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Corrosion in Metal Halide Lamps K. Hilpert, T. Karwath, T. Markus, and L. Singheiser Institute for Materials and Processes in Energy Systems, Research Centre J¨¹lich, 52425 J¨¹lich, Germany Metal halide
lamps contain a salt melt essentially made of iodides and bromides which in
part vaporise under operating conditions. The salt melt is present as condensed
phase at the cold spot of the inner wall of the discharge vessel. Important
constituents of the melt are the rare earth metal halides LnX3 (X =
Br, I) such as DyX3, HoX3, and TmX3 because
rare earth metals are efficient radiators in the visible range of the emission
spectrum. In metal halide lamps with a discharge vessels made of quartz glass
typical wall temperatures of the vessel range from 1000 K to 1300 K. These
temperatures range from 1300 K to 1600 K if the discharge vessel is made of
polycrystalline alumina. The two materials quartz glass and alumina show strong
corrosion attack by the salt melt. The processes leading to this attack are
unknown. In the
course of our systematic investigations of the chemistry in metal halide lamps
(1) experiments using Knudsen effusion mass spectrometry and annealing
experiments with closed ampoules made of quartz glass and alumina were carried
out (2). One and two chamber Knudsen cells as well as Knudsen cells with a gas
inlet system were used. The conditions in metal halide lamps could be simulated
in this way. This led to the identification of gaseous corrosion products such
as SiOBr2(g), SiOI2(g), SiI(g) and SiI2(g).
Annealing experiments with the closed ampoules showed for example chemical
reactions between the salt melt and alumina or quartz glass. XRD pattern before
and after the experiments showed the formation of amorphous phases and rare
earth aluminates. In addition, I(g) and Br(g) are formed by these reactions as
determined mass spectrometrically. Corrosion
processes are proposed which explain the phenomena in discharge lamps. 1. K. Hilpert und U. Niemann, Thermochimica Acta 299 (1997) 49 - 57. 2.
T. Karwath, K. Hilpert, and L. Singheiser,
Report of Research Centre J¨¹lich, J¨¹l-3725, J¨¹lich (1999), Germany. |
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