Lead-free piezoelectric (K,Na)NbO 3 -based ceramic with planar-mode coupling coefficient comparable to that of conventional lead zirconate titanate

We developed a (K,Na)NbO 3 -based lead-free piezoelectric ceramic with a KTiNbO 5 system, (K 1− x Na x ) 0.86 Ca 0.04 Li 0.02 Nb 0.85 O 3−δ –K 0.85 Ti 0.85 Nb 1.15 O 5 –BaZrO 3 –Fe 2 O 3 –MgO (K 1− x N x N–NTK–FM). K 1− x N x N–NTK–FM ceramic exhibits a very dense microstructure and a coupling coefficient of k p = 0.59, which is almost comparable to that of conventional lead zirconate titanate (PZT). The (K,Na)NbO 3 -based ceramic has the Γ 15 mode for a wide x range. The nanodomains of orthorhombic (K,Na)NbO 3 with the M 3 mode coexist within the tetragonal Γ 15 mode (K,Na)NbO 3 matrix. Successive phase transition cannot occur with increasing x . The maximum k p is observed at approximately the minimum x required to generate the M 3 mode phase. Unlike the behavior at the morphotropic phase boundary (MPB) in PZT, the characteristics of K 1− x N x N–NTK–FM ceramic in this region changed moderately. This gentle phase transition seems to be a relaxor, although the diffuseness degree is not in line with this hypothesis. Furthermore, piezoelectric properties change from “soft” to “hard” upon the M 3 mode phase aggregation.