Bulk MgB2 Research Articles

Enhanced superconducting properties in bulk MgB2 through spark plasma sintering of ball-milled and sieved crystalline boron

Enhanced superconducting properties in bulk MgB2 through spark plasma sintering of ball-milled and sieved crystalline boron

Investigation of Hot Press and Cold Press Effects on Structural, Electrical and Magnetic Properties of Pyrene Added Bulk MgB2

Investigation of Hot Press and Cold Press Effects on Structural, Electrical and Magnetic Properties of Pyrene Added Bulk MgB2

Estimation of magnetic levitation and lateral forces in MgB2 superconducting bulks with various dimensional sizes using artificial intelligence techniques

The advent of superconducting bulks, due to their compactness and performance, offers new perspectives and opportunities in many applications and sectors, such as magnetic field shielding, motors/generators, NMR/MRI, magnetic bearings, flywheel energy storage, Maglev trains, among others. The investigation and characterization of bulks typically relies on time-consuming and expensive experimental campaigns; hence the development of effective surrogate models would considerably speed up the research progress around them. In this study, we first produced an experimental dataset containing the levitation and lateral forces between different MgB2 bulks and one permanent magnet under different operating conditions. Next, we have exploited the dataset to develop surrogate models based on Artificial Intelligence (AI) techniques, namely Extremely Gradient Boosting, Support Vector Regressor (SVR), and Kernel Ridge Regression. After the tuning of the hyperparameters of the AI models, the results demonstrated that SVR is the superior technique and can predict levitation and lateral forces with a worst-case accuracy scenario 99.86% in terms of goodness of fit to experimental data. Moreover, the response time of these models for the estimation of new datapoints is ultra-fast.

Bulk MgB2 Superconducting Materials: Technology, Properties, and Applications.

The intensive development of hydrogen technologies has made very promising applications of one of the cheapest and easily produced bulk MgB2-based superconductors. These materials are capable of operating effectively at liquid hydrogen temperatures (around 20 K) and are used as elements in various devices, such as magnets, magnetic bearings, fault current limiters, electrical motors, and generators. These applications require mechanically and chemically stable materials with high superconducting characteristics. This review considers the results of superconducting and structural property studies of MgB2-based bulk materials prepared under different pressure-temperature conditions using different promising methods: hot pressing (30 MPa), spark plasma sintering (16-96 MPa), and high quasi-hydrostatic pressures (2 GPa). Much attention has been paid to the study of the correlation between the manufacturing pressure-temperature conditions and superconducting characteristics. The influence of the amount and distribution of oxygen impurity and an excess of boron on superconducting characteristics is analyzed. The dependence of superconducting characteristics on the various additions and changes in material structure caused by these additions are discussed. It is shown that different production conditions and additions improve the superconducting MgB2 bulk properties for various ranges of temperature and magnetic fields, and the optimal technology may be selected according to the application requirements. We briefly discuss the possible applications of MgB2 superconductors in devices, such as fault current limiters and electric machines.

Infiltration effect of Mg with different matter states in the reaction of MgB2 bulks

MgB2 superconductor bulks were fabricated using Mg and B precursors. The physical state of Mg (solid, liquid, or gas) significantly influences the final product, MgB2 bulk. In this study, the Solid-Solid, Liquid-Solid, and Gas-Solid methods were employed to fabricate MgB2 bulks. The superconducting properties and microstructure of various samples were systematically investigated. Notably, for the first time, nano-CT was utilized to comprehensively and non-destructively analyze the microstructure of MgB2 bulks, particularly the size and distribution of voids, which serve as the primary barrier affecting the superconducting properties. By examining the microstructure characteristics, the infiltration effect of Mg in different physical states during the reaction was analyzed, and a feasible reaction model was established. This research contributes to a better understanding of the reaction mechanism of MgB2, which may aid other researchers in enhancing the properties of MgB2 superconductors.

Effect of nano-Ag addition on the superconducting properties of MWCNT doped magnesium diboride superconductor

Bulk MgB2 superconductor added with nano silver (Ag), multiwalled carbon nanotube (MWCNT) and both the added (i.e., nano-Ag and MWCNT) samples were synthesized. The influence of nano-Ag and MWCNT addition on the structural and superconducting properties were studied. The XRD analysis confirms the formation of hexagonal MgB2 superconductor. Microstructural studies of all the synthesized samples are carried out by field emission scanning electron microscope (FE-SEM) and transmission electron microscope (TEM). The superconducting properties such as critical temperature (Tc), magnetic critical current density (Jc), irreversibility field (Birr) and flux pinning properties of all the synthesized samples were investigated at 10 K and field up to 7 T. Nano Ag added MWCNT doped MgB2 shows maximum enhancement in the critical current density i.e., 5.83 × 104 A/cm2 at 5 T and 10 K.

Modeling of levitation force by using Jc distribution approaches for bulk MgB2 fabricated with different methods

Studies in literature have shown that the bulk MgB2 structural characteristics and the associated superconducting properties are not homogeneous throughout the entire sample. This study focuses on understanding the effect of local differences in Jc distribution on the levitation force. For this purpose, numerical modeling was performed for four samples, two of which were previously fabricated, and the other two were fictionalized. Levitation forces for these samples were calculated with three different Jc distribution approaches: uniform, semi-uniform, and non-uniform. Comparing the four samples’ numerical results shows us that the position dependence of Jc should be considered when performing numerical modeling in cases where there are significant local differences because of the local defects, thermal origin stress cracks in the outer side, and impurity distribution. It is inferred that the obtained results will help to elucidate the physical background of the local features and shielding currents on the levitation force and improve the bulk properties of the MgB2 samples.

Very strong levitation force and stability achieved with a large MgB2 superconductor disc

This rapid communication highlights a remarkable achievement obtained with a 120 mm diameter large cylindrical MgB2 bulk: the measurement of magnetic levitation forces up to 700 N at 20 K, levitation being stable up to levitation forces exceeding 400 N. The investigated sample, which has one of the largest dimensions reported in the literature, was fabricated using the unconventional spark plasma sintering technique. This method is known as one of the most efficient methods for producing dense MgB2 samples. This breakthrough holds significant potential for advancing high-performance levitation systems and superconducting applications, including high-speed transportation systems and magnetic bearings.

The influence of the HIP process on the microstructure, critical temperature and critical current density of double-doped (with Dy2O3 + C, and Tb2O3 + C) MgB2 bulk compounds

ABSTRACT We present the results of the irreversibility field (B irr ), critical temperature (T c), and magnetic critical current density (J cm) for cylindrical samples encapsulated in carbon and doped with 1-3 at% dysprosium and terbium oxides. All samples were analyzed microstructurally by using a scanning electron microscope (SEM) and X-ray diffraction (XRD). The influence of annealing temperature and time and high isostatic pressure on J cm was investigated. Our results show that double-doped (C + Tb2O3 and C + Dy2O3) and low isostatic pressure heat treatment (0.1 MPa) decrease the T c by about 0.5 K compared to the undoped MgB2 (0.1 MPa). The magnetic measurements show that double-doped and heat treatment under high isostatic pressure (HIP - 1 GPa) leads to a decrease in T c of about 2.5 K compared to the undoped MgB2 sample after the HIP process. Our results show that the HIP process slightly increases J cm at 4.2 K and 20 K, and improves J cm at 30 K in undoped MgB2 samples. Furthermore, our results show that the HIP process and double-doped significantly increase J cm at 4.2K and decrease J cm at 30K. Our results indicate that the HIP process can produce high field and low-temperature pinning centers in doubly doped MgB2 materials.

The possible effect of surface barriers on the magnetic levitation of cylindrical superconductors

Abstract Superconducting magnetic levitation force measurements on large field cooled cylindrical MgB2 bulks with different diameters and thicknesses are reported. For these experiments, a special set-up permitting one to measure forces up to 500 N was used. In contradiction to previous measurements, the obtained force hysteresis cycles could not be reproduced with the analytical mean field model proposed by Bernstein et al (2017 Supercond. Sci. Technol. 30 065007). The failure of the model has been attributed to surface barriers effects which were not taken into account in the model. This last one was accordingly modified in order that the measured force cycles could be reproduced. Contrary to most other models describing surface barriers effects, the modified model suggests that above a threshold field anti-vortices and not vortices enter the superconductor. This behaviour is related to the storage by the superconductor of the mechanical work done by the operator. In addition, it has turned out that the threshold field is a decreasing function of the critical surface current density of the samples. As a consequence, the surface barriers effects occur only if this quantity and the critical current density are large enough. Otherwise, the internal magnetic field of the superconductor could be computed and was seen to be a decreasing function of the thickness of the superconductors.

Comparison of Superconducting Properties of Bulk MgB2 Samples with Pyrene Additive Produced by B Powder and C encapsulated B Powder

Comparison of Superconducting Properties of Bulk MgB2 Samples with Pyrene Additive Produced by B Powder and C encapsulated B Powder

Property-microstructure analysis and statistical qualification of MgB2 bulks: An investigation via micrograph feature extraction

In the pursuit of high-quality superconductors for stable operation at cryogenic temperatures, the understanding of process-structure-property relationships is crucial to device performance reproducibility and scalability. In this manuscript, we propose an indirect image-based method to evaluate statistical relationships between the ex-situ characterization results (i.e., superconducting, and mechanical properties) and microstructural distribution of MgB2 bulks produced via Spark Plasma Sintering. This study utilizes bulks processed under similar temperature profiles and sintering time, but different uniaxial loads and powder modalities. Considering the microstructure as the centerpiece between the MgB2 sintering thermodynamics and yielded macroscopic properties, multiple cross-sectional SEM micrographs of bulks MgB2 undergo an automated process that comprise image thresholding, segmentation, masking, and filtering to enable the identification of microstructural defects and byproducts. The statistical analysis establishes the relationship between processing conditions, bulk microstructural distributions, and ex-situ characterization results. Lastly, morphological analysis of the bulk cross-section microstructures indicates the effectiveness of the proposed method in identifying and statistically quantifying property variation in MgB2 bulk properties.

Overview of spark plasma synthesis and sintering of MgB2 superconductor

This study aims to investigate the sintering mechanisms of magnesium diborides using spark plasma sintering and optimize the sintering process conditions. Three different precursor materials were used: (i) commercial powder MgB2, (ii) mixtures of magnesium and nano boron powder (Mg + 2B), and (iii) mixtures of Mg and MgB4 powder for an ‘in-situ’ reactive synthesis. The density of the obtained bulks exceeds 90% of the theoretical density of the material. The structure and microstructure of the samples were analyzed using x-ray diffraction and scanning electron microscope, which were correlated with their superconducting properties. The critical current density (J c), pinning force (f p), and electrical resistivity (ρ) of the samples were also investigated. The results show that the in-situ reactive synthesis of MgB2 using magnesium and nano boron powder was the most effective method for producing high-quality samples with improved superconducting properties. The findings could lead to the development of more efficient methods for producing high-performance of MgB2 bulks.

Exfoliated Magnesium Diboride (MgB2) Nanosheets as Solid Fuels.

Magnesium diboride (MgB2) has been explored as an alternative fuel to boron (B) due to its high energy density and the additive effect of magnesium (Mg) to promote B combustion. However, the primary oxidation of MgB2 does not occur unless it decomposes at a high temperature (830 °C), which makes ignition difficult and the reaction slow. Recently, two-dimensional (2D) exfoliated MgB2 nanosheets have attracted increasing attention due to their unique properties and potential applications in various fields. In this study, we investigate the potential of 2D exfoliated MgB2 nanosheets as solid fuels for overcoming the challenges of MgB2 combustion. We analyzed their oxidation behavior and energetic performance through material characterization and combustion tests under slow- and fast-heating conditions and compared their performance with those of bulk MgB2, B nanoparticles, and a B/Mg nanoparticle mixture. This study highlights the potential of MgB2 nanosheets as promising solid fuels with superior energetic properties.

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.

Role of electron–phonon coupling in the superconducting state of MgB2 polycrystals fabricated at different conditions

In this study, the superconducting properties of MgB2 bulk samples fabricated at different conditions were investigated. Ex situ MgB2 samples were further mixed with Mg and B at different amounts and sintered at different times and temperatures. Using Raman spectroscopy, the bearing of electron-phonon (e-ph) coupling (EPC) has been investigated. The dependence of temperature on magnetization, M(T), along with the dependence of magnetic field on magnetization, M(H), were evaluated to analyze the superconducting characteristics. Using the McMillan formular, which is modified by Allen–Dynes, and the frequency of phonon located in the mid peak centre ω2 (E2g), the contribution of EPC, whose value is closely proportional to the variation of critical temperature was reckoned. The lowest EPC value was determined by Raman spectrum analysis to be 1.094 for the MgB2 sample added with 0.5 Mg and sintered at 1000 °C for 1 h. In addition, the sample showed the lowest critical temperature along with the highest critical current density. The small bundle field (Bsb) as well as the large bundle field (Blb) were obtained using the collective pinning theory, demonstrating the expansion of the small also the large bundle regimes under suitable sample fabrication conditions. Based on the dependence of temperature on normalized critical current density (j) and normalized small bundle field (Bsb), δTc pinning is the predominant flux pinning mechanism across totally MgB2 bulk samples. The model of Dew–Hughes confirmed that the addition of Mg along with B creates surface pinning in the matrix by analyzing the pinning properties.

Correlation between microstructure and superconducting properties of MgB2 bulk samples with Mg addition and Mg/hBN co-additions

Abstract The microstructure of polycrystalline MgB2 has a strong influence on the current carrying ability, with grain boundaries and non-superconducting nanoparticles acting as good flux pinning centres which improve the local (intrinsic) critical current density (J c) of the material, whereas porosity and poor connectivity between grains or particles adversely affect macroscopic current transport. Previous studies have found that hexagonal boron nitride (hBN) doping improves intrinsic J c by introducing nanoscale flux pinning centres, and Mg doping improves extrinsic J c by liquid-assisted sintering. Here we investigate the effect of co-doping with 5 wt.% Mg and 1 wt.% hBN with the aim of combining the improved intrinsic and extrinsic properties in bulk MgB2 samples fabricated using field assisted sintering. Additionally, the influence of ball milling and processing temperatures on MgB2 samples with only Mg additions is reported. By correlating microstructure with superconducting properties, we show that the presence of Mg liquid during processing of Mg-doped samples accelerates the reaction between BN and MgB2, forming an impurity phase, MgNB9, the presence of which is detrimental to superconducting performance. Nevertheless, we have achieved a considerable improvement in performance of samples doped only with Mg by increasing the sintering temperature.

Enhancement of the upper critical field in disordered nanocrystalline MgB2 bulks

Nanocrystalline superconducting MgB2 bulks were prepared by high pressure sintering of ball milled MgB2 powders. The disorder produced in these nanocrystalline materials shows a significant improvement in the upper critical field Hc2(0) of 38 T without severe suppression the superconducting transition temperature Tc. The band gap Δσ(0) decreases from 6.3 ± 0.1 meV to 5.4 ± 0.1 meV, while Δπ(0) keeps nearly fixed with the decrease in grain size to 20 nm. The ratios of the mean free path to the BCS coherence lengths li/ξ0,i(i = σ, π) turn out to be far less than 1, confirming that the two bands of the nanocrystalline bulks are both in the dirty limit. Strong intraband scatterings are responsible for the enhancement of Hc2(0).

Investigation of MgB2 bulk superconductors synthesized by in situ spark plasma sintering at different temperatures

In this research, the synthesis of MgB2 bulk superconductor with in-situ spark plasma sintering technique at different temperatures and the impacts of these sintering temperatures on the critical parameters (density, phase development, microstructure, transition temperature, critical current density and levitation force) of the superconductor were investigated. The optimum sintering temperature in terms of phase formation was found to be 850 °C. The maximum density was reached at 900 °C. It was determined that the critical temperatures of the samples sintered at 850 and 900 °C were around 39 K. It was found that the materials processed at different temperatures by in situ spark plasma sintering carry critical current densities of the order of 105 A/cm2 in the self-field and at 10 and 20 K. The maximum attractive and repulsive force are visible that there is not a linear relation with the sintering temperatures of samples.

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.