Piezoelectric structural sensor technology for extreme environments

High temperature sensors are desired for down-hole well monitoring, future propulsion components, as well as improving performance and maintainability of power production facilities and other rotary combustion engines. Recently discovered high temperature oxyborate crystals showed stable piezoelectric properties and high resistivity at temperatures close to its melting point (∼ 1500°C, or 2730°F), which is very promising for high temperature sensor applications. In this paper the feasibility of using oxyborate based high temperature piezoelectric crystal (HTPC) for high temperature piezoelectric sensor applications is demonstrated. Oxyborate HTPC with various crystal cuts and vibration modes were investigated to obtain high temperature resistivity, dielectric, piezoelectric and thermal expansion properties. YCa ₄O(BO₃)₃ crystals (YCOB) showed excellent piezoelectricity, low dielectric loss and high resistivity at temperatures up to 1000°C (> 1800°F). The measured thermal expansion coefficients of YCOB are about ∼3-8 ppm/K, depending on different orientations. High temperature accelerometers were demonstrated using YCOB HTPC at temperatures up to 1000°C with sensitivity remaining steady (∼ 2.4 pC/g) across the temperature range of 20°C - 1000°C.