Glasses containing very small particles of CdS are prepared and their properties studied. Changes in the optical absorption spectra of CdS glass result from quantum confinement effects. With very small particle size, large confinement energies of up to 4 eV are obtained, which have not been before. The blue-shifts in the optical absorption spectra show that the growth rate of the particles at short times are diffusion controlled, and the activation energy for growth is calculated. The band gaps of the CdS nanocrystallites in glass are determined from photoluminescence measurements, which show a high-energy peak, corresponding to the band gap, that had not been previously observed.

The Cima-Lopina equation for swelling in hydrogel networks with a bimodal distribution of cross-link densities is modified slightly for an end-linked system. The revised equation is verified with a controllable series of poly(ethylene oxide) hydrogels synthesized from linear and star PEO molecules cross-linked with a triisocyanate linker. Cross-link density in the gels is independently analyzed from elastic modulus and swelling equilibrium. Good correlation between these two methods confirms the accuracy of the revised Cima-Lopina equation.

Partially crystalline Si3N4, with nanosized crystals and a specific surface area greater than 200 m2/g, is obtained by pyrolysis of a commercially available vinylic polysilane in a stream of anhydrous NH3 to 1000°C. This polymer does not contain N initially. Crystallization to high-purity wSi3N4 proceeds with additional heating above 1400°C under Nz. The changes in crystallinity, powder morphology, infrared spectra, and elemental compositions, for samples annealed from 1000" to 1600°C under N,, are consistent with an amorpbous-to-crystalline transformation. Although macroscopic consolidation and local densification occur at 1400"C, volatilization and accompanying weight loss limit bulk densification. The effect of temperature on specific surface area is examined and related to the sintering process. These results are applicable to pyrolysis, decomposition, and crystallization studies of ceramics synthesized by polymeric precursor routes. [Key words: silicon nitride, precursor, polymer, pyrolysis, surface area.]

Partially crystalline silicon nitride, with a specific surface area greater than 200 m2/g, is obtained by the pyrolysis of an organometallic, polymeric precursor under NH3 to 1000 °C. Additional heating to 1400 °C under N2 produces alpha-Si3N4. The addition of up to 15% h-BN was found to affect the coarsening characteristics of amorphous silicon nitride by promoting surface area reduction and suppressing crystallinity. By combining Si3N4 and BN molecular and polymeric precursors prior to ceramic conversion, or incorporating Si, N, and B into a single preceramic polymer, the relative proportion and crystallinity of the ceramic phases can be controlled in the resulting Si3N4/BN composites.