Chris R. Bowen

Piezoelectric and ferroelectric materials and structures for energy harvesting applications

This review provides a detailed overview of the energy harvesting technologies associated with piezoelectric materials along with the closely related sub-classes of pyroelectrics and ferroelectrics. These properties are, in many cases, present in the same material, providing the intriguing prospect of a material that can harvest energy from multiple sources including vibration, thermal fluctuations and light. Piezoelectric materials are initially discussed in the context of harvesting mechanical energy from vibrations using inertial energy harvesting, which relies on the resistance of a mass to acceleration, and kinematic energy harvesting which directly couples the energy harvester to the relative movement of different parts of a source. Issues related to mode of operation, loss mechanisms and using non-linearity to enhance the operating frequency range are described along with the potential materials that could be employed for harvesting vibrations at elevated temperatures. In addition to inorganic piezoelectric materials, compliant piezoelectric materials are also discussed. Piezoelectric energy harvesting devices are complex multi-physics systems requiring advanced methodologies to maximise their performance. The research effort to develop optimisation methods for complex piezoelectric energy harvesters is then reviewed. The use of ferroelectric or multi-ferroic materials to convert light into chemical or electrical energy is then described in applications where the internal electric field can prevent electron–hole recombination or enhance chemical reactions at the ferroelectric surface. Finally, pyroelectric harvesting generates power from temperature fluctuations and this review covers the modes of pyroelectric harvesting such as simple resistive loading and Olsen cycles. Nano-scale pyroelectric systems and novel micro-electro-mechanical-systems designed to increase the operating frequency are discussed.

Pyroelectric materials and devices for energy harvesting applications

This review covers energy harvesting technologies associated with pyroelectric materials and systems. Such materials have the potential to generate electrical power from thermal fluctuations and is a less well explored form of thermal energy harvesting than thermoelectric systems. The pyroelectric effect and potential thermal and electric field cycles for energy harvesting are explored. Materials of interest are discussed and pyroelectric architectures and systems that can be employed to improve device performance, such as frequency and power level, are described. In addition to the solid materials employed, the appropriate pyroelectric harvesting circuits to condition and store the electrical power are discussed.

Fabrication of HA/TCP scaffolds with a graded and porous structure using a camphene-based freeze-casting method

A room temperature camphene-based freeze-casting method was used to fabricate hydroxyapatite/tricalcium phosphate (HA/TCP) ceramic scaffolds. By varying the solid loading of the mixture and the freezing temperature, a range of structures with different pore sizes and strength characteristics were achieved. The macropore size of the HA/TCP bioceramics was in the range of 100-200 microm, 40-80 microm and less than 40 microm at solid loadings of 10, 20 and 30 vol.%, respectively. The initial level of solid loading played a primary role in the resulting porosity of the scaffolds. The porosity decreased from 72.5 to 31.4 vol.% when the solid loading was increased from 10 to 30 vol.%. This resulted in an increase in the compressive strength from 2.3 to 36.4 MPa. The temperature gradient, rather than the percentage porosity, influenced the pore size distribution. The compressive strength increased from 1.95 to 2.98 MPa when samples were prepared at 4 degrees C as opposed to 30 degrees C. The results indicated that it was possible to manufacture porous HA/TCP bioceramics, with compressive strengths comparable to cancellous bone, using the freeze-casting manufacturing technique, which could be of significant clinical interest.

Processing and properties of porous piezoelectric materials with high hydrostatic figures of merit

Porous piezoelectric materials are of interest for applications such as low frequency hydrophones. This is due to their high hydrostatic figures of merit and low sound velocity, which leads to reduced acoustic impedance and enhanced coupling with water or biological tissue. A wide variety of methods are available to produce porous structures such as using reticulated polymer foams or volatile additives which are burnt out during the sintering process (e.g. polymer spheres). Each processing technique and additive produces its own distinctive microstructure, particularly in terms of pore size, morphology and porosity volume fraction. The aim of this paper is to manufacture a variety of porous microstructures and relate the structures to measured hydrostatic figures of merit.

Electrical characterization of hydroxyapatite-based bioceramics.

This paper studies the AC conductivity and permittivity of hydroxyapatite (HA)-based ceramics from 0.1 Hz-1 MHz at temperatures from room temperature to 1000 degrees C. HA-based ceramics were prepared either as dense ceramics or in porous form with interconnected porosity and were sintered in either air or water vapour. Samples were thermally cycled to examine the influence of water desorption on AC conductivity and permittivity. Surface-bound water was thought to contribute to conductivity for both dense and porous materials at temperatures below 200 degrees C. At temperatures below 700 degrees C the permittivity and AC conductivity of HA was also influenced by the degree of dehydration and thermal history. At higher temperatures (700-1000 degrees C), bulk ionic conduction was dominant and activation energies were of the order of approximately 2 eV, indicating that hydroxyl ions are responsible for conductivity.

Electrically Active Bioceramics: A Review of Interfacial Responses

Electrical potentials in mechanically loaded bone have been implicated as signals in the bone remodeling cycle. Recently, interest has grown in exploiting this phenomenon to develop electrically active ceramics for implantation in hard tissue which may induce improved biological responses. Both polarized hydroxyapatite (HA), whose surface charge is not dependent on loading, and piezoelectric ceramics, which produce electrical potentials under stress, have been studied in order to determine the possible benefits of using electrically active bioceramics as implant materials. The polarization of HA has a positive influence on interfacial responses to the ceramic. In vivo studies of polarized HA have shown polarized samples to induce improvements in bone ingrowth. The majority of piezoelectric ceramics proposed for implant use contain barium titanate (BaTiO3). In vivo and in vitro investigations have indicated that such ceramics are biocompatible and, under appropriate mechanical loading, induce improved bone formation around implants. The mechanism by which electrical activity influences biological responses is yet to be clearly defined, but is likely to result from preferential adsorption of proteins and ions onto the polarized surface. Further investigation is warranted into the use of electrically active ceramics as the indications are that they have benefits over existing implant materials.

Porous PZT ceramics for receiving transducers

PZT-air (porous PZT) and PZT-polymer (polymer impregnated porous PZT) piezocomposites with varying porosity/polymer volume fractions have been manufactured. The composites were characterized in terms of hydrostatic charge (d/sub h/) and voltage (g/sub h/) coefficients, permittivity, hydrostatic figure of merit (d/sub h/.g/sub h/), and absolute sensitivity (M). With decreasing PZT ceramic volume, g/sub h/ increased, and d/sub h/.g/sub h/ had a broad maximum around 80 to 90% porosity/polymer content. The absolute sensitivity was also increased. In each case, PZT-air piezocomposites performed better than PZT-polymer piezocomposites. Hydrophones constructed from piezocomposites showed slightly lower measured receiving sensitivities than calculated values for piezocomposite materials, which was due to the loading effect of the cable and the low permittivity associated with the piezocomposites.

Optimal configurations of bistable piezo-composites for energy harvesting

This paper presents an arrangement of bistable composites combined with piezoelectrics for broadband energy harvesting of ambient vibrations. These non-linear devices have improved power generation over conventional resonant systems and can be designed to occupy smaller volumes than magnetic cantilever systems. This paper presents results based on optimization of bistable composites that enables improved electrical power generation by discovering the optimal configurations for harvesting based on the statics of the device. The optimal device aspect ratio, thickness, stacking sequence, and piezoelectric area are considered. Increased electrical output is found for geometries and piezoelectric configurations, which have not been considered previously.

Composite dielectrics and conductors: simulation, characterization and design

Very large networks of randomly positioned resistors and capacitors have been used to simulate the microstructures of real two-phase (conductor–insulator) materials. These networks are found to exhibit fractional power law frequency dependences of dielectric properties and ac conductivity, of the type reported for a wide range of materials. The network results are related to the resistor and capacitor values by a simple logarithmic mixing rule. The same mixing rule is used to model the electrical characteristics of two-phase electrical composites. The results are tested using water impregnated lead zirconate titinate (PZT) ceramics samples that have a microstructure that forms a complex interconnected random array of conducting (water) and insulating regions. Excellent agreement is obtained between the experimental data and the modelling predictions based on the network simulation results. The power law exponents for ac conductivity and relative permittivity are found to be equal to the proportions of the composite occupied by the insulating and conducting phases, respectively. Studies of conducting polymer impregnated PZT are also presented which show less good agreement with modelling predictions.

The Frequency Dependent Permittivity and AC Conductivity of Random Electrical Networks

It has been shown that the universal dielectric response (UDR) of heterogeneous materials can be reproduced by electrical networks consisting of randomly positioned resistor and capacitors. The random network represents a microstructure that contains both insulating (the capacitor) and conductive regions (the resistor). This paper presents an investigation into the frequency dependent properties of large numbers of resistor-capacitor (R-C) networks. Parameters investigated include permittivity, conductivity and phase angle, with particular emphasis on observed power-law behavior and a comparison with previous work in this area.