Magneto-piezoresistance in elastomagnetic composites

Abstract

A model for the dependence of the electrical conductance, G, with the strain induced by external mechanical stress in conducting particles-polymer composites is presented. The model assumes that the percolation probability between neighboring particles must depart from a scale-invariant behavior but saturate at moderated-high strains, reaching percolation path's saturation, with sigmoid dependence. This dependence is obtained by proposing a dynamic picture where contacts or bonds between neighboring particles are created but also destructed when a stress is applied and relatively moderated or high strains, , are produced in the composite. The electrical conductance of prepared graphite-polydimethylsiloxane composites were measured as function of the applied pressure and fitted by the presented model. The elastic response to the uniaxial compression was studied using a texture analyzer. The possibility of nonuniversal effects in the conduction critical exponent, t, was taken into account. It is concluded that the saturation of the response in the G versus plots cannot be assigned to nonuniversal behavior of the exponent t, or to saturation of the elastic response. On the other hand, the presented model accounts for all the main experimental features observed in these systems and for previously reported data of elastomer composites. The simulated behavior of the piezoresistivity coefficient is also in qualitative agreement with previous reports.

Magneto-piezoresistivity in iron particle-filled silicone: An alternative outlook for reading magnetic field intensity and direction

“…The sample length can be easily reduced by a laboratory knife. All the process has been described in some detail in previous papers [9,20,21]. No methodology has been applied to induce a preferential orientation of the Fe microparticles; therefore they resulted homogeneously distributed and randomly oriented.…”

“…The peculiar morphology of magnetic particles used in this work is different from the one employed in previous investigations (pseudospherical morphology with moderate roughness). The reason of this choice is to improve composite material performances, delineated on the basis of previous studies [9,20,21], in order to achieve three specific characteristics: i) bigger size particles; ii) better silicone entrapping among particle cavities during the mechanical dispersion process to isolate each particle from nearest ones; iii) lower transversal contraction able to give particle contact as consequence of the irregular surface protrusions; iv) preferential in plane magnetization of a single particle due to shape anisotropy. The positive effects of these listed material characteristics were verified by means of the sample performance as will be shown and discussed in Section 3.…”

“…On the basis of this experimental expedient, a different MPR core elongation is obtained if the M i magnetic moment is directed in concordant or opposite direction with respect to M magnetization. According to the MPR model [20,21], a different sample elongation determines a different decrease of sample initial resistance R 0 . We will use the symbol R up for the sample resistance when M and M i magnetization directions are identical, while we will use R down in case they are opposite.…”

“…As a result of this strain, a decrease of average distance among the particles and a consequent decrease in resistivity occur (piezoresistivity). These are the fundamental mechanisms in the coupling of elastomagnetism and piezoresistivity which produce the magneto-piezoresistive (MPR) effect extensively treated in [20] and [21]. A magneto-piezoresistive sensitivity (change of resistivity on the inducing magnetic field gradient) higher than 10 11 !m/T has been both predicted by theoretical models and experimentally detected [20,21].…”

“…These are the fundamental mechanisms in the coupling of elastomagnetism and piezoresistivity which produce the magneto-piezoresistive (MPR) effect extensively treated in [20] and [21]. A magneto-piezoresistive sensitivity (change of resistivity on the inducing magnetic field gradient) higher than 10 11 !m/T has been both predicted by theoretical models and experimentally detected [20,21]. The main objective of this paper is to establish, by means of some basic experiments in the millimeter scale range, that the MPR effect can be used to detect the change of magnetic polarization direction at a magnetization intensity similar to that produced by the semi-permanent nanomagnets of a magnetic memory.…”