High temperature bulk acoustic wave properties of langasite

High temperature stable piezoelectric materials including langasite (La3Ga5SiO14, LGS) are, in principle, suitable for high temperature bulk acoustic wave applications such as resonant microbalances and gas sensors using specific surface affinity layers. The resonator material LGS has been shown to exhibit bulk oscillations at temperatures of up to at least 935 degreesC. However, the knowledge of the high temperature bulk acoustic wave properties is required for the evaluation of mass load dependent changes of the resonator characteristics, and, consequently, for the application of these materials as sensors. This paper presents and discusses the bulk acoustic wave resonator behaviour of LGS in the temperature range of up to 935 degreesC. Test devices are Pt contacted LGS resonators. Based on a one-dimensional description of the resonance behaviour, the material constants are determined as a function of temperature and oxygen partial pressure. The effective viscosity, eta, is found to control the resonator quality. At 600 degreesC and oxygen partial pressures down to ca. 10(-20) bar, an environment independent operation of LGS resonators is demonstrated. The mass sensitivities at 800degreesC are of the same order of magnitude as for quartz at room temperature.