Soft rubber as a magnetoelectric material—Generating electricity from the remote action of a magnetic field

Abstract

A magnetoelectric material is capable of converting a magnetic field into electricity. Wireless energy harvesting, drug delivery via remote action, multiple state memories are just some of the possible applications of this phenomenon. The magnetoelectric property is however rare and restricted either to certain hard exotic crystals that satisfy a stringent set of material symmetry constraints or painstakingly fabricated (still hard) composites. Soft materials that are capable of large deformations and are also magnetoelectric, do not exist. In this work, based on a simple mechanism predicated on a coupling facilitated by the universal electromagnetic Maxwell stress, deformability of soft matter and the embedding and stabilization of external charges, we experimentally demonstrate the transformation of silicone rubber into hitherto softest magnetoelectric material. Our material exhibits a room-temperature magnetoelectric coefficient as high as $193{\mathrm{m}\mathrm{V}\mathrm{c}\mathrm{m}}^{-1}{\mathrm{O}\mathrm{e}}^{-1}$ at the magnetic field of $\approx 600\mathrm{O}\mathrm{e}$ and the low frequency of $\approx 1\mathrm{H}\mathrm{z}$. This rivals the performance of some of the best single phase and composite materials but with a capability of significant deformation.

Introduction

The magnetoelectric (ME) coupling refers to the property of a material to respond electrically when subjected to a change of external magnetic field. Materials exhibiting ME effect have several applications including magnetic sensors, data storage, drug delivery via remote action, and wireless energy harvesting [1], [2], [3], [4], [5]. Simultaneous existence of both magnetic and electric polarization degrees of freedom in single phase crystals is rare due to both symmetry considerations as well as the mutually contradictory nature of the quantum mechanical aspects of the atomic bonding required for their existence [6], [7]. In addition, the ME coupling of single-phase materials (that do exist), such as BiFeO₃, Cr₂O₃, and YMnO₃, is very weak at room temperature [8], [9]. These issues pertaining to single-phase ME materials significantly limit their applications.

To overcome the drawbacks of single-phase ME materials, composites that combine piezoelectric and magnetostrictive materials have been proposed [10], [11], [12], [13]. Originally, BaTiO₃ and CoFe₂O₄ were used in such composites and the ME coupling coefficient was found to be slightly higher than that for single-phase ME materials [10], [14]. Later, Ryu et al. significantly improved the coupling coefficient by proposing a new design for the ME composites. In the new design, a PZT ceramic disc was sandwiched between two Terfenol-D discs [15]. The ME coupling in all these composites originates from the product effect of piezoelectricity and magnetostriction, two phenomena that are commonly observed in piezoelectric and magnetostrictive materials which are invariably (mechanically) hard materials [16], [17]. As shown by the middle sample marked with “ME composites” in Fig. 1, when a magnetic field is applied across a traditional ME composite, mechanical stresses ($\tau$) are induced in its magnetostrictive components and these stresses (marked by white arrays in Fig. 1) are transferred to the neighboring piezoelectric components which in turn generates electricity. In short, the performance of the ME composites critically depends on the properties of the piezoelectric and magnetostrictive components. In the past few decades, efforts are mostly focused on the improvement of these two components [15], [18], [19], [20]. Recently, Annapureddy et al. introduced a Fe-Ga alloy for the magnetostrictive phase and obtained strong ME coupling of the composite [21]. Li et al. successfully brought a new piezoelectric member, molecular-ionic ferroelectrics, to the family of ME composites [22]. Zong et al. replaced the commonly used piezoelectric component, PZT, with cellulose [23]. In order to ensure the high performance, another requirement is that the magnetic field induced stress in the magnetostrictive component should be large enough and efficiently passed to the piezoelectric component. Due to this requirement, magnetic active elastomers (MAE) which exhibit large strain but low stress in magnetic field are usually excluded when designing high performance ME composites [24], [25], [26], [27]. Finally, we remark that although polymers like PVDF are piezoelectric, their electromechanical coupling is quite weak and are ill-suited in the composite world to create a substantive magnetoelectric effect. In addition, at an elastic modulus of nearly 1GPa, PVDF (and materials of its kind) are not truly soft in the sense of large deformations that elastomers are capable of. In other words, truly soft materials which exhibit a substantial magnetoelectric effect do not exist. Thus, a ME material which is biocompatible, environmentally friendly, and able to endure large strain before failure, is critical for biomedical applications.

In this work, we exploit a novel mechanism predicted theoretically by us to engineer magnetoelectricity in ultra soft silicone rubber that is highly deformable and exhibits a magnetoelectric coupling rivaling that of hard composites.