High temperature gas phase chemistry for the development of advanced ceramic discharge lamps

The discharge vessels of advanced metal halide lamps are made of polycrystalline alumina (PCA). Such metal halide lamps contain a salt mixture, which in part vaporize under operating conditions. Important constituents of the fillings are metal iodides and bromides such as TmX3, HoX3, DyX3, and NaX (X=I, Br). The understanding of the complex high temperature chemistry in such lamps is an important aspect of the development. The identification of volatile species present in the gas phase and the determination of valuable thermodynamic data for these species is the purpose of our investigations. The determination of thermodynamic data for gaseous species and condensed phases by the elucidation of vaporization processes is taken out by the use of high temperature mass spectrometry, which is the most versatile method for these studies. Corrosion attack of the discharge vessels made of translucent alumina is observed during lamp operation. This can limit their life time. An essential aspect of this corrosion attack is the gas phase transport of Al2O3 in the PCA discharge vessel from the hot part to the cold side. Vaporization experiments were carried out using Knudsen effusion mass spectrometry. The mass spectrometric studies showed the formation of the vapor species AlX3(g) as gaseous corrosion product formed by the reaction of lanthanide halides (LnX3) with Al2O3. The gaseous species AlO(g) and AlOX(g) were identified besides others on leaking iodine and bromine into the Knudsen cell filled with alumina lumps.