Electrode loading effect and high temperature performance of ZnO thin film ultrasonic transducers

Nanocrystalline ZnO films of 5.8 μm thick were sputter-deposited on ferritic carbon steel plates (25 × 25 × 3 mm3) and characterized for use as ultrasonic transducers at both room temperature and high temperatures. Electrode loading effects have been studied using two types of electrodes, i.e., sputtered Cr/Au (5/50 nm) and silver paste, with electrode diameters 0.7-2.5 mm. Longitudinal and transverse waves were obtained in pulse-echo tests using both types of electrodes. With a silver paste top electrode, a dominant longitudinal mode was obtained, but with a thin Cr/Au film as the top electrode, shear waves were more dominant. Pulse-echo tests of the ZnO transducers were also performed at elevated temperatures up to 450 °C using a carbon paste electrodes. The sputtered ZnO films maintained a stable crystalline structure and orientation at the elevated temperatures, and ZnO devices on ferritic carbon steel could be used successfully up to 400 °C. However, when the temperature was increased further, rapid surface oxidation of the ferritic carbon steel caused the failure of the transducer.