Improving the connectivity of MgB₂ bulk superconductors by a novel liquid phase sintering process

Abstract

This work investigates a new processing method developed to improve the connectivity of ex-situ MgB2 bulks at low sintering temperatures. Mg additions (1 − 10 wt.%) were mixed to pre-synthesised MgB2 to make composite powders that were sintered at 900 ffiC by the Field Assisted Sintering Technique (FAST). Addition of 10 wt.% Mg resulted in a substantial increase in density from 68 to 79% and a dramatic reduction in MgB4from 11 to ∼ 0 wt.%. Pressure and dilatometry data recorded in-situ during the sintering process revealed that Mg additions led to different sintering mechanisms depending on the Mg fraction. For large Mg fractions (6 and 10 wt.%) a Mg liquid phase was formed and led to significant density improvements, and all pre-existing MgB4 was transformed into MgB2. A small amount of residual Mg remained in the bulks after the sintering process. Connectivity was improved with Mg additions, increasing 4 fold in the 10 wt.% Mg-MgB2 sample compared to unmodified MgB2. Jc values at low field were also significantly improved by Mg additions, in particular the 6 and 10 wt.% Mg-MgB2 specimens showed Jc(20 K, 0 T) values 4 − 5 times higher than for unmodified MgB2.

Effect of cubic and hexagonal boron nitride additions on the microstructure and properties of bulk MgB2 superconductors

“…The latter are more likely to serve as effective flux pinning centres owing to their size. Moreover, the formation of small Mg-N-B impurities also results in Mg as a by-product, which may improve grain connectivity and enhance low-field performance, similar to the effect of adding excess Mg to MgB2 powders before sintering [34]. Even though Mg-N-B impurities are situated at grain boundaries, the density is low enough not to impede macroscopic current transport.…”

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

“…Powders (∼3 g) after milling for 6 h were pressed into disc-shaped pellets for consolidation. Based on our preliminary work [25], 5 wt.% Mg addition was used in this study. Mg powder (purity: 99.8%, Alfa Aesar) was homogenously mixed by hand with ball-milled MgB 2 powder.…”

“…They found that liquid phases can form during the sintering which helps improve the bulk density. Therefore, the low-field J c was enhanced by Mg additions, in particular 6 wt.% and 10 wt.% Mg [25]. However, large pores up to hundreds of microns still remain in their best sample.…”

“…The addition of low melting point metals can lower the sintering temperature and improve the extrinsic connectivity of the MgB 2 particles by encouraging liquid phase sintering [15,16,23,24]. Specifically, we have shown previously that Mg additions to commercial MgB 2 powders could enhance J c over the whole range of fields in ex-situ samples prepared at 900 • C by Field Assisted Sintering Technology (FAST) [25]. However, even with 10 wt.% Mg addition, the MgB 2 microstructure is not fully dense, which suggests that there is the opportunity to further improve the J c values.…”

A Comparative Study of Experimental and Simulated Trapped Magnetic Field of MgB2 Bulks Prepared by the Magnesium Vapor Transport Process

“…Several processes have been developed for MgB2 bulk fabrication, including the in-situ method [4], [6]- [9], in which a mixed powder of magnesium and boron reacts; the high-pressure synthesis method [10]- [16]; the diffusion method using liquid magnesium [17], [18]; and liquid phase assisted sintering [19]. In the conventional in-situ method, the packing factor of MgB2 is as low as around 50%, since the density of reacted MgB2 is higher than the density of the Mg/B precursor powders [6], [7].…”