Abstract:
A container system including a vessel for holding a thixotropic semi-solid aluminum alloy slurry during its processing as a billet and an ejection system for cleanly discharging the processed thixotropic semi-solid aluminum billet. The crucible is preferably formed from a chemically and thermally stable material (such as graphite or a ceramic). The crucible defines a mixing volume. The crucible ejection mechanism may include a movable bottom portion mounted on a piston or may include a solenoid coil for inducing an electromotive force in the electrically conducting billet for urging it from the crucible. 
     During processing, a molten aluminum alloy precursor is transferred into the crucible and vigorously stirred and controlledly cooled to form a thixotropic semi-solid billet. Once the billet is formed, the ejection mechanism is activated to discharge the billet from the crucible. The billet is discharged onto a shot sleeve and immediately placed in a mold and molded into a desired form.

Description:
TECHNICAL FIELD OF THE INVENTION 
     The present invention relates generally to ceramics, and, more particularly, to a method and apparatus for containing and directing a flowable superconducting slurry. 
     BACKGROUND OF THE INVENTION 
     The history of superconductivity begins in the early twentieth century. The phenomenon of superconductivity was discovered in 1911 by Heike Onnes as part of an investigation of the physical properties of mercury at very low temperatures. In 1946, Ogg observed superconductivity in very low temperature metal-ammonia solutions. In the early 1970&#39;s, alloys of niobium metal were found to be superconductive at liquid helium temperatures. In 1986, Bednorz and Miller observed superconductivity in a La—Ba—Cu ceramic oxide lattice at about 35 K. Shortly thereafter, a research team lead by C. W. “Paul” Chu announced the first material to superconduct above the liquid nitrogen threshold, a ceramic oxide having the general formula Ba 2 YCu 3 O 7−x . Since 1988, a number of materials that exhibit superconductivity above the liquid nitrogen temperature have been identified, including various Ba—Sr—Ca—Cu oxide compositions and Th—Ca—Ba—Cu oxide compositions. 
     Superconductors having critical temperatures (T c &#39;s—temperatures below which they behave as superconductors) above the temperature of liquid nitrogen (about 78 K at standard pressures) are very advantageous since the costs of cooling with liquid nitrogen are much less than the costs of cooling with liquid helium. Moreover, liquid nitrogen cooling systems are safer, less complicated, and less hazardous than liquid helium cooling systems. 
     Superconductors made from ceramic oxides also share several disadvantages. One such disadvantage of oxide ceramic superconductors is the requirement of very high purity raw materials. Ceramic oxide superconductors are extremely sensitive to impurities in the parts-per-billion range, which tend to form local non-superconducting regions within single superconducting oxide grains, degrading or destroying the superconductivity thereof. This make is extremely difficult to produce oxide superconducting powders having consistent grain-to-grain properties, and even more difficult to form bodies having consistent intra-body and/or extra-body superconductor properties. 
     Another disadvantage of ceramic oxide superconductors is their extreme sensitivity to slight variations in their processing environment. Slight differences in furnace temperature and/or oxygen partial pressure during annealing can result in different electrical properties (such as T c , magnetic threshold H c , and the like) in pieces formed from the same superconducting oxide batch. Ceramic superconducting oxides are especially sensitive to oxygen partial pressure during processing, since most tend to have an oxygen-deficient perovskite structure. In other words, ceramic superconducting oxides such as Ba 2 YCU 3 O 7−x , are non-stoichiometric permutations of the stoichiometric perovskite-structured composition Ba 2 YCU 3 O 9 , wherein nearly ⅓ of the oxygen atoms have been removed. As a result, the material is very sensitive to the variations in processing occurring during the critical oxygenation step. 
     Still another disadvantage inherent in ceramic oxide superconductors is that they are relatively brittle. Even the “flexible” thin films or wires formed from oxide superconductor compositions are relatively brittle as compared to traditional metal wires. 
     Yet another disadvantage of ceramic oxide superconductors is that the superconducting oxide particles or grains tend to have anisotropic superconducting properties. Oxide superconductors have a multilayered crystal structure, and current tends to flow preferentially within the layers. Sintered ceramic oxide superconducting bodies tend to have randomized grain orientations, and so their current-carrying abilities are reduced to a fraction of the theoretical by the randomly oriented anisotropic grains. Moreover, the grain boundaries between the sintered grains also tend to be poor conductors, further limiting the current flow in a sintered ceramic oxide superconductor. 
     There is therefore a need for a ceramic oxide superconducting conduit having increased flexibility, homogeniety of properties, no grain boundary conductivity barriers. The present invention is addresses this need. 
     SUMMARY OF THE INVENTION 
     The present invention relates a flowable, formable high-temperature superconducting slurry. One form of the present invention includes a superconducting slurry formed of substantially spherical high-temperature superconducting oxide powder suspended in liquid nitrogen. 
     One object of the present invention is to provide an improved magnetic shielding apparatus. Related objects and advantages of the present invention will be apparent from the following description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a partial sectional side elevational view of a conduit filled with a superconducting slurry of a first embodiment of the present invention. 
     FIG. 2A is a partial sectional side elevational view of the conduit of FIG. 1 wherein the density of superconducting particles in the slurry is low. 
     FIG. 2B is a partial dectional side elevational view of the conduit of FIG. 1 wherein the density of superconducting particles in the slurry is high. 
     FIG. 3A is a perspective view of the conduit of FIG.  1 . 
     FIG. 3B is a schematic view of the conduit of FIG. 1 connected to a pump. 
     FIG. 4A is a partial sectional side elevational view of the conduit of FIG. 1 wherein the slurry is substantially monomodal. 
     FIG. 4B is a partial sectional side elevational view of the conduit of FIG. 1 wherein the slurry is substantially polymodal. 
     FIG. 5 is a partial sectional view of a double-walled vessel for containing superconducting slurry. 
     FIG. 6A is a partial sectional side elevational view of the conduit of FIG. 1 including a pair of screens defining a slurry-containing region therebetween. 
     FIG. 6B is a partial sectional side elevational view of the conduit of FIG. 1 including a pair of screens defining a slurry containing region therebetween and having a wire extending through the conduit and into the slurry containing region. 
     FIG. 7 is a partial sectional side elevational view FIG. 1 illustrating two spaced slurry containing regions. 
     FIG. 8 is a partial sectional perspective view of FIG. 1 illustrating the conduit formed into a coil. 
     FIG. 9 is a partial sectional side elevational view of FIG. 1 illustrating a switch formed in the conduit. 
     FIG. 10 is a partial sectional side elevational view of FIG. 1 illustrating a switch formed in the conduit. 
     FIG. 11 is a partial sectional side elevational view of FIG. 1 illustrating a resistor formed in the conduit. 
     FIG. 12A is a perspective view of a superconducting particle of FIG.  1 . 
     FIG. 12B is a perspective view of a coated superconducting particel of FIG.  1 . 
     FIG. 13 is a side elevational schematic view of illustrating the conduit of FIG. 1 passing through a series of permanent magnets alternating with coil windings. 
     FIG. 14 is a perspective view of the conduit of FIG. 1 formed into a coil and having a permanent magnet suspended therein. 
     FIG. 15 is a partial sectional side elevational view of the conduit of FIG. 1 having a pair of spaced inductors positioned therearound. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     For the purposes of promoting an understanding of the principles of the invention, reference will now be made to the embodiment illustrated in the drawings and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended, and alterations and modifications in the illustrated device, and further applications of the principles of the invention as illustrated therein are herein contemplated as would normally occur to one skilled in the art to which the invention relates. 
     FIGS. 1-15 illustrate a first embodiment of the present invention, a bulk superconductor system  20  including a superconductor slurry  22  contained within an elongated conduit  24 . The superconducting slurry  22  preferably includes particles  26  of a high T c  superconducting ceramic oxide, such as Ba 2 YCU 3 O 7−x  or the like, suspended in liquid nitrogen  28 . The concentration of superconductor particles  26  in the liquid nitrogen  28  matrix may be varied as desired. For example, it may be desired to maintain a high concentration of superconductor particles  26  in the slurry  22 , such that each of the particles  26  is in physical contact with at least two other particles  26  in order to provide a conductivity path through the slurry  22 . (See FIG.  2 A). Also, a high concentration of superconducting particles  26  in the slurry  22  would increase its bulk magnetic threshold H c , or the amount of magnetic flux necessary to break down superconductivity. Such a high concentration slurry  22  would be desirable if the system  20  were to be used for electrical power transfer or as a magnetic insulator. Alternately, a slurry  22  could be formed having lower concentration of superconducting particles  26 , if it were desired for the system to not have bulk electrical conductivity or if a specific H c  were desired. (See FIG.  2 B). 
     Referring to FIGS. 3A and 3B, the elongated conduit  24  is preferably a tube, pipe, or cryogenic hose that is at least somewhat flexible or ductile at room temperature. The elongated conduit  24  may itself be formed from an electrically conductive material, such as copper or steel, or may alternately be formed from an electrically insulating material such as rubber or polymer. FIG. 3B schematically illustrates the system  20 , in which the conduit  24  is fluidically connected to a pump  29  adapted to circulate liquid nitrogen  28  or slurry  22  therethrough. 
     FIGS. 4A and 4B illustrate slurries  22  having different particle size distributions (PSDs). The slurry  22  illustrated in FIG. 4A has a PSD that is characterized as substantially monomodal. In other words, there is substantially only one size superconducting particle  26  represented in the slurry  22  of FIG.  4 A. FIG. 4B illustrates a slurry  22  having a multimodal PSD which is made up of superconducting particles  26  having several distinct sizes. The PSD of the slurry  22  affects such parameters as particle density, slurry viscosity, and thixotropy. 
     In operation, as an electrical power conductor the slurry  22  could be pumped either mechanically or by gravitational flow to the point of use. The slurry  22  need only be cooled to the required T c  thus reducing the energy requirements. Since there are no Joule heating losses in an electrical superconductor (line resistance is zero), the only energy loss consideration is the energy input necessary to maintain the nitrogen in a liquid state. Moreover, the slurry  22  could be used to transmit either AC or DC power directly to the consumer. The primary advantage of AC power is that alternating current can be transmitted through electrically conducting cables with substantially less power loss from Joule heating (I 2 R) than can direct current. The superconducting slurry  22  can be used to transmit power with no line loss. Given a closed system having appropriate insulation, the energy required to maintain a liquid nitrogen based slurry is minimal compared to the power losses associated with transmitting AC current through conventional electrical cables. Likewise, the slurry  22  may be used to transmit information in the form of electrical signals or impulses with little or no attenuation or distortion. 
     FIG. 5 illustrates a double-walled conduit  24  having an exterior valve  30  communicating with a first conduit portion  32  defined between the outer and inner conduit walls  34 ,  36 . A second conduit portion  38  is defined radially inward of inner conduit wall  36  and may be accessed by a second valve  40 . Slurry  22  may be flowed through the first conduit portion  32 , the second conduit portion  38 , or both  32 ,  38 . 
     In operation as a magnetic shielding device, the first conduit portion  32  is filled with slurry  22 . The diamagnetic slurry  22  resists penetration of a magnetic field therethrough, thereby defining a magnetically insulated inner volume. The H c  of the slurry  22  is a function of the superconducting particle density therein, and may therefore be controlled by controlling the superconductor particle density of the slurry  22 , such that the critical breakdown strength of the magnetic shield thereby formed may be precisely controlled. 
     The container is illustrated as having the shape of a right circular cylinder, thereby defining an interior magnetically shielded volume of the same shape. However, the container can be formed having any desired size and shape (either regular or irregular) to define a magnetically shielded interior volume of the same desired shape. 
     FIG. 6A illustrates a conduit  24  having at least one slurry containing portion  42  defined by a pair of screens  44 . The slurry containing portion  42  of the conduit  24  is in fluid communication with the remaining portions of the conduit  24  (i.e., liquid nitrogen  28  passes substantially freely through the screens  44 , but superconducting particles  26  do not). The superconducting portion of the conduit is therefore the slurry-containing portion  42 . FIG. 6B illustrates the inclusion of an electrically conducting wire  46  extending through the conduit  24  and through the screen  44 , such that the superconducting portion  42  is in electrical communication with the exterior of the conduit  42 . The wire  46  may be formed from a superconductor material or from a conventional electrical conductor material. 
     In operation, the screens would separate the superconducting slurry  22  from the coolant, defining a predetermined area of use. This separation would reduce the amount of superconductor material required by restricting the superconductor material to the predetermined area of use. This configuration also enhances the shielding effects of the slurry  22  by allowing a single conduit  24  to define spaced area of shielding and non-shielding. 
     FIG. 7 illustrates a conduit  24  configuration including four screens  44  defining two slurry containing portions  42  positioned relatively close to each other. Each slurry-containing portion  42  includes a wire  46  extending therefrom through the conduit  24  to provide electrical communication therewith. 
     In operation, this configuration may be used to define a capacitative circuit element. Moreover, this configuration may be used to create a layering effect of magnetic shielding, i.e. partial magnetic shielding may be achieved to finely control the application of a magnetic field therethrough. One application for such a finely controlled magnetic field is in precision switching. Fine control can be achieved by varying such parameters as separation distance between the slurry-containing and non-slurry-containing portions of the conduit  24  and by varying the thicknesses of the slurry-containing and non-slurry-containing portions. Also, a magnetic oscillator could be built by placing a permanent magnet between the spaced diamagnetic (i.e., slurry-containing) portions of the conduit  24 . 
     FIG. 8 illustrates a conduit  24  formed as a coil or helix through which superconducting slurry  22  is flowed. The coil defines an interior volume at least partially shielded from external magnetic fields. 
     In operation, current is passed through the superconducting slurry  22  in the coiled conduit  24 . As the current density changed, the conducting coil behaves as an inductor. The magnetic field so generated within the inductor coil is especially well contained and shielded by the diamagnetic superconductor slurry  22 . 
     FIGS. 9 and 10 illustrate a conduit including a first and second superconducting partial screen  50  defining a switch portion  52  therebetween. The partial screens  50  are adapted to allow the passage of liquid nitrogen  28  therethrough and block the passage of superconducting particles  26 . Superconductor particles  26  fill the conduit  24 , including the switch portion  52 . A resistor coil  54  is positioned in thermal communication with the switch portion  52  and connected to a controlled electrical power source (not shown.) In FIG. 9, the resistor coil  54  encircles the switch portion  52 , while in FIG. 10 the resistor coil  54  is positioned adjacent the switch portion  52 . An inner conduit portion  56  free of superconducting particles  26  may be included through which liquid nitrogen  28  may flow. Additionally, the partial screens  50  may be adapted to pass liquid nitrogen only through the inner conduit portion  56 , such that the remaining volume of the switch portion is filled with dry superconductor particles  26 . 
     In operation, the resistor coil  54  is activated by the flow of electricity therethrough and generates heat. The resistor  54  provides heat to the slurry  22 , heating the superconductor particles above T c , such that they no longer superconduct. Thus, activation of the resistor coil  54  acts to switch the conduit  24  off, cutting the flow of power therethrough. Pulsing power through the resistor coil  54  could allow oscillations of current through the conduit  24 , as the temperature of the superconductor particles alternately exceeds and falls below T c . This configuration may therefore be used to transmit magnetic waves. 
     FIG. 11 shows a conduit  24  having a resistor portion  60  defined between two partial screens  62 . The partial screens have a central portion  64  through which liquid nitrogen  28  but not superconductor particles  26  may pass and an outer ring portion  66 . The outer ring portion  66  is preferably formed from an electrical conductor and is more preferably formed from a high T c  superconductor. A resistor layer  68  is formed in the resistor portion  60 , extending between the partial screens  62  in electric communication with the outer ring portions  66 . 
     In operation, the partial screens  62  allow liquid nitrogen  28  to pass through the resistor portion  60 , but prevent any superconductor particles  26  from passing therethrough. In other words, no slurry  22  is allowed to pass through the resistor portion  60 . When power is transmitted through the conduit  24 , the resistor potion  60  behaves as a resistor, i.e., current flow is resisted and heat is generated therein according to I 2 R. In this way, resistor circuit elements may be introduced into the superconducting conduit  24 . 
     FIG. 12A illustrates a typical substantially spherical superconductor particle  26 . Preferably, the superconducting particle is formed from a high T c  oxide superconductor such as Ba 2 YCU 3 ,O 7−x , but may be formed from any superconducting material with a T c  above the boiling point of liquid nitrogen. FIG. 12B illustrates a typical substantially spherical superconductor particle  26  having a nonconducting shell  70  formed thereabout. The nonconducting shell  70  is preferably formed from TEFLON, but may be formed from any convenient electrically insulating material. 
     The electrical conductivity of a slurry  24  formed from such particles is a function of the particle-to-particle contact or connectivity within the slurry. It is preferred that the superconducting particles  26  are substantially spherical to maximize contact and flowability. It is also preferred that the particle density of the slurry  24  be as great as possible while still allowing a flowable slurry  24 . 
     A superconducting slurry  24  formed using the coated particles  26  would exhibit the diamagnetic properties associated with a superconductor, but not the electrical current conducting properties. Such a slurry would be ideal for use in applications requiring magnetic and electrically insulative shielding. Such shielding could be used to insulate a powerful magnetic field without the generation of eddy currents in the insulator. The coating  70  also serves to protect the particles  26  from any dissolving agents they might otherwise come into contact with. 
     FIG. 13 illustrates another embodiment of the present invention, an alternating series of circular magnets  74  and wound coils  76  with a central axis  78  passing therethrough. A conduit  24  containing superconducting slurry  22  is positioned substantially coaxially with the axis  78  and extending through the magnets  74  and coil  76 . The magnets  74  may be electromagnets, permanent magnets, or a combination of both. The coils  76  are adapted to conduct electricity. 
     In operation, the magnetic field will resist the passage of the diamagnetic slurry  24  therethrough. The kinetic energy required to push the slurry  24  through the magnetic field established by the permanent magnets  74  is transduced into electrical current in the coil  76 . In other words, the change, due to the passage of the diamagnetic slurry material passing therethrough, in field strength in the field produced by the permanent magnets  74  induces an electric current in the coil. 
     FIG. 14 illustrates yet another embodiment of the present invention, a conduit  24  filled with superconducting slurry  22  and wound into a helix or coil having a central axis  80 . A cylindrical permanent magnet  82  having a cylindrical axis  84  is suspended coaxially with the coil. Preferably, the magnet  82  is spinning about its cylindrical axis  84 . The conduit  24  is connected to a current source (not shown). 
     In operation, the core of the helix defines a diamagnetic or magnetically shielded volume in which an appropriately sized and shaped permanent magnet may be suspended. The permanent magnet may be coupled to a variety of devices to achieve a variety of net effects. For example, the magnet may be used as a dashpot for precision measurement of forces or as a damper for absorption of forces (this would be especially conducive for use in spacecraft). The shock absorber could be actuated just prior to use (such as in landing or docking operations) by pumping the slurry  22  (or the coolant to form a slurry  22 ) through the helix. 
     FIG. 15 illustrates a conduit  24  carrying flowing superconducting slurry  22 . A first induction coil  90  is placed around one portion of the conduit  24 , and a second, spaced induction coil  92  is similarly wrapped around a second portion of the conduit  24 . 
     If an alternating current is passed through the first coil  90 , a similar current is generated in the slurry  22 . The current may be transmitted through the conduit  24  to the second coil  92 . The current flowing through the conduit  24  may then be used to induce a current in the second coil  92 . In this way, power may be transmitted through the superconducting slurry  22  in the conduit  24  with no energy loss from Joule heating or from leakage of the coolant  28  the power extraction point. 
     While the invention has been illustrated and described in detail in the drawings and foregoing description, the same is to be considered as illustrative and not restrictive in character, it being understood that only the preferred embodiment has been shown and described and that all changes and modifications that come within the spirit of the invention are desired to be protected.