Deterioration of metal halide arc tubes due to sodium iodide at high temperatures

Abstract

Bursting of arc tubes is one of the problems to be solved in metal halide lamps which contain sodium iodide as one of the additive compounds. Investigations have been made on a chemical reaction between sodium iodide and fused silica by heating fused silica capsules containing sodium iodide in the temperature range, 900∼1200°C. Two processes were found to be connected with such catastrophe. One was sodium dissolution into fused silica and the other was devitrification of fused silica, which was observed when the capsules contained water vapor as an impurity, or when the capsules contained water vapor as an impurity, or when the capsules were made of fused silica with OH redicals. Both processes were found to proceed remarkably above 1100°C to form many cracks on the inside surface of the capsules. The reaction products were compared with those found in the arc tube which had been operated horizontally at higher-than-rated wattage, and comparatively good agreements were obtained.