JPH0983024A - Persistent current switch employing oxide superconductor and structure thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a persistent current switch for oxide superconductor in which persistent current operation is realized by fixing a heater to a superconductor produced by adding specified vol.% of Bi-2201 crystal into a Bi-2212 crystal and then conducting or interrupting the heater. SOLUTION: A heater is fixed to a superconductor produced by adding 10-70vol.% of Bi-2201 crystal into a Bi-2212 crystal. The superconductor comprising Bi-2212 crystal is partially heat treated and decomposed to form the Bi-2201 crystal in the Bi-2212 crystal. When the superconductor is heated externally by means of a heater, the Bi-2201 makes a transition at first to normal conducting state because it has Tc as low as 30-40K or below and the current does not flow through the Bi-2201 but flows through the Bi-2212. When the current flows excessively to the Bi-2212 side, it makes a transition to normal conducting state thus turning off a persistent current switch.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an oxide superconducting magnet, and more particularly to a permanent current for an oxide superconductor which is indispensable for utilizing a magnet formed by winding a Bi-2212 material into a wire in a permanent current mode. The present invention relates to a switch and a persistent current switch structure.

[0002]

2. Description of the Related Art Most of metallic superconducting magnets operated at liquid helium temperature are NbTi alloy wire or Nb ₃ alloy wire.
Sn intermetallic compound wire has been used, and a technique for generating a magnetic field in a permanent current state has already been completed for a superconducting magnet configured by using these alone or in combination.

It is required that ordinary magnets can reduce power consumption, and in MRI and MRS (NMR),
Superconducting switches have become an indispensable technology for superconducting magnets because extremely high magnetic field stability is required.

The main technologies relating to the superconducting switch are those relating to the persistent current switch itself, Nb.
Connection of Ti or Nb ₃ Sn,
There is a superconducting connection technology between NbTi and Nb ₃ Sn, both of which have been completed.

Conventionally, a generally used persistent current switch using NbTi is heated or cooled by heating or cooling the superconducting transition temperature (hereinafter abbreviated as Tc) of NbTi as a superconductor to about 9.5K. It is possible to instantaneously switch between both ends of the persistent current switch from superconducting to normal conducting and normal conducting to superconducting, and it is important that the resistance when the persistent current switch is off is sufficiently high. The structure and material selection of a persistent current switch using this type of NbTi have been established in the prior art.

Further, in superconducting connection, a technique is required for connecting the permanent current switch portion and the magnet coil portion without loss when assembling them, and correspondingly, Pb alloy which is a low melting point metal is required. For example, a technique for connecting superconducting wires to each other has been already established with a loss that is practically negligible via the above. Thus, the reason why these technologies were established early in metal-based superconducting materials was
The liquid helium temperature of 4.2K and the operating temperature of the material are not so far apart, that is, Tc is up to 9.5K. Further, in connection technology, the coherent length of the metal-based superconductor is sufficiently long.

On the other hand, the oxide superconducting material has a Tc of liquid nitrogen temperature of 7 as is known as a high temperature superconductor.
It is not uncommon for the temperature to be higher than 7K, and when operating this at liquid helium temperature, in order to turn it on and off with, for example, a heater, it is necessary to energize for a long time, resulting in slow operation and economical efficiency. From a viewpoint, it is expected to be extremely difficult and has not been realized. Further, regarding the superconducting connection technique, the coherent length of the oxide superconductor is, for example, about 20Å (ab plane direction) in the Bi-2212 system, which is much shorter than that in the metal system. In other words, it becomes necessary to join the superconductors to each other within the coherent length. This means that it is necessary to control the unit cell level of the crystal, and in this regard, there is no example of successful superconducting connection between an oxide superconductor and a metal-based superconductor.

In view of the current situation, it is impossible to use this as a permanent current switch for an oxide superconducting magnet even if the technique of a metal-based permanent current switch that has already been completed is used. Therefore, the development of a persistent current switch for oxide superconductors is awaited.

Many reports have already been made on superconducting magnets using oxide superconductors. As an example of the Bi-2212 system related to the present invention, a report that 1.6T was generated at 4.2K (K. Shibutani et al .: IEEE Trans. Appli
ed Superconductivity 3 (1993) p.935-938) and a report that 0.7T was generated with a large diameter magnet at 20K by cooling the refrigerator (T.Hase et al.:Advances in Supercondu).
ctivity-VI (Proc. of ISS'93) p.621-624), and a report that produced a solenoid coil using a rectangular wire and generated 1.13T at 4.2K (Kazuyuki Shibuya et al .: 52nd 1994). The summary of lectures at the Low Temperature Engineering / Superconductivity Society of Japan in the fall of 2008 is p.58).

Connection between oxide superconductors, especially Bi-22
Almost the technology has been established for the connection of 12 devices (Takashi Hase et al .: 51st Spring 1994 Spring Low Temperature Engineering and Superconductivity Society Lecture Summary p.115), but there is no development example of the persistent current switch. There are no reports of operation under permanent current conditions.

[0011]

SUMMARY OF THE INVENTION The present invention has been made in consideration of the above circumstances, and it is an oxide superconducting magnet,
In particular, the present invention provides a permanent current switch and a permanent current switch structure for an oxide superconductor necessary for enabling a persistent current operation in a superconducting magnet which is formed by winding a Bi-2212 system material into a wire.

[0012]

A persistent current switch using an oxide superconductor of the present invention is a Bi-2212 crystal containing Bi.
-2201 A heater is attached to a superconductor in which the crystal volume fraction is 10% or more and 70% or less, and the normal conduction state and the superconduction state are switched by energizing or deenergizing the heater. The gist is that it is configured as. In the permanent current switch of the present invention, it is preferable that the superconductor has a tape shape or a linear shape, and the superconductor is covered with a covering material made of a silver-based alloy.

In the present invention, the superconductor is Bi-2212.
A Bi-2212 crystal is formed in a Bi-2212 crystal by decomposing a part of a crystal superconductor by heat treatment.

A permanent current switch structure using an oxide superconductor according to the present invention has the above tape-shaped or linear superconductor,
The gist of the present invention is to insert the heater wire through the hollow portion of the heater bobbin, wind the heater wire around the heater bobbin, enclose the heater wire with a resin cast material, and cover the heater wire with a heat insulating material to integrate them.

Further, when the superconductor has a tape shape or a linear shape, the Bi-2201 crystal is preferably present in a large amount in the longitudinal direction. With this configuration,
It is possible to increase the current capacity during superconducting conduction (when the permanent current switch is off).

The Bi-2212 crystal of the present invention basically means B
It shows that the molar ratio of i, Sr, Ca, Cu is approximately 2: 2: 1: 2, but not limited to this, for example, one in which an additive such as Ag is added, and the component composition is slightly Those adjusted are also included in the present invention.

[0017]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention includes Bi-2212 and Bi-22.
The superconducting magnet is formed by changing the abundance ratio of 01 and by appropriately selecting the critical current of the conductor by adjusting the cross-sectional area of the entire permanent current switch to turn on the superconducting magnet. Or, it can be turned off. That is, the present invention utilizes the temperature dependence of Jc of the conductor resulting from the difference in Tc between Bi-2201 and Bi-2212 as the operating principle.

According to the present invention, Bi-2212 crystals contain Bi.
3. A permanent current switch (with a superconducting magnet) in which the −2201 crystal is placed is placed in liquid helium.
When immersed in 2K and heated from outside with a heater,
Since the Tc of Bi-2201 is sufficiently low at 30 to 40 K or less, the superconducting state of Bi-2201 is broken to the normal conducting state, and the resistance of Bi-2201 becomes high, so that it tries to flow through the persistent current switch. The electric current to be applied flows through Bi-2212 while avoiding Bi-2201. However, Bi-22
Since the critical current density Jc of 12 is limited, if the current flows too much to the Bi-2212 side, the superconducting state of Bi-2212 is broken and the normal conducting state is obtained. As a result, the permanent current switch as a whole is in the normal conducting state, that is, it operates off.

Specifically, FIG. 12 is a schematic view of Bi-2201 existing in Bi-2212, and FIG. 13 is a sectional view of a persistent current switch in an equivalent circuit.
In FIG. 13A, in the vicinity of 4.2K, the permanent current switch as a whole is in the superconducting state, which gives the highest critical current density. In the same figure (b), Jc is an irreversible line of Bi-2201 as the temperature rises (in terms of temperature, irreversible temperature Tirr: 0.1 under a certain applied magnetic field).
When the temperature exceeds about 10 K near T, the Jc is only Bi-2212, and the Jc is rapidly reduced. In this portion, the entire persistent current switch is transferred to the normal conducting state. FIG. 6C shows a state in which the persistent current switch is off.

[0020]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described in detail below with reference to the embodiments shown in the drawings. Example 1 (Example of Persistent Current Switch Using Silver Alloy Sheath Tape ) In this example, the electrical resistivity near 4.2K has a high resistance so that shunting can be prevented when the permanent current switch is off. In order to realize this, it was decided to use an Ag-Au-Cu based alloy. Specifically, Katsumi Nomura et al .: 49th
1993 Spring Low Temperature Engineering / Superconductivity Society Lecture Summary p.5 and Kato Isomi: 50th Fall 1993 Fall Low Temperature Engineering / Superconductivity Society Lecture Summary p.31. In this example, Ag: 9
1.7%, Au: 5.0%, Cu: 3.3% (at.
%) Was adopted.

Further, in order to measure the resistivity, the cross-sectional area 1
3. Cut out a test piece of mm x 1 mm and length of 40 mm.
Apply current of 100 mA at 2K (in liquid helium),
The resistivity was measured by the 4-terminal method. As a result, it was confirmed that the electrical resistivity was 2.2 μΩ · cm at 4.2 K (without magnetic field).

Outer diameter made of the above-mentioned Ag-Au-Cu alloy
Bi in a pipe of 6 mm and inner diameter of 4 mm _2.1 , Sr_2.0 , C
a_1.0 , Cu_1.9 , Ag_0.1 I.e. Bi₂ O
_Three , SrCO_Three , CaCO_Three , CuO, Ag (powder)
Powder calcined in air as a raw material at 820 ° C for 100 hours
, Then sealed after classifying to less than 50 μm.
Rolling to a thickness of 0.1 mm using a rolling mill
Thus, a silver alloy sheath tape having a width of 10 mm was obtained.

The sheath tape thus obtained was crystallized in the heat treatment pattern shown in FIG. 1 to obtain a Bi-2212 superconducting wire. Furthermore, in order to thermally decompose a part of Bi-2212 into Bi-2201, heating temperature 6
It was kept at 00 ° C for 720 hours.

FIG. 2 shows an SEM photograph of the inner surface of the sheath tape, and FIG. 3 shows the temperature dependence of the magnetic susceptibility of the sheath tape. From the EDX analysis result and the temperature dependence of the magnetic susceptibility, it was found that the Bi-2201 phase was generated in Bi-2212 and the Tc of this portion was about 25K. Further, the ratio of Bi-2212 and Bi-2201 is estimated to be about 7: 3 in terms of volume fraction. Furthermore, from the morphological observation, it was confirmed that the Bi-2201 phase was present in a large amount in the form parallel to the Bi-2212 phase in the longitudinal direction of the sheath tape, and a parallel circuit of Bi-2212 and Bi-2201 was obtained equivalently. It is thought that it was done.

As shown in FIGS. 4 (a) to 4 (c), the sheath tape (or round wire) 1 which has been subjected to these heat treatments is
A heater lead wire 2 (about 5Ω at 4.2K) made of manganin wire was wound around an aluminum nitride insulator (heater bobbin) 3. For this aluminum nitride insulator, Au-Fe for monitoring the temperature of the permanent current switch
-Built-in chromel thermocouple 4. Further, a lead wire 5 for voltage detection was attached across the heater portion with a distance between terminals of 1 cm.

The permanent current switch thus obtained was encapsulated with epoxy resin 6 as a cast material, and its surface was covered with FRP 7. Further, the periphery of the FRP 7 was foam-molded by the Styrofoam 8 and heat-insulated.

This was immersed in 4.2K liquid helium,
The on-off characteristic as a permanent current switch was evaluated. FIG. 5 shows the switching characteristics of the (Bi-2212 + Bi-2201) / Ag tape.

For comparison, a tape conductor having only Bi-2212 phase and having a permanent current switch structure (heater, thermocouple, voltage tap) similar to the above is turned on.
The -off characteristic was also investigated. FIG. 6 shows the Bi-22121 / Ag.
It shows the switching characteristics of the tape. All the experiments were performed by immersing them in liquid helium. A current of 120 A is applied to the permanent current switch at the time of starting the measurement. The conductor Ic measured prior to the experiment was 4.
2K, 162A under self magnetic field.

In FIG. 5, 1 A (about 5 W) was applied to the heater 5 seconds after the start of measurement. The temperature change of the persistent current switch and the voltage across the switch at that time were recorded. As a result, when the temperature of the persistent current switch reaches 7.7K,
Both ends of the conductor show about 50 mV, after which the temperature rise is 1
Although the voltage gradually increased to 0.5K, the voltage across the conductor remained almost unchanged.

Next, when the heater current is set to zero 25 seconds after the start of the measurement, the voltage across the permanent current switch decreases to the value before the heater is energized as the temperature of the permanent current switch decreases, and the superconducting state is restored again. I'm back.

The rise and fall of the persistent current switch are both within 3 seconds, and the operating speed equivalent to that of the persistent current switch has been realized by using NbTi or the like which has been practically used in the conventional metal magnets. . Therefore, it was confirmed that the basic characteristics were practically sufficient.

On the other hand, in FIG. 6 showing a comparative example, Ic measured prior to the experiment was 4.2 K, which was 28 under the self magnetic field.
It was 5 A (0.1 μV / cm standard). 120A for conductor
After the start of the measurement in a state where electricity was applied to the heater, 1 A (about 5 W) was applied to the heater in 5 seconds. Bi-
Recorded as for 2212 + Bi-2201. Regarding the temperature change, the same behavior as in FIG. 5 was exhibited, but the voltage across the conductor did not change at all, and it was always in the superconducting state.

As is clear from the comparison of the two measurement results, a part of the Bi-2212 crystal was Bi-22.
It was confirmed that the silver alloy sheath tape conductor having the texture decomposed into 01 exhibited a property of sufficiently functioning as a permanent current switch of the superconducting magnet by heating with a heater, demonstrating the effectiveness of the present invention.

[Embodiment 2] (Example of permanent current switch using flat silver alloy wire) Above implementation
The same composition as in Example 1_2.1 , S
r_2.0 , Ca _1.0 , Cu_1.9 , Ag_0.1 I.e.,
Bi₂ O_Three , SrCO_Three , CaCO_Three, CuO, Ag
Using (powder) as a raw material, heating temperature 820 ° C x 100 hours
Enclose the powder after calcination in air after classifying it to 50 μm or less
Then, using a groove roll mill, expand it to an outer diameter of 1.4 mm.
I made a line. In addition, self-propelled turks from a round wire with an outer diameter of 1.4 mm
Using the head, obtain a rectangular wire with a width of 2.0 mm and a thickness of 0.5 mm.
Was.

These wires were crystallized under the heat treatment pattern conditions shown in FIG. The rectangular wire (2.0 x
0.5 mm) and a part of the crystal of round wire (outer diameter 1.4 mm) are pyrolyzed at a heating temperature of 60 in a stream of pure oxygen at 1 atm.
It was kept at 0 ° C for 1080 hours. Ic of the conductor thus obtained was 4.2K, 0T, and Ic of the rectangular wire was 210A,
The circular line Ic = 140 A (both 0.1 μV / c
m).

A heater, a voltage terminal and a thermocouple were integrated with these conductors in the same manner as in Example 1 and a switching operation test was conducted in liquid helium. Carrying current is rectangular wire 1
20A, round wire 100A. The heater current is 1
It was set to A (about 5 W). The test results are shown in Table 1 below.

[0037]

[Table 1]

As described above, it was confirmed that the tape functions as a permanent current switch as in the tape of Example 1.

[Embodiment 3] The critical current density of the tape conductor in which a part of Bi-2212 was decomposed into Bi-2201 in the Ag-Au-Cu alloy sheath by the same heat treatment as in Embodiment 1 was 4 terminals at each temperature. It was measured by the method. For the measurement, a small refrigerator (SRD-210 made by Sumitomo Heavy Industries) with a built-in magnetic regenerator that can cool up to 4K was used in combination with a heater to stably create various temperatures. In addition, the criteria for Jc was 0.1 μV / cm. FIG. 8 shows the temperature dependence of Jc when a magnetic field of 0.1 T was applied in a direction perpendicular to both the tape surface of the tape conductor and the current direction.

Below 10 K, Jc greatly decreases with increasing temperature, and it is considered that the switching action realized in Examples 1 and 2 reflects the temperature dependence of Jc in this portion. Here, the temperature dependence of this Jc is Bi-
2201 and Bi-2212 materials are assumed to be parallel equivalent circuits, and division is attempted by using the volume ratio of the superconducting phase obtained in Example 1, Bi-2212: Bi-2201 = 7: 3. The results are shown in FIGS. 9 and 10.

FIG. 9 is a diagram obtained by dividing FIG. 8 into Jc of Bi-2201 phase and Jc of Bi-2201 phase for convenience.
FIG. 10 shows the temperature dependence of Jc in the case where the Bi-2212 phase and the Bi-2201 phase are 100% each in consideration of the volume fraction.

The results shown in FIG. 10 are used as a reference (M. Akamatsu et a
l.Advances in Superconductivity-VI (proc.of ISS'93)
Irreversible line (Irreversiv) reported in p.131-134.)
Comparing with the result of the temperature dependence line of the ility curve) (see FIG. 11), it corresponds well with the characteristic that the irreversible temperature around 25 × 0.4 = 10 K in Bi-2201, that is, Jc = 0.

As described above, when it is considered that the Bi-2212 phase and the Bi-2201 phase are connected in parallel in an equivalent circuit (confirmed in the case of the tape wire in the SEM photograph of Example 1), the conductor Since the temperature dependence of Jc can be explained well by considering the cross-sectional area ratio, Bi-22
By adjusting the time and temperature for pyrolyzing the 12 phases, Bi
It was found that the cross-sectional area ratio of the −2212 phase and the Bi-2201 phase can be controlled, and thereby the temperature dependence of Jc in the persistent current switch conductor can be controlled to some extent.

The above examples are for more specifically explaining the present invention, and the above examples do not limit the present invention, and the constitution designed in light of the gist of the present invention is as follows. Both are included in the technical scope of the present invention. For example, the present invention includes not only the permanent current switch but also the case where the application is a current limiting device.

[0045]

As is apparent from the above description,
According to the persistent current switch using the oxide superconductor of the present invention, it is possible to enable the persistent current operation in the oxide superconducting magnet, particularly in the superconducting magnet wound by forming the Bi-2212 system material into a wire. Further, according to the persistent current switch of the present invention, the temperature dependence of Jc can be controlled to some extent by adjusting the cross-sectional area ratio of Bi-2212 and Bi-2201, so that the switching characteristic is controlled. It has the advantage of being able to.

[Brief description of drawings]

FIG. 1 is a graph showing a heat treatment pattern of a silver alloy sheath tape material according to the present invention.

FIG. 2 is an SEM showing the internal structure of a silver alloy sheath tape material.
It is a chart which shows the composition analysis by a photograph and EDX.

FIG. 3 is a graph showing temperature dependence of magnetic susceptibility of a silver alloy sheath tape material.

FIG. 4 is a perspective view showing a structure of a persistent current switch according to the present embodiment.

FIG. 5 is a graph showing a switching operation of the permanent current switch according to the present embodiment.

FIG. 6 is a graph showing the operation of a Bi-2212 / Ag tape conductor.

FIG. 7 is a graph showing a heat treatment pattern for crystallization of a silver alloy sheath rectangular wire.

FIG. 8 is a graph showing the temperature dependence of the critical current density of a silver alloy sheath tape material at 0.1T.

FIG. 9 shows the Jc-temperature dependence of the silver alloy sheath tape material as B
It is the graph which divided into each temperature dependence of i-2201 and Bi-2212.

FIG. 10: Virtual J of single conductors of Bi-2201 and Bi-2212
It is a graph which shows the temperature dependence of c.

FIG. 11 is a graph showing the temperature dependence of an irreversible line in the reference document.

FIG. 12 is a schematic diagram of Bi-2201 existing in Bi-2212.

FIG. 13 is an explanatory diagram showing the operation of the permanent current switch.

[Explanation of symbols]

 1 Sheath tape 2 Heater lead wire (manganin wire) 3 Insulator 4 Au-Fe-chromel thermocouple 5 Lead wire for voltage detection 6 Epoxy resin 7 FRP 8 Styrofoam

Claims (4)

[Claims] 1. A heater is attached to a superconductor obtained by allowing Bi-2212 crystals to exist in a volume fraction of 10% or more and 70% or less in Bi-2212 crystals, and energizing or deenergizing the heater. A permanent current switch using an oxide superconductor, characterized in that it is configured to switch between a normal conducting state and a superconducting state. 2. The persistent current switch according to claim 1, wherein the superconductor has a tape shape or a linear shape, and the superconductor is covered with a covering material made of a silver-based alloy. 3. The superconductor according to claim 1, wherein the superconductor is Bi-2212.
The permanent current switch according to claim 1, wherein a Bi-2212 crystal is formed in the Bi-2212 crystal by decomposing a part of a crystal superconductor by heat treatment. 4. The tape-shaped or linear superconductor according to claim 2 is inserted into a hollow portion of a heater bobbin, a heater wire is wound around the heater bobbin, and the heater wire is sealed with a resin cast material. Furthermore, a permanent current switch structure using an oxide superconductor characterized by being integrated with a heat insulating material.