ELSEVIER Synthetic Metals 71 (1995) 1773-1774 Pitch-based carbon fibers: bromination, resistivity, stability I.V.Klimenko, A.N.Shchegolikhin and T.S.Zhuravleva Institute of Chemical Physics, Russian Academy of Sciences, Kossygin st.4, Moscow, 117334, Russia Abstract. Blectro-physical chmctcristics of brominated pitch-based carbon fibers (CF) have been measured in the lo-300 K temperature range. The room-temperature elcctricaJ resistivity of these fibers has decreased by a fhctor of 7 relative to their initial state. The temperature dependencies of the resistmu: for both initial and Brz-treated CFs turned out to be similar. The temporary stability of the brominat& fibers resistance was monitored over one month. Thermal stability of Brs-treated fibers in 258OO’C temperature range has been studied by TGA-method. 1. INTRODUCIJGN Since it has been reported that bromination of pitch-based CFa (Thoml, P-100) by exposure to Brz-vapour has lowered the cIemical resistivity value of the CFs by a factor of 5 [l]. the bro&ted CFs have been the subject of intense studies [2-S]. 7Jn.q conductivity increase is of practical importance Since SUCh CFs could be usable in conducting composites production. As it has been shown by Gaier [2], a high degree of graphitization is needed for extensive bromination. and the pitch-b‘& CFS are known to be generally more graphitic than, e.g., PAN-based Cl+. imploring ways of lowering of the rcsistivities of various CFs contemporarily produced in Russia. we have chosen the pitch- based CFa for this particular work. Some preliminary results concerning preparation, evaluation of temporary or thermal stability, and measurements of electro-physical characteristics of brominated pitch-based CFs will be reported. 2.JXPERIMENTAL Domestic (produced in Russia) pitch-based carbon fibers with temperature of heat-treatment (THT) of 3000’ C were used throughout this study. The thickness of individual initial fibers was about 10 pm. The bromination was carried out at room temperature. The individual fibers were sealed in small glass vessel filled with Brz-gas, and allowed to react for 2-100 hrs. After the bromination has bcwn tern&ted by removal of the sample from the vessel, the brominated CFs were held under a firme hood, where bromine &sorption at room temperature has been accomplished for three days. Samples of the brominated CFs for electrical and weight measurements were additionally held under 10” Torr vacuum for 5 hrs. Tire electrical resistance was measured with a P3009 D.C. bridge by a two-point probe method in the temperature range of lo-300 K in Leybold cryostet model ROK-300. The eleczical contacts were made by using a silver paint. The standard deviation value of the measurements was within f 2 %. The fiber diameters were measured with the aid of POLAR P-3 II (Poland) polarization microscope. Weights of both untreated or Brz-treated CFs were measured with a P&n-Elmer AD-4 autobalance. Thermal stability of the CFs has been evaluated by using 0379-6779EdO9.50 0 1995 Elsevier Science S.A. All rights reserved SSDI 0379.6779(94)03046-9 thermogravirneIric analysis (TGA). A Perkin-Blmar TGA-7 analyser has been used. Typically, TGA axper&& were 8cco- mplished by using a flowing argon atmosphere, a heating rate of 10’ Urnin, and the sample weights of 4-6 mg. 3. RESULTS AND DISCUSSION. Fig. 1 shows the dependencies of weight (P) and resistivity (p, 300 K) versus Br+zatment time for the pitch-based CFs. 0.013 : : 13 0.009 0 . iooo, v, * Gi ;r” 0.00 1 3 0003 0 20 40 60 80 * 100 TIME OF BROMINATION, hrs Figure 1. Weight uptake (1) and electrical resistivity (2) of brominated pitch-based CFs versus time of brominarion, -3OOK The weight of the CFs incr& during the first 48 bra of the bromination. Further Errtreatment has not induced any changes in the CFs weight. Maximum weight uptaka was, of 22%. ‘Ibis value is typical for pitch-based carbon fibers [3,4]. As a result of bromination the diameter of CFs increased usually from 10 pm to about 12 pm This was taken into account in dculating of the resistivity values. The sharp resistivity drop during initial stages of the bromination (curve 2 in Fig.1) is followed by a region of steady resistivity (one large plateau without any discrete stages) at longer bromination times (over 48 hrs). The resistivity value was decreased by a factor of about 7 as a result of bromination (from 11.9 rn.Qmm to 1.7 rtQmm). This value is close to that found enrlier for vapourderived fibers but is greater than values