The Preparation Of Polyimide-Based Carbon Fibers And The Study Of Their Structure And Property

With the rapid development of aerospace technology, electronic components with compact designs and high power density have been developed. But these components often generate a large amount of heat while in operation, which must be immediately removed along the desired direction in order to make sure the components can work smoothly and efficiently. Graphite material not only has low density, excellent mechanical property and good thermal shock resistance, but also has a high thermal conductivity (TC). Its TC could be theoretically up to 2500W·m-1·K-1 along the graphite sheets direction, but only 6 W·m-1·K-1 in perpendicular direction. So it is thought to be an excellent candidate material with a high directional TC. It is well known that the aromatic polymer can be transformed into carbon materials by solid-phase carbonization, of which the polyimide (PI) material is one. And it is found that the polyimide films can be well graphitized and the TCs of their graphitizing films can reach as high as 1800 W·m-1·K-1.In this paper, PI fibers, as precursors, are turned to be carbon fibers by solid carbonization. We focus on the effects of structure and property of PI-based carbon fibers (PI-CFs) made by several factors such as heat treatment temperature (HTT), drawing conditions, heating rate and stabilization. And the results show that when the HTT is up to 1500℃, the carbon content of PI-CFs has already grown up to 93%.The internal structure of the fibers will becomes loose and surface defects become large if heated at higher temperature. The PI-based graphite fibers (PI-GFs) have high degree of orientation and graphitization, and the graphite sheets are arranged regularly. At the same time, with the increase in HTT, the tensile strength of PI-CFs grows in the beginning and then decreases, with a max tensile strength of 924.4MPa, but the breaking elongation decreases constantly. The PI-GFs exhibit a good potential for owning high TC. In order to obtain highly oriented PI-CFs, we need to apply a drawing function in carbonization to make sure that the PI-CFs could have a good mechanical property. Furthermore, we found that heating rate factor has little effect on the crystallite size and crystal improvement of the final products when carbonized at 1000℃, but a significant effect on the morphology and conductive properties of the final products. Therefore, in order to obtain the high-quality PI-CFs, we should set up the heating rate as slow as possible in the pyrolysis process, reducing the number of defects made by the release of small molecules gas.The PI fibers, stabilized under reflux of hot nitric acid, could form a large amount of intermolecular chemical bonds, making the fiber molecular chains transformed from the linear configuration to the cross-linked one. As a consequence, the exothermic interval widens with a milder curving and the total heat release is significantly reduced from 875.18mW/mg (non-stabilized) to 166.19mW/mg (stabilized for 15min). Thus, this stabilization method could effectively avoid a sudden jump in temperature and local heat storage during the carbonization, as well as help improve the stability of PI fibers. Simultaneously, the obtained PI-CFs will have the carbon yields improved remarkably. However, as the stabilization time further extends, some defects such as ruptures will appear on fiber surface, which would have some negative effects on the properties of the resulting products. So a moderate degree of stabilization is needed (recommended time 10-15min). Moreover, the conductive property of PI-GFs exhibits the same regularity related to the changes in structure, which increases in the beginning and then decreases, with a maximum thermal conductivity of 415.35 W·m-1·K-1 after graphitization at 2800℃.