Method of manufacturing an oxide superconductor with high critical current density

An oxide superconductor capable of realizing a high critical current density and its manufacturing method requiring only a low temperature heat treatment. An oxide superconductor has a superconductive layer with a composition of RE.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-x, where RE stands for any one of rare earth elements including Y, Eu, Gd, Dy, Ho, Er, and Yb, which is formed on the substrate by RE.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-x phase and CuO phase resulting from a decomposition of RE.sub.1 Ba.sub.2 Cu.sub.4 O.sub.8 phase, in which the CuO phase and micro-defects caused by the decomposition function as pinning centers. This superconductive layer is formed by applying a solution containing organic compounds of a plurality of metallic elements for constituting the oxide superconductive layer; calcining the substrate applied with the solution to obtain a calcined body in which the organic compounds contained in the solution are thermally decomposed; heating the calcined body to produce RE.sub.1 Ba.sub.2 Cu.sub.4 O.sub.8 phase; and decomposing the RE.sub.1 Ba.sub.2 Cu.sub.4 O.sub.8 phase into RE.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-x phase and CuO phase, to obtain the oxide superconductor.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an oxide superconductor in which a 
superconductive layer in a thin or thick film shape is formed on a 
substrate, and its manufacturing method. 
2. Description of the Background Art 
Conventionally known methods of manufacturing an oxide superconductor such 
as YBaCuO type superconductor include a sputtering method and a vacuum 
evaporation method. In these conventionally known methods, it is fairly 
common to have the composition of the obtained thin film to be somewhat 
different from the composition of the target or the evaporation source 
employed, so that the control of the composition of the oxide 
superconductor which includes at least three metallic elements presents 
unexpectedly difficult problem. 
In addition, these conventionally known methods are not suitable for the 
thick film formation as well as for the mass production, so that they are 
disadvantageous for the manufacturing of a large area superconductor. 
Moreover, in the thin film obtained by these conventionally known methods, 
it is possible to obtain the satisfactory crystal orientation property so 
that the sufficiently high critical temperature Tc and critical current 
density Jc can be realized, but it is difficult to form a thick film of a 
desired thickness without sacrificing this crystal orientation property. 
On the other hand, there is another conventionally known method of 
manufacturing an oxide superconductor called the CVD (Chemical Vapor 
Deposition) method. This CVD method is suitable for the mass production, 
and has a possibility of realizing a low temperature film formation. 
However, in this CVD method, there is a drawback concerning the difficulty 
to secure the sufficient amount of the necessary alkali metal materials 
including Ba materials and Sr materials at the appropriate vapor pressure 
in particular. 
There has also been a proposition of a potentially superior method of 
manufacturing an oxide superconductor called the MOD (Metal-Organic 
Deposition) method. This MOD method uses a solution in which 
organometallic complex salts of a plurality of metallic elements to 
constitute the oxide superconductor are dissolved in an organic solvent, 
to form a superconductive layer on a substrate by applying this solution 
on a surface of the substrate and then burning it. 
This MOD method has advantages in that it is easy to form a large area 
superconductor and a superconductive layer of desired thickness and film 
formation pattern can be manufactured. However, there is a drawback in 
this MOD method in that it is difficult to obtain an oxide superconductor 
with a high critical current density. 
Now, in any of these conventionally known methods of manufacturing an oxide 
superconductor, there is an unavoidable problem of realizing a 
sufficiently high critical current density in a form of a thick film such 
as a tape member. However, in any of these conventionally known methods, 
when the film is thickened to a level of a tape member, the critical 
current density inevitably decreases. 
In addition, it has been difficult in any of these conventionally known 
methods to control the thickness of the film while maintaining a desired 
superconductor property. 
In order to resolve these problems of conventionally known methods, there 
has been a proposition to raise the critical current density by 
introducing pinning centers for suppressing movements of magnetic fluxes 
which are entering into the oxide superconductor at a time of current 
conduction. 
Here, the pinning centers are portions which function to obstruct the 
movements of the magnetic fluxes due to the Lorentz's force exerted onto 
the magnetic fluxes entering into the superconductor which would generate 
the resistances in the superconductor, and it is known that this role of 
the pinning centers can be fulfilled by deposit particles or grain 
boundaries formed within the oxide superconductor. 
Conventionally, as a method of introducing the pinning centers to the balky 
oxide superconductor with the composition of Y.sub.1 Ba.sub.2 Cu.sub.3 
O.sub.7-x, there is a method which incorporates the Y.sub.2 Ba.sub.1 
Cu.sub.1 O.sub.x phase into the Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-x phase 
by utilizing the peritectic reaction from a state in which the Y.sub.2 
Ba.sub.1 Cu.sub.1 O.sub.x phase and the liquid phase are mixedly present. 
However, in this method, in order to introduce the Y.sub.2 Ba.sub.1 
Cu.sub.1 O.sub.x phase, there is a need to use a high temperature heat 
treatment with a temperature over 1000.degree. C. However, when such a 
high temperature heat treatment is applied with respect to the oxide 
superconductive layer formed on the substrate, the diffusion reaction at a 
boundary surface between the substrate and the superconductive layer is 
promoted, such that the composition of the oxide superconductive layer 
itself is affected and as a result the lowering of the critical current 
density is caused. 
Also, a conventional method of manufacturing a balky oxide superconductor 
includes a process for introducing the pinning centers by utilizing the 
decomposition process from the Y.sub.1 Ba.sub.2 Cu.sub.4 O.sub.8 phase to 
the mixture of the Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-x phase and the CuO 
phase, but this decomposition process requires the high temperature heat 
treatment with a temperature over 900.degree. C. under the usual ambient 
atmosphere with a significant oxygen partial pressure, so that it is 
difficult to utilize this decomposition process in the method of 
manufacturing an oxide superconductor on a substrate. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to provide an oxide 
superconductor capable of realizing a high critical current density along 
with a desired thickness and a superconductor property. 
It is another object of the present invention to provide a method of 
manufacturing such an oxide superconductor with a high critical current 
density, capable of introducing a pinning center by utilizing the 
decomposition process from the Y.sub.1 Ba.sub.2 Cu.sub.4 O.sub.8 phase to 
the mixture of the Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-x phase and the CuO 
phase, which only requires a heat treatment of a temperature significantly 
lower than that required in the conventional method. 
According to one aspect of the present invention there is provided an oxide 
superconductor, comprising: a substrate; and a superconductive layer with 
a composition of RE.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-x, where RE stands for 
any one of rare earth elements including Y, Eu, Gd, Dy, Ho, Er, and Yb, 
which is formed on the substrate by RE.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-x 
phase and CuO phase resulting from a decomposition of RE.sub.1 Ba.sub.2 
Cu.sub.4 O.sub.8 phase, in which the CuO phase and micro-defects caused by 
the decomposition function as pinning centers. 
According to another aspect of the present invention there is provided a 
method of manufacturing an oxide superconductor, comprising the steps of: 
(a) applying a solution containing organic compounds of a plurality of 
metallic elements for constituting an oxide superconductive layer with a 
composition of RE.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-x onto a substrate;. (b) 
calcining the substrate applied with the solution to obtain a calcined 
body in which the organic compounds contained in the solution are 
thermally decomposed; (c) heating the calcined body to produce RE.sub.1 
Ba.sub.2 Cu.sub.4 O.sub.8 phase; and (d) decomposing the RE.sub.1 Ba.sub.2 
Cu.sub.4 O.sub.8 phase into RE.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-x phase and 
CuO phase, to obtain the oxide superconductor having the superconductive 
layer formed on the substrate, where the CuO phase and micro-defects 
caused by this decomposing step are introduced into the superconductive 
layer as pinning centers. 
Other features and advantages of the present invention will become apparent 
from the following description taken in conjunction with the accompanying 
drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
First, the oxide superconductor and its manufacturing method according to 
the present invention will be outlined in general terms. 
Namely, the oxide superconductor according to the present invention has a 
superconductive layer with a composition of RE.sub.1 Ba.sub.2 Cu.sub.3 
O.sub.7-x, where RE stands for any one of the rare earth elements 
including Y, Eu, Gd, Dy, Ho, Er, and Yb, which is formed on a substrate by 
decomposing the RE.sub.1 Ba.sub.2 Cu.sub.4 O.sub.8 phase into the RE.sub.1 
Ba.sub.2 Cu.sub.3 O.sub.7-x phase and CuO phase such that the CuO phase 
and micro-defects caused by the decomposition process function as pinning 
centers. 
In this oxide superconductor according to the present invention, the CuO 
phase and the micro-defects introduced by the decomposition process 
function as the pinning centers to obstruct the movements of the magnetic 
fluxes entering into the superconductor due to the self-excitation at a 
time of current conduction through the superconductive layer, so that the 
critical current density of the superconductor can be improved. 
In addition, these CuO phase and micro-defects are produced by the 
decomposition of the RE.sub.1 Ba.sub.2 Cu.sub.4 O.sub.8 phase, so that 
they can be distributed uniformly over the entire superconductive layer. 
As a consequence, the pinning effects due to the pinning centers can be 
obtained uniformly from the entire superconductive layer, such that it is 
possible to exhibit the effect of the improved critical current density 
efficiently. 
Now, according to the manufacturing method according to the present 
invention, the above described oxide superconductor according to the 
present invention can be obtained by the following procedure. 
Namely, the superconductive layer is formed on the substrate from the 
solution containing organic compounds of a plurality of metallic elements 
for constituting the oxide superconductor with a composition of the 
RE.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-x which is applied onto the substrate, 
calcined to thermally decompose the organic salts, and heated to produce 
the RE.sub.1 Ba.sub.2 Cu.sub.4 O.sub.8 phase, where the produced RE.sub.1 
Ba.sub.2 Cu.sub.4 O.sub.8 phase is then decomposed into the RE.sub.1 
Ba.sub.2 Cu.sub.3 O.sub.7-x phase and CuO phase such that the CuO phase 
and the micro-defects caused by this decomposition process function as the 
pinning centers. 
In this manufacturing method, the RE.sub.1 Ba.sub.2 Cu.sub.4 O.sub.8 phase 
is produced by applying the solution to the substrate first and then 
heating it, so that the RE.sub.1 Ba.sub.2 Cu.sub.4 O.sub.8 phase can be 
produced on the substrate uniformly. 
In addition, in this manufacturing method, the CuO phase and the 
micro-defects are obtained from this RE.sub.1 Ba.sub.2 Cu.sub.4 O.sub.8 
phase by the decomposition process, so that they can be distributed 
uniformly over the entire superconductive layer, and consequently it is 
possible to manufacture the oxide superconductor with the improved 
critical current density. 
Moreover, in this manufacturing method, the oxide superconductor of a 
desired thickness can be manufactured as a thickness of the solution 
applied onto the substrate can be easily adjusted by appropriately 
controlling the viscosity of the solution and selecting the manner of 
application. 
Here, in the manufacturing method described above, before a calcined body 
is heated to produce the RE.sub.1 Ba.sub.2 Cu.sub.4 O.sub.8 phase, the 
calcined body should preferably be heated at the temperature in a range of 
700.degree. to 850.degree. C. under the ambient atmosphere of the purely 
inert gas or the inert gas with a low oxygen partial pressure, 
corresponding to the thermodynamically stable region for the RE.sub.1 
Ba.sub.2 Cu.sub.3 O.sub.7-x phase in the starting composition, to produce 
the RE.sub.1 Ba.sub.2 Cu.sub.3 O.sub.6 phase and the CuO phase, so as to 
ensure the production of the RE.sub.1 Ba.sub.2 Cu.sub.4 O.sub.8 phase by 
the further heat treatment. 
In this case, it is also possible to produce the RE.sub.1 Ba.sub.2 Cu.sub.3 
O.sub.6 phase to be c-axis oriented in a direction vertical with respect 
to the surface of the substrate, in order to obtain the c-axis oriented 
RE.sub.1 Ba.sub.2 Cu.sub.4 O.sub.8 phase. This procedure to obtain the 
c-axis oriented RE.sub.1 Ba.sub.2 Cu.sub.4 O.sub.8 phase from the c-axis 
oriented RE.sub.1 Ba.sub.2 Cu.sub.3 O.sub.8 phase and the CuO phase is 
effective because the RE.sub.1 Ba.sub.2 Cu.sub.4 O.sub.8 phase is highly 
unlikely to be c-axis oriented compared with the RE.sub.1 Ba.sub.2 
Cu.sub.3 O.sub.8 phase, so that when the RE.sub.1 Ba.sub.2 Cu.sub.4 
O.sub.8 phase is directly produced from the calcined body by the heat 
treatment, it is likely to obtain crystal grains in random orientations. 
It is to be noted here that it is preferable for the superconductive layer 
to be c-axis oriented because the superconductor currents flows in 
parallel to the surface of the substrate of the superconductor which is 
located on an ab-plane. 
It is to be noted here that the heating of the calcined body at the 
temperature below the above described range is not preferable as it will 
result in the incomplete decomposition of BaCO.sub.3 produced by the 
thermal decomposition of the organic salts. On the other hand, the heating 
of the calcined body at the temperature above the above described range is 
also not preferable as it will cause the diffusion reaction among the 
elements constituting the substrate and the elements, constituting the 
solution. 
Also, in this case, the further heat treatment to produce the RE.sub.1 
Ba.sub.2 Cu.sub.4 O.sub.8 phase from the RE.sub.1 Ba.sub.2 Cu.sub.3 
O.sub.6 phase and the CuO phase should preferably be made at the 
temperature in a range of 700.degree. to 850.degree. C. under the 
appropriate ambient atmosphere, corresponding to the thermodynamically 
stable region for the RE.sub.1 Ba.sub.2 Cu.sub.4 O.sub.8 phase in the 
starting composition, so as to produce the RE.sub.1 Ba.sub.2 Cu.sub.4 
O.sub.8 phase stably and efficiently. Here, as indicated in FIG. 1, the 
RE.sub.1 Ba.sub.2 Cu.sub.4 O.sub.8 phase becomes stable at the lower 
temperature side than the RE.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-x phase, so 
that the further heat treatment can be made at the temperature and the 
ambient atmosphere in the thermodynamically stable region for the RE.sub.1 
Ba.sub.2 Cu.sub.4 O.sub.8 phase in the starting composition by 
appropriately selecting the oxygen partial pressure and the temperature in 
the starting composition to be in the thermodynamically stable region for 
the RE.sub.1 Ba.sub.2 Cu.sub.4 O.sub.8 phase. Note here that a location of 
a straight line A-B indicated in FIG. 1 moves according to the starting 
composition. 
Furthermore, in the manufacturing method described above, the decomposition 
of the RE.sub.1 Ba.sub.2 Cu.sub.4 O.sub.8 phase into the Re.sub.1 Ba.sub.2 
Cu.sub.3 O.sub.7-x phase and CuO phase should preferably be achieved by 
the heating at the temperature in a range of 700.degree. to 850.degree. C. 
under the ambient atmosphere of the purely inert gas or the inert gas with 
a low oxygen partial pressure, corresponding to the thermodynamically 
stable region for the RE.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-x phase in the 
starting composition, in order to thermally decompose the RE.sub.1 
Ba.sub.2 Cu.sub.4 O.sub.8 phase into the RE.sub.1 Ba.sub.2 Cu.sub.3 
O.sub.7-x phase and CuO phase efficiently. Here, as indicated in FIG. 1 
described above, the RE.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-x phase becomes 
stable at the higher temperature side than the RE.sub.1 Ba.sub.2 Cu.sub.4 
O.sub.8 phase, so that this heating for the thermal decomposition of the 
RE.sub.1 Ba.sub.2 Cu.sub.4 O.sub.8 phase into the RE.sub.1 Ba.sub.2 
Cu.sub.3 O.sub.7-x phase and CuO phase can be made at the temperature and 
the ambient atmosphere in the thermodynamically stable region for the 
RE.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-x phase in the starting composition by 
appropriately selecting the oxygen partial pressure and the temperature in 
the starting composition to be in the thermodynamically stable region for 
the RE.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-x phase. In this case, it is 
further preferable to make this heating at as low temperature side as 
possible in order to prevent the grain boundary deposition of the CuO 
phase. 
Referring now to FIG. 2, one specific embodiment of the method of 
manufacturing an oxide superconductor according to the present invention 
summarized above will be described in detail. 
First, the substrate is prepared. Here, the substrate may be in any desired 
shape such as that of a plate, a wire, or a tape. It is preferable for 
this substrate to be made from a material with a high melting point which 
does not easily make the diffusion reaction with the elements constituting 
the oxide superconductive layer to be formed thereon, or a material having 
a crystalline structure similar to that of the oxide superconductive 
layer. More specifically, a monocrystalline substrate of strontium 
titanate (SrTiO.sub.3) or magnesium oxide (MgO), or a metallic substrate 
having a monocrystalline covering of either one of these can be used for 
this substrate, for example. 
Next, the solution to be applied to this substrate is prepared. As already 
mentioned above, this solution contains organic compounds of a plurality 
of metallic elements for constituting the oxide superconductor with a 
composition of the RE.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-x. 
Here, the organic compounds to be contained in this solution may includes Y 
acetylacetone complex salt [Y(acac).sub.3 ] where "acac" stands for 
acetylacetonato: CH.sub.3 COCHCOCH.sub.3 (C.sub.5 H.sub.7 O.sub.2), Ba 
acetylacetone complex salt [Ba(acac).sub.2 (OH.sub.2).sub.2 ], and Cu 
acetylacetone complex salt [Cu(acac).sub.2 ] for example. The organic 
compounds may also includes other acetylacetone complex salt such as Er 
triacetylacetonato complex salt [Er(acac).sub.3 ], as well as other 
organometallic compounds such as formate, acetate, naphthenate, etc. 
The solution can be prepared from these organic compounds by mixing the 
powders of these organic compounds at a predetermined mole ratio such that 
a relative rate of RE (rare earth) element, Ba element, and Cu element in 
the mixture becomes 1:2:3.1 to 4, respectively, and then dissolving the 
obtained mixture into the organic solvent. 
Here, the organic solvent can be a mixed solvent of pyridine (Py) and 
propionic acid (PA) for example, or a solvent of methanol, acetic acid, 
toluene, etc. 
Next, the solution is applied onto the surface of the substrate. Here, the 
manner of application can be any of the known solution application method 
such as the spin coating method, the screen printing method, the brushing 
method, and the dip coating method. 
Next, the entire substrate with the solution applied thereon is confined in 
a furnace such as a muffle furnace, to calcine it at the temperature of 
500.degree. to 700.degree. C. in the air, so as to thermally decompose the 
organic salts contained in the solution. As a result of this thermal 
decomposition, the calcined body containing the oxides of the elements 
contained in the solution such as RE.sub.2 O.sub.3, BaCO.sub.3, and CuO is 
produced. 
Next, the multi-stage heat treatment is applied to this calcined body in 
the furnace by appropriately controlling the temperature and the 
atmosphere inside the furnace according to the timing chart shown in FIG. 
2, as follows. 
First, the air inside the furnace is replaced by the inert gas such as the 
100% argon gas or the argon gas containing about 0.01% of oxygen, and the 
temperature inside the furnace is raised to the temperature T1 in a range 
of 700.degree. to 850.degree. C. under the appropriate ambient atmosphere, 
corresponding to the thermodynamically stable region for the RE.sub.1 
Ba.sub.2 Cu.sub.3 O.sub.7-x phase in the starting composition, over the 
period t1 in a range of several tens of minutes to several hours. Then, 
these temperature T1 and ambient atmosphere are maintained for the 
following period t2 in a range of several tens of minutes to several 
hours. 
As a result of this heat treatment, the solvent components in the solution 
are evaporated, and the organic components in the solution are 
sufficiently decomposed to produce the RE.sub.1 Ba.sub.2 Cu.sub.3 O.sub.6 
phase and the CuO phase. At this stage, as the oxygen is not supplied from 
the ambient atmosphere, the RE.sub.1 Ba.sub.2 Cu.sub.3 O.sub.6 phase is 
mainly produced. Also, the CuO phase is deposited at portions centered 
around the grain boundaries. 
Next, the temperature inside the furnace is set and maintained at the 
temperature T2 in a range of 700.degree. to 850.degree. C. while the 
oxygen partial pressure inside the furnace is increased by supplying 
oxygen gas into the furnace, so as to realize the temperature and the 
ambient atmosphere corresponding to the thermodynamically stable region 
for the RE.sub.1 Ba.sub.2 Cu.sub.4 O.sub.8 phase in the starting 
composition, over the period t3 in a range of several tens of minutes to 
several tens of hours. In the temperature and the ambient atmosphere 
corresponding to the thermodynamically stable region for the RE.sub.1 
Ba.sub.2 Cu.sub.4 O.sub.8 phase in the starting composition, the RE.sub.1 
Ba.sub.2 Cu.sub.3 O.sub.6 phase and the CuO phase at the grain boundaries 
react each other to produce the RE.sub.1 Ba.sub.2 Cu.sub.4 O.sub.8 phase, 
so as to realize the phase state of a mixture of the RE.sub.1 Ba.sub.2 
Cu.sub.4 O.sub.8 phase, the RE.sub.1 Ba.sub.2 Cu.sub.3 O.sub.6 phase, and 
the CuO phase. At this stage, as the oxygen gas is supplied from the 
ambient atmosphere, the RE.sub.1 Ba.sub.2 Cu.sub.3 O.sub.6 phase is 
believed to be actually in a form of RE.sub.1 Ba.sub.2 Cu.sub.3 
O.sub.6.0-7.0. 
Here, as already mentioned above, the RE.sub.1 Ba.sub.2 Cu.sub.4 O.sub.8 
phase is highly unlikely to be c-axis oriented compared with the RE.sub.1 
Ba.sub.2 Cu.sub.3 O.sub.6 phase, so that when the RE.sub.1 Ba.sub.2 
Cu.sub.3 O.sub.8 phase is directly produced from the calcined body by the 
heat treatment, it is likely to obtain crystal grains in random 
orientations. In contrast, in the procedure described above, by producing 
the RE.sub.1 Ba.sub.2 Cu.sub.3 O.sub.6 phase to be c-axis oriented in a 
direction vertical with respect to the surface of the substrate first, and 
then obtaining the RE.sub.1 Ba.sub.2 Cu.sub.4 O.sub.8 phase from the 
c-axis oriented RE.sub.1 Ba.sub.2 Cu.sub.3 O.sub.6 phase and the CuO 
phase, it is possible to secure the production of the c-axis oriented 
RE.sub.1 Ba.sub.2 Cu.sub.4 O.sub.8 phase. 
Next, the temperature inside the furnace is set and maintained at the 
temperature T3 in a range of 700.degree. to 850.degree. C. while the 
oxygen partial pressure inside the furnace is decreased by supplying argon 
gas into the furnace, so as to realize the temperature and the ambient 
atmosphere of the inert gas such as the 100% argon gas or the argon gas 
containing about 0.01% of oxygen, corresponding to the thermodynamically 
stable region for the RE.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-x phase in the 
starting composition, over the period t4 in a range of several tens of 
minutes to several hours. In the temperature and the ambient atmosphere 
corresponding to the thermodynamically stable region for the RE.sub.1 
Ba.sub.2 Cu.sub.3 O.sub.7-x phase in the starting composition, the 
RE.sub.1 Ba.sub.2 Cu.sub.4 O.sub.8 phase is thermally decomposed into the 
RE.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-x phase and CuO phase. 
Next, the temperature inside the furnace is set and maintained at the 
temperature T4 in a range of 700.degree. to 850.degree. C. while the 
oxygen partial pressure inside the furnace is increased by supplying 
oxygen gas into the furnace, over the period t5 in a range of several tens 
of minutes to several hours, during which period the oxygen is supplied 
from the ambient atmosphere to the RE.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-x 
phase. 
Next, the temperature inside the furnace is slowly decreased to the 
temperature T5 in a range of 400.degree. to 500.degree. C. under the same 
ambient atmosphere over the period t6 in a range of several tens of 
minutes to several hours. Then, these temperature T5 and ambient 
atmosphere are maintained for the following period t7 in a range of 
several hours to several tens of hours, such that the crystal structure of 
the complex oxide having a composition of RE.sub.1 Ba.sub.2 Cu.sub.3 
O.sub.7-x formed in the superconductive layer can be changed from the 
tetragonal structure to the rhombic structure during this period t7. As a 
result, the desired oxide superconductor which shows the superconductivity 
at approximately 90K can be produced. It is noted here that, the crystal 
structure can be changed from the tetragonal structure to the rhombic 
structure by taking the oxygen atoms into the crystal structure, so that a 
number of carriers is increased in the rhombic structure and the desired 
superconductivity characteristics can be realized. 
Finally, the temperature inside the furnace is slowly decreased further 
down to the room temperature under the same ambient atmosphere. 
In the oxide superconductor obtained by the procedure described above, the 
CuO phase remains within the grain boundaries of the RE.sub.1 Ba.sub.2 
Cu.sub.3 O.sub.7-x, and this remaining CuO phase and the micro-defects 
introduced by the decomposition process function as the pinning centers to 
obstruct the movements of the magnetic fluxes entering into the 
superconductor due to the self-excitation at a time of current conduction 
through the superconductive layer, so that the critical current density of 
the superconductor can be improved. 
Moreover, these CuO phase and micro-defects are obtained by utilizing the 
decomposition of the RE.sub.1 Ba.sub.2 Cu.sub.4 O.sub.8 phase, so that 
they can be distributed uniformly over the entire superconductive layer, 
and consequently, the pinning effects due to the pinning centers can be 
obtained uniformly from the entire superconductive layer and it becomes 
possible to exhibit the effect of the improved critical current density 
efficiently. 
Referring now to FIG. 3 to FIG. 6, one concrete example of an oxide 
superconductor and its manufacturing method according to the present 
invention will be described in detail. 
In this example, the solution to be applied to the substrate was prepared 
by mixing powders of Y acetylacetone complex salt [Y(acac).sub.3 ] with 
23% in weight of Y as the rare earth element, powders of Ba acetylacetone 
complex salt [Ba(acac).sub.2 (OH.sub.2).sub.2 ] with 41% in weight of Ba, 
and powders of Cu acetylacetone complex salt [Cu(acac).sub.2 ] with 23% in 
weight of Cu, at a mole ratio of Y: Ba: Cu=1: 2:3.5, and then dissolving 
the obtained mixture into the mixed organic solvent of the pyridine (Py) 
and the propionic acid (PA) with a weight percentage rate of PY: PA=5:3. 
Then, this solution was nearly completely evaporated, and dissolved again 
by adding methanol to obtain the uniform solution. 
The uniform solution so obtained was then applied onto the (100) surface of 
the monocrystalline substrate of strontium titanate (SrTiO.sub.3) by the 
spin coating method under the conditions of a rate of revolutions equal to 
3000 rpm and an application time equal to 5 sec. 
Then, the entire substrate with the solution applied thereon was confined 
in a muffle furnace, and calcined at the temperature of 600.degree. C. in 
the air for ten minutes, so as to carry out the rapid thermal 
decomposition of the organic salts. The resulting calcined body comprises 
microscopic crystal grains in Y.sub.2 O.sub.3 -BaCO.sub.3 -CuO phase. 
Then, the multi-stage heat treatment was applied to this calcined body in 
the furnace by appropriately controlling the temperature and the 
atmosphere inside the furnace according to the timing chart shown in FIG. 
8, as follows. 
First, in the argon gas atmosphere, the temperature inside the furnace was 
increased from the room temperature to 750.degree. C. over the period of 
80 minutes, and this temperature of 750.degree. C. was maintained for the 
following 3 hours. Then, the temperature inside the furnace was maintained 
at 750.degree. C. for further 15 hours while supplying the oxygen gas into 
the furnace. Then, after the atmosphere inside the furnace was replaced by 
that of the argon gas, the temperature inside the furnace was maintained 
at 750.degree. C. for further 1 hour. Then, the temperature inside the 
furnace was maintained at 750.degree. C. for further 30 minutes while 
supplying the oxygen gas into the furnace. Then, the temperature inside 
the furnace was slowly decreased to 450.degree. C. over the period of 1 
hour, and this temperature of 450.degree. C. was maintained for the 
following 3 hours. Finally, the temperature inside the furnace was slowly 
decreased further down to the room temperature. As a result, the oxide 
superconductor with the thickness of the superconductive layer equal to 
0.4 .mu.m was obtained. 
During this manufacturing procedure, the samples were taken out of the 
furnace at the stages (a), (b), and (c) indicated in the timing chart of 
FIG. 3, and the compositions of the samples were analyzed by using the 
X-ray diffraction method. The results of these X-ray diffraction analyses 
are shown in FIG. 4. 
It can be clearly seen in FIG. 4 that, at the stage (a), the c-axis 
oriented Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.6 phase is produced, while at the 
stage (b), the c-axis oriented Y.sub.1 Ba.sub.2 Cu.sub.4 O.sub.8 phase is 
produced as the c-axis oriented Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.6 phase 
and the CuO phase at the grain boundaries react with each other. Then, 
FIG. 4 also shows that, at the stage (c), the Y.sub.1 Ba.sub.2 Cu.sub.4 
O.sub.8 phase disappears while the Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-x 
phase appears, as a result of the thermal decomposition process of Y.sub.1 
Ba.sub.2 Cu.sub.4 O.sub.8 phase.fwdarw.Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-x 
phase+CuO phase carried out by the heat treatment under the argon gas 
atmosphere after the stage (b). 
On the other hand, as a comparative example, a conventional manufacturing 
method for producing the Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-x phase 
directly, without using the intermediate Y.sub.1 Ba.sub.2 Cu.sub.4 O.sub.8 
phase, was also carried out, by using the multi-stage heat treatment 
according to the timing chart shown in FIG. 5. Here, first, in the argon 
gas atmosphere, the temperature inside the furnace was increased from the 
room temperature to 750.degree. C. over the period of 30 minutes, and this 
temperature of 750.degree. C. was maintained for the following 19 hours. 
Then, the temperature inside the furnace was maintained at 750.degree. C. 
for further 30 minutes while supplying the oxygen gas into the furnace. 
Then, the temperature inside the furnace was slowly decreased to 
450.degree. C. over the period of 1 hour, and this temperature of 
450.degree. C. was maintained for the following 3 hours. Finally, the 
temperature inside the furnace was slowly decreased further down to the 
room temperature. 
Then, the critical current densities of the oxide superconductor obtained 
by the above described example of the manufacturing method according to 
the present invention (sample 1) and the oxide superconductor obtained by 
the above described comparative example of the conventional manufacturing 
method (sample 2) were measured at various temperatures. The result of 
these critical current density measurements are summarized in the table of 
FIG. 6. 
It can be clearly seen in FIG. 6 that, the oxide superconductor obtained by 
the manufacturing method of the present invention (sample 1) has the 
improved critical current density compared with the oxide superconductor 
obtained by the conventional manufacturing method (sample 2). In 
particular, at the temperature of 77 K, under 0 T (no external magnetic 
field), the sample 1 shows the critical current density Jc of 18000 
A/cm.sup.2, in contrast to the sample 2 which shows the critical current 
density Jc of only 2000 A/cm.sup.2, so that the considerable improvement 
of the critical current density can be achieved by the present invention. 
This considerable improvement of the critical current density in the 
present invention can be attributed to the pinning effect of the CuO phase 
and the micro-defects introduced by the decomposition process which 
function as the pinning centers to obstruct the movements of the magnetic 
fluxes entering into the superconductor due to the self-excitation at a 
time of current conduction through the superconductive layer. 
It is to be noted that, besides those already mentioned above, many 
modifications and variations of the above embodiments may be made without 
departing from the novel and advantageous features of the present 
invention. Accordingly, all such modifications and variations are intended 
to be included within the scope of the appended claims.