CA2601517C - Method for transiting a metal conductor into a superconducting state - Google Patents

Abstract

The invention relates to engineering, particularly to high-conducting materials and method for treatment thereof. Proposed is a method for transiting a metal conductor into a superconducting state, the method comprising plastically deforming the conductor by winding a wire into a single spiral or by twisting wires into a spiral, wherein a dislocation density in the conductor is brought up to a value of not less than 1.10 8 CM -2 by the deformation, and a further increase in the dislocation density in the conductor is carried out by heat treatment up to a value of 1-10 12- 1.10 15 cm 2. The metal conductor can be pre-twisted around its longitudinal axis, followed by winding into a single spiral.
The twisting of two wires into a spiral is carried out at an inclination angle of spiral turns to the longitudinal axis of the spiral in the range of 20 - 58°.
Each of the two wires can be pre-twisted around its longitudinal axis. It should be noted that the heat treatment is carried out by any known techniques at a temperature less than a melting temperature of the metal. Said heat treatment can be carried out simultaneously with passage of electric current through the spiral. The technical result is achievement of a dislocation density in a metal conductor which is required for transition thereof into a superconducting state and which cannot be achieved by plastic deformation.

Description

METHOD FOR TRANSITING A METAL CONDUCTOR
INTO A SUPERCONDUCTING STATE

Field of the Invention The invention relates to engineering and concerns high-conducting materials, in particular superconductors. Superconductors are substances whose electrical resistance drops down to zero when cooling down to below a certain critical temperature T
, i.e., the superconductivity is observed. A major portion of metals (lead, tantalum, tin, aluminum, zinc, tungsten, niobium) are superconductors. The transition into a superconducting state has been detected in several hundreds of metal alloys and compounds and in some highly-doped semiconductors (for example, the NT-50 alloy (niobium, tantalum, zirconium), V3Ga, PbMo6Sg, YBa2Cu3O7). The invention provides a method for transiting metal conductors to a superconductivity.

Background of the Invention Known is a number of methods for treating metals and alloys in order to increase the conductivity of metal conductors by combination of deformation and heat treatment.
Known is a method for treating an aluminum alloy (Japanese Application 61-44939, IPC C22F 1/04, publ. in 1986). The method consists in that a workpiece of the alloy is rolled at 450 - 350 C at a reduction coefficient of 10 - 40%, is heated up to 580 - 450 C, is rolled at a 40% reduction coefficient in progress of the heating, is further rolled at 350 - 100 C at a 60% reduction coefficient while cooling at a cooling rate of 100 C/s, and is subjected to a 70% cold drawing, thereby making it possible to increase the conductivity by 1 - 10% using a layered multi-stage treatment of the metal.
Known is a method for increasing a critical superconductivity temperature of a material (RU Patent 2,127,461, IPC6 HO1B 12/00, publ. in the Bulletin of Inventions, 7-99).
The method consists in deformation and annealing of the material. An annealing is preliminary performed till complete removal of plastic internal stresses, said deformation is carried out by compression above the yield strength of the material, thereby creating uniform internal tension stresses therein, and said annealing is performed till complete removal of plastic stresses.

Known is a method for generating the abnormal conductivity at elevated temperatures (RU Patent 1,826,744, IPC5 GO1R 19/30, C22F 1/00). The method consists in that deformed conductors are fixed on insulated hangers within a vacuum chamber, air is evacuated from the chamber, and the chamber is filled with an inert gas up to atmospheric pressure. Then constant current is supplied to current leads at a current density increase rate of not less than 100 A=cm Z=c' till stoppage of voltage growth at the conductor. The voltage growth stoppage is a beginning of transition of the conductor into the abnormal conductivity state.
Known is a method for generating the abnormal conductivity at elevated temperatures (RU Patent 2,061,084, IPC6 C22F 1/00, publ. in the Bulletin of Inventions, 15-96). The method consists in performing plastic deformation of a conductor by twisting two wires into a spiral at an inclination angle of turns of the spiral to its longitudinal axis in the range of 20 - 58 , and subsequent passing an electrical current.
Disclosure of the Invention An object being the basis of the invention is to achieve a high density of dislocations and their regular distribution in a metal conductor, which are necessary for the conductor to transit into a superconducting state at low temperatures but not achieved at plastic deformation by usual techniques such as work hardening, rolling, stretching, bending.
The object posed is accomplished by the provision of a method for transiting a metal conductor into a superconducting state, the method comprising plastically deforming the conductor by winding a wire into a single spiral or by twisting two wires into a spiral, followed by passing electrical current through the conductor, wherein a dislocation density in the conductor is brought up to a value of not less than 1= 10x cm 2 by the deformation, and a heat treatment of the conductor is carried out to further increase the dislocation density up to a value of 1.1012 - 1=1015 cm-2.

The object posed is accomplished also by that the metal conductor is pre-twisted around a longitudinal axis thereof, and the wire is then wound into a single spiral.

The object posed is accomplished also by that the plastic deformation of the conductor is carried out by twisting two wires into a spiral at an inclination angle of spiral turns to the longitudinal axis of the spiral in the range of 20 - 58 .

The object posed is accomplished also by that each of the two metal conductors is pre-twisted around its longitudinal axis, and then the two conductors are twisted into a spiral.
The object posed is accomplished also by that the heat treatment of the deformed conductors is carried out at a temperature below a melting temperature of the metal by any known and available techniques.
The object posed is accomplished also by that the heat treatment is carried out simultaneously with the passage of electric current.

The distinction of the claimed method from the prior art methods consists in a method of deforming by twisting and winding the conductors into a spiral. Due to this deformation method, a dislocation density in the conductor of up to 1.1015 cm2 and a uniform distribution of the dislocations throughout a length of the conductor are achieved.
Let us explain the technical essence of the invention.
To make a metal conductor able to transit into the superconducting state, it is necessary first to create a dislocation density of not less than 1= 108 em 2 in the conductor. It should be noted that such a dislocation density in the conductor is a critical density at which the conductor can transit into superconductivity. When the dislocation density is less than said value, the transition of the conductor into superconductivity is impossible. Such a dislocation density can be achieved by different deformation techniques: work hardening, bending, rolling, reducing, and stretching. Apart from the condition of formation of a high density of dislocations, their regular distribution along a length of the conductor is necessary, which is difficult to provide for by the metal deformation techniques listed above.

However, it is possible to achieve the density of dislocations and their regular distribution over the conductor length by winding a wire into a single spiral or by twisting two wires into a spiral. In case of winding a wire into a single spiral, it is necessary to aim at the achievement of an inner diameter of the single spiral, which is close to zero, while an outer diameter will be close to the double wire diameter in this case, and then achievement of a maximum packing density and a high degree of the dislocation density is possible.

The dislocation density obtained by the plastic deformation was calculated by a formula: n= k/b(r + h)-cos(3 (1) where n is a dislocation density, b is a Burgers vector, r is an average radius of a single spiral, h is a diameter of the conductor, (3 is an angle between a middle circumference line of the single spiral and a dislocation sliding plane, k is a ratio of the conductor diameter to an inner diameter of the single spiral. The testing for the dislocation density was carried out by an electronic microscope, and then a transition temperature of the conductor into the superconducting state was determined.
A determination of a critical temperature for transition into the superconducting state and the conditions for this transition were carried out by a formula:

t _ tõ~It (2) cr lg n X ir ncr icr where tcr is a critical temperature for transition into the superconducting state, tmeit is a melting temperature of the conductor, n r is a critical dislocation density for transition into the superconducting state, n is a dislocation density in the conductor, j r is a critical current density for transition into the superconducting state, jr - is a current density increase rate in the conductor. The formula (2), together with the above-mentioned formula (1), allows calculation of all parameters necessary for the conductor to transit into the superconducting state. For example, a transition temperature of 1,250 C is required. We calculate a required dislocation density for a tungsten wire by the formula (1) and determine that said density should be 5.7=1010 cm Z, then calculate a diameter of a cylindrical single spiral for a wire of two diameters, 0.0025 and 0.0050 cm, and determine that the single-spiral diameter should be 0.0081 and 0.0130 cm, respectively.
By plastically deforming, it is possible to achieve the dislocation density not higher than 1=1014 cm-2. With this dislocation density, a temperature for transition of the conductor into the superconducting state is sufficiently high. It should be noted that there is no transition of the conductor into the superconducting state at the dislocation density of less then 1.108 cm 2. If the dislocation density in the conductor is less then 1.101 2 cm2, then a temperature for transition into the superconductivity will be sufficiently high, e.g., said temperature for tungsten will be 1,750 C. Fig. 1 illustrates, in the tree-dimensional space, a transition temperature into the superconducting state versus a dislocation density and a current density increase rate. At the dislocation density of 1=108 cm-2 and the current density increase rate of 2=105 A cm-2=s-1 , a transition temperature into the superconducting state for tungsten is 3,410 C.
A
dislocation density value of 5=1015 cm-2 is an experimentally obtained result achievable in heat treatment of a metal conductor. With this dislocation density, the transition of a conductor into the superconducting state takes place at room temperature. A
dislocation 5 density value of 1= 1015 cm 2 is theoretically impossible; however, thanks to thermal treatment, we were successful to establish a transition into the superconducting state at - 20 Cin individual experiments at a current density increase rate of 1- 104 A=cm 2=s-1.
The stably obtained results correspond to parameters in the three-dimension diagram (Fig. 1) and are shown in Table 1.

10 Table 1 Sample number 1 2 3 4 Dislocation density, cm 1 10 1 10 1 10 1= 10 Current density increase rate, 1=104 2105 1=104 2105 A=cm 2 s-I

Superconductivity transition temperature, C

Thus, it has been found out that the provision of transition into the superconductivity is possible at -50 C when the dislocation density approaches 1= 1015 cm-2 and the current density increase rate is extreme. Increase in the dislocation density from 1= 108 cm 2 to 1= 10 15 cm-2 was succeeded to obtain by heat treatment of a conductor pre-deformed mechanically. Samples are specially deformed by twisting into a single spiral with an internal bending radius approaching zero. In doing so, geometrical redistribution of atoms occurs at inner (small curvature) and outer (large curvature) surfaces, wherein walls of dislocations are formed. When annealing a metal conductor twisted into a single spiral having a large radius of conductor curvature with the current density increase rate of more than 100 A/cm'=s, there is movement of electrons between the walls of dislocations like de Broil waves through waveguides. Similar phenomenon takes place, for example, with electromagnetic waves in microwave engineering.
When heating a conductor deformed by twisting up to above 3,000 C, single-crystal blocks are formed therein, said blocks being arranged along the conductor and having a length of from 0.5 to 2 times the diameter of the high dislocation density wire, which can be observed by microscope. In doing so, the wire becomes non-circular and represents a pack of the blocks arranged along the wire. These blocks are of different shapes depending upon a curvature degree and because of the high dislocation density.
If the spiral is heated by passing electrical current in an inert environment to achieve a heating temperature of 2,500 C, that is, a temperature below a melting temperature of the metal, then, the spiral transits into the superconducting state while the spiral temperature reduces down to room temperature, and the spiral is in the superconducting state as long as electric current passes through said spiral. To shorten a time for achieving transition of the conductor into the superconducting state at room temperature, it is necessary to use an external source for heating up to 3,000 C. A stability of the transition of the conductor into the superconducting state was experimentally tested at thousands of samples as a function of a heating temperature, a current density, and a dislocation density. To develop thermal annealing techniques for increasing the dislocation density due to the polygonization process and for monitoring the dislocation density, a transition temperature to the superconducting state was measured according to a volt-ampere characteristic, wherein the transition temperature for each particular sample remained at multiple (up to 10,000 cycles) transitions of the conductor into the superconducting state and back at an accuracy of 0.01%. It was found that the electrical heating of the spiral is unpractical because a lot of time is required to reduce the temperature for transition of the conductor into the superconducting state down to room temperature. For example, if a single spiral having parameters corresponding to the transition temperature of 3,000 C is used, many thousands of hours are then necessary to reduce the transition temperature down to 1,000-2,000 C, and the process becomes unpractical. In order to obtain a high degree of the dislocation density in heat treatment, it is necessary to achieve a temperature close to a melting temperature to reduce the process time.
Heat treatment techniques may be various: heating with electrical current, ion current, electron current, an electron gun, plasma discharges, a plasmatron, a laser beam, etc.; the only matter is that the heating should provide the achievement of a dislocation density necessary for the conductor to transit into the superconducting state.
In case of twisting two conductors into a spiral, it is also necessary to aim at that an inner diameter of the spiral be close to zero, then, an outer diameter will be equal to the double wire diameter as a result of which reduction in a total cross-section of the conductor as compared to a unitary single-spiral conductor takes place, i.e., a ratio of a total cross section diameter of the conductor to the cross-section of two conductors is twice reduced, and an angle of the twisting direction of the conductor relative to the longitudinal axis approaches 58 . As a result, a maximum packing density is achieved, i.e., the greater is said angle the greater is the dislocation density. This provides for a low transition temperature of the conductor into the superconducting state.
Values of angles are determined by the geometry of twisting and have been tested experimentally.

When passing electrical current through the conductor simultaneously with heat treatment, the electric current serves as a source of additional heating and is also used to monitor the state of the dislocation density according to a volt-ampere characteristic.
Other metals, such as copper, nickel, aluminum, tantalum, molybdenum, were tested as well.
Brief Description of Drawings The invention will be further explained by description of particular embodiments thereof and by accompanied drawings, wherein:
Fig. 1 illustrates, in the tree-dimensional space, a transition temperature into the superconductivity versus a dislocation density and a current density increase rate for a tungsten conductor;
Fig. 2 illustrates a volt-ampere characteristic of the heat treatment process for the tungsten conductor; and Fig. 3 illustrates a heat-treatment duration versus an applied voltage value.
A
voltage corresponding to the tungsten melting temperature is taken to be 100%
of the applied voltage.
Method Embodiments Example 1. Tungsten conductors of a 0.0025 cm diameter and having the i~ i~
dislocation densities of 1- 10 s, 2= 10io, 1.5 = 10~i, 1= 10 , 5= 10 after plastic deformation, and one reference tungsten conductor not subjected to plastic deformation, were placed into a chamber and connected to the current leads. A volt-ampere characteristic of the conductors was recorded by a two-coordinate self-recorder. A time-increasing current was supplied to the conductors, but said current was in a stabilization mode, that is, a current value remains unchanged irrespectively of a load at any time moment;
if the current supply was stopped, then the potential difference at the load was dependent only upon a conductor resistance. A variation of current in time was set by a current stabilization regulator. The current density increase rate was set to be equal to 2= 105 A=cm 2=s I, and the current density in the conductor was increased at such a rate till termination of the voltage dropping as monitored according to the volt-ampere characteristic plot, which termination is an evidence of the transition of the conductor into the superconductivity. Data is summarized in Table 2.

Table 2 Sample number 1 2 3 4 5 Reference Dislocation density, cm 1= 10 2 10 1.5 = 10 1= 10 5= 10 1.105 Superconductivity No 54 28 20 13.5 8 transition voltage, V transition It follows from Table 2 that the less is the dislocation density the greater is the superconductivity transition voltage. It is impossible to transit to the superconducting state with said current density increase rate at the dislocation density of less than 1- 10g cm 2, because the transition voltage will be yet more, but the conductor achieves the melting point at the voltage of 56 V. All conductors have transited into the superconducting state after plastic deformation, whereas the reference conductor has melted.

Example 2. A number of tungsten conductors having the dislocation density of 1.4= 101] cm"2 after plastic deformation were subjected to a test for transition to the superconductivity by setting a different current density increase rate for each conductor.
This experiment allows to clarify a dependency of the superconductivity transition temperature on the current density increase rate. Experiments were carried out with conductors having a diameter of 0.0025, 0.0035, 0.0052 cm. Results are summarized in Table 3.

Table 3 Sample number 1 2 3 4 5 6 Current density increase , q q a a 5 i 110' 0.5=10 2.5=10 5=10 7.5=10 1=10 rate, A=cm '=s Critical superconductivity 3300 3180 2470 1950 1600 1300 transition temperature, C

As a result of testing the reference conductors not wound into a spiral, it is found out that they do not transit into the superconductivity and are melted irrespectively of the current density increase rate. All the conductors of the above-mentioned diameters give the same readings in respect with a critical superconductivity transition temperature at the same current density increase rates.
Example 3. To verify the ability of large-diameter conductors to transit into the superconductivity, a single-spiral conductor having a diameter of 0.0825 cm was made of a tungsten conductor having a diameter of 0.0025 cm. A calculated dislocation density in the thus-obtained conductor was 2.81 = 108 cm 2. The experiment was carried out under conditions of Example 1. A current density increase rate was 2= 105 A=cm"2=s-1 while a superconductivity transition temperature was 3,100 C.

Example 4. A tungsten conductor was made of a tungsten wire of 0.005 cm in diameter by winding into a single spiral. The conductor was placed into a chamber with an inert gas and was connected to the current leads. A volt-ampere characteristic was recorded by a two-coordinate self-recorder. A time-increasing current was supplied to the conductor, but it was in a current stabilization mode, that is, any set current value remains unchanged irrespectively of a load at any time moment. A variation of current in time was set by a current stabilization regulator. Increase in current was carried out till stoppage of the voltage growth in the conductor. The voltage growth stoppage in the conductor is a beginning of transition into the superconducting state, and a further small increase in the current density results in reduction of the voltage down to 0, therefore, the current increase was terminated at a moment of the voltage growth stoppage, and the voltage was reduced by 5% in respect with a voltage fixed at the moment when the conductor transited into the superconducting state.
This operation was repeated many times, wherein the voltage was each time reduced by 5% through decrease in the current density. The plot in Fig. 2 in the form of the volt-ampere characteristic explains the stepped dynamic process for the heat treatment temperature and the treatment duration in order to reduce a transition temperature of the conductor into the superconductivity.

Example 5. A time dependency of the transition into the superconducting state upon a working voltage for heating a spiral of a tungsten conductor was studied. Fig. 3 illustrates a plot for dependency of the transition into the superconducting state upon a dislocation density and a current density increase rate, where a voltage variation in relation to a design voltage in % is laid off as abscissa, while the design voltage corresponding to the beginning of tungsten melting is taken to be 100%, and a heat-treatment duration in hours is laid off as ordinate.
5 It is possible to conclude based on the plot that heat treatment allows increase in the dislocation density up to 1.1013 cm 2 which results in reduction of the temperature for transition into the superconducting state.
Example 6. A spiral having a dislocation density of 1.1013 cm 2 was made of a tungsten conductor having a diameter of 0.0052 cm, and was placed into a vacuum 10 chamber to which a tungsten cathode was placed in parallel to said conductor to generate electron current, wherein the tungsten spiral served as an anode.
When voltage is applied, a bombardment of the spiral with the electron current occurs, which allows heating of the conductor in the spiral form up to any temperature depending upon an applied voltage. For example, if the conductor should be heated up to 3,000 C, then, a voltage between the cathode and the anode should be 2 kV. A tungsten spiral temperature was measured by a pyrometer. The electrical heating was turned off each 10 min, and a transition temperature of the conductor into the superconducting state was checked according to a volt-ampere characteristic. Such a heating makes it possible to obtain the dislocation density equal to 1=1015 cm 2 in the spiral conductor.

Example 7. Experiment conditions were similar to Example 1. Deformed conductors made of copper, tungsten carbide, nickel, aluminum, tantalum, molybdenum and graphite having different dislocation densities were heated while measuring a transition temperature of a conductor into the superconducting state at different current density increase rates. Results are summarized in Table 4.
Table 4 Material Cu Ni Al Ta Mo Dislocation density, cm - 1= 10 9 10 1 10 3 10 10 Current density increase 1=105 2=105 1=105 6=104 1=105 rate, A=cm-'=s ~

Superconductivity transition temperature, C

Thus, owing to the deformation of metal conductors by winding into a spiral, it is possible to generate a high density of dislocations in the metal conductor, which allows transition of the conductor into the superconducting state at low temperatures close to room temperature.

Industrial applicability Upon discovery of the superconductivity phenomenon in 1911, superconductors have found a wide application in engineering, particularly, in cables, lines and devices of different types, such as radiation receivers, magnetometers, magnets. Until now, the use of superconductors is limited by difficulties of their production due to super-low temperatures (-269,5 C) at which the metal superconductors transit into the superconductivity. The inventor offers a method for transiting metal conductors into the superconductivity at high temperatures, particularly, at temperatures close to rooin temperature and allowing use of the method under industrial conditions. This significantly simplifies the process of transiting conductors into the superconductivity and opens principally new perspectives to use these materials, for example, superconducting excitation coils for electrical machines and MHD generators, magnetic-cushion trains, energy accumulators, magnetic separators for beneficiation of weak-magnetic ores.

Claims (6)

1. A method for transiting a metal conductor into a superconducting state, the method comprising plastically deforming the conductor by winding a wire into a single spiral or by twisting two wires into a spiral, followed by passing electrical current through the conductor, characterized in that a dislocation density in the conductor is brought up to a value of not less than 1-10 8 cm-2 by the deformation, and a further increase in the dislocation density in the conductor is carried out by heat treatment up to a value of 1-10 12 - 1.10 15 cm -2.

2. The method according to claim 1, characterized in that the metal conductor is pre-twisted around its longitudinal axis and then is wound into a single spiral.

3. The method according to claim 1, characterized in that the plastic deformation of the conductor is carried out by twisting two wires into a spiral at an inclination angle of spiral turns to the longitudinal axis of the spiral in the range of 20 -58°.

4. The method according to any one of claims 1 - 3, characterized in that each of the two metal conductors is pre-twisted around a longitudinal axis thereof, and then the two conductors are twisted into a spiral.

5. The method according to any one of claims 1 - 4, characterized in that the heat treatment is carried out at a temperature less than a melting temperature of the metal by any known techniques.

6. The method according to any one of claims 1 - 5, characterized in that the heat treatment is carried out simultaneously with the passage of electric current.