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Author(s)
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Capra, Marco (Genoa U.) ; Bovone, Gianmarco (CNR, Italy) ; Loria, Federico (CNR, Italy) ; Bernini, Cristina (CNR, Italy) ; Hopkins, Simon C (CERN) ; Ballarino, Amalia (CERN) ; Tropeano, Matteo (ASG Supercond., Genova) ; Tumino, Andrea (ASG Supercond., Genova) ; Grasso, Gianni (ASG Supercond., Genova) ; Ferdeghini, Carlo (CNR, Italy) ; Putti, Marina (CNR, Italy ; U. Genoa) ; Vignolo, Maurizio (CNR, Italy) |
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Abstract
| We have established a new procedure for recovering boron contained in oxidized MgB$_2$ phase. Incorrect conditions of synthesis or conservation, or excess impurities in precursors, can lead to MgO formation with consequent degradation of superconducting properties, thus representing a possible problem in the industrial production of MgB$_2$ conductors following the ex-situ PIT process. A procedure has been developed in order to dissolve MgO (due to the oxidization of Mg in MgB$_2$ phase) and to decompose the MgB$_2$ phase into elemental B and a soluble Mg salt. The process can also be successfully applied to low-grade B in order to enhance the purity, providing that the B is first reacted into MgB2. In fact, MgB$_2$ synthesis converts residual B$_2$O$_3$ (present in B) and other impurities to leachable compounds. For the present work, an ill-preserved MgB$_2$ powder produced in 2008 was treated with glacial acetic acid. The reduction of impurities and secondary phases was monitored along each step of the proposed process by X-ray diffraction (XRD) technique and by energy dispersion spectroscopy (EDS) analysis. The purification process was successfully applied on two different MgB2 powders: the first was synthesized using commercial B (H. C. Starck), the second using a different magnesiothermic B with a low purity grade prepared by freeze-drying process at CNR-SPIN. An interesting result is that, after acidic treatment of MgB2, the resulting B particles are finer and more amorphous than pristine B particles. The increased surface area of B undoubtedly offers a more reactivity of powders versus the weak acid, because the impurities within the B particles are exposed to the direct action of the acid and not longer protected by B crust. |