Compositions: ceramic – Ceramic compositions – Yttrium – lanthanide – actinide – or transactinide containing
Reexamination Certificate
1998-12-15
2001-03-13
Marcantoni, Paul (Department: 1755)
Compositions: ceramic
Ceramic compositions
Yttrium, lanthanide, actinide, or transactinide containing
C313S635000, C313S636000, C313S640000
Reexamination Certificate
active
06200918
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a corrosion resistant ceramic and, in particular, a translucent ceramic suitable to high pressure discharge lamps such as metal halide lamp.
PRIOR ART
High pressure discharge lamps such as high pressure mercury lamp and high pressure sodium lamp are used in outdoor lighting for road, stadium, etc., in general-purpose lighting for shops, and as light sources of head lamps of vehicles, overhead projector, liquid crystal projector, etc. Presently, metal halide lamps are attracting much attention, that have a higher radiant efficiency and better color rendering properties than high pressure mercury lamp and high pressure sodium lamp.
In a metal halide lamp, a metal halide such as NaI, CsI or HgI2 is sealed into a discharge envelope or arc tube. A high voltage is applied between electrodes of the lamp to make electric discharges and, in turn, thermally evaporate the metal halide. The metal halide is dissociated into a metal and a halogen, and the metal emits its specific light. As for luminescent materials, halides of rare earth elements have a higher radiation efficiency than halides of Na, Hg, etc. Thus switchover from halides of Na, Hg, etc. to halides of rare earth elements is under consideration.
As for the discharge envelope materials, vitreous silica (SiO2) and translucent alumina (Al2O3) are used. Vitreous silica, however, is less corrosion resistant, and its heat resistance is not sufficient. Translucent alumina is better in heat resistance and corrosion resistance than vitreous silica, but its crystal system is hexagonal and its straight light transmittance is as low as 10% to 20% approximately.
Yttrium aluminum garnet (Y3Al15O12: YAG) has been proposed as a material for discharge envelope (for example, Japanese Provisional Patent Sho 59-207555). YAG has a cubic system and has a theoretical transmittance as high as 80% or over, and its mechanical strength and heat resistance are comparable to those of translucent alumina.
YAG, however, tends to react with halides of rare earth elements and is poor in chemical corrosion resistance. For example, YAG is stable against halides of Li, Na, Hg, Cs, Tl, etc., but it reacts with halides of rare earth elements. With the accumulation of lighting hours, the discharge envelop gets whitely turbid and the lamp properties deteriorate. This reaction of getting whitely turbid inside the discharge envelope is considered to advance with the following mechanism:
(M′-X)(
g
)+(M″-O)(
s
)⊖(M′-O)(
s
)+(M″-X)(
g
)
where (g) represents a gas, (s) a solid, X a halogen element, M′ and M″ rare earth elements, respectively. Under high temperature, the luminescent material being a metal halide (M′-X)(g) is dissociated into M′(g) and X(g), and the dissociated M′(g) snatches the oxygen element from the oxide ceramic (M″-O)(s) and a resulting (M′-O)(s) adheres to the internal wall of the discharge envelope. As a result, the discharge envelope gets whitely turbid.
To avoid the above-mentioned reaction of getting whitely turbid, it is conceivable to raise the filling pressure of a Hg gas, etc. to suppress the contact between the metal atoms dissociated from the metal halide and the envelope material, or to evenly heat the discharge envelope to make the halogen cycle smoother. However, the discharge envelop tends to rupture if the sealing pressure is raised or the envelope is heated up.
The related prior art includes Japanese Provisional Patent Hei 7-237983. It has proposed to provide an anti-corrosion layer of an oxide of rare earth elements on the internal surface of a translucent alumina discharge envelope. This anti-corrosion layer, however, react with a base. If the high pressure discharge lamp is used at 1000° C. or over, the anti-corrosion layer gradually reacts with a base to crystalize and form an opaque layer. Moreover, due to thermal fatigue resulting from turning on and off of the lamp, the anti-corrosion layer gradually peals off from a base differing in crystal structure, lattice constants, coefficient of thermal expansion, etc. Japanese Provisional Patent Hei 10-45467 has proposed to use YAG for a corrosion resistant vessel of a dry etching machine.
Terminology
In the present specification, ppm and % are weight ppm and weight % if not specified otherwise. Ca and Mg are substantially even to each other, and their quantities are indicated in principle as a total content of Ca and Mg. When a content of Ca, Mg or Si is set at 200 ppm or under or at 100 ppm or under, O is included, and the mean particle size does not include O. Corrosion resistance means corrosion resistance against a halide of rare earth elements in a metal halide lamp when the lamp is turned on, and corrosion resistance means no occurrence of getting whitely turbid.
SUMMARY OF THE INVENTION
One objective of the present invention is to provide a corrosion resistant ceramic having a high strength and an excellent corrosion resistance, in particular, to provide a translucent ceramic that does not get whitely turbid even if it is used for a discharge envelop using a halide of rare earth elements.
A secondary objective of the present invention is to form a high pressure discharge lamp in an inexpensive manner and to eliminate the needs of mirror-plane-polishing the interior surface of a high pressure discharge lamp.
In the corrosion resistant rare earth element aluminum garnet ceramic of the invention,
the total content of Tm, Yb and Lu in the constituent rare earth elements is from 10 to 100 mol %, and
in said ceramic, the content of Si is from 0 to 100 wt ppm in metal reduction, and the total content of Ca and Mg is from 0 to 200 wt ppm in metal reduction. Under these conditions, corrosion resistance of the ceramic is improved significantly, and the ceramic does not get whitely turbid even if it is used as a discharge envelope of a high pressure discharge lamp using a halide of rare earth elements over a long time. A constituent rare earth element other than Tm, Yb and Lu is, for example, Y.
Preferably, the total content in said constituent rare earth elements of Tm, Yb and Lu is from 10 to 50 mol %. Under this condition, the corrosion resistance of the ceramic can be increased while reducing the quantities of Tm, Yb and Lu used.
Preferably, said corrosion resistant ceramic is translucent and is an envelope for a high pressure discharge lamp.
Preferably, the mean particle size of said corrosion resistant ceramic is from 1 to 20 &mgr;m, and more preferably, from 1 to 15 &mgr;m, and most preferably, from 1 to 10 &mgr;m. Under these conditions, a high straight light transmittance is obtained, and the mean three-points bending strength can be 400 MPa or over, and Weibull coefficient can be 6 or over.
Preferably, the mean three-point bending strength of said corrosion resistant ceramic is 400 MPa or over and its Weibull coefficient is 6 or over. Under these conditions, a high resistance to thermal fatigue is obtained.
In a corrosion resistant ceramic according to the present invention, at least one face of a base being a rare earth element aluminum garnet ceramic is provided with a rare earth element aluminum garnet anti-corrosion layer wherein the constituent rare earth element is at least one of Tm, Yb and Lu, the Si content is from 0 to 100 wt ppm in metal reduction and the total content of Ca and Mg is from 0 to 200 wt ppm in metal reduction. Under this condition, the base can be constituted of inexpensive YAG, etc. and an anti-corrosion layer having excellent corrosion resistance can be obtained.
Preferably, said base is tubular, both said anti-corrosion layer and said base are translucent, said anti-corrosion layer is provided on the internal surface of said base, and the corrosion resistant ceramic is used as a discharge envelope for a high pressure discharge lamp.
Preferably, the Si content of said base is 4 ppm or under, its total content of Ca and mg is from 5 to 1000 wt ppm, and its mean particle size is from 1 to 15 &mgr;m.
Preferably, the
Imagawa Seiki
Kubo Hitoshi
Yagi Hideki
Yanagitani Takagimi
Arent Fox Kintner & Plotkin & Kahn, PLLC
Konoshima Chemical Co. Ltd.
Marcantoni Paul
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