Patent GB-968392-A (GB)
IPC: G21H1/10
Filed: 1960-12-02
Abstract: 968,392. Thermionic generators. NATIONAL RESEARCH DEVELOPMENT CORPORATION. Dec. 2, 1960 [Dec. 23, 1959; March 24, 1960], Nos. 43662/59 and 10540/60. Heading H1D. As shown, a thermionic generator comprises an emitter 1, Fig. 1, heated by a burner to 1500-1600 C., a foraminated collector 2 and an auxiliary anode 4 cooled by air, water or oil. To cool the collector 2 to 100-200 C., the anode 4 is formed with projections which are electrically insulated from collector 2 by thin layers 5 having good heatconducting properties. The device may contain A, Kr, Xe, Hg, Cd, Zn, Li, Na, K, Rb or Cs but a filling of A, at 1-2 mm. of Hg, and Hg saturated at 50-100 C. is preferred. The envelope may be of fused alumina or zirconia or recrystallized silicon carbide. Gettering materials, e.g. Ti or Zr, may be applied to a part of the surface to be heated. Electrons emitted by emitter 1 for the most part land on collector 2 but a small fraction penetrate through the holes into the auxiliary discharge space and ionize the gas or vapour filling. About 50% of these ions pass through the holes into the main discharge space provided that the electron mean free path is neither larger nor much smaller than the depth of the auxiliary discharge space. At large current densities the gas density in the auxiliary discharge space tends to drop because the discharge acts as an ion pump so that it is preferable to provide discharge-free communication channels between the auxiliary and main discharge spaces. A potential difference of 15 V. between collector 2 and anode 4 is sufficient to produce the required number of ions, the ratio of the currents I 1  and I 2  being about 50 : 1. The power expended in the auxiliary discharge can be reduced by using only a small D.C. voltage drop and superposing on this a very high frequency oscillation. To avoid arcing, the holes in collector 2 must be sufficiently small and numerous, e.g. 1000 holes of 0.01 mm.<SP>2</SP> clear area per cm.<SP>2</SP> The foraminous collector may be produced by photo-etching, or by electroplating a wire gauze, and has a low work function surface coating produced, for example, by oxidizing a coating of Cr, W or Mo, depositing a coating of an alkali metal, e.g. Cs, thereon and again oxidizing, the latter steps not being necessary if a vapour filling of Cs is to be used. To reduce heat radiation losses, a large area emitter 1, Fig. 3, in the form of a coating of Mo, W, Hf or thoriated tungsten on a ceramic casing 7, is used with a collector 2 surrounded by radiation shields 13. The casing 7 is secured by bolts 9 to the walls 10 of a combustion chamber fitted with heat shields 12. The copper collector 2 is spaced from the anode 4 by glass-enamel coated metal washers 17 fused to one another and to the electrodes. The casing 7 is vacuum-sealed to the lid 15 by metalclad asbestos washers enamel-sealed or brazed thereto. The collector 2 and anode 4 may be spaced by a ceramic body having perforations registering with those of the collector 2 which may be a coating on the ceramic body. The perforations may register with blind holes in the anode itself. The holes in the collector 2 may be conical rather than cylindrical. The emitter surface may be increased by shallow cellular divisions near the surface or by hemispherical gofferings or by roughening the surface, e.g. by etching or sandblasting. A plurality of such devices may be mounted in an octagonal, ceramic combustion chamber 10, Fig. 7, comprising ducts 22, 23 for the fuel and oxidant which are pre-heated by the exhaust gases in the chamber 24. A device for generating alternating current comprises two collectors 2, 2<SP>1</SP>, Fig. 9, and two anodes 4, 4<SP>1</SP> and a magnetic yoke 27 carrying two systems of windings, one 28, 28<SP>1</SP> energized by a direct current and the other 29, 29<SP>1</SP> by an alternating current. The energizing currents are such that they reinforce one another in the magnetic circuit associated with collector 2<SP>1</SP> and cancel one another at collector 2 in one half-cycle, and vice versa in the other half-cycle whereby electrons are directed alternately to collectors 2 and 2<SP>1</SP>. The effect can be enhanced by applying an accelerating voltage only to the anode backing the instantaneously active collector. To counteract the effect of the magnetic field parallel to the emitter surface produced by the current between emitter 1 and collector 2 in devices of the kind shown in Figs. 3 and 9, a magnetic field normal to the emitter surface may be provided, Figs. 10, 11 (not shown), by a magnet applied to the top of the device and perforated to allow access to the coolant. This magnetic field may also serve to prevent electrons from going to the radiation shields surrounding the collector which thus become positively charged and repel ions. The power required for the operation of the auxiliary discharge and for the magnetic controls may be derived from the output of the generators. As shown in Fig. 12, foraminous collectors 42<SP>1</SP>, 42<SP>11</SP> are secured to or integral with envelope 64 and are insulated by enamel glass from channel-shaped or cup-shaped anodes 44. A radiation screen 46, left electrically floating comprises a metal sheet 47 and metal foils 48 secured to one another or to an eyelet 49 around the periphery of each collector. Spacing of the foils is ensured by punch marks or corrugations therein, Fig. 21 (not shown). The foil nearest emitter 1 is blackened while the remaining foils and sheet 47 are highly polished or coated with Al 2 O 3  or MgO of high reflectivity. Instead of being planar the emitter 1 and envelope 64 may be of elliptical cross-section. Fig. 15 (not shown), the emitter 1 being a coating on an alumina tube and the envelope 64 being made in two halves by pressing out of sheet metal and being formed with projecting parts constituting the anodes 44. The collectors are in the form of a continuous gauze insulated from envelope 64 and radiation shield 46 by layers of enamel glass. The alumina tube is formed with a corrugated flange to which envelope 64 is secured by elastic metal bellows. A plurality of such elliptical generators may be assembled together in a hexagonal case through which is circulated a gaseous or liquid coolant. Concentric tubes for fuel and oxidant opening into flame nozzles at one end are provided in each generator, the emerging combustion gases being used to preheat the fuel and oxidant in a heat exchanger provided at the other end. In a generator designed to overcome the effects of the magnetic fields set up by the thermionic currents themselves, the emitters 41, Fig. 19, are formed from V-shaped metal sheets connected at their apices to a metal sheet 58 which may itself be electronemissive and which conducts the cathode current I. As shown, the main discharge spaces are outside the magnetic field set up by the strong currents circulating between sheet 58 and envelope 64, it being possible, e.g. by making the device in cylindrical form to ensure that the flux path is closed upon itself. External connections to the sheet 58 may be made around the whole of its periphery so as to avoid the building up of heavy currents in any one part of it. In manufacture the emitter sheets 41 may be coated with ceramic paste and pressed on to the envelope 57. In order to prevent the magnetic field from influencing the auxiliary discharge the anodes 44 are made of ferromagnetic material, e.g. soft iron. The radiation shield is electrically connected to the emitters. In a modified form of auxiliary discharge space, the anode 44, Fig. 22, is in the form of a cylinder surrounding the discharge. The top electrode 62 is connected with the cathode or left floating so that the electrons are reflected to and fro before landing on anode 44. In a generator designed for high current densities the collector is formed by a gauze 74, Fig. 23, an apertured plate 72 and a honeycomb structure 73 of highly polished metal strips, and is spaced 1 mm. from the emitter which is formed by mutually-insulated metal plates or coatings 71. The anode is in the form of a metal sheet 84 spaced from the collector by ribs 85 which are longitudinally staggered so that the auxiliary discharge space is in the form of a continuous meandering channel. The apertures in plate 72 need not be in precise register with the cells of honeycomb structure 73 and there may be 2 or 3 apertures per cell. Either the ribs 85 or the structure 73 may be integral with plate 72 or may be welded or brazed thereto. Output leads 78, secured to or in sliding contact with the emitter, pass through the collector and anode to an apertured terminal plate 79. These leads accurately determine the small gap between parts 84, 71 and by providing a sufficient number of leads the ceramic envelope 87 may be made thin and the self-magnetic fields reduced. In a device similar to that shown in Fig. 12, but with the omission of the radiation shield, the collector may be formed with a honeycomb structure to increase its surface area and thereby reduce the voltage drop adjacent the collector. In an alternative arrangement to that shown in Fig. 23, the collector may consist solely of the honeycomb structure 73, Fig. 28, and gauze 72 supported from the ribs 85. The leads 78 are independent of the ribs 85 and the collector. Two D.C. generators may be used in a push-pull arrangement to provide an A.C. output, Fig. 32 (not shown), by suppressing the auxiliary current in the two generators alternately. Specifications 890,617 and 968,393 are referred to.