Bulk superconductor and process of preparing same

A bulk superconductor including a plurality of units each composed of a substrate and a superconductive layer of R--Ba--Cu--O, where R is selected from La, Nd, Sm, Eu, Gd, Y, Dy, Ho, Er, Tm, Yb and mixtures thereof, formed on the substrate. The units are arranged in a row or in a matrix such that the superconductive layers of respective units are superconductively joined with each other.

BACKGROUND OF THE INVENTION 
This invention relates to a bulk superconductor such as a superconducting 
wire useful as electric cable and a superconducting plate useful as a 
magnetic shield plate and to a process of preparing same. 
The recent technology of superconductors can now produce superconductors 
having Tc of 90K or more, such as Y--Ba--Cu--O and Bi--Sr--Ca--Cu--O, and 
operable using liquid nitrogen. For the practical applications, however, 
it is necessary that such superconductors be shaped in the form of wires, 
tapes, plates, etc. Known such bulk superconductors, however, cannot be 
put into practice because the superconductive critical current is 
unsatisfactory. 
For instance, a Bi--Sr--Ca--Cu--O tape having a length of more than 100 m 
is known. This tape cannot be used in practice at a liquid nitrogen 
temperature. Further, Bi--Sr--Ca--Cu--O crystal has a high structural 
anisotropy, i.e. the critical current is high when a magnetic field is 
applied in the direction normal to the c-axis but is low in the direction 
parallel with the c-axis. Also known is a pancake coil of 
Bi--Sr--Ca--Cu--O capable of generating a magnetic field of more than 1 T 
at 20K. At a liquid nitrogen temperature, however, the magnetic field is 
only 0.1 T. 
When a Y--Ba--Cu--O having a Tc higher than the liquid nitrogen temperature 
is used for forming a plate in accordance with the powder-in-tube method 
suitably employed in Bi--Sr--Ca--Cu--O superconductors, the crystal grain 
boundaries form weak links so that superconducting critical current is 
extremely low. 
SUMMARY OF THE INVENTION 
It is, therefore, an object of the present invention to provide a bulk 
superconductor having no weak links and a high Superconducting critical 
current. 
Another object of the present invention is to provide an elongated 
superconductor material useful as a power cable. 
It is a further object of the present invention to provide a superconductor 
plate having a large area useful as an electromagnetic shield material. 
In accomplishing the foregoing object, one aspect of the present invention 
provides a bulk superconductor comprising a plurality of units each 
including a substrate and a superconductive layer of R--Ba--Cu--O, where R 
is selected from La, Nd, Sm, Eu, Gd, Y, Dy, Ho, Er, Tm, Yb and mixtures 
thereof, formed on the substrate, the units being arranged in a row or in 
a matrix such that the superconductive layers of respective units are 
superconductively joined with each other. 
In another aspect, the present invention provides a bulk superconductor 
comprising a first array of one or more, m-number of units and a second 
array of two or more, n-number of units, wherein m=n or m=n-1; 
each of the units of the first and second arrays including a substrate and 
a superconductive layer of R--Ba--Cu--O, where R is selected from La, Nd, 
Sm, Eu, Gd, Y, Dy, Ho, Er, Tm, Yb and mixtures thereof, formed on the 
substrate, 
the first and second arrays being assembled together such that the 
superconductive layer of each one of the units of the first array, except 
one of the terminal units when m=n, is in contact with and in 
superconductor joining with two superconductive layers of adjacent two 
units of the second array, so that the superconductive layers of the 
n-number of units of the second array are superconductively connected to 
each other through the superconductive layers of the m-number of units of 
the first array. 
In a further aspect, the present: invention provides a bulk superconductor 
comprising a first, m.times.p matrix of units with m-number of first to 
m-th columns arranged along the X-axis and p-number of first to p-th rows 
arranged along the Y-axis, wherein m is one or more and p is one or more, 
and a second, n.times.q matrix of units with n-number of first to n-th 
rows arranged along the X-axis and q-number of first to q-th rows arranged 
along the Y-axis, wherein m=n or m=n-1 and p=q or p=-1, 
each of the units of the first and second matrixes including a substrate 
and a superconductive layer of R--Ba--Cu--O, where R is selected from La, 
Nd, Sm, Eu, Gd, Y, Dy, Ho, Er, Tm, Yb and mixtures thereof, formed on the 
substrate, 
the first and second matrixes being assembled together such that the 
superconductive layer of each one of the units of the first to n-th 
columns, except one of the terminal units when m=n, is in contact with and 
in superconductor joining with two superconductive layers of adjacent two 
units of the first to m-th columns, respectively, and that the 
superconductive layer of each one of the units of the first to q-th rows, 
except one of the terminal units when p=q, is in contact with and in 
superconductor joining with two superconductive layers of adjacent two 
units of the first to p-th rows, respectively, so that the superconductive 
layers of the (n.times.q)-number of units of the second matrix are 
superconductively connected to each other through respective 
superconductive layers of the (m.times.p)-number of units of the first 
matrix. 
In a further aspect, the present invention provides a method of fabricating 
a bulk superconductor, comprising the steps of: 
providing a first array of one or more, m-number of units and a second 
array of two or more, n-number of units, wherein m=n or m=n-1, each of the 
units of the first and second arrays including a substrate and a 
superconductive layer of R--Ba--Cu--O, where R is selected from La, Nd, 
Sm, Eu, Gd, Y, Dy, Ho, Er, Tm, Yb and mixtures thereof, formed on the 
substrate; and 
assembling the first and second arrays together such that the 
superconductive layer of each one of the units of the first array, except 
one of the terminal units when m=n, is in contact with and in 
superconductor joining with two superconductive layers of adjacent two 
units of the second array, so that the superconductive layers of the 
n-number of units of the second array are superconductively connected to 
each other through the superconductive layers of the m-number of units of 
the first array. 
The present invention further provides a method of fabricating a bulk 
superconductor, comprising the steps of: 
providing a first, m.times.p matrix of units with m-number of first to m-th 
columns arranged along the X-axis and p-number of first to p-th rows 
arranged along the Y-axis, wherein m is one or more and p is one or more, 
and a second, n.times.q matrix of units with n-number of first to n-th 
rows arranged along the X-axis and q-number of first to q-th rows arranged 
along the Y-axis, wherein in m=n or m=n-1 and p=q or p=q-1, 
each of the units of the first and second matrixes including a substrate 
and a superconductive layer of R--Ba--Cu--O, where R is selected from La, 
Nd, Sm, Eu, Gd, Y, Dy, Ho, Er, Tm, Yb and mixtures thereof, formed on the 
substrate; and 
assembling the first and second matrixes together such that the 
superconductive layer of each one of the units of the first to n-th 
columns, except one of the terminal units when m=n, is in contact with and 
in superconductor joining with two superconductive layers of adjacent two 
units of the first to math columns, respectively, and that the 
superconductive layer of each one of the units of the first to q-th rows, 
except one of the terminal units when p=q, is in contact with and in 
superconductor joining with two superconductive layers of adjacent two 
units of the first to p-th rows, respectively, so that the superconductive 
layers of the (n.times.q)-number of units of the second matrix are 
superconductively connected to each other through respective 
superconductive layers of the (m.times.p)-number of units of the first 
matrix.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION 
Referring first to FIGS. 1 and 2, designated generally as U is a 
superconductor unit including a substrate 10 and a superconductive layer 
30 formed on the substrate 10. The substrate 10 is generally a single 
metal oxide crystal, such as ZrO.sub.2 and MgO, and the superconductive 
layer 30 is a R--Ba--Cu--O superconductor, where R is selected from La, 
Nd, Sm, Eu, Gd, Y, Dy, Ho, Er, Tm, Yb and mixtures thereof. 
The superconductor unit U may be prepared by, for example, applying a paste 
containing a heat-treated mixture of R.sub.2 O.sub.3, BaCO.sub.3, CuO and 
having a predetermined R:Ba:Cu atomic ratio onto a substrate 10. The paste 
may be obtained by dispersing the mixture in a suitable organic dispersing 
liquid, such as isopropanol, optionally containing an organic binder such 
as polyvinyl butyral. 
A seed crystal 20 such as NdBa.sub.2 Cu.sub.3 O.sub.x (Nd123) is then 
placed on one end of the coated paste. The resulting substrate 10 having 
the paste layer bearing the seed crystal 20 is introduced into a furnace 
having such a temperature gradient as shown in FIG. 3. The temperature is 
highest at a center where the superconductive particles are partly melted 
and is gradually decreased toward opposite directions with a temperature 
gradient of in the range of 1.degree. C./cm and 100.degree. C./cm. The 
substrate 10 is horizontally displaced within the furnace at a rate of 
0.1-10 cm/hour with the seed crystal bearing end being the leading end, so 
that the paste layer is successively subjected to a heat treatment from 
the seed crystal bearing end to the opposite end thereof. Thus, the 
superconductive phase gradually grows from the seed crystal bearing end to 
the opposite end during the passage through the furnace. 
In the embodiment shown in FIGS. 1 and 2, the substrate 10 is not entirely 
covered with the superconductive layer 30, namely a marginal portion of 
the substrate 10 remains uncovered with the superconductive layer 30. Such 
a structure is, however, not essential. One side of the substrate 10 may 
be entirely covered with the superconductive layer 30, if desired. 
In the present invention, a plurality of such units U constitute a bulk of 
superconductor such as in the form of a cable, tape and plate as will be 
described below. Since the units U used in the following embodiments have 
the same structure as that shown in FIGS. 1 and 2, similar reference 
numerals are used for designating the same parts in FIGS. 4-7. 
FIG. 4 illustrates the most simple structure according to the present 
invention, in which two units U1 and U2 each similar to the unit U shown 
FIGS. 1 and 2 are assembled together such that respective Superconductive 
layers 30 are superconductively joined or connected with each other. The 
term "superconductively joined" or "superconductively connected" used 
herein is intended to refer to the connection of the superconductive layer 
30 of one unit and the superconductive layer 30 of another unit such that 
the interface therebetween remains superconductive so that a 
superconducting current can flow from the one unit to the another unit. 
The assembling is preferably carried out by heating the two units U1 and 
U2 at a temperature of 900-1,000.degree. C. while pressing the two units 
U1 and U2 to each other at a pressure of 0.1 kgf/cm.sup.2 to 10 
kgf/cm.sup.2 with respective superconductive layers 30 being maintained in 
contact with each other. 
FIG. 5 depicts another embodiment of the present invention in the form of 
an elongated superconductor. The elongated superconductor includes a first 
array 1 of one or more, m-number of units (5 units in the illustrated 
embodiment) and a second array 2 of two or more, n-number of units (5 
units in the illustrated embodiment), wherein m=n or m=n-1. The first and 
second arrays 1 and 2 are assembled together such that the superconductive 
layer 30 of each one of the units of the first array 1, except one of the 
terminal units when m=n, is in contact with and in superconductor joining 
with two superconductive layers 30 of adjacent two units of the second 
array 2, so that the superconductive layers 30 of the n-number of units of 
the second array 2 are superconductively connected to each other through 
the superconductive layers 30 of the m-number of units of the first array 
1. The assembling of the first and second arrays 1 and 2 may be carried 
out in the same manner as that in the first embodiment, i.e. by heating 
the first and second arrays 1 and 2 at a temperature of 900-1,000.degree. 
C. while pressing the first and second arrays to each other at a pressure 
of 0.1 kgf/cm.sup.2 to 10 kgf/cm.sup.2. 
In the specific embodiment shown in FIG. 5, adjacent two units in the first 
and second arrays 1 and 2 are abutting engagement with each other. But 
this is not essential. Adjacent two units may be spaced apart from each 
other, if desired 
FIGS. 6 and 7 illustrate a further embodiment of the present invention in 
the form of a superconductive plate. The superconductive plate includes a 
first, m.times.p matrix 60a of units with m-number of first to m-th 
columns (first to fourth columns in the illustrated embodiment) arranged 
along the X-axis and p-number of first to p-th rows (first to third rows 
in the illustrated embodiment) arranged along the Y-axis, wherein m is one 
or more and p is one or more, and a second, n.times.q matrix 60b of units 
with n-number of first to n-th columns (first to fifth columns in the 
illustrated embodiment) arranged along the X-axis and q-number of first to 
q-th rows (first to fourth rows in the illustrated embodiment) arranged 
along the Y-axis, wherein m=n or m=n-1 and p=q or p=q-1. 
Each of the units V of the first and second matrixes 60a and 60b includes a 
substrate 10 and a superconductive layer 30 of R--Ba--Cu--O formed on the 
substrate. 
The first and second matrixes 60a and 60b are assembled together such that 
the superconductive layer of each one of the units of the first to n-th 
columns, except one of the terminal units when m=n, is in contact with and 
in superconductor joining with two superconductive layers of adjacent two 
units of the first to m-th columns, respectively, and that the 
superconductive layer of each one of the units of the first to q-th rows, 
except one of the terminal units when p=q, is in contact with and in 
superconductor joining with two superconductive layers of adjacent two 
units of the first to p-th rows, respectively, so that the superconductive 
layers of the (n.times.q)-number of units of the second matrix 60b are 
superconductively connected to each other through respective 
superconductive layers of the (m.times.p)-number of units of the first 
matrix 60a. The assembling of the first and second matrixes 60a and 60b 
may be carried out in the same manner as that in the foregoing embodiment. 
In the specific embodiment shown in FIG. 6, adjacent two units in each 
column and each row of the first and second matrixes 60a and 60b are 
abutting engagement with each other. But this is not essential. Adjacent 
two units may be spaced apart from each other, if desired, 
The following examples will further illustrate the present invention. 
EXAMPLE 1 
Three kinds of raw materials Y.sub.2 O.sub.3, BaCO.sub.3 and CuO were 
blended to provide a Y:Ba:cu atomic ratio of 1.8:2.4:3.4. The composition 
of this blend was such as to form an oxide superconductor including 
YBa.sub.2 Cu.sub.3 O.sub.x (Y123) phase and Y.sub.2 BaCuO.sub.5 (Y211) 
phase. The blend was calcined at 900.degree. C. for 24 hours and then 
melted at 1,400.degree. C. for 20 minutes. The melted mass was quenched by 
a nip of hammers made of copper and then finely pulverized in a mortar. 
The pulverized mass was mixed with the same amount of a binder solution 
(an isopropanol solution containing 10% by weight of polyvinyl butyral) to 
form a paste. The paste was applied with a screen printing device onto a 
surface of a ZrO.sub.2 substrate (10 in FIG. 1) having a length of 50 mm, 
a width of 10 mm and a thickness of 1 mm to form a paste layer of 0.2 mm 
thick. A seed crystal (20 in FIG. 1) of NdBa.sub.2 Cu.sub.3 O.sub.x 
(Nd123) cut to have an a-axis length of 1 mm, a b-axis length of 1 mm and 
a c-axis length of 0.1 mm was placed on one longitudinal end of the paste 
layer such that the c-axis of the crystal is oriented in the direction 
normal to the surface of the substrate. The substrate having the paste 
layer was then placed in an oven at 150.degree. C. for 30 minutes in air 
to evaporate the isopropanol and thereafter maintained at 600.degree. C. 
for 30 minutes to decompose the binder. 
The substrate having the paste layer was then heat-treated using a furnace 
as shown in FIG. 3. The furnace had a heating lamp 50 backed by light 
collecting mirrors 40. The inside of the furnace had a temperature 
distribution shown by the graph in FIG. 3. The center of the furnace 
provided the maximum temperature of 1,020.degree. C. The temperature 
decreased radially outward at a rate of 10.degree. C./cm. The substrate 
having the paste layer was horizontally displaced in the furnace at a rate 
of 1 cm/hour with the seed crystal-bearing side being the leading end, so 
that the superconductive particles were fused and grown to form a 
superconductive layer. Namely, during the passage of the paste layer 
through the furnace, the superconductive particles were partly fused when 
they reached the center of the furnace having the maximum temperature of 
1,020.degree. C. and the fused particles were then solidified upon leaving 
the center of the furnace, thereby growing the superconductive layer. The 
resulting heat-treated product was heated at 500.degree. C. for 100 hours 
in an oxygen atmosphere to obtain a Y123 superconductor unit (U in FIG. 1) 
having a superconductor layer (30 in FIG. 1). The superconductor layer had 
a critical current density of 55,000 A/cm.sup.2 at 77K in zero magnetic 
field when measured by the pulsed current method. 
A number of similar superconductor units were prepared. A pair of such 
units were each polished so that respective superconductor layers had a 
surface roughness of 1 .mu.m or less. The two units (U1 and U2 in FIG. 4) 
were assembled together such that the polished superconductor layers were 
in contact with each other. Thus, the two units were heated at 950.degree. 
C. for 10 minutes while applying a pressure of 1 kgf/cm.sup.2 to the 
contact surfaces. The assembly was cooled to room temperature and then 
annealed at 500.degree. C. for 100 hours in an oxygen stream at 1 atm to 
obtain a bulk superconductor having a critical current density of 45,000 
A/cm.sup.2 at 77K in zero magnetic field when measured by the pulsed 
current method. 
EXAMPLE 2 
Example 1 was repeated in the same manner as described except that the 
furnace used had such a temperature distribution that the temperature 
decreased radially outward at a rate of 1.degree. C./cm, and that the 
paste layer-bearing substrate was displaced through the furnace at rates 
of 0.1 cm/hour, 1 cm/hour and 10 cm/hour, thereby giving superconductor 
units having critical current densities of 30,000, 40,000 and 35,000 
A/cm.sup.2, respectively, at 77K in zero magnetic field, while the 
assembled bulk superconductors obtained from these superconductor units 
had critical current densities of 28,000, 35,000 and 30,000 A/cm.sup.2, 
respectively, at 77K in zero magnetic field 
EXAMPLE 3 
Example 1 was repeated in the same manner as described except that the 
furnace used had such a temperature distribution that the temperature 
decreased radially outward at a rate of 100.degree. C./cm, and that the 
paste layer-bearing substrate was displaced through the furnace at rates 
of 0.1 cm/hour, 1 cm/hour and 10 cm,/hour, thereby giving superconductor 
units having critical current densities of 32,000, 36,000 and 33,000 
A/cm.sup.2, respectively, at 77K in zero magnetic field, while the 
assembled bulk superconductors obtained from these superconductor units 
had critical current densities of 27,000, 34,000 and 29,000 A/cm.sup.2, 
respectively, at 77K in zero magnetic field. 
EXAMPLE 4 
Example 1 was repeated in the same manner as described except that a 
R--Ba--Cu--O (where R is Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm or Yb) 
superconductor layer was formed in place of Y--Ba--Cu--O. The bulk 
superconductors were found to have critical current densities of 50,000 
A/cm.sup.2 (R.dbd.Nd), 60,000 A/cm.sup.2 (R.dbd.Sm), 65,000 A/cm.sup.2 
(R.dbd.Eu), 35,000 A/cm.sup.2 (R.dbd.Gd), 40,000 A/cm.sup.2 (R.dbd.Dy), 
30,000 A/cm.sup.2 (R.dbd.Ho), 30,000 A/cm.sup.2 (R.dbd.Er), 25,000 
A/cm.sup.2 (R.dbd.Tm) and 20,000 A/cm.sup.2 (R.dbd.Yb) at 77K in zero 
magnetic field. 
EXAMPLE 5 
A number of bulk superconductors as shown in FIG. 4 were prepared and 
assembled to form an elongated superconductor as shown in FIG. 5. The 
assembling was carried out at 950.degree. C. for 10 minutes while applying 
a pressure of 1 kgf/cm.sup.2 to the contact surfaces. The assembly was 
cooled to room temperature and then annealed at 500.degree. C. for 100 
hours in an oxygen stream at 1 atm to obtain an elongated bulk 
superconductor. 
EXAMPLE 6 
A number of bulk superconductors as shown in FIG. 4 were prepared and 
assembled to form a plate-like superconductor as shown in FIG. 6. The 
assembling was carried out at 950.degree. C. for 10 minutes while applying 
a pressure of 1 kgf/cm.sup.2 to the contact surfaces. The assembly was 
cooled to room temperature and then annealed at 500.degree. C. for 100 
hours in an oxygen stream at 1 atm to obtain a bulk superconductor in the 
form of a plate. 
The invention may be embodied in other specific forms without departing 
from the spirit or essential characteristics thereof. The present 
embodiments are therefore to be considered in all respects as illustrative 
and not restrictive, the scope of the invention being indicated by the 
appended claims rather than by the foregoing description, and all the 
changes which come within the meaning and range of equivalency of the 
claims are therefore intended to be embraced therein.