MgB2 Samples Research Articles

Tackling the agglomeration of Mg2Si dopant in MgB2 superconductor using cast Mg–Si alloy

The work demonstrates a method to tackle the agglomeration of Mg2Si dopant in chemical doping in MgB2 superconductor to enhance its in-field critical current density. Mg-Si alloy which has uniformly distributed Mg2Si particles in it is prepared by casting technique. The alloy is used as the Mg source for the preparation of MgB2 samples doped with various weight percentage of nano carbon. XRD patterns show peaks of Mg2Si in all Si containing samples. SEM images show a microstructure with distinctly smaller grains and reduced crystallinity in doped samples. HRTEM analysis confirms that the method led to better distribution of Mg2Si particles within the MgB2 grains as compared to the normally prepared sample. It is found that doping has not severely affected the transition temperature of the samples and at the same time enhanced the in-field critical current density.

Refinement of MgB2 grains and the improvement of flux pinning in MgB2 superconductor through nano-Ni addition

Polycrystalline MgB2 samples with 4 at.% and 6 at.% Ni addition were prepared by an in-situ reaction process. Coarse (40 μm) and nano-size (40 nm) Ni powders were used for synthesis of the bulk samples. The phase formation, microstructure, and transport critical current density of the sintered samples were investigated as a function of the Ni particles size. A higher amount of nano-Ni addition (6 at.%) was found to dramatically decrease the reaction temperature of Mg and B, forming MgB2 fine grains with more defects and inferior crystallinity under the solid stage. Combined with the differential thermal analysis (DTA) and scanning electron microscopy (SEM) analysis, it was found that the nano-Ni particles is prone to form Mg-Ni eutectic liquid with Mg at 506 °C, which accelerates the formation of MgB2 at a significantly lower temperature with improved grain connectivity. The unique microstructure, with refined MgB2 grains, more grain boundaries and nano-inclusion inside matrix, is proposed to be responsible for the enhancement of flux pinning in the MgB2 bulk superconductor with nano-Ni doping. However, the positive effect of Ni was completely eliminated by doping of coarse-Ni particle due to the concentration of the inclusions with weak pinning and thus a strong reduction of Jc.

Role of double doping with C and RE 2 O 3 oxides on the critical temperature and critical current of MgB 2 phase

Abstract A series of MgB2 samples doped with C, RE2O3 oxides (RE = Ce, Nd), and C and RE2O3oxides was prepared by the hot isostatic pressing method at high pressure: 1 GPa under argon gas. Based on X-ray diffraction and magnetization measurements it was found that the RE2O3 is not incorporated into MgB2. A secondary REB4 phase is present after the synthesis. No decrease in the critical temperature (Tc) is observed after the RE2O3 doping. The Tc of C and RE2O3 doped MgB2 is decreased by 3 K. The doping decreases the diamagnetic response in comparison to pure MgB2 phase. The highest critical current density (Jc) was 3.4 × 105 A/cm2 at 4.2 K and 0.5 T for the bulk material, and the irreversibility field for the Nd2O3 doped sample was located at high field at T = 30 K. The results indicate that inside grains of pinning centers increase Jc from 4.2 K to 25 K and decrease Jc at 30 K. On the other hand, pinning centers between grains (precipitate) increase Jc at 30 K and decrease Jc from 4.2 K to 25 K in the middle and high magnetic fields. Our research shows that carbon-encapsulated boron creates a point pinning centers.

Calculation of Normalized Pinning Force and Nature of Pinning Mechanism for Nano-Al Doped MgB2 Superconductor

The Jc(H) of nano-Al doped MgB2 samples has been calculated from M(H) loop measurements at different temperatures. Normalized volume pinning forces as a function of the reduced field have been analyzed at different temperatures and doping level which was taken from the Jc(H) data. The modified scaling law was used as discussed by the Eisterer to analyze the pinning forces. This law was compared with the scaling law used by the Fietz and co-worker. The grain boundary pinning is found the dominant pinning mechanism in all the doped samples and exhibit the scaling behavior. The XRD and temperature dependence of resistivity confirms the successful substitution of nano-Al at Mg sites. The results endorse that the magnetic anisotropy decreased from pure to the dirty limit with the doping of the nano-Al. The enhancement in the Jc(H) of the sample with the nano-Al doping is due to decrease in the anisotropy and increase in the volume pinning forces. The 2 % nano-Al doped sample between 15–30K shows the highest Jc(H) among all the samples.

Neutron Microtomography of MgB2 Superconducting Multifilament Wire

Neutron imaging of sub-10-micrometres spatial resolution has been recently achieved in 2D mode within the framework of the Neutron Microscope project at the Paul Scherrer Institut. Here we report on the development of the PSI Neutron Microscope instrument and the results of the first microtomographic imaging experiment of multifilament superconducting MgB2 wire. The sample of MgB2 superconducting 37 multifilaments embedded in copper-nickel matrix was investigated –in microtomographic mode– with the scientific interest regarding the distribution of boron within the individual superconducting filaments (about 40μm in diameter). The resulting tomographic dataset revealed the distribution of boron within the entire 0.8mm thick multifilamental wire with the isotropic voxel size of 2.6 micrometres.

Microstructural Features of Magnetic NiCoB Nanoparticles Addition to MgB2 Precursor Powders

Magnesium diboride (MgB2) is a superconductor characterized by interesting properties like rather high superconducting transition temperature Tc = 39 K, long coherence length and low anisotropy. In addition, it has a very simple crystal structure and low density. Those properties make the MgB2 an ideal candidate for a wide range of applications. To improve the electromagnetic properties of MgB2, magnetic nickel-cobalt-boron (NiCoB) nanoparticles (mean grain size 17 ± 3 nm) were added to Mg and B precursor powders and sintered at 650 °C, i.e. the temperature of MgB2 superconductor formation. The nearly spherical NiCoB nanoparticles, as-prepared by the chemical reduction of metallic salts, were amorphous according to previous study. The resulting MgB2 sample, formed after the sintering at 650 °C, was subjected to detailed microstructural analysis which included the application of various experimental methods: XRD, FE-SEM, EDS, elemental mapping, TEM and SAED. The methods confirmed the formation of new crystal CoNi phase (due to heat treatment at 650C), consisting of spherical nanoparticles (~ 6 nm) with tendency to spherical agglomerates formation. Those nanosized magnetic particles (characterized by the single domain magnetic structure and blocking temperature TB below room temperature), located at MgB2 grain boundaries, could serve as effective magnetic pinning centers in MgB2, thus improving its electromagnetic properties

Effects of the size of Mg powder on the formation of MgB2and the superconducting properties

The effect of the size and shape of magnesium(Mg) powder on the formation of MgB2 and the critical current density(Jc,) of MgB2 bulk was studied. As a precursor for the formation of MgB2, Mg and MgB4 powder, which was synthesized through the reaction of boron (B) with Mg powders, was used. MgB4 was mixed with Mg powders of various sizes, pressed into pellets and heat-treated at 650℃-750℃ in flowing argon gas. The XRD analysis of the heat-treated MgB2 samples showed that the volume fraction of MgB2 was the highest as 92.74 % when spherical Mg powder with an average size of 25.7 μm was used, whereas the volume fraction was the lowest as 79.64 % when plate-like Mg powder with a size of 34.1 μm was used. The superconducting transition temperature (Tc) of MgB2 was not sensitive to the characteristics of the Mg powders used. All of the prepared MgB2 samples showed a high Tc of 38.3 K and a small superconducting transition width of 0.2 K-0.5 K. Jc (5 K and 1 T) of MgB2 was the highest as 3.93×104A/cm2 when spherical Mg powder with a size of 25.7 μm was used, whereas Jc was the lowest as 2.18×104 A/cm2 when plate-like Mg powder with a size of 34.1 μm was used. The relationship between the Jc of MgB2 and the characteristics of the Mg powders used was explained in terms of the volume fraction of MgB2 and the apparent density of the MgB2 pellets.

Enhancement of synthesis efficiency and critical current density in glycine-doped MgB2 bulks by two-step sintering

Glycine (Gly)-doped and Cu-and-Gly-co-doped MgB2 samples were two-step synthesized by a short 800 °C period followed by sintering at 600 °C for 2 h. In view of the depression of oxidation and the significant reduction of the synthesis time, the sintering efficiency was improved due to the instantaneous high-temperature process, in contrast with the one-step sintered samples at 800 °C. The critical current density was examined to be 5.1 × 103 and 8.4 × 103 A cm− 2 at 20 K and 3 T for the Gly-doped and the co-doped samples, respectively. Such enhancement, with respect to the one-step sintered un-doped sample, is attributed to the fine MgO (~20 nm) and Mg2Cu (~10 nm) pinning centers. The MgO and Mg2Cu in the co-doped sample precipitated at the grain boundary and within the grains, respectively. Besides, the residual Mg phase in the Gly-doped sample remained on the surface of the MgB2 grains, which proved the screw-dislocation growth of the MgB2 grains, i.e. the grains extended from inner center to outer zone at low temperature.

The Effect of Ar Ambient Pressure and Annealing Duration on the Microstructure, Superconducting Properties and Activation Energies of MgB2 Superconductors

MgB2 samples are produced by ex situ reaction method under vacuum, and various (0, 10, 20 Bar) Ar pressure for 0.5 and 1 h. The effect of ambient pressure and annealing duration on the microstructure and electrical properties of bulk samples are investigated. XRD, SEM, and magnetoresistance measurements are made. The a and c lattice parameters and the grain size values are the highest for the samples produced under vacuum and their values decrease with increasing Ar pressure. Moreover, these values increase when the annealing duration increases from 0.5 to 1 h. The increasing pressure reduces the bond lengths between the atoms thus the grain sizes decrease. Smaller grain size promotes the connection between grains which results in an increase of the critical current density (Jc). SEM micrographs reveal that the produced samples have granular structure which is a characteristic feature of MgB2. The decrease of grain sizes and thus enhancement in grain connectivity with increasing pressure is also confirmed by SEM images. Magneto resistivity measurements show that Tc values of the samples produced under vacuum are the highest. Tc values decrease with increasing ambient pressure and applied magnetic field. The activation energies (U0) of the samples are calculated using Arrhenius plots due to thermally activated flux flow theory. Existence and increase of Ar pressure causes increase of activation energies. The samples produced with 0.5-h annealing have higher activation energies than the ones produced with 1-h annealing.

The size effect on the magnetic levitation force of MgB2 bulk superconductors

In this study, the size effect on the magnetic levitation performance of disk-shaped MgB2 bulk superconductors and permanent magnets was investigated. MgB2 samples with varying diameters of 13mm, 15mm and 18mm, each of which were 2g in mass, were prepared by two-step solid state reaction method. Vertical levitation force measurements under both zero-field-cooled (ZFC) and field-cooled (FC) regimes were carried out at different temperatures of 20, 24 and 28K. It was determined that the levitation force of the MgB2 strongly depends on both the diameters of the sample and the permanent magnet. In ZFC regime, the maximum levitation force value for the permanent magnet and the sample 18mm in diameters reached to the 8.41N at 20K. In addition, in FC regime, attractive and repulsive force increased with increasing diameters of the sample and the permanent magnet. In that, the sample with 18 mm in diameter showed the highest attractive force value −3.46N at 20K and FC regime. The results obtained in this study are very useful in magnetic levitation devices as there is no detailed study on the size of superconductors and permanent magnets.

Electromagnetic behaviour of bulk MgB2 determined by numerical modelling using regional supercurrent properties

Previous studies in the literature have shown experimentally that the critical current density in bulk MgB2 depends on the position at which it is measured within the sample, and that the trapped magnetic field saturates above large values of sample diameter. To better understand this behaviour, we have carried out a detailed study of the trapped magnetic field and local critical current distribution in bulk MgB2 using numerical modelling with H-formulation. The properties of bulk MgB2 samples with radius-independent (i.e. uniform) and radius-dependent critical current were modelled, based on experimentally-determined parameters obtained from the literature. Radius-independent samples took properties originally derived from either the central or edge region of the original samples. In our study it was determined that the peak trapped magnetic field value of the sample with the central region properties was higher by 22.4% and 9.20% than the samples with edge region properties and that with radius dependent parameter, respectively. Additionally, to understand why the trapped magnetic field saturates as the diameter of sample increases, the trapped magnetic field was modelled using the radius dependent critical current density Jc (B,r) as input data rather than using a constant bulk critical current density. The peak trapped magnetic field values increase by 8.6% and 6.2% respectively for the central region radius-independent and radius-dependent property bulk MgB2 samples as the bulk diameter increases from 25 mm to 35 mm. This tendency to saturation in the peak trapped field indicates that increasing the bulk diameter alone does not have a significant effect on the value of the trapped magnetic field, unless the bulk superconducting current density is improved uniformly throughout the MgB2 bulk. These results enable us to understand how we can experimentally enhance the trapped magnetic field in bulk MgB2 by producing samples with uniform high critical current density distribution using fabrication methods such as graded doping.

The Effect of Fe Diffusion on Some Physical and Superconducting Properties of MgB2

The iron (Fe) diffusion in superconducting MgB2 bulk samples has been studied over the temperature range of 650–900°C for 1 h. Fe coating on bulk polycrstalline superconducting MgB2 samples was performed in two ways, i.e., on pressed pellets without sintering (set2) and on pressed and sintered pellets (set1). For both sets, a 50 μ m thick Fe layer was coated on MgB2 by evaporation in vacuum. Effects of Fe diffusion on the crystal structure and superconducting properties of MgB2 have been investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), and resistivity measurements. Fe diffusion coefficients were determined from lattice parameter c and room temperature resistivity values. The temperature dependence of the Fe diffusion coefficient in this temperature range is described by the Arrhenius relation. It has been found that the Fe diffusion coefficient increases with increasing sintering temperature, as expected. The plausible explanations for the observed improvement in microstructure and superconducting properties of the samples due to Fe diffusion are discussed.

Second-phase segregation and micro strain/lattice parameter dependent transition temperature in polycrystalline MgB2

Un-doped, metal-doped, and carbon-doped MgB2 samples were prepared by furnace cooling and quenching to investigate the second phase behavior and the resultant critical current density Jc performance under different heat treatment processes, which is infrequently mentioned, and to explore the strain/lattice parameter dependence of the superconducting transition temperature. To release the residual stress, quenching induced second-phase segregation in these MgB2 samples shows a negative effect on the Jc. Nevertheless, the dislocations and the lattice distortion assisted the enhancement of the high-field Jc in the un-doped and metal-doped MgB2 samples, which indicated that quenching could be technically applied for the fabrication of metal-sheathed MgB2 wires and tapes to obtain excellent Jc. After evaluating the micro strain and the lattice parameters’ (c and a for hexagonal lattice) variation, a dome was observed in the illustration of the strain/lattice parameter c/a dependence of Tc, which differed from the reported linear relation in previous work. This suggests that the c/a ratio and the strain may be the predominant parameters for scaling of the superconducting dome width in the superconducting phase diagram of MgB2.

Superior critical current density obtained in MgB2 bulks via employing carbon-coated boron and minor Cu addition

High performance Cu doped MgB2 bulks were prepared by an in-situ method with carbon-coated amorphous boron as precursor. It was found that the usage of carbon-coated boron in present work leads to the formation of uniformly refined MgB2 grains, as well as a high level of homogeneous carbon doping in the MgB2 samples, which significantly enhance the Jc in both Cu doped and undoped bulks compared to MgB2 bulks with normal amorphous boron precursor. Moreover, minor Cu can service as activator, and thus facilitates the growth of MgB2 grains and improves crystallinity and grain connectivity, which can bring about the excellent critical current density (Jc) at self fields and low fields (the best values are 7×105A/cm2 at self fields, and 1×105A/cm2 at 2T, 20K, respectively). Simultaneously, minor Cu addition can reduce the amount of MgO impurity significantly, also contributing to the improvement of Jc at low fields. Our work suggests that Cu-activated sintering combined with employment of carbon-coated amorphous boron as precursor could be a promising technique to produce practical MgB2 bulks or wires with excellent Jc on an industrial scale.

Influence of Metal Diboride and Dy2O3 Additions on Microstructure and Properties of MgB2 Fabricated at High Temperatures and under Pressure.

High temperatures and under pressure (HTP) processing has been used to study the effects of chemical doping in MgB2. ZrB2, TiB2 and NbB2 were selected as additives since, like MgB2, they have an AlB2-type structure and similar lattice parameters. Dy2O3 was selected as it has been reported to generate nanoscale, secondary intragrain phases in MgB2. While C is known to enter the B-sublattice readily, attempts to dope Zr and other elements onto the Mg site have been less successful due to slow bulk diffusion, low solubility in MgB2, or both. We have used high-temperature, solid-state sintering (1500 °C), as well as excursions through the peritectic temperature (up to 1700 °C), to investigate both of these limitations. Bulk MgB2 samples doped with MB2 (M = Zr, Ti and Nb) and Dy2O3 additions were synthesized and then characterized. Lattice distortion and high densities of crystal defects were observed in the MgB2 grains around nano-sized MB2 inclusions, this highly defected band contributed to a large increase in Bc2 but was not large enough to increase the irreversibility field. In contrast, distributed intragrain precipitates were formed by Dy2O3 additions which did not change the lattice parameters, Tc, Tc distribution or Bc2 of MgB2, but modified the flux pinning.

Superconductivity in MgB2 irradiated with energetic protons

A series of MgB2 samples were irradiated with protons of 11.3 and 13.2MeV. Magnetization data shows an insignificant reduction of the critical temperatures but a continuous decrease of the Meissner fraction with increasing fluence or energy. All samples show a consistent improvement of the critical current density compared to the virgin sample and an increase of the pinning energy at high fields as resulted from relaxation data.

Evaluation of quenching-induced lattice strain and superconducting properties in un-doped and glycine-doped MgB2 bulks

Bulk MgB2 samples with or without glycine doping were sintered at 800 °C followed by furnace cooling and quenching, respectively. The strain analysis and the microstructure observation revealed that the un-doped and glycine-doped MgB2 showed contrary response to the used quenching treatment, in terms of the crystallinity, the lattice parameters, and the superconducting properties. Accordingly, the critical current density of the quenched MgB2 is enhanced with respect to the furnace-cooled one, due to the pinning dislocations and the well-connected MgB2 net induced by the reserved strain. As for the glycine-doped samples, the impurity particles, which served as effective pinning centers in the furnace-cooled sample, segregated at the grain boundary under the driving force of the residual strain, and are destructive to the critical current density of the quenched one.

Al-doped MgB2 materials studied using electron paramagnetic resonance and Raman spectroscopy

Undoped and aluminum (Al) doped magnesium diboride (MgB2) samples were synthesized using a high-temperature solid-state synthesis method. The microscopic defect structures of Al-doped MgB2 samples were systematically investigated using X-ray powder diffraction, Raman spectroscopy, and electron paramagnetic resonance. It was found that Mg-vacancies are responsible for defect-induced peculiarities in MgB2. Above a certain level of Al doping, enhanced conductive properties of MgB2 disappear due to filling of vacancies or trapping of Al in Mg-related vacancy sites.

Improving magnetic properties of MgB2 bulk superconductors by synthetic engine oil treatment

The present study focuses on the effects of standby time of the MgB2 samples immersed in synthetic engine oil on the critical current density ( Jc(H)), magnetic field dependence of the pinning force density fp(b) and Tc performances of MgB2 bulk superconductors. Synthetic engine oil was used as a product which is cheap and a rich carbon source. Manufactured MgB2 pellet samples were immersed at different standby time of 30min, 120min, 300min and 1440min in synthetic engine oil after the first heating process. Finally, MgB2 samples immersed in synthetic engine oil were sintered at 1000°C and kept for 15min in Ar atmosphere. The critical current density of all of MgB2 samples immersed at different standby time in engine oil in whole field range was better than that of the pure MgB2 sample because of the number of the pinning centers. The MgB2 sample immersed at 300min standby time in synthetic engine oil has the best performance compared to other samples. The Jc value for the pure sample is 2.0×103A/cm2, whereas for the MgB2 sample immersed at 300min standby time in engine oil the Jc is enhanced to 4.8×103A/cm2 at 5K and 3T. The superconducting transition temperature (Tc) did not change with the increasing standby time of the samples in synthetic engine oil at all. The best diamagnetic property was obtained from the sample which kept in synthetic engine oil for 300min. Synthetic engine oil treatment results in remarkable improvement of the critical current density and pinning force performances of MgB2 superconductors. It was found that all MgB2 samples have a different pinning property at different measuring temperatures. Using synthetic engine oil as a product which is cheap and a rich carbon source in MgB2 bulk superconductors makes MgB2 samples immersed in synthetic engine oil a good candidate for industrial applications.

Processing of Bulk MgB<sub>2</sub> Superconductors for Application in Fault Current Limiters

Fault current limiters (FCL) require superconducting (SC) materials which can provide a definite rate of response to a fault event resulting in the SC – normal state reversible transition. The main characteristics determined the material suitability are the critical current density, jc, thermal conductivity and capacity which are strongly determined by manufacturing technology, in particular, of MgB2. In the paper we estimate the jc of bulk MgB2 samples by the vibrating magnetometer and inductive, contactless transformer, method using ring samples. The bulk MgB2 samples were produced under 30 MPa (hot pressing) and 2 GPa (quasihydrostatic pressing) at 800-1050 оС from different initial ingredients (Mg and B or MgB2 with and without additions). It is shown that the technology process and initial ingredients strongly influence the distribution of boron-and oxygen-enriched nanosized inhomogenities in MgB2 matrix, connectivity between SC grains, material porosity and, as result, the SC properties. The transformer method gives the jc in the range from 1.6·104 up to 6.3·104 A/cm2 at about 4 K while using magnetometer measurements the jc is estimated from 2.24·105 up to 5.1·105 A/cm2 at 10 K in self-magnetic fields. The contradictions in the jc estimated by different methods can be explained by instability of the SC state of MgB2, caused by variation of the applied magnetic field. Using the transformer method AC losses per a cycle before quenching for the best materials were estimated around 0.75-1 J/cm3, while the power of losses was about 200 W. The FCL model with rings cut out from SC MgB2 materials prepared using various technologies demonstrated that MgB2 is a promising material for application in inductive FCLs.