Paving the way for more environmentally friendly materials in future electromechanical devices, EU-backed researchers have found a new way to induce the piezoelectric effect in materials ordinarily not considered to be piezoelectric.
Piezoelectricity is much simpler than it sounds. It is the process of using crystals to convert mechanical energy into electrical energy, or vice versa, and is found in many everyday devices, from quartz watches and gramophones to microphones and speakers. However, while the concept of piezoelectricity may be simple, finding piezoelectric materials on which to apply this process has been a major obstacle for more than a century.
Researchers supported by the EU-funded BioWings and ESTEEM3 projects have now found a way to produce a piezoelectric response in materials not normally considered to be piezoelectric. Published in the journal ‘Science’, their findings could pave the way to a wide range of environmentally friendly and biocompatible electromechanical materials.
Piezoelectric materials have one fundamental prerequisite: their crystal structure must have no centre of symmetry. When pressure is applied to the non-centrosymmetric crystal structure, it deforms and its atoms get moved around, enabling the crystal to conduct an electric current. This makes piezoelectric materials attractive for a wide range of sensing applications.
The most well-known piezoelectric material is the quartz crystal. However, in microelectromechanical systems (MEMS), materials other than naturally occurring quartz must be used. These materials often contain harmful lead in the form of lead zirconate titanate. The research focuses on the development of new biomedical MEMS made with thin, lead-free films based on gadolinium-doped oxide materials. “Many micro-electromechanical systems already exist, but they often contain lead-containing materials that are harmful for human implantation. The BioWings project aims to develop biocompatible materials with properties similar to common lead-containing materials that do not contain lead or the other harmful materials,” notes study co-author Prof. Nini Pryds of BioWings project coordinator Technical University of Denmark in a ‘EurekAlert!’ news release.In their study, the researchers describe how they were able to induce a large and sustainable piezoelectric effect in centrosymmetric crystals – materials that usually do not allow such a response. They created piezoelectricity through the simultaneous application of alternating and direct current that resulted in the rearrangement of oxygen defects in the material and consequently led to polarisation. This broke the crystal symmetry of the material, achieving the sought-after piezoelectric effect.
By showing that it is possible to induce a piezoelectric effect in materials that are ordinarily not piezoelectric, the research team is laying the foundations for the design of lead-free, non-toxic piezoelectric materials. “The new development will provide a fundamental step towards environmentally friendly piezoelectric materials with high performance for use, e.g. in car technology and medical applications,” continues Prof. Pryds.
Today’s requirement of a non-centrosymmetric crystal structure in piezoelectric materials significantly limits the number of materials that can be used in modern devices. Prof. Pryds describes the results of the research as providing “a paradigm shift towards inducing piezoelectricity in centrosymmetric crystals, thereby expanding the number of possible materials used.” He concludes: “I expect it will have a significant effect on the design of new electromechanical devices with new biocompatible materials.” BioWings (Bio-compatible electrostrictive smart materials for future generation of medical micro-electro-mechanical systems) ends May 2022 and ESTEEM3 (Enabling Science and Technology through European Electron Microscopy) ends in June 2023.
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