| 摘要 |
<p>Polycrystalline translucent alumina having an average grain size of alumina crystal grain being not smaller than 20 .mu. and an average surface roughness of the fired body being not larger than 1/10 of the maximum grain size of alumina crystal grain and capable of being used for an envelope of a high pressure vapor discharge lamp, which is produced by shaping alumina composition powders consisting mainly of alumina into a given shape, primarily firing the shaped body under vacuum or in an atmosphere of hydrogen gas or dissociated ammonia gas at a temperature within 1,500.degree.C and then secondarily firing the primarily fired body at a temperature of 1,650-1,900.degree.C, at least the temperature from 1,400.degree.C to 1,700.degree.C being raised at a rate of not lower than 200.degree.C/hr.</p> |
