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The microstructure of high tensile strength （T Series） and high tensile modulus （M Series） polyacrylonitrile-based carbon fibers have been investigated by Wide Angle X-ray Diffraction, and the micropore structure of these carbon fibers was studied by Small Angle X-ray Scattering in this paper. Through the systems research on microstructure of high performance carbon fibers, obtained the data in crystalline state, orientation, defects and micropore structure, etc. Then achieved the relationship between the microstructure and mechanical properties, the characteristics of changes in the process of PAN-based carbon fibers, analysed the generating mechanism of defects and micropore, and provided a scientific basis of optimum technological conditions for the production of high performance carbon fibers. The main research works and results obtained are as follows:1. Wide Angle X-ray Diffraction results indicate that the structure of PAN-based carbon fibers is not belong to the common " two-phase structure " of crystalline and amorphous phase. In the strict sense, there is no crystallization parts or amorphous parts in it, and it’s structure is close to the graphite structure. The results indicate the following different structural characteristics between the two carbon fibers: （1） the interplanar spacing (d002 )of the T300, T700, T800, T1000, M35JB, M40JB, M55JB, M60JB gradually reduce and closely approximate the interplanar spacing of graphite, this proves that the heat treatment temperature of T300, T700, T800, T1000 and M35JB, M40JB, M55JB, M60JB has gradually increased, respectively; （2） the microcrystals in high modulus carbon fibers are more graphitic; （3） the crystallite size and orientation degree of high modulus carbon fibers is larger than that of the high strength carbon fibers; （4） the two dimensional layer stack of high modulus carbon fibers is more ordered than that of the high strength carbon fibers; （5） the high modulus carbon fibers develops to the orderly three-dimension structure or possibly has an existence of three-dimension graphite structure and another hexagonal crystallite graphite structure. The high orientation degree, large crystal dimension and orderly layer stack are beneficial to the increase of tensile modulus; at the same time the appropriate disorder stack of the layer, paracrystal and faults are favorable to increase the tensile strength of the carbon fibers.2. Small Angle X-ray Scattering results indicate that there existed apparent different structural characteristics about the micropore between the two fibers: （1） the tensile strength of the high strength carbon fibers grow or shrink, the average radius of gyration Rg following the same trend, but it is just the opposite for the high modulus carbon fibers; （2） the length of the minor axis is less than the major axis for the two carbon fibers showed that the micropore are ellipse-shaped and distributed between the layer stacks of the graphite; （3） the micropore size of the high strength carbon fibers is larger than the high modulus carbon fibers, however, the axial ratio of the high strength carbon fibers is smaller. And these reflect the more long and narrow micropore existed in the high strength carbon fibers, so these micropores are mainly located in a two-dimensional plane; （4） the micropore size increases when the tensile strength or tensile modulus increases for the two carbon fibers; （5） the higher tensile strength for the high strength carbon fibers, the smaller fractal dimension D, but the higher tensile modulus for the high modulus carbon fibers, the larger fractal dimension D.These results indicate that: in the process of high strength PAN-based carbon fibers, appropriate micropore size, disordered layer stack, paracrystals, less micropores and a smaller fractal dimension can improve the tensile strength; in the process of high modulus PAN-based carbon fibers, the appropriate stretching ratio and thermal treatment with larger orientation and crystal dimension, less micropores and a smaller micropore size can improve the tensile modulus.
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