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On the Possibility of Piezoelectric Nanocomposites without using Piezoelectric Materials

In a piezoelectric material an applied uniform strain can induce an electric polarization (or vice-versa). Crystallographic considerations restrict this technologically important property to non-centrosymmetric systems. It can be shown both mathematically and physically, that a non-uniform strain can potentially break the inversion symmetry and induce polarization even in non-piezoelectric dielectrics. The key concept is that all dielectrics (including non-piezoelectric ones) exhibit the aforementioned coupling between strain gradient and polarization-an experimentally verified phenomenon known in some circles as the flexoelectric effect. The coupling between strain gradients and polarization, and conversely between strain and polarization gradients, has been investigated in our prior work  (in addition to other researchers such as Mindlin, Yang, Tagantsev, Nowacki etc. who are duly cited). This flexoelectric coupling, however, is generally very small and evades experimental detection unless very large strain gradients (or conversely polarization gradients) are present. In the attached pre-print, based on a field theoretic framework and the associated Greens function solutions developed in the previous work, we quantitatively demonstrate the possibility of "designing piezoelectricity" i.e. we exploit the large strain gradients present in the interior of composites containing nanoscale inhomogeneities to achieve an overall non-zero polarization even under applied uniform stress. We prove that the aforementioned effect may be realized only if both the shapes and distributions of the inhomogeneities are non-centrosymmetric. In other words, the requirement of material non-centrosymmetry for naturally occurring piezoelectrics is transferred to a requirement of non-centrosymmetry in the shape/topology of the flexoelectric media. Our un-optimized quantitative results, based on limited material data and restrictive assumptions on inhomogeneity shape and distribution, indicate that apparent piezoelectric behavior close to 10% of Quartz may be achievable for inhomogeneity size regime of 4-5 nm. Considering that solely non-piezoelectric materials are used, these numbers are tantalizing and may be easily improved upon (and calculated using the developed model in our paper) for other materials. In future works, it is not unreasonable to expect enhanced performance based on optimization of shape, topology and appropriate material selection. This central notion was first (qualitatively) suggested by Cross and co-workers. One of their experimental findings is that ferroelectrics (even in their non-piezoelectric state) possess very high flexoelectric coefficients.

In future work we intend to quantitatively explore the effect of flexoelectricity in materials that are piezoelectric.

Please refer to the attached uncorrected proof copy of the pre-print (JMPS) for further details


PDF icon Proof_FlexoComposites.pdf691.7 KB


BoJing Zhu's picture

1.interesting idea. our group we use the similar idea to deal with crack(flaw )problem in functionally graded materials, and send our manuscript to JMPS about November 10 2006. but with the .........reason, our manuscript was declined



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