Patent GB-1152693-A (GB)
Title: Improvements in and relating to the Deposition of Particulate Materials
IPC: H01J9/227
Filed: 1966-05-03
Abstract: 1,152,693. Coating; coating apparatus. PHILCO-FORD CORP. 3 May, 1966 [3 May, 1965], No. 19442/66. Headings K, L. [Also in Divisions C4 and H1] Particulate solid material is deposited' on a plurality of discrete selected portions of a substrate by depositing solely over'each of these portions a layer of a composition in which the solid material can be embedded throughout the thickness of the layer by projecting particles of the solid material thereagainst, and projecting particles of the solid material against the layer with sufficient velocity to embed the particles throughout the thickness of the layer. The thickness of the layer is substantially greater than the longest rectilinear dimension of the particles of the solid material. In the manufacture of colour television picture tubes, the transparent glass faceplate of the tube is coated with a solution of polyvinyl alcohol photosensitised with ammonium dichromate, spun dry, the drying being completed with a draft of dry air, all these steps being carried out in yellow light. The polyvinyl alcohol layer is then locally exposed to actinic radiation through the shadow mask destined to be used in the final tube, the exposed parts thereby being rendered insoluble in water, the shadowmask and radiation source being so positioned that rays of say ultraviolet light impinge upon the shadow-mask at angles substantially the same as the angles at which in the completed tube one of the three electron beams will impinge the mask. The polyvinyl alcohol layer is then washed to remove the unexposed regions and to soak the exposed regions using sprays of deionised water, and then spun. Dry phosphor particles emissive of the desired colour. are then projected against the soaked regions from high-pressure air brushes. The selected. regions are then dried by a draft of dry air, and excess phosphor powder is washed from the face plate. The surfaces of the selected regions are further polymerised by wetting with a solution of ammonium dichromate and exposure to ultraviolet radiation. The above described steps are then repeated to deposit phosphor particles emissive of light of a different colour. Suitable phosphor particles emissive of green light are 10 to 15 micron sized particles of silver-activated zinc cadmium sulphide containing 40% cadmium coated with a silicate e.g. a barium silicate complex to make them free flowing; a suitable blue-light emissive phosphor is 10 to 15 microns sized silver-activated zinc cadmium sulphide coated with a silicate; suitable red-light emissive phosphor are 10 to 15 microns sized particles of zinc cadmium sulphide containing 80% cadmium coated with a silicate, 6 to 9 micron sized particles of yttrium vanadate activated with europium. The tube may be completed by lacquering and aluminising the face plate screened with three arrays of phosphor-containing polyvinyl alcohol dots. After the funnel portion is attached, the polyvinyl alcohol and the layer of lacquer are vaporised by pyrolysis. The phosphors may be deposited in other patterns e.g. parallel stripes. The. polyvinyl alcohol layer may be replaced by photosensitised gelatine such as dichromated albumin gelatin, the gelatin being preferably halogen-free. Different amounts of particulate material per unit area may be deposited on different portions of a substrate by using thicker photosensitive layers on which more material is to be deposited. Free flow of the phosphor particles may be obtained by admixture with aluminium oxide particles. Coating apparatus In Fig. 4, apparatus for projecting phosphor powder against surface 25 comprises two airbrushes 34 and 36 producing overlapping sprays of phosphor powder. To assist the gravity feeding of particles into the air-brushes, low pressure compressed air is supplied to cans 38 and 40 filled with powder, through solenoid valve 44, rigid pipe 42 and hoses 46 and 48. To actuate the air-brushes high pressure compressed air is supplied' through solenoid control valve 50. The air-brushes are rigidly secured to metal blocks 58 and 50 which are secured to rigid pipes 42 and 52. The pipes 42 and 52 are secured to a slide member 64 movable within a slot 62 in a support member 60 so that the air-brushes can be moved so as to impinge particles upon the entire area of surface 25. The slide member 64 is moved by a double-action pneumatic cylinder-piston assembly 66 comprising a cylinder 68 and' a piston rod 70 attached to the piston in the cylinder and to the slide member 64. Assembly 66 also comprises two pipes 76 and 78 connected to opposite ends of cylinder 68 and to the two outputs of a solenoid changeover valve 80 supplied with compressed air through pipe 82.