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Industrial Processes Title

Brighter, cheaper electric light

RELUM's successful combination of fundamental science and industrial know-how led to the optimisation of production processes in the manufacture of phosphor coatings for fluorescent lamps.
Philips Lighting and its key raw materials supplier, Rhône Poulenc, were supported by the Universities of Utrecht and Cadiz. Their project extended both the knowledge about the chemical and physical properties of rare earth oxide powders, and the understanding of the mechanisms by which these properties affect luminescence.

In the late 1980s Netherlands-based Philips Lighting had a problem. The company had pioneered fluorescent tube lighting, and more recently it had invented the new compact fluorescent lamp. However, it had lost its technological lead to Japanese competitors for the preparation of the phosphor coatings which give these lights their brightness.
Philips saw that the problem lay in the purity and the chemical and physical characteristics of the rare earth oxides which are the raw materials of phosphor production. Its supplier was the French chemicals manufacturer Rhône Poulenc. Together, the two companies developed a plan for the systematic optimisation of each stage of the material processing cycle.
The RELUM project set out to improve the purity and reactivity of the oxide powders, and to develop a new processing method which would yield powders of a more even grain size. Combined, these improvements would enhance the efficiency of fluorescent tubes and significantly lower the cost of producing them.

Fundamental understanding

What the two partners lacked was the detailed scientific knowledge on which to base the optimisation of their various production processes. Although it was clear that impurities in the oxides had some influence on luminescent efficiency, for example, Philips did not understand precisely the mechanisms by which they did so.
In specifying raw material quality standards, therefore, Philips was shooting in the dark. The relative significance of different properties was not clear. The company was probably demanding unnecessarily high standards in some particulars, while at the same time accepting impurities which had severe adverse impacts on efficiency.
The partners needed expert scientific help, and secured the collaboration of the Universities of Utrecht and Cadiz, which carried out a detailed investigation of the properties of a large number of rare earth oxide powders, produced by a range of different methods.
At Utrecht, a post-graduate student devoted his PhD studies to the examination of the influence of chemical compositions, production methods and impurities on the quality of precursor oxide powders and of the phosphor coatings in which they are used. This effort was supported by the analytical contribution of the University of Cadiz, which employed high resolution transmission electronic microscopy (HR-TEM) to determine impurity levels and to characterise materials in terms of their physical and chemical properties.

Very illuminating

The four-year research programme revealed both those characteristics of the oxide powders which were most desirable for the production of phosphor lamp coatings, and those which were least desirable. Most importantly, the research showed that optimisation of the production processes at Rhône Poulenc's plant in La Rochelle would make a real difference.
Traditionally, single oxides had been produced by a process of liquid-liquid extraction and oxalic precipitation, followed by firing of the oxalate. What the partners now discovered was that co-precipitation, the precipitation of two compounds at the same time, resulted in dramatically improved downstream processing performance.
By combining yttrium and europium in the liquid phase, for example, Rhône Poulenc could produce a far more uniformly mixed material than had been achievable by mixing two separately precipitated oxide powders. This change alone made possible a radical streamlining of Philips' phosphor production process.
The details of Philips' new method remain confidential, but it immediately offered such large benefits that within the four-year life of the BRITE-EURAM project they had switched to the use of co-precipitates across the entire range of luminescent rare earth oxides.

More economical coating

Another key factor turned out to be the variation in the grain size of the phosphor. For the best luminescent performance, the phosphor coating on the inner surface of a fluorescent tube needs to be three grains thick. However, it should also be smooth and dense and to achieve this efficiently, uniformity of particle size is required.
The more uniform the particle size, the less powder is needed to achieve a satisfactory coating, and the cheaper it is to produce each lamp. At the same time, the emission of UV light gets slightly stronger as grain size increases, so that a trade-off must be made between luminescence and production cost. The project enabled the partners to determine the optimal balance between these two factors.

Bridging the technology gap

Philips and Rhône Poulenc both retain underlying patents which predate RELUM, but because the project itself was concerned with new processes rather than new materials, no new ones have been created.
Many of Philips' European competitors are also supplied by Rhône Poulenc. They too were early beneficiaries of the switch to co-precipitation, although the new production technology has since been adopted by most producers of rare earth oxides.
What they still do not have access to is the parallel refinement of the phosphor production process made possible by the co-precipitates. However, this has enabled Philips to improve the luminescent efficiency of its fluorescent lamps by 2.5 per cent, in addition to achieving substantial savings in manufacturing costs.
Although the RELUM project focused specifically on process optimisation for fluorescent lamps, there have been spin-off benefits for the other phosphor-based technologies in which Philips is involved. These include both cathode ray tube (CRT) television screens and X-ray intensifying screens.
At the very least, Philips has regained the ground lost to its Japanese rivals during the 1980s, and the availability of more efficient and more price-competitive lamps will speed consumer take-up of energy saving fluorescent lighting.

Mutual respect, mutual benefit

Commercial confidentiality was a crucial issue both for Philips and for Rhône Poulenc. Each wanted a close collaboration, and knew that this would require considerable openness. At the same time, it was clear that there were aspects of their work about which they simply could not afford to be open.
The partnership's success in establishing and respecting boundaries of confidentiality which protected individual interests without inhibiting mutually beneficial joint work is amply demonstrated by the fact that the same partners are examining the possibilities of a new BRITE-EURAM application.



Project Title:  
Highly reactive rare earth oxide powders for more efficient luminescent materials.

Industrial and Materials Technologies (BRITE-EURAM/CRAFT/SMT)

Contract Reference: BE-3336

Cordis DatabaseFor more information on this project,
go to the CORDIS Database Record