Epitaxial oxide films via nitride conversion

Details Epitaxial oxide films via nitride conversion Epitaxial oxide films via nitride conversion

2001-08-16

2003-11-11

Jones, Deborah (Department: 1775)

Stock material or miscellaneous articles

Composite

Of metal

C428S472000, C428S697000, C428S698000, C428S701000, C428S702000, C428S692100, C505S230000, C505S238000

Reexamination Certificate

active

06645639

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates generally to epitaxial metal oxide films and, more particularly, to those films and related composite structures as can be prepared by oxidizing a previously deposited nitride layer.
BACKGROUND OF THE INVENTION
Recent emergence of high-temperature superconducting (HTS) thick-film tape technology is expected to meet the cost, size and performance requirements of superconducting components needed for advanced power applications for the defense and commercial sectors. One of the major potential HTS applications is in the electric power industry.
The YBa
2
Cu
3
O
7
and related ceramic materials (YBCO) have appropriate intrinsic properties in the liquid nitrogen temperature range. However, their properties are drastically affected by grain boundary misorientations. In order to provide high temperature and field applications, it is imperative that a biaxially textured, polycrystalline YBCO tape, or related article, be developed which contains few high angle grain boundaries.
A biaxially textured, flexible metal/alloy substrate is formed by conventional thermomechanical processing followed by epitaxial deposition of buffer layer(s), YBCO grown on such substrate often exhibited J
c
's over 1 MA/cm
2
at 77 K. To date, the preferred buffer layers have been the combination of CeO
2
and yttria stabilized zirconia (YSZ). However, these oxide buffer layers lack important properties, e.g., electrical and thermal conductivity and mechanical toughness. It has been a challenging engineering task to develop a large-scale continuous process for producing thick (>0.5 &mgr;m) crack-and pore-free oxide films. Microcracking in oxide films is commonly observed in thick films due to the brittle nature of the oxide materials. Microcracks in the oxide layer can serve as open paths for oxygen to diffuse and oxidize the underlying metal during subsequent YBCO processing. Finally, the oxide deposition step on the Ni substrates is difficult; high quality films are only obtained by using very low deposition rates. In addition, as with many HTS applications, conductive buffer layers are needed since they would provide electrical coupling of the HTS layer to the underlying metallic tape substrate. This is an important property in order to electrically stabilize the conductor during transient loss of superconductivity in some applications.
Conventional ceramic fabrication methods which can be used to make a long, flexible conductor result in materials with weak, if any, macroscopic or microscopic biaxial texture. In particular, YBCO materials fabricated using conventional techniques invariably contain numerous high angle grain boundaries. High angle grain boundaries act as Josephson coupled weak-links leading to a significant field-dependent suppression of the supercurrent across the boundary. For clean stoichiometric boundaries, the grain boundary critical current density depends primarily on the grain boundary misorientation. The dependence of J
c
(gb) on misorientation angle was first determined by Dimos et al. in YBCO for grain boundary types that can be formed in epitaxial films on bicrystal substrates. These include [001] tilt, [100] tilt, and [100] twist boundaries. In each case high angle boundaries were found to be weak-linked. The low J
c
observed in randomly oriented polycrystalline HTS fabricated using conventional methods can be understood on the basis that the population of tow angle boundaries is small and that frequent high angle boundaries impede long-range current flow. Hence, controlling the grain boundary misorientation distribution towards low angles is key to fabricating high-J
c
materials.
Successful fabrication of biaxially textured superconducting wire based on the coated conductor technology, requires optimization of the cost/performance of the HTS conductor. From a superconducting performance standpoint, a long, flexible, single crystal-like wire is required. From a cost and fabrication standpoint, an industrially scalable, low cost process is required. Both of these critical requirements are met by Rolling-assisted-biaxially-textured-substrates. However, in order for cost/performance for a conductor based on this technology to be optimized, further work needs to be done in the area of buffer layer technology. It is now clear that while it is fairly straightforward to fabricate long lengths of biaxially textured metals or alloys, it is quite difficult to deposit high quality buffer layers using low cost processes. Requirements of buffer layers include—it should provide an effective chemical barrier for diffusion of deleterious elements from the metal to the superconductor, provide a good structural transition to the superconductor, have a high degree of crystallinity, excellent epitaxy with the biaxially textured metal template, have good mechanical properties, high electrical and thermal conductivity and should be able to be deposited at high rates.
Buffer layers of the prior art include use of YSZ and CeO
2
, typically a configuration of CeO
2
(0.01 &mgr;m)/YSZ (0.5 &mgr;m)/CeO
2
(0.01 &mgr;m). The purpose of the first buffer layer is to provide a good epitaxial oxide layer on the reactive, biaxially textured Ni substrate without the formation of undesirable NiO. CeO
2
is special in its ability to very readily form single orientation cube-on-cube epitaxy on cube textured Ni. Deposition of CeO
2
using a range of deposition techniques is done using a background of forming gas (4%H
2
-96%Ar) in the presence of small amounts of water vapor. Under such conditions the formation of NiO is thermodynamically unfavorable while the formation of CeO
2
is thermodynamically favorable. The water vapor provides the necessary oxygen to form stoichiometric CeO
2
. It is not possible to deposit YSZ under such conditions with no evidence of undesirable orientations. In the case of CeO
2
one can readily obtain a single orientation, sharp cube texture. Ideally, it would be desired that the CeO
2
layer be grown thick such that it also provides a chemical diffusion barrier from Ni, followed by deposition of YBCO. However, when the CeO
2
layer is grown greater than 0.2 &mgr;m in thickness, it readily forms micro-cracks. Hence, an YSZ that does provide an excellent chemical barrier to diffusion of Ni and does not crack when grown thick is deposited on a thin initial template of CeO
2
. However, since there is a significant lattice mismatch between YSZ and YBCO (~5%), a second 45°-rotated orientation nucleates at times. In order to avoid the nucleation of this second orientation completely, a thin CeO
2
layer is typically deposited epitaxially on the YSZ layer, to complete the buffer layer structure. YBCO can now be deposited on the layer that has an excellent lattice match with YBCO (~0.1%).
The drawbacks of this buffer layer structure are that the deposition of the first CeO
2
layer is non-trivial. Strict control of deposition conditions in particular, the O
2
partial pressure is required to avoid formation of undesirable NiO (NiO typically nucleates in mixed orientations and is also very brittle). Furthermore, CeO
2
can have wide range of oxygen stoichiometries. It is brittle and is not conducting. It will be a challenging engineering task to develop a large-scale continuous process for producing thick (>0.5 &mgr;m) crack-and porosity-free oxide films. For example, in a continuous process involving reactive electron beam evaporation of Ce to form CeO
2
, issues relating to the formation of an oxide on the target complicate matters relating to rate of deposition as well as stability of the melt pool. Any change of conditions during deposition is known to have profound affects on the film microstructure. Moreover, any oxidation of the biaxially textured metal, even after the successful deposition of CeO
2
, can induce undesirable interfacial stresses leading to spallation or further cracking, thus deteriorating the material properties. Microcracks in the oxide buffer layer will adversely affect the

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