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New lead-free piezoelectric composites for high-power applications

Subject Area Synthesis and Properties of Functional Materials
Term from 2019 to 2024
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 414073759
 
Final Report Year 2024

Final Report Abstract

Piezoelectric resonance is the underlying physical phenomenon that enables a wide variety of modern devices, including ultrasonic transducers for cleaning or welding, miniaturized motors, voltage transformers, etc. Materials that are currently being used for these applications are facing increasingly stringent requirements and are approaching their limitations in terms of operating temperatures and vibration velocities. These so-called high-power conditions require the use of piezoelectrically hard materials, which rely on traditional hardening mechanisms based on point defects. However, these mechanisms have several inherent problems, mostly related to the mobility of point defects. To this end, fundamentally different hardening concepts have to be developed, which prerequisites a better understanding of the basic mechanisms and the microstructure of these materials. On the microscopic level, piezoelectric hardening is governed by the interaction of ferroelectric domain walls with various defects, purposely introduced into the material. The main hypothesis of this project was that strong hardening could be obtained by using specially designed second-phase inclusions as pinning defects. We thus initially prepared and studied a wide set of (3-0)-type composites, whereby the inclusion phases have been selected based on their thermal, mechanical, and electrical properties. Inclusions with the strongest influence on the hardening behavior were ZnO, ZrO2, and Ag. In order to provide a better view into these materials during high-frequency drive, we developed a new measurement approach combining electromechanical resonance measurement with in situ X-ray diffraction. This method enabled us to evaluate the contributions of different microscopic mechanisms and pinpoint the stress-driven domain wall motion as the dominant cause for losses. A detailed study of the material´s thermal stability revealed that the composites exhibit a much lower temperature dependence as compared to point-defect-hardened reference compositions, which was related to the internal residual stress state. During the investigation of the main project hypothesis, we additionally made other important findings, which enabled us to open up new research fields and to initiate additional projects. These include piezoelectric hardening by precipitated secondary phases, hardening by the application of DC electric fields, and observations of increased interactions between domain walls and 2D defects. In summary, the project established hardening by secondphase inclusions as a new piezoelectric hardening method. This resulted in two new leadfree materials, two new measurement and characterization methods, as well as an improved understanding of fundamental material mechanisms related to interactions of ferroelectric domain walls with defects.

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