Chemistry of inorganic compounds – Treating mixture to obtain metal containing compound – Alkaline earth metal
Reexamination Certificate
1999-07-23
2001-01-30
Kopec, Mark (Department: 1751)
Chemistry of inorganic compounds
Treating mixture to obtain metal containing compound
Alkaline earth metal
C423S518000, C252S30140S, C252S519400
Reexamination Certificate
active
06180073
ABSTRACT:
The present invention relates to a compound based on an alkaline-earth metal, on sulphur and on aluminium, or gallium or on indium, to a process for its preparation and to its use as luminophore.
The fields of luminescence and of electronics are currently experiencing significant developments. Mention may be made, as an example of these developments, of the development of cathodoluminescent systems for novel display and illumination techniques. One concrete application is that of the replacement of existing television screens by flat screens. These novel applications require luminophore materials exhibiting increasingly improved properties. Thus, in addition to their property of luminescence, these materials are required to have specific characteristics of morphology or of particle size, in order to facilitate in particular their employment in the desired applications.
More specifically, luminophores are requited which are micronic in size and which optionally have a narrow particle size distribution.
Alkaline-earth metal thiogallates are known as luminophores. These products are prepared from a mixture of salts or of oxides of the various constituents by heating at high temperature in the presence of a flux. This method of preparation results in products which are large in size and which have an often very broad particle size distribution.
The object of the invention is to provide products of this type with a small particle size.
With this aim, the compound of the invention, a first embodiment, corresponds to the formula AB
2
S
4
(1), in which A represents one or more alkaline-earth metals and B at least one element taken from the group comprising aluminium, gallium or indium, and it is characterized in that it is provided in the form of a powder with a residual oxygen content of not more than 1.5%, more particularly of not more than 1%.
According to a second embodiment, the compound of the invention corresponds to the formula (1) and it is characterized in that it is provided in the form of a powder composed of whole or unmilled particles with a mean size of not more than 10 &mgr;m.
The invention also relates to a process for the preparation of such a compound, which is characterized in that it contains the following stages:
a solution or a suspension comprising salts of the elements A and B and optionally of the doping element is formed;
the solution or the suspension is dried by atomization;
the product obtained in the preceding stage is reacted with carbon disulphide or with a mixture of hydrogen sulphide and of carbon disulphide.
Finally, the invention relates to the use as luminophore, in particular in cathodoluminescence, of a compound as described above.
Other characteristics, details and advantages of the invention will become still more fully apparent on reading the description which follows and the appended drawing, in which:
FIG. 1
is an X-ray spectrum of a compound according to the invention.
As explained above, the compound of the invention is provided in the form of a powder and it corresponds to the formula (1) AS
2
S
4
. In this formula, A is an alkaline-earth metal (group IIa of the Periodic Classification). The Periodic Classification of the elements to which reference is made is, here and throughout the description, that published in the Supplement to the Bulletin de la Soci{overscore (e)}t{acute over (e)} Chimique de France, No. 1 (January, 1966)
A can very particularly be strontium. A can also be magnesium, calcium or barium.
B can be aluminium, gallium or indium. B can more particularly be gallium. According to a specific embodiment, the compound of the invention is preferably a strontium thiogallate.
The invention also relates to the compounds in which A represents several alkaline-earth metals. Likewise, B can represent a combination of at least two of the elements aluminium, gallium or indium.
The compound of the invention can comprise one or more doping elements. Doping element is understood to mean, in this instance, any element which can confer, on the compound of formula (1) , properties of luminescence in a given application of the compound as luminophore. Without wishing to be restricted by a theory, it may be thought that the doping element can substitute for the alkaline-earth metal. The Count of doping element is usually not more than 10 atom % with respect to the alkaline-earth metal element. More particularly, this doping element can be chosen from divalent manganese, divalent rare-earth metals and the group comprising trivalent rare-earth metals in combination with an alkali metal. In the case of trivalent rare-earth metals, the presence of an alkali metal is necessary to compensate for the excess charge of the rare-earth metal. The alkali metal can more particularly be sodium.
Rare-earth metal is understood to mean the elements from the group composed of yttrium and the elements of the Periodic Classification with an atomic number of between 57 and 71 inclusive.
The doping element can more particularly be europium(II), ytterbium(II) or cerium(III) in combination with an alkali metal.
According to a first embodiment the compound of the invention is characterized by its low residual oxygen content. This content is in fact lower than that of the compounds of the prior art, This low residual oxygen level could be one of the reasons—for the advantageous luminescence properties of the product of the invention. As indicated above, this residual content is not more than 1.5%, more particularly not more than 1%. It is expressed as weight of oxygen with respect to the total weight of the compound.
According to a second embodiment of the invention, the compound of the invention is characterized by its morphology. According to this embodiment, the compound is composed of particles with a mean size of not more than 10 &mgr;m. Throughout the description, the characteristics of site and of particle size distribution are measured by the laser diffraction technique, using a particle sizer of the Cilas HR 850 type (distribution by volume).
This mean size can more particularly be not more than 5 &mgr;m and more particularly still not more than 4 &mgr;m.
The particles for which the size has been given above are unmilled or whole particles. Scanning electron microscopy photos make it possible to show that these particles do not have the broken or shattered appearance which is exhibited by particles which have been subjected to milling. Likewise, these photos show that these particles do not exhibit, adhering to their surface, particles of a markedly finer size, as can be the case as the result of milling, where fine particles created by the milling can become aggregated into bigger particles. However, it should be noted that the powder constituting the product of the invention may have been simply deagglomerated.
The compounds according to the first embodiment can, of course, exhibit the particle size characteristics which have just been given above, in combination with the characteristic of residual oxygen content. Likewise, those of the second embodiment can also exhibit, in combination with the particle size characteristics, the oxygen content of the compounds of the first embodiment.
All the additional characteristics which will now be given apply to both embodiments.
According to a preferred alternative form of the invention, the compound exhibits a narrow particle size distribution. Thus, the dispersion index &sgr;/m is not more than 0.7. It can more particularly be not more than 0.6.
Dispersion index is understood to mean the ratio:
&sgr;/
m
=(
d
84
−d
16
)/2
d
50
in which:
d
84
is the diameter of the particles for which 84% of the particles have a diameter below d
84
;
d
16
is the diameter of the particles for which 16% of the particles have a diameter below d
16
;
d
50
is the mean diameter of the particles.
The compounds of the invention can be composed of particles with a substantially spherical shape for which the diameter corresponds to the mean sizes which have been given above.
Another characteristic of the compounds of the i
Huguenin Denis
Macaudiere Pierre
Kopec Mark
Rhodia Chimie
Seugnet Jean-Louis
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