Processing of magnesium-boride superconductors
Processes for the fabrication of MgB 2 powder and wires are provided. Powders are produced by mechanically alloying magnesium- and boron-containing precursors under controlled conditions to avoid secondary phase and impurity formation. Powders are also prepared by vapor phase reaction of volatile magnesium- and boron-containing precursors. Wires, tapes, films and coatings are provided.
1. A method of making a magnesium boride superconductor, comprising:
combining magnesium-containing and boron-containing precursors to form a homogeneous mixture of magnesium- and boron-containing powders, said combining step carried out under an inert or reducing environment introducing the homogeneous mixture of the magnesium- and boron-containing precursor into a metal sheath; and
heating the homogenous mixture to convert the magnesium- and boron-containing powders into a superconducting magnesium boride.
2. The method of claim 1 , wherein the combining step is selected from the group consisting of ball milling, rod milling, and high energy ball milling.
3. The method of claim 2 , wherein the precursors comprise intermetallic compounds.
4. The method of claim 2 , wherein the precursors are selected from the group consisting of magnesium, boron, MgB 4 and MgB 7 .
5. The method of claim 2 , wherein the combining step comprises mechanically alloying the powders.
6. The method of claim 5 , wherein the mechanically alloyed powders have an average particle size in the range of about 5 nm to about 1 μm.
7. The method of claim 5 , wherein the powders are alloyed at temperatures of less than −20° C.
8. The method of claim 5 , wherein the precursor further comprises additives selected to modify the mechanical alloying process or to dope the resultant precursor powders.
9. The method of claim 2 , wherein the magnesium-containing precursor is a magnesium alloy.
10. The method of claim 9 , wherein the magnesium alloy is selected from the group consisting of Mg—Cu and Mg—Li alloys.
11. The method of claim 9 , wherein the alloying elements are selected from the group consisting of copper, sodium, lithium, calcium, silver, palladium and aluminum.
12. The method of claim 2 or 9 , wherein the precursors comprise metal hydrides.
13. The method of claim 1 , wherein the magnesium-containing and boron-containing precursors comprise dispersed boron particles and a reactive magnesium-containing phase.
14. The method of claim 1 , wherein the precursors comprise one or more of elemental magnesium and boron.
15. A method of making a precursor to a magnesium boride superconductor, comprising:
combining magnesium-containing and boron-containing precursors to form a homogeneous magnesium- and boron-containing powder, said combining step carried out under an inert or reducing environment,
wherein at least one of the magnesium-containing and boron-containing precursors is a vapor in the combining step.
16. The method of claim 15 , wherein the boron-containing precursor is a powder.
17. The method of claim 2 , 14 or 15 , further comprising introducing flux pinning sites into the precursor.
18. The method of claim 17 , wherein the flux pinning particles are selected from the group consisting of metal borides, metal carbides, oxides of aluminum, zirconium, magnesium, yttrium, ytterbium, lanthanum, thorium, boron and calcium, carbon, copper and boron.
19. The method of claim 17 , wherein the flux pinning particles are introduced during precursor fabrication.
20. The method of claim 17 , wherein the flux pinning particles are introduced after precursor fabrication.
21. The method of claim 2 , or 15 , further comprising:
introducing the homogenous mixture of the magnesium- and boron-containing precursor into a metal sheath.
22. The method of claim 15 , further comprising:
heating the homogenous mixture of the magnesium- and boron-containing precursor to convert the precursor into a superconducting magnesium boride.
23. The method of claim 21 , wherein the sheath is selected from the group consisting of copper, stainless steel, nickel alloys and oxide dispersion strengthened copper.
24. The method of claim 21 , wherein the sheath is selected from the group consisting of copper, tantalum-lined copper, niobium-lined copper, and iron-lined copper.
25. The method of claim 21 further comprising:
heating the homogenous mixture of the magnesium- and boron-containing precursor to convert the precursor into a superconducting magnesium boride.