Processing and Characterization of Air-Sprayed Bismuth Titanate Ultrasonic Transducers

Transducers for ultrasonic nondestructive evaluation of materials in harsh environments are needed to manage safe operations in a number of industrial applications including power generation, propulsion, and material and chemical processing. Bismuth titanate has a reasonably high Curie temperature and transduces electrical energy into elastic waves and vice versa. Herein, a slurry containing bismuth titanate powder is air-sprayed onto stainless steel substrates, functionalized, and characterized in terms of coating thickness, center frequency and bandwidth, and signal-to-noise ratio. Coatings 40- 70 μm thick had a center frequency of approximately 7 MHz and a broad frequency response range of 3-20 MHz. Transducers were thermally aged at 375 °C for seven days to assess their temperature tolerance. Post-aging analysis revealed a resonance frequency increase, thickness reduction, and microstructural changes, accompanied by a decrease in signal amplitude. Despite these changes, the aged transducers remained operational with good signal-to-noise ratio. Thermal cycling experiments showed that the response of pristine transducers is changed by cycling to 250 °C, while thermally aged transducers exhibited stable ultrasonic performance. Additional experiments on transducers pre-conditioned at 400 °C demonstrated improved thermal resilience after thermal aging at 350 °C. These field deployable air-sprayed BIT transducers are promising candidates for high-temperature NDE applications.