Ultrahigh thermal stability and piezoelectricity of lead-free KNN-based texture piezoceramics

The contradiction between high piezoelectricity and uniquely poor temperature stability generated by polymorphic phase boundary is a huge obstacle to high-performance (K, Na)NbO₃ -based ceramics entering the application market as Pb-based substitutes. We possess the phase boundary by mimicking Pb(Zr, Ti)O₃'s morphotropic phase boundary structure via the synergistic optimization of diffusion phase boundary and crystal orientation in 0.94(Na_0.56K_0.44)NbO₃-0.03Bi_0.5Na_0.5ZrO₃-0.03(Bi_0.5K_0.5)HfO₃ textured ceramics. As a result, a prominent comprehensive performance is obtained, including giant d₃₃ of 550 ± 30 pC/N and ultrahigh temperature stability (d₃₃ change rate less than 1.2% within 25-150 °C), representing a significant breakthrough in lead-free piezoceramics, even surpassing the Pb-based piezoelectric ceramics. Within the same temperature range, the d₃₃ change rate of the commercial Pb(Zr, Ti)O₃-5 ceramics is only about 10%, and more importantly, its d₃₃ (~ 350 pC/N) is much lower than that of the (K, Na)NbO₃-based ceramics in this work. This study demonstrates a strategy for constructing the phase boundary with MPB feature, settling the problem of temperature instability in (K, Na)NbO₃-based ceramics.