Study Of Nonlinear Conduction Behavior Of Silicone Rubber/Graphite Nanosheet Composites Under Pressure Field

Silicone Rubber (SR)/Graphite nanosheet (GN) nanocomposite is a novel stress-sensitive material, which possesses good electrical and mechanical properties. The nonlinear conduction behaviors depending on ambient pressure are studied and their intrinsic conducting mechanisms are also discussed in detail.The resulting exfoliated graphite nanosheets possess high aspect ratio with diameter ranging from 5 to 20μm and average thickness of 40 nm. Owing to this special morphology GN is very favorable to form conducting paths in the polymer matrix. As a result, very low graphite concentration is required to satisfy the critical percolation transition. A wet mixing method was used to fabricate samples to overcome the inhomogeneous distribution of GN in SR.A notable nonlinear resistance-pressure conduction behavior was found for the nanocomposite. The piezoresistive behavior, including the negative pressure coefficient (NPC) and the positive pressure coefficient (PPC), is directly attributed to compressive-stress-induced deformation of the conducting network. The transition from NPC to PPC effect is mainly due to the formation and destruction of conducting networks. However, SR/GN nanocomposite with the volume fraction of conductive nanosheets closest to the percolation threshold presents a sharp positive pressure coefficient effect of resistivity under very low pressure, in finger pressure range. Due to its low-percolation threshold, an effective conducting network of SR/GN nanocomposite could be formed by a little amount of GN, which makes the conductive network brittle, easy to be destructed and super sensitive. That is why the nanocomposite can show a remarkable positive piezoresistive effect under very low pressure. Examinations show that the tunneling theory could effectively account for the nonlinear resistance-pressure conduction behavior in the SR/GN system.The piezoresistive behavior of SR/GN nanocomposite is directly related to the variety and stability of conductive networks in the composite. Therefore, the pressure, filler content, compressive times, which can directly affect the conductive networks, play important roles in the piezoresistivity of the composites. The creep of the polymer matrix after unloading results the time dependence of resistance which is still related to the times of cyclic compressions and the speed of compression. On the other hand, the adding of another immiscible matrix changes the distribution of GN in the composite and thus influences the piezoresistivity of the composite.The different piezoresistive behaviors of SR nanocomposite under static and dynamic compression was explained successfully by analyzing correspondingly the macroscopical deformation and the microcosmic molecule motions of SR/GN matrix The resistance of silicone rubber/graphite nanosheet nanocomposite under static and dynamic compression changes in different ways which accords with the corresponding deformation characteristics: under static compression, resistance relaxation occurs, according to which it is concluded that the constant strain transforms over time from the initial elastic one to the viscoelastic one; under dynamic compression, the resistance rises with continuously increasing strain, in terms of which it is considered that short relaxation times may not allow the transformation of strain to happen and the elastic strain is dominative. A similar expression of Burgers equation was adopted here for describing resistance relaxation of SR/GN nanocomposite and the Burger's model without the dashpot in series for the cross-linked polymer network was also used to simulate the corresponding deformation characteristics of the polymer matrix.