CERAMIC/METAL AND A15/METAL SUPERCONDUCTING COMPOSITE MATERIALS EXPLOITING SUPERCONDUCTING PROXIMITY EFFECT

Abstract: A composite superconducting material (8) is made of ceramic superconducting particles (5) which are coated with a noble material (4) and surrounding by a metal matrix material (6). The metal matrix material (6) fills the regions between the coated ceramic particles (5). The noble material (4) is chemically nonreactive with the ceramic. Preferably, it is silver. The coating serves to chemically insulate the ceramic superconducting particles (5) from the matrix material (6). The matrix material is a metal that is susceptible to the superconducting proximity effect. Preferably, it is a NbTi alloy. The matrix material (6) is induced to become superconducting by the superconducting proximity effect when the temperature of the superoconducting material (8) is below the critical temperature of the ceramic superconducting particles (5). The superconducting material (8) has the improved mechanical properties of the matrix material (6). An alternative embodiment of the invention utilizes uncoated A15 compound superconducting particles in a metal matrix material which is preferably a NbTi alloy.

Description: CERAMIC/METAL AND A15/METAL SUPERCONDUCTING COMPOSITE MATERIALS EXPLOITING SUPERCONDUCTING PROXIMITY EFFECT The development of this invention was supported in part by grant number DEFG03-86ER45245-A012 from the U.S. Department of Energy. The United States Government has certain rights to the invention. RELATED APPLICATION INFORMATION This application claims priority from copending U.S. application Ser. No. 08/806,697 filed February 26, 1997. FIELD OF THE INVENTION This invention relates to high temperature ceramic superconductors and A15 superconductors and, more specifically, to improving the superconducting and mechanical characteristics of these superconducting materials . BACKGROUND OF THE INVENTION The discovery of high critical temperature (T c ) ceramic superconductors has inspired an enormous interest in their application. Conventional niobium alloy superconductors such as NbTi must be cooled well below a temperature of 10K to achieve useful superconductivity. High T c superconductors, on the other hand, have T c • s over 100K. Due to the great expense of cryogenic refrigeration, the high T c materials could find much wider application in electronic and laboratory devices . Of particular interest are materials which have T above 77K, because this is the temperature of liquid nitrogen, a common and relatively inexpensive refrigerant. Ceramic superconducting materials have not been used in many potential applications because they suffer from a number of shortcomings. The most severe problems with the ceramic superconductors are as follows : 1) They are brittle. They are not flexible and thus cannot be made into wires or other useful shapes . Cracks and boundaries between adjacent crystals severely limit supercurrent flow. 2) They are highly anisotropic. Supercurrents preferentially flow in certain directions with respect to the crystal lattice, reducing maximum supercurrent flow in randomly oriented multicrystalline pieces. 3) They are strong oxidizing agents. Many metals, such as copper, lead, tin, and niobium, are oxidized by contact with the ceramic superconductors, forming an insulating layer which impedes supercurrent flow. Only noble metals such as gold, silver, palladium and their alloys resist being oxidized by the ceramic superconductors . A less severe undesirable feature of the high temperature ceramic superconductors is that they can lose their superconducting properties. The superconducting structure inside the ceramics has an abundance of oxygen atoms which are necessary for superconductivity. Heating, grinding, etching, or prolonged exposure to ambient atmosphere or vacuum liberates the oxygen and destroys superconductivity. Oxygen content can be restored by annealing the ceramic in an atmosphere with a partial pressure of oxygen. The A15 superconductors such as Nb3S are also brittle materials, although they are not anisotropic and are relatively nonreactive. Their poor mechanical properties have rendered it difficult and expensive to use the A15 compounds in many applications requiring ductility. A superconducting wire made of a compressed powder of an A15 compound will have a low critical current density. This is because the supercurrent flow between neighboring A15 crystals can only travel through the small area of surface contact between neighboring crystals. This is unfortunate because A15 compounds generally have superior superconducting properties such as high T c ' s and high critical magnetic fields when in a single crystal form. To overcome the brittleness and anisotropy of the high T c ceramics, the superconducting proximity effect has been exploited in the prior art. For example, U.S. Patent 5,041,416 to Wilson describes a method for making a composite superconducting material . Powders of ceramic superconductor and normal metal are mixed and the mixture is subjected to heat and high pressure. The temperature is controlled during the process such that excessive oxygen is not lost from the ceramic . Silver is preferred as the normal metal. A severe problem with this method is that it attempts to use silver as a proximity superconductor. Silver has poor superconducting properties, and the resultant material therefore has poor superconducting properties . Another problem with this method is that when ceramic and silver powders are mixed, the silver particles tend to clump together, forming a macroscopically inho ogeneous material . U.S. Patent 5,194,420 to Akihama describes a composite ceramic/metal superconducting material consisting of superconducting ceramic particles dispersed in a metal matrix material. Many different metals are disclosed for use as the normal metal matrix. The noble metals (gold, silver) have poor superconducting properties, and are therefore not capable of carrying much supercurrent. If the non-noble metals are used, they are oxidized by the ceramic, forming an oxide layer which inhibits supercurrent. The oxide layer thus formed also reduces the critical magnetic field of the composite material. A composite material made according to Akihama will have a low supercurrent carrying capability. U.S. Patent 5,081,072 to Hosokawa et al . describes a method of preparing a superconducting ceramic powder and forming the powder into a superconducting material. Also discussed is the possibility of coating the ceramic particles with silver or gold resulting in a composite material consisting of ceramic particles fused together by the silver or gold. A composite material made only with silver or gold coated ceramic particles has poor superconducting properties, however, and the supercurrent carrying capability of the resultant material will be low. Further, because the silver or gold can only be made to be weakly superconducting, the material will behave poorly in modest magnetic fields. Therefore, there exists a need for a composite superconducting material that has mechanical properties superior to bulk ceramic materials, does not experience problems associated with the chemical reactivity of the ceramic superconductors, has a high critical temperature, and has a high supercurrent carrying capability. OBJECTS AND ADVANTAGES OF THE INVENTION It is an object of the present invention to provide a composite superconducting material that: 1) has a relatively high critical temperature; 2) has improved mechanical properties (such as ductility) compared to ceramic superconductors; 3) chemically insulates the ceramic superconducting material from the proximity superconducting material; 4) has increased isotropy compared to bulk ceramic superconductors ; 5) has increased critical current density compared to the prior art composite superconducting materials; and 6) has increased ability to remain superconducting in applied magnetic fields compared to the prior art superconducting materials . It is a further object of this invention to provide a composite A15 superconducting material that: 1) has improved mechanical properties compared to pure A15 materials ; and 2) retains the superior superconducting properties (relatively high T c , and high critical magnetic field) of the A15 materials. Further objects and advantages will be apparent upon reading the following description with reference to the drawings . SUMMARY OF THE INVENTION These objects and advantages are attained by a composite superconducting material comprising a plurality of ceramic superconductor particles. Each of the ceramic particles is coated with a thin layer of noble material which is chemically nonreactive with the ceramic particles. The coated ceramic particles are disposed in a metal matrix material which fills the regions between the particles. The metal matrix material is an intrinsic superconductor having a critical temperature lower than the critical temperature of the ceramic particles. In addition, the matrix material has an electron-phonon coupling coefficient greater than 0.5. The matrix material is thus susceptible to a superconducting proximity effect induced by the ceramic particles, and the ceramic particles are electrically coupled due to the proximity effect. A method for producing the composite superconducting material includes the step of forming a plurality of particles of a ceramic superconductor material. Each of the ceramic particles is coated with a thin layer of the noble material. The method also includes the step of providing a plurality of particles of the metal matrix material . The coated ceramic particles and the matrix material particles are then thoroughly mixed. The mixture is then fused to form a solid material. An alternative method for producing the composite superconductor material includes the step of providing a plurality of particles of the ceramic superconductor material. Each of the ceramic particles is coated with a thin layer of the noble material. A coating of the metal matrix material is then applied to each ceramic particle on top of the noble material coating. The coated ceramic particles are then fused to form a solid material. Possible metal matrix materials include niobium, indium, NbTi alloy, tin, lead, lead/bismuth alloys, and alloys thereof. In the preferred embodiment, the metal matrix material is NbTi alloy. The noble material is preferably silver, but other noble materials which are not reactive with the ceramic superconductor particles may also be used. For example, other noble materials which may be coated on the ceramic particles include gold, palladium, and alloys of gold, palladium or silver. The layer of noble material which is coated on each ceramic particle is preferably thinner than the decay length of the noble material. Also, the noble material should be sufficiently thick to chemically insulate the ceramic particles. Preferably, the thickness of the noble material layer is in the range of 5 to 3000 nanometers. If silver is used as the noble material, the thickness of the silver layer is preferably 5 nanometers. It is also preferred that the noble material be permeable to oxygen at least one temperature. The ceramic particles are preferably comprised of a cuprate superconductor ceramic material. The ceramic particles are also preferably larger than the superconducting coherence length of the ceramic material of which they are made. Preferably, the particles have physical dimensions in the range of 1.5 nanometers to 10 microns. An alternative embodiment of the invention presents a composite superconducting material comprising a plurality of A15 compound particles. The A15 compound particles are disposed in a metal matrix material which fills the regions between the particles. The metal matrix material is an intrinsic superconductor having a critical temperature lower than the critical temperature of the A15 compound particles. In addition, the matrix material has an electron-phonon coupling coefficient greater than 0.5. The matrix material is thus susceptible to a superconducting proximity effect induced by the A15 compound particles, and the A15 compound particles are electrically coupled due to the proximity effect. A method for producing the A15 compound composite superconducting material includes the step of providing a plurality of particles of an A15 compound and a plurality of particles of the metal matrix material . The matrix material particles and the A15 compound particles are mixed thoroughly. The resultant mixture is then f sed to form a solid material . An alternative method for producing the A15 compound composite superconducting material includes the step of providing a plurality of A15 compound particles. Each of the A15 compound particles is coated with a layer of the metal matrix material. The coated A15 particles are then fused to form a solid material . Possible A15 compounds include NbβSn, Nb3Ge, Nb3Si, or Nb3Al . Possible metal matrix materials include niobium, indium, NbTi alloy, tin, lead, lead/bismuth alloys, or alloys thereof. Preferably, the metal matrix material is NbTi. The A15 compound particles preferably have physical dimensions larger than the superconducting coherence length of the A15 compound of which they are made. The A15 compound particles, for example, can have physical dimensions in the range of 0.1 to 10 microns. BRIEF DESCRIPTION OF THE FIGURES Fig. 1A is a graph of the superconducting gap magnitude in a relatively high λ proximity superconductor. Fig. IB is a graph of the superconducting gap magnitude in a relatively low λ proximity superconductor. Fig. 2 is a cross sectional view of an intrinsic superconductor inside a proximity superconductor. Fig. 3 is a cross sectional view of a high T c cuprate ceramic embodiment of the composite superconducting material of the present invention. Fig. 4A is a graph of the conventional, incorrect understanding of the superconducting gap in a three layer junction with a low- λ metal in the middle . Fig. 4B is a graph of a new understanding of the superconducting gap in a three layer junction with a low-λ metal in the middle . Fig. 5 is a cross sectional view of a row of intrinsic superconducting particles in a metal matrix illustrating how a supercurrent can travel through the proximity superconducting regions. Fig. 6 is a cross sectional view of an A15 compound composite superconducting material according to an alternative embodiment of the present invention. Figs. 7A-7B are graphs of critical current density vs. composition for Nb3Sn/ Indium and Nb3Sn/Lead composite superconducting materials made in accordance with the present invention. Fig. 7C is a graph of critical current density vs. composition for Nb3Sn/Silver composite superconducting materials made in a manner analogous to the present invention. This graph illustrates the detrimental effect of silver on superconducting properties. DETAILED DESCRIPTION The present invention exploits the superconducting proximity effect to provide a composite superconducting material. The superconducting proximity effect occurs at the boundary between a normal conductor (a metal) and an intrinsic superconductor. The intrinsic superconductor induces the normal conductor to become superconducting near the boundary. The proximity superconductor has all the properties of a true superconductor. It expels magnetic fields, has a critical current density, and a superconducting gap. Also, the proximity superconductor is isotropic, and thus, is able to conduct supercurrent in any direction. The superconducting gap is an absence of electron energy states near the Fermi energy in a superconductor. It arises from the pairing of the electrons in the material which occurs at the superconducting critical temperature. The magnitude of the critical current density in any superconducting material is proportional to the superconducting gap. Thus, the larger the superconducting gap, the larger will be the critical current density of the material. Since the proximity effect has an exponential decay characteristic as a function of the distance from the intrinsic/proximity superconductor boundary, it is possible to specify a decay length. The decay length is the length over which the superconducting gap (and therefore the critical current density) of the proximity superconductor decreases by a factor of 1/e. The decay length is determined by the proximity superconductor and the temperature, to which it is inversely proportional. Silver, for example, has decay lengths of 22 nm and 44 nm at temperatures of 77K and 38.5K, respectively. Table 1 shows the decay lengths for a number of different metals. TABLE 1 A property of a proximity superconductor that is essential for understanding the proximity effect is the electron-phonon coupling coefficient, λ. While the decay length affects how rapidly (with distance) the superconducting gap decreases, λ affects the magnitude of the superconducting gap of the proximity superconductor. A high electron-phonon coupling coefficient results in a high superconducting gap and a high critical current density. The λ values for a number of different metals are given in Table 2. TABLE 2 The value of λ is a fundamental property of the electronic structure of a metal . Due to the nature of electron-phonon coupling, metals with a high value of λ (i.e., λ = 1 to 2 , such as lead, lead/bismuth alloys, tin, and NbTi alloys) generally have high resistivities and higher superconducting critical temperatures. Metals with low values for λ (i.e., λ = 0.1 to 0.2, such as silver and gold) generally have low resistivities and lower or no critical temperatures. Empirically, it is observed that high λ metals are more easily oxidized than low λ metals. All other factors being equal, high-λ proximity superconductors always have a larger superconducting gap, and therefore a larger critical current density, than low-λ proximity superconductors. It is noted that λ=0 for insulators . Figs. 1A and IB show the magnitudes of the superconducting gap for a high-λ proximity superconductor and a low-λ proximity superconductor, respectively. The high-λ proximity superconductor has a larger superconducting gap. Both curves are proportional to e"^ x , where k is the decay length and x is the perpendicular distance from the boundary. Large supercurrents can be carried in proximity superconductors with long decay lengths and large λ values. Since the decay length is inversely proportional to temperature, low temperatures enhance the proximity effect. It is noted that the critical current density is an important characteristic of superconducting materials because it is the maximum current density the material can carry. Many applications such as magnets demand high supercurrent density. It is desirable for superconducting materials to have as high a critical current density as possible. Fig. 2 shows a piece of an intrinsic superconductor 2 disposed inside a block of a proximity superconductor. The intrinsic superconductor 2 causes the proximity superconductor to become superconducting near the boundary of the two materials . The range of the proximity effect is represented by the circle 1. The range of the proximity effect is much larger for proximity superconductors having a long decay length and a high electron- phonon coupling coefficient. Fig. 3 shows a magnified cross sectional view of a ceramic composite superconducting material 8 according to a preferred embodiment of the present invention. The composite material 8 comprises superconducting high T c cuprate ceramic particles 5, each having a coating of a thin layer of noble material 4. The coated ceramic particles 5 are surrounded by a metal matrix material 6. The metal matrix material preferably completely fills the regions between the coated ceramic particles. Although the ceramic particles 5 are hard and brittle, the composite material 8 is relatively ductile because the metal matrix material 6 is ductile. The composite material 8 may be placed in a silver or copper tube and drawn to form a relatively flexible wire. The ceramic particles 5 have physical dimensions larger than the superconducting coherence length of the ceramic. Typically, the coherence length of high T c cuprate ceramics is about 1.5 nanometers. Preferably, the ceramic particles 5 are uniform in size and have physical dimensions in the range of 0.5 to 1.0 microns. More generally, the ceramic particles can have physical dimensions in the range of 1.5 nanometers to about 10 microns. The ceramic particles 5 may be single crystals or multicrystalline . The noble material 4 is selected to be chemically rionreactive with the superconducting ceramic particles 5. The cuprate ceramics are strong oxidizing agents and will oxidize all but the most noble metals such as silver, gold, palladium and their alloys. If a non-noble, oxidizable metal is contacted to the ceramic particles 5, an insulating metal oxide layer will form which blocks the superconducting proximity effect. For this and other reasons such as cost, silver is the preferred noble material. The layer of noble material 4 coated on each ceramic particle should be thinner than the proximity effect decay length of the noble material. For silver, a coating thickness of 5 nm is acceptable as the proximity effect decay length in silver is 22 nm (at 77K) . The maximum useful thickness of the noble material layer is about 3000 nanometers. Also, the layer of noble material 4 coated on each ceramic particle should be thick enough to chemically insulate the particle from the matrix material 6. Both the noble material/ceramic and noble material/metal matrix material interfaces should be clean and free of insulating contaminants to facilitate electrical contact across the interfaces. This is because insulating contaminants will block the superconducting proximity effect. The metal matrix material 6 is selected to have a large electron- phonon coupling coefficient λ, and preferably a long superconducting decay length. A large value of λ means that it is highly susceptible to the superconducting proximity effect. The combination of a long decay length and large electron-phonon coupling coefficient results in a large proximity effect extending deep into the metal matrix material 6. This is preferable as it increases the critical current density and improves the mechanical properties of the composite material 8 by allowing for greater distances between adjacent ceramic particles 5. The metal matrix material should have an electron-phonon coupling coefficient greater than about 0.5. The metal matrix material 6 should also be ductile to impart ductility to the composite material 8. Possible metal matrix materials include niobium, indium, NbTi alloy, tin, lead, lead/bismuth alloys, and alloys thereof. Preferably, the metal matrix material is NbTi alloy. The noble material 4 is selected to be chemically nonreactive with the ceramic particles 5. It has been empirically observed that materials having a high electron-phonon coupling coefficient are generally more reactive than materials having a low electron- phonon coupling coefficient. Therefore, a noble material which is sufficiently inert to remain unoxidized while in contact with a cuprate ceramic will likely have a low electron-phonon coupling coefficient. Silver, for example, has an electron-phonon coupling coefficient of 0.14. The composite material 8 as a whole is superconducting because the ceramic particles 5 cause the surrounding metal matrix material 6 to become superconducting via the proximity effect. The proximity effect extends from the ceramic particles 5 and through the layer of noble material 4 because the layer is substantially thinner than the decay length of the noble material. This is significant because, prior to the development of the present invention, conventional superconductor theory held that the superconducting proximity effect could not extend through even a thin layer of a low-λ metal such as the noble metals (silver, gold, palladium) . The conventional understanding of the proximity effect in a three layer junction is illustrated in the cross sectional view of Fig. 4A. The junction comprises an intrinsic superconductor 14, a thin layer of low-λ material 16, and a high-λ material 18. The superconducting gap clearly drops to nearly zero in the low-λ material 16 and stays at nearly zero in the high-λ material 18. Thus, the high-λ material 18 is not superconducting according to the incorrect conventional understanding. What actually happens in the three layer junction is shown in Fig. 4B. The superconducting gap rebounds in the high-λ material 18, provided that the low-λ layer 16 is thin compared to the decay length of the low-λ material. Thus, making a superconducting composite material 8 is possible even if the intrinsic superconductor is surrounded by a layer of low-λ material 16. The fact that the superconducting proximity effect can extend through a layer of a low-λ material was discovered by the present inventor. Returning to Fig. 3, it is noted that each layer of noble material 4 is somewhat detrimental to the superconducting properties of the composite material 8 because it likely has a low electron-phonon coupling coefficient. Therefore, it is best to make each layer of noble material as thin as possible, while still providing chemical insulation for the ceramic particles 5. The superconductivity of the surrounding metal matrix material 6 is enhanced via the proximity effect when the T c of the ceramic particles 5 is higher than the T c of the metal matrix material 6. This is the case for most combinations of cuprate ceramics and metal matrix materials (such as NbTi) . The composite material 8 should be designed such that adjacent ceramic particles 5 are electrically coupled due to the proximity effect. In other words, the nonzero superconducting gap functions induced in the metal matrix material 6 by adjacent ceramic particles 5 must overlap. If the ceramic particles 5 are too far apart, the critical current density of the composite material 8 will be reduced because the superconducting gap function will be smaller in the regions of the metal matrix 6 which are far from the ceramic particles 5. It is noted that if the ceramic particles are separated by a relatively large distance, then the T c of the composite material may be lower than the T c of the ceramic . Fig. 5 illustrates how a supercurrent 10 can travel through the composite material 8 by staying within the regions induced to be superconducting through the proximity effect. The circles 1 illustrate the approximate range of the proximity effect. The circles 1 should overlap in order to provide a continuous path for the supercurrent 10. The range of the proximity effect induced by the ceramic particles 5 will depend upon the temperature of the composite material 8, the properties of the matrix material 6 and of the noble material 4, and the thickness of the layer of noble material 4. A method for making the composite superconducting material 8 begins with selecting a suitable ceramic superconductor . Several possible ceramic superconductors are shown in Table 3 . YBa 2 Cu 3 θ 7 is preferred for many applications because of its high flux pinning strength and high T c . A high flux pinning strength allows the material to conduct supercurrent in high magnetic fields without energy loss . The superconducting ceramic is first formed into a powder which can be accomplished by many techniques known in the art, such as grinding. Preferably, the ceramic particles of the powder are in the size range of 0.5 to 1.0 microns. The best size range will depend upon the temperature at which the material is used, and the λ and decay length of the metal matrix material. The ceramic particles' of the powder are then cleaned by plasma etching. Cleaning prepares the surface of the ceramic particles to bond to the coating of noble material . Plasma etching techniques for the high T c cuprate ceramics are well known in the art . Each ceramic particle is then coated with a uniform layer of the noble material . The layer of noble material completely surrounds the ceramic particle. Preferably, silver is the noble material. Silver is preferred because it is easily applied as a coating, is permeable to oxygen, and is the least expensive of the metals that are not oxidized by the high T c cuprate ceramics. Silver can be deposited using a number of techniques well known in the art, such as chemical deposition and vapor deposition. A chemically deposited silver coating is formed by stirring the ceramic particles in a nonaqueous solution of a silver salt and adding a reducing agent. The silver is reduced and it precipitates on the surfaces of the ceramic particles. The silver solution should be nonaqueous because water adversely affects the ceramic particles. Alternatively, a vapor deposited coating may be formed by sifting the ceramic particles in a vacuum chamber containing a partial pressure of silver vapor. Both methods form clean silver coatings of accurate thickness. The silver coating is preferably 5 nm in thickness and completely surrounds the ceramic particle. The silver coating is thick enough to chemically insulate the ceramic particle. After the ceramic particles are coated with the noble material, the ceramic particles are replenished with oxygen. Replenishment is performed by heating the coated ceramic particles in an atmosphere with a partial pressure of oxygen. Since silver is permeable to oxygen at elevated temperatures (300°C and up) , oxygen reaches the ceramic. The best temperature, oxygen pressure, and annealing time is specific to each high T c ceramic and is selected to optimize the superconducting properties of each ceramic. Such annealing techniques for silver coated cuprate ceramics are well known in the art. The coated ceramic particles are then intimately mixed with particles of the metal matrix material. Preferably, mixing is performed at room temperature to avoid the formation of clumps. Preferably, the metal matrix material particles have physical dimensions in the range of 0.5 to 1.0 microns. Also preferably, the metal matrix material is a NbTi alloy. The volume ratio of ceramic particles to metal matrix material may be adjusted to optimize the superconducting properties of the composite material. Preferably, the composite material contains less than 50% by volume of the metal matrix material . Table 4 shows the decay lengths and electron-phonon couplings for several possible matrix material candidates . TABLE 4 The mixture is then compressed to fuse the particle mixture into a solid material. Applicable powder-fusing metallurgical techniques are well known in the art. Since the metal matrix material is ductile, it will flow under pressure to fill the regions between the coated ceramic particles. Preferably, no voids remain after the particle mixture is fused. Voids can be reduced by exposing the particle mixture to vacuum before fusing. Many different useful shapes can be created by disposing the particle mixture into a mold and compressing the mold. The particle mixture can be formed into a wire by placing it into a copper or silver tube and repeatedly drawing the tube to ever smaller diameters . According to an alternative method for making the composite superconducting material 8, the coated ceramic particles are covered with a layer of the metal matrix material instead of being mixed with particles of the metal matrix material. The thickness of the metal matrix material coating then determines the distance between adjacent ceramic particles. The matrix material coating can be formed with chemical or physical coating techniques similar to the techniques used to apply the coating of noble material. Such techniques are well known in the art. Ceramic particles having a two layer noble material and metal matrix material coating are then fused using the same powder metallurgical techniques described above . Fig. 6 shows a composite superconducting material 20 according to an alternative embodiment of the present invention. Composite superconducting material 20 comprises particles 22 of an A15 superconducting compound disposed in a metal matrix material 24. The A15 compound particles are not coated with a noble material because the A15 compounds are not particularly chemically reactive. The A15 particles should have physical dimensions larger than the coherence length of the A15 compound of which they are made. The A15 compounds typically have coherence lengths of about 0.1 micron, so the A15 particles should be at least this size. Preferably, the A15 particles have physical dimensions of about 0.5 to 1.0 microns . The A15 compound particles can be up to about 10 microns in size. If the A15 particles are substantially smaller than 0.5 microns, the superconducting properties of the composite material 20 may be degraded. The metal matrix material 24 is selected to be ductile and susceptible to the superconducting proximity effect. The metal matrix material should have an electron-phonon coupling coefficient greater than about 0.5. Possible metals which may be used as the metal matrix material include niobium, indium, NbTi alloy, tin, lead, lead/bismuth alloys, and alloys thereof. Preferably, the metal matrix material is a NbTi alloy. The composite material 20 has improved ductility as compared to pure A15 compounds, facilitating the manufacture of wire. The volume ratio of the A15 particles 22 to the metal matrix material 24 may be adjusted to optimize the superconducting properties of the composite material. The best volume ratio of A15 compound to metal matrix material will depend upon the metal matrix material used, among other factors. Preferably, the volume of the metal matrix material 24 comprises less than 50% of the total volume. The composite material 20 as a whole is superconducting because the A15 particles 22 induce the surrounding metal matrix material 24 to become superconducting by means of the proximity effect. Since the T c ' s of many A15 compounds are higher than the T c ■ s of many possible metal matrix materials (such as NbTi), the superconductivity of the surrounding metal matrix material is enhanced via the proximity effect. The composite material inherits the higher T c of the A15 compound. It is noted that if the A15 particles are separated by a relatively large distance, then the T c of the composite material may be lower than the T c of the A15 compound. The T c ' s of several possible A15 compounds are given in Table 5. TABLE 5 A method for making the composite superconducting material 20 includes the step of providing a quantity of particles of the A15 compound and a quantity of particles of the metal matrix material. The A15 compound particles and metal matrix particles are thoroughly mixed. The resultant mixture is then fused using high pressure to form a solid material. Preferably, no voids remain inside the composite material. Applicable powder metallurgical techniques are well known in the art. Alternatively, the particles of the A15 compound may be coated with a layer of the metal matrix material instead of being mixed with particles of the metal matrix material. This matrix material coating can be formed with chemical or physical coating techniques similar to the techniques used to apply the coating of noble material to the ceramic particles. Such techniques are well known in the art. The coated A15 particles are then fused into a solid material under high pressure. Applicable powder metallurgical techniques are well known in the art. Figs. 7A, 7B, and 7C show critical current densities for three A15 composite materials as a function of composition. All the measurements are taken at a temperature of 4.2K. The maximum critical current density is determined by the standard lμV/cm criterion. The composite materials are Nb3Sn/Indium, Nb3Sn/Lead, and Nb3Sn/Silver for Figs. 7A, 7B, and 7C, respectively. The b3Sn used in all the compositions has a 325 mesh size, which corresponds to particle sizes in the range of approximately 1-40 microns . The wires were made using the conventional powder-in-tube method. Powders of Nb3-Sn and the matrix metal were mechanically mixed in an inert atmosphere box and poured into a pre-cleaned and annealed copper billet. After filling the billet with the powder mixture, it was loosely fitted with a copper plug and evacuated to approximately 10 ~ 3 Torr . The plug was then forced at high pressure into the billet in a hydraulic press. The sealed billet was then reduced to a final diameter of about 300 microns through a series of cold rolling/drawing stages. Intermediate vacuum anneals at 250°C are required to soften the copper sheath which hardens during the cold working process . It can be seen that the indium composite material of Fig. 7A has a peak current density at approximately 15% indium content. Pure indium is not superconducting at 4.2K, so the critical current is zero for a wire consisting of 100% Indium. A pure Nb3Sn powder wire has a critical current density of about 11 KiloAmps/cm 2 . Including 15% indium increases this critical current almost fivefold to about 55 KiloAmps/cm 2 . The highest critical current density measured for the 15% Indium wire was 68 Kiloamps/cm 2 . This remarkable improvement is due to the proximity effect. Adding more than 15% Indium to the material results in deteriorating performance because the b3Sn particles become too far separated to be electrically coupled by the proximity effect. As can be seen in Fig. 7B, the lead composite materials having 50%, 65%, and 75% lead have critical current densities in excess of 100 KiloAmps/cm 2 , the maximum current density capability of the measuring apparatus. Pure lead has a critical current capability of about 55 KiloAmps/cm 2 . It can also be seen from Fig. 7B that only 20% by volume lead increases the critical current capability compared to pure Nb3Sn by over 8-fold to about 93 KiloAmps/cm 2 . Fig. 7C illustrates the detrimental effect of silver on composite superconductor materials. The critical current density of Nb3Sn/Silver decreases rapidly and monotonically with the addition of silver. Because silver is not a superconductor at any temperature, it is not susceptible to the proximity effect and so its addition adversely effects superconductivity. It will be clear to one skilled in the art that the above embodiment may be altered in many ways without departing from the scope of the invention. For example, lead, tin, indium or niobium may be used as the metal matrix material in either the high T c cuprate ceramic or A15 compound embodiment. Accordingly, the scope of the invention should be determined by the following claims and their legal equivalents .

Claims: CLAIMS What is claimed is: 1. A composite superconducting material comprising: a) a plurality of ceramic superconductor particles, wherein each of the particles is coated with a layer of noble material which is substantially chemically nonreactive with the particles; and b) a metal matrix material filling regions between the coated ceramic particles, wherein the matrix material is an intrinsic superconductor having a critical temperature lower than the critical temperature of the ceramic particles, the matrix material has an electron-phonon coupling coefficient greater than 0.5, the matrix material is susceptible to a superconducting proximity effect induced by the ceramic particles, and the ceramic particles are electrically coupled due to the proximity effect. 2. The composite superconducting material of claim 1 wherein the ceramic particles have physical dimensions larger than the superconducting coherence length of the ceramic material of which the particles are made. 3. The composite superconducting material of claim 2 wherein the physical dimensions range from 1.5 nanometers to 10 microns. 4. The composite superconducting material of claim 1 wherein the layer of noble material is sufficiently thick to chemically insulate the ceramic particle. 5. The composite superconducting material of claim 4 wherein the layer of noble material is 5 to 3000 nanometers thick. 6. The composite superconducting material of claim 1 wherein the noble material is selected from the group consisting of silver, gold, palladium, and alloys thereof. 7. The composite superconducting material of claim 1 wherein the noble material is relatively permeable to oxygen at least one temperature. 8. The composite superconducting material of claim 1 wherein the metal matrix material is selected from the group consisting of niobium, indium, NbTi alloy, tin, lead, lead/bismuth alloys, and alloys thereof. 9. The composite superconducting material of claim 1 wherein the ceramic particles comprise a material selected from the group consisting of YBa2Cu3θ7, Bi2Sr2Ca2Cu3θι o , Bi2Sr2CaιCu2θ8, (BiPb) 2Sr2Ca2Cu3θιo , Tl2Ba2Ca2Cu3θιo , and Tl2Ba2CaιC 2θ8. 10. The composite superconducting material of claim 1 wherein the thickness of the layer of noble material is substantially less than the decay length of the noble material . A method for producing a composite superconducting material , the method comprising the steps of : a) providing a plurality of particles of a ceramic superconductor material ; b) coating each of the ceramic particles with a layer of noble material , wherein the noble material is substantially chemically nonreactive with the ceramic superconductor material ; c) providing a plurality of particles of a metal matrix material, wherein the metal matrix material is an intrinsic superconductor having a critical temperature lower than the critical temperature of the ceramic particles, and wherein the metal matrix material has an electron-phonon coupling coefficient greater than 0.5; d) mixing the matrix material particles and the coated ceramic particles into a mixture; and e) fusing the mixture to form a solid material. 2. A method for producing a composite superconducting material, the method comprising the steps of: a) providing a plurality of particles of a ceramic superconductor material; b) coating each of the ceramic particles with a layer of noble material, wherein the noble material is substantially chemically nonreactive with the ceramic superconductor material; c) covering each of the coated ceramic particles with a layer of a metal matrix material, wherein the metal matrix material is an intrinsic superconductor having a critical temperature lower than the critical temperature of the ceramic particles, and wherein the matrix material has an electron-phonon coupling coefficient greater than 0.5; and d) fusing the ceramic particles to form a solid material. 13. A composite superconducting material comprising: a) a plurality of A15 compound particles; and b) a metal matrix material filling regions between the A15 compound particles, wherein the matrix material is an intrinsic superconductor having a critical temperature lower than the critical temperature of the A15 compound particles, the matrix material has an electron-phonon coupling coefficient greater than 0.5, the matrix material is susceptible to a superconducting proximity effect induced by the A15 compound particles, and the A15 compound particles are electrically coupled due to the proximity effect. 14. The composite superconducting material of claim 13 wherein the A15 compound particles comprise a material selected from the group consisting of Nb3Sn, Nb3Ge, Nb3Si, and Nb3Al . 15. The composite superconducting material of claim 13 wherein the metal matrix material is selected from the group consisting of niobium, indium, NbTi alloy, tin, lead, lead/bismuth alloys, and alloys thereof. 16. The composite superconducting material of claim 13 wherein the A15 compound particles have physical dimensions larger than the superconducting coherence length of the A15 compound of which they are made. 17. The composite superconducting material of claim 16 wherein the physical dimensions range from 0.1 to 10 microns . 18. A method for producing a composite superconducting material, the method comprising the steps of: a) providing a plurality of A15 compound particles; b) providing a plurality of particles of a metal matrix material, wherein the matrix material is an intrinsic superconductor having a critical temperature lower than the critical temperature of the A15 compound particles, and wherein the matrix material has an electron-phonon coupling coefficient greater than 0.5; c) mixing the matrix material particles and the A15 compound particles into a mixture; and d) fusing the mixture to form a solid material. 19. A method for producing a composite superconducting material, the method comprising the steps of : a) providing a plurality of A15 compound particles; b) coating the A15 compound particles with a coating of a metal matrix material, wherein the matrix material is an intrinsic superconductor having a critical temperature lower than the critical temperature of the A15 compound particles, and wherein the matrix material has an electron-phonon coupling coefficient greater than 0.5; and c) fusing the coated particles to form a solid material.

Inventors: HOLCOMB MATTHEW J, HOLCOMB, MATTHEW, J.
Applicants: UNIV LELAND STANFORD JUNIOR, THE BOARD OF TRUSTEES OF THE LELAND STANFORD JUNIOR UNIVERSITY
Application year: 1998
Main classification: H01L39
IPC codes: H01B12, H01B13, H01L39