Microstructural engineering for temperature-stable piezoresponse in KNN-based lead-free piezoceramics: a comprehensive review

Potassium sodium niobate (KNN) lead-free piezoceramics are among the most promising candidates to replace lead-based counterparts. However, the limited temperature stability of KNN ceramics remains a critical challenge for practical application. This review provides a comprehensive overview of recent advancements in both the performance and temperature stability of KNN-based piezoceramics. Special emphasis is placed on the correlation between microstructure and temperature stability, with a systematic analysis of key strategies, including diffuse phase transition with multiphase coexistence, polar nanoregions, domain engineering, multilayer gradient doping structure, atomic-scale local ferroelectric state design, and defect engineering. Furthermore, an objective evaluation of these advances is provided to examine the potential mechanisms underlying these strategies. Beyond summarizing recent progress in improving the properties and temperature stability of KNN-based ceramics, this review highlights the intricate interplay between microstructure and piezoelectric performance, offering valuable insights to guide future research and the rational design of high-performance, temperature-stable KNN-based lead-free piezoceramics.