Method of Making Terbium Activated Yttrium Aluminate Phosphor

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.

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.

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.

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 (>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 (>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.

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.

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.