ABSTRACT
Quartz metal halide lamps are often made with NaIScI3-ThE doses and tungsten electrodes containing Th02. ThO, reduces the work function of the electrode. During the operation of these lamps, it has been found that Th02 from the electrode is converted into ThI4 in the arc tube [1] and causes an unfavorable rise in lamp operating voltage. It is believed that ScI3 in the dose reacts with Th02 in the tungsten electrode to form volatile ThI4 and solid Sc703 in the electrode [2, 3]. ScI3 also reacts with the quartz wall (Si02) to produce SiI4. The SiI4 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 Th02 at the electrode surface available to react with ScI3. So, as the lamp ages, more SiI4 is produced, greater electrode melt back occurs and more free iodine and ThI4 are generated in the arc tube. Both excess iodine and ThI4 increase the operating voltage of the lamps. The present paper describes the performance of metal halide lamps prepared with Th02 free tungsten electrodes that contain small additions of a) La703-Y,03-Zr02 and b) CeO2 and compares their performance to lamps containing thoriated tungsten electrodes.
