J Clade et al. /Journal of the European Ceramic Sociery 25(2005)123-127 Fig 3.(Left) SEM image of SiC fibers obtained from a polysilane-polycarbosilane copolymer (Right)SEM image of a fracture surface of an SiC fiber containing 60-70 wt. of copolymer in toluene led to a mate- leading to an infusible copolymer which is still soluble in rial which could be spun continuously into green fibers with organic solvents and thus suitable for e.g. dry-spinning. No diameters of about 30 um. The further optimization of the curing of the green fibers before pyrolysis is required. Fur spinning conditions, aiming at even lower green-fiber diam- ther work, aiming at an optimization of the fiber properties eters, is in progress. shape, mechanical strength, microstructure), is in progress 3.3. Fiber properties Acknowledgements After pyrolysis at 1200C under nitrogen, the fibers show room-temperature tensile strengths of up to 800 MPa(diam- This work is based upon investigations performed in close eters of about 25 um)and Youngs moduli of up to 120 GPa; cooperation with the Institut fur Anorganische Chemie of the their oxygen content is less than I wt. %. Annealing up to Te hnische Universitat Bergakademie Freiberg, Germany 1900C leads to an increase of the Young's moduli to val- Therefore, Professor Dr. Gerhard Roewer, Dr. Dorit Mein- ues of up to 170 GPa at room temperature without a signif- hold and Thomas lange are gratefully acknowledged for this icant alteration of the tensile strength. In comparison, SiC cooperation. Professor Dr Martin Jansen and Dr. Thomas fibers obtained by Roewer et al. via the melt-spinning of a Jaschke of the Max Planck-Institut fur Festkorperforschung methylchloropolysilane, the curing of the green fibers un- in Stuttgart, Germany, are also gratefully acknowledged for der ammonia and the subsequent pyrolysis under argon up to the dTA/Tg measurements 1200C showed tensile strengths of up to 1000 MPa at room temperature; the Youngs modulus was estimated to be about 150GPa35 References During the heating process, the crystallization of p-SIC takes place, as is indicated by the X-ray powder diagrams in 1. Yajima, S, Hayashi, J. and Omori, M, Chem. Lett, 1975, 931 Fig 2 SEM photographs of the fibers(Fig 3)show that their 2. Yajima, S, Okamura, K. and Hayashi, J, Chem. Lett., 1975, 1209 cross-section is not perfectly circular, but oval or kidney 3. Yajima, S, Liaw, C, Omon, M. and Hayashi, J, Chem. Letf, 1976, shaped. Additionally, in most of the fracture surfaces little voids can be seen, which explains the relatively poor ten- 4. Yajima, S, Hayashi, J and Omori, M, US 4, 100, 233(1978) Burkhard. C. A.J. Am. Chem. Soc.. 1949.71.963 sile strengths. To overcome these disadvantages further work S, Omori, M, Hayashi, J. and Okamura, K, Chem. Lett. aiming at an optimization of the spinning, drying step, and 976.551 the pyrolysis process is in progress 7. Yajima, S, Hasegawa, Y, Hayashi, J and himura, M, J. Mater Sci. 978,13,2569 8. Hasegawa, Y, limura, M. and Yajima, S.,J. Mater: Sci., 1980, 15, 4. Conclusions 9. Verbeek, W. and Winter, G, DE 2, 236,078(197. 10. Fritz, G. and Raabe, B, Z. Norg. Allg. Chem., 1956, 286, 149 We have developed a new process which leads to Sic ce- IL. Fritz, G and Raabe, B, Z Anorg. Alg. Chem., 1959, 299, 232 ramic fibers with a low oxygen content, starting from readily 12. Fritz, G, Habel, D, Kummer, D and Teichmann, G, Z Norg. Allg available raw materials. The key step in this process consists 13. West, R, David, L. D, Djurovich, P. 1 and Yu, H, Bull. Am. Ceram of a thermal treatment of the initially synthesized polysilane Soc,1983,62,899126 J. Clade et al. / Journal of the European Ceramic Society 25 (2005) 123–127 Fig. 3. (Left) SEM image of SiC fibers obtained from a polysilane–polycarbosilane copolymer. (Right) SEM image of a fracture surface of an SiC fiber. containing 60–70 wt.% of copolymer in toluene led to a mate￾rial which could be spun continuously into green fibers with diameters of about 30
m. The further optimization of the spinning conditions, aiming at even lower green-fiber diam￾eters, is in progress. 3.3. Fiber properties After pyrolysis at 1200 ◦C under nitrogen, the fibers show room-temperature tensile strengths of up to 800 MPa (diam￾eters of about 25
m) and Young’s moduli of up to 120 GPa; their oxygen content is less than 1 wt.%. Annealing up to 1900 ◦C leads to an increase of the Young’s moduli to val￾ues of up to 170 GPa at room temperature without a signif￾icant alteration of the tensile strength. In comparison, SiC fibers obtained by Roewer et al. via the melt-spinning of a methylchloropolysilane, the curing of the green fibers un￾der ammonia and the subsequent pyrolysis under argon up to 1200 ◦C showed tensile strengths of up to 1000 MPa at room temperature; the Young’s modulus was estimated to be about 150 GPa.35 During the heating process, the crystallization of -SiC takes place, as is indicated by the X-ray powder diagrams in Fig. 2. SEM photographs of the fibers (Fig. 3) show that their cross-section is not perfectly circular, but oval or kidney￾shaped. Additionally, in most of the fracture surfaces little voids can be seen, which explains the relatively poor ten￾sile strengths. To overcome these disadvantages, further work aiming at an optimization of the spinning, drying step, and the pyrolysis process is in progress. 4. Conclusions We have developed a new process which leads to SiC ce￾ramic fibers with a low oxygen content, starting from readily available raw materials. The key step in this process consists of a thermal treatment of the initially synthesized polysilane, leading to an infusible copolymer which is still soluble in organic solvents and thus suitable for e.g. dry-spinning. No curing of the green fibers before pyrolysis is required. Fur￾ther work, aiming at an optimization of the fiber properties (shape, mechanical strength, microstructure), is in progress. Acknowledgements This work is based upon investigations performed in close cooperation with the Institut fur Anorganische Chemie of the ¨ Technische Universitat Bergakademie Freiberg, Germany. ¨ Therefore, Professor Dr. Gerhard Roewer, Dr. Dorit Mein￾hold and Thomas Lange are gratefully acknowledged for this cooperation. Professor Dr. Martin Jansen and Dr. Thomas Jaschke of the Max Planck-Institut f ¨ ur Festk ¨ orperforschung ¨ in Stuttgart, Germany, are also gratefully acknowledged for the DTA/TG measurements. References 1. Yajima, S., Hayashi, J. and Omori, M., Chem. Lett., 1975, 931. 2. Yajima, S., Okamura, K. and Hayashi, J., Chem. Lett., 1975, 1209. 3. Yajima, S., Liaw, C., Omori, M. and Hayashi, J., Chem. Lett., 1976, 435. 4. Yajima, S., Hayashi, J. and Omori, M., US 4,100,233 (1978). 5. Burkhard, C. A., J. Am. Chem. Soc., 1949, 71, 963. 6. Yajima, S., Omori, M., Hayashi, J. and Okamura, K., Chem. Lett., 1976, 551. 7. Yajima, S., Hasegawa, Y., Hayashi, J. and Iimura, M., J. Mater. Sci., 1978, 13, 2569. 8. Hasegawa, Y., Iimura, M. and Yajima, S., J. Mater. Sci., 1980, 15, 720. 9. Verbeek, W. and Winter, G., DE 2,236,078 (1972). 10. Fritz, G. and Raabe, B., Z. Anorg. Allg. Chem., 1956, 286, 149. 11. Fritz, G. and Raabe, B., Z. Anorg. Allg. Chem., 1959, 299, 232. 12. Fritz, G., Habel, D., Kummer, D. and Teichmann, G., Z. Anorg. Allg. Chem., 1959, 302, 60. 13. West, R., David, L. D., Djurovich, P. I. and Yu, H., Bull. Am. Ceram. Soc., 1983, 62, 899.