SYNOPSISBy following the progression of continuous carbonization (300-1250°C ) of polyacrylonitrile (PAN) -based oxidized fibers, variations in tensile mechanical properties and morphological structure are reported in detail along the carbonization line. The tensile strength and Young's modulus of the fibers generally increase throughout the carbonization stage. Meanwhile, the fiber diameter displays a significant decrease. The preferred orientation of carbon layer planes is observed to increase remarkably for temperatures over 400°C. In a similar manner, the stacking size increases significantly but reaches saturation around 600°C, a changing point corresponding to that observed for the variation of the Young's modulus. However, beyond about 1200"C, the stacking size again displays a marked increase. Results thus obtained are interpreted in relation to each other. Possible implications are also discussed.

Polyacrylonitrile (PAN) fibers of a special grade have been modified by a method of prestretching with various stretching ratios from negative to positive before the onset of stabilization. The effect of such pretreatments on the thermorhelogical and thermal behaviors of PAN fibers was followed by free shrinkage experiments and differential scanning calorimetry (DSC) analyses. It was found that prestretching had a significant influence on the physical shrinkage of PAN fibers. DSC results of PAN fibers showed dependence not only on atmospheric conditions but also on the extent of prestretching.

SYNOPSISThe progression of polyacrylonitrile (PAN) fibers on multistage stabilization stage has been monitored by following the variations in density, elemental composition, morphological aspect, and mechanical properties. The effects of various processing conditions during the multistage stabilization on the mechanical properties of final carbon fibers were evaluated in detail. To achieve an optimal extent of stabilization of PAN fibers is critical as the final mechanical properties of carbon fibers are concerned. This aim can be realized by adjusting and matching the temperatures in each step and the processing rates during multistage stabilization. Imposing stretching on multistage stabilization is beneficial to mechanical properties of carbon fibers. I NTRODU CTlO N Carbon fibers as reinforcements in the composites have gained wide range of applications from sports products to items used by the areospace industry. Among the many precursors for making carbon fibers, polyacrylonitrile fibers (PAN) have been established as a most suitable precursor for producing high-performance carbon fibers.'-5 The conversion of PAN fibers to carbon fibers usually include: (a) a low-temperature oxidative stabilization to yield a cyclized ladder structure, (b) higher temperature carbonization in inert environment to produce a graphitelike structure, and (c) an alternative "graphitization" treatment to improve the orientation of the carbon basal planes and the stiffness of fibers. Since the low-temperature stabilization (ZOO-300°C) step is usually time consuming and has significant effect on not only the mechanical properties but also the manufacturing cost of carbon fibers, tremendous efforts have been accumulated to understand the complex transformations during this intermediate step and to choose optimal processing conditions on

SYNOPSISPolyacrylonitrile (PAN)-based activated carbon fibers were developed with the idea of increasing their potential and efficiency in industrial applications. The PAN-based fibers were first oxidized in air in a continuous multistage stabilization process, and then subjected to a continuous, low temperature carbonization and activation treatment in a mixture of steam and argon. The effect of the activation condition on the specific surface area, elemental composition, as well as the morphological structure of activated carbon fibers was studied. The surface area of the carbon fibers increased remarkably after the steam activation. It was found that steam activation promoted the elimination of nitrogen from the fiber. The bulk oxygen content of the fibers increased upon activation, probably due to formation of carbon-oxygen functionalities on the surface of the activated carbon fibers. The surface oxygen level of activated carbon fibers was greater than the bulk analysis of oxygen. It was observed that activation decayed the order of the carbon structure. 0 1996 John Wiley & Sons, Inc.

SYNOPSISPolyacrylonitrile precursor fiber of a special grade for making carbon fibers was modified by stretching in the prestabilization stage to various extents. The effect of such stretching on tensile properties of the original precursor fiber, intermediate (oxidized) fiber, and resultant carbon fiber prepared through a continuous process was monitored. Improvements in tensile modulus of fibers at various stages were observed with increasing stretch ratios. However, no obvious enhancement of tensile strength of final carbon fibers was found.