Preparation of Cathode-Ray Phosphors

Much of what has been said in Sect. 6.3 on lamp phosphors, can be repeated here. Since fast electrons excite the host lattice and create mobile electrons and holes, the restrictions to the impurity level are usually even more severe. This is certainly the case for the frequently applied sulfides. In principle zinc sulfides are made by dissolving ZnO in sulfuric acid yielding an aqueous solution of ZnS04. Subsequently H2S is bubbled through the solution, converting ZnS04 into insoluble ZnS. The shape, size and crystal quality of this precipitate depends on the reaction conditions (pH, temperature, concentration). 

The raw material is then fired with a flux and the activator, and the product is sieved. Finally the flux is removed and the product milled. Suitable processes may be applied to obtain particles of the described size ( 5 jitm). 

Several methods are in use to fabricate the luminescent screen in the cathode-ray tube. Fbr conventional applications the screen thickness is IS-30 /zm, i.e. 2-4 particles deep, resulting in a weight of 3-7 mg/cm. The setting method has been used since the beginning of tube manufacturing and is still in use for monochrome 

Scanning election niticroscope photograph of panicles of the Y202S Eu phosphor, (reproduced with permission from Rch [I]) 

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