High-performance KNN-based ceramics: inter-granular coupling effect

In order to achieve the long-term objective of high-performance KNN-based ceramics, BiMnO₃-modified lead-free KNN-based ceramics were fabricated by conventional solid-state method in this work. The R–O–T multi-phases coexistence and the absence of Sb doping lead to a large piezoelectricity (d₃₃ = 440 pC/N), a high Curie temperature (T_C = 310 °C), and a good in situ temperature stability (d₃₃ is more than 300 pC/N below 100 °C). Based on the high performance, the effects of phase structure and microstructure on electrical properties were carried out by different sintering methods. All the results indicate that phase structure plays a major role on the excellent performance. And the sample with bimodal grain size distribution has a better electrical response than the one with uniform coarse grain size distribution, which can be elucidated by the inter-granular coupling effect. For the sample with bimodal grain size distribution, the large grains endure weaker inter-granular constraints from the surrounding refined grains, while much stronger constraints exist in the sample with uniformly coarse grains.