Svante Prochazka

Silicon oxycarbide glasses: Part II. Structure and properties

Silicon oxycarbide glass is formed by the pyrolysis of silicone resins and contains only silicon, oxygen, and carbon. The glass remains amorphous in x-ray diffraction to 1400 °C and shows no features in transmission electron micrographs (TEM) after heating to this temperature. After heating at higher temperature (1500–1650 °C) silicon carbide lines develop in x-ray diffraction, and fine crystalline regions of silicon carbide and graphite are found in TEM and electron diffraction. XPS shows that silicon-oxygen bonds in the glass are similar to those in amorphous and crystalline silicates; some silicons are bonded to both oxygen and carbon. Carbon is bonded to either silicon or carbon; there are no carbon-oxygen bonds in the glass. Infrared spectra are consistent with these conclusions and show silicon-oxygen and silicon-carbon vibrations, but none from carbon-oxygen bonds. 29 Si-NMR shows evidence for four different bonding groups around silicon. The silicon oxycarbide structure deduced from these results is a random network of silicon-oxygen tetrahedra, with some silicons bonded to one or two carbons substituted for oxygen; these carbons are in turn tetrahedrally bonded to other silicon atoms. There are very small regions of carbon-carbon bonds only, which are not bonded in the network. This “free” carbon colors the glass black. When the glass is heated above 1400 °C this network composite rearranges in tiny regions to graphite and silicon carbide crystals. The density, coefficient of thermal expansion, hardness, elastic modulus, index of refraction, and viscosity of the silicon oxycarbide glasses are all somewhat higher than these properties in vitreous silica, probably because the silicon-carbide bonds in the network of the oxycarbide lead to a tighter, more closely packed structure. The oxycarbide glass is highly stable to temperatures up to 1600 °C and higher, because oxygen and water diffuse slowly in it.

Silicon oxycarbide glasses: Part I. Preparation and chemistry

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.

Strength and Microstructure of Dense, Hot-Pressed Silicon Carbide

The microstructural and strength characteristics of dense SiC, hot-pressed from submicron SiC powders with small additions of boron, are shown to be strongly dependent on the control exercised over oxygen content during pressing. Uniform, equiaxed grain structures of SiC are obtained if sufficient oxygen is present to allow detectable particles of SiO2, whereas uniform, elongated grain structures of β SiC are obtained if excess carbon is utilized to reduce oxygen content to undetectable limits. At intermediate oxygen levels, free silicon was formed and large, tabular α-SiC grains in a fine-grained β-SiC matrix were obtained.

High-Pressure Hot-Pressing of Silicon Nitride Powders

Silicon nitride has been a prime candidate material for active components of future high temperature heat engines, particularly gas turbines. Its high temperature properties such as oxidation resistance, strength, creep and stress-rupture are critically dependent upon additives which are necessary for consolidation of Si3N4. powders by hot-pressing into pore free ceramics. Typical additions, MgO, Y2O3, ZrO2 at a few percent level, degrade high temperature properties severely.