Superparamagnetic anisotropic elastomer connectors exhibiting reversible magneto-piezoresistivity

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 pseudo-chains 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.