Compositions – Inorganic luminescent compositions – Compositions containing halogen; e.g. – halides and oxyhalides
Reexamination Certificate
1999-08-27
2001-06-12
Koslow, C. Melissa (Department: 1755)
Compositions
Inorganic luminescent compositions
Compositions containing halogen; e.g., halides and oxyhalides
C106S313000, C148S026000, C423S594120, C423S594120
Reexamination Certificate
active
06245260
ABSTRACT:
TECHNICAL FIELD
This invention relates to yttrium tantalate x-ray phosphors used as intensifier screens for x-ray imaging. More particularly, it relates to flux compositions for yttrium tantalate phosphor synthesis methods.
BACKGROUND ART
Unactivated and niobium activated monoclinic M′ yttrium tantalate (YTaO
4
) X-ray phosphors are used in x-ray intensifying screens for medical radiographic applications. Examples of these phosphors are given in U.S. Pat. Nos. 5,009,807, 5,112,524, and 4,225,653, which are incorporated herein by reference.
One property associated with x-ray phosphors which can cause serious problems is the presence of delayed fluorescence. This delayed fluorescence, also known as afterglow, lag, or persistence, is the emission of light from the phosphor after x-ray excitation is stopped. The presence of a large afterglow in a phosphor screen will compromise the quality of the radiographic images collected using that screen. Particularly, x-ray intensifier screens used in auto-changers in hospitals for routine x-ray procedures require low or zero-lag phosphor screens because the x-ray intensifier screens in automated changers are used many times over a short period of time. In such applications, a high lag phosphor screen can retain a part of the previous image which interferes with the new x-ray exposure. Yttrium tantalate phosphors commonly exhibit substantial levels of phosphor lag. Because these phosphors are used to prepare intensifier screens for use in automated rapid exposure X-ray devices, the availability of such phosphors with the lowest possible lag has become increasingly important in order to obtain high radiographic image quality.
Other properties associated with x-ray phosphors are their XOF (X-ray Optical Fluorescence) brightness and particle size. The use of a brighter x-ray phosphor in intensifier screens shortens exposure time and decreases x-ray dosage for patients needing medical X-ray imaging procedures. However, XOF brightness is usually accompanied by an increase the particle size of the phosphor grains. Increased particle size in intensifier screens reduces the resolution of the resulting x-ray images. Thus, preparation of an x-ray phosphor with improved XOF brightness without the concomitant increase in particle size is highly desirable.
Strontium is known to increase XOF brightness and decrease afterglow (persistence and lag) in yttrium tantalates when added to the formulation or to the commonly used Li
2
SO
4
or Li
2
SO
4
—LiCl fluxes as SrCO
3
, SrCl
2
, SrCl
2
.6H
2
O, or other Sr
2+
containing species. The use of eutectic Li
2
SO
4
—LiCl flux increases phosphor brightness but also tends to increase particle size and cause damage to crucibles and ovens. Although Sr increases XOF brightness and decreases persistence, the addition of Sr does not lead generally to the production of a zero lag phosphor and the increase in XOF brightness from Sr addition is not continuous. In particular, high levels of Sr form increased amounts of impurity phases which reduce XOF brightness and increase persistence. Thus, it would be beneficial to have a method to reduce persistence and increase XOF brightness without increasing particle size.
SUMMARY OF THE INVENTION
It is an object of the invention to obviate the disadvantages of the prior art.
It is another object of the invention to provide a method to reduce persistence and increase XOF brightness of yttrium tantalate x-ray phosphors without increasing particle size.
In accordance with one aspect the invention, there is provided a mixed flux for use in yttrium tantalate phosphor synthesis. The mixed flux comprises a mixture of lithium sulfate and sodium sulfate.
In accordance with another aspect of the invention, there is provided a method of making a yttrium tantalate phosphor. The method comprises forming an oxide mixture of yttrium oxide, tantalum oxide, and optionally strontium carbonate, adding a lithium sulfate-sodium sulfate mixed flux to the oxide mixture and firing the mixture at a temperature and for a time sufficient to form the phosphor.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
For a better understanding of the present invention, together with other and further objects, advantages and capabilities thereof, reference is made to the following disclosure and appended claims.
I have discovered that the use of a lithium sulfate-sodium sulfate (Li
2
SO
4
—Na
2
SO
4
) mixed flux during the synthesis of YTaO
4
phosphors leads to phosphors with increased XOF brightness and decreased afterglow relative to the same phosphor compositions prepared using the common Li
2
SO
4
flux. Use of the Li
2
SO
4
—Na
2
SO
4
mixed flux yields a YTaO
4
phosphor with a smaller particle size than can be prepared using pure Li
2
SO
4
flux in the same total amount.
Using a Li
2
SO
4
—Na
2
SO
4
mixed flux during synthesis reduces the persistence and lag, increases the XOF brightness, and decreases the particle size in yttrium tantalate X-ray phosphor both with and without the presence of Sr
2+
species. Powder lags of zero have been obtained with the Li
2
SO
4
—Na
2
SO
4
mixed flux. The Li
2
SO
4
/Na
2
SO
4
weight ratio in the flux ranges from about 0.5 to about 3.0. The total amount of Li
2
SO
4
—Na
2
SO
4
mixed flux added to the oxide mixture ranges from about 15 to about 50 weight percent (wt. %) of the total oxide mixture. Preferably, the amount of Li
2
SO
4
—Na
2
SO
4
mixed flux ranges from about 24 to about 32 wt. % of the oxide mixture and the Li
2
SO
4
/Na
2
SO
4
weight ratio ranges from about 1.0 to about 1.8 with the most preferred ratio being about 1.8.
REFERENCES:
patent: 4225653 (1980-09-01), Brixner
patent: 4543412 (1985-09-01), Murakawa et al.
patent: 4929384 (1990-05-01), Reddy
patent: 4929385 (1990-05-01), Reddy
patent: 4929386 (1990-05-01), Reddy
patent: 5009807 (1991-04-01), Reddy
patent: 5112524 (1992-05-01), Reddy et al.
patent: 5310505 (1994-05-01), Hedden et al.
patent: 5716546 (1998-02-01), Cox et al.
patent: 5900188 (1999-05-01), Marking et al.
Derwent Abstract for JP 72-001950 B, Apr. 25, 1969.*
Derwent Abstract for JP 73-022814 B, Nov. 12, 1965.
Clark Robert F.
Koslow C. Melissa
Osram Sylvania Inc.
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