Buffer layers on metal surfaces having biaxial texture as superconductor substrates

FIELD OF THE INVENTION
The present invention relates to structures upon which high current conductors may be epitaxially deposited on biaxially-textured rolled substrates of nickel and/or copper and other metals and alloys which can be characterized by a surface having biaxial texture. Also described are methods for fabricating said structures. The structures and methods for making the structures include but are not limited to Y.sub.2 O.sub.3 /SS (SS=Substrate comprising at least one metal and having a surface exhibiting biaxial texture), YSZ/Y.sub.2 O.sub.3 /SS (YSZ=yittria-stabilized zirconia), RE.sub.2 O.sub.3 /SS (RE=Rare Earth), RE.sub.2 O.sub.3 /Y.sub.2 O.sub.3 /SS, Y.sub.2 O.sub.3 /CeO.sub.2 /SS, RE.sub.2 O.sub.3 /Ce.sub.2 O.sub.3 /SS, Y.sub.2 O.sub.3 /YSZ/CeO.sub.2 /SS, RE.sub.2 O.sub.3 /YSZ/CeO.sub.2 /SS, and (RE').sub.x O.sub.y /SS (RE'=any combination which includes, but is not limited to Rare Earth elements, wherein x is a value near 2.0, believed to be between about 1.95 and about 2.05, (hereinafter stated as about 2.0), and wherein y is between about 3.0 and about 3.7).
BACKGROUND OF THE INVENTION
It has long been desired to grow biaxially oriented oxide buffer layers other than CeO.sub.2 directly on textured substrates, and also to have a single buffer layer on textured substrates. Also it has been desired to provide an alternative to pulsed laser deposition processes that may be easier to scale up for producing long length substrates.
Recent developments in deposited conductors, both rolling assisted biaxially textured substrates (RABiTS), and ion-beam assisted deposition (IBAD) based on YBa.sub.2 Cu.sub.3 O.sub.7 superconductors are promising, and are herein reported for the first time.
The "deposited conductor" approach described herein is useful for growing superconductors such as REBa.sub.2 Cu.sub.3 O.sub.7, (Bi,Pb).sub.2 Sr.sub.2 Ca.sub.n-1 CunO.sub.2n+4 (n=1-3), Tl.sub.1 Ba.sub.2 Ca.sub.n-1 Cu.sub.n O.sub.2n+3 (n=1-4), Tl.sub.2 Ba.sub.2 Ca.sub.n-1 Cu.sub.n O.sub.2n+4 (n=1-3), and Hg.sub.1 Ba.sub.2 Ca.sub.n-1 Cu.sub.n O.sub.2n+2+.delta. (n=1-4) with high critical-current densities. These high J.sub.c conductors will be suitable for transmission lines and various other applications. The demonstrated buffer layers may also be useful for photovoltaics, ferroelectrics, sensors, and electro-optic applications.
The following sections of publications also relate to the present invention, and are hereby incorporated by reference:
X. D. Wu, S. R. Foltyn, P. Arendt, J. Townsend, C. Adams, I. H. Campbell, P. Tiwari, Y. Coulter, and D. E. Peterson, Appl. Phys. Lett. 65 (15), Oct. 10, 1994, p1961.
M. Paranthaman et al., Physica C 275 (1997) 266-272.
OBJECTS OF THE INVENTION
Accordingly, it is an object of the present invention to provide new and improved biaxially oriented oxide buffer layers other than CeO.sub.2 directly on textured-Ni and/or Cu substrates.
It is another object to provide an alternative to pulsed laser deposition processes as well as improved buffer layer architectures that may be easier to scale up for producing long length substrates.
Further and other objects of the present invention will become apparent from the description contained herein.
SUMMARY OF THE INVENTION
In accordance with one aspect of the present invention, the foregoing and other objects are achieved by a biaxially textured article which comprises a substrate comprising at least one metal, the substrate having a surface exhibiting biaxial texture; and a buffer layer selected from the group consisting of Y.sub.2 O.sub.3 and RE.sub.2 O.sub.3, the buffer layer being epitaxially disposed upon the biaxially-textured surface of the substrate.
In accordance with a second aspect of the present invention, the foregoing and other objects are achieved by a biaxially textured article which comprises a substrate comprising at least one metal, the substrate having a surface exhibiting biaxial texture; a first buffer layer comprising CeO2, the first buffer layer being epitaxially disposed upon the biaxially textured surface of the substrate; and a second buffer layer selected from the group consisting of Y.sub.2 O.sub.3 and RE.sub.2 O.sub.3, the second buffer layer being epitaxially disposed upon the biaxially textured surface of the CeO.sub.2.
In accordance with a third aspect of the present invention, the foregoing and other objects are achieved by a biaxially textured article which comprises a substrate comprising at least one metal, the substrate having a surface exhibiting biaxial texture; a first buffer layer comprising CeO.sub.2, the first buffer layer being epitaxially disposed upon the biaxially textured surface of the substrate; a second buffer layer comprising YSZ, the second buffer layer being epitaxially disposed upon the biaxially textured surface of the CeO.sub.2 ; and a third buffer layer selected from the group consisting of Y.sub.2 O.sub.3 and RE.sub.2 O.sub.3, the third buffer layer being epitaxially disposed upon the biaxially textured surface of the YSZ.
In accordance with a fourth aspect of the present invention, the foregoing and other objects are achieved by a biaxially textured article which comprises a substrate comprising at least one metal, the substrate having a surface exhibiting biaxial texture; and a buffer layer comprising (RE').sub.x O.sub.y, wherein x is about 2.0, and y is between about 3.0 and about 3.7, the buffer layer being epitaxially disposed upon the biaxially textured surface of the substrate.
In accordance with a fifth aspect of the present invention, the foregoing and other objects are achieved by a method for making a biaxially textured article which comprises the steps of: providing a substrate comprising at least one metal, the substrate having a surface exhibiting biaxial texture; and epitaxially depositing upon the biaxially textured surface of the substrate a buffer layer selected from the group consisting of Y.sub.2 O.sub.3 and RE.sub.2 O.sub.3.
In accordance with a sixth aspect of the present invention, the foregoing and other objects are achieved by a method for making a biaxially textured article which comprises the steps of: providing a substrate comprising at least one metal, the substrate having a surface exhibiting biaxial texture; epitaxially depositing upon the biaxially textured surface of the substrate a first buffer layer comprising CeO.sub.2 ; and epitaxially depositing upon the biaxially textured surface of the CeO.sub.2 a second buffer layer selected from the group consisting of Y.sub.2 O.sub.3 and RE.sub.2 O.sub.3.
In accordance with a seventh aspect of the present invention, the foregoing and other objects are achieved by a method for making a biaxially textured article which comprises the steps of: providing a substrate comprising at least one metal, the substrate having a surface exhibiting biaxial texture; epitaxially depositing upon the biaxially textured surface of the substrate a first buffer layer comprising CeO.sub.2 ; epitaxially depositing upon the biaxially textured surface of the CeO.sub.2 a second buffer layer comprising YSZ, and epitaxially depositing upon the biaxially textured surface of the YSZ a third buffer layer selected from the group consisting of Y.sub.2 O.sub.3 and RE.sub.2 O.sub.3.
In accordance with an eighth aspect of the present invention, the foregoing and other objects are achieved by a method for making a biaxially textured article which comprises the steps of: providing a substrate comprising at least one metal, the substrate having a surface exhibiting biaxial texture; and epitaxially depositing upon the biaxially textured surface of the substrate a buffer layer comprising (RE').sub.x O.sub.y, wherein x is about 2.0, and y is between about 3.0 and about 3.7.

BRIEF DESCRIPTION OF THE DRAWINGS
In the drawings:
FIG. 1 is a schematic diagram of the various buffer layer architectures developed on textured-Ni and/or textured-Cu substrates.
FIG. 2 is the room temperature powder X-ray diffraction pattern for a 100 nm thick Y.sub.2 O.sub.3 film grown on textured-Ni substrates by e-beam evaporation.
FIG. 3 is the Y.sub.2 O.sub.3 (222) pole figure for Y.sub.2 O.sub.3 films grown directly on textured-Ni substrates by e-beam evaporation.
FIG. 4 is the room temperature powder X-ray diffraction pattern for a 100 nm thick Yb.sub.2 O.sub.3 film grown on textured Ni substrates by e-beam evaporation.
FIG. 5 is an X-ray .omega. scan for a 100 nm thick Yb.sub.2 O.sub.3 -buffered Ni substrates with FWHM of 11.7.degree. for Yb.sub.2 O.sub.3 (400).
FIG. 6 is an X-ray .phi. scan for a 100 nm thick Yb.sub.2 O.sub.3 -buffered Ni substrates with FWHM of 9.5.degree. for Yb.sub.2 O.sub.3 (222).
FIG. 7 is the room temperature powder X-ray diffraction pattern for a 400 nm thick YSZ film grown on Y.sub.2 O.sub.3 -buffered Ni substrates by rf magnetron sputtering.
FIG. 8 is an x-ray .omega. scan for a 400 nm thick YSZ film grown on Y.sub.2 O.sub.3 -buffered Ni substrates with FWHM of 9.5.degree. for YSZ (200).
FIG. 9 is an X-ray .phi. scan for a 400 nm thick YSZ film grown on Y.sub.2 O.sub.3 -buffered Ni substrates with FWHM of 11.7.degree. for YSZ (220).
FIG. 10 is the room temperature powder X-ray diffraction pattern for a 400 nm thick Yb.sub.2 O.sub.3 -film grown on Y.sub.2 O.sub.3 -buffered Ni substrates by rf magnetron sputtering.
FIG. 11 is the room temperature powder X-ray diffraction pattern for Yb.sub.2 O.sub.3 -films grown on CeO.sub.2 -buffered Ni substrates by rf magnetron sputtering.
FIG. 12 is an X-ray .omega. scan for YBCO film on Yb.sub.2 O.sub.3 /CeO.sub.2 -buffered Ni substrates with FWHM of 7.7.degree. for YBCO (006).
FIG. 13 is an X-ray .phi. scan for YBCO film on Yb.sub.2 O.sub.3 /CeO.sub.2 -buffered Ni substrates with FWHM of 9.5.degree. for YBCO (103).
FIG. 14 is an X-ray .omega. scan for YBCO film on Yb.sub.2 O.sub.3 /YSZ/CeO.sub.2 -buffered Ni substrates with FWHM of 6.5.degree. for YBCO (006).
FIG. 15 is an X-ray .phi. scan for YBCO film on Yb.sub.2 O.sub.3 YSZ/CeO.sub.2 -buffered Ni substrates with FWHM of 8.9.degree. for YBCO (103).
FIG. 16 is the temperature dependence resistivity plot for YBCO film on Yb.sub.2 O.sub.3 /CeO.sub.2 /Ni.
FIG. 17 is the temperature dependence resistivity plot for YBCO film on Yb.sub.2 O.sub.3 /YSZ/CeO.sub.2 /Ni.
FIG. 18 is the temperature dependence resistivity plot for YBCO film on Yb.sub.2 O.sub.3 /Y.sub.2 O.sub.3 /Ni.
FIG. 19 shows the temperature dependence of normalized critical current densities of YBCO deposited on Yb.sub.2 O.sub.3 /YSZ/CeO.sub.2 /Ni, Yb.sub.2 O.sub.3 /CeO.sub.2 /Ni, and Yb.sub.2 O.sub.3 /Y.sub.2 O.sub.2 /Ni. Also included is the result of YBCO deposited on standard YSZ/CeO.sub.2 /Ni architecture for comparison.
FIG. 20 shows the magnetic field dependence of normalized critical current densities of YBCO deposited on Yb.sub.2 O.sub.3 /YSZ/CeO.sub.2 /Ni and Yb.sub.2 O.sub.3 /CeO.sub.2 /Ni. Also included is the result of YBCO deposited on standard YSZ/CeO.sub.2 /Ni architecture for comparison.
FIG. 21 shows the room temperature powder X-ray diffraction pattern for a 90-nm thick Gd.sub.2 O.sub.3 film grown on textured-Ni substrates by e-beam evaporation.
FIG. 22 is X-ray .omega. scans for a 90-nm thick Gd.sub.2 O.sub.3 -buffered Ni substrate with FWHM of 7.1.degree. for Ni (200) and 5.2.degree. for Gd.sub.2 O.sub.3 (400).
FIG. 23 is X-ray .phi. scans for a 90-nm thick Gd.sub.2 O.sub.3 -buffered Ni substrate with FWHM of 10.1.degree. for Ni (111) and 9.2.degree. for Gd.sub.2 O.sub.3 (222).
FIG. 24 is a pole figure for Gd.sub.2 O.sub.3 film grown directly on textured-Ni (222) substrate by e-beam evaporation.

For a better understanding of the present invention, together with other and further objects, advantages and capabilities thereof, reference is made to the following disclosure and appended claims in connection with the above-described drawings.
DETAILED DESCRIPTION OF THE INVENTION
This invention describes how to grow biaxially oriented oxide buffer layers other than CeO.sub.2 directly on textured-Ni and/or Cu substrates and other metals and alloys which can be characterized by a surface having biaxial texture, and also describes methods for making buffer layers on textured substrates. High rate depositions are accomplished by the demonstrated reactive evaporation. Some of the buffer layers grown may be excellent diffusion barriers for Ni and/or Cu and may be chemically compatible with the high temperature superconductors. This invention also demonstrates the possibility of having a single buffer layer on textured-Ni and/or Cu substrates. This invention also demonstrates the possibility of obtaining predominantly the single texture component of YBCO superconductors. This invention opens up a wide variety of possibilities of growing several other buffer layers such as REAlO.sub.3 (RE=Rare Earth), AEZrO.sub.3, (AE=Alkaline Earth, Ca,Sr,Ba), RE.sub.2 Zr.sub.2 O.sub.7, Ca-stabilized Zirconia, Ti, Nb or Zr doped CeO.sub.2, and AECeO.sub.3 on either buffered-rolled metal substrates or directly on rolled metal substrates. These buffer layers may be of interest to other areas like photovoltaics, ferroelectrics, sensors and optoelectronic devices.
Buffer layers such as Y.sub.2 O.sub.3, RE.sub.2 O.sub.3, CeO.sub.2, and YSZ were deposited on RABiTS by vacuum processing techniques such as electron-beam evaporation and rf magnetron sputtering. FIG. 1 summarizes the architectures of various buffer layers developed in this invention.
The first buffer layer comprises an epitaxial laminate of Y.sub.2 O.sub.3, RE.sub.2 O.sub.3, RE.sub.2 O.sub.3 /Y.sub.2 O.sub.3 or YSZ/Y.sub.2 O.sub.3 deposited on a biaxially cube textured Ni and/or Cu substrate. The crystallographic orientation of the Y.sub.2 O.sub.3, RE.sub.2 O.sub.3, and YSZ were mostly (100). The second alternative buffer layer comprises an epitaxial laminate of Y.sub.2 O.sub.3 or RE.sub.2 O.sub.3 on CeO.sub.2 -buffered Ni and/or Cu substrates. The third alternative buffer layer comprises an epitaxial laminate of RE.sub.2 O.sub.3 (RE=Rare Earth)/YSZ/CeO.sub.2 /Ni (or Cu). The RE.sub.2 O.sub.3 films were grown epitaxially on YSZ-buffered Ni or Cu substrates. YBCO (YBa.sub.2 Cu.sub.3 O.sub.7-x) has also been grown on some of the buffer layers by pulsed laser deposition. An estimated J.sub.c of 0.7.times.10.sup.6 A/cm.sup.2 at 77.degree. K and zero field for a film with the architecture YBCO/Yb.sub.2 O.sub.3 /CeO.sub.2 /Ni. A J.sub.c of 1.4.times.10.sup.6 A/cm.sup.2 at 77.degree. K and zero field was also obtained for YBCO/Yb.sub.2 O.sub.3 /YSZ/CeO.sub.2 /Ni, and a J.sub.c of 0.8.times.10.sup.6 A/cm.sup.2 at 77.degree. K for YBCO/Yb.sub.2 O.sub.3 /Y.sub.2 O.sub.3 /Ni. The above-described buffer layers may also be useful for the subsequent growth of superconductors such as REBa.sub.2 Cu.sub.3 O.sub.7 (RE=Rare Earth), (Bi,Pb).sub.2 Sr.sub.2 Ca.sub.n-1 Cu.sub.n O.sub.2n+4 (n=1-3), Tl.sub.1 Ba.sub.2 Ca.sub.n-1 Cu.sub.n O.sub.2n+3 (n=1-4), Tl.sub.2 Ba.sub.2 Ca.sub.n-1 Cu.sub.n O.sub.2n+4 (n=1-3), and Hg.sub.1 Ba.sub.2 Ca.sub.n-1 Cu.sub.n O.sub.2n+2+.delta. (n=1-4) that are chemically and epitaxially compatible with Y.sub.2 O.sub.3, and RE.sub.2 O.sub.3 buffer layers.
For purposes herein, the following definitions apply: Biaxial(ly) Texture: The films are oriented in both out-of-plane (along the (001) direction) and in plane (along the (100) direction) directions. Epitaxial(ly): The films grown on a particular substrate will grow in the same orientation of the substrate is defined as epitaxial, for example, cube-on-cube orientation. Thus, a buffer layer as described herein as having been epitaxially deposited upon a biaxially-textured surface also exhibits a biaxially textured surface. RABiTS: Rolling-assisted biaxially-textured substrates; Rare Earth (RE): The group of elements which consists of: La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu. RE': any combination including but not limited to Rare Earth elements. (RE').sub.x O.sub.y. Any combination including but not limited to Rare Earth oxides, especially wherein x is about 2.0, and y is between about 3 and about 3.7. YBCO: YBa.sub.2 Cu.sub.3 O.sub.7-x. YSZ: yttria-stabilized zirconia. Table 1 summarizes the list of rare earth oxides and their structure type.
The specific processing conditions of the successful multi-layer sequences are described in detail hereinbelow.
EXAMPLE 1
Growth of Y.sub.2 O.sub.3 on Textured-Ni and/or Cu Substrates by E-beam Evaporation
An electron beam (e-beam) evaporation technique was used to deposit Y.sub.2 O.sub.3 films directly on Ni. The as-rolled Ni substrates were cleaned ultrasonically with both acetone and methanol and recrystallized to the desired (100) cube texture by annealing the substrates at 800.degree. C. for 2 hours in a vacuum of 10.sup.-6 Torr. Biaxially oriented Ni substrates were mounted on a substrate holder with a heater assembly in the e-beam system. After the vacuum in the chamber had reached 1.times.10.sup.-6 Torr at room temperature, a gas mixture of 4% H.sub.2 and 96% Ar was introduced until the pressure inside the chamber reached 1 Torr. The Ni substrates were annealed at 700.degree. C. for 1 hour at that pressure. The chamber was then pumped and maintained at a pressure of 2.times.10.sup.-5 Torr using a mixture of 4% H.sub.2 and 96% Ar. The gas flow was controlled by a dc-powered piezoelectric valve. The Y.sub.2 O.sub.3 layers were grown on the Ni substrates at temperatures ranging from 200 to 750.degree. C. The deposition rate for Y.sub.2 O.sub.3 was 1-5 .ANG./sec with the operating pressure of 10.sup.-5 Torr, and the final thickness was varied from 20 nm to 200 nm. The crucibles used were mostly of tungsten. Yttrium metal was used as the source. The XRD results from the .theta.-2.theta. scan (as shown in FIG. 2), .omega. and .phi. scans for as-deposited Y.sub.2 O.sub.3 (20 nm thick) on Ni at 700.degree. C. are as follows: The strong Y.sub.2 O.sub.3 (200) from FIG. 2 revealed the presence of a good out-of-plane texture. The FWHM for Ni (200) and Y.sub.2 O.sub.3 (200) are 7.2.degree. and 7.1.degree., and that of Ni (202) and Y.sub.2 O.sub.3 (222) are 11.7.degree. and 8.4.degree., respectively. The Y.sub.2 O.sub.3 (222) pole figure for Y.sub.2 O.sub.3 films grown at 700.degree. C. on Ni is shown in FIG. 3. From the XRD results, it can be concluded that Y.sub.2 O.sub.3 can be grown epitaxially on Ni. The CeO.sub.2 was also grown epitaxially on textured-Ni substrates at about 625.degree. C. using Ce metal as the source.
The pulsed laser deposition technique was used to grow YBCO at 780.degree. C. and 185 mTorr O.sub.2 on all the buffered-Ni substrates.
The methods described in Example 1 can also be used to grow Y.sub.2 O.sub.3 on textured-Cu substrates as well as on alloys of Cu and/or Ni and other metals and alloys which can be characterized by a surface having biaxial texture by e-beam evaporation.
EXAMPLE 2
Growth of Yb.sub.2 O.sub.3 on Textured-Ni and/or Cu Substrates by E-beam Evaporation
An electron beam (e-beam) evaporation technique was used to deposit Yb.sub.2 O.sub.3 films directly on Ni. The as-rolled Ni substrates were cleaned ultrasonically with both acetone and methanol and recrystallized to the desired (100) cube texture by annealing the substrates at 800.degree. C. for 2 hours in a vacuum of 10.sup.-6 Torr. Biaxially oriented Ni substrates were mounted on a substrate holder with a heater assembly in the e-beam system. After the vacuum in the chamber had reached 1.times.10.sup.-6 Torr at room temperature, a gas mixture of 4% H.sub.2 and 96% Ar was introduced until the pressure inside the chamber reached 1 Torr. The Ni substrates were annealed at 650.degree. C. for 1 hour at that pressure. The chamber was then pumped and maintained at a pressure of 2.times.10.sup.-5 Torr using a mixture of 4% H.sub.2 and 96% Ar. The gas flow was controlled by a dc-powered piezoelectric valve. The Yb.sub.2 O.sub.3 layers were grown on the Ni substrates at temperatures ranging from 200 to 750.degree. C. The deposition rate for Yb.sub.2 O.sub.3 was 1-5 .ANG./sec with the operating pressure of 10.sup.-5 Torr, and the final thickness was varied from 20 nn to 200 nm. The crucibles used were of graphite. Ytterbium oxide pellets were used as the source. The XRD results from the .theta.-2.theta. scan (as shown in FIG. 4), .omega. and .phi. scans (FIGS. 5 and 6) for as-deposited Yb.sub.2 O.sub.3 (100 nm thick) on Ni at 700.degree. C. is as follows: The strong Yb.sub.2 O.sub.3 (400) from FIG. 4 revealed the presence of good out-of-plane texture. The FWHM for Yb2O3 (400) are 11.7.degree., and that of Yb.sub.2 O.sub.3 (222) is 9.5.degree. respectively. From the XRD results, it can be concluded that Yb.sub.2 O.sub.3 can be grown epitaxially on Ni.
The methods described in Example 2 can also be used to grow Yb.sub.2 O.sub.3 on textured-Cu substrates as well as on alloys of Cu and/or Ni and other metals and alloys which can be characterized by a surface having biaxial texture by e-beam evaporation.
EXAMPLE 3
Growth of YSZ on Y.sub.2 O.sub.3 -buffered Ni and/or Cu by rf Magnetron Sputtering
Initially, Y.sub.2 O.sub.3 films were grown epitaxially by e-beam evaporation on rolled-Ni substrated as described in Example 1. The YSZ films were grown on these Y.sub.2 O.sub.3 -buffered Ni substrates by rf magnetron sputtering. The Y.sub.2 O.sub.3 -buffered Ni substrate was mounted on a heating block inside the sputter chamber. Prior to heating the substrate, the sputter chamber was evacuated to a pressure of 1.times.10.sup.-6 Torr. The chamber was then back-filled with a flowing mixture of 4% H.sub.2 and 96% Ar to a pressure of 5.times.10.sup.-2 Torr. The substrate was heated to 780.degree. C. for 15 min, and then the pressure was reduced to 1.times.10-2 Torr, and sputter deposited at about 780.degree. C. for 80 min with an on-axis YSZ target located 6 cm from the substrate. The plasma power was 75 W at 13.56 MHz. The resulting YSZ film was smooth, and its thickness was estimated to be approximately 400 nm. The accompanying .theta.-2.theta. X-ray scan (as shown in FIG. 7) shows good out-of-plane texture for the YSZ which is consistent with epitaxy. From the .omega. (as shown in FIG. 8) and .phi. (as shown in FIG. 9) scans for a 400 nm thick YSZ film grown on Y.sub.2 O.sub.3 -buffered Ni substrates, the FWHM for YSZ (200), and YSZ (220) are 9.5.degree. and 11.7.degree., respectively.
The methods described in Example 3 can also be used to grow YSZ on RE.sub.2 O.sub.3 -buffered Ni and Y.sub.2 O.sub.3 -buffered Cu as well as on alloys of Cu and/or Ni and other metals and alloys which can be characterized by a surface having biaxial texture by rf magnetron sputtering.
EXAMPLE 4
Growth of Yb.sub.2 O.sub.3 on Y.sub.2 O.sub.3 -buffered or CeO.sub.2 -buffered Ni and/or Cu by rf Magnetron Sputtering
Initially, Y.sub.2 O.sub.3 or CeO.sub.2 films were grown epitaxially by e-beam evaporation on Rolled-Ni substrates as described in Example 1. The Yb.sub.2 O.sub.3 films were grown on these Y.sub.2 O.sub.3 -buffered or CeO.sub.2 -buffered Ni substrates by rf magnetron sputtering. The buffered substrate was mounted on a heating block inside the sputter chamber. Prior to heating the substrate, the sputter chamber was evacuated to a pressure of about 1.times.10.sup.-6 Torr. The chamber was then back-filled with a flowing mixture of 4% H.sub.2 and 96% Ar to a pressure of 5.times..sup.-2 Torr. The substrate was heated to 780.degree. C. for 15 min, and then the pressure was reduced to 1.times.10.sup.-2 Torr, and sputter deposited at about 780.degree. C. for 60 min with an on-axis Yb.sub.2 O.sub.3 target located 6 cm from the substrate. The plasma power was 75 W at 13.56 MHz. The resulting Yb.sub.2 O.sub.3 film was smooth, and its thickness was estimated to be approximately 400 nm. FIG. 10 shows the .eta.-2.theta. X-ray scan for a 400 nm thick Yb.sub.2 O.sub.3 on Y.sub.2 O.sub.3 -buffered Ni substrates. The accompanying .theta.-2.theta. X-ray scan for a 400 nm thick Yb.sub.2 O.sub.3 on CeO.sub.2 -buffered Ni substrates (as shown in FIG. 11) shows good out-of-plane texture for the Yb.sub.2 O.sub.3 which is again consistent with epitaxy. From the (.omega. and .phi. scans for the Yb.sub.2 O.sub.3 /CeO.sub.2 /Ni film the FWHM for Ni (002), CeO.sub.2 (002), and Yb.sub.2 O.sub.3 (002) are 12.2.degree., 8.2.degree., and 10.7.degree., and that of Ni (111), CeO.sub.2 (111), and Yb.sub.2 O.sub.3 (222) are 10.3.degree., 8.7.degree. and 8.7.degree., respectively. From the XRD results, it can be concluded that Yb.sub.2 O.sub.3 can be grown epitaxially on Y.sub.2 O.sub.3 -buffered Ni and CeO.sub.2 -buffered Ni substrates. The Yb.sub.2 O.sub.3 films were also grown epitaxially on YSZ/CeO.sub.2 /Ni.
YBCO was then grown by pulsed laser deposition on three epitaxial laminates of Yb.sub.2 O.sub.3 /CeO.sub.2 /Ni, Yb.sub.2 O.sub.3 /YSZ/CeO.sub.2 /Ni, and Yb.sub.2 O.sub.3 /Y.sub.2 O.sub.3 /Ni. The typical YBCO thickness was about 300 nm for Yb.sub.2 O.sub.3 /CeO.sub.2 /Ni and Yb.sub.2 O.sub.3 /YSZ/CeO.sub.2 /Ni, and about 400 nm for Yb.sub.2 O.sub.3 /Y.sub.2 O.sub.3 /Ni. From the XRD results, the YBCO films grown on all architectures were found to be epitaxial. The typical .omega. and .phi. scans for YBCO on Yb.sub.2 O.sub.3 /CeO.sub.2 -buffered Ni substrates with FWHM of 7.7.degree. for YBCO (006) and 9.5.degree. for YBCO (103) films are shown in FIGS. 12 and 13. The typical .omega. and .phi. scans for YBCO on Yb.sub.2 O.sub.3 /YSZ/CeO.sub.2 -buffered Ni substrates with FWHM of 6.5.degree. for YBCO (006) and 8.9.degree. for YBCO (103) films are shown in FIGS. 14 and 15. The X-ray data (FIGS. 12-15) demonstrate the presence of a single texture component of YBCO on Yb.sub.2 O.sub.3 layers as compared to that of two texture components of YBCO grown directly on YSZ surface. The four probe resistivity plots showed a T.sub.c (zero resistance) of 90-92K for all three films (FIGS. 16, 17 and 18). The transport critical current-density J.sub.c was estimated to be 0.7.times.10.sup.6 A/cm.sup.2 for YBCO/Yb.sub.2 O.sub.3 /CeO.sub.2 /Ni and 1.4.times.10.sup.6 A/cm.sup.2 for YBCO/Yb.sub.2 O.sub.3 /YSZ/CeO.sub.2 /Ni, and 0.8 A/cm.sup.2 for YBCO/Yb.sub.2 O.sub.3 /Y.sub.2 O.sub.3 /Ni at 77.degree. K and zero field. The temperature and magnetic field dependence of normalized J.sub.c of these superconductors are shown in FIGS. 19 and 20. Also included are the data on films with YBCO deposited on standard YSZ/CeO.sub.2 /Ni architecture for comparison. As can be seen from these figures that the performance of YBCO deposited on alternative buffer architectures as disclosed in the present invention is similar to that of the standard RABiTS architecture, and demonstrate the presence of epitaxy and resulting strongly linked nature of the alternate buffer layers.
The methods described in Example 4 can also be used to grow Yb.sub.2 O.sub.3 on Y.sub.2 O.sub.3 -buffered or CeO.sub.2 -buffered Cu as well as on alloys of Cu and/or Ni and other metals and alloys which can be characterized by a surface having biaxial texture by rf magnetron sputtering.
EXAMPLE 5
Growth of Gd.sub.2 O.sub.3 on Textured-Ni Substrates by Reactive Evaporation
An electron beam (e-beam) evaporation technique was used to deposit Gd.sub.2 O.sub.3 films directly on Ni. The as-rolled Ni substrates were cleaned ultrasonically with both acetone and methanol and recrystallized to the desired (100) cube texture by annealing the substrates at 800.degree. C. for 2 hours in a vacuum of 10.sup.-6 Torr. Biaxially oriented Ni substrates were mounted on a substrate holder with a heater assembly in the e-beam system. After the vacuum in the chamber had reached 1.times.10.sup.-6 Torr at room temperature a gas mixture of 4% H.sub.2 and 96% Ar was introduced until the pressure inside the chamber reached 1 Torr. The Ni substrates were annealed at 700.degree. C. for 1 hour at that pressure. The chamber was then pumped and maintained at a pressure of 2.times.10.sup.-5 Torr using a mixture of 4% H.sub.2 and 96% Ar. The gas flow was controlled by a dc-powered piezoelectric valve. To understand the chemistry of the oxide formation, a Dycor Quadruple Gas Analyzer was mounted in the e-beam system. The background H.sub.2 O pressure (pH.sub.2 O) in the system was around 1.times.10.sup.-5 Torr. The crucibles used were usually graphite. Gadolinium metal was used as the source. The Gd.sub.2 O.sub.3 layers were grown on the Ni substrates at temperatures ranging from 200 to 800.degree. C. The deposition rate for Gd.sub.2 O.sub.3 was 1-5 .ANG./sec with the operating pressure of 10.sup.-5 Torr, and the final thickness was varied from 10 to 500 nm. During the deposition, the H.sub.2 O pressure slowly fell to 4.times.10.sup.-6 Torr (when Quartz Crystal Monitor was reading about 10 nm). The H.sub.2 O was introduced into the system through a valve to maintain the H.sub.2 O pressure around 1.times.10.sup.-5 Torr throughout the deposition. This H.sub.2 O pressure was sufficient to oxidize the film to form Gd.sub.2 O.sub.3 preferentially without oxidizing the Ni underneath. Also, just before the deposition, the Gd metal was evaporated for a few seconds with the shutter closed. This was critical in changing the partial pressure of from 10.sup.-7 Torr to 10.sup.-8 Torr. The Gd metal acts as an oxygen getter. The XRD results from the .theta.-2.theta. scan (as shown in FIG. 21), .omega. and .phi. scans for as-deposited Gd.sub.2 O.sub.3 (90 nm thick) on Ni at 700.degree. C. is as follows: The strong Gd.sub.2 O.sub.3 (400) from FIG. 21 revealed the presence of a good out-of-plane texture. The FWHM for Ni (200) and Gd.sub.2 O.sub.3 (400) are 7.1.degree. and 5.2.degree. (FIG. 22), and that of Ni (111) and Gd.sub.2 O.sub.3 (222) are 10.1.degree. and 9.2.degree. (FIG. 23), respectively. The Gd.sub.2 O.sub.3 (222) pole figure for Gd.sub.2 O.sub.3 films grown at 700.degree. C. on Ni is shown in FIG. 24. From the XRD results, it can be concluded that Gd.sub.2 O.sub.3 can be grown epitaxially on Ni, Cu, and alloys thereof.
TABLE 1__________________________________________________________________________ a/2.sqroot.2 Melting Temp. @ vap. pressureRE.sub.2 O.sub.3 Structure a or point (RE) 10.sup.-6 Torr 10.sup.-4 Torrtype type (.ANG.) a/.sqroot.2 data for pure metals (.degree. C.)__________________________________________________________________________Y.sub.2 O.sub.3 cubic 10.604 3.750 1509 973 1157La.sub.2 O.sub.3 cubic 11.327 4.005 920 1212 1388CeO.sub.2 cubic 5.411 3.827 795 1150 1380PrO.sub.1.83 cubic 5.47 3.868 931 950 1150Nd.sub.2 O.sub.3 cubic 11.08 3.918 1024 871 1062Sm.sub.2 O.sub.3 cubic 10.927 3.864 1072 460 573Eu.sub.2 O.sub.3 cubic 10.868 3.843 822 360 480Gd.sub.2 O.sub.3 cubic 10.813 3.824 1312 900 1175Tb.sub.2 O.sub.3 cubic 10.730 3.794 1357 950 1150Dy.sub.2 O.sub.3 cubic 10.665 3.771 1409 750 900Ho.sub.2 O.sub.3 cubic 10.606 3.750 1470 770 950Er.sub.2 O.sub.3 cubic 10.548 3.730 1497 775 930Tm.sub.2 O.sub.3 cubic 10.487 3.708 1545 554 680Yb.sub.2 O.sub.3 cubic 10.436 3.690 824 590 690Lu.sub.2 O.sub.3 cubic 10.390 3.674 1652 1300(Y.sub.0.95 Eu.sub.0.05).sub.2 O.sub.3 cubic 10.604 3.750Y.sub.2 Ce.sub.2 O.sub.7 cubic 5.370 3.798(Gd.sub.0.6 Ce.sub.0.4).sub.2 O.sub.3.2 cubic 10.858 3.839(Gd.sub.0.3 Ce.sub.0.7)O.sub.1.85 cubic 5.431 3.841__________________________________________________________________________
The present invention clearly demonstrates the epitaxial growth of of Y.sub.2 O.sub.3 and RE.sub.2 O.sub.3 layers on rolled-Ni and/or Cu substrates with or without additional buffer layers. The present invention also demonstrates new single and multi-buffer layer architectures for high current YBCO conductors.
Further, processing conditions such as those described hereinabove may be used to grow other buffer layers such as REAlO.sub.3 (RE=Rare Earth), AEZrO.sub.3 (AE=Ca,Sr,Ba), RE.sub.2 Zr.sub.2 O.sub.7 (RE=Rare Earth), Ca-stabilized Zirconia, Ti, Nb or Zr doped CeO.sub.2, and AECeO.sub.3 (AE=Ca,Sr,Ba) on rolled substrates. Other metallic substrates may also be used for growing these buffer layers.
Various techniques which can be used to deposit these buffer layers include but are not limited to: physical vapor deposition techniques which include electron-beam evaporation, rf magnetron sputtering, pulsed laser deposition, thermal evaporation, and solution precursor approaches, which include chemical vapor deposition, combustion CVD, metal-organic decomposition, sol-gel processing, and plasma spray.
While the preferred embodiments of the invention have been shown and disclosed, it will be obvious to those skilled in the art that various changes and modifications can be made therein without departing from the scope of the invention defined by the appended claims.

Buffer layers on metal surfaces having biaxial texture as superconductor substrates

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Wu et al, Appl. Phys. Lett., 65 (15) Oct. 10,1994, p. 1961.
Paranthaman et al, Physica, C 275, 1997, pp. 266-272, (no month).