A method of making a niobium-activated yttrium tantalate X-ray phosphor with improved brightness

Abstract: A niobium-activated yttrium tantalate x-ray phosphor which incorporates small molar amounts of a rubidium or cesium halide, a strontium or barium halide, and, optionally, a gallium or aluminum oxide, to effect improved brightness.

Description: The invention relates to the manufacture of niobium-activated yttrium tantalate x-ray phosphors. Such phosphors are disclosed in U.S. Patents 4,225,653 and 4,387,141. In both patents, the phosphor composition has the formula YNb x Ta 1-x O₄, where x is 0 to about 0.15. It has been discovered that phosphor brightness can be improved by the addition of small molar amounts of a halide of rubidium, cesium, strontium, or barium, and optionally, an oxide of gallium or aluminum. It is an object of the invention to provide a method of making a niobium-activated yttrium tantalate x-ray phosphor with improved brightness. This object is accomplished by the inventive method according to claim 1. The following non-limiting examples are presented. Example 1 (Control) Stoichiometric amounts of yttrium oxide, Y₂O₃, tantalum oxide, Ta₂O₅, and niobium oxide, Nb₂O₅, were weighed and blended together to form a phosphor composition of YTa 0.995 Nb 0.005 O₄. The molar concentration of niobium activator was 0.005 mole. A flux of lithium chloride in an amount of 33 weight percent was added to the mixture and blended using standard blending procedures. The blend was loaded into an alumina crucible and fired in an electric furnace for about 10 to 12 hours at about 1200-1300°C (2192-2372°F). The fired material was cooled in the furnace and the flux material leached out with deionized water washings. The flux-free material was then filtered, dried and classified. The sample was then measured for luminescence efficiency under x-ray excitation. This sample was the control for the following examples and the brightness value was normalized at 100. Example 2 The procedure of Example 1 was followed using a eutectic flux mixture of lithium chloride and lithium sulfate, (LiCl)₂-Li₂SO₄ (46.5 : 53.5 mole percent, respectively), in an amount of 33 weight percent of the phosphor raw material. The eutectic temperature of this flux composition is about 478°C (892°F). This sample was 8% brighter than the control. Example 3 The procedure of Example 1 was followed, with the addition of 0.01 mole of strontium fluoride, 0.001 mole of rubidium chloride, and 0.01 mole of gallium oxide as dopants. A lithium chloride-lithium sulfate eutectic flux mixture, 33 weight percent, was used. This sample was 22% brighter than the control. Example 4 The procedure of Example 3 was repeated to determine reproducibility of the results. This sample was 20% brighter than the control. Example 5 The procedure of Example 3 was used, except that instead of 0.01 mole of gallium oxide, 0.01 mole of aluminum oxide was used. This sample was 16% brighter than the control. Example 6 The procedure of Example 5 was repeated to determine the reproducibility of the results. This sample was 18% brighter than the control. Example 7 The procedure of Example 3 was used, except that no gallium oxide was used. This sample was 12% brighter than the control. X-ray optical fluorescence (XOF) brightness data for the examples are given in Table I. YTaO₄:Nb PHOSPHOR BRIGHTNESS DATA NO. FLUX DOPANT CONCENTRATION. MOLES XOF BRIGHTNESS SrF₂ RbCl Ga₂O₃ Al₂O₃ 1 LiCl -- -- -- -- 100 2 (LiCl)₂-Li₂SO₄ -- -- -- -- 108 3 (LiCl)₂-Li₂SO₄ 0.01 0.001 0.01 -- 122 4 (LiCl)₂-Li₂SO₄ 0.01 0.001 0.01 -- 120 5 (LiCl)₂-Li₂SO₄ 0.01 0.001 -- 0.01 116 6 (LiCl)₂-Li₂SO₄ 0.01 0.001 -- 0.01 118 7 (LiCl)₂-Li₂SO₄ 0.01 0.001 -- -- 112 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.

Claims: A method of making a niobium-activated yttrium-tantalate x-ray phosphor, comprising the steps of: forming a uniform mixture of the following ingredients in the following proportions: one mole of yttrium oxide, 0.995 mole of tantalum oxide, 0.005 mole of niobium oxide, from 0.001 mole to 0.01 mole of cesium halide or rubidium halide, from 0.01 mole to 0.10 mole of strontium halide or barium halide, and optionally, from 0.01 mole to 0.04 mole of gallium oxide or aluminum oxide; blending said mixture with a lithium chloride-lithium sulfate eutectic flux composition; firing said mixture at about 1200-1300°C (2192-2372°F) for about 10-12 hours to form said phosphor; washing said phosphor in deionised water to remove said flux; and filtering, drying, and classifying said phosphor. The method of claim 1, wherein said halide is selected from the group consisting of chloride, fluoride, and bromide. The method of claim 1, wherein said eutectic flux composition comprises about 33 weight percent of said mixture. The method of claim 1, wherein said phosphor comprises about 0.005 mole of niobium, about 0.01 mole of strontium fluoride, about 0.001 mole of rubidium chloride, and about 0.01 mole of gallium oxide.

Inventors: REDDY BUTCHI V, REDDY, BUTCHI V.
Applicants: GTE PROD CORP, GTE PRODUCTS CORPORATION
Application year: 1990
Main classification: C09K11
IPC codes: C09K11