Structure, ferroelectric and piezoelectric properties of Bi_0.5(Na_0.8K_0.2)_0.5TiO₃ modified BiFeO₃–BaTiO₃ lead-free piezoelectric ceramics

A new lead-free solid solution of (0.75 − x)BiFeO₃–0.25BaTiO₃–xBi_0.5(Na_0.8K_0.2)_0.5TiO₃ + 1 mol% MnO₂ has been prepared by a conventional ceramic technique and the effects of Bi_0.5(Na_0.8K_0.2)_0.5TiO₃ and sintering temperature on the structure, ferroelectric and piezoelectric properties of the material have been studied. The ceramics sintered at 960 °C for 2 h possess a pure perovskite structure and no second phases can be detected. After the addition of Bi_0.5(Na_0.8K_0.2)_0.5TiO₃, a morphotropic phase boundary of rhombohedral and orthorhombic phases is formed at x = 0.01. The addition of a small amount of Bi_0.5(Na_0.8K_0.2)_0.5TiO₃ can promote the grain growth, while excess Bi_0.5(Na_0.8K_0.2)_0.5TiO₃ causes an inhibition of grain growth. Sintering temperature has an important influence on the structure and electrical properties of the ceramics. The sintering temperature of 960 °C is a critical temperature to obtain the ceramics with good piezoelectric properties. For the ceramic with x = 0.01 sintered at/above 960 °C located at the morphotropic phase boundary, large grains, good densification, high resistivity and enhanced electrical properties are obtained.