US2538053A - Tantalum anode for gas filled tubes - Google Patents

Description

Jan. 16, 1951 1-:. K. SMITH 2,538,053

TANTALUM ANODE FOR GAS FILLED TUBES Filed March 3, 1948 3O FIG-1- IN V EN TOR.

" WM. m, ATTORNEY HA5 NON-EMISSIVE HEAT RADIATING COATING Patented Jan. 16, 1951 TANTALUM AN ODE FOR GAS FILLED TUBES Earl K. Smith, West Orange, N. J., assignor to Electrons, Incorporated, Newark, N. J a corporation of Delaware Application March 3, 1948, Serial No. 12,726

3 Claims. (01. 250-275) This invention relates to controllable electron discharge tubes, and more particularly to the anode structure for a rectifier or grid control gas filled tube.

One problem in the construction of rectifier and grid control gas filled tubes, more particularly in tubes for thehigher continuous current ratings, is that the anode tends to become overheated and emit electrons to a degree to interfere with the proper performance of the tube. Among other things, excessive electron emission from the anode while a negative or inverse voltage is applied thereto, tends to reduce the inverse voltage which the tube will stand without an are back or discharge through the tube in the wrong direction. Also, in case of a grid control tube, electron emission from the anode may modify the effective control of the grid, either by creating gas ions in the grid to anode space causing ionization and misflring of the tube when the anode assumes a relatively high positive potential immediately after the formation of such ions, or by establishing a cloud of gas ions near the grid tending to prevent the grid from assuming a negative potential by the voltage in its control circuit suffl cient to hold off conduction through the tube. Further, in the case of a rectifier or grid control tube using xenon or a similar gas filling, electron emission from the anode and creation of gas ions in the region adjacent the anode appears to be a factor contributing to ionic bombardment of the anode, which if excessive adversely affects the useful life of the tube by causing gas clean up and a reduction in the pressure of the gas filling.

Excessive electron emission from the anode of a tube of the type contemplated, which is objectionable for these and other reasons, depends of course upon the temperature the anode assumes in the operation of the tube, and also the nature of its surface as an emitter at such temperature level. During operation of the tube the anode is heated by the bombardment of electrons of the discharge current and by heat radiated fromthe hot cathode. The heat accumulated by the anode during operation of the tube has to be dissipated largely by radiation, assisted by conduction of heat to the outside of the tube through the supporting elements for the anode; and from the standpoint of temperature rise of the anode it is desirable that it should have a surface with good heat radiating qualities. Also, in addition to facilitating radiation of heat from the anode to lower its operating temperature, it is desirable that the electron emission from the anode at such operating temperature should be a minimum.

With these and other considerations in mind, the primary object of this invention is to provide an anode for gas filled tubes which is of a material, such as tantalum, which is suitable and adapted for an anode, and which is further provided with a surface coating or treatment to facilitate conduction and radiation of heat from the anode and to reduce the electron emission at the operating temperature the anode assumes, as compared with a plain metallic surface.

Generally speaking, and without attempting to define the nature and scope of the invention, it is proposed to use tantalum for the anode, and to provide a suitable coating on its exposed surface to keep its operating temperature and electron emission within tolerable limits even for a tube having a high continuous current rating. More specifically, in accordance with this invention the tantalum anode is treated so as to have a uniform and consistent oxidized surface, which has the desired qualities of low emissivity and superior heat radiating ability.

Various other characteristic features, attributes and advantages of the invention will be in part apparent and in part pointed out as the description progresses.

Although the improved anode of this invention may take a wide variety of forms and be employed in gas filled tubes of various types and sizes, it is convenient in discussing the nature of the invention to assume a typical tube structure for a rid control tube, such as illustrated in the accompanying drawings, in which the parts are shown more with a view of facilitating an explanation and understanding of the tube structure contemplated than for the purpose of showing in detail the particular construction and arrangement of parts preferably employed in practice.

In these drawings,

Fig. 1 is a general view in the nature of a longitudinal section through the typical tube assumed; and

Fig. 2 is a top plan view of the anode as a transverse section through the tube on the line 2-4 in Fig. 1.

The typical tube illustrated comprises in general a heat shielded cathode C of the oxide coated directly heated type, a control grid G, and the improved anode A, all enclosed and supported in an evacuated glass envelope E having a gas filling at the appropriate pressure of xenon or like rare gas, together with appropriate external leads or connections for the cathode, grid and anode.

In the typical tube structure illustrated, the cathode C is assumed to be of the slotted cylindrical type, such as disclosed in the prior patent to D. V. Edwards et al., No. 2,111,506, March 15, 1938, which is provided with an oxide coating on its inner surface preferably formed and treated in accordance with the disclosure of the prior patents to D. V. Edwards et al., No. 1,985,855, December 25, 1934, and No. 2,081,864, May 25, 1937.

This cathode C is surrounded by a cylindrical heat shield S having a circular discharge opening indicated at 5 in its top opposite the anode A. The heat shield S in the simple form illustrated, which in practice is preferably provided with multiple spaced metallic walls for heat insulation purposes, is formed by a bottom i having a peripheral flange welded to the lower edge of a heat shield can or cylinder 6, and a top 8 with its center discharge opening 5 similarly attached to the upper edge of this heat shield can 6. Tabs on the upper end of the cathode C are interposed and welded between the flange of the top 8 and the can 6 of the heat shield S, as indicated at 9, so that the cathode C is electrically connected at its upper end to the heat shield S. Similar tabs for the bottom of the cathode C are welded to a cross member I2; and a cathode supporting rod and lead-in connection 53, welded to this member 82, extends through a tubular insulator the bottom 1 of the heat shield S, and through a glass seal of the usual type in the stem l5 of the glass envelope E. The heat shield S is supported within the envelope E by a pair of supports ll, it, which are welded at "their upper ends to the bottom 1 of the heat shield S and are anchored at their lower ends in the stem E5 of the envelope, one of these supports ll extending through a glass seal in this stem to afford the desired ex ternal connection to the heat shield S and the upper end of the cathode C connected thereto.

The grid G in the typical structure shown comprises a flanged ring 2! to which spaced parallel grid bars 22 are welded, and to which a cylindrical skirt 23 is also attached. A plurality of supporting rods 24, 25 anchored in the stem I5 serve to support the grid G, one of these rods 2 2 extending through this stem to aiford the desired external connection to the grid. The grid bars 22 are preferably formed with a coating to inhibit electron emission, in the manner disclosed in the prior patent to D. V. Edwards et al., No. 2,012,339, August 27, 1935,

The anode A in the particular tube structure illustrated comprises a circular thin sheet of tantalum with a peripheral flange 2! and radial ribs or corrugations 28 (see Fig. 2) to give this tantalum sheet suflicient stillness or rigidity to prevent excessive warping or distortion when heated. A pair of brackets 29 of iron or steel are spot welded to the central portion of the anode A at a large number of closely spaced points in accordance with the disclosure of my prior application Ser. No. 674,953, filed June 7, 1946 which issued on Dec. 14, 1948, as Patent No. 2,456,540; and these brackets 29 are attached to an anode supporting rod 3B sealed in the upper end of the envelope E to afford an external anode connection.

It is contemplated that the dimensions and spacing of the parts of this tube structure, including the area of the discharge opening, cathode to grid and anode to grid spacing, and the spacing between the grid bars, will be chosen to conform with the control characteristics for the tube desired, in accordance with the teaching and disclosure of prior patents, such as D. V. Edwards et al., No. 1,905,692, April 25, 1933 and No. 2,068,- 539, January 19, 1937.

Considering now the improved structure of the anode A of this invention, it is desirable for reasons previously discussed that electron emission from this anode should be kept relatively low and within tolerable limits for proper performance and life of the type of tube contemplated. It is well known that electron emission from a metal depends upon the temperature to which it can be heated without excessive metal evaporation, as well as the nature or character of the metal itself and its so-called work function. In other words, the temperature which the anode A assumes in operation of the tube contemplated, together with the characteristics of its surface determine its electron emission.

In the tube of this invention tantalum is preferably used for the anode A, primarily because tantalum, although it gives up its occluded gases readily during the degassing and exhaust procedure 'for the tube, has a marked afrlnity for various gases other than inert or rare, such as xenon, preferably used as the final gas filling for the tube. For example, tantalum readily absorbs at low temperatures small traces of such gases as carbon monoxide, hydrogen and oxygen, and hence is a desirable material for the anode of a hot cathode tube, because it acts somewhat like a getter to absorb the gases that might otherwise have a deleterious or poisoning effect upon electron emission from the cathode. On account of the cost of tantalum, however, it is desirable to use a thin sheet for the anode A, suitably flanged and corrugated as indicated for stiffness; and in order to avoid localizing or overheating of such thin sheet of tantalum, due to its relatively poor ability to conduct heat from one part to another, and also to facilitate radiation and conduction of heat from the anode, it is desirable to employ iron supporting elements or brackets 29 as shown, in accordance with the disclosure and teaching of my prior application Ser. No. 674,953 above mentioned.

Although a substantial amount of heat supplied to such a tantalum anode A may be thus dissipated by radiation from such supporting brackets 29, and by conduction through these brackets and the anode supporting rod 30 to the outside of the tube, a still lower operating temperature for the anode may be obtained by improving the radiation of heat from the surface of the anode itself to and through the walls of the glass envelope E. It is well known that the amount of heat radiated from a heated body depends upon the character of its surface, a socalled black body being the most efficient in this respect and having the highest constant or coefiicient of heat radiation, While bright, shiny and polished surfaces are less eflicient in radiating heat and having lower heat radiating coefficients. Tantalum is a greyish white metal, and in the usual commercial form has a definite metallic luster causing its surface to have a low heat radiation coefficient.

In view of these facts, a tantalum anode of this invention is given a surface treatment to provide a film or coating which has a higher radiation coeificient and also lower electron emissivity at the same operating temperature than a plain untreated tantalum surface. In other words, the tantalum anode A of this invention is formed with a surface which has superior heat radiating qualities and inferior electron emissive characteristics for the same temperature, as compared with plain tantalum.

One type of surface treatment for a tantalum anode suitable to obtain these ends may be obtained by applying heat to the anode in such a way as to form on its exposed surface a film or coating of a dark color, which is apparently one of the lower Oxides of tantalum, presumably the tetroxide TazOr. It is found that such a film or coating resulting from the appropriate oxidation process of tantalum has a much higher heat radiation coefficient and much lower electron emission for the same operating temperature than a plain tantalum surface.

In forming this oxide coating for the plain tantalum surface of the anode, this surface is first thoroughly cleaned and degreased by any one of the well known procedures, and then is subjected to repeated cycles of alternating heating in a gas flame and cooling with a brief exposure to the atmosphere until the desired coating has been formed. The preferred method of treatment involves heating the tantalum to a moderate temperature generally approximating a dull red heat in the hot part of a gas flame, preferably adjusted to be slightly oxidizing, moving the heated part of the tantalum out of the hot part of the gas flame into exposure to an oxidizing atmosphere, until the temperature drops and the visible color disappears, and then repeating this cycle several times until a coating of the desired thickness and density is formed, in accordance with the method disclosed and claimed in my other application Ser. No. 115,104, filed September 10, 1949, as a division of this application.

It is found that this method of forming the surface coating gives a more consistent and uniform surface film and adequate bonding to the metal surface, without the tantalum becoming too brittle, than a single heating in a flame and exposure to the atmosphere, or by continuous heating in air by induction heating or the like. The reasons for this are not fully understood. In this connection, one characteristic of tantalum when heated in air to a dull red heat is that it absorbs nitrogen greedily and tends to become brittle, presumably on account of formation of an excessive amount of a nitride or other nitrogen-tantalum compound; yet temperatures of that order are needed to assure the formation of an oxide of tantalum. It may be that the brief exposures to the air in connection with moving the tantalum into and out of the hot part of a gas flame, in accordance with the preferred method of treatment, tends to limit the absorption of nitrogen and the formation of tantalum-nitrogen compounds tending to make the tantalum too brittle. There are indications that it may be desirable to form a nitride of tantalum to some extent for optimum heat radiation and minimum electron emission qualities of the surface coating, so that cyclic heating and brief exposures to the air in accordance with this invention is preferable to a process of oxidation in a nitrogen free environment. It may be added that the gas flame may contribute to the formation of the desired film in some manner not clearly disclosed or understood by my investigations to date, and also that it is likely that the repeated heating and exposure to the air tends to build up the desired thickness and density of the oxide film, including formation of crystals, with more cohesion and bonding to the body metal than a single heating.

From the foregoing it can be seen that the improved anode structure of this invention utilizes the advantages of tantalum as an anode material, and provides a film or coating for the surface of such anode which has better heat radiating qualities and lower electron emissivity than the plain metal, so that improved tube performance and life may be obtained.

It should be understood that the specific tube structure illustrated and described merely represents a typical embodiment of the invention, and that various modifications, adaptations and additions may be made in the arrangement and structure of the parts shown and described without departing from the invention.

What I claim is:

1. An electrode structure for electron discharge tubes having an extensive surface and supported within an envelope comprising, a thin sheet of tantalum having a dense coating of a dark colored oxide of tantalum, said tantalum sheet being characterized by an oxide layer closely bonded to a tough and non-brittle body capable of forming a strong Weld with a supporting element, said oxide coating having a higher heat radiation coefficient and lower electron emissivity at the same temperature than plain tantalum.

2. An electrode structure for electron discharge tubes affording an extensive surface having a higher heat radiation co-iiicient than plain tantalum and comprising, a thin sheet of tantalum having a peripheral flange and a plurality of spaced corrugations for stiffness, and a supporting element welded to said sheet, said tantalum sheet having a uniform coating of a dark colored oxide of tantalum formed and closely bonded on a tough and non-brittle body capable of forming a strong weld with said supporting element.

3. An anode structure for electron discharge tubes having an extensive surface supported within a tube envelope and comprising, a thin circular sheet of tantalum having a peripheral flange and a plurality of corrugations extending radially from a central portion, said tantalum sheet bein characterized by a uniform dense coating of a dark colored oxide of tantalum closely bonded to a tough and non-brittle body capable of forming a strong weld, and a supporting element welded to said central portion of said tantalum sheet.

EARL K. SMITH.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS FOREIGN PATENTS Country Date Great Britain Dec. 10, 1931 Number Date Number

Claims (1)

1. AN ELECTRODE STRUCTURE FOR ELECTRON DISCHARGE TUBES HAVING AN EXTENSIVE SURFACE AND SUPPORTED WITHIN AN ENVELOPE COMPRISING, A THIN SHEET OF TANTALUM HAVING A DENSE COATING OF A DARK COLORED OXIDE OF TANTALUM, SAID TANTALUM SHEET BEING CHARACTERIZED BY AN OXIDE LAYER CLOSELY BONDED TO A TOUGH AND NON-BRITTLE BODY CAPABLE OF FORMING A STRONG WELD WITH A SUPPORTING ELEMENT, SAID OXIDE COATING HAVING A HIGHER HEAT RADIATION COEFFICIENT AND LOWER ELECTRON EMISSIVITY AT THE SAME TEMPERATURE THAN PLAIN TANTALUM.

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