WO1991014287A1

WO1991014287A1 - High temperature superconducting controlled rectifier

Info

Publication number: WO1991014287A1
Application number: PCT/US1991/000683
Authority: WO
Inventors: Thomas Rendall Askew; Richard Bernard Flippen; Che-Hsiung Hsu
Original assignee: E.I. Du Pont De Nemours And Company
Priority date: 1990-03-05
Filing date: 1991-02-07
Publication date: 1991-09-19
Also published as: AU7255991A

Abstract

This invention relates to a new class of superconducting current control devices which possess voltage-current relationships similar to those of semiconducting diodes and controlled rectifiers. This is achieved by the application of weak magnetic fields to a polycrystalline high temperature superconducting element. A variety of connection schemes are possible which allow two terminal devices with assymmetric voltage-current relationships (diodes) and various three and four terminal devices with properties similar to semiconducting controlled rectifiers.

Description

TITLE HIGH TEMPERATURE SUPERCONDUCTING CONTROLLED RECTIFIER 5 FIELP OF THE INVENTION

This invention relates to a new class of superconducting current control devices which possess voltage-current relationships similar to those of semiconducting diodes and controlled rectifiers. 0 BACKGROUND OF THE INVENTION

A wide variety of superconducting devices are known which can store and manipulate information by controlled changes between several distinct, stable states. Included in this group are devices such as the cryotron 5 and Josephson junction logic elements. The phenomena exploited in these devices are generally limited to very low current flows of 1 milliampere or less. Although this is an advantage for digital circuit applications, it renders them unsuitable for power control where large 0 currents of 1 ampere or more must be controlled. Superconducting switches, which are used for power control applications, have two stable states, on and off, and change rapidly from the "on" state, i.e., passing a superconducting current, to the "off* state, 5 no superconducting current, by a process called quenching. This sudden transition occurs when the superconducting state in the switch is destroyed by the application of magnetic fields typically of 1000 Oe or more or the application of heat which warms the switch 0 above its superconducting transition temperature.

For the purposes of this invention superconducting compositions can be divided, into two broad classes according to whether the superconducting composition has a "high" transition temperature, that is above about

35. 40 K which is typical of the various compounds containing copper, oxygen and other elements, i.e., the recently discovered high temperature superconducting oxides, or a "low" transition temperature, that is below about 30 K which is typical of alloys of superconducting metals such as niobium. Another distinction is whether the superconducting material is a single.crystal or polycrystalline. The distinction between a single crystal and a polycrystalline material is important in the high temperature case because polycrystalline material exhibits a special sensitivity to very weak magnetic fields and useful electrical devices can be designed which take advantage of this effect. The same effect is not present in single crystals of high temperature superconducting material, nor in low temperature materials.

The devices which comprise this invention are like superconducting switches in that they can pass large currents, but different in that they make a continuous and controllable transition between.the fully on state and the fully off state. In many applications the devices of this invention would be used in the intermediate range between on and off where small changes in the flowing supercurrent could be effected. The devices of this invention also differ from superconducting switches in that an assymetric voltage- current relationship is realized which yields diode function or rectification. Superconducting devices are known which achieve rectification by controlled switching of arrays of individual off-on switches, see, for example, S. Kurita, Japn. Appl No. 1-21836KA). In contrast, this invention achieves rectification within a single device and in a continuous manner, much like a semiconducting rectifier. Y. Tzeng et al., Appl. Phys. Lett. 54. 949 (1989) disclose a high-temperature superconductor opening switch controlled by a magnetic field.

J. Bossaert et al., WO 88/10516, disclose an off-on switch using superconducting crystals which is controlled by applying a magnetic field parallel to the planes of the superconducting crystals.

SUMMARY OF THE INVENTION This invention provides a superconducting controlled rectifier with a continuous and controllable transition between the fully on state and the fully off state. The superconducting controlled rectifier is comprised of a polycrystalline superconducting element consisting essentially of grains of a high temperature superconducting oxide and grain boundaries of a nonsuperconducting material, means to pass the current to be controlled through the polycrystalline superconducting element, means to generate at least one magnetic field across part or all of the polycrystalline superconducting element, and means to cool the polycrystalline superconducting element below its superconducting transition temperature, wherein the grains are separated from one another by the grain boundaries and the thickness of the grain boundaries is from about 1/2 to about 100 times the coherence of length of the superconducting oxide. Preferably the thickness of the grain boundaries is from about 1/2 to about 10 times the coherence of length of the superconducting oxide. BRIEF DESCRIPTION OF THE FIGURES

Figure 1 is a illustrative drawing of the drawing of the superconducting controlled rectifier of this invention.

Figure 2 shows a typical current-voltage graph for a semiconducting diode. Figure 3 shows a variety of current-voltage graphs which descr.ibe the response of the polycrystalline superconducting element when small magnetic fields are applied. Figure 4 shows performance data for an example of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Surprisingly, the superconducting controlled rectifier of this invention has a polycrystalline superconducting element and provides a continuous and controllable transition between the fully on -state and the fully off state.

The superconducting controlled rectifier of this invention is comprised of four components, a polycrystalline superconducting element, means to pass the current to be controlled through the polycrystalline superconducting element, means to generate at least one magnetic field across part or all of the polycrystalline superconducting element, and means to cool the polycrystalline superconducting element below its superconducting transition temperature. The invention can be understood by considering the drawing shown in Figure 1 and all bold character references herein are to labels shown in Figure 1. The polycrystalline "superconducting element 1 has been processed in an appropriate way to achieve the desired sensitivity to weak magnetic fields. It may be a single object, as shown in the figure, or it may be an interconnected bundle or array of such objects, depending on ease of manufacture. Electrical connections A and B are provided to 1 in order that the electrical current to be controlled can be passed through it and be modulated by a weak magnetic field. The main field device 2 is a means for generating a weak magnetic field across part or all of the polycrystalline superconducting element 1. The main field device 2 has electrical connections C and D. The device 2 may be a solenoidal winding of electrical wire, together with supporting structures and electrical connections, or it may be a winding of electrical wire around a highly magnetic material such as iron which generates a magnetic field across the polycrystalline superconducting element. The wire used may be superconducting or nonsuperconducting. The auxiliary field device 3 is similar to 2 and also applies a magnetic field to the polycrystalline superconducting element. In some embodiments 3 may move with respect to 2 and 1 in linear or rotary fashion. Similarly 2 may move with respect to parts 1 and 3. Separate electrical connections E and F may be provided to 3 as shown, or 3 may be connected directly to 2 in series or parallel fashion. In some embodiments, 3 is omitted. The cryogenic enclosure 4 is a means to cool the polycrystalline superconducting element 1 to a temperature below the critical temperature for the existance of superconductivity in 1 and to maintain 1 at that temperature. Thus 4 may consist of an insulated enclosure and means to apply cryogenic liquids or gasses to the enclosed objects or to thermal shorts which are connected to the enclosed objects. 2 and 3 may be contained in the cryogenic enclosure, as shown in Figure 1, or one or both of them, together with supporting structures, may be outside the cryogenic enclosure.

The superconducting controlled rectifier is comprised of a polycrystalline superconducting element consisting essentially of grains of a high temperature superconducting oxide and grain boundaries of a nonsuperconducting material. The high temperature superconducting oxide is one of the copper oxdes with rare earth, bismuth and/or thallium and alkaline earth metals. The nonsuperconducting material is insulating, semiconducting, or metallic. The element may be completely solid or contain intersticies. The large majority of the grains must be structured so that the surface of the grain is not superconducting. Some grains may be entirely superconducting with little or no grain boundary materal between them. All portions of the current that flows from connection A to B in Figure 1 must flow across at least one nonsuperconducting grain boundary. Thus, any grains or clusters of grains which have superconducting surfaces must be sufficiently surrounded by grains with nonsuperconducting surfaces so that no current flow from A to B is possible except through paths which flow across grains with nonsuperconducting surfaces. Various means can be employed to make the grain surfaces nonsuperconducting. The chemical composition at the surface can be altered slightly from that in the center of the grain, the material at the surface can be in a different phase than that in the center, nonsuperconducting material can be separately introduced to coat the surfaces of the grains, or the atoms at the surface may be misalligned from those in the bulk of the grain to produce a nonsuperconducting surface. The thickness of this nonsuperconducting grain boundary should be from about 1/2 of the coherence length for the superconducting material to about 100 times the coherence length of the superconducting material, with the smaller values preferred. The coherence length for the known high temperature superconductors varies with material, direction in the crystal lattice, and temperature but is generally between 2 A (0.2 n ) and 200 A (20 nm) . The superconducting grains may contain internal structure or may be homogeneous except for the nonsuperconducting surface. Their size may range from about 100 A (10 nm) to 10⁸ A (1 cm) with the preferred range being from •about 1000 A (100 nm) to 10⁶ A (0.1 mm) . The crystal lattice of the individual grains may be oriented randomly or adjacent grains may have a similar orientation. Grains with extensive internal structure may not have an identifiable crystal lattice, and thus their direction of orientation may not be measureable.

The useful properties of the superconducting controlled rectifier of this invention are derived from the effects of weak magnetic fields on the electrical currents flowing across the thin nonsuperconducting grain boundaries between the grains of the polycrystalline superconducting element. These thin nonsuperconducting regions are often called "weak links". The term Josephson junction is usually reserved for the special case where the nonsuperconducting material is an insulator, and the superconducting material is large and flat with respect to the distance across the nonsuperconducting barrier. The effect of weak magnetic fields on Josephson junctions is well known in the prior art. Polycrystalline superconducting materials of the low temperature variety do not generally form weak links at the junctions between grains because the coherence length is much longer, generally longer than 200 A (20 nm) and the nature of their grain boundaries is different because they are metals or metal alloys. In contrast, the high temperature materials are oxides and have much shorter coherence lengths in at least one direction.

The weak links between the grains allow different amounts of supercurrent to pass depending on the magnitude of the applied magnetic field. With no applied magnetic field, all of the current passing from connection A to B in Figure 1 is a supercurrent, and there is no voltage developed between A and B. When a small magnetic field, typically 0.1-50 Oe, is applied some of the weak links in the material greatly reduce the amount of supercurrent which is allowed to pass and force the remainder to become a "normal" or resistive electrical current. As the applied field is increased, the fraction of the current flowing as a supercurrent is reduced and the fraction flowing as a resistive current is increased. Thus the polycrystalline superconducting element is actually a superconductor and normal conductor at the same time. The superconducting controlled rectifier of this invention functions by shifting current from one conduction process to the other and uses weak applied magnetic fields to modulate or effect the shifting process.

The polycrystalline superconducting element of this invention is constructed by synthesizing and/or forming the polycrystalline superconducting element so that is has the useful electrical response to weak applied magnetic fields and then constructing appropriate field devices to place around the element. The element and the one or more field devices can be connected in various ways to accomplich various electrical purposes. These various ways represent the various embodiments of the invention. The details of the synthesis and/or forming of the polycrystalline superconducting element may depend on the desired electrical performance and_. embodiment chosen to accomplish it.

Means to pass the current to be controlled through the polycrystalline superconducting element can be simply accomplished by forming contacts on the polycrystalline superconducting element and attaching wires to the contacts.

The main field device is a solenoidal winding of electrical conductor, either normal or superconducting, which generates a magnetic field when a current is passed from connection C and D of Figure 1. This generated field is applied to all or part of the polycrystalline superconductiong element either by placing the element close to or inside the solenoid or by placing the solenoid around some highly magnetic material such as iron and placing the element so that it is in the presence of the magnetic field. The number of turns of conductor in the solenoid and the geometry of the solenoid must be chosen so that the field generated is appropriate for the properties of the polycrystalline superconducting element, the particular embodiment of the device, and the desired electrical function. The performance of electrical devices is often described in terms of current-voltage graphs which show the behavior of the device under a varierty of electrical conditions, The relationship of the main field device, the polycrystalline superconducting element, and the resulting electrical performance is best described in terms of these current-voltage graphs. Figure 2 shows a typical current-voltage graph for a semiconducting diode, which is well known in the prior art. The utility of a diode results from the sharp bend in the current-voltage plot and various terms are used to describe it. Some portions of the curve can be approximated as straight lines for engineering purposes, and this range of the variables is called the quasi- linear control range. It is usually desirable to have a large quasi-linear control range. The slope of this linear approximation is called the forward or reverse conductance, depending on the direction of current flow. The utility of the diode depends on the two slopes being very different in magnitude. In general it is advantageous to have the extrapolated control point as close to the origin (V=0, 1=0) as possible.

Figure 3 shows a variety of current-voltage graphs which describe the typical response of the polycrystalline superconducting element when constant magnetic fields are applied. Curve H-0 shows the • response without any applied magnetic field. Curve H-l shows^' the response when a small, constant magnetic field of magnitude B is applied. Curve H-2 shows the response when the magnetic field magnitude is 2B. The magnitude of the magnetic field required to produce curves such as H-l and H-2 depends on the properties of the polycrystalline superconducting element but "B" is generally in the range of 1 Oe to 500 Oe with smaller values being preferred. A simple device with a current- voltage graph such as H-0, H-l, and H-2 is not very useful because there is essentially no quasi-linear control range or extrapolated control point. Furthermore, the curves are symmetric, that is they look the same for currents flowing in either direction, so there is no rectifying function. An essential part of the invention is the use of variable currents flowing through the main field device to produce variable magnetic fields which transform the current-voltage graph into a useful form.

-Curve D-l on Figure 3 is an example of an improved current-voltage graph.- This response is obtained under the following design conditions. The main field device is constructed with the proper number of turns of wire so that a current of magnitude J produces a field in the polycrystalline superconducting element of magnitude B. Connection points B and D of Figure 1 are connected electrically so that all current passing through the polycrystalline superconducting element 1 also passes through the main field device 2. When currents much smaller than J are flowing, the response will be approximately as curve H-0. When a current is flowing that produces a field of approximate magnitude B, the response will be approximately as curve H-l. When a current is flowing that produces a field of approximate magnitude 2B, the response will be approximately as curve H-2. The resulting curve is D-l which has a significant quasi-linear control range and an extrapolated control point close to the value (I^J,

V»0) . The resulting two terminal electrical device is a bidirectional current limiter. For currents less than value J it generates a negligible voltage drop, and for values greater than J a significant drop develops. It has an electrical function similar to the semiconductor configuration known as "back-to-back zener diodes". Note that the current-voltage graph is symmetric and there is no rectifying function.

The purpose of the auxiliary field device is to generate a magnetic field with constant magnitude.^' When this magnetic field is superimposed upon the variable magnetic field created by the main field device, rectification will occur. Curve D-2 in Figure 3 is an example of a current-voltage graph with rectification. This is achieved by passing a DC current from terminal E to terminal F of the auxiliary field device of sufficient magnitude that a magnetic field of amplitude 2B is created. Such a field could also be created by a permanent magnet of appropriate size. The device discussed in the above two paragraphs is now inserted into the constant magnetic field. When the variable current flowing between terminal A and terminal C is in the positive direction, i.e., the first quadrant of Figure 3, then the field generated by the variable current in the main field device and the field generated by the constant current in the auxiliary field device will be in opposition and the net field will be smaller than the individual fields. For variable current in the negative direction, i.e., the third quadrant of Figure 3, the two fields will add and the net field will be larger than either of the individual fields. This fundamental assymmetry'results in a net current-voltage graph such as D-2 and rectification. With appropriate design of the polycrystalline superconducting element and the two field devices, extrapolated control points near I«2J and near the origin (I»0, V«0) can be achieved along with rectification.

The same principles can be used to construct 3 and 4 terminal electrical devices, as described below. Means to cool the polycrystalline superconducting element below'its superconducting transition temperature i.e., cryogenic enclosures such as dewars, are generally well known and described in the prior art. EMBODIMENTS OF THE INVENTION The invention can be operated in various modes, depending on the interconnections of the six electrical connections shown in Figure 1 as described below and depending on whether DC or AC electrical signals are applied to the various connections. The term superconducting controlled rectifier has been used herein to include devices that achieve rectification in a continuous manner, much like a semiconducting rectifier, as well as the closely related devices such as current modulators or bidirectional current limiters which function much like the semiconductor configuration known as "back-to-back zener diodes". The specific function of each embodiment is indicated above in the Detailed Description of the Invention and in the discussion of various embodiments below. The invention is not limited to the specific embodiments discussed. ^• EMBODIMENT 1: Bidirectional Current Limiter The electrical connections for inputs for the current to be controlled are A and C. Terminals B and D are connected electrically. Terminals E and F are not used. This embodiment was. discussed in the Detailed Description of the Invention. The resulting current- voltage graph is illustrated in Figure 3 by curve D-l. EMBODIMENT 2: Superconducting Controlled Rectifier The electrical connections for inputs for rectification and current control of the current to be controlled are A and C. Terminals B and D are connected electrically. A DC current is applied to terminals E and F to generate a constant magnetic field. This embodiment was discussed in the Detailed Description of the Invention above. A typical current-voltage graph is given in Figure 3, curve D-2. The position of the extrapolated control points is adjustable by varying the current flowing between E and F.

EMBODIMENT 3; Three Terminal Superconducting Controlled Rectifier

The electrical connections for inputs for rectification and current control of the current to be controlled are A and C. Terminals B and D are connected electrically. Terminals E and F are not used. A DC current is applied to terminals C and D to create a componant of the applied magnetic field which is constant. The current flowing from A to C is added to the above-mentioned DC current in the main field device to produce a resultant current which produces the applied magnetic field. The resulting device performance is the same as Embodiment 2, but achieved through a different means. In Embodiment 2, two separately created magnetic fields are added, whereas in Embodiment 3 two separate currents, one constant and the other variable, are added to produce one magnetic field. EMBODIMENT 4: Bidirectional Superconducting Current Modulator This embodiment differs from those above in that the current to be controlled does not pass through the main field device, and passes only from connection A to B. A separate current is passed from C to D for the purpose of controlling the device. This separate current selects which of the various current-voltage graphs H-0, H-l, H-2 in Figure 3 will be followed by the device, depending oh how much magnetic field is generated. By changing the applied field, the device properties will sweep up and down the family of curves labelled "H" much like a semiconductor transistor sweeps up and down its family of characteristic curves. Depending on the design of the main field device, a variety of control functions can be achieved, as well as voltage or current gain.

EMBODIMENT 5: Electro-Mechanical Superconducting

Modulator It is possible for the various parts of Figure 1 to exhibit linear motion with respect to each other. The polycrystalline superconducting element must remain within the cryogenic enclosure. The magnetic field due to current flowing in a solenoid decreases as the the measurement point is moved away from the solenoid. Thus motion of the solenoid or motion of a magnetic shield within the solenoid will change the applied magnetic field at the superconducting polycrystalline element and cause modified device behavior. Linear mechanical motion of the various parts can be used for tuning of the device or for electrical control of an external device which is mechanically coupled to the device of this invention. Any of embodiments 1-4 can be used with linear motion applications. EMBODIMENT 6: Rotary Superconducting Modulator

It is possible for the various parts of Figure 1 to exhibit rotary motion with respect to each other. The principles discussed in Embodiment 5 are also applicable to this Embodiment. One or more polycrystalline superconducting elements can be mounted on the rotating portion of an electric motor or generator, usually called the armature or rotor, and function in the modes described in Embodiments 1 to . The main field device may be the field winding, somet-imes called the stator, of the motor or generator, or it may be a separate magnetic solenoid system added to the motor or generator. Similarly, the auxiliary field device may be the field winding of the motor or generator, or it may be a separate winding added for the purpose of acting on the polycrystalline superconducting element.

One or more polycrystalline superconducting elements can be mounted on the stationary portion of a motor or generator, usually called the field winding or stator, and function in the modes described in embodiments 1 to 4. The main field device or the auxiliary field device or both may be the ordinary armature windings of a motor or generator, or they may be special rotating windings added for the purpose of control. Since the device of this invention can achieve rectification, as well as switching, it can replace the mechanical device, commonly called a commutator, which periodically reverses the direction of current flow in the rotor of some types of motors and generators. EXAMPLES OF THE INVENTION

Fabrication of the Pol crvstalline Superconducting

Element The polycrystalline superconducting element using a YBa2Cu3θ7 superconductor was fabricated as follows. (a) Copper acetate monohydrate (230.0 g) was dissolved in 1.6 liter of purified water at about 75°C. Yttrium acetate hydrate (128.6 g) was dissolved in 0.6 liter of purified water at about 75°C. Barium hydroxide octohydrate (242.2 g) was dissolved in 0.8 liter of purified water at about 75°C. The copper acetate and yttrium acetate solutions were mixed together and then the barium hydroxide solution was added over a period of 5 minutes. The resulting brownish mixture was maintained at 75°C for one hour and then spray-dried using a Buchi 190 mini-spray dryer with a 0.7 mm nozzle to produce a dry powder.

(b) The dry powder produced in (a) was calcined in room air at 500^βC using the following firing schedule where the times given are the total time of firing. The material was fired so that it reached 100°C at.the end of 1 hour, 230°C at the end of 1 and 1/2 hours, 270°C at the end of 2 and 1/2 hours, 320^βC at the end of 2 and 3/4 hours, 400°C at the end of 3 and 3/4 hours, 480^βC at the end of 4 and 3/4 hours and 500°C at the end of 5 hours. The sample was then held at 500°C for 1 hour, and cooled back to room temperature over a period of several hours. The resulting calcined material is a blend of stable BaC03, CuO and amorphous yttrium compounds. (c) A mixture was formed from 12.0 g of

"Elvanol" HV (polyvinylalcohol, molecular weight about 115,000) and 69..3 g of purified water. This solution was then heated for 2 hours at 85°C. The material from part (b) was stirred into this solution in several steps until 117 g of material had been added. The dispersion was then transferred to a twin-cell mixer as described in U.S. 3,767,756. The mixer was operated at 80°C for 75 minutes. One of the cells was then connected to a 20 mil spinneret, with a 20 mesh screen used as a filter. Filaments were extruded, cut into lengths of about 8 cm and collected on a sheet for drying and firing.

.(d) The etruded material from (c) was placed in an alumina boat which was loaded in a quartz lined Fisher #497 furnace. Oxygen flowed through the furnace tube during the entire firing cycle with a flow rate of 1.0 SCFH (standard cubic feet per hour) . The firing schedule was room temperature to 160°C at l°C/min, held 1 hour at 160^βC for drying, 160°C to 850°C at 2°C/min., held 30 minute at 850°C, 850°C to 935°C at 2^βC/min, held 1 hour at 935°C, cooled to 600^βC at 5°C/min, held 1 hour at 600°C and then cooled to 35^βC at 5^βC/min. After firing, the average diameter of the extruded pieces was about 0.3 mm. As the samples were cooled below room temperature, magnetic evidence for the superconducting state appeared at 94 K. A supercurrrent of 0.75 amperes was observed to flow through the extruded structure at 77.3 K.

Main Field Device The main field device was a solenoid constructed from size 38 copper wire wound at a pitch of 42 turns per mm. The solenoid had a mean diameter of 2 mm and a length of 10 mm, giving a 5:1 aspect ratio and a reasonable approximation of the "long solenoid" design. It produced a magnetic field of 500 Oe per amp of current. The polycrystalline superconducting element was inserted into the main field device, and both were placed in a standard pyrex laboratory dewar.

Auxiliary Field Device The auxiliary field device was a "split pair" of coil windings, each with 175 turns and a coil diameter of 150 mm. The coils were spaced 125 mm apart, and the dewar was placed in a equidistant position between them. The magnetic field produced at the center of the assembly, where the polycrystalline superconducting element sits, was 12.7 Oe per amp of current. The resulting arrangement is identical to that in Figure 1, except the auxiliary field device is located outside the cryogenic enclosure, i.e., outside the dewar. Performance of Example Device The arrangement use in the first part of this Example of the invention was that of Embodiment 1 - the bidirectional current limiter. The performance data for the Example device are shown in Figure 4. Curves H-0 to H-4 show the response of the polycrystalline super¬ conducting element to constant applied magnetic fields. This data was obtained with the main field device shut off, and a constant current passing through the auxiliary field device to supply the appropriate magnetic field. The resulting current-voltage graphs are s.imilar to the corresponding curves in Figure 3. The 25 Oe graph (curve H-2 in Figure 4) shows a conductance of about 135 amperes/volt. Curve D-l of Figure 4 shows the graph obtained when the main field device is connected in series with the polycrystalline superconducting element and the auxiliary field device is disconnected. The conductance has improved by a factor of 4 to 33 Amps/volt and the increased quasi- linear control range can be observed. Curve D-2 was obtained under the following conditions: The main field device was adjusted so that it produced 250 Oe per amp of current and connected as in Embodiment 2 - controlled rectifier. The auxiliary field device was connected to a constant 2.1 amp current, producing a constant applied magnetic field of 27 Oe. A vertical shift of about 120 milliamperes is shown which moves the extrapolated control point from 160 milliamperess to 280 milli¬ amperes. The resulting curve is similar to curve D-2 in Figure 3. The reverse conductance is the same as that measured before, about 33 Amps/volt. The forward conductance is about 3000 amperes/volt for currents up to 0.2 amperes. The ratio of the two conductances shows significant assymmetry for the two current directions.

Claims

CLAIMS The Invention Being Claimed Is:

1. A superconducting controlled rectifier with a continuous and controllable transition between the fully on state and the fully off state, said superconducting controlled rectifier comprised of a polycrystalline superconducting element consisting essentially of grains^' of a high temperature superconducting oxide and grain boundaries of a nonsuperconducting material, means to pass the current to be controlled through said polycrystalline superconducting element, means to generate at least one magnetic field across part or all of said polycrystalline superconducting element, and means to cool said polycrystalline superconducting element below its superconducting transition temperature, wherein said grains are separated from one another by said grain boundaries and the thickness of said grain boundaries is from about 1/2 to about 100 times the coherence of length of said superconducting oxide.

2. A superconducting controlled rectifier of Claim 1 wherein the thickness of said grain boundaries is from about 1/2 to about 10 times the coherence length of said superconducting material.

3. A superconducting controlled rectifier of Claim 1 wherein said high temperature superconducting oxide is YBa2Cu3θ7.

4. A superconducting controlled rectifier of Claim 1 wherein the size of said grains is from about 100 A (10 nm) to 10⁸ A (1 cm) . 5. A superconducting controlled rectifier of Claim 4 wherein the size of said grains is from about 1000 A (100 nm) to 10⁶ A (0.1 mm) .

^' 6. A superconducting controlled rectifier of

Claim 1 wherein said means to generate a magnetic field is the passage of said current to be controlled through a solenoid, the electrical connections for inputs for the current to be controlled are one end of the polycrystalline superconducting element and one end of the wire winding of said solenoid, the other end of the polycrystalline superconducting element and the other end of said wire winding of said solenoid being connected electrically so that the current passing through said solenoid is said current to be controlled, and said superconducting controlled rectifier functions as a bidirectional current limiter.

7. A superconducting controlled rectifier of Claim 1 wherein two magnetic fields are generated, the first of which has a magnitude proportional to the current to be controlled and the second of which is a constant magnetic field, said means to generate first magnetic field is the passage of the current to be controlled through a solenoid, said two magnetic fields being either opposed to one another or adding depending on the direction of said current to be controlled, the electrical connections for inputs for the current to be controlled are one end of said polycrystalline superconducting element and one end of the wire winding of said solenoid, the other end of said polycrystalline superconducting element and the other end of said wire winding of said solenoid being connected electrically so that the current passing through said solenoid is said current to be controlled, and said superconducting controlled rectifier functions as a rectifier.

8. A superconducting controlled rectifier of Claim 7 wherein said means to generate said constant magnetic field is a DC current passing through a second solenoid.

9. A superconducting controlled rectifier of Claim 1 wherein said means to generate a magnetic field is the passage of the current to be controlled and a DC current through the same solenoid, the electrical connections for inputs for the current to be controlled are one end of the polycrystalline superconducting element and one end of the wire winding of said solenoid, the other end of the polycrystalline superconducting element and the other end of said wire winding of said solenoid being connected electrically so that the current to be controlled passes through said solenoid, the electrical connections for the input of the DC current are the two ends of the said wire winding of said solenoid, said current to be controlled and said DC current either adding or subtracting depending on the direction of said current to be controlled, and said superconducting controlled rectifier functions as a rectifier.

10. A superconducting controlled rectifier of Claim 1 wherein said means to generate a magnetic field is the passage of a controlling current through a solenoid, the electrical connections for inputs for the current to be controlled are the two ends of the polycrystalline superconducting element, the electrical connections for the input of controlling current are the two ends of the wire winding of said solenoid, and said superconducting controlled rectifier functions as a bidirectional current modulator.

11. A superconducting controlled- rectifier of

5 Claim 6 wherein said solenoid undergoes linear motion with respect to said polycrystalline superconducting element to thereby tune said superconducting controlled rectifier or control an external device which is mechanically coupled to said superconducting controlled 10 rectifier.

12. A superconducting controlled rectifier of Claim 8 wherein one or both of said solenoids undergoes linear motion with respect to said polycrystalline

15 superconducting element to thereby tune said superconducting controlled rectifier or control an external device which is mechanically coupled to said superconducting controlled rectifier.

^• 20

13. A superconducting controlled rectifier of Claim 9 wherein said solenoid undergoes linear motion with respect to said polycrystalline superconducting element to thereby tune said superconducting controlled rectifier or control an external, device which is 25 mechanically coupled to said superconducting controlled rectifier.

14. A superconducting controlled rectifier of Claim 10 wherein said solenoid undergoes linear motion

30 with respect to said polycrystalline superconducting element to thereby tune said superconducting controlled rectifier or control an external device which is mechanically coupled to said superconducting controlled rectifier.

35

15. A superconducting controlled rectifier of Claim 6 wherein one or more of said polycrystalline superconducting elements are mounted on the armature or rotor of an electric motor or generator, said solenoid being the stator of the electric motor or generator or a separate winding, and said polycrystalline superconducting elements undergo rotary motion with respect to said solenoid.

16. A superconducting controlled rectifier of Claim 8 wherein one or more of said polycrystalline superconducting elements are mounted on the armature or rotor of an electric motor or generator, one said solenoid being the stator of the electric motor or generator and the other said solenoid being a separate winding or both said solenoids being separate windings, and said polycrystalline superconducting elements undergo rotary motion with respect to said solenoid.

17. A superconducting controlled rectifier of Claim 9 wherein one or more of said polycrystalline superconducting elements are mounted on the armature or rotor of an electric motor or generator, said solenoid being the stator of the electric motor or generator or a separate winding, and said polycrystalline superconducting elements undergo rotary motion with respect to said solenoid.

18. A superconducting controlled rectifier of Claim 10 wherein wherein one or more of said polycrystalline superconducting elements are mounted on the armature or rotor of an electric motor or generator, said solenoid being the stator of the electric motor or generator or a separate winding, and said polycrystalline superconducting elements undergo rotary motion with respect to said solenoid.

19. A superconducting controlled rectifier of Claim 6 wherein one or more of said polycrystalline superconductaing elements are mounted on the stator of an electric motor or generator, said solenoid being the armature or rotor of the electric motor or generator or a separate rotating winding, and said solenoid undergoes rotary motion with respect to said polycrystalline superconducting elements.

20. A superconducting controlled rectifier of Claim 8 wherein one or more of said polycrystalline superconducting elements are mounted on the stator of an electric motor or generator, one said solenoid being the. armature or rotor of the electric motor or generator and the other said solenoid being a separate rotating winding or both solenoids being separate rotating windings, and said solenoids undergo a rotary motion with respect to said polycrystalline superconducting elements.

21. A superconducting controlled rectifier of Claim 9 wherein one or more of said polycrystalline superconducting elements are mounted on the stator of an electric motor or generator, said solenoid being the armature or rotor of the electric motor or generator or a separate rotating winding, and said solenoid undergoes rotary motion with respect to said polycrystalline superconducting elements.

22. A superconducting controlled rectifier of Claim 10 wherein one or more of said polycrystalline superconducting elements are mounted on the stator of an electric motor or generator, said solenoid being the armature or rotor of the electric motor or generator or a separate rotating winding, and said solenoid undergoes rotary motion with respect to said polycrystalline superconducting elements.