R. W. C. Lewis

Microstructural modelling of the polarization and properties of porous ferroelectrics

Micromechanical models of porous ferroelectric ceramics have often assumed that the material is fully polarized in a particular direction and/or consists of a single isolated pore. In this work the polarization state in three-dimensional porous polycrystalline ferroelectric networks has been modelled to eradicate the oversimplification of these idealized unit cells. This work reveals that microstructural network models more closely represent a porous ferroelectric microstructure since they are able to take into account the complex polarization distribution in the material due to the presence of high and low permittivity regions. The modelling approach enables the prediction of the distribution of poled and unpoled material within the structure. The hydrostatic figures of merit and permittivity were determined for a variety of porous lead zirconate titanate microstructures and found to be in good agreement with experimental data. The decrease in piezoelectric activity with porosity was observed to be associated with the complex polarization state within the material. Model results were shown to be much improved when compared to a model assuming a fully polarized model.

Nano-Imprinting of Highly Ordered Nano-Pillars of Lithium Niobate (LiNbO3)

Lithium niobate (LiNbO3) is used in applications such as in optoelectronics, surface acoustic wave devices and transducers. In this paper nano-patterning of LiNbO3 wafers at the wafer scale is investigated using a novel low cost and large-area nano-imprinting technique. The formation of nano-scale pillars is presented by nano-imprinting an ordered nickel etch mask by lift-off on a y-cut LiNbO3 wafer. The process lends itself to the development of novel sensors or high temperature nano-scale harvesting structures. Since one potential application for nano-pillars of piezoelectrics is energy harvesting, calculations of relevant figures of merit for LiNbO3 based composites are also presented.

Finite Element and Experimental Analysis of the Vibration Response of Radially Poled Piezoceramic Cylinders

The harmonic response and modal shapes of axially-symmetrical piezoceramic cylinders (tubes) polarised through the wall thickness have been predicted by finite element methods and determined experimentally. Analysis of ceramic cylinders has concentrated on the effects of the variation of diameter to thickness (d/t) ratios, and change in cylinder length (l). Investigation has taken into account material variance and vibration performance with relation to both ‘hard’ and ‘soft’ type ceramics. Computational finite element modelling (ANSYS) and numerical techniques has allowed for the prediction of the harmonic response and modal shapes, thus enabling the choice of cylinder geometry and performance. Resonant frequencies of piezoceramic cylinders have been determined experimentally by impedance analysis. The changes in resonant frequencies have been determined for a range of d/t and l/d ratios and for a variety cylinder of lengths. Predictions of harmonic response of the piezoceramic cylinders are shown to agree well with experimental results, with identification of the modal shapes.

The structure and properties of electroceramics for bone graft substitution

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.

Freeze cast porous barium titanate for enhanced piezoelectric energy harvesting

Energy harvesting is an important developing technology for a new generation of self-powered sensor networks. This paper demonstrates the significant improvement in the piezoelectric energy harvesting performance of barium titanate by forming highly aligned porosity using freeze casting. Firstly, a finite element model demonstrating the effect of pore morphology and angle with respect to poling field on the poling behaviour of porous ferroelectrics was developed. A second model was then developed to understand the influence of microstructure-property relationships on the poling behaviour of porous freeze cast ferroelectric materials and their resultant piezoelectric and energy harvesting properties. To compare with model predictions, porous barium titanate was fabricated using freeze casting to form highly aligned microstructures with excellent longitudinal piezoelectric strain coefficients, d 33. Both model and experimental data indicated that introducing porosity provides a large reduction in the permittivity () of barium titanate, which leads to a substantial increase in energy harvesting figure of merit, , with a maximum of 3.79 pm2 N-1 for barium titanate with 45 vol.% porosity, compared to only 1.40 pm2 N-1 for dense barium titanate. Dense and porous barium titanate materials were then used to harvest energy from a mechanical excitation by rectification and storage of the piezoelectric charge on a capacitor. The porous barium titanate charged the capacitor to a voltage of 234 mV compared to 96 mV for the dense material, indicating a 2.4-fold increase that was similar to that predicted by the energy harvesting figures of merit.

Understanding the effect of porosity on the polarisation-field response of ferroelectric materials

Abstract This paper combines experimental and modelling studies to provide a detailed examination of the influence of porosity volume fraction and morphology on the polarisation-electric field response of ferroelectric materials. The broadening of the electric field distribution and a decrease in the electric field experienced by the ferroelectric ceramic medium due to the presence of low-permittivity pores is examined and its implications on the shape of the hysteresis loop, remnant polarisation and coercive field is discussed. The variation of coercive field with porosity level is seen to be complex and is attributed to two competing mechanisms where at high porosity levels the influence of the broadening of the electric field distribution dominates, while at low porosity levels an increase in the compliance of the matrix is more important. This new approach to understanding these materials enables the seemingly conflicting observations in the existing literature to be clarified and provides an effective approach to interpret the influence of pore fraction and morphology on the polarisation behaviour of ferroelectrics. Such information provides new insights in the interpretation of the physical properties of porous ferroelectric materials to inform future effort in the design of ferroelectric materials for piezoelectric sensor, actuator, energy harvesting, and transducer applications.

Role of Single-Crystal Pillars in Forming the Effective Properties and Figures of Merit of Novel 1–3 Piezocomposites

This work examines the modelling and potential formation of submicron ferroelectric pillars which are exploited as an active component of modern 1–3 piezocomposites. These structures are of interest in sensor and actuator technologies, and most notably energy harvesting devices. This research area has seen recent growth in terms of interest with the advent of electronics with greater portability and wireless sensors. The fabrication of nanopillars on the single crystal surface is investigated using a nanoimprint lithography (NIL) approach. The use of a disposable master allows the user to reproduce large-area nanostructures and dry etching then allows for the nanopillars to be formed. Results on the prediction of effective electromechanical properties of 1–3-type composites either LiNbO3 single crystal or (1 – x)Pb(Mg1/3Nb2/3)O3 – xPbTiO3 (PMN-PT) single crystal are given for comparison.

Finite element modelling of bilayer porous PZT structures with improved hydrostatic figures of merit

A finite element model is presented in which bilayer lead zirconate titanate (PZT) structures that are formed from a dense layer and a porous layer are investigated for their hydrostatic sensing properties. The model simulates the poling of the porous ferroelectric material to determine the distribution of poled material throughout the structure. The fraction of PZT successfully poled is found to be closely related to resulting piezoelectric and dielectric properties of the composite. Structures with high layer porosity (>40 vol.%) and porous layer relative thickness (>0.5) were found to have a significantly improved hydrostatic piezoelectric coefficient, d<inf>h</inf>, hydrostatic voltage coefficient, g<inf>h</inf>, and hydrostatic figure of merit, d<inf>h</inf>.g<inf>h</inf>. The highest d<inf>h</inf>.g<inf>h</inf> of 7.74 × 10⁻¹² m²/N was observed in the structure with a porous layer relative thickness of 0.6 and porosity of 60 vol.%, which was more than 100 times higher than that for dense PZT (d<inf>h</inf>.g<inf>h</inf> = 0.067 × 10⁻¹² m²/N) and over three times that of PZT with 60 vol.% porosity with 3-3 connectivity (d<inf>h</inf>.g<inf>h</inf> = 2.19 × 10⁻¹² m²/N). The results demonstrate the potential for layered porous materials for use in hydrophones.

The influence of mechanical stress on piezoceramic tubes

Advanced piezoelectric devices increasingly demand materials that can provide both an improved electrical and mechanical response. For many applications, it is vital that the material engineer can characterize how stresses will influence the material and device response. In this paper measurement methods have been developed to simultaneously characterize the mechanical and piezoelectric performance of small lead zirconate titanate (PZT) tubes subjected at high stress. The influence of PZT composition and poling direction has been considered. The effects of stress on the non-linear mechanical response are discussed with respect to observations made by impedance analysis. These findings will guide material selection for enhanced sensor and actuator design.

Electrode ablation on piezoelectric ceramics by NS-pulsed laser ablation for sensor applications

This paper discusses the formation of complex geometry electrode structures on piezoelectric ceramic substrates using nanosecond pulsed laser ablation. Optical microscopy, scanning electron microscopy and surface profiling are used to examine the impact of the laser ablation process onto the substrate material. The influence of laser power, pulse energy and scanning speed on electrode removal and potential substrate damage is discussed. Fabricated prototypes with different electrode designs and potential sensing applications are presented.This paper discusses the formation of complex geometry electrode structures on piezoelectric ceramic substrates using nanosecond pulsed laser ablation. Optical microscopy, scanning electron microscopy and surface profiling are used to examine the impact of the laser ablation process onto the substrate material. The influence of laser power, pulse energy and scanning speed on electrode removal and potential substrate damage is discussed. Fabricated prototypes with different electrode designs and potential sensing applications are presented.