Fabrication And Mechanism Of Carbon Nanotube/epoxy Resin Composites With High Dielectric Constant And Low Percolation Threshold Through Microwave-assisted Curing Technology

Owing to high energy storage and the ability of homogenizing electric field,polymer matrix composites with high dielectric constant(high-k)show wide applications in producing capacitors,electronic storage devices,electrical engineer equipment,and so on.Inorganic nanoconductor/polymer composite is an important type of high-k composites,and has the advantages of maintaining good processing and mechanical properties of polymers due to its low percolation threshold(fc<1 wt%).However,inorganic nano-conductors generally aggregate in organic resin,while dielectric properties are sensitive to the structure of materials,these bring the difficulty of obtaining stable and desirable dielectric property.Therefore,how to fabricate polymer composites with higher dielectric constant at lower content of conductors based on commercial compositions is still an interesting topic with great challenge.By changing the spatial distribution of conductors in a composite,forming an order arrangement can greatly improve the dielectric constant and reduce the fc of the composite.The research work reported in this thesis mainly includes following two parts.First,based on multi-walled carbon nanotube(CNT)and epoxy resin(EP),high-k composite(m-CNT/EP)with much higher permittivity and lower fc was prepared with a microwave-assisted thermal-curing technology.Results show that the spatial structures and performances of CNT/EP composites are dependent on the curing process used.CNTs are orientated along the Z direction in m-CNT/EP composites,while those are randomly permutated in the composite(t-CNT/EP)produced with traditional thermal curing procedure.Accordingly,t-CNT/EP composites have isotropic dielectric properties,and m-CNT/EP composites exhibit anisotropic dielectric properties.fc values of m-CNT/EP composites in X,Y and Z directions are 0.29 wt%,0.29 wt%and 0.24 wt%,respectively,while those of t-CNT/EP composites are equal to 0.39 wt%.When the loading of CNTs(f)is 0.25 wt%,the dielectric constant in Z direction of m-0.25CNT/EP is as high as 673,about 20.7 times of that of t-0.25CNT/EP.The origin behind these interesting results was discussed from building the relationship between CNT distribution and dielectric properties using finite element and simulating equal circuits.Second,a new type of double-layer composite material([m-CNT/EP]2)with high dielectric constant and low dielectric loss was prepared with a microwave-assisted thermal-curing layering technique.Electrical and dielectric properties of[m-CNT/EP]2 composites were studied intensively and compared with those of m-CNT/EP.Results show that[m-CNT/EP]2 and m-CNT/EP composites have anisotropic dielectric properties.The fc values in the X,Y,and Z directions of m-CNT/EP are 0.29 wt%,0.29 wt%,0.24 wt%,respectively;while those of[m-CNT/EP]2 composites in X and Y directions are 0.34 wt%and 0.32 wt%,and there is no definite fc in the Z direction.Under the same f,the dielectric constants of[m-CNT/EP]2 composites in the X and Y directions are higher than those of m-CNT/EP composites.With the increase of f,the dielectric losses of m-CNT/EP composites in three directions increase,while the dielectric loss of[m-CNT/EP]2 composites in the Z direction is always at a very low level(<1),indicating that the double-layer composite material can greatly reduce the dielectric loss of composites.