MgB2 Samples Research Articles

Evaluation of in-plane and out-of-plane crystallinities with residual amorphous phases for MgB2 superconductor

We focus on in-plane and out-of-plane crystallinities of the MgB2 superconductor. These two structural properties were evaluated in terms of peak broadening at lower angles in the X-ray diffraction. The angular behavior of the in-plane and out-of-plane peaks was used to compare it with the corresponding behavior estimated in the case of several crystallite sizes (that affect the in-field superconductivity of MgB2). We propose that this comparison allows a simple evaluation to provide insight into the individual influences of in-plane and out-of-plane crystallinities on the in-field critical current density (without needing to calculate numerical values of crystallite sizes and lattice strains of fabricated MgB2 materials). For this study, we used MgB2 samples sintered at different temperatures. The sintering conditions affected not only the crystallinities but also the phase compositions. The behavior of the compositions, including amorphous phases, was also evaluated by using a quantitative analysis for X-ray diffraction results.

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.

Comparison of the Field Trapping Ability of MgB2 and Hybrid Disc-Shaped Layouts.

Superconductors have revolutionized magnet technology, surpassing the limitations of traditional coils and permanent magnets. This work experimentally investigates the field-trapping ability of a MgB2 disc at various temperatures and proposes new hybrid (MgB2-soft iron) configurations using a numerical approach based on the vector potential (A→) formulation. The experimental characterization consists in measurements of trapped magnetic flux density carried out using cryogenic Hall probes located at different radial positions over the MgB2 sample, after a field cooling (FC) process and the subsequent removal of the applied field. Measurements were performed also as a function of the distance from the disc surface. The numerical modelling of the superconductor required the evaluation of the critical current density dependence on the magnetic flux density (Jc(B)) obtained through an iterative procedure whose output were successfully validated by the comparison between experimental and computed data. The numerical model, upgraded to also describe the in-field behavior of ARMCO soft iron, was then employed to predict the field-trapping ability of hybrid layouts of different shapes. The most promising results were achieved by assuming a hollow superconducting disc filled with a ferromagnetic (FM) cylinder. With such a geometry, optimizing the radius of the FM cylinder while the external dimensions of the superconducting disc are kept unchanged, an improvement of more than 30% is predicted with respect to the full superconducting disc, assuming a working temperature of 20 K.

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.

Ex-situ spark plasma sintered MgB2 with Ge-based organometallic additions: Key ingredients for superconductivity enhancement

Dense samples of MgB2, with a relative density of 98.3–99.1 %, were prepared by ex-situ spark plasma sintering using a raw powder supplied by Alfa Aesar. Samples were added with Ge-based organometallics (repagermanium/RGe, propagermaniu/PGe, linear Ge-based organometallic/GeSP, and Na-doped octamer/8THGP-Na) for the starting composition (MgB2)(Ge-based organometallics)0.0014. Except for the 8THGP-Na, they have a similar chemical composition in respect to Ge and C, while the crystal structure differs. Below 20 K, all studied additions increased the critical current density, Jc, and the irreversibility field, Hirr. Samples added with GeSP and 8THGP-Na have marginal differences regarding superconducting characteristics. The best Jc and Hirr are obtained for the sample added with RGe. This sample has the highest amount of carbon in the crystal lattice of MgB2. Results point out that there is a complex influence of the additive on carbon behaviour. Although in general it is accepted that a higher amount of carbon produces higher Jc and Hirr, a comparison with the literature indicates that C-rich raw MgB2 powders (e.g. the C-enhanced MgB2 nanopowder produced by Pavezyum) are not suitable to promote strong superconductivity enhancement in the samples added with indicated organometallics, and the origin, states, and evolution of carbon should to be carefully assessed.

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

Kinetics mechanism on the efficiency of C substituting B in the MgB2 tape fabrication

Effective carbon doping is essential for MgB2 superconductors due to its enhancement on in-field critical current density. However, C-doped MgB2 samples prepared with different boron powders exhibit varying levels of C substitution efficiency, which directly affect the superconducting properties. In this article, to further investigate the factors influencing C substitution efficiency, a two-stage carbon doping process was established based on the in-situ X-ray diffraction (XRD) measurement. The process begins with the initial in-situ ternary reaction of Mg, B and C, followed by the subsequent diffusion of C into the MgB2 lattice. Through the analysis of thermodynamic equations, it can be inferred that the key factor in improving the C substitution efficiency for MgB2 is to align the activity of the B and C precursor powders, which enables a greater participation of carbon in the initial stage of the ternary reaction. According to the systematic study on nano-C doped MgB2 tapes, the C substitution efficiency in MgB2 tapes prepared using nano amorphous boron powder is notably lower compared to those prepared using micron crystalline boron powder. These findings suggest that high-activity amorphous B powder necessitates a C dopant with a lower activation energy to enhance its C substitution efficiency.

Influence of on-off pulsed current pattern on processes during spark plasma sintering of MgB2 superconductor

High density samples (92–94.5 %) of MgB2 were prepared by Spark Plasma Sintering (SPS). The on-off pulsed current patterns of SPS processing were 8–4, 12–2, 24–2, 99–1. Patterns with more on pulses favor formation of a higher amount of the secondary MgB4 phase through the decomposition of MgB2. They also promote enhancement of larger MgO crystallites without a strong increase in the amount of this phase. Densification rate and pressure in the SPS chamber show a similar behavior, but their amplitude varies and the temperatures defining different stages present some shifts. As-revealed differences induced by pulsed patterns impact superconducting properties in a complex manner. An attempt to assess correlations between the pattern and different superconducting parameters is presented.

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.

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.

Strong Influence of Pressure on the Magnetic Properties of MgB2 Bulk Superconductors

The influence of the pressure on the magnetic and superconducting properties of polycrystalline MgB2 bulks was studied. Bulk MgB2 samples were prepared using conventional in-situ solid state reaction and hot-pressing methods. The structural and electromagnetic properties of MgB2 samples were studied by using x-ray diffraction (XRD), scanning electronic microscope (SEM), magnetic hysteresis (M-H) and magnetic levitation force (Fz, Fx) measurements. XRD measurements proved high quality of MgB2 bulks with only small traces of MgO impurity phase. The zero-field Jc value reached 240 kA/cm2 for MgB2 sample produced by hot-press while 23 kA/cm2 for MgB2 sample produced by conventional in-situ at measurement temperature of 25 K. The max. levitation force values were obtained as 11.60 N and 15.42 N for MgB2 bulk samples produced by in-situ and hot-press methods at 25 K, respectively. All these magnetic measurements result indicate that pressure acts like driving force for manufacturing highly dense and high levitation capability MgB2 bulk superconductors.

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).

Experimental X-ray Charge-Density Studies─A Suitable Probe for Superconductivity? A Case Study on MgB2.

Case studies of 1T-TiSe2 and YBa2Cu3O7-δ have demonstrated that X-ray diffraction (XRD) studies can be used to trace even subtle structural phase transitions which are inherently connected with the onset of superconductivity in these benchmark systems. However, the utility of XRD in the investigation of superconductors like MgB2 lacking an additional symmetry-breaking structural phase transition is not immediately evident. Nevertheless, high-resolution powder XRD experiments on MgB2 in combination with maximum entropy method analyses hinted at differences between the electron density distributions at room temperature and 15 K, that is, below the Tc of approx. 39 K. The high-resolution single-crystal XRD experiments in combination with multipolar refinements presented here can reproduce these results but show that the observed temperature-dependent density changes are almost entirely due to a decrease of atomic displacement parameters as a natural consequence of a reduced thermal vibration amplitude with decreasing temperature. Our investigations also shed new light on the presence or absence of magnesium vacancies in MgB2 samples─a defect type claimed to control the superconducting properties of the compound. We propose that previous reports on the tendency of MgB2 to form non-stoichiometric Mg1-xB2 phases (1 - x ∼ 0.95) during high-temperature (HT) synthesis might result from the interpretation of XRD data of insufficient resolution and/or usage of inflexible refinement models. Indeed, advanced refinements based on an Extended Hansen-Coppens multipolar model and high-resolution X-ray data, which consider explicitly the contraction of core and valence shells of the magnesium cations, do not provide any significant evidence for the formation of non-stoichiometric Mg1-xB2 phases during HT synthesis.

The Inverse Magnetocaloric Effect of MgB2 Superconductor

The magnetocaloric effect-(MCE) of MgB2 is simulated, considering the effect of sintering temperature on MCE. The results show that MCE of MgB2 samples is an inverse type in a temperature range of diamagnetic-paramagnetic transition. Moreover, the MCE for MgB2 is improved with high sintering temperature. The comparison between MCE of MgB2 samples and other some reported magnetic materials has been done, showing that MCE parameters of MgB2 samples are larger than some MCE parameters of La1-xCdxMnO3, La1.25Sr0.75MnCoO6, Gd1−xCaxBaCo2O5.5, Ni58Fe26Ga28, Ni2+xMn1−xGe, Ge0.95Mn0.05 film, and (001)-oriented MnAs film. It suggested that MgB2 samples could be a promising sharing candidate for MR in cryogenic temperatures.

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.

Pengembangan Material Maju Superkonduktor Mg-B dengan Penambahan Graphene Oxide melalui Proses Powder in Sealed Tube

MgB2 is a high possible superconducting material that can be applied quite practically with the functionalization of Mg-B materials. Material development is carried out by adding carbon, namely Graphene Oxide (GO), which is a single atom layered material. The Powder in Sealed Tube (PIST) method is practically used to reduce oxidation. This study aims to analyze the effect of GO material doped with the PIST method made from MgB2 with a sintering temperature of 800℃ for 2 hours on its superconductivity, compound formation, and microstructure. The manufacturing process is carried out in a 1:2 ratio where 98% purity Mg is mixed with Boron, which is then added with 0, 0.3 and 3% wt GO doping, all ingredients are mixed stoichiometrically. The material that has been put in a tube and compacted sufficiently into SS316L which has been closed on one side to enter the powder, is then compacted with high pressure up to 1000 MPa. The material is sintered at a temperature of 800℃ for 2 hours which is then carried out by cooling in the furnace and taking bulk samples. The XRD results showed the formation of the dominant MgB2 phase and the formation of an impurity phase in the form of MgO and obtained a decent crystal size of 295 which was owned by the 3%wt GO PIST MgB2 sample. The SEM test shows the forms of formation (agglomeration) in each sample, with the presence of several axes. Cryogenic testing shows that with doping there is a movement of critical temperature to a lower direction where MgB2 0%wt GO has a TcOnset value of 39.4 K and a TcZero of 38.7 K, while MgB2 3%wt GO has a TcOnset value of 39.6 K and TcZero of 38 K.

The Effect of Sintering Number on Crystallographic and Microhardness Properties of MgB2 Samples Obtained by Mixing Ready MgB2 Powders and Mg and B Powders

Bu çalışmada hazır olarak satın alınan MgB2 tozları ile elementel Mg ve B tozlarının karışımı yolu ile elde edilen MgB2 numunelerinin ısıl işlem sayısının kristalografik ve mikrosertlik özellikleri üzerinde nasıl bir değişim meydana getirildiği karşılaştırmalı olarak incelenmiştir. Bu çalışma kapsamında hazır MgB2 tozları ve Mg-B tozları sitokiyometrik oranlarda karıştırılıp preslenerek 1’er gram halinde 4 adet tablete dönüştürülmüştür. Tabletler 1, 2, 3 ve 4 kez 670 OC’de 10 bar argon atmosferinde 60 dakika boyunca ayrı ayrı sinterlenip, kristalografik ve mikrosertlik analizine tabi tutulmuştur. Numune hazırlama aşamasında klasik katıhal reaksiyon yöntemi, kristalografik analizlerde, X-Işını Kırınım Yöntemi; Mikrosertlik analizlerinde, Vickers Yöntemi; Mikrosertlik modellemelerinde, Meyer’s Kanunu, Orantılı Numune Direnci Modeli (PSR), Hays-Kendall Yaklaşımı (HK) ve Elastik/Plastik Deformasyon Modeli (EPD) kullanılmıştır. Elde edilen analiz sonuçlarında tüm numunelerde MgB2’ye ait karakteristik piklerin baskın olduğu, genel olarak ısıl işlem sayısının artmasının numunelerde tanecik büyüklüğünü ve düzlemler arası mesafeyi azalttığı, örgü parametrelerinde ise genel bir artışa sebep olduğu görülmüştür. Aynı zamanda numunelerin mikrosertlik karakterini açıklamada en başarılı modelin Meyer’s Kanunu olduğu ve tüm numunelerin Çentik Boyut Etkisi (ISE) davranışı sergilediği gözlenmiştir.