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)NbO -based ceramics entering the application market as Pb-based substitutes. We possess the phase boundary by mimicking Pb(Zr, Ti)O's morphotropic phase boundary structure via the synergistic optimization of diffusion phase boundary and crystal orientation in 0.94(NaK)NbO-0.03BiNaZrO-0.03(BiK)HfO textured ceramics. As a result, a prominent comprehensive performance is obtained, including giant d of 550 ± 30 pC/N and ultrahigh temperature stability (d 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 d change rate of the commercial Pb(Zr, Ti)O-5 ceramics is only about 10%, and more importantly, its d (~ 350 pC/N) is much lower than that of the (K, Na)NbO-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)NbO-based ceramics.