Ternary oxide phosphor particles

Details Ternary oxide phosphor particles Ternary oxide phosphor particles

2001-10-15

2004-03-16

Koslow, C. Melissa (Department: 1755)

Compositions

Inorganic luminescent compositions

Compositions containing halogen; e.g., halides and oxyhalides

C252S30160R

Reexamination Certificate

active

06706210

ABSTRACT:

The present invention relates to rare earth activated phosphors. Such phosphors are known to possess excellent light output and colour rendering properties and have been utilized successfully in many display technologies. One particularly successful material, europium activated yttrium oxide (Y
2
O
3
:Eu
3+
), has shown particular promise in the field of field emission display.
The successful introduction of field emitting displays is dependent upon the availability of low voltage phosphors. As the phosphor exciting electrons have a comparatively low energy (less than 2 kV) as compared to conventional phosphors and one must avoid the use of sulphur to reduce contamination, new types of material have to be used. In particular, it is desirable to be able to make phosphor particles without a surface dead layer which occurs when fine particles are prepared using a conventional grinding technique. This dead layer is an important source of non-radiative luminescence routes for low energy electrons.
It is known that collodial chemical techniques may be used to provide sub 100 nm particles of compounds such as Y
2
O
3
and that these may be doped to form nanocrystalline red emitting Y
2
O
3
:Eu
3+
. However, binary oxide materials such as Y
2
O
3
and Gd
2
O
3
are not efficient hosts for elements other than europium. In particular, they cannot be used as host materials for blue emitting cerium based phosphors.
Accordingly, there is the need to obtain small, typically nanocrystalline, particles which will provide different emission colours.
It has now been found, according to the present invention, that as well as red emitting particles, other coloured emitting particles can be obtained when using as a host a ternary oxide, that is to say an oxide which is derived from another element apart from yttrium, gallium etc. Accordingly the present invention provides particles of a compound of formula:
Z
z
X
x
O
y
:RE
where Z is a metal of valency b
X is a metal or metalloid, of valency a, such that
2
y=b.z+a.x
, and
RE is a dopant ion of terbium, europium, cerium, thulium, samarium, holmium, erbium, dysprosium, praseodymium, manganese, chromium or titanium, having a size not exceeding 1 micron.
It will be appreciated that X must be such as will be capable of forming an anion with oxygen.
Generally the particles are nanoparticles, by which is meant particles having a size not exceeding 100 nm, generally not exceeding 50 nm and especially not exceeding 30 nm.
Z is typically trivalent or pentavalent and is preferably yttrium, gadolinium, gallium or tantalum with yttrium being particularly preferred. X is generally divalent or trivalent, preferably aluminium, silicon or zinc, with aluminium particularly preferred.
The rare earth element is preferably europium, terbium, cerium, thulium or dysprosium.
Particular particles of the present invention are those derived from yttrium and aluminium, yttrium and silicon, tantalum and zinc or zinc and gallium.
Specific compounds of the present invention include Y
3
Al
5
O
12
:Tb
3+
(referred to as YAG:Tb), Y
2
SiO
5
:Ce
3+
, Ta
2
Zn
3
O
8
and ZnGa
2
O
4
.
Thus the particles of the present invention can be green emitting as with Y
3
Al
5
O
12
:Tb
3+
or blue as in Y
2
S
i
O
5
:Ce
3+
although green emission may also be obtained from the yttrium aluminium compound if the concentration of terbium is reduced. Other colours can be obtained from other specified rare earth elements, for example, as follows: orange-samarium, blue-holmium, near infra-red-erbium or white-disprosium.
In general, the particles of the present invention are in the form of single crystals.
The particles of the present invention can generally be prepared by the coprecipitation of salts of the two metals (for simplicity X will be referred to hereafter as a metal) of, the ternary oxide and of the “rare earth” element in aqueous solution at elevated temperature which is then calcined to the oxide. According to the present invention there is provided a process for preparing the particles of the present invention which comprises:
preparing an aqueous solution of salts of Z, X and RE and a water-soluble compound which decomposes under the reaction conditions to convert said salts into hydroxycarbonate,
heating the solution so as to cause said compound to decompose,
recovering the resulting precipitate and
calcining it at a temperature of at least 500° C.
This process is similar to that disclosed in GB Application No. 9827860.9 The water-soluble compound which decomposes under the reaction condition is typically urea, which is preferred, or a weak carboxylic acid such as oxalic acid or tartaric acid. The urea and other water soluble compounds slowly introduce OH
−
ligands into the solution until the solubility limit has been reached. When the urea decomposes it releases carbonate and hydroxide ions which control the precipitation. If this is done uniformly then particles form simultaneously at all points and growth occurs within a narrow size distribution.
The nature of the salts of the metals is not particularly critical provided that they are water soluble. Typically, the salts are chlorides, but, for instance, aluminium perchlorate can also be used.
The reaction is carried out at elevated temperature so as to decompose the water soluble compound. For urea, the lower temperature limit is about 70° C.; the upper limit of reaction is generally 100° C.
The relative amounts of the two metal salts should be such as to provide the appropriate ratio of the metals in the mixed oxide. This can, of course, be found by simple experiment. Careful control of the relative amounts can be important as there is a tendency for the compounds to form a number of phases.
Doping with the “rare earth” metal salt can be carried out by adding the required amount of the dopant ion, typically from 1 to 10%, for example about 5% (molar).
The reaction mixture can readily be obtained by mixing appropriate amounts of aqueous solutions of the salts and adding the decomposable compound.
It has been found that rather than start the process by dissolving salts of the desired elements there are advantages to be obtained by preparing the salts in situ by converting the corresponding oxides to these salts. Apart from the fact that oxides are generally significantly cheaper than the corresponding chlorides or nitrates, it has also been found that the cathodoluminescence of the resulting particles can be superior.
It has been found that better results can generally be obtained by keeping the reaction vessel sealed. This has the effect of narrowing the size distribution of the resulting precipitate.
An important feature of the process is that decomposition takes place slowly so that the compounds are not obtained substantially instantaneously as in the usual precipitation techniques. Typically for urea, the reaction is carried out at, say, 90° C. for one to four hours, for example about 2 hours. After this time precipitation of a mixed amorphous
anocrystalline phase is generally complete. This amorphous stage should then be washed and dried before being calcined. Decomposition of urea starts at about 80° C. It is the temperature which largely controls the rate of decomposition.
Calcination typically takes place in a conventional furnace in air but steam or an inert or a reducing atmosphere such as nitrogen or a mixture of hydrogen and nitrogen can also be employed. It is also possible to use, for example, a rapid thermal annealer or a microwave oven. The effect of using such an atmosphere is to reduce any tendency the rare earth element may have from changing from a 3+ ion to a 4+ ion. This is particularly prone in the case of terbium and cerium as well as Eu
2+
. The use of hydrogen may also enhance the conductivity of the resulting crystals. Calcination generally requires a temperature of at least 500° C., for example 600° C. to 900° C., such as about 650° C. It has been found that by increasing the calcination temperature the crystallite size increases. Indeed it is

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