High temperature piezoelectric single crystals: Recent developments

High temperature piezoelectric materials serving at harsh environment without failure are desired for structural health monitoring and/or non destructive evaluation of the modern aerospace structures at elevated temperatures (above 600°C). In this report, different high temperature piezoelectric crystals, including the langasite (LGS) type crystals and GaPO4 in trigonal system, melilite type and fresnoite type crystals in tetragonal system, and oxyborate crystals in monoclinic system are surveyed, and their properties are compared. Among the LGS type piezoelectric crystals, the structural ordered trigonal Ca3TaGa3Si2O14 (CTGS) crystals as well as the Al substituted CTGS crystals shew relative low dielectric loss (<;5%@600°C), high electromechanical coupling factor (12%~18%@room temperature) and good temperature stability of piezoelectric properties up to 900°C, potential for usages in middle-high temperature range. Interestingly, the monoclinic RECa4O(BO3)3 (RECOB, Re: rare-earth elements) type crystal were found to own attractive dielectric, piezoelectric and electromechanical properties up to 1000°C, particularly, the YGdCOB crystals were determined to possess temperature independent behavior of piezoelectric properties (variations of piezoelectric d.26 <;-5% over the tested temperature range), promising for ultrahigh temperature piezoelectric applications.