Young Scientist
Ariane Keiko ALBESSARD
Display Materials and DevicesLaboratory, Toshiba Corp.
1, Komukai Toshiba-cho, Saiwaiku, Kawasaki 212-8582, Japan
Graduated from the Université
Paris-Sud, FranceMaster degree of Material Engineering from the University ofTsukuba, Japan“C’était très bien, je vous félicite!” said aman when I returned to my seat in the conference hall at Asia Display ’95, held in
Hamamatsu, Japan.
When I joined Toshiba, it had already been
decided that my work would be on the development of ‘spherical phosphors’. My boss, Dr
Masaaki Tamatani, a researcher with morethan 20 years of experience in the phosphor
field, had created these beautiful pearl shaped
phosphors (see the photograph) a few months
before, and decided to develop them for practical use. My work on spherical phosphors
covered a wide range of activities, including
preparation of the raw materials, theirspheroidization and subsequent evaluation,
phosphor screen preparation and simulation
calculations.
At first, we spheroidized several phosphor
materials by the thermal plasma method.(1)(2) In
this method, the phosphor particles, put in aplasma flame whose temperature is several
thousand degrees, melt and solidify into aspherical shape due to the surface tension.
Because a part of some particular elements ofthe material evaporated during this process,every phosphor acquired a body color, and XRD
measurements revealed that their stoichiometry had been distorted. As a result, their brightness decreased to half or less of the original
material - a critical problem for a phosphor.
This problem occurred even for the red-emitting europium doped yttrium oxide (Y2O3:Eu)
phosphor, which had the simplest composition.
However, by eliminating impurities in the raw
material and by optimizing the plasma conditions in cooperation with Neturen Co., Ltd., athermal plasma equipment manufacturer, theevaporation and the body coloring could besuppressed. Moreover, by doing an annealing
after the spheroidization, we were able to ob-tain a spherical Y2O3:Eu phosphor suitable forcommercialization.
For phosphors to be used as a layer, theirlayer properties are very important. Becauseof the spherical shape and the good dispersion of each particle, spherical phosphors areexpected to produce a thin, highly dense anduniform layer. In practice, we could decrease
the thickness of a phosphor layer composedof spherical phosphors by 30%, comparedwith a layer made of conventional phosphorshaving the same coating weight. In addition,
because of the good transparency of thespherical particles, the light coming out of thephosphor layer should have a minimal loss.
Therefore, we attempted to manufacture projection CRTs using spherical phosphors.
A projection CRT is a small-sized monochrome CRT used for projection TVs. Images
obtained from red-, green- and blue-emittingCRTs are enlarged with lenses and projected
on a screen measuring over 40 inches in diagonal. Thus, resolution and brightness are
crucial for projection CRTs. We found, withthe participation of Toshiba Hokuto Electronics Corp., a projection CRT manufacturer, thatboth characteristics could be increased by 10%
by using spherical phosphors. These data weresufficiently convincing that we decided topresent spherical phosphors to the world.
Eventually the day came for me to introduce spherical phosphors for the first time tothe public at Asia Display ’95.(3) This was also
my first oral presentation in an international
conference, and I must have looked so nervous that the man, an ex-colleague of my father, praised me. I was not expecting to hearFrench in Japan and was very surprised to meetsomeone who knew me, but I was so glad tobe honored like this when I needed confidence
in myself. As he told me, ‘spherical phosphors’
were certainly a success and created a great
sensation. Several companies called us laterto obtain information, and some phosphor research groups, from Japan and overseas, went
on to produce spherical phosphors by varioustechniques. Additionally, one year later, ourphosphor research group received the ‘Phosphor Award 1996’ from the Phosphor Re-search Society.
The next stage of my work was to create
spherical phosphors emitting colors other thanred. Because the green-emitting phosphor
makes a major contribution to the brightnessof projection TVs, we tried to spheroidize
green-emitting terbium doped yttrium silicate(Y2SiO5:Tb) phosphor. However, because of its
complex stoichiometry, we encountered more
serious problems than in the case of Y2O3:Eu.
By solving these problems one by one, we finally succeeded in obtaining a practical spherical Y2SiO5:Tb phosphor.

Once we obtained the required sphericalphosphors, we had to optimize the layer characteristics of the CRTs. Because projection CRTs
need a higher resolution (reflecting the recent
increasing demand for high-definition TVs), we
calculated the trajectory of the light in thephosphor screen, with the aid of a MonteCarlo simulation, to evaluate the resolution.(4)The electron beam reaching the phosphor
particles is converted into visible light, which
spreads out in the phosphor screen due toscattering. Thus, the width of the output light(called ‘spot size’ and represents the resolution) becomes larger than that of the electron
beam. Since the layer using spherical phosphors is thinner, the width of the spot size
becomes smaller than that of a thicker layer.
Unfortunately, while the light scattering lengthof spherical phosphors is larger than that of
the conventional phosphors (because of thesmaller surface area of the particles), light

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Young Scientist
spreads further along the lateral direction,
which leads to a larger spot size and cancels
the effect of the layer thickness. From the calculations, we found that by decreasing thephosphor secondary particle size by 20%, effect of the light scattering length and that of
the layer thickness can be minimized and maximized, respectively. As a result, projection CRTs
with improved resolution were obtained. To
perform this work, we collaborated with two
French students who worked as trainees in our
research group - that was a very pleasant experience for me, because it gave me the opportunity to be in contact with French people
and culture.
Five years after the spherical phosphors
were first made known to the public, Toshiba

started to use spherical Y2O3:Eu and Y2SiO5:Tb
phosphors in its 2000 model projection CRTs.(5)Thanks to the spherical phosphors and a newly
developed electron gun, the resolution of the2001 model projection CRTs increased by 40%
over that of the conventional 1999 model, andthe market share of Toshiba is growing.
Spherical phosphors are also expected toshow good characteristics in other light-emitting devices, such as PDPs (Plasma Display Panels), FEDs (Field Emission Displays), fluorescentlamps, X-ray detectors, etc. Because each device uses its own particular phosphors, the development of other spherical phosphors will
be difficult and time-consuming, but very interesting for the future of luminescent devices.

References(1) Matsuda, N., Tamatani, M., Okumura, M.,
Albessard, A. K., Inoue, Y. and Kawasaki, K. J.
SID 6, 2, 159-161 (1998).
(2) Tamatani, M., Matsuda, N., Okumura, M.,
Albessard, A. K., Inoue, Y., Yokota, S. andKawasaki, K. Proc. 6th Int. Conf. on Luminescent
Materials, 10-21 (1998).
(3) Albessard, A. K., Tamatani, M., Okumura, M.,
Matsuda, N., Hattori, H. and Motoki, S. Proc. Asia
Display ’95, 643-646 (1995).
(4) Albessard, A. K., Matsuda, N., Tamatani, M.,
Yokota, S., Inoue, Y., Terajima, A., Hattori, H.,
Akitsuki, K. and Suzuki, T. Proc. IDW ’00, 877880 (2000).
(5) Hattori, H., Akitsuki, K., Suzuki, T., Tamatani, M.,
Matsuda, N., Albessard, A. K., Yokota, S., Inoue,
Y. and Terajima, A. Proc. IDW ’00, 493-496(2000).

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