Spectroscopic Characterization of a Pre-Ceramic Polymer For Sic/Tic System
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SPECTROSCOPIC CHARACTERIZATION OF A PRE-CERAMIC POLYMER FOR SiC/TiC SYSTEM. FLORENCE BABONNEAU*, PATRICE BARRE* , JACQUES LTVAGE* AND MICHEL VERDAGUER**
* Laboratoire de Chimie de la Mati~re Condensie and **Laboratoire de Chimie des Mdtaux de Transition, Universiti Pierre et Marie Curie, 4 place Jussieu, 75005 Paris, France. ABSTRACT
Polytitanocarbosilane precursor for SiC/TiC ceramics was characterized by various spectroscopic techniques (29Si and 13 C CP MAS NMR, Ti K-edge XANES and EXAFS) in order to follow how titanium ions are incorporated into the polycarbosilane-based polymer. This polymer has been fired in argon atmosphere, and the evolution of the local environments of Si and Ti atoms, during the pyrolysis process, have been followed using 29Si MAS-NMR and Ti K-edge X-ray absorption: this study reveals the formation of Si-O bonds above 500'C, while Ti-C bonds clearly appear around 800'C. Above 1000°C, Si-O bonds are consumed by reaction with excess carbon and the system finally crystallizes into SiC/TiC mixture above 1200°C. INTRODUCTION The polymer route to non oxide ceramics has been greatly developped after the pioneering work of Yajima, who prepared SiC fibers from polycarbosilane (PC)[ 1]. This route offers a main advantage: fibers of refractory ceramics can easily been prepared. But other advantages have to be considered: soluble polymeric precursors can be chemically modified in order to get new systems with different properties. This point can be illustrated by the work of Yajima on polytitanocarbosilane[2].'By reacting a titanium alkoxide with polycarbosilane a new polymer is obtained that can be used to produce Si-Ti-C-O fibers with higher mechanical properties compared to the usual SiC fibers[3]. Several metallic atoms M such as Ti, Zr, Al or Nb, have been reacted with polycarbosilane, via a metallic alkoxide in order to get new polymeric precursors for Si-M-C-O systems. Spectroscopic characterizations of the precursors and of samples pyrolyzed in argon atmosphere have been undertaken with two main goals: (1) a better understanding of the reactions occuring during the preparation procedure between PC and the metallic alkoxide, (2) a study of the influence of M on the conversion process of the polymer into the final crystalline phases. This paper presents the spectroscopic study done on the Si-Ti-C-O system, mainly using 29 Si Magic Angle Spinning Solid State NMR (MAS NMR) and Ti K-edge X-ray absorption. EXPERIMENTAL SECTION
The polytitanocarbosilane, PTC, was prepared by refluxing polycarbosilane (Dow Coming) with titanium n-butoxide (Fluka) in toluene for 2 hours under argon. The solvent was distilled and then the temperature was raised up to 300°C to complete the crosslinking procedure. Chemical analysis gives SiC 3 .24Ti0 . 19H7 .57O1 .16 . The polymer was then fired under argon up to a given temperature with a heating rate of 10°C/min. MAS-NMR spectra were recorded on MSL 300 (29 Si) and MSL 400 (29Si CP and Mat. Res. Soc. Symp. Proc. Vol. 180. @1990 Materials Research Society
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