Piezoelectric and dielectric properties of polymer-ceramic composites for sensors

Abstract

The main objective of this PhD thesis is to develop new routes and concepts for manufacturing piezoelectric ceramic-polymer composites with adequate piezoelectric properties while retaining ease of manufacturing and mechanical flexibility and explore new possibilities to maximize especially the voltage sensitivity while taking into account environmentally relevant issues such as avoiding the use of the chemical element Lead in the piezoelectric ceramics. The starting status of the field and the targets are described in Chapter 1. The production of structured Lead Zirconium Titanate (PZT)–epoxy composites, fabricated using dielectrophoresis, is described in Chapter 2. The resulting thread-like arrangement of the PZT particles in the composites was found to enhance the piezoelectric and dielectric constants of the structured composites. The piezoelectric and dielectric properties of the composites as a function of PZT volume fraction were investigated and compared with the corresponding properties of unstructured composites. The experimentally observed piezoelectric and dielectric constants of the structured and unstructured composites could be described by existing theoretical models. It was found that an attractive combination of decent flexibility and good piezoelectric voltage sensitivity could be obtained for structured composites at around 10 vol.% PZT particles loading. In Chapter 3, a Zn based ionomer was used as the new polymer matrix because of its high flexibility, decent electrical conductivity, excellent adhesion to the ceramic phase and most importantly its self-healing potential. The effective poling conditions for PZT-Zn ionomer composites were investigated and the results were compared with those for the reference PZT-EMAA (ethylene methacrylic acid copolymer) composites and monolithic PZT ceramic. The experimentally observed dielectric and piezoelectric coefficient were compared with Yamada’s model. The tensile properties and high cycle fatigue of the composites for large strain levels were also studied. It was demonstrated that the partial loss of sensorial functionality of the composites after high cyclic tensile fatigue could be recovered by thermal healing, due to the self-healing character of the polymer matrix. The goal of Chapter 4 was to develop an approach to quantify the state of poling of the PZT granules inside the Zn-ionomer matrix by using high energy synchrotron X-ray diffraction. For this study, we used a 30 vol.% Zn-ionomer PZT composite which was optimally poled as described in Chapter 3. The poling efficiency, crystallographic texture and lattice strain of the PZT particles inside the polymer matrix were determined and compared with the values for corresponding bulk ceramics reported in literature. It was shown that for an applied macroscopic field of 15 kVmm-1 the PZT particles are effectively poled, leading to a maximum ?(002) domain reorientation volume fraction, of around 0.70. It is also found that a significant tensile lattice strain, ?{111}, of 0.6% occurs in the direction of the applied electric field, indicating the occurrence of residual stresses within the 2-4 µm size diameter particles. The PZT particles within the polymeric matrix were found to experience significant elastic constraints. In Chapter 5 the processing window of PZT based piezoelectric composites was unveiled over the entire phase diagram composition range and a material selection criterion for high g33 composites was formulated. The piezoelectric and dielectric properties for the complete set of PZT ceramics were reported and correlated to their microstructure, polarisation and strain hysteresis loops. The effect of the dielectric and piezoelectric properties of the filler particles on the effective properties of their composites was studied using theoretical models. It was demonstrated that the combination of low dielectric constant and moderately high d33 of the ceramic filler can lead to lead-free piezoelectric composites having sensorial properties not having been reported ever before for composite materials. Finally, in Chapter 6 we describe the preparation of regular single phase cubic Lead- free (K, Na)xLi1-xNbO3 (KNN) piezoceramic particles using a new solid state double calcination processing route. These particles were subsequently used to create random and structured KNN-epoxy composites. Using dielectrophoresis, these cubical KNN particles were structured into one dimensional chains in an epoxy matrix. Composites produced with these powders showed piezoelectric properties about a factor of 2 higher than those of composites processed with conventionally calcined powder. The dielectrophoretically structured KNN-epoxy composites with optimized particle size and morphology showed excellent piezoelectric properties which can replace lead -containing piezoelectric composite for sensor applications in future. In Figure S.1, we present an overview of the properties of the new composites created in the course of this graduation project and note with pleasure that in many cases the properties fall outside the domains of systems known at the start of the thesis project to yield attractive properties.