Reducing the Rare Earth Content in Red- and Green-Emitting Phosphors for Lighting Applications
Dr. W. Michael Chance and Dr. L. A. Boatner
Fluorescent lamps remain a relevant, energy-efficient branch of lighting technology. However, commercially available fluorescent bulbs currently rely on phosphor coatings predominately composed of expensive and scarce rare earth elements (REs). These phosphor materials are necessary for converting the mercury vapor emission within the lamp to visible white light. To steer next-generation lighting technologies away from unstable RE markets, extensive efforts have gone towards developing phosphor materials that require reduced or eliminated RE content. This work explored the synthesis and performance optimization of three classes of reduced-RE phosphors. We prepared red-emitting magnesium germanate and magnesium fluorogermanate phosphors by different routes and investigated the effects of synthetic method, constituent precursors, and anion doping on the phase purity, light yield, and photoluminescence of these RE-free materials. We also demonstrate enhanced Eu3+ red emission in yttrium aluminate hosts via Sm3+ co-doping. To develop reduced-RE green-emitting phosphors, zinc pyrophosphates were studied as hosts for low levels of RE dopants. While the material exhibited a quantum efficiency rivaling commercial standards, RE dopants were insoluble in the zinc pyrophosphate crystal lattice. Preliminary results suggest the enhanced performance of the zinc pyrophosphate phosphor may be due to the particle size or crystallinity of monazite precipitates. The results of these studies highlight the challenges associated with the synthesis of inorganic phosphors and showcased unexpected optical properties for further investigation.
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