Characterization of translucent polycrystalline alumina lamp envelopes

Sintering and sodium resistance of translucent polycrystalline alumina (PCA) lamp envelopes have been improved through better understanding of the controlling mechanisms. The atmosphere for sintering of translucent alumina doped with magnesiabased additives has evolved from pure H-2 to N-2-H-2 mixes. High-transmittance PCA can be accomplished by sintering in dry N-2-H-2 atmospheres containing as low as 2% H-2. Annihilation of pores during the sintering involves diffusion of gaseous species entrapped in pores via a point-defect pair mechanism including hydrogen interstitials and nitrogen solutes. Sodium reacts preferentially with spinel second phase in PCA of high-pressure sodium lamps. The rate of degradation in the PCA depends on the transport of a reaction product, Al, through the wall, which in turn hinges on the grain-boundary chemistry. The second phases, and the segregation, composition and structure of grain boundaries can be controlled by the dopants. A doping system consisting of MgO plus ZrO2 and Y2O3 significantly enhances the resistance of PCA to sodium attack.