Method of Treating Europium Activated Strontium Tetraborate Phosphor

This invention concerns europium activated strontium tetraborate UV emitting phosphors. Such phosphors are disclosed in U.S. Pat. No. 4,719,033.

In the process of making europium activated strontium tetraborate phosphor, SrB.sub.4 O.sub.7 :Eu, it occasionally happens that emission characteristics of particular phosphor lots are unsatisfactory. This invention is concerned with the treatment of such unsatisfactory phosphors in order to improve emission output.

In accordance with this invention, the phosphor is blended with strontium carbonate or strontium fluoride or ammonium fluoride and is then fired at an elevated temperature in a reducing atmosphere.

It is believed that the reason for poor emission characteristics in the unsatisfactory phosphors is the presence of a second phase of SrB.sub.6 O.sub.10. X ray diffraction analysis shows the presence Of such a phase in some phosphors having especially poor emission characteristics. It is believed that, because europium is soluble in the SrB.sub.6 O.sub.10 phase, the europium concentration in the SrB.sub.4 O.sub.7 host is reduced and therefore its UV emission is reduced. Treating the phosphor with SrCO.sub.3 or SrF.sub.2 or NH.sub.4 F converts the SrB.sub.6 O.sub.10 phase to SrB.sub.4 O.sub.7 and improves emission.

The firing can be done at a temperature of about 1500.degree. to 1800.degree. F. The reducing atmosphere can be hydrogen-nitrogen. When the reactant is SrCO.sub.3 or SrF.sub.2, at least about 0.4 mole percent should be used in order to improve emission characteristics. When the reactant is NH.sub.4 F, at least about 5 mole percent should be used.

In a specific example, a SrB.sub.4 O.sub.7 :Eu phosphor, lot number 052-54, had a 100 hour emission in fluorescent lamps of only 94.7% compared with a control.

971.44 grams of SrB.sub.4 O.sub.7 :Eu from lot 052-54 was blended with 9.45 grams of SrCO.sub.3 (1.6 mole %). The blend was fired at 1675.degree. F. for seven hours in 4%, hydrogen-96% nitrogen. The fired cake was broken up and screened. The 100 hour emission of this reacted phosphor had increased to 118.2% compared with the control.

971.44 grams of SrB.sub.4 O.sub.7 :Eu from lot 052-54 was blended with 8.20 grams of SrF.sub.2 (1.6 mole %). The blend was fired at 1675.degree. F. for seven hours in 4% hydrogen-96% nitrogen. The fired cake was broken up and screened. The 100 hour emission of this reacted phosphor had increased to 118.5% compared with the control.

971.44 grams of SrB.sub.4 O.sub.7 :Eu from lot 052-54 was blended with 18.96 grams of NH.sub.4 F (12.8 mole %). The blend was fired at 1675.degree. F. for seven hours in 4% hydrogen-96% nitrogen. The fired cake was broken up and screened. The 100 hour emission of this reacted phosphor had increased to 115.9% compared with the control.

In other examples, reacting lot 052-54 phosphor with 0.4 mole % SrCO.sub.3 increased 100 hour emission to 103%. Reacting lot 052-54 phosphor with 0.4 mole % SrF.sub.2 increased 100 hour emission to 104.9%.