Patent GB-695887-A (GB)
Title: Improvements to emissive electrodes
IPC: H01J17/06
Filed: 1950-10-13
Abstract: 695,887. Electrode materials and processing. SOC. ANON. POUR LES APPLICATIONS DE L'ELECTRICITE ET DES GAZ RARES-ETABLISSEMENTS CLAUDE-PAZ & SILVA. Oct. 13, 1950 [Oct. 27, 1949; April 18, 1950; July 3, 1950; Aug. 9, 1950], No. 25061/50. Class 39(i) An electrode for electrical discharge apparatus comprises a metallic support and an adherent coating of emissive material thereon containing an alkali-earth metal compound and being made from an activating substance obtained by reacting one of the metals tantalum and molybdenum upon matter containing an alkali-earth metal combination consisting mainly of barium dioxide. The emissive or activating material is obtained by bring- ing on a support barium dioxide in contact with molybdenum and/or tantalum and heating the support to a temperature corresponding to that of redheat in a non-oxidizing or reducing atmosphere. The duration of the reaction must be long enough to obtain a grey-black product. The temperature to which the support is heated does not exceed 1,500‹C and the heating is stopped before the barium volatilizes. Preferably the barium dioxide has been in contact with water before the heating commences and is mixed with a relatively inert substance, e.g. an alkali earth carbonate. As shown in Fig. 2 the electrode comprises a sleeve 1, disc 2, and cup element 3 all made of molybdenum and forming a cavity which is welded to a nickel yoke 5 and connected thereby to the current supply leads. A cap 6 of steatite is fitted over the cavity and prevents discharge. taking place from the surface of the cavity which it covers. An antiemissive layer of alumina 11 is provided on the yoke 5: The emissive layer 4 is formed by melting barium dioxide in the cavity, and heating the electrode to about 150‹C for half an hour in vacuum or in a non-oxidizing gas such as nitrogen or a rare gas, or a reducing gas such as hydrogen. The electrodes are mounted in the discharge apparatus in the usual way by evacuating the latter and introducing therein a rare gas, e.g. argon, under a pressure of a few millimetres of mercury, then arcing the apparatus. The electrode may be mounted so that the cavity faces away from the support, Fig. 1 (not shown), or towards the support, Fig. 3 (not shown), in which case the support 5 is replaced by a supporting nickel wire welded to the side wall 1, and the steatite is omitted. Fig. 4 shows an electrode having a wall 29 made of iron, a helical wire 30 of molybdenum being provided within the wall before the barium dioxide is introduced. The electrode is supported by lead wires 31, 32 a heating coil 28 being provided to effect the reaction between the molybdenum and the barium dioxide. In the embodiment shown in Fig. 5 the wall 33 is made of steatite, current being supplied by a lead 34 which extends into a helix 35. The molybdenum is introduced inside the electrode cavity, e.g. in the form of molybdenum grains, previously mixed with the barium dioxide. The reaction of the barium dioxide may also be effected on both molybdenum and tantalum, or on tantalum only. Also the materials generating the emissive substance may contain, beside the barium dioxide and molybdenum, other materials, e.g. strontium dioxide, molybdenum, thorium or tungsten compounds, or small amounts of metallic thorium or tungsten, or silica.