Silicon oxycarbide glasses: Part I. Preparation and chemistry
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Silicon oxycarbide glasses: Part I. Preparation and chemistry Gary M. Renlund and Svante Prochazka General Electric Corporate Research, Schenectady, New York 12301
Robert H. Doremus Materials Engineering Department, Rensselaer Polytechnic Institute, Troy, New York 12180-3590 (Received 10 December 1990; accepted 3 June 1991)
Silicone polymers were pyrolyzed to form silicon oxycarbides that contained only silicon, oxygen, and carbon. The starting polymers were mainly methyl trichlorosilane with a small amount of dimethyl dichlorosilane. NMR showed that the polymers had a silicon-oxygen backbone with branching and ring units. When the polymer was heated in hydrogen, toluene and isopropyl alcohol, used in production of the polymer, were given off in the temperature range 150 °C to 500 °C. Substantial decomposition of the polymer itself began only above about 700° by evolution of methane. The network of silicon-oxygen bonds and silicon-carbon bonds did not react and was preserved; the silicon-carbon bonds were linked into the silicon-oxygen network. The silicon oxycarbide was stable above 1000 °C, showing no dimensional changes above this temperature. The interior of the silicon oxycarbide was at very low effective oxygen pressure because oxygen diffused slowly in it. There was also a protective layer of silicon dioxide on the surface of the silicon oxycarbide.
I. INTRODUCTION Metal organic polymers are excellent starting materials for making ceramic powders and bulk materials, both crystalline and amorphous.1"5 For example, silicone polymers with a silicon-oxygen backbone react with ammonia to form silicon nitride. In the present work we found that certain silicone resins pyrolyze in hydrogen or helium to a black, hard, amorphous solid containing only silicon, oxygen, and carbon. This silicon oxycarbide glass has exceptional high temperature strength and chemical stability compared even to vitreous silica. It is resistant to crystallization and oxidation at temperatures above 1000 °C. From these properties and detailed structural studies we conclude that this glass is not just a mixture of silica and silicon carbide or carbon, but one of a series of homogeneous, probably metastable, amorphous solids in the silicon-oxygen-carbon system. Carbon has been added to silica by others in a variety of ways. Organic compounds or carbon were incorporated into porous silica that was a precursor to Vycor glass.4""6 Fibers made from polycarbosilanes7 under the trade name Nicalon are mainly composed of silicon, oxygen, and carbon in the ratio close to 3 : 1 : 4 . 8 Their properties degrade above 1000 °C. 9 A glass was prepared by pyrolysis of dimethyl diethoxysilane/etraethoxysilane copolymers that contained silicon, oxygen, and carbon.10 At temperatures above 1300 °C this glass reacted to form crystalline silicon carbide. In this paper the preparation of a stable silicon oxycarbide glass is described, as well as information 2716
J. Mater. Res., Vol. 6, No. 12, Dec 1991
about the pyrolytic reactions during its formation and its
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