Vol. 11, No. 4 December 2010 pp. 304-310 Formation of Isotropic Carbon Matrix in Carbon/Carbon Composites Derived from Pitch Chong-Jin Ahn 1 , In-Seo Park 2 and Hyeok-Jong Joo 1,♠ Department of polymer Science and Engineering, Chungnam National University, Daejeon 305-764, Korea Agency for Defense Development, Daejeon 305-600, Korea e-mail: [email protected] (Received August 30, 2010; Accepted December 7, 2010) Abstract To manufacture a carbon/carbon composite the coal tar pitch was used as the matrix precursor and the PAN (polyacrylonitrile)-based carbon fiber was used as the reinforcing material to weave 3-directional preform. For pressure carbonization HIP equipment was used to produce a maximum temperature of 1000 C and a maximum pressure of 100 MPa. The carbonization was induced by altering the dwell temperature between 250 C and 420 C, which is an ideal temperature for the moderate growth of the mesophase nucleus that forms within the molten pitch during the pressure carbonization process. The application of high pressure during the carbonization process inhibits the mesophase growth and leads to the formation of spherical carbon particles that are approximately 30 nm in size. Most particles were spherical, but some particles were irregularly shaped. The spread of the carbon particles was larger on the surface of the carbon fiber than in the interior of the matrix pocket. Keywords : Mesophase, Carbon/carbon composites, Pitch, Mesocarbon microbead, Pressure carbonization 1. Introduction The carbon material is normally produced through the heat treatment of organic materials such as phenolic resins, furan resin, or pitch. When a pitch is heat treated, it converts into cokes via a pyrolysis process with the volatilization of a low- molecular material in a liquid state. In this heat-treatment process, isotropic liquid pitch develops into mesophase [1]. The liquid crystal produced at this stage causes nucleation in liquid pitch, and the liquid crystal grows based on coalescence. In this process, the part of the pitch that is not converted into mesophase remains as isotropic pitch. The converted liquid mesophase forms bulk mesophase during the consistent heat treatment and converts into graphitizable solid soft cokes at a high temperature. Prior to the formation of bulk mesophase, the mesophase exists as a spherical, and this spherical mesophase separated from the isotropic pitch is known as mesocarbon microbead [2,3]. This mesocarbon microbead has attracted considerable attention lately as a precursor of a carbon material with an outstanding property. The mesocarbon microbead is used for manufacturing of high density carbon [4], column packing material [5], electrode material of lithium ion rechargeable batteries [6-8], super- active carbon [9,10], and carbon materials with improved mechanical strength [11,12]. A number of methods for manufacturing of the mesophase microbead have been introduced [13-22]. However, an important factor in producing this microbead was to decrease the distribution of particle sizes and high yield. Regarding the course of manufacturing a carbon/carbon composites derived from the pitch, a carbon-fiber preform is impregnated into molten pitch, and graphitization is performed after pressurized carbonization to form a carbon matrix in the preform, this graphitization to repeats the densification process to gain the composites. In this case, most of the pitch is converted into soft cokes through a bulk mesophase course and is converted into a graphite structure with strong anisotropy through the course of graphitization. However, the carbon matrix must exhibit isotropy when needed to be suitable for its applications. The carbon/carbon composites used in a rocket nozzles and nose cone in the space industry requires isotropy. Therefore, a reinforcing arrangement of carbon fiber, becomes more suitable for this use the more it exhibits multi- directional isotropic structures and the isotropy of a carbon matrix. Accordingly, the pitch fused in the course of the densification of a carbon fiber preform must allow the moderate growth of a liquid crystal. The further growth and the conversion of bulk mesophase must then be suppressed. When a consistent heat processing is performed, the liquid mesophase converts into fine grained cokes, and when the final graphitization is performed, the carbon matrix of the carbon/ carbon composites forms based on the fine grain. Thus, the Carbon Letters
Formation of Isotropic Carbon Matrix in Carbon/Carbon Composites Derived from Pitch
Jun 17, 2023
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