JPH05174646A - Manufacture of oxide superconductor - Google Patents

Abstract

PURPOSE:To realize an excellent superconductive characteristic in preferable reproducibility by reducing a residual carbon amount, and enhancing density of a superconductor. CONSTITUTION:Compounds not incorporating carbon, e.g. salt not including carbon such as nitrate and sulfate, halide, oxide and the like are used as starting materials including metal elements composing an oxide superconductor. The starting materials are mixed in a predetermined composition ratio, thus preparing material mixture powder. Calcination is applied to the material mixture powder as required, to mold it into a desired shape, followed by a heat treatment. Consequently, it is possible to obtain an oxide superconductor having a residual carbon amount of 0.03% or lower by weight.

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 an oxide superconductor having a high critical current density, particularly an oxide superconducting sheath wire.

[0002]

2. Description of the Related Art Since the discovery of La-based oxide superconductors in 1986, various oxide superconductors have been discovered one after another. Now, in Y-based oxide superconductors, it has become possible to realize superconducting magnetic levitation transport systems by using electronic devices centered on thin films and bulks with 211 phases finely dispersed to improve pinning force. .. In addition, in Bi-based oxide superconductors, it is easy to produce oriented crystals, so research into wire rods is progressing, and as an example of its application, a coil for high magnetic field generation was prototyped and relatively good results were obtained. Has been obtained. As described above, the performance of the oxide superconductor is surely improved for practical use.

By the way, as a starting material for producing an oxide superconducting wire, a compound powder such as an oxide or a carbonate, an oxalate coprecipitated powder or the like is generally used. Then, the starting materials as described above are mixed in a predetermined composition ratio,
After filling this mixed powder in a sheath material such as a silver pipe,
An oxide superconducting sheath wire is produced by firing the oxide superconductor. However, a large amount of carbon remains in the superconductor of the sheath wire produced in this way, and the current segregated at the grain boundaries hinders the current path, making it difficult to obtain a high critical current density.

Generally, when a bulk material is produced, even if a starting material containing carbon such as carbonate is used, the amount of residual carbon can be reduced because calcination and sintering are performed in an open system. Can be at least 0.10% by weight or less. Further, also in the case of producing a silver sheath wire, it is possible to reduce the amount of residual carbon to some extent by calcining the raw material powder in advance in an open system. However, in oxide superconducting wire, the final firing of the superconductor must be performed in a closed system, and since carbon does not decompose completely unless the temperature is higher than the crystallization temperature of the superconductor, firing is not performed. If the amount of carbon is not sufficiently reduced before performing, carbon tends to remain at the grain boundaries, and gas generated during sintering causes problems such as swelling and rupture of the sheath material. In particular, in a Bi-based oxide superconductor, the superconductor expands due to the effect of degassing due to carbon during firing at high temperature, and it is difficult to obtain a high-density fired body, and thus a high critical current density It was very difficult to produce the wire rods that they had.

[0005]

As described above, when an oxide superconducting wire is produced, since the superconductor is fired in a closed system, carbon is likely to remain in the superconductor. It was difficult to obtain a high critical current density because the residual carbon hinders the current path. Moreover, particularly in the Bi system, it is very difficult to manufacture a high-density fired body because the superconductor expands due to the effect of degassing such as carbon dioxide during firing at high temperature. As described above, the conventional method for producing an oxide superconducting wire has a problem that it is difficult to obtain a high critical current density because carbon easily remains in the superconductor and the density of the superconductor tends to decrease. Had.

Further, the problem regarding the residual carbon is not limited to the oxide superconducting sheath wire, and it is required to further reduce the residual carbon in the bulk material, for example.

The present invention has been made to address such a problem, and by reducing the amount of residual carbon and improving the density of the superconductor, it is possible to obtain excellent superconducting characteristics with good reproducibility. It is an object of the present invention to provide a method for producing the above oxide superconductor, particularly a method for producing an oxide superconducting wire.

[0008]

The method for producing an oxide superconducting wire of the present invention uses a compound containing no carbon as a starting material containing each constituent metal element of an oxide superconductor,
Characteristic is that these starting materials are mixed at a predetermined composition ratio to prepare a raw material mixed powder, and then this raw material mixed powder is heat-treated to produce an oxide superconductor having a residual carbon amount of 0.03% by weight or less. I am trying.

The starting material used in the production method of the present invention is not particularly limited as long as it is a compound containing no carbon, and for example, salts containing no carbon such as nitrates and sulfates, halides, An oxide or the like is used. From the viewpoint of reactivity, it is preferable to use nitrates or sulfates as at least a part of the starting materials.
It is also possible to use all of the starting materials as oxides, but in this case, it is preferable to promote the reaction in advance before the main firing.

By using the above-mentioned compound containing no carbon as a starting material, the amount of residual carbon after firing can be 0.03% by weight or less. Further, by setting the residual carbon amount to 0.03% by weight or less, not only is it possible to secure a sufficient current path, but also the gas amount generated during crystal growth of the superconductor is suppressed, so that the superconductor It is possible to increase the density of.

However, even if a compound containing no carbon is used, carbon may be contained as an impurity. Therefore, in the production method of the present invention, the following steps for reducing the amount of carbon are performed. After that, it is preferable to produce an oxide superconductor having a desired shape, for example, an oxide superconducting sheath wire.

That is, first, each starting material is weighed so as to have a desired composition, thoroughly mixed, and then pulverized well to make the particle diameter uniform to prepare a raw material mixed powder. For example, each starting material is dissolved in pure water, mixed with a hot stirrer or an evaporator, and dried. The sufficiently dried raw material mixed powder is pulverized and then sieved to have a uniform particle size. The particle size of the raw material mixed powder is preferably several μm or less.

When the raw material mixed powder thus produced is filled in the sheath material, it is desirable to pre-calcin at 750 ° C to 850 ° C. At this time, it is preferable to make the contact area as large as possible so that the raw material powder is well compatible with the atmosphere. Also, the calcination atmosphere needs to be set according to each superconductor, but air or oxygen is generally used. In particular, the amount of carbon can be significantly reduced in advance by calcination in oxygen. By passing through such a preliminary step, the amount of carbon in the raw material can be reduced to 0.03% by weight or less. Note that the calcination step can be omitted when manufacturing the bulk material. Rather, it may be desirable not to calcine in order to improve the density.

Next, the raw material mixed powder in which the amount of carbon is reduced in advance is molded into a target shape. For example, it is enclosed in a silver pipe, drawn, rolled, and the like to form a silver sheath wire or the like. After that, firing is performed in an atmosphere in which the oxygen concentration is controlled to obtain the target oxide superconductor. For example, in Bi-based oxide superconductors, the O ₂ concentration is 6%.
Bake for 10 to 100 hours in an atmosphere of ~ 10%. Here, degassing of NOx and the like due to nitrate decomposes at a much lower temperature than carbon dioxide, and therefore does not adversely affect the crystal growth of the superconductor. Therefore, the density of the superconductor can be increased. In addition, when manufacturing a sheath wire, in the process of drawing, 500 ℃ ~
By annealing at a temperature of 800 ° C, degassing from inside the sheath can be further promoted, and swelling and rupture during main firing can be prevented.

[0015]

In the method for producing an oxide superconductor according to the present invention, since a compound containing no carbo, such as nitrate or oxide, is used as a starting material, it is possible to avoid carbon contamination due to the material. It is possible to reduce the amount of carbon to some extent even if carbonate is used as the starting material, but in the case of sheath wire etc., firing is performed in a closed system, so the amount of carbon due to the raw material should be 0.03% by weight or less. Will be very difficult. Further, carbon is not completely decomposed unless the temperature is higher than the crystallization temperature of the superconductor, so that gas is generated at the crystal growth stage of the superconductor, which causes a decrease in the density of the superconductor.

In the oxide superconductor produced by the present invention, the amount of residual carbon can be extremely reduced to 0.03% by weight or less, and at the same time, the gas due to carbon is hardly generated during sintering. It is possible to prevent expansion of the superconductor, so that crystals grow and bond in a more dense state. In this way, it is possible to increase the density of Bi-based oxide superconducting wire, which was conventionally difficult to improve the density of the superconductor, and to reduce the amount of residual carbon to an extremely low level. can do. Therefore, it becomes possible to obtain an oxide superconducting wire having an extremely high critical current density.

[0017]

EXAMPLES Next, examples of the present invention will be described.

Example 1 An example of producing a silver sheath wire containing 2223 phase of Bi-based oxide superconductor in a volume fraction of 95% or more by the production method of the present invention will be described. First, as a starting material, Bi ₂ O ₃ ,
_{PbO, Sr (NO 3) 2} · 4H 2 O, Ca (NO 3) 2 · 4H 2 O,
Using Cu (NO ₃ ) ₂ · 3H ₂ O respectively, the starting composition is Bi
Weighed so as to be _1.8 Pb _0.4 Sr _2.0 Ca _2.0 Cu _3.0 O _x, and these were completely dissolved and mixed in 200 cc of pure water with a hot stirrer. After confirming that the mixture was sufficiently mixed, the temperature of the heater was raised to remove moisture at 95 ° C. and dried. The dried mixed powder was pulverized in a mortar and sieved to have a particle size of 2 μm or less, thereby obtaining a raw material mixed powder.

Next, outer diameter 7 mmφ, inner diameter 5 mmφ, length 200
A silver pipe of mm was filled with 8 g of the above raw material mixed powder. After that, wire drawing and wire drawing are repeated, and when the outer diameter reaches 1 mmφ, roll processing is performed and the final cross-sectional shape is 3 mm ×
It was a 0.1 mm tape-shaped silver sheath wire. This tape-shaped wire was heat-treated in an atmosphere with an O ₂ concentration of 7.7% under conditions of 835 ° C. for 50 hours to produce an oxide superconducting sheath wire.

The amount of residual carbon in the superconductor of the thus obtained sheath wire was measured by gas fusion analysis, and it was confirmed that the amount was 0.008% by weight, which was extremely low. Further, as a result of measuring the superconducting characteristics of the above Bi-based oxide superconducting sheath wire, the critical temperature T _c = 107.6K and the critical current density J _c = 4.6 × 10 ⁴ A / cm ² (77
K, 0T), which was a very high value.

Comparative Example 1 99.9% or more of Bi ₂ O ₃ , PbO and SrC were used as starting materials.
O ₃ , CaCO ₃ , and CuO powders were used, and these were weighed so that the starting composition was the same as in Example 1, mixed with a ball mill for 24 hours, and then mixed in the atmosphere at a temperature of 840 ° C. for 24 hours. I calcined for an hour.

The raw material mixed powder thus produced was filled in a silver pipe under the same conditions as in Example 1, and then processed and heat-treated under the same conditions as in Example 1 to produce an oxide superconducting sheath wire. The amount of residual carbon in the superconductor of the sheath wire thus obtained was measured by gas fusion analysis and found to be 0.044% by weight.
It became a higher value. As a result of measuring the superconducting characteristics, T _c = 106.4 K and J _c = 2.2 × 10 ⁴ A / cm ² , which were low values as compared with Example 1.

Further, when the densities of the respective superconductors of Example 1 and Comparative Example 1 were compared by SEM observation, the density of the oxide superconducting sheath wire of Example 1 was higher than that of the wire prepared in Comparative Example 1. High and dense crystals were obtained.

Example 2 Using a nitrate as a starting material for all metallic elements, Bi system 22
An example of producing a silver sheath wire rod having a volume fraction of 12 phases of 99% will be described. As each of the starting _{materials, Bi (NO 3) 3 ·} 5H 2
_{O, Pb (NO 3) 2} · 4H 2 O, Ca (NO 3) 2 · 4H 2 O, C
u (NO ₃ ) ₂ 3H ₂ O was used. These were weighed so that the starting composition was Bi _2.0 Sr _2.0 Ca _1.0 Cu _2.0 O _x , completely dissolved in 200 cc of water containing 20 cc of nitric acid, thoroughly mixed with a hot stirrer, and dried. .. The obtained mixed powder is crushed in a mortar and the particle size is 2μ.
It was sieved so as to be not more than m.

The raw material mixed powder obtained as described above was treated with an outer diameter of 7 mm.
After filling 8g of silver pipe with φ, inner diameter 5mmφ and length 200mm, wire drawing and wire drawing processes were repeated, and when the outer diameter became 1mmφ, roll processing was performed. By this roll processing, when the thickness became 0.8 mm, the intermediate annealing was performed at 700 ° C. for 24 hours. After that, further roll processing was performed to obtain a tape-shaped wire rod having a final cross-sectional shape of 3 mm × 0.1 mm. This tape-shaped wire was heat-treated at a temperature of 845 ° C. for 50 hours in an atmosphere having an O ₂ concentration of 7.7% to produce an oxide superconducting sheath wire.

When the amount of residual carbon in the superconductor of the sheath wire thus obtained was measured, it was suppressed to a very low value of 0.006% by weight. Further, as a result of measuring the superconducting characteristics, high characteristics such as T _c = 813K and J _c = 3.2 × 10 ⁴ A / cm ² were obtained.

Comparative Example 2 As starting materials, 99.9% or more of Bi ₂ O ₃ , PbO and SrC were used.
Powders of O ₃ , CaCO ₃ and CuO were used, weighed so that the starting compositions were the same as in Example 2, and mixed in a ball mill for 24 hours, then, in the atmosphere at a temperature of 840 ° C. for 24 hours. It was calcined for a time to obtain a raw material mixed powder. An oxide superconducting sheath wire was produced under the same conditions as in Example 2 except that this raw material mixed powder was used and intermediate annealing during the roll processing was omitted. The amount of residual carbon in this superconductor was 0.061% by weight, and the superconducting properties were T _c = 79.2K and J _c = 1.6.
The result was × 10 ⁴ , and only low values were obtained as compared with Example 2.

Example 3 Using an oxide as a starting material for all metallic elements, a Bi system 22
An example of producing a 23-phase silver sheath wire will be described.

Bi ₂ O ₃ , PbO, SrO, CaO, and CuO powders were used, and the starting compositions of these powders were Bi _1.84 Pb _0.34 Sr _1.91 Ca.
Each of them was weighed so as to have _2.03 Cu _3.06 O _x , dissolved in 200 cc of pure water, sufficiently mixed, and then dried by an evaporator. The obtained mixed powder is crushed in a mortar and the particle size is 1μ.
It was sieved so as to be not more than m. Then, this mixed powder was calcined in the atmosphere at 840 ° C. for 24 hours to obtain a superconductor powder.

The superconducting powder obtained as described above was applied to an outer diameter of 7 mm.
After filling 8g of silver pipe of φ, inner diameter 5mmφ, length 200mm, wire drawing and wire drawing processes were repeated, and when the outer diameter became 1mmφ, roll processing was performed and the final cross-sectional shape was 3mm ×
0.1mm tape wire was used. Then, this tape-shaped wire was heat-treated at a temperature of 835 ° C. for 50 hours in an atmosphere having an O ₂ concentration of 7.7% to produce an oxide superconducting sheath wire. The residual carbon of the superconducting wire is reduced to 0.015 wt%, it exhibited a T _c = 107.8K, very high characteristics and ^{_{J c = 4.2 × 10 4 A}} / cm 2.

Comparative Example 3 The oxalate coprecipitated powder adjusted to have the same composition as in Example 3 was mixed in a ball mill for 24 hours and then calcined in the air at 840 ° C. for 24 hours. An oxide superconducting sheath wire was produced under the same conditions as in Example 3 except that this calcined powder was used. The amount of residual carbon in this superconductor was 0.063% by weight, and the superconducting properties were T _c = 102.1K and J _c = 1.3 × 10 ⁴ , which were lower than those of Example 3.

The results of Examples 1 to 3 and Comparative Examples 1 to 3 are summarized in Table 1.

[0033]

[Table 1] Example 4 An example in which a Tl-based superconducting wire is manufactured by using nitrate for all the constituent elements will be described. Starting material is Tl (NO ₃ )
_{2, Ba (NO 3) 2} , Ca (NO 3) 2 · 4H 2 O, Cu (N
O ₃ ) ₂ · 3H ₂ O was used. These were weighed so that the starting composition was Tl _1.9 Ba _2.1. Ca _2.0 Cu _3.0 O _x , completely dissolved in 200 cc of pure water, thoroughly mixed with a hot stirrer, and dried. The obtained mixed powder was crushed in a mortar and sieved so that the particle size was 2 μm or less.

An outer diameter of 7 mmφ and an inner diameter of 5
After filling 8g of silver pipe with mmφ and length of 200mm, wire drawing and wire drawing processes were repeated, and when the outer diameter became 1mmφ, roll processing was performed to produce tape-shaped wire with final cross-sectional shape of 3mm × 0.1mm. .. This tape-shaped wire is in an oxygen flow
Heat treatment was performed at 870 ° C for 10 hours to prepare a Tl-based oxide superconducting sheath wire.

The residual carbon content in the superconductor of the sheath wire thus obtained was reduced to 0.009% by weight, and the superconducting characteristics were T _c = 106.3K and J _c = 3.7 × 10 ⁴ A / cm ^2. It showed a high value.

Comparative Example 4 Tl ₂ O ₃ , BaCO ₃ , CaCO ₃ , and CuO powders were used as starting materials, and they were weighed so as to have the same composition as in Example 4 and mixed sufficiently. After filling 8 g of the obtained powder into a silver pipe with an outer diameter of 7 mmφ, an inner diameter of 5 mmφ and a length of 200 mm, wire drawing and wire drawing processes were repeated, and when the outer diameter became 1 mmφ, roll processing was performed to obtain the final cross-sectional shape. 3mm x 0.1mm
Was used as a tape-shaped wire. This tape-shaped wire has an O ₂ concentration of
Heat treatment at 870 ℃ for 10 hours in 7.7% atmosphere,
An oxide superconducting sheath wire was produced. The residual carbon content in this superconductor was 0.056% by weight, and the superconducting property was T _c = 1.
The value was 24.5K, J _c = 1.2 × 10 ⁴ A / cm ² , and the value was lower than that in Example 4.

Example 5 Similar to the Tl-based silver sheath wire according to Example 4, Tl-based 22
A Tl-based oxide superconductor sheath wire containing 12 phases in a volume fraction of 99% or more was prepared. The starting composition is Tl _1.9 Ba _2.1.
It was Ca _1.0 Cu _2.0 O _x . The amount of residual carbon in this superconductor is reduced to 0.008% by weight, and the superconducting property is T
_c = 106.2K, showed an extremely high value of ^{_{J c = 4.3 × 10 4 A}} / cm 2.

Comparative Example 5 Comparative Example 4 was repeated except that the same starting materials as in Comparative Example 4 were weighed and mixed so as to have the same composition as in Example 5.
Under the same conditions as above, a Tl-based 2212 phase oxide superconducting sheath wire was prepared. The amount of residual carbon in this superconductor was 0.062% by weight, and the superconducting characteristics were T _c = 104.0K and J _c = 1.8 × 10.
^The value was ⁴ A / cm ² , which was lower than that in Example 5.

The results of Examples 4 to 5 and Comparative Examples 4 to 5 are summarized in Table 2.

[0040]

[Table 2]

[0041]

As described above, according to the method for producing an oxide superconductor of the present invention, a carbon-free compound is used as a starting material and the residual carbon amount is 0.03% by weight.
Because of the following, it is possible to extremely reduce the amount of carbon that is believed to remain at the grain boundaries and to suppress the influence of gas generated due to carbon. Therefore, expansion of the superconductor is suppressed, and an extremely high density oxide superconductor can be produced by sintering. These facilitate the production of a superconductor having a high critical current density. Thus, according to the present invention,
It is possible to stably provide a practical oxide superconductor having extremely high J _c and I _c .

[0042]

Claims (1)

[Claims] 1. A carbon-free compound is used as a starting material containing each constituent metal element of an oxide superconductor, and these starting materials are mixed in a predetermined composition ratio to prepare a raw material mixed powder. Heat treatment of the raw material mixed powder,
A method for producing an oxide superconductor, which comprises producing an oxide superconductor having a residual carbon content of 0.03% by weight or less.