The first objective of this project is to develop more reactive rare earth oxide powders with improved morphology and a reduced level of those impurities that have a detrimental effect on the luminescent performance.
The second objective is to use these improved rare earth oxide powders to develop a process for the preparation of more efficient luminescent powders with narrower particle size distribution.
1. By improving the quantum yield of the red phosphor YOX and the green phosphor CBT, an increase of 2.5% in luminous efficacy of /80 lamps was reached.
2. The luminous efficacy of YOX, for 254 nm excitation, is decreased, due to competitive absorption by impurities, c.q. transition metal ions and defects, like interstitial oxygen.
3. The green phosphors LAP and CBT are efficiently quenched by Eu3+, due to an electron transfer process.
4. In CBT transition metal ions decrease the luminous efficiency of Ce3+, Gd3+ and Tb3+, since they are incorporated in the lattice, energy transfer to these impurity ions is possible.
5. An amorphous surface layer on a phosphor particle is observed with HR TEM.
It is concluded that Cr3+ is not incorporated into either LAP or CBT, but is present in an amorphous second layer.
6. By changing process conditions during the production of the rare earth oxides it is possible to produce these starting materials with improved specifications.
7. Using mixed oxides instead of separate oxides as starting materials the highest quantum yield was found.
Mixed oxides have an optimal distribution of the activator, so no concentration quenching will occur on a micro scale.
8. By using mixed oxides in the production of phosphors the grainsize distribution has become narrower and better controlled and so the coating weight was lowered in fluorescent lamps resulting in costprice reduction.
9. Both higher efficacy (lm/W) and costprice reduction will lead to an acceleration of the replacement of incandescent lamps by (compact) fluorescent lamps and thus a considerable energy saving.
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