This invention relates generally to superconducting materials and processes for their manufacture, and more specifically relates to the manufacture of high temperature superconducting coils with electrical insulation.
The most important technological value of the high superconducting transition temperature superconductor Bi2Sr2CaCu2Ox (referred to herein as “Bi-2212”) may be as a round wire operated at “low temperatures”, i.e. 4.2K. That is because Bi-2212 is the only superconductor that can carry a significant supercurrent in the technologically useful form of a round wire in very high magnetic fields, i.e. above 23 Tesla (T). As high field uses inevitably involve construction of some form of coil, reliable Bi-2212 coil manufacture procedures are needed to maximize the potential of this material.
The coil fabrication technology used for the present high field superconductor material, Nb3Sn, is called the “wind-and-react” process, e.g., Taylor et al., “A Nb3Sn dipole magnet reacted after winding,” IEEE Trans. Magnetics Vol. MAG-21, No. 2, 1985, pp. 967-970. Typically a Nb3Sn precursor composite, either Nb filaments and Sn sources in a Cu matrix, or Nb filaments in a bronze matrix, is wiredrawn to a final diameter ˜1 mm and insulated with a glass yarn braid impregnated with a carbonaceous binder such as an organic resin. This wire is wound onto a coil former and heat-treated first to a temperature to burn off the carbonaceous binder, and then to the Nb3Sn formation temperature. This is typically done by burning the binder in air or oxygen at a relatively low temperature (˜300° C.) compared to the Nb3Sn reaction heat treatment temperature (˜650° C.). Any carbon that remains trapped within the windings after the binder is burned has no effect on the Nb3Sn phase formation.
It is very desirable to adopt this “wind-and-react” process for Bi-2212 coil fabrication, but in practice this has been difficult. The type of glass braid used for Nb3Sn coils fully melts at the reaction temperatures needed for Bi-2212 coils, so some combination of glass and ceramic, or pure ceramic is needed as the insulation material. Prior art Bi-2212 coils are plagued with many defects amongst the internal windings after reaction. The defects are often visually indicated by black stains (see Denis Markiewicz et al., “Perspective on a Superconducting 30 T/1.3 GHz NMR Spectrometer Magnet,” IEEE Trans. on Appl. Supercond., Vol 16, No. 2, 2006, pp. 1523-1526), and the defects result in coils delivering a fraction of the current they should be producing based on short sample testing. These coils are typically heat-treated in a furnace with continuous oxygen gas flow. The carbonaceous binder, known in the paper industry as “sizing,” is converted to CO2 during an initial low temperature heat treatment. The CO2 can be trapped in the tight winding pack, and even with a continuous flow of oxygen it is not possible to purge this trapped CO2 gas out of such a tightly wound pack. This presents a major problem, as the atmosphere adjacent to the wire surface is critical to the formation of the optimal phase of Bi-2212. The insulated wire is packed very densely into the coil former with the gas path in and out of coil pack only a series of many small orifices. It is very difficult to remove any unwanted gas, such as what might be produced from burning the binder, through such small orifices. A simple oxygen gas purge does not flush out the residual gas contaminants deep in the winding. One cause of a coil not carrying the expected current is the improper or incomplete formation of Bi-2212 due to contaminated atmosphere in even a small section of the coil during the reaction (high temperature) heat-treatment. Even if this only happens in a small section deep inside of the winding, the extracting and testing of the failed section from the coil is impractical as it may be only a short section of many thousands of meters.
One prior art investigator attempted to overcome this problem by using oxidized Hastelloy fibers as insulation material and a highly gapped weave, but the coil current was only 67% of the short sample (an uninsulated, uncoiled reference sample of the same wire) value. Watanbe, et al, “Ag-Sheated Bi2Sr2CaCu2O8 Square Wire Insulated with Oxidized Hastelloy Fiber Braid”, Advances in Cryo Engineering, Vol. 54, 2007, pp. 439-444. In addition, such a thin weave is not practical, in that such materials are both difficult to apply industrially and such wide gaps are highly susceptible to electrical shorting.
The present invention overcomes the problems above. In the present invention a round wire of Bi-2212 is manufactured as per the standard round wire powder-in-tube packing and wire drawing techniques (See Hasegawa et al, “HTS Conductors for Magnets”, IEEE Trans. on Appl. Supercond., Vol 12, No. 1, 2002, pp. 1136-1140), and then braided with a ceramic-glass yarn. The carbonaceous binder in the yarn is completely burned at a temperature lower than Bi-2212 partial melting point. This produces a byproduct of CO2 and other contaminants that are outgassed from the surface of other parts in the coil. After cooling the vessel to or approximately to room temperature, the CO2 and other contaminate gases are removed by evacuating the heat-treatment chamber containing the coil. After evacuation, the chamber is back-filled with pure oxygen gas or a desired mixture of gases. In this way all the contaminant gases are removed from the winding pack through the small orifices and completely replaced with the desired gas even in the most inaccessible areas in the winding. As the local atmosphere around the surface of the wire, particularly the concentration of oxygen, is critical to reaction sequence, high current Bi-2212 coils can now be obtained.
The process of burning of the binder insulation thus occurs by first evacuating the chamber of the initial furnace gas, which may be nitrogen, air, CO2, or some combination thereof, and then back-filling with a gas with oxygen, followed by the burning procedure at elevated temperature. The temperature is reduced to about room temperature and then the vessel is evacuated to remove the gaseous combustion products. The evacuation, refill with oxygen and burn-off cycle can be repeated one or more times. The back filling of oxygen can initially be of oxygen of a low partial pressure, followed by the burning procedure at elevated temperature, and during this burning procedure the pressure of oxygen can be gradually increased to ensure complete burn off of the binder.
In the drawings appended hereto:
A Bi-2212 wire is fabricated by the powder-in-tube or similar process and is insulated with a ceramic-glass yarn insulation. The yarn is applied either by braiding or serving. By necessity the yarn is treated with a carbonaceous organic binder, for example polyurethane resin, to ensure its flexibility and good handling properties. This insulated wire is wound as compactly as possible, creating a wind on a coil former at very high tension with minimum void spaces. Referring to
After the pump-out of CO2 from the system, the furnace chamber is back-filled with oxygen (at an oxygen concentration of from about 20% to 100%, preferably 100%), or the required gas mixture through a valved 15 port 16 and the temperature increased to the transition temperature of the powders to the high current superconducting phase. From this stage, the procedures can be the same as in any conventionally known Bi-2212 coil reaction sequence, typically a peak temperature of from 870° C. to 900° C., with more preferably a peak temperature of ˜890° C. with a 5° C./hr cool down to ˜830° C. held for 60-100 hours before furnace cooling.
The same procedures as above could be performed on a strand that has (Bi, Pb)2Sr2Ca2Cu3Ox, YBa2Cu3Ox or any other RE-123 compound (where RE=Y, Gd, Er, Ho, Nd, Sm, Eu, Yb, Dy, Tm, or Lu), as the superconductor instead of Bi-2212. The important concept is that this technique allows a superconductor that needs oxygen for proper phase formation to have access to oxygen while remaining electrically insulated from adjacent turns. When the superconductor wire is of the (Bi, Pb)2Sr2Ca2Cu3Ox family, a peak reaction temperature is typically from 870° C. to 900° C. When the ceramic superconductor wire is ReBa2Cu3Ox, where Re=one of the rare earths Y, Gd, Er, Ho, Nd, Sm, Eu, Yb, Dy, Tm, or Lu, the peak reaction temperature is typically 950° C. to 1050° C.
The invention is further illustrated by the following Example, which is intended to be illustrative of the invention and not delimitative thereof. In this Example, and elsewhere in the specification, the terms “witness sample” and “barrel sample” are usages that are common to those skilled in this art. Basically they refer to a small sample without insulation that is tested in parallel. It can be a straight sample or it can be mounted on the surface of a barrel. Mounting on a barrel surface gives a longer length in the testing region and thus a more accurate measurement. Because these witness or barrel samples do not have insulation, nor are they wound in layers, they don't experience the possible degradation issues that wire in coil form can experience.
Bi-2212 precursor powders with cation stoichiometry of Bi:Sr:Ca:Cu of 2.17:1.94:0.89:2.0 made by the melting-casting process were purchased from Nexans SuperConductors GmbH. As per
A 16 layer coil, with a total of 672 turns, was made from 112 m of 1.50 mm 85×19 wire. The coil was heat-treated in a flowing oxygen atmosphere using a partial melt-solidification process. The coil was annealed in the flowing oxygen gas at 450° C. for 10 hours with a heating rate of 100-150° C./hr., and this cycle was repeated twice to burn off the polyurethane resin binder. After cooling to room temperature the furnace was evacuated to a vacuum of <60 millitorr and held for 2 hours. Then the furnace was back-filled with pure oxygen. The furnace was ramped to a maximum temperature of 889° C. with a ramp rate of 40° C./hr and a cooling rate of 2.5° C./hr to 830° C. where it was held for 60 hours before a furnace cool-down to room temperature. No leakage was found on the coil surface after heat treatment. The coil was able to achieve a supercurrent of 425 at 4.2 K and 5 T applied field before quenching, equivalent to 90% of a 1 m witness test sample. The coil generated 3.98 T in 5 T background field as shown in
The temperature of the pre-reaction sequence needed to burn off the organic component of the braid depends on balancing two major factors. One factor is that the uses of specific temperatures have shown to have significant effects on the short sample Jc of Bi-2212. An experiment on short sample Ic optimization of strand without braid showed that a pre-reaction sequence of 320° C. for 2 hrs. gave ˜10-20% higher Ic than a pre-reaction sequence of 820° C. for 2 hrs. The other factor is that outgassing of undesirable gases is enhanced at higher temperatures. So one must balance the need to remove as much organic binder as possible by using high temperatures versus the need to use lower temperatures to optimize the intrinsic Ic of the strand.
While the present invention has been described in terms of specific embodiments thereof, it will be understood in view of the present disclosure, that numerous variations upon the invention are now enabled to those skilled in the art, which variations yet reside within the scope of the present teaching. Accordingly, the invention is to be broadly construed, and limited only by the scope and spirit of the claims now appended hereto.
Number | Name | Date | Kind |
---|---|---|---|
5531015 | Manlief et al. | Jul 1996 | A |
5902774 | Muranaka et al. | May 1999 | A |
6344430 | Duperray et al. | Feb 2002 | B1 |
6746991 | Rey et al. | Jun 2004 | B2 |
Entry |
---|
Miao, et al, IEEE Transactions on Applied Superconductivity (2007) 17(2): 2262-2265. |
Markiewicz, et al, IEEE Trans. on Appl. Supercond. 2006; 16(2): 1523-1526. |
Watanabe, et al., Advances in Cryo Engineering, 2007; 54: 439-444. |
Number | Date | Country | |
---|---|---|---|
20090325809 A1 | Dec 2009 | US |