Optimization of Carbon Encapsulated Boron Doping for High‐Performance Bulk Sintered MgB₂

Abstract

Carbon has been a standard doping agent in MgB 2 for long time. It, however, has also participated in a non-uniform distribution of the constituents throughout the bulk MgB 2. To address this issue, carbon encapsulated boron (CEB) was used instead of the manually added mixture of boron and carbon. The previous studies confirmed that only low concentrations of carbon in CEB were effective for synthesis of a high-performance bulk MgB 2. Here, a further step in optimization carbon content in CEB is reported. Carbon content in CEB varied as 1, 1.1, 1.35, 1.5, and 1.9 wt%. X-ray diffraction (XRD) results depict a slight shift in peaks corresponding to a-b plane, indicating carbon substitution into the lattice. High superconducting critical current density in self-field, such as 660, 550, and 435 kA cm À2 , was observed in the samples with 1.5 wt% CEB at 10, 15, and 20 K, respectively. In addition, J c of 75 kA cm À2 at 2 T and 20 K was observed in the 1.5 wt% CEB sample, which is thrice the value observed in the pure sample, with a minute tradeoff in T c (around 37.5 K). Scanning electron microscope (SEM) images reveal that small particles of size ranging from 50 to 200 nm contribute to J c improvement. Energy-dispersive X-ray (EDX) results show carbon uniformly distributed throughout the bulk.

Enhanced Flux Pinning Performance of Bulk MgB2 via Immersion of Synthetic Motor Oil

“…2). T c,onset of u-MgB 2 seem to be slightly lower than pure-MgB 2 , around 37.5 K which is to be expected from carbon substitution as suggested from earlier reports on carbon encapsulated boron powder for MgB 2 fabrication [22,30] (see Fig. 2).…”

“…5) for the u-MgB 2 as compared to pure-MgB 2 bulk. The peak position of u-MgB 2 is around 0.23, which points out to slight increase flux pinning contribution from carbon substitution, which is similar to bulk MgB 2 produced utilizing the carbon encapsulated boron powders [22].…”

“…Various studies also show that the T c depreciates in proportion with amount of carbon doped, for instance, 4 and 5% of nano-carbon substitution has dropped the T c to 34.68 (2) x = Δ(c∕a) × 7.5 and 34.35 K, respectively [21]. In general, the depreciation is proportional to carbon doping amount [22]. Critical current densities of MgB 2 bulks have shown to be improved via carbon doping in several instances such as using carbon nanotubes (2.5-10 at.%) where they found J c increase up to 5 × 10 4 A/cm 2 at 5 T and 5 K, similarly for 10% SiCdoped sample showed J c of 3.6 × 10 4 A/cm 2 at 4 T (20 K), and 3% GO-doped samples showed J c of 0.5 × 10 4 A/cm 2 at 4 T (20 K) [23][24][25].…”

Review on high-performance bulk MgB2 superconductors

“…The usage of CEB exhibited superior properties over MgB 2 prepared from conventional boron. Especially, for 1.5 wt.% CEB an optimum high J c of 435 kA cm −2 at self-field, 20 K was observed [131] as shown in the inset of figure 20 (right). This bulk demonstrated high J c values not only at low fields near self-field but also at high fields ranging from 2 to 4 T. The improvement was observed up to high magnetic fields (2-4 T) and was much more significant than in MgB 2 with other C-based dopants [133].…”

“…Microstructural studies depicted MgB 2 nanoparticles around 50-200 nm, likely due to the nano CEB powder (figure 19(b)). The qualitative analysis (energy dispersive xray (EDX) spectrum-area mapping shown in figure 19(c)) proved that the carbon distribution was uniform throughout the matrix in CEB using MgB 2 [131]. Using CEB at 1.5 wt.% looks beneficial for synthesizing stable and reliable highperformance bulk MgB 2 for use in everyday superconducting applications at high fields.…”