Rare earth oxide superconducting material and process for producing the same

Abstract: A rare earth oxide superconducting material represented by REBa₂Cu₃O y (RE is Y, Gd, Dy, Ho, Er or Yb), comprises oxide grains and at least one element selected from Rh and Pt, uniformly dispersed in the grain in a proportion of ⊘.⊘1-5% by weight (in terms of element) based on the rare earth oxide superconducting material. The rare earth oxide superconducting material can be produced by a melt processing and gives a high critical current density even in a highly magnetic field.

Description: The present invention relates to a rare earth oxide superconducting material and a process for producing said superconducting material. More particularly, the present invention relates to a rare earth oxide superconducting material which comprises grains of an oxide superconducting material represented by REBa₂ Cu₃ O y (RE is Y, Gd, Dy, Ho, Er or Yb) and at least one element selected from Rh and Pt, uniformly dispersed in said grain and which gives a high critical current density even in a highly magnetic field, as well as to a process for producing said superconducting material by melting the raw materials. Oxide superconducting materials have high critical temperatures and vigorous researches are under way in order to put them into practical use. In obtaining an oxide superconducting material as a bulk material, sintering has heretofore been used generally. The oxide superconducting materials obtained by sintering have a microstructure in which the grains are fine and a number of grain boundaries exist inside; when observed under an optical microscope, they show, in some cases, the presence of cracks along the grain boundaries and impurities at the grain boundaries. In these oxide superconducting materials obtained by sintering, the superconducting grains are combined with each other by a weak-link, and the critical current density (Jc) is controlled by the weak-link, making it impossible to obtain a high Jc. JP-A-1/028219 suggested increasing the Jc (e.g. to about 1200 A/cm²) by using Pt and Pd additives in the raw material for sintering. Meanwhile, in superconducting materials of single crystal structure, it is known that no grain boundary problems as mentioned above exist and that a high Jc is obtained even in a highly magnetic field. Hence, it was investigated to allow the superconducting material of microstructure obtained by sintering to approximate a single crystal structure and it was proposed to disperse microstructure particles of superconducting phase in a single crystal phase to fix the magnetic flux line coming into, that is, to form pinning centers. For example, a melting process represented by a MTG process (a Melt Textured Growth process) was proposed. In the MTG process, a rare earth oxide superconducting material, for example, is slowly cooled generally from the incongruent melting point of 123 phase [REBa₂ Cu₃ O y (RE is a rare earth element including Y)] to give rise to a peritectic reaction between 211 phase (RE₂ BaCuO₅) and liquid phase to cause crystal growth; the 211 phase exists inside the crystals because of incomplete peritectic reaction after growth and acts as pinning centers; as a result, the rare earth oxide superconducting material obtained shows a high Jc even in a magnetic field. The oxide superconducting material obtained by the MTG process, however, has various disadvantages in that the particles of the 211 phase are large and their distribution is non-uniform and that cracks exist along the direction of crystal growth. Also, a QMG process (a Quench and Melt Growth process) was proposed in EP-A-374 263 and JP-A-1538⊘3/199⊘, as an improved process for the MTG process. In the QMG process, raw materials for rare earth oxide superconducting material are subjected to melting, quenching and solidification to obtain an intermediate comprising a Ba-Cu oxide phase and an Y₂O₃ phase or the like of 5⊘ µm or less dispersed in said oxide phase, or Y₂O₃ and a Ba-Cu oxide are mixed to obtain a plate-like material of 5 mm or less in thickness or a linear material; the intermediate or the plate-like or linear material is heated at the incongruent melting point of 123 phase to convert to a semi-molten state and then is cooled slowly from that temperature at a given cooling rate to give rise to a peritectic reaction between 211 phase and liquid phase to grow a 123 phase in which a 211 phase of 2⊘ µm or less is finely and uniformly dispersed. According to the disclosure in the above document, the superconducting material obtained by the QMG process exhibits a very high pinning effect and gives an excellent Jc in a highly magnetic field. Further, a MPMG process (a melt powder and melt growth process) was proposed. In the MPMG process, a material obtained by melting, quenching and solidification according to the QMG process is ground for higher shapability and the obtained superconducting material is said to give the same high Jc as in the QMG process. The QMG process and the MPMG process, as compared with the melt processing, can exhibit a high pinning effect and give an excellent Jc, but must conduct melting in two stages making the procedure complex. It would be desirable to provide a novel rare earth oxide superconducting material showing, preferably at least to the extent obtainable by the QMG process and the MPMG process, a very high pinning effect and an excellent Jc in a highly magnetic field. In another aspect we aim to provide a novel process for producing a rare earth oxide superconducting material showing an excellent Jc, by a melt processing employing a simple procedure. In one aspect of the invention, there is provided a rare earth oxide superconducting material represented by REBa₂Cu₃O y (RE is Y, Gd, Dy, Ho, Er or Yb), comprising oxide grains and at least one element selected from Rh and Pt, contained in said grains in a proportion of ⊘.⊘1-5% by weight (in terms of element) based on the rare earth oxide superconducting material. In a second aspect of the invention, there is provided a process for producing a rare earth oxide superconducting material represented by REBa₂Cu₃O y (RE is Y, Gd, Dy, Ho, Er or Yb), comprising oxide grains and at least one element selected from Rh and Pt, contained in said grains in a proportion of ⊘.⊘1-5% by weight (in terms of element) based on the rare earth oxide superconducting material, which process comprises adding, to powders containing RE, Ba and Cu [these powders are raw materials for REBa₂Cu₃O y ], at least one element selected from Rh and Pt or at least one compound thereof so that the addition amount becomes ⊘.⊘1-5% by weight (in terms of element) based on the rare earth oxide superconducting material to be finally obtained, shaping the resulting mixture, subjecting the shaped material to a heat treatment at a temperature equal to or higher than the incongruent melting point of the REBa₂Cu₃O y oxide superconducting material, and subjecting the resulting material to slow cooling and a heat treatment. In the present technique, at least one element selected from Rh and Pt or at least one compound thereof is added to raw materials for REBa₂Cu₃O y oxide superconducting material, whereby the resulting oxide superconducting material contains Rh and/or Pt in a uniformly dispersed state. A high Jc similarly to the oxide superconducting materials obtained by the QMG process and the MPMG process, uniformity in every portion and excellent superconductivity, have been obtained thereby. The process for producing a rare earth superconducting material according to the present invention enables melting in one stage as in the conventional melting process and can provide a rare earth oxide superconducting material having excellent superconductivity. The REBa₂Cu₃O y oxide superconducting material according to the present invention may be obtained by adding given amount(s) of platinum group element(s) or compounds(s) thereof to raw material powders and then subjecting the resulting mixture to melting, slow cooling and heat treatment in substantially the same manners as in the conventional melt processing. The REBa₂Cu₃O y oxide superconducting material containing given amount(s) of platinum group element(s) in a uniformly dispersed state, obtained according to this method can exhibit uniform superconductivity in every portion, as compared with the rare earth oxide superconducting materials lacking in overall uniformity, obtained by the conventional melt processing, and give a high Jc even in a highly magnetic field similarly to the rare earth oxide superconducting materials obtained by the conventional melt processing. These features, although the reasons are not clear, are apparently caused by use of given amount(s) of platinum group element(s) or compound(s) thereof as a material and have been found by the present inventors for the first time. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a microphotograph showing the microstructure of the crystals of an example of the rare earth oxide superconducting material of the present invention. Fig. 2 is a microphotograph showing the microstructure of the crystals of a rare earth oxide superconducting material obtained by the conventional process. DETAILED DESCRIPTION Embodiments and preferred features are described in detail. The REBa₂Cu₃O y oxide superconducting material of the present invention has a multi-layered perovskite structure containing, as a rare earth element (RE), Y, Gd, Dy, Ho, Er or Yb and is represented by, for example, YBa₂Cu₃O₇. The raw material powders containing RE, Ba ad Cu elements, used for obtaining the REBa₂Cu₃O y oxide superconducting material have no particular restriction as long as they are a mixed oxide powder of an oxide of RE (i.e. Y, Gd, Dy, Ho, Er or Yb), a carbonate or oxide of Ba and an oxide of Cu, a calcined powder obtained from said mixed oxide powder, a frit powder obtained from said mixed oxide powder, or the like, in which powder the individual raw materials are contained so as to constitute, after firing, REBa₂Cu₃O y and RE₂BaCuO₅. The particle diameters of the raw material powders have no particular restriction, either, but is generally 2-2⊘ µm. In the present invention, at least one platinum group element selected from Rh and Pt or at least one compound thereof (e.g. PtO₂) is added to the above raw material powders in a proportion of ⊘.⊘1-5% by weight (in terms of element) based on the REBa₂O₃O y oxide superconducting material to be obtained finally, to obtain a mixed powder. Addition of a single metal powder of Rh or Pt is generally preferred. When the platinum group element or compound thereof is added in a proportion less than ⊘.⊘1% by weight (in terms of element) based on the REBa₂O₃O y oxide superconducting material to be obtained finally, no effect is obtained by the addition. When the addition proportion is more than 5% by weight, impurities appear in a larger amount and an undesirable effect is incurred. The platinum group element or compound thereof is added to the raw material powders for rare earth oxide superconducting material, preferably in the form of powder for easier mixing. Addition in a fine powder having particle diameters of about 2⊘ µm or less is preferred generally. When the particle diameters are more than about 2⊘ µm, the powder remains in the final product as agglomerates, reducing the homogeneity of the final product. The reduction in homogeneity causes fluctuation of superconductivity. The mixed powder consisting of the raw material powders and the platinum group element or compound thereof is thoroughly mixed to uniformly disperse the element or compound in the raw material powders; the resulting powder is shaped into a desired shape; the shaped material is heated to a temperature equal to or higher than the incongruent melting point of the REBa₂Cu₃O y oxide superconducting material to be obtained finally; the resulting material is cooled slowly as in the known melting process and then subjected to a heat treatment in an oxygen atmosphere to obtain a rare earth oxide superconducting material of the present invention. The shaping can be conducted according to a known method such as doctor blade method, press molding method, casting method or the like, to obtain a shaped material as a bulk material of rare earth oxide superconducting material. It is also possible to obtain a shaped material by spray- or powder-coating the above-mentioned mixed powder on a substrate made of a metal, a ceramic or the like to form a layer of rare earth oxide superconducting material on the substrate. The temperature equal to or higher than the incongruent melting point of desired rare earth oxide superconducting material, used in the present invention varies depending upon the kind of the RE component (either of Y, Gd, Dy, Ho, Er or Yb) employed. Said temperature is about 1⊘⊘⊘-12⊘⊘°C when the RE component is Y, about 1⊘5⊘-125⊘°C when the RE component is Gd, about 1⊘⊘⊘-12⊘⊘°C when the RE component is Dy, about 1⊘⊘⊘-115⊘°C when the RE component is Ho, about 95⊘-11⊘⊘°C when the RE component is Er, and about 9⊘⊘-11⊘⊘°C when the RE component is Yb. Said temperature can be appropriately selected within the above range, depending upon the kind of the RE component used, the heating conditions employed, the size of the shaped material prepared, etc. The heating is conducted by keeping the shaped material in the above temperature range for a given length of time. The time is not particularly restricted and, similarly to the temperature range, can be appropriately selected depending upon the heating conditions employed, etc. The time is 2⊘ minutes to 2 hours generally. The material after heating is cooled slowly as in the conventional melting process and then subjected to a heat treatment in an oxygen atmosphere at a given temperature to obtain a REBa₂Cu₃O y oxide superconducting material. The cooling is conducted at a cooling rate of preferably about 1-5°C/min. The heat treatment is preferably conducted in an oxygen atmosphere generally at 65⊘-4⊘⊘°C for about 1⊘-5⊘ hours. EXAMPLES A further explanation is now given with reference to Examples. Example 1 Powders of Y₂O₃, BaCO₃ and CuO were mixed so that the atomic ratio of Y, Ba and Cu became 1.5⊘ : 2.25 : 3.25. The mixture was calcined at 8⊘⊘°C for 1⊘ hours. The calcined product was ground in isopropyl alcohol in a rotary mill containing zirconia flint pebbles, for 15 hours to obtain a calcined powder having an average particle diameter of about 5 mm. To the calcined powder was added a platinum (Pt) powder having an average particle diameter of about 3 µm, so that the fired body (oxide superconducting material) has the content of Pt in term of element as shown in Table 1; the mixture was made uniform in isopropyl alcohol in the same rotary mill using zirconia flint pebbles. Each of the resulting mixed powders was press-molded to obtain pellets of 7 mm in thickness and 2⊘ mm in diameter. The pellets were placed in an electric furnace containing the air and kept at 115⊘°C for 1.5 hours to give rise to incongruent melting point. Each of the resulting materials was cooled slowly from 1⊘2⊘°C to 92⊘°C in 8⊘ hours, and then heat-treated at 5⊘⊘°C for 24 hours with the atmosphere in the furnace changed to oxygen, to obtain oxide superconducting materials each in pellet form. The oxide superconducting materials each in pellet form were polished and observed under an optical microscope. Fig. 1 is a microphotograph showing the microstructure of the crystals of the oxide superconducting material of sample No. 1-12, and Fig. 2 is a microphotograph showing the microstructure of the crystals of the superconducting material of sample No. 1-1 containing no platinum powder. In Fig. 1, the white matrix is an YBa₂Cu₃O y superconducting material and the fine black spots dispersed in the matrix is an Y₂BaCuO₅ non-superconducting material. Meanwhile, in Fig. 2, the black portions which are an Y₂BaCuO₅ superconducting material have larger particle diameters than in Fig. 1 and are dispersed non-uniformly. Five pellets were selected at random from each oxide superconducting material, and about 1⊘⊘ mg each was cut out from the pellets and measured for hysteresis of magnetization using a SQUID magnetometer. From the measurement was calculated a Jc (A/cm²) at a magnetic field of 1T at a temperature of 77K. The calculation results of Jc and its ranges expressed in ± are shown in Table 1. Sample No. Content of Pt (Wt.%) Jc (A/cm²) 1-1 0.0000 2800 ± 150 1-2 0.0025 3300 ± 170 1-3 0.0050 3200 ± 140 1-4 0.0075 3200 ± 110 1-5 0.0100 8700 ± 180 1-6 0.0125 11000 ± 250 1-7 0.0150 9200 ± 230 1-8 0.0175 9800 ± 230 1-9 0.02 10000 ± 310 1-10 0.05 11000 ± 200 1-11 0.10 12000 ± 190 1-12 0.50 12000 ± 230 1-13 1.00 14000 ± 420 1-14 2.00 16000 ± 500 1-15 3.00 11000 ± 260 1-16 4.50 9900 ± 270 1-17 4.75 9500 ± 240 1-18 5.00 9000 ± 320 1-19 5.25 3400 ± 200 1-20 5.50 3300 ± 210 1-21 5.75 3400 ± 220 1-22 6.00 3100 ± 160 Example 2 A calcined powder was obtained in the same manner as in Example 1 except that Gd₂O₃ was used in place of Y₂O₃ so that the atomic ratio of Gd, Ba and Cu became 1.3 : 2.⊘: 3.⊘. To the calcined powder was added a rhodium (Rh) powder having an average particle diameter of 15 µm, so that the fired body (oxide superconducting material) has the content of Pt in term of element as shown in Table 2, and mixed powders were prepared in the same manner as in Example 1 and then shaped into pellets of 7 mm in thickness and 2⊘ mm in diameter in the same manner as in Example 1. The pellets were treated in the same manner as in Example 1 except that they were kept at 112⊘°C for 1 hour to give rise to incongruent melting point and then cooled slowly from 98⊘°C to 93⊘°C in 5⊘ hours, whereby oxide superconducting materials in pellet form were obtained. The microstructure of the crystal of the oxide superconducting material of sample No. 2-12 in pellet form was the same as that of Fig. 1 of Example 1. In the same manner as in Example 1, five samples each were cut out from the pellets of the oxide superconducting materials and measured for hysteresis of magnetization to calculate the Jc (A/cm²) of each oxide superconducting material at a magnetic field of 1T at a temperature of 77 K. The calculation results of Jc and its ranges expressed in ± are shown in Table 2. Sample No. Content of Rh (Wt.%) Jc (A/cm²) 2-1 0.0000 3600 ± 260 2-2 0.0025 3200 ± 270 2-3 0.0050 4000 ± 340 2-4 0.0075 3000 ± 150 2-5 0.0100 9000 ± 380 2-6 0.0125 9100 ± 330 2-7 0.0150 8900 ± 310 2-8 0.0175 12000 ± 350 2-9 0.02 11000 ± 370 2-10 0.05 10000 ± 240 2-11 0.10 10000 ± 280 2-12 0.50 11000 ± 330 2-13 1.00 10000 ± 420 2-14 2.00 13000 ± 360 2-15 3.00 8900 ± 360 2-16 4.50 12000 ± 270 2-17 4.75 9900 ± 430 2-18 5.00 13000 ± 260 2-19 5.25 2800 ± 320 2-20 5.50 3700 ± 250 2-21 5.75 2900 ± 330 2-22 6.00 3000 ± 360 Example 3 BaCO₃ and CuO powders were weighed so that the atomic ratio of Ba and Cu became 1 : 1, mixed and calcined in an oxygen flow at 1⊘⊘⊘°C for 1⊘ hours. The calcined product was ground in isopropyl alcohol in a rotary mill containing zirconia flint pebbles, for 1⊘ hours to obtain a barium-copper oxide compound powder having an average particle diameter of about 5 µm. To the powder were added copper oxide and, as a rare earth element oxide (RE₂O₃), one of Dy₂O₃, Ho₂O₃, Er₂O₃ and Yb₂O₃ so that the atomic ratio of RE : Ba : Cu became 1.8 : 2.4 : 3.4. To the resulting powder was added a platinum powder having an average particle diameter of about 1 µm, so that the fired body (oxide superconducting material) has the content of Pt in term of element as shown in Tables 3-6. They were mixed and press-molded to obtain pellets of 1⊘ mm in thickness and 2⊘ mm in diameter. The pellets were placed in an electric furnace containing the air, and kept for 3⊘ minutes under the following temperature conditions which differed depending upon the kind of rare earth element. 115⊘°C (Dy₂O₃, Ho₂O₃) 11⊘⊘°C (Er₂O₃) 1⊘5⊘°C (Yb₂O₃) The resulting pellets were cooled slowly for 1⊘⊘ hours in the following temperature range. 1⊘⊘⊘°C to 9⊘⊘°C (Dy₂O₃, Ho₂O₃) 95⊘°C to 85⊘°C (Er₂O₃) 9⊘⊘°C to 8⊘⊘°C (Yb₂O₃) The pellets were further heat-treated at 5⊘⊘-3⊘⊘°C for 5⊘ hours in the same furnace with the atmosphere changed to oxygen, to obtain oxide superconducting materials in pellet form. In the same manner as in Example 1, 5 samples each were cut out from the pellets of the oxide superconducting materials and measured for hysteresis of magnetization to calculate the Jc (A/cm²) of each oxide superconducting material at a magnetic field of 1T at a temperature of 77 K. The calculation results of Jc and its ranges expressed in ± are shown in Tables 3-6. Sample No. Rare earth element Content of Pt (Wt.%) Jc (A/cm²) 3-1 Dy 0.0000 4000 ± 220 3-2 Dy 0.05 4400 ± 180 3-3 Dy 0.1 9600 ± 300 3-4 Dy 0.5 9500 ± 260 3-5 Dy 2 9800 ± 350 3-6 Dy 4 9500 ± 320 3-7 Dy 5 9000 ± 110 3-8 Dy 5.5 6600 ± 270 3-9 Dy 6 5300 ± 310 Sample No. Rare earth element Content of Pt (Wt.%) Jc (A/cm²) 4-1 Ho 0.0000 3800 ± 260 4-2 Ho 0.05 4500 ± 300 4-3 Ho 0.1 8900 ± 240 4-4 Ho 0.5 9500 ± 370 4-5 Ho 2 10000 ± 310 4-6 Ho 4 11000 ± 270 4-7 Ho 5 9000 ± 250 4-8 Ho 5.5 6500 ± 110 4-9 Ho 6 4200 ± 370 Sample No. Rare earth element Content of Pt (Wt.%) Jc (A/cm²) 5-1 Er 0.0000 4500 ± 330 5-2 Er 0.05 4300 ± 210 5-3 Er 0.1 8800 ± 320 5-4 Er 0.5 9000 ± 380 5-5 Er 2 9200 ± 200 5-6 Er 4 9300 ± 190 5-7 Er 5 8500 ± 300 5-8 Er 5.5 6400 ± 270 5-9 Er 6 5200 ± 190 Sample No. Rare earth element Content of Pt (Wt.%) Jc (A/cm²) 6-1 Yb 0.0000 3300 ± 150 6-2 Yb 0.05 4100 ± 260 6-3 Yb 0.1 8300 ± 390 6-4 Yb 0.5 8800 ± 230 6-5 Yb 2 8500 ± 180 6-6 Yb 4 9000 ± 410 6-7 Yb 5 8900 ± 370

Claims: A process for making a rare earth oxide superconducting material representable by the formula REBa₂Cu₃Oy, in which RE is selected from Y, Gd, Dy, Ho, Er and Yb, comprising providing a powder mixture comprising powders which contain RE, Ba and Cu and are raw materials for REBa₂Cu₃Oy; shaping the powder mixture, and subjecting it to a heat treatment at a temperature equal to or higher than the incongruent melting point of the REBa₂Cu₃Oy superconducting material, followed by slow cooling at a rate of 1 to 5 C°/minute; characterised in that an additive which is Pt and/or Rh or compound thereof is included in the powder mixture in a quantity giving from 0.01 to 5 wt% of the Pt and/or Rh (on an elemental basis) in the finally-obtained superconducting material, and the heat treatment and slow cooling are followed by a final heat treatment in oxygen at from 650 to 400°C for from 10 to 50 hours. A process according to claim 1 in which the powders have particle diameters of from 2 to 20µm. A process according to claim 1 or claim 2 in which the additive is provided as a metal powder of one of Pt and Rh. A process according to any one of the preceding claims in which the additive is a fine powder of particle size 20µm or less. A process according to any one of the preceding claims in which the additive comprises Rh or compound thereof. A process according to any one of the preceding claims in which the additive is included in a quantity giving from 0.05 to 3 wt% of the Pt and/or Rh (on an elemental basis) in the finally-obtained superconductivity material. A process according to any one of the preceding claims in which RE is Y and the incongruent melting point is from 1000 to 1200°C. A process according to any one of claims 1 to 6 in which RE is Gd and the incongruent melting point is from 1050 to 1250°C. A process according to any one of claims 1 to 6 in which RE is Dy and the incongruent melting point is from 1000 to 1200°C. A process according to any one of claims 1 to 6 in which RE is Ho and the incongruent melting point is from 1000 to 1150°C. A process according to any one of claims 1 to 6 in which RE is Er and the incongruent melting point is from 950 to 1100°C. A process according to any one of claims 1 to 6 in which RE is Yb and the incongruent melting point is from 900 to 1100°C. A rare earth oxide superconducting material obtainable by a method according to any one of claims 1 to 12, comprising oxide grains consisting essentially of REBa₂Cu₃Oy where RE is selected from Y, Gd, Dy, Ho, Er and Yb, having fine particles consisting essentially of RE₂BaCuO₅ dispersed uniformly in them, and containing Rh and/or Pt additive in an amount of from 0.01 to 5 wt% (on an elemental basis) of the superconducting material. A rare earth oxide superconducting material according to claim 13 having a critical current density (Jc) at 77K of at least 8700A/cm² under a magnetic field of 1T. A rare earth oxide superconducting material according to claim 13, obtainable by a method according to claim 6 and having a critical current density (Jc) at 77K of at least 11000A/cm² under a magnetic field of 1T.

Inventors: HIRABAYASHI IZUMI, OGAWA NAOYUKI, SAKAI TAKENOBU, HIRABAYASHI, IZUMI, OGAWA, NAOYUKI, SAKAI, TAKENOBU, HIRABAYASHI, IZUMI, INT. SUPERCONDUCTIVITY
Applicants: INT SUPERCONDUCTIVITY TECH, NGK INSULATORS LTD, TOYOTA MOTOR CO LTD, INTERNATIONAL SUPERCONDUCTIVITY TECHNOLOGY CENTER, NGK INSULATORS, LTD., TOYOTA JIDOSHA KABUSHIKI KAISHA
Application year: 1991
Main classification: C04B35
IPC codes: H01L39, C01G1, H01B12, C04B35, C01G3