US2836491A - Tungsten-titanium-nickel cathodes - Google Patents

Description

May 27, 1958 A. M. soumo's- ETAL 2,836,491

TUNGSTEN-TITANIUM-NICKEL CATHODES Filed June 17, 1957 4 Sheets-Sheet 1 aoo I500 I800 May 27, 1958 A. M. BOUNDS EIAL 2,836,491

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TUNGSTEN-TITANIUM-NICKEL CATHODES 4 Sheets-Sheet 4 Filed June 17, 1957 United rates Patent 8 TUNGSTEN-TITANIUM-NICKEL CATHODES Ardrey M. Bounds, Lava-rock, md Richard L. Hoff, Norristown, Pa., assignors t Superior Tube Company, Norristown, Pa., a corporation of Pennsylvania Application June 17, 1957, Serial No. 666,053

4 Claims. (Cl. 755-170) This invention relates to electron tube cathodes of the indirectly-heated type.

In general, the object of the invention is to provide indirectly-heated cathodes which have, as compared to prior nickel alloy cathodes, substantially enhanced resistance to deformation at cathode-operating temperatures and which have emission and sublimation characteristics substantially equivalent to or better than those of prior nickel or nickel alloy cathodes.

In accordance with the present invention, such objective is attained by making the cathode sleeves or the like from nickel cathode alloys which include the additives tungsten and titanium within the low and narrow limits hereinafter specified.

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

Fig. l is a group of curves exemplary of the yield strength, at cathode-operating temperatures, of various tungsten-titanitmi-nickel alloys and of a reference-nickel cathode alloy; and

Figs. 2A, 2B, 3A, 3B and 4A, 4B comprise groups of curves referred to in discussion of the emission characteristics of indirectly-heated cathodes of the referencenickel alloy and of tungsten-titanium-nickel alloys.

In general, indirectly-heated cathodes consist of a nickel alloy base element, such as a sleeve or cup, having thereon a coating of alkaline earth metals, such as barium, strontium or the like. The fabrication of the alloy stock into the 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, reactions between the base element materials and the coating materials convert the coating into a combination of complex oxides suited to emit electrons when heated to cathode-operating temperatures. In the more usual types of service, the life of a tube is considered terminated when its cathode emission is definitely subnormal at normal heater current. For many uses, including field service where the available supply Voltage may be low or fluctuating, tubes are considered unfit for use when the cathode emission is substantially affected by low or varying heater temperature.

The operating life of a tube is also affected by cathode characteristics other than electron emission. Eruptive flaking or peeling of its cathode coating shortens the normal life of tubes, particularly high-voltage rectifier tubes. The formation and growth of a high-impedance interface between the cathode base element and its oxide coating adversely au' ects tube operation. The resistive component of such interface is damaging, particularly in pulse type service, and 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-otf condi- ICC tions. The operating life of a tube may also be effectively terminated by the formation, from material sublimed from the cathode, of a leakage path between electrodes of the tube. In addition to such electrical characteristics, the operating life of a tube is also determined by the physical or mechanical characteristics of its cathode. Cathode sleeves made of the usual nickel alloys often bowed when subjected to high activation temperatures, causing internal short-circuits or change in interelectrode spacing. Also in services where the tubes were subjected to severe mechanical shock, as in airborne missile equipment, buckling or deformation of their cathode sleeves rendered the tubes inoperative before performance of their intended function.

We have determined that the addition of tungsten and titanium within the narrow percentage ranges later herein specified, to nickel cathode alloys, provides indirectlyheated cathodes, which have, at cathode-operating temperatures, a hot strength at least several times that of nickel-cathode alloys and which have a high level of stable emission equal to or better than that of nickel cathodes. Furthermore, such tungsten titanium-nickel alloys are amenable to hot and cold metal working steps incident to fabrication of cathode sleeves and the like from the alloy stock. Such cathodes also exhibit the characteristics of virtual freedom from sublimation and of negligible interface impedance.

Considering first the enhancement of yield strength at cathode-operating temperatures of about 1600 F., it is known that the addition of tungsten alone effects an increase in hot strength. Such increase is substantially proportional to the percentage of tungsten added. Increasing the percentage of tungsten from 2% to 4%, for example, increases the hot strength of a nickelcathode alloy from about 3800 p. s. i. (pounds per square inch) to 4800 p. s. i. as compared to a value of 2500 p. s. i. for the reference-nickel alloy. It is not considered practical however, to increase the amount of tungsten much above 4% because of cathode fabrication difficulties. On the other hand, we have found that the hot strength of such a tungsten-nickel alloy may be further substantially increased without encountering fabrication difiiculties by the addition of titanium. Specifically, the addition of 0.15% titanium to a 4% tungsten-nickel alloy increases the hot yield strength to 6250 p. s. i. and the addition of 0.4% titanium to the tungstennickel base alloy increases the yield strength to 7650 p. s. i. However, the addition of 2.6% titanium to the tungsten-nickel base increases the hot yield strength to only 7450 p. s. i., indicating that the increase in hot strength is not proportional to the addition of titanium. The strength of a tungsten-titaniurn-nickel alloy, containing titanium in the range of 0.05% to 3%, at cathodeoperating temperatures, is about twice that of the 4% tungsten-nickel alloy and over three times the hot strength of the nickel base alloy.

From tests on binary titanium-nickel cathode alloys, it was determined that addition of titanium to nickel increases the activation so rapidly with increase of titanium that a cathode having only .5 titanium activates quickly to extremely high levels, but quickly deteriorates to unsatisfactory low levels. However, in combination with tungsten as an additive to nickel, the effect of the addition of titanium in the range of 0.05% to 3% is both substantially to increase the hot strength and to provide good emission characteristics throughout a long cathode-operating life. In short, tungsten and titanium interact favorably with regard both to the mechanical and the electrical properties of indirectly-heated cathodes.

From metallographic investigation, it appears that the addition of tungsten and titanium produces precipitation hardening at temperatures below 1200 F. However, at

cathode-operating temperatures of '1600" F. or higher, this second phase is dissolved into the solid solution.

. For this reason, the marked enhancement of hot strength resulting from small additions of titanium to the tung- 'son and because of its general shortage, cobalt need not be included, or, added,-in amounts more than 1%. The

amount of aluminum added was found not to affect the p'eratures in the .neighborhoodofffloOt)? F. For this re'a-.

stem-nickel alloy is attributed to the mechanism of solid cathode-operating temperatures, although possibly also solution hardening resulting from the additional presence serving to result in precipitation hardening at lower temof titanium. W peratures. r Data compiledon the emission characteristics of indir The emission characteristics of oxide-coated cathodes rectly-heated cathodes made of nickel alloys, tungsten using the nickel ,and tungsten-titanium-nickel alloys of nickel alloys, titanium-nickel alloys and tungsten-titanium- Table A are shown in Figs. 2A, 213, 3A, 3B, 4A and 4B. nickel alloys indicates interaction of the tungsten and ti in these figures, each curve is identified by the respective tanium in enhancement of, or'maintenance of, the emisalloy designation presented in Table A. For these emis sion characteristics of nickel cathodes as well as a subsion tests, the cathodes were utilized in the standard diode stantial increase in their resistance to deformation when structure defined in Spec.F270-,-52T of ASTM (Ameri subjected to severe mechanical shock while at cathodecan Society of Testing Materials). operating temperatures. Such effects were not predictable The cathode sleeves were 0.45 inch, 0. D. x 0.002 inch from knowledge of theprior art; wall 1; 27 mm. long. The life burning conditions were For nickel-cathode alloys containing from about 1% an anode=cathode supply voltage (E f :100 wo'lt a to about 5% tungsten by weighhgt a ll may be added heater voltage (E;) of 6.5 volts, and a load resistance in the range of from about 095% to EXCept for 29 (R of1000' ohms. Anode current readings weretaken residuals and one or more activating agents, the balance at 0, 5, 56, 100, 200, 350 d 500 h nd ifi'gm 0f h i y isiessefltially BiCkel ly then every 250 hours to the end of the test.- At each of ing 6017831101 s 1636855 of la q ig y il bp these'test times, the anode current 'was read for plate Cefltagfis of Co a fi 1 p to 0 0f a5 een voltage of volts for a number of heatervolta'ges in found to have little eifect upon the emission characterisl di h normal voltage (E =6.5 volts) andsub b tics or upon the mechamcal strength at cathode-operatmg ma} voltages i l di E 11 S h anodecurtempeaturets. t n k 1. m d an rent readings plotted against time constitute the curves of n. 688 uI g n-. 311111914110 6 Ca 0 y Figs. 2A, 213, 3A and 3B. The I PM or direct-current nesium and/ or aluminum maybe used in Small controlled o emission figure f merit mfFi 4 4B are quanmles as j g ld zi i f T 3333 3; 0 x rived from these anode-current vs. heater-voltage'readings i f magnetmm s g i lf O f as. described in detail in an article of Briggs and Richard a if s i 9 z 2 g??? in the ASTM bulletin for January 1951. Briefly, 'the f t on n e u m o 0 1 PM value is the ratio of the I E coordinates at the avoid .peehng of the oxide coatmg. Because of its effect kne f the anoda current/heater Volta 6 IV h {h upon interface impedance, use of silicon as an activating 3 1. t h c a e agent should be avoided; if present, the concentration of l u anges 1 space 0 3 mlte c011- smcen Should not exceed drtron to a temperaturemuted condition (subnormal in determination of the limits .of tungsten and :titanium heater a flcompansoll P p i the lon forobtaining both enhanced hot strength of indirectly- Curves t'ubes havmg #220 mckel cathode's' are also heated cathodes and preservation or enhancement of shown In 3 8 The Correspondlng CUIVBS emission properties, tests were conducted on a number of for tubes havlllg the cfithaloy 13-331 l y reference tungsten-titanium-nickel cathode -alloys. Specific examodes are not shown since they are substantially similar' ples of such alloys are listed .below in Table A. 5 to those of the #220 alloy cathodes. Emission test curves Table 0 Alloy Ti W Al M I St I Fe Mn 0 on 00 Ni 3.94 .132 .057 .015 .020 .009 .114 .013 4.72 4.05 .127 .060 .013 .025 .048 .113 .009 4.64 4. 67 .112 .037 .015 .105 .130 .10 .052 Essentially 3.37 .140 .06 .018 .125 .105 .085 .013 .127 Remainder.

*Rcference nickel alloy. Reference tungsten-nickel alloy. p 7 p 7 As shown by the test curves of Fig. l, the yield for the #54053 alloy were not obtained vsincethis heat strengths of the tungsten-titanium-nickel cathode alloys of was essentially a duplicate of #5402 7. Table A are significantly higher than those of both the Referring to Figs. 2A, 2B, and 4B, the indirectlyreference-nickel and the reference tungsten-nickel cathode heated cathodes ofalloys #552 and #5514 activated more alloys throughout a high temperature range including rapidly than the #220..alloy' reference cathodes. Specathode-operatingtemp r ill the vicinity of 1650 c'ifi'cally,.the #552 cathodes were activated .by 14 hours F. in gener l, s determined y these tests. the hot y of life with an FM value of 14.5: the #5514 cathodes strength of the tungsten-titanium-nickel cathode alloys is were activated byl9 hours of life with an FM value of a u 21/2 to 3 fi i hg i t g flh i gl iggi 14.8: the reference cathodes required, 22 hours of life to alloy #220 and signi cant y 'g er an ato e reach an FM value of 14.5. tungsten-nickel all Y- shocktests 011 a Production tube After activation and ,until near the end ,of the life yp Q Show a difecicorrelatioll between the Shock tests, the emission characteristics of the #552 cathodes deformation characteristics of indirectly-heated cathodes re 1ower th tho f th eferen thode, artieu. yi Strength f the Same Cathode 3 at a larly at .subnorm'al heater voltage. Near the. endof the similar temperature}.l1 h b 1 th #55 V d life tests, the emission, for normal heater voltage of the The presence of 'g er co a tcontent in e 2 an #552 cathodes, slowly rose to match or cross the falling #5514 alloys increases theyieldstrength at temperatures emission of 'thejreference cathodes (Fig. 2A). Throughbelowabout 9 .'F b 8 little or 110 effect 'attemout the' flife tests, the emission characteristics of the' #5514 cathodes were consistently higher than those of the reference cathodes. This is shown by Figs. 2A, 2B and 4B for all three methods of evaluation.

Examination of the cathodes after life testing indicated some peeling of the oxide coating of the #552 cathodes. This is attributed to the slightly higher aluminum content of the #552 cathodes and is explanatory of the difference between the emissions of the #552 and #5514 cathodes during life. The beneficial effect upon emission of the lower percentage of titanium in the #552 alloy was obscured, until late in life, by the peeling effect due to excess aluminum. The beneficial effect, both upon mechanical and emission characteristics of addition of titanium in amounts as small as 0.15% in the #552 alloy can be realized by properly limiting the concentration of any aluminum included for its activation effects.

It was determined from examination of the life-tested cathodes and by tests conducted with uncoated cathodes in special sublimation tubes that the additives tungsten and titanium in the #552 and #5514 cathodes did not contribute to formation of sublimation deposits and did not result in harmful interface-impedance.

Referring to Figs. 3A, 3B and 4A the indirectly-heated cathodes of alloy #54027 activated more slowly than the #220 reference cathodes, requiring 50 hours to reach an FM value of 15.3, whereas the reference cathodes required 32 hours to reach an FM value of 15.5. However, after the first 350 hours of testing, the emission characteristics of the #54027 cathode surpassed and remained higher than those of the #220 cathodes for the remainder of the life test period. At the end of the test period, the FM value of the #54027 cathodes was 14.8, whereas the FM value of the reference cathodes had fallen to 14.2.

It was determined by examination of the life-tested cathodes and by tests conducted with uncoated cathodes in special sublimation tubes that the additives tungsten and titanium in the #54027 cathodes did not contribute to the formation of sublimation deposits or result in harmful interface-impedance. In fact, the #54027 alloy containing the highest amount of titanium had the lowest sublimation rate of all, indicating a favorable reaction between titanium and one or more of the subliming elements, nickel, magnesium, copper and/or manganese, thereby reducing the sublimation rate of the alloy.

The tungsten-titanium-nickel cathodes have a resistance to deformation, which at cathode-operating temperatures, is at least several times that of the reference-nickel cathodes, and which also when the aluminum content, if any, is held to suitably low controlled values, have emission characteristics which are stable and equal to or better than those of the reference-nickel cathodes. Since the cathodes have nearly the same physical strength throughout the range of about 0.15% to 3% titanium, and since titanium serves both as an activating and a strengthening agent, the more exact selection of the titanium percentage may be made on the basis of its effect on the electrical properties of the cathodes. As indicated above, for titanium percentages in the range of from about 0.4% to 3%, the emission characteristics are superior to those of the reference cathodes and for the higher percentages of this range, the sublimation rate is definitely lower than that of the reference cathodes. For good coating adherence and low interface-impedance, the tungsten-titanium-nickel cathode alloys should not contain aluminum in excess of about 0.15% or silicon in excess of about 0.04%; neither of these properties is impaired by addition of tungsten and titanium throughout the ranges herein given.

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 tungsten in the range of 1% to 5% by weight, titanium in the range of 0.05% to 3% 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 composed of an alloy containing 1% to 5% tungsten, 0.05% to 3% titanium; at least one of the activating agents aluminum, magnesium and silicon in the range of not more than about 0.07% magnesium, 0.15% aluminum, 0.05 silicon; and the balance essentially nickel with not more than about 0.2% iron, 0.20% manganese, 0.08% carbon, and 0.20% copper as residuals.

3. An indirectly-heated cathode structure characterized by high strength at cathode-operating temperatures, sustained high level of emission and negligible sublimation and composed of an alloy containing from about 5% tungsten, about 3% titanium, and the balance essentially nickel with not more than about 0.20% iron, 0.20% manganese, 0.08% carbon, 0.20% copper as residuals, and not more than about 0.07% magnesium, 0.15% aluminum and 0.05% silicon for cathode activation.

4. An indirectly-heated cathode structure characterized by a strength at cathode-operating temperatures of 2 /2 to 3 times that of unalloyed cathode nickel, and having a composition of about 4% tungsten, 0.4% titanium, 08% aluminum, 0.08% maximum carbon, 0.04% maximum magnesium, 0.04% maximum silicon, 0.10% maximum manganese, and the balance substantially nickel.

References Cited in the file of this patent UNITED STATES PATENTS 1,899,623 Lowry 2 Feb. 28, 1933 2,103,267 Mandell Dec. 28, 1937 2,172,967 De Boer Sept. 12, 1939 2,323,173 Widell June 9, 1943 2,396,977 Widell Mar. 19, 1946 2,720,458 Kates Oct. 11, 1955 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 2,836,491 May 2'7, 1958 Ardrey M. Bounds et a1.

It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 4, line 21, for "56" read 5O Signed and sealed this 14th day of October 1958.,

SEAL Attestz KARL I-IO AXLINE ROBERT C. WATSON Attesting Officer Commissioner of Patents

Claims (1)

1. 2. AN INDIRECTLY-HEATED CATHODE STRUCTURE CHARACTERIZED BY HIGH STRENGTH AT CATHODE-OPERATING TEMPERATURES, SUSTAINED HIGH LEVEL OF EMISSION, AND NEGLIGIBLE SUBLIMATION AND COMPOSED OF AN ALLOY CONTAINING 1% TO 5% TUNGSTEN, 0.05% TO 3% TITANIUM, AT LEAST ONE OF THE ACTIVATING AGENTS ALUMINUM, MAGNESIUM AND SILICON IN THE RANGE OF NOT MORE THAN ABOUT 0.07% MAGNESIUM, 0.15% ALUMINUM, 0.05% SILICON, AND THE BALANCE ESSENTIALLY NICKEL WITH NOT MORE THAN ABOUT 0.2% IRON, 0.20% MANGANESE, 0.08% CARBON, AND 0.20% COPPER AS RESIDUALS.

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