Local tensile stress field induced thermal stability of piezoelectric properties in BNT-based composite ceramics

Sodium bismuth titanate (BNT)-based lead-free piezoelectric ceramics exhibit favorable electrical properties; however, the thermal depolarization effect limits their operational temperature range. Traditional chemical doping methods encounter difficulties in enhancing the depolarization temperature ( T d ) while preserving piezoelectric properties. In this work, magnesium oxide (MgO) was incorporated into 0.845Na 0.5 Bi 0.5 TiO 3 –0.055BaTiO 3 –0.1Li 0.5 Bi 0.5 TiO 3 (BNLT) ceramics to construct a zero to three composites. The difference in thermal expansion coefficients between the secondary phase and BNLT resulted in a localized stress field, which significantly delayed the thermal depolarization of the BNLT ceramics. Phase and microstructural analyses revealed the formation of a Mg 2 TiO 4 phase, which was distributed at the grain boundaries of the BNLT matrix. Electrical performance tests demonstrated that the T d increased from 61°C to 89.4°C with the addition of MgO. Assessment of interplanar spacing using transmission electron microscopy indicated the presence of tensile strain, which is proposed to stabilize the ferroelectric phase and enhance T d . Furthermore, the properties of composite ceramics BNLT: x %MgO, BNLT: y %Si 3 N 4 , and BNLT: z %Al 2 O 3 , each characterized by different thermal expansion coefficients of the secondary phase, were compared to validate the proposed mechanism.