Pressure-induced percolation transitions in composites

In physical experiments the percolation threshold for conductivity of conductor-insulator mixtures can be achieved by addition of conductive inclusions to the insulating embedding matrix, keeping the volume of the cell constant, or by compression of a mixture containing an initially nonpercolating quantity of conducting inclusions. The latter case we define as a pressure-induced percolation transition (PIPT). Two limiting cases of PIPT may be envisioned, namely rigid conducting particles in a deformable insulator and deformable conducting particles in a less deformable insulator. In both cases the percolation state is achieved at some critical pressure (strain) which depends on the initial concentration of the conducting phase. The experimental results on conductivity of powder conductor-insulator mixtures show that pressure-driven percolation differs substantially from the usual percolation model: even in the simplest case of a rigid conductor in a deformable insulator the critical index of conductivity mu close to the critical strain is much less than the expected value mu approximate to 2. The value mu approximate to 2 is achieved at larger strains. The low value of mu approximate to 0.38 close to the percolation threshold can be explained by considering the competition of processes of destruction and creation of contacts between conductive inclusions and clusters under the applied stress. In the case of deformable conductive inclusions interpretation of the PIPT is rather complicated. The strain sensitivity of the conductivity is maximal close to the percolation threshold; we consider some practical implications of anomalous strain sensitivity.