Millennial Special Leading Papers on Ceramics in the 20<sup>th</sup> Century: the Best of JCerSJ Some Properties of PAN-Based Carbon Fiber

Abstract

In the late 19th century, T. A. Edison, a great inventor, applied a carbon filament made from bamboo fibers grown in Kyoto to an incandescent lamp, which was one of the earliest technical applications of carbon fiber. The need for carbon fiber for use in military aircraft originated shortly after World War II in the U.S.A. as carbon fiber reinforced structural materials. Since the early 1950s, many varieties of reinforcing fibers have been developed and the UCC demonstrated high-performance carbon fiber made from rayon. However, its tensile strength and elastic modulus were not particularly high. A technical and commercial breakthrough for high-performance carbon fibers occurred in the mid 1960s-the production of carbon fibers from polyacrylonitrile (PAN) precursor fibers. This process proved to be more economical due to the lower cost of the PAN precursor fiber and the simpler process required to convert PAN fiber to carbon fiber. Shindo first reported the use of PAN as a precursor for carbon fibers. At that time almost no military application of engineering materials was considered in Japan. The history of these carbon fibers can be found in “Carbon Reinforcements and Carbon/Carbon Composites, ” by E. Fitzer and L. M. Manocha, Springer-Verlag (1998).Shindo's invention appeared as Japanese Patent No. 304892 (1963, applied for in 1959). His claim was a method of manufacturing carbon or graphite materials, which comprises heating PAN polymer up to 350°C in an oxygen-rich atmosphere followed by heating to a temperature above 800°C. The heating PAN fiber at lower temperatures in an oxidizing atmosphere does not cause melting or deformation of the fiber during subsequent heating. This heat treatment was referred to as a stabilization process and became a fundamental process for conversion of organic precursors to inorganic fibers.Shindo's paper describes, (1) the growth of crystallites in two kinds of PAN-based carbon fibers, (2) changes in the mechanical properties and (3) changes in the electrical resistance by heat treatment in the temperature range, 1000 to 3000°C. From the viewpoint of the remarkable development of carbon fiber applications subsequently, the available data on mechanical and crystallite growth must be significant. High grade of preferred orientation of graphite layer planes parallel to the fiber axis, which is shown in X-ray diffraction photos and electron diffraction pattern, could be due to the high-strength and high-modulus of the fiber. Furthermore, after heat treatment at a temperature as low as 1000°C, the fiber assumed significantly higher preferred orientation. This shows that a high-modulus carbon fiber can be more easily manufactured from PAN fiber than rayon-based fiber.The density of the carbon fibers vs. heat treatment temperature curve (Fig. 8) shows a fairly large increase above 2000°C. The electrical resistivity vs. heat treatment temperature curve (Fig. 9) shows three regions: a steep decrease below 1000°C, a slight decrease from 1000 to 2300°C and a very small decrease above 2300°C. Figure 10 shows that the tensile strength of the fibers decreases steadily from 5000-10000 to 2000-5000kgf/cm2 with raising heat treatment temperature from 1000 to more than 2500°C. The elongation at the breaking point decreases from 1 to 0.3% with raising heat treatment temperature, as shown in Fig. 12. Young's modulus, as determined from these curves, increases from about 11000kgf/mm2 at a heat treatment temperature of 1000°C to 15000kgf/mm2 at 3000°C with a maximum at 2000°C, as shown in Fig. 13. The content of the Shindo's paper appeared in Govt. Res. Inst., Osaka, Report No. 317 (1961) in English, which has been widely cited in many articles concerning carbon.