(363h) Rheo-Electric Behavior of Carbon Black Suspensions in Shear Flow

Carbon black is a ubiquitous additive used to modify the optical, mechanical and electrical properties of polymers, fluids and gels. There is recent renewed interest in the properties of specialty, high-structured, conductive carbon blacks due to their use as the electrical current collector in electrochemical flow technologies. Despite its important role in system performance, there is little understanding of the electron transfer dynamics in such materials particularly when subjected to transient and steady shear flows.

In this work, the rheo-electric behavior of a high-structured, conductive carbon black (Vulcan XC-72) is studied in mineral oil. Mineral oil is a model dielectric fluid that exhibits two important characteristics for this study: 1.) its viscosity is sufficient to induce significant structural erosion at shear rates accessible in commercial rheometers, and 2.) it is not ionogenic so that the electrical transport properties of the carbon black can be isolated from double-layer capacitance. The rheological and electrical response of the suspension under steady shear flow is thoroughly characterized. These measurements are complemented with small angle neutron scattering measurements made in a commercial stress-controlled rheometer (RheoSANS). We find that these suspensions, when sheared above their macroscopic yield stress, undergo shear-thinning and reversible agglomerate erosion. This structural erosion has a dramatic effect on the electrical properties of the suspension and the resulting complex impedance is fit to an equivalent circuit that describes the evolution of the charge transfer dynamics. This model reveals the important role of colloidal forces in the electron transport process in such suspensions.