Compositionally Graded KNN-Based Multilayer Composite with Excellent Piezoelectric Temperature Stability

The inherent disadvantage of lead-free potassium sodium niobate (KNN)-based ceramics is the severe temperature instability of piezoelectric charge coefficient (d₃₃) caused by the polymorphic phase boundary. Herein, a new concept of structural gradient is proposed by designing compositionally graded multilayer composites with multiple successive phase transitions, to solve the challenge of the inferior temperature stability. The structural gradient ceramics exhibit a superior temperature reliability (d₃₃ remains almost unchanged in the temperature range of 25–100 °C), far outperforming the previously reported KNN counterparts with d₃₃ variation above 27% over the same temperature range. The synergistic contribution of the continuous phase transition, the strain gradient, and the complementary effect of each constituent layer leads to the excellent temperature stability, which is also confirmed by phase-field simulation. These findings are expected to provide a new paradigm for functional material design with outstanding temperature stability.