Studies of PMN-PT/polymer composites

Abstract

Lead magnesium niobate-lead titanate (PMN-PT) wit nominal composition of 0.65PMN-0.35PT (abbreviated as PMN-PT) is studied in this project. This composition of PMIN-PT with around 35mo1 % PT is at its morphotrophic phase boundary (MPB) region where it has very high dielectric and piezoelectric properties. In this work, four kinds of samples were fabricated. These include PMN-PT ceramic discs and fibres, PMN-PT / vinylidene fluoride-trifluoroethylene P(VDF-TrFE) 0-3 composites and PMN-PT/ epoxy 1-3 composites. PMIN-PT ceramic powders were prepared by the Columbite method and by the modified sol-gel method. PMIN-PT discs were fabricated using the dry pressing and sintering method. The dielectric and piezoelectric properties of the ceramics were evaluated and the data were used as references in the composite modeling. PMN-PT/epoxy 1-3 composites with ceramic volume fractions ranged from 0.43 to 0.86 were fabricated by a dice-and-fill method. The resonance characteristics of the 1-3 composites were studied by progressively thinning down the composite samples. The modified parallel and series model calculation was used to compare with the measured parameters of the dice-and-fill 1-3 composites. PMN-PT ceramic fibres were fabricated by a modified sol-gel method in which metal alkoxides were used as the starting precursor solutions. Fine oxide powders were mixed with the solution and the gel fibres containing oxide powders were extruded. With this method, crack-free PMN-PT fibres (~100um diameter) could be fabricated after proper heat treatments. Crack-free PMN-PT fibres were incorporated into an epoxy matrix to form 1-3 composites. Ceramic volume fractions ranged from 0.12 to 0.42 were achieved. Using the measured parameters of the epoxy and the 1-3 composites, the relative permittivity, elastic and piezoelectric properties of a single PMN-PT fibre were found. The PMN-PT / P(VDF-TrFE) 0-3 composites were prepared using solvent casting. PMIN-PT powders prepared by the Columbite method were dispersed uniformly in the copolymer matrix and bulk composites are fabricated by hot pressing. Composites with PMN-PT volume fraction of 0.05 to 0.4 have been fabricated. Since the piezoelectric coefficients of ceramics and copolymer are opposite in sign, properties of the 0-3 composites can be modified using different poling conditions. The electric fields acting on the ceramic inclusions and the copolymer matrix were estimated by model calculation. The variation of the coercive field of the 0-3 composites against the PMN-PT volume fraction was also calculated. Original results on the study of PMN-PT ceramics and several kinds of composites have been reported in the thesis.