US2872028A - Getter structure - Google Patents

Description

Feb. 3, 1959 E. S. THALL GETTE'R STRUCTURE Filed Dec. 29, 1954 L-co @iz/r5.5

FIEL

IN V EN TOR.

- EARLE S. THALL BY GETTER STRUCTURE Earle S. Thall, East Grange, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application December 29, 1954, Serial No. 478,369

5 Claims. (Cl. 20G-.4)

This invention relates to an improved getter structure and to an improved method of making the same. More particularly, the invention provides an improved getter structure having a core containing a material which eX- hibits gettering properties and a sheath surrounding the core and provides an improved method of making such a structure. The structure of the invention may be in wire form and proves useful as a getter, that is, as a clean-up agent for removing residual gases within an evacuated electron discharge device, when activated in what is known as a getter flash. The structure of the invention is adapted to control the direction of the activation flash of the getter structure and protects the core of the Wire from an atmosphere which is chemically reactive with the material of the core before the activation ash thereof.

Metals which prove ideally suited as getter materials, due to their relatively high chemical reactivity with gases, are relatively difficult to handle because they also react rapidly with moisture and some of the gases in the atmosphere. and hydrated oxides on exposure to the atmosphere.

Due to the oxidizability of such highly reactive metals, these metals have heretofore been introduced into electron tubes as alloys. However, unless such alloys contain a relatively small percentage of the highly reactive getter material, they are still attacked by the oxygen and water vapor of the atmosphere. Due to the necessary high percentage of alloying stable material required, a relatively high temperature is usually needed to liberate the getter material. But even at high temperatures the getter material is given olf relatively slowly. Protracted high temperature treatment of the alloy accompanied by vaporization of undesired constituents may be deleterious. For example, when an aluminum-barium alloy which is stable in the atmosphere is introduced into an electron tube the barium is liberated only at a higher temperature than that needed to activate or flash a getter material composed of barium alone. By reason of the higher heating required there is also the risk that the heating may be carried so high that although aluminum has a higher temperature of volatilization than barium, part of the aluminum volatilizes and deposits on those parts of the tube where the barium is to be precipitated.

Space limitations often permit only a limited amount of getter material within an electron tube. The need for a relatively large amount of alloying stable material reduces the yield of the reactive material within the tube. In some cases the yield has been inadequate for a thorough gettering action.

Previous attempts to mechanically encase a core of a getter material in a sheath have resulted in the introduction of gases between the outside surface of the core material and the inside surface of the sheath. When such a structure is activated Within an evacuated tube for gettering purposes, the entrapped gases are released. This increases the amount of gas within the tube.

Barium, for example, forms barium oxide n 2,872,028 Patented Feb. 3, vi959 An object of the invention is to provide an improved getter structure free from entrapped gases.

Another object of the invention is to encase free alkaline earth metal material without entrapping gases.

A further object of the invention is to provide an improved structure for isolating atmospherically reactive material and for controlling the direction of the activation flash of the reactive material.

It is yet another object of the invention to provide an improved getter having a core material and a protective sheath react exothermically to flash all of the material of the getter.

lt is still another object of the invention to provide an improved getter structure having a core of barium, a sheath of aluminum, and a tube, having an eccentric bore therethrough, around the sheath, and wherein said core completely fills the space within the sheath so that the region between the outer surface of the core and inner surface of the sheath is free of entrapped'gases.

It is yet a further object of the invention to provide an impro-ved method of casting, into a container having a relatively thin walled portion, a material which has a melting point above that of the container.

A still further object of the invention is to provide an improved method of making a getter structure for use within an electron tube and wherein a core of an alkaline earth metal is encased within a container having an eccentric bore therethrough without the entrapment of gases within the container.

According to the invention a structure and a method of making the same are provided for attaining the foregoing objects. i While the invention is pointed out with particularity in the appended claims, it may be best understood from the following detailed description and drawing wherein like numerals refer to like parts, and wherein:

Figure l is a flow chart of a method of making an aluminum-clad alkaline earth metal getter structure according to the invention;

Figure 2 is a vertical cross-section of a mold assembly illustrating a casting step according to the invention;

Figure 3 is a cross-sectional view taken on line 3 3 of the mold assembly shown in Figure 2;

Figure 4 is a side view partly in section of a structure produced by the mold apparatus shown in VFigures 2 and 3;

Figure 5 is a flow chart of a method of making another getter structure according to the invention; and

Figure 6 shows an enlarged perspective view, partly in section, of a getter wire made according to the method illustrated in Figure 5. y

Referring now to the drawing in greater detail, there is shown in Figure 1 a-ow chart of a method according to the invention of making an aluminum-clad alkaline earth metal structure that is adapted to control the direction of the getter flash thereof. The material to be used for the core of the structure, which includes a metal of the alkaline earth group and which may have a melting point above that of aluminum, is melted and poured into an aluminum container having an eccentric bere therethrough. The aluminum container is maintained at a temperature below its melting point while the core material in the container cools. The core material completely iills the space within the container by virtue of its being poured into the container; consequently, the resultant structure is substantially free of entrapped gases. The operations or steps shown in the flow chart of Figure l will be described in greater detail below in connection with a description of one embodiment of the invention.

A vacuum melting furnace (not shown) may be used to melt'the core material. The core material used in this embodiment of the invention is composed of an alkaline 'a Il as the core material it is heated to a temperature of y above 850 C. and in an atmosphere of relatively pure argon at a pressure of about 0.2 atmosphere. The crucible usedin containing the barium during the melting operation should be of a material which is not reactive with barium. One material suitable for constituting the crucible and which is readily available in the trade is known as Armco Iron. A thermocouple gauge (not shown) may be used for the reading of relatively low pressures and a mercury manometer (not shown) may be used for reading higher pressures.

The melting may be accomplished by first placing the barium metal within the crucible in the chamber of a vacuum furnace and evacuating the latter to a relatively low pressure, say 25 microns of mercury. The barium is then slowly heated until most of the barium reactive gases therein are driven off. The chamber is then flushed out a number of times with an inert gas such as argon to remove substantially all traces of such gases. Two ushings have proved sucient. The pressure of argon is then adjusted to about 100 microns of mercury so as to provide a vapor pressure of argon which is at least as great as the vapor pressure of barium at its melting point to prevent the barium from boiling off into the furnace. The temperature of the furnace is then increased to melt the barium. After the barium is completely molten, the heat is increased and the melt is held at an elevated temperature for about iive minutes in order to completely degas the melt and to provide an elevated pouring temperature. The melt is then poured into a mold assembly of the type shown in Figure 2, in an inert atmosphere. The mold assembly is then allowed to cool to room temperature in the same inert atmosphere. After the core material has cooled to room temperature the structure produced by the mold is removed.

An alternate method of removing the barium reactive gases may be used. In the alternate method the barium is deliberately allowed to react with the residual gases in the furnace. As a consequence a slag is formed; the slag floats to the top of the melt. Relatively pure barium may then be poured from below the layer of slag. This alternate method, however, it not preferred since it results in a waste of some of the relatively expensive barium.

There is illustrated in Figures 2 and 3 an apparatus useful in practicing the method of the invention and cornprising a mold assembly 14 adapted to receive a sheath or container 12, having an eccentric bore therethrough and a core of molten material 10. The mold assembly i4, which may comprise a copper body 16 closed off at the bottom thereof by a suitable metal plug 18, is relatively massive compared to the sheath and molten material received therein. In one example the copper body 16 had a length of 7% inches and a wall thickness of one inch. The body 16 may be split, as shown in Figure 3, to facilitate removal of the composite structure produced by the mold assembly. While the use of a split copper body is preferred, a body of any other material may be used having a thermal conductivity and capacity such that the temperature of the inside surface of the container is maintained at a temperature below its melting point.

A funnel 20, which may be of a material such as graphite so as to reduce any oxides that may be formed, may be disposed around an opening at the top of the mold assembly as viewed in Figure 2 in order to direct the flow of barium into the container 12. The container 12 may have an inside diameter of about one-half inch and a Wall thickness of the order of about one-sixteenth of an inch at the thickest portion of the wall and about tion working the structure to wire form.

t one-thirty-second of an inch at the thinnest portion of the wall.

The sheath or container 12 is preferably made of aluminum. The use of an aluminum sheath is preferred since, on flashing a getter structure comprising a core material including an alkaline earth metal, the sudden and marked increase in temperature of the getter structure seems to initiate a reaction between part of the core materialaand a portion of the aluminum sheath adjacent the core material. This reaction is apparently exothermic and insures that substantially all of the core material is heated to ashing temperature. The aluminum sheath may be provided with a difference in thickness between portions of the sheath wall such that the thinnest portion of the wall is consumed in the getter flash while parts of the thicker portions thereof control the direction of the flash.

It will be noted from the foregoing that an inside diameter of One-half inch is used. It has been found that if the inside diameter of the aluminum container is reduced to appreciably below one-half inch the poured material is cooled to solid state before it reaches the bottom of the container and prevents the formation of the desired composite structure. If the inside diameter of the aluminum container is increased appreciably beyond onehalf inch the larger mass ofpoured material will possess a magnitude of heat such that special cooling means are needed to maintain the inside surface of the aluminum container below its melting point. Such cooling means are relatively expensive.

Figure 4 is a view of a structure produced by the mold apparatus shown in Figures 2 and 3. It will be seen that the ends of the aluminum container 12 are pinched together in order'to seal the core material lil from the atmosphere.

The outside surface of the structure produced by the mold is then marked along the thinnest portion of the wall of the container. A getter wire which is made by working this structure to wire form may then be positioned within an electron discharge device in orientation such that the reactive portion of the getter wire is flashed in the desired direction within the device. The marking referred to is chosen of a character such that it is retained by the outside esurface of the container during the opera- The marking may be made -by painting, along the desired surface, a chemical which is reactive with the material of the surface to produce a discoloration thereof. Chemicals known in the trade as machinists marking fluid may bc used for this purpose.

The structure may be drawn down to the desired diameter by means of wire-drawing dies. Since the aluminum container is relatively ductile, any spaces formed between the outside surface of the core material and the inside surface of the aluminum container are substan-V tially eliminated in the drawing operation by the pressure forcing the aluminum sheath against the core. The sheath of the wire thus formed retains the eccentric orientation of the bore of the aluminum container and the marking along the outside surface of the thinnest portion of the wall thereof. The wire may be cut into desired lengths by means of a pinching operation so that an aluminum coating is retained around the barium at the severed ends.

It has been shown that barium, which has a melting point of about 850 C., may be cast into an aluminum container which has a melting point of about 660 C., without melting the container. Since aluminum exhibits a boiling point of about 2056 C. and barium exhibits a boiling point of about ll40 C., barium will boil oif or flash at a lower temperature than aluminum. Therefore, barium rather than aluminum will be more likely to be deposited on surfaces within an electron discharge device for providing gettering action within the device.

Similarly, other metals of the alkaline earth group,

namely, strontium-melting pointcf about 800 C., calcium-melting point of about 810 C., and magnesiummelting point of about 651 C., may be cast in an aluminum containerwhich has a melting point that is lower than that of the melt which is cast into the container. When magnesium is used as the casting material the melt is ordinarily heated to a temperature substantially above 660 C., the melting point of aluminum, in order to assure a free ow of the melt into the container.

While the method of the invention has been described with regard to relatively pure alkaline earth metal cores, the method of the invention may be used in the casting of other materials which exhibit gettering action on ashing. For example, it is often desired to use a bariumaluminum alloy as a getter material for certain high temperature flash getters wherein the barium-aluminum alloy used is relatively unstable in air. Then, too, it is often desirable to 4alloy a relatively small quantity of Valuminum with an alkaline earth metal getter material in order to improve the workability of the getter material so that a slug of the getter material may be more easily rolled, swaged, or drawn through wire-forming dies to produce getter material in the form of relatively thin wire.

A barium-aluminum alloy may be cast by the method of the invention to produce an aluminum-clad bariumlaluminum alloy core structure. One getter material made according to the method of the invention has a core of an alloy of barium of 99% purity and aluminum of 99.6% purity in a ratio of about 99% barium to 1% aluminum by weight. In order to prevent the barium from reacting with the atmosphere during the measuring step, the barium may be weighed in paratline oil and rinsed in toluene before being placed in the melting crucible with the aluminum. The melting crucible is placed in a vacuum melting furnace of the aforedescribed type and the vacuum chamber is evacuated to a relatively low pressure. The charge is then slowly heated until most of the gases therein are driven olf. The chamber is then flushed out with argon. The pressure of argon is then adjusted to about 150 millimeters of mercury and the charge quickly melted.

After the charge is completely molten, the heat is further increased and the charge held for about 5 minutes at the increased temperature in order to completely degas the melt and to achieve an elevated pouring temperature. The alloy is then poured into the aluminum container in the mold assembly of the aforedescribed type and allowed to cool to room temperature in the argon atmosphere.

Instead of the argon atmosphere called for in the above description, any other inert atmosphere may be used provided the gas of the inert atmosphere is not absorbed by the core material or the aluminum liner. For example, helium or neon may be used as the inert atmosphere. However, argon is preferred for reasons of economy. Instead of the inert atmosphere a vacuum may be used; but, as mentioned before, the use of a vacuum is not preferred.

There is shown in Figure 5 a flow chart of a method according to the invention of making another flashable getter structure. This structure is also adapted to control the direction of the getter flash from the structure. This getter structure also employs a metal of the alkaline earth group. While the .material used for the core of the structure may be any material which exhibits gettering action on flashing, such as for example lithium or cerium, the use of an alkaline earth metal with a sheath of aluminum around and in contact with the core material is preferred. As has been explained before, n etals of the alkaline earth group, on being heated to flashing temperature, react with aluminum in a reaction which is apparently exothermic. This insures that all of tie core material is flashed.

The alkaline earth metal core material is melted and poured'into an aluminum container to form an aluminum-clad slug according to the method described with respect to Figures l, 2, 3, and 4. The slug, which is characterized in being substantially free from entrapped gases, is then inserted into a tube having an eccentric bore therethrough for controlling the direction of the getter ash of the structure. The eccentrically bored tubing, containing thealuminum-clad core material, is then Worked to wire form to produce a getter structure in wire form and which is adapted to be used within an electron discharge device.

Figure 6 shows a getter wire made according to a method of the invention illustrated in Figure 5. While the method illustrated in Figures l through 4 utilizes a container having an eccentric bore, it is feasible in practicing the method of Figure 5 to provide the container in immediate contact with the core that has a central bore. Thus, as shown in Figure 6, the getter wire includes a core 22 comprising an alkaline earth metal material surrounded by an inner sheath 2.4 of aluminum having a central bore. The inner sheath is in turn surrounded by an outer sheath 26 having an eccentric bore therethrough. The outer sheath 26 may be of a material which has a melting point above that of the inner sheath 24 so that the inner sheath may be consumed by the getter flash while the outer sheath controls the direction of the flash. The outer sheath is preferably chosen to be of a material which exhibits a relatively low vapor pressure at the normal operating temperature of the device within which the getter wire is to be used. Iron, nickel, cobalt, titanium, and alloys including any one or more of these materials have proven desirable for the outer sheath.

The outer surface of the getter wire is provided with a marking 28 along the thinnest portion of the wall of the outer sheath 26 for identification of the direction in which the getter ilash of the wire will be directed on activation of the wire. As previously mentioned with respect to the marking of the thinnest wall portion of the aluminum container having an eccentric bore, the outside surface of the outer sheath 26 may be marked by painting, along the desired surface, a chemical which may be reactive With the material of the surface to produce a discoloration thereon. When machinists marking fluid is used as the marking chemical, vthe surface along which the mark is to be made may be inscribed and the impression thus made lled with the marking fluid.

Other forms of enclosures for the reactive material of the core may be used instead of those wherein the inner sheath has the inner and outer surfaces thereof concentric with the long axis of the core, and thus has a uniform wall thickness along all portions thereof, and the outer sheath has its inner surface concentric and its outer surface eccentric with the long axis of the core. For example, both the inner and outer sheaths may be provided with eccentric bores therethrough and with the thinnest wall portions thereof in registry with each other. In this example both the inner and the outer sheaths contribute to the control of the direction of the getter ash of the wire.

While the getter structure made according to the method of the invention is useful as a getter material within electron tubes, it will be appreciated that the invention is equally useful in other applications wherein -a core of a highly reactive getter material is desired which is substantially free of entrapped gases.

What is claimed is:

l.-A flashable getter structure for an electron discharge device comprising a core of a material reactive with the ordinary atmosphere and exhibiting gettering action on activation by flashing, and an aluminum casing around said core having an outer surface of uniform curvature, said casing having a bore radially eccentric with respect to said curvature, whereby said casing has a relatively thin wall portion, and a-relatively thick wall portion extending in parallelY relation and longitudinally of said casing, said relatively thin vwall portion of said casing being adapted to be consumed by the getter flash of said structure before said relatively thick portion thereof thereby providing control over the direction of said getter ash, said core completely filling the space within said casing whereby the entire region of said structure within said casing is substantially free of entrapped gases.

2. A structure adapted to be used as a getter within an electron discharge device comprising an elongated core of a ashable getter material including an element selected from the class consisting of alkaline earth metals and alkaline earth metal alloys, and an aluminum sheath around said core and having the inner surface of said sheath concentric with the long axis of the core and the outer surface thereof of uniform curvature throughout a cross-section of said sheath and eccentric with said axis, said core completely lling the space withil in said sheath, the thinnest wall portion of said sheath being adapted to be consumed by the getter flash of said Y structure thereby providing control over the direction of said getter flash, said thinnest wall portion extending longitudinally of said sheath, and having a coating of marking material thereon, whereby said sheath may be oriented to cause said thinnest portion thereof to face said direction.

3. A ashable getter structure comprising a core of a material including an element selected from the class consisting of alkaline earth metals and alkaline earth metal ailoys and reactive with the ordinary atmosphere and exhibiting gettering action on activation by hashing, and inner and outer elongated sheaths around said core, said core completely lling the space within said inner sheath, said inner sheath being of a material that is exothermically reactive with said core material when said core material attains a temperature sulicient to effect a getter ash thereof, thereby insuring the flashing of all of the material of said core when a portion of said core is flashed, the entire region of said structure within said inner sheath being substantially free of entrapped gases, said outer sheath encasing said inner sheath and being of a material including an element selected from the class consisting of iron, nickel, cobalt, and titanium and having a longitudinally extending portion of the wall thereof that is thinner than other portions thereof, the outer surface of said outer sheath having the same curvature at said portion and said other portions, whereby said structure is adapted to control the activation flash thereof to a direction extending from said thinner wall portion.

4. A ilashable getter structure comprising a core material reactive with the ordinary atmosphere and exhibiting gettering action on activation by dashing, and elongated inner and outer sheaths, said core material cornpletely filling the space within said inner sheath, said inner sheath surrounding said core material and being of a material that is exothermically reactive withsaid core material when said core material attains a temperature suiiicient to effect a getter ilash thereof thereby providing means for flashing all of the material of said core when a portion of said core is flashed, the region of said structure within said inner sheath being substantially free of entrapped gases, said outer sheath encasing said inner sheath, said inner and outer sheaths each having a longitudinally extending wall portion that is thinner than other portions of the wall, the wall of the outer sheath having an outer surface of uniform curvature completely around said outer sheath, the thinner wall portions of the sheaths being in registry with each other, whereby said structure is adapted'to control the direction of the activation flash thereof, said thinner wall portion having a marking on its outer surface for indicating its location on said outer sheath, whereby said direction may be predetermined.

5. A method of making a structure adapted to be used as a getter within an electron discharge device having an aluminum sheath, and a core material exhibiting gettering action on flashing and having a melting point above that of aluminum; and comprising the operations of heating said material to a temperature above the melting point thereof, pouring said material at said temperature into an aluminum container, said container being characterized by a thermal conductivity and capacity suihcient to maintain the temperature thereof below the melting point of aluminum thereby providing an aluminum clad slug, inserting said slug into a tube having an eccentric bore and an outerlsurface of uniform curvature around said tube, marking the outside surface of said tube along the thinnest wall portion thereof to provide a reference indication, and working said tube and slug to wire form while preserving said reference indication, whereby spaces formed during the cooling of said core material, between the outside surface of said core and the inside surface of said container, are substantially eliminated and a structure is provided having the region thereof within said container substantially free of entrapped gases and adapted to be oriented for disposing said thinnest wall portion in the direction of a desi-red flash.

References Cited in the tile of this patent UNITED STATES PATENTS 1,682,590 Austin Aug. 28, 1928 1,973,550 Todt Sept. 11, 1934 2,100,257 Larson Nov. 23, 1937 2,329,317 Atlee Sept. 14, 1943 2,624,450 Britten et al. Jan. 6, 1953 2,657,452 Veenemans et al. Nov. 3, 1953 2,837,207 Solet et al. June 3, 1958 FOREIGN PATENTS 567,291 Great Britain Feb. 7, 1945

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