US3113236A - Oxide dispenser type cathode - Google Patents

Description

Dec. 3,51963 H. J. LEMMr-:Ns ETAL. 3,113,236

oxIDE DISPENSER TYPE cATHoDE Filed June 17. 1960 INVENTUR HENDRIKUS QJ. LEMMENS PIETER ZALM ANTONIUS .|.A.V. STRATUM JOHANNES R. BLATTER FRIEDRICH H. R. AIMER BY M -L i. A ENT United States Patent O 3,113,236 @KEER DHSPENSER TYPE CA'IHGDE Hendrikus `loliannes Lemmens, Pieter Zalm, Antonius .lohannes Alberta Van Stratum, Johannes Reinier Blatter, and Friedrich Hermann Raymiud Almere, all

of Eindhoven, Netherlands, assigner-s to North American Philips Company, inc., New York, NSY., a corporation oi Delaware Filed .lune 17, 1960, Ser. No. 36,027 Claims priority, application Netherlands June 23, 1959 6 Claims. (Cl. 313-346) This invention relates to cathodes -for electron devices or electric discharge tubes. In particular, it relates to a thermionic, oxide, dispenser-type cathode, in which a non-metallic emissive layer activated by barium is separated from a source or layer dispensing barium by means of a permeable metal layer.

A known cathode comprises an electron emissive layer which consists of alkaline-earth oxides sprayed onto a porous `metal layer having beneath it a layer of bariumaluminum which contains a metal binding the aluminum and which in turn is supported by a carrier of, for example, ceramic material. The barium-aluminum dispenses barium at a comparatively high vapo-r pressure, which diffuses through the porous metal layer into the alkaline-earth oxide layer. The evaporation of barium from such a cathode will be considerable.

Another known cathode is made by spraying onto a support a mixture of carbonyl nickel powder, bariumstrontium carbonatos and zirconium hydride as a reducing agent. The layer is pressed under high pressure and sintered. Since the layer contains a high percentage of nickel powder, the cathode has a metallic appearance and its emission density is lower than that of ordinary oxide cathodes.

The chief obiect of the invention is to provide a cathode capable of high continuous emission densities at relatively low operating temperatures and possessing low evaporation of active material.

According to the invention, the cathode comprises a non-metallic emissive layer activated by barium, and possibly strontium, separated `from a layer or source dispensing the alkaline-earth metal by a metal layer permeable to said alkaline-earth metal. The permeable metal layer constitutes the current supply means or conductor for the activated layer and is of a composition such that this metal layer, together with the non-metallic activated layer, substantially does not form any insulating interfaces. rThe layer or supply ydispensing the alkaline-earth metal consists of alkaline-earth metal compounds from which tree alkaline-earth metal is produced or generated by a metal surrounding it or contained in it.

A simple embodiment of a cathode according to the invention comprises a layer of barium-strontium oxide provided on a sprayed layer of inactive or passive nickel powder having beneath it a layer of barium-strontium oxide on a support of active nickel, which serve as the dispensing source, the nickel powder layer making electric contact with the active nickel support. rIhe term active and inactive or passive nickel is to be understood herein to mean nickel containing or substantially not containing respectively reducing impurities such as aluminum, silicon, tungsten, or magnesium, which function to -free by a reducing process barium from bariumstrontium oxide. ln general, active nickel contains an irnpurity content exceeding, in weight percent, 0.01% of aluminum or 0.5% of tungsten or 0.01% of silicon or 0.01% of magnesium, or combinations of these impurities equivalent to the specified value of one of them. Pass-ive nickel contains a much lower impurity content than that specified above, and when it contains no reducing impurities at all, it may be described as inactive, which is 3,113,236 Patented Dec. 3, 1963 ice preferred -in the invention. Since the nickel layer which supports the emissive layer consists `of preferably inactive or passive nickel, compounds such as silicates Kwhich exhibit high electric resistance and for many oxide cathodes give rise to the very undesirable so-called interface layer resistance are not formed between this nickel layer and the emissive layer. It has been found that cathodes of the invention are capable of a continuous (DC. emission, in contrast to pulsed emission) current density of 1.5 a./m.2 at a temperature of 760 C., which is several tens of degrees lower than is usual for oxide cathodes.

ln the cathode oi the invention, a tine nickel gauze or mesh or a perforated nickel foil may be employed as the permeable metal member instead of the porous sprayed nickel layer, although the workability of these materials with the required purity is comparatively poor. Instead of nickel as the material for the permeable metal layer, it is alternatively possible, -for example, to use molybdenum or tungsten or, if the cost is not objectionable, platinum. For the dispensing source, use may alternatively be made of a barium oxide compound mixed, for example, with reducing material, for example Til-l2 (which during heating decomposes to `form Ti), which alfords the advantage that it is not necessary to use carbonates to form the oxides previously suggested, in which latter event a fairly large quantity of gas is always evolved.

Instead of barium-strontium oxides, obtained from the corresponding carbonates, it is possible to -use for the eniissive layer, for example, barium oxide or strontium oxide or calcium oxide, or barium zirconate, which also affords the advantage that it `does not itself evolve gas upon activation of the cathode and is not harmed by contact with air.

In order that the invention may be readily carried into effect, it will now be described in detail, by =way of example, with reference to the accompanying drawing, in which FIGS. 1, 2, and 3 are cross-sectional views of three embodiments of a cathode according to the invention.

FlG. 1 illustrates a planar type cathode of the invention, comprising a nickel cylinder 1 having a reentrant upper side 2 forming a cavity 12 containing bariumstrontium oxide 3 in the form of a pellet. A gauze or mesh d of pure nickel wire, for example, of 35 microns thickness, is welded on top of the cylinder enclosing the cavity l2. A carbonate layer 5, for example, of 70 microns thickness, to be later decomposed into bariumstrontium oxide, is sprayed onto the nickel gauze 4. The cathode contains a heating lilament 6. A lead-inconductor 1.3 is welded to the bottom end of the nickel support 1. The nickel cylinder ll consists of nickel and several hundredths percent (0.03%) of aluminum and silicon, and is thus active nickel. The nickel mesh l is of high purity and contains no detectable amount of any of the reducing impurities referred to earlier, and is thus inactive nickel. In the manufacture of such a cathode, the nickel cylinder 1 may be formed to the shape illustrated in the drawing by a drawing operation. Next, the oxide pellet 3 is placed inside the cavity 12. The pellet 3 may have dimensions of 2 millimeters in diameter and 0.3 millimeter thick, and may comprise a mixture of barium and strontium carbonates in the ratio of 55 to 45 by weight. These carbonates, which are available in powder form, may be heated at 1050 C. in vacuum to convert them to the corresponding oxides and the resultant oxide powder then compressed to the pellet form shown in the drawing. After the oxide pellet 3 has been introduced into the cavity 12, the mesh d, whose mesh openings are approximately 0.05 millimeter by 0.05 millimeter, is welded over the cavity 12 so as to completely enclose the pellet 3, except, of course, for the mesh openings. Then, a mixture of barium and strontium carbonates, in a Weight ratio of 55 to 45, suspended in a liquid vehicle, is sprayed onto the mesh 4l and surrounding cylinder portions to the thickness previously mentioned. A suitable spraying mixture is provided by ball milling 50 grams of powder in 100 millilitres of amyl acetate after which 40 millilitres of butanol with nitrocellulose is added. Finally, the terminal i3, which constitutes a current supply lead for the cathode from the external circuit, is welded in place and the ilament 6 provided within the hollow support 1. As will be noted, the supply conductor l, by which the cathode is connected in the external circuit, provides a direct current connection through the nickel cylinder l directly to the mesh d, to which it is in direct electrical contact and on which the emissive layer is located. The structure below the mesh comprises the barium dispensing source. After suitable activation of the cathode, which includes heating the cathode to a temperature of about 1050 C. in vacuum for about 5 minutes, which, in the usual way, serves to convert the carbonates in the structure to the corresponding oxides, the cathode may be operated in the usual electron device at temperatures ranging from about 700 C. to 850 C. ln this range of temperatures, the impurities in the active nickel portion 2 reduce the barium-strontium oxides 3 in the cavity producing free barium, which, by a didusion process, passes outwardly through the openings in the mesh 4 into the oxide layer 5 thereby activating it and causing it in turn to be capable of very high electron emission densities. Because of the constant supply of barium to the emissive layer 5, high continuous or D.C. emission becomes possible, in sharp contrast to the usual oxide cathode, which is incapable of sustaining high emission densities for any extensive period. For this reason, where oxide cathodes have been required to produce large amounts of current, pulsing of the cathode has been necessary. Thus, the cathode of the invention is capable of operation at the low temperatures of the oxide cathode, but without being restricted to the usual pulse operation to produce high emission densities. Moreover, it has been found that barium or strontium evaporation from the cathode is relatively small.

FlG. 2 shows a modification of the cathode illustrated in FIG. 1. In this modification, the pellet 3 consists of barium aluminate, in which the mole ratio of barium oxide (BaO) to aluminum oxide (M203) is 21/2 to 1.0, to which has been added 5% by weight of titanium hydride (Til-I2). In this case, the titanium hydride serves as a reducing agent for the barium compound which, when heated to about 800 C., will result in the generation of free barium. As a reducing compound is mixed directly in with the barium compound, the support l, 2 need not contain any reducing impurities. Thus, it may consist of passive nickel or molybdenum. However. since the usual nickel employed in electron tubes does contain some impurities, that material is most conveniently used for the support 1. The pellet 3 fills the cavity 12 and forms with the surrounding portions of the cylinder l a relatively smooth surface. Onto this surface is sprayed a layer 7 of pure nickel powder, which thus has an inactive character, for example, to a thickness of 20 microns. As will be noted, the layer '7 is directly electrically connected to the support l. Finally, onto the pure nickel layer '7 is provided an oxide layer 8, which may be a mixture of barium and strontium oxides in the ratio of 55 to 45. The layer 3 may be, for example, 40 microns thick. ln order to increase the resistance of the cathode to poisoning, the barium aluminate may be replaced with a mixture of barium and strontium oxides to which the titanium hydride is added. This combination produces a higher vapor pressure of barium, which means then that the dispensing rate is increased. This renders the cathode less susceptible to poisoning.

FIG. 3 shows a cylindrical type cathode of the invention. ln this case, a hollow support lli with a rectangu- ,liasse lar cross-section, and which may be of active cathode nickel, supports on opposed surfaces sprayed layers of mixtures of barium and strontium oxides 9, which, as usual, are provided in the carbonate form and which are converted to the oxides during activation, and on top of the oxide layers 9 is sprayed a layer of pure, inactive, nickel powder lll, which encloses the oxide layers 9. Such sprayed layers are porous and permeable to diffusing barium atoms. Finally, on top of the pure nickel layers lll are provided layers ll of strontium oxide, which, as usual, are initially provided in the carbonate form and converted to the oxide during activation. As will be noted, the nickel layer l0 contacts the nickel support 14. As described earlier, as the current supply member or permeable layers 4, 7, and l0 are of inactive material, they do not form insulating interfaces with the emissive layer, which is responsible for the high D.C. emission capabilities of the cathode of the invention.

While we have described our invention, in connection with specific embodiments and applications, other modifications thereof will be readily apparent to those skilled in this art without departing from the spirit and scope of the invention as detined in the appended claims.

What is claimed is:

l. A therm'ionic icathode for an electron device operable at temperatures of about 700 C. to 850 C., cornprising a support, a rst layer on said support off a barium oxidic compound associated Wit-h a reducing substance and capable when heated of reacting together to dispense continuously free barium metal, a barium-permeable electrically-conductive grid-like metal member over and enclosing the said rst layer, means providing an external circuit connection -to said grid-like metal member, an oxidic emissive layer on the grid-like metal member on the side remote from said lirst layer, and means for heating said cathode whereby barium dis Jensed by said iirst layer diffuses through the grid-like metal member into the oxidic emissive layer activating same to produce copious electrons, said grid-like metal member consisting essentially of inactive nickel free of any element selected from the group consisting of aluminum, silicon, tungsten and magnesium and thus substantially unreactive with the emissive layer to form an insulating interface.

2. A thermionic cathode for an electron device operable at temperatures of about 700 C. to 850 C., coni- :rising a conductive support, a iirst ylayer on said support of a barium oxidic compound associated with a metallic reducing substance and capable when heated of reacting together to dispense continuously free barium metal, a barium-permeable electrically-conductive grid-like metal member over and enclosing the said iirst layer and connected to the support, means connected to said support to provide an external circuit connection to said grid-like metal member, an emissive layer comprising principally strontium oxide on the grid-like metal member on the side remote `from said first layer, and means for heating said cathode whereby barium dispensed by said iirst layer diffuses through the grid-like metal member into the oxide emissive layer activating same to produce copious electrons, said grid-like metal member :consisting essentially or inactive nickel free of any element selected from the group consisting of aluminum, silicon, tungsten and magnesium and thus substantially unreactive with the emissive layer to form an insulating interface.

3. A cathode as set forth Ain claim 1 wherein the gridlike metal member comprises a thin preforated nickel foil.

4. A cathode as set for-th in claim V1 wherein the emissive layer consists of a member selected from the group consisting of strontium oxide and calcium oxide.

5. -A cathode as set -forth in claim l wherein the emissive layer consists of barium Zirconate.

6r. A cathode as set Iforth in claim 1 wherein the gridlike member comprises a line nickel mesh.

(References on following page) References Cited in the le of this patent UNITED STATES PATENTS Waldschmidt Sept. 27, 1938 Laerty Feb. 28, 1950 Lemmens et al. Feb. 27, 1951 Espersen et al. Jan. 4, 1955 Watts Nov. 19, 1957 Coppola Dec. 9, 1958 Germany Dec. 24, l1958 December 3, 1963 Patent Noa 3,113,236

Hendrikus Johannes Lemmens et a1,

in the above numbered pathat error appears ers Patent should read as It is hereby certified t that the said Lett ent requiring correction and corrected below.

Column t2v line 10, for "a./m.2" read A/cm2 Signed and sealed this 28th day of April 1964.

(SEAL) Attest: ERNEST VL SWIDER EDWARD J. BRENNER Attesting Officer Commissioner of Patents

Claims (1)

1. A THERMIONIC CATHODE FOR AN ELECTRON DEVICE OPERABLE AT TEMPERATURES OF ABOUT 700*C. TO 850*C., COMPRISING A SUPPORT, A FIRST LAYER ON SAID SUPPORT OF A BARIUM OXIDIC COMPOUND ASSOCIATED WITH A REDUCING SUBSTANCE AND CAPABLE WHEN HEATED OF REACTING TOGETHER TO DISPENSE CONTINUOUSLY FREE BARIUM METAL, BARIUM-PERMEABLE ELECTRICALLY-CONDUCTIVE GRID-LIKE METAL MEMBER OVER AND ENCLOSING THE SAID FIRST LAYER, MEANS PROVIDING AN EXTERNAL CIRCUIT CONNECTION TO SAID GRID-LIKE METAL MEMBER, AND

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