YTTRIUM-BARIUM-COPPER OXIDE POWDER AND PROCESS FOR MANUFACTURING YTTRIUM-BARIUM-COPPER OXIDE SUPERCONDUCTING SINTERED BODY

Abstract: An yttrium-barium-copper oxide powder consisting of a powdery mixture of Y₂BaCuO₅ with Ba₃Cu₅O₈, which can be formed into a dense sintered body having excellent superconducting characteristics. An yttrium-barium-­ copper oxide superconducting body is manufactured by firing the above powder preferably at 820-980°C under oxidizing atmosphere having an oxygen partial pressure of 0.01-0.5 atm.

Description: The present invention relates to an yttrium-barium-copper oxide powder from which a dense sintered body having excellent superconducting characteristics is obtainable and a process for manufacturing an yttrium-barium-copper oxide superconducting sintered body by using the above powder. In superconducting bodies, to heighten the critical current density that is one of superconducting characteristics is an indispensable requirement for putting the superconducting bodies to practical use. In superconducting sintered bodies, since high densification of the sintered bodies can increase the critical current density, extensive studies for obtaining dense sintered bodies have been carried out. Heretofore, as a manufacturing process of YBa₂Cu₃O₇ (oxygen nonstoichiometry omitted) superconducting sintered bodies, there have been known (1) processes wherein a YBa₂Cu₃O₇ powder is synthesized and then sintered, (2) processes wherein at least two kinds of powders are mixed together and YBa₂Cu₃O₇ is manufactured by reaction sintering, etc. See for example WO 88/05029 (pages 8 and 9) and JP-A-1-160 825 (published 23 June 1989). In the above processes (1) the YBa₂Cu₃O₇ powder is generally obtained by mixing predetermined amounts of Y₂O₃ powder, BaCO₃ powder and CuO powder, followed by calcining at 900∼950°C in the ambient atmosphere or oxygen. Other than the above, there are a manufacturing process of YBa₂Cu₃O₇ comprising synthesis by a co-precipitation method of a nitrate, oxalate or the like containing a predetermined amount of Y-Ba-Cu, followed by calcination and also a process comprising a gas phase synthesis method to obtain a YBa₂Cu₃O₇ powder directly, etc. The above processes (2) include a process wherein a mixture of a Y₂BaCuO₅ powder with one kind or more of unknown phase powders (hereinafter referred to 16 as "A-powder") is obtained by mixing predetermined amounts of Y₂O₃ powder, BaCO₃ powder and CuO powder, followed by calcining at 900∼950°C in vacuum (under an oxygen partial pressure of 10 Pa (10⁻⁴ atm.) or less). In the above-described processes (1) there has been presented a problem that dense sintered bodies cannot be obtained due to a poor sintering ability of YBa₂Cu₃O₇. Although the densification is possible to a certain extent by providing a YBa₂Cu₃O₇ fine powder by a co-precipitation method, a gas phase synthesis method or the like, there have been disadvantages such as a difficulty in composition control and an extreme cost increase. In the case of the above-described processes (2) where the mixture of a Y₂BaCuO₅ powder with A-powder is used, dense sintered bodies can be obtained owing to an excellent sintering property. However, A-powder has such a high reactivity with moisture, carbon dioxide and oxygen in the ambient atmosphere that difficulties have been encountered in storage and handling of A-powder. An object of the present invention is to provide an yttrium-barium-copper oxide powder which is excellent in sintering property, low in reactivity with the ambient atmosphere and easy to manufacture, and obviates the above-described disadvantages and difficulties. Another object of the present invention is to provide a process for manufacturing, by using the above powder, an yttrium-barium-copper oxide superconducting sintered body which is dense and excellent in superconducting characteristics. The yttrium-barium-copper oxide powder according to the present invention is characterized by a mixture of a Y₂BaCuO₅ powder with a Ba₃Cu₅O₈ powder. Preferably this is an equimolar mixture, giving a ratio of Y:Ba:Cu = 1:2:3. The process according to the present invention for manufacturing an yttrium-barium-copper oxide superconducting sintered body is set out in claim 2. Here, the oxidizing atmosphere preferably has an oxygen partial pressure of 1 x 10³ ∼ 50 x 10³ Pa 0.01∼0.5 atm. ). Preferably the mixture is equimolar. The invention will now be described with reference to the accompanying drawing wherein: Fig. 1 is a chart showing an X-ray diffraction pattern by Cu-Kα-ray of the powder of the present invention. Although various processes are conceivable for manufacturing a mixture of Y₂BaCuO₅ with Ba₃Cu₅O₈, they are not essential in the present invention. There will be no problem either in a process wherein the Y₂BaCuO₅ powder and Ba₃Cu₅O₈ powder are prepared severally and then mixed mechanically or a process wherein a powdery mixture of Y₂BaCuO₅ with Ba₃Cu₅O₈ is directly obtained. The Ba₃Cu₅O₈ powder does not exist as a stable phase under a high oxygen partial pressure such as in the air or oxygen, so that its manufacturing process necessarily comprises mixing of predetermined amounts of a BaCO₃ powder and a CuO powder and then calcining the resultant mixture at 850∼950°C in an atmosphere having an oxygen partial pressure of 100 Pa (10⁻³ atm.) or less, followed by a temperature decrease to 300∼500°C and an oxygen partial pressure increase to 1 x 10³ Pa (10⁻² atm.) or more. Although the crystal structure and lattice constant of Ba₃Cu₅O₈ have not so far been reported, a Ba₃Cu₅O₈ phase could be identified by similarity of the X-ray diffraction pattern shown in Fig. 1 to X-ray diffraction patterns of Sr₃Cu₅O₈ and Sr₂BaCu₅O₈ reported by Ikeda, et al. (Jpn. Soc. Powder and Powder Metall., Spring Conference, 1988, Preliminary Manuscript of lectures, p. 31). However, the composition of oxygen was not determined precisely so that there was a possibility that the oxygen content was not exactly 8, whereas in the present invention the oxygen nonstoichiometry is not material. It has been found from the result of assiduous studies that excellent sintering properties of the powdery mixture of Y₂BaCuO₅ and Ba₃Cu₅O₈ are attributable to the following sintering process. Ba₃Cu₅O₈ decomposes at 500∼600°C into BaCuO₂ and an unknown phase (B-phase). The B-phase is a CuO-rich phase and, at 820°C or more, forms a liquid phase. The densification progresses as Y₂BaCuO₅ and BaCuO₂ reaction-sinter with the aid of this liquid phase. The firing temperature is 820∼980°C. This is because the liquid formation at below 820°C will be too little for the densification to progress, while at above 980°C YBa₂Cu₃O₇ will melt or decompose. More preferably it is 920∼950°C. The reason why the firing atmosphere is defined to be an oxidizing atmosphere, preferably having an oxygen partial pressure of 1 x 10³ ∼ 50 x 10³ Pa (0.01∼0.5 atm.), is because in a reducing atmosphere having an oxygen partial pressure of less than 1 x 10³ Pa (0.01 atm.), YBa₂Cu₃O₇ will be so unstable that Y₂BaCuO₅ and BaCuO₂ will not react with each other. Alternatively, if the oxygen partial pressure exceeds 50 x 10³ Pa (0.5 atm.), Ba₃Cu₅O₈ will not decompose at below 900°C and Y₂BaCuO₅ reacts directly with Ba₃Cu₅O₈ to produce YBa₂Cu₃O₇, so that the liquid phase formation is too little to facilitate the densification. However, in this case if the temperature is increased to a high temperature, for example, 950°C, the densification can be achieved satisfactorily. Ba₃Cu₅O₈ is inherently not in a stable phase in the atmosphere. However, it does not decompose at below 500°C and remains stable at room temperature or thereabouts substantially without reacting with moisture, carbon dioxide and oxygen in the ambient atmosphere. The invention will be explained hereinafter in more detail by way of example. Example 1 A Y₂O₃ powder (average particle diameter: 0.4 µm), a BaCO₃ powder (average particle diameter: 0.8 µm) and a CuO powder (average particle diameter: 2.5 µm), each having a purity of 99.9% were compounded at a molar ratio of 1:4:6 and then mixed for 15 hours with nylon coated steel balls in a pot-mill using water as a solvent. The mixed solution was passed through a 44 µm mesh sieve and then dried at 100°C in a hot-air dryer. The dried powder was put into an alumina crucible and kept at 900°C for 10 hours in nitrogen gas stream (oxygen partial pressure: 1 Pa (10⁻⁵ atm.)) of 0.4 ℓ/min. Then the temperature was reduced at a rate of 2°C/min. to 500°C which temperature was kept for 1 hour, replacing the atmosphere with oxygen gas stream. Then the temperature was reduced at a rate of 2°C/min. to room temperature. The resultant calcined powder was pulverized for 20 min. with an agate mortar. An X-ray diffraction pattern by Cu-Kα-ray of the obtained powder is shown in Fig. 1. In Fig. 1, the denotation a is a diffraction line from Y₂BaCuO₅, shown by C. Michel and B. Raveau in an article (C. Michel et al, J. Solid State Chem., 43 , 73-80, 1982) and the denotation b is a diffraction line from Ba₃Cu₅O₈. According to the result of ICP spectrometry, this powder had a cation molar ratio, Y:Ba:Cu, of 1.0:2.0:3.0. Example 2 The powder prepared in Example 1 was left to stand for 3 days in the ambient atmosphere with a humidity of 50∼80%. An X-ray diffraction measurement was conducted on the powder after standing and, however, no change in the X-ray diffraction pattern was observed. Likewise, a mixture of Y₂BaCuO₅ powder which had been calcined at 900°C for 10 hours in a vacuum and A-powder (described above) was left to stand in the ambient atmosphere with a humidity of 50∼80%. From a few minutes after standing, the powder changed its color from green to brown and absorbed moisture. From an X-ray diffraction pattern of the powder after standing for 1 hour, it was recognized that a majority of the A-powder decomposed to BaCO₃ and CuO. Example 3 The powder prepared in Example 1 was put into a toluene solution with zirconia grinding media and pulverized and mixed for 6 hours in a pot-mill. The mixed solution was passed through a 74 µm mesh sieve and then dried. This powder had an average particle diameter of 4 µm. This powder was molded into a sample of 40mmx30mmx5mm with an iron mold and rubber-pressed under a hydrostatic pressure of 7 tons/cm². The temperature was increased at a rate of 5°C/min., kept at 920°C and 950°C respectively for 3 hours and reduced at a rate of 2°C/min. The oxygen partial pressure of the atmosphere was controlled by mixing oxygen gas with nitrogen gas at an arbitrary ratio through flowmeters and monitored by a zirconia oxygen sensor at an exit port of the electric furnace. The obtained sintered body was measured for bulk density by the Archimedes' method using kerosine as a solution. With respect to the critical current density, the sample was finished into 20mm×4mm×2mm and determined by the four-terminal method in liquid nitrogen at 77°K. The relation between the oxygen partial pressure and the bulk density of the sintered body is shown in Table 1. As shown in Table 1, when the firing temperature was 920°C, the samples Nos. 1∼5 of the present invention yielded dense sintered bodies having a bulk density of 5.6 g/cm³ or more at an oxygen partial pressure of 1 x 10³ ∼ 50 x 10³ Pa (0.01∼0.5 atm.), while sample No. 13 of Comparative Example wherein the oxygen partial pressure was less than 1 x 10³ Pa (0.01 atm.) did not form YBa₂Cu₃O₇ at less than 1 x 10³ Pa (0.01 atm.), which showed a critical current density of 0 and had no superconducting characteristics. When the oxygen partial pressure exceeded 50 x 10³ Pa (0.5 atm.) at 920°C, densification was barely effected even in Examples of the present invention such as Samples Nos. 11 and 12. At 950°C, the bulk density exceeded 5.6 g/cm³ over all oxygen partial pressures. When the oxygen partial pressure was 10 x 10³ Pa ∼ 50 x 10³ Pa (0.1∼0.5 atm.), the density was so high as 5.8 g/cm³ or more. In the foregoing Examples, only YBa₂Cu₃O₇ was taken up as a superconducting sintered body. However, it is needless to say that those substituting another rare-earth element for Y will be able to provide sintered bodies equivalent to the present invention, on the chemical analogy of rare earth elements. As explained above, since the yttrium-barium-copper oxide powder of the present invention is a superconducting material powder that is readily densifiable and stable in the atmosphere, sintered bodies high in density and excellent in superconducting characteristics can be obtained from the above material powder according to the process of the invention.

Claims: A yttrium-barium-copper oxide powder consisting of a mixture of a Y₂BaCuO₅ powder with a Ba₃Cu₅O₈ powder. A process for manufacturing an yttrium-barium-copper oxide superconducting sintered body which process comprises firing a mixture of a Y₂BaCuO₅ powder with a Ba₃Cu₅O₈ powder in an oxidizing atmosphere having an oxygen partial pressure of at least 1 x 10³ Pa (0.01 atm.) in a temperature range of 820°C to 980°C. The process claimed in claim 2, wherein said oxidizing atmosphere has an oxygen partial pressure between 1 x 10³ and 50 x 10³ Pa (0.01 atm. and 0.5 atm.). The process claimed in claim 2 or claim 3, wherein said firing is conducted at a temperature between 920°C and 950°C.

Inventors: MATSUHIRO, KEIJI, SAKAI, HITOSHI, YOSHIDA, MANABU
Applicants: NGK INSULATORS, LTD.
Application year: 1989
Main classification: C04B35
IPC codes: C04B35, C01G3, H01L39, C01G1, H01B12