Advances in Applied Ceramics: Structural,Functional and Bioceramics

The perovskite-like layer structured praseodymium titanate (Pr₂Ti₂O₇; PTO) is a good candidate for high temperature piezoelectric application because of its high Curie point. Single phase, dense, textured PTO ceramic was prepared by spark plasma sintering using a two-step method for the first time. The Lotgering orientation factor was 0·87. The Curie point of PTO is >1555±5°C. The dielectric constant increases with grain size due to the decreasing effect of grain boundaries and increasing extrinsic contribution from the ferroelectric domain wall movement. For the first time, PTO was shown to be ferroelectric because it showed piezoelectric activity after poling (d₃₃ = 0·5±0·1 pC N^–1).

A high frequency transducer for medical imaging (25 MHz) was fabricated using a pad printing process to deposit a curved lead zirconate titanate (PZT) thick film on electroded backing (porous PZT). This piezoelectric thick film was characterised, and a thickness coupling factor (47%) comparable with that of a bulk ceramic with similar composition was measured. This transducer was successfully modelled with a numerical tool previously published and specifically adapted to curved shapes. The experimental axial and lateral resolutions are 40 and 230 μm respectively. Moreover, the sensitivity is sufficiently high to consider this transducer to be integrated in an echographic system for high frequency imaging such as skin.

This paper describes the development of screen printed vibration energy harvesters developed at the University of Southampton. The mark 1 harvester developed very low levels of power (2 μW) due to the poor piezoelectric properties of the printed film. Properties were improved by blending particle sizes and optimising firing and poling conditions. The new piezoelectric paste was applied to harvesters developed for the EU funded project TRIADE. Power outputs have improved to 240 μW from an excitation vibration of 0·29g_rms (g = 9·8 m s⁻²) at 67 Hz. Multilayer structures also demonstrate further improvements, and the harvester has been demonstrated powering an autonomous wireless sensor system for condition monitoring.

A strategy to establish a model that takes into account the different pitfalls relative to the simulation of ZnO nanowire (NW) based devices is proposed. Starting from the governing equations of the problem, an equivalent circuit of a ZnO NW working in quasi-static 33 mode is established. It takes into account the semiconducting properties of the NW and the effect of the Schottky contact between the tip of the NW and an electrode. Such a circuit model is expected to become a valuable tool to evaluate the performances of NWs as active elements in integrated systems. As an application, a microgenerator based on an NW array under a compressive strain has been modelled. The influence of the electrode material and the NW dimensions on the microgenerator performances is investigated. It is shown that the NW aspect ratio is of critical importance to achieve a high efficient energy conversion.

In this contribution, we present a review on the various bulk piezoelectric ceramic transformers based on planar vibration modes. The studied transformer shapes include rectangular bar, rings and discs (e.g. disc ring dot) homogeneously poled in the thickness direction. Electrical parameters like transformation ratio, efficiency and input and output power are measured and modelled as a function of frequency and resistive load in the secondary circuit. Analytical modelling has been performed for the ‘Rosen type’ and ‘ring dot’ disc or ring transformers as well as for the bar transformers and several other geometries. Finite element analysis modelling has been performed for the set of other more complicated disc transformer geometries. An overview of the results on the studied transformers is presented, including data on transformation ratio, efficiency and optimum load.

Polypropylene based ferroelectret films exhibit a strong electromechanical activity and provide a promising solution for the air coupled ultrasonic (ACUS) transducers. Ultrasonic transmission between two air coupled ferroelectret transducers in dependence on the amplitude and polarity of the high voltage exciting pulse revealed a strongly non-linear electromechanical response of the ferroelectret transmitter which provides an increase in the transmitter efficiency. The authors present a simple model describing both promotion and competition of the piezoelectric and electrostriction contributions, as well as increase in the transducer constant under high voltage excitation. Enlargement of the inverse transducer constant of the polypropylene ferroelectret film by a factor of 4 was demonstrated. The non-linear properties of the polypropylene ferroelectrets result in a strong increase in their ACUS figure of merit under the high voltage excitation, which exceeds the results of their technological optimisation. Consequently, enhancement of the ACUS system transmission by 12 dB and signal to noise ratio by 32 dB was achieved.

The purpose of this study is to model the non-linear hysteretic behaviour of piezoceramic materials under an external (mechanical or electrical) stress. The model is constructed by bridging the characteristics of microscopic domain distribution into the macroscopic behaviour. Choosing the remanent strain and remanent polarisation as internal variables, a domain orientation distribution is used to describe the evolution of these variables and to develop a phenomenological model of ferroelasticity for electromechanical loading histories. This model is able to reproduce the (longitudinal and transversal) strain and electric displacement in function of uniaxial electrical and mechanical loading. This formulation is validated through a comparison with experimental data of the literature. The evolution of the elastic and piezoelectric constants under mechanical stress is successfully compared as well.