The invention relates to a phosphor composition of a sulphide-containing phosphor and a coating, in particular a phosphor composition of an oxysulphide phosphor.
Oxysulphides of yttrium, lanthanum, gadolinium and the like are used as the starting material for luminescent materials, such as in particular phosphors for cathode ray tubes. Particularly europium-activated yttrium oxysulphide is used on a large scale for the manufacture of color display screens. In the customary methods of manufacturing color display screens, the phosphor is processed either as a light-sensitive, aqueous powder suspension (flow-coating process) or as a flowable powder or a solid powder which can be applied by dusting (dusting process). In these processes, it is very important that the phosphor powder contains only individual desagglomerated phosphor particles and is free of agglomerates because agglomerates adversely affect the quality of the display screen. Therefore, only if a phosphor can be completely desagglomerated is it fit to be processed. In customary desagglomeration processes, the raw, still agglomerated phosphor is subjected to a wet-grinding operation in an ammoniacal solution. This results in an ammoniacal suspension of the phosphor which, when it is stirred constantly, can resist reagglomeration for many hours. In this desagglomerated state, customarily also the surface of the phosphor powder is subjected to refining treatments, such as latex coating, pigmentation etc. If an acid phosphor suspension is necessary for the further manufacture of the display screen, then this can be prepared by carefully acidifying the ammoniacal phosphor suspension, while stirring continually. However, the acid phosphor suspension is much more unstable relative to reagglomeration than the ammoniacal suspension and keeps only for a very short time.
However, the phosphor powder desagglomerated by wet-grinding in an ammoniacal medium cannot be isolated and stored. Once the powder suspension has sedimented, desagglomeration requires a new wet-grinding operation. As a result, the preparation of storable, desagglomerated phosphor powder, which can be resuspended without having to be subjected to another wet-grinding operation, requires a suitable surface conditioning. In known methods, this is achieved by coating the phosphor particles with fine inorganic particles, such as silicon dioxide, aluminum oxide, titanium dioxide, iron oxide and the like, and/or organic polymers, such as acrylates, which preclude reagglomeration. However, these inorganic substances lead to a reduction of the luminescent efficiency of the phosphor. In the manufacturing process for the display screen, the organic coating substances are often incompletely pyrolyzed, thus causing greying of the finished phosphor layer and a reduction of the light efficiency of the phosphor screen.