actuators were investigated as a means to shape change, or morph, composite structures. A carbon fibre cantilever was examined along with its response to applied strains (from piezoelectric actuators), with emphasis on the characterisation of shape/deflection and the reproducibility of the shape/deflection of the structure.