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Production of Sapphire Tubes for High-Pressure Sodium Lamps Using The Stepanov Method at High Rates of Growth

The process of the Stepanov growth of sapphire tubes is studied at the growth rates up to 20 mm · min−1 in a Mo crucible with graphite succeptor. It is established that the main factor preventing the production of high-quality samples is contamination of the melt due to the gas transport reactions the growth chamber atmosphere and formation of oversaturated carbon solid solution in the Al2O3 matrix. The decomposition of this solid solution gives rise to the second-phase-particle precipitations. The production of highquality sapphire tubes at the growth rates up to 20 mm · min−1 is possible with a careful elimination of water vapours and oxygen from the growth chamber atmosphere.

Sapphire arc tubes useful as the arc chamber for HPS lamps have been made by a number of processes, including a modified Czochralski process known as the edge-defined, film-fed growth or (EFG) process developed by Tyco Laboratories, Inc. This process uses a seed crystal and a die on the surface of molten alumina where a hollow tube is continuously pulled out of the melt through the die. This process is expensive and slow. Another process used to produce single crystal alumina or sapphire is called the floating zone process in which a PCA feed rod is introduced at a predetermined velocity into a heating zone wherein one or more lasers or other concentrated heat source is focused on the rod to melt the alumina in the zone to produce a "melt volume" of molten alumina. A sapphire fiber is continuously drawn from the melt volume at the desired velocity and the feed rod is moved simultaneously, but at a slower rate so that the process is a continuous one. This process is used primarily to produce sapphire fibers and does not readily lend itself to production of single crystal alumina tubing, although its use for such is disclosed in U.S. 3,943,324. Japanese Patent Publication 62-28118 discloses a furnace process for converting PCA tubes to sapphire tubes, but does hot show the use of a monolithically joined seed starter for forming sapphire.


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