Granular material conductivity increases in mysterious ways under pressure

Scientists reveal how electrical resistance in metallic granular media decreases as the pressure on the micro-contact interface between the grains increases

New York | Heidelberg, 9 June 2017

What happens when you put pressure on bunch of metallic microbeads? According to physicists, the conductivity of this granular material increases in unusual ways. So what drives these changes? The large variations in the contact surface between two grains or the rearranging electrical paths within the granular structure? In a recent study published in EPJ E, a French team of physicists made systematic measurements of the electrical resistance - which is inversely related to conductivity - of metallic, oxidised granular materials in a single 1D layer and in 3D under compression. Mathieu Creyssels from the Ecole Centrale of Lyons, Ecully, France, and colleagues showed that the granular medium conducts electricity in a way that is dictated by the non-homogenous contacts between the grains. These finding have implications for industrial applications based on metallic granular material.

In this study, the authors examined the resistance of a metallic powder packed in 3D volumes made of cylinders of various sizes and that of a one- dimensional layer of metallic beads. In both cases, they found that the resistance decreases with the pressure in a non-standard way . To elucidate this anomaly, they studied how the presence of a homogeneous oxide layer at the grains' surfaces influences conductivity. They also looked at how micro-contact between the grains changes conductivity, depending on their ability to change the shape of the granular material under pressure.

They found that in one dimension, the total electrical resistance of a chain of grains is always the sum of the individual resistances of the elementary contacts. This is unlike what happens in 3D, which is far more complicated. They explain this complex conductivity using a simple model showing that the contact regions between two grains is akin to a set of micro-contacts with consistent conductivity in parallel.

Reference: Pressure dependence of the electrical transport in granular materials. M. Creyssels, C. Laroche, E. Falcon, and B. Castaing (2017), Eur. Phys. J. E 40: 56, DOI 10.1140/epje/i2017-11543-3