Study On Fabrication Technology Of MgB₂ Superconducting Wires And Heat-conducting Property Of Interface

The discovery of superconductivity in MgB₂ by Japanese scientists in 2001 has impact in all of physics because of its 39K critical transitional temperature which set a new record in inter-metallic compound with superconductivity. Although MgB₂ superconducting materials have high critical transition temperature, large coherence length and have no week connection effect and a series of other excellent properties, the exist preparation technology of MgB₂ superconducting wire still can’t meet the commercial applications requirements.In this paper, the MgB₂ superconducting wires were fabricated by the in-situ Powder-in-tube （PIT） method, to explore the preparation technology of MgB₂ superconducting wires, including the single core of MgB₂/Cu and MgB₂/Monel superconducting wires. The 7 core and 19 core of MgB₂/Fe/Fe, MgB₂/Fe/Monel and MgB₂/Fe/Cu superconducting wires were also fabricated. Meanwhile, the heat-conducting property of MgB₂/Fe and MgB₂/Monel superconducting wires was researched to confirm the effect of diffusion layer on wires.The single core results showed that the MgB₂/Cu wires demand intermediate annealing when the deformation of metal sheath reached 82% during the fabrication process by PIT method. The fully recrystallization temperature of the metal sheath is 450℃ and the intermediate annealing time is 10 minutes. With the process of annealing, the Mg element which diffuse into the metal sheath is seldom so that the diffusion layer is thin while the thickness is only 5μm. The impurity phase generated and the loss of Mg element is few. During the fabrication process of MgB₂/Monel wires, once intermediate annealing is need to densify the powder when the diameter reaches Φ1.75mm. Compared with the wires with twice annealing, the wires with once annealing own higher density, finer grain size and higher tensile strength. The Mg element which diffuse into the metal sheath is seldom so that the diffusion layer is thin while the thickness is only 25 μm. Its critical current density is as large as the the original wire and reached 3.5×105A/cm2.The results of heat-conducting research showed that the presence of Nb can significantly reduce the spread and reaction between the elements from sheath and the Mg elements from the core so that the utilization of Mg was improved. For MgB₂/Monel wires, inter-diffusion and reaction between Cu element and Mg element can be observed, and there is a thicker brittle reaction layer between the sheath material and core of wire. The presence of the layer results in poor interface combination and lower thermal diffusion coefficient. For MgB₂/Fe wires, the brittle reaction layer is thin because the inter-diffusion and reaction between core and metal sheath is hard. So the influence of diffusion layer on the heat conductionperformance is not apparent.Multicore test results showed that 7-filamentary MgB₂/Fe/Fe wires have good grain connection and higher density. And its yield strength is low while its tensile strength is high so that the wires were not easy to break in the process of drawing. As the ferromagnetic materials, Fe can effectively shield the external magnetic field, reduce the infiltration of the external magnetic field. So the critical current density reached 4.0 × 105A/cm2 at 10K,0T and fell slowly as the change of the external magnetic field. It can reach 1.5 X 10⁴A/cm² at 10K,2T.