US2912611A - Thermionic cathodes - Google Patents

Thermionic cathodes Download PDF

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Publication number
US2912611A
US2912611A US446206A US44620654A US2912611A US 2912611 A US2912611 A US 2912611A US 446206 A US446206 A US 446206A US 44620654 A US44620654 A US 44620654A US 2912611 A US2912611 A US 2912611A
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United States
Prior art keywords
cathode
nickel
layer
cathodes
emission
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Expired - Lifetime
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US446206A
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English (en)
Inventor
Beck Arnold Hugh William
Cutting Alan Butler
Brisbane Alan Douglas
King George
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International Standard Electric Corp
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International Standard Electric Corp
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
    • H01J1/00—Details of electrodes, of magnetic control means, of screens, or of the mounting or spacing thereof, common to two or more basic types of discharge tubes or lamps
    • H01J1/02—Main electrodes
    • H01J1/13—Solid thermionic cathodes
    • H01J1/20—Cathodes heated indirectly by an electric current; Cathodes heated by electron or ion bombardment
    • H01J1/28—Dispenser-type cathodes, e.g. L-cathode
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
    • H01J3/00—Details of electron-optical or ion-optical arrangements or of ion traps common to two or more basic types of discharge tubes or lamps
    • H01J3/02—Electron guns
    • H01J3/027—Construction of the gun or parts thereof
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
    • H01J9/00—Apparatus or processes specially adapted for the manufacture, installation, removal, maintenance of electric discharge tubes, discharge lamps, or parts thereof; Recovery of material from discharge tubes or lamps
    • H01J9/02—Manufacture of electrodes or electrode systems
    • H01J9/04—Manufacture of electrodes or electrode systems of thermionic cathodes
    • H01J9/042—Manufacture, activation of the emissive part

Definitions

  • the present invention relates to thermionic cathodes.
  • a cathode For the purpose of providing a cathode having long life and copious thermionic emission, it has been proposed to construct a cathode from sintered metal, such as tungsten, forming a housing for a cathode heater and a reservoir in which materials of high thermionic emissivity are contained.
  • sintered metal such as tungsten
  • a cathode heater With barium oxide as the emissive material, it is believed that atoms of barium migrate through the sintered cathode wall during operation and liberate electrons from the cathode surface.
  • dispenser type cathodes as they may be called, has not been very successful in the past thirty or more years during which they have been investigated.
  • a dispenser type cathode in which the active cathode material is formed from a pressed and sintered mass of mixed powders of the metals nickel or cobalt and thermionically emissive alkaline earth material.
  • the compacted thermionically emissive mass may be used as a surface layer, having any desired thickness, applied to a sintered nickel base; or be used to form a plug, held in a suitable sleeve, or even as a directly heated self supporting cathode, the compacted material being sintered during manufacture.
  • the important features of the cathode ofFig. 1 which enable its successful production are believed to be, be sides the processing details, the use of zirconium hydride as a reducing agent, carbon having been found to be unsuitable with thicknesses of emissive layers exceeding 0.002 inch, and the presence of the sintered nickel layer 4, which enables satisfactory adhesion of the active layer 5 onto the base 3 of the cathode cup 1.
  • a mixture of 69% nickel powder, 30% mixed barium and strontium carbonates, and 1% of a zirconium reducing agent is prepared.
  • the nickel powder is purchased in the form of carbonyl nickel powder having a particle size of 1-5 microns diameter, and, before use, is stoved in vacuum at 400 C.
  • the barium-strontium carbonate mixture is a standard double carbonate powder such as commonly used in the manufacture; of thermionic cathreference to the accompanying diagrammatic drawings in Fig. 3 shows a cross-section through a plug type of cathode having layers of different composition;
  • Fig. 4 shows curves relating to emission current and temperature for cathodes of the invention and previously known types of cathode
  • Fig. 5 shows a modification of the cathode of Fig.2.
  • the end surface 3 of the cup carries a coating 4 of nickel powder sintered toit, and an outer layer 5 of what we shall refer to as active material pressed on to the layer 4 and finally sintered.
  • This layer of active material 5, whose preparation will be described more fully below, is formed initially of an intimate mixture of finely divided nickel, alkaline earth carbonate or carbonates and zirconium hydride, a
  • zirconium is in the form known commercially as zirconium hydride, whichis really a solution of hydrogen in zirconium. 30 grams of the nickel, carbonate and zirconium hydride mixture is ball-milled in 50 millilitres of amylacetate for 30 minutes, and is then filtered and dried in air at 110 C. The resultant powder is then stored for further use.
  • the surface 3 of the cathode cup of Fig. 1 is covered with a thin layer of nickel by spraying or brushing a suspension of suitable nickel powder onto the surface.
  • the same carbonyl nickel powder mentioned above is used, suspended in amylacetate.
  • This nickel is sintered onto the surface by stoving in vacuum or hydrogen at 1000 C. to form the layer 4, whose purpose, as mentioned'above, is primarily to improve the tenacity of the surface layer 5 onto the cathode cup.
  • the layer 4 should essentially provide a rough surface, and from this point of view a thin layer, such as produced by brushing, is to be preferred.
  • On to this surface is brushed or sprayed emissive powder, prepared as described above, suspended in amylaceta'te.
  • the cathode is completely dried at 110 C. and is then pressed against a polished plate, an applied pressure 'of between 20 to 100 tons per square inch being satisfactory.
  • the final thickness of the emissive layer is not critical and may be from 0.001 inch to more than 0.004 inch. If it is desired to store the cathode for any length of time it is advisable to give it a protective coating such as may be obtained by dipping it in a solution of methyl methacrylate plastic dissolved in acetone. A plastic known commercially as Diakon has been successfully used.
  • the active'layer 5 is of metallic appearance and may be machined, although it is possible, and in some circumstances it may be desirable, to postpone lathe or like cutting operations until after activation.
  • the activation process of the layer type of cathode shown in Fig. 1 is essentially the same as that required for the plug type of cathode shown in Fig. 2, which will now be described, the details of the activation process being discussed subsequently.
  • a solid metal backing plate for the plug 7 such as is indicated at 8 in Fig. 2. This backing plate not only provides additional mechanical support for the plug 7, but also prevents evaporation of thermionically emissive material into the heater space.
  • cathodes When required for activation, cathodes, formed as described above in connection with Figs. 1 or 2, and provided with heaters 2, are assembled into envelopes either together with the other electrodes with which they will eventually cooperate, or each with a convenient anode to form diodes from which the cathodes will be extracted after activation.
  • These assembled tubes are mounted on a pumping station in normal manner, and are baked for one hour, during which, at a temperature between 350 and 400 C., the Diakon, if present, depolymerises With-considerable evolution of gas.
  • the anode and any other electrodes are out-gassed in normal manner by heating to a higher temperature and are maintained hot during the subsequent break-down of the carbonates into oxides, which is carried out in the normal way by heating at a considerably higher temperature until carbon dioxide ceases to be evolved.
  • the cathode temperature is lowered somewhat and electronic emission is drawn from the cathode by applying a suitable voltage to the anode and any other electrodes. In the initial stages of activation, if too high a current density is drawn from the cathode it readily be comes poisoned. At low current densities the thermionic emission increases with time at a rate depending upon the emission current density.
  • the cathode is therefore activated by drawing the maximum current, depending on the state of activation of the cathode and the temperature, which avoids the above mentioned poisoning effect until the desired emission density is obtained.
  • This activation does not result in any appreciable amount of gas being evolved'and may be performed, if desired, after the valve is sealed off, provided the electrode assembly includes a getter which is fired before activation.
  • the active material becomes sintered.
  • the cathodes are quite stable, and may be exposed to the normal atmosphere without permanent poisoning.
  • the cathodes may be activated, as suggested above, in temporary diode structures from which they are subsequently recovered.
  • the activation process is effective for substantially the whole thickness of the active material, and the latter may be machined or ground, if desired, at this stage. Since the surface is now sintered, it may be machined more readily than before activation, and without permanent deterioration. Nevertheless, as mentioned previously, the active material may be machined before the activation process, if desired, although not such a high surface polish can be obtained as when the machining is performed after activation.
  • the activated and sintered material can also be welded.
  • the cathode is assembled into the discharge tube for which it is finally required, the tube then being outgassed in normal manner. Only a short reactivation process is required.
  • a plug type of cathode in which the porosity of the sintered material is varied so as to be comparatively small at the cathode surface, and larger in the interior, is shown in Fig. 3, in which a composite plug 9 is inserted in a cylinder 6 of nickel or other refractory metal.
  • the plug is shown formed of four layers of active material.
  • the layer 10 contains 40% of alkaline earth carbonate in the original powder mixture, the layer 11 20%, and the layer 12 10% of carbonate.
  • the pore size will be larger with the larger percentage of carbonate.
  • the under surface of the plug comprises a layer 13 of nickel powder without any admixed thermionically emissive material. Each of these layers may vary in thickness from, say, 0.010 inch to 0.020 inch.
  • the various layers may be made one at a time, for ex ample, by first fitting the sleeve 6 with a suitable backing plate, putting in nickel powder to form the layer 13, then compressing this layer, then adding the next layer of active material, pressing this, and so on until the plug is formed, after which it is activated as described.
  • the loose powders can be inserted one at a time and the plug compacted in a single operation.
  • the provision of the layer 13 of nickel powder substantially prevents emission into the cathode heater space. If desired, it may be replaced by a solid metal backing plate, such as the plate 8 of Fig. 2.
  • the cathodes of the present invention provide, at a given operating temperature, values of thermionic emission density considerably greater than those published-for other types of dispenser cathode, or, to put it another way, they give the same emission as known types but at a lower temperature. Life tests reveal, as mentioned above, that very long lives, even at remarkably high emission densities, are achieved. An indication of the relationship between emission and cathode temperature for cathodes of the present invention and some other known types of cathode is shown in Fig.
  • Curve A shows the variation of pulsed emission obtainable at various temperatures with known types of oxide coated cathode, a maximum temperature of about 900 C. being indicated for a useful life of 1000 hours.
  • Curve B refers to the pulsed emission obtainable from a type of dispenser cathode involving diffusion through porous tungsten, and is based on published figures; a much higher maximum temperature of some 1200 C. may be used.
  • the shaded area'C gives the approximate range of cathodes of the present invention based upon tests up to the present time. It will be seen that the operating temperatures are intermediate between those of curves A and B. No indication of any limiting maximum temperature has yetbeen obtained below the melting point of nickel. The limitations in this respect have been due to difficulty in cooling the anodes'of experimental diodes in which the cathodes have been incorporated.
  • the continuous emission-temperature curve for the conventional oxide cathode is shown at D.
  • the curve extends up to 1 amp/cm. at which the life of the cathode is normally considerably less than 1000 hours.
  • Curve E relates to cathodes of the present invention giving continuous emission.
  • the upper limit of the curve is not yet known, nor is the maximum temperature for active material 1 a reasonable life; on the other hand, while early samples of cathodes to which the curve relates have had lives of 2500 hours when operated continuously at 3 amps./ cm. later samples show even longer lives without any sign of being over-run.
  • a cathode of the type illustrated in Fig. 2 had a nickel cylinder 6 of length 0.236 inch, 0.180 inch outer diameter and 0.158 inch internal diameter, accommodating a plug 7, 0.025 inch'thick.
  • a heater power of 7.1 watts was required.
  • of about e watts would be required to raise a nickel cylinder of the same dimensions to the same temperature, if conduction cooling by the necessary supports is completely ignored.
  • cathodes according to the present invention owing to their high emission capabilities, will find immediate applcation in the field of cathode ray tubes, high voltage rectifiers and beam tubes for high frequency use.
  • the cathode described in the present specification have all had a planar emitting surface. Surfaces of a shape such as required for some types of electron guns may readily be made by employing dies of suitable configuration in the pressing of the active material of Furthermore, in certain applications, a rod or tube of the active material, may be usecias a directly or indirectly heated cathode.
  • the material requires sintering be fore assembly into a discharge tube with other electrodes, i.e. sintering must be done before activation and not durprocess, connections to the cathode being made by welding.
  • a directly heated cathode was made up in the form of a isfactory emission was obtained.
  • a cathode otherwise similar to that of Fig. 2 is shown but having an outer layer 14 of sintered metal of lower work function such as tantalum, tungsten, zirconium, titanium, thorium or silicon.
  • the layer 14 is first formed and sintered separately.
  • the layer 14, the active material of the plug 7 in the initial powder form and the backing plate 8 are ina power troduced into the sleeve 6 and are compressed together
  • the cathode is then processed as has previously been de scribed.
  • an outer layer such as 14, Fig. 5 may be included or may replace the upper layer 1'2 containing the 10% carbonate mixture.
  • An indirectly heated cathode comprising a sintered of mixed powders of metallic nickel, thermionically emissive alkaline earth material, and a a metal support member to which said united, said support including a heater compartment, the said mixed powders containing from 10% to 40% by weight of alkaline earth carbonate, a proportion not exceeding 1.5% of said reducing agent and the remainder pure nickel.
  • the cathode of claim 1 wherein the nickel powder has a particle size of between 1 and 5 microns, and the ratio of nickel to alkaline earth material is in the range of 2:1 by weight.
  • the cathode of claim 1 further comprising "an intermediate nickel layer between said mass and said support 4.

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  • Engineering & Computer Science (AREA)
  • Manufacturing & Machinery (AREA)
  • Solid Thermionic Cathode (AREA)
  • Microwave Tubes (AREA)
  • Powder Metallurgy (AREA)
US446206A 1953-08-14 1954-07-28 Thermionic cathodes Expired - Lifetime US2912611A (en)

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Application Number Priority Date Filing Date Title
GB328590X 1953-08-14
GB3013171X 1955-06-23

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US582595A Expired - Lifetime US3013171A (en) 1953-08-14 1956-05-03 Thermionic cathodes

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BE (3) BE548876A (zh)
CH (3) CH328590A (zh)
DE (1) DE1015941B (zh)
FR (3) FR1113771A (zh)
GB (1) GB750339A (zh)
NL (1) NL97850C (zh)

Cited By (17)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3056061A (en) * 1959-03-06 1962-09-25 Philips Corp Method of manufacturing nickel supports for oxide cathodes and cathodes provided with such supports
US3076915A (en) * 1954-12-24 1963-02-05 Egyesuelt Izzolampa Cathode assembly and method of making same
US3113236A (en) * 1959-06-23 1963-12-03 Philips Corp Oxide dispenser type cathode
US3159461A (en) * 1958-10-20 1964-12-01 Bell Telephone Labor Inc Thermionic cathode
US3170772A (en) * 1961-01-05 1965-02-23 Tokyo Shibaura Electric Co Oxide coated cathodes for electron tubes
US3186786A (en) * 1961-06-01 1965-06-01 Bell Telephone Labor Inc Method for processing oxide coated cathodes
US3307241A (en) * 1963-10-14 1967-03-07 Litton Prec Products Inc Process for making cathodes
US3374385A (en) * 1963-07-10 1968-03-19 Rca Corp Electron tube cathode with nickel-tungsten alloy base and thin nickel coating
US3384511A (en) * 1963-09-19 1968-05-21 Bell Telephone Labor Inc Cathode structures utilizing metal coated powders
US3458749A (en) * 1966-06-24 1969-07-29 Philips Corp Dispenser cathode made of tungsten powder having a grain size of less than three microns
US3465400A (en) * 1967-02-01 1969-09-09 Varian Associates Method of making cylindrical mesh electrode for electron tubes
US3974414A (en) * 1975-07-09 1976-08-10 Gte Sylvania Incorporated Cathode ray tube cathode
US4855637A (en) * 1987-03-11 1989-08-08 Hitachi, Ltd. Oxidation resistant impregnated cathode
US5131878A (en) * 1990-09-22 1992-07-21 Samsung Electron Devices Co., Ltd. Process for manufacturing dispenser cathode
US5171180A (en) * 1991-04-23 1992-12-15 Gold Star Co., Ltd. Method for manufacturing impregnated cathodes
EP0685868A1 (en) * 1994-05-31 1995-12-06 Nec Corporation Cathode member and electron tube having the cathode member mounted thereon
US6033280A (en) * 1995-09-21 2000-03-07 Matsushita Electronics Corporation Method for manufacturing emitter for cathode ray tube

Families Citing this family (15)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
GB904505A (en) * 1958-11-25 1962-08-29 Harries Electronics Corp Ltd Improvements in or relating to electron discharge tubes for television, radar and the like
US3160780A (en) * 1961-01-17 1964-12-08 Philips Corp Indirectly heated cathode
US3263115A (en) * 1962-05-23 1966-07-26 Gen Electric Dispenser cathode and method of manufacture
US3303378A (en) * 1964-06-17 1967-02-07 Chorney Paul Monolithic cathode structure
US3436584A (en) * 1966-03-15 1969-04-01 Gen Electric Electron emission source with sharply defined emitting area
US3436583A (en) * 1966-03-15 1969-04-01 Gen Electric Electron gun
BE759247A (fr) * 1969-11-22 1971-05-21 Philips Nv Dispositif muni d'un tube electronique, et tube electronique destine a un tel dispositif
US3803677A (en) * 1972-04-28 1974-04-16 Air Liquide Method for making cathodes for electron guns
US3835327A (en) * 1972-05-08 1974-09-10 United Aircraft Corp Triode electron gun for electron beam machines
DE2808134A1 (de) * 1978-02-25 1979-08-30 Licentia Gmbh Vorratskathode
FR2525808A1 (fr) * 1982-04-23 1983-10-28 Thomson Csf Cathode thermoelectronique
EP0146383B1 (en) * 1983-12-20 1992-08-26 Eev Limited Apparatus for forming electron beams
JPH0298921A (ja) * 1988-10-05 1990-04-11 Fujitsu Ltd 電子銃およびその製造方法および該電子銃を備えた露光装置および該露光装置を用いる半導体装置の製造方法
FR2647257B1 (fr) * 1989-05-19 1991-07-05 Thomson Tubes Electroniques Cathode impregnee avec capacite reduite avec application aux tubes de visualisation du type indexation de faisceau, et tubes electroniques comprenant un tel dispositif
JPH11339633A (ja) * 1997-11-04 1999-12-10 Sony Corp 含浸型陰極およびその製造方法、並びに電子銃および電子管

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US1552310A (en) * 1923-07-24 1925-09-01 Gen Electric Electrode for discharge tubes
US1931254A (en) * 1928-02-28 1933-10-17 Electrons Inc Electronic tube
US2147447A (en) * 1936-09-21 1939-02-14 Siemens Ag Glow cathode
US2275886A (en) * 1941-04-30 1942-03-10 Gen Electric Process of activating cathodes
US2492142A (en) * 1945-10-17 1949-12-27 Kenneth J Germeshausen Electric system embodying coldcathode gaseous discharge device
US2619706A (en) * 1947-04-14 1952-12-02 Gen Electric Electrode for electric discharge devices
US2673277A (en) * 1949-10-25 1954-03-23 Hartford Nat Bank & Trust Co Incandescible cathode and method of making the same
US2700118A (en) * 1951-11-29 1955-01-18 Philips Corp Incandescible cathode
US2700000A (en) * 1952-02-27 1955-01-18 Philips Corp Thermionic cathode and method of manufacturing same
US2722626A (en) * 1953-02-16 1955-11-01 Philips Corp Thermionic cathode
US2733378A (en) * 1956-01-31 Thermionic cathodes for electronic discharge devices
US2741717A (en) * 1951-06-14 1956-04-10 Siemens Ag Dispenser type cathode having gettercoated parts
US2813220A (en) * 1954-12-06 1957-11-12 Philips Corp Indirectly heated cathode

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US2543439A (en) * 1945-05-02 1951-02-27 Edward A Coomes Method of manufacturing coated elements for electron tubes
US2640949A (en) * 1951-02-07 1953-06-02 Atomic Energy Commission Electron source
US2640950A (en) * 1951-06-06 1953-06-02 Atomic Energy Commission Point electron source

Patent Citations (13)

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US2733378A (en) * 1956-01-31 Thermionic cathodes for electronic discharge devices
US1552310A (en) * 1923-07-24 1925-09-01 Gen Electric Electrode for discharge tubes
US1931254A (en) * 1928-02-28 1933-10-17 Electrons Inc Electronic tube
US2147447A (en) * 1936-09-21 1939-02-14 Siemens Ag Glow cathode
US2275886A (en) * 1941-04-30 1942-03-10 Gen Electric Process of activating cathodes
US2492142A (en) * 1945-10-17 1949-12-27 Kenneth J Germeshausen Electric system embodying coldcathode gaseous discharge device
US2619706A (en) * 1947-04-14 1952-12-02 Gen Electric Electrode for electric discharge devices
US2673277A (en) * 1949-10-25 1954-03-23 Hartford Nat Bank & Trust Co Incandescible cathode and method of making the same
US2741717A (en) * 1951-06-14 1956-04-10 Siemens Ag Dispenser type cathode having gettercoated parts
US2700118A (en) * 1951-11-29 1955-01-18 Philips Corp Incandescible cathode
US2700000A (en) * 1952-02-27 1955-01-18 Philips Corp Thermionic cathode and method of manufacturing same
US2722626A (en) * 1953-02-16 1955-11-01 Philips Corp Thermionic cathode
US2813220A (en) * 1954-12-06 1957-11-12 Philips Corp Indirectly heated cathode

Cited By (20)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3076915A (en) * 1954-12-24 1963-02-05 Egyesuelt Izzolampa Cathode assembly and method of making same
US3159461A (en) * 1958-10-20 1964-12-01 Bell Telephone Labor Inc Thermionic cathode
US3056061A (en) * 1959-03-06 1962-09-25 Philips Corp Method of manufacturing nickel supports for oxide cathodes and cathodes provided with such supports
US3113236A (en) * 1959-06-23 1963-12-03 Philips Corp Oxide dispenser type cathode
US3170772A (en) * 1961-01-05 1965-02-23 Tokyo Shibaura Electric Co Oxide coated cathodes for electron tubes
US3186786A (en) * 1961-06-01 1965-06-01 Bell Telephone Labor Inc Method for processing oxide coated cathodes
US3374385A (en) * 1963-07-10 1968-03-19 Rca Corp Electron tube cathode with nickel-tungsten alloy base and thin nickel coating
US3384511A (en) * 1963-09-19 1968-05-21 Bell Telephone Labor Inc Cathode structures utilizing metal coated powders
US3404034A (en) * 1963-09-19 1968-10-01 Bell Telephone Labor Inc Preparation of metal-coated powders and cathode structures
US3307241A (en) * 1963-10-14 1967-03-07 Litton Prec Products Inc Process for making cathodes
US3458749A (en) * 1966-06-24 1969-07-29 Philips Corp Dispenser cathode made of tungsten powder having a grain size of less than three microns
US3465400A (en) * 1967-02-01 1969-09-09 Varian Associates Method of making cylindrical mesh electrode for electron tubes
US3974414A (en) * 1975-07-09 1976-08-10 Gte Sylvania Incorporated Cathode ray tube cathode
US4855637A (en) * 1987-03-11 1989-08-08 Hitachi, Ltd. Oxidation resistant impregnated cathode
US5131878A (en) * 1990-09-22 1992-07-21 Samsung Electron Devices Co., Ltd. Process for manufacturing dispenser cathode
US5171180A (en) * 1991-04-23 1992-12-15 Gold Star Co., Ltd. Method for manufacturing impregnated cathodes
EP0685868A1 (en) * 1994-05-31 1995-12-06 Nec Corporation Cathode member and electron tube having the cathode member mounted thereon
US5757115A (en) * 1994-05-31 1998-05-26 Nec Corporation Cathode member and electron tube having the cathode member mounted thereon
US6033280A (en) * 1995-09-21 2000-03-07 Matsushita Electronics Corporation Method for manufacturing emitter for cathode ray tube
US6222308B1 (en) * 1995-09-21 2001-04-24 Matsushita Electronics Corporation Emitter material for cathode ray tube having at least one alkaline earth metal carbonate dispersed or concentrated in a mixed crystal or solid solution

Also Published As

Publication number Publication date
FR68247E (fr) 1958-04-09
FR1113771A (fr) 1956-04-04
BE548876A (zh)
NL97850C (zh)
CH328590A (fr) 1958-03-15
CH337954A (fr) 1959-04-30
CH338908A (fr) 1959-06-15
US3013171A (en) 1961-12-12
BE544065A (zh)
FR71337E (fr) 1959-12-22
GB750339A (en) 1956-06-13
DE1015941B (de) 1957-09-19
BE531122A (zh)

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