Method for manufacturing MgB2 superconductor, and MgB2 superconductor

Abstract

Provided are a method for manufacturing MgB.sub.2 superconductor by pressure molding a mixture of Mg powder or MgH.sub.2 powder and B powder and heat-treating the mixture, the method including (I) a step of adding a polycyclic aromatic hydrocarbon to the B powder, while heating the mixture to a temperature higher to or equal to the melting point of the polycyclic aromatic hydrocarbon at the time of this addition, and thereby covering the surface of the B powder with the polycyclic aromatic hydrocarbon; and (II) a step of mixing the B powder having the surface covered with the polycyclic aromatic hydrocarbon, with the Mg powder or the MgH.sub.2 powder, or a step of combining the B powder having the surface covered with the polycyclic aromatic hydrocarbon, with an Mg rod; and an MgB.sub.2 superconducting wire which has high critical current density (Jc) characteristics and less fluctuation in the critical current density (Jc).

Claims

1. A method for manufacturing an MgB.sub.2 superconductor comprising: (I) adding a polycyclic aromatic hydrocarbon to a B powder, while heating to a temperature higher than or equal to the melting point of the polycyclic aromatic hydrocarbon at the time of this addition, and thereby covering a surface of the B powder with the polycyclic aromatic hydrocarbon; (IIa) mixing the B powder having the surface covered with the polycyclic aromatic hydrocarbon, with an Mg powder or an MgH.sub.2 powder, and then packing a metal tube with the resulting mixture; or (IIb) packing a metal tube with the B powder covered with the polycyclic aromatic hydrocarbon and an Mg rod; and (III) pressure molding and heat-treating the packed metal tube, thereby producing the MgB.sub.2 superconductor.

2. The method for manufacturing an MgB.sub.2 superconductor according to claim 1, wherein the polycyclic aromatic hydrocarbon is at least one selected from the group consisting of coronene, anthanthrene, benzo(ghi)perylene, circulenes, corannulene, dicoronylene, diindenoperylene, helicene, heptacene, hexacene, kekulene, ovalene, zethrene, benzo[a]pyrene, benzo[e]pyrene, benzo[a]fluoranthene, benzo[b]fluoranthene, benzo[j]fluoranthene, benzo[k]fluoranthene, dibenz[a,h]anthracene, dibenz[a,j]anthracene, olympicene, pentacene, perylene, picene, tetraphenylene, benz[a]anthracene, benzo[a]fluorene, benzo[c]phenanthrene, chrysene, fluoranthene, pyrene, tetracene, triphenylene, anthracene, fluorene, phenalene, and phenanthrene.

3. The method for manufacturing an MgB.sub.2 superconductor according to claim 1, wherein the polycyclic aromatic hydrocarbon is solid at room temperature and atmospheric pressure and has a melting point that is lower than the decomposition temperature, and a ratio between the number of carbon atoms and the number of hydrogen atoms in the polycyclic aromatic hydrocarbon is such that the ratio C:H is in the range of 1:0.5 to 1:0.8.

4. The method for manufacturing an MgB.sub.2 superconductor according to claim 1, wherein an amount of addition of the polycyclic aromatic hydrocarbon is in the range of 0.05 mol % to 40 mol % relative to a theoretical amount or an amount of experimental production of MgB.sub.2.

5. The method for manufacturing an MgB.sub.2 superconductor according to claim 1, comprising steps (I), (IIa) and (III).

6. The method for manufacturing an MgB.sub.2 superconductor according to claim 1, comprising steps (I), (IIb) and (III).

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 is an explanatory diagram for the production process for single core MgB.sub.2 wire according to an internal Mg diffusion method as an embodiment of the present invention.

(2) FIG. 2 is a diagram illustrating the production process for single core MgB.sub.2 wire according to a powder-in-tube (PIT) method.

(3) FIG. 3 is a diagram illustrating the magnetic field-dependency of the critical current density Jc at 4.2 K of the wire obtained in Example 1 of the present invention.

(4) FIG. 4 is a diagram illustrating the magnetic field-dependency of the critical current density Jc at 4.2 K of the wire obtained in Example 2 of the present invention.

(5) FIG. 5 is an explanatory diagram for the addition of coronene and the Jc-B curve in connection with the MgB.sub.2 wire produced by a PIT method, which was obtained in an Example of the present invention.

(6) FIG. 6 is a diagram illustrating the chemical formulae of the chemical substances used as the polycyclic aromatic hydrocarbon according to an embodiment of the present invention.

(7) FIG. 7 is a diagram illustrating the chemical formulae of the chemical substances used as the polycyclic aromatic hydrocarbon according to an embodiment of the present invention.

(8) FIG. 8 is a diagram illustrating the chemical formulae of the chemical substances used as the polycyclic aromatic hydrocarbon according to an embodiment of the present invention.

(9) FIG. 9 is a diagram illustrating the chemical formulae of the chemical substances used as the polycyclic aromatic hydrocarbon according to an embodiment of the present invention.

(10) FIG. 10 is a diagram illustrating the chemical formulae of the chemical substances used as the polycyclic aromatic hydrocarbon according to an embodiment of the present invention.

(11) FIG. 11 is a diagram illustrating the chemical formulae of the chemical substances used as the polycyclic aromatic hydrocarbon according to an embodiment of the present invention.

DESCRIPTION OF EMBODIMENTS

(12) Hereinafter, embodiments of the present invention will be explained in detail using the drawings or tables. Meanwhile, for the terms used in the present specification, definitions thereof will be described below.

(13) The “internal Mg diffusion method” is a method for manufacturing wire by disposing an Mg rod inside a metal tube, packing the gaps between the metal tube and the Mg rod with B powder, processing this composite into wire, and then heat-treating the wire.

(14) The “powder-in-tube method” is a method for producing wire by packing a metal tube with a raw material powder of a superconductor, processing the composite into wire, and then heat-treating the wire.

(15) The “critical current density Jc” refers to the maximum superconducting current density that can be passed per unit area of a superconducting wire. Usually, the critical current density refers to a value per unit area of the superconductor core in the wire.

(16) In regard to the Mg powder, MgH.sub.2 powder, and B powder that are used as raw materials, powders having purities or particle sizes similar to those of the conventional powders such as those described in Patent Literatures 1 to 3, which are related to the proposal of the present applicant, can be used by appropriately regulating the mixing ratio. For example, in regard to the particle size, the average particle size of the Mg powder or the MgH.sub.2 powder is preferably in the range of 200 nm to 50 μm, and the average particle size of the B powder is preferably in the range of 0.2 μm to 1 μm. In regard to the mixing ratio, it is preferable that Mg or MgH.sub.2 and B are mixed at a molar ratio in the range of Mg or MgH.sub.2/B=0.5/2 to 1.5/2, and more preferably at a molar ratio in the range of 0.8/2 to 1.2/2. Then, appropriate amounts of a polycyclic aromatic hydrocarbon and SiC are added to the mixture of Mg or MgH.sub.2 powder and B powder, and the resulting mixture can be sufficiently mixed in a ball mill or the like.

(17) In regard to the polycyclic aromatic hydrocarbon, various compounds among those compounds having a carbocyclic or heterocyclic ring, each being tricyclic or higher-cyclic, may be considered, and the number of carbon atoms of the polycyclic aromatic hydrocarbon is not particularly limited; however, the number of carbon atoms is preferably in the range of 18 to 50. The polycyclic aromatic hydrocarbon may have various functional groups as long as the functional groups do not impair the operating effect of the present invention, and the polycyclic aromatic hydrocarbon can be appropriately selected in consideration of easy availability, handleability, price and the like. A typical example of the substituent is an alkyl group having 1 to 8 carbon atoms, and particularly 1 to 4 carbon atoms. More specific examples include coronene, anthracene, perylene and biphenyl disclosed in Table 1, Table 2 and FIG. 6 to FIG. 11; an alkyl-substitute carbocyclic aromatic hydrocarbon; and a heterocyclic aromatic hydrocarbon such as thiophene. Furthermore, in regard to the amount of addition of the polycyclic aromatic hydrocarbon, it is preferable to add the polycyclic aromatic hydrocarbon at a proportion of 0.05 mol % to 40 mol % relative to the theoretical amount or the amount of experimental production of MgB.sub.2.

(18) Meanwhile, in regard to the boiling points and the melting points of the polycyclic aromatic hydrocarbons (nanographenes) of Table 1 and Table 2, the database of SciFinder (American Chemical Society; https:://scifinder.cas.org/scifinder/) may be referred to, and in a case in which there are no measured values, calculated values of the boiling point and the melting point may be used (Calculated using Advanced Chemistry Development (ACD/Labs) Software V11.02).

(19) TABLE-US-00001 TABLE 1 Chemical Melting point Boiling point Name formula [° C.] [° C.] Coronene (or superbenzene) C.sub.24H.sub.12 438 525 Anthanthrene C.sub.22H.sub.12 261 497 Benzo(ghi)perylene C.sub.22H.sub.12 278 500 Circulene C.sub.28H.sub.14 295 604 Corannulene C.sub.20H.sub.10 268 438 Dicoronylene C.sub.20H.sub.10 Not published Not published Diindenoperylene C.sub.32H.sub.16 Not published >330  Helicene C.sub.26H.sub.16 Not published Not published Heptacene C.sub.30H.sub.18 Not published 677 Hexacene C.sub.26H.sub.16 Not published 604 Kekulene C.sub.48H.sub.24 Not published Not published Ovalene C.sub.32H.sub.14 475 Not published Zethrene C.sub.24H.sub.14 262 583 Benzopyrene C.sub.20H.sub.12 179 495 Benzo(a)pyrene C.sub.20H.sub.12 179 495 Benzo(e)pyrene C.sub.20H.sub.12 178 493 Benzo(a)fluoranthene C.sub.20H.sub.12 150 468 Benzo(b)fluoranthene C.sub.20H.sub.12 168 481 Benzo(j)fluoranthene C.sub.20H.sub.12 165 480 Benzo(k)fluoranthene C.sub.20H.sub.12 217 480 Dibenz(a,h)anthracene C.sub.22H.sub.14 262 524 ± 17 Dibenz(a,j)anthracene C.sub.22H.sub.14 196 524 ± 17

(20) TABLE-US-00002 TABLE 2 Chemical Melting point Name formula [° C.] Boiling point [° C.] Olympicene C.sub.19H.sub.12 Not published 511 Pentacene C.sub.22H.sub.14 268 524 ± 17 Perylene C.sub.20H.sub.12 276 497 Picene C.sub.22H.sub.14 366 519 Tetraphenylene C.sub.24H.sub.16 232 578 ± 17 Benz(a)anthracene C.sub.18H.sub.12 158 438 Benzo(a)fluorene C.sub.17H.sub.12 189.5 405 Benzo(c)phenanthrene C.sub.18H.sub.12 159 436 ± 12 Chrysene C.sub.18H.sub.12 254 448 Fluoranthene C.sub.16H.sub.10 110.8 375 Pyrene C.sub.16H.sub.10 145 404 Tetracene C.sub.18H.sub.12 357 437 ± 12 Triphenylene C.sub.18H.sub.12 198 438 Anthracene C.sub.14H.sub.10 218 340 Fluorene C.sub.13H.sub.10 116 295 Phenalene C.sub.13H.sub.10 70-75 290 Phenanthrene C.sub.14H.sub.10 101 332

(21) A mixture such as described above is processed into a bulk material or a wire, and methods and conditions similar to the methods and conditions of conventional cases may be employed. A bulk material can be produced by pressure molding and heat treating the mixture, and for example, pressing using a conventional mold or the like may be used, while the pressure is preferably 100 kg/cm.sup.2 to 300 kg/cm.sup.2. A wire can be produced by, for example, packing the mixture in a metal tube made of iron or the like, processing the metal tube into tape or wire with a mechanical rolling or the like, and then heat treating the tape or wire. Regarding the conditions, conditions similar to the conditions of conventional cases may be employed. That is, according to the usage, the mixture can be heat-treated under the conditions of a temperature and a time period sufficient to obtain the MgB.sub.2 superconducting phase, in an inert atmosphere such as argon or a vacuum.

(22) Furthermore, the metal tube used, the heat treatment temperature, and the heat treatment time are not intrinsic to the substitution of B sites with C, and therefore, various metal tubes, heat treatment temperatures, and heat treatment times can be selected.

(23) The MgB.sub.2 superconductor of the present invention obtained as described above is useful for enhancing the capacity of superconducting linear motor cars, MRI medical diagnostic apparatuses, semiconductor single crystal pulling apparatuses, superconducting energy storages, superconducting rotating machines, superconducting transformers, superconducting cables and the like.

(24) Thus, the present invention will be explained in more detail by describing Examples below. The present invention is not intended to be limited to the following Examples.

EXAMPLES

Example 1

(25) B powder and a 5 mol % solid powder of coronene: C.sub.24H.sub.12 as C atoms were sufficiently mixed, and the mixture was vacuum-sealed in a quartz tube. This was heat-treated for 5 minutes at 440° C., which was a temperature higher than or equal to the melting point of coronene: C.sub.24H.sub.12, and then the quartz tube was cooled to room temperature. During the heat treatment, coronene: C.sub.24H.sub.12 melted and infiltrated into the B powder, and the coronene covered (coated) the surface of the B powder. Using this powder coated with coronene: C.sub.24H.sub.12, MgB.sub.2 wire was produced by an internal Mg diffusion method (FIG. 1). An Mg rod having a diameter of 2 mm was disposed at the center of an iron tube having an inner diameter of 4 mm and an outer diameter of 6 mm, and the gap between the Mg rod and the iron tube was packed with this B powder coated with C.sub.24H.sub.12. Thereafter, the packed iron tube was processed into a wire having a diameter of 0.6 mm by means of a groove rolling and wire drawing. This wire was subjected to a heat treatment for 6 hours at 670° C. in an argon atmosphere, and thus MgB.sub.2 superconducting wire was obtained. Mg diffused into the B layer during the heat treatment, and Mg reacted with B to produce MgB.sub.2. At that time, C.sub.24H.sub.12 is decomposed, and a portion of carbon substituted for B of MgB.sub.2. Furthermore, for a comparison, a mixed powder obtained by adding 10 mol % of SiC nanoparticles to B, and an additive-free B powder were used to produce MgB.sub.2 wires according to the internal Mg diffusion method under the same conditions.

(26) For these wires, the critical current density Jc was measured at 4.2 K in various magnetic fields. The results are presented in FIG. 3. As can be seen from this, the MgB.sub.2 wire produced using a B powder coated with coronene: C.sub.24H.sub.12 had its Jc increased to a large extent compared to the wire produced by adding SiC or the additive-free wire. Thus, the superiority of the coronene: C.sub.24H.sub.12 coating for the B powder raw material became clearly known. In regard to the addition of SiC, Mg.sub.2Si is precipitated out as an impurity inside the MgB.sub.2 layer after the heat treatment, and this becomes an inhibitory factor for the superconducting current. Thus, a sufficiently high Jc may not be obtained. On the other hand, in regard to the addition of coronene: C.sub.24H.sub.12, it is speculated that since a precipitate of such an impurity is not incorporated, a Jc that is high as such may be obtained.

Example 2

(27) A B powder coated with coronene: C.sub.24H.sub.12 was produced in the same manner as in Example 1, and MgB.sub.2 wire was produced by a powder-in-tube (PIT) method (FIG. 2). The B powder coated with coronene: C.sub.24H.sub.12, and Mg powder were sufficiently mixed at a molar ratio of 2:1, and an iron tube having an inner diameter of 4 mm and an outer diameter of 6 mm was packed with this mixture. This powder-packed iron tube was processed into a wire having a diameter of 1 mm by means of a groove rolling and wire drawing. This wire was subjected to a heat treatment for one hour at 700° C. in an argon atmosphere, and thus MgB.sub.2 superconducting wire was produced. The Mg and B powder thus packed in the iron tube reacted with each other by the heat treatment, and thus MgB.sub.2 was produced. At that time, C.sub.24H.sub.12 was decomposed, and a portion of carbon substituted for B of MgB.sub.2. Furthermore, similarly to Example 1, an MgB.sub.2 wire was produced, for a comparison, according to the PIT method under the same conditions, using a mixed powder obtained by adding 10 mol % of SiC nanoparticles to a mixed powder of B and Mg, and an additive-free B powder.

(28) For these wires, the critical current density Jc was measured at 4.2 K in various magnetic fields. The results are presented in FIG. 4. As can be seen from this, the MgB.sub.2 wire produced using a B powder coated with coronene: C.sub.24H.sub.12 had its Jc increased to a large extent compared to the wire produced by adding SiC or the additive-free wire. Thus, the superiority of the coronene: C.sub.24H.sub.12 coating for the B powder raw material became clearly known. In regard to the addition of SiC, Mg.sub.2Si is precipitated out as an impurity inside the MgB.sub.2 layer after the heat treatment, and this becomes an inhibitory factor for the superconducting current. Thus, a sufficiently high Jc may not be obtained. On the other hand, in regard to the addition of coronene: C.sub.24H.sub.12, it is speculated that since a precipitate of such an impurity is not incorporated, a Jc that is high as such may be obtained.

(29) FIG. 5 is an explanatory diagram for the addition of coronene: C.sub.24H.sub.12 and the Jc-B curve for the MgB.sub.2 wire produced by a PIT method, which was obtained in an Example of the present invention. The wire diameter Φ of the MgB.sub.2 wire produced by the PIT method was 1.0 mm, and the heat treatment temperature was 700° C., while the heat treatment time was 1 hour. The addition of coronene: C.sub.24H.sub.12 was adjusted to 4 types, namely, no addition, 2% by mass, 5% by mass, and 10% by mass. With regard to the critical current density Jc at 4.2 K, which is the temperature of liquid helium, in a case in which the amount of addition of coronene: C.sub.24H.sub.12 was 10% by mass in a magnetic field of 10 T, the critical current density Jc was 1.8×10.sup.4 [A/cm.sup.2]. As the amount of addition of coronene: C.sub.24H.sub.12 increased, the critical current density Jc increased. Furthermore, measured values of the critical temperature Tc are shown in Table 3. As the amount of addition of coronene: C.sub.24H.sub.12 increased, the critical temperature Tc decreased. However, there is a sufficient margin to the critical temperature at 20 K, which temperature can be easily achieved by refrigerator conduction cooling. Therefore, even if liquid helium is not used, superconductivity can be realized, and even the current circumstances for the supply of liquid helium can be safely coped with.

(30) TABLE-US-00003 TABLE 3 Change T.sub.c (K): calculated T.sub.c (K): measured in axial value induced value induced length X in Mg from reference from resistance a (nm) (B.sub.1−xC.sub.x).sub.2 value [x] value No addition — — 38.8 38  2% C.sub.24H.sub.12 0.0005 0.016 38 36.9  5% C.sub.24H.sub.12 0.0014 0.045 35 35.1 10% C.sub.24H.sub.12 0.0015 0.048 34.4 34.2

(31) Particular embodiments of the present invention have been illustrated and explained; however, it will be obvious to those ordinarily skilled in the art that various other modifications and alterations can be made without departing from the spirit and the scope of the present invention. Therefore, the present invention includes all of such modifications and alterations.

INDUSTRIAL APPLICABILITY

(32) According to the method for manufacturing MgB.sub.2 superconductor of the present invention, addition of a polycyclic aromatic hydrocarbon with excellent uniformity to MgB.sub.2 superconducting wire can be realized, and thereby, an MgB.sub.2 superconducting wire having high critical current density (Jc) characteristics and a critical current density (Jc) with less fluctuation can be provided. The MgB.sub.2 superconductor thus produced is suitable for the use in superconducting linear motor cars, MRI medical diagnostic apparatuses, semiconductor single crystal pulling apparatuses, superconducting energy storages, superconducting rotating machines, superconducting transformers, superconducting cables, and the like.