DE3856529T2 - Process for producing a superconducting elongated body made of oxide composite material - Google Patents

Description

Background of the invention Field of the invention

The present invention relates to a method for producing an elongated article from a sintered ceramic material that is superconducting.

It relates in particular to a method for producing a superconducting wire from a sintered ceramic material made of a compound oxide, which can be used for producing superconducting coils or the like.

The present invention particularly relates to a method for manufacturing a superconducting wire from a sintered ceramic material made of a compound oxide having a higher critical current density and a higher critical superconductivity transition temperature.

Description of the related art

Superconductivity is a phenomenon in which the electrical resistance becomes zero and can therefore be used to manufacture power cables and a variety of devices and apparatuses that are desired to reduce the consumption of electrical energy, and Several proposals have already been made for its application, which exploit the phenomenon of superconductivity.

Superconductivity is in fact applicable in a wide range of industrial fields, for example in the field of electrical energy supply, e.g. fusion energy, MHD energy generation, energy transport or storage of electrical energy; in the field of transport engineering, e.g. magnetic levitation trains, magnetic propulsion of ships; in the medical field, e.g. in an irradiation unit with high energy radiation; in the field of science, e.g. NMR or high energy physics; or in the field of sensors or detectors for determining very weak magnetic fields, microwave or radiation energy or the like, and in the electronic field, e.g. for Josephson element devices and high-speed computers with lower energy consumption.

However, their practical application has been limited so far because the phenomenon of superconductivity can only be observed at very low cryogenic temperatures. Among the known superconducting materials, a group of materials having a so-called A-15 structure exhibits a higher Tc (critical temperature of superconductivity) than others, but even the peak value of Tc in the case of Nb3Ge, which has the highest Tc, does not exceed a maximum value of 23.2ºK.

This means that liquefied helium (boiling point 4.2ºK) is the only cryogenic agent that can provide such a low Tc temperature. However, helium is not only a limited, expensive resource, but also requires a large-scale liquefaction system. It is therefore desirable to find other superconducting materials with a much higher Tc value. However, research over the past 10 years has not yet found any material with a higher Tc value than the above-mentioned one.

It is known that certain ceramic materials made of compound oxides have the property of superconductivity. For example, US Patent No. 3,932,315 describes a compound oxide of the Ba-Pb-Bi type which has superconductivity. However, this type of superconductor has a rather low transition temperature of less than 13ºK and thus the use of liquefied helium (boiling point 4.2ºK) as a cryogenic agent is essential to achieve superconductivity.

The possibility of the existence of a new type of superconducting material with a much higher Tc value was demonstrated by Bednorz and Muller, who found a new oxide-type superconductor in 1986 [Z. Phys. B64 (1986), p. 189]. However, in this document the use of such an oxide material to make a superconducting wire from it is neither described nor mentioned.

This new oxide-type superconducting material is [La,Ba]2CuO4 or [La,Sr]2CuO4, which are so-called K2NiF4-type oxides, which have a crystal structure similar to that of perovskite-type superconducting oxides already known (see, for example, BaPb1-xBixO3, described in U.S. Patent No. 3,932,315). The K2NiF4-type oxides have higher Tc values of about 30°K, which are much higher than those of the already known superconducting materials.

As compound oxide type superconductors consisting of oxides of elements of groups IIa and IIIa of the Periodic Table of Elements, there can be mentioned those having a quasi-perovskite structure, which can be regarded as having a crystal structure similar to that of the perovskite type oxides, and includes an orthorhombic distorted perovskite or an oxygen-deficient distorted perovskite such as Ba₂YCu₃O7-δ, in addition to the above-mentioned K₂NiF₄ type oxide such as [La,Ba]₂CuO₄ or [La,Sr]₂CuO₄. Since these superconducting materials have a very high Tc value of 30 to 90ºK, it is possible to use liquefied hydrogen (bp = 20.4ºK) or liquefied neon (bp = 27.3ºK) as cryogenic agents to achieve superconductivity in practice. Hydrogen in particular is an inexhaustible source, but it has the disadvantage of being explosive.

However, the above-mentioned new type superconducting materials that have just emerged have so far only been studied and developed in the form of sintered bodies, a shapeless product made from powders, but no attempts have been made to form them into a wire. The reason is that the new type superconductors are ceramic materials made of a compound oxide, which do not have good plasticity or processability compared with the generally known metal type superconducting materials such as Ni-Ti alloy, and therefore cannot be formed into an elongated object such as a wire, or can only be formed with difficulty, using a conventional method such as a wire drawing method in which the superconducting metal is drawn into a wire directly or embedded in copper.

In Japanese Patent Laid-Open No. 61-131307 and also in "IEEE Transactions on Magnetics", Vol. MAG-19, No. 3, May 3, 1983, p. 402, a method for producing a superconducting wire from a metal-type superconducting material which can be oxidized and is very brittle, such as PbMo0.35S₈ or PbMo₆S₈, has been proposed, which comprises filling the material powder in a metal shell, extruding the metal shell filled with the material powder at a temperature of more than 1000°C, and then drawing the extruded composite body. However, this metal processing method cannot be directly applied to a ceramic material made of a compound oxide because the compound oxide type superconducting materials cannot exhibit superconductivity unless a specific or predetermined crystal structure is created. In other words, a superconducting wire with a higher critical temperature and a higher critical current density suitable for practical applications cannot be obtained outside the specified optimal conditions. In particular, if the shell is not selected from suitable materials, the resulting compound oxide may be reduced by a chemical reaction with the shell metal, resulting in poor or degraded superconductivity properties.

In the field of forming ceramic materials, it is a general practice that, in order to produce an elongated article, such as wires or rods, an organic binder is added to the ceramic material powder to facilitate the shaping or deformation of the powder material. In this way, a mixture of the powder material and the organic binder is formed into a rod by means of an extruder or a press and thereafter the formed rod is transferred directly or via a trimming or cutting step to an intermediate sintering step to remove the organic binder before being subjected to the final sintering step.

The above-mentioned compression molding and trimming or cutting operations lead to large losses of expensive ceramic material, so that the material is not only uneconomical, but also the dimensional ratio of the rod’s longitudinal direction to its cross-sectional direction cannot be increased. This process cannot therefore be used in practice.

The extrusion process is much better than compression molding in terms of material economy and productivity, but it requires large amounts of organic binder added to the powder material. This organic binder is difficult to completely remove during the intermediate sintering stage and therefore remains in the final sintered article, which is the cause of defects in the product that affect its strength and flexibility. It is therefore difficult to produce a thin rod of a ceramic material with higher dimensional ratios from longitudinal direction to cross-sectional direction using the extrusion process.

DE-B-12 57 436 describes the production of a superconducting wire by using a powder metallurgy process. In this process, a metal tube is filled with Nb₃Sn. The diameter of the tube is reduced and the narrowed tube is then heat-treated at a temperature sufficient to make the powder filled into the tube superconducting.

To produce a reliable and practical superconducting structure, it is essential that the structure has sufficient strength and toughness to withstand bending forces during use and also has as small a cross-sectional dimension as possible so that it can transmit electric current at a higher critical current density and at a higher critical temperature.

An object of the present invention is therefore to provide a method for producing a superconducting wire made of sintered ceramics which has a sufficient length for practical applications, namely has a higher dimension ratio of the longitudinal direction to the cross-sectional direction, without using an organic binder which is the cause of the decrease in the strength and toughness of the product.

Another object of the present invention is to provide a method for producing a thin superconducting wire from a compound oxide type sintered ceramic which has a higher resistance to breakage even when the diameter of the wire is greatly reduced, in other words, at higher cross-sectional dimension reduction ratios.

Still another object of the present invention is to provide a method for producing a fine superconducting wire made of a compound oxide type sintered ceramic having a higher critical current density and a higher critical temperature.

Summary of the invention

The present invention relates to a method for producing a superconducting elongated article as defined in claim 1.

The elongated articles that can be produced by the method according to the invention include rods, wires, strands, ribbons, strips or any other articles whose dimensional ratio between the longitudinal direction and the cross-sectional direction is more than 30, wherein the cross-section of the article is not limited to a circle but can have any configuration, for example a rectangular shape.

The ceramic powder material consisting of a compound oxide exhibiting superconductivity includes any compound oxide exhibiting superconductivity after the heat treatment of the present invention.

In general, the ceramic powder material that can be used in the inventive process can have the general formula:

AaBbCc

wherein "A" stands for at least one element selected from a group comprising groups IIa and IIIa of the Periodic Table of the Elements, "B" stands for at least one element selected from a group comprising groups Ia, IIa and IIIa of the Periodic Table of the Elements, "C" stands for at least one element selected from a Group comprising oxygen, carbon, nitrogen, fluorine and sulphur, and "a", "b" and "c" represent the atomic ratios of the elements "A", "B" and "C". They preferably satisfy the following equation:

"a"x (average valence of "A") + "b"x (average valence of "B") = c"x (average valence of "C")

The elements of group Ia of the Periodic Table of Elements are H, Li, Na, K, Rb, Cs and Fr. The elements of group IIa of the Periodic Table of Elements can be Be, Mg, Ca, Sr, Ba and Ra. The elements of group IIIa of the Periodic Table of Elements can be Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Ac, Th, Pa, U, Np, Pu, Am, Cm, Bk, Cf, Es, Fm, Md, Mo and Lr. The elements of group Ib of the Periodic Table of Elements can be Cu, Ag and Au. The elements of Group IIb of the Periodic Table of Elements can be Zn, Cd and Hg. The elements of Group IIIb of the Periodic Table of Elements can be B, Al, Ga, In and Ti.

The ceramic powder material is preferably a powder mixture containing oxides of metals that have a higher oxygen potential for the formation of the oxide than copper.

Among the superconducting ceramic materials, there may be mentioned those containing at least two elements selected from Groups Ia, IIa and IIIa of the Periodic Table of Elements as "A", at least copper as "B" and oxygen as "C", such as Y-Ba-Cu-O type ceramic materials, Y-Sr-Cu-O type ceramic materials, La-Sr-Cu-O type ceramic materials and La-Ba-Cu-O type ceramic materials.

The ceramic material powder may be a compound oxide (mixed oxide) having the crystal structure of the K2NiF4 type oxides such as [La, Ba]2CuO4 or [La,Sr]2CuO4.

In addition, the ceramic material powder can be a compound oxide (mixed oxide) which has a perovskite type crystal structure, exhibits superconductivity and has the general formula:

(α 1-x, βx) γy Oz

wherein α represents an element selected from the elements of Group IIa of the Periodic Table of the Elements, β represents an element selected from the elements of Group IIIa of the Periodic Table of the Elements, �gamma represents an element selected from the elements of Group Ib, IIb, IIIb, IVa or VIIIa of the Periodic Table of the Elements, x, y and z represent numbers which lie within the following respective ranges:

0.1 ≤ x ≤ 0.9

0.4 ≤ y ≤ 4.0 and

1 ≤ z ≤ 5

In particular, it can be a compound oxide in which α stands for Ba, β for Y and γ for Cu.

It is also preferable to use a ceramic material powder prepared by mixing Bi₂O₃, SrCO₃, CaCO₃ and CuO powders, drying and then pressing the powder mixture, sintering the pressed mass and then pulverizing the sintered mass.

The ceramic material powders are preferably granulated before being filled into the metal tube.

The metal tube can be selected from a group comprising the metals Ag, Au, Pt, Pd, Rh, Ir, Ru, Os, Cu, Al, Fe, Ni, Cr, Ti, Mo, W and Ta and alloys comprising these metals as their base.

The heat treatment can be carried out over a temperature range of 700 to 1000ºC.

The plastic deformation of the metal pipe filled with the ceramic material powder can be carried out in such a way that the cross section of the metal pipe is reduced in a dimension reduction ratio in the range of 16 to 92%. The plastic deformation can be carried out by wire drawing, which is carried out using dies, cylinder dies or extruders.

Plastic deformation can be carried out by forging, for example by means of a drop forging unit or by rolling.

It is also preferable to allow the metal tube containing the sintered ceramic material powder inside it to cool slowly at a rate of less than 50°C/min, after which the heat treatment is completed.

A device which can be used to carry out the above-described method according to the invention is described in more detail below with reference to the accompanying drawings, which, however, do not limit the present invention.

Short description of the drawings

Figures 1A to 1J illustrate a series of steps for fabricating a superconducting elongated article according to the invention;

2A to 2C show variations of the superconducting elongated articles according to the invention, wherein Fig. 2A shows an explanatory perspective view of the article, Fig. 2B shows a cross-section thereof and Fig. 2C shows an explanatory view of another embodiment of the article;

Fig. 3A and 3B show cross sections of the variations according to Fig. 2;

Fig. 4 shows another embodiment of the present invention, wherein Fig. 4A is a cross-sectional view and Fig. 4B is a plan view of the elongated article according to the invention;

Fig. 5 shows another embodiment of the present invention, wherein Fig. 5A shows a cross-sectional view and Fig. 5B shows a perspective view of the elongated article according to the invention;

Fig. 6 shows an explanatory view of an apparatus for continuously producing a composite body according to the invention having a belt-shaped configuration.

In Fig. 1, Figs. 1A to 1J illustrate the steps for producing an elongated article according to the invention.

First, a metal tube 1 having a predetermined cross-sectional configuration (outer diameter "L" and inner diameter "I") is filled with a ceramic material powder 2 as shown in Fig. 1B.

Then, the resulting metal tube filled with the ceramic powder material is subjected to a wire drawing operation, which can be carried out by means of cylindrical dies 3 as shown in Fig. 1C, or by means of a die or a series of dies 4 as shown in cross section in Fig. 1D. The wire drawing can be carried out by means of a die forging unit 5 as shown in Fig. 1E, or by means of a wire drawing machine of the extruder type. type (Fig. 1F shows the cross section of an extruder head). In case the elongated article has a rectangular cross section, the metal tube can be rolled by means of rollers 7 as shown in Fig. 1H.

An annealing step may be incorporated into the wire drawing step to facilitate wire drawing. It is also preferable to seal one end or the opposite ends of the metal tube before subjecting it to the wire drawing operation, as shown in Fig. 1H, to prevent the powder material from escaping from the metal tube.

Fig. 1I illustrates a perspective view of the resulting wire drawn product comprising an inner core obtained from the material powder 2 and having a reduced diameter "I'" so that the final product obtained after the subsequent sintering step as described below has the same configuration as Fig. 1I.

Fig. 1J illustrates the case where the outer tube is removed.

Fig. 2 shows a variation of the elongated article made according to the invention, in which perforations or through holes are provided in the metal tube 11. In an embodiment, as shown in perspective view in Fig. 2A, the fine holes 13 are cut out of the metal tube over the entire surface by means of a CO2 laser or the like. Fig. 2B shows in the form of a cross-sectional view the tube shown in Fig. 2A. The holes 13 can be replaced by a slot 13a, as shown in Fig. 2C. The slot 13a can have a dimension in width of about 200 µm.

It is known that the resulting superconducting wire is affected in an oxygen-containing atmosphere, for example in air, so it is preferable to close the holes 13 cut in the metal tube 11. For this purpose, the holes 13 may be filled with a sealant 14 to isolate the sintered wire of the superconductor 12 from the surrounding atmosphere, as shown in Fig. 3A. However, the practical implementation of this method is difficult or results in low productivity, so it is practically preferable to cover the entire outer surface of the perforated metal tube with a suitable airtight cylindrical covering, for example a heat-shrinkable plastic tube which is chemically stable, as shown in Fig. 3B. The covering may consist of a metal layer applied by vacuum deposition to the entire surface of the metal tube, and more preferably a coating of low melting point glass applied to the metal tube to seal it completely.

Fig. 4A illustrates an elongated sintered superconductor having a rectangular cross section according to an embodiment of the present invention, and Fig. 4B shows a plan view thereof. This superconductor can be manufactured by a method comprising molding the superconducting material to form it into the shape of a rectangular body 21 and then covering the molded article 21 with a metal shell 22.

Fig. 5 illustrates another embodiment of the elongated article having the same circular cross-section as that of Fig. 11 of the present invention, but in this case the superconductor 21 is covered with a sheath 24 in the form of a mesh (lattice). Fig. 5A shows a cross-sectional view and Fig. 5B a perspective view of the resulting superconductor.

The above variation can be used for various applications. The presence of the above porous outer shell or cover provides the following advantages:

First, the surface of the shell 22 and 24 is oxidized by the oxidative treatment to form copper oxide, so that the oxygen content in the sintered superconductor is not affected or changed by the oxidation of the shell 22 or 24. In particular, in the case of Fig. 2A and Fig. 4, the through holes 13 are distributed over the entire surface of the metal tube 11 and 22 so that the sintered superconductor can communicate with the external atmosphere. The mesh-like shell 24 can achieve a larger contact surface between the superconductor and the surrounding atmosphere.

Fig. 6 illustrates an apparatus for continuously producing an elongated object according to the invention. In this case, the ceramic material powder is preferably mixed with an organic binder.

The apparatus comprises a continuous furnace equipped with two heaters in a binder removal zone 112 and in a sintering zone 113. An elongated strip or wire 114 is fed to the inlet of the binder removal zone 112 from a reel 115. The elongated article 114 unwound from the reel 115 is continuously fed to the binder removal zone 112 where the elongated article 114 is heated to a temperature of 400 to 700°C to remove the binder from the elongated article 114.

After the binder removal zone 112, the elongated article 114 is introduced into a continuous wrapping station 116, which is arranged downstream of the binder removal zone 112. The continuous wrapping station 116 is equipped with a drum 118 for introducing a foil 117 made of metal or an alloy into a guide 119 in which the foil 117 is wrapped around the elongated article 114. The edge (Seam) of the wrapped foil 117 is welded by means of a laser welding device 120 so that the elongated object 114 is enveloped by the metal foil 117.

The resulting composite body comprising the elongated article 114 and the covering film or outer shell 117 is then introduced into the sintering zone 113 where the composite body is heated to a temperature of 850 to 950°C to sinter the elongated body. The longitudinal dimension or length of the sintering zone 113 and the speed of advancement of the composite body can be adjusted so that the sintering is fully accomplished.

The product 121 obtained in this way can be wound on a drum 122 for storage. In this embodiment, the product has sufficient flexibility and sufficient self-supporting properties since the elongated object 114 contains the binder. The device shown in Fig. 6 allows the sintering to be carried out continuously with a higher productivity.

In a modification of the present invention, the composite body comprising the elongated article and the outer film (shell) can be shaped or deformed to have the desired configuration, for example, the shape of a coil (roll) or the like, because of the higher flexibility and self-supporting properties, so that the sintering can be carried out in the state of the rolled-up configuration or in a state in which the coil (roll) is supported by another electrically conductive body. The presence of the shell made of metal or a metal alloy also increases the bending strength.

Description of the preferred embodiments

In a preferred embodiment of the present invention, the following powders can be used as ceramic material powder:

(1) A ceramic powder containing compound oxides having the crystal structure of K2NiF4 type oxides and possessing superconducting properties, particularly powder of [La,Ba]2CuO4 or [La,Sr]2CuO4;

(2) Compound oxides with a perovskite-type crystal structure that are superconducting and have the general formula:

(α 1-x, βx) γy Oz

wherein α represents an element selected from elements of Group IIa of the Periodic Table of Elements, β represents an element selected from elements of Group IIIa of the Periodic Table of Elements, �gamma represents an element selected from elements of Group Ib, IIb, IIIb, IVa or VIIIa of the Periodic Table of Elements, x, y and z represent numbers having values within the following respective ranges:

0.1 ≤ x ≤ 0.9

0.4 ≤ y ≤ 4.0, and

1 ≤ z ≤ 5

in particular a compound oxide of the Ba-Y-Cu-O type, in which α stands for Ba, β for Y and γ for Cu.

It is also possible to use another type of ceramic material powders, for example a compound oxide consisting of at least two elements selected from a group comprising groups IIa and IIIa of the Periodic Table of the Elements, an element selected from a group comprising group Va of the Periodic Table of the Elements, Cu and oxygen, e.g. a compound oxide of the Sr-Ca-Bi-Cu-O type, which is prepared, for example, by mixing Bi₂O₃, SrCO₃, CaCO₃ and CuO powders, drying and then pressing the powder mixture, Sintering the pressed mass and subsequent pulverization of the sintered mass.

The material powders used in the invention are not limited to those mentioned above.

The ceramic powder materials are preferably granulated in advance before being filled into the metal tube. If the material powder in the metal tube cannot be pressed into a higher packing density, it is convenient to granulate the material powder in advance to facilitate filling into the metal tube and to achieve a higher packing density.

In a preferred embodiment, the material powder is granulated into particles having an average particle size of less than 0.1 mm and then heat-treated before being filled into the metal tube. This heat treatment corresponds to the final sintering used in the conventional method. In this case, the material powder may be subjected to further heat treatment if necessary after the powder has been pressed in the metal tube. If the heat treatment of the powder material results in coagulation of the powders to form large particles having an average particle size of more than 0.1 mm, the heat-treated powder may be pulverized before being pressed (compacted) in the metal tube. In this case, a conventional final heat treatment is carried out in the pulverized state having an average particle size of less than 0.1 mm. The resulting heat-treated powder therefore has a crystal structure as a whole that has superconductivity and therefore no part remains that is not superconducting.

In addition, a higher packing factor or packing density is achieved in the metal tube and a higher elongation ratio of the wire is ensured. The The resulting wire is converted into an elongated superconducting wire with a higher critical current density.

In this way, a wire made of a compound oxide having the crystal structure of K2NiF4 type oxides, for example, a powder of [La,Ba]2CuO4 or [La,Sr]2CuO4 can be obtained by sintering a material powder mixture of oxides, carbonates, nitrates, sulfates or the like of the elements constituting the compound oxide, for example, a powder mixture of La2O3, BaO2, SrO2 and CuO.

The metal tube may be made of a metal selected from the group comprising Ag, Au, Pt, Pd, Rh, Ir, Ru, Os, Cu, Al, Fe, Ni, Cr, Ti, Mo, W and Ta, and an alloy comprising these metals as a base.

In particular, it is preferable to select the metal from Ag, Au, platinum group metals including Pt, Pd, Rh, Ir, Ru and Os, and alloys containing them as a base metal. The metals Ag, Au and the platinum group metals are almost inert to the superconducting ceramic materials under the heating conditions, and therefore the heat treatment can be carried out at a sufficiently high temperature so as to accelerate the sintering or solid-solid reaction between the superconducting ceramic particles in the metal tube to obtain a uniform elongated article. When a metal tube made of copper is used instead of one made of the platinum group metals, there is a possibility of a reaction between the superconducting material and the copper of the metal tube, resulting in the composition in the resulting wire being changed or fluctuating. Since the copper tube can be oxidized, it is difficult to carry out the heat treatment at a high temperature. These problems can be avoided by using a tube made of a platinum metal that is chemically inert to the ceramic filling of the tube, and as a result the resulting wire has a composition that is evenly distributed along the length direction. The superconducting wire, whose outer metal tube is made of a platinum group metal, therefore has almost the same critical temperature as the inner body or mass made by sintering the same ceramic material powder, and it has a much higher critical current density than a wire whose outer metal tube is made of copper.

According to the invention, it is also possible to arrange another metal, e.g. copper, a copper alloy or stainless steel, around the outer metal tube. This additional metal layer increases the flexibility of the resulting wire, which is obtained by plastic deformation.

The step of plastically deforming the metal tube filled with the ceramic metal powder is preferably carried out under such conditions that the cross section of the metal tube is reduced in a dimension reduction ratio in the range of 16 to 92%, more preferably 20 to 90%. If the reduction ratio exceeds 92%, the material powder filling the tube cannot follow or accompany the movement of the inner surface of the metal tube, resulting in breakage of the sintered ceramic wire inside the metal tube at several points. On the other hand, if the reduction ratio is less than 16%, a satisfactory packing density in the metal tube cannot be expected, so that complete sintering cannot be carried out.

The plastic deformation is preferably carried out using a wire drawing process, in particular by means of a drawing die or a series of drawing dies, a cylinder drawing die or a series of cylinder drawing dies, or an extruder or a series of extruders. The plastic deformation can be carried out by forging, which is preferably achieved by means of a die forging unit or by rolling.

It is also possible to combine more than two operations of the above-mentioned plastic deformation by extrusion, rolling, drop forging and/or other wire drawing. It is also possible to form the deformed wire into the desired configuration, for example into the shape of a coil (roll) which can be applied to a superconducting magnet or the like, before the coil (roll) is heat treated.

The heat treatment of the metal tube filled with the ceramic material powder, which is carried out after the plastic deformation, is preferably carried out at a temperature in the range of 700 to 1000°C, which is selected as a function of the elements constituting the ceramic material. In this way, the superconducting powders filled in the metal tube are kept in a state in which they are in contact with each other, but do not fuse together to form a continuous body even after the plastic deformation. The heat treatment promotes sintering or reaction between the powders to form a uniform product.

Generally, the temperature at which sintering of the compound oxide powders is carried out is below the melting point of the sintered body, which is the upper limit, and preferably above a temperature lower than the melting point by 100ºC. If the sintering temperature is below the temperature lower than the melting point by 100ºC, complete sintering reaction cannot be achieved and therefore the resulting product does not have practical strength. On the other hand, if the sintering temperature exceeds the upper limit of the melting point, a liquid phase is formed, so that the sintered body melts or decomposes, resulting in a decrease in Tc.

In one embodiment, the metal tube, after being filled with the material powder, is deformed or drawn into a wire to achieve the desired configuration, the metal tube is subjected to sintering at a temperature at which the compound oxide superconductor is not formed but which is greater than or equal to half the reaction temperature in absolute temperature, to such an extent that the grain boundaries of the material powders diffuse into each other. After the wire drawing step, an intermediate annealing is carried out, which is followed by further wire drawing. If necessary, the combination of wire drawing and intermediate annealing can be repeated as often as desired. Thereafter, the sintered composite body is subjected to the final treatment which comprises slow cooling at a rate of less than 50ºC/min and rapid cooling at a rate of more than 50ºC/min. to obtain the finished superconductor product. The reasons why the above method is preferred are that the Y-Ba-Cu-O type compound oxide superconducting ceramic does not exhibit superconducting properties unless it is sintered at a temperature higher than about 900°C, and thereby the constituent element Cu of the ceramic is reduced by reacting with the metal constituting the outer tube, resulting in deterioration of superconductivity. In order to solve this problem, it is preferable to use as the material powder a powder prepared by pulverizing a sintered ceramic mass which is itself superconductive and carrying out sintering at a temperature at which no reductive reaction occurs after the wire drawing process.

It is also preferable that after completion of the sintering or heat treatment, the metal tube containing a sintered ceramic body inside it is slowly cooled at a rate of less than 50°C/min. When the method of the present invention is applied to a sintered ceramic wire made of a compound oxide of Y-Ba-Cu-O type or the like, an improvement in superconducting properties can be achieved by a heat treatment which comprises slowly cooling the sintered body at a rate of less than 50°C/min and rapid cooling thereof at a rate exceeding 50ºC/min.

The outer metal tube or the outer metal shell may be removed after completion of sintering, but if necessary, the outer metal tube may be left as it is on the outer surface of the sintered ceramic body to improve safety for the magnetic field and to provide a heat dissipation path as a precaution in case the superconductivity breaks unexpectedly. On the other hand, when the inherent properties of a sintered ceramic body, such as chemical resistance and abrasion resistance, are required, it is preferable to remove the metal tube from the sintered body (Fig. 1J). The outer metal may be removed from the sintered body by mechanical means, such as grinding, or by chemical means, such as using an etching liquid such as nitric acid.

In a variation of the method of the invention, almost all of the metal of the metal tube can be removed during the sintering step, namely when sintering and removal of the metal are carried out simultaneously, leaving a very thin skin layer on the surface of the sintered body. The thickness of the skin layer remaining on the surface of the sintered body is less than 500 µm, preferably less than 200 µm, so that even if the metal melts during sintering due to its own surface tension, the skin layer remaining on the surface is retained on the surface without dripping of the molten metal.

It is preferred that the plastic deformation is carried out by metalworking or metal processing in such a way that the object to be processed is subjected to a compressive stress, for example by Wire drawing and forging to cause the material powder filled into the metal tube to be pressed (compacted).

According to the invention, a sequence of steps is used which include, after plastic deformation, for example by wire drawing to reduce the cross-sectional dimension of the finished metal tube, performing an intermediate annealing on the deformed tube at a temperature at which the metal tube is annealed, further performing plastic deformation, for example by wire drawing on the annealed tube, and then performing a final heat treatment on the resulting tube to sinter the ceramic material powder filled in the metal tube. In this case, the metal tube can be removed after the intermediate annealing and the first annealing, but before the final annealing of the ceramic material powder, in order to prevent an undesirable reaction between the ceramic powder and the metal of the metal tube at a high sintering temperature. It is also possible to repeat another intermediate annealing after the first wire drawing, which follows the first intermediate annealing, so that the metal tube is then removed after the repeated second intermediate annealing before the final annealing. In this sequence of operations comprising the first annealing, the wire drawing and the second annealing, the second annealing advantageously provides sufficient strength to withstand a force applied externally to the annealed article and/or to give the annealed body the desired configuration before it is fed to and held in the final sintering furnace in which the annealed article is annealed without the external metal tube. The combination of the wire drawing and the intermediate annealing can be repeated with the desired number of times to increase the dimensional reduction ratio in the cross-sectional direction without wire breakage occurring, and thus the resulting wire has a small diameter and higher strength.

The intermediate annealing is carried out in a temperature range in which the metal tube is annealed but the ceramic powder is not sintered. Since the intermediate annealing following the wire drawing is carried out in a temperature range in which the metal tube is annealed but the ceramic powder is not sintered, a higher dimensional reduction ratio can be achieved to form a thin ceramic wire with a satisfactory bending strength without breaking. The intermediate annealing can be carried out at an appropriate temperature selected as a function of the type of metal constituting the metal tube and the components and composition of the ceramic powder. It is also possible to carry out a cold working step before and/or after the hot working step. In addition, it is also possible to repeat the series of steps comprising the above hot working and sintering steps several times.

In a specific variation of the method of the invention, the through holes traversing a wall of the metal tube are created after completion of the plastic deformation, so that the ceramic material powder filled into the perforated metal tube is sintered in an open state.

In this variation, the wall of the metal tube of the wire is perforated after wire drawing by means of a laser, an electron beam or a micro-drill or the like so as to allow the passage of a gas, particularly an oxygen-containing gas, through the perforations or holes. When the metal tube is not perforated, the ceramic powder material is sintered in a closed or sealed state in the outer metal tube in the sintering step, so that the oxygen deficit in the compound oxide is too large to obtain a product with improved superconductivity. It is therefore preferable to provide through holes in the wall and to carry out the sintering of the metal tube in an oxygen gas-containing atmosphere in order to provide an appropriate amount of oxygen. to the compound oxide in the metal tube. The general property of the superconducting compound oxides is that the superconductivity is affected by the oxygen deficiency in the crystal structure. In other words, the oxygen deficiency as well as the crystal structure of the resulting sintered wire are key factors for the superconductivity. It is therefore important to adjust the atomic ratios of the elements to meet the above-mentioned ranges of the elements defined in connection with the general formula: (α 1-x, βx) γy Oz and to follow the above-mentioned method of the invention to adjust the oxygen contents defined by the general formula. Otherwise, the Tc value cannot be improved due to a different crystal structure and an inappropriate oxygen deficiency.

In the special variant described above, sufficient oxygen can be supplied through the through holes or slots provided in the outer metal layer so that the resulting sintered body made of compound oxide has the crystal structure of a so-called quasi-perovskite crystal structure, e.g. an orthorhombic structure or the like, for which there is a higher possibility of the formation of Cooper pairs.

The through holes or slots are preferably closed by filling them with sealing material or by covering the entire external surface of the metal tube with another (further) metal sheath or covering in order to protect the compound oxide against deterioration as a result of attack by the surrounding moist gas.

It is also preferable to increase the partial pressure of the oxygen gas during the sintering step to promote the penetration of the oxygen gas through the holes or slits.

The present invention will now be described in more detail with reference to illustrative examples, but it should be noted that the scope of the invention is not limited thereby.

example 1

20.8 wt% of commercial Y2O3 powder, 54.7 wt% of commercial BaCO3 powder and 24.5 wt% of commercial CuO powder were mixed together in an attritor in a wet state and then dried. The dried powder was sintered in air at 880°C for 24 hours. The sintered body was pulverized and passed through a sieve to obtain a powder having a particle size of less than 100 mesh. The steps of sintering, pulverizing and sieving were repeated three times.

After the granulation treatment, the material powder was pressed (compacted) in an iron pipe with an outer diameter of 5 mm, an inner diameter of 4 mm and a length of 1 m and the opposite ends of the pipe were sealed.

When the tube filled with the material powder was subjected to a series of wire drawing operations in such a way that its outer diameter was reduced at a reduction ratio of 19% per pass, the tube broke when its outer diameter was reduced to 1.2 mm.

Wire drawing was therefore terminated when the outside diameter had been reduced to a value of 1.5 mm. The tube was then subjected to a series of operations including an intermediate annealing for 25 hours at 750ºC, a plurality of wire drawing operations each of which was carried out at a reduction ratio of 18% per pass so that the diameter of the tube was reduced to 0.6 mm, and sintering carried out at 930ºC for 3 hours. The measured value of the critical temperature (Tc) was 38ºK.

Claims (16)

1. A method of manufacturing a superconducting elongated article comprising the steps of: Filling a metal tube with a material powder, cold plastically deforming said metal tube filled with said material powder to reduce the cross-section of said metal tube, and Heating the deformed metal tube to sinter the said material powder, characterized, that the metal tube is filled with a ceramic material powder, which consists of a superconducting compound oxide, that the deformed metal tube is subjected to an intermediate annealing at a temperature such that said metal tube is annealed but said material powder is not sintered, and that the pipe is subjected to further plastic deformation before the heating step mentioned is carried out. 2. A method according to claim 1, characterized in that after sintering said ceramic powder, said metal tube containing the sintered material powder in its interior is slowly cooled at a rate of less than 50°C/min. 3. A method according to claim 1 or 2, characterized in that said ceramic material powder is a compound oxide having a K₂NiF₄ type crystal structure. 4. Process according to claim 3, characterized in that the compound oxide is (La,M)₂CuO₄, where M is Ba or Sr. 5. A method according to any one of claims 1 to 3, characterized in that said ceramic material powder is a compound oxide with a perovskite-type crystal structure, which is superconducting and has the general formula: (α 1-x, βx) γy Oz where: α is an element selected from the elements of Group IIa of the Periodic Table of Elements, β is an element selected from the elements of group IIIa of the Periodic Table of Elements, γ Cu and x, y and z numbers that satisfy the following ranges: 0.1 ≤ x ≤ 0.9 0.4 ≤ y ≤ 4.0 and 1 ≤ z ≤ 5. 6. Process according to claim 5, characterized in that α stands for Ba and β stands for Y. 7. A method according to claim 5, characterized in that said ceramic material powder has been prepared by mixing powders of Bi₂O₃, SrCO₃, CaCO₃ and CuO, drying and then pressing the Powder mixing, sintering of the pressed mass and subsequent pulverization of the sintered mass. 8. Method according to one of claims 1 to 7, characterized in that the said metal tube is made of a metal selected from a group comprising Ag, Au, Pt, Pd, Rh, Ir, Ru, Os, Cu, Al, Fe, Ni, Cr, Ti, Mo, W and Ta and alloys of these metals. 9. A method according to any one of claims 1 to 8, characterized in that said cold plastic deformation of the metal tube filled with the ceramic material powder is carried out in such a way that the cross section of said metal tube is reduced in a dimensional reduction ratio in the range of 16 to 92%. 10. Method according to one of claims 1 to 9, characterized in that the said cold plastic deformation is carried out by wire drawing. 11. A method according to claim 10, characterized in that said wire drawing is carried out with at least one device selected from the group consisting of drawing nozzles, cylinder drawing nozzles and an extruder. 12. Method according to one of claims 1 to 9, characterized in that the cold plastic deformation is carried out by forging. 13. A method according to claim 12, characterized in that said forging is carried out by means of a drop forging unit or rollers. 14. A method according to any one of claims 1 to 13, characterized in that said ceramic material powder is granulated before it is filled into said metal tube. 15. Method according to one of claims 1 to 14, characterized in that after the cold plastic deformation, but before the heating, openings are created which pass through a wall of said metal tube. 16. A method according to any one of claims 1 to 15, characterized in that said metal tube is removed from said sintered ceramic material powder after completion of cooling.