The strange case of enhancing the resistivity by extended sintering time in (Bi0.5Na0.5)TiO3–BaTiO3 solid solution

Abstract

With the presence of volatile elements in the composition of a ceramic material, longer sintering time presumably creates more vacancies. In the case of (Bi0.5Na0.5)TiO3–BaTiO3 lead-free piezoelectric ceramics and when batched and calcined with a Bi2O3 deficient stoichiometry, there can be a surprising increase in resistivity with increasing sintering time. It is deduced that the extended sintering time reduces the concentration of bismuth vacancies (and the ionically compensating oxygen vacancies). This is also associated with a decrease of depolarization temperature, Td, and possibly a change in dielectric polarization phenomena from normal to relaxor ferroelectric. These observations do not occur in pure (Bi0.5Na0.5)TiO3, suggesting that they are actually facilitated by the Ba2+ content in the solid solution BNT−BT. Microstructural observations indicate the formation of BaTi2O5 secondary phase which considerably grows as sintering goes on. With more Ti4+ being incorporated to the secondary phase, the stoichiometry of the solid solution can be restored, so is the resistivity and the observed trends in decreased Td.