The characterization of high-performance PAN-based carbon fibers developed by continuous carbonization and air oxidation
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The properties of four kinds of Type II carbon fibers, which had been precarbonized at 300 °C, 400 °C, 500 °C, and 600 °C, respectively, during two-stage continuous carbonization, were measured after being air oxidized for periods of 1 to 6 min at 550 °C. The effects of precarbonization temperature on mechanical properties, density, morphology, elemental composition, and microstructure of the carbon fibers during the air oxidation are discussed in this article. The precarbonization process strongly affected the surface properties and mechanical properties of the final oxidized carbon fibers. The carbon fibers developed from the different precarbonization temperatures all had different structures. The carbon fibers that had been precarbonized at 300 °C had a more ordered structure than other fibers after air oxidation. These carbon fibers also had a higher performance than the other fibers. Carbon fibers also showed different oxidation behaviors caused by differences in surface morphology resulting from each different precarbonization temperature. Optimum conditions not only improved the tensile strength and modulus, but also increased the density and oxygen content. Experimental results showed that the tensile strength of the carbon fibers precarbonized at 300 °C increased from 2.4 GPa to 4.3 GPa (80%) after 6 min oxidation at 550 °C.
I. INTRODUCTION Carbon fibers can be made in a variety of grades and forms each with their own characteristics. During the manufacturing process of PAN-based carbon fibers, a stabilization process is necessary to ensure a high-quality product. In this process, a lower temperature treatment (200-300 °C) in an oxygen-containing atmosphere leads to the formation of a ladder polymer in the stabilized fibers. Forming a ladder polymer is a very important step since it allows the subsequent polymer degradation reactions during carbonization to proceed without collapse of the fiber structure or loss of orientation,1"3 which influences the physical properties and the microstructure of the final carbon fibers.4 Subsequent to stabilization, carbonization is carried out at a temperature of from 800 to 3000 °C in an inert atmosphere. Figure 1 summarizes the main steps in the manufacture of carbon fibers from PAN fibers. The maximum temperature depends upon the type of carbon fibers ultimately required. Carbon fibers heat-treated up to 1000 °C exhibit low modulus and tensile strength; such fibers are defined as Type A carbon fibers. For these fibers, a heat treatment to approximately 1400 °C yields higher tensile strength and a medium Young's modulus; these fibers are defined as Type II carbon fibers. Treatment temperatures higher than 2500 °C result in a high modulus and medium strength; these fibers are defined as Type I carbon fibers. Commonly, the mechanical properties of composites depend greatly upon the interaction between the J. Mater. Res., Vol. 10, No. 6, Jun 1995 http://journals.cambridge.org
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components of the composite, i.e., carbon fibers and resin matrix, at
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