Defect chemistry and electromechanical properties of piezoelectric Ca3TaGa3Si2O14 single crystals at extreme temperatures

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Piezoelectric sensors are very promising for monitoring industrial processes in harsh environments (e.g. for energy conversion) and increasing their efficiency. The great economic importance is supported by application of systems with wireless interrogation. To exactly determine the operating frequency of related devices, low electromechanical loss as well as high stability at extreme temperatures are required. Consequently, investigation, understanding and minimization of electromechanical loss effects in piezoelectric crystals are key objectives.The choice of piezoelectric CTGS (Ca3TaGa3Si2O14) crystals made in the initial application is justified since it exhibits very low electromechanical loss as confirmed during the reporting period. Starting from the determination of fundamental properties such as thermal expansion, thermal conductivity and temperature dependence of the resonance frequency as well as of characteristic phase velocities of acoustic waves, a detailed investigation of the atomic transport mechanisms at temperatures up to 1000°C are carried out. A defect model reflecting the dominant atomic transport mechanism is developed. Furthermore, the complete set of electromechanical material constants in an extremely wide temperature range from 4.2 K to 900°C is obtained, allowing device simulations. The electromechanical loss is determined in detail together with the National Institute of Standards and Technology and described in the frame of a model that assigns the fundamental loss mechanisms. In conclusion, CTGS shows very low loss and very good long-term stability at 1000°C, which underlines its relevance for high-temperature systems.Based on the results obtained during the first funding period, there are open questions in the field of atomic transport at low oxygen partial pressures, the contribution of protons and/or OH- groups to the charge transport, the contribution of point defects to electromechanical loss, the influence of non-linear piezoelectric coefficients on the behavior of electromechanical devices as well as in the polarization dependence of attenuation of surface and bulk acoustic waves. The work packages derived from the objectives will again be investigated according to the complementary competencies of the applicants, taking into account new aspects of the relationship between defect chemistry, atomic transport and electromechanical loss in the range from room temperature to 1300°C at TU Clausthal. For the identification of defects, the optical absorption will be measured additionally at the Lviv National Polytechnic University. The main research at Leibniz IFW Dresden will include the investigation of temperature dependent BAW and SAW damping for an increased frequency range, the determination of the viscosity tensor and of specific SAW parameters as well as the non-linear piezoelectric behavior.