Combined effects of Li content and sintering temperature on polymorphic phase boundary and electrical properties of Li/Ta co-doped (Na, K)NbO_3 lead-free piezoceramics

摘要

Crystallographic structure, phase transition and electrical properties of lead-free (Na_(0.535)K_(0.485)_(1-x)Li_x (Nb_(0.942)Ta_(0.058))O_3 (x = 0.042-0.098) (NKL_xNT) piezoelectric ceramics were investigated. The experimental results show that both Li content and sintering temperature strongly affect the orthorhombic-tetragonal polymorphic phase boundary (PPB), which results in remarkable differences of the piezoelectric property and its temperature stability in the NKL_xNT ceramics. Chemical analysis indicates that sodium volatilizes more seriously than potassium and lithium with increasing sintering temperature. Due to the comprehensively optimized effects of Li content and sintering temperature, an enhanced piezoelectric constant d_(33) (276 pC/N) was obtained at room temperature in the ceramics with x = 0.074 sintered at 1000℃. In the same composition, a further high d_(33) up to 354 pC/N was obtained at 43℃, which is close to its T_(o-t) temperature. Furthermore, better temperature stability can be obtained when x = 0.082 sintered at 1000℃, whose piezoelectric constant d_(33) (236 pC/N) keeps almost constant from room temperature to 100℃. Such a temperature-independent piezoelectric property is available in the NKL_xNT ceramics with high Li content because its T_(o-t) was moved below room temperature.