US2975050A - Zirconium-aluminum-nickel cathodes - Google Patents

Description

Yield Strength PS].

March 14, 1961 BOUNDS ET AL 2,975,050

ZIRCONIUM-ALUMINUM-NICKEL CATHODES Filed June 4, 1957 4 Sheets-Sheet 1 March 14, 1961 ET AL 2,975,050

ZIRCONIUM-ALUMINUM-NICKEL CATHODES 4 Sheets-Sheet 2 Filed June 4, 1957 OmVN 3 929w BE (v n l March 14, 1961 A, BOUNDS ET AL 2,975,050

ZIRCONIUM-ALUMINUM-NICKEL CATHODES 4 Sheets-Sheet 3 Filed June 4, 1957 l wf 4.5)vs.Life

Hours of Life Hours of Life Hours of Life March 14, 1961 Filed June 4, 1957 A. M. BOUNDS ET AL 5,050

ZIRCONIUM-ALUMINUM-NICKEL CATHODES 4 Sheets-Sheet 4 Fig. 4

l FM vs,Life

Hours of Life I FM vs. Life 4600 s'ooo 6000 Hours of Life lo'oo 2600 3000 barium, strontium or the like.

coating adversely affects the tube operation.

United. States PatentlfO ZIRCONIUM-ALUMENUM-NICKEL CATHODES Ardrey M. Bounds, Laverock, and Richard L. Hoif, Norristown, Pa., assignors to Superior Tube Company, Norristown, Pa., a corporation of Pennsylvania Filed June 4, 1957, $81. No. 663,525

4 Claims. cl. 75-170 This invention relates to electron tube cathodes of the indirectly heated type as distinguished from cathodes of the directly-heated filamentary type.

In general, the object of the invention is to provide indirectly-heated cathodes which, as compared to prior nickel alloy cathodes, have substantially increased resistance to deformation at cathode-operating temperatures and which have emission and sublimation characteristics substantially equivalent to or better than those of the prior 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 zirconium and aluminum within the low, narrow percentage limits hereinafter specified.

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

Fig. 1 is a group of curves exemplary of the yield strength, at cathode-operating temperatures, of several zirconium-aluminum-nickel alloys and of a reference nickel cathode alloy; and

Figs. 2, 2A, 3A, 3B, 3C, 4 and 4A each comprises groups of curves referred to in discussion of the emission characteristics of indirectly-heated cathodes of the reference nickel alloy and of zirconium-aluminum-nickel allow.

In general, indirectly-heated cathodes consists of a nickel alloy base element, such as a sleeve or cup, having thereon a coating of alkaline earth metals, such as 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 materials and the coating materials which convert the coating to 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 heater supply voltage is 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 afiected by cathode characteristics other than electron emission. Eruptive flaking or peeling of the 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 The re sistive component of such interface is damging, particularly in pulsed type service, even at ordinary frequencies: the capacitive component of such interface impedance is Patented Mar. 14, 9 1

particularly damaging at high frequencies-even when the tube is not operated under pulsed or cut-off conditions. 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 changes 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 addition of zirconium and aluminum, within low, narrow percentage ranges later specified, to nickel cathode alloys provide indirectlyheated cathodes, which at cathode-operating temperatures, have a hot strength from about two to over three and one-half times that of the nickel cathode alloy; which have, during the normal life expectancy of nickel cathodes, a high level of stable emission substantially equivalent to or better than that of nickel alloy cathodes; and some of which have high level emission for life ex-v tending substantially beyond that of nickel alloy cathodes.

Furthermore, such zirconium-aluminum-nickel alloys are amenable to hot and cold metal-working steps incident to fabrication of cathode sleeves and other indirectlyheated cathode elements from the alloy stock. Such zirconium-aluminum-nickel alloy cathodes also exhibit the characteristics of virtual freedom from sublimation and of negligible interface impedance.

Considering firstthe enhancement of yield strength at cathode-operating temperatures of about 1650 F.,

bled by increasing the percentage of zirconium substantially beyond 0.l%-up to about 0.5%. Within such range of addition of zirconium, aluminum may also be added, but not in excess of about 0.2%. From metallographic investigation, it appears that addition of zirconium and aluminum produces precipitation-hardening. However, the increased strength, due to such effect, falls off at temperatures of 1500 F. or higher. The 0.5% upper limit specified for addition of zicronium should not be greatly exceeded as there is danger of incipient melting during the exhaust schedules used to activate the cathodes. The upper limit specified. for addition of aluminum should not be appreciably exceeded because of flaking of the cathode coating and the re- I sultant impairment of the bond which forms at the interface between the cathode sleeve and its emissive oxide coating.

Except for residuals, the balance of the new cathode alloys is essentially the nickel base usually including cobalt not in excess of 1%. Higher percentages of. cobalt,

up to 10% or 15% in the nickel base, have .been found to have little efiect upon the emission characteristics or upon the mechanicalstrength at cathode-operating temperatures. I I

We have found that zirconium, besides being a very effective strengthening agent for nickel cathode alloys,

is also an efiective activating agent for such alloys. Consequently, the use of magnesium and silicon as activating agents may be minimized or avoided. This is of advantage because magnesium is largely responsible for formation of sublimation deposits leading to interelectrode leakage paths and because silicon is largely responsible for formation of a high interface-impedance between cathode sleeves and their coatings- In the zirconiumaluminum-nickel alloys, both the zirconium and aluminum are activating agents. Considering both the mechanical and emission characteristics of indirectlyhcated cathodes, the preferred alloy'should contain about 0.25% zirconium and 0.08% aluminum, balance nickel.

In determination of the limits of addition of zirconium and aluminum, both for obtaining enhanced hot-strength of indirectly-heated cathodes and for preservation or enhancement of their electrical properties, tests were conducted on a series of zirconium-nickel and zirconiumaluminum-nickel alloys: specific examples of such alloys For comparison purposes, the emission curves of concurrently tested tubes having #220 nickel cathodes are also shown in Figs. 2-4A.

Referring to Figs. 2-4A, the indirectly-heated cathodes of the #54016 and #54018 alloys activated as rapidly or more rapidly than the #220 nickel alloy cathodes, although containing much lower percentages of the activating agents, magnesium and silicon, than the reference alloy.

After activation, the emission and FM characteristics of the #54016 alloy cathodes remained high and stable, somewhat surpassing those characteristics of the #220 nickel cathodes for the usual life expectancy of 1000 hours (Figs. 3, 3A, 4). During an extended 6500 hour life test (Figs. 2A, 3B, 4A), the emission characteristics of the #54016 cathodes remained relatively high and stablesubstantially surpassing those of the #220 cathodes at all readings beyond 2000 hours of life. The #54016 cathodes exhibited good coating adherence durare listed below in Table A. ing the 6500-hour life test, whereas during that extended Table A Alloy Zr Al Mg Si Fe Mn 0 Cu Co Ni #54016 .137 .005 .009 .005 .02 .06 .05 .009 .071 Essentially #54018 .11 .11 .608 .000 .079 .08 .038 .008 .05 Remain #5525 .43 .12 .025 .006 .017 .05 .042 .004 .62 der.

#220 0 .009 .025 .014 .032 .05 .04 .014 .103 Essentially (Iitemain- As shown by the test. curves of Fig. 1, the yield strengths of the zirconium-bearing alloys of Table A are significantly higher than that of the reference nickel cathode alloy throughout a rangeof high temperatures including the usual cathode-operating temperatures of about 1600 F. In general, as determined by these and other tests, the hot yield strength of the zirconium-aluminum-nickel alloys was at least twice that of the nickel cathode alloy #220; the hot yield strength of alloy #5525 was nearly four times that of reference alloy #220 at 1650 F. As confirmed by shock tests on tubes with cathodes at operating temperatures, there is direct correlation between the shock deformation characteristics of indirectly-heated cathodes and the hot yield strength of the cathode alloy.

The emission characteristics of oxide-coated indirectlyheated cathodes using the alloys of Table A for the cathode sleeves are shown in Figs. 2-4A. In these figures, the curves are identified by the respective alloy designations of Table A. For these emission tests, the cathodes were incorporated in the standard diode structure defined in Spec. F-270-52T of 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 for the emission tests were: an anode-cathode supply voltage (E of 100 volts; a heater voltage (E) of 6.5 volts, and a load resistance (R of 1000 ohms.

Anode current readings were taken at 0, 5, 25, 50, 100, 200, 350 and 500 hours, and then every 250 hours to the end of the test. At each test period, the anode current was read at a plate voltage of volts for a series of heater voltages, including the normal voltage (E v.) and subnormalvoltage (including E =4.5 v.). Such anode current readings plotted against time constitute the curves of Figs. 2-3C. The I FM or direct-current Figure of Merit curves of Figs. 4, 4A are derived from the anode current vs. heater'voltage readings as described in detail in an article of Briggs and Richard in the ASTM Bulletin of January 1951. Briefly, the I FM value is the ratio. of the I ,E coordinates at the knee of the anode current/heater voltage curve where the anode-current changes from a space charge limited condition to a temperaturerlimited condition (subnormal heater voltage).

time the coating adherence of the #220 nickel cathodes suffered severe depreciation. For the #54016 cathodes, no sublimation deposit could be detected until after 2700 hours. Even after 6500 hours, only a faint trace was discernible, whereas for the #220 nickel cathodes a sublimation deposit was visible at 25 hours, and became a heavy deposit at the end of the 6500-hour test. The interface-impedance of the #54016 cathodes remained negligible throughout the extended life test.

After activation, the emission and FM characteristics of the #54018 alloy cathodes remained high and stable and were closely equivalent to or better than those of the #220 nickel cathodes for the usual life expectancy of 1000 hours (Figs. 2, 3C, 4). The #54018 alloy cathodes exhibited good coating adherence and negligible interfaceimpedance throughout the 1000-hour life test period. For the #54018 cathodes, no sublimation deposit was visible until after 500 hours of life and there still was only a trace at 1000 hours of life. For the #220 nickel cathodes, a sublimation deposit was visible at 25 hours and was quite heavy at 1000 hours.

Although the #5525 alloy activated rapidly to a favorably high emission level, the emission dropped quickly due to coating peel. Such coating peel was attributed to interaction between the aluminum and zirconium which in this alloy were respectively 0.12% aluminum and 0.43% zirconium. Such interaction was also indicated by the lower amenability to cold drawing characteristics of this higher aluminum alloy. It was concluded that the proportion of aluminum should be reduced as the zirconium content is increased to obtain long cathode life and to avoid fabrication difiiculties.

In brief rsum, the zirconium-aluminum-nickel cathodes have a resistance to deformation which is from about two to four times greater than that of the reference nickel cathodes; their emission characteristics are equivalent to or better than those of the reference nickel cathodes for the normal life expectancy of the latter and continue to be at high level substantially beyond the normal life expectancy of the reference nickel cathodes; and their sublimation characteristics are substantially better than those of the reference nickel cathodes.

What is claimed is:

1. A cathode structure of the indirectly-heated nonfilarnentary type including sleeves and cups characterized by high strength at cathode-operating temperatures, rapid activation, and good emission and sublimation characteristics and composed of an alloy tree of tungsten and containing zirconium in the range of 0.05% to 0.5% by weight, aluminum 0.005% to 0.2% by weight, and the remainder essentially nickel.

2. An indirectly-heated cathode structure as in claim 1 in which the alloy compositionby weight is:

Percent Zirconium 0.05 to 0.5 Aluminum 0.005 to 0.2 Magnesium not more than 0.07 Silicon not more than 0.05 Carbon not more than 0.08 Manganese not more than 0.15 Cooper not more than 0.05

and the remainder essentially nickel.

3. A cathode structure of the indirectly-heated nonfilamentary type including sleeves and cups in which the composition by weight is about 0.25% zirconium, 0.08% aluminum, balance substantially a tungsten free nickel base.

minum being in the range of 0.005% to 0.2% by weight, the maximum percentage of aluminum for a given percentage of zircomium varying from 0.2% for the lower limit of zirconium to 0.005% for the upper limit of zitconium.

References Cited in the file of this patent UNITED STATES PATENTS 4. A cathode structure of the indirectly-heated nonfilamentary type including sleeves and cups composed of a nickel-base alloy which is free of tungsten and which, contains zirconium and aluminum, the zirconium being in the range of 0.05% to 0.5% by weight and the al-u-

Claims (1)

1. A CATHODE STRUCTURE OF THE INDIRECTLY-HEATED NONFILAMENTARY TYPE INCLUDING SLEEVES AND CUPS CHARACTERIZED BY HIGH STRENGTH AT CATHODE-OPERATING TEMPERATURES, RAPID ACTIVATION, AND GOOD EMISSION AND SUBLIMATION CHARACTERISTICS AND COMPOSED OF AN ALLOY FREE OF TUNGSTEN AND CONTAINING ZIRCONIUM IN THE RANGE OF 0.05% TO 0.5%

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