The inherent disadvantage of lead-free potassium sodium niobate (KNN)based ceramics is the severe temperature instability of piezoelectric charge coefficient (d 33 ) 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 (d 33 remains almost unchanged in the temperature range of 25-100 °C), far outperforming the previously reported KNN counterparts with d 33 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.

The prominent advances in both piezoelectricity and temperature stability of potassium sodium niobate-based ceramics make this material system the most potential alternative to toxic lead-based families. However, previous studies have shown that the excellent temperature stability of the electrostrain can be obtained only under a high electric field. This issue can be well solved by our new proposed strategy of constructing multilayer composite ceramics, where an extremely low electric-field-dependent temperature stability of the strain can be achieved, far outperforming the results reported so far. The synergistic contributions from stacking components with different strain responses under different temperatures and electric field strengths realize the dynamic balance of electrostrain of the multilayer composite ceramics, which is also revealed by phasefield simulation. This work provides new ideas for the artificial structural design for the development of stable and reliable high-performance piezo/ferroelectric ceramics.

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Niobate-based lead-free piezoceramics have attracted wide attention due to their excellent piezoelectric properties. Although the temperature sensitivity of piezoelectricity or strain in one sample has been solved to a certain extent, how to simultaneously improve the temperature stability of both in one sample is still an issue. Herein, by constructing multilayer composite ceramics and doping Ho element, both improved piezoelectric and strain temperature stability (the variations are below 3% under 30-100 °C) are achieved, showing great property advantage compared with previous reports. Different from the compositionally graded composite ceramic design, the Ho doping can not only increase orthorhombic-tetragonal phase transition temperature (T O-T ) and then create the condition for the formation of successive phase transition, but also stabilize the oriented domain state. Therefore, the excellent temperature stability of both piezoelectricity and strain can be attributed to the multistep phase transition induced by the multilayer design, the fine regulation of T O-T interval by the optimization of lamination combination, and the stabilized polarization induced by Ho doping. The new strategy for solving both piezoelectric and strain temperature sensitivity can further promote the commercial application of potassium sodium niobate-based lead-free piezoelectric ceramics.

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