Attrition milling and wire processing effects on a commercial MgB2 powder

C. E. SOBRERO; M. T. MALACHEVSKY; A. SOLDATI; A. SERQUIS

Genova

Congreso; Eucas 2013; 2013

ESAS

Magnesium diboride is a candidate for replacing NbTi and Nb3Sb in appliances operating in cryogen-free systems. Among the existing processing methods to obtain wires and tapes, the most employed at a commercial scale is the ex-situ technique, i.e. the powder-in-tube (PIT) technique employing pre-reacted powder. Commercial MgB2 has a very inhomogeneous granulometry, with several large agglomerates of about 100 µm in diameter, that preclude a good packing density of the powder inside PIT tapes. When ex-situ methods are employed, the superconducting core does not densify during further heat treatments, and a good starting density is needed to improve superconducting properties. An intensive attrition milling, achieved by using CW balls, effectively reduces the agglomerates and grains sizes. Besides improving the compaction of the superconducting cores and the homogeneity of the metal-magnesium diboride interfaces, it has been observed an improvement of the critical current densities. In this work, a commercial MgB2 powder was attritioned at 300 rpm during 35 hours under nitrogen atmosphere to reduce oxidation. The resulting powder was observed in a TEM. Crystallite size and micro-strain within the powder grains were estimated from the broadening of Cu-Kα x-ray diffraction peaks. The integral breadths method, with a Cauchy profile was employed, giving a mean grain size of about 15 nm, in accordance with the TEM observation. An increase of the micro-strain within the grains and crystallinity loss was observed. Changes in the superconducting properties were assessed by measuring the magnetic moment as a function of temperature in a SQUID magnetometer, after zero field cooling. A slight reduction in the Tc accompanied by a broader transition was observed. Stainless steel and titanium sheathed single filament wires were prepared with this powder. Some powder was removed from each sheath to observe at the TEM if any microstructural change produced by the mechanical strain can be detected. Short samples were heat treated for 1 hour at 800 C in argon, and powders from the superconducting cores were observed in the TEM to examine if thermal relaxation can be detected. The superconducting properties were characterized by determining the magnetic Jc in a SQUID.