Simultaneous Enhancement of Piezoelectricity and Temperature Stability in KNN-Based Lead-Free Ceramics Via Layered Distribution of Dopants

The past two decades have seen a great enhancement of piezoelectric coefficients (d(33)) to higher than 570 pC/N in (K, Na)NbO3 (KNN) piezoelectrics, but one notoriously unresolved issue is their severe temperature instability, obstructing them toward practical applications. The present work demonstrates a facile approach to overcome this problem by introducing a layered distribution of key dopants (Li and Sb) in a monolithic ceramic, featuring stepwise varied polymorphic phase transition (PPT) temperatures along the thickness direction. The dopant-graded ceramic exhibits an outstanding d(33) of 508 pC/N and a very large piezoelectric strain (S-uni, of 0.18%). More importantly, an excellent temperature stability (d(33) variation within 13% over the temperature range of 25-150 degrees C) is achieved, which is superior to that of most state-of-art KNN counterparts. These are attributed to the construction of spatially diffused PPT in combination with enhanced polarization, permittivity, and piezoresponse through interfacial effect, including the Maxwell-Wagner effect and intergranular stress by gradient doping. The results offer an alternative strategy for designing high-performance piezoelectric materials with desirable temperature reliability.