Since the past half century, active devices represented by integrated semiconductors and active or passive devices based on various functional ceramic materials have profoundly changed human society. As the basic materials which are widely used in distributed intelligent sensors, energy suppling or transformation systems, and precisely micro-nano actuators, piezoelectric ceramics and their application technology play a key role in both the civil and defense fields.
However, due to the intrinsic 6mm point group symmetry, in the piezoelectric strain matrix of natural piezoelectric ceramics (represent the ability of piezoelectric materials to convert electric field into strain output; third-order tensor; with 18 matrix elements, namely, strain coefficient dij), there are only five non-zero matrix elements. For many years, almost all researches have always focused on how to synthesize new materials and how to improve the values of five existing piezoelectric coefficients; although some scholars have begun to realize the existence of individual non-zero piezoelectric coefficient matrix elements, there is lack of a basic theoretical and systematic study. Therefore, in the fundamental theory of piezoelectricity, every important progress is very tough, which has limited further designs and development of piezoelectric devices.
Novel electromechanical metamaterial design can create overall 18 nonzero piezoelectric coefficients
Recently, in the form of Research Article-Physical Sciences, Science Advances, a journal of Science group, reported on the unique research exploration and the important advances in this field by Prof. Shuxiang Dong’s research group in the College of Engineering at Peking University. The title of the thesis is: Designing electromechanical metamaterial with full nonzero piezoelectric coefficients (DOI: 10.1126/sciadv.aax1782).
Different from the traditional chemical method of doping-modification to improve piezoelectric performance, inspired by the symmetry principle in condensed matter physics, the research work introduced the design idea of ordered functional structures in metamaterials to piezoelectric ceramic design.
Through delicately topological and geometrical design, the novel electromechanical metamaterials obtained apparently quasi-broken symmetry instead of the intrinsic 6mm symmetry of naturally occurring piezoelectric ceramics, and for the first time all non-zero 18 piezoelectric strain coefficient elements are created. Moreover, some elements have shown enhanced piezoelectric values by an order of magnitude compared with the natural piezoelectric coefficients.
This progress breaks the traditional knowledge that the piezoelectric ceramic material has only five non-zero piezoelectric coefficients dij in the past 70 years, which bring a brand-new design idea to the future development of piezoelectric devices.
This work was evaluated by reviewers as "will have a significant impact on various piezoelectric technologies". The broken symmetry, an extensive mechanism in physics, leads to many charming phenomena such as antiferromagnetism and superconductivity. Usually, novel broken symmetry is pretty tough to controllably build in the atomic level or micro-level, while the results demonstrate that some equivalent effects may be architected through macroscopic metamaterial design to obtain unnatural parameters and counterintuitive properties. The design principle should be inspirational to create unnatural apparent properties of other multiphysics coupling metamaterials.
Based on quasi-symmetry breaking of 6mm point group, novel electromechanical metamaterial design can create overall 18 nonzero piezoelectric coefficients
The first author of the paper is Jikun Yang, a 2016-grade Ph.D. candidate in College of Engineering, Peking University. Professor Shuxiang Dong is the only corresponding author. Professor Xiaohui Wang and Academician Ji Zhou of Tsinghua University participated in the research of this subject; Academician Longtu Li of Tsinghua University advised the work. The research was funded by the National Natural Science Foundation of China (51772005, 51132001), and supported by Beijing Key Laboratory for Magnetoelectric Materials and Devices (BKL-MEMD).