M.. Prauchner et al. Carbon 43(2005)591-597 y=19+33 matter is important because it allows the use of the whole tar generated as a by-product during charcoal production, therefore stimulating volatile recovery in industrial chimneys and aggregating revenue to the charcoal-making industry, which is particularly interest ing because planted biomass is an environmentally friendly and renewable energy source Although the tensile properties make clear that the produced fibers are not useful as structural reinforce- ment, other properties give rise to different potential 31y=61+0.63X applications. For example, the reduced carbon yield of the precursor and the high isotropic and disordered structure of the fibers suggest a great potential for the production of activated carbon fibers, besides using them in the manufacture of thermal insulation carbon In o(MPa) felts. Efforts have been made to improve the process Fig. 5. Weibull distribution for tensile strength measurements of so far employed mainly aiming to reduce the period of eucalyptus tar pitch-based carbon fibers. time necessary to stabilize the fibers porosity in carbonized material and even to crack for- mation, as already discussed. In that respect, Derbyshire Acknowledgments et al. [4] demonstrated that fiber tensile strength in creases with increasing precursor carbon yield. The sec The authors thank Fundacao de amparo a Pesquisa ond reason is that the relatively large diameters of the do estado de minas gerais(FAPEMIG), Empresa Bra- fibers produced in the present work is certainly contrib- sileira de Pesquisa Agropecuaria(EMBRAPA) and uting to reduce the tensile strength because thicker fibers Fundacao de empreendimentos cientificos e Tecnolog contain more flaws per unit length compared to smaller icos(FINATEC)for financial support diameter fibers. Fitzer and KOnkele [36] have reported that larger diameters imply lower tensile strengths. In turn, Mora et al. [5] found that the tensile strength of References coal tar pitch-based isotropic carbon fibers varies from 143 MPa to 414 MPa when the fiber diameter vari Edie DD, Diefendorf rj. Carbon fiber manufacturing. In: from 35 um to 16um Buckley JD, Edie DD, editors. Carbon-carbon materials and The tensile modulus of the carbon fibers obtained was composites. Park Ridge, NJ: Noyes: 1993. p 18-30 [2]Savage G. In: Carbon-carbon composites. London: Chapman 14+3 GPa. This value is lower than the usual for Hal;1993.p.53-7 GPCF. For example, the values reported by Mora et 3 Edie DD. The effect of pr structure and proper al. [S] in the previously quoted paper fall in the range of carbon fibers. Carbon 1998: 36: 345-62 29-36GPa. Besides the flaws. this result reflects the mis- [4] Derbyshire F, Andrews R, Jacques D, Jagtoyen M, Kimber G oriented structure of the fibers derived from the three- Antell T. Synthesis of isotrop arbon fibers from pitch precursors. Fuel 2001; 80: 345-56 dimensional structure of eucalyptus tar pitch molecules 5 Mora E, Blanco C, Prada V, Santamaria R. Granda M As expected, eucalyptus tar pitch-based carbon fiber Menendez r. A study of pitch-based precursors for general oresented a moderately low electrical resistivity, rpose carbon fibres. Carbon 2002: 40: 2719-25 2 x 10-Q2m. However, this value is by far higher than [6 Alcaniz-monge J, Cazorla-Amoros D, Linares-solano A, Oya A usual for HPCF produced from mesophase pitch, typi Sakamoto A, Hoshi K. Preparation of general purpose carbon fibers from coal tar pitches with low softening point. Carbon cally in the order of 10Q2m, and even higher than that 1997:35:1079-87 reported by Fu et al. [8] for carbon fiber produced from [7] Donnet JB, Bansal RC. In: Carbon fibers. New York: Marcel an isotropic petroleum pitch, 3 x 10 Q2m. Once more, Dekker: 1990. p. 367-446 [ chapter 7) the result reflects the disordered. misoriented and flawed [8 Fu x, Lu w, Chung DDL. Ozone treatment of carbon fiber for structure of the fibers, which partially hinders electro conduction along the filaments [9] Roh YB Jeong KM, Cho HG, Kang HY, Lee YS. Ryu SK, et al. Unique charge/discharge properties of carbon materials with different structures. J Power Sources 1997: 68: 271-6 [10 Egashira M, Takatsuji H, Okada S, Yamaki J. Properties of 4. Conclusions activated carbon fiber fo electrode in lithium batteries. J Power Sources 2002- 107- 56-60 (1 Mochida l, Kisamori S, Hironaka M, Kawano S, Matsumura Y, The present work shows that eucalyptus tar pitches Yoshikawa M. Oxidation of no into no, over active carbon are potential precursors of isotropic carbon fibers. This fibers. Energy Fuels 1994: 8: 1341-4porosity in carbonized material and even to crack for￾mation, as already discussed. In that respect, Derbyshire et al. [4] demonstrated that fiber tensile strength in￾creases with increasing precursor carbon yield. The sec￾ond reason is that the relatively large diameters of the fibers produced in the present work is certainly contrib￾uting to reduce the tensile strength because thicker fibers contain more flaws per unit length compared to smaller diameter fibers. Fitzer and Ko¨nkele [36] have reported that larger diameters imply lower tensile strengths. In turn, Mora et al. [5] found that the tensile strength of coal tar pitch-based isotropic carbon fibers varies from 143MPa to 414MPa when the fiber diameter varies from 35lm to 16lm. The tensile modulus of the carbon fibers obtained was 14 ± 3GPa. This value is lower than the usual for GPCF. For example, the values reported by Mora et al. [5] in the previously quoted paper fall in the range 29–36GPa. Besides the flaws, this result reflects the mis￾oriented structure of the fibers derived from the three￾dimensional structure of eucalyptus tar pitch molecules. As expected, eucalyptus tar pitch-based carbon fibers presented a moderately low electrical resistivity, 2 · 10
4 Xm. However, this value is by far higher than usual for HPCF produced from mesophase pitch, typi￾cally in the order of 10
6 Xm, and even higher than that reported by Fu et al. [8] for carbon fiber produced from an isotropic petroleum pitch, 3 · 10
5 Xm. Once more, the result reflects the disordered, misoriented and flawed structure of the fibers, which partially hinders electron conduction along the filaments. 4. Conclusions The present work shows that eucalyptus tar pitches are potential precursors of isotropic carbon fibers. This matter is important because it allows the use of the whole tar generated as a by-product during charcoal production, therefore stimulating volatile recovery in industrial chimneys and aggregating revenue to the charcoal-making industry, which is particularly interest￾ing because planted biomass is an environmentally friendly and renewable energy source. Although the tensile properties make clear that the produced fibers are not useful as structural reinforce￾ment, other properties give rise to different potential applications. For example, the reduced carbon yield of the precursor and the high isotropic and disordered structure of the fibers suggest a great potential for the production of activated carbon fibers, besides using them in the manufacture of thermal insulation carbon felts. Efforts have been made to improve the process so far employed mainly aiming to reduce the period of time necessary to stabilize the fibers. Acknowledgments The authors thank Fundac¸a˜o de Amparo a Pesquisa do Estado de Minas Gerais (FAPEMIG), Empresa Bra￾sileira de Pesquisa Agropecua´ria (EMBRAPA) and Fundac¸a˜o de Empreendimentos Cientı´ficos e Tecnolo´g￾icos (FINATEC) for financial support. References [1] Edie DD, Diefendorf RJ. Carbon fiber manufacturing. In: Buckley JD, Edie DD, editors. Carbon–carbon materials and composites. Park Ridge, NJ: Noyes; 1993. p. 18–39. [2] Savage G. In: Carbon–carbon composites. London: Chapman & Hall; 1993. p. 53–7. [3] Edie DD. The effect of processing on the structure and properties of carbon fibers. Carbon 1998;36:345–62. [4] Derbyshire F, Andrews R, Jacques D, Jagtoyen M, Kimber G, Rantell T. Synthesis of isotropic carbon fibers and activated carbon fibers from pitch precursors. Fuel 2001;80:345–56. [5] Mora E, Blanco C, Prada V, Santamarı´a R, Granda M, Mene´ndez R. A study of pitch-based precursors for general purpose carbon fibres. Carbon 2002;40:2719–25. [6] Alcan˜iz-monge J, Cazorla-Amoro´s D, Linares-solano A, Oya A, Sakamoto A, Hoshi K. Preparation of general purpose carbon fibers from coal tar pitches with low softening point. Carbon 1997;35:1079–87. [7] Donnet JB, Bansal RC. In: Carbon fibers. New York: Marcel Dekker; 1990. p. 367–446. [chapter 7]. [8] Fu X, Lu W, Chung DDL. Ozone treatment of carbon fiber for reinforcing cement. Carbon 1998;36:1337–45. [9] Roh YB, Jeong KM, Cho HG, Kang HY, Lee YS, Ryu SK, et al.. Unique charge/discharge properties of carbon materials with different structures. J Power Sources 1997;68:271–6. [10] Egashira M, Takatsuji H, Okada S, Yamaki J. Properties of containing Sn nanoparticles activated carbon fiber for a negative electrode in lithium batteries. J Power Sources 2002;107:56–60. [11] Mochida I, Kisamori S, Hironaka M, Kawano S, Matsumura Y, Yoshikawa M. Oxidation of NO into NO2 over active carbon fibers. Energy Fuels 1994;8:1341–4. 3.5 4.0 4.5 5.0 5.5 -4 -3 -2 -1 0 y = -19 + 3.3 x y = -6.1 + 0.63 x ln{ln[1/(1-P)]} ln [σ (MPa)] Fig. 5. Weibull distribution for tensile strength measurements of eucalyptus tar pitch-based carbon fibers. 596 M.J. Prauchner et al. / Carbon 43 (2005) 591–597