Process for producing a superconductor of an oxide system from acetylacetonates

A process for producing a superconductor of an oxide system, which comprises uniformly mixing metal elements for constituting the oxide system at least partly in the form of acetylacetonates in a solvent with the rest, if any, being in the form of alkoxides, carboxylates and/or inorganic salts, hydroxides and/or oxides to obtain a homogeneous mixture, and sintering the mixture.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a process for producing a superconductor 
of an oxide system. 
2. Discussion of Background 
Conventional superconductors are most commonly of a metallic type. Among 
them, Nb.sub.3 Ge had the highest transition temperature (critical 
temperature) for superconductivity at a level of 23.2 K. 
On the other hand, with superconductors of a metal oxide system, the 
critical temperature was usually lower than the superconductor of a 
metallic system and was at a level of 13 K even with BaPb.sub.1-x Bi.sub.x 
O.sub.3 which had the highest critical temperature. 
Recently, however, as a superconductor of an oxide system having a high 
critical temperature, a material of a La-Sr-Cu-O system (critical 
temperature: about 40 K) and a material of a Y-Ba-Cu-O system (critical 
temperature: about 90 K) have been discovered and have created a boom for 
the development of materials having high temperature superconductivity. 
For the preparation of these superconductors of an oxide system, a 
so-called dry (powder) method and a coprecipitation method have been 
commonly and widely used as disclosed in Zeitschrift for Physik 
B-Condensed Matter, Vol. 64, p. 189 (1986) and Japanese Journal of Applied 
Physics, Vol. 26, No. 3, PL 196 (1987) and ditto, Vol. 26, No. 4, PL 314 
(1987). 
The dry method is a method wherein powder materials of oxides or carbonates 
of e.g. La, Y, Ba, Sr and Cu are mechanically mixed by means of a mortar 
or a mill, followed by sintering to obtain a sintered product of oxides. 
The coprecipitation method is a method wherein nitrates of the 
above-mentioned metals were uniformly mixed and dissolved in an aqueous 
medium, and then oxalic acid or ammonia is added to simultaneously form 
the respective precipitates in the form of a mixture 
The conventional dry method as mentioned above has drawbacks such that even 
when guaranteed reagents are used as the respective powders, their purity 
is not so high at a level of from 98 to 99.9% by weight, and impurities 
are included in the superconductor after sintering. There is a limitation 
in the uniformity of the mixed state attainable by mechanical mixing of 
the respective characteristics such that the critical temperature is low, 
the transition temperature range is wide, and the critical current density 
is small. Further, the sintering temperature is required to be high, and 
it takes a long time for the sintering. 
In the coprecipitation method, since alkaline earth metal ions hardly 
precipitate unless the aqueous solution of the mixture is made alkaline, 
ammonia or the like is added to facilitate the precipitation of alkaline 
earth metal ions. However, it has a drawback that if ammonia or the like 
is added, copper forms complex ions, which can hardly be precipitated. 
Therefore, it has been pointed out that the coprecipitation method is not 
suitable to obtain a superconductor of an oxide system having a specific 
desired composition (Applied Physics, Vol. 56, No. 5, p. 606 (1987)). 
Thus, the coprecipitation method also has a problem in obtaining a 
sintered product having good superconducting characteristics. 
Recently, a new high temperature oxide superconductor (constituting 
elements: Bi-Sr-Ca-Cu-O) containing no rare earth elements has been 
reported at a press conference on Jan. 21, 1988 and published on Jan. 22, 
1988 by Kinzoku Zairyo Gijutsu Kenkyusho, and has created a further drive 
for the research of new superconducting superconductor of a Y-Ba-Cu-O 
system discovered by professor Chu of Houston University and contains no 
rare earth elements, and it shows superconducting characteristics even 
when dipped in water and is stable and readily reproducible. Further, it 
does not contain Ba as opposed to the oxide superconductor of a YBCO 
system and is free from the possibility that Ba turns into BaCO.sub.3 
during the sintering. It is therefore possible to set the sintering 
temperature at a low level. Thus, it is considered to be a practical 
superconductor. However, this superconductor of a Bi-Sr-Ca-Cu-O system is 
also produced by a dry system having the above-mentioned problem. 
Further, a new superconductor of an oxide system containing thallium has 
been discovered by professor Haman of Arkansas University in the United 
States, which has further promoted the research for new superconducting 
materials. This superconductor of a Tl-Ca-Ba-Cu-O system has a critical 
temperature higher than the superconductor of a Y-Ba-Cu-O system and can 
be regarded as a more practical superconducting material. 
However, this superconductor was also prepared by a dry method wherein 
powder materials of oxides or carbonates of thallium, calcium, barium and 
copper were mixed by means of a mortar or a mill, followed by sintering to 
obtain a sintered product of oxides. 
SUMMARY OF THE INVENTION 
The present invention has been made to overcome the above-mentioned 
problems 
More specifically, it is an object of the present invention to provide a 
process for producing a superconductor of an oxide system having excellent 
superconducting characteristics such that as compared with the 
conventional methods, the transition temperature (critical temperature) 
for superconductivity is high, the transition temperature range is narrow 
and the critical current density (current density at the critical 
temperature or below) can be made high. 
It is another object of the present invention to provide a process for 
producing an oxide superconductor of a Bi-Sr-Ca-Cu-O system having 
excellent superconducting characteristics, whereby the metal elements are 
homogeneously mixed as compared with the conventional method for the 
production of such a superconductor. 
A further object of the present invention is to provide a process for 
producing a superconductor of a Tl-Ca-Ba-Cu-O system having excellent 
superconducting characteristics by a low temperature sintering. 
The present invention provides a process for producing a superconductor of 
an oxide system, which comprises uniformly mixing metal elements for 
constituting the oxide system at least partly in the form of 
acetylacetonates in a solvent with the rest, if any, being in the form of 
alkoxides, carboxylates and/or inorganic salts, hydroxides and/or oxides 
to obtain a homogeneous mixture, and sintering the mixture.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In a first embodiment of the present invention, the metal elements for 
constituting the oxide system are at least one element selected from the 
group consisting of Mg, Ca, Sr and Ba of Group IIa of the Periodic Table, 
at least one element selected from the group consisting of Sc, Y and 
lanthanoides of Group IIIa of the Periodic Table, and Cu, and such metal 
elements are mixed wholly in the form of acetylacetonates. The homogeneous 
mixture of the acetylacetonates is then hydrolyzed with an alkali, and the 
hydrolyzed mixture is washed, followed by sintering to obtain the desired 
superconductor. 
In the present invention, metal acetylacetonates which can readily be 
uniformly dissolved, dispersed or suspended in a solvent, are employed, 
whereby a homogeneous mixture can readily be obtained. The metal 
acetylacetonates are hydrolyzed by an addition of an alkali to the 
homogeneous mixture, whereby the hydrolyzing speed is very high, and the 
hydrolysis can readily and completely be conducted. The product thereby 
obtained is usually gel-form metal hydrates (hydroxides) or oxides, which 
are hardly soluble, whereby a homogeneous gel mixture is obtained. 
Further, the hydrolyzed mixture is washed, whereby adverse effects of 
alkali metal ions to the superconductivity will be eliminated. (If the 
hydrolyzed mixture is not washed, the transition temperature will be lower 
by from 1 to 5 K, the transition temperature range will be wider by from 3 
to 16 K, and the critical current density will be lower by from 2 to 20 
A/cm.sup.2.) 
The metal acetylacetonates can be prepared in a high purity usually at a 
level of from 99.999 to 99.99999%. By using such pure materials and highly 
pure water (such as deionized water or distilled water), it is possible to 
obtain metal hydrates (hydroxides) or oxide particles (powder) with a 
purity substantially higher than the case where inorganic reagents such as 
metal oxides or carbonates are employed (a dry method, or a 
coprecipitation method). 
The present inventors have confirmed by experiments that high performance 
superconductors of an oxide system can be obtained by the process of the 
present invention wherein acetylacetonates of at least one element 
selected from the group consisting of Mg, Ca, Sr and Ba of Group IIa of 
the Periodic Table, at least one element selected from the group 
consisting of Sc, Y and lanthanoides as the starting materials. The 
hydrolyzates obtained by this analyzed by the X-ray analysis. It has been 
confirmed by experiments that such metal hydrates can be all converted to 
metal oxides by sintering. 
In this embodiment, an acetylacetonate of at least one element selected 
from the group consisting of Mg, Ca, Sr and Ba (hereinafter referred to as 
a Group IIa acetylacetonate), an acetylacetonate of at least one element 
selected from the group consisting of Sc, Y and lanthanoides (hereinafter 
referred to as a Group IIIa acetylacetonate) and Cu acetylacetonate are 
employed. 
A typical Group IIa acetylacetonate may be represented by the formula 
(CH.sub.3 COCHCOCH.sub.3).sub.2 M.sup.1 wherein M.sup.1 is Mg, Ca, Sr or 
Ba. A typical Group IIIa acetylacetonate may be represented by the formula 
(CH.sub.3 COCHCOCH.sub.3).sub.3 M.sup.2 wherein M.sup.2 is Sc, Y or a 
lanthanoid. A typical Cu acetylacetonate may be represented by (CH.sub.3 
COCHCOCH.sub.3).sub.2 Cu. However, the acetylacetonates are not limited to 
such specific examples, and may be any acetylacetonates so long as at 
least one acetylacetonate group is attached to the above-mentioned 
respective metal elements. 
There is no particular restriction as to the ratio of the Group IIa 
acetylacetonate, the Group IIIa acetylacetonate and the Cu 
acetylacetonate. They may be mixed in any ratio so long as the desired 
superconductor of an oxide system can be obtained. However, when Y is used 
as the Group IIIa element, it is preferred to mix them in a ratio of the 
Group IIa acetylacetonate/Y acetylacetonate/Cu acetylacetonate 
=2-10/1/3-10 (atomic ratio of metals). When La is used as the Group IIIa 
element, it is preferred to mix them in a ratio of (the Group IIa 
acetylacetonate+La acetylacetonate)/Cu acetylacetonate=2/1 (atomic ratio 
of metals). There is no particular restriction as to the atomic ratio of 
metals as between the Group IIa acetylacetonate and the La 
acetylacetonate. 
The above-mentioned metal acetylacetonates are uniformly dissolved, 
dispersed or suspended in a solvent such as methanol, ethanol, 
isopropanol, benzene, toluene, xylene, tetrahydrofuran, diethyl ether, 
diphenyl ether or dioxane and then hydrolyzed by an addition of an alkali 
such as KOH, NaOH or LiOH, if necessary, together with water. 
In this specification, the term "uniformly mixed" is a concept which covers 
not only the uniformity like a solution but also a substantially uniformly 
mixed state of an emulsion or a dispersion. 
There is no particular restriction as to the conditions for the hydrolysis 
such as the concentration of the metal acetylacetonates, the amount of the 
alkali and the amount of water. However, the amount of the alkali for the 
hydrolysis is preferably from 0.1 to 5 mol% relative to the total molar 
amount of the metal acetylacetonates used, whereby the alkali can be 
removed substantially completely in the subsequent washing step, and a 
mixture of high purity can readily be obtained. The amount of water may be 
at any level in excess of the stoichiometric amount for the hydrolysis of 
the metal acetylacetonates and is preferably in large excess. The 
temperature is preferably at least 60.degree. C. 
The hydrolyzed mixture thus prepared, is washed with distilled water or 
deionized water, or a water-containing solvent wherein an organic solvent 
well missible with water, such as methanol, ethanol, propanol or acetone 
is added. 
When the hydrolyzed mixture (metal oxides or hydrates) is in a liquid form, 
it is cleaned, for example, by filtration and drying followed by washing 
with water, by evaporating the solvent, followed by washing, or by 
repeating the operation of centrifugal separation and addition of a 
solvent. When the hydrolyzed mixture is in a gel form, the alkali metal 
ions may be removed, for example, by washing it as it is, or in some 
cases, washing it after evaporating the solvent or after filtration and 
drying. 
The hydrolyzed homogeneous mixture thus washed, is sintered to obtain a 
superconductor of an oxide system. 
The hydrolyzates may be metal oxides depending upon the elements. However, 
hydrolyzates are usually amorphous hydrates (inclusive of hydroxides) in 
many cases, and such amorphous hydrates can readily be converted to metal 
oxides at a relatively low temperature (200-500.degree. C.) by sintering. 
The organic components are thermally decomposed by sintering. However, the 
atomic ratio of the metal components in the sintered product are 
substantially the same as the atomic ratio of metals in the metal 
acetylacetonates used as the starting materials. 
There is no particular restriction as to the sintering conditions (such as 
the temperature, the number of times and the atmosphere). The sintering is 
conducted usually at a temperature of from 800 to 1,000.degree. C., 
preferably from 900.degree. to 950.degree. C. for at least 2 hours, 
preferably at least 4 hours. At the time of sintering, the hydrolyzed 
mixture may be presintered at a temperature of from 800 to 1,000.degree. 
C., preferably from 900.degree. to 950.degree. C., for at least 2 hours, 
preferably at least 4 hours, then pulverized and molded and finally 
sintered at a temperature of from 850.degree. to 1000.degree. C., 
preferably from 900.degree. to 950.degree. C. for at least 2 hours, 
preferably at least 4 hours. 
There is no particular restriction as to the pressure for molding. However, 
the pressure is preferably high and is usually preferably at least 0.5 
kg/cm.sup.2 G. The atmosphere for sintering may be an oxygen atmosphere or 
air. However, in order to obtain a product having good superconductivity, 
it is preferred to employ an atmosphere rich in oxygen. 
There is no particular restriction as to the cooling conditions after the 
sintering. The sintered products may be naturally cooled in air unless 
cracking occurs, or may be cooled in an oxygen stream over a period of 
about 8 hours. 
In a second embodiment of the present invention, the same metal elements as 
in the first embodiment are uniformly mixed in a solvent in the form of 
the respective alkoxides or acetylacetonates, followed by hydrolysis to 
obtain a mixture of oxides or hydrages (hydroxides) of such metal 
elements, and the mixture is then sintered to obtain a superconductor of 
an oxide system. 
The alkoxides or acetylacetonates of the metal elements used in this 
embodiment are uniformly dissolved, dispersed or suspended in a solvent 
and then hydrolyzed by an addition of water, whereby they usually undergo 
a change from a sol to a gel and finally form the metal hydrates 
(hydroxides) or oxide particles. This method is a so-called alkoxide 
process and has such features that it is thereby possible to obtain super 
fine particles of metal hydrates (hydroxides) or oxides and that it is 
possible to obtain a homogeneous mixture of two or more different kinds of 
metal hydrates (hydroxides) or oxides. 
The conditions for the hydrolysis and the speed of hydrolysis are usually 
substantially different as between the metal alkoxides and the metal 
acetylacetonates. In order to obtain a uniform mixture of plurality of 
oxides, it is advisable to use one of the two types. 
The metal alkoxides or acetylacetonates to be used in this embodiment are 
alkoxides or acetylacetonates of Mg, Ca, Sr, Ba, Sc, Y, lanthanoides 
and/or Cu. They may be of any structures or forms. Namely, the alkoxy 
group for the metal alkoxides may have any number of carbon atoms and may 
be an alkoxy group from a polyhydric alcohol. Preferred examples of such 
an alkoxy group include, for example, a methoxy group, an ethoxy group, a 
propoxy group, an isopropoxy group, a butoxy group, a tertiary butoxy 
group or a secondary butoxy group. However, the alkoxy group is not 
limited to such specific examples. There is no particular restriction to 
the number of alkoxy groups or acetyl groups bonded to a metal element. At 
least one such a group may be bonded to a metal element. 
There is no particular restriction as to the ratio in mixing an alkoxide or 
acetylacetonate of at least one element selected from the group 
.consisting of Mg, Ca, Sr and Ba (hereinafter referred to as a Group IIa 
compound), an alkoxide or acetylacetonate of at least one element selected 
from the group consisting of Sc, Y and lanthanoids (hereinafter referred 
to as a Group IIIa compound and an alkoxide or acetylacetonate of Cu 
(hereinafter referred to as a Cu compound). They may be mixed in any ratio 
so long as a desired superconductor of an oxide system can be obtained. 
However, when Y is used as the Group IIIa element, it is preferred to mix 
them in a ratio of the Group IIa compound/the Y-containing compound/the Cu 
compound=2-10/1/3-10 (atomic ratio of metals). When La is used as the 
Group IIIa element, it is preferred to mix them in a ratio of (the Group 
IIa compound+the La-containing compound)/the Cu compound=2/1 (atomic ratio 
of metals). There is no particular restriction as to the ratio between the 
Group IIa compound and the La-containing compound. 
The above-mentioned metal alkoxides or metal acetylacetonates are uniformly 
dissolved, dispersed or suspended in a solvent such as methyl alcohol, 
ethyl alcohol, isopropyl alcohol, benzene, toluene, xylene, 
tetrahydrofuran, diethyl ether, diphenyl ether, anisole or ethyl acetate 
and then hydrolyzed. The hydrolyzates may be metal oxides depending upon 
the elements. However, the hydrolyzates are usually amorphous hydrates 
(inclusive of hydroxides) in many cases. Such hydrates can in most cases 
be converted to metal oxides at a relatively low temperature 
(200.degree.-500.degree. C.) by sintering. 
There is no particular restriction as to the conditions for the hydrolysis. 
The hydrolysis may be conducted under the conditions as described with 
respect to the first embodiment. 
The hydrolyzed mixture is treated and sintered in the same manner as 
described above with respect to the first embodiment. 
There is no particular restriction as to the cooling conditions after the 
sintering. The sintered products may be naturally cooled in air unless 
cracking occurs, or may be cooled in an oxygen stream over a period of 
about 5 hours. 
In a third embodiment of the present invention, a part of the metal 
elements which are the same as in the first and second embodiments, are 
dissolved, dispersed or suspended in a solvent in the form of 
acetylacetonates. Then, the metal acetylacetonates are hydrolyzed by an 
addition of an alkali to obtain a hydrolyzate. The rest of the metal 
elements are mixed with the hydrolyzate in the form of carboxylates to 
obtain a homogeneous mixture, which is then sintered to obtain a desired 
superconductor of an oxide system. 
In this embodiment, the hydrolyzate of the metal acetylacetonates is mixed 
with the metal carboxylates, whereby the respective components can be 
uniformly mixed in the atomic or molecular order. Accordingly, the 
sintered product obtainable from such a mixture will be homogeneous, and a 
superconductor of an oxide system having excellent superconductivity will 
be obtained. 
There is no particular restriction as to the metal acetylacetonates to be 
used. The metal acetylacetonates may be the same as described above with 
respect to the first embodiment. 
Likewise, there is no particular restriction as to the metal carboxylates 
to be used in this embodiment. They may have any structures or forms so 
long as they have at least one carboxyl group bonded to a metal. Namely, 
the carboxylic acids to form the above-mentioned metal carboxylates may 
have any number of carbon atoms, may be polybasic carboxylic acids, or may 
contain other groups like an amino acid or oxy acid. 
In the case where the metal is Cu, specific examples for such metal 
carboxylates include, for example, dicarboxyates of the formula (R.sup.1 
COO) Cu wherein R.sup.1 is a hydrogen atom or a substituted or 
unsubstituted hydrocarbon group having from 1 to 50 carbon atoms, 
preferably the propionates, acetates, butyrates, oxalates, naphthenates, 
stearates and citrates. When the metal is Mg, Ca, Sr or Ba, the 
carboxylates include, for example, dicarboxylates of the formula (R.sup.2 
COO.sub.2 M wherein M is Mg, Ca, Sr or Ba, and R.sup.2 is a hydrogen atom 
or a substituted or unsubstituted hydrocarbon group having from 1 to 50 
carbon atoms, preferably the respective acetates, butanoates, 
naphthenates, benzoates, stearates, citrates and pentanoates. Likewise, 
when the metal is Sc, Y or a lanthanoid, the carboxylates include, for 
example, tricarboxylates of the formula (R.sup.3 COO).sub.3 M' wherein M' 
is Sc, Y or a lanthanoid, and R.sup.3 is a hydrogen atom and a substituted 
or unsubstituted hydrocarbon group having from 1 to 50 carbon atoms, 
preferably the cyclohexane carboxylates, acetates, butanoates, 
naphthenates, benzoates, citrates, stearates and octenoates. 
In this embodiment, acetylacetonates of at least one type of metal elements 
among the three types of metal elements i.e. the Group IIa elements, the 
Group IIIa elements and Cu, are dissolved, dispersed or suspended in a 
solvent, and then the metal acetylacetonates are hydrolyzed by an addition 
of an alkali to obtain a hydrolyzate. Then, carboxylates of metal elements 
of the type other than the type contained in the above metal 
acetylacetonate, are added to the hydrolyzate to obtain a homogeneous 
mixture. 
There is no particular restriction as to the method for the preparation of 
such a homogeneous mixture. For example, the predetermined metal 
acetylacetonates and metal carboxylates may be dissolved, dispersed or 
suspended in a solvent, and then an alkali is added, if necessary, 
together with water, to hydrolyze the metal acetylacetonates, whereby a 
uniform mixture of the hydrolyzate of the metal acetylacetonates and the 
metal carboxylates may be prepared. 
It is of course possible that the metal acetylacetonates are dissolved, 
dispersed or suspended in a solvent, then hydrolyzed by an addition of an 
alkali, if necessary together with water, and then the metal carboxylates 
are added and stirred, so that the hydrolyzate and the metal carboxylates 
are thoroughly mixed to obtain a homogeneous mixture. 
There is no particular restriction as to which metal elements among the 
three types should be used in the form of metal acetylacetonates and which 
should be used in the form of metal carboxylates. However, from the 
viewpoint of e.g. the stability, it is preferred to use Sc, Y and 
lanthanoids of Group IIIa in the form of metal acetylacetonates. It is of 
course unnecessary to use all of a certain metal in the form of a metal 
acetylacetonate. Namely, a part of a metal may be used in the form of a 
metal acetylacetonate, and the rest may be formed in the form of a metal 
carboxylate. 
Further, there is no particular restriction as to how much among the total 
metal atoms should be used in the form of the metal acetylacetonates. An 
optional ratio may be employed so long as a homogeneous mixture can be 
prepared. However, the proportion of the metal atoms used in the form of 
metal acetylacetonates in the total metal atoms is preferably from 30 to 
90 atomic %, more preferably from 50 to 80 atomic.%, whereby a homogeneous 
mixture can readily be prepared. 
Further, there is no particular restriction as to the ratio of the three 
types of the metal elements. The ratio of the three types of the metal 
elements may be the same as described with respect to the first and second 
embodiments. 
The solvent may be methanol, ethanol, isopropanol, tetrahydrofuran, ethyl 
ether, benzene or toluene. In the hydrolysis of the metal 
acetylacetonates, an alkali is added, whereby the metal acetylacetonates 
can readily be hydrolyzed. The amount of the alkali may be small and is 
usually at least 0.01 mol%, preferably from 0.1 to 1 mol%, relative to the 
metal acetylacetonates. 
There is no particular restriction as to the alkali. Specific examples 
include sodium hydroxide, potassium hydroxide, ammonia, an amine, lithium 
hydroxide, barium hydroxide and magnesium hydroxide. 
There is no particular restriction as to the conditions for the hydrolysis, 
which may be the same as described in the first embodiment. 
The hydrolyzate of the metal acetylacetonates used in this embodiment may 
not necessarily be the one wherein the acetylacetonate groups are 
completely hydrolyzed, so long as it is capable of forming a uniform 
mixture with the metal carboxylates. Namely, it may be the one wherein 
acetylacetonate groups are partially hydrolyzed. 
The homogeneous mixture thus prepared may be sintered as it is. However, it 
is possible to further improve the superconductivity of the resulting 
product by removing e.g. alkali metal ions which adversely affect the 
superconductivity, by washing the homogeneous mixture with distilled water 
or deionized water, or a water-containing solvent wherein an organic 
solvent well missible with water, such as methanol, ethanol, propanol or 
acetone, is added, prior to the sintering. It has been confirmed by the 
present inventor that by such washing prior to the sintering, the critical 
temperature can be made higher by from 1 to 5 K, the transition 
temperature range can be made narrower by from 2 to 10 K, and the critical 
current density can be improved by from 2 to 6 A/cm.sup.2. 
The treatment of the homogeneous mixture and the subsequent sintering may 
be conducted in the same manner as described in the foregoing embodiments. 
In a fourth embodiment of the present invention, a part of the same metal 
elements as in the preceeding embodiments are dissolved, dispersed or 
suspended in the form of acetylacetonates and then hydrolyzed with an 
alkali to obtain a gelled hydrolyzate, and the rest of the metal elements 
are added to the hydrolyzate in the form of inorganic salts, hydroxides 
and/or oxides. The homogeneous mixture thus obtained is dried and sintered 
to obtain a desired superconductor of an oxide system. 
An acetylacetonate of at least a part of at least one metal element used in 
this embodiment is uniformly dissolved, dispersed or suspended in a 
solvent and then hydrolyzed in the presence of an alkali to obtain a 
hydrolyzate wherein particles of the metal hydrate (hydroxide) or oxide 
are uniformly dispersed. To this hydrolyzate, the rest of the metal 
elements are dissolved, dispersed or suspended in the form of inorganic 
salts, hydroxides and/or oxides to obtain a mixture, whereby a homogeneous 
mixture can be obtained. Accordingly, a sintered product obtained from 
such a mixture will be homogeneous, and a high performance superconductor 
of an oxide system can be obtained. 
The metal acetylacetonates used in this embodiment may be the same as 
described above with respect to the preceeding embodiments. 
The inorganic salts of metals used in this embodiment are salts comprising 
an anion of Mg, Ca, Sr, Ba, Sc, Y, a lanthanoid or Cu and an anion such as 
NO.sub.3.sup.-, CN.sup.-, Cl.sup.-, SO.sub.4.sup.2-, CO.sub.3.sup.2-, 
Br.sup.-, F.sup.- or I.sup.-. Specifically, they include, for example, 
YCl.sub.3, Y(NO.sub.3).sub.3, BaCl.sub.2, Ba(CN).sub.2, CuSO.sub.4, 
CuCO.sub.3, MgF.sub.2, Mg(CN).sub.2, CaCO.sub.3, CaF.sub.2, La.sub.2 
(CO.sub.3).sub.3, La(NO.sub.3).sub.3 and SrCO.sub.3. 
Preferred specific examples of such inorganic salts include nitrates, 
carbonates and halides of Cu, Sr, Ca, Ba, La, Y and lanthanoids. 
The metal hydroxides used in this embodiment are compounds wherein at least 
one hydroxyl group is bonded to Mg, Ca, Sr, Ba, Sc, Y, lanthanoids or Cu. 
Typical hydroxides in the case of Mg,.Ca, Sr, Ba and Cu are Mg(OH).sub.2, 
Ca(OH).sub.2, Sr(OH).sub.2, Ba(OH).sub.2 and Cu(OH).sub.2. Typical 
hydroxides in the case of Sc, Y and lanthanoids are represented by M'(OH) 
wherein M' is Sc, Y or a lanthanoid. 
The oxides of metals to be used in the present invention include CuO in the 
case where the metal is Cu, MO wherein M is Mg, Ca, Sr or Ba in the case 
where the metal is Mg, Ca, Sr or Ba, and M'.sub.2 O.sub.3 wherein M' is 
Sc, Y or a lanthanoid in the case where the metal is Sc, Y or a 
lanthanoid. 
In this embodiment, at least one part of at least one type of the 
above-mentioned metal acetylacetonates is uniformly dissolved, dispersed 
or suspended in a solvent such as methyl alcohol, ethyl alcohol, isopropyl 
alcohol, butyl alcohol, hexane, petroleum ether, diethyl ether, benzene, 
toluene, xylene or tetrahydrofuran and then hydrolyzed in the presence of 
an alkali. Then, inorganic salts, hydroxides and/or oxides of the rest of 
the elements are dissolved, dispersed or suspended to the hydrolyzate to 
obtain a homogeneous mixture. 
There is no particular restriction as to the alkali. Any alkali may be used 
so long as it is capable of facilitating the hydrolysis of the metal 
acetylacetonates. Specific examples of such an alkali include, for 
example, sodium hydroxide, potassium hydroxide, ammonia, lithium 
hydroxide, barium hydroxide, magnesium hydroxide and primary, secondary 
and tertiary amines. The metal acetylacetonates can readily be hydrolyzed 
by conducting the hydrolysis in the presence of such an alkali. 
The alkali may be used in a small amount at a level of at least 0.1 mol% 
relative to the metal acetylacetonates. 
There is no particular restriction as to which metals among those used in 
this embodiment should be used in the form of metal acetylacetonates or 
which should be used in the form of inorganic salts, hydroxides and/or 
oxides. However, from the viewpoint of the operation efficiency and the 
uniformity of the resulting mixture, it is preferred that metals which can 
readily be uniformly dissolved, dispersed or suspended in a solvent as 
metal acetylacetonates should be used in the form of metal 
acetylacetonates, and those which can hardly be uniformly dissolved, 
dispersed or suspended as metal acetylacetonates should be employed in the 
form of inorganic salts, hydroxides and/or oxides. It is of course 
unnecessary that all of a certain metal be used in the form of the metal 
acetylacetonate. Namely, a part of a metal may be used in the form of a 
metal acetylacetonate, and the rest may be used in the form of an 
inorganic salt, hydroxide and/or oxide. 
The conditions for the hydrolysis and the nature of the hydrolyzate of the 
metal acetylacetonates are as described above with respect to the 
preceeding embodiments. The mixture containing the hydrolyzate of the 
metal acetylacetonates thus prepared usually has a nature of a sol. The 
above-mentioned inorganic salts, hydroxides and/or oxides are mixed 
thereto, whereby a homogeneous mixture is obtained. 
There is no particular restriction as to the ratio of the metal 
acetylacetonates and the inorganic salts, hydroxides and/or oxides used. 
However, the ratio of the metal acetylacetonates/the inorganic salts, 
hydroxides and/or oxides is preferably from 1/1 to 5/1 by atomic ratio of 
metals with a view to preparing a homogeneous mixture. 
There is no particular restriction as to the method for the addition of the 
inorganic salts, hydroxides and/or oxides. Any method may be employed so 
long as a homogeneous mixture is obtainable. For example, they may be 
mixed in the form of powders, or they may be added in the form of an 
aqueous solution or an alcohol solution, followed by mixing. 
There is no particular restriction as to the ratio for mixing three types 
of compounds i.e. a compound containing at least one element selected from 
the group consisting of Mg, Ca, Sr and Ba (a Group IIa compound), a 
compound containing at least one element selected from the group 
consisting of Sc, Y and lanthanoids (a Group IIIa compound) and a compound 
containing Cu. Such a ratio may be the same as described above with 
respect to the ratio of the three types of metal elements in the 
preceeding embodiments. The homogeneous mixture thus prepared may be 
washed prior to the sintering as described in the foregoing. 
The mixture thus treated is then sintered to obtain a superconductor of an 
oxide system. The sintering conditions and the cooling conditions after 
the sintering may be the same as described above with respect to the 
preceeding embodiments. 
In a fifth embodiment of the present invention, a reaction product obtained 
by reacting a mixture comprising at least one member selected from the 
group consisting of alkoxides of Mg, Ca, Sr and Ba, at least one member 
selected from the group consisting of alkoxides of Sc, Y and lanthanoids 
and an alkoxide of Cu, with acetone and ethyl acetate or with 
acetylacetone, is hydrolyzed, and the product thereby obtained is sintered 
to obtain a superconductor of an oxide type. 
In this embodiment, the metal elements in a predetermined combination are 
uniformly dissolved, dispersed or suspended in a solvent in the form of 
metal alkoxides, and then the metal alkoxides are reacted with acetone and 
ethyl acetate or with acetylacetone and thereby converted to the 
respective metal acetylacetonates, whereby it is possible to obtain a 
mixture more uniform than the mixture obtained by merely mixing the 
respective metal acetylacetonates in a predetermined ratio. The metal 
acetylacetonates thus formed are hydrolyzed to obtain a hydrolyzate. The 
hydrolyzate is accordingly very homogeneous, and the superconductor of an 
oxide system obtained by sintering it will have excellent 
superconductivity. 
In this embodiment, a homogeneous solution, dispersion or suspension is 
prepared which comprises an alkoxide containing at least one metal 
selected from the group consisting of Mg, Ca, Sr and Ba (hereinafter 
referred to as a Group IIa alkoxide), an alkoxide containing at least one 
metal selected from the group consisting of Sc, Y and lanthanoids 
(hereinafter referred to as a Group IIIa alkoxide) and an alkoxide of Cu. 
There is no particular restriction as to the structures or forms of the 
metal alkoxides. They may be of any structures or forms so long as they 
can be uniformly dissolved, dispersed or suspended in a solvent used and 
they are capable of forming metal acetylacetonates when reacted with 
acetone and ethyl acetate or with acetylacetone. Namely, the alkoxy group 
to form the metal alkoxide may have any number of carbon atoms and may be 
an alkoxy group from a polyhydric alcohol. Preferred examples of such an 
alkoxy group include, for example, a methoxy group, an ethoxy group, a 
propoxy group, an isopropoxy group, a butoxy group, a tertiary butoxy 
group and a secondary butoxy group. However, the alkoxy group is not 
limited to such specific examples. There is no specific restriction to the 
number of alkoxy groups bonded to a metal element, so long as at least one 
such group is bonded to a metal element. 
There is no particular restriction as to the mixing ratio of the Group IIa 
alkoxide, the Group IIIa alkoxide and the Cu alkoxide. They may be mixed 
in any ratio so long as the desired superconductor of an oxide system can 
be obtained. For example, when Y is used as the Group IIIa element, it is 
preferred to mix them in a ratio of the Group IIa alkoxide/Y alkoxide/Cu 
alkoxide=2-10/1/3-10 (atomic ratio of metals). When La is used as the 
Group IIIa element, it is preferred to mix them in a ratio of (the Group 
IIa alkoxide+La alkoxide)/Cu alkoxide=2/1 (atomic ratio of metals). There 
is no particular restriction as to the ratio between the Group IIa 
alkoxide and the La alkoxide. 
The metal alkoxides are uniformly dissolved, dispersed or suspended in the 
predetermined ratio as mentioned above in a solvent such as methyl 
alcohol, ethyl alcohol, isopropyl alcohol, benzene, toluene, xylene, 
tetrahydrofuran, diethyl ether, diphenyl ether or benzyl alcohol and then 
reacted with acetone and ethyl acetate or with acetylacetone to obtain 
metal acetylacetonates. 
The mixture of metal acetylacetonates thus prepared is more homogeneous 
than the one obtained by mixing the respective metal acetylacetonates 
prepared separately by reacting the respective alkoxides with acetone and 
ethyl acetate or with acetylacetone. The superconductor of an oxide system 
prepared from the hydrolyzate of more homogeneous metal acetylacetonates 
shows superior superconductivity than the superconductor prepared from a 
mixture obtained by mixing the respective metal acetylacetonates 
separately produced. 
The reason why the characteristics of the superconductor of an oxide type 
prepared from the homogeneous mixture of metal acetylacetonates obtained 
by mixing the respective metal elements in the state of metal alkoxides, 
are superior to the characteristics of the superconductor of an oxide 
system prepared from a mixture obtained by mixing separately produced 
acetylacetonates of the respective metal elements, is not adequately 
understood. However, it is considered that when the metal acetylacetonates 
are formed from the metal alkoxides, not only the respective metal 
acetylacetonates are separately formed, but a part thereof may form a 
composite containing a linkage represented by the formula: 
##STR1## 
wherein M.sup.1 and M.sup.2 are different metal atoms, or may form a 
solvate, thus serving as a link between the different metal 
acetylacetonates, whereby the entire mixture acts as a unitary substance 
in the state of a solution, dispersion or suspension, or during the 
hydrolysis. 
There is no particular restriction as to the concentration of the mixture 
obtained by dissolving, dispersing or suspending the metal alkoxides. The 
concentration may be at any level so long as the reaction with acetone and 
ethyl acetate or with acetylacetone can be conducted without trouble. It 
is usual, however, that a mixture of metal alkoxides is prepared at a 
concentration of from 5 to 45% by weight, preferably from 10 to 20% by 
weight, and acetone and ethylacetone, or acetylacetone is added thereto 
for reaction. 
The ratio of acetone to the metal alkoxides when acetone and ethyl acetate 
are used for the preparation of the metal acetylacetonates, is usually 
from 0.3 to 2 mols, preferably from 0.8 to 1.2 mols per mol of the alkoxy 
group of the metal alkoxides. The ratio of ethyl acetate to the metal 
alkoxides, is usually from 0.5 to 2 mols, preferably from 0.8 to 1.2 mols 
per mol of the alkoxy group in the metal alkoxides. 
The reaction may be conducted in such a manner that for example, acetone is 
added to the mixture of metal alkoxides, and the mixture is heated at a 
temperature of from 25.degree. to 70.degree. C. for from 1 to 8 hours. 
Then, ethyl acetate is added, and the mixture is stirred at a temperature 
of from 25.degree. to 70.degree. C. for from 30 minutes to 10 hours. 
On the other hand, when acetylacetone is used for the preparation of the 
metal acetylacetonates, the ratio of acetylacetone to the metal alkoxides 
is usually from 0.5 to 2 mols, preferably from 0.8 to 1.2 mols per mol of 
the alkoxy group of the metal alkoxides. 
The reaction may be conducted, for example, by adding acetylacetone to a 
mixture of the metal alkoxides, followed by heating at a temperature of 
from 25.degree. to 75.degree. C. for from 1 to 20 hours. 
The metal acetylacetonates thus prepared are hydrolyzed in the same manner 
as described above with respect to other embodiments. The hydrolyzate is 
then treated and sintered also in the same manner as described above. 
In a sixth embodiment of the present invention, the metal elements for 
constituting the oxide system are Bi, Sr, Ca and Cu, and they are 
dissolved, dispersed or suspended in a solvent at least partly in the form 
of acetylacetonates with the rest, if any, being in the form of alkoxides, 
followed by removal of the solvent to obtain a homogeneous mixture, which 
is then sintered to obtain a superconductor of a Bi-Sr-Ca-Cu-O system. The 
uniformly dissolved, dispersed or suspended mixture may be hydrolyzed 
prior to the removal of the solvent. 
The metal alkoxides and the metal acetylacetonates to be used in this 
embodiment can be prepared usually in a high purity of from 99.999 to 
99.99999%. By using such pure materials and by uniformly mixing them in a 
solvent, it is possible to obtain a homogeneous superconductor having a 
high purity. 
The alkoxides of Bi, Sr, Ca and Cu to be used in this embodiment may be of 
any structures or forms. Namely, the alkoxy group for such a metal 
alkoxide may have any number of carbon atoms and may be an alkoxy group 
from a polyhydric alcohol. Preferred specific examples of such an alkoxy 
group includes, for example, a methoxy group, an ethoxy group, a propoxy 
group, an isopropoxy group, a butoxy group, a t-butoxy group, a sec-butoxy 
group and ethylene glycol. However, the alkoxy group is not limited to 
such specific examples. Further, there is no particular restriction to the 
number of alkoxy groups bonded to a metal element, and at least one alkoxy 
group may be bonded to a metal element. 
The acetylacetonates of Bi, Sr, Ca and Cu to be used in this embodiment are 
compounds wherein at least one acetylacetonate group is attached to such a 
metal atom. They may be of any structures or forms. Namely, so long as the 
basic structure is acetylacetonate, for example, the hydrogen atom may be 
substituted by a fluorine atom or a hydrocarbon group. 
A part or some of Bi, Sr, Ca and Cu may be used in the form of the 
respective metal alkoxide. Further, one metal may be used in the form of 
two types of metal compounds as in the case where a Bi alkoxide and a Bi 
acetylacetonate are used as the Bi component. 
There is no particular restriction as to the ratio of Bi, Sr, Ca and Cu. 
They may be used in any ratio so long as the desired superconductor of an 
oxide system can be obtained. However, it is preferred to use them in a 
compositional ratio to form a high Tc phase (Tc=about 120 K (onset)). 
Usually, a compositional ratio of Bi.sub.1 Sr.sub.0.5-3 Ca.sub.0.5-3 
Cu.sub.0.5-3.5 is preferred. 
In this embodiment, the metal acetylacetonates are uniformly dissolved, 
dispersed or suspended, optionally together with the above-mentioned metal 
alkoxides, in a solvent such as methyl alcohol, ethyl alcohol, isopropyl 
alcohol, butyl alcohol, benzene, toluene, xylene, tetrahydrofuran, diethyl 
ether, diphenyl ether or DMF. Then, the solvent is removed directly or 
after the hydrolysis, and the resulting homogeneous mixture is sintered. 
There is no particular restriction as to the concentration of the metal 
alkoxides or the metal acetylacetonates in the above solvent, so long as 
such metal compounds can be uniformly dissolved, dispersed or suspended in 
the solvent. 
When at least one of Ba, Sr, Ca and Cu is used in the form of a metal 
alkoxide or a metal acetylacetonate, and such a metal alkoxide or a metal 
acetylacetonate dissolved, dispersed or suspended in the solvent is 
subjected to hydrolysis, it undergoes a change from a sol to a gel and 
finally forms the metal hydrate (hydroxide) or metal oxide particles. This 
method is a so-called sol-gel method and has features such that it is 
thereby possible to obtain super fine particles of the metal hydrate 
(hydroxide) or oxide and that it is thereby possible to obtain a 
homogeneous mixture of two or more metal hydrates (hydroxides) or oxides. 
Further, it is thereby possible to set the sintering temperature at a 
level lower than the dry method and the sintering time at a level shorter 
than the dry method. 
By using highly pure metal alkoxides or metal acetylacetonates as mentioned 
above and highly pure water (such as deionized water or distilled water), 
it is possible to obtain particles (powder) of the metal hydrate 
(hydroxide) or oxide having a purity substantially higher than that 
attainable by the dry method. 
The resulting sol and gel are usually amorphous as analyzed by X-ray 
analysis. However, it has been confirmed by experiments that they will be 
all readily converted to the corresponding metal oxides by sintering. 
There is no particular restriction as to the concentration of the metal 
alkoxides or the metal acetylacetonates during the hydrolysis, the manner 
of addition of water or the conditions for the hydrolysis. The amount of 
water added for the hydrolysis may be at any level in excess of the 
stoichiometric amount for the hydrolysis of the metal alkoxides or the 
metal acetylacetonates, but is preferably in large excess. The reaction 
temperature is preferably at a level of the boiling point of the solvent, 
and the reaction time is preferably from 5 to 15 hours. 
For the hydrolysis, an acid or a base, such as methoxy ethanol, 
ethanolamine, n-methyl ethanolamine, triethylamine, HCl, HNO.sub.3 or 
H.sub.2 SO.sub.4, may be added in a small amount, preferably in an amount 
of from 0.1 to 10 times the molar amount of the total of the starting 
materials to regulate the rate of the hydrolysis, to facilitate the 
sol-gel transformation or to increase the solubility prior to the 
hydrolysis. 
The resulting hydrolyzates may be composed solely of metal oxides, but they 
may usually be amorphous hydrates (inclusive of hydroxides) in many cases, 
which can readily be converted to metal oxides by sintering at a 
relatively low temperature (200.degree.-500.degree. C.). 
There is no particular restriction as to the manner of removing the 
solvent. The solvent may be removed by evaporation or filtration followed 
by drying. 
There is no particular restriction as to the sintering conditions (such as 
the temperature, the number of times and the atmosphere). However, it is 
usual to employ a temperature of from 700.degree. to 950.degree. C., 
preferably from 800.degree. to 900.degree. C. and a time of from 1 to 20 
hours, preferably from 1 to 8 hours, whereby the sintering can be 
conducted at a low temperature for a short period of time as compared with 
the conventional dry method. 
In a seventh embodiment of the present invention, the metal elements for 
constituting the oxide system are Tl, Ca, Ba and Cu, and they are 
dissolved, dispersed or suspended in a solvent at least partly in the form 
of acetylacetonates with the rest, if any, being in the form of alkoxides, 
followed by removal of the solvent to obtain a homogeneous mixture, which 
is then sintered to obtain a superconductor of a Tl-Ca-Ba-Cu-O system. The 
uniformly dissolved, dispersed or suspended mixture may be hydrolyzed 
prior to the removal of the solvent. 
The alkoxides and the acetylacetonates of the metal elements to be used in 
this embodiment are uniformly dissolved, dispersed or suspended in a 
solvent and then hydrolyzed by an addition of water, whereby they undergo 
a change from a sol to a gel and finally form particles of the metal 
hydrates (hydroxides) or oxides. This method is a so-called sol-gel 
method, which has been described in the foregoing. 
As mentioned above, the metal alkoxides and the metal acetylacetonates may 
usually be obtained in a high purity at a level of from 99.999 to 
99.99999%. Therefore, by using such high purity metal compounds and highly 
pure water (such as deionized water or distilled water), it is possible to 
obtain particles (powder) of metal hydrates (hydroxides) or oxides having 
a purity substantially higher than that attainable by the conventional dry 
method. Further, since uniformly mixed super fine particles are 
obtainable, there is a merit such that the sintering temperature can be 
made low and the reaction time can be made short as compared with the dry 
method. 
The sol and the gel obtained by this method are usually amorphous as 
analyzed by the X-ray analysis, and it has been confirmed by experiments 
that they will be all converted to the metal oxides by sintering. 
The alkoxides and the acetylacetonates of thallium, calcium, barium and 
copper used in this embodiment may be of any structures or forms. Namely, 
the alkoxy group constituting such a metal alkoxide may have any number of 
carbon atoms, and may be an alkoxy group from a polyhydric alcohol. 
Preferred specific examples of such an alkoxy group include a methoxy 
group, an ethoxy group, a propoxy group, an isopropoxy group, a butoxy 
group, a t-butoxy group, a sec-butoxy group and ethylene glycol. However, 
the alkoxy group is not limited to such specific examples. 
Whereas, the acetylacetonates may be of any structures or forms so long as 
the basic structure is acetylacetonate. Namely, the hydrogen atom of the 
basic structure may be substituted by e.g. a halogen atom or an oxygen 
functional group. And they may have any number of carbon atoms. 
Further, there is no particular restriction as to the number of alkoxy 
groups or acetylacetonate groups bonded to a metal element. At least one 
such group may be bonded to a metal element. 
The solvent useful in this embodiment includes, for example, methyl 
alcohol, ethyl alcohol, isopropyl alcohol, butanol, benzene, toluene, 
xylene, tetrahydrofuran, diethyl ether, diphenyl ether, DMF, a primary 
amine, a secondary amine and a tertiary amine. 
The hydrolysis and the sintering can be conducted in the same manner as 
described in the foregoing. 
Now, the present invention will be described in further detail with 
reference to Examples. However, it should be understood that the present 
invention is by no means restricted to such specific Examples. 
EXAMPLE 1 and COMATIVE EXAMPLES 1 and 2 
So as to bring the composition of the desired superconductor of an oxide 
system to (La.sub.0.9 Sr.sub.0.1).sub.2 CuO.sub.4, acetylacetonates of La, 
Sr and Cu (each having a purity of at least 99.999%) were added in a 
predetermined ratio in a total amount of 50 g in 1 liter of 
tetrahydrofuran, and uniformly dispersed (partially dissolved). This 
dispersion was adjusted to 80.degree. C. and subjected to hydrolysis by an 
addition of deionized water (200 ml) containing 0.1 g of NaOH, whereby 
gelled blackish brown precipitates were formed. The precipitates were 
collected by filtration, then washed with 30 ml of a solvent mixture of 
distilled water/acetone in a volume ratio of 1/1, dried and analyzed by an 
X-ray diffraction apparatus, whereby the precipitates were found to be 
composed mainly of a mixture of hydroxides of La, Sr and Cu and contain an 
amorphous substance of the respective oxides. 
The dried product of the precipitates were presintered in an oxygen stream 
at 900.degree. C. for 4 hours to obtain a porous presintered product. The 
presintered product was pulverized in a mortar and then formed into a 
pellet having a diameter of 10 mm and a thickness of 1.5 mm by means of a 
pelletizer. This pellet was again sintered in an oxygen stream at 
920.degree. C. for 6 hours to obtain a densely sintered product. 
For the purpose of comparison, the above precipitates without being 
subjected to washing and a mixture of La.sub.2 O.sub.3, SrCO.sub.3 and CuO 
powders which were all guaranteed reagents, were, respectively, 
presintered and sintered in the same manner as above to obtain a pellet 
having the above washing step omitted and a pellet according to the 
conventional dry method (Comparative Examples 1 and 2). 
With respect to three types of samples, the temperature dependency of the 
resistivity was measured by a four terminal method by placing each sample 
provided with four indium electrodes with a space of 1.5 mm from an 
another in a criostat and gradually cooling it with a liquefied helium. 
The results are shown in FIG. 1. 
In FIG. 1, curve 1 shows the characteristics of the sample prepared by the 
process of the present invention, curve 2 shows the characteristics of the 
sample obtained in the same manner except that the washing step was 
omitted, and curve 3 shows the characteristics of the sample according to 
the conventional dry method. 
From the results of FIG. 1, it is evident that as compared with the product 
of the conventional dry method, the superconductor of an oxide system 
prepared by the process of the present invention undergoes a quick 
transition to the superconducting state with its resistivity rapidly 
dropping to 0. Further, it is evident that the superconductivity is 
superior also as compared with Comparative Example 1 wherein the washing 
was omitted. The onset temperature at which the resistivity of each sample 
has started to rapidly decrease to the superconducting state, the offset 
temperature at which the resistivity has become 0 and the difference 
between the two temperatures (the transition temperature range) are shown 
in Table 1. Further, at the liquefied helium temperature (4.3 K), the 
voltage applied to each sample was raised to gradually increase the 
current, and the critical current density at which the superconducting 
state has been broken and turns to a normal conducting state, was measured 
and shown in Table 1. 
TABLE 1 
__________________________________________________________________________ 
Critical 
Method for the Transition 
current 
preparation of 
Onset Offset 
temperature 
density 
Example No. 
the sample 
temp. (K) 
temp. (K) 
range (K) 
(A/cm.sup.2) 
__________________________________________________________________________ 
Example 1 
Process of the 
43.8 41.9 1.9 98.6 
present 
invention 
Comparative 
Process 43.7 40.0 3.7 60.4 
Example 1 
wherein 
washing was 
omitted 
Comparative 
Dry method 
43.6 37.3 6.3 27.4 
Example 2 
__________________________________________________________________________ 
From the results of Table 1, it is evident that the superconductor of an 
oxide system prepared by the process of the present invention has 
practically excellent superconducting characteristics such that as 
compared with the product having the washing step omitted and the product 
obtained by the conventional dry method, not only the critical temperature 
at which the resistivity becomes 0 is high, but also the range for 
transition from the normal conducting state to the superconducting state 
is narrow, and the critical current density is high. 
With respect to the three types of the sintered samples, the structural 
analysis was conducted by e.g. the X-ray diffraction, whereby the sample 
by the process of the present invention was found to be a sintered product 
comprising a substantially uniform single phase of so-called K type 
(La.sub.0.9 Sr.sub.0.1).sub.2 CuO.sub.4. On the other hand, with the 
sample having the washing step omitted, the alkali metal was taken in as 
an impurity, and other phases were observed in addition to the 
superconductive phase. Further, the sample prepared by the conventional 
dry method was found to contain a perovskite structure of ABO.sub.3 type 
(wherein A and B are metal elements) and relatively many other phases in 
addition to the K.sub.2 NiF.sub.4 type phase. 
The gelled hydrolyzate composed mainly of a mixture of hydroxides of La, Sr 
and Cu formed by the hydrolysis according to the process of the present 
invention and the sintered product thereof obtained by sintering it at a 
temperature of from 200.degree. to 500.degree., were found to be composed 
of pure and uniform super fine particles at a level of from a few tens to 
few hundreds .ANG., respectively, by the analysis (observation by means of 
a scanning type electron microscope). Namely, such a nature is considered 
to effectively serve to obtain the sintered product of the desired 
composition. 
EXAMPLE 2 and COMATIVE EXAMPLES 3 and 4 
So as to bring the composition of the desired superconductor of an oxide 
system to YBa.sub.2 Cu.sub.3 O.sub.7, acetylacetonates of Y, Ba and Cu 
were uniformly dispersed (partially dissolved) in a total amount of 6.0 g 
in 500 ml of ethyl alcohol at 80.degree. C. To this dispersion, 500 ml of 
distilled water containing 40 mg of NaOH was dropwise added over a period 
of one hour to conduct the hydrolysis, whereby a gel of a mixture of the 
reaction products was obtained. 
The gel thus obtained was dried and washed with 30 ml of a solvent mixture 
of ethanol/distilled water in a volume ratio of 3/1, and the dried product 
was preliminarily calcined in air at 500.degree. C. for 3 hours. Then, the 
calcined product was presintered in an oxygen stream at 950.degree. C. for 
3 hours to obtain a coarse presintered product. The presintered product 
was pulverized and then formed into a pelletized sample in the same manner 
as in Example 1. This pellet was sintered in an oxygen stream at 
950.degree. C. for 9 hours in the same manner as in Example 1 to obtain a 
densely sintered product. 
For the purpose of comparison, samples were prepared by using a mixture 
having the above washing step omitted and a mixture of Y.sub.2 O.sub.3, 
BaCO.sub.3 and CuO powders in the same manner as in Example 1 and 
subjected to the presintering and the sintering in the same manner as 
above to obtain two types of sintered products (Comparative Examples 3 and 
4). 
With respect to the three types of samples, the temperature dependency of 
the resistivity and the critical current density were measured in the same 
manner as in Example 1. The results are shown in FIG. 2 and Table 2. 
In FIG. 2, curve 4 shows the characteristics by the process of the present 
invention, curve 5 shows the characteristics by the process wherein the 
washing step was omitted, and curve 6 shows the characteristics by the 
conventional dry method. The critical current density is (77.4 K). 
TABLE 2 
__________________________________________________________________________ 
Critical 
Method for the Transition 
current 
preparation of 
Onset Offset 
temperature 
density 
Example No. 
the sample 
temp. (K) 
temp. (K) 
range (K) 
(A/cm.sup.2) 
__________________________________________________________________________ 
Example 1 
Process of the 
98.4 96.1 2.3 127.0 
present 
invention 
Comparative 
Process 98.3 92.2 6.1 67.0 
Example 3 
wherein 
washing was 
omitted 
Comparative 
Dry method 
97.4 76.1 21.3 22.4 
Example 4 
__________________________________________________________________________ 
From the results in FIG. 2 and in Table 2, it is evident that as compared 
with the product by the method wherein the washing step was omitted and 
the product by the conventional dry method, the superconductor of an oxide 
system prepared by the process of the present invention has practically 
excellent superconducting characteristics such that the critical 
temperature for the transition to the superconducting state is high, the 
transition temperature range is narrow, and the critical current density 
is high. Whereas, with the products by the process wherein the washing 
step was omitted and by the conventional dry method, the critical 
temperature, the transition temperature range and the critical current 
density are inadequate for practical application. 
The poor characteristics of the product by the process wherein the washing 
step was omitted are attributable to the alkali metal taken in as an 
impurity and other phases are included in the superconductive phase. The 
poor characteristics of the product by the conventional dry method were 
found to be attributable to relatively many unwanted phases present in 
addition to the high temperature superconducting phase, as a result of the 
analysis such as the same X-ray diffraction method as used in Example 1. 
Further, with respect to each sample used in Examples 1 and 2, the 
susceptibility was measured, whereby in each case, Meissner effects were 
observed at a temperature of not higher than the temperature at which the 
resistivity became 0. 
EXAMPLE 3 and COMATIVE EXAMPLES 5 and 6 
So as to bring the composition of the desired superconductor of an oxide 
system to YBa.sub.2 Cu.sub.3 O.sub.7, acetylacetonates of Y, Ba and Cu 
were uniformly suspended (partially dissolved) in a total amount of 10 g 
in 500 ml of ethyl alcohol at 80.degree. C. To this suspension, 500 ml of 
distilled water containing 5 ml of ammonia was dropwise added over a 
period of one hour to conduct the hydrolysis, whereby a gel of the mixture 
of products was obtained. 
The gel thus obtained, was preliminarily baked in air at 500.degree. C. for 
2 hours and then presintered in an oxygen stream at 950.degree. C. for 3 
hours to obtain a coarse presintered product. The presintered product was 
pulverized in a 
mortar and formed into a pellet having a diameter of 10 mm and a thickness 
of 1.5 mm by means of a pelletizer. This pellet was again sintered in an 
oxygen stream at 900.degree. C. for 8 hours to obtain a densely sintered 
product. 
For the purpose of comparison, samples by the conventional dry method and 
by the coprecipitation method were prepared by using a mixture of Y.sub.2 
O.sub.3, BaCO.sub.3 and CuO powders and nitrates of Y, Ba and Cu, which 
were all guaranteed reagents, respectively, and they were, respectively, 
presintered and sintered in the same manner as above to obtain two types 
of sintered products. 
With respect to the three types of samples, the temperature dependency of 
the resistivity and the critical current density were measured in the same 
manner as in Example 1. The critical current density is a value measured 
at the liquefied nitrogen temperature (77.4 K). The results are shown in 
FIG. 3 and Table 3. 
In FIG. 3, curve 7 shows the characteristics by the process of the present 
invention, curve 8 shows the characteristics by the conventional dry 
method, and curve 9 shows the characteristics by the conventional 
coprecipitation method. 
TABLE 3 
__________________________________________________________________________ 
Critical 
Method for the Transition 
current 
preparation of 
Onset Offset 
temperature 
density 
Example No. 
the sample 
temp. (K) 
temp. (K) 
range (K) 
(A/cm.sup.2) 
__________________________________________________________________________ 
Example 3 
Process of the 
98.6 95.5 3.1 127.4 
present 
invention 
Comparative 
Dry method 
97.5 80.1 17.4 37.6 
Example 5 
Comparative 
Coprecipita- 
92.7 42.5 50.2 -- 
Example 6 
tion method 
__________________________________________________________________________ 
From the results in FIG. 3 and Table 3, it is evident that as compared with 
the products of the conventional methods, the superconductor of an oxide 
system prepared by the process of the present invention has practically 
excellent superconducting characteristics such that the critical 
temperature for the transition to the superconducting state is high, the 
transition temperature range is narrow, and the critical current density 
is high. Whereas, with the products prepared by the conventional two 
methods, the critical temperature, the transition temperature range and 
the critical current density are inadequate for practical application. 
Especially the product by the coprecipitation method did not show 
superconductivity at the liquefied nitrogen temperature (77.4 K) although 
the composition was the same as the other two samples. The poor 
characteristics of the superconductors of an oxide system according to the 
conventional methods were found to be attributable to relatively many 
unwanted phases present in addition to the high temperature 
superconductive phase, as a result of the analysis such as X-ray 
diffraction. 
EXAMPLE 4 and COMATIVE EXAMPLE 7 
By using acetylacetonates of at least one element selected from the group 
consisting of Mg, Ca, Sr and Ba of Group IIa of the Periodic Table, at 
least one element selected from the group consisting of Sc, Y and 
lanthanoids of Group IIIa of the Periodic Table and Cu, experiments were 
conducted in the same manner as in Example 3. The combinations used 
include, for example, La-Sr-Ba-Cu, Y-La-Ba-Ca-Cu, Sc-Ba-Cu, Y-La-Ba-Cu, 
Y-Ba-Mg-Cu, Ho-Ba-Cu (Ho is holmium), Er-Ba-Cu (Er is erbium) and Yb-Ba-Cu 
(Yb is ytterbium). 
At the same time, for the purpose of the comparison, sintered products 
having the same intended compositions as above were prepared by the 
conventional dry method and the coprecipitation method and evaluated. 
As a result, it has been found that in each of the above combinations, a 
superconductor of an oxide system having a critical temperature of from 20 
to 95 K can be prepared by the process of the present invention. In each 
case, the product by the process of the present invention had superior 
superconductivity as compared with the products by the conventional 
methods, as shown by the comparison between Example 3 and Comparative 
Examples 5 and 6. 
With respect to each of the samples used in Examples 3 and 4, the 
susceptibility was measured, whereby in each case, Meissner effects were 
observed at a temperature of not higher than the temperature at which the 
resistivity became 0. 
EXAMPLES 5 and 6 and COMATIVE EXAMPLES 8 and 9 
So as to bring the composition of the desired superconductor of an oxide 
system to YBa.sub.2 Cu.sub.3 O.sub.7, yttrium acetylacetonate (purity: at 
least 99.999%), barium acetylacetonate (purity: at least 99.999%) and 
copper stearate (purity: at least 99.9%) were added in a predetermined 
ratio in the total amount of 10 g to 1 liter of butanol and uniformly 
dispersed (partially dissolved). To this dispersion, 20 ml of distilled 
water containing 30 mg of NaOH was added, and the mixture was heated at 
80.degree. C. for 1 hour to hydrolyze the yttrium acetylacetonate and the 
Ba acetylacetonate, whereby a gelled mixture was obtained. 
The gelled mixture was washed with deionized water until the water used for 
washing was substantially neutral and presintered in an oxygen atmosphere 
at 900.degree. C. for 6 hours to obtain a porous presintered product. The 
presintered product was pulverized in a mortar and formed into a pellet 
having a diameter of 10 mm and a thickness of 2.0 mm by means of a 
pelletizer. This pellet was sintered in an oxygen atmosphere at 
940.degree. C. for 3 hours to obtain a densely sintered product (Example 
5). 
A sintered product was obtained in the same manner as in Example 5 except 
that washing with deionized water prior to the sintering was omitted 
(Example 6). 
For the purpose of comparison, a mixture of Y.sub.2 O.sub.3, BaCO.sub.3 and 
CuO powders and a precipitate formed by adding small amounts of ammonia 
and oxalic acid to an aqueous solution of nitrates of Y, Ba and Cu, each 
prepared by using guaranteed reagents, were, respectively, presintered and 
sintered in the same manner as above, to obtain sintered products by the 
conventional dry method and the coprecipitation method (Comparative 
Examples 8 and 9). 
With respect to each of the four types of samples, the temperature 
dependency of the resistivity was measured by a four terminal method. The 
results are shown in FIG. 4. 
In FIG. 4, curve 10 shows the characteristics of the sample prepared by the 
process of the present invention wherein the washing was conducted prior 
to the sintering, curve 11 shows the characteristics of the sample 
prepared by the process of the present invention wherein the washing prior 
to the sintering was omitted, curve 12 shows the characteristics of the 
sample by the conventional dry method, and curve 13 shows the 
characteristics of the sample by the conventional coprecipitation method. 
From FIG. 4, it is evident that as compared with the products by the 
conventional methods, the superconductor of an oxide system prepared by 
the process of the present invention undergoes a sharp transition to a 
superconducting state with the resistivity abruptly dropping to 0 when 
cooled. With respect to each sample, the onset temperature, the offset 
temperature and the transition temperature range are shown in Table 4. 
Further, the critical current density of each sample in the 
superconducting state (77.4 K) was obtained and shown in Table 4. 
TABLE 4 
__________________________________________________________________________ 
Critical 
Method for the Transition 
current 
preparation of 
Onset Offset 
temperature 
density 
Example No. 
the sample 
temp. (K) 
temp. (K) 
range (K) 
(A/cm.sup.2) 
__________________________________________________________________________ 
Example 5 
Process of the 
97.9 96.8 1.1 98.0 
present 
invention 
including 
washing 
Example 6 
Process of the 
97.2 94.2 3.0 92.4 
present 
invention 
without 
washing 
Comparative 
Dry method 
94.2 90.0 4.2 36.5 
Example 8 
Comparative 
Coprecipita- 
91.7 79.2 12.5 10.6 
Example 9 
tion method 
__________________________________________________________________________ 
From the results in Table 4, it is evident that as compared with the 
products by the conventional methods, the superconductors of an oxide 
system prepared by the process of the present invention have practically 
excellent superconducting characteristics such that not only the 
temperature at which the resistivity drops completely to 0 is high, but 
also the range for the transition from the normal conducting state to the 
superconducting state is narrow, and the critical current density is very 
high. Whereas, with the products prepared by the conventional methods, 
particularly by the coprecipitation method, the offset temperature, the 
transition temperature range and the critical current density are all 
inadequate for practical application. 
Further, among the products of the present invention, the one obtained by 
the process including the washing prior to the sintering is superior in 
the offset temperature, the transition temperature range and the critical 
current density to the one obtained by the process wherein the washing 
step was omitted. 
With respect to each of the four types of sintered product samples, the 
structural analysis was conducted by e.g. the X-ray diffraction method, 
whereby each of the products obtained by the process of the present 
invention was found to be a sintered product composed of a substantially 
uniform single phase of a so-called oxygen-deficient type perovskite 
structure. On the other hand, the products by the conventional dry method 
and the coprecipitation method were found to contain relatively large 
proportions of other phases in addition to the oxygen-deficient type 
perovskite structure. Especially the product by the coprecipitation method 
was found to have the ratio of elements deviated from the desired 
composition. Thus, the poor characteristics of the superconductors of an 
oxide system according to the conventional methods as shown in FIG. 4 and 
Table 4, are considered to be attributable to the deviation of the 
composition and the presence of other phases than the high temperature 
superconductive phase. 
It has been found by the analysis (observation by means of a scanning type 
electron microscope) that in the process of the present invention, the 
gelled hydrolyzate composed mainly of yttrium hydroxide, barium hydroxide, 
copper stearate and copper oxide formed by the hydrolysis in Example 5 and 
the sintered product thereof obtained by sintering at a temperature of 
from 200.degree. to 500.degree. C., are composed of pure uniform super 
fine particles of a few tens to a few hundreds .ANG.. Namely, such a 
nature is considered to effectively serve to obtain the sintered product 
of the desired composition. 
EXAMPLES 7 and 8 and COMATIVE EXAMPLE 10 
So as to bring the composition of the desired superconductor of an oxide 
system to (La.sub.O.92 Sr.sub.0.08).sub.2 CuO.sub.4, copper 
acetylacetonate, strontium acetylacetonate and lanthanum oleate were 
uniformly dispersed (partially dissolved) in a total amount of 8 g in 1 
liter of butanol. This dispersion was subjected to hydrolysis by an 
addition of 10 ml of distilled water containing 30 mg of KOH, whereby a 
gelled mixture was obtained. 
The gelled mixture was washed with deionized water until the water used for 
the washing was substantially neutral, then dried at 100.degree. C. for 4 
hours and presintered in an oxygen atmosphere at 950.degree. C. for 6 
hours. The presintered product thus obtained was pulverized and formed 
into a pelletized sample in the same manner as in Example 5. Then, the 
pelletized sample was sintered in an oxygen atmosphere at 950.degree. C. 
for 2 hours to obtain a densely sintered product (Example 7). 
Further, a sintered product was prepared in the same manner as in Example 7 
except that the washing with deionized water prior to the sintering was 
omitted (Example 8). 
For the purpose of comparison, a sample by the conventional dry method was 
prepared by using La.sub.2 O.sub.3, SrCO.sub.3 and CuO powders in the same 
manner as in Example 5 and presintered and sintered in the same manner as 
above to obtain a sintered product (Comparative Example 10). 
With respect to the three types of samples, the temperature dependency of 
the resistivity and the critical current density were measured in the same 
manner as in Example 5. The results are shown in FIG. 5 and Table 5. 
In FIG. 5, curve 14 shows the characteristics by the process of the present 
invention wherein the washing was conducted prior to the sintering, curve 
15 shows the characteristics by the process of the present invention 
wherein the washing prior to the sintering was omitted, and curve 16 shows 
the characteristics by the conventional dry method. The critical current 
density is the value measured at the liquefied helium temperature (4.2 K). 
TABLE 5 
__________________________________________________________________________ 
Critical 
Method for the Transition 
current 
preparation of 
Onset Offset 
temperature 
density 
Example No. 
the sample 
temp. (K) 
temp. (K) 
range (K) 
(A/cm.sup.2) 
__________________________________________________________________________ 
Example 7 
Process of the 
47.9 47.0 0.9 79.9 
present 
invention 
including 
washing 
Example 8 
Process of the 
47.0 44.1 2.9 75.0 
present 
invention 
without 
washing 
Comparative 
Dry method 
43.6 37.3 6.3 27.4 
Example 10 
__________________________________________________________________________ 
From the results in FIG. 5 and Table 5, it is evident that as compared with 
the product by the conventional method, the superconductors of an oxide 
system prepared by the process of the present invention have practically 
excellent superconducting characteristics such that the critical 
temperature is high, the transition temperature range is narrow and the 
critical current density is high. Whereas, with the product prepared by 
the conventional dry method, the critical temperature, the transition 
temperature range and the critical current density are all inadequate for 
practical application. 
The poor characteristics of the superconductor of an oxide system prepared 
by the conventional dry method have been found to be attributable to the 
relatively large content of unwanted phases other than the high 
temperature superconductive phase. 
Further, with respect to each of the samples obtained 
susceptibility was measured, whereby in each case Meissner effects were 
observed at a temperature of not higher than the temperature at which the 
resistivity became 0. 
EXAMPLE 9 and COMATIVE EXAMPLES 11 and 12 
So as to bring the composition of the desired superconductor of an oxide 
system to (La.sub.0.9 Sr.sub.0.1).sub.2 CuO.sub.4, copper acetylacetonate 
(purity: at least 99.999%), lanthanum acetylacetonate (purity: at least 
99.999%) and SrCO.sub.3 (purity at least 99.9%) were used in a 
predetermined ratio. Ten mmol of copper acetylacetonate and 18 mmol of 
lanthanum acetylacetonate were added to 1 liter of tetrahydrofuran and 
uniformly dispersed (partially dissolved). This dispersion was adjusted to 
60.degree. C., and 0.5 ml of triethylamine was added thereto. Then, the 
hydrolysis was conducted by dropwise adding deionized water in large 
excess (200 ml) over a period of hour, whereby a blackish brown gel was 
formed. To this gel, 2 mmol of SrCO.sub.3 was added and uniformly mixed. 
Then, the mixture was heated to 80.degree. C. under stirring to evaporate 
the solvent and water to obtain a dried mixture. 
The dried mixture was presintered in oxygen at 900.degree. C. for 4 hours 
to obtain a porous presintered product. The presintered product was 
pulverized in a mortar and formed into a pellet having a diameter of 10 mm 
and a thickness of 1.5 mm by means of a pelletizer. This pellet was again 
sintered in oxygen at 930.degree. C. for 2 hours to obtain a densely 
sintered product. 
For the purpose of comparison, a mixture of La.sub.2 O.sub.3, SrCO.sub.3 
and CuO powders and a precipitate formed by adding small amounts of 
ammonia and oxalic acid to an aqueous solution of nitrates of La, Sr and 
Cu, each prepared by using guaranteed reagents, were, respectively, 
presintered and sintered in the same manner as above to obtain sintered 
products by the conventional dry method and the coprecipitation method. 
With respect to each of the three types of samples thus obtained, the 
temperature dependency of the resistivity was measured by a four terminal 
method. The results are shown in FIG. 6. 
In FIG. 6, curve 17 shows the characteristics of the sample prepared by the 
process of the present invention, curve 18 shows the characteristics of 
the sample prepared by the conventional dry method, and curve 19 shows the 
characteristics of the sample prepared by the conventional coprecipitation 
method. 
From FIG. 6, it is evident that as compared with the products by the 
conventional methods, the superconductor of an oxide system prepared by 
the process of the present invention undergoes a sharp transition to the 
superconducting state with the resistivity abruptly dropping to 0 when 
cooled. With respect to each sample, the onset temperature, the offset 
temperature and the transition temperature range were measured and shown 
in Table 6. Further, the critical current density of each sample at the 
liquefied helium temperature (4.3K) was obtained and shown in Table 6. 
TABLE 6 
__________________________________________________________________________ 
Critical 
Method for the Transition 
current 
preparation of 
Onset Offset 
temperature 
density 
Example No. 
the sample 
temp. (K) 
temp. (K) 
range (K) 
(A/cm.sup.2) 
__________________________________________________________________________ 
Example 9 
Process of the 
44.2 42.2 2.0 60.7 
present 
invention 
Comparative 
Dry method 
43.6 37.3 6.3 27.4 
Example 11 
Comparative 
Coprecipita- 
42.7 28.0 14.7 15.1 
Example 12 
tion method 
__________________________________________________________________________ 
From the results in Table 6, it is evident that as compared with the 
products by the conventional methods, the superconductor of an oxide 
system prepared by the process of the present invention has practically 
excellent superconducting characteristics such that the temperature at 
which the resistivity drops to 0 is high, the range for the transition 
from the normal conducting state to the superconducting state is very 
narrow and the critical current density is very high. Whereas, with the 
products by the conventional methods, particularly by the coprecipitation 
method, the offset temperature, the transition temperature range and the 
critical current density are all inadequate for practical application. 
With respect to each of the above three types of sintered product samples, 
the structural analysis was conducted by e.g. the X-ray diffraction 
method, whereby the product prepared by the process of the present 
invention was found to be composed of a substantially uniform single phase 
of a so-called K type (La.sub.0.9 Sr.sub.0.1). On the other hand, the 
products by the conventional dry method and the coprecipitation method 
were found to contain relatively large proportions of a perovskite 
structure of ABO.sub.3 type (wherein A and B are L metal elements) and 
other phases in addition to the K.sub.2 NiF.sub.4 type phase. Especially 
the product by the coprecipitation method was found to have the ratio of 
elements substantially deviated from the desired composition. Thus, the 
poor characteristics of the superconductors of an oxide system prepared by 
the conventional methods as shown in FIG. 6 and Table 6, are considered to 
be attributable to the presence of such other phases and such 
compositional deviation. 
The gelled hydroxides of Cu and La formed by the hydrolysis according to 
the process of the present invention were found to be pure uniform super 
fine particles of from a few tens to a few hundreds .ANG. by the analysis 
(observation by a scanning type electron microscope). Namely, such a 
nature is considered to effectively serve to obtain the sintered product 
of the desired composition. 
EXAMPLES 10 and 11 and COMATIVE EXAMPLES 13 and 14 
So as to bring the composition of the desired superconductor of an oxide 
system to YBa.sub.2 Cu.sub.3 O.sub.7, yttrium acetylacetonate, barium 
acetylacetonate and CuO were used in a predetermined ratio. Ten mmol of 
yttrium acetylacetonate and 20 mmol of barium acetylacetonate were 
uniformly dispersed (partially dissolved) in 500 ml of benzene at 
80.degree. C. Then, hydrolysis was conducted by adding 30 ml of distilled 
water containing 5 mg of NaOH to this dispersion and heating it for 5 
hours at 80.degree. C., whereby a CuO was uniformly mixed. The homogeneous 
mixture thus obtained was washed with deionized water until the water used 
for washing was substantially neutral. Then, the washed mixture was left 
to stand at room temperature and in an atmosphere of 80.degree. C. to 
evaporate water and the solvent. The dried product thus obtained was 
preliminarily calcined at 900.degree. C. for 4 hours in oxygen and then 
presintered in oxygen at 940.degree. C. for 6 hours to obtain a coarse 
presintered product. The presintered product was pulverized and formed 
into a pelletized sample in the same manner as in Example 9. This pellet 
was sintered in oxygen at 950.degree. C. for 2 hours to obtain a densely 
sintered product (Example 10). 
Further, a sintered product was prepared in the same manner as in Example 
10 except that the washing with deionized water prior to the sintering was 
omitted. 
For the purpose of comparison, samples by the conventional dry method and 
the coprecipitation method were prepared by using a mixture of Y.sub.2 
O.sub.3, BaCO.sub.3 and CuO powders and nitrates of Y, Ba and Cu, 
respectively, in the same manner as in Example 9 and presintered and 
sintered in the same manner as above to obtain two types of sintered 
products. 
With respect to each of the four types of samples, the temperature 
dependency of the resistivity and the critical current density were 
measured in the same manner as in Example 9. The results are shown in FIG. 
7 and Table 7. In FIG. 7, curve 20 shows the characteristics by the 
process of the present invention wherein the washing was conducted prior 
to the sintering, curve 21 shows the characteristics by the process of the 
present invention wherein no washing was conducted prior to the sintering, 
curve 22 shows the characteristics by the conventional dry method, and 
curve 23 shows the characteristics by the conventional coprecipitation 
method Further, the critical current density is the value as measured at 
the liquefied nitrogen temperature (77.4 K). 
TABLE 7 
__________________________________________________________________________ 
Critical 
Method for the Transition 
current 
preparation of 
Onset Offset 
temperature 
density 
Example No. 
the sample 
temp. (K) 
temp. (K) 
range (K) 
(A/cm.sup.2) 
__________________________________________________________________________ 
Example 10 
Process of the 
97.5 96.8 0.7 136 
present 
invention 
including 
washing 
Example 11 
Process of the 
97.4 96.0 1.4 108 
present 
invention 
without 
washing 
Comparative 
Dry method 
94.0 90.0 4.0 31.0 
Example 13 
Comparative 
Coprecipita- 
92.5 83.0 9.5 30.0 
Example 14 
tion method 
__________________________________________________________________________ 
From the results in FIG. 7 and Table 7, it is evident that as compared with 
the products by the conventional methods, the superconductors of an oxide 
system prepared by the process of the present invention have practically 
excellent superconducting characteristics such that the critical 
temperature for the transition to the superconducting state is high, the 
transition temperature range is narrow and the critical current density is 
high. It is also evident that among the products of the present invention, 
the one prepared by the process wherein washing was conducted prior to the 
sintering was superior to the one prepared by the process wherein the 
washing was omitted, in the offset temperature, the transition temperature 
range and the critical current density. Whereas, with the products 
prepared by the conventional two methods, the critical temperature, the 
transition temperature range and the critical current density are all 
inadequate for practical application. 
The poor characteristics of the superconductors of an oxide type by the 
conventional methods were found to be attributable to the relatively large 
content of unwanted phases other than the high temperature superconductive 
phase, as a result of the analysis by e.g. the X-ray diffraction method as 
in Example 9. 
Further, with respect to each of the samples used in Examples 9 to 11, the 
susceptibility was measured, whereby in each case, Meissner effects were 
observed at a temperature of not higher than the temperature at which the 
resistivity became 0. 
EXAMPLE 12 and COMATIVE EXAMPLES 15 and 16 
So as to bring the composition of the desired superconductor of an oxide 
system to YBa.sub.2 Cu.sub.3 O.sub.7, 10 mmol of (CH.sub.3 CH.sub.2 
CH.sub.2 O).sub.3 Y, 20 mmol of 
##STR2## 
and 30 mmol of (CH.sub.3 CH.sub.2 CH.sub.2 CH.sub.2 CH.sub.2 O).sub.2 Cu 
were suspended in 300 ml of tetrahydrofuran (THF) under an argon gas 
stream. Then, 100 ml of a THF mixed solution containing 130 mmol of dry 
acetone was added thereto at 40.degree. C., and the mixture was stirred at 
40.degree. C. for 3 hours. 
Then, 130 mmol of ethyl acetate was added thereto, and the mixture was 
stirred at room temperature for 1 hour. Then, 50 ml of THF containing 200 
mmol of H.sub.2 O was gradually added thereto for hydrolysis, whereby a 
mixture containing a gelled product was obtained. 
The solvent was distilled off, and the dried mixture was presintered in an 
oxygen stream at 900.degree. C. for 4 hours to obtain a porous presintered 
product. The presintered product was pulverized in a mortar and formed 
into a pellet having a diameter of 10 mm and a thickness of 1.5 mm by 
means of a pelletizer. This pellet was again sintered in an oxygen stream 
at 900.degree. C. for 8 hours to obtain a densely sintered product. 
For the purpose of comparison, a mixture obtained in the same manner as in 
Example 12 except that a mixture of yttrium acetylacetonate, barium 
acetylacetonate and Cu acetylacetonate was used and a precipitate formed 
by adding small amounts of ammonia and oxalic acid to an aqueous solution 
of nitrates of Y, Ba and Cu to bring the composition of the desired 
superconductor of an oxide system to YBa.sub.2 Cu.sub.3 O.sub.7, were, 
respectively, presintered and sintered in the same manner as above to 
obtain the sintered products by the process wherein a mixture of the 
respective metal acetylacetonates was used and by the conventional 
coprecipitation method. 
With respect to each of the three types of samples, the temperature 
dependency of the resistivity was measured by a four terminal method. The 
results are shown in FIG. 8. 
In FIG. 8, curve 24 shows the characteristics of the sample prepared by the 
process of the present invention, curve 25 shows the characteristics of 
the sample prepared by the process wherein a mixture of the respective 
acetylacetonates was used, and curve 26 shows the characteristics of the 
sample prepared by the conventional coprecipitation method. 
From FIG. 8, it is evident that as compared with the products of 
Comparative Examples, the superconductor of an oxide system prepared by 
the process of the present invention undergoes a sharp transition to the 
superconducting state with an abrupt drop of the resistivity to 0 when 
cooled. With respect to each sample, the onset temperature, the offset 
temperature and the transition temperature range were measured and shown 
in Table 8. Further, the critical current density of each sample at the 
liquefied nitrogen temperature (77.4K) was measured and shown in Table 8. 
TABLE 8 
__________________________________________________________________________ 
Critical 
Method for the Transition 
current 
preparation of 
Onset Offset 
temperature 
density 
Example No. 
the sample 
temp. (K) 
temp. (K) 
range (K) 
(A/cm.sup.2) 
__________________________________________________________________________ 
Example 12 
Process of the 
98.8 97.9 0.9 157.6 
present 
invention 
Comparative 
Process 98.6 95.5 3.1 127.4 
Example 15 
wherein a 
mixture of the 
respective 
metal acetyl- 
acetonates 
was used 
Comparative 
Coprecipita- 
91.2 86.3 4.9 32.0 
Example 16 
tion method 
__________________________________________________________________________ 
From the results in FIG. 8 and Table 8, it is evident that as compared with 
the process wherein the respective metal elements are mixed in the form of 
the metal acetylacetonates or the coprecipitation method, the 
superconductor of an oxide system prepared by the process of the present 
invention has practically excellent characteristics such that the 
temperature at which the resistivity drops to 0 is high, the range for the 
transition from the normal conducting state to the superconducting state 
is narrow, and the critical current density is high. Whereas, especially 
with the product by the coprecipitation method, the offset temperature, 
the transition temperature range and the critical current density are all 
inadequate for practical application. 
With respect to each of the above three types of sintered product samples, 
the structural analysis was conducted by e.g. the X-ray diffraction 
method, whereby the product by the process of the present invention was 
found to have a uniform single phase with a three layer oxygen-deficient 
perovskite structure. proportion of other phases in addition to the high 
temperature superconductive phase, and the deviation in the elemental 
ratio from the desired composition was found to be substantial. 
The poor characteristics of the superconductor of an oxide system prepared 
by the conventional coprecipitation method, as shown in FIG. 8 and Table 
8, are considered to be attributable to such a compositional deviation. 
The hydrolyzate composed mainly of a gelled mixture of hydroxides of Y, Ba 
and Cu formed by the hydrolysis according to the process of the present 
invention and the sintered product thereof obtained by sintering it at a 
temperature of from 200.degree. to 500.degree. C., were found to be 
composed of pure uniform super fine particles of from a few tens to a few 
hundreds .ANG. by the analysis (observation by a scanning type electron 
microscope). Namely, such a nature is considered to effectively serve to 
obtain the sintered product of the desired composition. 
EXAMPLE 13 and COMATIVE EXAMPLE 17 
So as to bring the composition of the desired superconductor of an oxide 
system to (La.sub.0.9 Sr.sub.0.1).sub.2 CuO.sub.4, 18 mmol of La(OC.sub.2 
H.sub.5) 2 mmol of Sr(OC.sub.2 H.sub.5).sub.2 and 10 mmol of was stirred 
at 50.degree. C. for 10 hours. Then, the hydrolysis was conducted by 
gradually adding 50 ml of an ethanol solution containing 10 g of distilled 
water, whereby a mixture containing a gelled product was obtained. 
The solvent was removed, and the mixture thereby obtained was preliminarily 
baked at 500.degree. C. for 2 hours, and presintered in an oxygen 
atmosphere at 950.degree. C. for 3 hours to obtain a coarse presintered 
product. The presintered product thus obtained was pulverized and then 
formed into a pelletized sample in the same manner as in Example 12. This 
pellet was sintered in an oxygen atmosphere at 950.degree. C. for 8 hours 
to obtain a densely sintered product in the 
For the purpose of comparison, a mixture obtained in the same manner as in 
Example 13 except that a mixture of lanthanum acetylacetonate, strontium 
acetylacetonate and copper acetylacetonate was used to bring the 
composition of the desired superconductor of an oxide system to 
(La.sub.0.9 Sr.sub.0.1).sub.2 CuO.sub.4, was presintered and sintered in 
the same manner as above to obtain a sintered product. 
With respect to each of the two types of samples, the temperature 
dependency of the resistivity and the critical current density were 
measured in the same manner as in Example 12. The results are shown in 
FIG. 9 and Table 9. 
In FIG. 9, curve 27 is the characteristics of the product prepared by the 
process of the present invention, and curve 28 is the characteristics of 
the product prepared by the process wherein the mixture of 
acetylacetonates was used. The critical current density is the value 
measured at the liquefied helium temperature (4.3 K). 
TABLE 9 
__________________________________________________________________________ 
Critical 
Method for the Transition 
current 
preparation of 
Onset Offset 
temperature 
density 
Example No. 
the sample 
temp. (K) 
temp. (K) 
range (K) 
(A/cm.sup.2) 
__________________________________________________________________________ 
Example 13 
Process of the 
45.6 44.9 0.7 160.4 
present 
invention 
Comparative 
Process 43.8 41.8 2.0 98.7 
Example 17 
wherein a 
mixture of 
acetyl- 
acetonates 
was used 
__________________________________________________________________________ 
From the results in FIG. 9 and Table 9, it is evident that as compared with 
the product prepared by the process wherein a mixture of the respective 
metal acetylacetonates is used, the superconductor of an oxide system 
prepared by the process of the present invention has superior 
superconducting characteristics. The poor superconducting characteristics 
of the superconductor of an oxide system prepared by the process wherein a 
mixture of the respective metal acetylacetonates was used, are considered 
to be attributable to the non-homogeneous mixing of the metal elements. 
With respect to each of the samples used in Examples 12 and 13, the 
susceptibility was measured, whereby in each case, Meissner effects were 
observed at a temperature of not higher than the temperature at which the 
resistivity became 0. 
EXAMPLE 14 and COMATIVE EXAMPLE 18 
So as to bring the composition of the desired Bi-Sr-Ca-Cu-O superconductor 
to Bi.sub.1 Sr.sub.1 Ca.sub.1 Cu.sub.2 Ox, Bi(OC.sub.2 H.sub.5).sub.3, 
Sr(OC.sub.4 H.sub.9).sub.2, Ca(OC.sub.4 H.sub.9).sub.2 and Cu(CH.sub.3 
COCHCOCH.sub.3).sub.2 (each having a purity of at least 99.999%) were 
added in a predetermined ratio in a total amount of 4 g in 500 ml of 
butanol. Further, 0.1 ml of methoxyethanol and 0.1 ml of ethanolamine were 
added thereto. Then, the mixture was refluxed for 24 hours, and then 10 ml 
of distilled water was added thereto over a period of 30 minutes. The 
mixture was further refluxed for 24 hours to obtain blackish brown 
particles. 
The product thus obtained was analyzed by means of an X-ray diffraction 
apparatus and found to be an amorphous mixture of Bi, Sr, Ca and Cu. 
The product was concentrated under reduced pressure to remove the solvent, 
dried at 300.degree. C. for 1 hour and then formed into a pellet having a 
diameter of 10 mm and a thickness of 1.5 mm by a pelletizer. This pellet 
was sintered in an oxygen stream at 900.degree. C. for 8 hours to obtain a 
densely sintered product. 
For the purpose of the comparison, a mixture of Bi.sub.2 O.sub.3, 
SrCO.sub.3, CaO and CuO powders which were all guaranteed reagents, was 
sintered in the same manner as above to obtain a sintered product 
according to the conventional dry method. 
With respect to the two types of samples, the temperature dependency of the 
resistivity was measured by a four terminal method. The results are shown 
in Table 10 and FIG. 10. 
In FIG. 10, curve 29 shows the characteristics of the sample prepared by 
the process of the present invention, and curve 30 shows the 
characteristics of the sample prepared by the conventional dry method. 
TABLE 10 
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Example No. Onset (K) Offset (K) 
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Example 14 125 86 
Comparative 120 70 
Example 18 
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From the results in Table 10 and FIG. 10, it is evident that as compared 
with the product prepared by the conventional method, the Bi-Sr-Ca-Cu-O 
superconductor prepared by the process of the present invention has a high 
critical temperature and exhibits superior superconductivity. 
With respect to the above two types of sintered product samples, the 
structural analysis was conducted by the X-ray diffraction method, and 
they were considered to be polymorphic. However, the intensity in the 
X-ray diffraction was distinctly stronger with the product prepared by the 
process of the present invention, thus indicating that the crystallization 
was more advanced. (When measured under the same conditions, the intensity 
of the peak of the product prepared by the dry method was about 1/4 time.) 
Further, a study has been made by changing the sintering temperature to 
500.degree. C., 600.degree. C., 700.degree. and 800.degree. C. From the 
results of the X-ray diffraction, it is evident that according to the 
process of the present invention, the crystallization was adequately 
advanced even at 700.degree. C. Whereas, in the dry method, heating at a 
temperature of about 900.degree. C. is required to obtain a sintered 
product capable of undergoing a transition to superconductivity. Thus, it 
is evident that the process of the present invention wherein a 
superconductor is synthesized from a liquid phase is far superior. 
Sintered products were prepared in the same manner as in Example 14 and 
Comparative Example 18 except that the sintering temperature was changed 
to 700.degree. C., and the resistance of such sintered products were 
measured. The results are shown by curves 31 and 32 in FIG. 10. From FIG. 
10, it is evident that by the synthesis in accordance with the dry method, 
the product sintered at 700.degree. C. shows .eta.-T characteristics of a 
semiconductor behavior and does not show a transition to 
superconductivity, whereas the sintered product according to the process 
of the present invention shows an onset temperature of 95 K and an offset 
temperature of 60 K. 
With respect to the samples obtained in Example 14 and Comparative Example 
18, the susceptibility was measured, whereby in each case, Meissner 
effects were observed at the offset temperature.