Oxide superconductor and process for preparation thereof

The present invention relates to an oxide superconductor comprising a composite oxide of RE , Ba and Cu, wherein the superconductor comprises a micro structure comprised of a monocrystalline REBa.sub.2 Cu.sub.3 O.sub.7-x phase (123 phase) and a RE.sub.2 BaCuO.sub.5 phase (211 phase) finely dispersed therein, the 123 phase being formed in a plurality of domains respectively for individual RE compositions and in the order of the 123 phase forming temperatures in respective layers. The present invention relates also to a process for the preparation of an oxide superconductor, characterized by forming a layer from a mixed powder of the RE, Ba and Cu compounds, forming another layer(s) of a mixed powder of RE, Ba and Cu compounds having another RE composition(s) different from the above-mentioned RE composition in the 123 phase forming temperature to form a multi layer structure, putting said plurality of layers on top of one another so that the 123 phase forming temperatures in respective layers continue towards a higher temperature side or a lower temperature side, subjecting the assembly to press molding to form a precursor, putting said precursor on a supporting material with the layer having the highest 123 phase forming temperature being located at the highest position, heating said precursor to a temperature range in a solid liquid coexisting region to bring said precursor into a semi molten state, and either gradually cooling said precursor in a 123 phase temperature range or inoculating the precursor with a seed crystal and gradually cooling the inoculated precursor in the above mentioned temperature range to grow a 123 phase crystal at a growth rate of 5 mm/hr or less.

TECHNICAL FIELD 
The present invention relates to an oxide superconductor and a process for 
the preparation thereof. 
BACKGROUND ART 
A YBa.sub.2 Cu.sub.3 O.sub.7-x type (123 phase type) superconductor 
prepared by a QMG (quench and melt growth) method, that is, a kind of a 
melt method, apparently has a critical current density (Jc) of 10.sup.4 
A/cm.sup.2 or more under conditions of 77K and 1T which suffices for 
practical use (see "New Superconducting Materials Forum News", No. 10 
(1988), p. 15). Studies have been made also on an increase in the size of 
the QMG material and a combination of RE elements (see "Physica C", 
162-164 (1989), pp. 1217-1218 or Preprints of the 50th Symposium of the 
Japan Society of Applied Physics, Autumn, 1989, 29a-P-10). In these 
methods, an oxide superconductor material containing Y element alone or 
various RE elements is unidirectionally grown in a temperature gradient to 
increase the size of a crystal. 
In a polycrystalline structure, a grain boundary acts as a weak link to 
decrease superconducting properties. The above-mentioned study on the 
increase in the size of the crystal has been made for the purpose of 
solving this problem. 
The above-mentioned prior art technique, however, can provide only a single 
crystal material having a size of 0.3 cm.sup.3 at the largest, and it was 
very difficult to unidirectionally grow the 123 phase structure to form a 
large single crystal. 
That is, the prior art could not solve problems regarding the means for 
forming a nucleus of a crystal, a method of regulating the growth of a 
crystal, etc. 
An object of the present invention is to provide a material comprising a 
REBa.sub.2 Cu.sub.3 O.sub.7-x phase in a large-size single crystal form 
(hereinafter referred to as "123 phase") in an oxide superconductor 
comprising a composite oxide of at least two elements selected from the 
group consisting of Y, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb and Lu (hereinafter 
referred to as "RE elements"), Ba and Cu. 
Another object of the present invention is to provide a material having a 
structure such that a RE.sub.2 BaCuO.sub.5 phase (hereinafter referred to 
as "211 phase") having a very small grain diameter is dispersed in a large 
size 123 phase. 
CONSTITUTION OF THE INVENTION 
In order to attain the above-mentioned objects, the present invention 
provides a technique for controlling the growth of a crystal by varying 
the kinds of the RE elements or the kinds of the RE elements and the 
mixing ratio of the RE elements (hereinafter referred to as "composition 
of RE") through the utilization of the difference in the crystal forming 
temperature inherent in individual components of RE, and a process for the 
preparation of a large size crystal through inoculation of a seed crystal. 
Specifically, the present invention-provides a superconductor comprising a 
composite oxide of a RE element(s), Ba and Cu, wherein said superconductor 
is an oxide high temperature superconductor having a micro structure 
comprising a monocrystalline 123 phase and a 211 phase finely dispersed 
therein; said 123 phase being formed in a plurality of domains 
respectively for individual RE compositions and in the order of the 123 
phase forming temperatures in respective domains. The invention also 
provides a process for the preparation of the oxide high temperature 
superconductor, characterized by mixing oxides (including composite 
oxides) of RE, Ba and Cu so that the molar ratio of metal elements (RE, 
Ba, Cu) falls within a region defined by connecting points (10, 60, 30), 
(10, 20, 70) and (50, 20, 30) to each other, forming another mixed powder 
having a RE composition different from said mixed powder in the 
temperature of formation of the 123 phase of the RE composition in said 
mixed powder so as to have a composition falling within said region, 
putting said plurality of mixed powders in the order of the 123 phase 
forming temperature in the RE composition to form a multi-layer structure, 
subjecting the assembly to press molding to form a precursor, heating said 
precursor to a temperature region wherein a solid phase (211 phase) 
determined by the RE composition and a liquid phase determined by an oxide 
of Ba and Cu coexist (a solid-liquid coexisting region) to thereby bring 
the precursor into a semi molten state, and cooling the precursor in a 123 
phase forming temperature region determined by the RE composition under 
predetermined conditions in a gradual or stage holding manner, or cooling 
the precursor from the solid liquid coexisting region to a seed crystal 
inoculation temperature, inoculating a seed crystal (a single crystal of a 
123 phase according to RE composition having a 123 phase forming 
temperature above the maximum 123 phase forming temperature in the RE 
composition of the precursor) into the precursor at said temperature and 
cooling said inoculated precursor under the same conditions as those 
described above in a gradual or stage holding manner. The above-mentioned 
process enables an oxide superconductor having the abovementioned features 
to be produced through the control of the nucleation of the 123 phase to 
increase the size of a grown crystal.

BEST MODE OF CARRYING OUT THE INVENTION 
The best mode of carrying out the present invention will now be described 
in detail. 
The 123 phase forming temperature, Tg, in an oxide superconductor is 
determined by the composition of rare earth elements (including Y) 
contained in the oxide superconductor. In the air, the 123 crystal forming 
temperature of RE elements used in the present invention among the rare 
earth elements is approximately as given in Table 1. 
TABLE 1 
__________________________________________________________________________ 
RE Y Sm Eu Gd Dy Ho Er Tm Yb Lu 
__________________________________________________________________________ 
.degree.C. 
1000 
1060 
1050 
1030 
1010 
990 
980 960 
900 880 
__________________________________________________________________________ 
The smaller the atomic number, that is, the larger the radius of the RE 
element, the higher the crystal forming temperature. 
When a plurality of RE elements are mixed with each other, the formation 
temperature, Tg, of a crystal having a composition wherein the molar 
fraction of RE.sub.1, the molar fraction of RE.sub.2, . . . . . . in the 
whole RE element are m.sub.1, m.sub.2, . . . . . . , respectively RE.sub.1 
(m.sub.1), RE.sub.2 (m.sub.2), . . . . . ], can be approximately expressed 
by the following equation. 
EQU Tg=(Tg(RE.sub.1).times.ml+Tg(RE.sub.2).times.m.sub.2 +. . . . ) 
Among the rare earth elements, each of the Ce, Pr and Tb elements, as such, 
does not form a 123 phase structure, so that these elements are not used 
as the rare earth elements according to the present invention. Further, 
with respect to La, an initial crystal from the molten state comprises 
(La.sub.1-x Ba.sub.x).sub.2 CuO.sub.4, and with respect to Nd, an initial 
crystal from the molten state comprises Nd.sub.1+Y Ba.sub.2-Y Cu.sub.3 
O.sub.7-X. That is, they provide no genuine 123 phase intended in the 
present invention. For this reason, they, as such, are not used in the 
present invention. Since, however, the addition of La and Nd in a minor 
amount to other RE systems can enhance the 123 phase forming temperature, 
they can increase the temperature range in the selection of RE. Further, 
it is also possible to use them for enhancing the 123 phase forming 
temperature of the seed crystal. 
The process for the preparation of a superconductor according to the 
present invention based on the above-mentioned findings will now be 
described. Oxides (including composite oxides) of RE, Ba and Cu are mixed 
with each other so that the molar ratio of metal elements (RE, Ba, Cu) 
falls within a region defined by connecting to each other a point A (10, 
60, 30), a point B (10, 20, 70) and a point C (50, 20, 30) in a ternary 
equilibrium phase diagram shown in FIG. 1, and a layer is formed from the 
mixture. When the composition is outside the abovementioned region, the 
shape of a precursor, which will be described later, cannot be maintained 
at the time of heating of the precursor to a solid liquid coexisting 
region and, further, the formation of the 123 phase does not proceed 
smoothly during cooling steps after heating. 
The molar ratio preferably falls within a region defined by connecting, to 
each other, a point D (30, 33, 37), a point E (15, 38, 47), a point F (15, 
30, 55) and a point G (30, 25, 45) of the phase diagram. 
In this case, RE.sub.2 O.sub.3, BaCuO.sub.3, BaO, CuO, CuO.sub.2, 
BaCuO.sub.2, RE.sub.2 BaCuO.sub.5, REBa.sub.2 Cu.sub.3 O.sub.7-X, etc. are 
considered as starting materials. 
Then, other RE elements having a 123 phase forming temperature different 
from that of the RE elements in the above mentioned mixed powder, for 
example, RE elements having a 123 phase forming temperature above or below 
that of the above-mentioned powder mixture layer, are mixed with each 
other so as to have a composition falling within the above mentioned 
region, and this mixture is put in layer form on the mixed powder layer to 
form a double layer structure. 
The above-mentioned procedure is repeated to put mixed powders comprising a 
plurality of RE compositions on top of one another to form a plurality of 
layers. In this case, the lamination is conducted so that the 123 phase 
forming temperatures of the RE compositions in respective layers continues 
towards a higher temperature side or a lower temperature side. 
The layer thickness is preferably about 2 cm or less from the viewpoint of 
the effect attained by the present invention, the working efficiency, etc. 
After individual layers are put on top of one another, the resultant 
assembly is pressed and molded to form a precursor. 
The precursor may be prepared by a lap quenching process. The lap quenching 
process comprises heating the above-mentioned mixed powder layer to a 
temperature of 1200.degree. C .or above to form a melt, pressing the melt 
against a cooled material, for example, a cooled mass made of a metal 
having high heat conductivity, to rapidly cool the layer (hammer quenching 
process), thereby forming a molding, subjecting a mixed powder layer 
containing other RE elements different from said molding in the 123 phase 
forming temperature to a molding in the same manner as that described 
above, thereby forming another molding, and putting a plurality of these 
moldings on top of one another so that the 123 phase forming temperatures 
of individual moldings continue towards a higher temperature side or a 
lower temperature side, thereby forming a precursor. 
The composition of the precursor produced by the powder lamination process 
or the lap quenching process has such a composition that an oxide of RE is 
finely dispersed in oxides of Ba and Cu. 
The process for the preparation of a superconductor by using the above 
mentioned precursor will now be described. 
At the outset, the precursor is put on a support in such a manner that the 
layer having the highest 123 phase forming temperature is located on the 
highest position, and then heated within a temperature range in a solid 
liquid coexisting region, that is, in the range of from a temperature 
above the 123 phase forming temperature (Tg) to below the 211 phase 
dissolution temperature (Td) (that is, the lower limit of the heating 
temperature is a temperature capable of sufficiently decomposing a Lu 
based 123 phase while the upper limit of the heating temperature is a 
temperature at which the precursor cannot maintain its shape due to the 
decomposition of a Sm-based 211 phase (about 900.degree. to 1300.degree. 
C.)) for 15 to 45 min to bring the precursor into a semi molten state, 
thereby forming a 211 phase (a solid phase) in a liquid phase (Ba, Cu 
oxides). 
Then, the precursor is cooled at an arbitrary cooling rate from the 
above-mentioned temperature range to a temperature of (Tg(H)+10).degree. 
C. and subjected to gradual cooling or stage holding cooling substantially 
equivalent to the gradual cooling in the temperature range of from 
(Tg(H)+10).degree. C. to (Tg(L) 40).degree. C. to grow a 123 phase at a 
growth rate of 5 mm/hr or less. 
Tg(H) represents the highest 123 phase forming temperature of the RE 
compositions in the precursor, and Tg(L) represents the lowest 123 phase 
forming temperature. 
In the 123 phase crystal, since the structure is complicated, the entropy 
change in the crystallization is so large that it is difficult to bring 
about nucleation. This often renders the growth incomplete. For this 
reason, it is necessary to conduct gradual cooling or stage holding 
cooling in a temperature range of from 1070.degree. C. at which the Sm 
based 123 phase begins to form to 840.degree. C. at which the growth of 
the Lu based 123 phase is fully completed (see Table 1). 
The substantially average cooling rate, R (.degree. C./hr), in the above 
mentioned gradual cooling or stage holding cooling is determined by the 
following formula: 
EQU R.ltoreq.k..DELTA.Tg/D 
wherein k represents a target grain growth speed (mm/hr); 
.DELTA.Tg represents a maximum Tg deviation, provided that 
.DELTA.Tg=(Tg(H)+10).degree. C.-(Tg(L)-40).degree. C.; and 
D represents a total thickness (mm). 
The gradual cooling or stage holding cooling at a cooling rate represented 
by the above mentioned formula enables cooling to be successively 
conducted according to the 123 phase forming temperatures of respective 
layers. In particular, when use is made of the following technique wherein 
a seed crystal is inoculated, it is possible to produce a monocrystalline 
123 phase in each layer. 
The gradual cooling may be conducted in an atmosphere having a temperature 
gradient of 2.degree. C. or more. Alternatively, the precursor may be 
moved so as to provide the above mentioned temperature gradient. 
In the treatment at the 123 phase forming temperature, the formation of a 
nucleus is allowed to begin with a layer having the highest 123 phase 
forming temperature, Tg(H), in the precursor, and the crystal is grown 
while allowing the crystal growth direction to succeed to layers having a 
lower 123 phase forming temperature. The formation of a multi-layer 
structure in the above-mentioned manner enables the effective prevention 
of the occurrence of other crystal nucleuses compared with a single layer 
structure having the same dimension. 
A technique for attaining a better effect of the present invention wherein 
the precursor is inoculated with a monocrystalline seed crystal containing 
a 123 phase and having a 123 phase forming temperature above the highest 
123 phase forming temperature, Tg(H), in the precursor will now be 
described. 
At the outset, as described above, the precursor is put on a support 
material and then heated to a solid liquid coexisting temperature range, 
thereby bringing the precursor into a semi molten state. 
Then, the precursor is cooled from the above-mentioned temperature range to 
the seed crystal inoculation temperature, that is, to a temperature in the 
range of from 900.degree. to 1100.degree. C. In this temperature range, 
the precursor is inoculated with a seed crystal. The lower limit in the 
above-mentioned temperature range was determined based on the fact that it 
is possible to use as the seed crystal a Yb-based 123 phase, while the 
upper limit was determined based on the fact that it is possible to use as 
the seed crystal a Sm-based 123 phase crystal containing La and Nd. 
The seed crystal comprises a RE composition having a 123 phase forming 
temperature higher than the 123 phase forming temperature of the RE 
composition in the precursor, and is a crystal wherein at least a face in 
contact with the precursor has a monocrystalline structure. 
After the precursor is inoculated with a seed crystal, it is subjected to 
gradual cooling or stage holding cooling in a temperature region of from 
(Tg(H) +10).degree. C. to (Tg(L)-40).degree. C. 
The inoculation of the precursor in a semi molten state having a 
temperature close to the 123 phase forming temperature with the above 
mentioned seed crystal (a single crystal or a polycrystal having a 
contacting face in a single crystal form) enables a crystal nucleus of a 
123 phase to be formed from the seed crystal and, at the same time, the 
crystal direction of the 123 phase is more strictly controlled in a 
direction capable of providing a higher current density through the growth 
of a 123 phase in an arbitrary direction identical to that of the seed 
crystal, so that it is possible to attain a very high critical current 
density in combination with an increase in the size of the 123 phase 
crystal. 
It is also possible to add at least one of Pt and Rh to the above-mentioned 
precursor components. This enables the grain diameter of the 211 phase in 
the 123 phase to be reduced. Pt and Rh may be added in respective amounts 
of 0.2 to 2.0% by weight and 0.005 to 1.0% by weight. This enables the 
effect of the present invention to be efficiently exhibited. 
When the precursor is subjected to a heat treatment, it should be supported 
by some material. At the present time, platinum is mainly used as the 
supporting material. Since, however, the liquid phase component in a semi 
molten state (oxides of Ba and Cu) has high reactivity, the contact of the 
liquid component with the supporting material for a long period of time 
brings about deviation of the liquid phase component and contamination 
from an impurity element, so that the crystallinity and superconductive 
properties are spoiled. The present inventors have found that the 123 
phase per se can serve as a stable supporting material. Specifically, 
between the above-mentioned precursor (hereinafter referred to as 
"precursor M") and the supporting material supporting the precursor M are 
provided another precursor H comprising a RE composition having a crystal 
forming temperature above that of the RE composition of the 123 phase in 
the precursor M and a further precursor L comprising a RE composition 
having a crystal forming temperature below that of the RE composition of 
the 123 phase in the precursor M. That is, an assembly comprising 
precursor M-precursor L-precursor H-supporting material disposed in that 
order is provided, and these precursors are utilized as a barrier to the 
supporting material. The precursor H is used as a barrier for preventing 
the liquid phase portion of the precursor M from flowing out to the 
supporting material, while the precursor L is used as a barrier for 
preventing the 123 phase crystal derived from the precursor H from growing 
to inhibit the crystal growth of the precursor M. If the 123 phase of the 
lowermost layer in the precursor M has the same function as that of the 
precursor L, it is possible to omit the precursor L. The provision of 
these barriers enables the crystal to be more efficiently grown. 
The present inventors have made the following experiment on the inoculation 
of the precursor of the present invention with a seed crystal. 
Y(Yb)Ba.sub.2 Cu.sub.3 O.sub.7-x powders and Y(Yb).sub.2 BaCuO.sub.5 
powders with the proportion of Y to Yb being varied in 10 % increments as 
specified in Table 2 were prepared, and 20% by mole of the Y(Yb).sub.2 
BaCuO.sub.5 powder was added to the Y(Yb)Ba.sub.2 Cu.sub.3 O.sub.7-x 
powder so that the ratio RE : Ba : Cu was 6:9:13. The mixtures were heat 
melted at 1400.degree. C., and lap hammer quenching was conducted 9 times 
so that the Y component was successively replaced with Yb to prepare 
precursors. Due to the difference in the RE component, the 123 phase 
forming temperatures become as given in Table 2. 
TABLE 2 
______________________________________ 
Y:Yb 
(%) 100:0 90:10 80:20 
70:30 
60:40 
50:50 
40:60 
30:70 
20:80 
______________________________________ 
.degree.C. 
1000 990 980 970 960 950 940 930 920 
______________________________________ 
These layers were provided on a supporting material with the layer having a 
Y content of 100 % being located at the lowermost position. Thereafter, 
the assembly was heated to 1100.degree. C. After one end of the precursor 
was cooled to 1020.degree. C., a Sm-based single crystal was put as a seed 
crystal on one end of the precursor in a semi molten state. Then, the 
assembly was gradually cooled at a rate of 3.degree. C./hr until the 
temperature of the one end became 910.degree. C. FIG. 2 shows the micro 
structure of a junction between the seed crystal and the resultant 
crystal. From the drawing, it is apparent that the crystal orientation of 
the seed crystal has succeeded. 
Thereafter, the following experiment was conducted by using the above 
mentioned precursor. The precursor was put on a supporting material 
containing Sm as RE with the Y side being upward. The assembly was heated 
to 1100.degree. C. Then, a temperature gradient of 5.degree. C./cm was 
provided within a furnace. After the Y side of the precursor was cooled to 
1020.degree. C., a previously prepared Sm-based single crystal was put as 
a seed crystal on the Y side of the precursor in a semi molten state. 
Then, the precursor was gradually cooled at a rate of 3.degree. C./hr 
while maintaining the temperature gradient at 5.degree. C./cm until the 
temperature of the one end of the precursor became 910.degree. C. FIG. 3 
shows the structure of a crystal around the surface of the resultant 
superconductor. From the drawing, it is apparent that the nucleation is 
also prevented at the surface of the superconductor. FIG. 4 shows the 
structure of a Sm barrier inserted between the precursor and the 
supporting material, that is, a crystal of the precursor contact portion. 
It is apparent that the Sm-based superconductor is polycrystalline while 
the Y Yb based superconductor is monocrystalline. 
As described above, the superconductor produced by the seed metal 
inoculation method has a texture such that a 211 phase is finely dispersed 
in a monocrystalline 123 phase. Further, the above-mentioned 123 phase is 
has a plurality of domains respectively for individual RE compositions in 
such a manner that the 123 phase forming temperatures of respective 
domains continue towards the higher temperature side. The resultant 
monocrystalline superconductor is a cylindrical material having an 
approximate size of an average diameter of 50 mm and a height of about 30 
mm, that is, has a size 30 to 50 times larger than the prior art material 
reported in the above-mentioned Physica C. 
The 211 phase in the superconductor is dispersed in the 123 phase in a 
volume fraction in the range of from 5 to 50%, preferably in the range of 
from 10 to 30%. The grain diameter is as small as 20 .mu.m or less. In 
particular, when Pt or Rh is added, the average grain diameter is 2 .mu.m 
or less (maximum value: about 5 .mu.m). 
In the superconductor having the above-mentioned structure, although 123 
phases different from each other in the RE composition are put on top of 
one another, since no grain boundary exists between individual domains, no 
disconnection of the superconductive state occurs, so that a very large 
current can flow. 
Further, the 211 phase can impart toughness to the 123 phase and, at the 
same time, can hold a guantum magnetic flux passing through the 
superconductor (pinning action). In the present invention, since the 211 
phase is dispersed in a very fine state over the whole texture, it is 
possible to hold the guantum magnetic flux. Therefore, even when the 
superconductor is placed in a magnetic field, the guantum magnetic flux is 
pinned, so that it is possible to obtain a large current density. 
The material of the present invention can exhibit excellent superconductive 
properties by virtue of the above mentioned effect. 
EXAMPLES 
Example 1 
Dy.sub.2 O.sub.3 and Er.sub.2 O.sub.3 were kneaded with each other to 
prepare 6 kinds of RE compositions with the ratio Dy : Er being varied in 
20% increments [(100:0), (80:20), (60:40), (40:60), (20:80) and (0:100)], 
and BaCuO.sub.2 and CuO were mixed with the RE compositions so as to have 
a ratio RE : Ba : Cu of 25:35:40 to prepare 6 kinds of mixed powders. The 
mixed powders were successively each put in the form of a layer having a 
thickness of about 6 mm on top of one another from the Dy layer to the Er 
layer by using a mold having a diameter of 30 mm to prepare a cylindrical 
precursor having a height of about 35 mm. FIG. 5 shows the precursor thus 
prepared. 
The precursor was supported by a Pt sheet with the Er side located down, 
heated in the air from room temperature to 1180.degree. C. over a period 
of 2 hrs, maintained at that temperature for 30 min, cooled to 
1020.degree. C. at a rate of 100.degree. C./hr, and further cooled to 
940.degree. C. at a rate of 0.5.degree. C./hr. Thereafter, the precursor 
was cooled to room temperature within the furnace, reheated to 800.degree. 
C. in an oxygen gas stream, and gradually cooled to 200.degree. C. at a 
cooling rate of 8.degree. C./hr. 
The resultant sample comprised three large crystal grains, and the largest 
crystal grain had a volume of about 15 cm.sup.3. That is, a very large 
superconductor material could be prepared. 
Example 2 
Dy.sub.2 O.sub.3, Ho.sub.2 O.sub.3 and Er.sub.2 O.sub.3 were kneaded with 
each other to prepare 5 kinds of RE compositions having respective ratios 
Dy:Ho:Er of (100:0:0), (50: 50:0), (0:100:0), (0:50:50) and (0:0: 100), 
and BaCuO.sub.2 and BaCu.sub.2 O.sub.3 were mixed with the RE compositions 
to have a ratio RE:Ba:Cu of (25:28 :47) to prepare 5 kinds of mixed 
powders. The mixed powders were each successively put in the form of a 
layer having a thickness of about 6 mm on top of one another from the Dy 
layer to the Er layer by using a mold having a diameter of 50 mm to 
prepare a cylindrical precursor having a height of about 30 mm. 
The precursor was supported by a Pt sheet with the Er side located down, 
heated in the air from room temperature to 1180.degree. C. over a period 
of 2 hr, maintained at that temperature for 30 min, and cooled to 
1040.degree. C. at a rate of 100.degree. C./hr. At 1040.degree. C., the 
cleavage plane of a seed crystal having a RE composition such that the 
ratio Sm : Nd was 7:3 was put on the precursor in a semi molten state to 
inoculate the precursor with a seed crystal. The precursor was cooled from 
1020.degree. C. to 940.degree. C. at a cooling rate of 0.5.degree. C./hr. 
Thereafter, the precursor was cooled to room temperature within a furnace, 
reheated to 800.degree. C. in an oxygen gas stream, and gradually cooled 
to 200.degree. C. at a cooling rate of 8.degree. C./hr. 
As shown in FIG. 6, the resultant sample was in the form of a cylinder 
having a diameter of about 43 mm and a height of about 27 mm, and two 
small crystals having an orientation different from that of the seed 
crystal were observed, near the platinum, as the supporting material. A 
major part of the sample, however, comprised a monocrystalline grain 
having the same orientation as that of the seed crystal, and a 
superconductor material having a size of 35 cm.sup.3 could be obtained. 
Example 3 
RE.sub.2 BaCuO.sub.5 having 6 kinds of RE compositions with the ratio Y : 
Er being varied in 20% increments [(100 :0), (80:20), (60:40), (40:60), 
(20:80) and (0:100)] was prepared, and BaCuO.sub.2 and BaCu.sub.2 O.sub.2 
were mixed with the RE compositions so as to have a ratio RE:Ba:Cu of 
(18:35:47). Further, 0.5% by weight of a Pt powder was added thereto to 
prepare 8 kinds of mixed powders. The mixed powders were each successively 
put in the form of a layer having a thickness of about 5 mm on top of one 
another from the Y layer to the Er layer by using a mold having a diameter 
of 50 mm to prepare a cylindrical precursor having a height of about 30 
mm. 
The precursor was supported by a Pt sheet with the Er side located down, 
heated in the air from room temperature to 1150.degree. C. over a period 
of 2 hr, maintained at that temperature for 30 min and cooled to 
1030.degree. C. at a rate of 100.degree. C./hr. At 1030.degree. C., the 
cleavage plane of a seed crystal having such a RE composition that the 
ratio Sm : Nd was 7:3 was put on the precursor in a semi molten state to 
inoculate the precursor with the seed crystal. The precursor was cooled 
from 1010.degree. C. to 940.degree. C. at a cooling rate of 0.5.degree. 
C./hr. Thereafter, the precursor was cooled to room temperature within a 
furnace, reheated to 700.degree. C. in an oxygen gas stream, and gradually 
cooled to 250.degree. C. at a cooling rate of 5.degree. C./hr. 
As shown in FIG. 7, the resultant sample was in the form of a cylinder 
having a diameter of about 46 mm and a height of about 28 mm, and two 
small crystals having an orientation different from that of the seed 
crystal were observed, near the platinum, as the supporting material. A 
major part of the sample, however, comprised a single crystal grain having 
the same orientation as that of the seed crystal, and a superconductor 
material having a size of 40 cm.sup.3 could be obtained. 
The present sample was subjected to a measurement of magnetic 
susceptibility, Jc, by means of a vibrating sample magnetometer and found 
to have a Jc value of 1.4.times.10.sup.4 A/cm.sup.2. 
Example 4 
RE.sub.2 BaCuO.sub.5 having 6 kinds of RE compositions with the ratio Y:Er 
being varied in 20% increments [(100 :0), (80:20), (60:40), (40:60), 
(20:80) and (0:100)] was prepared, and BaCuO.sub.2 and BaCu.sub.2 O.sub.2 
were mixed with the RE compositions so as to have a ratio RE:Ba:Cu of 
(18:30:52). Further, 0.02% by weight of a Rh powder was added thereto to 
prepare 6 kinds of mixed powders. The mixed powders were each successively 
put in the form of a layer having a thickness of about 5 mm on top of one 
another from the Y layer to the Er layer through the use of a mold having 
a diameter of 50 mm to prepare a cylindrical precursor having a height of 
about 30 mm. 
The above-mentioned precursor with the Er side located down was put on 1.5 
mm-thick precursors having respective RE compositions of Sm and Yb 
prepared by the hammer quenching process to prepare an assembly comprising 
(precursor having a composition of Y Er)-(precursor having a composition 
of Yb) (precursor having a composition of Sm) (Pt sheet) disposed in that 
order. The assembly was heated in the air from room temperature to 
1150.degree. C. over a period of 2 hr, maintained at that temperature for 
30 min, and cooled to 1030.degree. C. at a cooling rate of 100.degree. 
C./hr. At 1030.degree. C., the cleavage plane of a seed crystal having 
such a RE composition that the ratio Sm : Nd was 7:3 was put on the 
precursor in a semi molten state to inoculate the precursor with the seed 
crystal. The precursor was cooled from 1010.degree. C. to 940.degree. C. 
at a cooling rate of 0.5.degree. C./hr. Thereafter, the precursor was 
cooled to room temperature within a furnace, reheated to 700.degree. C. 
in an oxygen gas stream, and gradually cooled to 250.degree. C. at a 
cooling rate of 5.degree. C./hr. 
As shown in FIG. 8, the resultant sample was in the form of a cylinder 
having a diameter of about 46 mm and a height of about 28 mm and 
comprising a single crystal grain having the same orientation as that of 
the seed crystal, and a very large superconductor material having a volume 
of 45 cm.sup.3 could be obtained. The present sample was subjected to a 
measurement of Jc value under the same condition as that of Example 3 and 
found to have a Jc value of 1.5.times.10.sup.4 A/cm.sup.2. 
Example 5 
Y(Yb)Ba.sub.2 Cu.sub.3 O.sub.7-x powders and Y(Yb).sub.2 BaCuO.sub.5 
powders with the proportion of Y to Yb being varied in 10% increments as 
specified in Table 2 were prepared, and 20% by mole of the Y(Yb).sub.2 
BaCuO.sub.5 powder was added to the Y(Yb)Ba.sub.2 Cu.sub.3 O.sub.7-x 
powder so that the ratio RE:Ba: Cu was 6:9:13. The mixtures were 
heat-melted at 1400.degree. C., and lap hammer quenching was conducted 9 
times so that the Y component was successively replaced with Yb to prepare 
a molding having a thickness of about 2 mm in each layer and a total 
thickness of about 20 mm. The molding was cut into a size of 20 mm square 
to prepare a precursor. Due to the difference in the RE component, the 123 
phase forming temperatures became as given in Table 2. The precursor was 
once heated at 1100.degree. C. for 20 min for partial melting, and 
subjected to unidirectional growth in a temperature gradient of 30.degree. 
C./cm with the Y composition side facing the higher temperature side by 
cooling the Y side from 1010.degree. C. to 850.degree. C. through cooling 
of the whole furnace at a rate of 6.degree. C./hr, thereby preparing a 
superconductor. In this case, the average crystal growth rate was about 
0.8 mm/hr. Oxygen annealing was conducted by gradually cooling the 
superconductor in an oxygen gas stream from 700.degree. C. to 300.degree. 
C. at a cooling rate of 10.degree. C./hr. As a result, as shown in FIG. 9, 
a sample comprising three crystal grains was obtained, and the largest 
crystal grain had a volume of about 4 cm.sup.3. 
Example 6 
REBa.sub.2 Cu.sub.3 O.sub.7-x powders and RE.sub.2 BaCuO.sub.5 powders with 
the RE composition being changed to Sm, Eu, Gd, Dy, Ho, Er, Tm and Yb in 
that order, and 20% by mole of the RE.sub.2 BaCuO.sub.5 powder was added 
to the REBa.sub.2 Cu.sub.3 O.sub.7-x powder so that the ratio RE:Ba:Cu was 
6:9:13. The mixtures were heat melted at 1450.degree. C., and lap hammer 
quenching was conducted 8 times so that the Sm component was successively 
replaced with Yb to prepare a molding having a thickness of about 2 mm in 
each layer and a total thickness of about 17 mm. The molding was cut into 
a size of 20 mm square to prepare a precursor. The precursor was heated to 
1150.degree. C., cooled to 1060.degree. C. at a cooling rate of 50.degree. 
C./hr and further cooled to 910.degree. C. at a cooling rate of 2.degree. 
C./hr, thereby conducting unidirectional growth. In this case, the average 
crystal growth rate was about 0.2 mm/hr. 
FIG. 10 is a sketch of an EPMA image showing a distribution of Dy, Ho and 
Er elements. It is apparent that these elements are distributed in a 
lamellar form. Further, in the resultant sample, the orientation was over 
the whole sample. 
Industrial Applicability 
As described above in detail, the present invention facilitates an increase 
in the size of a bulk material having a high critical current density and 
can be applied to various fields, which renders the present invention very 
effective from the viewpoint of industry. Specific examples of the 
application of the oxide superconductor include a superconductive coil, a 
superconductive magnetic shielding material and a substrate for a 
superconductive device.