High temperature ReCOB piezocrystals: Recent developments

Piezoelectric sensors for high temperature applications have attracted attention due to their simplistic structure, fast response time and ease of integration. In this article, oxyborate ReCa₄O(BO₃) ₃ (Re: rare earth element; abbreviated as ReCOB) piezoelectric crystals were surveyed for their potential use in high temperature sensing applications. In contrast to quartz and GaPO₄ crystals, no phase transformation(s) are observed prior to their melting points, being in the order of ∼1500 °C. The electrical resistivity, dielectric, piezoelectric properties and resonance-impedance characteristics were studied as a function of temperature over the range of Room Temperature (RT) to 950 °C. The resistivity of ReCOB was found to be ∼2×10⁸ Ohm cm at 800 °C, two orders higher than langasite, another widely studied crystal system. The electromechanical coupling factors k₂₆ and piezoelectric coefficients d₂₆ were found to be >20% and >10 pC/N, respectively, with the variation being <20% over the studied temperature range. The resonance frequency for width shear vibration was found to decrease linearly with increasing temperature for YCOB crystals, with the Temperature Coefficient of Frequency (TCF) in the order of 70 ppm/K, while for NdCOB crystals, a nonlinear behavior was observed, demonstrating a potential zero TCF crystal cut. The high resistivity, high piezoelectric properties and low mechanical and dielectric losses, together with temperature independent characteristics, demonstrate that oxyborate crystals are promising candidates for high temperature sensing applications. © 2010 Elsevier B.V. All rights reserved.