Sol–gel synthesis of 0.94(Bi_0.5Na_0.5)TiO₃–0.06BaTiO₃ lead-free piezoelectric films: effect of pyrolysis temperature on phase evolution and electrical properties

Sol–gel processing was applied to fabrication of 0.94(Bi_0.5Na_0.5)TiO₃–0.06BaTiO₃ (BNT–BT6) lead-free ferroelectric thin films on Pt(111)/Ti/SiO₂/Si(100) substrates, whose phase structure and electrical properties were found to be greatly affected by the pyrolysis temperature. It was found that a large amount of pyrochlore phase formed in the films if pyrolysis was conducted around 350 °C; pyrolyzing at lower and higher temperatures in the range from 300 to 450 °C could avoid its formation even though other conditions are the same. The reasons are revealed by the DSC and TG experiments conducted in inert gas and air, and a phenomenological model was proposed with a focus on the thermal evolution of carbonaceous in pyrolysis and crystallization process. High-quality BNT–BT6 films with a relatively low surface roughness and a dense cross section were obtained by conducting the pyrolysis at the optimized temperature (400 °C), which also show the best ferroelectric and piezoelectric properties.

The effects of pyrolysis temperatures on BNT–BT6 films deposited by the sol–gel technique were investigated. A phenomenological model was built to clarify why phase structures and electrical properties of BNT–BT6 lead-free thin films are greatly affected by pyrolysis temperature.