Duane Ray Bujalski

Insights into the oxidation chemistry of SiOC ceramics derived from silsesquioxanes

We have undertaken a systematic study of the oxidation chemistry for a range of SiOC ceramics derived from silsesquioxane polymeric precursors. This study examines the oxidation for 500 hours at 600, 800, 1000 and 1200°C for four SiOC powders. The material changes upon oxidation were characterized qualitatively by color change and optical microscopy and quantitatively by weight and composition change. In this study we employ a very easy method that uses the weight change upon oxidation and a carbon analysis after oxidation to arrive at the composition of the oxidized SiOC. Combined these qualitative and quantitative techniques have shown that on oxidation at 800 and 600°C the SiOC composition is more rapidly changed to that of silica than oxidation over the same time frame at 1000 or 1200°C. The data indicates that this difference is due to the relative rates of oxidation of the excess carbon versus the Si—C bonds in the SiOC. At lower temperatures initially the carbon oxidation predominates which leads to higher porosity throughout the material and an increase in the surface area with eventually ‘complete’ oxidation to silica. At higher temperatures the Si—C bond oxidation rate is comparable to the rate of oxidation of carbon. This allows a silica-like surface to build up on the SiOC, which slows all subsequent reactions due to the necessity to diffuse O2 in and COx out of the bulk. Under these oxidation conditions materials that originally contain high amounts of excess carbon are more quickly oxidized to silica than those that contain minimal amounts of excess carbon, as confirmed by elemental analysis and optical microscopy. Regardless of the time or temperature of the oxidation conditions no materials were found to be completely stable to oxidation. SiOC materials with low levels of excess carbon showed the best resistance to change upon oxidation.

Fine-diameter polycrystalline SiC fibers

Abstract Various organosilicon polymers have been converted to small-diameter, polycrystalline silicon carbide fibers by melt-spinning, cross-linking and pyrolyzing to high-temperature in argon. Several wt% boron was doped into the fibers before pyrolysis. Use of polycarbosilane precursor gave 8–10 μm diameter fibers having up to 2.6 GPa tensile strength, 450 GPa elastic modulus, 3.1–3.2 g/cm3 density. The microstructure consists of >95 wt% β-SiC crystallites of 30–40 nm average crystallite size. Stoichiometric fibers or fibers having excess carbon content have been prepared. Fiber has been thermally aged under inert conditions at 1800°C for 12 h with minimal strength and microstructural change. Stoichiometric fiber maintains higher strength after oxidative aging at 1370°C. Current processing efforts are aimed at preparing the fiber in continuous tow form.

Stoichiometry control of SiOC ceramics by siloxane polymer functionality

The guidelines, or empirical rules, previously described in the literature to estimate ceramic compositions from preceramic polymer compositions have been refined and quantified. Thermogravimetric and residual gas analyses of the pyrolysis of organosilsesquioxane polymers have identified the organic degradation products at various temperatures and indicated that essentially all of the silicon and oxygen atoms of these highly branched polymers are retained in the 1200 °C ceramic residue. Series of organosilsesquioxanes with systematically varied amounts of organosilsesquioxane and endcapping components were synthesized, cured, and pyrolyzed to 1200 °C under an inert atmosphere. Multiple linear regression analysis was used to quantify the relationships between the amount of carbon retained in the ceramic residues and the mole fractions of the various organic components of the preceramic polymer, allowing for retention of the silicon and oxygen of the silsesquioxane. Specifically, a phenylsilsesquioxane fragment contributes an average of 3.94 carbons to the resulting ceramic material, vinylsilsesquioxane, 1.52 carbons, methylsilsesquioxane, 0.59 carbons and a vinyldimethylsilyl endcapping group, 2.75 carbons. The utility of the model was shown by employing this information to predict a select set of candidate precursors to an SiOC with a carbon content near a desired 18 wt.% level. One of the candidate precursors (MeSiO1.5)0.84(Me2ViSiO0.5)0.16 (predicted to afford an SiOC at 18.1 wt.% carbon) was then prepared, cured, pyrolyzed and analyzed to test the accuracy of the model. The 1200 °C ceramic was found to have 18.4 wt.% carbon, indicating good agreement between the actual and predicted values.

Vinyl ether-modified poly(hydrogen silsesquioxanes) as dielectric materials

Vinyl ether-modified poly(hydrogen silsesquioxanes) or PHSQ were prepared via a platinum-catalyzed hydrosilylation reaction of PHSQ with an alkyl vinyl ether (VE) in toluene. The product formed in a near quantitative yield and its composition was characterized by multinuclear magnetic resonance spectroscopy. Multi-detector size exclusion chromatography revealed that relative to the PHSQ starting material, the PHSQ–VEs increased in molecular weight and radius of gyration, and the relationship between intrinsic viscosity and molecular weight suggested a branched structure. Thermal analyses indicated a cure onset around 100 °C; an onset of thermal decomposition at ca. 230 °C; and mass loss completed by 550 °C. Evolved gas analysis from thermogravimetric experiments revealed the initial elimination of the ethylene linkage, followed by cleavage of the carbon–carbon bonds. The materials prepared by pyrolysis at 425 °C were porous. Nitrogen porosimetry measured an increase in microporosity—from 0.187 to 0.295 cm3 g−1 (<5 nm)—when the VE content was increased from 10 to 50 wt%. The PHSQ–VEs were spin-coated onto silicon wafers and cured either at 400, 425, or 450 °C. The dielectric constant of the spin-coated films ranged from 2.3 to 3.0, and the modulus was between 2.2 and 12.9 GPa depending on material composition.