HIGH-TEMPERATURE ACOUSTIC EMISSION SENSING USING ALUMINUM NITRIDE SENSOR

We developed a heat-resistant acoustic emission (AE) sensor using AlN thin films and detected AE signals from molten-salt attack of Type-304 stainless steel pipe. Aluminum nitride (AlN) is a promising AE sensor element for high-temperature environment, such as gas turbines and other plants because AlN maintains its piezoelectricity up to 1200°C. Highly c-axis-oriented AlN thin-film sensor elements were prepared on silicon single crystals by rf magnetron sputtering. The AE sensor sensitivity increased with d33 and thickness of AlN elements and the heat-resistant AE sensors were developed using AlN elements with high d33 values and thickness of 9 μm. The sensor showed a constant sensitivity across a frequency range of 100 kHz to 1 MHz at ambient temperature. The sensor characteristics were evaluated at elevated temperatures up to 700°C. It was confirmed that the AE sensor works well up to 700°C and does not deteriorate. The AE sensor detected AE signals from frequent cracking of oxide scales produced by molten-salt attack (85% V2O5 + Na2SO4) at 700°C during cooling of the pipe.