Piezoelectricity in Excess of 800 pC/N over 400 °C in BiScO₃–PbTiO₃–CaTiO₃ Ceramics

Ultrasonic sensors are widely applied in industries near room temperature; however, their application at high temperature is still a challenge mainly due to the lack of high-performance piezoelectric ceramics. Here, the 0.364BiScO₃–0.636PbTiO₃–0.005CaTiO₃ ceramic exhibits excellent piezoelectric performances at 20–440 °C. Its piezoelectric coefficient d₃₃ increases from 475 pC/N at 20 °C to 853 pC/N at 360 °C, and then it gradually decreases to 669 pC/N at 440 °C. Furthermore, the planar electromechanical coupling factor k_p gradually increases from 0.59 at 20 °C to 0.67 at 200 °C, and then it remains at a stable value of 0.65–0.67 at 150–350 °C. These achievements are because the ceramic morphotropic phase boundaries have a flat Gibbs free energy versus polarization curve and a wide temperature range. Since the piezoelectric ceramic shows satisfactory piezoelectric properties at 20–440 °C, the corresponding ultrasonic sensors can in situ monitor many high-temperature devices, such as engines, wheels, drills, boilers, etc.