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In this work, ex situ MgB2 was mixed with 0.5 mol of Mg and sintered. The sintering conditions were varied over a temperature range of 600–1000 °C for 1, 3, and 7 h, respectively. The addition of Mg during the sintering increased the partial pressure of Mg and thus suppressed the decomposition of MgB2. Onset of critical temperature, Tc, was retained at ∼ 38 K even after the addition of Mg. By increasing the sintering temperature, magnetic critical current density, Jc at self-field, and 20 K of the ex situ samples increased consistently. With the addition of Mg for 1 h sintering, self-field Jc (20 K) was enhanced more than 20 times to 104 A cm−2 as the sintering temperature was increased. Such significant enhancement in the Jc is mainly due to the improved grain coupling aided by Mg during the short sintering.

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.

In this work, magnesium diboride, MgB2 samples were prepared by using magnesium, Mg powder and carbon-encapsulated boron, B powder (1.35 wt% carbon) with addition of dysprosium oxide, Dy2O3 and lanthanum oxide, La2O3 as dopants. Different weight percentages with the ratio of x wt.% Dy2O3: x wt.% La2O3 were used where x = 0.00, 0.25, 0.50, 0.75, 1.00, respectively. X-ray diffraction (XRD) results confirmed MgB2 as the major phase for all the samples. Magnetization measurement showed a slight decrease of critical temperature, Tc from 38.1 K to 37.6 K with the co-addition of Dy2O3 and La2O3. Self-field critical current density, Jc at 20 K increased with the increasing of co-addition levels probably due to improved grain coupling. The highest self-field Jc obtained is 433 kA cm−2 with 1.00 wt% co-addition level. Field dependent Jc (≤4 T) at 20 K of the co-added samples is higher compared to that of the pure one. The present results show that a small amount of Dy2O3 and La2O3 co-addition into MgB2 is effective to enhance flux pinning and Jc.

The large coherence length of MgB2 material allows large defects such as grain boundaries to act as strong pinning centres. As an alternative to the high-cost commercial nano precursors, we used a novel ultra-sonication technique to refine a cheap rough boron precursor and avoid the use of the expensive commercial nano amorphous boron powder. Recent novel low-cost ultra-sonication technique proved to improve the critical current density, J c, of MgB2. Here, we discuss the consequences of ultra-sonication duration in hexane medium on the boron precursor as well as the superconducting properties of MgB2 bulks. XRD revealed small peaks corresponding to unreacted Mg. First derivative of susceptibility was plotted as a function of temperature to explain the Tc,onset and ΔTc. J c was found to grow if the MgB2 bulk was prepared of boron ultra-sonicated for up to 30 minutes and then reduced with further boron ultra-sonication (60 minutes). SEM micrographs showed that boron powder subjected to longer ultra-sonication resulted in agglomeration of broken boron particles, forming large MgB2 grains. Based on the comparison of superconducting properties with previous results, we speculate that viscosity of the ultra-sonication medium might have a crucial role in the refinement of boron precursor powder.

In this work, we report an optimum amount of Mg precursor required for obtaining high critical current density (J c ) of bulk MgB 2 superconductor synthesized via solid state sintering. We add a slight excess of Mg precursor to balance its losses while formation of pinning phases and impurities such as Ag-Mg phases and MgO, respectively. Several amounts of Mg had been varied such as x = 1.05, 1.075, 1.1, 1.125, and 1.15 in MgxB 2 . To obtain the best critical current density values, we used carbon encapsulated boron (1.5% carbon) and added 4 wt% Ag. The x-ray diffraction analysis showed the presence of AgMg and AgMg 3 phases and a minute amount of MgO. The SQUID measurements indicated that highest J c of 350 kA/cm 2 at 20 K and self-field in sample with x = 1.075. All results analyzed explain the improved critical current performance based on nanometer-sized Ag-Mg particles in the final product.

MgB2 bulks were produced by in situ solid-state reaction in Ar gas using high-purity commercial powders of Mg metal and amorphous nano-B mixed in a fixed ratio of Mg:B = 1:2. Single-phase materials have been obtained and grain structure and its implications are reported. All samples were sintered at 775 °C for 3 h in Ar atmosphere. X-ray diffraction results show that high-purity MgB2 phase has been fabricated, which as well is reflected in critical temperature (Tc = 37.8 K). Magnetic measurements reveal a high critical current density (Jc) value such as 408 kA/cm2 when nano-amorphous boron is used. SEM studies show that nano-ordered MgB2 grains are formed, thereby creating large number of grain boundaries responsible for high critical current densities. Analysis of normalized pinning force curve as a function of reduced magnetic field indicates that dominant pinning is caused by grain boundaries.

High-density MgB2 bulks with superior mechanical and superconducting properties were fabricated using spark plasma sintering (SPS). The sharp superconducting transition at 37.5 K proved high quality of the superconductors. Critical current density significantly exceeded that of conventionally sintered bulks. Flux pinning diagrams indicated dominance of grain boundary pinning, with peak position at 0.2. SPS bulks showed improved mechanical properties with 8-times higher bending strength compared to dense hot isostatic pressed bulks. Trapped field was measured at 14 K and 20 K, 1 mm above the bulk’s surface, with applied pulse field up to 2 T. A local overheating together with a complex trapped field formation in the MgB2 bulks during pulse-field magnetization were studied to elucidate limitations of Pulse Field Magnetization.

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.

The electromagnetic properties of YBa2Cu3O7-δ (Y-123) were improved by reducing size of the secondary phase Y2BaCuO5 (Y-211) via ultra-sonication and optimization of the top seeded melt growth (TSMG). Ultra-sonication a clean and low-cost technique, which excels over other techniques in producing refined Y-211 particles with sharp edges, uniform size, and contamination-free surfaces. The Y-211 powder was ultra-sonicated for various intervals ranging from 0 to 100 min, with a constant power (300 W) and frequency (20 kHz). Single grain Y-123 bulks were prepared from these powders and their electromagnetic and superconducting performance was compared. The superconducting transition temperature of all the bulk samples was around 91 K and the transition width was less than 1.5 K. At 77 K, the highest critical current density (Jc) and trapped field was found in the sample with Y-211 ultra-sonicated for 100 min. Flux pinning studies revealed a strong contribution from δTc pinning. The present results demonstrate that the superconducting performance can be improved by ultra-sonication process producing nanometer-sized Y- 211 particles in bulk Y-123. This technique can help to produce large bulk superconductors for commercial exploitation.

In this study, we focus on methods to further improve the flux pinning and critical current density (J c) of disk shaped MgB2 bulk superconductors by a comparative study with locally obtained charcoal at optimized processing conditions. The doping of charcoal was varied at 0, 0.2, 0.4 and 0.6 wt%. Bulk MgB2 samples were produced by in-situ solid state reaction in Ar gas and ambient pressures at 775 °C, employing high purity of Mg metal and nano B powder mixed in a fixed ratio of Mg:B = 1:2 and optimal conditions. The phase has been investigated by X-ray diffractometer (XRD). The effects of the dopants on superconducting properties especially the field dependence of J c have been determined by superconducting quantum interference device (SQUID) and critical temperature (T c). Measurements of the magnetic susceptibility showed sharp superconducting transitions (ΔT c ∼ 1 K). J c at self-field and 20 K was determined to be ~280 and 467 kA/cm2 for the 0 and 0.4 wt% charcoal added samples respectively. The structural and superconducting results were correlated with the addition of charcoal in the final product.