Reactive spark plasma sintering of MgB₂in nitrogen atmosphere for the enhancement of the high-field critical current density

Abstract

High density bulks (97%–99%) of MgB2 were prepared by spark plasma sintering (SPS) in nitrogen (N2) atmosphere for different heating rates (10, 20 and 100 °C min−1) and compared with reference samples processed in vacuum and Ar. N2 reacts with MgB2 and forms MgB9N along the MgB2 grain boundaries. The high-field critical current density is enhanced for the sample processed in N2 with a heating rate of 100 °C min−1. At 2–35 K, this sample shows the strongest contribution of the grain boundary pinning (GBP). All samples are in the point pinning (PP) limit and by increasing temperature the GBP contribution decreases.

The discrepancies in different facets of MgB2 bulk superconductors prepared under various sintering durations by spark plasma sintering

“…The values of h 0 and k n show an increasing trend with increasing temperature, changing from h 0 = 0.26 and k n = 0.60 at 15 K to h 0 = 0.33 and k n = 0.63 at 30 K in the PeF, and from h 0 = 0.24 and k n = 0.57 at 15 K to h 0 = 0.33 and k n = 0.61 at 30 K in the PaF, which indicated the PP becomes stronger at higher temperature. We can find similar phenomena from other studies [12,[35][36][37][38]. Since the existence of the second phase of MgB 4 particles has been confirmed from the XRD analysis, and MgB 4 particles can be effective pinning centers [9], we can speculate that the MgB 4 particles provide PP centers in this study.…”

“…Since the existence of the second phase of MgB 4 particles has been confirmed from the XRD analysis, and MgB 4 particles can be effective pinning centers [9], we can speculate that the MgB 4 particles provide PP centers in this study. The h 0 and k n values rising with the temperature is not a monotonous trend, but rather a fluctuation, which can be verified from some studies with small temperature intervals [12,35]. This temperature dependence of the pinning mechanism may be due to the change of the anisotropy or the percolation with the temperature.…”

“…The peak intensity of the secondary phase increases to a certain extent with the sintering duration, which reflects the slightly increasing content of the secondary phase with the sintering duration. It is inevitable that oxidation reaction occurs once the sample is in contact with oxygen during the preparation process [12,20]. As the sintering duration is prolonged, MgB 4 is more likely to form under high temperature and pressure conditions [21,22].…”

“…This method can suppress the grain coarsening and prepare highdensity samples in a short time by the high heating and cooling rates. The literature about the preparation of the SPSed MgB 2 samples has studied their mechanical properties [10,11] and the influence of preparation conditions on the performance of samples [12][13][14]. However, there is no study to compare the performance differences between the facets perpendicular and parallel to the compression direction of the SPSed MgB 2 bulk samples.…”

Grain boundary characteristics of Fe-based superconductors

“…To date, disk-shaped MgB 2 bulks prepared by a hot-pressing technique have been prepared by many groups [65][66][67][68], and the resultant trapped field reached 3 T [65]. Nearly fully dense MgB 2 bulks can be obtained by spark plasma sintering  in a relatively short time (less than an hour) [69][70][71][72][73][74]. The magnesium diffusion/infiltration technique is known as an alternative way to obtain dense MgB 2 wires/ bulks without applying high pressure during heating [75][76][77][78][79].…”