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

We developed a (K,Na)NbO3-based lead-free piezoelectric ceramic with a KTiNbO5 system, (K1− x Na x )0.86Ca0.04Li0.02Nb0.85O3−δ–K0.85Ti0.85Nb1.15O5–BaZrO3–Fe2O3–MgO (K1− x N x N–NTK–FM). K1− 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)NbO3-based ceramic has the Γ15 mode for a wide x range. The nanodomains of orthorhombic (K,Na)NbO3 with the M3 mode coexist within the tetragonal Γ15 mode (K,Na)NbO3 matrix. Successive phase transition cannot occur with increasing x. The maximum k p is observed at approximately the minimum x required to generate the M3 mode phase. Unlike the behavior at the morphotropic phase boundary (MPB) in PZT, the characteristics of K1− 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 M3 mode phase aggregation.