Abstract
The sheath (3) is a material, which includes an aluminium (Al) matrix, in which nanometric aluminium oxide particles (Al.sub.2O.sub.3) are homogenously dispersed, the content of Al.sub.2O.sub.3 is 0.25 to 5 vol. % and the balance is Al. It is preferred that Al.sub.2O.sub.3 originates from the surface layer present on Al powder used as feedstock material for consolidation. The superconductor based on magnesium diboride (MgB.sub.2) core (1) is fabricated by powder-in-tube or internal magnesium diffusion to boron technology, while the tube is the Al+Al.sub.2O.sub.3 composite, which is a product of powder metallurgy. A loose Al powder is pressed by cold isostatic pressing, and then the powder billet is degassed at elevated temperature and under vacuum, and then is hot extruded into a tube. A thin diffusion barrier (2) tube filled up with a mixture of Mg and B powders or Mg wire surrounded with B powder is placed into the Al+Al.sub.2O.sub.3 composite tube under inert gas or vacuum. Such composite unit is cold worked into a thin wire and then annealed at 625-655° C. for 8-90 min, what results in a formation superconducting MgB.sub.2 in a wire's core (1).
Claims
1. A superconductor wire comprising: a superconductive core (1) based on magnesium diboride (MgB.sub.2); an outer sheath (3) based on aluminium (Al); wherein the superconductor wire has a density of less than 2.9 g.cm.sup.−3; wherein the superconductor includes at least one core (1) and a sheath (3) fully covers outer surface of the wire, wherein the outer sheath (3) is a composite material, which includes a metallic matrix of a pure Al and stabilizing component of aluminium oxide (Al.sub.2O.sub.3) in a volume of Al+Al.sub.2O.sub.3 composite sheath (3): the Al.sub.2O3.sub.3 component occupies from 0.25 to 5 vol. %, the Al matrix occupies from 95 to 99.75 vol. %, and impurities occupy up to 1 vol. %, preferably up to 0.3 vol. %, wherein the Al.sub.2O.sub.3 component is homogenously dispersed in an entire volume of Al+Al.sub.2O.sub.3 composite.
2. The superconductor wire based on MgB.sub.2 core with Al based sheath according to claim 1, wherein the Al.sub.2O.sub.3 component occupies from 0.5 to 3 vol. % in a volume of Al+Al.sub.2O.sub.3 composite.
3. The superconductor wire based on MgB.sub.2 core with Al based sheath according to claim 1, wherein the wire has an overall density less than 2.7 g.cm.sup.−3.
4. The superconductor wire based on MgB.sub.2 core with Al based sheath according to claim 1, wherein the sheath (3) is made of Al+Al.sub.2O.sub.3 composite tube, which is a direct product of powder metallurgy or is produced by machining of the powder metallurgy product, wherein the powder metallurgy product is fabricated by consolidating of atomized Al powders.
5. The superconductor wire based on MgB.sub.2 core with Al based sheath according to claim 4, wherein the Al powder has a mean particle size from 0.5 to 20 μm.
6. The superconductor wire based on MgB.sub.2 core with Al based sheath according to claim 1, wherein the Al.sub.2O.sub.3 component stems from a thin native surface layer, which passivates the Al powder used for consolidation.
7. The superconductor wire based on MgB.sub.2 core with Al based sheath according to claim 1, wherein the sheath (3) is made of Al+Al.sub.2O.sub.3 composite tube, which is a direct product of powder metallurgy or is produced by machining of the powder metallurgy product, wherein the powder metallurgy product is fabricated by consolidating of mechanically milled mixture of Al and Al.sub.2O.sub.3 powders.
8. The superconductor wire based on MgB.sub.2 core with Al based sheath according to claim 1, wherein the Al.sub.2O.sub.3 component in Al+Al.sub.2O.sub.3 composite sheath (3) has at least one characteristic dimension less than 250 nm, preferably less than 50 nm.
9. The superconductor wire based on MgB.sub.2 core with Al based sheath according to claim 1, wherein the Al grains in a final Al+Al.sub.2O.sub.3 composite sheath (3) has transversal grain size less than 10 μm, preferably less than 1 μm and even more preferably less than 900 nm.
10. The superconductor wire based on MgB.sub.2 core with Al based sheath according to claim 1, wherein the Al.sub.2O.sub.3 component are nanometric crystalline Al.sub.2O.sub.3 dispersoids, located at the Al grain boundaries and/or in the Al grain interiors.
11. The superconductor wire based on MgB.sub.2 core with Al based sheath according to claim 1, further comprising a diffusion barrier (2), which has a form of a layer at the interface between the core (1) and the sheath (3).
12. The superconductor wire based on MgB.sub.2 core with Al based sheath according to claim 11, wherein the diffusion barrier (2) is based on the elements from a group of titanium (Ti), tantalum (Ta), niobium (Nb), vanadium (V), iron (Fe), or is based on a combination of these elements, preferably based on Ti and/or V.
13. The superconductor wire based on MgB.sub.2 core with Al based sheath according to claim 1, wherein on the surface of an outer sheath (3) is a thin insulating Al.sub.2O.sub.3 layer, preferably formed by anodic oxidation.
14. The superconductor wire based on MgB.sub.2 core with Al based sheath according to claim 13, wherein the insulating Al.sub.2O.sub.3 layer formed by anodic oxidation has the thickness up to 10 μm.
Description
DESCRIPTION OF DRAWINGS
(1) The invention is further described in more detail with the aid of FIGS. 1 to 22. The cross-sections of the superconductor wires are shown schematically, the core/the cores size and the sheath ratio are being illustrative only. Particularly shown structures of Al particles and Al.sub.2O.sub.3 are not to be interpreted as a narrowing range of protection.
(2) FIG. 1 is a schematic cross-section of a single core superconductor with a barrier layer.
(3) FIG. 2 is a schematic cross-section of a multifilament superconductor with a barrier layer.
(4) FIG. 3 is a cross-sectional image of an Al+Al.sub.2O.sub.3 superconductor with a composite sheath, Ta barrier and MgB.sub.2 core.
(5) FIG. 4 is a longitudinal cross-sectional image.
(6) FIG. 5 a transversal microstructure of the Al+Al.sub.2O.sub.3 sheath composite with deformed Al grains directed to the deformation direction after cold working are shown. Images are obtained in a transmission electron microscope. The white arrow in the microstructure of FIG. 4 represents the direction of the deformation axis. The red arrows in the microstructure show crystalline Al.sub.2O.sub.3 nanoparticles within the Al grains interiors as well as the Al grain boundaries.
(7) FIG. 6 shows a combined elemental map demonstrating the presence of Al.sub.2O.sub.3 particles in the cross-sectional structure of the composite Al after cold working. The dark contrast areas (in the original format shown in red) indicate the presence of oxygen (O), a light contrast areas (in the original format shown in green) of the presence of Al. The white arrow shows the Al.sub.2O.sub.3 particles present within the Al grains interiors, dark (in the original format shown as red) arrow shows the Al.sub.2O.sub.3 particles present at the Al grain boundaries. Images are obtained in transmission electron microscope using EDS analysis.
(8) FIG. 7 shows the high resolution γ-Al.sub.2O.sub.3 particle (left) and EDS elemental analysis (right), and
(9) FIG. 8 shows a corresponding FTT pattern confirming the presence of γ-Al.sub.2O.sub.3 obtained in transmission electron microscope.
(10) FIG. 9 is a longitudinal microstructure of the Al+Al.sub.2O.sub.3 sheath in superconductor heated at high temperature of 645° C. for 30 min with coarsened Al grains and a distinct substructure of low angle Al grain boundaries decorated with forest dislocations. The microstructure of the composite is similar in the transversal direction. Images are obtained in transmission electron microscope.
(11) FIG. 10 shows the interface between the Al+Al.sub.2O.sub.3 sheath and the diffusion Ta barrier in the annealed superconductor with MgB.sub.2 core in the transversal direction obtained in the scanning electron microscope.
(12) FIG. 11 shows the hardness (H) and reduced Young's modulus (Er) along the composite sheath, the interface and the Ta barrier (the hardness indents are shown in the microstructure of FIG. 10).
(13) FIG. 12 shows the mechanical properties (ultimate tensile strength and Vickers hardness) of the Al+Al.sub.2O.sub.3 composite at a temperature of 300 K as a function of the Al.sub.2O.sub.3 content given by the thickness of the native am-Al.sub.2O.sub.3 layer on the surface of the Al powder used and the relative surface area of the used Al powder.
(14) FIG. 13 illustrates the tensile stress-strain curves of the Al+Al.sub.2O.sub.3 composite at a temperature of 77 K as a function of the Al.sub.2O.sub.3 content given by the thickness of the native am-Al.sub.2O.sub.3 layer on the surface of the Al powder used and the relative surface area of the used Al powder. In the chart insert, the function of 0.2% offset strain stress (YS.sub.0.2) is displayed as a function of the Al.sub.2O.sub.3 phase content at the temperature of 77 K.
(15) FIG. 14 illustrates the temperature dependence of the electrical resistance of the Al+Al.sub.2O.sub.3 composite sheath for different concentrations of the Al.sub.2O.sub.3 phase.
(16) FIG. 15 is a graph showing the temperature evolution over time during the annealing of the Al+Al.sub.2O.sub.3/Ta/Mg+B wire prepared from Al of 99.8 wt. % powder of d.sub.50=3 μm.
(17) FIG. 16 shows the critical currents at 4.2 K of the Al+Al.sub.2O.sub.3/Ta/Mg+B wire prepared from Al of 99.8 wt. % powder of d.sub.50=3 μm heat treated by the various annealing to form MgB.sub.2 core.
(18) FIG. 17 shows a change in the Al grain size at cryogenic temperatures for Al+1.6 vol % Al.sub.2O.sub.3 composite wire prepared from Al 99.996% powder of size d.sub.50=1.9 μm after 30 min annealing realized at the temperatures 595-650° C.
(19) FIG. 18 shows a microhardness at cryogenic temperatures for Al+1.6 vol % Al.sub.2O.sub.3 composite wire prepared from Al 99.996% powder of size d.sub.50=1.9 μm after 30 min annealing realized at the temperatures 595-650° C.
(20) FIG. 19 shows an electrical resistance at cryogenic temperatures for Al+1.6 vol % Al.sub.2O.sub.3 composite wire prepared from Al 99.996% powder of size d.sub.50=1.9 μm after 30 min annealing realized at the temperatures 595-650° C.
(21) FIG. 20 shows the interface between the Al+Al.sub.2O.sub.3 composite and the barrier Ti layer with a thin Al.sub.3Ti interface layer in the superconductor heat treated at 628° C. for 10 min. obtained in scanning electron microscope.
(22) FIG. 21 shows a fine-grained transversal microstructure of the Al+Al.sub.2O.sub.3 sheath of a single MgB.sub.2 core superconductor with the Ti barrier heat treated at 628° C. for 10 min. The images are obtained in transmission electron microscope.
(23) FIG. 22 shows a fine-grained transversal microstructure of the Al+Al.sub.2O.sub.3 sheath of a single MgB.sub.2 core superconductor with the Ti barrier heat treated at 628° C. for 10 min. Black arrows illustrate crystalline Al.sub.2O.sub.3 nanoparticles in Al grain interiors and at high angle grain boundaries. The images are obtained in a dark field in high-resolution transmission electron microscope.
THE EXEMPLARY EMBODIMENT
Example 1
(24) In this example, according to FIGS. 1, 3 to 11, 13 and 16, the single-core superconductor is fabricated from the Mg 99.99 wt. % wire with the diameter of 3 mm, which is surrounded by the B 99.8 wt. % powder in the Ta 99.9 wt. % tube with an inner diameter of 5.5 mm and an outside diameter of 7.1 mm. The Mg+B/Ta semi-product is rotatory swaged to the tube with the diameter of 6 mm and then inserted into the Al+Al.sub.2O.sub.3 composite tube. The Al+Al.sub.2O.sub.3 composite is manufactured using the nitrogen atomized Al 99.8 wt. % powder with d.sub.50=3 μm. The atomized powder is cold isostatically pressed into a green billet, which is subsequently degassed at 420° C. for 12 h under vacuum. The degassed powder billet is extruded at 420° C. using a reduction ratio of 8:1 into a rod with the diameter of 10 mm, from which a tube with an internal diameter of 6.3 mm and an outside diameter of 9.1 mm is machined. Such an assembly unit of Al+Al.sub.2O.sub.3/Ta/Mg+B is then cold rotatory swaged to a diameter of 7.5 mm and cold groove rolled to a wire with approximate cross-section of 1×1 mm.sup.2.
(25) The Al+Al.sub.2O.sub.3 composite sheath mechanically stabilizes the Mg+B/Ta core 1 during intense plastic deformation, ensures homogeneous deformation along the cross-section and length, compaction of Mg+B components in the core, and assures the integrity of Al+Al.sub.2O.sub.3/Ta/Mg+B assembly unit without the formation of undesirable cracks other flows. The intensively deformed Al+Al.sub.2O.sub.3/Ta/Mg+B wire is subsequently subjected to annealing at 635° C. (FIG. 15) for a total time of 60 min and the heating rate of 25° C..Math.min.sup.−1 under a protective atmosphere of argon (Ar), during which reaction between Mg wire and B powder takes place, followed by formation of the MgB.sub.2 superconducting core 1 (as shown in FIG. 3). Due to the exothermic reaction between Mg and B, overheating occurs and the temperature increases shortly (up to about 8 min) up to about 655° C. (FIG. 11). In the resulting superconductor, MgB.sub.2 core 1 forms approximately 36 vol. %, Ta barrier 2 approximately 5 vol. %, and Al composite sheath 3 approximately 59 vol. %.
(26) The Al sheath 3 efficiently stabilized by Al.sub.2O.sub.3 nanoparticles (FIGS. 4 and 5) maintains its Al grain structure formed by submicrometric grains with an average transversal grain size of about 480 nm and which are extensively elongated along to the direction of deformation throughout the wire forming process as well as after the annealing reaction performed at a temperature close to the melting temperature of the Al+Al.sub.2O.sub.3 composite (i.e., approximately 656° C. and 653° C., respectively). Thus, the Al+Al.sub.2O.sub.3 composite sheath retains to a large extent also its advantageous mechanical properties (e.g., Vickers hardness >60, ultimate tensile strength >200 MPa) (FIG. 12). In this example the content of crystalline Al.sub.2O.sub.3 nanoparticles, which have a size of about 25 or 28 nm and are evenly distributed in Al matrix, is about 1.4 vol. %. As shown in FIG. 10, the Ta/Al+Al.sub.2O.sub.3 interface is metallurgically clean without the presence of undesired porosity, cracks and intermetallic phases, which confirms the course of hardness at the interface (FIG. 11).
(27) The Al+Al.sub.2O.sub.3 composite exhibits a preferred residual-resistance ratio R.sub.300K/R.sub.25K=20 and an electrical resistance of approximately 9.5.Math.10.sup.−8 Ohm at 25K (FIG. 14). At cryogenic temperatures, the Al+Al.sub.2O.sub.3 composite retains the advantageous mechanical strength required to stabilize the MgB.sub.2 superconducting core (for example the yield strength equals to 260 MPa at 77 K). The superconductor exhibits high critical current values at 4.2 K, for example 100 and 1000 A in the magnetic fields 6 and 2.2 T, respectively.
(28) By anodizing the surface of the Al+Al.sub.2O.sub.3 composite sheath 3, a stable Al.sub.2O.sub.3 film of a thickness of several microns was formed on the surface of the superconductor, which provide sufficient electrical insulation by increasing the breakdown voltage from about 1 to 300 V.
(29) If the deformed Al+Al.sub.2O.sub.3/Ta/Mg+B wire is intentionally subjected to annealing at a temperature above 635° C., for example at 645° C. for 30 min, the temperature of the superconductor may increase above the melting point of Al+Al.sub.2O.sub.3 composite (i.e., above 656° C., resp. above 653° C.). Despite the local melting of the Al grains, the heat-treated wire remains stable and does not lose its shape integrity. The significant coarsening of Al grain is observed in such Al+Al.sub.2O.sub.3 composite sheath (FIG. 9). A typical texture of severely elongated Al grains disappears and transforms to equiaxed Al grains with several hundred pm in size. Nevertheless, a distinct substructure, formed by low angle grain boundaries and decorated by a dense dislocation network, retained in Al matrix. This microstructural change does not have a major effect on the critical current values, which are mostly affected only by the MgB.sub.2 structure (FIG. 16). Obviously, the mechanical properties of such heat-treated composite decrease significantly (Vicker hardness deteriorates down to ˜35).
Example 2
(30) In this example, according to FIGS. 20 to 22, the single-core superconductor is fabricated from the Mg 99.99 wt. % wire with the diameter of 2.9 mm, which is surrounded by the B 99.8 wt. % powder with the size below 1 μm in the Ti 99.99 wt. % tube with an inner diameter of 5.5 mm and an outside diameter of 7.2 mm. The Mg+B/Ti semi-product is rotatory swaged to the tube with the diameter of 6.2 mm and then inserted into the Al+Al.sub.2O.sub.3 composite tube. The Al+Al.sub.2O.sub.3 composite is manufactured using the nitrogen atomized Al 99.8 wt. % powder with d.sub.50=3 μm. The atomized powder is cold isostatically pressed into a green billet, which is subsequently degassed at 420° C. for 12 h under vacuum. The degassed powder billet is extruded at 420° C. using a reduction ratio of 8:1 into a rod with the diameter of 10 mm, from which a tube with an internal diameter of 6.3 mm and an outside diameter of 9.1 mm is machined. Such assembly unit of Al+Al.sub.2O.sub.3/Ti/Mg+B is then cold rotatory swaged to a diameter of 7.5 mm and cold groove rolled to a wire with approximate cross-section of 1×1 mm.sup.2.
(31) The Al+Al.sub.2O.sub.3 composite sheath mechanically stabilizes the Mg+B/Ti core 1 during intense plastic deformation, ensures homogeneous deformation along the cross-section and length, compaction of Mg+B components in the core, and assures the integrity of Al+Al.sub.2O.sub.3/Ti/Mg+B assembly unit without the formation of undesirable cracks other flows. The intensively deformed Al+Al.sub.2O.sub.3/Ti/Mg+B wire is subsequently subjected to annealing at 628° C. for a total time of 10 min and the heating rate of 25° C..Math.min.sup.−1 under a protective atmosphere of Ar, during which reaction between Mg wire and B powder takes place, followed by formation of the MgB.sub.2 superconducting core 1 (as shown in FIG. 3). Due to the exothermic reaction between Mg and B, overheating occurs and the temperature increases shortly up to about 642.5° C. In the resulting superconductor, MgB.sub.2 core 1 forms approximately 23 vol. %, Ti barrier 2 approximately 27 vol. %, and Al composite sheath 3 approximately 50 vol. %.
(32) The Al sheath 3 efficiently stabilized with crystalline Al.sub.2O.sub.3 nanoparticles with the size of approximately 28 nm and a total content of 1.4 vol. % (FIG. 22) retains its fine-grained structure of submicrometric Al grains with an average transversal grain size of approximately 800 nm (FIG. 21) after the heat treatment was performed. In this way, the Al+Al.sub.2O.sub.3 composite sheath also preserves to a large extent its advantageous mechanical properties (e.g., Vickers microhardness of 56 Pa). As shown in FIG. 20, on the Ti/Al+Al.sub.2O.sub.3 interface, which is free of undesired porosity and cracks, exists a thin, approximately 1 μm thick layer of the intermetallic Al.sub.3Ti phase. The formation of an undesirable Al.sub.3Ti layer, which may be detrimental in respect to thermal and electrical conductivity of the superconducting wire, is significantly reduced by the short annealing mode realized at a relatively low temperature and fast heating rate and reduced diffusivity of Ti in Al+Al.sub.2O.sub.3. The superconducting wire exhibits a reasonable current density at a temperature of 4.2 K, for example, 10.sup.4 and 10.sup.5 A.Math.cm.sup.−2, in the fields 5.6 and 2 T, respectively, and a good tolerance of critical currents to deformation, for example, up to the tensile strain of 0.21% at 4.2 K in the 6 T magnetic field.
Example 3
(33) In this example according to FIG. 2, the multifilament superconductor is fabricated such that seven cores 1 with barrier 2 are manufactured similarly to Example 1 and then the cores 2 are inserted into a Al composite tube with larger diameter and formed into a wire similarly to single-core wire.
Example 4
(34) In this example, as shown in FIGS. 11 to 13, the composites with various portions of Al.sub.2O.sub.3 were tested: 0.0 vol. % (i.e., wrought Al, not powdered composites), 0.57 vol. %, 1.51 vol. %, 2.12 vol %, and 3.12 vol. %. The changes in Al.sub.2O.sub.3 content were obtained by using gas atomized Al 99.8 wt. % powders of different mean particle size d.sub.50=0.8 to 21 μm. The resulting mechanical properties of the sheath 3 are shown in FIG. 12 and the tensile stress-strain curves varying content of Al.sub.2O.sub.3 are shown in FIG. 13. The mechanical strength increases proportionally with the Al.sub.2O.sub.3 content, which is a function of the average transversal Al grain size in the composite i.e., higher yield strength and hardness are achieved with a smaller grain size. As shown in FIG. 14, the electrical resistance of the composite at low operating temperatures increases with Al.sub.2O.sub.3 content that is inversely proportional to the Al grain size in the composite. However, even for Al powder with d.sub.50=0.8 μm and with Al.sub.2O.sub.3 content of 3.12 vol. % it still achieves an acceptable value of 2.6.Math.10.sup.−9 Ωm.
Example 5
(35) In this example, as shown in FIGS. 17-19, the structural stability of Al+1.6 vol. % composite wire prepared from Al 99.996 wt. % powder with d.sub.50 of 1.9 μm was verified after annealing held for 30 minutes at 595-650° C. As shown in FIG. 17, the transversal Al grain size changes only slowly from 604 nm to 860 nm as the annealing temperature increases from 595° C. to 650° C., respectively. This finding is confirmed by only subtle changes to the micro-hardness (FIG. 18) and the electrical resistance at cryogenic temperatures (FIG. 19) determined for annealed Al+Al.sub.2O.sub.3 composites.
INDUSTRIAL APPLICABILITY
(36) Industrial applicability is obvious. According to the invention, it is possible to reproducibly fabricate ultra-lightweight superconductors based on Al.
LIST OF REFERENCE LABELS
(37) 1—the core of a superconductor wire 2—the diffusion barrier in a superconductor wire 3—the sheath of a superconductor wire PIT—powder-in-tube IMD—internal magnesium diffusion to boron PM—powder metallurgy
