Electrical and Physical Properties of Lead-Free (Na_0.5K_0.5)NbO₃- Bi_0.5(Na_0.90K_0.10)_0.5TiO₃ Ceramics

Abstract

Extending the investigations on (Na 0.5 K 0.5 )NbO 3 -based solid solution for lead-free piezoelectric ceramics, this paper consider the complex solid-solution system (Na 0.5 K 0.5 )NbO 3 Bi 0.5 (Na 0.90 K 0.10 ) 0.5 TiO 3 [NKN-BNK. (Na 0.5 K 0.5 )NbO 3 with 1 ~ 5 mol% Bi 0.5 (Na 0.90 K 0.10 ) 0.5 TiO 3 has been prepared following the conventional mixed oxide process. It can be concluded that the NKN-BNKT ceramics have orthorhombic structures in the case x 0.03. With increasing BNKT content (x=0.04 to 0.05), however, the structure changes from orthorhombic to rhombohedral phase. Above results demonstrated that the MPB between orthorhombic and rhombohedral phases exits in the solid solution with the BNKT content of x=0.03. At the MPB composition, the cryctalline structure of ceramics is considered to be a coexistence of orthorhombic and rhombohedral phase. Owing to the phase coexistence at the phase boundary, there exists a different symmetry regions (DSR) near the MPB. The DSR boundary motion increases the dielectric permittivity and piezoelectric coefficients. The electromechanical coupling factor and dielectric constant are higher for compositions near the MPB. The dielectric constant (K T 33 ), planar coupling coefficient ( k p ), thickness coupling coefficient ( k t ) and piezoelectric constant (d 33 ) of 0.98NKN-0.02BNKT ceramics were 1180, 30%, 58%, and 180, respectively.