Abnormal grain growth in (K, Na)NbO3-based lead-free piezoceramic powders

Abnormal grain growth (AGG) is frequently observed in sintered (K, Na)NbO3 (KNN)-based piezoceramics. However, in the present study, abnormal grain growth was unexpectedly discovered in calcined KNN-based powders. To explain the phenomenon, three well-established models that account for the AGG in sintered ceramics were discussed, including (a) liquid-phase-assisted grain growth, (b) two-dimensional nucleation grain growth, and (c) complexion coexistence. However, the AGG in calcined powders was concluded to be none of them, but a consequence of the A-site compositional inhomogeneity in the K2CO3-Na2CO3-Nb2O5 ternary system. Since repeated calcination and ball milling have low efficiency on solving AGG and the accompanied compositional inhomogeneity, abnormal grains were found to coexist with normal grains at a very high calcination temperature, that is, 1000 degrees C. The compositional inhomogeneity is believed to be remaining even after sintering and consequently deteriorate the comprehensive performances, which might be a determinant for the unstable reproduction of KNN-based piezoceramics.