JP2003207526A - Method and device for measuring critical current density of semiconductor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a measuring method and device for nondestructively evaluating a local value of critical current density and its distribution in no contact in a large-area and large-size superconductor such as large-area membrane, lengthy wire rod, or bulk material, particularly, with a large thickness. <P>SOLUTION: In this method for measuring critical current density of a superconductor, the superconductor is divided to blocks of a prescribed shape, a coil is arranged in one block, AC current is carried to the coil, the current carried to the coil and a third harmonic induced voltage induced in the coil are detected, and the critical current density is determined from a prescribed arithmetic expression showing the relation between the current and the voltage under a prescribed condition. The position of the coil is successively moved to other blocks, and the above operations are repeated to determine the distribution of critical current density every block. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention TECHNICAL FIELD OF THE INVENTION

The present invention provides local values of critical current densities and distributions of large-area and large-sized superconductors such as large-area superconducting films, long superconducting tape wire materials, and large-sized superconducting bulk materials.
The present invention relates to a measuring method and a measuring device for nondestructive and noncontact evaluation.

2. Description of the Related Art Large-area and large-scale superconductors such as large-area superconducting films, long superconducting tape wire materials, and large-sized superconducting bulk materials are used in various electric power devices such as fault current limiters, power transmission cables and bearings Is expected to be applied. The critical current density in superconductors is an important parameter that determines the performance when applied to these power devices. In order to develop and fabricate a large area, large size, and high-quality superconductor in which the high critical current density is uniformly distributed, the local critical current density and its distribution are non-destructive, non-contact, and simple. It was desired to develop a method for evaluation.

[002]

(1) One of the most commonly used methods for evaluating the critical current density of a superconductor is to measure the current-voltage characteristics by attaching electrodes to the superconductor. It is a four-terminal method. In order to use the four-terminal method, it is necessary to process the superconductor, and the deterioration of superconducting properties at that time becomes a problem. In addition, this method can evaluate only the critical current density of a relatively small superconductor. One of the most commonly used methods for non-destructively and non-contact evaluation of the critical current density is a method of measuring direct current magnetization. It can only evaluate the average critical current density over a relatively small superconductor. (2) As a method of evaluating the distribution of the local critical current density of the superconducting film, a method of measuring an alternating current flowing in the coil and a third harmonic induction voltage generated in the coil has already been proposed. That is, the alternating current value I when the third harmonic induced voltage starts to increase largely as the alternating current increases from zero.
_This is a method of evaluating the critical current density from _th . (Reference [1]: JH Classen, ME Reev.
es, and R.S. J. Soulen, Jr. , A c
ontactless method for mea
maintenance of the critical
current density and criti
cal temperature of superc
onducing films, Rev. Sci. I
nstrum. 62, 996 (1991). )

However, this alternating current value I _th is proportional to the product J _cd of the critical current density J _c and the thickness d of the superconductor, and in a superconductor having a large d, the coil current I ₀ is I _th. In some cases, J _c cannot be measured and J _c cannot be measured. (3) As another method for evaluating the critical current density from the third harmonic induction voltage, there is a method used for a relatively small columnar or plate-shaped superconductor. (Reference [2]: CP Bean, Magnetizatio.
no of high-field supercond
octors, Rev. Mod. Phys. 36,31
See (1964). ) In this method, it is necessary to arrange the coil so as to surround the superconductor, so this method cannot be used for large-area and large-sized superconductors. Only the current density can be evaluated.
In other words, this method cannot locally evaluate the critical current density of large-area / large-scale superconductors, and examines whether or not high-quality superconductors with evenly distributed critical current density are produced. Was impossible.

In order to develop and fabricate a large-area / large-sized superconductor in which a high critical current density is uniformly distributed, which is expected to have various applications, some local values of the critical current density are required.
It is also essential to develop a method for nondestructively evaluating whether or not the critical current density is evenly distributed. The method shown in the above (2) is used for nondestructively evaluating the local critical current density and its distribution of a large area superconducting film. In this method, the thickness d Is big,
(D> I ₀ / kJ _c : where k is a proportional constant determined by the shape and arrangement of the coil.) The critical current density of a superconductor capable of shielding a magnetic field could not be evaluated.
In view of these problems, the object of the present invention is to provide a local value of the critical current density in a large-area / large-sized superconductor such as a large-area film, a long wire, or a bulk material, especially when the thickness is large. It is an object of the present invention to provide a measuring method and a measuring device that evaluate the distribution in a non-destructive and non-contact manner.

[0095]

The present invention achieves the above object.
In order to do so, the following solution is adopted. (1) In the method for measuring the critical current density of a superconductor,
Virtually divide the conductor into blocks of a certain shape,
Place the coil in the block, let the alternating current flow through the coil,
Current I flowing in this coil₀And this current I₀By carp
Third harmonic induced voltage V induced in the_ThreeAnd detect
The current I₀And the voltage V _ThreeSeki between
The critical current density is calculated from a predetermined formula that expresses the
Move the position of to another block and repeat the above operation
Then, calculate the distribution of the critical current density for each block. (2) Measurement of critical current density of the superconductor described in (1) above
In the method, (I) Small current 0 <I₀-I_c1<< I_c1in the case of: V_Three= (G₁f / J_c) I_c1 ^-B(I₀-I_c1)
^{2 + b}， (Ii) Large current I_c1<< I₀<I_thin the case of: V_Three= (G_Twof / J_c) I₀ ^Two And the predetermined condition is J_cIs the critical current density, f is the coil
Frequency of flowing alternating current, I_c1Is the third harmonic induction voltage
The threshold value I that starts to take a value other than zero, I_thIs J_cproportional to d
Current value, where d is the thickness of the superconductor, b is 0 <b <
A parameter that takes a value of 1, G_i(I = 1, 2) is a coil
It should be a coefficient determined by the geometric shape and arrangement of the.

(3) In a device for measuring a critical current density of a superconductor, an alternating current magnetic field generated by an alternating current flowing through the current detector and the current detector is generated and a magnetic flux induced by the superconductor is detected. And a means for extracting the third harmonic induced voltage from the induced voltage detected by the coil. (4) In the apparatus for measuring the critical current density of a superconductor according to (3) above, the superconductor according to the above (2) is calculated from the current and the third harmonic induction voltage under a predetermined condition. Equipped with arithmetic means for obtaining the critical current density for each divided block.

[0097]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention comprises a plurality of large superconductors.
The surface of each block that is virtually divided into blocks
A detection coil is placed nearby and an alternating current is passed through the coil.
I₀And the third harmonic induction voltage V induced in the coil_ThreeTo
Measure and calculate the critical current for each block from the given formula
Measurement method for calculating density and measurement therefor
On the device. Specifically, the alternating current I₀Is increased from zero
The current I₀There is a threshold I _c1Until you reach
Voltage V_ThreeDoes not appear. This is the alternating current I₀Caused by
AC magnetic field H₀Is the lower critical magnetic field H_c1If less than
Can magnetic flux lines (vortex) enter the superconductor?
It is. That is, 0 <I₀<I_c1When V_Three= 0
Is. On the other hand, I₀> I_c1Current I at₀And voltage V
_ThreeIs related to the current I₀Depending on the size of the following (1)
Alternatively, it is given by the equation (2). (I) Small current: 0 <I₀-I_c1<< I_c1(0 <H₀
-H_c1<< H_c1)in the case of:     V_Three= (G₁f / J_c) I_c1 ^-B(I₀-I_c1)^{2 + b}, (1) (Ii) Large current I_c1<< I₀<I_th(H_c1<< H
₀<J_cIn case of d):     V_Three= (G_Twof / J_c) I₀ ^Two                          (2) Where J_cIs the critical current density, d is the thickness of the superconductor, f
Is the frequency of the alternating current flowing in the coil, b is a value of 0 <b <1
Parameter that takes_iIs the coil geometry and placement
It is a coefficient determined by.

The critical current density J _c can be evaluated from the measured values of the detected current I ₀ and voltage V ₃ using the equation (1) or (2). The measuring operation of this measuring device is performed in a non-contact manner without processing the superconductor. Moreover, the distribution of the local critical current density can be evaluated by scanning and measuring the surface of the superconductor using a minute coil. Although it was already mentioned, in the literature [1], the current _{I 0} is greater than the threshold _{I t h} (i.e., an alternating magnetic field _{H 0} is greater than _{J c} d) if the method of measuring the critical current density It is shown. However, when the thickness d of the superconductor is large, this condition may not be satisfied, and the critical current density cannot be evaluated.

The third harmonic induced voltage is derived as follows.
It AC current I flowing in circular coils arranged at z> 0₀
Due to cos (2πft), H is given to the superconductor surface z = 0.
₀A magnetic field of cos (2πft) is generated. Where k is
As a proportional coefficient determined by the shape and arrangement of the ill, H₀=
kI₀It can be expressed as. Current amplitude I₀Is the threshold I_c1
= H_c1/ K does not exceed the lower critical magnetic field H
_c1Magnetic flux line (vortex) penetrates into the superconductor
I can't. In this case, the third harmonic induction in the coil
No voltage is generated. That is, I₀<I _c1(H₀<H
_c1) Is V_Three= 0. Below, I₀> I_c1(H₀
> H_c1) Consider the case. Critical state model (literature
According to [2]), the gradient of the magnetic field H in the superconductor
The magnitude of ∂H / ∂z is the critical current density J_cbe equivalent to. critical
Penetration length of magnetic flux line (H₀-H_c1) / J
_cIs larger than London's penetration length λ, and
Consider the case where the thickness is smaller than the conductor thickness d. Conventional criticality
∂H / ∂z = ± J, like the state model_cMagnetic field by integrating
The distribution of H (z, t) is obtained. | H (z, t) | <H
_c1In the region of, magnetic flux lines cannot exist. You
That is, the magnetic flux density B is | H | <H_c1Then B = 0
It H> H_c1In the case of, the relational expression between B and H is B = μ
₀(H^{1 / b}-H_c1 ^{1 / b})^bGiven by μ₀Is true
Air permeability, b is a parameter having a value of 0 <b <1
It (Reference [3]: EH Brandt, Geome.
tric barrier and current
string in type-II superco
nducers obtained from co
ntinuum electrodynamics,
Phys. Rev. B. 59,3369 (1999) visit
Teru. ) The relational expression between B and H is H >> H_c1Then B =
μ₀Is approximated to H, while 0 <H-H_c1<< H_c1so
Is B = μ₀H_c1[(H / H_c1-1) / b]^bAnd approximation
To be done. Thus, from the distribution of H (z, t), B (z,
The distribution of t) is obtained, and the shielding at z <0 inside the superconductor is obtained.
Shadowing current density j (z, t) = (1 / μ₀) ∂B / ∂z
A cloth is obtained. Coil generated by this j (z, t)
The induced voltage V generated in the coil from the time derivative of the interlinkage magnetic flux
(T) is required. Fourier transform this induced voltage
And the third harmonic component is V_Three‘Cos (6πft) +
V_Three"Sin (6πft)", the absolute value of the third harmonic
Value V_Three= [(V_Three’)^Two+ (V_Three")^Two]^{1 / Two}Calculation of
The result is given by the above equation (1) or (2). Na
The equations (1) and (2) are the DC bias magnetic field.
Only the alternating magnetic field generated by the coil is applied to the superconductor
I was guided as if it were hanging. On the other hand, H _c1More than ten
A large DC bias magnetic field is applied to the superconductor.
AC magnetic field H₀If cos (2πft) is superposed,
I₀(2) regardless of the size of. (Example)

Embodiments of the present invention will be described in detail below with reference to the drawings. FIG. 1 is a block diagram of the detection apparatus of the present invention. The detection device 10 has a constant voltage and a fixed frequency f.
A signal generator 11 for generating a reference AC signal (for example, 1 kHz), a current amplifier 13 for amplifying the output current of the signal generator 11, a resistor 14 for detecting the output current,
A multimeter 15 for measuring an output current, a coil 16 that also serves to generate an AC magnetic field and detect an induced voltage, a filter 18, a frequency tripler 12 that triples the frequency f, and a lock-in amplifier 19 that locks to the frequency of 3f. , A computer 20 for obtaining the critical current density from the detected current and voltage under a predetermined condition based on a predetermined arithmetic expression. The current I ₀ flowing through the coil 16 is detected by the multimeter 15 via the resistor 14. This multimeter 1
The detected current value I ₀ of 5 is compared with the reference value by the computer 20 and is used to determine the critical current density Jc. The computer 20 feeds back the deviation ΔI from the reference value to the signal generator 11 and controls the output current of the signal generator 11 to the reference value. Current I ₀ flowing in coil 16
The frequency f is detected by the frequency tripler 12 and is output as a signal of the frequency 3f tripled by the frequency tripler 12. The lock-in amplifier 19 locks the frequency to the tripled frequency 3f and outputs it to the computer 20.

As for the induced voltage detected by the coil 16, only the third harmonic induced voltage V ₃ is taken out through the filter 18 and the lock-in amplifier 19 and input to the computer 20 to obtain the critical current density J _c. used. During the detection operation, the large superconductor 17 is virtually divided into a plurality of blocks in the order of each block, and the coil 16 is arranged near the surface of the block, and the coil 16 is controlled by the signal generator 11 and the current amplifier 13. Apply current. An induction current flows through the superconductor 17 due to the magnetic flux generated by the coil 16. A secondary magnetic flux is generated by this induced current,
The secondary magnetic flux is detected by the coil 16 as an induced voltage. Of this induced voltage, only the third harmonic induced voltage is extracted. The computer 20 obtains the critical current density J _c of the corresponding block from the equation (1) or (2) based on the measured current I ₀ and the third harmonic induction voltage V ₃ under a predetermined condition. As the predetermined conditions, J _c is the critical current density, f is the frequency of the alternating current flowing in the coil, I _c1 is the threshold value at which the third harmonic induction voltage starts to take a value other than zero, and I
_th is a current value proportional to J _cd , where d is the thickness of the superconductor, b is a parameter with a value of 0 <b <1, and G _i is a coefficient determined by the geometrical shape and arrangement of the coil. To do. Thereafter, the same operation is performed to sequentially obtain the critical current densities of the other divided blocks in the large-sized superconductor 17.

FIG. 2 is a characteristic diagram of the third harmonic induction voltage V ₃ generated in the coil 16 with respect to the AC current I ₀ flowing in the coil 16. The data of this characteristic diagram is obtained for a superconducting YBa ₂ Cu ₃ O _7-d film having a diameter of 2 inches and a thickness of d = 400 nm under the conditions of liquid nitrogen temperature 77K and frequency f = 1 kHz. FIG. 4 is a characteristic diagram of the third harmonic induction voltage V ₃ generated in the coil 16 with respect to the alternating current I ₀ flowing in the coil 16. The data of this characteristic chart is 17
mm × 16 mm, thickness d = 6 mm, superconducting YBa ₂ Cu
₃ O _7-d bulk sample, liquid nitrogen temperature 77K,
It is obtained under the condition of frequency f = 1 kHz.

FIG. 3 shows a case where a superconducting thin film is fed to a coil.
AC current I₀Third harmonic induction in the coil against
Square root of voltage V_Three ^1/2FIG. 2 and 3
The same data is used for the current value and the voltage value. Figure 3
0 <I₀The region of <0.48 mA is the above formula (1).
Equation (2) is a valid area. That is, equation (2)
Of I_thIs about 0.48 mA. In Figure 3, the measurement
It's hard to see because it's buried in Iz, but this 0 <I₀<I_th=
If you analyze the data in the 0.48 mA area precisely, the principle
Specifically J_cCan be asked. 4 and 5
Since it is easy to understand, a detailed description will be given next. Figure 4 is super
AC current I flowing in the coil in the conductive bulk sample₀
In the characteristic diagram of the third harmonic induced voltage generated in the coil for
is there. Figure 5 shows the flow in the coil of a superconducting bulk sample.
Alternating current I₀The third harmonic induced in the coil against
Square root of conductive pressure V_Three ^1/2FIG. 4 and 5
The same data is used for the current value and voltage value of. Figure
From 5, the square root of the third harmonic induction voltage V_Three ^1/2Interact with
Current I₀It can be seen that the relationship with and is linear. Previous
The expression (2) is V_Three ^1/2= (G_Twof / J_c)^1/2I
₀Can be rewritten as Therefore, the characteristic song of FIG.
The linear dependence passing through the origin of the line shows that equation (2) is correct.
In addition, the slope of the straight line of the characteristic curve in FIG._Two
f / J_c)^{1 / 2}be equivalent to. Coefficient G in equation (2)_Two
If you know the geometry and arrangement of the coil,
It is possible to calculate these numerical values. But up
A simpler method is another method (example
For example, four-terminal method, DC magnetization method, etc.)
J_cAC current I in a superconductor whose₀
And the third harmonic induction voltage V_ThreeAnd the coefficient G_TwoDecide
It is to keep. Superconductor surface using the same coil
As long as you measure the same distance from
G _TwoUsing the slope (G_Twof / J_c)
^1/2And the value of frequency f, the critical current density J_cAsk for
be able to.

According to the present invention, from the measured values of the alternating current I ₀ flowing in the coil and the third harmonic induced voltage V ₃ generated in the coil, the above equation (1) or (2) is used to locally It becomes possible to evaluate various critical current densities. Thus
It is possible to correctly evaluate the critical current density of each block, which is an important parameter when applying large area and large size superconductors to electric power equipment. The distribution of the critical current density can be evaluated by scanning the coil on the superconductor and performing the measurement. Since the resolution of the distribution is determined by the size of the coil, if a minute coil is used, measurement with higher resolution becomes possible.

[Brief description of drawings]

FIG. 1 is a block configuration diagram of a detection device of the present invention.

FIG. 2 is a characteristic diagram of the third harmonic induction voltage V ₃ generated in the coil with respect to the alternating current I ₀ flowing in the coil in the superconducting thin film.

FIG. 3 is a characteristic diagram of the square root V ₃ ^1/2 of the third harmonic induction voltage generated in the coil with respect to the alternating current I ₀ flowing in the coil in the superconducting thin film.

FIG. 4 is a characteristic diagram of a third harmonic induction voltage V ₃ generated in a coil with respect to an alternating current I ₀ flowing in the coil in a superconducting bulk sample.

FIG. 5 is a characteristic diagram of a square root V ₃ ^1/2 of a third harmonic induction voltage generated in a coil with respect to an alternating current I ₀ flowing in the coil in a superconducting bulk sample.

[Explanation of symbols]

10 Detection device 11 signal generator 12 frequency tripler 13 Current amplifier 14 Resistance 15 Multimeter 16 coils 17 Large superconductor 18 filters 19 Lock-in amplifier 20 computers

Continued front page    (Declaration by the applicant) Patents related to the results of consigned research in countries etc. Application (2001 Ministry of Economy, Trade and Industry "AC superconducting electromotive force equipment platform Technical R & D, high current density superconducting film fabrication technology research development From Japan, subject to Article 30 of the Act on Special Measures for Revitalizing Industrial Vitality of) F term (reference) 2G035 AA12 AA15 AB07 AC15 AD10                       AD18 AD55                 2G053 AA00 AB14 BA16 BB11 BB17                       BC02 BC14 CA03 CA18 CB12                       DA01 DB19                 2G060 AA10 AE40 AF03 HA02 HC13

Claims (4)

[Claims] 1. A superconductor is virtually divided into blocks of a predetermined shape, a coil is arranged in one block, an alternating current is passed through the coil, a current I ₀ is passed through this coil, and this current causes the coil to pass through the coil. The induced third harmonic induced voltage V ₃ is detected, and the current I ₀ and the voltage V ₃ are detected under a predetermined condition.
The critical current density is calculated from a predetermined arithmetic expression expressing the relationship with
A method for measuring a critical current density of a superconductor, which comprises sequentially moving a position of a coil to another block and repeating the above operation to obtain a distribution of the critical current density for each block. 2. The critical current density of the superconductor according to claim 1,
In the measurement method, (I) Small current 0 <I₀-I_c1<< I_c1in the case of: V_Three= (G₁f / J_c) I_c1 ^-B(I₀-I_c1)
^{2 + b}， (Ii) Large current I_c1<< I₀<I_thin the case of: V_Three= (G_Twof / J_c) I₀ ^Two And the predetermined condition is J_cIs the critical current density, f is the coil
Frequency of flowing alternating current, I_c1Is the third harmonic induction voltage
The threshold value I that starts to take a value other than zero, I_thIs J_cproportional to d
Current value, where d is the thickness of the superconductor, b is 0 <b <
A parameter that takes a value of 1, G_i(I = 1, 2) is a coil
The feature is that it is a coefficient determined by the geometrical shape and arrangement of
A method for measuring the critical current density of superconductors. 3. A current detector, a coil for generating an AC magnetic field generated by an AC current flowing through the current detector and detecting a magnetic flux induced by a superconductor, and an induction voltage detected by the coil. An apparatus for measuring a critical current density of a superconductor, comprising: means for extracting a third harmonic induced voltage. 4. The apparatus for measuring the critical current density of a superconductor according to claim 3, wherein the superconductor is calculated from the current and the third harmonic induced voltage under a predetermined condition based on the arithmetic expression according to claim 2. An apparatus for measuring a critical current density of a superconductor, characterized by comprising arithmetic means for determining a critical current density for each divided block.