Reaserch On Percolation Model And Piezoresistivity Of Conductive Polymer Composites

With the rapid development of robotic technology, applications of robots have been expanded to many areas. Robots with higher intelligence and safety are demanded largely. In such circumstances, development of flexible soft-sensors which can cover most parts of robot body have attracted extensive attention. Conductive polymer composites (CPCs) are piezoresistive materials consisting of polymer matrix and conductive fillers, in which the flexibility of polymer and electrical conductivity of fillers are combined together. The traditional piezoresistive materials using in sensing technology, such as metals and semi-conductors, are stiff and fragile. CPCs are more stretchable and flexible which are much more suitable for soft sensor applications.This dissertation focuses on development of CPCs with low filler volume fraction and high piezoresistive properties. The behaviors of CPCs in term of percolation threshold and the piezoresistive effect are investigated. Both theoretical and experimental methods are performed to study the relationship between the conductive network structure and the electrical resistivity of CPCs.Following by the literature review of chapter one, in the second chapter, we investigate the electrical properties of CPCs filled with spherical carbonyl iron particle(CIP). We align CIPs in the direction of magnetic field to form chain-like structures,and obtain the electrically anisotropic CPCs. The effect of the intensity of magnetic field on the length of CIP chains is investigated with statistical method. In addition,CPCs with various CIP volume fractions are prepared, their electrical resistivity and the change of resistance with pressure force are tested and the percolation threshold and piezoresistive sensitivity are obtained. A 2D conductive network model filled with rods is proposed to investigate the effects of average length, length distribution and orientation of CIP chains on the structure of conductive network and the percolation threshold of CPCs. The model predicted percolation threshold is well agreement with the experimental data.In the third chapter, we prepare CPCs filled with rod-like multi-walled carbon nanotube (MWCNT) and a few amount of CIPs. The CIPs move along the direction the magnetic field and induce the MWCNTs to arrange in some degree, so the CPCs would be anisotropic. A 3D conductive network model filled with rods and Monte Carlo simulation method are proposed to investigate the effects of orientation of MWCNTs on the structure of conductive network and the percolation threshold of CPCs.Moreover, an 8-chain model is proposed to evaluate the electrical resistivity of CPCs,which is influenced by the degree of MWCNTs alignment. The percolation threshold and piezoresistive sensitivity are obtained experimentally to validate the theoretical results.In fourth chapter, we study disk-like graphene nanoplatelet (GNP) filled anisotropic CPCs, in which CIPs move under the effect of magnetic field and induce GNPs to arrange in some degree. A 3D conductive network model filled with disks is proposed based on lattice model and excluded volume theory. The orientation distribution of GNPs would change the critical contact distance of adjacent GNPs. The critical contact distance increases with the increase of GNP alignment, while the GNP volume fraction decreases accordingly. The experimental results are obtained to validate the analytical model. Meanwhile, the piezoresistive sensitivity of GNP filled anisotropic CPCs is also investigated.