US2938785A - Tungsten-niobium-nickel cathodes - Google Patents

Description

y 1, 1960 A. M.'BOUNDS ETAL 2,938,785

TUNGSTEN-NIOBIUM-NICKEL CATHODES Filed July 12, 1957 s Sheets-Sheet 1 'I'S'd 000! m ns mm y 1, 1960 A. M. BQUNDS ETAL 2,938,785

TUNGSTEIN-NIOBIUM-NICKEL. CATHODES Filed July 12, 1957 3 Sheets-Sheet 2 3 3 95: 32 31. 3 a cm 23 B 2.6: one. on 03 c3 e3 e2 3 (DMSI y 1960 A. M. BOUNDS ETAL 2,938,785

TUNGSTEN-NIOBIUM-NICKEL CATHODES 3 Sheets-Sheet;

Filed July 12, 1957 Unite tates Patent TUNGSTEN-NIOBIUM-NICKEL CATHODES Ardrey M. Bounds, Laverock, and Richard L. Hoff, Norristown, Pa., assignors to Superior Tube Company, Norristown, Pa., a corporation of Pennsylvania Filed July 12, 1957, Ser. No. 671,655

4 Claims. (Cl. 75-170) This invention relates to cathodes of the indirectly heated type for electron tubes.

In general, the object of the invention is to provide indirectly-heated cathodes which have substantially enhanced resistance to deformation when subjected to shock or vibration at elevated operating temperature and which have good emission and sublimation characteristics sub stantially equivalent to or better than those of other nickel alloy cathodes.

In accordance with the present invention, such'objective is obtained by making the cathode sleeves, cups or the like from a nickel alloy which includes, within narrowpercentage ranges herein specified, the additives tungsten and niobium. Considering all of the factors involved, the preferred composition contains about 4% tungsten, 0.3% niobium and 0.1% aluminum with the remainder of the alloy being essentially nickel. p

In the following description, reference is made to the accompanying drawings in which:

Fig. 1 is a group of graphs exemplary of the yield strength, at cathode-operating temperatures, of tungstenniobium-nickel alloys and of reference nickel and tungsten-nickel cathode alloys; and

Figs. 2A, 2B and 3 comprise groups of curves referred to in discussion of the emission characteristics of indirectly-heated cathodes made from the reference nickel alloy and from nickel alloys including the additives tungsten and niobium.

In general, indirectly-heated cathodes consist of a nickel alloy base element, such as a sleeve or cup, having thereon a thin coating of alkaline earth metals such as barium, strontium or the like. The fabrication of the alloy stock into cathode base elements involves hot and cold working steps, such as forging, rolling, drawing, stamping and the like. After assembly of the coated cathode, including its heater and other electrodes within an envelope to form an electronic tube, the'cathode is activated by temporarily heating it substantially above its normal operating temperature. During activation, there are reactions between the base element alloy materials and the coating materials which convert the coating to a combination of complex oxides which emit electrons when heated to cathode-operating temperatures in the neighborhood of 1600" F. In the more usual services, the effective life of the tube is terminated when its cathode ernission is definitely subnormal at normal heater current. However, for many uses, including field applications where the available source voltage is low or fluctuating, tubes are considered unfit for use when their cathode emission is substantially aifected by low or varying heater current.

The operating life of a tube is also afiected by cathode characteristics other than electron emission. For example, the normal life of high-voltage rectifier tubes has often abruptly terminated because of eruptive flaking or peeling of the cathode coating from the cathode sleeve. Also, the operation of amplifier, oscillator and mixer tubes has been adversely afiected byformation and conice tinued growth of a high-impedance interface between the cathode sleeve and its coating. The resistive component of such interface impedance is damaging, particularly in pulse-type operation, even at ordinary frequencies; the

capacitive component of such interface impedance is particularly damaging at high frequencies, even when the tube is not operated under pulsed or cut-01f conditions. The life of a tube may also be terminated by the forma tion, from material sublimed' from its cathode, of a leak-1 age path between electrodes of the tube. The operating. life of a tube is also determined by the mechanical properties of its cathode element; for example, cathode sleeves made of the usual nickel alloys often bowed whensubjected to high temperatures during their activation; period, so causing internal short-circuits or significant changes in interelectrode spacing. Also in services where" the tubes are subjected to severe mechanical shock, as in,

aiford rapid activation, sustained high emission levels,-

virtual freedom from sublimation, negligible interface intpedance, and a hot strength substantially exceeding that of usual nickel-cathode alloys.

Considering first the enhancement of hot-strength at cathode operating temperatures in the neighborhood of 1600 F., the addition of a small amount of niobium effects a great change in hot-strength as compared to the addition of a muchlarger amount of tungsten. For example, a binary tungsten-nickel cathode alloy containing about 2% tungsten has a yield strength of about 3800 psi (pounds per square inch) at a test loading rate of- 0.004 inch per minute. Increasing the tungsten per-j centage to 4% increases the hot yield strength to only- 4100 psi. and it is not feasible much further to increase the hot-strength by addition of tungsten because the alloy then becomes unsuited for fabrication into cathode sleeves by the usual metal-working techniques. On the contrary, the addition of even as small an amount 0.008% niobium produces a pronounced strengthening effect, and by using both tungsten up to about 5% and niobium up to 2%, there may be produced alloys suited for fabrication into indirectly-heated cathodes and whose yield strength at cathode-operating temperatures substantially exceeds that of prior nickel cathodes and tungsten nickel cathodes. The combinde strengthening effect of tungsten and niobium involves both solid-solution strengthening and precipitation hardening; the effect upon yield strength at cathode-operating temperatures ofniobium and tungsten could not be predicted.

Data compiled on the emission characteristics of indif rectly-heated cathodes made of nickel alloys, tungsten nickel alloys, and tungsten-niobium-nickel alloys indicates interaction of the tungsten and niobium in enhancement or maintenance of the electrical characteristics of nickel cathodes as well as a substantial increase in their resist: ance to deformation when subjected to severe mechanical shock at cathode-operating temperatures. Such effects were not predictable from the prior state of the art. For nickel-cathode alloys containing about 1% to 5% by weight of tungsten, niobium may be added in the range of from about 0.05% to 2%. Except for residuals and one or more activating agents, the balance of the alloy is es sentially the nickel base usually including cobalt not in excess of about 1%; higher percentages of cobalt in the nickclbase havebecn foundto have little effect upon Patented May 31, 196p the electrical characteristics of the cathodes or upon their mechanical strength at operating temperatures.

In these tungsten-niobium-nickel alloys, magnesium and/or aluminum may be presentin small percentages as ASTM-(American Society of Testing Materials). The

cathode sleeves were 0.045" OD x 0.002" wall it 27 mm. long. The life burning conditions were an anode-cathode supply voltage (E of 100 volts, a heater voltage (E cathode-activating agents. There is reason to believe the of 6.5 volts and a load resistance (R of 1000 ohms. niobium interacts with aluminum and probably also with Anode current readings were taken at 0, 5, 25, 50, 100, magnesium in efie'ctingrapid activation of the cathode 200, 350 and 500 hours and thenevery 250 hours to the coating; The percentage of magnesium should not be end of the test. Ateach test period, the anode current excess of about 0.07% to insure a low rate of sublimawas read for a plate voltage of 40 volts for a series of ti'olL The percentage of aluminum, at least for the heater 'volt'ages' including the normal voltage ."(i.e., 6.5 higher percentages of niobium, should not be in excess volts) and sub-normal voltages including 4.5 volts; Such of 0.1%, to avoid shortening of high emission life due anode current readings plotted against time'constitute the to peeling of the oxide coating, but it may be as high as curves of Figs. 2A,-2B and 3. The I FM or direct- 0.25% for special applications Where the very high initial current emission figure of merit curves of Fig. 3 are deemission resulting from interaction with niobium can be rived from the anode current vs; heater voltage readings used to advantage. Because of its effect upon interface as described in detail in an article of Briggs and Richard impedance, silicon should not be used as an activating in the ASTM Bulletin for January 1951. Briefly, the agent; if present, its concentration should not exceed I FM value is the ratioof the I E coordinates at the about 0.05 knee of the anode current-heater voltage curve, where the In determination of the limits of tungsten and niobium anode current changes from a space charge limited confor obtaining enhanced hot-strength of indirectly heated dition to a, temperature-limited condition (sub-normalcathodes and preservation or enhancement of electrical heater voltage).- For comparison purposes, the emission properties, tests were conducted on alloys listed below curves for tubes having nickel cathodes are also shown inTable A. in Figs.-2A3. The corresponding curves for tubes hav- Table A Alloy Nb A1 Mg Si Fe Mn 0 Cu 00 N1 #5510 .425 4.17 13 .005 .03 .141 .098 .027 .009 4.64 Eslsaentially l V m inder. #553 1.71 3.70 .14 .041 .011 .112 .061 .119 .008 .175 Do. #220 nil nil .009 .011 .008 .086 .114 .040 .008 .054 Do. A31" nil 3. 93 .025 .025 .014 .032 .05 .04 .014 .103 Do.

Reference nickel alloy.

"Reference tungsten-nickel alloy.

As shown by the test curves of Fig. 1, the yield strengths of the tungsten-niobium-nickel alloys of Table Aare significantly higher than those of the referencenickel alloy throughout a high-temperature range including cathode-operating temperatures approaching 1600 F. With the tungsten content held at about 4% (which approaches the sate maximum for cathode fabrication by conventional techniques),'the significant increase of hot-strength with increase of niobium is evident from comparison of the yield strength curves of the tungstennickel alloy A31 and the tungsten-niobium-nickel alloys #5516 and #553. Moreover, in the range of about 400 F. to 1200 F., the yield strengths of the alloys #5516 and #553 are very similar, but for cathodeoperating temperatures approaching 1600 F., the yield strength of alloy #553, having the higher percentage of niobium, falls 011 much more slowly with increasing temperature than does the yield strength of alloy #5516 having the lower percentage of niobium. In general, the hot yield strength of the base-nickel alloy can be increased to better than 2 /2 times by addition of tungsten and niobium within the limits above specified. As confirmed by shock tests on tubes with cathodes at operating temperatures, there is direct correlation between the shock deformation characteristics of the cathodes and the yield strengths of the cathode alloy at the same temperature.

From tests on these and other nickel cathode alloys, ithas been determined that cobalt has little or no eifect, even when present in concentrations up to 15%, on yield strength at temperatures above about 1500" F. For this reason and because of the general shortage of cobalt, it need not be included or added.

The emission characteristics of oxide coated cathodes using the #553 alloy of Table A for cathode sleeves are shown in Figs. 2A, 2B and 3. In these figures, the curves are identified by the alloy designations of Table A. For these emission tests, the cathodes were utilized in the standard diode structure defined in Spec. F27052T of ing the #A-31 alloy cathodes are not shown since they are substantially similar to those of the #220 alloy cathodes.

As best shown in Fig. 3, the #553 alloy cathodes activated very rapidly and in 25 hours had an FM value higher than the #220 reference-nickel cathodes. Throughout the initial life, the emission of the #553 alloy cathodes was essentially the same as that of the reference-nickel cathodes both at normal and sub-normal heater voltages (Figs. 2A, 2B). Later in the life tests, the emission of the #553 alloy cathode declined but this was found, upon examination of the cathodes after completion of the life tests, to be due to peeling of the oxide coating. Such peeling and consequent loss of emission was directly attributed to inclusion of a somewhat too high percentage of aluminum for an alloy having this much niobium. For high emission and long life, the percentage of aluminum should not exceed about 0.1% when niobium is present in concentrations of about 1% or higher. Since niobium is itself an effective activating agent, the cathode activation is rapid even with the lower aluminum percentage, insuring high emission throughout a long life. The #553 alloy cathodes had substantially less sublimation deposits than the #220 reference-nickel cathodes with correspondingly greater freedom from formation of interelectrode leakage paths.

In brief summary, tungsten-niobium-nickel cathodes may be activated more rapidly than reference-nickel cathodes; by suitably limiting the percentage of aluminum, tungsten-niobium-nickel' cathodes will have high, sustained emission substantially equivalent to that of the reference-nickel cathodes; the sublimation characteristics of tungsten-niobium-nickel cathodes are not adversely affected by the additives tungsten and niobium but by an unduly high percentage of magnesium, manganese or copper; for good coating adherence and low interface impedance, the tungsten-niobium-nickel alloys should not contain in excess of ,about.0.07-% orsilicon in excess of about 0.02%; the tungsten-niobium-nickel cathodes have an enhanced resistance to shock deformation, which at cathode-operating temperatures, may be as much or more than 2 /2 times that of the reference-nickel cathodes. A preferred composition affording substantially enhanced mechanical strength at cathode-operating temperatures and good electrical characteristics is about 0.3% niobium, 0.1% aluminum, 4% tungsten, and the balance essentially nickel.

What is claimed is:

1. An indirectly-heated cathode structure characterized by high strength at cathode-operating temperatures, sustained high level of emission and low sublimation and composed of an alloy containing niobium in the range of 0.05% to 2% by Weight, tun sten in the range of from 1% to 5% by Weight, and the remainder essentially nickel.

2. An indirectly-heated cathode structure characterized by high strength at cathode-operating temperatures, sustained high level of emission and negligible sublimation and interface impedance and composed of an alloy containing, by Weight, 0.05% to 2% niobium; 1% to 5% tungsten; at least one of the activating agents aluminum and magnesium in the range of not more than 0.07% magnesium, 01% aluminum; and the balance essentially nickel with not more than about 0.05% silicon, 0.1% iron, 0.1% manganese, 0.08% carbon, 0.05% copper, as residuals.

3. An indirectly-heated cathode structure characterized by high-strength at cathode-operating temperatures, sustained high level of emission and low sublimation and composed of an alloy containing, by weight, about 0.3%

niobium, 4% tungsten, 0.07% aluminum and the balance essentially nickel with not more than about 0.02% silicon, 0.1% iron, 0.1% manganese, 0.08% carbon, 0.05% copper, as residuals.

4. An indirectly-heated cathode structure characterized by high strength at cathode-operating temperatures, sustained high level of emission and low sublimation and composed of an alloy containing, by weight, about 0.3% niobium, 4% tungsten, 0.03% magnesium, and the balance essentially nickel with not more than about 0.02% silicon, 0.1% iron, 0.1% manganese, 0.08% carbon, 0.05% copper, as residuals.

References Cited in the file of this patent UNITED STATES PATENTS 1,445,253 White Feb. 13, 1923 1,588,518 Brace June 15, 1926 1,926,846 G-iard Sept. 12, 1933 2,266,318 Heller Dec. 16, 1941 2,323,173 Widell June 29, 1943 2,370,395 Cooper Feb. 27, 1945 2,396,977 Widell Mar. 19, 1946 2,720,458 Kates Oct. 11, 1955 2,809,890 Bounds Oct. 15, -7

FOREIGN PATENTS 142,663 Australia Aug. 2, 1951 OTHER REFERENCES Journal of Metals, Sept. 1953, Trans. AIME, pages 1149-4165.

Claims (1)

1. AN INDIRECTLY-HEATED CATHODE STRUCTURE CHARACTERIZED BY HIGH STRENGTH AT CATHODE-OPERATING TEMPERATURES, SUSTAINED HIGH LEVEL OF EMMISSION AND LOW SUBLIMATION AND COMPOSED OF AN ALLOY CONTAINING NIOBIUM IN THE RANGE OF 0.05% TO 2% BY WEIGHT, TUNGSTEN IN THE RANGE OF

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