GB801067A - Improvements in or relating to methods of forming luminescent screens coated with thin films of organic material - Google Patents

Abstract

801,067. Luminescent screens. PHILIPS ELECTRICAL INDUSTRIES, Ltd. March 21, 1956 [March 24, 1955], No. 8829/56. Class 39(1). A method of forming a luminescent screen, e.g., for a cathode ray tube or an electron microscope, coated with a thin film of organic material includes the steps of settling luminescent material in an aqueous alkaline settling bed having a pH value greater than 9À5 and containing in solution potassium and/or sodium silicate and at least one alkaline compound, after the luminescent material has settled adding an amount of acidifying agent calculated to reduce the pH value of the settling bed to a value between 5 and 8À5, e.g., about 7, providing on the acidified settling bed a solution of the organic material having a specific gravity lower than that of the settling bed, and thereafter removing the settling bed. The luminescent material may be settled as a suspension in the solution of potassium and/or sodium silicate. The alkaline compounds are an alkali metal compound, an alkaline-earth metal compound or an ammonium salt, e.g., potassium sulphate, barium nitrate, calcium acetate or ammonium sulphate. The pH value is reduced to a value between 5 and 8À5 by adding to the settling bed an acid, e.g., phosphoric acid, or acid reacting salts, e.g., sodium di-hydrogen phosphate. The organic film materials may be nitro-cellulose or esters of polymetacrylic acid. Solvents for these materials may be butyl acetate, ethyl acetate or mixtures thereof. Softening agents may be added, e.g., dibutylphthalate. Aluminium may be provided by evaporation on the organic film which may subsequently be removed by heating. A portion of the settling bed can be removed after the settling of the luminescent material, whereafter the remaining portion is acidified until its pH value is reduced to between 5 and 8À5. The solution of the organic material can contain more organic material than is necessary for forming the film and after this solution has been provided the settling bed and the solution of organic material floating thereon are decanted in such manner that the solution of organic material passes along the luminescent screen leaving thereon a thin film. of the solution from which the solvent is subsequently evaporated. Alternatively the solution of the organic material can contain exactly the amount of organic material required for forming the film, the solvent being evaporated after the solution has been provided on the settling bed, so that a solid film of organic material floating on the settling bed is formed, whereafter the settling bed is removed in such manner that the film is deposited on the luminescent screen.