US2678486A - Grid for electron discharge devices - Google Patents

Description

A. J. CHICK ETAL 2,678,486

GRID-FOR ELECTRON DISCHARGE DEVICES May 18, 1954 "5 Sheets-Sheet 2 Filed Oct. 11 1949 FIG. 2

l I l I 500 600 700 8 I l I 00 J00 400 TEMPERATURE 0v c.

. I vlll AJ. CHICK INVENTORS LJ. SPECK ATTORNEY y 18, 1954' A. J. CHICK ETAL GRID FOR ELECTRON DISCHARGE DEVICES Filed Oct. 11 1949 v 3 Sheets-Sheet 3 AJ CHICK INVENTORS- L. J. SPEC BY v 4a, ATTORNEY Patented May 18, 1954 UNITED STATES PATENT I QEF WE GRID'FOR ELECTRON DISCHARGE DEVHJES A .AlthlllJuChiOk; Summit, and Lawrence J. Speck,

Basking Ridge, N. J., iassignors toBelltTelephone Laboratories, Incorporated, New York,

N. Y.', acorporation 'of1N ew York myApplication.October 11, 1949; Serial No...120,652

f Claims. 1

This invention relates toaelectron. discharge devicesandmore particularly .to. the-fabrication of a grid for such devices.

.w'In such devices particularly-suitable for use a .in broad-band frequency-systems =with.high gain, .rdifiiculty has been .-encountered due to grid .wire ..-.v'ibration.u .Because 01 113118. very small; spacings involved intsucn devices ibetween thecathode -emitting surface.;and the. grids even, slight vibrations :in. the grid 'wireslcause. large changes and distortions in the characteristic-output ofthe device: Such Vibrations. may be caused bynon- .nuniformity ofelectron emission-which causes unbalanced (magnetic-forces between adjacent Wires,

by internal electrostatic forces, or by mechanie cal shock-orwibration of L the tube. However,

. basically in eachcase these troublesome vibrationsuareimost severeewhenethe grid wires are 7 loose.

In-tubesofthe typeabovereferred to,..an ill'llSw trative example of which is "that. described in .PatentlNo. 2,521,127; granted October"24,..1950, to

I R. S.-Gormley, :C Magga: and L. F,.-.Moose,.planar wound over some brazing materialvandl-then heated. to-a brazing temperature, the permanent deformation still takes place. When the brazing wtakes place the Wires are held by thesolidified -r brazing material in this elongated position. The molybdenum frame will then contract morethan the wires so. that the final product is still a grid with wires below the initial tension prior to i brazing.

- Oneprimary object .of this invention. isto improve grid assemblies for electron discharge devices.

A further object of thisinvention is to provide grid assemblies wherein the grid wires are tighter so that deleterious vibrations are substantially reduced.

vA still furtherwobject is to facilitate theiabrication of grids for electron discharge devices.

A still further object is to provide a method wherein the grid wiresare not ruptured or materially destroyed'during fabrication.

.wln accordance. with one feature of this in- .vention, a separator isinserted between the grid grids; are-used forwery accurate. spacingsof. the

cathode, grid. and. anode assemblies in..pi1e-up Such; wiresrare-eoften :wound. on 1- two. grid ;...irames, back to back, atwthesamee time. If the wires. and. framesare of. difierent. materials, .the

. different expansivitieseause ,the initial tension with. which the wires were .Wound to. drop.. .very .drastically ,-following .the... brazing Loperation. Even ifthe wires and the.frames.areqofthesame material the.differenumasses..invo1vedresult in different expansions givinglrise vto .the. sam deleterious result. -Ifthe wires such asofitungsten wereinitially ,.tightly..woundlontwo grid .frames -such raslmolyb idenum, and; then... merely sheated to aghrazing temperature. and cooled, .but not.,lbrazed,=\.the.grid

wires would zbewstretched the,process,heyond their .elastic; -.limit and ;a -permanent, set...intro-, ;1-. -.duced.-. I n-cooling the. wiresrwould.=contract .but

would retain sthisfipermanentidefonmation causingthe :grid wiresaoibe wound-.very loosely around he frames.

Im one f-known processnwhen rthei-lwireslare framesprior to the winding of the grid.

In accordance with another. feature of this invention, the separator has a larger'thermal expansion over a range of temperatures of moment, than the grid frame or wires.

In accordance with a further feature of this invention, the separator has a higher value of "relaxation timethan either thegrid frame or "In' accordance with a still "further feature of :this invention, an additional stress is given to the sy'grid' wires'just'prior to the solidification of the bracing material to increase the final tension of the'grid.

: As is knownythe grid wires are very-taut on the frame immediately following the brazing,

"while the frames and wiresremain at the elevated' temperatures. 'lhis'is becausethe wires have been held'to theframe in their elongated position; so that the permanent set has no harmful effect. It is-a'feature of devices constructed inaccordance with-this invention: that a further stress is introduced into the 'wire', 'beyond that which is given by theexpansion of thejframe, to cause a taut grid at these elevated'temperaturesand a resultant tauter grid following the contraction of the materials.

mliMore specifically, it is a feature of .thisinven- Nation. that following the flowing or" the brazing -i material andwbefore thelheatingmsource is withdrawn to allow the brazing material tog solidify,

an :theseparator. expandsumore than the grid frame to give an additional small stress to the grid wires, which causes a large percentage increase in the grid tension at room temperatures.

The separator may be of any one of a variety of materials, either insulator or metal, though if of metal it must be coated with a ceramic material, such as aluminum oxide, to prevent adhesion to the frame during the brazing operation. It is not essential that the expansivity of the material be greater than that of the frame or the wire. It is, however, important that the total expansion be greater. Thus, while materials having higher expansivities, or incremental changes per unit length, have been found to be advantageous, a material having a lower expansivity, but a larger mass or thickness may also be used as it is the total expansion that is of major importance.

Further, it is to be realized that the expansivity should not be too greatly above that of the frame and wire materials lest the greater expansion during the brazing cause the grid wires to rupture.

It is also important that the separator temperature lag that of the grid frame. This it will tend to do to some extent regardless of the relative thermal characteristics of the materials because normally the heat is applied for the brazing operation from outside the wires and the frame and must therefore be transferred to the separa tor. However, it has been found advantageous to have a separator whose thermal characteristics are such that it will take a longer time to attain a certain temperature; this may also be ex: pressed by saying that the separator must have a higher relaxation time. The relaxation time is a constant for any material which is defined by the expression where 1- is the relaxation time, x is the distance from one side to the center of a slab of the material of infinite length, and h is the thermal diffusivity of the material which is defined by the expression wherein K is the conductivity of the material, p its density, and 0 its specific heat. It is thus seen that the relaxation time is a constant for a given material within the ranges in which the significant terms are constant. It can be shown that the relaxation time may be interpreted as the time for a heat pulse to travel into the center of the slab, a distance .rs. value of 7', the relaxation time, is of course, determined by the thickness of the slab chosen; it is generally followed that the slab is of twocentimeter thickness so that are is one centimeter,

and values given herein for relaxation time are The exact numerical charge device employing a grid assembly fabricated in accordance with this invention;

Fig. 2 is a perspective view of a grid assembly such as may be fabricated in accordance with this invention;

Fig. 3 is a front view of one dual grid sandwich during the fabrication;

Fig. 4 is a sectional view taken along the line 44 of Fig. 3;

Fig. 5 is an exploded view of the sandwich of Fig. 3, showing particularly the internal separator;

Fig. 6 is a graph showing the expansivities of various materials that may be employed in the method in accordance with this invention;

Fig. 7 is a sectional view of a sandwich, similar to Fig. 4, and showing one modification of this invention;

Fig. 8 is a part view of another dual grid sandwich during the fabrication; and

Fig. 9 is an exploded view of the sandwich of Fig. 8, showing particularly the interval separator.

Referring now to the drawings, Fig. 1 shows one device suitable for use in microwave applications in which high transconductance, high gain, low capacitance losses, and high power output are of paramount importance. This device H, which is described in detail in Patent No. 2,527,127, granted October 24, 1950, to S. Gormley, C. Maggs, and L. F. Moose, comprises ananode button l2, a planar grid l3, and a cathode disc I4 having an emissive coating applied thereto and being supported in a sleeve member or supporting housing I5.

The planar grid l3, as shown in Fig. 2, is a cross-lateral grid, the windings l6 and I7 being brazed to a circular frame |8 having a central aperture l9; either the wires or frame may advantageously be of tungsten or molybdenum. In the fabrication of this planar grid as shown clearly in Fig. 5, two rectangular frames I8 are placed back to back, the frames having their edges rounded off as by bending and the apertures l9 centrally located therein. A separator 2| is placed in the space between the two frames I8 and the fine wire wound over the pair of plates by the winding methods disclosed in Patent No. 2,566,848, granted September 14, 1951, to J. A. Morton. A foil of brazing material or alloy 22 having an aperture 23 coinciding with aperture is in frame l8 is then placed on each frame on top of the wound wires [1. Wire I6 is then wound over the brazing foil 22 at right angles to the wire H. The dual-grid assembly, which combination will hereinafter be referred to as a sandwich, then appears as shown in Figs. 3 and 4.

In accordance with a feature of this invention, the separator 2| is of a material having particular thermal characteristics with respect to the materials of the grid frame and wires. Specifically,

the separator has a larger expansion at the brazing temperature and a higher value of relaxation time. A fuller appreciation of these limitations on the separator 2| and of its effect in the brazing operation whereby a final tauter grid is produced can be had from the following description of one specific arrangement that has been advantageously employed. The grid wires l6 and I! in this illustrative embodiment are of tungsten strands, of 0.00032-inch diameter. The separator 2| is of steatite, while the frames l8 are of molybdenum and are first properly degreased and cleaned and then given a flash coating of copper by plating. This copper coating is sintered to a smooth even flowirate of frorn75xto 100.:cubic'feet' per ihour is were not: dependent on this theory or explanation.

' 'arator 2 l.,:having a muchahigherlvalu'e ofirelaxaallows the slower heating separator to continue to increase in both temperature and expansion,

.i1nti1 it passes the point 30, atwhich itsexpansion andtha't of the molybdenum arethe same, and reaches a point, such 'a's3l, where the sep- 4 5' 5 yer. eThej'zwiresallfizzandrld are woundrat165 'per centrof their: breaking: strength.

.Forrbrazingr theisandwichais .supportedialong twoiedges' ina framework l'ocatedxwithin a heater coilrcontained in a hydrogen. bottle. A hydrogen a used;:-;:l?ower is: applied .in rapproximately. second: steps untilrtthe. brazing1metah is visually observed toliflow asbrazing alloyeknown: aszfBT all'oyylwhich has a .-:oon'rpositio1 1i- 0137:128 peni'cent 1.: SflVGf-fllid'FfZBiZ per cent. copper.risiemployedm alloyimelts at:778 C.",i.bl1t' abrazing: temperature ofijapproximately 8509 6. is employed.

Referring...now;.tox.Fig; 6, the..expansivities of plottedcasa against temperatures. awhile. the. following explanation ishnot strictlyl .quantitatively accurate; it. is :believed; tothe:clearlyrvsexplanatory oflthe operationofrthe separator; in the sandwich. However; it is tobeiundarstood thatithisinven- J3 ration. and the known beneficial results therefrom Aswheat is appiied to. the sandwich; the.molybdenum frames, havingiatvery low relaxationztime, heat up-rapidly along curve 2fiiandequickly. reach thetexpansivity at point flwhen thextemperature ismaintainediat 850 C. '5 The :steatiteiseptionstimelfand also I being shielded. fror l the heat upply by theluframes, progressesorhuchzvmore slowly alonglits curve: 28 sotha t' at the time :that heframesare. initiallyat point 2?; theseparator mayhave only :reachediapoint:suchtas 2th As is aeasily seen, the expansion-tat thisxzpoint iszhelow t that of the molybdenum frames. However, heat 'is supp'lied to the sandwichfor about l5 seconds at thishighest; temperature of 850 C., during which time there is no additional expansion or" Wthe molybdenum frames, as. they have already ..reachedf. .this temperature; this additional .heat

arator has forced the molybdenum frames to expand further and thus has introduced a further tension on the tungsten wires. At this point, the power supplied to the heating coil is turned 01f and the sandwich allowed to cool down, with the tungsten Wire brazed and held to the molybdenum frame in this last elongated condition. The separator thus gives, in eifect, an additional elongation to the wire, as represented graphically in Fig. 6 by the distance along the ordinate between the points 30 and 3|, which causes a tauter final grid structure than is otherwise obtained.

When the sandwich has cooled down, the two frames are separated by cutting the wires 5 E and I! along the edges of the frames and the separator 2i removed for use in another operation. The frames are then punched to proper circular size and have the appearance shown in Fig. 2.

In the specific illustrative embodiment described above, the materials employed have the following characteristics:

Tests made with other separators illustrate terials may be employed as the separator:

. .r,1marelativeiexpansion and asto lag in heating of the separator are essential; .these conditions are wgenerally tobemet intermsof aihigherr expansivity and. higher relaxation time, butpas'explaineduabovanmay. betotherwise met- In -one test; a 30-milLplatinum. separator, which has the same thermalsexpansion as 40 2mi1s ofixsteatite, .was employed; however, the wires-broke: due to the-expansion of-..the. separator even before the brazing temperature. wasmeached' Thiszis be- 5 czcause the :.relaxation time for platinum is only 1.69 seconds; On the otherflhand a separator of al:zircon porcelain'body, commerciallyilznown as theLvarious. materials;ofzthissandwich arettherein Alsimag 4'25, which has a relaxation tiine 'of 23 seconds andis thus less than that of steatiteg produced no improvement in wire tension because the thermal expansion of Alsimagd-lfi is even less 4 :than that of tungsten and. onlyabout 13.73 that of the steatite.

It is of course to be realized i that othenma- Thus, a=forsterite bodygknown commercially as-Alstmag1243, which'has amuch highen enpansivity than steatite anda relaxation time of zilrseconds may be employed, the thickness of the cer-amic being. proportioned to provide the propen-total "1 expansion. Further the separatorneed not be 1 .a single. substance or element, but may 'its'elf be composed of .different layers of -materials;=:provided that the composite thermal characteristicsoi' the bOdy arawithin the requirements explained above. Thus, the poor espansivity of Alsimag i'iB may be compensated forbyusing a thin layer of molybdenum between tvvodayers of this ceramid- Other possiblemateriaia and combinationswill be apparentto those skilled in I the art.

Further control of the expansion Of-"the-sepaseveral apertures 3 3 which may be drilled'through the separator. A heater element 36' is' threaded 7 through these apertures and brought out through the sandwich I to externalconnections?No insulation problem need be considered when the separator is a ceramic. In the case of this modification, when the brazing temperature has been reached and the brazing material observed to flow, the internal heater 3 1 may be energized to cause additional heat to flow to the separator 2! to overcome its slow relaxation time and cause sufficient expansion and subsequent elongation of the grid wires. The time that power is applied to the heater element 34 is of course so chosen that the grid wires will expand to their maximum advantage without rupturing. This localized heating of the separator then causes the additional thermal expansion to further tighten the grid wires.

If both the grid frames and wires are of tungsten a different sandwich may be employed, as shown in Figs. 8 and 9. As it is very diificult to work with tungsten or to punch out circular portions, the frames 36 are initially made in the desired ring shape. The frames are placed in apertures 31 on the surfaces of the rectangular separator member 33, the apertures being of the same size as the frames, as shown in Fig. 9. A grid 39 is then wound around the separator over the grid 35. The brazing foil, which may be in the form of a circular ring at of the same size as the grid frames 36, is placed over the grid Wires 39, and a second grid ii! is wound around the separator 38 on top of the foil 40 and at right angles to the grid 39. Because the expansivities of the grid frames and wires are the same and because of the particular shape of the separator member, very tight grids may be achieved. It is of course to be understood that this sandwich is not limited in its use to these materials, but may also be employed when the grid frame is molybdenum or any other advantageous material and the wires are of tungsten molybdenum or other material.

It is to be understood that the above-described arrangements are illustrative of the principles of this invention and that numerous other arrangements may be devised by those skilled in the art without departing from the spirit and scope of the invention.

. What is claimed is:

1. The method of fabricating grids for electron discharge devices comprising winding grid wires around two frames having a separator between them, heating the wires and the frames so that the frames expand and'introduce a permanent set into the wires, additionally heating the separator while maintaining the temperature of the frames and wires substantially constant until the separator causes said frames to separate causing an additional elongation of the wires, and attaching the wires to the frames following the separation of the frames and this additional elonga 15 tion.

2. The method of fabricating grids for electron discharge devices comprising winding very fine wire around two frames having a separator between them, the tightness of the winding apwire around two frames, heating the wire and frames so that the frames expand and extend the wire, separating the frames while maintaining the temperature of the frames and the wire substantially constant to cause an additional extension of the wire, and attaching the wire to the frames following this additional extension.

4. The method of fabricating grids for electron discharge devices comprising winding very fine wire around two frames having a separator between them, said separator. having a larger thermal expansion and a longer relaxation time than said grid wire and said frames, heating said frames and wires to a brazing temperature, continuing to heat said separator while maintaining the temperature of said frames substantially constant to cause said frames to separate and additionally expand said wire, and attaching the wire to said frames following this additional expansion.

5. The method of fabricating planar grids for electron discharge devices comprising placing a separator member between two grid frames, said separator member having a higher thermal expansion and a longer relaxation time than said frames, placing brazing material on said frames, winding a grid wire on said frames in one direction, winding a second grid wire on said frames in the direction perpendicular to said first grid wire, heating said wires and frames to a temperature sufficient to melt said brazing material, maintaining said temperature substantially constant to expand said separator member and cause said frames to separate, and cooling said frames and wires following the separation of said frames.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 535,321 Delany Mar. 5, 1895 1,472,505 Trimble Oct. 30, 1923 2,261,350 Epstein Nov. 4, 1941 2,451,360 Skehan Oct. 12, 1948 2,527,127 Gormley et al Oct. 24, 1950 2,610,387 Borland Sept. 16, 1952

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