Substrate materials

Details Substrate materials Substrate materials

1999-04-23

2001-06-26

Beck, Shrive (Department: 1762)

Superconductor technology: apparatus, material, process

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

Substrate for supporting superconductor

C505S230000, C505S236000, C505S238000, C427S062000

Reexamination Certificate

active

06251834

ABSTRACT:

The present invention relates to substrate materials for oxide superconductor layers. The present invention also relates to superconducting laminates comprising a substrate and a layer of an oxide superconductor on the substrate, and to processes for the manufacture of such superconducting laminates.
Oxide superconductors such as YBa
2
Cu
3
O
7
(YBCO) and Bi
2
Sr
2
CaCu
2
O
8
(BSCCO) have been found to exhibit superconductivity at temperatures above the boiling point of liquid nitrogen (−196° C.). Unfortunately, it has been found that the bulk oxide superconductors exhibit rather low critical current densities, rendering them unsuitable for a number of potential applications. The low critical current densities are thought to be due in part to poor conduction across the grain boundaries in the ceramic oxide materials, in particular because the materials have highly anisotropic layered structures.
Greatly improved critical current densities have been achieved by depositing the superconducting oxides as highly oriented thin films on oxide substrates, such as oriented single-crystal oxide substrates. Preferably, the texture of the epitaxially grown superconductor layer should be within 1-4 degrees from grain to grain, with the (001) plane of the superconductor lying in the plane of the substrate. The resulting superconducting layers can be fabricated into electronic devices, but the oxide substrates are clearly unsuitable for high-field windings and power transmission applications.
A. Goyal et al. in
J. Mater. Res
. Vol 12, pages 2924-2940 (1997) review recent studies on epitaxially grown thin films of oxide superconductors having controlled grain boundary misorientation distributions (GBMD's).
R. Hawsey and D. Peterson in
Superconductor Industry
, 1996, pages 23-29 describe a process for the deposition of the oxide superconductor YBCO on a substrate of cubic textured nickel with CeO and yttria stabilized zirconia (YSZ) buffer layers. The nickel substrate has a highly cube-textured surface. That is to say, a high proportion of the nickel grains are oriented with their (100) crystallographic axis perpendicular to the substrate surface and with the (010) and (001) axes of the grains parallel. This results in epitaxial growth of the oxide buffer layers and superconductor layers such that the YBCO grains are highly oriented with their (001) crystallographic axis perpendicular to the surface, thereby maximising critical current density in the superconducting layer.
The cubic nickel substrates described above are magnetic at liquid nitrogen temperatures, and are therefore not preferred for some AC superconducting applications. Furthermore, the nickel substrates provide a poor thermal match to YBCO superconductor layers. The quality of thermal expansion matching is relevant to the processability of such superconducting laminates, because the laminates must undergo extensive thermal excursions. Firstly, in production, the superconductor is deposited on the substrate at about 700° C. Secondly, the YBCO layer must be oxidised by treatment in oxygen at 500° C. for an extended period to render it superconducting. Thirdly, the YBCO superconductor must be cooled to liquid nitrogen temperatures (−196° C.) for superconducting operation. It has now been found that improved thermal matching is desirable between the superconductive oxide layer and the substrate in order to minimise damage during the thermal excursions.
Accordingly, it is an object of the present invention to provide a substrate for a superconducting oxide layer that provides better thermal matching to the oxide superconducting layer than pure cubic nickel.
It is a further object of the present invention to provide a non-magnetic alloy substrate for a superconducting oxide layer whereby the resulting superconducting laminate is more suited to some AC engineering applications.
The present invention provides a substrate for an oxide superconductor layer, the substrate comprising an alloy layer consisting essentially of either: (1) a cube texture FeNi alloy in the composition range 47% Ni to 58% Ni; or (b) a cube texture NiCu alloy in the composition range 41% Ni to 44% Ni.
The cubic &agr;-phase alloys of Ni with Fe or Cu provide a good substrate for epitaxial growth of oxide superconductor layers, such as YBCO. The alloy compositions are selected to give improved thermal matching with YBCO over the temperature range −196° C. to +500° C. Furthermore, the NiCu alloys are nonmagnetic, and therefore preferred for use in AC engineering applications.
Preferably, the alloy layers of the substrate according to the invention consist essentially of Fe and Ni, or of Cu and Ni. That is to say, only Ni, Fe and Cu are present in the alloys apart from minor constituents (up to 5%, preferably up to 2%) of impurities or additives such as Mn or Si, for example to regulate grain growth. All percentages are in atomic %.
Preferably, the substrate according to the present invention further comprises an oxide buffer layer covering a surface of the alloy layer. The buffer layer substantially prevents any chemical reactions between the alloy layer and the superconducting oxide layer, and also blocks oxidation of the alloy layer during the oxygen annealing step that must be carried out following deposition of a YBCO layer in order to render it superconductive. More preferably, the oxide buffer layer comprises a refractory oxide such as CeO, MgO or yttria stabilized zirconia (YSZ). Preferably, the buffer layer has a thickness of no more than 5 micrometers.
The surface of the substrate onto which it is intended to deposit the oxide superconductor layer should be smooth, in order to ensure that the oxide is deposited with a high degree of crystallographic orientation and large grains. Preferably, the surface roughness is less than 1 micrometer, more preferably 200 nanometers or less, and most preferably 50 nanometers or less, as determined by profilometric techniques.
The grain size of the alloy substrate should be large for optimal DC applications to minimise percolative effects. For AC applications, the grain size should be small. The grain morphology can be modified by chemical composition adjustment and also by the annealing procedure. The annealing can be carried out in two stages in order to avoid secondary recrystallisation causing development of other non-cubic texture orientations. The first stage annealing is carried out at about 300° C. to establish the cubic texture. This is followed by a high temperature annealing at 700° C. when grain growth takes place. The cube texture and grain size are preferably determined by X-ray diffraction texture goniometry.
In order to achieve highly oriented epitaxial growth of the oxide superconductor layer, the alloy layer of cubic a-phase alloy is cube-textured in orientation with respect to the substrate surface on which deposition of the superconducting layer will take place. That is to say, a substantial fraction of the grains of the alloy should be cube-textured with respect to the substrate surface. The term cube-textured, sometimes called {100}<100> orientation, signifies that the alloy grain is oriented with its (001) crystallographic plane parallel to the alloy layer surface, and with its (100) crystallographic axis parallel to the rolling direction of the alloy layer. Preferably, at least 75% of the grains of the alloy are textured within 6°, more preferably 4°, of ideal cube-texture, more preferably at least 90% of the grains are so oriented.
For high current engineering applications it is preferable for the substrate alloy layer to be as thin as possible in order to maximise the average critical current of the laminate, since the substrate does not itself superconduct under the operating conditions. On the other hand, some structural strength is needed in the alloy layer to enable coils to be wound, and to resist Lorentz forces in operation, especially in high current DC applications. Accordingly, the thickness of the alloy substrate layer is preferably 5-25 m

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