Development of high-temperature stable piezoelectric materials and electrodes

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Electrical losses in piezoelectric materials and the stability of the electrodes were identified by our group as the limiting factors of piezoelectric devices in the temperature range up to 1300 °C. Highly attractive sensor and actuator applications are not feasible due to temperature limitations. The most promising high temperature piezoelectric materials are langasite (La3Ga5SiO14) and gallium orthophosphate (GaPO4) exhibiting a phase stability up to about 1470 °C and 930 °C, respectively. Our group is presently investigating the high temperature properties of langasite. Therefore, the emphasis of the proposed research is on the latter material. Despite the temperature limit of 930 °C, the study of gallium orthophosphate is justified by the extremely low electrical losses at temperatures up to 900 °C. The first part of the project includes the investigation of the piezoelectric behavior, of the microstructural and compositional changes during operation and of their correlation with the crystal growth conditions. The electrodes of resonant devices experience extremely high accelerations. They must represent a good compromise on high conductivity and low mass. The mechanical properties must be nearly gas atmosphere and temperature independent to avoid disturbances of the resonance behavior. Consequently, the demands on electrodes for resonant devices exceed those for common high temperature electrodes. Therefore, the objective of the second part of the project is the investigation and improvement of the electrode properties including chemical and thermal stability, electrical conductivity, adhesive strength as well as electrode oxidation and non-stoichiometry in the temperature range up to 1300 °C. Proposed starting materials are Pt-Rh alloys, TiN and La0.3Sr0.7CrO3.