Development of embeddable piezoelectric transducers for high-temperature structural health monitoring applications [electronic resource]
Keywords: Materials Science and Engineering.

Utilizing transducers (sensors and actuators) to achieve structural health monitoring (SHM) for structures used in high-temperature environments is desirable but still challenging because most of the transducers cannot survive the harsh environments when temperatures go above 200°C. Recent research in high-temperature piezoelectric materials has facilitated the development of high-temperature ultrasound-based SHM systems that can maintain full functionality after being exposed in high-temperature environments. However, the trade-off between the piezoelectric responses and the maximum working temperature confines the potential applications. To solve the trade-off issue of the piezoelectric elements used in the ultrasonic SHM techniques, we first studied the manufacturing development of a high-temperature piezoelectric material system, BiScO3-PbTiO3 (BS-PT), at its morphotropic phase boundary (MPB). The unique phase combination of BiScO3 and PbTiO3 at the MPB provides a good starting point with a sufficiently high depoling temperature and fair piezoelectric responses. For further enhancing the piezoelectric responses of the BS-PT system, we conducted a series of studies on compositional effects around the MPB composition and then identified PbO as the critical factor of controlling the electric charge leakage characteristics in this material system. With controlled PbO deficiency, our newly developed BS-PT system demonstrates higher piezoelectric coupling coefficients and better thermal depoling resistance than commercial PZT-based systems, and shows its potential to detect damage up to 350°C. In addition to material characterizations, we also investigated the SHM signals and damage detection capability of our in-house fabricated BS-PT transducers. Results from multiple tests on the temperature-dependence of the transducer signals indicated that the new BS-PT transducers were able to generate clear and strong signals on various test substrates. In these tests, we observed a very promising trend of increasing signal amplitudes up to approximate 2 - 3 times stronger at 300°C than the original signals at room temperature. Within the depoling temperature range, the mechanical and piezoelectric responses of our new BS-PT transducers at high temperatures show favorable potentials in signal transduction for SHM applications in harsh environments. Last, to make the BS-PT piezoelectric transducers more versatile for more SHM applications like bondline monitoring or embedded SHM systems for composite structures, we developed a microfabrication process with screen-printing techniques to fabricate miniaturized BS-PT transducer arrays. This process enables mass-production of thinner piezoelectric transducers with more complex shapes at a lower cost, compared to the traditional solid-state reaction method. We also demonstrated that by releasing the sintered transducers onto the flexible polyimide substrates, we are able to fabricate a compact piezoelectric transducer array on a wafer and then deploy the array to cover a much larger area or a more complex shape for potential applications on complex structures in high-temperature environments (e.g., hypersonic aircraft, industrial gas turbines, aircraft engines, and automotive engines).