Recent Developments in Phosphor Materials

Abstract

Solid-state luminescent materials in powder form are generally labeled as “phosphors” in technological applications involving the production of light (usually in the visible spectral region) as a result of excitation by UV radiation, by x-rays, or by electron-beam impact. Since the widest utilization of phosphors occurs in the lighting industry, in the conversion into visible light of UV radiation from low-pressure Hg plasma, the emphasis in the following presentation will be on lighting phosphors.

In the last decade, the development of new phosphor materials for lighting applications has centered on refractory host matrices (hexaaluminate and yttrium oxide) activated with trivalent rare earths. In view of the resulting high materials costs, the question of efficient sensitization of rare-earth emission has become technologically very important. Present models of the sensitization process are inadequate in providing detailed guidelines for the optimization of this type of excited-state energy transfer. In this overall context, it appears that Gd³⁺ can play an important role as sensitization intermediate, by tunneling the excitation energy from the sensitizer, excited by 254 nm radiation,to the specific emitter present in the lattice. The detailed operation of Gd³⁺ as sensitization intermediate is currently under investigation.

Efficient phosphor emission under 254 nm excitation is a necessary but not sufficient condition for a technically viable product: in effect, the emission efficiency of the material must remain unaltered after the harsh lamp-baking fabrication step, and especially during the prolonged exposure (up to several thousand hours) to the Hg plasma.

Operationally, only a handful of efficient phosphors are found to satisfy these exacting requirements; the mechanisms whereby efficient phosphors dramatically degrade in performance in the baking process, and especially in the lamp environment, are at present poorly understood.