U.S. patent application number 09/409120 was filed with the patent office on 2002-09-26 for method of depositing buffer layers on biaxially textured metal substrates.
Invention is credited to BEACH, DAVID B., CHIRAYIL, THOMAS G., GOYAL, AMIT, MORRELL, JONATHAN S., PARATHAMAN, MARIAPPAN, SPECHT, ELIOT D..
Application Number | 20020134300 09/409120 |
Document ID | / |
Family ID | 27020509 |
Filed Date | 2002-09-26 |
United States Patent
Application |
20020134300 |
Kind Code |
A1 |
BEACH, DAVID B. ; et
al. |
September 26, 2002 |
METHOD OF DEPOSITING BUFFER LAYERS ON BIAXIALLY TEXTURED METAL
SUBSTRATES
Abstract
A laminate article comprises a substrate and a biaxially
textured (RE.sup.1.sub.xRE.sup.2.sub.(1-x)).sub.2O.sub.3 buffer
layer over the substrate, wherein 0<x<1 and RE.sup.1 and
RE.sup.2 are each selected from the group consisting of Nd, Sm, Eu,
Ho, Er, Lu, Gd, Tb, Dy, Tm, and Yb. The
(RE.sup.1.sub.xRE.sup.2.sub.(1-x)).sub.2O.sub.3 buffer layer can be
deposited using sol-gel or metal-organic decomposition. The
laminate article can include a layer of YBCO over the
(RE.sup.1.sub.xRE.sup.2.sub.(1-x)).sub.2O.sub.3 buffer layer. A
layer of CeO.sub.2 between the YBCO layer and the
(RE.sup.1.sub.xRE.sup.2.sub.(1-x- )).sub.2O.sub.3 buffer can also
be include. Further included can be a layer of YSZ between the
CeO.sub.2 layer and the (RE.sup.1.sub.xRE.sup.2.-
sub.(1-x)).sub.2O.sub.3 buffer layer. The substrate can be a
biaxially textured metal, such as nickel. A method of forming the
laminate article is also disclosed.
Inventors: |
BEACH, DAVID B.; (KNOXVILLE,
TN) ; MORRELL, JONATHAN S.; (KNOXVILLE, TN) ;
PARATHAMAN, MARIAPPAN; (KNOXVILLE, TN) ; CHIRAYIL,
THOMAS G.; (KNOXVILLE, TN) ; SPECHT, ELIOT D.;
(KNOXVILLE, TN) ; GOYAL, AMIT; (KNOXVILLE,
TN) |
Correspondence
Address: |
GREGORY A NELSON
AKERSON, SENTERFITT & EIDSON
222 LAKEVIEW AVENUE Suite 400
PO Box 3188
WEST PALM BEACH
FL
33402-3188
US
|
Family ID: |
27020509 |
Appl. No.: |
09/409120 |
Filed: |
September 30, 1999 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09409120 |
Sep 30, 1999 |
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08922173 |
Sep 2, 1997 |
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6077344 |
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Current U.S.
Class: |
117/3 |
Current CPC
Class: |
C23C 18/1216 20130101;
C30B 25/02 20130101; C23C 18/1225 20130101; C21D 2201/05 20130101;
C21D 8/12 20130101; C30B 29/22 20130101; C23C 18/1275 20130101;
C30B 5/00 20130101; C23C 14/028 20130101; H01L 39/2461 20130101;
C23C 18/1254 20130101; C30B 29/16 20130101; C30B 23/02 20130101;
C23C 18/1241 20130101; C21D 2201/04 20130101; C23C 18/1279
20130101 |
Class at
Publication: |
117/3 |
International
Class: |
C30B 001/00; C30B
003/00; C30B 005/00; C30B 028/02 |
Goverment Interests
[0002] This invention was made with government support under
contract DE-AC05-96OR22464, awarded by the United States Department
of Energy to Lockheed Martin Energy Research Corporation, and the
United States Government has certain rights in this invention.
Claims
What is claimed is:
1. A laminate article, comprising: a substrate; a biaxially
textured (RE.sup.1.sub.xRE.sup.2.sub.(1-x)).sub.2O.sub.3 buffer
layer over said substrate, wherein 0<x<1 and RE.sup.1 and
RE.sup.2 are each selected from the group consisting of Nd, Sm, Eu,
Ho, Er, Lu, Gd, Tb, Dy, Tm, and Yb.
2. The laminate article according to claim 1, wherein x=1 .
3. The laminate article according to claim 1, wherein said
(RE.sup.1.sub.xRE.sup.2.sub.(1-x)).sub.2O.sub.3 buffer layer is
deposited using sol-gel.
4. The laminate article according to claim 1, wherein said
(RE.sup.1.sub.xRE.sup.2.sub.(1-x)).sub.2O.sub.3 buffer layer is
deposited using metal-organic decomposition.
5. The laminate article according to claim 1, further comprising a
layer of YBCO over said
(RE.sup.1.sub.xRE.sup.2.sub.(1-x)).sub.2O.sub.3 buffer layer.
6. The laminate article according to claim 5, wherein said YBCO
layer is on a surface of said
(RE.sup.1.sub.xRE.sup.2.sub.(1-x)).sub.2O.sub.3 buffer layer.
7. The laminate article according to claim 5, further comprising a
layer of CeO.sub.2 between said YBCO layer and said
(RE.sup.1.sub.xRE.sup.2.sub- .(1-x)).sub.2O.sub.3 buffer layer.
8. The laminate article according to claim 7, further comprising a
layer of YSZ between said CeO.sub.2 layer and said
(RE.sup.1.sub.xRE.sup.2.sub.- (1-x)).sub.2O.sub.3 buffer layer.
9. The laminate article according to claim 1, wherein said
substrate is biaxially textured.
10. The laminate article according to claim 9, wherein said
substrate is selected from the group consisting of nickel, copper,
iron, aluminum, and alloys containing any of the foregoing.
11. A laminate article, comprising: a substrate ; a biaxially
textured buffer layer over said substrate, wherein the buffer layer
is selected from the group consisting of Y.sub.2O.sub.3 and
CeO.sub.2.
12. The laminate article according to claim 11, wherein said buffer
layer is deposited using sol-gel.
13. The laminate article according to claim 11, wherein said buffer
layer is deposited using metal-organic decomposition.
14. The laminate article according to claim 1, wherein said metal
substrate is biaxially textured.
15. The laminate article according to claim 9, wherein said metal
substrate is nickel.
16. A method of forming a buffer layer on a metal substrate,
comprising the steps of: coating a substrate with a coating
solution; pyrolyzing said coating solution to form a biaxially
textured (RE.sup.1.sub.xRE.sup.2.sub.(1-x)).sub.2O.sub.3 buffer
layer over the substrate, wherein 0<x<1 and RE.sup.1 and
RE.sup.2 are each selected from the group consisting of Nd, Sm, Eu,
Ho, Er, Lu, Gd, Tb, Dy, Tm, and Yb.
17. The method according to claim 16, further comprising the step
of cold rolling the metal substrate to form a biaxially textured
metal substrate, said cold rolling step before said coating
step.
18. The method according to claim 16, further comprising the step
of sonification of the metal substrate before said coating
step.
19. The method according to claim 16, wherein the coating solution
is a rare earth methoxyethoxide in 2-methoxyethanol.
20. The method according to claim 19, wherein the rare earth is
selected from the group consisting of Nd, Sm, Eu, Ho, Er, Lu, Gd,
Tb, Dy, Tm, and Yb.
21. The method according to claim 16, wherein said pyrolyzing step
includes heating the coating solution to between about
600-800.degree. C.
22. The method according to claim 16, wherein RE.sup.1 and RE.sup.2
are each selected from the group consisting of Sm, Eu, Ho, Er, Lu,
Gd, Tb, Dy, Tm, and Yb and said pyrolyzing step includes heating
the coating solution to between about 600-1000.degree. C.
23. The method according to claim 16, wherein RE.sup.1 and RE.sup.2
are each selected from the group consisting of Eu, Ho, Er, Lu, Gd,
Tb, Dy, Tm, and Yb and said pyrolyzing step includes heating the
coating solution to between about 600-1100.degree. C.
24. The method according to claim 16, wherein RE.sup.1 and RE.sup.2
a are each selected from the group consisting of Ho, Er, Lu, Gd,
Tb, Dy, Tm, and Yb and said pyrolyzing step includes heating the
coating solution to between about 600-1200.degree. C.
25. The method according to claim 16, wherein RE.sup.1 and RE.sup.2
are each selected from the group consisting of Ho, Er, Lu, Tb, Dy,
Tm, and Yb and said pyrolyzing step includes heating the coating
solution to between about 600-800.degree. C.
26. The method according to claim 16, wherein x=1.
27. The method according to claim 16, wherein the substrate is
biaxially textured.
28. The method according to claim 27, wherein the substrate is
selected from the group consisting of nickel, copper, iron,
aluminum, and alloys containing any of the foregoing.
29. The method according to claim 16, wherein said pyrolizing step
is in a reducing atmosphere.
30. The method according to claim 29, wherein said pyrolyzing step
further includes introducing at least one of water or oxygen gas
into the atmosphere to reduce processing temperatures during said
pyrolizing step.
31. A method of forming a buffer layer on a metal substrate,
comprising the steps of: coating a substrate with a coating
solution; pyrolyzing said coating solution to form a biaxially
textured buffer layer over the substrate, wherein the buffer layer
is selected from the group consisting Of Y.sub.2O.sub.3 and
CeO.sub.2.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This is a Continuation-In-Part of application Ser. No.
08/922,173 filed Sep. 2, 1997.
FIELD OF THE INVENTION
[0003] This invention, relates to biaxially textured metal oxide
buffer layer on metal substrates. More specifically, the invention
relates to a non-vacuum process for depositing single epitaxial
films of rare-earth oxides on metal substrates.
BACKGROUND OF THE INVENTION
[0004] Biaxially textured metal oxide buffer layers on metal
substrates are potentially useful in electronic devices where an
electronically active layer is deposited on the buffer layer. The
electronically active layer may be a superconductor, a
semiconductor, or a ferroelectric material.
[0005] For example, the next generation of superconducting wire to
be used for power transmission lines will have a multi-layer
composition. Such deposited conductor systems consist of a metal
substrate, buffer layer, and a superconducting layer. The metal
substrate, such as Ni, Ag, or Ni alloys, provides flexibility and
support for the wire. Metal oxide buffer layers, such as cerium
oxide (CeO.sub.2), or yttria-stabilized zirconia (YSZ), comprise
the next layer and serve as chemical barriers between the metal
substrate and the top layer, the high-temperature
superconductor.
[0006] For a superconducting film to carry a high current, a
certain degree of alignment between grains of the superconductor is
required. Most preferably, the grains should be aligned both
perpendicular to the plane of the substrate (c-axis oriented) and
parallel to the plane of the substrate (a-b alignment). To achieve
this alignment, high T.sub.c superconductors have generally been
deposited on (100) oriented single-crystal oxide substrates.
However, single-crystal substrates are generally too expensive and
have poor mechanical properties. As such, single-crystal substrates
are presently unsuitable as practical conductors.
[0007] A method to develop practical coated conductors is disclosed
in U.S. Pat. No. 5,741,377 ('377) by Goyal et al. This method
called RABiTs, short for rolling assisted biaxially textured
substrates, uses roll-texturing of metal to form a metallic tape
with a {100}<001> cubic structure. However, if the metal is
nickel or a nickel alloy, a buffer layer between the metal it
substrate and the ceramic superconductor is necessary to prevent
interdiffusion of the ceramic superconductor and the metal
substrate and also to prevent the oxidation of nickel substrate
during the deposition of the superconducting layer. Useful buffer
layers include cerium oxide, yttrium stabilized zirconia (YSZ),
strontium titanium oxide, rare-earth aluminates and various
rare-earth oxides.
[0008] To achieve high critical current densities, it is important
that the biaxial orientation be transferred from the substrate to
the superconducting material. As stated, a biaxially textured metal
substrate can be provided by the method disclosed in the '377
patent. The conventional processes that are currently being used to
grow buffer layers on metal substrates and achieve this transfer of
texture are vacuum processes such as pulsed laser deposition,
sputtering, and electron beam evaporation. Researchers have
recently used such techniques to grow biaxially textured
YBa.sub.2Cu.sub.3O.sub.x (YBCO) films on metal substrate/buffer
layer samples that have yielded critical current densities
(J.sub.c) between 700,000 and 10.sup.6 A/cm.sup.2 at 77.degree. K
(A. Goyal, et al., "Materials Research Society Spring Meeting, San
Francisco, Calif., 1996; X. D. Wu, et al., Appl. Phys. Lett.
67:2397, 1995). One drawback of such vacuum processes is the
difficulty of coating long or irregularly shaped substrates, and
the long reaction times and relatively high temperatures
required.
[0009] A further consideration during the fabrication process is
the undesirable oxidation of the metal substrate (for example, when
using Ni). If the Ni begins to oxidize, the resulting NiO will
likely to grow in the (111) orientation regardless of the
orientation of the Ni (J. V. Cathcart, et al., J. Electrochem. Soc.
116:664, 1969). This (111) NiO orientation adversely affects the
growth of biaxially textured layers and will be transferred,
despite the substrate's original orientation, to the following
layers.
[0010] For producing high current YBCO conductors on
{100}<001> textured Ni substrates, high quality buffer layers
are necessary. Buffer layers such as CeO.sub.2 and YSZ have
previously been deposited using pulsed laser ablation, e-beam
evaporation, and sputtering. In addition, solution techniques have
been used to deposit films of rare-earth aluminates on biaxially
textured nickel substrates. However, the rare-earth aluminates had
c-axis alignment but has always given a mixture of two epitaxies
(100) [001] and (100) [011]. This is a structure believed to be
unsuitable for growth of high critical current YBCO films.
SUMMARY OF THE INVENTION
[0011] It is an object of the invention to provide a new and
improved method for fabricating alloy and laminated structures
having epitaxial texture.
[0012] It is another object of the invention to provide a method to
produce epitaxial superconductors on metal alloys and laminated
structures having epitaxial texture.
[0013] It is yet another object of the invention to provide a
non-vacuum process to produce epitaxial buffer layers on metal
substrates.
[0014] It is a further object of the invention to provide a process
for growing rare-earth oxide buffer layers with single in-plane
epitaxy.
[0015] Another object of the invention is to provide an epitaxial
textured laminate using rare-earth oxides.
[0016] Still another object of the invention is to provide an
epitaxial textured superconducting structure having a J.sub.c of
greater than 100,000 A/cm.sup.2 at 77 K and self-field.
[0017] Yet another object of the invention is to provide a solution
process for producing single cube oriented oxide buffer layers,
such as cerium oxide.
[0018] These and other objects of the invention are achieved by the
subject method and product.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] There are shown in the drawings embodiments of the invention
that are presently preferred, it being understood, however, that
the invention is not limited to the precise arrangements and
instrumentalities shown, wherein:
[0020] FIG. 1 is a flow diagram illustrating method steps according
to a first embodiment of the invention.
[0021] FIG. 2 is a theta-2-theta scan of the c-axis oriented
Gd.sub.2O.sub.3 film on Ni {100}<001> substrate.
[0022] FIG. 3 is an omega scan of the Ni (002) reflection
(FWHM=7.12.degree.) of a 600 .ANG. thick Gd.sub.2O.sub.3 film on a
roll-textured Ni substrate.
[0023] FIG. 4 is an omega scan of the Gd.sub.2O.sub.3 (004)
reflection (FWHM=8.56.degree.) of a 600 .ANG. thick Gd.sub.2O.sub.3
film on a roll-textured Ni substrate.
[0024] FIG. 5 is a phi scan of the Ni (111) reflection
(FWHM=8.39.degree.) of a 600 .ANG. thick Gd.sub.2O.sub.3 film on
roll-textured Ni substrate.
[0025] FIG. 6 is a phi scan of the Gd.sub.2O.sub.3 (222) reflection
(FWHM=9.74.degree.) of a 600 .ANG. thick Gd.sub.2O.sub.3 film on a
roll-textured Ni substrate.
[0026] FIG. 7a is the (222) pole figures observed for a 600 .ANG.
thick Gd.sub.2O.sub.3 film on Ni (100) substrate.
[0027] FIG. 7b is the orientation image micrograph of the film in
FIG. 7a.
[0028] FIG. 7c is the orientation image micrographs on the film in
FIG. 7b with different gray scale shadings.
[0029] FIG. 8 is a theta-2-theta scan of c-axis oriented
Yb.sub.2O.sub.3 film on Ni (100) substrate.
[0030] FIG. 9 is an omega scan of the Ni (002) reflection
(FWHM=7.06.degree.) of a 2000 .ANG. thick Yb.sub.2O.sub.3 film on a
roll-textured Ni substrate.
[0031] FIG. 10 is an omega scan of the Yb.sub.2O.sub.3 (004)
reflection (FWHM=9.89.degree.) of a 2000 .ANG. thick
Yb.sub.2O.sub.3 film on a roll-textured Ni substrate.
[0032] FIG. 11 is a phi scan of the Ni (111) reflection
(FWHM=8.45.degree.) of a 2000 .ANG. thick Yb.sub.2O.sub.3 film on a
roll-textured Ni substrate.
[0033] FIG. 12 is a phi scan of the Yb.sub.2O.sub.3 (222)
reflection (FWHM=10.11.degree.) of a 2000 .ANG. thick
Yb.sub.2O.sub.3 film on a roll-textured Ni substrate.
[0034] FIG. 13 is the Yb.sub.2O.sub.3 (222) pole figure of a 2000
.ANG. thick sol-gel Yb.sub.2O.sub.3 film on roll-textured Ni
substrate.
[0035] FIG. 14 is a structural stability relationship graph for
Re.sub.2O.sub.3 wherein symbol A represents the hexagonal
structure, symbol B represents the monoclinic structure, and symbol
C represents the cubic structure.
[0036] FIG. 15 is a schematic of a reel-to-reel continuous
dip-coating unit.
[0037] FIG. 16 is a schematic of a high J.sub.c YBCO film having as
layers: YBCO, CeO.sub.2, YSZ, Re.sub.2O.sub.3, and Ni.
[0038] FIGS. 17a and 17b are an omega scan of the YBCO (006)
reflection (FWHM=6.9.degree.), and a phi scan of the YBCO (103)
reflection (FWHM=10.5.degree.), respectively of the film in FIG.
16.
[0039] FIGS. 18a and 18b are an omega scan of the YSZ (002)
reflection (FWHM=7.9.degree.), and a phi scan of the YSZ (111)
reflection (FWHM=10.9.degree.), respectively of the film in FIG.
16.
[0040] FIGS. 19a and 19b are an omega scan of the EU.sub.2O.sub.3
(004) reflection (FWHM=8.0.degree.), and a phi scan of the
EU.sub.2O.sub.3 (222) reflection (FWHM=10.8.degree.), respectively
of the film in FIG. 16.
[0041] FIGS. 20a and 20b are an omega scan of the Ni (002)
reflection (FWHM=7.8.degree.), and a phi scan of the Ni (111)
reflection (FWHM=10.3.degree.), respectively of the film in FIG.
16.
[0042] FIG. 21 is a graph illustrating the field dependence of the
JC for 300 nm thick film having an architecture according to FIG.
16 where RE=Eu.
[0043] FIG. 22 is a schematic of a high J.sub.c YBCO film having as
layers: YBCO, RE.sub.2O.sub.3, and Ni.
[0044] FIG. 23 is a schematic of a high J.sub.c YBCO film having as
layers: YBCO, CeO.sub.2, RE.sub.2O.sub.3, and Ni.
DETAILED DESCRIPTION OF THE INVENTION
[0045] Referring to FIG. 1, a method for depositing single
epitaxial films of rare-earth oxides on metal substrates, according
to the present invention, is illustrated. The method comprises
preparing a biaxially textured metal substrate, preparing a
rare-earth oxide coating solution, coating the metal substrate with
the coating solution, and heat treating the metal substrate and
solution to pyrolyze the coating solution and to form a rare-earth
oxide on the substrate.
[0046] Prepare Substrate
[0047] Any method of preparing a biaxially textured metal substrate
is acceptable for use with this invention. However, the presently
preferred method of preparing the biaxially textured metal
substrate is disclosed in U.S. Pat. No. 5,741,377 by Goyal et al.,
which is incorporated herein by reference. The biaxial texture is
achieved, for example, by cold rolling high purity (99.99%) nickel
rod in a rolling mill until the length of the rod had been
increased by a factor of about 20 (deformation was over 95%). The
desired cubed texture {100}<001> was developed by
recrystallization of the rolled Ni at 800.degree. C. for 60-120
minutes at a pressure less than 10.sup.-7 torr or at 900.degree. C.
for 60 minutes in a flow of 4% H.sub.2/Argon gas mixture. Other
preferred materials include, but are not limited to copper, iron,
aluminum, and alloys containing any of the foregoing, including
nickel.
[0048] Clean substrate
[0049] Prior to coating the metal substrate with the rare-earth
oxide coating solution, the metal substrate is preferably cleaned
to remove any organics on the metal substrate. Any method of
removing organics from the metal substrate is acceptable for use
with this invention. For example, the organics can be removed by
methods such as vacuum annealing, electro-polishing, or reverse
sputtering. However, the presently preferred method of removing
organics from the metal substrate is to ultrasonically clean the
metal substrate in a cleaning solution.
[0050] Any cleaning solution capable of being used during
ultrasonification is acceptable for use with this invention;
however, the presently preferred cleaning solution is isopropanol.
The invention is not limited as to a particular length of time in
which the metal substrate is ultrasonically cleaned so long as the
organics are removed from the metal substrate. A preferred range of
time is between about 5-60 minutes, and a most preferred length of
time is about 60 minutes.
[0051] Prepare Solution
[0052] Many different methods of preparing a coating solution for
use with the invention are known. Three commonly used solution
preparation techniques are as follows: (i) sol-gel processes that
use metal alkoxide complexes in alcohol solution; (ii) hybrid
processes that use chelating agents such as acetylacetonate or
diethanolamine to reduce alkoxide reactivity; and (iii)
metal-organic decomposition (MOD) techniques that use
high-molecular-weight precursors and water-insensitive
carboxylates, 2-ethyl-hexanoates, naphthanates, etc. in an organic
solvent. Although the coating solution can be prepared using any of
these methods, any method capable of producing a coating solution
capable of being coated on a metal substrate and subsequently
capable of forming a rare-earth oxide on the substrate is
acceptable for use with this invention. Additionally, the coating
solution can be prepared using any combination of the three methods
discussed above or with any other method that requires solution
precursors.
[0053] In the presently preferred embodiment of invention,
rare-earth alkoxide precursors were used in 2-methoxyethanol. The
preferred alkoxide being rare-earth methoxyethoxides. An
illustrative example of the method is as follows. The rare-earth
isopropoxides is reacted with 2-methoxyethanol under an inert
atmosphere. After refluxing, a portion of the solution is removed
by distillation. The remaining solution is then cooled and
additional 2-methoxyethanol is added. The solution was again
refluxed, and further portion of the solution was removed by
distillation. The process of dilution, reflux, and distillation is
repeated for a total of three cycles to ensure the complete
exchange of the methoxyethoxide ligand for the isopropoxide
ligand.
[0054] The final concentration of the solution is adjusted to
obtain a 0.5 M solution of rare-earth methoxyethoxide in
2-methoxyethanol. The final coating solutions is prepared by
reacting 1 part of a 1.0 molar solution of water in
2-methoxyethanol with 4 parts of the 0.5 M rare-earth
methoxyethoxide solution. Hydrolysis was not necessary in some
instances.
[0055] Applying the Coating Solution to the Metal Substrate
[0056] Any method of applying the coating solution to the metal
substrate is acceptable for use with this invention. However, two
preferred methods of applying the coating solution to the metal
substrate are (i) spin coating and (ii) dip coating. For either of
the two preferred methods, the metal substrate can be dipped in a
controlled atmosphere or in air.
[0057] Spin coating involves spinning the metal substrate at high
revolutions per minute (RPM), for example approximately 2,000 RPM,
applying the solution onto the metal substrate. Equipment capable
of spin coating is known in the art as a spinner. For example
spinners are used during semiconductor manufacturing to apply
photo-resist to semiconductor wafers. However, the invention is not
limited as to a particular type of spinner. Any spinner capable of
applying a coating solution to the metal substrate is acceptable
for use with this invention. Additionally, so long as coating
solution is applied to the metal substrate with the desired
thickness and uniformity, the invention is not limited as to any
particular process parameters for use with the spinner. In a
preferred embodiment of the invention, however, the spinner is
operated at about 2000 RPM for a period of about 30 seconds to
obtain a continuous coating.
[0058] Although any equipment can be used to dip coat the coating
solution onto the metal substrate, the preferred equipment is a
reel-to-reel dip coating unit as illustrated in FIG. 18. The
reel-to-reel dip-coating unit 20 includes a pay-out reel 22, a
solution container 24, pulleys 26, and a take-up reel 28. The
pay-out reel 22 provides the metal substrate 30 for dipping. The
solution container 24 contains the coating solution 32, and the
pulleys 26 direct the metal substrate 30 into the coating solution
32 and onto the take-up reel 28.
[0059] Also included can be a furnace 34 for heat treatment of the
metal substrate 30 and coating solution 32. The furnace 34 is
disposed between the solution container 24 and the take-up reel 28.
The take-up reel 28 acts to retrieve the metal substrate 30 after
being coated with the coating solution 32.
[0060] The rate at which the metal substrate 30 is withdrawn from
the coating solution 32 depends upon the desired thickness and
concentration of the coating solution 32 on the metal substrate 30.
As the rate of withdrawal increases, at a given point, depending on
the solution and the substrate, the amount of coating solution 32
applied to the metal substrate 30 increases. However, so long as
the coating solution 32 is applied to the metal substrate 30 with
the desired thickness and consistency, the invention is not limited
as to any particular withdrawal rate. In a preferred embodiment of
the invention, however, the metal substrate is withdrawn at a rate
of about 3cm/min.
[0061] Heat Treatment
[0062] The heat treatment process pyrolyzes the coating solution
thereby leaving the rare-earth oxide remaining on the metal
substrate. The enclosure containing the metal substrate is
preferably purged with a reducing atmosphere prior to the beginning
of the heat treatment process. Purging the container prior to heat
treatment removes undesirable contaminants from the atmosphere
within the enclosure. During the heat treatment process, the metal
substrate is preferably maintained in a reducing atmosphere to
prevent any oxidation of the metal substrate. An inert atmosphere
may also be preferably maintained around the metal substrate during
cooling. Also, by maintaining the reducing atmosphere around the
metal substrate during cooling, oxidation of the metal substrate
can be prevented.
[0063] The heat treatment process is for a combination of time and
temperature sufficient to pyrolyze the coating solution and leaves
the desired crystal structure of the rare earth oxide. Any time and
temperature combination sufficient to pyrolyze the coating solution
and leave the desired crystal structure of the rare earth oxide is
acceptable for use with the invention. A more detailed discussion
as to the preferred temperature ranges for the various rare earth
oxide compounds is included below.
[0064] During the heat treatment process, low partial pressures of
water and/or oxygen gas can be introduced into the atmosphere
surrounding the metal substrate. The addition of water and/or
oxygen gas acts as a catalyst for pyrolyzing the coating solution
at lower temperatures. Thus, the introduction of low partial
pressures of water of oxygen gas into the atmosphere advantageously
allows for a lower processing temperature.
[0065] Hydrogen containing atmospheres are the preferred
atmospheres for the heat treatment of the coated substrates, with
4% v/v hydrogen in argon, helium, or nitrogen the most preferred
atmosphere for safety reasons. Mixtures of 2-6% v/v hydrogen are
commonly referred to as "forming gas" and are not generally
combustible under most conditions. Carbon monoxide/carbon dioxide
mixtures are also commonly used as gaseous reducing its agents.
[0066] Any furnace capable of producing the desired temperature and
time parameters is acceptable for use with this invention.
Additionally, any enclosure for the metal substrate capable of
preventing contamination of the metal substrate is acceptable for
use with this invention. However, the presently preferred enclosure
is equipped with gas fixtures for receiving the reducing
atmosphere.
[0067] An illustrative example of the preferred heat treatment
process follows, it is being understood that the practice of the
invention is not limited in this manner. The coated metal substrate
is placed in a quartz tube equipped with a gas inlet and outlet. A
bottled gas mixture containing 4% hydrogen in 96% argon is then
allowed to flow into the quartz tube for 20-30 minutes at room
temperature. At the same time, the furnace is preheated to the
desired temperature. The quartz tube is then introduced into the
furnace and heated for a period of approximately one hour. After
heating, the metal substrate is quenched to room temperature by
removing the quartz tube from the furnace. During quenching the
flow of 4% hydrogen in 96% argon gas mixture is maintained.
[0068] Crystal Structure
[0069] Three types of crystal structures are known for the rare
earth oxides (Henry R. Hoekstra, Inorg. Chem. 5, 755 (1966)). These
crystal structures are hexagonal (type A), monoclinic (type B), and
cubic (type C). Type A (hexagonal) has a space group of p3ml and
contains one molecule per unit cell or P63/MMC, containing two
molecules per unit. Each trivalent cation is bonded to seven oxygen
atoms (four short bonds and three long bonds), while the two types
of oxygen atoms are bonded to five and four metal atoms,
respectively.
[0070] Type B (monoclinic) has a space group of C2/m, and contains
six molecules per unit cell, and also shows seven-fold cation
coordination. The crystal lattice has three different cation sites
and five different anion sites which bond to four, five or six
metal atoms.
[0071] Type C (cubic) has a space group of Ia3, with 16 molecules
per unit cell. It is derived from the fluorite (CaF.sub.2)
structure by doubling the lattice parameter and by removing one
fourth of the oxygen ions to maintain the charge neutrality between
RE.sup.3+and Q.sup.2-. The crystal lattice has two kinds of six
fold coordinated RE ions.
[0072] Structural stability relationships for these three structure
types for RE.sub.2O.sub.3 are shown in FIG. 17. The symbol A
represents the hexagonal structure, symbol B represents the
monoclinic structure, and symbol C represents the cubic structure.
The A-B phase boundary line is vertical, independent of temperature
and lies between neodymium and samarium. The B-C phase boundary is
fairly well understood, and further extension of the line to higher
temperatures is prevented by fusion of the rare earth oxides. Thus,
Y.sub.2O.sub.3 and the five heaviest rare earth oxides namely
Ho.sub.2O.sub.3, Er.sub.2O.sub.3, Tm.sub.2O.sub.3, Yb.sub.2O.sub.3,
and Lu.sub.2O.sub.3 exist in only the cubic form at ambient
pressures. Also, the B-C phase boundary is completely reversible
for pure Dy.sub.2O.sub.3, Tb.sub.2O.sub.3, and Gd.sub.2O.sub.3, and
for Eu.sub.2O.sub.3, and Sm.sub.2O.sub.3 in the presence of water
as catalyst. The exact locations of these phase boundaries for
RE.sub.2O.sub.3 films are under further investigation.
[0073] Temperature Table
[0074] Table 1 illustrates the approximate temperature ranges to
which the heat treating process will heat the rare-earth oxide
solution. The temperature range is equivalent to the temperature
range at which the as-grown oxide film has a cubic crystal
structure.
1TABLE 1 Temperature Temperature Composition Range (.degree. C.)
Composition Range (.degree. C.) Nd.sub.2O.sub.3 600-800
Gd.sub.2O.sub.3 600-1200 Sm.sub.2O.sub.3 600-1000 Tb.sub.2O.sub.3
600-1455 Eu.sub.2O.sub.3 600-1100 Dy.sub.2O.sub.3 600-1455
Ho.sub.2O.sub.3 600-1455 Tm.sub.2O.sub.3 600-1455 Er.sub.2O.sub.3
600-1455 Yb.sub.2O.sub.3 600-1455 Lu.sub.2O.sub.3 600-1455
CeO.sub.2** 600-1455 Y.sub.2O.sub.3* 600-1455 *Not rare-earth
oxides **Does not have RE.sub.2O.sub.3 structure.
EXAMPLE 1
[0075] Gadolinium isopropoxide was synthesized using the method of
Brown et al. The Gd.sub.2O.sub.3 precursor solution was prepared by
reacting 4.85 g (15 mmole) of gadolinium isopropoxide with 50 ml of
2-methoxyethanol under an inert atmosphere. After refluxing for
approximately 1 hour, two-thirds of the solution (isopropanol and
2-methoxyethanol) was removed by distillation. The flask was
allowed to cool, and 20 ml of 2-methoxyethanol was added. The flask
was refluxed for 1 hour, and 2/3 of the solution was then removed
by distillation. This process of dilution, reflux, and distillation
was repeated for a total of three cycles to ensure the complete
exchange of the methoxyethoxide ligand for the isopropoxide ligand.
It was estimated that this occurs when the boiling point of the
solution reaches 124.degree. C. The final concentration of the
solution was adjusted to obtain a 0.5 M solution of gadolinium
methoxyethoxide in 2-methoxyethanol. A partially hydrolyzed
solution suitable for spin coating or dip-coating was prepared by
reacting 1 part of a 1.0 molar solution of water in
2-methoxyethanol with 4 parts of the 0.5 M gadolinium
methoxyethoxide solution.
[0076] Strips of roll-textured nickel were cleaned by
ultrasonification for 1 hour in isopropanol that had been dried
over aluminum isopropoxide and distilled under inert gas. The
coating of the nickel was accomplished using spin coating at 2000
RPM for 30 seconds or dip coating with a withdrawal velocity of 3
cm/min.
[0077] The nickel substrates were placed in a quartz tube equipped
with a gas inlet and outlet. A bottled gas mixture containing 4%
hydrogen in 96% argon was allowed to flow for 20-30 minutes at room
temperature. At the same time, a tube furnace was preheated at the
desired temperature of 1160.degree. C. This temperature could be
reduced to 800-900.degree. C. by the addition of approximately 150
ppm of O.sub.2. The quartz tube containing the coated substrate was
then introduced into the furnace and heated for periods varying
from 5 minutes to 1 hour.
[0078] After heat treatment, the coated substrate was quenched to
room temperature by removing the quartz tube out of the furnace.
During this time, a gas mixture of 4% hydrogen and 96% argon was
continually flowing through the quartz tube.
[0079] Highly crystalline gadolinium oxide (Gd.sub.2O.sub.3) film
on roll-textured Ni substrates was obtained. The texture of films
were analyzed by X-ray diffraction (XRD), and film microstructure
was analyzed using scanning electron microscope (SEM), and electron
back scatter Kikuchi patterns (EBKP).
[0080] FIGS. 2-7 illustrate the XRD data for a 600 .ANG. thick
sol-gel grown Gd.sub.2O.sub.3 film that was heat-treated at
1160.degree. C. in a flowing gas mixture of 4% H.sub.2 and 96% Ar
on roll-textured Ni substrate. The strong (004) peak of
Gd.sub.2O.sub.3 in FIG. 2 indicates the presence of a strong c-axis
aligned film. The omega and phi scans of FIGS. 3-6 and the
Gd.sub.2O.sub.3 (222) pole figures of FIG. 7 indicate the presence
of a single in-plane textured Gd.sub.2O.sub.3 film. A SEM
micrograph indicated the presence of a dense and crack-free
microstructure. FIG. 7 shows the(222) pole figures observed for
Gd.sub.2O.sub.3 on textured Ni. A single orientation of
Gd.sub.2O.sub.3 film is evident. The orientation image micrographs
for the Gd.sub.2O.sub.3 film shown in FIG. 7 indicate the grains
are well connected by boundaries less than 6 degrees.
EXAMPLE 2
[0081] Ytterbium isopropoxide was synthesized using the method of
Brown et al. A flask containing 4.85 g (15 mmole) of ytterbium
isopropoxide was reacted with 50 ml of 2-methoxyethanol under an
inert atmosphere. After refluxing for approximately 1 hour,
two-thirds of the solution (isopropanol and 2-methoxyethanol) was
removed by distillation. The flask was allowed to cool, and 20 ml
of 2-methoxyethanol was added. The flask was refluxed for 1 hour,
and 2/3 of the solution was then removed by distillation. This
process of dilution, reflux, and distillation was repeated for a
total of three cycles to ensure the complete exchange of the
methoxyethoxide ligand for the isopropoxide ligand. It was
estimated that this occurs when the boiling point of the solution
reaches 124.degree. C. The final concentration of the solution was
adjusted to obtain a 0.5 M solution of ytterbium methoxyethoxide in
2-methoxyethanol. Coating solutions were prepared by reacting 1
part of a 1.0 molar solution of water in 2-methoxyethanol with 4
parts of the 0.5 M ytterbium methoxyethoxide solution.
[0082] Strips of roll-textured nickel were cleaned by
ultrasonification for 1 hour in isopropanol that had been dried
over aluminum isopropoxide. The coating of the nickel was
accomplished using spin coating at 2000 RPM for 30 seconds or dip
coating with a withdrawal velocity of 3 cm/min.
[0083] The nickel substrates were placed in a quartz tube equipped
with a gas inlet and outlet. A bottled gas mixture containing 4%
hydrogen in 96% argon was allowed to flow for 20-30 minutes at room
temperature. At the same time, a tube furnace was preheated at the
desired temperature of 1160.degree. C. This temperature could be
reduced to 800-900.degree. C. by the addition of approximately 150
ppm of O.sub.2. The quartz tube with the coated substrate was then
introduced into the furnace and heated for periods varying from 5
minutes to 1 hour.
[0084] After heat treatment, the coated substrate was quenched to
room temperature by removing the quartz tube out of the furnace.
During this time, a gas mixture of 4% hydrogen and 96% argon was
continually flowing through the quartz tube.
[0085] Highly crystalline ytterbium oxide (Yb.sub.2O.sub.3) film on
roll-textured Ni substrates was obtained. The texture of films was
analyzed by X-ray diffraction (XRD), and film microstructure was
analyzed using scanning electron microscope (SEM), and electron
backscatter Kikuchi patterns (EBKP).
[0086] FIGS. 8-13 illustrate the XRD data for a 600 .ANG. thick
sol-gel grown Yb.sub.2O.sub.3 film that was heat-treated at
1160.degree. C. in a flowing gas mixture of 4% H.sub.2 and 96% Ar
on roll-textured Ni substrate. The strong (004 peak of
Yb.sub.2O.sub.3 in FIG. 8 indicates the presence of a strong c-axis
aligned film. The omega and phi scans and Yb.sub.2O.sub.3 (222)
pole figures indicate the presence of a single in-plane textured
Yb.sub.2O.sub.3 film.
EXAMPLE 3
[0087] In a preferred embodiment of the invention, europium oxide
Eu.sub.2O.sub.3 films were epitaxially grown on textured Ni
substrates at around 1050.degree. C. in the presence of a 4%
hydrogen and 96% argon gas mixture. The metal substrate was dip
coated from a 0.1 to 0.5 M solution of europium methoxyethoxide in
2-methoxyethanol. The dip-coated substrate was placed in a quartz
tube and purged with a gas mixture of 4% hydrogen and 96% argon for
20-30 min at room temperature. At the same time, the furnace was
preheated to 1050.degree. C. The quartz tube with the metal
substrate was introduced into the furnace and heated for
approximately 1 hour.
[0088] After heat treatment, the coated substrate was quenched to
room temperature by removing the quartz tube out of the furnace.
During this time, a gas mixture of 4% hydrogen and 96% argon was
continually flowing through the quartz tube.
[0089] By introducing water (dew point of approximately 25.degree.
C.; wet gas) or low partial pressures of oxygen gas (preferably 100
mTorr oxygen) into the quartz tube along with 4% hydrogen and 96%
argon gas mixtures, textured Gd.sub.2O.sub.3 and Eu.sub.2O.sub.3
films have been produced at 800.degree. C.
EXAMPLE 4
[0090] FIG. 16 illustrates the following architecture used to
demonstrate the growth of a high J.sub.c YBCO film 40 having as
layers: YBCO 42, CeO.sub.2 44, YSZ 46, Eu.sub.2O.sub.3 48, and Ni
50. The YBCO layer 42 was applied using an ex-situ BaF.sub.2
process. The CeO.sub.2 44 and the YSZ 46 layers were applied using
sputtering. The Eu.sub.2O.sub.3 48 layer was applied by dip
coating.
[0091] After preparing a biaxially textured Ni substrate 50, the
substrate 50 was immersed in an Eu.sub.2O.sub.3 precursor solution
(Europium methoxyethoxide) and then withdrawn at a rate of 3 cm/min
in a linear dip-coating unit. After coating, the Ni substrate 50
was annealed in a mixture of 4% H.sub.2 and 96% Ar at a temperature
of 1060.degree. C. for one hour and quenched to room
temperature.
[0092] The rf magnetron sputtering technique was used to grow YSZ
46 and CeO.sub.2 44 cap layers on the Eu.sub.2O.sub.3-buffered Ni
substrates 48, 50 at 780.degree. C. The plasma power was 75 W at
13.56 MHz. The resulting YSZ 46 and CeO.sub.2 44 films were smooth
and dense. Precursor YBCO films 42 were grown on the
CeO.sub.2-buffered YSZ/Eu.sub.2O.sub.3 (dip-coated)/Ni substrates
44, 46, 48, 50 by electron beam co-evaporation of Y, EBaF.sub.2,
and Cu at a combined deposition rate of approximately 6
.ANG./sec.
[0093] The .theta.-2.theta. scan of the YBCO film 50 indicated the
presence of a c-axis aligned film. FIGS. 17a-20a and 17b-20b
respectively illustrate the XRD results from omega and phi scans on
the YBCO/CeO.sub.2/YSZ/Eu.sub.2O.sub.3 (dip-coated)/Ni. The FWHM
values for Ni (002) , Eu.sub.2O.sub.3 (004) , YSZ (002) and YBCO
(006) are 7.8.degree., 8.0.degree., 7.9.degree. and 6.9.degree.,
and those of Ni (111), Eu.sub.2O.sub.3 (222), YSZ (111) and YBCO
(103) are 10.3.degree., 10.8.degree., 10.9.degree. and
10.5.degree., respectively. The simulation indicates that the film
thickness for Eu.sub.2O.sub.3 48, YSZ 46, CeO.sub.2 44, and YBCO 42
are 60 nm, 295 nm, 10 nm and 300 nm, respectively. The room
temperature resistivity of the 300 nm thick YBCO film 40 on
CeO.sub.2/YSZ/Eu.sub.2O.sub.3 (dip-coated) /Ni was low and the
T.sub.c measured was about 90 K. The field dependence of J.sub.c
for the same film 40 is; shown in FIG. 21. The zero field I.sub.c
measured was 16 A which translates to a J.sub.c of 1.1 MA/cm.sup.2.
The J.sub.c at 0.5 T is about 20% of the zero field J.sub.c.
[0094] As illustrated in FIGS. 22 and 23, in addition to the
architecture illustrated in FIG. 16
(YBCO/CeO.sub.2/YSZ/RE.sub.2O.sub.3/Ni) described above, it. is
possible to develop two new alternative architectures: (A) YBCO 60,
RE.sub.2O.sub.3 62, and Ni 64 (FIG. 22) and (B) YBCO 70, CeO.sub.2,
72, RE.sub.2O.sub.3, 74, and Ni 76 (FIG. 23)
OTHER EXAMPLES
[0095] Textured Sm.sub.2O.sub.3 has been grown on Ni substrates at
900.degree. C. in the presence of 4% hydrogen and 96% argon gas
mixture using sol-gel alkoxide precursors. C-axis aligned mixed
rare earth oxides, (RE.sup.1RE.sup.2).sub.2O.sub.3 (for example,
(Sm.sub.0.6Eu.sub.0.4).sub.2O.sub.3) have also been grown at around
900.degree. C. on textured Ni substrates. In general, any solid
solution of any rare-earth element oxides may be used in this
process as long as the solution has a cubic structure in the
temperature range of 600 to 1455.degree. C. Also, more than two
rare earth element oxides with the cubic structure can be used. In
addition, epitaxial CeO.sub.2 films on EU.sub.2O.sub.3 (sol-gel)
have been grown on buffered Ni substrates at 1050.degree. C. using
Ce methoxyethoxide precursors. Epitaxial CeO.sub.2 was also grown
directly on textured Ni.
[0096] Epitaxial RE.sub.2O.sub.3 layers can also be applied using
multiple coating steps in which a first epitaxial RE.sub.2O.sub.3
layer is applied to a substrate and subsequent epitaxial
RE.sub.2O.sub.3 layers are applied on top of the first, layer using
the method according to the invention. Alternatively, epitaxial
islands of RE.sub.2O.sub.3 templates can be produced on textured Ni
substrates followed by a continuous layer of epitaxial
RE.sub.2O.sub.3 layers using the method according to the
invention.
* * * * *