High-Temperature Piezoelectric Resonators: Electroacoustic Properties and Applications of Langasite-Type Crystals

Abstract

Piezoelectric devices based on high-temperature stable single crystals are of increasing interest due to a wide range of potential applications, such as actuators and resonant gravimetric sensors. The latter require highly accurate resonant-frequency measurements at temperatures as high as 1000 °C, and this can only be achieved with narrow resonance peaks and correspondingly low acoustic loss at these temperatures. Relevant atomistic defects in langasite (La3Ga5SiO14, LGS) and catangasite (Ca3TaGa3Si2O14, CTGS) are reviewed with respect to their impact on electrical conductivity and acoustic loss. The focus on these crystals also enables a comparison of electroacoustic properties of partially disordered (LGS) and ordered (CTGS) crystals and the formulation of approaches for reducing acoustic loss and enhancing device performance.Starting with defect chemistry models for LGS and CTGS, electrical conductivities as a function of temperature and oxygen partial pressure are considered. One result of this analysis is that contributions of oxygen ions to the total conductivity are determined to be relatively small in the studied crystals. Temperature-dependent acoustic loss of LGS and CTGS crystals is also studied. Two independent techniques, contact resonant piezoelectric spectroscopy, and a contactless tone-burst excitation technique, are applied. Contributions to the measured acoustic loss are determined by fitting loss-related functions for point defects and conductivity-related relaxations. The extracted fit parameters, in particular the activation energies of the processes, are discussed and compared with that of the electrical conductivity. The losses are caused by a superposition of several mechanisms including conductivity-related relaxations, point-defect relaxations and intrinsic phonon-phonon loss.The long-term stability of the crystals is also evaluated at 1000 °C with respect to fundamental materials properties including electrical conductivity and resonant frequency.After one year of thermal treatment, the resonance frequency of CTGS resonators from different crystal sources is found to decrease by no more than 0.4 %. Thus, the applicability of CTGS as a piezoelectric resonator that operates for long periods at extreme temperatures, such as a resonant nanobalance, is demonstrated.Application examples to be presented include resonant nanobalances for in-situ monitoring of non-stoichiometry and phase transformations in thin films during growth at high temperatures. In this context, the key application properties of the sensors at high temperatures, such as mass sensitivity and resolution, are discussed. As an application example, the attached figure shows the oxygen non-stoichiometry x of thin praseodymium-cerium oxide films (Pr0.1Ce0.9O2-x) at 700 °C as a function of oxygen partial pressure determined by LGS and CTGS nanobalances, which agrees very well with literature data (chemical capacitance: D. Chen et al., Chem. Mater. 2014, 26, 22, 6622–6627). Figure 1