Ultrasonic sensor is suitable as a next-generation autonomous driving assist device because its lower price compared to that of other sensors and its sensing stability in the external environment. Although Pb(Zr, Ti)O₃ (PZT)-relaxor ferroelectric system has excellent piezoelectric properties, the change in capacitance is large in the daily operating temperature range due to the low phase transition temperature. Recently, many studies have been conducted to improve the temperature stability of ferroelectric ceramics by controlling the grain size and crystal structure, so it is necessary to study the effect of the grain size on the piezoelectric properties and the temperature stability of PZT-relaxor ferroelectric system. In this study, the piezoelectric properties, phase transition temperature, and temperature coefficient of capacitance (TCC) of 0.9 Pb(Zr_1-xTi_x)O₃-0.1 Pb(Zn_1/3Nb_2/3)O₃ (PZT_x-PZN) ceramics with various grain sizes were investigated. PZT_x-PZN ceramics with larger grain size showed higher piezoelectric properties and temperature stability, and are expected to be suitable for ultrasonic devices in the future.

The piezoelectric properties of 0.65Pb(Zr_1-xTi_x)O₃-0.35Pb(Zn_1/6Ni_1/6Nb_2/3)O₃ (PZT_x-PZNN) ceramics with 0.530≤ x≤0.555 were investigated for application to piezoelectric energy harvesters. Although a morphotropic phase boundary (MPB) was found at approximately x＝0.545, the ceramic with the highest figure of merit (FOM) (d₃₃×g₃₃) was observed at a composition of x＝0.540. Values of this figure of merit, d₃₃×g₃₃, of 19.6 pm²/N and 20.2 pm²/N were obtained from PZT_0.540-PZNN ceramics sintered at 920℃ and 950℃, respectively. A high output power of 937 μW and a high power density of 3.3 mW/cm³ were obtained from unimorph-type piezoelectric energy harvesters fabricated using PZT_0.540-PZNN ceramic sintered at 920℃ for 4h.

In a Pb-included piezoelectric composition, Sr_yPb_1-y[(Zn_1/3Nb_2/3)_x-(Ni_1/3Nb_2/3)_0.2-(Zr_0.46Ti_0.54)_0.8-x]O₃ was selected in order to attain high piezoelectric properties. According to the PZN ratio (x) and the amount of Sr doping (y), the crystal structure, microstructure and piezoelectric properties were measured and evaluated. In the case of Sr 4 mol% doping, the piezoelectric properties were the highest for a PZN ratio of 0.1. In this condition, the grain size was larger and the intensity was higher. With the PZN ratio fixed and varying the Sr doping, the piezoelectric properties increased until 10 mol% doping and then decreased for over 12 mol% doping. In the case of x＝0.1 and y＝10 mol%, the best piezoelectric properties were obtained, i.e., d₃₃＝660 pC/N and k_p＝68.5%, and these values seem to be related to the grain size and crystal structure.

Pb(Zr, Ti)O₃ (PZT) is a piezoelectric material applied in a typical actuator and has been actively studied. However, in order to overcome the limitations of PZT, piezoelectric ceramics comprising mixed solid solutions of PZT with various relaxer electric materials have been studied. The Pb(Zn_1/3Nb_2/3)-Pb(Ni_1/3Nb_2/3)-Pb(Zr, Ti)O₃ (PZN-PNN-PZT) piezoelectric ceramic, known to have high piezoelectric constant and electromechanical coupling coefficient, was studied herein. The piezoelectric characteristics with various Zr contents (Zr/Ti ratios), PZN molar ratios, and sintering temperatures were compared. The piezoelectric properties of d₃₃＝580 pC/N and kP＝0.68 were obtained with the 0.1PZN-0.2PNN-0.7PbZr_0.46Ti_0.54O₃ composition sintered at 1,290℃.