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POLYSILANES AS POSSIBLE PRECURSORS TO SILICON CARBIDE R. A. Sinclair, Central Research Laboratory, 3M Center, St. Paul, MN 55144 Robert West, Department of Chemistry, University of Wisconsin, Madison, WI 53706 Organometallic Polymer Routes to Ceramics The continuing search for new types of high-strength materials, and for performance improvements in existing ceramics, has encouraged several nonconventional approaches to ceramics synthesis. Some of the advantages of polymeric routes have already been demonstrated and include new fabrication procedures leading to continuous fibers, coatings and infiltrated porous structures. At a more fundamental level, polymer pyrolysis can allow control over the microstructure of the final ceramic, with important consequences for both physical and chemical properties (1). Although the first practical indications for polymer to ceramic conversion can be found in the production of high performance carbon fiber from organic polymers, the possibility of making ceramics from linear inorganic polymers was recognized two decades ago (2), (3). High strength continuous silicon carbide fiber, derived from polysilane starting material, has now become available commercially, largely as a result of the pioneering effort of S. Yajima (4). Polysilane Synthesis Polysilanes were originally synthesized by Kipping (5), but were not characterized until 1949 when Burkhard identified permethylpolysilane in the condensation reaction of dimethyldichlorosilane with sodium (6): Me 2 SiCl

2

Na )

(Me 2 Si)n

+

NaCl

(n = 55)

This highly crystalline polymer is too intractable to be processed directly but can be converted to a soluble, fusible low molecular weight polycarbosilane by thermolysis at moderate temperature (4): (Me2Si)n

-0

Ar

(MeHSi-CH2)y

y

Subsequently, the spun polycarbosilane was oxidatively crosslinked prior to a final high temperature conversion to high strength SiC fiber. Several other types of polysilanes are being evaluated as silicon carbide precursors, including those derived from disilaryl (7) and vinylsilane (8) starting materials. Also, organosilane copolymers are a further class of compounds having desirable polymer properties useful in ceramic technology (9). The first examples of these copolymers contained only dimethylsilane and

mat. RSe. Soc.

Symp. Proc.

Vol. 32 (1984)

Published by Elsevier Science Publishing Co., Inc.

388

phenylmethylsilane units with the Me2 Si/PhMeSi ratios between They were infiltrated into porous silicon 3:1 and 20:1. nitride ceramic specimens and then pyrolyzed in situ to silicon carbide, with a resultant increase in strength for the comPolymer having a comonomer ratio near 1, known as posite (10). "polysilastyrene", is soluble in common organic solvents such as toluene and tetrahydrofuran and readily characterized using GPC, NMIR, MS, UV and IR techniques.

re

Final produce

.2

7 hour:

MWn 10

Fig. 1. Gel permeation chromatograph of polysilastyrene.

Fig. 2. Ultraviolet spectrum of a high. molecular-weight polysilastyrene fraction.

A con