Study Of Relationship Between Density And Microscopic Structure Of Carbon Fibers During Carbonization And Graphitization

The density of carbon fiber is a macroscopic quantity which is surely influenced by the microscopic structures. Moreover, the carbon fiber is a kind of graphite-like structure which is composed of crystalline regions and amorphous regions. Both the density of crystalline region and the density of amorphous region influence the density of carbon fiber. Therefore, it is very important to study the reason why the density of crystalline region and the density of amorphous region change.The thesis studies the reason why the density of crystalline region and the density of amorphous region change during the carbonization and graphitization and their influence to the density of carbon fiber by density gradient measurements, X-ray diffraction, Raman spectra, elemental analysis and thermogravimetric analysis. The results indicated that:1. The density of carbon fiber increased with the development of heat treatment during low temperature carbonization, and decreased slowly during high temperature carbonization while rose up during graphitization. 2. The density of crystalline region increased during the carbonization and graphitization processes. From350℃to1000℃, graphene planes were formed continually and got closer which resulted in the increasing of the crystalline density. From1000℃to1500℃, the size of crystallite increased constantly, but the d-spacing of (002) plane had not much variation which led to the little change of the density of crystalline region. From1500℃to2300℃, the d-spacing of (002) place had reduced drastically and the crystalline grains grew and stacked together. At the same time, the density of crystalline region rose rapidly.3. The density of amorphous region increased gradually before1000℃and decreased sharply after1000℃. The reason for this phenomenon mentioned above was that the cyclic structures formed gradually in amorphous region and enlarged constantly which caused the amount of orderly region increased in the amorphous region. Consequently, it made the density of amorphous region increased. After1000℃, the reduction of the density of the amorphous region was due to the elimination of the nitrogen element, the emergence of pores and the disorder-order transition in amorphous region to crystal.4. The contribution of crystalline region to density had been increasing and the contribution of amorphous region to density had been decreasing during carbonization and graphitization. In the thesis, the mechanism for the variation of the density of crystalline region, the density of amorphous region and their influence to the density of carbon fiber have been investigated. It is significantly meaningful for controlling the structure of carbon fiber in the process of carbonization and graphitization and improving the properties of carbon fiber.