Thorn Electrical Industries Limited

Doreen Yvonne Hobbs

Peter Whitten Ranby

Phosphors having useful flourescent emission properties comprise aluminates of magnesium, zinc and/or lithium having an hexagonal or so-called '.beta.-alumina' crystal structure and activated by cerium and terbium. In preferred phosphors the ratio of Mg:Al is from 0.01 to 0.5:1, Ce:Al from 0.001 to 0.1:1 and Tb:Al from 0.005:1 to 0.25:1. Where zinc or lithium wholly or partially replace the magnesium, chemically equivalent amounts may be used. A particuarly useful phosphor conforms substantially to the approximate formula (Ce,Tb).sub.1 Mg.sub.1-2 Al.sub.22 O.sub.x, where x is the number of atoms of oxygen required to satisfy the valencies of the metals. The invention also provides a valuable process for the preparation of such aluminate phosphors, giving a product of fine grain which may not require milling or other severe grinding (which is expensive and tends to harm emission properties) to bring it into the preferred particle size range for lamp manufacture. In this process the source of alumina is aluminium ammonium sulphate (ammonium alum) and this is mixed with oxides of the other constituent metals (or decomposable compounds thereof) and fired at temperatures above 1200.degree. C and preferably at 1450.degree. C and even higher. An increase in brightness may be obtained by a final heating in hydrogen in the region of 1050.degree. C.

Method of preparing cerium and terbium activated aluminate phosphors

Phosphors having useful fluorescent emission properties comprise aluminates of magnesium, zinc and/or lithium having an hexagonal or so-called '.beta.-alumina' crystal structure and activated by cerium and terbium. In preferred phosphors the ratio of Mg:Al is from 0.01 to 0.5:1, Ce:Al from 0.001 to 0.1:1 and Tb:Al from 0.005:1 to 0.25:1. Where zinc or lithium wholly or partially replace the magnesium, chemically equivalent amounts may be used. A particularly useful phosphor conforms substantially to the approximate formula (Ce,Tb).sub.1 Mg.sub.1.sub.-2 Al.sub.22 O.sub.x, where x is the number of atoms of oxygen required to satisfy the valencies of the metals. The invention also provides a valuable process for the preparation of such aluminate phosphors, giving a product of fine grain which may not require milling or other severe grinding (which is expensive and tends to harm emission properties) to bring it into the preferred particle size range for lamp manufacture. In this process the source of alumina is aluminium ammonium sulphate (ammonium alum) and this is mixed with oxides of the other constituent metals (or decomposable compounds thereof) and fired at temperatures above 1200.degree. C and preferably at 1450.degree. C and even higher. An increase in brightness may be obtained by a final heating in hydrogen in the region of 1050.degree. C.

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Doreen Yvonne Hobbs