Buffer layers on biaxially textured metal substrates

Details Buffer layers on biaxially textured metal substrates Buffer layers on biaxially textured metal substrates

1999-02-11

2001-05-22

Jones, Deborah (Department: 1775)

Stock material or miscellaneous articles

Composite

Of metal

C428S469000, C428S471000, C428S697000, C428S700000, C428S701000, C428S702000, C117S944000

Reexamination Certificate

active

06235402

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to the growth of a biaxially textured metal oxide buffer layer on metal substrates. More particularly, it relates to a sol-gel technique for growing such buffer layers on metal substrates.
2. Background of the Art
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.
For example, superconducting wire to be used for power transmission lines has a multi-layer composition
10
(FIG.
1
). Such deposited conductor systems consist of a metal substrate
11
, buffer layer(s)
12
, and a superconducting layer
13
. The metal substrate, such as Ni, Ag, or Ni alloys, provides flexibility for the wire and can be fabricated over long lengths and large areas. Metal oxide buffer layers, such as LaAlO
3
, CeO
2
, or yttria-stabilized zirconia (YSZ), comprise the next layer and serve as chemical barriers between the metal substrate and the last layer, the high-temperature superconductor.
To achieve high critical current densities from the wire, it is important that the superconducting material be biaxially oriented and strongly linked. The orientation necessary begins with the texture of the metal substrate which must be maintained in the buffer layer and thus transferred to the superconductor. 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
2
Cu
3
O
X
(YBCO) films on metal substrate/buffer layer samples that have yielded critical current densities between 700,000 and 10
6
A/cm
2
at 77° 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.
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 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.
Thus, it would be advantageous to have a process which avoids the above disadvantages of vacuum-based processes.
SUMMARY OF THE INVENTION
We have discovered a new non-vacuum based process by which biaxially textured buffer layers can be grown on metal substrates under conditions that reduce the potential for oxidation of the metal. This process also avoids the drawbacks of scaling up the prior art vacuum processes. This can be achieved using our novel sol-gel method described below.
Sol-gel is a technique where alkoxide solutions are spin-cast, dip-coated, or sprayed onto substrates in air. Sol-gel is advantageous because it is a relatively low-cost procedure, is capable of coating long length conductors or irregularly shaped substrates, and yields the desired orientation usually after shorter reaction times and lower temperatures than necessary in other processes.
One aspect of the invention provides a method of forming a buffer layer on a metal substrate, comprising the steps of: coating a biaxially oriented metal substrate with a sol-gel solution of a metal oxide; and heating the coated metal substrate in a reducing atmosphere under such conditions that the sol-gel solution forms a buffer layer which has an epitaxial orientation to that of the metal substrate. Preferably, the reducing atmosphere has low oxygen partial pressures (i.e., preferably no more than about 0.1% v/v). More preferably, the reducing atmosphere comprises an inert gas and hydrogen. Most preferably, the reducing atmosphere comprises argon and about 4% hydrogen.
The metal substrate is preferably selected from the group consisting of nickel, silver, copper, and non-magnetic substrates such as nickel alloys and copper alloys. The metal oxide of the sol-gel solution is preferably selected from the group consisting of alkaline earth zirconium oxides, lanthanum nickel oxide, yttrium europium oxide, strontium aluminum lanthanum tantalates, zirconium doped cerium oxide, cerium oxide, yttria-stabilized zirconia, strontium titanium oxide, lanthanum aluminum oxide, rare earth aluminum oxides, and yttrium aluminum oxides. Most preferably, the metal oxide of the sol-gel solution is lanthanum aluminum oxide or neodymium aluminum oxide.
Another aspect of the invention provides a method of making an electronic device, comprising the steps of: coating a biaxially oriented metal substrate with a sol-gel solution of a metal oxide; heating the coated substrate in a reducing atmosphere under such conditions that the sol-gel solution forms a buffer layer which has an epitaxial orientation to that of the metal substrate; and depositing an electronically active layer on the buffer layer. Preferably, the reducing atmosphere has low oxygen partial pressures (i.e., preferably no more than about 0.1% v/v). More preferably, the reducing atmosphere comprises an inert gas and hydrogen. Most preferably, the reducing atmosphere comprises argon and about 4% hydrogen. Advantageously, the electronically active layer can be superconductor, semiconductor, or ferroelectric materials.
The metal substrate is preferably selected from the group consisting of nickel, silver, copper, and non-magnetic substrates such as nickel alloys and copper alloys. The metal oxide of the sol-gel solution is preferably selected from the group consisting of alkaline earth zirconium oxides, lanthanum nickel oxide, yttrium europium oxide, strontium aluminum lanthanum tantalates, zirconium doped cerium oxide, cerium oxide, yttria-stabilized zirconia, strontium titanium oxide, lanthanum aluminum oxide, rare earth aluminum oxides, and yttrium aluminum oxides. Most preferably, the metal oxide of the sol-gel solution is lanthanum aluminum oxide or neodymium aluminum oxide.
A further aspect of the invention provides a metal having a biaxially oriented buffer layer and a superconductor made by methods of the above kind.
The objects of the invention, therefore, include providing a method of the above kind:
(a) which avoids the metal substrate oxidation problems of the prior art;
(b) which offers a relatively low cost procedure for coating long length conductors or irregularly shaped substrates;
(c) which yields the desired phase on the buffer layer with shorter reaction times and lower temperatures than necessary in the prior art; and
(d) which allows the orientation of the texture of the metal substrate to be maintained in the buffer layer and thus transferred to the electronically active layer.
These and still other objects and advantages of the present invention will be apparent from the description below. However, this description is only of the preferred embodiments. The claims should, therefore, be looked to in order to assess the whole scope of the invention.


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J.V. Cathcart, et al., “The structure of Thin Oxide Films Formed on Nickel Crystals”,J. Electrochem. Soc.116:664-668, 1969. (no month).
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