Densification of tungsten-bronze KBa2Nb5O15 lead-free piezoceramics

Densification of tetragonal tungsten-bronze (TTB) structured KBa2Nb5O15 ceramics was studied as a function of extra K2CO3 amounts (δ) ranging from 0 to 5 mol%. The lattice constant c-axis, which corresponds to the spontaneous polarization direction for TTB structure, was remarkably increased compared to a-axis, when δ was increased up to 3. In this case, both of the sintered density and tetragonality of KBa2Nb5O15 ceramics were also enhanced, but were deteriorated in the specimens with higher δ than 3, because of the secondary phase precipitation and inhomogeneous microstructure. The enhanced tetragonality observed in the δ = 3 sample resulted in the increase of the Curie temperature (Tc) i.e., the δ = 3 sample demonstrated the highest Tc (376 °C) which corresponds to about 96% of Tc for the single crystal. Furthermore, diffuse phase transition at Tc was suppressed for the δ = 3 sample, because the optimized amount of excess K2CO3 could compensate effectively for the K-loss defect formed in the TTB structure during sintering process.