Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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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.
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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
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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
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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
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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
77
Current density increase rate,
1=104 2105 1=104 2105
A=cm 2 s-I
Superconductivity transition
500 20 100 -50
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
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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
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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
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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
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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
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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
900 1300 600 2000 2000
temperature, C
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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.
