GB889027A - Photoconductive pick-up tube and method of manufacture - Google Patents

Abstract

889,027. Television pick-up tubes. RADIO CORPORATION OF AMERICA. Feb. 26, 1958 [March 25, 1957], No. 6284/58. Class 39(1) In a television pick-up tube having a photoconductive target 28 scanned by an electron beam, the optically flat polished face plate 31 of the tube is sealed to the main envelope portion 12 by means of an interposed ring of indium 42, the seal being made solely by the application of axial pressure without heating at all or at least without heating sufficient to melt the parts. The indium ring 42 has opposite 45 degrees bevels so as to have a triangular cross-section and is centrifugally cast when in a molten state on to the inside of a brass or stainless steel ring 40 and then trimmed to form the bevels. If 40 is of stainless steel it is copper plated before the indium is deposited. The axial pressure of 600 1bs./sq. inch (e.g. 170 lbs. for the envelope cross-sectional solid area) applied while the parts are held in a jig squeezee in each side of the indium ring to form a T-shaped cross-section as shown in Fig. 2(d) the leg of the T forming an electrical contact for the conductive transparent underlayer 33 of tin oxide or tin chloride. The envelope shell end is ground flat, polished and cleanedbefore the seal is made to the face plate. The face plate is of glass or ceramic formed as an optically flat window by grinding and polishing followed by cleaning with chromic acid, rinsing with distilled water and baking to remove grease. The signal layer 33 is coated so as not to reach the periphery of the face plate and the photoconductive layer of uniform thickness 32 covers part of the area of the signal layer and no area outside it. The precise areas of layers 32, 33 are controlled by apertured masks. The signal layer and photoconductive layer are deposited before the face plate is sealed to the envelope. The indium seal must be formed at a temperature of less than 155‹C. (the melting point of indium) and if the photoconductive layer is of antimony trisulphide, of less than 150 degrees to avoid harming the latter. The photoconductive layer is made by evaporation in an argon atmosphere at a pressure of 1 mm. Hg. to cause the layer to be porous. During subsequent processing of the tube the indium seal and photoconductive layer must be kept cool e.g. by blowing air on to the face plate. Alternatively the gun is sealed in first, then the indium seal is made, then the final processing is carried out. The focusing and deflection means are magnetic. The electron beam is accelerated by three tubular electrodes and scans the photoconductive screen through a fine mesh screen. An exhaust tube is built into the gun end of the tube. The indium ring may be a lead for the transparent tin oxide or tin chloride layer.