9.4 Chemistry of metal halide discharge lamps
Aim: to enlight some high temperature materials chemistry issues in action in these energy systems
Topic description and teaching suggestions: Metal halide gas discharge lamps are increasingly used in many kinds of applications. The study of total lighting systems, in which the lamp considered as a reactor is one important element, is highly interdisciplinary. Knowledge from several major domains, e.g. electrical engineering, plasma physics and chemistry, materials chemistry is necessary in order to optimize a given light system for a particular application. Considering the discharge vessel as a reactor, it is clear that high temperatures and large temperature gradients, in combination with the presence of corrosive compounds such as metal halides (typically mixtures of selected alkali and rare earth element halides) can lead to complex transport phenomena and corrosion processes. In this context major issues are, from the basic research point of view, thermochemistry and problems connected with material transport and corrosion within the lamp bulb and the electrode. Thermodynamic modelling can help in understanding what processes are going on in these high temperature devices. In summary, there is a lot of high temperature materials chemistry to be discussed in the operation of a gas discharge metal halide lamp: composition of the gas phase, thermodynamic and spectroscopic properties of the gaseous species and their volatility, diffusion phenomena and reactions of the hot gas with the bulb material (glass or alumina), etc. This topic may be included as optional and could perhaps be exploited in a tutorial class.
Markus, U. Niemann, K. Hilpert, “High temperature gas phase chemistry for the development of advanced ceramic discharge lamps” J. Phys. Chem. Solids 66 (2005) 372.
van Erk, “Trasport properties in metal halide gas discharge lamps”, Pure Appl. Chem, 72(11)2159-2166 (2000) and literature cited therein.
Hilpert, U. Niemann, “High temperature chemistry in metal halide lamps”, Thermochimica Acta 299(1997)49.