On the study of phase formation and critical current density in superconducting MgB2 (original) (raw)
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Ceramics International, 2019
In this study, the critical current density and mechanical performance of MgB 2 superconductors prepared using different magnesium sources (Mg and MgH 2 with 99.9 % and 96.5 % purities, respectively) were analyzed. When the samples were characterized structurally and electrically, we found that the grain sizes were about 40 nm and transition temperature was around 30 K being lower than the literature. Critical current densities of all samples were calculated using Bean's model and our calculations yielded critical current density values higher than 10 6 A.cm-2 in self-field. The highest critical transition temperature value belongs to M800 (Mg with 99.9% purity) sample and when the critical current density and transition temperature values of this sample are considered, this sample appears to have the potential for practical use. Vicker's microhardness measurements were performed and yield stress (Y), elastic modulus (E), brittleness (B i), ductility (D) and fracture toughness (K IC) were calculated. All samples exhibited Indentation Size Effect (ISE) and microhardness measurements were compared with some microhardness models with were compatible with ISE behavior.
Structural characterization and superconducting properties of MgB2 prepared by SHS-method
International Journal of Self-Propagating High-Temperature Synthesis, 2007
Self-propagating high-temperature synthesis (SHS) of bulk MgB 2 superconductor from Mg-2B powder blend is reported. This reaction proceeds violently at 100 A under a protective atmosphere. Since the heat of reaction of Mg and B was not enough for chain reaction, then (Ti + C) mixed powders were used as the ignition agent to assist the reaction (Mg + 2B). In this case, the combustion front moved without any difficulty. The diffraction lines of the product can be indexed to a hexagonal MgB 2 phase, with lattice constants a = 3.0845 Å, and c = 3.5259 Å. For comparison, the direct synthesis of (Mg + 2B) mixture was carried out at (800 ° C-1000 ° C). It can be found, that high-temperature sintering (1000 ° C) will induce the formation of impurities. The MgB 2 grains are fine, well compacted and more homogenous. The structure of materials was studied using XRD, FESEM and EDX. M-H curvatures were measured under the magnetic fields between -80 kOe and 80 kOe. J c was calculated from width of magnetization hysteresis loops based on the extended Bean Model.
Journal of Superconductivity and Novel Magnetism, 2019
In this paper, the results of the superconductivity evaluation of MgB 2 intermetallic-based superconducting materials prepared through mechanical alloying are reported. The formation of the MgB 2 intermetallic phase took place through an intermediate phase reaction at the solid-state reaction temperature, which tends to favor single-phase materials. The annealing of mechanically milled Mg and B powders resulted in the highest mass fraction of MgB 2 observed in this study, reaching more than 90% after annealing at 900°C for 1 h. At that point, there were still second phases present consisting of Mg, MgO, and Fe 2 B. Improved superconducting properties were obtained in samples sintered at temperatures of 850°C and 900°C that had a low resistivity, low critical temperature gap values, and a high residual resistivity ratio (RRR).
Preparation and characterization of MgB2 superconductor
Pramana, 2002
Superconducting MgB 2 crystallites having a size up to 3 X 2 X 1 mm 3 have been grown using a liquidassisted-sintering process under atmospheric pressure of argon. The magnetization measurements have been carried out on MgB 2 samples to determine upper critical field, critical current density and irreversibility line. The obtained values of critical current density and upper critical field were ~10 6 A/cm 2 (at 1 T and 5 K) and 12 T, respectively. The Andreev reflection studies carried on metallic MgB 2 /Ag planar junctions revealed a novel two-energy gap like structure for MgB 2. Surface analysis using X-ray photoelectron spectroscopy revealed that MgB 2 is sensitive to environmental degradation.
Physica C-superconductivity and Its Applications, 2010
The structural, transport and magnetic properties of MgB2 superconductor heavily blended with Mg is studied. The samples are synthesized with a new approach in both, pressed carbon environment and in flowing argon. The excess magnesium used is observed to play dual role: one being the prevention of Mg losses during the synthesis process and hence maintaining the stoichiometry of MgB2 phase, and second being the formation of Mg milieu probably all around the MgB2 grains to give a dense structure. Excess Mg also improves the grain connectivity by going into the pores and there by minimizing the insulating junctions. The residual resistivity of the sample is observed to decrease from 57.02 μΩ cm to 10.042 μΩ cm as it is progressively filled with superconductor–normal–superconductor (SNS) type junctions amongst the grains by the virtue of increased magnesium content. The synthesized samples devoid of porosity show the superconducting transition, Tc in the range of 39–34 K as of clean MgB2 samples, though overloaded with Mg. The excess Mg resulted in enhanced critical current density, Jc from 6.8 × 103 A cm−2 to 5.9 × 104 A cm−2 at 20 K and 10 kOe, with reasonable decrease in the superconducting transition. Thus our samples being overloaded with Mg impart mechanical strength and competitive superconducting properties, which forms a part of interest.
2007
The phenomenon of suddenly decrease of electrical resistance at negligible values was emphasized for a lot of metals and alloys in low temperature conditions and was named superconductivity. The paper presents an overview regarding the evolution of the superconductor materials and the actual stage of these materials used in electrotechnics and in other different domains, being presented the methods through which the superconductors were obtained and the properties achieved for each one. The paper refers to obtaining MgB2 superconducting material. The evolution of formation superconducting phases was monitored by mean of X-ray diffraction and scanning electron microscopy respectively.
Physical Review B, 2010
The effects of high-temperature vacuum-annealing-induced Mg deficiency in MgB 2 single crystals grown under high pressure were investigated. As the annealing temperature was increased from 800 to 975°C, the average Mg content in the MgB 2 crystals systematically decreased while T c remains essentially unchanged and the superconducting transition slightly broadens from ϳ0.55 to ϳ1.3 K. The reduction in the superconducting volume fraction was noticeable already after annealing at 875°C. Samples annealed at 975°C are partially decomposed and the Mg site occupancy is decreased to 0.92 from 0.98 in as-grown crystals. Annealing at 1000°C completely destroys superconductivity. X-ray diffraction analysis revealed that the main final product of decomposition is polycrystalline MgB 4 and thus the decomposition reaction of MgB 2 can be described as 2MgB 2 ͑s͒ → MgB 4 ͑s͒ +Mg͑g͒. First-principles calculations of the Mg 1−x ͑V Mg ͒ x B 2 electronic structure, within the supercell approach, show a small downshift of the Fermi level. Holes induced by the vacancies go to both and bands. These small modifications are not expected to influence T c , in agreement with observations. The significant reduction in the superconducting volume fraction without noticeable T c reduction indicates the coexistence, within the same crystal, of superconductive and nonsuperconductive electronic phases, associated with regions poor and rich in Mg vacancies.
Physica C Superconductivity, 2004
Various heat-treatment schedules have been applied to the optimization of Tc and Jc,m in bulk MgB 2. Samples were prepared by first mixing in a ‘V’ shaped mixer and then planetary milling a stoichiometric mixture of Mg and B powders. These powders were then compacted and heat-treated at different schedules under 200 Torr of Ar. The heat-treating schedules investigated involved different time-temperatures, heating rates, and cooling rates. Magnetic critical current densities (at 20 K) of more than 2 × 10 5 A/cm 2 in 10 kOe and 4 × 10 5 A/cm 2 in 5 kOe were obtained. The influence of sodium silicate additions on the bulk properties has been studied.
Role of MgO impurity on the superconducting properties of MgB2
Physica C: Superconductivity, 2014
We address the effect of MgO impurity on the superconducting properties of MgB 2 . The synthesis of MgB 2 is very crucial because of sensitivity of Mg to oxidation which may lead to MgO as a secondary phase. Rietveld refinement was performed to determine the quantitative volume fraction of MgO in the samples synthesized by two different techniques. Both the samples were subjected to magnetization measurements under dc and ac applied magnetic fields and the observed results were compared as a function of temperature. Paramagnetic Meissner effect has been observed in a sample of MgB 2 having more amount of MgO (with T c = 37.1K) whereas the pure sample MgB 2 having minor quantity of MgO shows diamagnetic Meissner effect with T c = 38.8K. M-H measurements at 10K reveal a slight difference in irreversibility field which is due to MgO impurity along with wide transition observed from ac magnetic susceptibility measurements. The magnetotransport measurements ρ(T) using ρ N = 90%, 50% and 10% criterion on pure sample of MgB 2 has been used to determine the upper critical field whereas the sample having large quantity of MgO does not allow these measurements due to its high resistance.
Al-alloyed MgB2: correlation of superconducting properties, microstructure, and chemical composition
Superconductor Science & Technology, 2005
Al-alloyed MgB 2 was prepared by solid state reaction of Mg 1−x Al x alloy and crystalline B powder particles. The nominal composition of the samples was Mg 1−x Al x B 2 , with x = 0, 0.1, 0.2, and 0.3. Microstructure and chemical composition have been investigated by scanning electron microscopy (SEM), electron probe microanalysis (EPMA), and preliminary transmission electron microscopy (TEM). The Al concentration of the matrix is close to, but less than, the nominal Al concentration and the difference increases with increasing nominal Al concentration. Al is incorporated into MgB 2 grains of ∼1 µm size by substitution of Mg lattice sites with Al. Al was found to be distributed inhomogeneously, which partially explains the broadening of the superconducting transition width ( T c ) with increasing Al mole fraction. Al-alloyed samples contain large (∼15 µm) and small (∼2 µm) secondary phases embedded in the (Mg, Al)B 2 matrix. The composition of large secondary phases is found to be (Mg, Al)B 7+δ , with the mole fraction of excess boron (δ) increasing from 0.99 at.% in the pure sample to 4.14 at.% in the highest alloyed sample. The Al to Mg mole fraction ratio in these large secondary phases is about half of that in the matrix. The size and density of the large secondary phases increased with increase in Al mole fraction. The secondary phases constitute less than 4% of the total sample volume and are thus not likely to affect the bulk superconductivity. Magnesium diffusion and evaporation governed the formation of secondary phases and can be explained by considering the effects of annealing temperature, annealing time, and boron powder particle size.