Method of producing composite metal oxide material

A composite material is disclosed which includes a substrate, an oriented film provided on a surface of the substrate and formed of a crystal of a Y123 metal oxide of the formula LnBa.sub.2 Cu.sub.3 O.sub.y wherein Ln stands for Y or an element belonging to the lanthanoid and y is a number of 6-7, and a layer of a Y123 metal oxide of the formula LnBa.sub.2 Cu.sub.3 O.sub.y wherein Ln stands for Y or an element belonging to the lanthanoid and y is a number of 6-7 formed on the oriented film.

BACKGROUND OF THE INVENTION 
This invention relates to a composite material having a metal oxide later 
of a Y123 crystal structure. The present invention is also concerned with 
a method of preparing such a composite material. 
One known method for the preparation of a metal oxide crystal having a Y123 
crystal structure includes contacting a seed material with a molten 
mixture of metal oxides to allow the desired crystal to grow on the seed 
material. The term "Y123 crystal structure" used in the present 
specification and claims is intended to refer to a crystal structure 
similar to that of YBa.sub.2 Cu.sub.3 O.sub.7-x where x is a number of 
0-1. The term "Y123 metal oxide" is intended to refer to a metal oxide 
having a Y123 crystal structure. 
As the seed crystal, a magnesia single crystal, a Sm123 metal oxide crystal 
or a Y123 metal oxide crystal has been hitherto used. The use of a 
magnesia single crystal has a problem that the crystal which has grown on 
the magnesia seed crystal forms a Y123 polycrystal rather than a single 
Y123 crystal. When a Y123 crystal is used as the seed crystal, the molten 
mixture climbs along the seed crystal so that a holder thereof will be 
wetted and reacted with the molten mixture. 
SUMMARY OF THE INVENTION 
It is, therefore, an object of the present invention to provide a composite 
material having a Y123 metal oxide crystal layer grown on a thin Y123 
metal oxide film and having excellent crystallinity and crystal 
orientation. 
It is another object of the present invention to provide a method for 
preparing such a composite material. 
In accomplishing the foregoing objects, there is provided in accordance 
with one aspect of the present invention a composite material comprising a 
substrate, an oriented film provided on a surface of said substrate and 
formed of a metal oxide which has a crystal structure of a Y123 metal 
oxide and which has the formula: 
EQU LnBa.sub.2 Cu.sub.3 O.sub.y 
wherein Ln stands for Y or an element belonging to the lanthanoid and y is 
a number of 6-7, and a layer of a metal oxide crystal provided over a 
surface of said oriented film and having the formula: 
EQU LnBa.sub.2 Cu.sub.3 O.sub.y 
wherein Ln stands for Y or an element belonging to the lanthanoid and y is 
a number of 6-7. 
In another aspect, the present invention provides a method of producing a 
composite material having a layer of a first metal oxide having the 
formula: 
EQU LnBa.sub.2 Cu.sub.3 O.sub.y 
wherein Ln stands for Y or an element belonging to the lanthanoid and y is 
a number of 6-7, said method comprising the steps of: 
providing a seed material including a substrate, and an oriented film 
having a thickness of 0.01-1 .mu.m and provided over a surface of said 
substrate, said oriented film being formed of a second metal oxide which 
has a crystal structure of a Y123 metal oxide and which has the formula: 
EQU LnBa.sub.2 Cu.sub.3 O.sub.y 
wherein Ln stands for Y or an element belonging to the lanthanoid and y is 
a number of 6-7; and 
contacting said oriented film with a liquid phase which contains a melt 
composed of Cu, Ba and O and in which a solid phase is present at a 
position separated by said liquid phase from the position at which said 
oriented film contacts said liquid phase, said solid phase providing said 
liquid phase with solutes which constitute said first metal oxide so that 
said solutes are transported to the position at which said oriented film 
and said liquid phase contact, thereby permitting said first metal oxide 
to grow on said oriented film as primary crystals. 
The composition of the first metal oxide may be the same as or different 
from that of the second metal oxide.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION 
The composite material according to the present invention includes a seed 
material on which a layer of a metal oxide crystal having a Y123 crystal 
structure is formed. The seed material is a composite material composed of 
a substrate, and an oriented film provided on a surface of the substrate 
and formed of a metal oxide which has a crystal structure of a Y123 metal 
oxide and which has a composition expressed by the formula: 
EQU LnBa.sub.2 Cu.sub.3 O.sub.y 
wherein Ln stands for Y or an element belonging to the lanthanoid 
(including La, Ce, Or, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu) 
and y is a number of 6-7. 
The substrate is preferably formed of a material which is substantially 
unwettable with a melt composed of Ba, Cu and O. Examples of suitable 
substrates include oxides containing an alkaline earth metal, such as 
alkaline earth metal oxides and mixed oxides containing an alkaline earth 
metal. The use of a MgO crystal is particularly preferred. 
Whether or not the substrate is wettable may be determined as follows: 
A sample having a length of 50 mm, a thickness of 3 mm and a width of 15 mm 
is immersed by a depth of 5 mm for 15 hours in a melt of a mixture of BaO 
and CuO (Ba/Cu molar ratio of 3:7) contained in a 10 ml crucible, with the 
longitudinal axis of the sample being oriented normal to the surface of 
the melt. The melt is maintained at 1,050.degree. C. When the distance 
through which the melt climbs along the sample after 15 hours immersion is 
not greater than 5 mm, then the sample is regarded as being unwettable. 
The oriented film of a Y123 crystal is provided over a surface of the 
substrate. The direction of the orientation of the film is not 
specifically limited. For example, an oriented film whose c-axis is 
oriented in the direction normal to the plane of the substrate or whose 
a-axis is oriented in the direction normal to the plane of the substrate 
may be suitably used. 
When the seed material has an a-axis oriented film, a layer of a crystal 
which is grown thereon is also a-axis oriented. Thus, the use of a seed 
material having an a-axis oriented film is advantageous in the preparation 
of a good quality SIS junction, since the thickness of the I-layer can be 
made nearly equal to the coherence length of the S-layer. 
The thickness of the oriented film of a Y123 crystal is preferably 0.01-1 
.mu.m. The crystallinity of a Y123 crystal layer grown on the oriented 
film generally depends upon the thickness of the oriented film. Thus, a 
thickness of the oriented film greater than 1 .mu.m is not preferable 
since the Y123 crystal layer tends to form a polycrystal. Too small a 
thickness below 0.01 .mu.m also results in the formation of a polycrystal. 
The oriented film may be easily formed on the substrate by a plasma 
deposition method in which a raw material oxide powder of Ln--Ba--Cu--O is 
fed into a high frequency plasma to vaporize same and to permit the vapors 
to deposit on a surface of the substrate. 
The thus obtained composite seed material is used for forming thereon of a 
layer of a Y123 metal oxide crystal having the formula: 
EQU LnBa.sub.2 Cu.sub.3 O.sub.y 
wherein Ln stands for Y or an element belonging to the lanthanoid and y is 
a number of 6-7. The symbol Ln in the Y123 metal oxide crystal layer may 
be the same as or different from that in the oriented film of the seed 
material. Such a Y123 crystal layer may be suitably obtained by contacting 
the oriented film of the seed material with a melt of a mixture of metal 
oxides containing solutes or components which constitute the Y123 crystal 
under conditions so that the Y123 crystal is grown as primary crystals. In 
this case, even when the oriented film of the seed material is not 
completely oriented, the Y123 crystal grown thereon has good crystallinity 
and crystal orientation. 
FIG. 1 schematically shows the construction of the thus obtained product. 
Designated as 101 is the substrate, as 102 the oriented film and as 103 
the Y123 crystal layer grown on the oriented film 102. 
The formation of the Y123 crystal layer 103 on the seed material may be 
suitably performed by a method including a step of contacting the oriented 
film 102 with a liquid phase which contains a melt composed of Ba, Cu and 
O and in which a solid phase is present at a position separated by the 
liquid phase from the position at which the oriented film 102 contacts the 
liquid phase, the solid phase providing the liquid phase with solutes or 
components which constitute the desired Y123 metal oxide crystal 103 so 
that the solutes are transported to the position at which the oriented 
film and the liquid phase contact, thereby permitting the desired metal 
oxide crystal 103 to grow on the seed material as primary crystals. 
The solid phase to be used in the method of the present invention is 
preferably a Y-series 211 metal oxide. The term "Y-series 211 metal oxide" 
used in the present specification and claims is intended to refer to a 
metal oxide having the formula: 
EQU Ln.sub.2 BaCuO.sub.5-x 
wherein Ln is as defined above and x is a number of 0-1. 
As the liquid phase, any desired metal oxide mixture may be used. 
Generally, a melt composed of Ba, Cu and O is used. Such a liquid phase 
may be obtained by admixing at least two oxide sources selected from a 
source of copper oxide, a source of barium oxide and a source of BaCu 
oxide to form an admixture, the admixture being subsequently heated at a 
temperature of, for example, 880.degree. C. for 40 hours. In this case, it 
is desirable that the molar ratio of Ba to Cu should be 3:5 to 3:8. 
The weight ratio of the liquid phase to the solid phase is preferably 2-10 
parts by weight per part by weight of the solid phase. 
The solid phase and the liquid phase are contained in a crucible formed of 
a material which does not react with the liquid phase and which does not 
adversely affect the properties of the desired metal oxide crystal, such 
as yttria, magnesia, alumina or stabilized zirconia. The crucible is 
placed within a furnace or oven. The seed material is immersed in the 
liquid phase while gradually lowering the temperature of the liquid phase 
so as to cause the crystal of the desired Y123 metal oxide to deposit on 
the seed material and to grow. For the acceleration of the growth of the 
crystal, the temperature of the liquid phase adjacent to the seed material 
is made lower by about 20.degree. C. at maximum than the pertectic 
temperature. In an alternative, the substrate is slowly pulled up at a 
rate of 0.01-10 mm per hour. 
The following examples will further illustrate the present invention. 
EXAMPLE 1 
Preparation of Seed Material: 
A magnesia single crystal (substrate) in the form of a rod was heated so 
that the surface temperature was in the range of 600.degree.-1,000.degree. 
C. According to the plasma deposition method, a raw material oxide powder 
of Ln--Ba--Cu--O was vaporized and allow to deposit on the heated surface 
of the substrate. The substrate was then cooled to 300.degree. C. in the 
atmosphere of oxygen to form an oriented film on the cleavage plane of the 
substrate. The plasma deposition was performed under the conditions 
summarized in Table below. 
______________________________________ 
Raw Material Powder 
Composition: Y.sub.1.3 Ba.sub.2.2 Cu.sub.3.2 O.sub.7-d 
Average particle diameter: 
2-3 .mu.m 
Carrier Gas (feed rate): 
Ar (2 liter/minute) 
Vacuum: 200 torr 
High Frequency Power: 
32 kW 
Plasma Gas (feed rate): 
Ar (2 liter/minute) 
Sheath Gas (feed rate): 
O.sub.2 (50 liter/minute) 
______________________________________ 
The X-ray diffraction pattern of the oriented film thus obtained is shown 
in FIG. 2. From the diffraction pattern, the c-axis of the oriented film 
is found to be oriented in the direction normal to the plane of the 
substrate. Some peaks oriented in the other directions also exist, 
suggesting that the orientation of the film is not uniform. The film has a 
Y123 crystal structure. 
Formation of Y123 Crystal Layer: 
As shown in FIG. 3, into a yttria crucible 11 having an inside diameter of 
30 mm and a height of 50 mm were charged Y.sub.2 BaCuO.sub.5 (solid phase) 
12 and a calcined mass (liquid phase) 13 obtained by calcining a mixture 
of barium carbonate and copper oxide (a molar ratio of Ba/Cu of 3:5) at 
880.degree. C. for 40 hours. The crucible 11 was placed inside of a 
furnace having a platinum susceptor 16 and a high frequency coil 17 and 
the contents in the crucible 11 was heated to about 1,000.degree. C. to 
melt the mixture of BaO and CuO (liquid phase 13) beneath which the solid 
phase 12 was present. Then, the above seed material having the oriented 
film 14 formed on the cleavage plane of the magnesia single crystal 15 was 
contacted with the melt in the crucible and, after lowering the 
temperature of the crucible by 20.degree. C., was continuously pulled up 
at a rate of 0.2 mm per hour for about 50 hours while being rotated at 100 
revolutions per minute (rpm), whereby a single crystal of YBa.sub.2 
Cu.sub.3 O.sub.7-x was allowed to grow on the seed crystal bar as primary 
crystal. 
The thus grown Y123 crystal layer had a mirror surface. The polarized 
microscope revealed that the Y123 crystal layer had good crystallinity and 
was free of grain boundaries though a twin plane was present. 
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.