High-temperature and broadband immersion ultrasonic probes

Immersion ultrasonic probes for measurements and imaging at high temperature are presented. The probes consist of sol-gel-sprayed thick films as piezoelectric ultrasonic transducers (UTs) directly deposited onto steel buffer rods. They operate in pulse-echo mode at temperatures up to 500degC. The operating ultrasonic frequency is between 5 MHz and 20 MHz, controlled by the film thickness. The ultrasonic thickness measurement of a steel plate with the probe fully immersed in molten zinc at 450degC was demonstrated using ultrasonic plane waves. For imaging purposes, the probing end of the steel buffer rod was machined into a semispherical concave shape to form an ultrasonic lens and achieve high spatial resolution with focused ultrasound in liquids. Ultrasonic surface and subsurface imaging using a mechanical raster scan of the focused probe in silicone oil at 200degC was also carried out. The importance of the signal-to-noise ratio (SNR) in the pulse-echo measurement is discussed