Refractory metals: crucial components for light sources

Abstract

More than 40 elements of the periodic table are used in the lighting industry as auxiliary materials and as light emitting compounds in the solid as well as in the gaseous state. The so-called ‘refractory metals’ are of crucial importance for all existing types of industrial light sources. They comprise the elements tungsten, molybdenum, niobium, tantalum, rhenium, zirconium and hafnium. Wire and coils made out of potassium-doped non-sag tungsten became an indispensable part of all commercial incandescent lamps. Coils in halogen incandescent lamps represent parts with the highest energy load to a solid-state material in technology. The demands are still rising to increase efficacy and lamp life in many respects. Thoriated tungsten became the preferred material for electrodes in nearly all makes of discharge lamps which contribute immensely to energy saving in lighting. The replacement of thoria in these applications is not a trivial task. Electrical power supply into the silica vessels of halogen incandescent lamps or metal halide discharge lamps would be impossible without molybdenum feather-edged ribbon of some 10 μm in thickness; niobium is used in form of small cups in high-pressure sodium lamps. Tungsten metallurgy initiated a tremendous progress in vacuum technology and powder metallurgy. The introduction of gas-filled lamps made it necessary to develop separation methods to make rare gases available and, by adding halogen compounds to avoid wall blackening, ppm-technologies were first applied for highly reactive gases on industrial scale. In addition, a new understanding about lamp quality requires other features, like surface finish, to be taken into account. It is beyond any doubt that the investigations on refractory metals in the lighting industry will be a pacemaker for other technologies in the future.