JP7449763B2 - Oxide superconducting wire and method for producing oxide superconducting wire - Google Patents

Description

The present invention relates to an oxide superconducting wire and a method for manufacturing the oxide superconducting wire.

Patent Document 1 discloses an oxide superconductor in which voids are dispersed. When this oxide superconductor is subjected to oxygen annealing treatment, the oxygen diffusion rate increases due to the presence of voids, and the time required for the treatment can be shortened.

Japanese Patent Application Publication No. 10-92234

In the configuration of Patent Document 1, there is a problem that if the number of voids is too large, the cross-sectional area of the oxide superconductor decreases, and the critical current value decreases on the contrary.

The present invention has been made in consideration of these circumstances, and an object of the present invention is to provide an oxide superconducting wire that can be subjected to oxygen annealing treatment in a short time and that suppresses a decrease in critical current value.

In order to solve the above problems, an oxide superconducting wire according to a first aspect of the present invention includes a substrate, an intermediate layer laminated on the substrate, and an oxide superconducting wire laminated on the intermediate layer and containing a plurality of voids. The superconducting layer includes a superconducting layer formed of a physical superconductor and a protective layer laminated on the superconducting layer, and the void area ratio, which indicates the ratio of the area of the voids in the superconducting layer observed from the stacking direction, is 0. It is within the range of 8 to 3.7%.

According to the above aspect, since the void area ratio is 0.8% or more, good critical current characteristics can be obtained even if oxygen annealing is performed in a short time. Further, by setting the void area ratio to 3.7% or less, it is possible to suppress a decrease in the critical current value due to insufficient volume (cross-sectional area) of the oxide superconductor contained in the superconducting layer.

Here, a standard deviation of the area of the voids in the superconducting layer observed from the stacking direction may be 1618 nm ² or less.

In this case, since the variation in the size of each void is within an appropriate range, oxygen is uniformly distributed throughout the oxide superconductor, making it possible to further stabilize the critical current characteristics.

A method for producing an oxide superconducting wire according to a second aspect of the present invention includes the steps of: laminating an intermediate layer on a substrate; and applying a superconducting layer formed of an oxide superconductor including a plurality of voids to the intermediate layer by a vapor deposition method. and a step of laminating a protective layer on the superconducting layer, and the void area ratio, which indicates the ratio of the area of the voids in the superconducting layer observed from the stacking direction, is 0.8 to 0.8. The evaporation rate when laminating the superconducting layer is set so as to fall within the range of 3.7%.

According to the manufacturing method of the above embodiment, the void area ratio can be easily set within the range of 0.8 to 3.7% by adjusting the deposition rate of the superconducting layer. Therefore, it is possible to perform oxygen annealing treatment in a short time, and it is possible to manufacture an oxide superconducting wire in which a decrease in critical current value is suppressed.

According to the above aspect of the present invention, it is possible to provide an oxide superconducting wire in which oxygen annealing treatment can be performed in a short time and a decrease in critical current value is suppressed.

FIG. 1 is a cross-sectional view of an oxide superconducting wire according to the present embodiment. 1 is a transmission electron micrograph of a superconducting layer of a sample prepared in Experimental Example 1. 3 is a transmission electron micrograph of the superconducting layer of the sample prepared in Experimental Example 2. 3 is a transmission electron micrograph of the superconducting layer of the sample prepared in Experimental Example 3. 3 is a transmission electron micrograph of the superconducting layer of the sample prepared in Experimental Example 4. 3 is a transmission electron micrograph of the superconducting layer of the sample prepared in Experimental Example 5. 3 is a transmission electron micrograph of the superconducting layer of the sample prepared in Experimental Example 6.

Hereinafter, the oxide superconducting wire of this embodiment will be explained based on the drawings.
As shown in FIG. 1, the oxide superconducting wire 10 has a laminate 15 configured by laminating a substrate 11, an intermediate layer 12, a superconducting layer 13, and a protective layer 14 in this order.
The substrate 11 is a tape-shaped metal substrate. Specific examples of the metal constituting the metal substrate include a nickel alloy represented by Hastelloy (registered trademark), stainless steel, and an oriented Ni--W alloy in which a texture is introduced into a nickel alloy.

The intermediate layer 12 may have a multilayer structure, and may include, for example, a diffusion prevention layer, a bed layer, an alignment layer, a cap layer, etc. in the order from the substrate 11 side to the superconducting layer 13 side. These layers are not necessarily provided one by one; some layers may be omitted, or two or more layers of the same type may be repeatedly laminated. Intermediate layer 12 may be a metal oxide. By forming the superconducting layer 13 on the intermediate layer 12 having excellent orientation, it becomes easy to obtain the superconducting layer 13 having excellent orientation.

The superconducting layer 13 is made of an oxide superconductor. Examples of the oxide superconductor include RE-Ba-Cu-O-based oxide superconductors represented by the general formula REBa ₂ Cu ₃ O _y (RE123). Examples of the rare earth element RE include one or more of Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu. In the general formula of RE123, y is 7−x (oxygen vacancy amount). A plurality of voids exist in the superconducting layer 13. Details of the void will be described later.

The protective layer 14 has functions such as bypassing overcurrent that occurs during an accident and suppressing chemical reactions that occur between the superconducting layer 13 and a layer provided on the protective layer 14. Examples of the material constituting the protective layer 14 include silver (Ag), copper (Cu), gold (Au), or alloys containing one or more of these (eg, Ag alloy, Cu alloy, and Au alloy). The protective layer 14 may be composed of two or more metals or two or more metal layers. The protective layer 14 can be formed by a vapor deposition method, a sputtering method, or the like.

As shown in FIG. 1, a stabilizing layer 16 may be provided on the outer periphery of the laminate 15. The stabilizing layer 16 has functions such as bypassing overcurrent generated in an accident and mechanically reinforcing the superconducting layer 13 and the protective layer 14. As the material of the stabilizing layer 16, for example, copper can be used.

Next, an example of a method for manufacturing the oxide superconducting wire 10 will be described. Note that the manufacturing method described below is an example, and other manufacturing methods may be employed.

First, a substrate 11 is prepared.
Next, the intermediate layer 12 is laminated on the substrate 11. The intermediate layer 12 can be formed using a vapor deposition method such as an IBAD (Ion-Beam-Assisted Deposition) method, a PLD (Pulsed Laser Deposition) method, or an MOCVD (Metal Organic Chemical Vapor Deposition) method.
Next, a superconducting layer 13 is laminated on the intermediate layer 12. The superconducting layer 13 can be formed using a vapor deposition method such as an IBAD method, a PLD method, or an MOCVD method. At this time, the size and number of voids included in the superconducting layer 13 can be adjusted by changing the deposition rate. For example, increasing the deposition rate tends to increase the size and number of voids included in the superconducting layer 13. Conversely, if the deposition rate is reduced, the size and number of voids included in the superconducting layer 13 tend to decrease.

Next, a protective layer 14 is laminated on the superconducting layer 13. The protective layer 14 can be formed by sputtering or the like. Thereby, a laminate 15 is obtained.
Next, oxygen annealing treatment is performed. More specifically, the laminate 15 is heated to 300 to 500° C. in an oxygen atmosphere. By performing the oxygen annealing treatment, air can be introduced into the superconducting layer 13 and necessary critical current characteristics (critical current value, etc.) can be obtained. By including voids in the superconducting layer 13, the oxygen diffusion rate can be increased and the time required for oxygen annealing treatment can be shortened.
Next, a stabilizing layer 16 is formed around the outer periphery of the laminate 15, if necessary. The stabilizing layer 16 can be formed by plating or the like.

As described above, the method for manufacturing the oxide superconducting wire 10 of this embodiment includes the step of laminating the intermediate layer 12 on the substrate 11, and the step of depositing the superconducting layer 13 formed of the oxide superconductor including voids by a vapor deposition method. The method includes a step of laminating a protective layer 14 on the intermediate layer 12 and a step of laminating a protective layer 14 on the superconducting layer 13.

Next, a preferable ratio of voids included in the superconducting layer 13 will be explained using a specific experimental example.
As shown in Table 1, samples of oxide superconducting wire 10 in Experimental Examples 1 to 6 were prepared.

In Experimental Examples 1 to 6, Hastelloy tape was used as the substrate 11. An intermediate layer 12 was laminated on the substrate 11 using the PLD method. A superconducting layer 13 was laminated on the intermediate layer 12 using the PLD method. As the oxide superconductor constituting the superconducting layer 13, GdBa ₂ Cu ₃ O _y was used. The thickness of the superconducting layer 13 in the stacking direction was 2 μm. The above points are common to each of Experimental Examples 1 to 6.
Here, as shown in Table 1, Experimental Examples 1 to 6 have different "evaporation rates". The vapor deposition rate is the rate at which the thickness of the superconducting layer 13 increases due to vapor deposition. For example, in Experimental Example 1, the superconducting layer 13 was deposited so that its thickness increased at a rate of 5 nm per second.

For each of the prepared samples of Experimental Examples 1 to 6, a specimen for observation with a thickness of 100 nm was cut out from the central portion of the superconducting layer 13 in the stacking direction. The cut observation sample was observed using a transmission electron microscope (TEM) from the lamination direction of the superconducting layer 13, and a TEM image was taken in a predetermined field of view. The photographed TEM image was imported into image processing software "ImageJ", and the area of the void reflected in the TEM image was measured using the area measurement function of the image processing software. The "total void area" is the total area of all voids shown in the TEM image. "Void area standard deviation" is the degree of variation in the area of all voids shown in a TEM image, expressed as a standard deviation.

"Void area ratio" in Table 1 indicates the ratio of the total void area to the visual field area. For example, the sample of Experimental Example 1 had a total void area of 377.0 nm ² and a visual field area of 783535.6 nm ² . Therefore, the void area ratio of the sample of Experimental Example 1 was 377.0÷783535.6×100=0.05%. Note that the void area ratios for Experimental Examples 2 to 6 are shown to the first decimal place (rounded to the second decimal place), but the void area ratio for Experimental Example 1 is smaller than the others, so the values are shown to the second decimal place (rounded to the second decimal place). (rounded to the third decimal place).

As mentioned above, the "void area ratio" and "void area standard deviation" are calculated from an image obtained by cutting out an observation sample from a part of the superconducting layer 13 and observing a partial area of the cut out observation sample. are doing. However, according to the vapor deposition method, voids are formed almost uniformly in the oxide superconductor, so the values of "void area ratio" and "void area standard deviation" in Table 1 are based on the values of the sample prepared in the experimental example. This value is substantially the same as the "void area ratio" and "void area standard deviation" of the entire superconducting layer 13.

Note that the higher the deposition rate in the vapor deposition method, the larger the number of voids included in the superconducting layer 13, and the larger the size of each void tends to be. Therefore, as shown in Table 1, the higher the deposition rate, the higher the void area ratio. It is considered that the smaller the standard deviation of the pore area, the smaller the variation in the pore size, so that oxygen is evenly supplied throughout the oxide superconductor through the pores. On the other hand, it is considered that the larger the standard deviation of the pore area, the more uneven the oxygen supply through the pores becomes, making it difficult for oxygen to diffuse throughout the oxide superconductor. In the sample of Experimental Example 1, the number of voids observed was 1, so the void area standard deviation was set to 0.

2A to 2F are images of the superconducting layer 13 of each sample prepared in Experimental Examples 1 to 6, observed with a transmission electron microscope from the stacking direction. As shown in FIGS. 2A to 2F, the superconducting layer 13 includes an oxide superconductor 13a and a plurality of voids 13b. For example, it can be seen that in FIG. 2F (Experimental Example 6), the number of voids 13b is larger than in FIG. 2A (Experimental Example 1), and the area of each void 13b is also larger.

Ic/Icmax shown in Table 1 indicates the ratio of the critical current value Ic of each sample to the maximum critical current value Icmax. "Critical current value Ic" is the critical current value after each sample of Experimental Examples 1 to 6 was heated to 500° C. in an oxygen atmosphere and slowly cooled to room temperature over 10 hours. The "maximum critical current value Icmax" is determined by heating to 500°C in an oxygen atmosphere and slowly heating to room temperature over 100 hours so that oxygen is completely introduced into the superconducting layer 13 of each sample in Experimental Examples 1 to 6. This is the critical current value after cooling.

That is, the maximum critical current value Icmax is the maximum critical current that can be exhibited by the superconducting layer 13 in which oxygen has been completely introduced by subjecting each sample of Experimental Examples 1 to 6 to oxygen annealing over a sufficient period of time. It is a value. That is, it is the critical current value of the ideal oxide superconducting wire 10. The value of Ic/Icmax is an index for determining how close to the ideal oxide superconducting wire 10 the superconducting layer having voids is in a state when oxygen annealing is performed in a short time (10 hours). becomes. Note that the values of Ic and Icmax were measured using a four-terminal method with the sample in a low temperature state in liquid nitrogen.

As shown in Table 1, in the sample with a void area ratio of less than 0.1% (0.05% in Experimental Example 1), the value of Ic/Icmax was 70%. This means that a critical current characteristic of only about 70% of the ideal oxide superconducting wire 10 can be obtained. A possible cause is that the voids 13b included in the superconducting layer 13 were insufficient, and oxygen was not sufficiently distributed to the oxide superconductor during the short oxygen annealing process.

On the other hand, in samples with a void area ratio of 0.8 to 3.7% (Experimental Examples 2 to 5), the value of Ic/Icmax was 93% or more. This means that even if oxygen annealing is performed for a short time, critical current characteristics substantially equivalent to those of the ideal oxide superconducting wire 10 can be obtained. The reason why such results were obtained is that the value of the void area ratio was appropriate. In other words, this is considered to be because the presence of appropriate voids in the superconducting layer 13 promoted the supply of oxygen, and oxygen was sufficiently distributed in the oxide superconductor.

On the other hand, in the sample with a void area ratio of 5.6% (Experimental Example 6), the value of Ic/Icmax was 87%, which resulted in a decrease in critical current characteristics when the void area ratio was too large. A possible cause is that the volume (cross-sectional area) of the oxide superconductor contained in the superconducting layer 13 was insufficient.

From the above, the oxide superconducting wire 10 whose void area ratio, which indicates the area ratio of the voids 13b of the superconducting layer 13 observed from the stacking direction, is within the range of 0.8 to 3.7% can be produced in a short time. Even if oxygen annealing treatment is performed, it is possible to exhibit good critical current characteristics.
Further, as a manufacturing method for obtaining the oxide superconducting wire 10 as described above, the superconducting layer 13 is laminated by a vapor deposition method so that the void area ratio is within the range of 0.8 to 3.7%. It is recommended to set the deposition rate.

In addition, in the samples of Experimental Examples 2 to 5, the value of Ic/Icmax was 93% or more, and good results were obtained, but one reason for this was that the value of the standard deviation of the void area (420 to 1618 nm ² ) was not appropriate. There are many things that can be mentioned.
For example, if the standard deviation of the void area is too large, it is conceivable that the size of each void 13b will vary greatly and oxygen will not be uniformly distributed throughout the oxide superconductor 13a. Therefore, the value of the void area standard deviation is preferably 1618 nm ² or less, which is the maximum value in Experimental Examples 2 to 5.
In Experimental Examples 2 to 5, the minimum value of the standard deviation of the void area was 420 ^nm2 , but the smaller the standard deviation of the void area, the more uniform the size of each void 13b becomes, and the more uniformly oxygen spreads through the oxide superconductor 13a. Conceivable. That is, the value of the void area standard deviation may be smaller than 420 nm ² .

Therefore, the standard deviation of the area of the voids 13b of the superconducting layer 13 (standard deviation of void area) observed from the stacking direction is preferably 1618 nm ² or less. As a result, the variation in size of each void 13b is within an appropriate range, so that oxygen is uniformly distributed in the oxide superconductor 13a, making it possible to further stabilize the critical current characteristics.

Note that the technical scope of the present invention is not limited to the above embodiments, and various changes can be made without departing from the spirit of the present invention.

For example, even if conditions other than the void area ratio in the above embodiment are changed, it is considered that the same effect can be obtained as long as the void area ratio is within the range of 0.8 to 3.7%. Specifically, an oxide superconductor other than GdBa ₂ Cu ₃ O _y may be used as the superconducting layer 13, or the superconducting layer 13 may contain artificial pins. The intermediate layer 12 may be laminated by a method other than the PLD method. Alternatively, the laminate 15 may include layers other than the substrate 11, intermediate layer 12, superconducting layer 13, and protective layer 14.

In addition, the components in the above-described embodiments may be replaced with well-known components as appropriate without departing from the spirit of the present invention, and the above-described embodiments and modifications may be combined as appropriate.

10...Oxide superconducting wire 11...Substrate 12...Intermediate layer 13...Superconducting layer 13b...Void 14...Protective layer

Claims (3)

A substrate made of any one of nickel alloy, stainless steel, and oriented Ni-W alloy ;
an intermediate layer laminated on the substrate;
a superconducting layer laminated on the intermediate layer and formed of an oxide superconductor containing a plurality of voids;
a protective layer laminated on the superconducting layer,
An oxide superconducting wire having a void area ratio, which indicates a ratio of the area of the voids in the superconducting layer observed from the stacking direction, within a range of 0.8 to 3.7%. The oxide superconducting wire according to claim 1, wherein a standard deviation of the area of the voids in the superconducting layer observed from the stacking direction is 1618 nm2 or less. Laminating an intermediate layer on a substrate made of one of nickel alloy, stainless steel, and oriented Ni-W alloy ;
a step of laminating a superconducting layer formed of an oxide superconductor containing a plurality of voids on the intermediate layer by a vapor deposition method;
laminating a protective layer on the superconducting layer,
The evaporation rate when stacking the superconducting layer is set so that the void area ratio, which indicates the area ratio of the voids in the superconducting layer observed from the stacking direction, is within the range of 0.8 to 3.7%. A method for producing an oxide superconducting wire.

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