JPH11264064A - Production of superconductive material and superconductive material obtained therefrom - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a production method of a superconductive material having a high critical temp. and applicable to various applications. SOLUTION: A high purity niobium containing niobium and impure contents of, for example <10 ppm oxygen (O). <10 ppm nitrogen (N), <10 ppm carbon (C) is subjected to heat treatment at 450-1500 deg.C in an atmosphere containing at least one of oxygen, nitrogen and carbon or in the atmosphere. The obtained high purity niobium exhibits a superconductive phenomenon at about 30K. The superconducting material having a desired shape in accordance to object and application is obtained by processing previously the high purity niobium in a desired shape.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a superconducting material using niobium as a base material and a superconducting material obtained by the method, and more particularly, to heating a high-purity niobium material in a predetermined atmosphere. The present invention relates to a novel method for producing a superconducting material by processing and a superconducting material obtained thereby.

[0002]

2. Description of the Related Art There is known a so-called superconductivity phenomenon in which the electric resistance of a substance becomes zero at a predetermined temperature. There is a superconducting magnet having a structure in which a wire made of superconducting material is coiled as a device utilizing this superconducting phenomenon, such as an accelerator for high energy experiments, a magnetic levitation train,
It is used for MRI-CT (Magnetic Resonance Imaging Apparatus) and the like. Examples of the substance exhibiting superconductivity include metal element superconductors such as Nb and Pb, alloy superconductors such as NbTi, niobium nitride (NbN), niobium carbide (NbC), and Nb ₃ S.
n, compound superconductors such as Nb ₃ Al and YBaCuO
Oxide-based oxide superconductors are known. As a material used for the above-described superconducting magnet, a superconducting material which has a high temperature in a superconducting state (hereinafter referred to as a critical temperature) and is easily processed into a wire is desired.

[0003]

Among the superconducting materials described above, NiTi is excellent in workability and is widely used as a wire for superconducting magnets, but its critical temperature is as low as about 9K. . On the other hand, Nb ₃
Sb and Nb ₃ Al both had high critical temperatures of about 18 K, but were hard and brittle and had insufficient workability. For this reason,
In order to use these materials as wires for superconducting magnets, special processing is required, resulting in poor yield and high manufacturing costs. In addition, the manufactured wires were insufficient in terms of brittleness. On the other hand, niobium nitride (NbN) and niobium carbide (NbC) can be formed only in a film form using a sputtering method or the like, and it is extremely difficult to perform a processing such as wire drawing for use as a superconducting wire. Was.

The present invention has been made to solve the above-mentioned problems of the prior art, and an object of the present invention is to provide a method of manufacturing a superconducting material having a high critical temperature and excellent workability (manufacturing of a superconducting material having difficulty in processing) Method). Also,
Another object of the present invention is to provide a superconducting material having a high critical temperature and excellent workability.

[0005]

According to the present invention, there is provided a method for producing a superconducting material comprising niobium as a base material, wherein the niobium is placed in an atmosphere containing at least one of oxygen, nitrogen and carbon or in the atmosphere. And heating temperature 450 ° C ~ 150
There is provided a method for producing a superconducting material, characterized by performing a heat treatment at 0 ° C.

In the method for producing a superconducting material according to the present invention, niobium is used as a starting material, and the niobium is heated at a temperature of 450 ° C. to 1500 ° C. in an atmosphere or atmosphere containing at least one of oxygen, nitrogen and carbon. To process. At least one of niobium oxide, niobium nitride and niobium carbide is formed on the surface of the heat-treated niobium, and the heat-treated niobium becomes superconductive at about 30K. As described above, according to the present invention, a superconducting material exhibiting a high critical temperature can be manufactured by a simple process of heat treatment of niobium in a predetermined atmosphere.

In the present invention, the heating temperature is preferably from 450 ° C. to 1500 ° C., because it is possible to synthesize niobium oxides, nitrides, and carbides, and more preferably, the reason for increasing the rate of compound synthesis. From 700
C. to 1500C. When the heating temperature is lower than 450 ° C., the synthesis rate of the compound is slow and requires a long time. When the heating temperature is higher than 1500 ° C., the sublimation rate is large in the form of niobium oxide, resulting in material loss.

In the method for producing a superconducting material according to the present invention, the heat-treated niobium material may be rolled. In this case, since the electrical resistance measured along the rolling direction becomes zero at a higher temperature than the electrical resistance measured in the direction perpendicular to the rolling direction, the rolled niobium material is cut out along the rolling direction. Is preferred. As a result, the niobium material cut out along the rolling direction enters a superconducting state at a higher temperature than the niobium material cut out in a direction perpendicular to the rolling direction.

In the present invention, the atmosphere for the heat treatment is preferably an atmosphere containing at least one of oxygen, nitrogen and carbon, or the atmosphere. For example, O ₂ , N ₂ , C ₂
A one-component gas atmosphere such as O or CO ₂ , or N ₂ / O ₂ ,
A mixed gas atmosphere such as N ₂ / Ar, O ₂ / Ar, (N ₂ + O ₂ ) / Ar is preferable. It is preferable that the concentration (partial pressure) of oxygen, nitrogen and / or carbon in the gas atmosphere is higher.

In the method for producing a superconducting material according to the present invention,
Niobium used as a starting material is an impurity of oxygen (O) <10 ppm, nitrogen (N) <10 ppm, and carbon (C) <10 ppm because high purity increases environmental resistance and prevents diffusion into the inside. High purity niobium having a content is preferred.

According to the present invention, the niobium material is preliminarily processed into a predetermined shape in accordance with the purpose and application, and is heated to 450 ° C. in an atmosphere containing at least one of oxygen, nitrogen and carbon or in the atmosphere. By performing the heat treatment at a temperature of 1500 ° C., a superconducting material having a predetermined shape can be manufactured. For example, niobium as a starting material is processed into a linear shape having a desired thickness or a plate shape having a desired thickness, and is heated at 450 ° C. in an atmosphere or atmosphere containing at least one of oxygen, nitrogen, and carbon. When the heat treatment is performed at a temperature of about 1500 ° C., a linear or plate-shaped superconducting material having the same dimensions as before the heat treatment can be obtained. Therefore, the method for manufacturing a superconducting material of the present invention can solve the problem of workability of the superconducting material in the prior art.

According to the manufacturing method of the present invention, a superconducting material having a desired shape can be obtained through a heat treatment by processing niobium as a raw material into a desired shape. The obtained superconducting material can be used, for example, for applications such as superconducting magnets for high magnetic fields, and is particularly suitable for wires and shielding materials for superconducting magnets, as well as high-temperature corrosion-resistant materials for applications other than superconductivity. .

[0013]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be specifically described below with reference to the drawings, but the present invention is not limited thereto.

Example 1-1 First, a starting material was 100 mm in diameter and 350 m in length.
m high-purity niobium ingot was prepared. This niobium ingot was manufactured by the method of Japanese Patent Application Laid-Open No. 8-165528 by the present inventor. The impurity content of this high-purity niobium ingot is oxygen (O) <10 ppm,
Nitrogen (N) <10 ppm and carbon (C) <10 ppm. This high-purity niobium ingot is about 2 mm in size.
It cut out using the precision cutting machine so that it might become a 2x22mm column shape. The surface of the sample immediately after being cut out had a silver-white color showing a niobium metal color. The sample cut into a column shape was subjected to heat treatment at 450 ° C. for 1800 seconds in an air atmosphere. The surface of the sample after the heat treatment was a white-yellow powder. The surface of the sample after removing the white-yellow powder on the surface had a pale yellow color. As a result of analyzing the content of impurities in the sample after removing the powder, oxygen (O)
<10 ppm, nitrogen (N) <10 ppm, carbon (C) <
It was 10 ppm.

FIG. 1 shows an SEM image of the surface of the heat-treated sample.
(A) and (b). FIG. 1A is an SEM image of the most oxidized portion when observing the surface of the sample, and FIG. 1B is the SEM image of the least oxidized portion.
It is an EM image. From these figures, it can be seen that in the case of low-temperature oxidation of high-purity niobium material, the thickness of the oxidized portion on the crystal surface is 10 μm or more.
It was about 30 μm, indicating that the oxidized portion was very thin. In FIG. 1 (b), each vertical line indicates a trace of a line analysis when EPMA observation was performed, and the thickness of the oxidized portion was evaluated from a line analysis of niobium and oxygen and an SEM image.

FIG. 2A shows an SEM of a grain boundary of the same sample.
FIG. 2 (b) shows an enlarged image of the central part of the crystal grain boundary.
3 shows an EM image. When observing the grain boundary, it is considered that niobium is oxidized in a triangular shape from the surface to the inside along the grain boundary, and this grain boundary is considered to play a role of a pipe for oxygen diffusion. The oxygen diffusion depth was about 20 μm further from the surface portion of niobium. The oxidized portion was confirmed to be an oxide layer by EPMA line analysis and mapping. Therefore, in low-temperature oxidation of ultrahigh-purity niobium, diffusion of oxygen into the interior is considered to be largely affected by the grain boundary.

Next, the electric resistance of the heat-treated sample was measured. The method and conditions for measuring the electric resistance are described below. A normal four-terminal method was used to measure the electric resistance, and a polarity-reversal DC current was used to reduce the induced electromotive force induced by the measurement circuit and the atmosphere and improve the accuracy.
Note that a copper enamel wire having a diameter of 1 mm was used for the terminal, and the current value was set to 100 mA to 150 mA in order to minimize the influence of Joule heat. Further, in order to prevent Joule heat from being generated during the measurement of the electric resistance, the measurement was started after applying a current 5 seconds before the measurement. Further, in order to cancel the thermoelectromotive force, a process is performed in which a time interval of 25 seconds is provided between the measurement of the electric resistance at one point and the measurement of the electric resistance at the next point, and then the polarity is reversed and the current is supplied for 5 seconds. Was done. The temperature range measured was from room temperature to about 10K. FIG. 3 shows the results of the measurement of the electric resistance measured under the above conditions by white circles. In the figure, for comparison,
The electric resistance of the high-purity niobium material not subjected to the heat treatment is indicated by a white cross mark. In FIG. 3, the horizontal axis indicates the absolute temperature,
The vertical axis shows the resistance value measured at each temperature divided by the resistance value measured at room temperature and normalized. As can be seen from the figure, in the sample subjected to the heat treatment of this example, the electric resistance value sharply decreases around 30K. This seems to indicate that the sample started to be in a superconducting state from around 30K.

Observation of Changes in Oxidation Amount and Oxidation Rate with Heat Treatment Time Thirteen samples prepared by the same method as in Example 1-1 were prepared, and these thirteen samples were subjected to the same atmosphere and heating as in Example 1-1. Heat treatment was performed at the temperature. After a lapse of 30 minutes from the start of the heat treatment, one sample was taken out every 5 minutes, the amount of oxygen was measured for each sample taken out, and the oxidation rate was calculated. FIG. 4 shows the amount of oxygen with respect to the oxidation time (black circles in the figure).
And the results of the oxidation rate (black squares in the figure). In the figure, the horizontal axis represents time, the left vertical axis represents oxygen content,
The right vertical axis indicates the oxidation rate, respectively. As can be seen from the figure, the oxidation rate of the high-purity niobium material is high in the initial stage, and the oxidation rate decreases and becomes almost saturated after about 40 minutes. It can be understood that as the oxidation proceeds on the surface of the high-purity niobium material and the entire surface is covered with an oxide film to form a stable oxide layer, the internal supply of oxygen (= diffusion-controlled) becomes slower. it can. From this, it is presumed that the oxidation of the high-purity niobium material is progressing by supplying oxygen to the “oxide-niobium” interface through the oxide layer, and the oxidation of the high-purity niobium material is formed on the surface of the high-purity niobium material. This implies that there is a limit to the thickness of the oxide layer to be formed.

Example 1-2 Example 1 was repeated except that the heat treatment time was changed to 5400 seconds.
A sample was prepared in the same manner as in Example 1, and the obtained sample was measured for electrical resistance under the same conditions as in Example 1-1. The measurement result is shown by a white cross mark in FIG. FIG. 5 shows Example 1-
The measurement results of the sample No. 1 are also shown by white squares. As can be seen from FIG. 5, similarly to the sample of Example 1-1, the electrical resistance of the sample of the present example sharply decreased at around 30 K, and it was suggested that the sample began to become superconductive at about 30 K. It seems to be.

Example 1-3 A sample was prepared and heat-treated in the same manner as in Example 1-1 except that the heating temperature was changed to 450 ° C. Then, the electrical resistance of this sample was measured by the same method as in Example 1-1. As a result, also in this sample, a sharp decrease in the electrical resistance was observed at about 30 K, as in Example 1-1. 3
It seems that this suggests that the superconducting state has started from around 0K.

Example 1-4 A sample was prepared and heat-treated in the same manner as in Example 1-1 except that the heating temperature was 700 ° C. Then, the electrical resistance of this sample was measured by the same method as in Example 1-1. As a result, also in this sample, a sharp decrease in the electrical resistance was observed at about 30 K, as in Example 1-1. 3
It seems that this suggests that the superconducting state has started from around 0K.

Example 2-1 A high-purity niobium ingot was prepared as a starting material in the same manner as in Example 1-1. This high-purity niobium ingot,
Prior to warm forging, heat treatment was performed at 800 ° C. for 1 hour at 10 ⁻⁵ mbar, and immediately thereafter, 36 mm × 2 in air.
The sheet was warm-pressed into a 00 mm × 290 mm plate. Then, the plate-like high-purity niobium ingot was placed in a 2.5 mm ×
It was rolled until it became 400 mm x 2100 mm. Further, the rolled plate material is set to a predetermined size of 2.5 mm × 4.
It was cut out to a size of 00 mm × 400 mm and heat-treated at 800 ° C. for 1 hour at 10 ⁻⁵ mbar. And about the obtained board | plate material, the electric resistance of the direction parallel to the direction perpendicular to the rolling direction and the direction parallel to the direction were measured by the same method as Example 1-1. FIG. 6 shows the measurement results. In FIG. 6, a white square mark indicates a measurement result in a direction perpendicular to the rolling direction, and a white circle mark indicates a measurement result in a direction parallel to the rolling direction. As can be seen from FIG. 6, the electrical resistance measured in a direction parallel to the rolling direction is:
As in the case of Example 1-1, the value rapidly decreased at around 30K, which seems to indicate that the superconducting state started at around 30K. On the other hand, the electric resistance measured in the direction perpendicular to the rolling direction does not show a sharp decrease even at around 30K, and it seems that the superconducting state has not yet been reached at around 30K.

[0023]

According to the present invention, niobium, for example, high purity niobium having an impurity content of oxygen (O) <10 ppm, nitrogen (N) <10 ppm, and carbon (C) <10 ppm, is used to convert oxygen, nitrogen and carbon to high purity. A superconducting material having a high critical temperature can be manufactured by performing heat treatment at a temperature of 450 ° C. to 1500 ° C. in an atmosphere containing at least one of them or in the air. In particular, in the method of the present invention, high-purity niobium is previously processed into a predetermined shape, and only by performing the above-described heat treatment, a superconducting niobium having a predetermined shape can be obtained. Very useful for the production of superconducting materials or products.

[Brief description of the drawings]

FIG. 1 shows an SEM image of a surface of niobium subjected to a heat treatment, FIG. 1 (a) is an SEM image of a portion most oxidized during surface observation, and FIG. 1 (b) is a degree of oxidation. Is the SEM image of the least part.

FIG. 2 is an SEM image showing an oxidation state of a crystal grain boundary portion of the heat-treated niobium. FIG. 2 (a) is an SEM image of the crystal grain boundary portion, and FIG. 2 (b) is a crystal grain. 5 shows an SEM image in which a central portion of the boundary is enlarged.

FIG. 3 is a graph showing measurement results of electric resistance of niobium subjected to heat treatment in Example 1-1.

FIG. 4 is a graph showing the results of the amount of oxygen and the oxidation rate with respect to the oxidation time of niobium.

FIG. 5 is a graph showing the measurement results of electric resistance of niobium subjected to heat treatment in Example 1-2.

FIG. 6 is a graph showing measurement results of electric resistance of a niobium rolled in Example 2-1 in a direction parallel to a direction perpendicular to the rolling direction and a direction parallel to the rolling direction.

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁶ Identification code FI C22F 1/00 661 C22F 1/00 661A 682 682 691 691Z 1/02 1/02 C23C 8/20 C23C 8/20 8/24 8 / 24 8/28 8/28 30/00 30/00 C C23F 17/00 C23F 17/00 H01B 13/00 561 H01B 13/00 561Z

Claims (6)

[Claims] 1. A method for producing a superconducting material containing niobium as a base material, comprising: heating the niobium in an atmosphere containing at least one of oxygen, nitrogen and carbon or in an atmosphere at a heating temperature of 450 ° C. to 1 ° C.
A method for producing a superconducting material, comprising performing heat treatment at 500 ° C. 2. The method according to claim 1, wherein the niobium is a high-purity niobium having an impurity content of oxygen <10 ppm, nitrogen <10 ppm, and carbon <10 ppm. 3. The heating temperature is 700 ° C. to 1500 ° C.
The method for producing a superconducting material according to claim 1, wherein: 4. The method according to claim 1, further comprising rolling the heat-treated niobium material, and cutting out the rolled niobium material in a rolling direction. A method for producing the superconducting material according to the above. 5. A superconducting material produced by the method according to claim 1. 6. The superconducting material according to claim 5, wherein at least one of niobium oxide, niobium nitride and niobium carbide is formed on a surface.