US2362468A - Getter - Google Patents

Description

NOV. 14, 11944. R, U CLARK 2,362,468

GETTERS Filed Sept. 27, 1941 jesi/em.' fwd/W54@ @zgmw Patented Nov. 14, 1944 estates GETTER.

Richard U. Clark, Fort Wayne, Ind., assignoi to Fansteel Metallurgical Corporation, North Chicago, Ill., a corporation of New York VApplication September 27, 1941, Serial No. 412,601

7 Claims.

The present invention has for its purpose the provision of an improved means for cleaning up the residual gas in vacuum devices. It also provides for continued clean-up of such occluded gases as may normally be released from the walls and other parts of the vacuum tube elements with which it is used over Very extended periods of time. The present invention also further exhibits desirable selective gettering, in that it does not materially affect the noble gases.

A wide variety of gettering materials have been used in the past in `vacuum devices, but most of them have exhibited one or more disadvantages, viz., a tendency to migrate from place to place within the vacuum device, poor or incomplete gettering action in the presence of mercury vapor, excessively high vapor pressure at relatively low temperatures of operation, and an inability to exist in their active form in a normal atmosphere, requiring that the getter salts be produced by reaction within the vacuum device with other materials. Often, too, the getter cannot be stored `for indefinite periods unless special precautions are employed for excluding the air. Most of these disadvantages are particularly characteristic of l the low temperature active getters.

Certain other metallic getter materials have been suggested in sheet, wire and loose powder form, sheets and Wire, being usually metals that are costly in finished form. are not economical and require considerable material for good results. The loose powder tends to migrate, is not self-supporting and is difficult to handle in the necessary out-gassing cycle.

There also exists a class of high temperature active getters which includes the refractory metals `of `the Fourth and Fifth Groups of the Periodic Table, i. e., titanium, zirconium, hafnium, thorium, yttrium, columbium and tantalum. The employment of these metals in sheet, wire or loose powder form has been suggested, but the use of sheet and wire has not been realized on a large scale because the use of the amount of material required rendered the devices too expensive for wide commercial exploitation. Similarly disadvantageous was the powder form of the metal because of its tendency to migrate, failure to bel gassing cycle.

peratures. Of the refractory metals named above, I have found columbium and tantalum to be particularly excellent in porous pellet form.

In order to obtain a self-supporting porous pellet that will be an effective getter, I have found it desirable to press the metal powder into a pellet under considerable pressure, and to sinter Asame under vacuum conditions so that the metal particles and the pellet are fairly gas-free when cooled. After fabrication in this form the pellet may be readily Welded to a suitable wire lead, and mounted as desired in the vacuum device.

When mounting the porous getter in the vacuum device it is desirable that it be so positioned that it may be out-gassed during the exhaust cycle at a fairly high temperature. When a porous getter of columbium is used, the pellet should be heated to above 1600c C. during the out- In order to accomplish this the pellet may be included as part of an active electrode of the tube and heated by internal bombardment, or by means of high frequency excitation. If preferred it may be heated by passing an electric current through the body by means of wires connected directly thereto and leading to an external source of power.

The efficiency of the novel getter has been repeatedly demonstrated in standard bulbs having a volume averaging 215 ccs. After out-gassing, the tubes have been sealed off at pressures of about 10"4 mm. Porous columbium getter pellets weighing from 0.11 to 0.12 grams enable the attainment of vacuums of the order of 2 to 5 l08 In order to obtain the utmost economy it may be desirable to employ the porous getter as part of one or more of" the electrodes of the vacuum device, as a stray electron shield, as an anode end plate, as a grid collar, or in various other positions where the necessary out-gassing temperature may be reached, and also it is to be desired that the pellet willpass through cycles of varying temperature up t0 about 600 C. during operating cycles. Although such a cycle is recommended for columbium due tothe fact that its gettering action is thus increased, other metals would require ranges of temperature suited to their particular natures.

The invention will readily be understood from the accompanying drawing in which:

Fig. l is an elevation partly in section of a thermionic tube embodying my invention; and

Fig. 2 is a similar View showing a different embodiment.

Referring to the drawing, the reference numeral IU designates the envelope of the tube. This envelope includes a heated cathode I I which may suitably be a filament, a cylindrical plate I2 and a grid I3. These elements are, of course, electrically connected to prongs in the base of the tube in the usual manner.

In the embodiment of the invention shown in Fig. 1, the grid I3 is mounted upon a grid collar I4 which is a collar of self-supporting, porous material, including a refractory metal from the Fourth and Fifth Groups of the Periodic Table.

In the embodiment of Fig. 2, the cylindrical plate or anode I2 is provided with an anode end plate I5 which may be of the same material. In this case, the grid collar I4 may be of any suitable solid metal since suicient gettering action can be attained with the end plate I5.

In the embodiment shown in Fig. 1 and Fig. 2, respectively, the location of the grid collar rI4 and the end plate I5, respectively, is such that during operation of the tubes, they heat up to about 600 C. In the degassing operation during the manufacture of the tubes, the grid collar and the plate I5, respectively, may be heated up to 1600 C. in any suitable manner, for example, by

`high frequency excitation.

One of the great points of difference in my getter in comparison with other forms is that by x providing a porous sponge-like material I greatly n 'increase the active surface of the getter; also I provide a type of surface that is most adaptable to optimumgetterin'g. Under the present accepted theory of gettering, a very considerable proportion of gettering action takes place as an adsorption effect wherein a monatomic layer of gas is attracted and held by the surface of the metal. It is believed therefore that this adsorption effect is greatly increased in the present invention due to the great metal area exposed. Since the occluded gas is supposed to be also more readily held in pocket-like conformations, it is believed to account in part for the excellent results obtained with the porous getter, which contains innumerable small pockets.

A further considerable clean-up effect is known to be due to actual solution of thegas within the metal, dueto the gradual infiltration of the gas within the crystal lattice of the metal. In metal structures in which a protective external gas film of uniform and unbroken nature tends to form on the surface, this reaction with the inner metal is found to be greatly retarded. In the case of a highly porous metal structure the external socalled protective film has been shown to be much' more subject to rupture than is the case with metal having smooth surfaces, hence it is believed that added reaction with the interior of the metal is obtained with my porous getter.

As to the preferred cycle for using my getter in conventional thermionic tubes, best results appear to be had by the usual out-gassing cycle applied in the order of filament, grid and anode, followed by complete out-gassing of the pellet to over 1600 C. An additional bombardment of the tube elements with the getter raised to over 1600 C. when the tubes other elements appear fairly well de-gassed, also seems to improve the eventual clean-up after the tube is sealed olf.

When the vacuum device has been sealed off from the pump and system, the pellet getter is raised in temperature to around 500 to 600 C. for several cycles of five to ten minutes duration, with subsequent cooling between the cycles for at least one or more minutes. The 'result of these cycles is to lower the vacuum gradually to a point of very low pressure as previously pointed out.

It is not necessary for me to use pellets made up For example, I have stances, but for the larger number of instances its vapor pressure is too high.

I claim: 1. In a high vacuum thermionic tube having a cathode and an anode, a getter element consisting of a self-supporting, sintered, porous body including a refractory metal from the Fourth and Fifth Groups of the Periodic Table.

2. In a high vacuum thermionic tube having a cathode and an anode, a getter element consisting of a self-supporting, sintered, porous body including a refractory metal from the Fourth and Fifth Groups of thePeriodicTable and another refractory metal of low vapor pressure.

3. Ina high vacuum thermionic tube comprising an envelope, a cathode and an anode within said envelope, and a self-supporting, sintered, porous body including a refractory metal from the Fourth and Fifth Groups .of the Periodic Table, said body being arranged for heating to high degassing temperatures within said envelope.

4. In a high vacuum thermionic tube comprising an envelope, a heated cathode and an anode within said envelope, anda self-supporting, sintered, porous body including a refractory metal from the Fourth and Fifth Groups of the Periodic Table, said body being arranged for heating to high degassing temperatures within said envelope and arranged to be heated to relatively low temperatures during use.

5. In a high vacuum thermionic tube having a cathode and an anode, a getter element consisting of a self-supporting, sintered, porous body of a refractory metal of the Fifth Group of the Periodic Table.

6. In a high vacuum thermionic tube having a cathode and an anode, a getter element cons isting of a self-supporting, sintered, porous body of columbium.

7. In a high vacuum thermionic tube having an envelope and electrical elements therein including a cathode and an anode', agettering elem'ent consisting of a self-supporting, sintered, porous body including a refractorymetal from the Fourth and Fifth Groups of the Periodic Table, said body constituting at least a part 'of one of said elements.

RICHARD U. CLARK.

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