JP2013163615A - Whisker crystal of iron-based superconductor and production method of the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a whisker crystal of iron-based superconductor which exhibits superconductivity at an absolute temperature of about 30 K, and to provide a production method which is easy to cope with toxicity and can industrially advantageously produce the whisker crystal.SOLUTION: A mixed powder comprising raw materials constituted of a metal arsenide powder and a metal powder and an additive for promoting crystal growth is filled in a capsule-shaped reaction vessel made of metal. Then, a pressure of 240-380 MPa is mechanically applied via two punches arranged oppositely to each other. After depressurizing, the temperature of the reaction vessel is kept within the range of 700-1,000°C for ≥48 h. Successively, the reaction vessel is furnace cooled to 700°C or less at a fixed temperature lowering rate of ≤0.3°C/min.

Description

  The present invention relates to an iron-based superconductor whisker crystal and a method for producing the same.

  In more detail, the present invention is a whisker crystal in the form of a needle needle containing iron and arsenic in the composition and further containing two or more other elements, and exhibits superconductivity at an absolute temperature of about 30K. The present invention relates to a crystal and a method for producing the whisker crystal.

  An iron-based superconductor is known to have the highest superconducting transition temperature when it contains iron and arsenic as essential elements and further contains two or more additional elements (Non-patent Document 1). Although such thin films and bulk crystals of iron-based superconductors have already been produced (Non-patent Document 1), whisker-like crystals having a high aspect ratio which is industrially important have been difficult to produce.

  One reason for this is the response to the toxicity of the essential element arsenic. In a general whisker crystal manufacturing method, a raw material element is vaporized, transported by a reaction gas or a carrier gas, and grown on a substrate by a gas phase reaction. In this manufacturing method, the raw material element is contained in a crystal growing apparatus. Because of the widespread diffusion of arsenic, it was difficult to cope with toxicity.

  Another reason is that the superconducting properties of the iron-based superconductor are sensitive to the crystal composition. For example, an iron-based superconductor having a superconducting transition temperature of 30 K or higher in absolute temperature is composed of four or more elements including iron and arsenic (Non-Patent Document 1). In the known method, it is difficult to synthesize whisker crystals by arbitrarily controlling four or more kinds of constituent elements including iron and arsenic. Actually, whisker crystals of iron-based superconductors have not been manufactured so far.

  Patent Document 1 describes a method for producing needle-like crystals containing iron. In this manufacturing method, iron arsenide is used as a part of the raw material, but finally needle-like crystals containing both elements of iron and arsenic are not manufactured. Furthermore, since the aspect ratio of the length and the diameter of the crystal obtained by this production method is 100 or less (Patent Document 1), it is difficult to apply this production method to the production of whisker crystals of iron-based superconductors.

  Although the manufacturing method of the whisker crystal | crystallization of the copper oxide superconductor which does not contain the element which has toxicity is disclosed by the nonpatent literature 2, for the reason regarding toxicity, the manufacturing method disclosed by the nonpatent literature 2 is used. It is difficult to apply to the manufacture of whisker crystals of iron-based superconductors containing arsenic as a raw material element.

Similarly, a method for producing a whisker crystal of a magnesium diboride (MgB ₂ ) superconductor is disclosed in Non-Patent Document 3, for example, but is disclosed in Non-Patent Document 3 for reasons relating to control of the crystal composition. It is difficult to apply this manufacturing method to the production of whisker crystals of iron-based superconductors containing arsenic as a raw material element.

JP 2001-342014 A

Maeda Kyogo, Imai Yoshimune, Takahashi Hideyuki, Solid State Physics 46 (2011) 453. Nagao, M .; Sato, M .; Maeda, H .; Kim, S. J .; Yamashita, T. Appl. Phys. Lett. 2001, 79, 16. Y. Wang, C. Zhuang, J. Gao, X. Shan, J. Zhang, Z. Liao, H. Xu, D. Yu, and Q. Feng, J. Am. Chem. Soc. 131, 2436 (2009 ). A. Iandelli, E. Franceschi, Journal of the Less Common Metals, Volume 30, Issue 2, February 1973, Pages 211-216. Satomi KAKIYA, Kazutaka KUDO, Yoshihiro NISHIKUBO, Kenta OKU, Eiji NISHIBORI, Hiroshi SAWA, Takahisa YAMAMOTO, Toshio NOZAKA, and Minoru NOHARA, Journal of the Physical Society of Japan 80 (2011) 093704. F. Izumi and T. Ikeda, Mater. Sci. Forum 321-324, 198 (2000).

  In the widely disclosed method for producing whisker crystals, the raw material element is transported by utilizing the vaporization or evaporation of the reaction gas and the raw material material, so that the element diffuses extensively inside the synthesis apparatus. For this reason, in this manufacturing method, when manufacturing an iron-based superconductor containing arsenic, in order to ensure the safety of the manufacturing process, the apparatus configuration and the process become complicated, and it must be manufactured under various constraints. Don't be. Furthermore, it is difficult to manufacture an iron-based superconductor crystal having a high superconducting transition temperature of 30 K or higher and comprising four or more constituent elements including iron and arsenic to a desired component composition. Moreover, it is difficult to produce a high aspect ratio whisker crystal having an aspect ratio (L / d) of 200 or more in length (L) and diameter (d). It is an object of the present invention to provide a whisker crystal of an iron-based superconductor having a superconducting transition temperature of about 30 K, which is easily toxic and a manufacturing method capable of industrially manufacturing the whisker crystal. It is said.

  As a result of repeated investigations in view of the above-mentioned present situation, the present inventors have mixed an additive that promotes crystal growth into the raw material, filled this mixed powder into a capsule-shaped metal reaction vessel, and mechanically We succeeded in producing whisker crystals of iron-based superconductors by applying an appropriate pressure to optimize the density of the mixed powder and then releasing the pressure and applying an appropriate heat treatment.

  Furthermore, the present inventors confirmed that the whisker crystal of the iron-based superconductor thus obtained transitions to a superconducting state at an absolute temperature of 33K.

In addition, the whisker crystal has a needle-like shape, has a SrZnSb ₂ type crystal structure, an aspect ratio (L / d) of length (L) to diameter (d) is 200 to 10,000, and diameter (d) is It was also confirmed that the size was 1 × 10 ⁻³ mm or less.

  The present invention has been completed based on these new findings and further studies. That is, the present invention provides the following iron-based superconductor whisker crystals and a method for producing the same.

The whisker crystal of the iron-based superconductor of the present invention includes M1 (one or more elements selected from magnesium, calcium, strontium or barium), Fe (iron), M2 (one or more elements selected from platinum group elements). Element) and As (arsenic), and the molar ratio of the elements in each group is
M1: Fe: M2: As = 10: (10-5x) :( 4 + 5x): 18 (0.1 ≦ x ≦ 0.36)
It is characterized by having a SrZnSb ₂ type crystal structure.

In the iron-based superconductor whisker crystal of the present invention, the aspect ratio (L / d) of the length (L) and the diameter (d) of the needle-like crystal is 200 or more, and the diameter (d) is 1 × 10. It is preferably ⁻³ mm or less.

  The method for producing an iron-based superconducting whisker crystal of the present invention comprises metal arsenide powder and metal powder, and the constituent elements are M1 (one or more elements selected from magnesium, calcium, strontium or barium), Fe ( Iron), M2 (one or more elements selected from platinum group elements) and As (arsenic), and the molar ratio of the elements in each group is M1: Fe: M2: As = 10: (10 −5x): (4 + 5x): 18 (0.1 ≦ x ≦ 0.36) A mixed powder of a raw material and a powder additive that promotes crystal growth is filled into a capsule-shaped metal reaction vessel. The reaction vessel is charged into a die, and a pressure of 240 to 380 MPa is mechanically applied through two punches arranged opposite to each other. After depressurization, the reaction vessel is placed in a temperature range of 700 to 1000 ° C. for 48 hours. Hold for more than an hour and then react Vessels and is characterized in that the furnace cooled to 700 ° C. or less at a constant cooling rate of 0.3 ° C. / min or less.

  In the method for producing an iron-based superconducting whisker crystal of the present invention, the powder additive is preferably at least one selected from metal arsenide or metal halide.

  In the method for producing an iron-based superconducting whisker crystal of the present invention, it is preferable to install an inner container in a metal reaction vessel and separate the mixed powder and the metal reaction vessel.

The whisker crystal of the iron-based superconductor of the present invention has significant anisotropy in the crystal shape as compared with the conventional bulk crystal and thin film crystal. For example, the length reaches 2 mm, while the diameter is 1 × 10 ⁻³ mm or less. Further, the whisker crystal of the iron-based superconductor of the present invention exhibits superconductivity even at a temperature exceeding the absolute temperature of 30K. For this reason, it can be applied as an iron-based superconducting wire for a device or an iron-based superconducting junction element material.

  Whisker crystals having a superconducting transition temperature exceeding 30 K have been produced even with copper oxide superconductors, but copper oxide superconductors have limited applications due to the inherent brittleness of ceramics. On the other hand, iron-based superconductors are closer to alloys than ceramics, and thus are not as brittle as copper oxide superconductors, and can be used for a wider range of applications.

  In the method for producing a whisker crystal of an iron-based superconductor according to the present invention, a mixed powder of a raw material and an additive is filled in a capsule-like metal reaction vessel, and crystal growth is performed by a solid phase reaction. Easy to control the structure. In addition, since a device that transports elements by reaction gas or evaporation is not used, it is simple and simple, and safety is maintained despite including toxic substances. From such a viewpoint, it is considered that the method for producing a whisker crystal of an iron-based superconductor according to the present invention is useful for industrial application.

It is the figure which showed the external appearance and its cross section of the capsule-shaped metal reaction container before sealing used for manufacture of the whisker crystal | crystallization of an iron-type superconductor. It is the figure which showed the punch and die for sealing of a capsule-shaped metal reaction container used for manufacture of the whisker crystal | crystallization of an iron-type superconductor. It is the figure which showed the cross section of the capsule-shaped metal reaction container after sealing used for manufacture of the whisker crystal | crystallization of an iron-type superconductor. It is a scanning electron micrograph of a whisker crystal of an iron-based superconductor. It is a scanning electron micrograph (high magnification) of a whisker crystal of an iron-based superconductor. It is the figure which showed the temperature dependence of the magnetic susceptibility of the whisker crystal of an iron-type superconductor. It is the figure which showed the dependence of the electrical resistivity of the whisker crystal | crystallization of an iron-type superconductor on temperature and crystal | crystallization size (diameter). 3 is a powder X-ray diffraction chart of a whisker crystal of an iron-based superconductor. It is the high-resolution image by the transmission electron microscope of the whisker crystal of an iron-type superconductor, and the electron beam diffraction pattern corresponding to this high-resolution image.

  In the method for producing an iron-based superconducting whisker crystal according to the present invention, a capsule-shaped metal reaction vessel is filled with a mixed powder of a metal arsenide powder and a raw material composed of the metal powder and a powder additive for promoting crystal growth. Finally, a whisker crystal of an iron-based superconductor is obtained.

  The whisker crystal of an iron-based superconductor is composed of iron and arsenic as constituent elements, and at least one element selected from M1 and a platinum group element selected from magnesium, calcium, strontium, or barium. It contains four groups of element M2. The molar ratio of the elements in each group is M1: Fe: M2: As = 10: (10-5x) :( 4 + 5x): 18 (0.1 ≦ x ≦ 0.36). A typical example of M1 is calcium. However, calcium belongs to the second group of the periodic table and is likely to be a divalent cation, and the ionic radius and covalent bond radius of magnesium, strontium, or barium are close to calcium. It is possible to replace some or all of them with one or more. Platinum is typically exemplified as M2, but platinum has a similar atomic radius and electron distribution to other platinum group elements, that is, ruthenium, rhodium, palladium, osmium, or iridium, and therefore a part of platinum. Or it is possible to replace the whole with one or more of other platinum group elements. In addition, although iron is one of the constituent elements, as described in Non-Patent Document 1, some of them are other iron group elements having similar ionic radii and magnetic properties in the crystal, that is, It can be replaced with cobalt or nickel.

  In order to promote the growth of whisker crystals of iron-based superconductors, melting or decomposition in a sealed metal reaction vessel at a temperature of 1000 ° C. or less, preferably 700 to 1000 ° C., more preferably 800 to 900 ° C. Use the additive. Whisker crystals are not grown when the temperature is out of the range or the additive is not added.

  For such an additive, for example, a metal arsenide such as calcium arsenide can be used in order to minimize mixing of impurities into the whisker crystal. As described in Non-Patent Document 4, calcium arsenide is preferably used as an additive because it decomposes at less than 900 ° C. when the composition is CaAs. In addition to metal arsenides, metal halides are widely used as additives for promoting crystal growth. Metal halides can also be used to grow whisker crystals of iron-based superconductors.

  Among the constituent elements of whisker crystals of iron-based superconductors, calcium is a dangerous substance that reacts violently when it comes into contact with water to generate hydrogen. In the powder state, oxidation easily proceeds in the air. For this reason, the mixed powder containing calcium needs to be stored in a vacuum or in an inert gas, and is difficult to handle as a raw material.

  In order to avoid using such difficult-to-handle calcium as a raw material, calcium arsenide is synthesized in advance, and the synthesized calcium arsenide is used as a raw material. Further, when calcium arsenide is used in excess of the amount necessary for crystal growth, CaSe thermally decomposes at less than 900 ° C. as described above, so that the excess calcium arsenide functions as an additive.

  When arsenic (melting point is 630 ° C.) is used as a raw material, the melting point is 200 ° C. or more lower than that of other raw materials. To avoid this problem, use alternative source materials. For example, among the metal arsenides of the constituent elements, one or more of calcium arsenide, iron arsenide, platinum arsenide and the like whose melting point or decomposition temperature is higher than that of arsenic is used.

  Calcium arsenide is pre-synthesized by direct reaction of arsenic and calcium. Each raw material is weighed in argon gas so as to have a stoichiometric composition, and is vacuum-sealed in a quartz tube. This quartz tube is put into an electric furnace such as a muffle furnace, and heated at a constant heating rate. When the temperature reaches 500 ° C., the temperature is maintained. In general, the rate of temperature rise is desirably 0.1 to 10 ° C./min, preferably 0.1 to 1 ° C./min, and more preferably 0.1 to 0.4 ° C./min. When the rate of temperature rise exceeds 10 ° C./min, a uniform reaction cannot be obtained. The holding time is desirably 48 hours or longer in order to sufficiently promote the reaction. After completion of the holding, the quartz tube is taken out from the electric furnace and rapidly cooled in air to room temperature. After the cooling is completed, the pre-synthesized calcium arsenide is taken out from the quartz tube, pulverized and mixed using a mortar or the like, and vacuum sealed again in the quartz tube. The same heat treatment is repeated with the holding temperature changed and the other conditions being the same, and finally calcium arsenide is obtained. The final holding temperature is preferably 700 to 800 ° C, more preferably 800 ° C. The synthesis of iron arsenide and platinum arsenide can also be performed in the same manner as the synthesis of calcium arsenide as described above.

According to Non-Patent Document 5, the bulk iron-based superconductor has a constituent element molar ratio of Ca: Fe: Pt: As = 10: (10-5x) :( 4 + 5x): 18, and x = 0. In the case of 36, it crystallizes into a SrZnSb ₂ type structure and exhibits superconductivity. Referring to this result, even in the production of whisker crystals, the molar ratio of the raw material elements is Ca: Fe: Pt: As = 10: (10-5x) :( 4 + 5x): 18 (x is 0.1 or more and 0) The raw material can be weighed and mixed so as to have an arbitrary value of .36 or less. As the raw material, for example, iron arsenide, calcium arsenide, pure iron, and platinum powders can be selected.

  In general, 1 to 20%, preferably 5 to 10%, and more preferably 10% of an additive by mass ratio is added to the raw material mixed powder and mixed until the whole becomes uniform.

If the particle size of the mixed powder is too small, the melting point of some of the particles will decrease and the reaction will not proceed uniformly.If it is too large, the uniform reaction will not proceed, so it may be distributed in an appropriate range. desirable. Suitable particle size range is generally ^{0.5 × 10 -3 mm~10 × 10 -3} mm, preferably ^{1 × 10 -3 mm~7 × 10 -3} mm, and more preferably 2 × ^{10 -3} mm to 5 × 10 ⁻³ mm.

  The purity of the mixed powder is generally 99.9% or higher, preferably 99.99% or higher. If the purity is too low, impurities are mixed in and the superconducting properties are adversely affected.

  The mixed powder is filled into a capsule-shaped metal reaction vessel, this reaction vessel is loaded into a die, and a pressure of 240 to 380 MPa is mechanically applied through a punch arranged facing the top and bottom, etc. Thereafter, the reaction vessel is appropriately heat-treated. The reaction vessel is made of metal. Those made of glass or ceramic are excellent in heat resistance and chemical stability, but are easily damaged in the process of applying pressure. The metal material forming the reaction vessel is preferably a high melting point noble metal, tantalum, niobium, etc., more preferably tantalum or niobium, and even more preferably tantalum.

  If the mixed powder and the metal reaction vessel are in direct contact with each other, the reaction vessel may be damaged during the heat treatment. Therefore, it is preferable to avoid contact using the inner vessel. As shown in FIG. 1, first, a mixed powder 13 of a raw material and an additive is packed in a bottomed cylindrical inner container 14, and then packed in a bottomed cylindrical reaction container 15 made of metal. Thereafter, the mixed powder 13 is covered with a lid 12 made of the same material as that of the inner vessel 14, and further covered with a metal lid 11 of the reaction vessel 15, and the end of the reaction vessel 11 protruding from the lid 11 is bent inward. The reaction vessel 11 is made into a capsule. The inner container 14 may be made of, for example, boron nitride or silicon nitride, which has a higher melting point than the mixed powder 13 and hardly reacts with the reaction container 15 or the mixed powder 13 during heat treatment.

  Next, as shown in FIG. 2, a uniaxial load (in the cylinder axis direction) is applied to the capsule-shaped reaction vessel 22 using the dice 23 and the punches 21 and 24 disposed so as to face each other vertically. The bent portion located at the end of the container 22 is crimped. Further, after applying an appropriate pressure to the reaction vessel 22 to increase the density of the mixed powder 13, the pressure is released. If the applied pressure is too low, the crimping of the bent portion of the reaction vessel 22 becomes insufficient, the contents are likely to leak during the heat treatment, the synthesis reaction does not proceed sufficiently, and whisker crystals do not grow. On the other hand, if the applied pressure is too high, the space in which the whisker crystal grows decreases, and the growth of the whisker crystal is hindered. For this reason, the pressure applied to the reaction vessel 22 is set to 240 MPa or more and 380 MPa or less. Preferably it is 300 MPa or more and 380 MPa or less, More preferably, it is 380 MPa.

  Then, as shown in FIG. 3, a sealed capsule-like reaction vessel 33 is sealed in a vacuum vessel 32 such as a transparent quartz tube or a refractory metal tube and heated in a vacuum 31 using a heating device such as a muffle furnace. To do. In general, the rate of temperature rise is desirably 0.1 to 10 ° C./min, preferably 0.1 to 1 ° C./min, and more preferably 0.1 to 0.4 ° C./min. If it is too fast, a uniform reaction cannot be obtained. The heat treatment requires two steps of temperature holding and cooling, and whisker crystals cannot be grown properly if the following conditions are not met during temperature holding and cooling. In the temperature maintenance, which is the first stage of the heat treatment, a constant temperature is maintained in the range of 700 to 1000 ° C. The holding temperature is preferably in the range of 900 to 1000 ° C, more preferably 1000 ° C. In addition, in the temperature holding, if the holding time is too short, the reaction does not proceed sufficiently, so that 48 hours or more are necessary. If kept for 72 hours, the reaction proceeds more fully.

Cooling, which is the second stage of the heat treatment, is performed by furnace cooling after maintaining the temperature. If the furnace cooling rate is too fast, whisker crystals will not grow, so the furnace cooling rate is 0.3 ° C./min or less, preferably 0.1 ° C./min or less. It is sufficient that the furnace is cooled to a temperature below the temperature at which whisker crystal growth stops, that is, 700 ° C. or below. Preferably, the furnace is cooled to 500 ° C. or lower. After the furnace cooling, the reaction vessel 33 is taken out into the air and returned to room temperature. Stick needle-like whisker crystals are formed on the surface of the sample taken out from the reaction vessel 33. By the two-stage heat treatment as described above, the reaction and sintering of the raw material proceed, and a bulk crystal or sintered body is generated inside the capsule-like reaction vessel 33. With the generation of the bulk crystal or sintered body, a gap is generated between the bulk crystal or sintered body and the inner wall of the reaction vessel 33. Whisker crystals are formed on the surface of the bulk crystal or sintered body located in the gap. As such a whisker crystal, an aspect ratio (L / d) of length (L) and diameter (d) is 200 or more and diameter (d) is 1 × 10 ⁻³ mm or less. . The crystal structure of the whisker crystal is a SrZnSb ₂ type crystal structure.

<Example 1>
Arsenic (purity 99.9%) and calcium (purity 99.9%) were weighed in an argon gas so that the molar ratio was 1: 1, mixed, and vacuum-sealed in a quartz tube. This quartz tube was put into an electric muffle furnace and heated to 500 ° C. at a temperature rising rate of 0.2 ° C./min. This temperature was maintained for 48 hours. After completion of the temperature holding, the quartz tube was taken out from the electric muffle furnace and cooled in air to room temperature. After cooling, the sample was taken out from the quartz tube, pulverized and mixed using a mortar, packed in the quartz tube, and vacuum-sealed again. A similar heat treatment was performed at a holding temperature of 800 ° C. to finally obtain calcium arsenide (CaAs).

  Arsenic and iron (purity 99.9%) were weighed so as to have a molar ratio of 1: 1, mixed, and vacuum-sealed in a quartz tube. This quartz tube was put in an electric muffle furnace and heated to 500 ° C. at a temperature rising rate of 0.2 ° C./min. This temperature was maintained for 48 hours. After completion of the temperature holding, the quartz tube was taken out from the electric muffle furnace and cooled in air to room temperature. After cooling, the sample was taken out from the quartz tube, pulverized and mixed using a mortar, packed in the quartz tube, and vacuum-sealed again. A similar heat treatment was performed at a holding temperature of 800 ° C. to finally obtain iron arsenide (FeAs).

As raw materials, synthesized calcium arsenide (CaAs), synthesized iron arsenide (FeAs), iron and platinum (purity 99.9%) in a molar ratio of CaAs: FeAs: Fe: Pt = 12.81: 8: Weighed to 1: 5 and mixed using a mortar until they were uniform. Particle size range of the mixed powder was ^{1 × 10 -3 mm~10 × 10 -3} mm. In addition, the amount of calcium arsenide contains an additive equivalent to 10% by mass.

  This mixed powder has a bottomed cylindrical shape and is packed in a boron nitride inner container (outer diameter 6.4 mm × inner diameter 5 mm × height 6 mm), and a boron nitride lid (diameter 4.9 mm × thickness 1 mm). ) And was packed in a tantalum reaction vessel (outer diameter 6.8 mm × inner diameter 6.4 mm × height 6.5 mm). Further, a tantalum lid (diameter: 6.3 mm x thickness: 0.2 mm) is put on, the end of the reaction vessel protruding from the lid is folded inward, and a die and a punch placed vertically are used. A uniaxial load was applied until a pressure of 380 MPa was reached, and the bent portion was crimped. The capsule-like reaction vessel was vacuum sealed in a transparent quartz tube.

  The quartz tube sealed in a vacuum was placed in an electric muffle furnace and heated. The heating rate was 0.8 ° C./min. After the furnace temperature reached 1000 ° C., the furnace temperature was kept constant and held at that temperature for 72 hours. Subsequently, the furnace was cooled down to 700 ° C. at a constant cooling rate (0.1 ° C./min) over 48 hours. After reaching 700 ° C., the quartz tube in which the reaction vessel was vacuum-sealed was taken out of the furnace into the air and room temperature was reached. Until cooled. When the sample was taken out from the reaction container after the heat treatment, whisker crystals were formed on the surface of the sample as shown in FIG.

As a result of observing the produced whisker crystal with a scanning electron microscope, as shown in FIG. 5, the whisker crystal has a long and narrow needle shape, the length (L) is 0.1 to 2 mm, and the diameter (d) is 0.1. It was distributed in the range of 2 × 10 ⁻³ mm to 5 × 10 ⁻³ mm. The aspect ratio (L / d) was 200 to 10,000.

  The produced whisker crystals were collected and the total weight was 2 mg. The temperature change of magnetic susceptibility was measured for whisker crystals. As shown in FIG. 6, a clear diamagnetic transition was confirmed at an absolute temperature of 33K. Next, one whisker crystal was taken out and the temperature change of the electrical resistivity was measured by a four-terminal method. As shown in FIG. 7, a rapid decrease in resistivity was confirmed at an absolute temperature of 33K. Furthermore, it was observed that the electric resistance becomes zero at 10K or less. From the above, the transition at 33K is considered a superconducting transition.

In addition, whisker crystals were collected and well pulverized and mixed in a mortar, and the powder was subjected to structural analysis by X-ray diffraction at room temperature. FIG. 8 shows the result of analyzing the diffraction chart by the Rietveld method. As a result of analysis with reference to the crystal structure analysis result of the bulk sample already published in Non-Patent Document 5, the crystal structure of the whisker crystal was successfully analyzed as an SrZnSb type ₂ crystal structure (tetragonal crystal). Note that the lowermost curve in FIG. 8 shows the difference between the actually measured value and the calculated value. Also, the position of the diffraction peak due to the SrZnSb ₂ type crystal structure phase and impurities is shown below the chart. It is presumed that the impurities were mixed because the portion other than the crystals could not be completely separated when collecting the crystals. From the analysis result by the Rietveld method, it is considered that the whisker crystal has the same crystal structure as a known bulk crystal. Moreover, the result of the composition analysis by Rietveld method (refer nonpatent literature 6) was Ca: Fe: Pt: As = 10: 9: 5: 18. This analysis result corresponds to Ca: Fe: Pt: As = 10: 9: 5: 18, which is the composition of the raw material elements excluding the additive.

FIG. 9 shows a high-resolution image of the whisker crystal generated by a transmission electron microscope and an electron beam diffraction pattern corresponding to the high-resolution image. It was observed that clear stripe-type contrast was repeated with a period of 1 nm. This contrast period coincides with the stacking period in the c-axis direction of the SrZnSb ₂ type crystal structure. Furthermore, the stacking direction is considered to be perpendicular to the a-axis direction from the distribution of diffraction spots of the electron diffraction pattern taken with the same incident axis. Since the major axis direction of the whisker crystal is perpendicular to the stacking direction, the generated whisker crystal is considered to have grown in the a-axis direction of the SrZnSb _2- type crystal structure.

FIG. 7 shows the dependence of electrical resistivity on temperature and diameter. It can be seen that the superconducting transition temperature decreases as the diameter of the whisker crystal decreases. When the diameter is 1 × 10 ³ nm or more, the superconducting transition temperature is 33K, and when the diameter is reduced to 0.45 × 10 ³ nm, the superconducting transition temperature is lowered to 25K.

<Example 2>
The molar ratio of the raw material elements not including the additive component is Ca: Fe: Pt: As = 10: (10-5x) :( 4 + 5x): 18, and x is 0,0.1,0.2,0.3. , 0.36, and production of whisker crystals was attempted in the same manner as in Example 1. The formation of whisker crystals was confirmed when x ≧ 0.1. When x = 0.2, whisker crystals were formed most densely. When x = 0.36, the size of the whisker crystal was the largest, but the superconducting transition temperature was 25K.

<Comparative Example 1>
An attempt was made to produce whisker crystals under the same conditions as in Example 1 except that 10% by mass of calcium arsenide was not added. However, whisker crystals were not formed.

<Comparative Example 2>
Attempt to produce whisker crystals under the same conditions as in Example 1 except that the heat treatment of holding at 1000 ° C. for 72 hours and furnace cooling to 700 ° C. over 48 hours at a constant cooling rate is omitted and immediately quenched in air. It was. However, whisker crystals were not formed.

<Comparative Example 3>
An attempt was made to produce whisker crystals under the same conditions as in Example 1 except that the pressure applied to the capsule-shaped metal reaction vessel before the heat treatment was 420 MPa. However, whisker crystals were not formed.

According to the present invention, it is easy to cope with toxicity, and it is easy to control the crystal composition of four or more elements including iron and arsenic. Furthermore, a whisker crystal having a high aspect ratio in which the aspect ratio (L / d) of the length (L) and the diameter (d) of the whisker crystal is 200 or more and the diameter is 1 × 10 ⁻³ mm or less. Can do. The resulting whisker crystal exhibits superconductivity at an absolute temperature of about 30K, and is not as brittle as a whisker crystal of a known copper oxide superconductor. For this reason, the use which can be applied can be expanded and it is industrially useful. In particular, it may be applicable to new industrial technical materials such as superconducting wires for devices and superconducting junction element materials.

11 Lid of reaction vessel 12 Lid of inner vessel 13 Mixed powder of raw material and additive 14 Inner vessel 15 Reaction vessel 21 Punch (upper part)
22 Capsule-shaped metal reaction container (before crimping)
23 Dice 24 Punch (Bottom)
31 Vacuum 32 Vacuum container 33 Capsule-shaped metal reaction container (after crimping)

Claims (5)

A whisker crystal of an iron-based superconductor, M1 (one or more elements selected from magnesium, calcium, strontium or barium), Fe (iron), M2 (one or more elements selected from platinum group elements) ) And As (arsenic), and the molar ratio of the elements in each group is
M1: Fe: M2: As = 10: (10-5x) :( 4 + 5x): 18 (0.1 ≦ x ≦ 0.36)
A whisker crystal of an iron-based superconductor characterized by having a SrZnSb type ₂ crystal structure. The whisker crystal of the iron-based superconductor according to claim 1, wherein the aspect ratio (L / d) of the length (L) and the diameter (d) of the needle-like crystal is 200 or more, and the diameter (d) is A whisker crystal of an iron-based superconductor characterized by being 1 × 10 ⁻³ mm or less.   The method for producing a whisker crystal of an iron-based superconductor according to claim 1 or 2, comprising a metal arsenide powder and a metal powder, wherein the constituent element is selected from M1 (magnesium, calcium, strontium or barium). It consists of four groups of one or more elements), Fe (iron), M2 (one or more elements selected from platinum group elements) and As (arsenic), and the molar ratio of the elements in each group is M1: Fe : M2: As = 10: (10-5x) :( 4 + 5x): 18 (0.1 ≦ x ≦ 0.36) mixed powder of raw material and powder additive that promotes crystal growth In a capsule-shaped metal reaction vessel, this reaction vessel is loaded into a die, mechanically applied with a pressure of 240 to 380 MPa through two opposed punches, and after depressurization, The reaction vessel is kept at a temperature range of 700 to 1000 ° C. In and held for 48 hours or more, followed method for producing whisker crystals of iron-based superconductors, characterized by furnace cooling to 700 ° C. or less at a constant cooling rate of 0.3 ° C. / min or less the reaction vessel.   The whisker crystal of the iron-based superconductor according to claim 3, wherein the powder additive is at least one selected from metal arsenide or metal halide. Manufacturing method.   The method for producing a whisker crystal of an iron-based superconductor according to claim 3 or 4, wherein an inner container is installed in the metal reaction vessel, and the mixed powder and the metal reaction vessel are separated. A method for producing whisker crystals of iron-based superconductors.