Method of making terbium activated yttrium aluminate phosphor

A Y.sub.3 Al.sub.5 O.sub.12 :Tb and method for producing the phosphor are disclosed. The method invloves forming a uniform mixture of source materials for Y.sub.3 Al.sub.5 O.sub.12 :Tb phosphor and barium fluoride with the barium fluoride being present in the mixture at a level of from about 0.254 to about 7.61 weight percent, and firing the mixture at an elevated temperature to react the source material to form the phosphor. The phosphor has an improved brightness of at least about 75% over a phosphor of the same type produced without the barium fluorides.

This invention relates to a method of making Y.sub.3 Al.sub.5 O.sub.12 :Tb 
phosphors in which barium fluoride in controlled concentrations is used in 
the initial mixture. This results in improved luminescence and in the 
desired small particle size and narrow particle size distribution. 
BACKGROUND OF THE INVENTION 
The phosphor Y.sub.3 Al.sub.5 O.sub.12 :Tb exhibits green emission under 
electron excitation and is used in special CRT applications as heads up 
displays and projection TV systems. The phosphor is useful in that it 
exhibits linear burn resistance, more linear than Y.sub.3 (Al,Ga).sub.5 
O.sub.12 :Tb and thus is utilized under high beam current densities. The 
phosphor is not as efficient in luminescence output as Y.sub.3 
(Al,Ga).sub.5 O.sub.12 :Tb, therefore is not significantly utilized in 
these applications. 
A method by which Y.sub.3 Al.sub.5 O.sub.12 :Tb can be made more efficient 
would be desirable. 
U.S. Pat. No. 4,479,886 relates to making Y.sub.3 Al.sub.5 O.sub.12 :Ce 
phosphor in which barium fluoride is added to the finished phosphor to 
increase the efficiency of converting UV radiation to visible radiation. 
U.S. Pat. Nos. 4,070,301 and 4,141,855 disclose the addition of a barium 
compound to a cerium activated phosphor prepared from yttria and alumina 
to enhance the intensity of UV emission of the YAlO.sub.3 :Ce. 
A publication entitled "Effect of BaF.sub.2 on the Synthesis of the Single 
Phase Cubic Y.sub.3 Al.sub.5 O.sub.12 :Tb", by K. Ohne and T. Abe, J. 
Electrochem. Soc., Vol. 133, No. 3 (March 1986) discusses a high 
concentration of BaF.sub.2 (20%) on Y.sub.3 Al.sub.5 O.sub.12 :Tb phosphor 
but does not consider particle size. 
SUMMARY OF THE INVENTION 
In accordance one aspect of this invention, there is provided a method for 
producing a Y.sub.3 O.sub.5 O.sub.12 :Tb phosphor which involves forming a 
uniform mixture of source materials for Y.sub.3 Al.sub.5 O.sub.12 :Tb 
phosphor and barium fluoride with the barium fluoride being present in the 
mixture at a level of from about 0.254 to about 7.61 weight percent, and 
firing the mixture at an elevated temperature to react the source 
materials to form the phosphor. The phorphor has an improved brightness of 
at least about 7.5% over a phosphor of the same type produced without the 
barium fluoride. 
In accordance with another aspect of this invention, there is provided the 
phosphor produced by the above described method.

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. 
The phosphor of the present invention is a cathode ray phoshor having the 
formula Y.sub.3 Al.sub.5 O.sub.12 :Tb. The approximate content of the 
BaO:6.6Al.sub.2 O.sub.3 phase is from about 0.4 to about 5 weight percent. 
In the method for making this phosphor, the first step is forming a uniform 
mixture of source materials for Y.sub.3 Al.sub.5 O.sub.12 :Tb phosphor and 
barium fluoride. These source materials are typically yttrium oxide, 
aluminum oxide, and terbium oxide. Relative amounts of these materials 
will be apparent in the Example that follows. The amount of barium 
fluoride in the mixture is from about 0.254 to about 7.61 weight percent. 
This amount of barium fluoride results in an increase in luminescence of 
at least about 7.5% over that of the phosphor produced without barium 
fluoride. The preferred amount of barium fluoride is from about 0.254 to 
about .761 weight percent of the mixture. This preferred amount of barium 
fluoride results in increasing the luminescence by at least about 7.5% 
with a slightly reduced dispersed weight due to an improved particle size 
and distribution. 
The mixture is formed by conventional methods for mixing and blending dry 
solids. 
The blended mixture is fired at an elevated temperature to react the source 
materials to form the Y.sub.3 Al.sub.5 O.sub.12 :Tb phosphor. The firing 
is done usually at about 1600.degree. C. for about 6 hours in air. The 
barium fluoride acts as a flux. 
The resulting phosphor cake can then be classified if desired, to remove 
the out of size material. This is done most typically by screening the 
phosphor through a 200 mesh screen. 
The luminescence characteristics after CRT excitation of a logarithmic 
BaF.sub.2 addition are shown in Table 1. The addition of barium fluoride 
can increase the relative brightness of the phosphor by as much as about 
12.8% with a three-fold increase in dispersed weight. The preferred level 
of barium fluoride is from about 0.254% to about 0.761% by weight of the 
mixture. It has been found that these levels result in from about a 7.5% 
to about 11.2% increase in the brightness of the phosphor with a slight 
decrease in dispersed weight as opposed to a phosphor produced without 
barium fluoride. 
The particle size characteristics as measured by Coulter Counter on the 
logarithmic BaF.sub.2 addition are shown in Table 2. At the preferred 
levels of BaF.sub.2 (0.254 to 0.761 weight percent), the particle size 
(50%) is reduced by about 1 micrometer to the 5 to 7 micrometer range and 
the particle size distribution is improved as exhibited by a narrower QD 
of 0.31 and by reducing the coarse fraction (&gt;about 20 micrometers) to 
1.0-6.0% range. This improved particle size and distribution results in a 
reduced dispersed weight by about 0.4 mg/cm.sup.2 as shown in Table 1. 
Higher additions of BaF.sub.2 from about 2.54 to about 7.61 weight percent 
result in larger particle sizes (50%) and more coarse fractions (&gt;20 
micrometers) although the overall distribution (QD) is further improved. 
These larger particle sizes are reflected in higher dispersed weights. 
Further characterization by x-ray diffraction of the phosphor of this 
invention is exhibited in Table 3. From these analyses it can be seen that 
addition of barium fluoride decreases the formation of YAlO.sub.3 and 
Al.sub.2 O.sub.3 phases and slightly increases the formation of 
BaO:6.6Al.sub.2 O.sub.3 phase. The x-ray diffraction intensity at 2.69 A 
from the Y.sub.3 Al.sub.5 O.sub.12 phase (the luminescent material) is 
increased up to about 0.761% by weight BaF.sub.2 with severe drop off at 
higher concentrations. This is indicative of the improved crystallinity of 
the luminescent phase. 
To more fully illustrate this invention, the following nonlimiting example 
is presented. All parts, portions, and percentages are on a weight basis 
unless otherwise stated. Example 
A mixture of about 541.94 g Y.sub.2 O.sub.3, about 795.92 g Al(OH).sub.3 
and about 44.86 g Tb.sub.4 O.sub.7 is intimately blended. To about 23.05 g 
of this mixture is added various amounts of BaF.sub.2 as listed below and 
the material is further blended. 
______________________________________ 
Weight Weight 
Dry Blend BaF.sub.2 
Wt. % 
Test grams grams BaF.sub.2 
______________________________________ 
1 23.05 0 0 
2 23.05 0.0585 0.254 
3 23.05 0.1753 0.761 
4 23.05 0.5845 2.54 
5 23.05 1.753 7.61 
______________________________________ 
This blend is then fired in a programmed furnace at about 1600.degree. C. 
for about 6 hours in air. This firing is done by ramping up at about 
367.degree. C./hour to about 1600.degree. C., and held at about that 
temperature for about 6 hours, and the furnace is then cooled. The 
resulting material is broken up and sieved through a 200 mesh screen. 
These materials result in CRT luminescence, particle size. and x-ray 
diffraction characteristics as listed in Tables 1, 2, and 3 respectively. 
TABLE 1 
______________________________________ 
CRT Luminescence Characteristics of Y.sub.3 Al.sub.5 O.sub.12 :Tb 
Weight Color 
Sam- Dispersed Trans- 
Relative 
Coordinates 
ple % BaF.sub.2 
(mg/cm.sup.2) 
mission 
Brightness 
x y 
______________________________________ 
1 0 3.2 20 75.8 0.344 
0.519 
2 0.254 2.8 20 83.3 0.348 
0.531 
3 0.761 2.9 18 87.0 0.353 
0.544 
4 2.54 10.5 22 88.6 0.355 
0.551 
5 7.61 10.5 20 88.6 0.355 
0.551 
6* 0 3.2 20 100 0.347 
0.535 
______________________________________ 
*Y.sub.3 (AlGa).sub.5 O.sub.12 :Tb 
TABLE 2 
______________________________________ 
Y.sub.3 Al.sub.5 O.sub.12 :Tb 
Particle Size Characteristics 
Coulter Counter 
100 micrometers 
Sonic 
Sample % BaF.sub.2 
50% Q.D. % &gt;20 micrometers 
______________________________________ 
1 0 7.0 0.48 11.9 
2 0.254 5.6 0.35 6.0 
3 0.761 6.1 0.31 1.0 
4 2.54 19.0 0.26 47.2 
5 7.61 13.4 0.25 12.1 
______________________________________ 
TABLE 3 
______________________________________ 
Y.sub.3 Al.sub.5 O.sub.12 :Tb 
X-ray Diffraction Characteristics 
Peak Height Intensities in " counts/sec" 
Y.sub.3 Al.sub.5 O.sub.12 
YAlO.sub.3 
Al.sub.2 O.sub.3 
BaO:6, 6Al.sub.2 O.sub.3 
Sample 
% BaF.sub.2 
2.69.ANG. 
2.62.ANG. 
2.55.ANG. 
2.51.ANG. 
______________________________________ 
1 0 4182 732 314 -- 
2 0.254 5445 15 213 23 
3 0.761 5762 -- 75 49 
4 2.54 3180 -- 65 85 
5 7.61 3237 -- -- 155 
______________________________________ 
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.