MgB2 Superconductor Research Articles

Various effects of aliphatic amino acids on the critical current of the MgB2 superconductor

Carbon-doped MgB2 polycrystalline bulk samples were prepared by reacting mixtures of Mg and B powders, with and without 0.5 wt.% of the amino acids glycine, alanine, valine, leucine or isoleucine. The amount of carbon introduced into the MgB2 structure depends on the amino acid and is surprisingly higher for glycine, which has the lowest carbon content among the aliphatic amino acids. The critical temperatures of samples doped with all kinds of additives were lower than for the undoped sample. The critical current density was influenced by the additives to different extents, alanine, valine and leucine yielding the best results. The pinning force is also larger for these additives at T < 30 K. An analysis of the normalized flux pinning force behavior leads to the conclusion that point pinning is predominant in all samples.

First-principles investigation of the superconducting properties of MgXB4 (X = Al, Li, Na, K)

The discovery of superconductivity of MgB2 has attracted considerable research attention. Further exploration and discovery of MgB2-like superconductors are important to determine the properties of MgB2 and enhance our understanding of the superconducting mechanism. Four MgB2-like MgXB4 (X = Al, Li, Na, K) superconductors based on an MgB2 structure were designed in this study. The electronic structure, phonon dispersion relationship, Eliashberg spectral function, electron-phonon coupling constant (λ), and superconductivity of MgXB4 were investigated using a first-principles approach. The results indicated that MgXB4 (X = Al, Li, Na, K) were dynamically stable and demonstrated metallic characteristics. Further study of the Eliashberg spectral function and electron-phonon coupling constant indicated that these compounds were superconductors. The calculated superconducting transition temperatures (TC) of MgXB4 (X = Al, Li, Na, K) were 0.12, 22.7, 28.1, and 30.4 K, respectively.

Phase Evolution and Critical Temperature of MgB&lt;sub&gt;2&lt;/sub&gt; Superconductor Made of β-Rhombohedral Boron with Temperature Sintering Variation

We report the phase formation and electrical resistivity of MgB2 superconductor samples. Method of synthesis was conventional solid-state reaction employing crystalline β-rhombohedral boron (B) and magnesium (Mg) with a stoichiometric ratio of Mg:B=1:2. The two precursors were mixed and sintered at various temperatures from 600°C to 900°C for an hour. All samples were characterized employing XRD, SEM, and Cryogenic Magnetometer. It is shown that the MgB2 phase begins to form at a sintering temperature of 700°C. The highest peak intensity of the MgB2 phase was observed in a sample sintered at 900°C indicates the largest fraction of the superconducting phase among all synthesized samples. Electrical resistivity values were carried out to investigate the superconducting properties of the samples. It is also shown the samples sintered at a temperature of 800°C and 900°C possess superconducting properties with a critical temperature of ~ 43 K

Nb sheathed MgB2 superconducting tape with addition of VB2

MgB2 superconductor is a strategic material for practical applications, due to its superconducting properties as well as the low cost of the raw materials. For many technological applications cable, wire or tape format are required. The addition of VB2 in the MgB2 superconducting bulk had already been used in previous works as an efficient method to enhance the critical current density of the MgB2. In the present work, MgB2 superconducting tapes were developed and characterized using different amount of VB2 addition. Niobium was used as sheath material. The goal of this work is to create an efficient method to improve the transport properties of MgB2 superconducting tapes to be useful for practical application. The results include microstructural, crystallographic, magnetic and transport characterizations. All analyzes were performed together to study the influence of the VB2 addition on the superconducting behavior of MgB2 tapes.

Heat treatment influence on microstructure and properties of MgB2 superconductor

Study results of 1 mm diameter MgB2 superconductors fabricated by “powder in tube” method are presented. Ti and Nb were used as diffuse barriers materials. The mixture of magnesium and boron, powder of MgB2, and also mixture of magnesium, boron and MgB2 were used to form superconducting compound. Heat treatments in temperature range of 650-700 °C and 750-900 °C for 1h were conducted. It is established that 1 h heat treatment at 700 °C or 1 h at 900 °C heat treatment are sufficient for formation of MgB2 superconducting phase (Tc=38 K) in superconductors with titanium barrier and powder mixture of magnesium and boron or magnesium, boron and magnesium diboride. The Tc of in-situ process Nb - sheated superconductors was 37.6 K, the same Tc were measured for ex-situ superconductors heat treated at 900 °C for 1 h. The critical current measurement results of fabricated superconductors were presented, optimal heat treatment modes were determined for fabricated superconductors.

Electromagnetic design of actively shielded MRI magnet for dedicated mice imaging

A 3.0 T MRI magnet using MgB2 superconductors with conduction cooling for dedicated mice imaging was designed. The magnet configuration is determined by the central magnetic field strength, maximum magnetic field, magnetic field uniformity, uniformity volume, stray field, and coil costs. Linear programming was used to optimize the superconducting magnet of the MRI system. The optimized superconducting magnet can generate a central field of 3.0 T with field homogeneity better than 10 ppm in a 50 mm diameter of spherical volume (DSV). Actively shielded coils with opposite current densities were adopted to reduce the stray field. Finite element methods were used to analyze the mechanical stress and strain in the MgB2 coils during winding, cool-down from room temperature and electromagnetic excitation. The analysis results showed that the strain on the coils is within the design criteria. In this paper, the coil optimization design, electromagnetic and stress analysis were given.

Development and Superconducting Properties of Practical 18- and 30-Filamentary MgB2 Long Wires with Twisting by the In Situ Way

In this work, practical kilometer-length 18- and 30-filament Monel/Nb/MgB2 wires have been prepared by in situ powder-in-tube (PIT) method, combing in the twisting of wires with twisting pitches (TP) of 5 mm, 10 mm, and 20 mm. Transport critical current density (Jc) properties at various temperatures and magnetic fields for MgB2 wires with different TP and the effects of the heat-treatment conditions and wire diameter size on the Jc of 30- filament wire have been investigated in detail. The results suggest the Jc of MgB2 wire without twisting at 4.2 K, 4 T, and 20 K, 3 T separately reaches 1.9 × 105 and 5.87 × 104 A/cm2 for 30-filament wire, and 1.48 × 105 and 4.4 × 104 A/cm2 for 18-filament wire. After twisting, as the TP reduction, the Jc degradation of MgB2 wire is much slower in 30-filament than that in 18-filament at various magnetic fields. At the TP of 5 mm, the Jc degradation comparing with the untwisted wires at 4.2 K and 4 T is up to 30% in 18-filament wire but only about 10% in the 30-filament. This work indicates that the twisting route is very promising to the engineering application of MgB2 superconductor.

Enhancement of Jc in C6H16N2 added MgB2 superconductor without carbon substitution for boron

The influence of C6H16N2 addition on the microstructural and superconducting properties of MgB2 was systematically investigated. Unlike previously reported carbon sources, the C6H16N2 additive had no effect on the lattice parameters of MgB2. However, the obvious improvement of critical current density (Jc) was obtained for C6H16N2 added MgB2 without carbon substitution for boron. The average grain size was reduced with the addition of C6H16N2. Additionally, different numbers of spherical MgB2 grains appeared inside the C6H16N2 added MgB2. As the added amount of C6H16N2 increased, the active cross-sectional area fraction (AF) values first increased then decreased. The possible reasons for these results have been discussed. The additive C6H16N2 resulted in a small depression in Tc and slightly increased the ΔTc of MgB2 at 0 T. The Hc2 (T) and Hirr (T) values of C6H16N2 added MgB2 samples are larger than those of pure MgB2 at temperatures below 25 K. It was concluded that the reduced grain size and the improved grain connectivity were mainly correlated with the enhancement of the Jc, Fp, Hc2, and Hirr performances in C6H16N2 added MgB2 superconductors.

Determination of magnetic levitation force properties of bulk MgB2 for different permanent magnetic guideways in different cooling heights

Abstract In our study as different from the literature, the magnetic levitation force between a bulk MgB2 and two different permanent magnetic guideway (PMG) arrangements, which are Halbach and Conventional PMGs, were investigated in different cooling heights (CHs) and Field-Cooled (FC) condition at the temperatures of 37 K and 33 K. The cylindrical bulk MgB2 superconductor was fabricated by in-situ solid state reaction process with the diameter of 18 mm and the height of 5 mm. The XRD data indicates well developed MgB2 phase and the Jc value was obtained as 68 kA/cm2 at 30 K in the self-field. Experimental results show that MgB2 bulk above the Halbach PMG can exhibit better load capability at all the cooling heights between the bulk MgB2 and the PMG due to a more suitable magnetic field distribution. The maximum levitation force for Halbach PMG corresponds to 16.86 N whereas the conventional PMG shows 9.02 N at 37 K in cooling height of 77 mm. Additionally, the maximum levitation force increases while the CH increases because flux exclusion is more effective for larger CHs. It is considered that the experimental results obtained in study are very useful for future Maglev applications, because there are limited number of studies on magnetic levitation force of MgB2 bulk for different MgB2-PMG arrangements.

Enhanced magnetic field dependent critical current density of MWCNT doped magnesium diboride superconductor

Abstract MgB2 bulk samples added with multiwall carbon nanotube (MWCNT) were synthesized by powder–in–sealed tube (PIST) method. The effect of MWCNT doping on the structural and superconducting properties of MgB2 superconductor were investigated. The XRD analysis reveals the hexagonal structure of MgB2 in all the samples. FESEM, TEM and Raman analysis indicate the microstructural changes due to MWCNT doping. Raman spectra show the shifting of the E 2 g peak is more for lightly doped samples. DC magnetization measurement shows a sharp superconducting transition in all the samples and a slight reduction in critical temperature ( T c ) for the doped samples. The J c (B) characteristics of doped samples show a significant enhancement of J c compared with pristine MgB2 at 10 and 20 K. MgB2 doped with 1 and 2 wt% MWCNT show maximum enhancement in J c i.e., 1.3 × 10 5 and 1.1 × 10 5 A/cm2 at 5 T and 10 K, respectively, while heavily doped (5 wt%) sample shows lower J c (B) characteristics. The flux–pinning properties of MWCNT doped MgB2 have been enhanced due to the formation of point pinning centers.

Predicting doped MgB2 superconductor critical temperature from lattice parameters using Gaussian process regression

Magnesium boride, MgB2, has attracted much attention since the discovery of its superconductivity in 2001. The absence of weak-links in grain boundaries, less prominent anisotropy, rather simple powder-in-tube wire fabrication techniques, and much lower prices of raw materials, have made this new superconducting material a promising candidate for high-field magnet applications. Furthermore, it has been demonstrated that various methods, such as chemical doping, irradiations, and different processing parameters, can lead to lattice disorders in the materials and thus alter physical properties. Empirical results have shown that changes in lattice parameters through various methods correlate with changes in Tc but correlations are merely general tendencies and obviously not universal. In this work, the Gaussian process regression model is developed to predict critical temperature based on lattice parameters among disordered MgB2 in various materials systems. This modeling approach demonstrates a high degree of accuracy and stability, contributing to efficient and low-cost predictions of Tc and understandings of disorders and superconductivity in MgB2 superconductors.

Research Progress of Electromagnetic Properties of MgB2 Induced by Carbon-Containing Materials Addition and Process Techniques

For the high transition temperature (Tc) and low cost taking both raw materials and fabrication process into account, MgB2 has been a competitive candidate to replace the conventional NiTi superconductor for high-temperature application in fault current limiters, transformers, motors, magnetic resonance imaging, adiabatic demagnetization refrigerators, generators, etc. The carbon-containing materials addition induced high critical current density (Jc) is reviewed based on their influences on the upper critical field (Hc2), flux pinning force, and connectivity. The doping effects were compared in the overview focusing on SiC, organic dopants, and graphene-related dopants. SiC doping is featured for the high-field critical current density, which is caused by the increased Hc2 attributed to the substitution of carbon on boron site and the strong flux pinning force offered by the nanosized secondary phases in the MgB2 matrix. Organic dopants have the advantage over SiC dopant for their relatively homogeneous distribution in the MgB2 matrix based on wet mixing of the organics and the raw boron powders. Low doping level of two-dimensional materials can improve the superconducting properties in all measured fields because of the combined advantages of carbon substitution effect and grain connectivity. MgB2 fabricated with carbon-encapsulated boron also introduces strong flux pinning centers in MgB2, which show weak destruction of the connectivity of the MgB2 grains as reflected by the low-magnetic-supercurrent behavior. High-pressure treatment and diffusion method can fabricate high-density MgB2 superconductors with better connectivity and increase the Jc compared with the in situ and ex situ methods.

Effects of silver doping on magnetic levitation performance of MgB2 superconductors

MgB2 bulk samples with the silver contents of 0, 1.5, 3.0, 4.5, 6.0, 9.0 and 12.0 wt% were fabricated at 775 °C for 3 h in Ar atmosphere by solid-state reaction method. The structural and electromagnetic properties of MgB2 samples were studied using X-ray diffraction (XRD), scanning electronic microscope (SEM), energy dispersive spectrometry (EDS), magnetic hysteresis (M-H and M-T), magnetic levitation force (Fz) and lateral force (Fx) measurements. The effect of silver addition on the levitation force performance of MgB2 has been studied for the first time in this work. All the samples indicate the onset of superconducting critical temperature (Tc,onset) in the temperature range 37.97–38.11 K. The Jc values of the 3 wt% of silver doped MgB2 sample at 15 and 25 K were 98 and 49 kA/cm2 at 0.5 T as the highest remnant Jc value was around 110 and 65 kA/cm2 at 15 and 25 K, respectively. The repulsive and attractive levitation force at 25 K increases with rising x content up to 3 wt% silver and then decreases with increasing silver addition up to 12 wt%. The maximum attractive (repulsive) levitation force values at 25 and 30 K were observed for 3 wt% silver addition as −7.09 (+19.03) and −6.03 (+15.22), respectively. These results reveal that the silver addition is the effective metallic dopant for MgB2 and can be beneficial for improving the magnetic levitation achievement of MgB2 superconductors for high field engineering applications.

The Effect of Holding Time Variations in MgB2 Superconductor with Addition Nickel Prepared by Powder in Sealed Tube Method

In this research, the effect holding time on the synthesis of MgB2 superconducting material with the addition of nickel has been carried out using the powder in sealed tube method. MgB2 is a superconductor which has a critical temperature of ~ 39 K. The addition of 0%, 4%, and 8% Ni was added to study its effect on the superconductivity characteristics of MgB2. The sample preparation process begins by weighing the raw materials in the form of Mg, B and Ni powders according to stoichiometric calculations. The material is then crushed for 1 hour using mortar agate then put into a stainless steel tube SS 316L and the PIST pressing process is carried out with a pressure of 5 MPa, then the sintering process is carried out using a muffle furnace with a temperature of 800ºC and holding time for sintering temperature for 1 hour, 3 hours and 5 hours. Samples were characterized by means of XRD, SEM-EDS and Cryogenic Magnet. The results of phase identification through XRD showed that the phases formed were MgB2, MgNi2.5B2, MgO, Mg and Ni. Morphological structure and elemental composition were seen through SEM-EDS. From the Cryogenic Magnet test, pure MgB2 samples with a holding time of 1 hour had a critical temperature Tc of 36.29 K and did not have Tc. Meanwhile, pure MgB2 at a holding time of 5 hours has a Tc of 42.18 K and a Tc of 25.55K.

Out-of-plane ionicity versus in-plane covalency interplay and electron–phonon coupling in MgB2 superconductor

Using the Density Functional Theory (DFT) within the Generalized Gradient Approximation (GGA) pseudopotential and plane wave basis method along with the frozen-phonon approach that starts from the ab initio evaluation of the total energy Etot of the solid with frozen-in atomic displacements, it is found that a superposition of A2u and the E2gvibrations modes is the key factor in the superconducting mechanism in MgB2 compound. Electron–Phonon coupling to these A2u and E2g phonon modes especially at the zone-boundary A point of the hexagonal Brilliouin zone leads to an interband hole charge transfer (and transfer back) between in-plane σ bond to the out-of-plane π bond along with an interatomic electron charge transfer (and transfer back) between the Magnesium s-states to the Boron out-of-plane pz-state. The direction of the electronic current is opposite to that of hole current so that it reinforces the polarization associated with these currents and may generate a large dynamical charge at a given critical temperature Tc that drives the compound into the superconducting state.

Pinning mechanism and engineering critical current density considerations in the design of MgB2superconducting coils

Superconducting coils are designed for the generation of specified magnetic fields (low, intermediate, or high). To function in different field regimes, it is important to analyze the pinning mechanics and the types of pinning centers in the superconductor in a precisely defined magnetic field. Correct identification of pinning centers would allow for the formulation of approaches to increase the critical current density and critical magnetic field, ultimately making it possible to reduce the cost of producing superconducting coils (with smaller amounts of wire needed). This paper will present a method to analyze pinning centers in MgB2 superconductor wires in low, intermediate, and high magnetic fields at 4.2 K, 20 K, 25 K, and 30 K. This method allows us to elucidate the influences of doping, isostatic pressure, and reduction of the wire diameter on individual pinning centers in a specific magnetic field. In this article, we also show a method to analyze the types of pinning centers and their impact on the engineering critical current density (Jec). This method not only allows for the identification of the types of pinning centers, but also identifies which type allows for a shift in the Jec criterion of 100 A/mm2 to enable higher magnetic fields.

Carbon-Coating Layers on Boron Generated High Critical Current Density in MgB2 Superconductor.

Boron particles with a homogeneous carbon-coating layer were employed as the precursor to fabricate MgB2 superconductors to generate artificial two-dimensional (2D) flux-pinning centers. Systematic microstructure investigation reveals that the carbon layers are well-distributed in the MgB2 matrix without agglomeration. The thickness of the carbon layers is smaller than the MgB2 coherent length, which makes them transparent to supercurrent. The critical current density is increased because of the strong flux-pinning effects of the 2D carbon layers in the superconductor as highly efficient flux-pinning centers and the increased irreversibility field due to the carbon-doping effects.

The effects of sintering temperature on superconductivity of MgB2 prepared by hot pressing

The [Formula: see text] superconductor bulks were prepared by hot-pressing under the pressure of 55 MPa at different temperatures of 650[Formula: see text]C, 700[Formula: see text]C, 800[Formula: see text]C, 900[Formula: see text]C, 1000[Formula: see text]C, respectively, and the hot-pressing effects on the superconducting properties of [Formula: see text] bulks were investigated. The density of bulk samples increased with the rise of sintering temperature and the density reached to the maximum value when the sintering temperature rose to 1000[Formula: see text]C. For the sample sintered at 1000[Formula: see text]C, the critical transition temperature ([Formula: see text] reached the highest value of 38.8 K despite the high content of [Formula: see text] (30.62 wt.%). However, the critical current density ([Formula: see text] did not increase with the increase in the density and [Formula: see text]. The sample sintered at 700[Formula: see text]C maintained the high superconducting volume and had a small grain size, thus providing the best magnetic flux pinning force and [Formula: see text]. An effective hot-pressing process to improve [Formula: see text] might be the way to prepare by further increasing the density of sintering samples at 700[Formula: see text]C.

Critical current density and pinning mechanism in Hetero structure of Bi-2212 and MgB2 superconductors

Critical current density and pinning mechanism in Hetero structure of Bi-2212 and MgB2 superconductors

On the origin of the sharp, low-field pinning force peaks in MgB2 superconductors

Various MgB2 thin films and single crystals were found in the literature to exhibit a sharp, narrow peak at low fields in the volume pinning force, Fp(H)-diagrams. The origin of this peak is associated with a steep drop of the current density when applying external magnetic fields and is ascribed to sample purity. We show here that bulk MgB2 prepared by spark-plasma sintering also shows the sharp, narrow peak in Fp. The peak is also seen in the volume pinning force scaling, Fp/Fp,max vs h = H/Hirr. Furthermore, polycrystalline bulk MgB2 samples prepared close to the optimum reaction temperature reveal this peak effect as well, but other samples of the series show a regular scaling behavior. The combination of magnetization data with data from electric transport measurements on the same samples demonstrates the origin of this peak effect. On increasing preparation temperature, the pinning force scaling changes from grain boundary pinning to point pinning and the grain connectivity gets worse. Hence, the sharp, low-field peak in Fp vanishes. Therefore, the occurrence of the peak effect in Fp gives important information on the grain coupling in the MgB2 samples.