Superconductor technology: apparatus – material – process – Processes of producing or treating high temperature... – Heating – annealing – or sintering
Reexamination Certificate
2001-07-26
2003-07-29
Kopec, Mark (Department: 1751)
Superconductor technology: apparatus, material, process
Processes of producing or treating high temperature...
Heating, annealing, or sintering
C505S737000, C505S742000
Reexamination Certificate
active
06599862
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of preparing a bismuth-based high temperature superconductor suitable for preparing a raw material for a high temperature superconducting wire or for forming high temperature superconducting films such as a superconductor device made up of stacked superconductor films.
2. Description of the Related Art
There have recently been discovered various oxide-based high temperature superconductors one after another, and there have since been seen vigorous activities to develop techniques for fabricating wires or superconductor devices from these materials, which have a poor plastic workability. For example, with reference to fabrication of wires, techniques for using a yttrium-based high temperature superconductor and a bismuth-based high temperature superconductor have recently come to be in the mainstream of R and D efforts.
Among these materials, the bismuth-based high temperature superconductor is receiving a lot of attention as a material quite advantageous in the fabrication of wires because this superconductor tends to undergo crystal growth oriented in a direction in which electric current can flow with ease, and has a crystal structure wherein flake-like substances are bonded with each other in a direction of thickness.
The reason for this is as follows. For the fabrication of wires from high temperature superconductors, there has usually been adopted a method of filling superconductor powders as adjusted into a silver tube, and applying thereto plastic working such as drawing, rolling, and so forth, thereby fabricating long products. Accordingly, the bismuth-based high temperature superconductor having the properties as described above has advantages in that its crystals tend to be oriented in a direction in which electric current can flow with ease when subjected to compression working in the axial direction by rolling, and so forth, and further, the powders thereof tend to be closely packed with ease.
However, there has been pointed out a problem with the bismuth-based high temperature superconductors having high temperature superconductivity such as Bi
2
Sr
2
Ca
2
Cu
3
O
z
(hereinafter referred to as Bi-2223), and (Bi, Pb)
2
Sr
2
Ca
2
CU
3
O
z
(hereinafter referred to as Bi/Pb-2223) in that these superconductors has a drawback of poor manufacturability, and tends to generate substances showing non-superconductivity (non-superconductive phase) during a manufacturing process even though the crystals of these superconductors can be oriented with greater ease when subjected to plastic working as compared with the case of the yttrium-based high temperature superconductor.
That is, With reference to the Bi-2223, various methods have been tried such as a method of sintering (a sintering temperature: on the order of 890° C.) a starting material composed of powders of raw materials mixed at a composition ratio (a non-stoichiometric ratio) deviating from the stoichiometric ratio on purpose in order to increase the amount of Bi-2223 generated because a Bi-2223 phase is hardly obtainable by mixing of raw materials according to the stoichiometric ratio, and a method of devising process conditions such as changing of the partial pressure of oxygen during sintering (refer to, for example, “Solid State Commun., 110 (1999), p. 287, and “Supercond. Sci. Technol.”, 11 (1998), p. 288). However, with either of the above-described methods according to the “solid-state reaction process”, there has not occurred sufficient progress in the final reaction to arrive at the Bi-2223 phase from Bi
2
Sr
2
CaCu
2
O
z
(hereinafter referred to as Bi-2212), which is an intermediate product phase, thereby failing to succeed in obtaining crystals of the Bi-2223 single-phase.
Accordingly, there has been practiced a process of adding Pb to the bismuth-based high temperature superconductor to substitute Pb for a portion of the Bi content, and turning a superconductive phase into a (Bi, Pb)
2
Sr
2
Ca
2
Cu
3
O
z
composition (that is, the Bi/Pb-2223), thereby facilitating the superconductive phase to be turned into a single phase in order to enhance properties of the superconductor described above.
In preparing the Bi/Pb-2223, there has been applied the “solid-state reaction process” whereby a starting material composed of powders of raw materials mixed at a non-stoichiometric ratio is sintered (a sintering temperature: on the order of 850° C.) in the atmosphere (refer to, for example, “Material Research Bulletin”, 31 (1996), p. 979). However, it is to be pointed out that the reaction process requires a lot of time (on the order of 100 hours or longer), and the Bi/Pb-2223 purity of the final product prepared by such means as described has failed to exceed 96%.
This is because the process inevitably had to be started from mixed powders of raw materials blended at a non-stoichiometric ratio, so that elements not subjected to reaction as well as elements not evaporated remained as impurities, and the reaction to form the Bi/Pb-2223 via the Bi-2212 phase, which is a non-superconductive phase, did not fully proceeds, thereby leaving out a portion of the Bi-2212 between phases.
Thus, since it has been extremely difficult to obtain crystals of the Bi-2223 single-phase according to conventional techniques, there has been practiced a process of adding Pb to a bismuth-based high temperature superconductor, and turning a superconductive phase into Bi/Pb-2223, thereby facilitating the bismuth-based high temperature superconductor to be turned into a single phase. Nevertheless, it has still been difficult to turn the bismuth-based high temperature superconductor fully into a single phase, and in addition, it has taken considerable time to prepare the superconductor.
However, in order to enhance the superconductive properties of the bismuth-based high temperature superconductor (for example, in order to increase a critical temperature Tc thereof), it is considered a very important subject to cause the bismuth-based high temperature superconductor to “turn into a single-phase” wherein a second phase which is a non-superconductive insulating substance is not allowed to reside.
Further, an attempt has so far been made to turn the bismuth-based high temperature superconductor into a single-phase of the superconductive phase by adding Pb to the bismuth-based high temperature superconductor. However, Pb is a toxic element and, as such, is a raw material of which it is preferable to refrain from using as much as possible. In addition, among superconductors having an identical performance, a superconductor composed of varieties less in the number of raw materials, even by one, is evidently desirable, and consequently, even from this point of view, it is quite significant to remove Pb from the bismuth-based high temperature superconductor composed of a relatively large number of elements.
Further, in a bismuth-based high temperature superconductor, Pb is an element affecting not only stabilization of the 2223 phase but also electrical and magnetic properties thereof. For example, it is known that, with a complex copper oxide-based superconductor of a lamellar structure, such as a bismuth-based high temperature superconductor, conductivity in a direction perpendicular to a main conductive face thereof has profound effects on the most important properties of superconductive materials such as a critical magnetic field, and a critical current, and Pb exerts effects corresponding to its content on the conductivity in a direction perpendicular to the main conductive face.
Accordingly, if means for preparing easily and stably varieties of bismuth-based high temperature superconductors with a Pb content varying in a range of from 0 to a variety of values can be established, this makes it possible to implement preparation of bismuth-based high temperature superconductors having variously controlled properties, thereby largely contributing to progress in research on physical properties thereof and expansion in application thereof. Even for such r
Lee Sergey
Tajima Setsuko
Flynn ,Thiel, Boutell & Tanis, P.C.
Kopec Mark
Superconductivity Research Laboratory
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