US3524098A - Aluminum anode power tube - Google Patents

Description

1'. D. KIRKENDALL ETAL 3,524,098

ALUMINUM ANODE POWER TUBE Aug. 11, 1970 Filed May 15, 1968 INVENTORS THGVAS Q KIRKENDALL PETER F. VARAD/ may US. Cl. 313--311 4 Claims ABSTRACT OF THE DISCLOSURE An electron tube comprising a gas-tight envelope closed at one end by an aluminum anode cup and having electrodes therein attached to support structures, which may be aluminum, that terminate in annular disc-type terminals, which also may be aluminum, and form a part of the gas-tight envelope.

BACKGROUND OF THE INVENTION This invention is related generally to electron discharge devices and is concerned more particularly with an electron tube having an aluminum anode as a structural part of the tube envelope.

A conventional power tube of the prior art comprises a gas-tight envelope closed at one end by a deep metallic cup, usually copper, which serves as the anode of the tube. Usually, the open end of the anode cup is peripherally attached to one end of a Kovar sleeve which is circumferentially sealed at the opposite end to one end of a dieelectric cylinder, such as glass or ceramic for example. Other electrodes, such as the cathode and grid for example, are disposed longitudinally within the cavity of the anode cup, insulatingly spaced from one another and from the internal surface of the anode. Generally, these internal electrodes are supported at one end of relatively heavy metallic structures, usually made of copper, which terminate at the opposite ends in annular flanges that protrude through the wall of the gas-tight envelope and form a part thereof. The radially extended flanges serve'as the external connecting terminals for the electrodes and are commonly referred to as disc terminals. These disc terminals are longitudinally spaced apart by intervening dielectric cylinders, such as glass or ceramic for example, and connecting Kovar sleeves which also form parts of the gas-tight envelope.

One problem encountered when using copper is electron tubes is the elimination of copper oxides from the copper material. Because copper oxides have higher vaporization pressures than copper, the copper oxides will vaporize out of the copper material in the vacuum environment of an electron tube during high temperature operation. The copper oxide molecules will migrate in the gastight envelope of the tube and deposit on surfaces that are at a lower temperature than the copper, such as the surface of dielectric cylinders, for example. Thus leakage paths will form on the surfaces of insulators and will deteriorate their dielectric properties. A further complication arises when the copper oxide molecules are fractured by impinging electrons. The free oxygen, thus released, will poison the emitting surface of the cathode and thereby shorten the elfective life of the tube. In order to eliminate copper oxides as much as possible prior to assembly of the tube, all copper parts are given an extensive acid cleaning and are stored in nitrogen-filled enclosures.

In the production of power tubes, a problem of major concern is the removal of volatile gases from the component materials of the tube before sealing off from the exhaust system. Gases that are evolved from the elements United States. Patent 3,524,098 Patented Aug. 11, 1970 of a tube during high temperature operation can be ionized by electron collisions and result in intermittent arcing or voltage breakdown of the tube. Furthermore, the gas ions will bombard the cathode and may damage the electron-emitting surfaces of the cathode. Gas ions are also drawn to the negative grid and cause an undesirable increase in grid current. In order to remove these volati e gases from the heavy copper anode as much as possible prior to assembly of the tube, the copper anode, after acid cleaning and bright dipping, is heated in a vacuum furnace at high temperature for an extended period of time, such as 600 C. for approximately six hours, for example. Additionally, during tube processing, the assembled tube is puumped continuously and heated at a high temperature for an extended period of time, such as 400 C. for about twelve hours, to remove volatile gases from all the component parts of the tube. During this out-gassing stage of tube fabrication, careful processing procedure requires that the copper anode be torch heated to a higher temperature, such as 600 C. for example, for a time interval of about two to four hours. This torch heating operation is required to further out-gas the copper anode and to eliminate any copper oxides that may have formed on the inner surface of the anode.

Exterior copper surfaces which are exposed to air become discolored at high temperatures due to the formation of surface oxides. Consequently, after the out-gassing cycle, the exterior copper surfaces of the power tube, such as the outer surface of the copper anode cup for example, are covered with a thick, brown scale. This coating of copper oxide not only has an objectionable appearance but it also interferes with the electrical and thermal conductivity of the copper. Therefore, after the tube is sealed off, the exterior copper oxide scale is removed by immersing the tube in an acid cleaning bath. Further oxidation is avoided by the application of a protective coating such as a plating which is applied by immersing the tube in a plating bath.

Power tubes of the described type are quite heavy and cumbersome due primarily to the weight of the copper anode cup at one end of the tube. Another contributing factor to the excessive weight of the tube is the relatively heavy copper structures which support the other electrodes of the tube. Difiiculty in handling these tubes has resulted in damage to the filament wires and jarring the grid out of alignment with the closely spaced cathode structure; A significant factor tobe considered in reducing the weight of the tube is that the density of copper is 555 pounds per cubic foot.

From the foregoing discussion, it can be seen that a suitable substitute for the copper material in power tubes would be a decided improvement if it reduced the weight of the tube, did not form oxides that become volatile at high operating temperatures, and could be out-gassed in a shorter length of time.

SUMMARY OF THE INVENTION This invention relates to an electron tube having a gastight envelope closed at one end by an aluminum anode cup. Internal metallic structures for supporting the other electrodes of the tube may be made of aluminum also. The density of aluminum, 168 pounds per cubic foot, is approximately one-third the density of copper and reduces the weight of the tube correspondingly. The thin oxide layer that forms on aluminum surfaces is very stable at high temperatures and does not become volatile in the vacuum environment of an electron tube. Power tubes made with aluminum parts give off less gas during the out-gassing cycle and pass through this stage of tube production in less time than required for power tubes made with copper parts. Aluminum anode power tubes have operated satisfactorily in place of copper anode power tubes indicating that aluminum is a satisfactory substitute for copper in power tubes.

BRIEF DESCRIPTION OF THE DRAWINGS For better understanding of this invention, reference is made to the drawings wherein:

FIG. 1 is an elevational view, partly in axial section, of a typical electron tube having the aluminum anode of this invention;

FIG. 2 is an enlarged axial fragmentary view showing the brazing method of forming a joint between an aluminum anode and the adjacent end of a glass bulb;

FIG. 3 is an enlarged axial fragmentary view showing the corolled means of forming a joint between an aluminum anode and the adjacent anode of a glass bulb; and

FIG. 4 is an enlarged axial fragmentary view showing the coextruded means of forming a joint between an aluminum anode and the adjacent end of a glass bulb.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to the drawing, wherein like characters of reference designate like parts throughout the. several views, there is shown in FIG. 1 an illustrative tube comprising a gas-tight envelope closed at one end by a deep cylindrical anode cup '10 which is made of aluminum material. Anode encloses an open-ended cavity 12 and, externally, may carry a plurality of radially extending fins 14 which encircle the anode cup 10 and provide radiational cooling for the anode. An external flange 16, adjacent the open end of anode 10, extends radially outward from the wall of the anode cup and provides the anode terminal of the tube. As shown more clearly in 'FIG. 2, a peripheral groove 18 in the wall of the anode 10, adjacent terminal flange 16, receives one end of a Kovar sleeve 20. The aforesaid end of Kovar sleeve 18 is aluminized and hermetically brazed to aluminum anode 10* by one of the methods well known in the art of plating. Because over fifty percent of the composition of Kovar is iron, the remainder being cobalt and nickel, Kovar sleeve 20 is treated as a ferrous alloy material in the aluminizing process. .A hot-dip coating in liquid aluminum is one of the methods used to aluminum-coat ferrous materials, as described on pages 490 and 491 of Metals Handbook, volume 2, 8th edition. When the selected surface area of Kovar sleeve 20 has been aluminized, it can be hermetically attached to aluminum anode 10* by a dip-brazing technique described in Military Specification MIL- B- 7883A which comprises the steps of applying a stop-off solution to areas that are not intended to be brazed, placing Kovar sleeve 20 and anode 10 into a fixture that holds the aluminized surface of Kovar sleeve 20 in close fitting relationship with the selected surface of anode 10 and immersing the fixtured assembly in a molten bath of aluminum-silicon filler alloy. Alternatively, the aluminum surface area of Kovar sleeve 18 may be vacuum brazed to the selected surface area of aluminum anode 10 by a process described in NASA Tech Brief 66-10241.

An outwardly extending annular flange 22 at the other end of Kovar sleeve 20 is hermetically attached by conventional means, such as heli-arc welding for example, to a similar flange 23 at one end of another Kovar sleeve 26. This joint is advantageously made the final seal in constructing the tube. The opposite end of Kovar sleeve 26 is peripherally sealed in the conventional manner, to one end of a dielectric bulb 28 and a portion 24 of anode cup 10 extends beyond the glass-to-metal seal to shield it from electron bombardment. The other end of bulb 28 is peripherally sealed to one end of a Kovar sleeve 30 which, at the opposite end, is hermetically attached, as by brazing for example, to one side of an outwardly extending, annular flange 32. Flange 32 serves as the grid terminal of the tube and is an integral part of a longitudinally extending grid support cylinder 34 which terminates at the other end in an inwardly extending, annular flange or deck 36. One end of a Kovar sleeve 38 is circumferentially attached, as by brazing for example, to the inner periphery of grid support cylinder 34 adjacent grid terminal flange 32. Flange 32 and support cylinder 34 are usually made of copper which can be brazed to Kovar by a method already well known in the tube-making art.

The other end of Kovar sleeve 38 is peripherally sealed to one end of a dielectric ring '40 which is similarly sealed, at the opposite end, to one end of a Kovar sleeve 42. The other end of Kovar sleeve 42 is hermetically attached, as by brazing for example, to an outwardly extending, annular flange 44, which is usually made of copper. Flange 44 provides one of the cathode terminals of the tube and is an integral part of a longitudinally extending support cylinder 46 which terminates at the opposite end in an inwardly extending annular flange or deck 48'. One end of a Kovar sleeve 50 is circumferentially attached, as by brazing for example, to the inner periphery of cathode support cylinder 46 adjacent cathode terminal 44. Cathode terminal 44 and support cylinder 46 are usually made of copper and are brazed to adjacent ends of Kovar sleeves 42 and 50 respectively, in the conventional manner. The other end of Kovar sleeve '50 is peripherally sealed to one end of a vitreous ring 52 which is similarly sealed, at its opposite end, to one end of a Kovar sleeve 54. The opposite end of Kovar sleeve 54 is hermetically attached, as by brazing for example, to one side of a metallic disc 56 which provides the other cathode terminal of the tube and which is attached, as by brazing for example, to one end of a cathode support cylinder 58. Cylinder 58 is terminated at the other end by an inwardly extending annular flange or deck 60. An exhaust tubulation 62 is circumferentially sealed over a central aperture in disc 56 and is pinched off to close the aperture after processing of the tube is completed, thereby closing the other end of the described gas-tight envelope. Disc 56, cylinder 58 and exhaust tubulation 62 are usually made of copper which can be brazed to Kovar or copper by a method well known in the art of tube manufacturing. Exhaust tubulation 62 is enclosed by a protective cap 64 which is peripherally attached to the external surface of cathode terminal disc 56 by conventional means, such as high temperature solder for example.

Within the gas-tight enclosure described above, a plurality of grids rods 66 have respective ends secured in a conventional manner, such as brazing for example, to the annular surface of flange 36 and form a cylindrical, cage-like structure which extends longitudinally into anode cavity 12. Within anode cavity 12, a filament cage (not shown) is coaxially disposed within the grid cage and in spaced relationship therewith. The filament cage comprises a plurality of longitudinally extending, parallel filament wires (not shown) which have respective ends attached to one end of respective filament support rods 69 and 70, alternate filament wires connecting to support rods 68 and the intervening filament wires connecting to support rods 70. The other ends of the longitudinally extending support rods 98 are attached by conventional means, such as brazing for example, to the annular surface of flange 48. Support rods 70 extend longitudinally and insulatin gly through respective aligned holes (not shown) in flange 48 and are attached at the other respective ends, to the annular surface of flange 58.

Prior to assembly of the tube, the aluminum anode is given an alkaline cleaning to remove surface contaminants and it is stored in an ordinary air atmosphere. In a very short time, after the alkaline cleaning, a thin film of aluminum oxide forms on the surfaces of the aluminum anode. However, unlike copper oxides, the thin oxide layer that forms on aluminum surfaces is very stable at high temperatures. It is well known that aluminum oxides, such as alumina ceramic for example, can be used in high temperature environments without deteriorating. Furthermore, the varporization pressure of the aluminum is much higher than that of the aluminum oxide layer. In fact, the aluminum oxide is stable up to temperatures much higher than the melting point of the aluminum itself. Consequently, an aluminum oxide layer does not become volatile in the vacuum environment of an electron tube envelope during high temperature operation of the tube. Therefore, a power tube having an aluminum anode does not have the problems associated with vaporization of metallic oxides from the anode material, such as formation of leakage paths on dielectric surfaces and oxygen poisoning of the cathode emitting surface.

After heating a number of aluminum anodes at 400 C. for several hours, the exterior surfaces of the aluminum anodes which were exposed to the air showed no visible evidence of discoloration or formation of a thick oxide layer. Therefore, the external surface area of an aluminum anode does not require a final acid cleaning and application of a protective plating or other coating for the purpose of preventing heavy oxidation. The resistance of aluminum to heavy oxidation and the extreme stability of the thin aluminum oxide layer eliminates the requirements for initial acid cleaning and storage in a nitrogen atmosphere that are required for copper anodes.

A study of the degassing properties of high purity aluminum and high purity copper indicated that aluminum, in reaching a relatively gas-free condition, released less gas than copper. In terms of time required to liberate a specific quantity of gas from aluminum and from cop per, it was observed that the aluminum released the specified quantity of gas in less time than the copper. After two hours in a vacuum environment at 600 C., it was found that copper was releasing twice as much gas as aluminum. It was also found that aluminum outgassed at 350 became relatively gas free in less time than copper out-gassed at 600 C. These results indicate that power tubes made with an aluminum anode can be out-gassed at a lower temperature and in a shorter time period than power tubes made with a copper anode. For this reason, the aluminum anodes do not require vacuum firing at high temperature prior to assembly in a power tube. Furthermore, the aluminum anodes do not require torch heating duing the out-gassing cycle of the assembled tube. Tests have indicated that the out-gassing cycle for aluminum anode power tubes can be shortened considerably.

Electrical tests have shown that the aluminum anode power tubes have better voltage stability and lower field emission than copper anode power tubes. Aluminum anode power tubes exhibit good cathode emission, low gas and no measurable grid emission. Impinging electrons penetrate through the thin oxide layer on the internal surfaces of the anode and are conducted out of the tube through the anode terminal flange 16. It has been found that the thin aluminum oxide [film does not interfere with the thermal radiation efiiciency or the electrical conductivity of the aluminum anode. The aluminum anode does not overheat and the lower electrical conductivity of aluminum as compared to that of copper does not have a detrimental eflect on the electrical characteristics of the tube. These results indicate that the electrical and thermal conductivity of copper is higher than required for normal operation of the tube.

A number of power tubes were fabricated having an aluminum anode attached to an otherwise conventional power tube. The weight of a typical power tube having a copper anode is about forty-three pounds and the weight of an aluminum anode power tube having the same configuration is about nineteen pounds. Thus, the weight of the aluminum anode power tube is less than half the weight of the copper anode power tube. The reduced weight and consequent ease in handling of the aluminum anode power tube results in a reduced likelihood of damaging the filament or jarring the grid out'of alignment. An additional advantage resulting from the reduced weight of the aluminum anode power tube is a considerable savings in shipping costs.

An alternative means for hermetically attaching the aluminum anode 10 to Kovar sleeve 26 is shown in FIG. 3 of the drawing. A laminated collar 72 comprises a layer of ferrous material 74, such as stainless steel for example, which has been corolled to a layer of aluminum 76 as described on pages 495 and 496 of Metals Handbook, volume 28, 8th edition. The resulting joint between the longitudinally adjacent layer of ferrous metal 74 and aluminum 76 is vacuum-tight and mechanically strong. This corolled material is manufactured and can be obtained commercially. At one end of the collar 72, the aluminum layer 76 is hermetically attached by conventional means, such as brazing for example, to a rib 78 which extends radially outward from the longitudinally extended portion 24 of aluminum anode 10. The other end of the laminated collar 72 extends radially outward to form an annular flange portion 77 and the stainless steel surface thereof is attached by conventional means, such as heli-arc welding for example, to the outwardly extending flange end 23 of Kovar sleeve 26.

Another alternative means for hermetically attaching the aluminum anode 10 to Kovar sleeve 26 is shown in FIG. 4. Metallic sleeve 80 comprises a tubular section of aluminum 82 at one end, a tubular section of ferrous metal 84, such as stainless steel for example, at the other end and a transition joint 83 therebetween. The transition joint 83 is a metallurgical bond made by forcing the respective metals through a die at elevated temperatures. These co-extruded aluminum-to-stainless steel transition sleeves are made commercially, as shown on page 27 of Materials Engineering, February 1966, and are presently available in sizes suitable for use in power tubes. The aluminum end of the transition sleeve 80 is inserted into the peripheral groove 18 of anode 10 and is attached thereto by conventional means, such as dip-brazing, for example. The ferrous metal end of the transition sleeve 80 is provided with an outwardly extending flange 86 which is hermetically attached by conventional means, such as heli-arc welding for example, to the flange 23 on the adjacent end of Kovar sleeve 26.

It is obvious that the weight of the described power tubes could be reduced still further by making the respective support cylinders 34, 46, and 58 from aluminum instead of the usual copper material. In such a device, the ends of the respective support cylinders are attached to the respective inner peripheries of annular flanges 32, 44, and 56, respectively, which flanges are made of the usual copper material and attached to adjacent ends of respective Kovar sleeves in the usual manner. Alternatively, the respective flanges 32, 44 and 56 are also made of aluminum and form integral parts of respective aluminum support cylinders 34, 46 and 58. In this case, the aluminum flanges are hermetically attached to adjacent ends of respective Kovar sleeves by one of the methods shown in FIGS. 2, 3, and 4.

Although the word aluminum has been used throughout the specification, alloys of aluminum are intended to be included within the meaning of the term aluminum in the claims appended hereto. These and other modifications which may occur to those skilled in the art may be made without departing from the spirit and scope of this invention, as expressed in the appended claims.

We claim:

1. An electron discharge device comprising a gas-tight envelope having at least one dielectric portion, an anode which is formed substantially entirely of aluminum and which comprises a portion of said envelope, at least one additional electrode within the envelope and having terminal means extending through the envelope, and means for hermetically attaching said aluminum anode to the dielectric portion whereby a vacuum-tight seal is formed therebetween, said means comprising a Kovar ring sealed 7 to said dielectric member, and an intervening laminated member having longitudinally adjacent layers of dissimilar materials hermetically attached to said aluminum an ode and another of the layers being hermetically attached to said Kovar ring.

2. An electron discharge device as set forth in claim 1 wherein said laminated member comprises a layer of aluminum and a layer of ferrous material, the aluminum layer being hermetically attached to said aluminum anode and the ferrous material being hermetically attached to said Kovar ring.

3. An electron discharge device comprising a gas-tight envelope having at least one dielectric portion, an anode which is formed substantially entirely of aluminum and which comprises a portion of said envelope, at least one additional electrode within the envelope and having terminal means extending through the envelope, and means for hermetically attaching said aluminum anode to the dielectric portion whereby a vacuum-tight seal is formed therebetween, said means comprising a Kovar ring sealed 20 to said dielectric portion, and an intervening member having longitudinally disposed portions of dissimilar material hermetically joined by a transition joint therebetween, one member being hermetically attached to said aluminum anode and the other member being hermetically attached to said Kovar ring.

4. An electron discharge device as set forth in claim 3 wherein said intervening member comprises an aluminum portion and a ferrous portion hermetically joined at said transition jont, the aluminum portion being hermetically attached to said aluminum anode and the ferrous portion being hermetically attached to said Kovar ring.

References Cited UNITED STATES PATENTS 3,189,677 6/1965 Anthony et a1. 17450.61 3,112,185 11/1963 Ochsner 29183.5 3,209,195 9/1965 Schade 313--246 3,127,537 3/1964 Horsting 3l3-346 3,419,744 12/1968 Kerstetter 313-346 2,812,466 11/1957 Murdock 313317 JOHN W. HUCKERT, Primary Examiner B. ESTRIN, Assistant Examiner U.S. Cl. X.R. 313352, 355, 317

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