Process for providing hydrolysis resistant phosphors

A process is disclosed for making a rare earth oxysulfide phosphor resistant to hydrolysis. The process involves adding to a water slurry of the phosphor a sufficient amount of a source of silicon dioxide and a water soluble salt which can be an alkaline earth metal salt or a transition group IIB metal salt with agitation for a sufficient time to form a silicate coating on the phosphor with the cation of the water soluble salt becoming the cation of the silicate coating, the amount of the coating being sufficient to impart hydrolysis resistance thereto. The resulting silicate coated phosphor is then separated from the resulting liquor and heated at a sufficient temperature for a sufficient time in ambient atmosphere to remove essentially all of the water therefrom and to produce the final hydrolysis resistant phosphor. A hydrolysis resistant rare earth oxysulfide phosphor is disclosed consisting essentially of a host which can be gadolinium oxysulfide, yttrium oxysulfide, gadolinium-yttrium oxysulfide, and lanthanum oxysulfide, from about 0.001 to about 0.10 moles of terbium per mole of host as an activator, and a sufficient amount of a silicate coating on the phosphor to impart hydrolysis resistance thereto.

BACKGROUND OF THE INVENTION 
This invention relates to a process for making rare earth oxysulfide 
phosphors resistant to hydrolysis and to the hydrolysis resistant 
phosphors thus produced. More particularly it relates to a process for 
making rare earth oxysulfide phosphors resistant to hydrolysis by 
imparting a silicate coating thereon, and to the silicate coated 
hydrolysis resistant phosphor thus produced. 
Rare earth oxysulfide phosphors have become successful x-ray intensifier 
phosphors. In this application the phosphor is on a screen called an 
intensifier screen. The screen is mounted in a cassette where in operation 
the phosphor thereon is exposed to x-rays. The phosphor converts the 
x-rays into visible or near visible radiation to which a photosensitive 
film is exposed resulting in an image being produced on the film. 
One of the problems that has developed in the above application is that if 
the phosphor comes in contact with water, a hydrolytic reaction can occur 
and hydrogen sulfide is released. Additionally, if water is inadvertently 
dropped on an intensifying screen in an x-ray cassette and a film is 
placed in the cassette, a reaction between hydrogen sulfide and the silver 
halide in the film emulsion occurs. This reaction causes a brown stain on 
the intensifying screen that reduces the speed of the screen resulting in 
inferior radiographs. 
Therefore, if the phosphors could be made resistant to hydrolysis so that 
they consistently produce good radiographs, it would be highly desirable 
and an advancement in the art. 
SUMMARY OF THE INVENTION 
In accordance with one aspect of this invention there is provided a process 
for making a rare earth oxysulfide phosphor resistant to hydrolysis. The 
process involves forming a slurry of the phosphor in water. To the slurry 
is added a sufficient amount of a source of silicon dioxide and a water 
soluble salt which can be an alkaline earth metal salt or a transition 
group IIB metal salt with agitation for a sufficient time to form a 
silicate coating on the phosphor with the cation of the water soluble salt 
becoming the cation of the silicate coating, the amount of the coating 
being sufficient to impart hydrolysis resistance to the phosphor. The 
resulting silicate coated phosphor is then separated from the resulting 
liquor and heated at a sufficient temperature for a sufficient time in an 
ambient atmosphere to remove essentially all of the water therefrom and to 
produce the final hydrolysis resistant phosphor. 
In accordance with another aspect of this invention there is provided a 
hydrolysis resistant rare earth oxysulfide phosphor consisting essentially 
of a host which can be gadolinium oxysulfide, yttrium oxysulfide, 
gadolinium-yttrium oxysulfide, or lanthanum oxysulfide, from about 0.001 
to about 0.10 moles of terbium per mole of host as an activator, and a 
sufficient amount of a silicate coating on the phosphor to impart 
hydrolysis resistance thereto. 
DETAILED DESCRIPTION OF THE INVENTION 
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 in connection with 
the above description of some of the aspects of the invention. 
By the process of this invention, a terbium activated rare earth oxysulfide 
phosphor is made resistant to hydrolysis by coating the phosphor with a 
silicate. 
The starting rare earth oxysulfide phosphor to be made hydrolysis resistant 
consists essentially of a host which is preferably gadolinium oxysulfide, 
yttrium oxysulfide, gadolinium-yttrium oxysulfide, and lanthanum 
oxysulfide, and from about 0.001 moles to about 0.10 moles, and preferably 
from about 0.001 moles to about 0.006 moles of terbium per mole of the 
host as an activator. The starting phosphor can be made by any well known 
solid state sintering technique. 
A slurry is first formed of the phosphor in water, preferably deionized 
water. Generally from about 1 to about 10 weight parts of water are used 
per part of the starting phosphor. 
To the slurry is added a sufficient amount of a source of silicon dioxide 
and a water soluble salt which can be an alkaline earth metal salt or a 
transition group IIB metal salt, with agitation for a period of time, 
preferably from about 5 minutes to about 15 minutes to form a silicate 
coating on the phosphor. The amount of the coating is sufficient to impart 
hydrolysis resistance to the phosphor. 
The source of silicon dioxide can be any source which will react with the 
soluble metal salt to form the silicate coating provided it is of 
sufficient purity so that the final coated phosphor is not contaminated. 
The preferred source of silicon dioxide is relatively pure potassium 
silicate. Especially preferred is potassium silicate supplied by the 
Chemical and Metallurgical Division of GTE Products Corporation under the 
name of PS-6 which is an aqueous solution of potassium silicate having a 
specific gravity of about 1.267 and a purity of at least about 99.99%. 
The water soluble salt is added as a precipitating agent for the silicate 
with the cation of the salt becoming the cation of the silicate which 
coats the phosphor. The preferred cations of the salt are zinc and 
magnesium. The water soluble salts are preferably zinc sulfate, zinc 
chloride, and magnesium sulfate which is preferably in either the 
anhydrous or the heptahydrate form, the criterion for choosing the salt 
being that of economics, convenience, and availability. 
The resulting preferred coatings from the above preferred salts are 
therefore zinc silicate and magnesium silicate. 
The silicon dioxide and the water soluble salt are added in amounts so that 
the silicon dioxide content of the coating makes up in percent by weight 
from about 0.05% to about 1.0% and preferably from about 0.1% to about 
0.3% of the phosphor. If the coating level falls below about 0.05%, 
although there can still be hydrolysis resistance, the flow properties of 
the phosphor powder can be adversely affected. Therefore, in actual 
practice, the coating level is at least about 0.05%. Preferably the water 
soluble salt is added in an amount to give a weight ratio of from about 4 
to 1 of the water soluble salt to the silicon dioxide. It has been found 
that this amount is generally sufficient to give the desired coating. It 
will be obvious to those skilled in the art how to compute these values. 
Some preferred relative amounts of silicon dioxide, water soluble salt, 
and phosphor will become apparent in the examples that ensue. 
The resulting coated phosphor is then separated from the resulting liquor 
by any standard technique such as filtration. 
The silicate coated phosphor is then heated at a sufficient temperature for 
a sufficient time in ambient atmosphere to remove essentially all of the 
water therefrom. Generally temperatures of from about 100.degree. C. to 
about 150.degree. C. and heating time periods of from about 1 hour to 
about 3 hours are sufficient to dry the phosphor. The resulting dried 
silicate coated phosphor is then heated at a sufficient temperature for a 
sufficient time to produce the final hydrolysis resistant phosphor. 
Heating temperatures are generally from about 400.degree. C. to about 
800.degree. C. Heating time periods are generally from about 0.5 hours to 
about 3.0 hours. The heating is done in ambient atmosphere. Heating of 
this type of phosphor under these conditions generally further improves 
the hydrolysis resistance of the silicate coated phosphor. 
The hydrolysis resistant rare earth oxysulfide phosphors of this invention, 
produced by the process of this invention consist essentially of a host 
which can be gadolinium oxysulfide, yttrium oxysulfide, gadolinium-yttrium 
oxysulfide, and lanthanum oxysulfide, from about 0.001 moles to about 0.10 
moles, and preferably from about 0.001 moles to about 0.006 moles of 
terbium per mole of host as an activator, and a sufficient amount of a 
silicate coating to impart hydrolysis resistance to the phosphor. 
The silicate coating can be an alkaline earth metal silicate or a 
transition group IIB metal silicate with zinc and magnesium silicates 
being especially preferred. The preferred hydrolysis resistant phosphors 
of this invention therefore are terbium activated gadolinium oxysulfide 
with a zinc silicate coating and terbium activated gadolinium oxysulfide 
with a magnesium silicate coating. 
The SiO.sub.2 content of the coating makes up generally in percent by 
weight from about 0.05% to about 1.0% and preferably from about 0.1% to 
about 0.3% of the phosphor, the amount of coating being sufficient to 
impart hydrolysis resistance to the phosphor. 
The preferred use of these coated phosphors is as x-ray intensifier 
phosphors. By virtue of their resistance to hydrolysis, the screens which 
they make up can be continually used without noticeable degradation to the 
screen due to hydrolytic breakdown of the phosphor. To more fully 
illustrate this invention, the following non-limiting examples are 
presented. All parts, portions, and percentages are on a weight basis 
unless otherwise stated.

EXAMPLE 1 
About 122.5 parts of terbium activated gadolinium oxysulfide phosphor is 
slurried in about 340 parts of deionized water. About 0.245 parts of 
SiO.sub.2 are added as a potassium silicate solution to give a coating on 
the phosphor which makes up about 0.2% of the phosphor on a SiO.sub.2 
basis. The resulting solution is agitated for about 5 minutes after which 
time about 0.97 parts of MgSO.sub.4.7H.sub.2 O are added. Agitation is 
continued for about an additional 5 minutes. The resulting slurry is then 
filtered and the resultant coated phosphor is dried for about 2 hours at 
about 150.degree. C. The resulting dried coated phosphor powder is then 
transferred to a crucible and heated for about 2 hours at about 
500.degree. C. in ambient atmosphere. The resulting magnesium silicate 
coated phosphor is subjected to the following lead acetate paper test 
procedure. About 10 to about 20 grams of the coated phosphor is placed in 
a twenty milliliter glass vial with a screw cap closure. Several drops of 
water are added to the phosphor and a wetted lead acetate strip is placed 
across the mouth of the vial. The cap is then screwed tightly onto the 
vial and it is allowed to set for about 24 hours before it is opened. Upon 
removal, the lead acetate paper is examined for evidence of disoloration 
caused by the reaction of lead acetate and hydrogen sulfide vapor. There 
is no evident discoloration of the test paper indicating that the above 
coated phosphor is resistant to hydrolysis. 
EXAMPLE 2 
About 115 parts of terbium activated yttrium oxysulfide phosphor is 
slurried in about 320 parts of deionized water. To this slurry is added 
about 0.455 parts of SiO.sub.2 as a potassium silicate solution to give a 
coating on the phosphor which makes up about 0.4% by weight of the coated 
phosphor on a SiO.sub.2 basis and about 1.8 parts of MgSO.sub.4.7H.sub.2 
O. The coated phosphor is obtained as described in Example 1. The 
magnesium silicate coated phosphor when subjected to the above described 
lead acetate paper test shows no discoloration indicating that it is 
hydrolysis resistant. 
EXAMPLE 3 
About 100 parts of terbium activated gadolinium oxysulfide is slurried in 
about 280 parts of deionized water. To this slurry is added about 0.200 
parts of SiO.sub.2 as a potassium silicate solution to give a coating on 
the phosphor which makes up about 0.2% by weight of the phosphor on a 
SiO.sub.2 basis, and about 0.79 parts of anhydrous ZnSO.sub.4. The 
procedure followed is the same as in Example 1. The resultant zinc 
silicate coated phosphor shows no discoloration of the lead acetate paper 
when tested. 
EXAMPLE 4 
A terbium activated gadolinium oxysulfide phosphor is coated with magnesium 
silicate to give varying SiO.sub.2 contents and tested for hydrolysis by 
the lead acetate paper procedure. The visual results are given in Table 1 
along with the results for a SiO.sub.3 coated phosphor for purposes of 
comparison. 
TABLE 1 
______________________________________ 
Lead Acetate Test 
% SiO.sub.2 
Coating Visual Results 
______________________________________ 
0.0 None Moderate Brown 
0.2 Magnesium Silicate 
White 
0.4 " " 
0.6 " " 
0.8 " " 
1.0 " " 
0.0 None Moderate Brown 
0.005 Colloidal Silica 
Very Dark 
0.10 " Dark 
0.20 " " 
0.60 " " 
1.00 " " 
______________________________________ 
Only the magnesium silicate coated material shows no discoloration of the 
test paper indicating no hydrolysis. 
EXAMPLE 5 
A terbium activated yttrium oxysulfide phosphor is coated with magnesium 
silicate to give varying SiO.sub.2 contents and tested for hydrolysis by 
the lead acetate paper procedure. The results are given in Table 2. 
TABLE 2 
______________________________________ 
Lead Acetate Test 
% SiO.sub.2 
Coating Visual Results 
______________________________________ 
0.0 None Very Dark 
0.2 Magnesium Silicate 
White 
0.4 " " 
0.8 " " 
______________________________________ 
The samples which are coated show no noticeable hydrolysis as indicated by 
the white paper, whereas the uncoated sample shows hydrolysis as indicated 
by the dark paper. 
While there has been shown and described what are at present considered the 
preferred embodiments of the invention, it will be obvious to those 
skilled in the art that various changes and modifications may be made 
therein without departing from the scope of the invention as defined by 
the appended claims.