Modifications of structural, dielectric and ferroelectric properties induced by porosity in BaTiO3 ceramics with phase coexistence

The modifications of structural and functional properties induced by the gradual increase of porosity from 4 vol% to 26 vol% produced by addition of polymeric sacrificial templates in BaTiO3 ceramics were investigated in detail. With respect to other previous studies concerning porous ceramics, the present ones show a phase superposition around room temperature, whose relative amount impact on their functional properties. Low/high porosity favours the orthorhombic/tetragonal symmetry, respectively. Permittivity and polarisation (both irreversible and reversible ones) regularly decrease when reducing ceramic density, while dc tunability and P(E) loops of denser ceramics have shown peculiar features at intermediate fields, i.e. modifications of hysteretic-nonhysteretic character of ε(E) dependences at about± (8–10) kV/cm and wasp-waisted P(E) loops. To explain such effects, a simplified Landau-based approached considering variable shear stress contributions (higher in dense ceramics, lower in porous ones) was developed to examine the stability of orthorhombic/tetragonal states as a function of stress and applied field levels. The simulations indicated for dense ceramics the possibility of a field-induced structural transformation at intermediate field range overlapped on the general trend of stress-induced increasing of orthorhombic amount. The porosity-induced modification of properties in BaTiO3 ceramics is related to the value of porosity itself which reduces the amount of ferroelectric active phase, to possible modifications of pore connectivity at various porosity levels, but also by modifications of internal stress level induced by porosity, which impact on the polymorph phase composition.