JPH04501188A - Improvements regarding superconducting composite conductors - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed description of the invention] Improvements regarding superconducting composite conductors The present invention relates to composite conductors that exhibit superconductivity at relatively high critical temperatures and in high magnetic fields. It concerns a method for manufacturing ceramic composites that can generate high currents. be. Such composite conductors can be simple or multi-bonded monoliths. It may also be stretched into a wire or tape.

One of the important properties of superconducting materials is their ability to conduct current without resistance. It is power. This characteristic can be used, for example, in the manufacture of magnetic solenoids and machines, or in It is used in monolithic materials used in applications and magnetic screens.

Once a closed superconducting circuit is formed, the resulting current (known as supercurrent) is , will flow without attenuation. This current is called a persistent current, and its circuit is permanently It is said to be in a mode. When the current density in a superconducting material exceeds a certain value, the superconducting material Since the conductor exhibits electrical resistance, the current is no longer a true supercurrent and requires superconductivity. It has been found that this makes it less useful for applications where This limiting current density is This is called the critical current density, and its specific value for any material is the depends on the black structure, applied magnetic field, and sample temperature. Design and optimization of superconducting materials The main objective of optimization is to reduce the supercurrent that can flow by maximizing its critical current density. The goal is to maximize the

Ceramic superconductors traditionally tend to have low critical current density characteristics, especially at high magnetic fields. was there. This feature is a significant drawback for its widespread use. these There are three main factors that cause low critical current densities in materials.

First, ceramic superconductors tend to contain narrow planar regions that are non-superconducting. low magnetic field , these regions still generate a small amount of supercurrent called the Josephson supercurrent. It can flow.

However, this current is greatly reduced by the applied magnetic field. the Such regions are called weak links, and they occur in polycrystalline ceramic superconductors. Weak links occur at grain boundaries and other parts of the microstructure. Substance is a weak link When the material is composed of a collection of superconducting regions separated by The critical current that can flow through a superconductor is determined by the network structure of weak links. determined. In the design and fabrication of ceramic superconductors, such weak links are should be removed or spatially distributed so as not to unduly impede current flow. Ru.

A second factor leading to low critical current densities is the criticality that often occurs in these materials. It is an extreme anisotropy of current. Therefore, in the case of ceramic YBaCuO, the crystallographic The critical current density in the C-axis direction is much lower than that in the crystallographic a-b plane.

This is because the current is forced to pass along the C-axis in some regions. Polycrystalline materials with irregular grain arrangements tend to exhibit low critical current densities. of current Anisotropy becomes even more complex when there is an applied magnetic field perpendicular to the direction of flow.

If the magnetic field is parallel to the C-axis direction, the critical current density is decreases more rapidly with increasing applied magnetic field.

The third factor that appears to determine the critical current density is the microstructure and defect structure. properties and how its current density interacts with quantized magnetic flux lines, The magnetic flux lines are sometimes called magnetic flux vortices. Regarding materials that flow high supercurrents in high magnetic fields. In other words, it has the property of having a strong interaction with magnetic flux lines and preventing their movement. is extremely important for the microstructure. Such a feature is called a pinned center. and carefully control the microstructure to improve the efficiency of superconductor pinning centers. This has been the main objective for a long time. Nucleus of magnetic flux vortex in ceramic-tsuk superconductor Because the size of the region is characteristically small, pinning by point defects seems to be weaker. . In stretched planar structures, pinning becomes more pronounced, especially at the interface with non-superconducting regions. Become stronger. This is because magnetic interactions contribute to flux vortex bin stoppage.

In this specification, the term particle means a single crystal. Therefore, multiple Polycrystalline materials consisting of vertical single crystals can be said to be composed of particles.

According to the present invention, particles exhibiting superconductivity and having a relatively low critical current region and a superconducting path adjacent to the particle and bridging the region of relatively low critical current. and further particles arranged to provide comprising regions of crystalline material and further disposed between said particles as defined herein. and a composite conductor comprising an elongated pinning center of non-superconducting material to increase said superconductivity. The body is provided.

A substance with an elongated crystal structure is introduced in such a way as to cause organized growth, thereby A composite ceramic that produces a corresponding elongated grain structure in a superconducting material that crystallizes A method of manufacturing a superconductor is also provided. Base for organized growth of superconducting materials The material is either a non-superconductor or has a lower critical temperature than the superconducting material that is subsequently deposited. It is a material that exhibits superconducting properties and is therefore suitable for acting as a pinning center. Ru. As used herein, the term non-superconducting material has a lower critical temperature; It is therefore considered to contain a superconducting material suitable to act as a binning center.

(The lines of magnetic flux are pinned at sharp differences in physical properties. Therefore, they are clearly It is desirable to have grain boundaries. Ordinary metals or insulators are different from superconductors. Therefore, it would be satisfactory as a pinning material. However, superconductors also Also, strong pinning is possible as long as its properties are significantly different from the rest of the superconducting material. make it possible. ) In this way, the particles are substantially in the direction of the superconducting current. Therefore, the grain structure and orientation of superconducting materials can be controlled, thereby increasing the superconducting current. The need for cross-grain boundaries is reduced.

It is sufficient that the elongated particles of the superconducting material overlap, but preferably one particle It is preferable to arrange the structure so that the ends of the particles are close to the central regions of adjacent particles.

As a result, the current progressively crosses regional grain boundaries aligned near the direction of current flow. , flows from one elongated particle to another. The smell of such substances This is because supercurrent flowing across grain boundaries is known to be a problem. is important.

Therefore, the method of the present invention ensures that the crystallographic order and structure of the grain structure maintains high current. Ensure that the crystallographic direction of the current flow is oriented in the direction of current flow.

The invention relates to monolithic (simply or multiply bonded) conductors; It can be similarly applied to conductors in the form of drawn wires or tapes.

The method of the invention results in a superconductor comprising a finely distributed arrangement of non-superconducting regions; The non-superconducting region acts to define the crystal coordination and morphology of the superconducting material during manufacturing. In addition, when the composite exhibits superconductivity, it is possible to function, thereby maximizing the pinning of magnetic flux vortices.

According to a preferred embodiment of the invention, the distribution of superconducting particles is divided into elongated pinning centers. There is an epitaxial relationship with the fabric and orientation, and the crystal structure at the pinning center is due to the manufacturing process. It causes and stimulates the crystal orientation of superconducting particles at the critical stage of .

According to a preferred feature of the invention, the superconducting composite comprises an aggregate of grains of non-superconducting material. nothing. The grains are superelectrified prior to a heat treatment step that determines the final grain structure of the composite. included or produced within the conductive composite at an intermediate stage in its manufacture. .

Grains of non-superconducting materials are typically filamentary. i.e. the stretched shape fibers or elongated flakes or plates, ribbons, or tapes It is in a state of The grain is at least three vortex spacings long, but preferably should be several millimeters or longer. (In a magnetic field of 1 Tesla, the vortex spacing is It will be about 0.1 μm. The spacing is the minimum length for grains on the order of 1 μm. show. ) In one specific embodiment, the non-superconducting material has a precise crystal structure. Each grain is a superconductor on which the surface atoms grow. Preferably, it is in the form of a single crystal with an orientation equal to the atomic arrangement. Under the following conditions The grains can be bi-crystalline or polycrystalline. Ru. (1) Grain spacing is heavy or polycrystalline aggregates (sometimes called particle groups) It is small compared to the size of an individual single crystal, and (2) has a grain structure (called a crystal structure). ) is appropriate (i.e. superconducting structures on which surface atoms grow) (equivalent to the atomic arrangement on the base of

One example of a non-superconducting material that can be used is magnesia (MgO), which is When used, it assumes a flake with (1,O,O) planes parallel to the flake surface. It may be obtained or formed by

The crystal structure and orientation of non-superconducting single crystals can be adjusted to the desired shape in the composite after appropriate heat treatment. A single crystal is preferred, although it is preferably such that it gives rise to a microstructure. The grains are distributed in an aligned manner, with adjacent grains lined up parallel to each other, and the edges of each grain are arranged and distributed so that they are located near the center of each adjacent filament. Preferably.

Preferably, the long axis of each non-superconducting grain is such that the supercurrent flows through the final composite. It is best to face the direction in which the camera is facing.

In some monolithic composite conductor designs, the average non-superconducting grain direction is must vary from point to point.

For the operation of superconductors by choosing the smallest particle size (i.e. thickness) The flux pinning in the chosen magnetic field of can be optimized.

The grain spacing of non-superconducting materials facilitates the epitaxial growth of superconductors across the intergrain spaces. Should be close enough to ensure. Preferred microstructure according to the invention In the structure, each grain is a superconductor with an orientation determined by the grain's atomic structure. surrounded by bodies. Then, the continuity of epitaxy of adjacent grains of a non-superconductor or the non-superconductor. One of the largest grain boundaries that separates adjacent superconductors from adjacent grains of superconductors exists.

Non-superconductor grains must not degrade the superconductor through internal diffusion or reaction, and should be selected The material preferably has elastic properties and thermal expansion adapted to the shape of the superconducting composite. It should have a tensile modulus.

A preferred non-superconducting material is magnesia (MgO). Magnesia is hot Conveniently inert to most ceramic superconductors and also well oriented It has a crystal structure suitable for epitaxial growth of superconducting particles.

Alternatively, the non-superconducting material may be one described in BPA88120:38°1, for example. In the form of fibers coated with an epitaxial loose layer of magnesia by It can be a single crystal material such as some sapphire.

In other embodiments, the precursor of magnesia, magnesium metal, is The filament may be coated with silver cladding, and the filament may be coated with silver clad magnesium. Forming (composite assembly) by swaging and stretching into a wire shape. before standing) can be done. The magnesia is then passed through the silver coating during a low temperature anneal. It is synthesized by diffusing oxygen, and the filament thus prepared has a silver coating or It can be incorporated into either composite after the coating has been removed.

The filaments mentioned above can also be formed in situ during the deformation of the powder composite; Magnesia is also formed during the pre-low temperature anneal in oxygen.

It is understood that the invention is not limited to the use of magnesia or magnesia coating materials. Should. Other suitable materials are strontium titanate, ittria, and ittria. Umbarium copper oxide (Y2BaCuO5 - so-called green phase), Rear stabilized zirconia, lanthanum gallate, silver fluoride or magnesium fluoride It is mu.

Manufacturing begins by forming the desired grains of non-superconducting material and the superconducting matrix. make a composite by combining one or more other materials to The method may include reacting the complex to obtain a superconducting phase.

If the grains are synthesized prior to the formation of the complex, it is necessary to arrange them in a properly aligned manner. A number of different methods can be used to include the compound in the form of a complex. − In one method, the grains are mixed with a powder material to form a superconducting matrix. and filled into a suitable ductile pressure vessel such as a silver-palladium alloy. The container is , may be clad with stainless steel. This preform is swaging , can be reduced to the required size by stretching, extrusion, forging or rolling. can.

In the second method, the powder superconductor is an oxide, nitrate, carbide, or metal powder. Possibly in the form of a mixed precursor powder.

In the third method, the matrix powder is mixed with a viscous binder material and viscous processed It can be processed with technology to create the required aligned microstructure.

In a fourth method, particles are included in a liquid during sol-gel processing and placed on a suitable substrate. Arrangements can be made by flow coating or spin coating processes. And this process can be repeated until the material forms the required thickness.

In a fifth method, the grains are added simultaneously to the liquid precursor either separately or through a common nozzle. and then sprayed onto the substrate. Spray method is thermal spraying method or plasma spraying method It may be.

In a sixth method, the complex can be produced by liquid infiltration, where the A fiber in which the superconductor or precursor is in the form of a wire, tape, or sheet. It is penetrated into the fibrous mat.

Alternatively, manufacturing may involve the use of precursors that produce the desired grains after processing. So Examples of precursors such as magnesium metal or magnesium coated substrates are Fire fiber. If a precursor is used to generate grains, the precursor However, it is necessary to generate grains before the reaction to form the superconducting matrix. .

In one such manufacturing route, grain precursors are processed into composite preforms prior to deformation. produced by including ductile metal powders like magnesium in the form Ru. The highly organized metal filament is then swaged, drawn, and pressed. It can be formed by extrusion or rolling, and then the filament is oxidized. before the final reaction step to give a superconducting microstructure with the desired morphology and crystal structure. The grains can be generated during the heat treatment step.

A final heat treatment, whatever route is used to obtain the desired sequence, prior to the final reaction. The steps configure the superconductor to produce the desired microstructural morphology and crystalline texture. It is designed to react and modify or control the microstructure of the target material.

As previously described, this is a solid-phase epitaxy, tactile epitaxy (tac to-epitaxy), or melting, or partial melting, and recrystallization; The growth of the grains is controlled by the crystal structure and distribution of the nucleated grains.

The exact combination of heat treatment time and temperature can vary from ceramic superconductor to ceramic superconductor. It has been found that it also depends on the type of grain and the size of the complex. for example , in some cases in a temperature gradient or magnetic field, or under predetermined filling conditions It is advantageous to carry out the reaction under In the final stage of this reaction process, Low-temperature annealing in oxygen, nitrous oxide, or other gases appears to be required .

The above method is suitable for rare earth metals (e.g. YBaCuO), bismuth metals (e.g. B 1Pb5rCaCuO) and thallium metal (TIBaCaCuO). It has been widely applied to ceramic superconductors.

The invention also allows the addition of selected additives, particularly silver, gold, or hafnium, to This may include controlling the particle structure and distribution of impurities during coalescence.

Previous Patent Application No. 8710113.8714993 and 8718506 A complex constructed according to the invention to enable titration of the described chemical species. It is also envisioned that the combination will include an electrode and a solid electrolyte.

We now describe the present invention as a cross-sectional view of a superconducting composite wire, and a microstructure or particle The invention will now be described by way of example with reference to the accompanying drawings, which show details of construction. In the figure, FIG. 1 is a cross-sectional view of a superconducting composite wire precursor.

FIG. 2 is a cross-sectional view showing the microstructure of the composite after treatment.

FIG. 3 is a cross-sectional view of the composite after subsequent coatings have been applied.

Figure 4 shows the microstructure of the composite wire.

In the method according to a specific embodiment of the present invention, YBa2Cu with an average particle size of 0°5 μm The 3o7 powder 11 is heated to SrTiO3 or slightly Mixed with filamentary grains 10 in the form of elongated flakes of MgO. Phila The volume fraction of ment is 20%. Disperse this mixture well in viscous mineral oil and coat it. forming a lloid suspension, which is then viscous processed to obtain the preferred alignment of the filaments. obtain. Finally, this is molded to produce a tape with a cross-sectional area of 5 x 0.5 mm. Fi The average spacing between the filaments is 2 μm (see Figure 1).

The sample was then heat treated in oxygen flow at 500°C for 12 hours to thermally decompose the binder material. Let me understand. Thereafter, residual carbonaceous matter is removed at 850° C. for 1 hour. Finally, in the air Sinter at 980° C. for up to 16 hours. Nucleation of favorable microstructures during this step happens. The sample was then heated in oxygen at 25°C/min to 880°C, then at 1°C/min. to 500°C, where the samples were kept for 200 hours for isothermal oxygen annealing. and finally cooled to room temperature at 1°C/min. Single crystal grain with boundary 13 The final microstructure containing 12 is shown in FIG.

In another manufacturing route embodying the invention, the same filament is made of mixed nitrates. The superconductor is suspended in a liquid solution 15 in the form of a superconductor. 0.5 ml of this solution was spin-coated onto the MgO'7 wafer substrate 14 to form a coating film with a thickness of 5 to 10 μm. Form. After drying at 75°C for 10 minutes, subsequent coatings were applied until the composite thickness was 0. Spin coat onto the same sample until 1 mm. (See Figure 3) The heat treatment was as follows. The sample was gradually heated to 870°C and placed in oxygen for 30 minutes. Keep it. The sample was then sintered at 980°C in air for 2-5 minutes and 88°C at 25°C/min. to 0°C, then cooled at 1°C/min to 500°C, where the sample was subjected to isothermal oxygen annealing. The mixture was kept for 100 hours and finally cooled to room temperature at 1°C/min. final miku The structure is again as shown in FIG.

In yet another production route embodying the invention, powdered YBa with an average particle size of 1-2 μm is used. 2Cu3O7 is mixed with 20% volume fraction of magnesium powder with an average particle size of 20 μm. Ru. This mixture was poured into a cylindrical silver plate with an outer diameter of 15 mm, an inner diameter of 12 mm, and a length of 20 cm. Insert into reform. The composite was then hydrostatically extruded to an outer diameter of 1 mm. shall be. Heat treatment was performed at 300°C in oxygen for 200 hours, then at 980°C for 2-5 minutes. The sample was then cooled to 500°C at 25°C/min, where it was subjected to isothermal oxygen annealing. The sample is kept for 100 hours and then finally cooled to room temperature at 1°C/min. push Including filaments 17 in precursor matrix 18 after extrusion and before heat treatment The microstructure is shown in FIG.

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Claims (20)

[Claims] 1. Particles that can exhibit superconductivity and have a region of relatively low critical current and a superconducting path adjacent to the particle and bridging the region of relatively low critical current. and further particles arranged to provide an elongated grain structure (12). A composite conductor comprising a region of polycrystalline material, the composite conductor as defined herein As shown in FIG. Composite conductor further comprising an elongated pinning center (10) of superconducting material . 2. The elongated pinning centers (10) are arranged so that the long axes of adjacent elongated pinning centers are mutually real. are arranged so that they are qualitatively parallel, with the end of each elongated pinning center connecting each adjacent field. The compound according to claim 1, characterized in that the compound is located near the central portion of the filament. Combined conductor. 3. The long axis of each elongated pinning center (10) is such that the supercurrent flows through the final complex. The composite conductor according to claim 2, characterized in that the conductor is in the right direction. 4. The distribution of particles (12) is epitaxial to the distribution and orientation of elongated pinning centers. Composite conductor according to claim 1, characterized in that the composite conductor is physically associated. 5. Ensure that the extended pinning center (10) is at least three vortex spacings long. A composite conductor according to any one of claims 1 to 4 characterized by: 6. Claim characterized in that the length of the elongated pinning center is at least 1 μm. Composite conductor according to range 5. 7. Non-superconducting materials include magnesia, strontium titanate, and yttrium burr. copper oxide, yttria stabilized zirconia, lanthanum gallate, silver fluoride and and magnesium fluoride. A composite conductor according to any one of claims 1 to 7. 8. Claims characterized in that the non-superconducting material is magnesia (MgO) Composite conductor according to item 7. 9. Claims characterized in that magnesia is deposited on a substrate of a single crystal material. Composite conductor according to range 8. 10. Claim 8, wherein the single crystal material is sapphire. composite conductor. 11. Grains of a substance with an elongated structure are held in an elongated bottle between the particles of a superconducting material. of a composite conductor comprising particles of superconducting material, characterized in that it is introduced to provide Production method. 12. The grains are mixed with a powder material forming particles of superconducting material and placed in a suitable ductile container. Characterized by filling to create a preform, which is then reduced to the required dimensions. A method for manufacturing a composite conductor according to claim 11. 13. Claim 1, characterized in that the powder material is in the form of a mixed precursor powder. A method for manufacturing a composite conductor according to item 12. 14. The powder material is from the class containing oxides, nitrates, carbides or metal powders. The composite conductor according to claim 13, characterized in that the composite conductor comprises at least one of . manufacturing method. 15. Powder material is mixed with viscous binder material and processed with viscous process technology for distribution. A composite conductor according to claim 11, characterized in that it produces an arrayed microstructure. How the body is manufactured. 16. Granules of powder material are included in the liquid during sol-gel processing and applied during the coating process on the substrate. The composite conductor according to claim 11, characterized in that the composite conductor is arranged by a process. Production method. 17. Claims characterized in that the particles are sprayed onto the substrate simultaneously with the liquid precursor. A method for manufacturing a composite conductor according to Scope 11. 18. Liquid infiltration method in which liquid superconductors or precursors are infiltrated into fibrous mats Therefore, the composite conductor according to claim 11, characterized in that it forms a composite body. How the body is manufactured. 19. Additives can be added to composite conductive materials to improve the particle structure and distribution of impurities within them. 12. The method of manufacturing a composite conductor according to claim 11, wherein the method comprises: controlling. 20. that the additive is selected from a class including gold, silver, and hafnium; A method for manufacturing a composite conductor according to claim 19.