Enhancing depolarization temperature and thermal stability of KNN-based piezoelectric ceramics through Li substitution and Al2O3 doping

Obtaining both large piezoelectric responses and broad service temperature range of ferroelectric ceramics is a key challenge for practical applications due to the so-called piezoelectric coefficient (d33)-Curie temperature (TC) trade-off. Here we report the strategy of employing Li substitution and Al2O3 doping in order to enhance the depolarization temperature (Td) and thermal stability of (K,Na)NbO3 (KNN)-based lead-free piezoelectric ceramics. All KNN-based ceramics are fabricated using conventional solid-state reaction techniques. Optimal comprehensive properties of d33 = 205 pC/N, kp = 46.5%, ε33T/ε0 = 998, tan δ = 0.047, Td = 470 °C are obtained with Li substitution of 6.5 mol%. Furthermore, doping 0.2 mol% Al2O3 improves the thermal stability in the whole working temperature range of 25 – 470 °C with a d33 decrement less than 10%, keeping the relatively high d33 of 200 pC/N even at high temperature (400 – 470 °C). The Al2O3 doping shows a further effect of inhibiting grain growth, which significantly reduces the grain size of ceramics. The results obtained in this work pave the way for the practical application of KNN-based ceramics at high temperature, such as aerospace sensors, oilfield logging, engine monitoring and injection nozzles in automobiles.