Patent GB-696056-A (GB)
Title: Improvements in and relating to methods of electrode activation
IPC: H01J9/12
Filed: 1947-11-19
Abstract: 696,056. Cathode materials and processing. BRITISH THOMSON-HOUSTON CO., Ltd. Nov. 19, 1947 [Jan. 28, 1944], No. 30708/47. Class 39(i) [Also in Group XI] In the activation of a cathode of a discharge device such as a photoelectric cell, by alkali metal contained in a capsule arranged in the exhaust tube, the alkali metal is vaporized and at least part is deposited on the inner surface of the exhaust tube and the deposit is subsequently again vaporized, preferably after sealing off the exhaust tube long, so that the metal vapour is driven to the cathode surface which is heated to reaction temperature, and the alkali metal is in excess so that the additional amount serves subsequently as a getter. In one form, Fig. 2, a metal capsule 18a containing a pellet 19 of caesium producing reaction mixture is placed in exhaust tube 15. The device is baked and exhausted in an oven at a temperature of 300-340‹C, the tube 15 is cooled by air jets from one or more nozzles 23, the charge 19 is heated by a high frequency coil 24 to flash it and to deposit caesium at 25 and the tube 15 is sealed off long at 26. The cathode 13 surfaced with antimony thermally deposited thereon is then activated by placing the cell in one oven at 250-260‹C and the exhaust tube in another oven at 215-225‹C and evaporating the caesium on to the heated cathode surface, Fig. 5 (not shown). The cell is then tipped off short at 27 and baked again at 180‹C for 20 minutes so as to utilize the slight excess of caesium in the cell as a getter. The device has a glass envelope 10 sealed to headers 11, 12 of chrome iron or 'Allegheny 55', and the cathode 13 is of nickel. The capsule 18a is of nickel foil tubing with pinched ends and having one or more apertures 21 to permit escape of caesium vapour and may be held in place by a spiral spring 22. A number of photocells may be processed on a single rotary machine, in which case the procedure is slightly different. In Fig. 6 photocells X are carried on an intermittently rotating turret 30 from a loading station A through swing doors 34 to oven 33a to heat them up and then to oven 33b to bake and exhaust them. The ovens 33a, 33b may be at 450‹C and 330‹C respectively and are heated by resistance heaters 35. The cells X then pass through swing doors 37 to station J, where the caesium pellet is flashed by a high frequency coil 24 and the caesium not only deposits on tube 15 but also starts to react with the cathode surface. The cells then pass through swing doors 43 into another oven 44 which heats up the cathode and exhaust tube 15 to the desired different temperature of 250-260‹C and 215-225‹C respectively to complete the reaction. The cells then pass through swing doors 47, are sealed off short, and removed at P. Slight modifications of cell construction are exemplified in Figs. 7-9 (not shown). In all these figures the exhaust tube is attached to the anode instead of to the cathode, and in Figs. 8 and 9 (not shown) the cathode surface is formed on a header instead of on a separate disc. In Fig. 8 the caesium producing capsule is supported by a constriction in the exhaust tube instead of by a spring. The invention may also be applied to electron multipliers, and instead of antimony and caesium, bismuth and caesium may be used; caesium may be replaced by another alkali metal such as rubidium. In the case of a photocell, the envelope may be a spherical glass bulb instead of tubular. The Specification as open to inspection under Sect. 91, states that the cathode disc 13 may be of material known under the R.T.M. "Nichrome ". The cathode surface may be of any metal from the fifth group of the periodic system. This subjectmatter does not appear in the Specification as accepted.