Corrosion in Metal Halide Lamps

摘要

Metal halide lamps contain metal halides (e.g. TmX_3, DyX_3, HoX_3, NaX; X = I, Br) as major component of the lamp fill. Corrosion attack of the discharge vessels made of quartz glass and polycristalline alumina (PCA) is observed during lamp operation and this can limit their life time. An essential aspect of this corrosion attack is the gas phase transport of SiO_2 in the quartz vessels and of Al_2O_3 in the PCA vessels from the hot part of the vessel to the cold part. Vaporisation experiments were carried out using Knudsen effusion mass spectrometry and annealing experiments using quartz and PCA ampoules with different fills in order to elucidate the corrosion attack. Annealing experiments in the Knudsen cell of the mass spectrometer -Knudsen cell system were carried out additionally. The characterisation after the annealing was carried using ceramography, XRD, and SEM/EDX. The mass spectrometric studies showed the formation of the vapour species SiX_4(g) and SiX2(g) as well as AlX_3(g) as gaseous corrosion products formed by the reaction of a lanthanide halide (LnX_3) with quartz and Al_2O_3, respectively. A solid solution phase between quartz and TmX_n as well as the phases TmAlO_3(s) and Tm_4Al_2O_9(s) were identified as solid corrosion products in the attack of quartz and alumina, respectively. The gaseous species SiO(g) and SiOX_2(g) were identified besides other on leaking SiX_4(g) into the Knudsen cell filled with quartz lumps. The vapour species AlO(g) and AlOBr(g) were observed in the same way if iodine and bromine is leaked into a cell filled with Al_2O_3 lumps. The observed gas phase transport of SiO_2 and Al_2O_3 in lamps can be explained by the mentioned gaseous oxides and oxihalides. It is driven by the temperature difference and in the case of quartz glass by the crystallisation energy of cristobalite.