Superconductor technology: apparatus – material – process – Processes of producing or treating high temperature... – Process of making wire – tape – cable – coil – or fiber
Reexamination Certificate
1999-11-30
2001-04-24
Kopec, Mark (Department: 1751)
Superconductor technology: apparatus, material, process
Processes of producing or treating high temperature...
Process of making wire, tape, cable, coil, or fiber
C505S491000, C505S492000
Reexamination Certificate
active
06221813
ABSTRACT:
The invention relates to a process for producing shaped bodies comprising textured superconducting material and the further processing of these shaped bodies to form massive high-temperature superconducting components such as power leads or high-temperature superconducting strips and wires by the oxide-powder-in-tube (OPIT) method.
Owing to the still too low current-carrying capacity of many high-temperature superconducting components, their use is limited. Further development of such components is necessary so that higher superconduction currents can flow through these components. To produce high-temperature superconductors having a high current-carrying capacity, it is necessary to optimize the high-temperature superconducting material in respect of purity, phase purity, phase composition, degree of crystallization and orientation.
The shaping of longitudinal bodies is customarily carried out using cold isostatic pressing (CIP) by the wet bag CIP method in which the pressing forces act on all sides of the shaped bodies, so that only weak or nonuniform alignment of the particles occurs.
In the wet bag method, a thin-walled, flexible rubber or plastic mold is filled with powder and, after being closed, introduced into a pressure vessel. Compaction to form shaped bodies is achieved by means of a liquid pressure transfer medium at a predetermined pressure. Tight dimensional tolerances as are required, in particular, for introduction of the shaped bodies into silver sheathing tubes in the OPIT method cannot be achieved in this way, so that further shaping, e.g. by turning the shaped bodies on a lathe, is necessary. This additional step increases the complication of the process and increases costs and also entails the risk of increasing the residual carbon content in the product, particularly in the case of small particle sizes, as a result of interaction with the CO
2
in the surrounding atmosphere during mechanical forming. However, it is especially small particles sizes which are desirable for the OPIT method.
Since pressure is applied on all sides in the wet bag method, uniform pretexturing of the platelet-shaped particles cannot be achieved. Following shaping, attempts are made to compensate for the nonuniform pretexturing by means of the mechanical treatment steps, e.g. extruding and/or rolling in the case of strip and wire production. This causes many flaws, cracks and fractures in and between the inorganic powder particles. These lead to comparatively low current densities or require long annealing times to heal these defects because unimpaired current flow can occur only in the case of oriented, directly adjoining defect-free grains of the high-temperature superconducting phase.
In uniaxial pressing of longitudinal shaped bodies, e.g. in a hydraulic press, the platelet-shaped powder particles are essentially aligned perpendicular to the pressing direction and thus perpendicular to the longitudinal direction of the shaped body. The texture is rotated precisely by 90° from the optimum alignment and has a particularly adverse effect on the current flow. When the texture is aligned optimally, the crystallographic (a,b) plane which is oriented parallel to the plane of the platelets and to the plane of maximum current-carrying capacity is in the direction of the current flow in the component. During the thermal treatments following shaping, the preferred orientation is essentially retained.
When metal tubes are filled manually with oxidic powders and when compaction is achieved by ramming rather than mechanical pressing, not only the increased effort but also the inhomogeneity of the pressed density and the low density of the shaped body are particularly disadvantageous. This leads to low superconducting contents in the component and to particularly low current carrying capacities.
EP-A-0 611 737 reports a process for producing superconductors based on (Pb,Bi)—(Sr,Ca)—Cu—O, in which an oxidic melt is first produced and quenched and the solid obtained is particularly finely comminuted. EP-A-0 396 581 describes a process for producing wires or strips of superconducting materials, in which process the compaction of the oxidic powder is carried out by wet bag cold isostatic pressing. WO-94/00886 likewise discloses a process in which isostatic compaction is achieved by the wet bag method.
WO-96/21951 teaches a process for producing an elongated superconducting body either by extrudation or by two successive isostatic compaction steps between which a heat treatment step is carried out or by a combination of first an extrusion procedure and two successive isostatic compaction steps between which a heat treatment step is carried out.
Further significant disadvantages of the high-temperature superconducting shaped bodies of the prior art are that the shaped bodies pressed by the wet bag method do not have a sufficiently accurate shape, that this method of shaping is particularly labor intensive and time consuming and that the process cannot be automated.
Saleable shaped blanks for further processing to produce massive high-temperature superconducting components such as power leads or to produce high-temperature superconducting strip or wire therefore have to have properties which can be transferred to the product by the user by means of comparatively few and/or short energy-saving thermal and/or mechanical treatments. In addition, the objective of the optimization of these blanks has to be to make the entire production process up to the finished products as simple and inexpensive as possible while also achieving improved product properties.
It is an object of the invention to provide a process for producing a superconducting material, by means of which an improved texture can be generated in a simple manner so that the heat-treated shaped bodies which can be produced by the process have fewer defects which impair the electrical properties and also have homogeneous compaction, a high density and a higher current-carrying capacity. A further object is to propose a method which is economical to carry out and can be used for an industrial process. In particular, it is an object to provide blanks in the form of un-heat-treated or heat-treated shaped bodies such that their subsequent further processing to produce massive high-temperature superconducting components such as power leads or to produce high-temperature superconducting strips and wires can be carried out particularly simply, quickly and with low energy consumption compared to the prior art.
These objects are achieved by a process for producing a shaped body, in which a mixture of oxidic starting powders or a superconducting material, which comprises at least 30% by volume of platelet-shaped primary particles and has such a composition that a high-temperature superconducting material is formed on later, suitable thermal treatment, is comminuted by milling, shearing and/or rolling in such a way that the comminuted power has a powder particle size distribution having a d
90
of ≦20 &mgr;m, and in which the powders which have been comminuted in this way are isostatically compacted by the dry bag method.
For the purposes of the present invention, a shaped body is a body which has been shaped and can have been compacted according to the invention and may have been treated by means of further process steps such as heat treatment. This shaped body can accordingly consist essentially of a mixture of oxidic starting powders, of a superconducting material or of a heat-treated high-temperature superconducting material. The term shaped body is therefore retained in various stages of the production procedure.
The starting material can be a mixture of oxidic starting powders or a superconducting material. If organic salts such as a mixture of a plurality of oxalates or if a mixture of inorganic compounds such as carbonates or/and sulfates are used as starting material, it is necessary to calcine these first before use in the process of the invention, since calcination liberates very large amounts of gas which could destroy a pressed body on h
Bock Joachim
Gauss Stephan
Neumann Jurgen
Riedel Günther
Aventis Research & Technologies GmbH & Co. KG
Frommer & Lawrence & Haug LLP
Kopec Mark
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