Silicon oxycarbide (SiOC) glasses with controlled amounts ofSiOC bonds and free carbon have been produced via the pyrolysis of suitable preceramic networks. Their chemical durability in alkaline and hydrofluoric solutions has been studied and related to the network structure and microstructure of the glasses. SiOC glasses, because of the character of the SiOC bonds, exhibit greater chemical durability in both environments, compared with silica glass. Microphase separation into silicon carbide (SiC), silica (SiO 2 ), and carbon, which usually occurs in this system at pyrolysis temperatures of >1000°-1200°C, exerts great influence on the durability of these glasses. The chemical durability decreases as the amount of phase separation increases, because the silica/silicate species (without any carbon substituents) are interconnected and can be easily leached out, in comparison with the SiOC phase, which is resistant to attack by OH ؊ or F ؊ ions.

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23Na multiple-quantum (MQ) MAS NMR is applicable for monitoring the chemical and structural changes resulting from atmospheric exposure of a series of alkali aluminoborosilicate glasses with compositions RNa2O:1B2O3:1SiO2:0.25Al2O3 (where R = 0.5−2.5). Glasses with high alkali concentrations possess greater numbers of nonbridging oxygens within the bulk structure and presumably at the initial surface of a fresh sample, and for three samples with R ⩾ 1.5 sharp resonances are revealed in the isotropic dimension of an MQMAS NMR experiment conducted after prolonged atmospheric exposure. The MQMAS NMR experiments, combined with 1H → 23Na cross-polarization magic-angle spinning (CPMAS) NMR measurements, indicate that these resonances arise from sodium cations no longer participating in the glass network. Two new phases are formed as corrosion products and have been identified as an anhydrous Na2CO3 phase and a NaBO2·1H2O phase through comparison with 23Na MQMAS and 1H → 23Na CPMAS NMR spectra of crystalline samples. Due to an inherent difficulty with direct quantification of populations based on MQMAS spectra, a simplified approach for quantification of the amount of the new carbonate phase is presented. Values are then calculated for relative amounts of corrosion product formation for different exposure times and bulk glass compositions.

Inert and radioactive markers have been used to study the mechanism of diffusion during Pd2Si formation. With the aid of Ti as an inert marker it has been shown that silicon is the dominant diffusing species during polycrystalline Pd2Si formation. When radioactive silicon is used as a marker it is found that the radioactive silicon is uniformly distributed throughout the Pd2Si after silicide formation. The self-diffusion of silicon in Pd2Si was investigated and found to be much lower than that necessary to produce a uniform radioactive silicon distribution, had silicon diffused by a grain boundary or pure interstitial mechanism. It is therefore proposed that silicon diffuses by a vacancy mechanism during silicide formation.