Spark Plasma Sintering (SPS) is a promising rapid consolidation technique that allows a better understanding and optimization of the sintering kinetics and therefore makes it possible to obtain MgB2 bulk superconductor with tailored microstructures consisting of grains with either spherical or elongated morphology. In this contribution, the role of the precursor powders on the superconducting properties of MgB2 is investigated. Three sets of bulk MgB2 material were processed from: (i) a commercially available MgB2 powder; (ii) a mixture of Mg metal and amorphous B using a single‐step solid‐state reaction process and (iii) a mixture of amorphous boron coated with carbon and Mg metal. The samples were prepared in the same SPS processing conditions. The microstructure of the samples was investigated by X‐ray diffraction, SEM and TEM and correlated to their superconducting properties. The critical current density of the best sample at 20K was Jc = 500 kA/cm2 in self‐field, which is one of the highest critical current density reported for MgB2 bulk superconductors.

No abstract

Critical current density, J c, in superconductors is strongly connected with size of defects in the material. Frequently, the smaller defects, the higher J c. In this work, we tried to reduce the size of cheap commercial boron precursor powder using high energy ultra-sonication in ethanol media. The resulting powder was then utilized in synthesizing bulk MgB2 via sintering at 775 °C. Effect of boron powder ultra-sonication on superconducting properties of the bulk MgB2 was studied and discussed. SEM of ultra-sonicated boron showed fine particles with sharp edges (high-energy surfaces), irregular shapes and clustering of fine particles occurred for longer ultra-sonication durations. XRD proved a high quality of MgB2 with only small traces of MgO. Around 36% improvement in Jc at 20 K and Tc close to 39 K were observed in MgB2 bulk prepared with boron ultra-sonicated for 15 min. Microstructure studies showed numerous nanometre sized MgB2 grains in the bulk. Other bulks (made of boron ultra-sonicated longer, for 30, 45, and 60 min) have larger grains. It resulted in slightly lower J c, anyway, still by 22% higher than in reference bulk. The present results demonstrate that the high performance bulk MgB2 can be achieved without reduction in T c via employing a cheap boron, reduced in size by high-energy ultra-sonication.

No 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.

Silver addition has been known to improve mechanical and superconducting performance of bulk magnesium boride (MgB2) superconductor synthesized via solid state sintering. While formation of pinning phases and impurities such as Ag–Mg phases and MgO, respectively, responsible for high performance, part of the Mg in initial mixture will be expected to be consumed by silver. In this work, the authors add varied amounts of Mg systematically such as x: 2 (while x = 1.05, 1.075, 1.1, 1.125, 1.15) ratio of Mg:B instead of usual stoichiometric ratio of 1:2. To obtain high superconducting properties, the authors use carbon encapsulated boron (1.5% carbon) along with 4 wt% Ag. X‐ray diffraction analysis shows the presence of Ag–Mg phases and minute amount of MgO. Superconducting quantum interference device (SQUID) measurements indicate that high irreversibility field (4.76 T) and large Jc of 520, 440, and 347 kA cm−2 at 10, 15, and 20 K, respectively, at self‐field was exhibited in sample with x = 1.075. Flux pinning force studies are done to analyze the effect of secondary phases formed. All results analyzed explain the improved critical current performance based on nanometer‐sized Ag–Mg particles in the final product.

No abstract

We successfully refined a cheap commercial boron powder by means of high-energy ultra-sonication and utilized it in synthesis of bulk MgB2. Rietveld phase analysis of X-ray diffraction pattern revealed completely formed MgB2 with a low amount of MgO. MgB2 bulk prepared of boron ultra-sonicated in ethanol for 15 min showed self-field Jc of around 300 kA/cm2 at 20 K without any compromise in Tc (~39 K). Pinning analysis based on Dew-Hughes expression showed major pinning contribution from grain-boundary pinning (~95.5%), along with a slight contribution from point pinning (4.5%). The microstructure study detected a system of large MgB2 grains (hundreds nm large) and 10–20 nm sized particles, possibly Mg-B-O, formed at MgB2 grain boundaries.