Hydrothermal Synthesis of Sodium Bismuth Titanate Ferroelectrics

Abstract

We have studied phase formation processes during sodium bismuth titanate, Na0.5Bi0.5TiO3 (NBT), synthesis under hydrothermal conditions at a temperature of 230°C for 24 and 48 h using stoichiometric mixtures of titanium oxide and bismuth hydroxide and NaOH solutions in the concentration range 5–30 M. The results demonstrate that, at sodium hydroxide concentrations in the range ≤10 M, there is almost no reaction between the starting reagents. In the concentration range 10–15 M, two-phase products are formed, containing a perovskite-like phase and the Bi12TiO20 bismuth titanate. Single-phase samples with the perovskite structure are formed at NaOH concentrations from 18 to 20 M, but their composition differs considerably from the stoichiometric one (Na0.61–0.69Bi0.39–0.31TiO2.89–2.81δ0.11–0.19, where δ stands for an oxygen vacancy). In the NaOH concentration range 22–30 M, first partial and then complete Bi3+ dissolution occurs. Increasing the synthesis time helps to stabilize the perovskite phase. The single-phase NBT ceramics have resistivity of 1.3 × 109 Ω cm, dielectric permittivity e/e0 = 360, dielectric loss tangent tan δ = 0.15, longitudinal electromechanical coupling coefficient Kp = 0.068, piezoelectric charge coefficients –d31 = 6.7 pC/N and d33 = 9 pC/N, and piezoelectric voltage coefficients –g31 = 2.07 mV m/N and g33 = 2.84 mV m/N. The relatively low piezoelectric performance of the ceramics is obviously due to the imperfect structure of the synthesized phases.