Development and Testing of High-temperature Piezoelectric Wafer Active Sensors for Extreme Environments

Development of high-temperature piezoelectric wafer active sensors (HT-PWAS) using high-temperature piezoelectric material for harsh environment applications is of great interest for structural health monitoring of high-temperature structures such as turbine engine components, airframe thermal protection systems, and so on. This article presents a preliminary study with the main purpose of identifying the possibility of developing PWAS transducers for high-temperature applications. After a brief review of the state of the art and of candidate high-temperature piezoelectric materials, the article focuses on the use of gallium orthophosphate (GaPO4) samples in pilot PWAS applications. The investigation started with a number of confidence-building tests that were conducted to verify GaPO4 piezoelectric properties at room temperature and at elevated temperatures in an oven. Electromechanical (E/M) impedance measurements and material characterization tests (scanning electron microscopy, X-ray diffraction, energy dispersive spectrometry) were performed before and after exposure of HT-PWAS to high temperature; it was found that GaPO4 HT-PWAS maintain their properties up to 1300°F (∼705°C). In comparison, conventional PZT sensors lost their activity at around 500 F (∼260°C). Subsequently, HT-PWAS were fabricated and installed on metallic specimens in order to conduct an in situ evaluation of their high-temperature performance. A series of in situ tests were performed using the E/M impedance and pitch-catch methods; the tests were conducted in two situations: (a) before and after exposure to high temperature and (b) inside the oven. The experimental results show that the fabricated HT-PWAS can survive high oven temperatures up to 1300°F (∼705°C) and still present piezoelectric activity. The article also discusses fabrication techniques for high-temperature PWAS applications, including the wiring of the sensor ground and signal electrodes, bond layer adhesive selection, and preparation.