Patent GB-722082-A (GB)
Title: Improvements in or relating to fluorescent screens
IPC: H01J29/28
Filed: 1952-11-27
Abstract: 722,082. Luminescent screens. WESTING- HOUSE ELECTRIC INTERNATIONAL CO. Nov. 27, 1952 [Dec. 5, 1951]. No. 30074/52. Class 39 (1). [Also in Group XXIII]. A method of making a fluorescent screen comprises forming a layer of fluorescent material and alkali silicate binder on a glass backing plate, depositing a very thin layer of powdered glass having a melting point below 650‹C. on the first layer, and heating the resulting structure to a temperature not exceeding 650‹C. in an inert atmosphere such as nitrogen to cause the powdered glass to melt and flow to a smooth layer. The phosphor must be inorganic and capable of withstanding a temperature of 650‹C. for up to an hour without substantial deterioration. As shown, a soda lime glass plate 1 in the shape of a spherical segment is coated on one face with zinc sulphide 2 by placing the plate concave side up in a water solution of potassium silicate and sprinkling the phosphor powder on to the solution. A wetting agent may be added to prevent the phosphor powder from floating on the surface. Alternatively the phosphor powder may be suspended in additional silicate solution which is then added to the main solution. The solution contains 3-12 per cent, preferably 6 per cent of potassium silicate by weight the ratio of silica to potassium oxide being 2¢ : 1. Other phosphors such as cadmium tungstate, calcium tungstate and barium lead sulphate and other alkali silicate binders such as sodium silicate can be used. The phosphor particle size is 10-40 microns. After the settling, the silicate solution is removed by draining or siphoning and the phosphor layer dried slowly and then heated to 400‹C. for a few minutes to remove the moisture from the binder. Glass powder 3 of low melting point is then deposited as a thin layer on the phosphor layer 2. The glass powder comprises B 2 O 3  60-70 per cent, preferably 67 per cent, SiO 2  15-25 per cent, preferably 20 per cent, Al 2 O 3  1-4 per cent, preferably 2 per cent, MgO 1-3 per cent, preferably 2 per cent, Na,O, K 2 O and Li 2 O 7-15 per cent in the ratio of about 2 : 1 : 1 preferably 5 per cent Na 2 O, 2 per cent K 2 O, 2 per cent Li 2 O. The glass powder may be suspended in carbon tetrachloride, trichloroethylene or ether and sprinkled or flowed on to the phosphor layer; this is preferable to dry sprinkling as the deposited powder is more closely packed and further smoothing of the surface is unnecessary. The screen is now placed on an iron mould 4 (preheated to 220‹C.) and the assembly heated slowly in an oven to 650‹C. and held there for one hour in the presence of nitrogen. As a result the surface glass melts and flows to make a smooth layer. On removal from the oven this fused glass layer is sprayed with a solution of tin chloride comprising equal parts of tin chloride, acetic acid and methanol. A layer of photoemissive material such as cesiated antimony is then deposited on the tin oxide layer. The glass powder may alternatively be such as to flow at 600‹C. so as to reduce the deterioration of the phosphor and may in general have a melting point of 400-650‹C. The glass plate 1 may originally be a flat disc which when heated on iron mould 4 sags to the shape of the mould. The screen is preferably used in X-ray image intensifiers or other exciting radiations such as X-rays may be used. The photo emissive and/or tin oxide layers may however be omitted. A prior art screen is referred to in which zinc sulphide particles are mixed with glass particles and applied to a glass plate, a photoemissive layer of cesiated antimony being deposited on the particle layer: Specifications 652,451 and 719,762 are referred to.