Superconductor technology: apparatus – material – process – Processes of producing or treating high temperature... – Process of making wire – tape – cable – coil – or fiber
Reexamination Certificate
1997-04-02
2001-02-20
Talbot, Brian K. (Department: 1762)
Superconductor technology: apparatus, material, process
Processes of producing or treating high temperature...
Process of making wire, tape, cable, coil, or fiber
C505S740000, C505S432000, C427S062000, C264S177110, C264S211110, C029S599000
Reexamination Certificate
active
06191074
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the fabrication of superconducting wires and rods. More particularly, the present invention primarily relates to the fabrication of superconducting wires and rods made from Yttrium Barium Copper Oxide and Bismuth Strontium Calcium Copper Oxide.
2. Brief Description of the Related Technology
High T
c
superconductivity was first discovered in Yttrium Barium Copper Oxide (YBCO) in 1987. Since then, much progress has been made in discovering other compounds and in understanding the fundamental properties of these materials. At the same time, significant effort has been directed towards identifying the potential applications of these new materials, and developing the processing conditions for fabrication of useful forms of these materials, such as wires and rods.
Some of the large scale applications of superconducting wires are in power generation and transmission, high field magnets, magnetic energy storage, and current leads for low T
c
superconducting magnets and magnetic energy storage systems. These materials are also used in other applications like electronic devices and magnetometers.
Conventionally, wires of ductile materials are made by extrusion or by rolling and drawing processes. These processes cannot be directly applied to the high T
c
superconductors, which are ceramic materials and not capable of plastic deformation. Several variations of these processes are being developed, however, to adapt them to ceramic materials. These include the powder-in-tube process used to fabricate metal-clad composite and the plastic extrusion process used to fabricate bare wires and rods. Other processes like surface coating techniques are also being pursued. See S. Jin, R. C. Sherwood, R. B. Van Dover, T. H. Tiefel and D. W. Johnson, Jr.,
Appl. Phys. Lett.,
51, 203 (1987); M. T. Lanagan, R. B. Poeppel, J. P. Singh, D. I. Dos Santos, J. K. Lumpp, U. Balachandran, J. T. Dusek and K. C. Goretta,
J. Less Common Metals,
149, 305 (1989); S. R. Su, M. O'Connor, M. Levinson and P. G. Rossoni,
Physica C,
178, 81 (1991); L. D. Woolf, W. A. Raggio, F. E. Elsner, M. V. Fisher, R. B. Stephens, T. L. Figueroa, C. H. Shearer, J. D. Rose, K. M. Schaubel, R. A. Olstad, T. Ohkawa, D. M. Duggan, M. DiMartino and R. L. Fagaly,
Appl. Phys. Lett.,
58, 534 (1991); and H. Shimizu, H. Kumakura and K Togano,
Jpn. J. Appl. Phys.,
27, L414 (1988), which are incorporated herein by reference.
The powder-in-tube process uses a preform composed of a nominal ductile metal tube packed with fine superconducting powder. A wire is rolled, drawn, or both rolled and drawn in successive stages until the required final dimensions are achieved. The resulting composite superconducting wire is easily wound into coil shapes. When the radius of the resulting superconducting wire is small, the wire is first wound into a coil and then the coil is heat treated to sinter the powder.
In the case of the plastic extrusion process, superconducting powder is mixed with a set of organics to prepare a paste, which is then extruded to form the desired cross sections. Some of the advantages of this process over the powder-in-tube process are the following: (i) there is no problem of degradation or contamination from other material; (ii) oxygen annealing is comparatively easy, since the surface is exposed; (iii) the monolithic nature of the wire makes it useful for applications like current leads, where metallic cladding is undesirable; and (iv) this process can be applied to the fabrication of different sections and large cross-section area rods.
The plastic extrusion process consists of several stages like paste preparation, extrusion, drying, binder burn-out, sintering and oxygenation. Each one of these stages involves several parameters which affect the quality of the final product and its microstructure. The plastic extrusion process itself is used in the fabrication of certain conventional ceramic shapes. However, in prior attempts at applying the plastic extrusion process to the fabrication of superconducting wires and rods, the additives and processing parameters used have not achieved wires and rods having desired and consistent electrical/superconducting and mechanical properties.
The present invention provides the additives, processing parameters, and processing stages necessary to achieve desired and consistent electrical/superconducting and mechanical properties. For example, the present invention: (i) uses polymeric additives which form an extrudable paste but do not degrade the final properties of superconductor; (ii) optimizes the amount of each of the additives; (iii) optimizes extrusion parameters like compaction load and extrusion rate; and (iv) provides for the removal of solvent first and other polymeric additives later through proper heat treatment and sintering parameters.
SUMMARY OF THE INVENTION
The present invention relates to the fabrication of superconducting wires and rods having desired and consistent electrical and mechanical properties. The fabrication of superconducting wires and rods based on Yttrium Barium Copper Oxide (YBCO) and Bismuth Strontium Calcium Copper Oxide (BSCCO) is the primary focus. The present invention employs a plastic extrusion-type process.
The fabrication process for YBCO wires and rods has several process steps in common with the fabrication process for BSCCO wires and rods. The first step in the fabrication of YBCO and BSCCO superconducting wires and rods is the preparation of an extrudable paste. Extrudable paste is formed by mixing superconducting powder, comprising YBCO or BSCCO, with a set of organic additives, which include binder, plasticizer, lubricant, dispersant, and a solvent. Preferably, the binder is ethyl cellulose, the plasticizer is glycerol, the lubricant is stearic acid, and the dispersant is triolein. The solvent may be butyl carbitol, ethyl alcohol or various mixtures of both.
The next step in the fabrication process is to use a piston extruder to extrude the superconducting wire or rod. A suitable piston extruder comprises a die, barrel, port for connecting a vacuum pump, and a ram. The die includes a finishing tube and extrusion nozzle. The dimensions of the piston extruder are designed to promote slip flow and avoid laminations in the extrusion.
The use of the piston extruder involves three main steps. In the first main step, the extrusion nozzle is first closed, or plugged with a plug, and the extrudable paste is loaded in the barrel in the form of small, loose granules. The granules are preferably made by cutting the paste into small cubes having sides of about 2 to 3 mm. The barrel, filled with the granules, is then connected to a vacuum pump through the port, and is subjected to a vacuum of about 200 millitorr. The vacuum removes air in the extrudable paste and thereby reduces the defects and improves the extrusion behavior of the wires and rods. The second main step is to compact the paste by using the ram to apply a compaction stress of about 100 MPa. The third and final main step is to open or unplug the nozzle and then move the ram at a constant rate of about 0.02 to 0.1 inch per minute to extrude the wire or rod.
The next step in the process of fabricating YBCO and BSCCO wires and rods is to dry the extruded wire or rod to remove the solvent. Since this drying results in shrinkage, the drying conditions are chosen so as to avoid cracking of the wire or rod. The extruded wire or rod is first held in still air at room temperature until about 20% of the solvent is lost. The wire or rod is then transferred to a convection dryer, where it is exposed to flowing warm air at 60° C. The time periods for drying depend upon the diameter of the wire or rod.
The next fabrication step involves subjecting the dried wire or rod to a binder burn-out treatment for removing the remaining organic additives. The dried wire or rod is first heated in a furnace to a temperature of about 100° C. Then, in the temperature range of 100° C. to 300° C., wires are heated in flowing
Ponnusamy Devamanohar
Ravi-Chandar Krishnaswamy
Salama Kamel
Fulbright & Jaworski
Talbot Brian K.
University of Houston
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