MgB2 Superconductor Research Articles

Tuning of superconductivity of bulk MgB2 by ball milling and sieving the boron precursor

This study aims to explore the impact of sieving the crystalline boron precursor applied after its ball milling on the critical current density (Jc) of MgB2 produced by the in-situ reaction. Various analytical techniques, such as X-ray diffraction, magnetization measurements, and microstructure analysis were employed to systematically investigate the bulk polycrystalline MgB2 samples. SEM revealed that grains of ball-milled B powder had a nanoscale size. Sieving made the powder more uniform in size. The purity of the refined powder was tested using X-ray diffraction, and four different precursor combinations were evaluated. MgB2 bulk had a homogeneous, dense microstructure composed of MgB2 nanocrystals, in contrast to the bulk MgB2 prepared of as-purchased B. The latter exhibited uneven microstructure containing both crystalline and porous non-crystalline portions. The improved inter-grain connectivity and microstructural homogeneity after ball milling led to an increase of self-field Jc at 20 K from 71.2 kA/cm2 to 144.6 kA/cm2. After sieving the B precursor before and after ball milling, a further increase of Jc by 18% and 8% was achieved.

Effect of wire diameter on structure and electrical properties of (Al + Al2O3)-sheathed MgB2 with Nb barrier

Effect of wire diameter and annealing on the properties of internal magnesium diffusion (IMD)-processed MgB2 composite wires is studied by establishing a correlation with the characteristics of the interfacial reaction zones developed at Mg − B and barrier (Nb) – sheath (Al + Al2O3) interfaces. The MgB2 superconductor phase grows at the Mg − B interface along with the B-rich MgB4 phase. Formation of MgB2 prior to MgB4 is anticipated owing to a relatively lower Gibbs free energy at annealing temperatures studied. An intermetallic NbAl3 phase has developed at the barrier – sheath interface whose thickness decreases with an increase in the wire diameter. Development of NbAl3 further provides mechanical reinforcement and aids at yielding a dense superconductor phase. Resistivity – voltage characteristics of wire indicates the formation of MgB2 through a huge drop in resistivity values across the core of the wire. An annealing temperature of 645 °C near the melting point of pure Mg (650 °C) and annealing time of 120 min results in highest critical current of the MgB2 wire. Annealing time beyond 120 min has shown an impediment to the flow current due to crack propagation possibly because of accumulation of stresses within the developed superconductor phase due to grain growth. A significantly greater resistance for the wire with the smallest diameter is attributed to the formation of a thicker NbAl3 phase. However, highest engineering current density has also been attained by the wire with the smallest diameter annealed at 645 °C for 60 min.

Interfacial reactions and critical current density of Cu-sheathed Cu-doped MgB2 wire with Ti diffusion barrier

Development of reaction zones at the core Mg(Cu) – boron (B) and barrier (Ti) – sheath (Cu) interfaces of an internal magnesium diffusion (IMD) manufactured MgB2 composite wire has been studied at varying temperatures of 560–665 °C for 30 min and varying time periods of 30–180 min at 560 °C. MgB2 has predominantly formed at the Mg(Cu) – B interface with little solubility of Cu of 1–3 at%. A second phase enriched with Cu, Mg2Cu, has also formed at random locations within the MgB2 phase matrix, indicating the advantage of doping Cu in the Mg rod. However, a layer of another B-rich phase, MgB4, has also formed at a higher temperature of 665 °C with negligible solubility of Cu at the MgB2 – B interface. At the Ti – Cu interface, the formation of CuTi2, CuTi, Cu4Ti and Cu4Ti3 intermetallic phases took place. However, the Cu4Ti and Cu4Ti3 phases develop after annealing the wire at a relatively higher temperature of 620 °C and at 560 °C with progress in annealing time. Critical current density (Jc) decreases with annealing time; however, the sensitivity of Jc towards annealing time is low. Critical current density is found to be strongly dependent on the annealing temperature of the composite wires for the formation of the MgB2 superconductor phase. A maximum critical current density of ∼ 2.5 × 104 A/cm2 at 5 T and 4.2 K is attained for the Cu-doped MgB2 composite wire when annealed at 665 °C for 30 min. A maximum shift of only 0.5 K is measured for MgB2 when formed at the highest annealed temperature compared to the lowest one in this study.

Point-contact spectroscopy in the Centre of Low Temperature Physics Košice (Review article)

Point-contact spectroscopy offers a unique straightforward possibility to study the electronic properties of metals. Soon after the invention of this technique by Igor Yanson in the B. Verkin Institute for Low Temperature Physics and Engineering of the NAS of Ukraine [Sov. Phys. JETP39, 506 (1974)], multiple laboratories adopted this technique and applied it to various topical problems in modern solid-state physics. Here, we offer a brief review of how point-contact spectroscopy has been developed and used in the Centre of Low Temperature Physics Košice. By this technique, we were able to obtain for example the spectrum of the electron-phonon interaction in an unprecedented large energy scale up to 160 meV in LaB6. The Zeeman splitting of the Pr3+ ion levels in the crystal-electric field has been detected for the first time in PrNi5. “Inverse” point-contact spectra of the electron-phonon interaction found in semimetallic arsenic were explained by the weak localization in the point-contact area. The point-contact Andreev reflection spectroscopy enabled to detect not only the superconducting energy gap in YB6, but also the Einstein-like phonon mode responsible for superconductivity. The first spectroscopic evidence of the two-gap superconductivity in MgB2 has been provided in our experiments. High spin polarization in Co2FeSn Heusler nanowires for spintronics has been obtained.

Bulk MgB2 superconductor for levitation applications fabricated with boron processed by different routes

Bulk MgB2 discs were prepared by an in situ route from mixtures of magnesium and boron powders. The boron powders were produced by two methods. The first one consisted of a self-propagating high temperature magnesiothermic synthesis (SHS) process followed by acid and fluorine cleaning and a heat treatment in inert atmosphere. This approach produced boron with purities between 86 % and 97 %, where the main impurity was Mg. Depending on the final heat treatment, these boron powders were amorphous or crystalline. In the second route, high purity nano powders (99 %) of boron were obtained by a diborane pyrolysis process. Bulks of MgB2 were characterized by structural, microstructural, and magnetic measurements. Critical current density, pinning force aspects and levitation force (including guiding force) details were assessed. Amorphous lower purity boron (86–97 %) obtained by the first processing route was found to promote the largest levitation forces of the MgB2 bulks and, among these samples, the best levitation results were recorded when using boron with a purity of 95–97 %. Use of a lower purity boron that decreases the cost of MgB2 promotes large scale production at industrial level of bulk MgB2 superconducting magnets for levitation applications and enhances the applicability potential of MgB2 superconductor. The relationship between levitation force and specific features of the samples such as pinning force details are discussed.

Correlations between volume defects and grain boundaries in flux-pinning of MgB[formula omitted] superconductor

In order to enhance flux-pinning effect of grain boundaries, we quenched 5 wt% (Fe, Ti) doped MgB2 superconductor in water. During the study, we discovered that there was a correlation between pinned fluxes at volume defects and pinned fluxes at grain boundaries. We present mechanisms of the correlation and details of relative calculations. Pinned fluxes at a volume defect are leak-out-depinned through grain boundaries because grain boundaries are connected to the volume defect. Several equations were derived, which are push-out-depinned force of flux quanta from a volume defect, flux-pinning force of flux quanta at grain boundaries, and the number of leak-out-depinned fluxes from a volume defect. TEM images proved that many grain boundaries would exist on MgB2 grains which are contact on (Fe, Ti) particles. A result was obtained by comparing M–H curve of air-cooled 5 wt% (Fe, Ti) doped MgB2 with that of water-quenched one on the base of the equations, which shows approximately 34% of the number of flux quanta pinned at the volume defect was leak-out-depinned from the volume defect and pinned at the grain boundaries. The study suggested that amount of pinned fluxes at grain boundaries could be calculated by M–H curve of superconductor.

The Influence of Preparation Temperature on the Different Facets of Bulk MgB2 Superconductors.

Two MgB2 samples were prepared using the spark plasma sintering (SPS) technique at different temperatures-950 °C (S1) and 975 °C (S2)-for 2 h under 50 MPa pressure to study the influence of preparation temperature on different facets, namely those perpendicular (PeF) and parallel (PaF) to the compression direction of uniaxial pressure during the SPS of MgB2 samples. We analyzed the superconducting properties of the PeF and PaF of two MgB2 samples prepared at different temperatures from the curves of the critical temperature (TC), the curves of critical current density (JC), the microstructures of MgB2 samples, and the crystal size from SEM. The values of the onset of the critical transition temperature, Tc,onset, were around 37.5 K and the transition widths were about 1 K, which indicates that the two samples exhibit good crystallinity and homogeneity. The PeF of the SPSed samples exhibited slightly higher JC compared with that of the PaF of the SPSed samples over the whole magnetic field. The values of the pinning force related to parameters h0 and Kn of the PeF were lower than those of the PaF, except for Kn of the PeF of S1, which means that the PeF has a stronger GBP than the PaF. In low field, the most outstanding performance was S1-PeF, whose critical current density (JC) was 503 kA/cm2 self-field at 10 K, and its crystal size was the smallest (0.24 µm) among all the tested samples, which is consistent with the theory that a smaller crystal size can improve the JC of MgB2. However, in high field, S2-PeF had the highest JC value, which is related to the pinning mechanism and can be explained by grain boundary pinning (GBP). With an increase in preparation temperature, S2 showed a slightly stronger anisotropy of properties. In addition, with an increase in temperature, point pinning becomes stronger to form effective pinning centers, leading to a higher JC.

Self-Propagating High-Temperature Synthesis of MgB2 superconductor: A Review

Self-Propagating High-Temperature Synthesis of MgB2 superconductor: A Review

Pressure-induced superconductivity at 32K in MoB2.

Since the discovery of superconductivity in MgB2 (Tc ∼ 39K), the search for superconductivity in related materials with similar structures or ingredients has never stopped. Although about 100 binary borides have been explored, only a few of them show superconductivity with relatively low Tc. In this work, we report the discovery of superconductivity up to 32K, which is the highest Tc in transition-metal borides, in MoB2 under pressure. The Tc of MoB2 in the α phase can be well explained by theoretical calculations in the framework of electron-phonon coupling. Furthermore, the coupling between the d electrons of Mo and the out-of-plane Mo-phonon modes are the main driving force of the 32K superconductivity of MoB2. Our study sheds light on the exploration of high-Tc superconductors in transition metal borides.

Calculation of AC Losses in a 500 kJ/200 kW Multifilamentary MgB2 SMES Coil

SMES technology based on MgB2 superconductor and cryogenic-free cooling can offer a viable solution to power-intensive storage in the short term. One of the main obstacles to the development of dry-cooled SMES systems is the heat load removal due to the AC loss of the superconductor during fast charging and discharging cycles at high power. Accurate knowledge of the amount and distribution of AC loss in the coil is of paramount importance for the sizing and the design of the cooling system and for assessing the possible operational limits of the technology. In this manuscript, the AC loss of a 500 kJ/200 kW multifilamentary MgB2 SMES during charge–discharge cycling at full power is numerically investigated. The methodology and assumptions of the calculation are briefly resumed, and numerical results are reported and discussed in detail. In particular, the time profile and the distribution of the dissipated power all over the coil are reported. An average dissipation of 85.5 mW/m is found all over the coil during one charge–discharge cycle at full power, with a peak of 150.1 mW/m in the turns lying at the ends of the coil.

Essential factors that can tremendously influence the grain connectivity of ex-situ sintered MgB2 superconductor

Extending the holding time and slightly excessive Mg addition and Mg-Cu co-addition are introduced to the MgB2 superconductors prepared by ex-situ sintering method aiming to improve their grain connectivity and Jc. The influence of which on superconducting performance and microstructure of ex-situ sintered MgB2 was also systemically studied. The AF value of Mg-added samples is obviously improved, especially the AF value of the Mg-Cu co-addition sample has reached the level of the in-situ sintering samples. The Jc of two samples with Mg addition is improved by comparing to others within the entire whole field.

Enhancement for phonon-mediated superconductivity up to 37 K in few-hydrogen metal-bonded layered magnesium hydride under atmospheric pressure.

The discovery of superconductivity in layered MgB2 has renewed interest in the search for high-temperature conventional superconductors, leading to the synthesis of numerous hydrogen-dominated materials with high critical temperatures (Tc) under high pressures. However, achieving a high-Tc superconductor under ambient pressure remains a challenging goal. In this study, we propose a novel approach to realize a high-temperature superconductor under ambient pressure by introducing a hexagonal H monolayer into the hexagonal close-packed magnesium lattice, resulting in a new and stable few-hydrogen metal-bonded layered magnesium hydride (Mg4)2H1. This compound exhibits superior ductility compared to multi-hydrogen, cuprate, and iron-based superconductors due to its metallic bonding. Our unconventional strategy diverges from the conventional design principles used in hydrogen-dominated covalent high-temperature superconductors. Using anisotropic Migdal-Eliashberg equations, we demonstrate that the stable (Mg4)2H1 compound is a typical phonon-mediated superconductor, characterized by strong electron-phonon coupling and an excellent Tc of 37 K under ambient conditions, comparable to that of MgB2. Our findings not only present a new pathway for exploring high-temperature superconductors but also provide valuable insights for future experimental synthesis endeavors.

Higher harmonics of the ac-susceptibility for pristine MgB2 superconductor variation with ac-field and frequency: analysis of irreversibility line

Higher harmonics of the ac-susceptibility for pristine MgB2 superconductor variation with ac-field and frequency: analysis of irreversibility line

The effect of C and (Er, Sm, Eu)2O3 doping and high isostatic pressing on superconducting properties of MgB2 phase and volume of cylindrical MgB2 ceramic samples

We present our research on the influence of pressure on the synthesis of the MgB2 phase and, consequently, on the superconducting properties of the MgB2 system. We investigated undoped, C-doped, and double-doped, i.e. C and Re2O3 Re = Sm, Eu, Er), MgB2 samples. We conducted comparative studies of critical temperature, irreversibility field, critical current and pinning force for ceramic samples annealed at ambient pressure and 1 GPa. Heat treatment under high isostatic pressing (HIP) of 1 GPa reduced the volume of the undoped MgB2 cylinder by 40%. This significantly increases the density, homogeneity and number of connections between the grains, and also eliminates voids in the undoped material. However, we did not observe any reduction in the volume of C-doped and double-doped MgB2 cylinders. Our research shows that the critical parameters of the MgB2 superconductor can be increased more effectively by doping C and the HIP process than by double doping C with Er, Sm or Eu and the HIP process. The HIP process improved the efficiency of pinning centers created by magnetic admixtures with a low magnetic moment (Sm and Eu), but on the other hand, reduced this efficiency for admixture with a high magnetic moment (Er).

Synthesis of Three-Dimensional Carbon Nanosheets and Its Flux Pinning Mechanisms in C-Doped MgB2 Superconductors.

Three-dimensional carbon nanosheets (3D-CNS) were synthesized by salt template spray-drying method in order to solve the agglomeration of 2D nanocarbon by a traditional mixing method. MgB2 bulks doped with 3D-CNS with molar ratio composition of MgB2-x(3D-CNS)x (x = 0, 0.1 and 0.2) have been prepared by in situ sintering process. The microstructure, critical current density and flux pinning of the sintered samples have been investigated. Differing from the structure in previous studies, the 3D-CNS doping is more efficient for the refinement of the MgB2 grains due to the 3D network structures. The results clearly show that more active C releasing from 3D-CNS at high temperature can provide effective flux pinning centers by the substitution of C for B in MgB2 lattice. Furthermore, the lattice distortion and increased grain boundaries should be responsible for the enhancement of critical current density (Jc) at high magnetic fields as well as the increased irreversible magnetic field (Hirr). However, the positive action in Jc at low field has been extremely offset by the concentration of impurities at MgB2 grain boundaries such as released extra C without substitution and MgO, which is considered to further deteriorate the grain connectivity.

Towards an effective mass model for the quasi-1D magnesium diboride superconducting nanostructures

We developed a parabolic effective mass model with the aim to describe the in-plane constricted superconductivity in few-monolayer MgB$_2$. The model was employed to study a constricted quasi-$1$D superlattice with the simplest version of the Anderson approximation. Resonant Fano-Feshbach-like effect was found, between a miniband of a continuous normal state dispersion cutting across $E_F$ and a miniband of quasi-discrete dispersion, even though the latter is significantly energetically distanced from the Fermi level. The origin of the phenomenon was identified to be the presence of characteristic strongly localised eigenstates, due to the specific material parameters. Moreover, the mechanism leading to a "precursor" pseudogap region in a system with a non-zero phase difference is described, when the strength of the coupling is still not sufficient to open a superconducting gap, but enough to introduce electron-hole mixing into the system.

Effects of Sn-ion irradiation on local structure and flux pinning properties of MgB2 thin films

We studied the effects of tin (Sn) ion irradiation on the local structure and flux pinning properties of 412-nm-thick MgB2 thin films. Ions with 2 MeV incident energy were irradiated onto MgB2 thin films at room temperature and various dose levels from 2 × 1012 to 7 × 1013 atoms/cm2. X-ray diffraction and extended X-ray absorption fine-structure results confirmed extended average bond lengths and increased mean-square relative displacement that correlated with crystalline disorder serving as artificial pinning centers induced by ion irradiation. While the superconducting transition temperature (Tc) exhibited a decreasing trend, the magnetization Jc (H) of the irradiated films measured at both 5 K and 20 K were remarkably enhanced compared to that of pristine films at high applied magnetic fields. The decreases in exponent β from 1.1 and 1.13 to 0.69 and 0.62 after irradiation at 5 K and 20 K, respectively, as the dose increased to 5 × 1013 atoms/cm2 indicates a shift in the pinning mechanism from weak collective pinning to strong plastic pinning according to collective pinning theory. In addition, the scaling behavior of the flux pinning force density (Fp) in the irradiated MgB2 films showed a crossover from surface pinning to normal point pinning. These results suggest that intermediate-energy irradiation by heavy ions such as Sn generates effective pinning sources for improving Jc(H) in MgB2 superconductors.

Second harmonic AC calorimetry technique within a diamond anvil cell.

Tuning the energy density of matter at high pressures gives rise to exotic and often unprecedented properties, e.g., structural transitions, insulator-metal transitions, valence fluctuations, topological order, and the emergence of superconductivity. The study of specific heat has long been used to characterize these kinds of transitions, but their application to the diamond anvil cell (DAC) environment has proved challenging. Limited work has been done on the measurement of specific heat within DACs, in part due to the difficult experimental setup. To this end, we have developed a novel method for the measurement of specific heat within a DAC that is independent of the DAC design and is, therefore, readily compatible with any DACs already performing high pressure resistance measurements. As a proof-of-concept, specific heat measurements of the MgB2 superconductor were performed, showing a clear anomaly at the transition temperature (Tc), indicative of bulk superconductivity. This technique allows for specific heat measurements at higher pressures than previously possible.

Superconductivity and Hydrogen Economy: A Roadmap to Synergy

Hydrogen as an energy carrier is a promising alternative to fossil fuels, and it becomes more and more popular in developed countries as a carbon-free fuel. The low boiling temperature of hydrogen (20 K or −253.15 °C) provides a unique opportunity to implement superconductors with a critical temperature above 20 K such as MgB2 or high-temperature superconductors. Superconductors increase efficiency and reduce the loss of energy, which could compensate for the high price of LH2 to some extent. Norway is one of the pioneer countries with adequate infrastructure for using liquid hydrogen in the industry, especially in marine technology where a superconducting propulsion system can make a remarkable impact on its economy. Using superconductors in the motor of a propulsion system can increase its efficiency from 95% to 98% when the motor operates at full power. The difference in efficiency is even greater when the motor does not work at full power. Here, we survey the applications of liquid hydrogen and superconductors and propose a realistic roadmap for their synergy, specifically for the Norwegian economy in the marine industry.

Mg gas infiltration for the fabrication of MgB2 pellets using nanosized and microsized B powders

MgB2 superconductor pellets were synthesized through Mg gas infiltration method using nanosized- and microsized B powders. There was a marked difference in the superconducting properties of the two samples, particularly in the pinning force and dominant pinning mechanism. The microstructures of the samples were observed using HR-TEM and STEM-HAADF, and the results showed that the primary reason for the difference in the superconducting properties is the distribution of the nanosized second-phase particle MgO. Additionally, a feasible reaction model for the Mg gas infiltration method was established. Compared to the Mg liquid infiltration method, the gas infiltration showed better penetrability ability with a small amount of residual Mg. This study presents a novel synthesis process to fabricate an MgB2 pellet with superior density and superconducting properties. This method can be used in multiple applications such as superconducting bearings, compact superconductor magnets, and magnetic shielding.