Reaction of sodium with polycrystalline alumina (PCA) arc tubes in high-pressure sodium lamps can limit lamp performance. The rate of degradation depends on the grainboundary diffusion of a reaction product, aluminum, through the PCA wall. The effects of sintering aids and other dopants on the sodium resistance of PCA are investigated via accelerated lamp tests and microstructural analyses. Key material parameters for sodium resistance are the spinel (MgAl 2 O 4 ) second phase and the grain-boundary MgO level. Eliminating spinel in the sintered body, doping with tetravalent cations to charge-compensate the Mg 2؉ solutes, and creating a second phase to absorb MgO in situ, significantly improve the resistance of PCA to sodium attack.
Digital imaging and analysis methods are applied to the quantitative study of microstructural changes which occur during hot-pressing of yttria-doped silicon nitride. Effects of processing changes upon the grain growth and microstructural anisotropy are described. Relationships between grain cross-sectional area and mechanical properties are established. It was found that, over the range of processing conditions used, increases in grain size correlated strongly with an increase in fracture toughness. The grain size distribution broadened significantly with hot-press time, resulting in reduced flexural strength. Although no significant change in the mean grain shape factor was observed, the variance in shape factor decreased as the hot-press time was extended.
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