The effect of porosity on the structural and functional (dielectric, ferroelectric, non-linear, and piezoelectric) properties in Ba_0.85Ca_0.15Ti_0.90Zr_0.10O₃ (BCTZ) ceramics were investigated. Various levels of microporosity in the range of 3% to 31% have been produced using poly(methyl-methacrylate) (PMMA) microspheres as sacrificial templates. The structural investigation indicates a phase coexistence, as expected for this composition at room temperature. The maximum permittivity decreases with increasing porosity, from around 7000 (ceramic with 3% porosity) down to 3500 (ceramic with 31% porosity), and Curie temperature shifts from 47°C to 67°C when increasing porosity, related to the possible porosity-induced structural and internal stress modifications. An enhanced piezoelectric response was found in the BCTZ ceramic with intermediate porosity around 18 vol.%, with the highest value of piezoelectric response of 470 pC/N and figure of merit of 65 (pC/N)2. The optimum piezoelectric properties at the intermediate porosity level are related to the contributions related to the field inhomogeneity, as predicted by finite element calculations, but also from other factors such as microstructural changes (pore shape and connectivity) and possible field-induced structural modifications.
Acknowledgment: This work was supported by a grant from the Romanian Ministry of Education and Research, CNCS – UEFISCDI, project no PN-III-P4-ID-PCE-2020-1988 ”Engineering of lead-free porous ceramic materials for piezo-, pyroelectric sensors with energy harvesting applications” (acronim: EnginPOR).