Research Progress on Sintering Aids for (K, Na)NbO₃ Based Lead-Free Piezoelectric Ceramics

（K, Na）NbO₃ （KNN） based piezoelectric ceramics are currently the most promising lead-free piezoelectric ceramic systems. However, these materials are difficult to achieve densification due to reasons such as the volatilization of alkali metal ions during sintering at high temperatures. Adding sintering aids can effectively lower the sintering temperature and inhibit the volatilization of alkali metal ions. It is one of the common methods to improve the compactness and piezoelectric properties of KNN based lead-free piezoelectric ceramics. The working mechanism of sintering aids is briefly described, and the commonly used sintering aids in KNN based lead-free piezoelectric ceramics at present and their action mechanisms are comprehensively summarized. The influence of various sintering aids on the sintering characteristics, microstructure, piezoelectric characteristics of KNN based lead-free piezoelectric ceramics is reviewed. Finally, the future development direction of sintering aids for KNN based lead-free piezoelectric ceramics is given.