GB646533A - Improvements in and relating to ultra-violet radiation emitting phosphors - Google Patents

Abstract

646,533. Luminescent materials. BRITISH THOMSON-HOUSTON CO., Ltd. April 14, 1948, No. 10366. Convention date, April 15, 1947. [Class 39 (i)] A phosphor emitting ultra-violet radiations of longer wave length when excited by ultraviolet radiations of shorter wave length, par. ticularly for use in low pressure mercury discharge lamps, comprises magnesium phosphate of approximately pyrophosphate composition activated by both cerium and thorium. The thorium may be present in quantities varying from 0.001 to 15 per cent or more of ThO 2 and the cerium in quantities varying from 0.001 to 25 per cent of Ce 2 O 3 , preferably about 12 per cent ThO 2 and 13 per cent Ce 2 O 3 . The phosphor may be prepared by cold co-precipitation of magnesium, cerium and thorium phosphates as MgNH 4 PO 4 plus activators, followed by firing at about 900-1200‹ C. in steam and hydrogen to decompose the ammonium containing compounds with formation of activated Mg 2 P 2 O 7 . In another method 510 grs. Mg(NO 3 ) 2 .6H 2 O, 118 grs. Ce(NO 3 ) 3 .6H 2 O and 90 grs. Th(NO 3 ) 4 .4H 2 O are dissolved in 2 litres of distilled water and under vigorous agitation a cold solution of 400 grs. of (NH 4 ) 2 HOP 4 in 2 litres of distilled water is added followed by about 400 c.cs. of concentrated ammonia. After standing the precipitate is filtered, washed, dried and then fired in an atmosphere of steam and hydrogen for 1 hour at between 900-1200‹ C., preferably 1050‹ C. The material is then cooled in hydrogen and then sieved. Silica quantities of from 0.001 to 10 per cent, preferably 1 to 5 per cent, can be added to the phosphor before firing, either by adding finely divided silicic acid powder to the solution of nitrate before the phosphates are precipitated or by adding ethyl silicate, dissolved in alcohol, to the nitrate solution. After firing the silica may remain free, it may form some magnesium silicate or it may form a double compound with the phosphate. The Specification refers to cerium activated magnesium pyrophosphate and to ceriumthorium activated magnesium silicate. Specifications 550,744, 631,415 and 633,891 are referred to. The Specification as open to inspection under Sect. 91 refers to the use of bismuth, lead and tin as activators alone and in combination with thorium. This subject-matter does not appear in the Specification as accepted.