| With the development of morden technology,heat dissipation is becoming a main problem on the development of many industries.For example,with the increasing integration level of electronic component,the equipment will produce a large sum of heat in a certain time.If the heat can't be conducted imidiately and properly,then the equipment will be overheated,which may cause disturbtion on the stability and safety of the equipment.Finding materials with high conductivity is the key point to solve the problem.Graphite crystal have extremely high thermal conductivity along its layer plane,as high as 2000W·m-1·K-1,and very low across its layer plane,as low as 6-9 W m·1·K-1.This anisotropy property makes graphite the ideal thermal management materials.Polyimide film can be converted into high crystal graphite film with high thermal conductivity as 1800W·m-1·K-1 through simple thermal treatment.It can be inferred that polyimde fiber can be converted into graphite fiber through a similar carbonization process.Polyimide based graphite fibers with high thermal conductivity is a promising material to solve the heat dissipation issues.In this work,we prepared polyimide fibers by "Two step method" and through a dry-jet wet spinning process.Then,polyimide fibers were heated at high temperature to prepare carbon fibers and graphite fibers.We studied the influence of different polymerization time,tensile stress on polyimide fibers during carbonization process,molecular structure of polyimide fibers,physical cross-linking and chemical cross-linking on the structure and property of final graphite fibers.Then we studied the chemical reaction and structure evolution during carbonization and graphitization process,and point out the degration mechanism of polyimide fiber.The structure and property of PI based carbon fibers depends on the molecular structure on polyimide.With the increasing of ODA component,the thermal stability of co-polyimide fiber is improved,the heat release during carbonization process is getting lower which improved the mechanical property of carbon fiber.With the increase of ODA content,the thermal conductivity of polyimide based graphite fiber increased first and then decrease,the thermal conductivity of POP-20%is the highest,which is 305.2W*m-1*K-1.Then we studied the processing conditions on the properties of polyimide fiber and polyimide based graphite fiber.With the increasing of polymerization time,the molecular weight of PAA solutions becomes lower and the distribution of molecular weight becomes homogenous.With the increasing of polymerization time,the crystallization degree,mechanical property,thermal conductivity becomes better.The polymerization time of 60min is the best.The orientation degree of PI based carbon fiber can be induced by exerting strain on PI fibers during carbonization process.The strain induced PI based carbon fibers have a certain degree promotion on crystallization,thermal conductivity,mechanical property with the adding of strain.The thermal conductivity of graphite fiber prepared by 160g tensile forces is the highest,which is 128.2W*m-1*K-1.To improve the thermal stability of PI fibers during carbonization process,co-polyimide fibers is fabricated by adding DABA component to PMDA/p-PDA PI fibers.Then cross-linking polyimide fiber is prepared by direct heat cross-linking and chemical cross-linking method.The polyimide fibers prepared by direct heat treatment form physical crosslinking due to the interaction of hydrogen bonds between molecular chains containing-COOH bond.The thermal stability,carbon yield and graphitization yield of physical cross-linking polyimide fibers are higher than chemical cross-linking method.At last,the process and mechanism of the structure evolution of PI fibers heated from 300? to 2800? have been analysed by TG-MS and XPS test.This work laid a solid foundation on the study of making carbon fibers through aromatic PI fiber,which may have potential use in heat management field. |
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