Structures and electric properties of pitch-based carbon fibers heat-treated at various temperatures
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M. Endo Faculty of Engineering, Shinshu University, 500, Wakasato, Nagano 380, Japan (Received 6 July 1989; accepted 13 October 1989) Four kinds of pitch precursor fibers possessing different mean sizes of mesophase domains in their transverse sections, but having the same chemical constituents, were prepared by controlling only the spinning conditions. The fibers were thermoset and heat-treated at various temperatures from 600 °C to 3300 °C. Lc (002) or dOO2 vs THT curves showed that pitch precursor fibers with finer transverse microstructures have a lower degree of graphitizability. Further, these four kinds of fibers exhibited systematically different dOo2 vs Lc (002) curves. The resistivity at room temperature is dependent on dOo2, being independent of the mean size of the mesophase domains in the transverse section of the pitch precursor fiber. The transverse magnetoresistance at 77 K and 10 kG, (Ap/p),, also snowed a universal dependence on J002 if (Ap/p), was positive. However, the (Ap/p), vs dom curves seemed to be slightly split in the negative region of (Ap/p), for each of the four kinds of fibers.
I. INTRODUCTION
The heat treatment temperature (THT) dependences of dOo2 or Lc (002) of various carbonaceous materials have been studied.1'2 These materials exhibit different THT dependences and unique d002 vs Lc (002) curves. Carbonaceous materials which yield carbons by heat treatments at high temperatures are not only different physically but also chemically different from one another, i.e., have different molecular structures. The relations between the molecular structures and the graphitizabilities of carbonaceous materials have been investigated.3"5 The molecular structure of a carbonaceous material affects the optical texture of the resultant carbon, and the larger optical texture corresponds to the higher graphitizability of the carbon.3"5 On the other hand, the graphitizabilities of pitch precursor fibers spun under various conditions have been systematically studied.6"9 Controlling the spinning conditions enabled us to change the optical texture of the transverse section of the pitch precursor fiber, keeping the molecular structure and the preferred orientation of the molecules along the fiber axis constant. The optical texture of the transverse section of the pitch precursor fibers was quantitatively estimated by introducing as a parameter the mean size of the mesophase domains in the transverse section of the pitch precursor fiber.7'9 These studies qualitatively showed that a larger mean size of mesophase domains in the transverse section of a pitch precursor fiber makes the graphitizability of the pitch precursor fiber 570
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J. Mater. Res., Vol. 5, No. 3, Mar 1990
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higher, under the strongly restricted conditions of a constant molecular structure and a constant molecular alignment along the fiber axis. Such restricted conditions have not been investigated in earlier work.1"5 In this article, four kinds of pitch precursor fibers were prepared,
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