The pyrolysis process of a polytitanocarbosilane into SiC/TiC ceramics: An XPS study
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A. Glisenti and G. Granozzi Dipartimento di Chimica Inorganica, Metallorganica ed Analitica, Universita di Padova, via Loredan 4, 35131 Padova, Italy
F. Babonneau Chimie de la Matiere Condensee, Universite Paris 6, Tour 54, 4 place Jussieu, 75005 Paris, France
J. D. Mackenzie UCLA, Department of Materials Science and Engineering, Boelter Hall 6531, Los Angeles, California 90024 (Received 7 December 1989; accepted 6 June 1990)
The pyrolysis process of polytitanocarbosilane, precursor for SiC/TiC ceramics, has been followed step by step using x-ray photoelectron spectroscopy. This study shows that Ti-O bonds, present in the precursor, are stable up to 700 CC. Above this temperature, Ti-C bonds, precursor for TiC network, start to form. From a detailed analysis of Ti(2p) and Si(2p) peaks, the formation of intermediate species such as and TiC^O,, has been detected.
I. INTRODUCTION
Non-oxide ceramics prepared from polymers constitute a research field of growing interest: carbides and nitrides can now be prepared by firing suitable metallorganic polymer precursors.1'2 The method offers several advantages with respect to traditional powder processing; among the most attractive are low firing temperature, good control of impurities, and the possibility of producing fibers or coatings. However, a stepby-step characterization of the pyrolysis is needed to exploit to the utmost the possibilities offered from this method. As an example, high-tech NICALON SiC fibers are currently produced from a polycarbosilane (PC) according to the process developed by Yajima.3 After exposure at high temperatures, these fibers show strength degradation phenomena consequent to a crystallization process of the amorphous silicon carbide phase.4 This problem has been partially overcome by using a different metallorganic polymer, polytitanocarbosilane (PTC), precursor of SiC/TiC ceramics. The fired PTC exhibits an amorphous state stable up to higher temperatures, so that strength degradation starts only above 1400 °C.5 Some of us have already studied the conversion from PTC to SiC/TiC ceramics following the evolution of the local environment around Si atoms during the pyrolysis by using 29Si MAS-NMR measurements.6 From this study it has been shown that, starting at 500 °C, the oxygen, introduced into the system via the titanium alkoxide, reacts with the Si-C bonds, pro1958
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J. Mater. Res., Vol. 5, No. 9, Sep 1990
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ducing SiO4, SiCO3, and SiC2O2 units. The amount of Si-O bonds in the system increases with the firing temperature and reaches a maximum value near 1000 °C. Above this temperature the total amount of Si-O bonds starts to decrease, probably due to a carbothermal reduction with the carbon in excess. At 1500 °C Si-O bonds are no longer present in the system that is now constituted, as revealed by x-ray diffraction analysis, by a microcrystalline mixture of SiC and TiC. From the 29Si MAS-NMR spectra, however, no information was obtained about the environment around Ti atoms. This pr
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