US2958836A - Multiple-characteristic superconductive wire - Google Patents

Description

Nov. 1, 1960 H. o. MoMAHoN 2,958,836

MULTIPLE-CHARACTERISTIC SUPERCONDUCTIVE WIRE Filed July 11, 1957 2 sheets-sheet 1 looo LLI .3 soo- Ni soo- 9 `28 C 7 K5 g C c D K6 w INVENTOR.

HOWARD O. McMAHON ATTORNEYS.

Nov. 1, 1960 H. o. MCMA-xoN MULTIPLE-CHARACTERISTIC SUPERCONDUCTIVE WIRE Filed July l1, 1957 2 Sheets-Sheet 2 F C. 7 v 44h 44h 44b U U u 44a. u u U42@ dfi? \42a AAA/ 42a 42h 44 42: b,.

F G. aA 7' MQ; l FIG. 5

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c 36 C K2 c C /-4O f/ t/ F3a INVENTOR. HOWARD O. McMAHON v ATTORNEY;

United States atent MULTIPLE-CHARACTERISTIC SUPERCON- DUCTIVE WIRE Howard O. McMahon, Lexington, Mass., assignor to Arthur D. Little, Inc., Cambridge, Mass.

Filed July 11, 19.57, Ser. No. 671,329

8 Claims. (Cl. 338-32) This invention yrelatesto an improved construction for cryotron circuits. More particularly, it relates to, as an article of manufacture, a-conductor having zones of different superconductive properties formed along its length and to constructions utilizing conductors of this type to form cryotron circuits. For example, such a conductor may be intertwined with a second similar conductor to form a plurality of connected cryotrons.

My improved construction may best be understood from the following description taken in connection with the accompanying drawingsin which:

Figure l is a family of curves for different materials showing how the temperature at which a material becomes superconductive changes as a function of applied `magnetic field, the materials being superconductive when maintained under the conditions represented by the areas to the left of and below the respective curves.

Figure 2 is a diagrammatic representation of a cryotron,

Figure 3l is aschematic drawing of a cryotron Hip-flop circuit made from ciyotrons of -the type illustrated in Figure 2,

Figure 4 is a section-through a base conductor having a coating ofdifferent superconductive properties deposited on portions thereof,

,'Figure 5 is asection through the conductor of Figure 4 with the coating diffused into the conductor to form a composite cryotron conductor made according to my invention,

Figure 6 is a sectional view of another embodiment of a compositey cryotron conductor made accordingttomy invention,

Figure 7 is adiagrammatic representation illustrating how a plurality of connected cryotrons may be formed from conductors .of the type `illustrated in Figures 3 and 4, and

Figure 8 .is aschematic diagram of the ,cryotron `iiipflop circuit of Figure 3 fabricated from a pluralityof cryotrons arranged as in Figure V7.

Similar reference characters refer `to similar parts throughoutthe several viewsof the drawings.

The cryotron, which lis ya switching element kuseful in digital computers, depends for its operation on the changes vin properties ofcertainelectrical conductors when subjected to temperatures approaching absolutezero. As the temperature approaches absolutezero, in the absence of a magnetic lield, thesematerials change suddenly from a resistive state to Ka superconductive state invwhich their resistance :is identically zero. vThe temperature at which this change occurs is known as the transition temperature. Whena magnetic `irield'irsapplied to the conductor, the transitionternperature is lowered, the relationship between applied magnetic held and transition temperature for ,anurnber/ofthese materials being shown in Figure l. As shownin thisiigure, inthe absencevof a magnetic field tantalum loses all electrical resistance when reduced to ajtemperature of 4.4 K. or below, lead does so at r17.2 K., and niobium at 8 K.

fi eje In all, there are twenty-one elements, in addition to many alloys and compounds, which undergo transition to the super-conductive state at temperatures ranging between O and 17 K. The presence of a magnetic field causes the transition temperature to move to` a lower value, or, if a constant temperature is maintained, an increase in the magnetic field to a certain level will cause the superconductive material to revert to its normal resistive state. From Figure l it is apparent that a magnetic eld of between 50 and 100 oersteds will cause a tantalum wire held at 4.2 K. (the boiling point of liquid helium at atmospheric pressure) to change from the superconductive to the resistive state.

The cryotron is a circuit element which makes use of the shift between the superconductive and resistive states of these materials when held at constant temperatures. For example, Figure 2 illustrates a cryotron having a central or gate conductor 2 about which is wound a control coil 4, both the gate conductor and the coil being of materials which are normally super-conductive at depressed temperatures. The ent-ire unit is immersed in liquid helium to render the gate conductor 2 and the control conductor 4 superconductive. If a current of suilicient magnitude is applied to the control conductor, the magnetic eld produced thereby causes the gate conductor Vto transfer from the superconductive to the resistive state. Thus, the control coil and gate wire form an electrically-operated switch in which the gate can be changed from the superconductive to the resistive state by the application of current to the control coil.

Tantalurn is a desirable material for gate conductors, since its transition temperature in the SO-to-lGO-oersted region is 4.20 K, the boiling point of helium at a pressure of one atmosphere. This temperature is attainable without the use of complicated pressure or vacuum equipment for raising or lowering the temperature of helium. Niobium, which has a relatively high quenching field (the held strength required to render a super-conductive material resistive), is usually used as the material for the control coil, since it is desirable, and in many cases necessary, that the control conductor remain superconductive throughout the operation of the cryotron, and this coil is subject to substantially the same magnetic iields as those imposed on the gate conductor. Additionally, in most applications it is desirable to have the control conductor in the form Vof a coil such as coil 4 in Figure 2 in order to minimize the current necessary to produce a quenching iield. L

in cryotron circuitry, the gate conductor of one cryotron is often connected -in series with the control conductor of another, and therefore the cryotron must provide a current gain for successful operation of the circuit, i.e. the current controlled by the gate conductor of the cryotron should be larger than that required to energize its control coil. if the control conductor is not in the form of a coil, the current through the conductor required to quench the tantalum gate may produce a held large enough to cause self-quenching of a tantalum gate connected in series with it. In practice it has been found that suitable current gain is obtained in a cryotron having a .G09 inch tantalum gate conductor with a single layer control coil of .003 inch niobium wire having 250 turns per inch.

A simple bi-stable element, the basic unit of a binary digital computer, may be formed with cryotrons by connecting the gates of two of the umts in parallel and arranging to have one gate or the other conduct all the current through the combination in the same manner as a vacuum tube flip-op. Thus, as shown in Figure 3, a cryotron ilip-op may comprise two cryotrons K1 and i2 whose gate conductors Klg and KZg are connected together at one end to a power supply, illustratively shown as a battery 6, in series with a limiting resistor R1. Preferably, resistor R1 is of much higher resistance than the flip-flop circuit, so that the power supply is essentially a constant current source. Gate conductor Klg is connected in series with control coil KZC, and gate conductor K2g is similarly connected to control coil K10. The control conductors are returned to ground to complete the circuit through read-in cryotrons K3 and K4, respectively, and read-out cryotrons K5 and K6, respectively, in a manner to be described. When conductor Klg is resistive and conductor IQZg is superconductive, an entirely superconductive path is formed through gate conductors KZg and K3g and control conductors K1c and Kc. The path through the series combination including conductor Klg is resistive at this time. Thus, all the current from the power supply will flow through gate conductor K2g and none through conductor Klg, and this same current flowing through control coil Klc will keep the gate conductor Klg enclosed therein in the resistive state. The flip-flop is thus stable in this position, much as a conventional bi-stable vacuum tube flip-flop is stable in one position or the other. If the available current from the power supply is less than twice that required to quenchone of the gate conductors, and if a pulse is applied to control winding KBC to quench gate conductor K3g to make the path including conductor KZg resistive, the current divides approximately equally between the two paths. There is now insufficient current flow through control conductor Klc to maintain the gate conductor Klg in its resistive state. As gate conductor Kg changes from the resistive to the superconductive state, a superconductive path is formed through it and control conductor KZC, gate conductor K4g, and control conductor Kc; thus, all the available current ows through this path to quench gate conductor KZg. The flip-flop thus reaches its other stable position.

Similarly, a current pulse of suilcient magnitude, applied to control conductor K4c, will cause the Hip-flop to revert to the former position. Gate conductor KSg or VKtg will be quenched, depending on whether the superconductive path through the flip-flop is through control conductor KSC or Kc, and therefore the conductive states of these gate conductors are indicative of the position of the Kl-KZ ip-op.

Because of its small size and low cost, the cryotron is an ideal basic switching element for use in large data processing equipment, which may utilize many thousands of such elements, connected together in a number of basic circuits, such as the ip-flop illustrated in Figure 3. Prior to my invention, the small size of the individual cryotron, in most respects an important advantage, has presented a serious problem in the fabrication of the various circuits in which it is employed. The four terminals of .each cryotron are generally connected to other -superconductive circuit elements by welding, to preserve superconductivity at the point of connection, and thus cryotron circuits require an average of approximately two welded joints per cryotron, e.g. the flip-flop of Figure 3 has ten internal welded connections, as indicated by the reference characters 7 through 24. The individual gate and control conductors of the cryotrons may be as small as one or two mils in diameter, and, therefore, extreme care must be used in handling these elements and forming the welded connections between them, in order to prevent breakage and to assure correct connection. In fact, the fabrication of cryotron circuits is generally performed under a microscope and is a time-consuming, tedious job.

Accordingly, it is a principal object of my invention to provide an improved cryotron circuit construction having a minimum of internal connections, and cryotron gate and control conductors adapted for ecient use in such construction. It is another object of my invention to provide a cryotron circuit construction of the above character whose assembly requires minimum handling of 4 individual cryotrons. It is a further object of my invention to provide cryotron conductors of the above character capable of use in mechanized assembly operations. It is yet another object of my invention to provide cryotron conductors of the above character susceptible of low-cost manufacture. lt is a still further object to provide a cryotron flip-flop using a circuit construction of the above character. Other objects of my invention will in part be obvious and in part appear hereinafter.

The invention accordingly comprises an article of manufacture and the features of construction, combina-r tions of elements, and arrangements of parts which will be exempliiied in the constructions hereinafter set forth, and the scope of the invention will be indicated in the claims.

in general, my invention utilizes composite superconductive wires having alternate gate and control portions which require different magnetic field strengths to render them resistive at the temperature of operation. Thus, a conductor made according to my invention may have a gate segment with a relatively low quenching field followed by a control segment with a relatively high quenching iield, in turn followed by a second gate segment, and so on. A conductor having many such segments may be cut according to the number of cryotrons to be made from it and the manner in which they are to be interconnected.

Accordingly, one composite conductor having two gate and two control segments might be substituted for gate Klg, control coil KZC, gate K4g, and control K6c in Figure 3; similarly, a conductor of this type might be substituted for the other gate and control conductors. The two composite conductors would be connected together at 7 and 24, and thus the number of internal connections in Figure 3 would be reduced to two, 2O percent of the number previously required.

My composite cryotron conductor may comprise a tantalum wire having niobium-coated portions, with the uncoated portions serving as gate segments and the coated ones, which have a substantially greater quenching iield, serving as control segments. Alternatively, a wire having a base material with a given superconductive property may have another material diiused into it along alternate segments to alter the properties of those segments so that the treated and untreated portions have different quenching ield strengths.

In Figure 5 I have illustrated a composite cryotron conductor made according to my invention. As shown therein, a conductor, generally indicated at 26, comprises ialternate control segments 28 of niobium and gate segments 30 of a niobium-tantalum alloy. The' conductor 26 may be made, as illust-rated in Figure 4, by iirst forming coatings 32 of tantalum at spaced intervals along a niobium wire 34. The coatings 32 may be formed by -any desirable process such as evaporation, electro-deposition, etc. The coated wire is then heated `at a suitable temperature for a suiiicient period of time to diffuse the tantalum coatings int-o the niobium base conductor. This results in the composite conductor 26 of Figure 5, in which the untreated niobium segments 28 have control conductor characteristics, i.e. stronger quenching fields, and the segments 30 of the niobium-tantalum ialloy have gate characteristics, i.e. weaker quenching iields.

Another composite cryotron conductor embodying the features of my invention is illustrated in Figure 6. As shown therein, a conductor, generally indicated at 36, comprises a tantalum base conductor having niobiumcoated control segments 38 and uncoated tantalum gate segments 40. The niobium coatings may be formed lby evaporation or electro-deposition in similar fashion to the coatings 32 of Figure 4.

Conductors 26 and 36 are preferably as small in diameter as possible, the lower size limit being determined mainly by problems in handling. Thus the base conductors V34 and 40 are preferably tive-mil Wires, with the untreated segments of the finished composite conductors having the same diameter; the treated segments 30 and 38 may have somewhat greater diameters, resulting from the introduction of additional material into them. The tantalum coatings 32 of Figure 4 should be thick enough to provide segments 30 having quenching characteristics sufficiently different from those of the niobiurn control segments 28 to provide vefficient circuit operation, i.e. the disparity in quenching fields 'between the segments 28 and 30 should `be at least 3 'to l. Theniobium coatings on the segments 38 should be thick enough to insure complete coverage of the portions of the base conductor beneath.

While the conductors 26 and 36 have been described using niobium and tantalurn, it will be understood that other materials may be provided if they have widely different quenching characteristics, to insure that, in circuit operation, the control segments will remain superconductive duringthe quenching of the gate `segments associated with them. It will also be understood that composite conductors having segments o-f more than two quenching vcharacteristics may be provided Within the `purview of my invention. Thus, a conductor similar to the conductor 36 may be formed with lead-coated segments in addition to the niobium-coated and untreated tantalum segments. The vtantalum segments would be quenched by relatively low quenching fields, the leadcoated segments by stronger fields, and the niobiumcoated ones by still stronger fields.

My composite superconductive wire may be used to form a plurality of connected cryotrons, as illustrated in Figure 7. As shown therein, composite conductors 42 and 44 have gate and control segments 42a and 42b, and 44a and 44b, respectively. The conductors may be of the type illustrated in Figures 5 or 6, Iand they yare preferably wound by suitable machinery to form a stock material having any desirable number of sections, each section being an individual cryotron, with a control segment of one conductor formed into a coil around `a corresponding gate segment of the .other conductor. Portions of the control segments `are also used las the oonnecting wires between the sections, so that in subsequent use these connecting wires will remain superconductive throughout the operation of the circuit in which they are incorporated.

The superconductive stock serves as a convenient material for the fabrication of all types of cryotron circuits, including elementary cryotron switches as well as the most complex circuits. Thus, as will be described, a piece of stock of the fright length may lbe cut off and various operations may be performed on it to form a cryotron iiip-liop. It will be noted that, while the lengths of the various segments may vary, in general, it is desirable that all the segments of each quenching characteristic be of equal length to facilitate the formation of the stock.

In Figure 8 I have illustrated a flip-liep made from the cryotron stock of Figure 7. Six cryotrons are detached from the stock as a single unit, the cryotrons serving as K1', K2', K3', K4', K5', and K6' in Figure 8. The conductors are welded together at one end to form a connection 7 similar to connection 7 and are connected through a resistor R1' to `a battery `6'. Proceeding downwardly (Figure 8) from connection 7', cryotrons K1' and K2' are connected with the control coil of each in series with the gate of the other, as in Figure 3. At cryotron K4', however, the stock is altered by cutting, as indicated by the dotted lines 46 and 48, to isolate the control coil KA'C, 'whose terminals then serve as read-in terminals. Gate K4'g remains in series with gate Klg tas in Figure 3. The connection between gate K3g `and control coil K'c is severed as indicated at 50, and the gate is connected in series with gate KZg -by a welded connection 52. Control ycoil K3c is isolated from gates KS'g and K6'g, as indicated by the dotted lines 54 and 156, and cryotron K3' thus serves as the other read-in cryotron. Gate K'g, isolated at 48 and 54, and gate KSg, isolated at 56, may thus serve as read-out gates, their control coils K6'c and KSc being in series with read-in gates K4g and K3'g, respectively. The control coils lKSc and K'c are also connected together at la welded junction 24' similar to junction 24 of Figure 3.

Thus, the elements of Figure 8 -are connected in ythe sarne circuit relationship as are their counterparts in Figure 3, and the operation of the two circuits is identical. However, only three internal connections are required in the construction of Figure 8, as compared with the ten which are required when the circuit is constructed from individual cryotrons.

'Ihus I have described a composite cryotron conductor having segments with different quenching characteristics. Segments along the same conductor may thus serve as gate and control elements in various superconductive circuits, thereby eliminating many of the welded connections between gate and control elements heretofore required in the fabrication of such circuits. These composite superconductive conductors Ymay be manufactured in any suitable Ifashion, and I have illustrated two forms which they kmay take, although others may also be found suitable.

I have .also described a cryotron .stock formed .from a vpair of composite conductors. The superconductive stock `shown is very useful in such cryotron applications yas flipiiops, and I have illustrated a simple hip-flop in which the number of internal welded connections is reduced from ten to three through the use of this stock. Although the above description is specifically directed to stock comprising two composite conductors, it will be noted that three or more such conductors, with segments having any desirable number of quenching field values, may be combined in a similar manner for various circuit applications.

It will thus be seen that the objects set forth above, among those made apparent from the preceding description, are efficiently attained and, since certain changes may be made in the above article and constructions without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

It is also to be understood that the following claims are intended to cover all of the generic and specific features of the invention herein described, and all statements of the scope of the invention which, as a matter of language, might be said to fall therebetween.

I claim:

l. As an article of manufacture, a conductor for use in the manufacture of the combination of superconductive electric circuit elements and means for maintaining a low temperature environment therefor, said conductor being superconductive in said environment in the absence of an applied magnetic field and having gate portions requiring a given magnetic field strength to be rendered resistive and control portions requiring a greater magnetic field strength to be rendered resistive, said gate and control portions being periodically disposed along the length of said conductor in end-to-end electrical superconducting relationship.

'2. 'I'he combination defined in claim 1 in which said gate segments are of a niobium-tantalum alloy and said control segments are of niobium.

3. As an article of manufacture, a conductor for use in the manufacture of the combination of superconductive electric circuit elements and means for maintaining a low temperature environment therefor, said conductor being superconductive in said environment in the absence of an applied magnetic field and comprising a base conductor requiring a given magnetic field strength to be rendered resistive, said base conductor having formed on its periphery at spaced intervals along its 7 length coatings of a material requiring a different magnetic eld strength to be rendered resistive, the ends of each of said coatings being in electrical superconductive relationship tosaid base conductor.

4. The combination dened in claim 3 in which said base conductor is of tantalum and said coatings are niobium.

5. As an article of manufacture, a conductor for use in the fabrication of the combination of superconductive electric circuit elements and means for maintaining a low temperature environment therefor, said conductor being supercondnctive in said environment in the absence of an applied magnetic field and having a set of -rst portions requiring a given magneticield strength to be rendered resistive and a set of second portions requiring a greater magnetic eld strength to be rendered resistive, said iirst and second portions being alternately disposed along the length of said conductor in end-to-end superconducting relationship, one of said sets of portions being of a material having one of said field strengths and the other of said sets of portions being of an alloy including said material and a material having a difIerent quenching eld strength.

6. As an article of manufacture, cryotron stock for use in the fabrication of the combination of superconductive electric circuit elements and -means for maintaining a low temperature environment therefor, said stock comprising a pair of conductors, each of which is superconductive in said environment in the absence of an applied'magnetic field, said conductors each having a central base portion with a given magnetic eld strength and a plurality of spaced coatings disposed along said base portion, said coatings being of a material having a greater magnetic quenching field strength than said base portion, the ends of said coatings being in electrical superconductive relationship with said base portion, the coated portions of each conductor being in the form of coils wound about the uncoated portions of the other conductor.

7. The combination defined in claim 6 in which said base portions are of tantalum and said coatings are of niobium. f

8. The combination defined in claim 6` in which said coated and uncoated portions are periodically disposed along said conductors.

References Cited in the le of this patent UNITED STATES PATENTS Matthies et al June 13, 1939 Buck Apr. 29, 1958 OTHER REFERENCES

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