Investigation On Dielectric Properties In MMT/SiO₂/LDPE Composites

Low Density Polyethylene?LDPE?is widely used as an insulating material in the electrical field because of its good insulativity and economy.However,under the long-term effect of the electrical field,the material is apt to be electrical tree aging,and even lead to breakdown.The nano-particles with different dimensions are added into the polyethylene matrix according to a certain mass fraction,and the synergistic effect between them can be used to improve the dielectric properties of the insulating material.In this dissertation,the matrix is LDPE,the organic montmorillonite after the ultrasonic treatment and nano-silica modified by silane coupling agent as inorganic fillers,to prepared four kind of materials about LDPE,MMT/LDPE,SiO₂/LDPE and MMT/SiO₂/LDPE by melt blending.The microstructure of the four materials were characterized by XRD,DSC and FTIR tests.Then the dielectric strength,polarization behavior,dielectric loss and volt-ampere characteristics of the four materials are compared.Finally,electrical treeing experiments are carried out on four specimens to study the growth morphological characteristics of the electrical trees under the power frequency voltage.The experimental results of XRD,DSC and FTIR show that:nano-SiO₂ has heterogeneous nucleation effect,but it disrupts the crystallization process of LDPE matrix and introduces more crystalline-amorphous interface.The lamellar structure of MMT blocks the movement of LDPE matrix molecular chains,inhibit the crystallization rate of the composite material,and reduce its crystallinity.The addition of the surface-modified nano-SiO₂ and MMT into LDPE does not change the structure of the LDPE molecular chains,nor the formation of new chemical bonds,but rather mixes with the LDPE matrix molecular chain through the modifier long chain in a physically entangled form in LDPE.Dielectric properties test results show that:the addition of nano-SiO₂can significantly increase the LDPE threshold electric field and dieletric strength;the current density in the composite materials are reduced and the material's dielectric loss are increased.Nano-MMT and LDPE have poor compatibility,so the dieletric strength of the composite material can be reduced.Due to the synergistic effect between the nano-SiO₂ and nano-MMT,the dielectric properties of the composite materials have been effectively improved.In the comparative study on the initiation experiment of electrical trees,compared with pure LDPE samples,SiO₂/LDPE shows no obvious inhibition effect,while the electrical tree lengths of MMT/LDPE and MMT/SiO₂/LDPE,only up to 87.7%and 67.0%of the pure LDPE,and the fractal dimension of the latter is significantly higher than the other samples,so it shows the best performance on the resistance.In summary,while adding nano-SiO₂ and nano-MMT,the dielectric properties of the composite is effectively improved.Moreover,the heterogenous nucleation of nano-SiO₂ and the blocking effect of the MMT in the amorphous region are mutually synergistic,which remarkably inhibits the aging of electrical tree in the LDPE-matrix composite.