Sintering of dense alumina

The material constituting the envelope must be colorless and transparent at the wavelengths considered, it must resist the chemical aggressions due to plasma and, lastly, it must be able to tolerate this temperature of 1,200°C for a very long lifespan (more than 20,000 hours). The performance of the most refractory glasses - like silica glass of halogen lamps - does not satisfy these specifications, so the only solution is to use a sintered ceramic alumina has therefore become indispensable. 

The optical applications of ceramics demand a material that is transparent and not just translucent. Any inclusion, even a transparent one, but with an optical index different from that of the matrix, causes a scattering of the light - an effect illustrated by depolished glass. Owing to the variation between the ophcal index of gases inside a pore (n = 1) and that of alumina (n = 1.76), a porous alumina ceramic is not transparent. The effect of porosity is extremely marked and a porosity of 0.3% is enough to reduce transparency by 90% compared to a dense body [KIN 76] the manufacture of alumina envelopes for sodium vapor lamps consequently requires that the densification prachcally reaches 100%. 

To obtain a perfectly dense material is the major difficulty in the sintering of ceramics. As indicated in Chapter 1, it is generally easy to eliminate opened porosity, whose canal morphology allows various matter transport mechanisms 

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