Research On Preparation And Electrical Properties Of Carbon-nitride/Polyimide Composite Films

Polyimide(PI),as an insulating material,has been widely used in life and industry.Compared with other electrical insulating materials,it has outstanding mechanical properties,higher electrical breakdown strength,high temperature and low temperature resistance.PI composites prepared by adding nano-fillers are expected to further improve their mechanical and electrical properties and be better applied in the field of electrical insulation.By high temperature calcination melamine after ultrasonic dispersion was prepared by carbon nitride nanometer(CNNS),and take hot water and the method of poly(dopamine packages increase groups on the surface of the carbon nitride to improve its dispersion in polymer matrix,and two kinds of modified carbon nitride or hydrothermal method modified carbon nitride(HCNNS)and poly(dopamine wrapped carbon nitride(DCNNS).Infrared analysis showed that the modification was successful,that is,the content of hydroxyl group was increased on HCNNs and the content of amino group was increased on DCNNs.Composite films were prepared by thermoimination in situ polymerization.Polyamidoic acid(PAA)solution was prepared from phthalic acid dianhydride(PMDA)and 4,4-diaminotetraphenyl ether(ODA)by adding fillers in N,N-dimethylacetamide(DMAC)solvent.Three series of CNNs/PI,HCNNs/PI and DCNNs/PI composite films were prepared by thermal imination.Compared with pure PI films,the mechanical and electrical properties of CNNs/PI composite films are slightly improved.SEM analysis results show that when the content of CNNs is lower than 0.75wt%,CNNs can be relatively uniformly dispersed in the PI matrix.However,when the content of CNNs is higher than 0.75wt%,filler particles agglomeration phenomenon and the formation of pores appear in the composite film.Compared with PI films,the mechanical properties of CNNs/PI composites were significantly improved:the tensile strength and elongation at break of CNNs/PI composites were increased by about 45%and 10%,respectively,by adding 0.75wt%CNNs.In terms of electrical properties,the dielectric loss ratio of CNNs/PI composites is reduced.At the same time,when the content of CNNs is 0.5wt%,the breakdown strength of the composites reaches the maximum of 203 kV/mm,which is 15%higher than that of pure PI.The dispersion of HCNNs is better than that of CNNs,and the electrical and mechanical properties of HCNNs/PI are improved more obviously than that of CNNs/PI.SEM analysis showed that compared with CNNs,HCNs were better dispersed in the PI matrix,and agglomeration occurred only when the content of HCNs reached 1wt%.In terms of mechanical properties,the elongation at break of HCNNS/PI composite film is 100%higher than that of PI.In terms of electrical properties,the dielectric loss of HCNNS/PI composite film is still at a low value(<0.006),the maximum breakdown strength is 280 kV/mm,which is about 60%higher than pure PI.The dispersion of DCNNs is better than that of CNNs,and the electrical and mechanical properties of DCNNs/PI are improved more obviously than that of CNNs/PI.In the DCNNs/PI composite film,the dispersion of DCNNs in PI matrix is good,and the elongation at break of the composite film is about 120%higher than that of PI.This is because the compatibility between DCNNs and PI matrix is better than that of CNNs or HCNNs,so that the elongation at break of the composite film is greatly increased.In terms of electrical properties,DCNNs improved the dielectric constant and breakdown strength of PI composite films more significantly than CNNs and HCNNs.When the breakdown strength reaches the maximum(297kV/mm),the decrease rate of PI composite film is also significantly less than that of PI composite film with CNNs and HCNNs added.It shows that the phenolic hydroxyl group and amino group on the surface of DCNNs make it better dispersed in the PI matrix,so as not to cause too large pores between DCNNs and PI and prevent the breakdown voltage from falling too fast.