Electrical resistivity mechanism in magnetorheological elastomer

Magnetorheological elastomers (MRE) are smart materials made by aligning magnetic microparticles inside a liquid polymer. Once the polymer is cured, this anisotropic structure is kept, giving to the composite new properties such as a large change of electrical resistivity with applied pressure. In order to understand the conduction mechanism in such composite, the influence of pressure on the electrical resistivity of metal powders without polymer was first investigated. It was found that the initial resistivity of metal powder at zero pressure is about 10Ω.cm for pure nickel powder and 10Ω.cm for silver coated nickel particle. The piezoresistivity of the powders follows a power law with a coefficient close to (-1) at high compression, which allows to determine the thickness of the oxide layer. The change of resistance with pressure was found to be an order of magnitude larger for a MRE composite than for the same volume fraction of fillers dispersed randomly in the polymer. The filler particles have a high surface roughness, and when particles are brought into contact under pressure, the electric current takes place via microcontacts between asperities. The model of tunnel resistance developed in this study includes the roughness parameters and the thickness of the oxide layer found with the powder and introduces the thickness of the polymer layer as a new parameter. This model well reproduces experimental curves for piezoresistivity of composites informing on the thickness of the insulating polymer layer strongly adsorbed on the surface of particles.