Constrained sintering and electrical properties of BNT–BKT lead-free piezoceramic thick films

Bi0.5(Na0.82K0.18)0.5TiO3 (BNKT) thick films were prepared by screen-printing on the alumina substrate. Grain growth behavior, constrained sintering processing, dielectric properties, ferroelectric properties and piezoelectric properties were studied. The results show that BNKT thick films exhibit a much lower grain growth rate and smaller grain size than bulk samples due to the constrained sintering on rigid substrates. The XRD measurement revealed that the residual tensile stress formed during sintering resulting from the constrained in-plane shrinking. The typical 80 μm thick films demonstrate the relative dielectric constant of 775, dielectric loss of 3.6% at 1 kHz, remnant polarization of 18.6 μC/cm2, coercive field of 61 kV/cm, and longitudinal effective piezoelectric coefficient, d33 of 105 pm/V at room temperature. Compared with BNT-based piezoelectric bulk ceramics, the smaller longitudinal piezoelectric coefficient is related to the clamping effect of the substrate and the limited extrinsic piezoelectric contributions due to the reduced non-180° domain wall motions.