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
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.
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.
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
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
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.