US1695845A - Alloy base for oxide-coated cathodes - Google Patents

Description

Patented Dec. 18, 1928.-

4 UNITED STATES PATENT. OFFICE.

GEORGE W. HALLOCK, OF BLOOMIIELD, JERSEY, ASSIGNOR J30 WESTINGHOUSE LAMP COMPANY, A CORPORATION OF PENNSYLVANIA.

ALLOY BASE FOB OXIDE-COATED CATHODES.

No Drawing.

This invention relates to electron-emittingcathodes of the oxide coated type, such as are used in thermionic discharge devices, and more particularly to an electrically conduc- 5 tive core or base for such cathodes.

Heretofore in the manufacture of oxide coated cathodes'ithas been the usual practiceto employ a core of nickel or platinum-iridium alloy. Nickel has very largely supersededthe use of platinum-iridium, due to the excessive expense of the platinum. Nickel has the d1S advantage, however, in that its specific resistance is such that the voltage-ampere characteristics required for the large size filaments cannot be readily obtained without reducing the thickness of the core to such an ex- 'tent that it is mechanically weak or, if proportioned for proper tensile strength, causing it to o crate at an excessively high tem-. 2 perature, elther of which result in short life. For instance, with large sizes of ribbon, in order to obtain the required voltage and amperage characteristics, and the requisite surface area to maintain the proper operating temperature, it is necessary to roll the core into'a ribbon of considerable width and of an extreme thinness, below 1 mil. Great difficulty is involved in uniformly rolling the material to this degree of thinness and the edges become frayed and feathered so that the ribbon is easily torn and does not have 7 sufficient strength to withstand the strain required to maintain it taut on the mount. The ribbon is, therefore, made thicker and narrower than theoretically required in order to increase the tensile strength but due to the decrease in} heat radiating surface thus occasioned, the cathode operates at an excessively high temperature. This high filament temperature results in bombardment of the cathode by back emission from the grid or plate, caused by the thermionically active coating on the cathode being thrown ofi at the high operating temperature thereof and depositing on the grid, and the heating of the grid, to an electron emitting temperature by heat radiated .from the overly heated. This back bombardment rapidly:

cathode. destroys the filament.

Difliculty hasalso been experienced in the smaller size filaments when employing nickel cores, in causing the oxide coating to adhere, since in order to obtain the required voltageamperage characteristics, it is necessar to v go to the other extreme, and make the ribbon Application filed October 27, 1927. Serial No. 229,276.

narrow and thick, rather than broad and thin. The oxlde coating does not adhere as readily 'to the narrow ribbon as it does to the broader surface.

The hot strength of nickel is low, moreover,

which contributes materially to the failures due to breakage of both the smaller sized and the extremely thin nickel ribbons.

One of the objects of the present invention 1s to overcome the above mentioned diiiiculties and to produce a metal core for an oxide coat-' ed cathode which will have the proper specific resistance to enable the required voltage-amperage characteristics to be readily obtained.

at the proper operating temperature and whlch will be strong and rugged.

A further object is to provide an electrical- I ly conductive core which will have a high tensile strength while hot.

A stlll further object is to provide a thermionlcally active cathode which will have a large electron-emitting surface area for av ploy an alloy having a higher specific resistance than pure nckel, whereas, for the smaller slze'filaments, Iemploy an alloy having a lower specific resistance than nickel.

Thus, for instance, in producing cores for cathodes of large size,'that is, having a filament weight corresponding to an 8 mil wire, 1n order to meet the voltage and amperage re-' qulrements which have become standardized in the radio industry and maintain proper operat-mg temperature of the filament, 'it would be necessary to flatten an 8 mil nickel wire to a width of about 40 mil and a thickness of less than 1 mil. This is not practical, however, due to the low tensile strength of nickel, and the extreme fragility of a ribbon of this thinness. I have found, however, that certain alloys, such as an alloy of silicon and nickel or manganese and nickel in the proper proportions have a higher specific resistance than pure nickel and, therefore, that it is possible to employ a thicker ribbon of substantially the same width and which will, therefore, have increased tensile strength in proportion to the increase in size. Siliconnickel and manganese-nickel have greater hot strength than ure nickel moreover so that the strength of tlie filament is still further increased.

For the. large'size wires, silicon-nickel is preferred since its change in specific resistance for various percentages is very slight. An alloy containing about 3% of silicon I have found very satisfactory, but the percentage may run from 2 to 6% without materially a tering the electrical conductivity of the metal. This isadvantageous since it eliminates the necessit of maintaining exact proportions in the fabrication of the wire. Manganese nickel with a manganese content of 4% is also satisfactory for large filaments,

but the proper percentage of manganese must be more carefully maintained, since the con- .ductivity varies considerably with the change in manganese content. I

Pure nickel ribbon of a weight corresponding'to a 6 or 8' mil Wire has a specific resist-. ance of about 5.35 ohms per sq. cm., whereas, the alloys which I prefer for filaments of this: weight have a specific resistance of about 6.

For small size wires, it is desirable that the specific resistance of' the metal core be lower 'meet the standard requirements.

than that afiorded by pure nickel in order to of pure nickel, as the Wei ht of the filament is decreased, for use 1n tubes of smaller output, the thickness of the filamentary core increases and the .width decreases so that for the smallest size filaments the thickness approaches' the width. When these conditions are obtained,-diificulty is experienced in causing the oxide coating to properly adhere to the small flat surface presented by the narrow ribbon. In such case, I have found tlrat an alloy such as an alloy of manganese and nickel containing about 2% of manganese,

which has a lower specific resistance than nickel, may be shapedinto a relatively broad and thin ribbon while maintaining the required voltage and amperage conditions and the proper operating temperature. This broad ribbon presents a more extensive sur-' face to which the oxide coating will adhere more tenaciously. It also increases the electron emitting area of the oathode and thereby enhances the electron emit-.

ting property of the coated cathode. In these small size wires the increased tensile strength afforded by the alloy core increases very-materially the life of the device in which it is employed.

I have further found that the silicon or jmanganese-nickel alloy cores are not as susceptible to variations 1n their specific resistances by internal changes in the core as is nickel. 4 In thecase of pure nickel, the conductivity,variesaccording' to the particular In the caseheat treatment and mechanical working to which it is subjected; It is the practice in the manufacture of electron discharge devices employing oxide sary to take into consideration the change in conductivity effected by this high temperature treatment. Since this treatment is not uniform in all tubes, the electrical conductivity of filament .cores of the same size is notuniform and this non-uniformity has caused considerable difliculty.

In the case of silicon-nickel or manganese nickel alloy, I have found that this high temperature treatment does not materially vary the electrical conductivity and much greater uniformity in the electrical characteristics of the cathodes is obtained and the design of the I core considerabl simplified.

One of the difficulties, which has'been mentioned before, when em loy ing large oxide coated cathodes with n ckel cores has been the high temperature at which the cathodes necessarily operate by virtue of the fact that they must be made thicker and narrower than they should be theoretically, in order to impart the necessary mechanical strength thereto. The proper o erating temperature, particularly in tubes avin a large power output, must be carefully 0 served, since if the v I temperature is too high the oxide coating sputters or otherwise becomes transferred over to the other electrodes. These other electrodes become highly heated by radiation from the cathode so that they themselves become thermionically active and the electrons emitted therefrom bombard the filament and rapidly disintegrate the same so that it quick- 1y urns out. Due to the greater specific resistance of silicon-nickel or manganese-nickel when employed as a core for filament of large size, the heat radiating area is increased for the same voltage and current conditions and consequently the filament operates at a lower temperature, whereby the difficulties due to back emission are eliminated.

It should be understood that while siliconand manganese have'been specified as the particular alloying ingredients in definite proportions, that the invention is not limited to these specific details, butequivalent metals may be employed andother proportions, de-

5 thermionically active cathode comprising an alloy of nickel and silicon.

2. An electron-emitting cathode comprising an alloy of nickel and silicon and a coating thereon of thermionically active ma- 10 terial.

3. A metal core for an electron-emitting cathode of the oxide coated type composed of an alloy of nickel and about 3% of silicon.

4. An electron-emitting cathode comprising a core composed of an alloy of a metal of the group containing nickel and up to 6% of silicon.

In testimony whereof, I have hereunto sub scri lzied my name this 26th day of October, 192

GEORGE W. HALLOCK.