Enhanced Extrinsic Piezoelectric Contribution and Microscopic Origin of Grain Size Effect in Lead-Free KNN-Based Ceramics.

(K,Na)NbO3 (KNN)-based ceramics are considered among the most promising lead-free piezoelectric materials, despite the numerous challenges encountered during the sintering process. To better understand the sintering behavior of KNN-based ceramics, we adopted an innovative approach combining powder activity modulation with a two-step sintering process, successfully obtaining samples with diverse grain morphologies in the KNNTa-BNN ceramics, including uniformly distributed small grains, large grains, and even single-crystalline-like oversized grains. Comprehensive structural analysis reveals that the formation of large grains is associated with the abnormal grain growth. In this process, surface activity factors of calcined powder serve as the primary determinants of grain evolution pathways, while elemental segregation acts as a superimposed perturbing factor. Owing to the differences in T phase content and internal stress between large and small grains, Raman spectroscopy technology was employed to effectively distinguish them. The increase in grain size leads to a rise in long-range ordered non-180° domains, significantly enhancing the extrinsic contribution. As a result, the 6Ta ceramics with appropriately large grain sizes (∼31.65 μm) exhibit excellent comprehensive piezoelectric performance (d33 ∼ 315 pC/N, TC ∼ 390 °C), making it highly competitive among KNN-based ceramics. When the single-crystalline-like oversized grains were developed, the piezoelectric performance (d33 > 500 pC/N) can be further enhanced. This work enriches the grain growth theory of KNN-based ceramics and offers a potential route to high-performance single crystals.