A RE-123-based oxide superconductor (REBa2Cu3O7-x, in which RE represents a rare earth element including Y) which has been found recently exhibits superconductivity at a temperature higher than a liquid nitrogen temperature. It is therefore expected as a promising material from a practical standpoint. There is a strong demand to use the linearly-processed RE-123-based oxide superconductor as a conductor for electric power supply.
As a method for processing an oxide superconductor into a linear material, the following method is under consideration. A metal material which is highly strong, heat resistive and is easily processed into a linear material is first formed as an elongated tape. An oxide superconductor is then deposited as a thin film on the tape-shaped metal substrate.
The oxide superconductor has electric anisotropy such that the flow of electricity is promoted along a and b crystal axes of the crystals itself, but the flow of electricity is impeded along a c crystal axis of the crystals itself. Accordingly, if the oxide superconductor is provided on a substrate, it is necessary to orient the a and b crystal axes so as to promote the flow of electricity and orient the c crystal axis along another direction.
However, the metal substrate itself is an amorphous material or a polycrystalline material which has a crystal structure significantly different from that of the oxide superconductor. It is therefore difficult to form an oxide superconductor film having high crystal orientation on the substrate. A difference in coefficients of thermal expansion and lattice constants between the substrate and the superconductor may cause distortion in the superconductor or peeling-off of the oxide superconductor film from the substrate during a cooling process to a superconducting critical temperature.
As an approach to address these problems, an intermediate layer (i.e., a buffer layer) is first provided on the metal substrate and an oxide superconductor film is formed on the intermediate layer. The intermediate layer consists mainly of MgO, YSZ (yttria-stabilized zirconium), SrTiO3 or the like each having a physical characteristic value, such as a coefficient of thermal expansion and a lattice constant, intermediate with respect to the values of the substrate and the superconductor.
In the intermediate layer, the c crystal axis of each crystal thereof are oriented at a right angle with respect to a surface of the substrate, but the a crystal axis (or the b crystal axis) of each crystal thereof are not in-plane oriented along the substantially same direction. Accordingly, the oxide superconductive layer to be provided on the surface of the substrate also has the a crystal axis (or the b crystal axis) each of which are not oriented the substantially same direction in-plane direction, thereby failing to increase critical current density Jc.
This problem is eliminated by the ion beam assist method (IBAD method). In the IBAD method, constituent particles ejected from a target by sputtering are deposited on a substrate while being irradiated with argon ions, oxygen ions or other ions at the same time emitted from an ion gun at a tilted angle (e.g., 45 degrees). According to the IBAD method, an intermediate layer having high orientation of the c crystal axis and high in-plane orientation of the a crystal axis with respect to a film deposition surface on the substrate can be provided.
FIGS. 6 and 7 illustrate an example in which a polycrystalline thin film as an intermediate layer is formed on the substrate by the IBAD method. In FIG. 6, a reference numeral 100 denotes a plate-like substrate and 110 denotes a polycrystalline thin film formed on an upper surface of the substrate 100.
The polycrystalline thin film 110 is formed by plural fine crystal grains 120, each having a cubic structure, joined together at grain boundaries. The c crystal axis of each crystal grain 120 is oriented at a right angle with respect to the upper surface (i.e., a film deposition surface) of the substrate 100. The a crystal axis and the b crystal axis of each crystal grain 120 are in-plane oriented along the same direction with each other. The c crystal axis of each crystal grain 120 is oriented at a right angle with respect to the film deposition surface (i.e., the upper surface) of the substrate 100. The crystal grains 120 are joined together with the a crystal axis (or the b crystal axis) being angled (i.e., grain boundary angle K illustrated in FIG. 7) at 30 degrees or less.
Although the IBAD method is considered as a highly practical method in that it provides linear materials with excellent mechanical properties and that stable high characteristics are easy to obtain, the intermediate layer formed by the IBAD method (hereinafter, also referred to as a “IBAD intermediate layer”) has been considered not to have high orientation unless the thickness is not less than about 1000 nm. A deposition rate in the IBAD method is as slow as about 3 nm/min since crystal orientation is controlled by ion beam impact on a non-orientated metal tape. This may take a longer time for film deposition and thus is inferior in productivity.
As a method for addressing this problem, development and study have been made intensively to use a fluorite-based oxide, such as YSZ and GdZrO (see, for example, Patent Documents 1 and 2) and a rock-salt-based oxide, such as MgO (see, for example, the Patent Document 1).
In the former method, because of the simple lamination structure and wider conditions for film deposition, elongation has been achieved earlier. However, since it is necessary to make the intermediate layer thicker, a production rate becomes low. There is also a problem that the internal stress of the film became large which may cause warpage of the substrate.
The latter method is expected to fundamentally solve the above-described problem. Since plural extremely thin films having a thickness of not more than several tens of nanometers are laminated together in this method, however, various kinds of technical know-how have been needed in order to keep the same narrow conditions for film deposition over the elongated substrate.    [Patent Document 1] U.S. Pat. No. 6,933,065    [Patent Document 2] International publication No. 2001-040536