Decoupling densification from grain growth: Two-step sintering for high-performance KNN-based piezoelectric-transparent ceramics

Lead-free potassium sodium niobate (KNN)-based piezoelectric-transparent ceramics exhibit challenges including insufficient densification, excessive grain growth, and severe K/Na volatilization during conventional sintering. Here, we systematically compare one-step sintering with two-step sintering for the (1- x %)[(K 0.5 Na 0.5 ) 0.97 Ca 0.03 Nb 0.99 Zn 0.01 O 3 ]- x %Sr(Sc 0.5 Nb 0.5 )O 3 ceramics. Two-step sintering decouples densification from grain growth by leveraging the kinetic difference between grain boundary diffusion and migration, achieving refined grains, improved relative density, and effective suppression of K and Na volatilization. Moreover, it promotes the orthorhombic-to-tetragonal phase transition, increasing the fraction of tetragonal and pseudocubic phases and enhancing overall lattice symmetry. Consequently, the optimal ceramic achieves superior optical transmittance (52.2% at 800 nm) and improved ferroelectric/dielectric/piezoelectric properties, with a piezoelectric coefficient of 128 pC/N, approximately 21% higher than that of the one-step sintering counterpart. This work demonstrates that two-step sintering constitutes a highly effective strategy for simultaneously achieving high transparency and strong piezoelectric response in KNN-based ceramics.