Superconducting Mg-MgB2 and Related Metal Composites and Methods of Preparation (21029)
This invention provides superconducting magnesium diboride (MgB2) materials and methods for their production that overcome present day synthesis limitations. Magnesium diboride has recently been found to be superconducting at the critical temperature Tc of 39°K, much higher than the best low-temperature intermetallic superconductors. However, widespread use of MgB2 as an economical superconducting material has been limited because it is a brittle ceramic and difficult to use in bulk form as a single phase. This new process creates a composite consisting of MgB2 within a robust metallic phase having excellent properties and processability suitable for commercial manufacture. Applications in power transmission, medical diagnosis, communications and high performance electronics are projected in the growing high temperature superconducting market
ADVANTAGES: Efficient and economical fabrication of MgB2/metal composites that retain the unique superconductivities properties of MgB2 while overcoming its brittleness, low strength and lack of processability.
SUMMARY: Magnesium diboride is superconducting at the critical temperature Tc of 39°K, higher than the best low-temperature intermetallic superconductors (Tc ~ 23°K). Unlike cuprate superconductors, MgB2 shows excellent conduction across grain boundaries and is a simple, stoichiometric compound, easy and inexpensive to synthesize.
However, widespread use of MgB2 as a superconducting material has been limited by its brittle nature and low strength, making it difficult to use in bulk as a single phase, e.g. as wire. One approach, the powder-in-tube (PIT) process, creates an MgB2 wire encased in a metallic outer phase. While showing improved strength and toughness, such wires are very fragile, as cracks in the diboride phase interrupt the superconducting pathway. Accordingly, there is a need for process and fabrication techniques, and compositions to better utilize the superconductivity of such materials.
This invention provides magnesium diboride composites that overcome the above shortcomings. Composites, consisting of an intimate mixture between the superconducting phase and a metallic matrix, were prepared efficiently and economically. MgB2 materials with Tc ~ 37°K were synthesized. The superconducting properties of MgB2 are retained, while the metallic phase provides in a tough, ductile and robust matrix providing, good thermal conductivity, strength, durability, processability and resistance to environmental degradation.
One approach is to infiltrate with liquid Mg a preform of MgB2 made from inexpensive tapped or packed MgB2 powders, (Appl. Phys. Lett. 79, 4186, 2001). Al/ MgB2, and Zn/ MgB2 composites have also been prepared. The composite can be subjected to subsequent processing operations or used directly.
Alternatively, B powders or B fibers have been transformed rapidly to MgB2 after infiltration with liquid Mg, affording Mg/MgB2 composites after solidification. Composite wires, consisting of several hundreds continuous MgB2 fibers embedded within an Mg matrix, were produced (Figure). The MgB2 fibers exhibited superconducting properties (Tc = 39°K and Jc = 360 kA/cm2 at 5°K) comparable to the best results published for bulk MgB2. The fibers are cylindrical and straight, allowing high packing densities within a mechanically tough, thermally dissipating, electrically conductive Mg matrix. The process is scalable to continuous lengths of superconducting Mg/MgB2 wires Appl. Phys. Lett. 83, 120, 2003).
STATUS: U. S. Patent numbers 6,630,427 and 6,995,119 have issued.
David C. Dunand
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