Correlation between microstructure and superconducting properties of MgB₂ bulk samples with Mg addition and Mg/hBN co-additions

Abstract

The microstructure of polycrystalline MgB2 has a strong influence on the current carrying ability, with grain boundaries and non-superconducting nanoparticles acting as good flux pinning centres which improve the local (intrinsic) critical current density (Jc) of the material, whereas porosity and poor connectivity between grains or particles adversely affect macroscopic current transport. Previous studies have found that hBN doping improves intrinsic Jc by introducing nanoscale flux pinning centres, and Mg doping improves extrinsic Jc by liquid-assisted sintering. Here we investigate the effect of co-doping with 5 wt.% Mg and 1wt.% hBN with the aim of combining the improved intrinsic and extrinsic properties in bulk MgB2 samples fabricated using field assisted sintering. Additionally, the influence of ball milling and processing temperatures on MgB2 samples with only Mg additions is reported. By correlating microstructure with superconducting properties, we show that the presence of Mg liquid during processing of Mg-doped samples accelerates the reaction between BN and MgB2, forming an impurity phase, MgNB9, the presence of which is detrimental to superconducting performance. Nevertheless, we have achieved a considerable improvement in performance of samples doped only with Mg by increasing the sintering temperature. The main aims of this study are not only the optimisation of the processing conditions, but also an attempt to create a roadmap for optimizing superconducting properties of bulk to MgB2 samples by summarising how the various microstructures are developed during processing.

Bulk MgB2 Superconducting Materials: Technology, Properties, and Applications

“…As mentioned in Introduction, for more than 20 years, since the discovery of the superconductivity in MgB 2 , scientists have been exploring the possibility of increasing the pinning and, hence, the critical current density using various additives [ 38 , 39 , 40 , 41 , 42 , 43 , 44 , 45 , 46 , 47 , 48 , 49 , 50 , 51 , 52 , 53 , 54 , 55 , 56 , 57 , 58 , 59 , 60 , 61 , 62 , 63 , 64 , 65 , 66 , 67 , 68 , 69 , 70 , 71 , 72 , 73 , 74 , 75 , 76 , 77 , 78 , 79 , 80 , 81 , 82 , 83 , 84 , 85 , 86 , 87 , 88 , 89 , 90 ,…”

“… ( a )—The schemes of an SFCL model and a testing circuit for the simulation of a fault event. ( b )—Typical oscilloscope traces of the current in a protected circuit (black, solid curve) and the voltage drop across the primary coil of the SFCL model (red, dashed curve) at 50 Hz and about 4 K (from [ 90 ]). The experiment details are described in [ 120 ].…”

Investigation of Layered Structure Formation in MgB2 Wires Produced by the Internal Mg Coating Process under Low and High Isostatic Pressures

“…However, PIT MgB 2 wires have several disadvantages, such as a large number of voids, poor connections between grains, low density of pinning centers, low irreversible magnetic field, and large grains [ 8 , 9 , 10 , 11 , 12 , 13 , 14 ]. Current research has shown that doping allows for increasing the density of pinning centers [ 15 , 16 , 17 , 18 , 19 , 20 , 21 ].…”