Piezoelectric Materials for High Temperature Sensors

Piezoelectric materials that can function at high temperatures without failure are desired for structural health monitoring and/or nondestructive evaluation of the next generation turbines, more efficient jet engines, steam, and nuclear/electrical power plants. The operational temperature range of smart transducers is limited by the sensing capability of the piezoelectric material at elevated temperatures, increased conductivity and mechanical attenuation, variation of the piezoelectric properties with temperature. This article discusses properties relevant to sensor applications, including piezoelectric materials that are commercially available and those that are under development. Compared to ferroelectric polycrystalline materials, piezoelectric single crystals avoid domain-related aging behavior, while possessing high electrical resistivities and low losses, with excellent thermal property stability. Of particular interest is oxyborate [ReCa4O (BO3)(3)] single crystals for ultrahigh temperature applications (>1000 degrees C). These crystals offer piezoelectric coefficients d(eff), and electromechanical coupling factors k(eff), on the order of 3-16 pC/N and 6%-31%, respectively, significantly higher than those values of alpha-quartz piezo-crystals (similar to 2 pC/N and 8%). Furthermore, the absence of phase transitions prior to their melting points similar to 1500 degrees C, together with ultrahigh electrical resistivities (>10(6) Omega.cm at 1000 degrees C) and thermal stability of piezoelectric properties (< 20% variations in the range of room temperature similar to 1000 degrees C), allow potential operation at extreme temperature and harsh environments.