DFG - Piezoelectric - New lead-free piezoelectric composites for high-power applications

Piezoelectric ceramics enable conversion between electrical and mechanical signals and are widely used in various electronic applications. High energy density and low power consumption make them indispensable in high-power applications, such as small voltage transformers and ultrasonic devices. Moreover, their miniaturization potential enables the development of new portable electronic devices and electronic body implants. High-power applications require piezoelectrically hard materials, whereby the hardening is conventionally achieved by pinning the ferroelectric domain walls with defect complexes. This hardening mechanism, however, suffers degradation of electromechanical properties at high vibration velocities and elevated temperatures, which considerably limits the output power and therefore represents a vital drawback for applications. Moreover, the state-of-the-art piezoelectrics contain large amounts of hazardous lead, placing them on the watch list of many environmental regulations. The main goal of the proposed project is to develop an alternative hardening mechanism, which will result in a new group of lead-free piezoceramics with higher and more stable high-power piezoelectric properties. Instead of using classical hardening with defect complexes, a new approach based on engineering the microstructure will be utilized. To this end, lead-free (3-0)-type piezoelectric composites will be designed using relaxor matrix and various rigid non-perovskite inclusions. It is hypothesized that in these composites the pinning of domain walls can be obtained by mechanical stresses and charged carriers from inclusions. To resolve the different mechanisms, composites with either semiconductive or insulating inclusions will be investigated. The pinning strength will be evaluated for small- and large-signal electromechanical excitation regimes over a broad frequency and temperature range. The high-power properties and depolarization behaviour will be compared to the state-of-the-art hard Pb(Zr,Ti)O3 materials. To understand the macroscopic electromechanical response, the project will additionally focus on simultaneous investigation of microstructural and crystallographic parameters. The crystallographic structure and residual stresses will be investigated using X-ray diffraction, nuclear magnetic resonance, and neutron diffraction. Moreover, in situ time-resolved measurements using high-energy X-ray diffraction will be utilized to determine the contributions from domain walls and lattice strains. Development of in situ high-power measurements will improve the general understanding of the non-linear behaviour of hard piezoelectrics. In summary, this project will introduce a new class of sustainable hard piezoelectric materials, provide basic scientific understanding of the novel hardening mechanism, and give guidelines for the design of other hard piezoelectrics utilizing the composite approach.