Role of excess Mg and heat treatments on microstructure and critical current of MgB2 wires

Abstract

We present a detailed analysis of the effect of heat treatments on the microstructure, magnetization, and transport properties of MgB2 wires produced by the powder-in-tube method. We have used commercial MgB2 powder with 5 at. % Mg powder added as an additional source of magnesium and stainless steel as sheath material. We measure the dc transport critical current that can be increased or decreased by more than one order of magnitude as compared with the as-drawn wire, depending on the annealing parameters. We correlate the changes in the critical current with changes in the microstructure, as determined from scanning and transmission electron microscopy analysis. We show through magnetization measurements of short annealed wires that inappropriate annealing conditions result in a deterioration of the connectivity due to the loss of Mg and in inhomogeneous weak-link limited current flow, rendering the critical state model inapplicable. We discuss the optimization of the annealing conditions that strongly improve the connectivity by eliminating most of the microcracks present in the unannealed wires, where excess Mg promotes the recrystallization. The loss of Mg during the heat treatment may be precluded by annealing long wire lengths with a high heating rate.

MgB2 coated superconducting tapes with high critical current densities fabricated by hybrid physical–chemical vapor deposition

“…The former employs powder starting materials [27], whereas the latter employs a powder of synthesized superconducting material [28]. The superconducting properties of PIT-processed MgB 2 conductors are very sensitive to the quality of the starting powder [29][30][31][32][33], particle size [34][35][36], porosity of the MgB 2 core [37,38], heat treatment temperature [34,[39][40][41], and the sheath material [27,42]. In general, ex situprocessed MgB 2 tapes show lower J c than in situ-processed tapes.…”

Synthesis of dense bulk MgB2by an infiltration and growth process

“…Giunchi et al also observed the presence of several grains of size 100 m after processing at 950 ºC in a similar experiment [30], suggesting that such fine grain size is due potentially to the use of a lower reaction temperature (750 ºC), which limits grain growth. Such homogeneous microstructure is attributed presence of excess Mg during reaction, which is known for improving grain connectivity, elimination of microcracks, facilitating recrystallization and avoiding B-rich impurities [35].…”

Large field generation with a hot isostatically pressed powder-in-tube MgB2coil at 25 K

“…We packed MgB 2 powder into stainless steel tubes (inner and outer diameters 4.6 and 6.4 mm) and cold-drew them into round wires with external diameter in the range of 0.8-1.4 mm, with one intermediate annealing. This results in MgB 2 cores of very uniform, circular cross section, with diameters of 0.5-0.9 mm corresponding to f ∼ 45% and no reaction between the sheath and the superconductor [8,9].…”

“…We have shown [8,9] that, by adding 5 at.% Mg to the initial powder, heat-treating long lengths of wire with sealed ends to avoid Mg loss, and by choosing appropriate annealing conditions, the microcracks produced by the drawing (the most severe current-limiting factor in the as-drawn wires) can be healed due to a recrystallization promoted by the excess Mg.…”