Conductive and robust nitride buffer layers on biaxially...

Details Conductive and robust nitride buffer layers on biaxially... Conductive and robust nitride buffer layers on biaxially...

2001-06-29

2004-08-31

Silverman, Stanley S. (Department: 1754)

Superconductor technology: apparatus, material, process

High temperature devices, systems, apparatus, com- ponents,...

Superconductor next to two or more nonsuperconductive layers

C505S238000, C428S698000, C428S702000

Reexamination Certificate

active

06784139

ABSTRACT:

CROSS-REFERENCE TO RELATED APPLICATIONS
The following relate to the present invention and are hereby incorporated by reference in their entirety: U.S. Pat. No. 5,739,086 Structures Having Enhanced Biaxial Texture and Method of Fabricating Same by Goyal et al., issued Apr. 14, 1998; U.S. Pat. No. 5,741,377 Structures Having Enhanced Biaxial Texture and Method of Fabricating Same by Goyal et al., issued Apr. 21, 1998; U.S. Pat. No. 5,898,020 Structures Having biaxial Texture and Method of Fabricating Same by Goyal et al., issued Apr. 27, 1999; U.S. Pat. No. 5,958,599 Structures Having Enhanced Biaxial Texture by Goyal et al., issued Sep. 28, 1999; U.S. Pat. No. 5,964,966 Method of Forming Biaxially Textured Substrates and Devices Thereon by Goyal et al., issued Oct. 21, 1999; and U.S. Pat. No. 5,968,877 High To YBCO Superconductor Deposited on Biaxially Textured Ni Substrate by Budai et al., issued Oct. 19, 1999.
FIELD OF THE INVENTION
The present invention relates to electrically conducting and mechanically robust nitride buffer layers which can be deposited epitaxially on biaxially textured surfaces, metals and alloys. The invention also discloses a method to form such epitaxial layers using high rate deposition methods.
BACKGROUND OF THE INVENTION
Recent emergence of the 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. Hence, in order to enable applications of HTS at high temperature and high fields, it is imperative that a biaxially textured, polycrystalline YBCO tape, or related article, be developed which contains a minimal number of high angle grain boundaries.
One of the industrially scalable processes for producing biaxially textured YBCO conductors is by using Rolling Assisted Biaxially Textured Substrates (RABiTS). As described more fully below, 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 77K. To date, the preferred buffer layers for the RABiTS approach have been the combination of CeO
2
and 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.
Numerous applications of high temperature superconductors, such as transformers, generators and motors require high current carrying, flexible conductors which can sustain magnetic fields above 0.1T. Due to the thermally activated flux flow, the critical current density of most of the highly anisotropic superconducting compounds, such as the Bi-based compounds, rapidly drops at 77K in the presence of an externally applied magnetic field. Moreover, it is not clear if the uniaxially textured, Bi-based wires which typically contain numerous high angle grain boundaries and have approximately ⅔ of their cross-sectional area occupied by silver, will ever reach adequate cost/performance levels for large scale commercial applications. Hence, the development of a viable and low cost processing route based on (Y or Re)Ba
2
Cu
3
Ox (YBCO) is of great interest currently and forms a central research thrust in the area of high temperature superconductivity, YBCO compounds have favorable intrinsic properties. Epitaxial YBCO thin films on single-crystal substrates yield critical current densities (J
c
's) in the range of 10
6
-10
7
A/cm
2
at 77K, 0T. YBCO films also have a high irreversibility field of ~6T at 77K.
Conventional ceramic fabrication methods which can be used to make a long, flexible conductor result in materials with weak, if any, macroscopic or microscopic in 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 which 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 weaklinked. The low J
c
observed in randomly oriented polycrystalline HTS fabricated using conventional methods can be understood on the basis that the population of low 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. Practically speaking, this limitation entails the fabrication of biaxially textured superconductors.
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 (RABiTS). 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 backgrou

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