Effect of K content to lead-free SrBi₄Ti₄O₁₅–(Na_0.5Bi_0.5)Bi₄Ti₄O₁₅ piezoelectric ceramics

The high-performance lead-free piezoelectric ceramics based on a mixed bismuth layer-structured ferroelectric Sr_0.9((Na_1−xK_x)_0.5Bi_0.5)_0.1Bi₄Ti₄O₁₅ (x = 0.0, 0.12, 0.16, 0.2, 0.24, 0.28, 0.32) (abbreviated as SNKBT) were synthesized by conventional solid-stated processing. The phase structure, microstructural morphology, and electrical properties of SNKBT ceramics with different K content were studied carefully. With the increase of K content, the lattice parameters, a and b (supposing b > a), were found to be almost unchanged, however, the lattice parameters c was found to increase monotonously. Therefore the unit cell volume increased monotonously. The plate-like grain size of SNKBT ceramics was observed to decrease with the increase of K content. The piezoelectric activities of SNKBT ceramics is improved by the modification of K content. The Curie temperature (T_c) and piezoelectric constant (d₃₃) for the 0.24 mol% K-contented SNKBT solid solutions were found to be 569 °C and 20 pC/N, respectively. Meanwhile the piezoelectric ceramics possess stable aging performance and thermal annealing properties, which, together with the high T_c and d₃₃ demonstrating that the SNKBT materials are promising candidates for high-temperature applications.