X-ray diffraction patterns of ~-SiC (3C or the cubic polytype of SiC) powders often exhibit an addi tion a l peak at d = 0.266 nm, high background intensity arou nd the (Ill) peak, and relative intensities for peaks which differ from those predicted from the crystal structure. Com puter simulations were used to show that aH these features are du e to stacking faults in the powders and not due to the presence of other polyty pes in the powders. Such simulations a llow diffraction patterns to be gener ated for different types, freq uencies, and spatial d istribu tio n of faults. Co mpa rison of the simulation results to the XRD data indicates that the B-SiC pa rticles consist either of heavil y faulted clusters distributed irregularly between regio ns that have only occasional faults or twins, or the powders consi t of two types of particle with different populations of faults: tho e with a hi oh density of faults and those with only twins or occasion a l faults. Additiona l information is necessary to determine which description is correct. However, th e simul a ti o n results can be used to rule out certa in fault configurations. ' Ibiden Company. Tokyo. Japa n. The data were obtained fro m a diffraclomeler (Philips mode l ADP 3250) wi lh a CuKa X•ray ge nerat or and Ni filter. u in g a 0.02° step scan wilh I s at each slep.

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Oxygen diffusion in vitreous silica glass is studied using the gas exchange technique. The tracer concentration profiles are consistent with a model based on two mechanisms, one network and the other interstitial. These processes are coupled through limited network-interstitial exchange. Nuclear reaction analysis and secondary ion mass spectrometry techniques are performed and compared. These results are compared to experiments on transport in thin silica films grown on single-crystal silicon. [

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