Mixed Carbides via Polymer Route
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MIXED CARBIDES VIA POLYMER ROUTE 3
2 1 GIAN D. SORARU , FLORENCE BABONNEAU AND JOHN D. MACKENZIE IDipartimento di Ingegneria dei Materiali, University of Trento, 38050 Mesiano, Trento, Italy. 2 Chimie de la Mati~re Condensde, Universitd Paris 6, 4 place Jussieu, 75005 Paris, France. 3 Department of Materials Science and Engineering, Boelter Hall 6531, University of California,
Los Angeles, CA 90024, USA. ABSTRACT Several polymetallocarbosilanes, pre-ceramics precursors for Si-M-C-O systems, have been prepared from polycarbosilane and metallic alkoxides, M(OR)n with M = Ti, Zr and Al. Polymers have essentially been characterized by Magic Angle Spinning Nuclear Magnetic Resonance (MAS-NMR). The pyrolysis process has been followed for each system with X-Ray Diffraction (XRD) and MAS-NMR. The role of the metallic element M on the transformation process of these systems will be discussed. INTRODUCTION Among the new methods for processing non-oxide ceramics with improved properties, pyrolysis of metal-organic polymers is taking an increasing important role, especially for the production of fibers [ 1]. One of the most attractive and challenging features of this process is the possibilities to control the microstructure and the properties of the final ceramic material by chemically modifying the polymer composition and/or its structure. Reacting a polycarbosilane precursor for SiC fibers with a titanium alkoxide, a new polymer is obtained that gives, after pyrolysis, ceramic materials in the Si-Ti-C-O system [2]. Commercially available TYRANNO fibers produced from this polytitanocarbosilane, display superior tensile strength compared with SiC NICALON fibers [3]. Similar procedures can give the opportunity to modify the polycarbosilane with different elements just by changing the nature of the metal alkoxide. Using this strategy, a large variety of new metal-organic polymers, precursors for Si-M-C-O systems, can be prepared. This paper will present results concerning Si-Ti (PTC), Si-Zr (PZC) and Si-Al (PAC) systems. EXPERIMENTAL SECTION The polymers PTC, PZC and PAC have been prepared from polycarbosilane and metal alkoxides (Ti(OBun) 4 , Zr(OBun)4 BuOH and AI(OBuS)3) according to a procedure already published[2]. Chemical analysis (M/Si) gives the following results for PTC (0.15), PZC (0.12) and PAC (0.24).The MAS-NMR spectra have been recorded on Bruker MSL 300 and 400 spectrometers and the XRD patterns on a Philips equipment using a CuKct radiation. EXPERIMENTAL RESULTS AND DISCUSSION
Characterizationof modifiedpolycarbosilanes The problem in this characterization was to understand how the metallic element was Mat. Res. Soc. Symp. Proc. Vol. 180. @1990 Materials Research Society
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incorporated into the new PC based polymer. Previous works[2] suggested the formation of Si-O-M bridges between the PC chains and the alkoxide part. The
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Si CPMAS-NMR spectrum of pure PC (Figure la) shows two peaks at -0.8 ppm
and - 17.6 ppm assigned respectively to SiC4 units in which Si atoms are bonded with four C atoms and SiC
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