Ferroelectric and piezoelectric properties of (Na, K)0.5Bi0.5TiO3 lead free ceramics

(1 - x)Na0.5Bi0.5TiO3 - xK0.5Bi0.5TiO3 (NKBT100x; x being the mole ratio) piezoelectric ceramics were prepared by a conventional solid reaction method. The microstructure and ferroelectric properties of NKBT100x solid solution ceramics with compositions of Na0.5Bi0.5TiO3 (NBT)-rich and K0.5Bi0.5TiO3 (KBT)-rich were investigated by SEM, XRD, temperature dependence of dielectric constant, and hysteresis loop, respectively. XRD studies showed that the pure perovskite structure existed in NKBT100x ceramics with x ≤ 0.50, while a small amount of a second phase, K4Ti3O8, appeared in the ceramics with x ≥ 0.70 due to the structural instability of KBT-rich ceramics. The grain size decreased with increasing KBT content, showing that the KBT-rich solid solutions were very difficult to densify fully. The temperature dependence of the dielectric constant of NKBT100x ceramics showed that the depolarization temperature (Td) reached a minimum value at the composition of x = 0.16 (NKBT16) near the morphotropic phase boundary, which suggested that the ferroelectric state of co-existence between the rhombohedral phase of NBT and tetragonal phase of KBT were not as stable as the single phase of NBT and KBT, respectively. The poled density NKBT50 (x = 0.50) showed high piezoelectric properties, high Td and low dielectric loss, which was a good candidate for lead free piezoelectric ceramics working at relatively high temperatures