Research On Conductive Percolation Of Filled Conductive Composite Under Loading And Temperature

Electrical properties of filled conductive composite are closely related to the filler contents.A common phenomenon is that when the filler increases to a certain critical content,the resistivity of the conductive composites will change dramatically which make it change from an insulator to a conductor,which means conductive percolation occurs.Research on conductive percolation mainly focus on the observation of the percolation phenomenon,the proposing of the percolation theory and the calculation of the percolation model.Since the conductive composites are often used under the condition of the external field,the characteristic of the conductive percolation and the permeation threshold will change under the influence of the external field.In this thesis,the conductive percolation of PP/CB/CNTs composites under force or temperature will be studied.First,polypropylene(PP)/carbon black(CB)/carbon nanotube(CNTs)composites were prepared.The matrix materials PP,conductive filler CB and CNTs were weighed,proportioned,dried in the oven,and then melt blending them through a twin screw extruder.Finally,PP/CB/CNTs conductive composites with different filler contents were injection molded by injection molding machine.Secondly,SEM scanning electron microscopy was used to observe the microstructure of the specimens,and the dynamic thermodynamic properties of the composites were measured by DMA.The SEM results show that the CB/CNTs filler is dispersed evenly in matrix PP,and CNTs fiber acts as a“bridge”between CB particles.With the increase of filler contents,the distance between particles will decreases,and the probability of the conductive network formed through particle contact will increases.With the increase of temperature,energy storage modulus E'decreases,and the loss factor Tan?increases gradually,while the loss modulus E"increases first and then decreases with the appearance of loss modulus peak,where the temperature is the glass transition temperature,T_g of the composites.Then,uniaxial compression experiments of PP/CB/CNTs composites at different temperatures were carried out on the MTS universal testing machine.The experiment results show that the strain of the composites increases with the increase of the stress,and fibers reinforce the matrix with the increasing filler.As the temperature increases,the yield strength of the material decreases.By comparing PP/CB and PP/CB/CNTs composite systems,it could be found that a small amount of CNTs would enhance the strength of the composites.Finally,the piezoresistive experiments of the composites at different temperatures were carried out by MTS testing machine.Influences of load and temperature on the percolation curves and the conductive percolation characteristics were studied under the action of force/temperature.The results show that the percolation curves move to the direction of low resistance under small strain at room temperature.When strain increases to a certain value,percolation curves will move to the direction of high resistance and decline rate of resistivity will slow down.At high temperature,the percolation curves also move to the direction of low resistivity under small strain.Strain increases the movement to the direction of high resistance,and the percolation curves will be more gentle and the decline rate of resistivity will be slower.Under the combined action of stress and temperature,stress plays a dominant role when the strain is small,while the strain is large,the percolation curves will be more complex.By comparing PP/CB and PP/CB/CNTs,it could be found that CNTs will weaken the affect of external field to percolation curve.