PASTE FOR FORMING SUPERCONDUCTIVE CERAMIC FILM

Abstract: An improved paste for forming superconductive ceramic film comprising two powders. The first powder is composed of a complex oxide contg. as components at least one element selected from among Sc, Y and La, at least one element selected from the alkaline earth metal group, and Cu. The second powder is composed of a carbonate salt of an alkaline earth metal. The alkaline earth metal is the same in the two powders. The amt. of carbonate salt of the alkaline earth metal lies between 0.1 and 10.0 wt.% of the amt. of the paste (preferred 2.0 wt.%, best 5 wt.%). The complex oxide is represented by the molecular formula (LaBa)2CuO. The carbonate salt of the alkaline earth metal is represented by the formula BaCO3 or SrCO3. The paste contains an organic medium selected from among ethylcellulose, butylcarbinolacetate or a mixt. of these. The complex oxide can be (LaSr)2CuO, YBa2Cu3O6.5, LuBa2Cu2O6.5, YbBa2Cu3O6, TmBa2Cu3O6.5, ErBa2Cu3O6.5, TbBa2Cu3O6.3, GdBaCu3O6.5, EuBa2Cu3O6.5, SmBa2Cu3O6.5, or LaBa2Cu3O7.

Description: TECHNICAL FIELD This invention relates to a superconducting ceramic film and, more particularly, to a paste used for formation of a superconducting ceramic film capable of maintaining a high critical temperature under large electric current densities. BACKGROUND ART A typical example of the superconducting ceramic films printed on various kinds of substrate is disclosed by Hideomi Koinuma et al in " High Tc Superconductivity in Screen Printed Yb-Ba-Cu-O Films ", Japanese Journal of Applied Physics, Vol. 26, No. 5, May 1987, pages L761 to L762. For forming the superconducting ceramic films, two powders with respective nominal compositions of (Yb 0.25 Ba 0.75 )₃Cu₃O 9-δ ( which is referred to as powder A ) and (Yb 0.32 Ba 0.68 )₃Cu₃O 9-δ ( which is referred to as powder B ) were prepared by mixing the prescribed amounts of Yb₂O₃, BaCO₃ and CuO in ethanol, calcining to form a dried mixture at 900 degrees in centigrade for 12 hours, pulverizing the calcined product, then further calcining the powder at 900 degrees in centigrade for 2 days to produce the powder B. The powder B was mixed into hepthyl alcohol to produce a paste which was printed on several kinds of substrate through a 150 mesh silk screen. After being dried in vacuo, the paste was sintered in air at 900 degrees in centigrade for 0.5 to 5 hours to produce a superconducting ceramic films. Koinuma et al reported in the paper that the superconductive ceramic films had shown zero resistivities at high temperatures. Namely, one of the specimens had a temperature of zero resistivity as high as 77.2 degrees in Kelvin under an electric current density of 0.02 ampere/cm², but when the electric current density was increased to 14.2 amperes/cm², the superconducting ceramic film merely showed the zero resistivity at 15.8 degrees in Kelvin. Thus, the paste disclosed by Koinuma has a drawback in critical temperature under large electric current densities. It is therefore an important object of the present invention to provide a paste used in a screen printing which results in a superconducting ceramic film capable of maintaining a high critical temperature under a large electric current density. DISCLOSURE OF INVENTION To accomplish these objects, the present invention proposes to mix a powder of an alkaline earth metal carbonate into a powder of superconducting oxide. In accordance with the present invention, there is provided a paste used for printing a superconducting film on a substrate, comprising: a) a first powder of an oxygen compound containing at least one element selected from the group consisting of scandium, yttrium and lathanides, at least one element selected from the group consisting of alkaline earth metals, and copper; and b) a second powder of an alkaline earth metal carbonate. The oxygen compound may be selected from the group consisting of (LaBa)₂CuO, (LaSr)₂CuO, (LaCa)₂CuO, YBa₂Cu₃O 6.5 , LuBa₂Cu₃O 6.5 , YbBa₂Cu₃O₆, TmBa₂Cu₃O 6.5 , ErBa₂Cu₃O 6.5 , DyBa₂Cu₃O 6.4 , TbBa₂Cu₃O 6.3 , GdBa₂Cu₃O 6.5 , EuBa₂Cu₃O 6.5 , SmBa₂Cu₃O 6.5 and LaBa₂Cu₃O₇. On the other hand, the alkaline earth metal carbonate may be selected from the group consisting of BaCO₃, SrCO₃ and CaCO₃. In an implementation, the alkaline earth metal carbonate has a proportion ranging between about 0.1 % and about 10.0 % by weight with respect to the paste. The paste is allowed to contain an organic vehicle which may be selected from the group consisting of ethyl cellulose, butyl carbitol acetate and a mixture thereof. The oxygen compound may contain a plurality of elements selected from the group consisting of scandium, yttrium and lathanides, a plurality of elements selected from the group consisting of alkaline earth metals and copper. The inventors discover that a carbonate affects a superconducting oxide upon the sintering process and, accordingly, improves the electric current density thereof. Particularly, an alkaline earth metal carbonate does not react with the superconducting oxide, so that the alkaline earth metal carbonate is the most preferable additive to improve the electric current density of the superconducting ceramic material without the sacrifice of the critical temperature or the temperature of zero resistivity. Experimental results are shown in Table I in which a powder of an alkaline earth metal carbonate is added to a powder of a superconducting oxide to form a paste for printing on a substrate, then sintering the paste, thereby obtaining each of samples No. 1 to 14. However, samples No. 15 to 19 are formed without the powder of alkaline earth metal carbonate. Each sintering temperature and each period of time are shown in Table II. Comparing the samples No. 1 to 14 with the samples No. 15 to 19, it is understood that relatively high critical temperatures are maintained even if the electric current densities are increased. This is because of the fact that the powder of the alkaline earth metal carbonate is mixed into the powder of superconducting oxide. No. Sintering Temp. ( in Kelvin scale ) Sintering Time ( hour ) 1 1123 2 2 1123 2 3 1123 2 4 1173 2 5 1123 2.5 6 1173 1 7 1123 1 8 1173 3 9 1173 3 10 1073 4 11 1173 2 12 1173 2 13 1173 3 14 1123 2 15 1123 2 16 1173 2 17 1173 1 18 1123 1 19 1173 3 BEST MODE FOR CARRYING OUT THE INVENTION First, a superconducting ceramic material of an oxygen compound, an alkaline earth metal carbonate and an organic vehicle are prepared to form a paste. The oxygen compound is represented by the molecular formula of YBa₂Cu₃O 6.5 , and the alkaline earth metal carbonate is represented by the molecular formula of BaCO₃. The organic vehicle is formed by mixing ethyl cellulose into butyl carbitol acetate and is proportioned to ethyl cellulose-butyl carbitol acetate ratio 1:4. The oxygen compound and the alkaline earth metal carbonate are respectively pulverized to form a first powder of the oxygen compound and a second powder of the alkaline earth metal carbonate, and the first and second powders are sifted by using a 325 mesh screen. The first and second powders are mixed into the organic vehicle to form the paste which contains about 81.5 % of the oxygen compound, about 8.5 % of the alkaline earth metal carbonate and 10 % of the organic vehicle by weight. The paste thus prepared is printed on a top surface of an yttrium stabilized zirconia substrate through a 250 mesh screen in Japanese Mesh Screen Standard to form a thin film, and the printed film is dried in air, then being further dried in a high temperature ambient at about 125 degrees in centigrade for about 10 minutes. The yttrium stabilized zirconia substrate is manufactured by Toyo Soda Corporation and has a generally square-shaped top surface measuring about 25 mm x 25 mm. The yttrium stabilized zirconia substrate has a thickness of about 1 mm. Then, the yttrium stabilized zirconia substrate with the printed film is placed in a high temperature ambient of about 930 degrees in centigrade for about 4 hours, thereby forming a superconducting ceramic film on the yttrium stabilized zirconia substrate. The superconducting ceramic film is a generally rectangular shape measuring about 20 mm x about 10 mm and having a thickness of about 15 micrometers. The resistivity and its temperature dependence are measured by the dc four probe method. The superconducting ceramic film stays in the superconducting state and the critical temperature or the temperature of zero resistivity as high as about 91 degrees in Kelvin is achieved. When the electric current density is increased to about 750 amperes/ cm², no substantial difference takes place in the critical temperature. Preferred modes of carrying out the invention are described in the claims. INDUSTRICAL APPLICATION As described hereinbefore, the paste used for formation of a superconducting ceramic film is useful in formation of a superconducting ceramic film. Moreover, the paste is used in a screen printing process. The superconducting ceramic film thus formed is capable of keeping a high critical temperature even if the current density is increased.

Claims: A paste used for printing a superconducting film on a substrate, comprising: a) a first powder of an oxygen compound containing at least one element selected from the group consisting of scandium, yttrium and lathanides, at least one element selected from the group consisting of alkaline earth metals, and copper; and b) a second powder of an alkaline earth metal carbonate. A paste as set forth in claim 1, in which an alkaline earth metal used for forming said alkaline earth metal carbonate is identical with said element selected from the group consisting of alkaline earth metals used for forming said first powder. A paste as set forth in claim 2, in which said alkaline earth metal carbonate has a proportion ranging between about 0.1 % and about 10.0 % by weight with respect to said paste. A paste used for printing a superconducting film on an yttrium stabilized zirconia substrate , comprising: a) a first powder of an oxygen compound represented by the molecular formula of YBa₂Cu₃O 6.5 which is about 81.5 % by weight with respect to said paste; b) a second powder of an alkaline earth metal carbonate represented by the molecular formula of BaCO₃ which is about 8.5 % by weight with respect to said paste; and c) an organic vehicle containing about 2 % of ethyl cellulose and about 8 % of butyl carbitol acetate by weight with respect to said paste, said first and second powders being mixed into said organic vehicle.

Inventors: KANDA, YOSHIO MITSUBISHI K.K.K. CHUO KENKYUJO, MORI,SATORU MITSUBISHI KINZOKU K.K. CHUO KENKYUJO, TAKESHITA,TAKUO MITSUBISHI K.K.K. CHUO KENKYUJO, YOSHIDA,HIDEAKI MITSUB. K.K.K. CHUO KENKYUJO
Applicants: MITSUBISHI MATERIALS CORPORATION
Application year: 1988
Main classification: C01G3
IPC codes: H01L39, H01B12, C01G9, C04B35, C01G1, C01G3, H01B13