Preparation And Interfacial Strenthing Mechanism Of Aramid Fiber-Mica Paper Based Insulation Materials

High performance fiber paper-based insulating material has the advantages of low density,high specific strength,high specific stiffness,and high temperature resistance,etc,which is an important basic raw material to realize the development of electrical equipment towards miniaturization,lightweight and high capacity.As a new kind of paper-based insulating material,aramid-mica paper is a functional material made from aramid fiber and mica.It has high temperature resistance,corona resistance,excellent strength and flexibility,which can be used as high-end electrical insulating material such as high voltage/frequency conversion motor and traction motor.However,due to the low surface chemical activity of aramid fiber and mica,the interfacial bonding of aramid fiber and mica is weak,and the overall properties of composite materials are poor,which limits its application in the high-end field.Therefore,the key to prepare high performance aramid-mica paper-based insulating materials is to study the mechanism of improving the interface between aramid fiber and mica.In order to improve the overall properties of aramid-mica paper,the optimization of mica particle size,the adjustment of the distribution of mica in Z-direction of paper,and the enhancement with nanofibers are studied in this thesis.The mechanism of interfacial bonding between aramid fiber and mica is revealed in order to improve the dielectric properties of aramid-mica paper.Firstly,the effects of mica particle sizes on the mechanical and electrical properties of paper-based materials were studied using aramid fiber and two kinds of micas（mica A and mica B）as raw materials.The results showed that when the particle size of mica（D50）decreased from 155μm to 98μm by ball milling,the particle size distribution span first increased and then decreased,and the specific surface area increased 3.6 times,while the paper thickness increased and the tensile index and dielectric strength decreased with mica particle size decreasing.Compared with mica B,the ratio of diameter to thickness of mica A was larger,and the shedding was more complete,which was helpful to improve the mechanical and insulating properties of paper.When the content of mica in insulator paper was 50%and the average diameter of mica was 130¹40μm,the tensile index of insulating paper containing mica A could be increased by 10.6%,and the dielectric strength could be increased by 28.3%compared with insulator paper containing mica B,respectively.When the particle size of mica was larger,the laminated structure of mica was easily formed in the fiber network system,while the smaller the particle size of mica,the overlapping structure was formed between the mica particles.Therefore,the effect of mica particle size on the interface depended on the stacking structure of mica particles in the paper network structure.Secondly,using mica A as raw material,the distribution of mica in laminated paper was regulated by stratification method,and the Z-direction distribution of mica was quantitatively characterized by discrete factor（F_d）.The effects of Z-direction distribution of mica on paper properties and interfacial bonding were studied.The results showed that when the quantity of the paper was 90 g·m^-2,the composite method of pure aramid paper and pure mica paper was adopted,and the mica paper was used as the intermediate layer to form a"sandwich"structure with aramid paper.The tensile strength of the paper was better than that of the mica paper distributed in the surface layer.The highest tensile index and dielectric strength of paper were 7.88 N·m·g^-1 and 10.34 kV·mm^-1,respectively.When laminating with aramid-mica paper,F_d had a significant negative correlation with tensile index（R²=0.97996）,dielectric strength（R²=0.90836）and interlaminar bonding strength（R²=0.57659）.When F_d=0,which meant that mica was uniformly distributed in laminated paper,the comprehensive properties of the paper were the best,and the tensile index,dielectirc strength and interlaminar bonding strength were increased by 4.90%,15.35%and 1.95%compared with the traditional primary forming method,respectively.Therefore,adjusting the Z-direction distribution of mica in paper improved the material interface difference and compatibility,thus reducing the damage ability of mica to the interface bonding of aramid fibers,and improving the comprehensive properties of paper.In order to further improve the interfacial bonding of aramid-mica insulation materials,plant fiber based nanofibers（NFC）and aramid nanofibers（ANFs）were used to enhance the properties of paper-based insulating materials.NFC/mica composites were prepared by mechanical ball milling method and were used to reinforce aramid-mica paper.The properties of the paper were compared with those of conventional NFC reinforced aramid-mica paper.The results showed that when the dosage of NFC was 6%,the tensile index of paper added with NFC and NFC/mica composites increased by 88.9%and 108.1%compared with that of unadded paper,respectively.The interlaminar bonding strength was increased by56.92%and 61.15%respectively,while the dielectric constant,dielectric loss and heat resistance decreased slightly.SEM results showed that NFC was uniformly coated on the surface of mica by mechanical ball milling,and the interlaminar overlap porosity of mica was reduced.Therefore,as an interface bond bridge,NFC could improve the interfacial bonding of aramid fiber and mica by winding and coating,and improved the comprehensive properties of aramid-mica paper effectively.ANFs was prepared by alkali solution method and then used to strengthen aramid-mica paper.Meanwhile,ANFs/mica binary composite insulator paper was prepared.The results showed that the ANFs retains the fiber bundle structure and have a micro-nano size,and the intertwined fibers formed the network structure.The self-reinforcement of aramid-mica paper was realized by improving the interfacial compatibility of ANFs and a large number of active groups brought about by its high specific surface area.With the increase of ANFs content,the properties of aramid-mica paper were improved significantly.When the amount of ANFs was 15%,the tensile index of paper was 31.21 N·m·g^-1,the dielectric strength was 16.39 kV·mm^-1,which were 268.4%and 66.51%higher than that of blank pattern,respectively.The interlaminar bonding strength of paper increased by 0.8 times.ANFs/mica composite insulator papers were prepared by insitu deposition successfully.The results showed that the transparency of composite paper increased with the increasing of ANFs ratio.When the ratio of ANF to mica was 1:1,the breaking strength of the paper reached 96.78 MPa,and the elongation at break was 2.19%,the dielectric strength was 47.6 kV·mm^-1.ANFs and mica was enhanced by self-coating and hydrogen bonding,and the mechanical strength and dielectric strength of the material were improved.