Method of Making Europium Activated Yttrium Oxide Phosphor

This invention concerns europium activated yttrium oxide phosphors. The manufacture of such phosphors is disclosed in the following U.S. Pat. Nos.: 3,322,682; 3,449,258; 3,574,131; 3,582,493; 3,635,658; 4,032,471; 4,191,662. In these patents, yttrium and europium compounds are dissolved in nitric or hydrochloric acid, and are then co-precipitated as the oxalates or sulfates. Or, as disclosed in U.S. Pat. No. 3,759,835, yttrium oxide and europium oxide powders are blended with suitable dopants, with subsequent processing steps to form the phosphor.

This invention discloses a process which does not involve co-precipitation or the blending of yttrium oxide and europium oxide powders. In this invention, europium oxide is dissolved in nitric acid. Ammonium hydroxide is then added to precipitate europium hydroxide, which is then dissolved in citric acid monohydrate to form europium citrate solution. Yttrium oxide powder is then dispersed in the europium citrate solution to form a slurry which is then spray dried. The spray dried powder is then fired at a high temperature to form europium activated yttrium oxide phosphor. This phosphor showed improved maintenance in fluorescent lamps.

In a specific example, 400 ml of HNO.sub.3 was added to 133.78 grams of Eu.sub.2 O.sub.3 in 2000 ml of water to form Eu(NO.sub.3).sub.3. NH.sub.4 OH was added to precipitate Eu(OH).sub.3. The Eu(OH).sub.3 was dissolved in 2000 ml of warm water and 350 grams of citric acid monohydrate to form europium citrate solution, which was then adjusted to a pH of 6.0 with NH.sub.4 OH. 2174 grams of Y.sub.2 O.sub.3 powder were added to the europium citrate solution, along with water, to form a slurry. The slurry was then spray dried at an inlet temperature of 310.degree. C. and an outlet temperature of 115.degree. C. The spray dried powder was first fired to an oxide at 1000.degree. C. for two hours in an air atmosphere furnace, and later second fired at 1600.degree. C. for four hours in air using alumina crucibles. The fired powder was then lightly mortared, washed three times with water, filtered, dried, and sieved using 400 mesh screens. The thus-produced phosphor was compared in fluorescent lamps with a prior art co-precipitated phosphor. The respective maintenance results at 100 hours, 550 hours and 1000 hours were: 103.2% versus 97.8%; 103.9% versus 94.9%; 102.9% versus 94.1%.