MgB2 superconductive wire material, and production method therefor

Abstract

An MgB.sub.2 superconducting wire includes a core containing MgB.sub.2 and a metal sheath which surrounds the core. The core includes at least a first MgB.sub.2 core positioned on the center side, and a second MgB.sub.2 core positioned outside the first MgB.sub.2 core, and the density of the second MgB.sub.2 core is lower than the density of the first MgB.sub.2 core.

Claims

1. An MgB.sub.2 superconducting wire comprising a core containing MgB.sub.2 and a metal sheath which surrounds the core, wherein the core comprises at least a first MgB.sub.2 core positioned on the center side, and a second MgB.sub.2 core positioned outside the first MgB.sub.2 core, and the density of the second MgB.sub.2 core is lower than the density of the first MgB.sub.2 core, and the density of the second MgB.sub.2 is less than 70% of the true density of MgB.sub.2, there are voids which extend in the longitudinal direction of the MgB.sub.2 superconducting wire, in the first MgB.sub.2 core, and the void in the first MgB.sub.2 core has the same shape as a void present in the second MgB.sub.2 core, and the ratio of the void in the first MgB.sub.2 core is smaller than the ratio of the void in the second MgB.sub.2 core.

2. The MgB.sub.2 superconducting wire according to claim 1, wherein the density of the first MgB.sub.2 core is 70% or more of the true density of MgB.sub.2 (2.62 g/cm).

3. The MgB.sub.2 superconducting wire according to claim 1, wherein the cross sectional area of the first MgB.sub.2 core is 50% or more of the whole cross sectional area of the core.

4. The MgB.sub.2 superconducting wire according to claim 1, wherein the ratio of the void in the first MgB.sub.2 core is 20% or less.

5. The MgB.sub.2 superconducting wire according to claim 1, wherein the MgB.sub.2 superconducting wire is used as a superconducting cable or a superconducting magnet.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1A is an example of a schematic view of a cross section perpendicular to the longitudinal direction (the extending direction of a wire) of a conventional MgB.sub.2 superconducting wire prepared by PIT method.

(2) FIG. 1B is an example of a schematic view of a cross section along the longitudinal direction of a conventional MgB.sub.2 superconducting wire prepared by PIT method.

(3) FIG. 2A is an example of a schematic view of a cross section perpendicular to the longitudinal direction of an MgB.sub.2 superconducting wire in the first example of the present invention.

(4) FIG. 2B is an example of a schematic view of a cross section along the longitudinal direction of an MgB.sub.2 superconducting wire in the first example of the present invention.

(5) FIG. 3A is an example of a schematic view of a cross section perpendicular to the longitudinal direction of an MgB.sub.2 superconducting wire in the second example of the present invention.

(6) FIG. 3B is an example of a schematic view of a cross section along the longitudinal direction of an MgB.sub.2 superconducting wire in the second example of the present invention.

(7) FIG. 4 is a diagram showing a powder filling method in the MgB.sub.2 superconducting wire of the present invention.

(8) FIG. 5 is a diagram showing a step of producing the MgB.sub.2 superconducting wire of the first example.

(9) FIG. 6 is a diagram showing a step of producing the MgB.sub.2 superconducting wire of the second example.

DESCRIPTION OF EMBODIMENTS

(10) Hereinbelow, examples of the present invention will be described with reference to the attached drawings. The attached drawings show specific examples based on the principle of the present invention, and these are for understanding the present invention, and are certainly not used for restrictively interpreting the present invention.

(11) First Embodiment

(12) FIG. 2A and FIG. 2B are an example of a schematic cross-sectional view of an MgB.sub.2 superconducting wire in the first example of the present invention.

(13) An MgB.sub.2 superconducting wire 1 in the present example includes an MgB.sub.2 core 2, and a metal sheath 5 which surrounds the MgB.sub.2 core 2. The metal sheath 5 is constituted by a barrier layer 6 positioned in the side of the core 2 and a stabilizing layer 7 positioned outside the barrier layer 6.

(14) As a characteristic of the present example, the MgB.sub.2 core 2 is constituted by a high-density MgB.sub.2 core 3 positioned at its center, and a low-density MgB.sub.2 core 4 having a density lower than that of the high-density MgB.sub.2 core 3, and positioned outside the high-density MgB.sub.2 core 3. Therefore, the low-density MgB.sub.2 core 4 is arranged adjacent to the barrier layer 6 of the metal sheath 5. The low-density MgB.sub.2 core 4 has a density lower than that of the high-density MgB.sub.2 core 3, and hereinbelow, details of these cores 3 and 4 will be described.

(15) The high-density MgB.sub.2 core 3 is a part filled with the mixed powder of Mg and B subjected to mechanical milling using a planetary ball mill apparatus (hereinafter referred to as strongly mixed powder) or MgB.sub.2 powder. In the high-density MgB.sub.2 core 3, only minute voids 8 of several m or less are present, as shown in an enlarged view in FIG. 2B. Considering conduction characteristics and the like, the density of the high-density MgB.sub.2 core 3 is preferably 70% or more of the true density of

(16) MgB.sub.2 (2.62 g/cm).

(17) The low-density MgB.sub.2 core 4 is a part filled with a powder obtained by slightly mixing powders of Mg and B (hereinafter referred to as weakly mixed powder). During the wire-drawing process, a soft Mg powder is present in the part of the low-density MgB.sub.2 core 4, thus the low-density MgB.sub.2 core 4 functions as a cushioning material between the part of the high-density MgB.sub.2 core 3 and the barrier layer 6. Whereby, deformation of the barrier layer 6 is suppressed, thus the break of the barrier layer 6 and disconnection of wire can be prevented. However, after the heat treatment, the part where Mg was present becomes a flat void 9 which extends in the longitudinal direction of an MgB.sub.2 superconducting wire. The void 9 has a size of about several tens m.

(18) For preparing the MgB.sub.2 superconducting wire of the present example, it is necessary to constitute as such, when a raw material powder is filled in a metal tube. FIG. 4 is a view showing a powder filling method in the MgB.sub.2 superconducting wire. In an example of FIG. 4, a raw material powder of the high-density MgB.sub.2 core 3 and a raw material powder of the low-density MgB.sub.2 core 4 are filled in a metal tube having the barrier layer 6 and the stabilizing layer 7. At that time, for realizing the constitution of the cross section as designed, it is desirable to fill each powder as a pressure-molded body. A columnar strongly mixed powder molded body 41 for the high-density MgB.sub.2 core 3 is inserted into a cylindrical weakly mixed powder molded body 42 for the low-density MgB.sub.2 core 4. A plurality of molded bodies each including the molded bodies 41 and 42 are filled in a metal tube, so as to have a desired length. Such molded bodies are filled, whereby it is possible to control initial powder arrangement and powder density, and it leads to improve long length uniformity.

(19) Next, the method for producing an MgB.sub.2 superconducting wire of the present invention will be specifically described.

(20) <Preparation of Metal Tube>

(21) A metal tube (metal sheath 5) is made to have a two-layer structure of the barrier layer 6 and the stabilizing layer 7.

(22) The stabilizing layer 7 may not be present when only experimentally energizing to a short wire, but is essential in the case of being used as a coil or cable. In the present example, iron (Fe) was used in the barrier layer 6 and copper (Cu) was used in the stabilizing layer 7. The barrier layer 6 is required not to react with the filled Mg and B during heat treatment, and to have good processability since the wire needs to be drawn to a thin wire diameter. As such a material, other than Fe, niobium (Nb), tantalum (Ta), nickel (Ni) or an alloy containing them can be used.

(23) The stabilizing layer 7 is required to have a low resistivity so as to be used as a current path when the superconductor is quenched (normal conduction transition), and have a high thermal conductivity, particularly in the case of conduction-cooling without refrigerant. As such a material, other than Cu, aluminum (Al), silver (Ag), gold (Au) or an alloy containing them can be used. The barrier layer 6 and the stabilizing layer 7 are desirably integrated in advance. As the procedure, there are a method of integrating the layers by mechanically applying a pressure and a method of electrochemically coating (plating). In the present example, a pipe obtained by plating a Fe tube with Cu was used.

(24) <Raw Material Powder>

(25) In the present example, using the high-density MgB.sub.2 core 3 as a raw material powder, the mixed powder (strongly mixed powder) of Mg and B subjected to mechanical milling was filled in a metal tube. Specifically, the powders of Mg and B were weighed so as to be a stoichiometric composition, and put in a container made of tungsten carbide (WC) that was a hard metal, together with a ball made of WC, and treated at 400 rpm6 hrs, using a planetary ball mill apparatus. When the B particles are sunk into the Mg particles, the mixing method may not be the method described above. In addition, while the strongly mixed powder of Mg and B was used in the present example, it is also effective to fill the MgB.sub.2 powder for increasing the density. However, heat treatment at a high temperature for a long period of time is necessary for increasing binding of particles.

(26) As a raw material powder of the low-density MgB.sub.2 core 4, a powder obtained by slightly mixing Mg and B (weakly mixed powder) was used. Specifically, the powders of Mg and B were weighed so as to be a stoichiometric composition, and put in a plastic container, together with a ball made of SUS304, and mixed for 6 hrs, using a ball mill apparatus. It may be mixed using a V mixer or mortar, not limited to this method. In the case of using the above powder, the operation was carried out in a glove box for preventing oxidation. It is desirable that the water content and the oxygen amount in the glove box are both controlled to 10 ppm or less. In the case of improving conduction characteristics in a high magnetic field, it is effective to add a third element such as carbon, to the raw material powder of the high-density MgB.sub.2 core 3 and the raw material powder of the low-density MgB.sub.2 core 4.

(27) <Powder Molding and Filling>

(28) The raw material powder obtained as described above is pressurized to prepare a molded body. The strongly mixed powder is filled in a columnar mold. Also, the weakly mixed powder is filled in a cylindrical mold. Then, these molds were pressurized by a press machine. The molded body of strongly mixed powder and molded body of weakly mixed powder prepared herein were filled in a metal tube, then the density is improved by wire-drawing process. Therefore, it is not necessary to pressurize by large force herein, and the pressure that can be handled without breaking the molded body (several tens MPa) is enough.

(29) A columnar strongly mixed powder molded body 41 (molded body for the high-density MgB.sub.2 core 3) was inserted into a cylindrical weakly mixed powder molded body 42 (molded body for the low-density MgB.sub.2 core 4), and this was filled in a metal tube. For realizing the constitution of the cross section of wire as designed, it is desirable to fill both weakly mixed powder and strongly mixed powder as a molded body, but it is not essential. For example, for reducing labor of preparing a molded body, after filling the cylindrical weakly mixed powder molded body 42 in the metal tube, it is also possible to fill a strongly mixed powder into a center hole by tapping.

(30) <Wire-Drawing Process>

(31) The metal tube filled with the raw material powder as described above was repeatedly processed using a wire-drawing apparatus, so as to have an area reduction ratio (cross-sectional area reduction ratio) per a path in a range of 8 to 12%, to be processed to a desired diameter ( 0.5 mm in the present example). As the wire-drawing apparatus, a hydrostatic pressure extruder, a draw bench, a wire-drawing machine, a swager, a cassette roller die, a grooved roll, or the like can be used.

(32) <Heat Treatment>

(33) Finally, the drawn wire is subjected to heat treatment (sintering heat treatment) of holding it in a non-oxidizing atmosphere, at a temperature of 600 C. and more and 850 C. or less, for several minutes to several ten hours. Whereby, an MgB.sub.2 phase is produced to form an MgB.sub.2 superconducting wire. The heat treatment is desirably performed in a non-oxidizing atmosphere, for preventing undesirable oxidation of the filler powder. Specifically, an inert gas such as argon (Ar) or nitrogen (N.sub.2), or a vacuum having a degree of vacuum of medium vacuum or more (collectively called as non-oxidizing atmosphere) is desirable, and in any case, it is desirable that the contents of water and oxygen are both 10 ppm or less.

(34) <Evaluation of Long Length Uniformity>

(35) A wire of 0.5 mm300 m in length prepared as described above was cut by 30 m in length, and each end part was sampled (total of 11 pieces), embedded with resin, and polished, and the cross section was observed with an optical microscope. Whereby, long length uniformity of the MgB.sub.2 core shape was evaluated. Specifically, in each cross section, circularity (maximum value of deviation from the average radius) of the MgB.sub.2 core and variety of the MgB.sub.2 core area were evaluated. For comparison, the same evaluation was performed for a wire prepared by a conventional method of filling only strongly mixed powder. As a result, in the wire by the conventional method, the maximum value of deviation from the average radius of the MgB.sub.2 core was about 15% on average, and the variety of the MgB.sub.2 core area was about 5% on average. On the other hand, in the wire by the present example, the maximum value of deviation from the average radius of the MgB.sub.2 core was about 5% on average, and the variety of the MgB.sub.2 core area was about 1% on average. Whereby, the effect of improving long length uniformity by two layer filling of strongly mixed powder and weakly mixed powder was confirmed.

(36) <Evaluation of Conduction Characteristics>

(37) For improving conduction characteristics, it is desirable that the cross sectional area of the high-density MgB.sub.2 core 3 (the area of the cross section perpendicular to the longitudinal direction of the wire, corresponding to the cross section in FIG. 2A) is wide, in the range that the long length uniformity can be secured. Particularly, it is preferred that the cross sectional area of the high-density MgB.sub.2 core 3 is 50% or more of the whole cross sectional area of the core 2. In the present example, the outer diameters of each layer when the final wire diameter was 0.5 mm were as follows; Cu: 0.5 mm, Fe: 0.4 mm, the low-density MgB.sub.2 core 4 (weakly mixed powder): 0.3 mm, and the high-density MgB.sub.2 core 3 (strongly mixed powder) : 0.23 mm, and the cross-sectional area ratio of the low-density core : the high-density core was about 4:6. Also, the density of the high-density MgB.sub.2 core 3 in this example is 70% or more of the true density of MgB.sub.2 (2.62 g/cm).

(38) For comparison with the present example, a wire filled with only the weakly mixed powder (comparative example) was prepared. For the wire of the present example and the wire filled with the weakly mixed powder of the comparative example, the critical current density (Jc) of the MgB.sub.2 core was evaluated though magnetization measurement, and consequently, Jc improvement of about 15% on average was confirmed at 20 K and 0 to 5 T. When the cross sectional area ratio of the high-density MgB.sub.2 core is increased, further Jc improvement is expected.

(39) FIG. 5 is a diagram showing a method for producing the MgB.sub.2 superconducting wire of the first example. First, as described above, a metal tube of a two-layer structure of a barrier layer 6 and a stabilizing layer 7 is prepared (510). Next, an Mg powder and a B powder are slightly mixed by any of the methods shown above, to form a weakly mixed powder of Mg and B (521). Thereafter, a molded body of a weakly mixed powder is prepared using a cylindrical mold (522), then the cylindrical weakly mixed powder molded body is filled in a metal tube (520).

(40) Also, the Mg powder and the B powder are mixed by mechanical milling to form a strongly mixed powder of Mg and B (531). As a raw material powder of the high-density MgB.sub.2 core 3, an MgB.sub.2 powder may be used. Next, a molded body of a strongly mixed powder (or MgB.sub.2 powder) is prepared using a columnar mold (532). Thereafter, the columnar strongly mixed powder molded body is filled in a metal tube (530). Here, a procedure of filling the weakly mixed powder molded body in a metal tube and then filling the strongly mixed powder molded body in a metal tube was used, and as described above, a procedure of filling the strongly mixed powder molded body in the weakly mixed powder molded body, then filling the molded body integrating the weakly mixed powder molded body with the strongly mixed powder molded body in a metal tube may be used. Thereafter, the metal tube is subjected to wire drawing by the method described above (540), and the drawn wire is subjected to heat treatment (550).

(41) <Second Embodiment>

(42) Next, the second embodiment will be described. The high-density MgB.sub.2 core 3 has a difficulty that the binding of particles is poor, considering that the filling rate is high. For improving it, it is effective to add weakly mixed powder to the raw material powder of the high-density MgB.sub.2 core 3. When the weakly mixed powder is added, small B particles enter between particles of hard strongly mixed powder or MgB.sub.2 powder. Then, MgB.sub.2 is produced there, whereby the contact area between particles is expanded, and sintering is promoted.

(43) FIG. 3A and FIG. 3B are an example of a schematic cross-sectional view of an MgB.sub.2 superconducting wire in the second example of the present invention. The constitution other than the high-density MgB.sub.2 core 3 is the same as in the first example, thus the explanation is omitted. In the present example, the weakly mixed powder is added to the raw material powder of the high-density MgB.sub.2 core 3 to prepare the high-density MgB.sub.2 core 3. In this case, voids 9 are generated in the high-density MgB.sub.2 core 3 by diffusion of Mg, as shown in an enlarged view in FIG. 3B. Therefore, in the high-density MgB.sub.2 core 3, the minute voids 8 of several m or less and the voids 9 of several tens m or so mixedly exist. While the filling rate is decreased by generation of the voids 9, the effect of binding improvement is larger than that, thus conduction characteristics are improved.

(44) The void 9 has a flat shape which extends in the longitudinal direction of an MgB.sub.2 superconducting wire, that is the same shape as the void 9 present in the low-density MgB.sub.2 core 4. The weakly mixed powder is added to the highly mixed powder for the high-density MgB.sub.2 core 3, thus the ratio of the void 9 in the high-density MgB.sub.2 core 3 is smaller than the ratio of the void 9 in the low-density MgB.sub.2 core 4. Considering conduction characteristics and the like, the ratio of the void 9 in the high-density MgB.sub.2 core 3 is preferably 20% or less.

(45) FIG. 6 is a diagram showing a method for producing the MgB.sub.2 superconducting wire of the second example. The same number as in FIG. 5 is assigned for the step same as the content described in the first example, and the explanation is omitted. In the following, only a step of preparing a molded body of the mixed powder for the high-density MgB.sub.2 core 3 will be described.

(46) First, the Mg powder and the B powder are mixed by mechanical milling to prepare a strongly mixed powder of Mg and B (first mixed powder) (533). As a substitute for this, an MgB2 powder may be used. Also, the Mg powder and the B powder are slightly mixed using any of a ball mill apparatus, a V mixer, and a mortar as described above for the weakly mixed powder, to prepare a weakly mixed powder of Mg and B (second mixed powder) (534). Thereafter, the strongly mixed powder of Mg and B and the weakly mixed powder of Mg and B are slightly mixed to form a mixed powder for the high-density MgB2 core 3 (535). Then, a molded body of the mixed powder prepared in step 535 is prepared using a columnar mold (536).

(47) <Evaluation of Conduction Characteristics>

(48) The evaluation was performed also for the second embodiment. As the second embodiment, 10% by mass of the weakly mixed powder was added to the highly mixed powder described in the first example (highly mixed powder for the high-density MgB.sub.2 core 3), and other procedures were carried out in the same manner as in the first embodiment to prepare a wire. As a result of evaluating Jc of the MgB.sub.2 core in the same manner as in the first embodiment, Jc improvement of about further 15%, as compared to the wire of the first embodiment, could be confirmed.

(49) According to the examples above, a superconducting wire having high conduction characteristics uniformly over a whole length can be provided. Also, by using the MgB.sub.2 superconducting wire of the example above, a superconducting cable or superconducting magnet (applied to MRI, NMR or the like) which has high performance as compared with conventional ones can be realized.

(50) The present invention is not limited to the examples described above, but includes various modifications. For example, the examples described above describe in detail to easily understand the present invention, and it is not necessarily limited to the example having all constitutions described above. In addition, a portion of the constitution of an example can be substituted by the constitution of another example, and also, the constitution of another example can be added to the constitution of an example. Also, another constitution can be added to, removed from, and substituted for a portion of the constitution of each example.

(51) In the above examples, the MgB.sub.2 core 2 was prepared with two cores, but may be prepared with three or more cores. In this case, it may be constituted so that the core density is reduced from the center toward the outside.

(52) In the above examples, it was described about a round wire having a circular cross-sectional shape, and a single core wire having a single MgB.sub.2 core. However, the same effect can be expected by the present invention, also for a square wire or tape wire having a square cross sectional shape and a multi-core wire having a plurality of MgB.sub.2 cores.

REFERENCE SIGNS LIST

(53) 1 MgB.sub.2 superconducting wire 2 MgB.sub.2 core 3 high-density MgB.sub.2 core (first MgB.sub.2 core) 4 low-density MgB.sub.2 core (second MgB.sub.2 core) 5 metal sheath 6 barrier layer 7 stabilizing layer 8 void 9 void 41 strongly mixed powder molded body 42 weakly mixed powder molded body