Characterization of translucent polycrystalline alumina (PCA) ceramics

Optical transmittance, mechanical strength, and sodium resistance, three important properties of PCA, improved over the years. These properties are related to microstructure including grain size, residual pores, second phases, grain boundary segregation, and impurities, as well as to point defects such as oxygen vacancies. Sintering of PCA to translucency was achieved in N-2-H-2 atmospheres. Oxygen vacancies and pairs of hydrogen interstitials and nitrogen solutes are involved in the sintering mechanism. Reaction of sodium with PCA arc tubes in lamps can limit lamp performance. Eliminating spinel second phase in the sintered body and doping with tetravalent cations to charge-compensate the magnesium solutes, significantly improved the resistance of PCA to sodium attack. The rate of sodium attack depends on the grain boundary transport of a reaction product, aluminum species, through the PCA wall, which in turn is related to diffusion of oxygen vacancies along grain boundaries.