Stock material or miscellaneous articles – Composite – Of inorganic material
Reexamination Certificate
1999-06-18
2001-07-10
Lorin, Francis J. (Department: 1775)
Stock material or miscellaneous articles
Composite
Of inorganic material
C428S689000, C428S699000
Reexamination Certificate
active
06258472
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
The invention relates to a product having a substrate of a partially stabilized zirconium oxide and a buffer layer of a fully stabilized zirconium oxide. The invention further relates to a process for producing such a product.
Such a product is disclosed in an article by K. S. Harshavardhan et al., Appl. Phys. Lett. 59 (1991) 1638.
The invention relates in particular to such a product whose buffer layer is used as a base for a layer of a ceramic superconductor, in particular a high temperature superconductor. A product supplemented in this way by a functional layer of a high temperature superconductor is, in particular, used in a superconducting current limiter for an electrical power distribution network. In such a current limiter, use is made of the fact that, at an appropriately low temperature, a superconductor maintains its superconductivity only as long as the current density of a current flowing through it remains below a particular limit value. This limit value is in principle depends on the temperature of the superconductor and any magnetic field that may be flowing through it. For a high temperature superconductor relevant to a technical application, the temperature is usually the temperature of liquid nitrogen at standard pressure; the magnetic field is conventionally assumed to be zero if no other specific indication is given.
The high temperature superconductor relevant for use in a superconducting current limiter, in particular a compound having the chemical formula YBa
2
Cu
3
O
7−x
, or one of the thallium-containing compounds (Tl, Pb)n (Ba, Sr)m Ca
k
Cu
l
O
y
with (nmkl)=(2212) or (1223) may be prepared for the indicated purpose in the form of a layer or an arrangement of layers, the layer or each layer having a thickness of the order of 1 mm and the layer or all layers having an area of several square meters. In a three-phase power distribution network according to conventional practice three such current limiters would need to be installed in parallel.
The product disclosed by the aforementioned article comprises a substrate of a polycrystalline partially stabilized zirconium oxide and a buffer layer of a fully stabilized zirconium oxide applied thereon. Zirconium oxide is distinguished by thermomechanical properties, in particular a coefficient of thermal expansion, which correspond well to the corresponding thermomechanical properties of the high temperature superconductor YBa
2
Cu
3
O
7−x
. Zirconium oxide is therefore in principle particularly suitable as a substrate for a layer of a high temperature superconductor. Pure zirconium oxide must, however generally be ruled out since pure zirconium oxide is subject to phase transitions at certain temperatures and is accordingly not stable enough. It is, however, possible to hinder or fully prevent these phase transitions by adding another oxide, for example an oxide selected from the group consisting of yttrium oxide, alkaline earth metal oxides and rare earth oxides, to the zirconium oxide. A fully stabilized zirconium oxide is obtained, for example, by adding from 8 to 12, in particular 9 mol % of yttrium oxide, and a partially stabilized zirconium oxide, which has been found to be suitable as a substrate for high temperature superconductors, is obtained by adding 3 mol % of yttrium oxide to pure zirconium oxide.
An article by Y. Iijima et al., Appl. Phys. Lett. 60 (1992) 769 describes a product having a substrate of a nickel alloy, described in detail, which is coated with a biaxially textured high temperature superconductor of the aforementioned composition. The term “biaxial” texture is in this case intended to mean a texture in which each crystallite of the high temperature superconductor has crystallographic axes that are aligned in a particular way with regard to the macroscopic geometry of the layer. In this case, each crystallite has a crystallographic c-axis, relative to which the crystal structure of the crystallites has maximally high symmetry, which is aligned approximately parallel to a main axis aligned perpendicular to the layer. In this context, “approximately parallel” permits quite noticeable deviations of the orientation of a c-axis from the main axis; a full width at half maximum for the angular distribution for the orientation of the c-axes should however remain limited to a reasonable value. In the present context, this full width at half maximum may be up to 30°; a full width at half maximum of up to at most 10° is preferred. A c-axis can moreover be identified in tetragonal, hexagonal, rhombohedric and cubic crystal systems. In a tetragonal crystal system, the c-axis is four-fold, and in the other crystal systems it is six-fold; the cubic crystal system moreover has a plurality of c-axes. Other crystallographic axes, which form an orthogonal system together with the c-axis, are commonly referred to as the a-axis and b-axis. A further requirement of a biaxial texture is that the a-axes of the crystallites be approximately parallel with one another, the term “approximately parallel” again permitting a possibly significant deviation from parallel in the strict sense. A particularly preferred biaxial texture would be distinguished by a full width at half maximum for the angular distribution of the c-axes of below 5°, and a full width at half maximum for the angular distribution of the a-axes of below 10°. Functionally, a biaxial texture therefore to some extent reproduces a specially oriented monocrystalline structure in a polycrystalline structure.
In order to obtain the biaxial texture of the superconductor in the product disclosed by the article just mentioned, the metallic substrate of this product (polycrystalline and untextured as usual) is coated with a buffer layer of fully stabilized zirconium oxide. In the special coating process used to deposit the buffer layer, in which the fully stabilized zirconium oxide is evaporated by a first ion beam and is transported to the substrate as a beam of atoms or molecules, the substrate with the buffer layer growing on it is exposed to a second ion beam, incident at a particular angle. The effect of this second ion beam is that the buffer layer is deposited with the desired biaxial texture. On this biaxially textured buffer layer, the functional layer of the high temperature superconductor is then grown, to which end the high temperature superconductor is deposited with a conventional laser ablation process. The biaxial texture of the buffer layer is continued into the functional layer substantially by itself and therefore gives the latter as well the desired biaxial texture with a full width at half maximum or the aforementioned angular distribution of between 20° and 30°. The critical current strength of this functional layer is stated as 1.4×10
4
A/cm
2
.
For further information about the production of a biaxially structured layer of fully stabilized zirconium oxide, reference may be made to an article by Iijima et al., Proc. of the 4th Intern. Symposium of Superconductivity, Tokyo, Oct. 14th-17th, 1991, p. 517 et seq., and an article by Iijima et al., Proc. of the 4th Intern. Symposium on Superconductivity, Tokyo, Oct. 14th-17th, 1991, p. 679 et seq. While the deposition of a biaxially structured buffer layer of fully stabilized zirconium oxide on a metallic substrate as described is successful, the deposition of such a buffer layer on a substrate of unstructured partially stabilized zirconium oxide fails.
SUMMARY OF THE INVENTION
Accordingly, the object of the invention is to provide a product of the type mentioned at the outset, which overcomes the cited disadvantages and drawbacks and which has a buffer layer of biaxially structured fully stabilized zirconium oxide.
With this and other objects in view, there is provided, in accordance with this invention, a product having a substrate of a partially stabilized zirconium oxide and a buffer layer of a fully stabilized zirconium oxide, in which the substrate is untextured and the buffer layer is biaxially textur
Dzick Jürgen
Freyhardt Herbert-Carl
Heinemann Klaus
Hoffmann Jorg
Neumuller Heinz-Werner
Greenberg Laurence A.
Lerner Herbert L.
Lorin Francis J.
Siemens Aktiengesellschaft
Stemer Werner H.
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