Metal Halide Lamps Containing Electrodes Doped with La_2O_3-Y_2O_3-ZrO_2 and CeO_2

摘要

Quartz metal halide lamps are often made with Nal-SCI3-TI1I4 doses and tungsten electrodes containing TI1O2. ThO2 reduces the work function of the electrode. During the operation of these lamps, it has been found that ThOa from the electrode is converted into ThL| in the arc tube [1] and causes an unfavorable rise in lamp operating voltage. It is believed that Sclj in the dose reacts with ThO2 in the tungsten electrode to form volatile TI1I4 and solid SC2O3 in the electrode [2, 3]. SCI3 also reacts with the quartz wall (SiO2) to produce SiL;. The SiL| reacts with hot tungsten electrodes to form low melting point tungsten silicide phases and releases free iodine. These reactions result in electrode melt back (deformation) and may increase the amount of TI1O2 at the electrode surface available to react with Scl3. So, as the lamp ages, more SiLi is produced, greater electrode melt back occurs and more free iodine and TI1I4 are generated in the arc tube. Both excess iodine and TI1I4 increase the operating voltage of the lamps. The present paper describes the performance of metal halide lamps prepared with ThO2 free tungsten electrodes that contain small additions of a) La2O3-Y2O3-ZrO2 and b) CeO2 and compares their performance to lamps containing thoriated tungsten electrodes.