The effect of processing conditions on the morphology, thermomechanical, dielectric, and piezoelectric properties of P(VDF-TrFE)/BaTiO3 composites

In this study (0–3) P(VDF-TrFE)/BaTiO3 composites containing up to 60 vol% of ceramic phase were prepared by solvent casting or compression molding. Their thermomechanical, dielectric, and piezoelectric properties were investigated, and discussed in the light of the properties of the basic components, the processing route and the resulting morphology. The crystalline structure of the P(VDF-TrFE) matrix was found to be highly dependent on the processing route, while the structure of BaTiO3 was not affected by any of the processing steps. The mechanical properties of the solvent cast materials showed a maximum at 30 vol% BaTiO3, while they increased monotonically with BaTiO3 content for compression molded materials. This difference was attributed to a higher amount of porosity and inhomogeneities in the solvent cast composites. Permittivity as high as 120 and piezoelectric coefficient d33 up to 32 pC/N were obtained for compression molded composites, and the observed decrease in d33 with aging time was attributed to the effect of mechanical stress release in the polymer matrix. Introduction Composites combining a piezoelectric ceramic phase and a polymer phase, either inactive or piezoelectric, have attracted great interest as they offer a unique combination of properties and design flexibility [1, 2]. Piezoelectric and pyroelectric activity, a wide range of dielectric constants and high-breakdown strength are combined with mechanical flexibility, formability, and low acoustic impedance. Furthermore the properties of piezoelectric composites may be tailored by the judicious choice of the polymeric matrix and ceramic filler, of their volume fraction, and of the type of connectivity, making these materials very attractive for applications as sensors and actuators. Binary piezoelectric composites may have different connectivity patterns, which were classified by Newnham et al. [3] and designated by the notation (m–n), where m and n indicate the connectivity of each one of the two phases. According to this nomenclature, composites in which piezoelectric ceramic particles are completely surrounded by a threedimensionally connected polymeric phase have (0–3) connectivity. As (0–3) composites are easier to manufacture and shape than composites with other connectivity patterns, a large research effort is taking place to improve their piezoelectric coefficient. Several polymers have been used as matrices for such composites, including, e.g., epoxy resins [4, 5], chloroprene [6], polyethylene oxide [7], and polyamides [8, 9]. Polyvinylidene fluoride (PVDF) and its copolymer with trifluoroethylene, P(VDF-TrFE), have raised particular interest as they have large relative permittivity and high dielectric strength, and exhibit themselves a relatively large piezoelectric activity. It has been demonstrated for this type of composites that it is possible to polarize independently the polymeric matrix and/or the ceramic inclusions [10–13], therefore adding an additional S. D. Vacche F. Oliveira Y. Leterrier (&) V. Michaud J.-A. E. Månson Laboratoire de Technologie des Composites et Polymères (LTC), Ecole Polytechnique Fédérale de Lausanne (EPFL), 1015 Lausanne, Switzerland e-mail: [email protected] D. Damjanovic Ceramics Laboratory (LC), Ecole Polytechnique Fédérale de Lausanne (EPFL), 1015 Lausanne, Switzerland 123 J Mater Sci (2012) 47:4763–4774 DOI 10.1007/s10853-012-6362-x