JP2000247795A - Production of re123 oxide superconductive bulk body - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a simplified and stabilized production process for a large sized bulk body of high orientation by decomposing and melting raw materials for the bulk body at a temperature higher than the decomposition and melting point of an oxide to be prepared, then reducing the temperature lower than the decomposition and melting point and sowing a seed crystal to the melt. SOLUTION: The bulk body of RE123 oxide superconductor that is represented by a general formula: REBa2Cu3Ox (RE is one or two or more kinds of rare earth elements) and has a 123 phase crystal structure is prepared by melting and coagulating method. In this preparation, raw materials (or their precursors) for the bulk body are held at a temperature higher than the decomposition and melting point of the 123 phase of REBa2Cu3Ox oxide to be prepared. Then, the molten product is cooled down below the crystallization temperature. By utilizing the time until the crystallization is initiated, seed crystals are brought into contact with the treated raw materials and they are kept at the crystal growth temperature or they are slowly cooled down along a temperature gradient, thereby obtaining the objective bulk body of RE123 oxide superconductor mainly comprising the RE123 phase from the seed crystals.

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 large-sized oriented RE123-based oxide superconducting bulk material by a melt-solidification method, and in particular, a "crystal of the same kind as the bulk material to be produced" can be used as a seed crystal. The present invention relates to a method for stably manufacturing an RE123-based oxide superconducting bulk body capable of simplifying all manufacturing steps.

In recent years, oxide superconducting materials having a high critical temperature (Tc) have been discovered one after another, and practical application to various fields such as strong superconducting magnets, flywheels, and superconducting magnetic bearings has been studied.

However, in order for a superconducting material to be put to practical use in these fields, it is important to show a high critical current density (Jc) in addition to a high critical temperature (Tc), which is practically advantageous. Considering use at a liquid nitrogen temperature, among superconducting materials known at present, “REBa ₂ Cu ₃ O _x -based oxide superconducting material (RE is a rare earth element and Y, La, Pr, N
One or more of d, Sm, Eu, Gd, Dy, Ho, Er, Tm and Yb) ”has achieved high critical current density in a magnetic field by development and improvement of its manufacturing technology. And
It is considered to be one of the most noticeable superconducting materials.

[0004]

2. Description of the Related Art In order to achieve a high critical current density (Jc) in an oxide superconductor, a) improvement in crystallinity, b) improvement in crystal grain orientation, c) crystal grain boundaries, impurity phases, and vacancies. It is considered necessary to remove weak bonds due to holes, cracks, etc., d) control the amount of oxygen in the superconducting phase, e) densify, f) introduce flux pinning points. As one of the suitable methods for taking this measure, a technique for producing a superconductor crystal by a “melt solidification method” has been attracting attention. According to this melting and solidification process, in addition to improving the control and compactness of the crystal orientation is relatively easy, it is also advantageous to introduce the magnetic flux pinning centers, REBa ₂ Cu ₃ O _x based oxide It has achieved great results in increasing the magnetic field current density of superconductors.

[0005] The above-mentioned "melt solidification method" is a process utilizing solidification from a semi-molten state, and is generally a process for producing a bulk body in which the composition of an RE123-based oxide superconductor is obtained in the air or in a low oxygen partial pressure atmosphere. R to make the raw material
After elevating the temperature of the 123 phase of EBa ₂ Cu ₃ O _x oxide (hereinafter referred to as RE123 phase) to the temperature at which it decomposes and melts (generally, the peritectic point) and melt it, it is subjected to a temperature gradient or isothermal field. In this method, bulk crystals are grown by slow cooling slowly or by maintaining the supercooled temperature range in which the RE123 phase can grow.
It is relatively easy to prepare a 123 crystal, and a fine RE-Ba-Cu-O oxide 211 phase (R
When E is La and Nd, the phase is 422.
211 phases) can also be dispersed. RE12
When the fine RE211 phase is dispersed in the three crystal grains, this acts as a magnetic flux pinning point, so that the critical current density (Jc)
Is improved.

[0006] The RE formed in the RE123 crystal grains
In the case where the 211 phase particles are positively used as a pinning point to increase the critical current density, it is necessary to finely disperse the RE211 phase particles. It is also known that the addition of a small amount (approximately 0.1 to 2% by weight) of an element such as Pt, Rh or Ce to the fine dispersion of the RE211 phase particles is effective. Further, in order to prevent the occurrence of cracks in the bulk oxide superconductor and improve the mechanical strength, the addition of an Ag element (10 to 30
(About% by weight) is known to be effective, but there is a report that addition of an Ag element is also effective for improving the critical current density.

In the precursor in a semi-molten state, since a large amount of the RE211 phase is present as a second phase in the liquid phase, when growing the RE123 phase crystal, R2 is added on the second phase.
Since a plurality of nuclei are generated in the E123 phase and these tend to be nuclei to cause crystal growth, it is difficult to obtain oriented single crystal grains. It is widely practiced to use a “seed crystal” to control the starting point of growth and the crystal orientation. Note that, as the seed crystal, a crystal mainly including the RE123 phase having the same crystal structure as the RE123 phase crystal to be manufactured is often used.

Conventionally, this seed crystal is placed on the surface of a bulk material (bulk precursor) before heating as shown in FIG. 1 (method 1) or the bulk material (bulk precursor) is heated by heating. When the body reaches its maximum temperature, it is placed on its surface (method 2) and then slowly cooled (see Fig. 1 (a)).
Alternatively, it has been used as a growth starting point of oriented crystals in the process of maintaining a supercooled region (see FIG. 1B). Therefore, the seed crystal has not only the same crystal structure as the RE123 phase crystal to be produced, but also the “RE123 phase” having a higher decomposition temperature than the RE123 phase crystal to be produced so as not to be eliminated during the process. A crystal as the main phase "is used. For example, a Y123 phase crystal (decomposition temperature: about 1010
° C), Sm1 with a higher decomposition temperature
23-phase crystals (decomposition temperature: about 1060 ° C.), Nd123 phase crystals (decomposition temperature: about 1090 ° C.), and the like are often used as seed crystals.

[0009]

As described above, when a RE123-based superconducting bulk body is produced by a melt-solidification method using a seed crystal, a RE123 phase composition having a high decomposition temperature, which is different from that of the crystal to be produced, is required. Holding oriented seed crystals ”
Must be prepared separately. Therefore, the number of overall steps for fabricating the desired superconducting bulk body increases, and a great disadvantage is imposed in terms of time and cost. RE1 having a higher decomposition temperature than Nd123 phase crystals
Since there are no 23-phase crystals, in order to produce a bulk body of Nd123-phase crystals by the melt-solidification method, it is necessary to use another seed crystal material having a different crystal structure, such as a MgO single crystal. However, the growth of a bulk Nd123 phase crystal using a MgO seed crystal lacks certainty, and has not been sufficiently satisfactory as a stable production technique for a large bulk body.

In view of the above, an object of the present invention is to provide an R-phase having a RE123 phase as a main phase by a melt-solidification method.
In producing the “E123-based oxide superconducting bulk material”, “a crystal of the same type as the RE123 phase crystal of the bulk material to be produced” or “RE1 of the bulk material to be produced” is used as a seed crystal.
A crystal having the same structure as the 23-phase crystal and having the same decomposition temperature "can be used, thereby simplifying the entire manufacturing process, and stably producing a large-sized bulk body having a desired orientation. The aim was to establish a method that would allow them to be raised.

[0011]

Means for Solving the Problems As a result of intensive studies to achieve the above object, the present inventor has obtained the following findings. a) After the raw material (precursor) for producing the RE123-based oxide superconducting bulk is decomposed and melted in accordance with the process of the melt-solidification method, the decomposed and molten raw material is heated to a temperature below the temperature at which RE123 phase crystals can be crystallized (generally, the peritectic point). Even if it is kept, the crystal growth does not start immediately if there is no seed crystal. B) Therefore, the crystal growth from the raw material (precursor) in the semi-molten state which has been cooled to the crystallization temperature of the RE123 phase crystal or less. Utilizing the grace period until the start of, the seed crystal is brought into contact with the raw material (precursor) in the semi-molten state for the first time at this time, and then the crystal growth temperature that has been conventionally employed is maintained, Alternatively, when slowly cooled under a temperature gradient or in an isothermal field, even in this case, a large RE123-based oxide superconducting bulk material having the same orientation as the seed crystal having the RE123 phase as the main phase and having the same orientation as that of the seed crystal is more stable. Can be bred,

C) Therefore, according to this method, it is not always necessary to use a seed crystal having a high decomposition temperature,
An oriented crystal having the same composition as the RE123 phase of the bulk oxide superconductor to be produced can be used as a seed crystal. D) Therefore, a “step of growing a crystal of another composition” for preparing the seed crystal Can be omitted, and
In growing a RE123 crystal (for example, Nd123 crystal or the like) having a high generation temperature, it is not necessary to use a MgO single crystal or the like having a different crystal structure, which is inferior in reliability, as a seed crystal.

The present invention has been made on the basis of the above findings and the like, and provides a "method for producing a RE123-based oxide superconducting bulk material" described in the following items 1) to 5). 1) The chemical composition is represented by the general formula REBa ₂ Cu ₃ O _x (RE is a rare earth element and Y, La, Pr, Nd, Sm, Eu, Gd, Dy, Ho, E
r, one or more of Tm and Yb: hereinafter the same) and having a 123-phase crystal structure.
In producing a 123-based oxide superconducting bulk body by a melt-solidification method, first, a bulk body producing raw material is “RE
After decomposing and melting at a temperature higher than the temperature at which the “123 phase of Ba ₂ Cu ₃ O _x oxide decomposes and melts,” the 123 phase crystal is cooled in a temperature range lower than the decomposition melting temperature while the temperature is lowered. A method for producing a RE123-based oxide superconducting bulk material, comprising: bringing a seed crystal having a structure into contact with its surface for seeding, and subsequently growing the crystal at a crystal growth temperature. RE-Ba which further contains one or more of Pt, Rh, Ce and Ag which are characteristic improving elements in addition to the one represented by ₂ Cu ₃ O _x , and has a 123-phase crystal structure. in the -cu-O-based oxide superconducting bulk body prepared by melting and solidification method, first holding a bulk body prepared raw material to a temperature above the temperature at which is incongruent melting "Preparation for REBa ₂ Cu ₃ O _x 123 phase oxide" After decomposing and melting, while lowering the temperature, A RE123-based oxide superconducting bulk material characterized in that a seed crystal having a 123-phase crystal structure is brought into contact with its surface in a temperature range lower than the melting temperature to seed the crystal, and then the crystal is grown at a crystal growth temperature. 3) The method for producing a bulk RE123-based oxide superconductor according to the above item 1) or 2), wherein the seed crystal is preheated and used at the time of seeding. The RE123-based oxide superconducting bulk material according to any one of the above 1) to 3), wherein a crystal having the same chemical composition as that of the “REBa ₂ Cu ₃ O _x oxide bulk material to be produced” is used. 5) In seeding, the seed crystal supported by the wire of “pure metal or alloy having a melting point equivalent to the seeding temperature” is brought close to the seeding surface, and the seed crystal is melted by the temperature near the seeding surface. Seeding by dropping on the seeding surface Characterized the door, said 1) to 4) The method for manufacturing a RE123-based oxide superconducting bulk body according to any one of items.

Here, "RE123" applied to the method of the present invention.
As a starting material for producing a bulk oxide superconducting body, it is preferable to use a mixed powder obtained by mixing oxides of RE, Ba and Cu or a carbonate compound or the like in the usual manner. The raw material powder mixed so that the ratio of Cu becomes a predetermined ratio is fired at a temperature of 800 to 1000 ° C. to be a precursor, and the precursor powder is obtained by simultaneously firing the above three elements. For example, a mixture of Ba and Cu oxides or carbonate compounds, etc.
And the like obtained by mixing oxides of the above. The obtained precursor powder is further subjected to molding by, for example, a uniaxial press or the like, and is subjected to a melt-solidification process.

As described above, elements such as Pt, Rh, and Ce have the effect of finely dispersing the RE211 phase particles generated in the RE123 phase crystal to improve the critical current density. Both are effective property improving elements, such as contributing to the improvement of the mechanical strength and critical current density of the oxide superconducting bulk material to be obtained. Therefore, if necessary, one or more of them may be used as a precursor powder. It is added and mixed.
In this case, the form of the element to be added (additive form) may be any of metal fine powder, oxide powder and the like. The addition and mixing may be performed before or after firing the precursor powder.

Hereinafter, the method of the present invention will be described in more detail together with its operation and effects. Now, in the melt-solidification process, first, the molded body (precursor) is heated to a temperature higher than the decomposition temperature of the “RE123 phase to be produced” and is decomposed and melted. Next, as shown in FIG.
The seeding is performed while the temperature is lowered to a temperature lower than the decomposition temperature of the "23 phase". In view of the easiness of the processing operation, the seeding is preferably performed after the seeding temperature is maintained at a predetermined seeding temperature. The “seed crystal” used for seeding may be RE123.
As long as it has a phase crystal structure may be different in chemical composition, but as described above, to produce "capable simplification of the whole process REBa ₂ Cu ₃ O _x oxide bulk body The use of "crystals of the same chemical composition" offers significant benefits.

What is important during this seeding is the temperature of the seed crystal to be brought into contact with the precursor in the semi-molten state. When the temperature of the seed crystal is low, a plurality of nuclei are generated in the precursor in a semi-molten state due to a decrease in the temperature of the precursor due to contact with the seed crystal, and a desired uniformly oriented RE123 phase crystal cannot be grown. Therefore, the seed crystal should be preheated to the vicinity of the seeding temperature at the time of seeding. However, if the size of the seed crystal is small, its heat capacity is also small, so the influence of heat removal is small,
Therefore, in this case, preheating of the seed crystal is not necessarily required.

In order to seed the precursor surface in a semi-molten state, as shown in FIG. 3, for example, an Ag wire or an Ag-Pd alloy wire, an Ag-Pt alloy wire, or the like has a melting point corresponding to the seeding temperature. Suspend the seed crystal with a wire or strip of pure metal or alloy,
It is preferable that the seed crystal is dropped on the surface of the precursor by melting the wire and brought into contact with the precursor as shown in FIG. Here, the alumina rod shown in FIG. 3 is a holder for the wire, and the thermocouple is for measuring the temperature of the wire portion. According to this method, accurate control of the seeding timing becomes possible.

After the seeding, the precursor in the semi-molten state is maintained at a predetermined crystal growth temperature, which is a supercooled region, or is slowly cooled slowly in this crystal growth temperature region, and the seed crystal is started from the seed crystal. A RE123 crystal having the same orientation is grown.

As described above, after the precursor of the RE123-based oxide superconducting bulk material is decomposed and melted, the temperature is lowered, and R
If seeding is performed in a temperature range lower than the decomposition melting temperature of the E123 phase, it is not necessary to use a seed crystal having a high decomposition temperature as a seed crystal. Since it can be used as a seed crystal, the manufacturing process of the bulk oxide superconducting body can be significantly simplified. In addition, among the RE123 phases, the Nd123 phase having the highest decomposition temperature has a main phase of Rd123.
In producing an E-Ba-Cu-O-based oxide superconducting bulk, the Nd123 phase crystal can be used as a seed crystal as it is, so that the production stability of the superconducting bulk is greatly improved.

Furthermore, even in the case where a "crystal having a composition different from the RE123 phase of the bulk oxide superconductor to be produced" is used as the seed crystal in the method of the present invention, the seeding is sufficiently low in the method of the present invention. Since it is performed at a temperature, the seed crystal dissolves in the liquid phase of the precursor, and the desired uniformly oriented crystal is not generated, or the impurity element due to the seed crystal is mixed into the crystal, and the characteristics are deteriorated. In addition, it is possible to avoid disadvantages that cannot be avoided in the conventional method.

Next, the present invention will be described more specifically with reference to examples.

[Example 1] First, powders of yttrium oxide, barium carbonate and cupric oxide were prepared by mixing Y: Ba: Cu = 1.8: 2.
The mixture was mixed at a ratio of 4: 3.4, calcined at 900 ° C., and further pulverized and mixed to obtain YBa ₂ Cu ₃
The O _x oxide superconductor material of the precursor powder was produced. Next, this precursor powder was molded by a uniaxial press to obtain a molded precursor having a diameter of 40 mm and a thickness of about 10 mm.

Next, the molding precursor is subjected to 115 in an electric furnace.
After heating at 0 ° C. for about 1 hour to decompose and melt, the temperature is lower than the decomposition temperature of the Y123 phase (about 1010 ° C.).
The temperature was lowered to 5 ° C. When the temperature in the furnace is lowered to 1005 ° C., an oriented Y123 phase seed crystal (a 1.5 mm square, 1 mm thick crystal) having the same composition as the YBa ₂ Cu ₃ O _x oxide superconducting material to be prepared is formed. Using a Pt wire, the precursor was directly placed on the surface of the semi-molten precursor from the upper part of the electric furnace without preheating.

Thereafter, the temperature in the furnace was fixed at 1000 ° C. and maintained for 300 hours, and then gradually cooled to 970 ° C. at a cooling rate of 1 ° C./h. YBa ₂ Cu ₃ O composed of oriented Y123 crystals
_x oxide superconducting bulk material could be grown.

Example 2 Y: Ba: Cu = 1.2: 2.1: powder of yttrium oxide, barium carbonate and cupric oxide
The mixture was mixed at the ratio of 3.1, calcined at 900 ° C, pulverized and mixed. Then, 15% by weight of silver oxide powder and 0.5% by weight of Pt powder were added to the calcined powder and mixed. Thus, a precursor powder of the YBa ₂ Cu ₃ O _x oxide superconducting material was produced. Next, this precursor powder was formed by a uniaxial press to obtain a formed precursor having a diameter of 40 mm and a thickness of about 15 mm.

Next, the molding precursor is subjected to 115 in an electric furnace.
After heating at 0 ° C. for about 1 hour to decompose and melt, the temperature was lowered to 965 ° C., which is lower than the decomposition temperature of the Y123 phase when Ag was added (about 970 ° C.). At this time, when the temperature in the furnace falls to 970 ° C. or less, the Y
Oriented Y12 with the same composition as Ba ₂ Cu ₃ O _x oxide superconductor
A three-phase seed crystal (crystal having a size of 8 mm square and a thickness of 3 mm) was coated on an Ag wire (melting point: 960.5) attached to the tip of an alumina rod as shown in FIG.
° C) and inserted from the upper part of the electric furnace. And
The inserted seed crystal was held just above the surface of the central part of the precursor in a semi-molten state, and was preheated. Subsequently, the indicated temperature of the thermocouple attached to the seed crystal (see FIG. 3) is the melting point of Ag.
After confirming that the temperature reached 960.5 ° C. or higher, the alumina rod was pulled out of the furnace. As described above, the fact that the indicated temperature of the thermocouple becomes equal to or higher than the melting point of Ag means that the seed crystal is dropped on the surface of the precursor by the dissolution of the Ag wire and seeding is performed.

After keeping the seeded seed at a furnace temperature of 962 ° C. for 200 hours, the precursor was gradually cooled at a cooling rate of 1 ° C./h for 15 hours.
Thus, it was possible to grow an Ag-containing YBa ₂ Cu ₃ O _x oxide superconducting bulk composed of oriented Y123 crystals having a thickness of about 12 mm.

Embodiment 3 Nd: Ba: Cu = 1.6: 2.8: 3.3 was prepared by mixing powders of neodymium oxide, barium carbonate and cupric oxide.
, And calcined at 900 ° C., and further pulverized and mixed to prepare a precursor powder of an NdBa ₂ Cu ₃ O _x oxide superconducting material. Next, this precursor powder was formed by a uniaxial press, and the diameter was 30 mm and the thickness was 1 mm.
A molding precursor of about 0 mm was obtained.

Next, the molding precursor is subjected to 115 in an electric furnace.
After decomposing and melting by heating at 0 ° C. for about 2 hours, the temperature is lower than the decomposition temperature of the Nd123 phase (about 1090 ° C.).
The temperature was lowered to 0 ° C. At this time, detects the time when the temperature in the furnace was dropped to _{1060 ℃, NdBa 2 Cu 3 O} x oxide oriented Nd123 phase seed crystal of the same composition as the superconductor material (thickness in 4mm angle to be produced is 2mm Was suspended on an Ag-10% by weight Pd alloy wire (melting point: 1050 ° C.) attached to the tip of an alumina rod, and inserted from the upper part of the electric furnace. Then, the inserted seed crystal was held immediately above the surface of the central part of the precursor in a semi-molten state, and this was preheated. Subsequently, after confirming that the indicated temperature of the thermocouple attached to the vicinity of the seed crystal was equal to or higher than 1050 ° C., which is the melting point of the Ag—Pd alloy, the alumina rod was pulled out of the furnace to complete the seeding.

After completion of the seeding, the precursor is placed at 1 ° C.
The cooling was performed slowly at a cooling rate of 900 ° C. to 900 ° C., but this process allowed the growth of an NdBa ₂ Cu ₃ O _x oxide superconducting bulk composed of oriented Nd123 crystals of about 20 mm square and about 9 mm thick. Was.

[0031]

As described above, according to the present invention, it is not necessary to use a seed crystal having a high decomposition temperature as a seed crystal.
Since the oriented crystal having the same composition as the three phases can be used as the seed crystal, not only can the manufacturing process of the bulk oxide superconductor be simplified, but also the high-performance RE1 with few impurities can be used.
Industrially extremely useful effects are obtained, for example, the production stability of the 23 type oxide superconducting bulk body can be significantly improved.

[Brief description of the drawings]

FIG. 1 is a heating / cooling curve diagram of a bulk material producing raw material illustrating a conventional seeding time when producing a RE123-based oxide superconducting bulk material by a melt-solidification method.
(b) shows different examples of the cooling method.

FIG. 2 is a heating / cooling curve diagram of a raw material for producing a bulk body, illustrating a seeding time in the method of the present invention when producing a RE123-based oxide superconducting bulk body by a melt solidification method.

FIG. 3 is a schematic diagram illustrating an example of a seeding method according to the method of the present invention.

FIG. 4 is an explanatory diagram relating to a contact state of a seed crystal with a raw material (precursor) for producing a bulk body.

Claims (5)

[Claims] The chemical composition is represented by the general formula REBa ₂ Cu ₃ O _x (RE
Is one or more rare earth elements) and 12
In a 3-phase crystal structure RE123-based oxide superconducting bulk body made produced by melt solidification method, 12 of REBa ₂ Cu ₃ O _x oxide first bulk body preparing raw material "to produce
After decomposing and melting at a temperature higher than the temperature at which the “three phases” decompose and melt, the seed crystal having a 123-phase crystal structure in the temperature range lower than the decomposing and melting temperature is cooled while the temperature is lowered. A method for producing an RE123-based oxide superconducting bulk material, comprising: 2. The chemical composition has a general formula of REBa ₂ Cu ₃ O _x (RE
Contains one or more rare earth elements), and further contains one or more of Pt, Rh, Ce and Ag, which are characteristic improving elements, and has a 123 phase crystal structure. When producing a RE-Ba-Cu-O-based oxide superconducting bulk body by melt-solidification method, first, "123 phase of REBa ₂ Cu ₃ O _x oxide to be produced" is decomposed and melted as a bulk body production raw material. After decomposing and melting at a temperature equal to or higher than the temperature, while lowering the temperature, a seed crystal having a 123 phase crystal structure is brought into contact with its surface in a temperature range lower than the decomposition melting temperature to seed the seed crystal, Subsequently, a crystal is grown at a crystal growth temperature, and a method for producing an RE123-based oxide superconducting bulk material. 3. The method for producing an RE123-based oxide superconducting bulk material according to claim 1, wherein a seed crystal is preheated and used at the time of seeding. 4. REBA ₂ Cu ₃ O _{x to be} prepared as a seed crystal
The RE1 according to any one of claims 1 to 3, wherein a crystal having the same chemical composition as that of the "oxide bulk body" is used.
Manufacturing method of 23 type oxide superconducting bulk body. 5. In seeding, a seed crystal supported by a wire of “pure metal or alloy having a melting point corresponding to the seeding temperature” is brought close to the seeding surface, and the seed crystal is melted by the temperature near the seeding surface. The RE1 according to any one of claims 1 to 4, wherein the RE1 is dropped on a seeding surface and seeded.
Manufacturing method of 23 type oxide superconducting bulk body.