US2837207A - Getter structure - Google Patents

Description

June 3, 1958:

Filed Dec. 29, 1954 l. S- SOLET El' AL GETTER STRUCTURE 2 Sheets-Sheet 1 IN VEN-TORJ HUBERT L .'WHER Ina S. SDLET rah/iv June 3, 1958 vl. s. soLET ETAL INVENTORS HUBER-.r I .WHER a l IRR S. SDLET www i and to an improved method of making the same.

United States GETTER STRUCTURE Application December 29, 1954, Serial No. 478,352

Claims. (Cl. 2G6.4)

This invention relates to an improved getter structure More particularly, the invention provides an improved getter wire having a core containing a material which exhibits gettering properties and a sheath surrounding the core and provides an improved method of making such a wire. The wire made according to the method of the invention proves useful as a getter, that is, as a clean-up agent for removing residual gases within an evacuated electron discharge device. The structure of the invention protects the core of the wire from an atmosphere which is chemically reactive with the material of the core. The method vof making the structure includes the casting of a core material in a container.

Metals which prove ideally suited as getter materials, due to their relatively high chemical reactivity with gases, are relatively diiiicult to handle because they also react rapidly with moisture and some of the gases in the atmosphere. Barium, for example, forms barium oxide 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 suchalloys 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, a relatively high temperature is usually needed to liberate the getter material. Even at high temperatures the getter material is given oi relatively slowly. Protracted high temperature treatment of the alloy accompanied by vapoi-ization of undesired constituents may be deleterious. For example, when an aluminum-barium alloy which is stable in the atmosphere is introduce-d into an electron tube the alloy is decomposed and the barium liberated only at a higher temperature than that needed to activate or flash the barium. 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.

An object of the invention is to provide an improved atent ICC structure comprising a container having therein an atrnospherically reactive material.

A further object is to provide a structure comprising an atmospherically reactive alkaline earth metal and a container shielding said metal from the atmosphere and which is free of entrapped gases.

It is another object of the invention to provide an improved getter structure having a core of barium and a sheath of aluminum and wherein said core completely lills 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 a further object of the invention to provide an improved method of casting, into a container, a material which has a melting point above that of the container and which is reactive with the atmosphere.

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 sheath of aluminum without the entrapment of gases within the sheath.

It is yet another object of the invention to provide an improved aluminum sheathed wire comprised substantially of an alkaline earth metal and Which is adapted to be used as a getter material within an electron tube.

According to the invention a structure and a method of making the same are provided for attaining the foregoing objects.

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. The embodiments described are presented solely for illustrative purposes and not by way of limitation.

In the drawings:

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

Figure 2 is a liow chart showing in greater detail the steps of a method of the invention.

Figure 3 is a vertical cross-sectional view of a mold assembly illustrating a casting step according to the invention.

Figure 4 is a cross-sectional view taken on line 4-4 of the mold assembly shown in Figure l.

Figure 5 is a side view partly in section of a structure produced by the mold apparatus shown in Figure 3.

Figure 6 shows an enlarged perspective view partly in section of an aluminum sheathed barium wire made according to the method of the invention.

Referring now to the drawings in greater detail there is shown in Figure l a flow chart of a method, according to the invention, which is used in making an aluminum-clad alkaline earth metal structure. 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. The aluminum container is maintained at a temperature below its melting point while the core material in the container cools. The core material completely fills 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 ydescription of one embodiment of the invention.

Figure 2 depicts an embodiment of the method illustrated in Figure l. 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 earth in metal form such as barium having a purity of the order of 99 barium. The core material is melted under vacuum or in an atmosphere of an inert gas at a relatively low pressure and at a temperature of above 850 C., the melting point of barium. For example, at atmosphere of relatively pure argon at a pressure of about 0.2 atmosphere may be used. The vCrucible used in containing the barium during the melting operation may be of a material 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 resultant gas evolution ceases. The chamber is then flushed out a number of times with an inert gas such as argon to remove substantially all traces of oxygen; two flushings have proven sulicient. 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 lprevent 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 five minutes in order to degas the melt and to achieved an elevated pouring temperature. The melt is then poured, in an inert atmosphere, into a mold assembly of the type shown in Figure 3; the mold assembly is then allowed to cool to room temperature in the same inert atmosphere.

There is illustrated in Figure 3 an apparatus showing the position of the core material and the aluminum sheath or container 12 within a mold assembly 14. The mold assembly 14, which may comprise a copper mold support 16 closed off at the bottom thereof with a suitable metal plug 18, is relatively massive compared to the aluminum and barium material within the support. A copper mold support having a length of 71A inches and a Wall thickness of one inch has been used. The support may be split, as shown in Figure 4, to facilitate removal of the composite structure produced by the mold assembly. While the use of a split copper mold support is preferred, a mold support of any other material may be used having a thermal conductivity and capacity such that the temperature of the inside surface of the container 7 is maintained at a temperature below the melting point of aluminum throughout the pouring and cooling steps.

A funnel 20, which may be of a material such as graphite so as to reduce any alkaline earth metal oxides that may be formed, may be disposed around an opening at the top of the mold assembly as viewed in Figure 3 inorder to direct the ow of barium into the aluminum container 12. The aluminum container 12 may have an inside diameter of about one-half inch and a wall thickness of the order of three sixty-fourths of an inch.

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 of poured material will possess a magnitude of heat such that special cooling means are neededto maintain the inside surface of the aluminum container below its melting point. Such cooling means are relatively expensive. K

After the core material has cooled to room temperature, the structure produced by the mold is removed. Figure 4 5 is a view partly in section of such a structure. The ends of the aluminum container 12 are pinched together in order to seal the core material 10 from the atmosphere.

The structure may be drawn down to the desired diameter by means of wire-drawing dies. Since the aluminurn container is relatively ductile any spaces formed between the outside surface of the core material and the inside surface of the aluminum container are substantially eliminated in the drawing operation by the pressure forcing the aluminum sheath against the core. The Wire thus formed may then be cut into the desired lengths by means of a pinching operation so that an aluminum coating is retained around the barium'at the severed ends. Thus, it is seen 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 1140a C., and barium exhibits a boiling point of about 2056 C., barium will boil off 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 point of about 800 C., calciummelting point of about `810" C., and magnesium-melting point of about 651 C., may be cast in an aluminum container which has a melting point which 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 lassure a free llow 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 alloys containing a metalfrom. the group of alkaline earth metals. For example, it is often desired touse a barium-aluminum alloy as a getter material for certain high temperature flash getters wherein the barium-aluminum alloy used is relatively unstablein air. it is often desirable to alloy a relatively small quantity of aluminum 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 maybe more easily rolled, swaged, or drawn through wire-forming dies to produce getter material in the form of relatively thin wire.

The barium-aluminum alloy may be cast by the method of the invention to produce an aluminum-clad bariumaluminum alloy core structure. One getter material made according to the method of the inventionv has a core of an alloy of barium of 99% purity and aluminum of 99.6% purity in a ratio of 99.0 grams of barium to 1.0 gram aluminum i. e. 99% barium and 1% aluminum by weight. The barium may be weighed in paraine 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 afore-described type and the vacuum chamber is evacuated to a relatively low pressure so as to remove barium reactive gases. The charge is then slowly heated until the evolution of gases ceases and the chamber flushed out with argon. The pressure of argon is then adjusted to about 150 millimeters of mercury and the charge quickly melted.

An alternative method of removing the 4barium reactive gases may be used. In the alternate method the barium v is deliberately allowed to react with the residual gases in Then, too,

5 the increased temperature in order to degas the melt and to achieve an elevated pouring temperature. 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. In stead of the inert atmosphere a vacuum maybe used; but, as mentioned before, the use of a vacuum is not preferred.

Figure 6 shows a portion of a wire adapted to be used as a getter material within an electron tube and which was made according to the method of the invention. While the drawing shows a sectional view of the wire for purposesy of illustrating its structure, the core material 22, which is of a material including an alkaline earth metal, is preferably completely sheathed by a coating of aluminum 24. l

While the aluminum-clad 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 application where a core of a highly reactive material is desired which is substantially free of entrapped gases.

What is claimed is:

1. A structure adapted to be used as a getter within an electron discharge device comprising a sheath of a material including aluminum, and a solid core member including barium within said sheath, said core member completely lling the space within said sheath.

2. A structure adapted to be used as a getter within an electron discharge device comprising a solid core member including an alloy consisting of aluminum and at least 50% barium by weight, `and a sheath of aluminum surrounding said core member, said core member completely filling the space within said sheath.

3. A structure adapted to be used as a getter within an electron discharge device comprising a solid core member including an alloy containing barium and aluminum in the ratio of about 99% barium to about 1% aluminum by weight, and a sheath of aluminum surrounding said core member, said core member completely filling the space within said sheath.

4. A method of making an aluminum-clad core structure having a core material including a metal selected from the class consisting of alkaline earth metals and alkaline earth metal alloys and characterized in being reactive with the ordinary atmosphere, and comprising the operations of heating said material to a temperature below the melting point thereof and in an atmosphere at below ordinary atmospheric pressure whereby said material is substantially degassed, further heating said material in an atmosphere of an inert gas at a pressure at least as high as that of the vapor pressure of said material and The alloy to a temperature sufficient to melt said material whereby said material is melted while evaporation of said material is reduced, still further heating said material in said last named atmosphere for achieving a pouring temperature of said material which is above ythe melting pointthereof, pouring said material in an inert gas at said last named pressure into an aluminum container which has a melting point below said melting point while drawing heat from the inside surface of said container through said container to maintain said inside surface at a temperature below the melting point thereof and thereby preventing any alloying between said core material and said container, cooling said material in an inert gas to a temperature below the melting point of said container, and mechanically working said container to remove any gases entrapped therein with said core material and to `close an opening in said container, whereby an aluminum-clad core structure is provided having a substantial freedom from entrapped gases.

5. A method of making aluminum-clad wire having a core material wherein the major constituent is barium, and comprising the operations of heating said material to a temperature below the melting point thereof and in an atmosphere at a pressure below that of the ordinary atmosphere for degassing said material, further heating said material in an atmosphere of argon at a pressure of at least 0.2 atmosphere whereby evaporation of said barium is reduced, raising the temperature of said material to pouring temperature within said atmosphere of argon, pouring said material into a mold assembly including an aluminum container and in said atmosphere of argon, said mold assembly being characterized by a thermal conductivity andcapacity sucient to maintain said container at a temperature below the melting point of aluminum, cooling said material in said atmosphere of argon to a temperature below that of the melting point of aluminum thereby forming a structure comprising an aluminum container and a solid core member of said material, removing said structure from said mold assembly, and working said structure into wire, whereby spaces formed during the cooling of said assembly, between the outside surface of said core member and the inside surface of said container, are substantially eliminated and an aluminum-clad barium wire is provided which is substantially free of entrapped gases.

References Cited in the le of this patent UNITED STATES PATENTS 1,682,590 Austin Aug. 28, 1928 2,100,257 Larson Nov. 23, 1937 2,100,746 Miller et al Nov. 30, 1937 2,329,317 Atlee Sept. 14, 1943 2,624,450 Britten et al. Ian. 6, 1953 'FOREIGN PATENTS 567,291 Great Britain Feb. 7, 1945

Claims (1)

1. 4. A METHOD OF MAKING AN ALUMINUM-CLAD CORE STRUCTURE HAVING A CORE MATERIAL INCLUDING A METAL SELECTED FROM THE CLASS CONSISTING OF ALKALINE EARTH METALS AND ALKALINE EARTH METAL ALLOYS AND CHARACTERIZED IN BEING REACTIVE WITH THE ORDINARY ATMOSPHERE, AND COMPRISING THE OPERATIONS OF HEATING SAID MATERIAL TO A TEMPERATURE BELOW THE MELTING POINT THEREOF AND IN AN ATMOSPHERE AT BELOW ORDINARY ATMOSPHERIC PRESSURE WHEREBY SAID MATERIAL IS SUBSTANTIALLY DEGASSED, FURTHER HEATING SAID MATERIAL IN AN ATMOSPHERE OF AN INERT GAS AT A PRESSURE AT LEAST AS HIGH AS THAT OF THE VAPOR PRESSURE OF SAID MATERIAL AND TO A TEMPERATURE SUFFICIENT TO MELT SAID MATERIAL WHEREBY SAID MATERIAL IS MELTED WHILE EVAPORATION OF SAID MATERIAL IS REDUCED, STILL FURTHER HEATING SAID MATERIAL IN SAID LAST NAMED ATMOSPHERE FOR ACHIEVING A POURING TEMPERATURE OF SAID MATERIAL WHICH IS ABOVE THE MELTING POINT THEREOF, POURING SAID MATERIAL IN AN INERT GAS AT SAID LAST NAMED PRESSURE INTO AN ALUMINUM CONTAINER WHICH HAS A MELTING POINT BELOW SAID MELTING POINT WHILE DRAWING HEAT FROM THE INSIDE SURFACE OF SAID CONTAINER THROUGH SAID CONTAINER TO MAINTAIN SAID INSIDE SURFACE AT A TEMPERATURE BELOW THE MELTING POINT THEREOF AND THEREBY PREVENTING ANY ALLOYING BETWEEN SAID CORE MATERIAL AND SAID CONTAINER, COOLING SAID MATERIAL IN AN INERT GAS TO A TEMPERATURE BELOW THE MELTING POINT OF SAID CONTAINER, AND MECHANICALLY WORKING SAID CONTAINER TO REMOVE ANY GASES ENTRAPPED THEREIN WITH SAID CORE MATERIAL AND TO CLOSE AN OPENING IN SAID CONTAINER, WHEREBY AN ALUMINUM-CLAD CORE STRUCTURE IS PROVIDED HAVING A SUBSTANTIAL FREEDOM FROM ENTRAPPED GASES.

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