Effects of quenching and annealing on conductivity and ferroelectric properties of Na_0.5Bi_0.5TiO₃-NaNbO₃  ceramics

Quenching procedures are effective in enhancing the low depolarization temperature of sodium bismuth titanate (NBT) relaxor ferroelectrics. In the present work, the influence of thermal processing parameters on the structure and electrical properties of NBT ceramics modified with 5 mol% sodium niobate (NBT-5NN) are evaluated. It is shown that quenching leads to higher electrical conductivity due to reduction at high temperatures, resulting in a higher oxygen vacancy concentration. The quenched materials also exhibit enhanced rhombohedral distortion and ferroelectric properties. The reduction process is largely reversible by reoxidation during annealing at intermediate temperatures in the region of 550 °C, which helps to recover the bulk resistivity to values close to those of the as-sintered materials. On the other hand, the rhombohedral distortion and ferroelectric properties were not affected significantly by the annealing procedure, indicating that the mechanisms responsible for the modification of electrical conductivity and structural distortion/ferroelectric properties have a different origin.