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 °C. 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 °C. 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, η, is found to control the resonator quality. At 600 °C and oxygen partial pressures down to ca. 10⁻²⁰ bar, an environment independent operation of LGS resonators is demonstrated. The mass sensitivities at 800 °C are of the same order of magnitude as for quartz at room temperature.

Electrical conductivity and acoustic loss Q⁻¹ of single crystalline Li(Nb,Ta)O₃ solid solutions (LNT) are studied as a function of temperature by means of impedance spectroscopy and resonant piezoelectric spectroscopy, respectively. For this purpose, bulk acoustic wave resonators with two different Nb/Ta ratios are investigated. The obtained results are compared to those previously reported for congruent LiNbO₃. The temperature dependent electrical conductivity of LNT and LiNbO₃ show similar behavior in air at high temperatures from 400 to 700 °C. Therefore, it is concluded that the dominant transport mechanism in LNT is the same as in LN, which is the Li transport via Li vacancies. Further, it is shown that losses in LNT strongly increase above about 500 °C, which is interpreted to originate from conductivity-related relaxation mechanism. Finally, it is shown that LNT bulk acoustic resonators exhibit significantly lower loss, comparing to that of LiNbO₃.

Recently, a laboratory setup for microwave-based characterization of powder samples at elevated temperatures and different gas atmospheres was presented. The setup is particularly interesting for operando investigations on typical materials for exhaust gas aftertreatment. By using the microwave cavity perturbation method, where the powder is placed inside a cavity resonator, the change of the resonant properties provides information about changes in the dielectric properties of the sample. However, determining the exact complex permittivity of the powder samples is not simple. Up to now, a simplified microwave cavity perturbation theory had been applied to estimate the bulk properties of the powders. In this study, an extended approach is presented which allows to determine the dielectric properties of the powder materials more correctly. It accounts for the electric field distribution in the resonator, the depolarization of the sample and the effect of the powder filling. The individual method combines findings from simulations and recognized analytical approaches and can be used for investigations on a wide range of materials and sample geometries. This work provides a more accurate evaluation of the dielectric powder properties and has the potential to enhance the understanding of the microwave behavior of storage materials for exhaust gas aftertreatment, especially with regard to the application of microwave-based catalyst state diagnosis.