Magnetorheological elastomers, MREs, based on elastic organic matrices displaying anisotropic magnetoresistance and piezoresistivity at room temperature were prepared and characterized. These materials are dispersions of superparamagnetic magnetite forming cores of aggregated nanoparticles inside silver microparticles that are dispersed in an elastomeric polymer (poly(dimethylsiloxane), PDMS), curing the polymer in the presence of a uniform magnetic field. In this way, the elastic material becomes structured as the application of the field induces the formation of filaments of silver-covered inorganic material agglomerates (needles) aligned in the direction of the field (parallel to the field). Because the magnetic particles are covered with silver, the MREs are not only magnetic but also electrical conductors. The structuration induces elastic, magnetic, and electrical anisotropic properties. For example, with a low concentration of particles in the elastic matrix (5% w/w) it is possible to obtain resistances of a few ohms when measured parallel to the needles or several megaohms in the perpendicular direction. Magnetite nanoparticles (Fe(3)O(4) NP) were synthesized by the coprecipitation method, and then agglomerations of these NPs were covered with Ag. The average size of the obtained magnetite NPs was about 13 nm, and the magnetite-silver particles, referred to as Fe(3)O(4)@Ag, form micrometric aggregates (1.3 μm). Nanoparticles, microparticles, and the MREs were characterized by XRD, TEM, SEM, EDS, diffuse reflectance, voltammetry, VSM, and SQUID. At room temperature, the synthesized magnetite and Fe(3)O(4)@Ag particles are in a superparamagnetic state (T(B) = 205 and 179 K at 0.01 T as determined by SQUID). The elastic properties and Young's modulus of the MREs were measured as a function of the orientation using a texture analysis device. The magnetic anisotropy in the MRE composite was investigated by FMR. The electrical conductivity of the MRE (σ) increases exponentially when a pressure, P, is applied, and the magnitude of the change strongly depends on what direction P is exerted (anisotropic piezoresistivity). In addition, at a fixed pressure, σ increases exponentially in the presence of an external magnetic field (H) only when the field H is applied in the collinear direction with respect to the electrical flux, J. Excellent fits of the experimental data σ versus H and P were achieved using a model that considers the intergrain electron transport where an H-dependent barrier was considered in addition to the intrinsic intergrain resistance in a percolation process. The H-dependent barrier decreases with the applied field, which is attributed to the increasing match of spin-polarization in the silver covers between grains. The effect is anisotropic (i.e., the sensitivity of the magnetoresistive effect is dependent on the relative orientation between H and the current flow J). In the case of Fe(3)O(4)@ Ag, when H and J are parallel to the needles in the PDMS matrix, we obtain changes in σ up to 50% for ...

An anisotropic magnetorheological composite formed by dispersions of silver-covered magnetite microparticles (Fe3O4@Ag) in polydimethylsiloxane (PDMS) displaying electrical conduction only in one preferred direction is presented. A set-up for applying and detecting electrical conduction through the composite is described and applied to characterize the behavior of the system in on-off commutation cycles. The composite is obtained by loading the polymer with relatively low concentration of fillers (5% w/w of the total weight) and curing it in the presence of a uniform magnetic field. The fillers appear in the final composite as an array of needles, i.e. pseudo-chains of particles aligned in the direction of the magnetic field. Using Fe3O4 nanoparticles (13 nm) it is possible to obtain cured composites in a superparamagnetic state, that is, without magnetic hysteresis at room temperature. Hysteresis is not found in the elastic properties either; in particular, Mullins effects (change of physical properties after the first strain-stress cycle) were not observed. No measurable transversal electrical conduction was detected (transversal resistivity larger than 62 Mohms•cm). Thus, significant electrical conductivity is present only between contact points that are exactly facing each other at both sides of the composites in the direction parallel to the needles. The I-V curves in that direction have ohmic behavior and exhibit both piezoresistance and magnetoresistance, that is, the electrical conductivity in the direction parallel to the pseudochains increases when a pressure (i.e. compressive stress) is applied at constant magnetic field and/or when a magnetic field is applied at constant pressure. The materials do not exhibit magnetoelectric or piezoresistive hysteresis. These characteristics illustrate the high potentiality of these systems in elastic connectors where electrical conduction can be varied by external mechanical or magnetic forces. List of changes following suggestions of the reviewerWe improved the language following all the suggestions of the reviewer, listed here: Table 1 Figure 7 shows the absence of significant transversal conductivity for an elastomeric connector of 2 mm thickness. Measurements were performed using the set-up shown in Figure 1.This result was observed also for samples of 1 and 3 mm thickness, which is consistent with the no variation of the resistivities (  and   ) with the sample thickness". Page 14: another sentence was rephrased: "On the other hand, no measurable current was detected between contacts that do not coincide vertically". 1) A glossary of terms is included asPage 15: "…the desired anisotropic properties, which, in this case, are stabilized by the curing of the PDMS resin".Page 15: "The fillers and the matrix appear contactless after curing."Page 15: another sentence was rephrased: "The absence of hysteresis in the elastic, piezoresistive and magnetoresistive properties can be associated to the lack of relevant adhesion between the inorganic fillers ...

In this article we explore how structural parameters of composites filled with one-dimensional, electrically conducting elements (such as sticks, needles, chains, or rods) affect the percolation properties of the system. To this end, we perform Monte Carlo simulations of asymmetric two-dimensional stick systems with anisotropic alignments. We compute the percolation probability functions in the direction of preferential orientation of the percolating objects and in the orthogonal direction, as functions of the experimental structural parameters. Among these, we considered the average length of the sticks, the standard deviation of the length distribution, and the standard deviation of the angular distribution. We developed a computer algorithm capable of reproducing and verifying known theoretical results for isotropic networks and which allows us to go beyond and study anisotropic systems of experimental interest. Our research shows that the total electrical anisotropy, considered as a direct consequence of the percolation anisotropy, depends mainly on the standard deviation of the angular distribution and on the average length of the sticks. A conclusion of practical interest is that we find that there is a wide and well-defined range of values for the mentioned parameters for which it is possible to obtain reliable anisotropic percolation under relatively accessible experimental conditions when considering composites formed by dispersions of sticks, oriented in elastomeric matrices. arXiv:1405.0634v1 [cond-mat.soft]