MgB2 Superconductor Research Articles

Biaxial strain engineering on the superconducting properties of MgB2 monolayer

The effect of biaxial strain on the superconducting properties of MgB2 monolayer was studied by first-principles calculations. The stability analyses by phonon dispersions show that a biaxial strain of as much as 7% can be applied onto MgB2 without inducing any imaginary frequency. The superconducting property calculations based on the frame of Migdal-Eliashberg theory successfully reproduce the two-gap superconductivity of MgB2. The results show that the tensile biaxial strain can increase the critical temperature of MgB2 by as much as ∼20% while the compressive biaxial strain would decrease the critical temperature by ∼29%. The detailed microscopic mechanism of the biaxial strain effect on the superconducting properties was studied by calculations of electronic structures and phonon dispersions. The increased Tc is a combining result of the increased electron density at the Fermi level and the in-plane boron phonon softening. By means of high-throughput screening of proper substrates, it is found that most of the substrates would result in tensile strain in MgB2 film, some of which can result in tensile strain of higher than 10%, consistent with many experimental works that the MgB2 thin film has increased Tc. The results in this work provide a detailed understanding of the biaxial strain engineering mechanism and demonstrate that biaxial strain engineering can be an effective way of tuning the superconducting properties of MgB2 and other similar materials.

Enhanced Flux Pinning Performance of Bulk MgB2 via Immersion of Synthetic Motor Oil

The present investigation focuses on the incorporation of synthetic motor oil as an inexpensive, rich carbon source in bulk MgB2 superconductor and its effect on superconducting and flux pinning properties. A set of three MgB2 bulk samples were prepared from commercial high-purity powders of Mg metal and amorphous B powder utilizing a conventional in situ solid-state reaction process. Before sintering, the MgB2 samples were immersed in used and new synthetic motor oil for a standby time of 30 min and sintered in pure Ar atmosphere at 775 °C for 3 h. X-ray powder diffractometer (XRD) analysis confirmed that single-phase formation of MgB2 with a small shift in X-ray diffraction peaks especially at (110) towards the peak position due to the effect of carbon substitution into the boron sites in lattice for samples immersed in new and used synthetic oil. The magnetization measurements indicated the Tc (onset) value to somewhat decrease to 37.5 K as a result of carbon doping. Microstructural observations with scanning electron microscopy (SEM) suggested that fine nano-sized MgB2 grains improved self-field critical current density around 3.8 × 105 A/cm2 at 20 K for all samples studied. Further, the high-field critical current density (Jc) was improved especially for the sample immersed in used synthetic motor oil with the value of 2.7 × 104 A/cm2, 6 × 103 A/cm2 at 20 K, and at 3 T and 4 T, which is higher as compared to pure-MgB2 sample. In essence, the results signify that the bulk MgB2 samples immersed with used synthetic motor oil would improve the bulk performance at high magnetic fields indicating to be a viable option for industrial applications.

Enhancing Critical Current Density of Bulk MgB2 via Nanoscale Boron and Dy2O3 Doping

Moderate critical current density (Jc) has been a long‐lasting problem in bulk MgB2 superconductors. We show a certain increment in Jc of bulk MgB2 via the use of amorphous boron precursor together with Dy2O3 doping. Dy2O3 dopant concentration varies from 0 to 2 wt%. X‐Ray diffraction (XRD) shows the formation of DyB4 particles. The critical temperature (Tc) is not affected by Dy2O3 doping and stands close to 38 K, showing that there is no Dy interaction with the MgB2 lattice. Microstructural studies show nanometer‐sized MgB2 grains. A high self‐field Jc of around 380 kA cm−2 is achieved at 20 K within the Dy2O3 doping range of 0.5–1.5 wt%. At around 1 wt% Dy2O3 doping an improved high‐field performance, 90 kA cm−2 at 2 T, 20 K, is observed. In the flux pinning diagram, 1 wt% Dy2O3 doping caused a peak shift from 0.19 (0 wt%) to 0.23. This indicates secondary pinning by DyB4 and lattice strains. Raman studies show the increase in the phonon density of states (PDOS) with increasing Dy2O3 doping.

Effect of cubic and hexagonal boron nitride additions on the microstructure and properties of bulk MgB2 superconductors

MgB2 is a promising material for intermediate temperature applications where conventional low temperature superconductors cannot be used, especially if the range of magnetic fields over which is has acceptable current carrying performance can be expanded. However, its applicability is limited by poor properties at elevated magnetic fields. Carbon-based dopants can be used to dramatically improve the high-field performance of MgB2, but at the cost of a reduction in the superconducting transition temperature (T c) that limits the operation temperature to 20 K or below. Here we report an enhancement of superconducting performance of MgB2 with the addition of cubic and hexagonal boron nitride (BN), without any significant reduction in T c. Ex-situ bulk samples of MgB2 with two forms of BN addition were manufactured by the field assisted sintering technique after high energy ball milling of powder mixtures. We find that hexagonal BN (hBN) nanoparticles mixed homogenously with MgB2 powder react much more easily to produce Mg–N–B impurities than larger cubic BN (cBN) particles (∼10 µm) under the same processing conditions. The addition of 1 wt% hBN or 5 wt% cBN combined with 6 h of milling has been demonstrated to improve the critical current density (J c) of MgB2 over the entire magnetic field range. It is proposed that the nano-sized Mg–N–B impurities, that typically reside at MgB2 grain boundaries, increase pinning strength by introducing additional flux pinning centres. In addition, excess Mg may benefit the low-field performance by improving the connectivity. This work shows the significance of microstructural characterization on inhomogeneous superconducting materials to analyse their performance.

Effects of magnetic and non-magnetic doping on the vortex lattice in MgB2

Small-angle neutron scattering has been used to study the vortex lattice in superconducting MgB2 doped with either manganese or carbon to achieve a similar suppression of the critical temperature. Measurements were performed with the magnetic field applied along the c axis, where the vortex lattice in pure MgB2 is known to undergo a field- and temperature-driven 30° rotation transition. For Mn doping, the vortex lattice phase diagram remains qualitatively similar to that of pure MgB2, indicating only a modest effect on the vortex–vortex interaction. In contrast, the vortex lattice rotation transition is completely suppressed in the C-doped case, probably due to a change in the electronic structure which affects the two-band/two-gap nature of superconductivity in MgB2. The vortex lattice longitudinal correlation length shows the opposite behavior, remaining roughly unchanged between pure and C-doped MgB2 while it is significantly reduced in the Mn-doped case. However, the extensive vortex lattice metastability and related activated behavior, observed in conjunction with the vortex lattice transition in pure MgB2, are also seen in the Mn-doped sample. This shows that the vortex lattice disordering is not associated with a substantially increased vortex pinning.

Synthesis and characterization of MgB2 superconductors with carbon nanotubes (CNTs) and tin (Sn) addition

MgB2/CNT is a promising candidate for superconducting wire application due to its excellent mechanical properties and carbon nanotube’s low density. However, strong interfacial adhesion between the CNT reinforcement and the MgB2 matrix is difficult to manage. Therefore, this study examines the synthesis and characterization of magnesium diboride (MgB2) superconductors with carbon nanotubes (CNTs) and tin (Sn) addition. Determining the proper method and combination of CNT & Sn affects MgB2 superconductors is crucial. Raw materials of magnesium (Mg), boron (B), Sn, and multi-walled carbon nanotubes (MWCNTs) were used for a solid-state reaction process to determine the proper synthesis method and the effect of CNT on superconductors’ critical temperature. Each sample was obtained by weighing the raw material first, followed by hand grinding with agate mortars for 3 hours. The pelletization was then conducted by using a compact pressing machine with a pressure of 350 MPa. The compacted samples were then sintered at 800 °C for 2 hours either through the vacuum or PIST process. Finally, all were characterized, and MgB2 was discovered to be the dominant phase with minor impurity phases such as MgO, Mg, Mg2Sn, C, and Sn. Based on SEM morphological analysis, the grain boundaries of sample A1 were more precise than B2. In both, the grain size also varies, and the distribution of elements is uneven. Subsequently, Cryogenic Magnet Characterization indicated that at 40 K, almost all samples possess superconducting characteristics. For future studies, the potential impact of MgB2 on critical current density (Jc) and magnetic density (Hc) in several commercial applications such as Magnetic Resonance Imaging (MRI), magnetic levitation, and transformers needs to be investigated

Machine learning doped MgB2 superconductor critical temperature from topological indices

Abstract Due to the absence of weak-links in grain boundaries, less anisotropy, and high availabilities at reasonable cost, magnesium boride, MgB2, has been studied extensively in the past decade. It has relatively high critical temperature, which is correlated to crystallographic and electronic structures. Two topological indices, the electric connectivity index and valence energy level connectivity, are characteristics of compound branching. We develop the Gaussian process regression (GPR) model to shed light on the relationship between topological descriptors and superconducting transition temperature for doped MgB2 superconductors. The model is highly accurate and stable, which contributes to fast predictions of superconducting transition temperature.

Comparisons of magnetic behaviors between pure MgB2 and 33 wt% (Fe, Ti) particle-doped MgB2 superconductor

Comparisons of magnetic behaviors between pure MgB2 and 33 wt% (Fe, Ti) particle-doped MgB2 superconductor

Disordered MgB[formula omitted] superconductor critical temperature modeling through regression trees

Magnesium boride superconductors are promising candidates in high energy physics applications. These superconductors have several advantages as compared to other high temperature superconductors, such as the absence of weaklinks in grain boundaries, abundant availability of raw materials, simple wire fabrication process, and increased isotropy in microstructure and material performance. Critical temperature, Tc, is the most critical parameter in superconductor characterizations. Various factors have impacts on Tc, including the chemical doping, irradiation, and synthesis conditions. It has been shown that disorders in the crystal structure lead to changes in normal state resistivities and Tc, which are correlated with each other. Here, the regression tree model is developed to predict Tc of disordered magnesium boride superconductors through room temperature resistivities. This modeling approach manifests a high degree of accuracy and stability, contributing to efficient low-cost estimations of critical temperature and understandings of disorder and superconductivity in MgB2 superconductors.

High-temperature phonon-mediated superconductivity in monolayer Mg2B4C2

A two-dimensional material – Mg2B4C2, belonging to the family of the conventional superconductor MgB2, is theoretically predicted to exhibit superconductivity with critical temperature Tc estimated in the 47–48 K range (predicted using the McMillian-Allen-Dynes formula) without any tuning of external parameters such as doping, strain, or substrate-induced effects. The origin of such a high intrinsic Tc is ascribed to the presence of strong electron-phonon coupling and large density of states at the Fermi level. This system is obtained after replacing the chemically active boron-boron surface layers in a MgB2 slab by chemically inactive boron-carbon layers. Hence, the surfaces of this material are inert. Our calculations confirm the stability of 2D Mg2B4C2. We also find that the key features of this material remain essentially unchanged when its thickness is increased by modestly increasing the number of inner MgB2 layers.

Enhanced flux pinning by magnetic CrB2 nanoparticle in MgB2 superconductor

Different from the non-magnetic pinning centers, the magnetic pinning centers have stronger flux pinning force due to the direct magnetic interaction between the magnetic pinning centers and the magnetic vortices, thus have been widely used to enhance the critical current density of various superconductors. In this article, nanoparticles of antiferromagnet CrB2 were introduced as magnetic pinning center to improve the flux pinning force density (Fp), irreversible field (Hirr), and critical current density (Jc) of MgB2. Compared to the pristine sample, the CrB2 doped MgB2 with low doping levels shows a significantly improved Jc and Fp, especially in the high field region. Hirr is also substantially enhanced. It is revealed that the magnetic flux pinning behavior of CrB2 doped samples obviously deviates from the surface pinning law followed by most pure MgB2. Through theoretical analysis and curve fitting, an extra pinning force superimposed on the surface pinning background is separated and identified, which follows the rule of point pinning. Such a pinning scenario is supported by the microstructure analyses which demonstrate that CrB2 nanoparticles are highly dispersed and embedded in MgB2 matrix, acting as effective pinning centers.

The effect of strain and pressure on the electron-phonon coupling and superconductivity in MgB2—Benchmark of theoretical methodologies and outlook for nanostructure design

Different theoretical methodologies are employed to investigate the effect of hydrostatic pressure and anisotropic stress and strain on the superconducting transition temperature (Tc) of MgB2. This is done both by studying Kohn anomalies in the phonon dispersions alone and by explicit calculation of the electron–phonon coupling. It is found that increasing pressure suppresses Tc in all cases, whereas isotropic and anisotropic strain enhances the superconductivity. In contrast to trialed epitaxial growth that is limited in the amount of achievable lattice strain, we propose a different path by co-deposition with ternary diborides that thermodynamically avoid mixing with MgB2. This is suggested to promote columnar growth that can introduce strain in all directions.

Extreme Learning Machine Approach to Modeling the Superconducting Critical Temperature of Doped MgB2 Superconductor

Magnesium diboride (MgB2) superconductor combines many unique features such as transparency of its grain boundaries to super-current flow, large coherence length, absence of weak links and small anisotropy. Doping is one of the mechanisms for enhancing these features, as well as the superconducting critical temperature, of the compound. During the process of doping, the MgB2 superconductor structural lattice is often distorted while the room temperature resistivity, as well as residual resistivity ratio, contributes to the impurity scattering in the lattice of doped samples. This work develops three extreme learning machine (ELM)-based empirical models for determining MgB2 superconducting critical temperature (TC) using structural distortion as contained in lattice parameters (LP) of doped superconductor, room temperature resistivity (RTR) and residual resistivity ratio (RRR) as descriptors. The developed models are compared with nine different existing models in the literature using different performance metrics and show superior performance over the existing models. The developed SINE-ELM-RTR model performs better than Intikhab et al. (2021)_linear model, Intikhab et al. (2021)_Exponential model, Intikhab et al. (2021)_Quadratic model, HGA-SVR-RRR(2021) model and HGA-SVR-CLD(2021) model with a performance improvement of 32.67%, 29.56%, 20.04%, 8.82% and 13.51%, respectively, on the basis of the coefficient of correlation. The established empirical relationships in this contribution will be of immense significance for quick estimation of the influence of dopants on superconducting transition temperature of MgB2 superconductor without the need for sophisticated equipment while preserving the experimental precision.

Critical Current Density and Meissner Effect of Smart Meta-Superconductor MgB2 and Bi(Pb)SrCaCuO.

The smart meta-superconductor MgB2 and Bi(Pb)SrCaCuO increase the superconducting transition temperature (TC), but the changes in the transport critical current density (JC) and Meissner effect are still unknown. Here, we investigated the JC and Meissner effect of smart meta-superconductor MgB2 and Bi(Pb)SrCaCuO. The use of the standard four-probe method shows that Y2O3:Eu3+ and Y2O3:Eu3++Ag inhomogeneous phase significantly increase the JC, and JC decreases to a minimum value at a higher temperature. The Meissner effect was measured by direct current magnetization. The doping of Y2O3:Eu3+ and Y2O3:Eu3++Ag luminescent inhomogeneous phase causes a Meissner effect of MgB2 and Bi(Pb)SrCaCuO at a higher temperature, while the non-luminescent dopant reduces the temperature at which samples have Meissner effect. The introduction of luminescent inhomogeneous phase in conventional MgB2 and copper oxide high-temperature Bi(Pb)SrCaCuO superconductor increases the TC and JC, and Meissner effect is exerted at higher temperature. Therefore, smart meta-superconductivity is suitable for conventional and copper oxide high-temperature superconductors.

Theoretical study of superconducting parameters of Al and Li Co-doping substitution on MgB2

In the present study we investigated the theoretical calculation of Superconducting state parameters by Al as electron doping and Li as hole co-doping on MgB2 superconductor for four different concentrations (Mg0.962Al0.018Li0.02B2, Mg0.854Al0.096Li0.05B2, Mg0.812Al0.138Li0.05B2, Mg0.77Al0.14Li0.09B2). The well-known Empty Core (EMC) model of Ashcroft potential was used to study the theoretical investigation of the superconducting state parameters (SSP) viz. Electron–Phonon Coupling Strength (λ), Coulomb Pseudopotential (µ*), Transition Temperature (Tc), Isotope Effect Exponent (α) and Effective Interaction Strength (N0V) of Mg1-x-yAlxLiyB2 system. It is observed that addition of Al & Li doping to superconductor MgB2 causes the parameters λ, Tc, α and N0V decreases. This suggests decrease in superconducting behaviour of MgB2 is due to effect of Al and Li co-doping in MgB2 which also shows strong coupling to intermediate coupling behaviour. Whereas Coulomb Pseudopotential (µ*) remains approximately same. In this paper it is also observed that Transition Temperature (Tc) for Mg1-x-yAlxLiyB2 alloys for different concentration is almost same with the experimental values.

I-V characteristics of MgB2 conductors with different metallic sheaths

I-V characteristics of MgB2 superconductors with different metallic sheaths were measured in the range of voltages from 10−7 V up to 10−1 V at liquid helium temperature and external magnetic fields 4.75–8.0 T. R-T measurements of compared superconductors between 20 K and 300 K were also done, which allows to estimate the conductor’s temperature after quench. Quench power density of MgB2 superconductors with different sheaths is compared and analysed. It was found that the tape with Ni sheath reaches higher quench power density in contrast to Ti sheathed one in entire measured range. The quench properties and transport ability were enhanced with adding high conductive strip of metal stabilization. The effect of the metallic sheaths and additional stabilization on the thermal stability of MgB2 conductors carrying DC currents is discussed.

Two-gap to single-gap superconducting transition on a honeycomb lattice in Ca1−xSrxAlSi

It is a well-established fact that the physical properties of compounds follow their crystal symmetries. This has especially pronounced implications on emergent collective quantum states in materials. Specifically, the effect of crystal symmetries on the properties of superconductors is widely appreciated, although the clarification of this relationship is a core effort of on-going research. Emergent phenomena on honeycomb lattices are of special interest, as they can give rise to spectacular phenomenology, as manifested by the recent discovery of correlated states in magic-angle graphene, or by the high-temperature superconductivity in MgB$_2$. Here, we report on the structural and microscopic superconducting properties of a class of ternary superconductors with Al/Si honeycomb layers, i.e. Ca$_{1-x}$Sr$_{x}$AlSi. We show that this solid solution is a remarkable model system with a highly tunable two-gap to single-gap superconducting system on a honeycomb lattice, where the superconductivity is enhanced by a subtle structural instability, i.e. the buckling of the Al/Si layers.

Levitation force of Graphene added bulk MgB2 superconductor

The effect of Graphene addition on levitation/suspension force of bulk MgB2 superconductor has been studied in both zero field cooling (ZFC) and field cooling (FC) conditions at different temperatures. For this, the pristine, 0.5, 1 and 3 wt% of Graphene added bulk MgB2 samples are prepared by solid state reaction method under argon atmosphere. X-Ray diffraction (XRD) and scanning electron microscopy (SEM) of prepared samples have been performed for structural characterization and surface morphology respectively. The structural characterization shows well developed phase of MgB2 while hexagonal shaped grains are evident in SEM photographs. Electrical resistivity measured between 20 K and 300 K confirms a decrease in superconducting transition temperature (TC) and increase in normal state resistivity of pristine MgB2 due to Graphene addition. The levitation force measurement under zero field cooling condition reveals that low level (0.5 wt%) of Graphene addition enhances the levitation force throughout the investigated temperature range. However the levitation force measurements performed at different initial field height under field cooling condition confirm the presence of both attractive and repulsive force. Further, the maximum levitation force (FMLF) and maximum attractive force (FMAF) estimated from the field cooling data of pure and Graphene added MgB2 is found to be a strong function of initial field cooling height (HFC). The maximum repulsive/levitation force (FMLF) increases exponentially with increase in initial field cooling height, while an exponential decrease in attractive force (FMAF) of MgB2 has been observed with increase in initial field cooling height.

Enhanced critical current density and flux pinning of anthracene doped magnesium diboride superconductor

The present investigation reports the structural, superconducting and flux pinning properties of anthracene doped MgB2 superconductor prepared by powder-in-sealed-tube (PIST) method. The formation of the hexagonal structure of MgB2 is confirmed from the XRD analysis of all the synthesized samples. The microstructural changes by anthracene doping in the MgB2 superconductor are studied using FESEM, TEM and Raman analysis. Raman spectra show the change in the E2g peak frequency in all the doped samples. All the samples show a sharp superconducting transition and the critical temperature (Tc) of anthracene doped samples decreases. The Jc(B) behavior at 10 and 20 K shows a significant enhancement in the anthracene doped samples as compared to undoped MgB2. 3 wt% anthracene doped MgB2 shows maximum enhancement in Jc at 10 and 20 K, i.e., 7.03 × 105 and 2.5 × 105A∕cm2 at 2 T, while 5 wt% anthracene doped sample (MBA5) shows lower Jc(B) characteristics. The grain boundary pinning is the dominant pinning mechanism in all the synthesized samples.

CVD Yöntemi ile Karbon Katkılanmış Bor Kullanılarak Üretilen MgB2 Süperiletkeninin Yapısal ve Manyetiksel Özelliklerine Sıcaklık Etkisinin İncelenmesi

Carbon, which is known to play a improvement role on the superconductivity properties of MgB2, was doped into MgB2 by CVD (chemical vapor deposition) method, and the change of structural and magnetic properties observed in MgB2 in relation to the change of sintering temperature was investigated. In this context, in the sample preparation step, amorphous nano boron powders with carbon added by chemical vapor deposition method were mixed with magnesium powders at certain mass ratios and sintered at four different temperatures (700-800-900-1000 0C) with the classical solid state reaction method and converted into carbon-added MgB2 samples. SEM photographs of amorphous nano-boron powders with carbon added were taken and subjected to elemental analysis. The structural and magnetic properties of the samples obtained were examined. X-ray diffraction graph, magnetization values and Magnetic Field-Magnetic Moment (M-H) graphs of the samples obtained by XRD method and the critical current density values with Bean Method were found and Magnetic Field-Critical Current Density (Jc-H) graph was created. It was determined that, in all the samples obtained, the MgB2 superconductor structure was formed and the carbon structures included in the structure caused changes in the lattice parameters due to the ion radius difference, that fracture occurred in the magnetization curve of the sample prepared by sintered at 1000 0C due to the intense impurities caused by the phase transitions due to the high temperature, in samples prepared at low temperatures, and that magnetization curves that could not expand sufficiently were observed, flux jumping occurs in the magnetization curve and this also affected the critical current density. In addition, the change in the critical current density value from 7.0×10^3 A/cm2 to 2.8×10^4 A/cm2 showed that the reaction temperature was an effective parameter on the experimental results. As a result of the examinations, when compared between the 4 different sintering temperature values used, it was found that the sintering temperature of 900 0C was the temperature at which the best physical results were obtained in MgB2.