Ko Tse-Hao et al / New Carbon Materials, 2006, 21(4): 297-301 unit(Lc: 13 nm, La: 35-39 nm) and a higher degree of even islands and complete interlinking among lattices graphitization(R: 0.20)after heat treatment at 2700C. So, which causes an increase in tensile strength by 20-30% the deposited film on the surface of carbon fibers may A good correlation exists between preferred orientation duce high graphitization of PAN-based carbon fibers be and modulus in carbon fibers: more highly oriented layer ause of its good graphitizability, thereby improving the planes mean higher tensile modulus [18]. All batch fibers degree of graphitization of the resultant carbon fibers af exhibit similar tensile modulus as shown in Fig 4. Therefore heat treatment at 2700C it could be deduced that the deposited carbon film may too thin to control elongation of carbon fibers regardless of 3.3 Mechanical and electrical properties of carbon whether it is uniformly distributed Mechanical properties are the most important properties that determine the performance of carbon fibers. Mechanical properties of the fibers strongly depend not only on the pre cursor but also on heat-treatment conditions, mainly on creases with heat treatment, but decreases beyond 18000 in. temperature. The strength of PAN-based carbon fibers in- Fig 3 shows tensile strength of the as-received carbon fi- bers and their modified forms after 2700C heat treatment Fibers d and e of the modified forms after 2700C exhibit higher tensile strength than those of the as-received carbon fibers and the other modified carbon fibers(a, b and C) after being graphitized, which are attributed to two factors Batches N First, CVD modification with retention of more than 25 min causes carbon film to spread on fibers and fills the surface Fig 4 Tensile of as-received carbon fibers and their modified canals of carbon fibers. It reduces stress and defects. Second forms(A-E)after heat treatment at 2700C e structural unit (such as La and Lc)of carbon fibers is enlarged after the fibers are modified. Before a crack can Fig5 shows resistivity of the as-received carbon fibers propagate through a fiber to cause failure, certain conditions and their modified forms after heat treatment at 2700C must be fulfilled. Crystallite size along one direction of Modified fibers possess lower electrical conductivity than crack propagation must exceed a critical flaw value for fail- the as-received ones after being graphitized. Improvement ure under tension[17]. As shown in Fig. I and Fig. 2, carbon of the degree of graphitization of carbon fibers through film deposited on D and E fibers is more evenly distributed CVD modification results in good electrical conductivity of han A, B, and C fibers, which means that the deposition on graphite carbon fibers the surfaces of A, B, and C fibers cannot completely attain higher lattice thickness and diameter. In contrast, deposition f呈且 2.6 42086420 100 A-2700° C-2700E B-2700°c D-2700° Batches No Batches No Fig5 Resistivity of as-received carbon fibers and their modified orms(A-E)after heat treatment at 2700C Fig 3 Tensile strength of as-received carbon fibers and thei modified forms(A-E)after heat treatment at 2700CKo Tse-Hao et al. / New Carbon Materials, 2006, 21(4): 297–301 unit (Lc: 13 nm, La: 35–39 nm) and a higher degree of graphitization (R: 0.20) after heat treatment at 2700℃. So, the deposited film on the surface of carbon fibers may in￾duce high graphitization of PAN-based carbon fibers be￾cause of its good graphitizability, thereby improving the degree of graphitization of the resultant carbon fibers after heat treatment at 2700℃. 3.3 Mechanical and electrical properties of carbon fibers Mechanical properties are the most important properties that determine the performance of carbon fibers. Mechanical properties of the fibers strongly depend not only on the pre￾cursor but also on heat-treatment conditions, mainly on temperature. The strength of PAN-based carbon fibers in￾creases with heat treatment, but decreases beyond 1800℃. Fig.3 shows tensile strength of the as-received carbon fi￾bers and their modified forms after 2700℃ heat treatment. Fibers D and E of the modified forms after 2700℃ exhibit higher tensile strength than those of the as-received carbon fibers and the other modified carbon fibers (A, B and C) after being graphitized, which are attributed to two factors. First, CVD modification with retention of more than 25 min causes carbon film to spread on fibers and fills the surface canals of carbon fibers. It reduces stress and defects. Second, the structural unit (such as La and Lc) of carbon fibers is enlarged after the fibers are modified. Before a crack can propagate through a fiber to cause failure, certain conditions must be fulfilled. Crystallite size along one direction of crack propagation must exceed a critical flaw value for fail￾ure under tension [17]. As shown in Fig.1 and Fig.2, carbon film deposited on D and E fibers is more evenly distributed than A, B, and C fibers, which means that the deposition on the surfaces of A, B, and C fibers cannot completely attain higher lattice thickness and diameter. In contrast, deposition Fig.3 Tensile strength of as-received carbon fibers and their modified forms (A–E) after heat treatment at 2700℃ even islands and complete interlinking among lattices, which causes an increase in tensile strength by 20–30%. A good correlation exists between preferred orientation and modulus in carbon fibers: more highly oriented layer planes mean higher tensile modulus [18]. All batch fibers exhibit similar tensile modulus as shown in Fig.4. Therefore, it could be deduced that the deposited carbon film may be too thin to control elongation of carbon fibers, regardless of whether it is uniformly distributed. Fig. 4 Tensile of as-received carbon fibers and their modified forms (A–E) after heat treatment at 2700℃ Fig.5 shows resistivity of the as-received carbon fibers and their modified forms after heat treatment at 2700℃. Modified fibers possess lower electrical conductivity than the as-received ones after being graphitized. Improvement of the degree of graphitization of carbon fibers through CVD modification results in good electrical conductivity of graphite carbon fibers. Fig.5 Resistivity of as-received carbon fibers and their modified forms (A–E) after heat treatment at 2700℃