MgB2 Bulk Samples Research Articles

Raman spectroscopy of carbon doped MgB2 prepared using carbon encapsulated boron as precursor

We have studied the surface topographical changes and the Raman spectroscopic modifications for carbon-doped MgB2 that was fabricated with carbon-encapsulated boron as precursor. Bulk MgB2 samples of 20 mm diameter and 7 mm thickness were prepared by an in-situ one-step solid state sintering technique. It was found that optimum doping led to the reduction of the average grain size, which was then effective in enhancing flux pinning in MgB2. For the optimum doped sample, the self-field critical current density Jc at 20 K reached 375 kA/cm2. However, Tc values were suppressed with further carbon doping, presumably due to an increase in the disorder in the system, which was demonstrated by a decrease in full width and half maxima of the E2g mode in the Raman spectra.

Improved Critical Current Densities of Bulk MgB2 Using Carbon-Coated Amorphous Boron

In this study, we report on a further improvement of the critical current density of the sintered bulk MgB 2 material utilizing the optimized sintering temperature combined with a varying content of carbon in carbon-encapsulated boron. The MgB 2 bulk was prepared from high-purity commercial powder of Mg metal and a carbon-encapsulated boron with 0 wt.%, 2.8 wt.%, 4.5 wt.%, and 7.3 wt.% of carbon, using a single-step solid-state reaction at 805°C for 3 h in pure argon atmosphere. The magnetization measurements confirmed a sharp superconducting transition with onset T c at around 38.5 K, decreasing with increasing carbon content. For 7.3 wt.% of carbon, the bulk MgB 2 reached the superconducting transition at around 33 K. Scanning electron microscopy of the fractured bulk MgB 2 cross section showed a dispersion of 100-200 nm large grains. Due to the carbon doping and optimized processing, the critical current density (J c ) in bulk MgB 2 samples with the carbon-coated boron was improved both in low and high magnetic fields. The highest J c values at 20 K, of 375 and 220 kA/cm 2 , in the self-field and 1 T, respectively, were achieved in the MgB 2 sample with 2.8 wt.% of carbon in the carbon-encapsulated boron. The present results clearly demonstrate that the optimized sintering temperature combined with the appropriate amount of carbon in carbon-coated boron is able to improve the entire J c performance of the bulk MgB 2 material.

Time-Dependent Diffusion Coefficient of Fe in MgB2 Superconductors

The iron (Fe) diffusion in superconducting MgB2 bulk samples has been studied for sintering time durations of 15 min, 30 min, 1 h, 2 h, and 4 h at 900∘C. Fe coating bulk polycrstalline superconducting MgB2 samples for Fe coating were prepared by pelletizing and used in the diffusion experiments with initial sintering at 800∘C for 1 h. A thin layer of Fe was coated on MgB2 pellets by evaporation in vacuum. Effects of Fe diffusion on the structural, electrical, and superconducting properties of MgB2 have been investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), infrared spectroscopy (IR), energy-dispersive X-ray spectroscopy (EDS), and resistivity measurements. Fe diffused samples have slightly increased critical transition temperatures and have larger lattice parameter c values, in comparison with bare samples. Fe diffusion coefficients were calculated from depth profiles of c parameter and room temperature resistivity values. Depth profiles were obtained by successive removal of thin layers from Fe diffused surfaces of the samples. Our results have shown that the Fe diffusion coefficient decreases with increasing sintering time and resistivity measurements can be utilized for determination of diffusion coefficient.

Effect of laser irradiation on superconducting properties of bulk MgB2 sintered at different temperatures

The surfaces of MgB2 bulk samples produced by a pellet/closed tube method at two different sintering temperatures of 650 and 850 °C, after hot pressing at 200 °C, were irradiated with the same irradiation dose by using an Nd:YVO4 laser in order to study the possible potentiality of laser irradiation to improve pinning performances and critical current density of MgB2 superconductor. The measurements showed that the magnetic field dependence of the critical current density values of irradiated sample sintered at 650 °C slightly increased with a narrowing in superconducting transition region as compared to the reference sample sintered at same temperature. However, irradiated sample sintered at 850 °C showed a decrease in pinning performance and similar critical temperature values as compared to the corresponding reference sample. From these results it can be said that the same laser irradiation dose affects superconducting properties of bulk MgB2 in different ways depending on sintering temperature of the superconductor.

The Effects of Structural, Thermal, and Magnetic Properties of Hexylbenzene-Doped MgB2 Superconductor

The effects of the amount of hexylbenzene additive (C12H18) on the structural, thermal, and magnetic properties of MgB2 superconductor are examined in this study. Pure and hexylbenzene-doped MgB2 bulk samples were produced with in situ solid-state reaction method. X-ray diffraction patterns of MgB2 doped with MgB2 and hexylbenzene at different ratios were determined to have MgB2 as the main phase and consisted of a small amount of MgO. Pure and different ratios of hexylbenzene-doped Mg and B starting powders were heat-treated by a differential scanning calorimeter between room temperature and 800 °C. It was determined from the differential scanning calorimetry curves obtained that the first exothermic peak pointed the MgB2 phase emerging with a solid–solid (Mg–B) reaction, and this temperature shifted towards the low temperatures as the hexylbenzene addition rates increased. It was observed that there was dependency to the applied field in all samples from the ac susceptibility measurements as a function of the temperature in pure and hexylbenzene-doped MgB2 superconductor materials, and shift towards the lower temperatures in Tc, superconducting transition temperature, with increasing content. It was observed that the changes occurred in in-phase (\(\chi ^{\prime })\) and out-off-phase (\(\chi ^{\prime \prime }\)) components of ac susceptibility both weakened the MgB2 phase structure of hexylbenzene content and, as a result of this, led to changes in the pinning mechanism.

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.

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.

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.

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.

Tellurium addition as a solution to improve compactness of ex-situ processed MgB2-SiC superconducting tapes

Ex-situ spark plasma sintering (SPS) was used to obtain dense MgB2-based tapes in a Fe sheath with the starting composition (MgB2)0.975 + (SiC)0.025 + Te0.01. Prior to the SPS procedure of tape formation, the samples were submitted to a series of cold working processes typical for the powder-in-tube technique. The tapes were compared with optimal doped bulk samples (having the same starting composition) and a pristine MgB2 tape. The morphology of the composite samples, the phase structure of both the core and the inner face of the metallic sheath shows the formation of a plethora of traces as a result of interaction between MgB2, additives, and the Fe sheath. Important critical parameters, like critical current density and the irreversibility field, show that there is a field and temperature range where the SiC and Te-added tapes display better critical parameters comparative to either pristine MgB2 tapes in the Fe sheath or SiC and Te doped MgB2 bulk samples.

Effect of different-sized h-BN nano-particles on some properties of MgB2 superconductors

The present study reports the effect of hexagonal boron nitride (h-BN) doping on the superconducting properties of bulk MgB2 samples. Structural, electrical and magnetic properties of 3 wt% h-BN doped MgB2 samples are studied in terms of three different sized h-BN nanoparticles. The results show that h-BN doping has no significant change on critical transition temperature (±0.5 K deviation), but decreases the magnetic Jc with increasing the h-BN particle size. Among the doped samples, a highest Jc (T = 10 K) value of 1.8 × 104 A/cm2 is achieved for the sample doped with the smallest sized h-BN at zero external magnetic field but it is still lower than the 2.5 × 104 A/cm2 obtained for the un-doped sample. In the scope of this study, it is investigated whether or not the nano-sized h-BN particle is a possible candidate to improve the pinning properties of the MgB2 sample and we suggest that particle size is very important parameter in h-BN doping into MgB2.

Low-Temperature Fracture Strength of MgB2Bulk Processed by Spark Plasma Sintering

Mechanical properties of a high packing ratio MgB 2 bulk sample processed by spark plasma sintering (SPS) were evaluated at 77 K through bending tests for specimens cut from the bulk sample. The effects of the packing ratio and temperature on the fracture strength of MgB 2 bulk are discussed. The fracture strength at 77 K of the MgB 2 bulk processed by SPS was higher than that at room temperature. The packing ratio of the MgB 2 bulk processed by SPS was higher than those of other MgB 2 bulks processed by sintering under the ambient pressure and hot isostatic pressing. The fracture strength was improved by increasing the packing ratio. The fracture strength at a very low temperature was estimated from the bending test results at 77 K and room temperature. The fracture strength at 20 K of the MgB 2 bulk processed by SPS was estimated to be around 425 MPa.

EBSD analysis of MgB2 bulk superconductors

The grain orientation, the texture and the grain boundary misorientations are important parameters for the understanding of the magnetic properties of the bulk MgB2 samples intended for super-magnet applications. Such data can be provided by electron backscatter diffraction (EBSD) analysis. However, as the grain size (GS) of the MgB2 bulks is preferably in the 100–200 nm range, the common EBSD technique working in reflection operates properly only on highly dense samples. In order to achieve a reasonably good Kikuchi pattern quality on all samples, we apply here the newly developed transmission EBSD (t-EBSD) technique to several bulk MgB2 samples. This method requires the preparation of TEM slices by means of focused ion-beam milling, which are then analyzed within the SEM, operating with a specific sample holder. We present several EBSD mappings of samples prepared with different techniques and at various reaction temperatures.

A trapped magnetic field of 3 T in homogeneous, bulk MgB2 superconductors fabricated by a modified precursor infiltration and growth process

The wetting of boron with liquid magnesium is a critical factor in the synthesis of MgB2 bulk superconductors by the infiltration and growth (IG) process. Poor wetting characteristics can therefore result potentially in non-uniform infiltration, formation of defects in the final sample structure and poor structural homogeneity throughout the bulk material. Here we report the fabrication of near-net-shaped MgB2 bulk superconductors by a modified precursor infiltration and growth (MPIG) technique. A homogeneous bulk microstructure has subsequently been achieved via the uniform infiltration of Mg liquid by enriching pre-reacted MgB2 powder within the green precursor pellet as a wetting enhancer, leading to relatively little variation in superconducting properties across the entire bulk sample. Almost identical values of trapped magnetic field of 2.12 T have been measured at 5 K at both the top and bottom surfaces of a sample fabricated by the MPIG process, confirming the uniformity of the bulk microstructure. A maximum trapped field of 3 T has been measured at 5 K at the centre of a stack of two bulk MgB2 samples fabricated using this technique. A steady rise in trapped field was observed for this material with decreasing temperature down to 5 K without the occurrence of flux avalanches and with a relatively low field decay rate (1.5%/d). These properties are attributed to the presence of a fine distribution of residual Mg within the bulk microstructure generated by the MPIG processing technique.

Effects of Bi-2212 addition on the levitation force properties of bulk MgB2 superconductors

We present a detailed investigation of the effects of Bi2Sr2Ca1Cu2O8+κ (Bi-2212) adding on the levitation force and magnetic properties of bulk MgB2 obtained by hot press method. The amount of Bi-2212 was varied between 0 and 10 wt% (0, 2, 4, 6, 10 wt%) of the total MgB2. Moreover, we present MgB2 bulk samples fabricated by using different production methods including hot pressing method to our knowledge. All samples were prepared by using elemental magnesium (Mg) powder, amorphous nano-boron (B) powder and Bi-2212 powder which are produced by hot press method. As a result of hot press process, compact pellet samples were manufactured. The vertical and lateral levitation force measurements were executed at the temperatures of 20, 24 and 28 K under zero-field-cooled (ZFC) and field-cooled (FC) regimes for samples with various adding levels. At 24 K and 28 K under ZFC regime, the 2 wt% Bi-2212 added sample exhibits a higher vertical levitation force than the pure sample. Bi-2212 added MgB2 samples compared to the pure sample have lower attractive force values in FC regime. The magnetic field dependence of the critical current density J c was calculated from the M-H loops for Bi-2212 added MgB2 samples. The 2 wt% Bi-2212 added sample has the best levitation and critical current density performance compared to other samples. The critical temperature (T c ) has slightly dropped from 37.8 K for the pure MgB2 sample to 36.7 K for the 10 wt% of Bi-2212 added sample. The transition temperature slightly decreases when Bi-2212 adding level is increased.

Optimization of sintering conditions in bulk MgB2 material for improvement of critical current density

The present investigation focuses on methods to further improve the Jc values of disk-shaped bulk MgB2 superconductors by optimizing the sintering conditions. We prepared two sets of bulk MgB2 material from commercial high-purity powders of Mg metal and amorphous B using a single-step solid-state reaction process. To optimize the sintering time, a set of samples was sintered at 775 °C with sintering duration ranging between 1 and 10 h (pure Ar atmosphere). A second set of samples was produced similarly at 775, 780, 785, 795, 800 and 805 °C (3 h, pure argon atmosphere). X-ray diffraction analysis showed that both sets of samples were single phase MgB2. Magnetization measurements confirmed a sharp superconducting transition with Tc,onset ≈ 38.2 K–38.8 K. The critical current density (Jc) values for MgB2 samples produced for 1 h were the highest in all processed materials, i.e., the high Jc value of 270,000 A/cm2 and 125,000 A/cm2 (20 K, self-field and 1 T) were achieved in the sample produced at 775 °C, without any additional doping. In contrast, the second series of samples clearly indicated that at 805 °C (3 h) the highest Jc of 245,000 A/cm2 and 110,000 A/cm2 (20 K, self-field and 1 T) were achieved. AFM and EBSD observations indicated that largest amount of fine grains do exist in the sample sintered at 775 °C, but the narrowest distribution of grains does exist in the sample sintered at 800 °C. The present results clearly demonstrate a strong relation between the microstructure and the pinning performance. The optimization of the sintering conditions is crucial to improve the performance of bulk MgB2 samples.

Numerical modelling and comparison of MgB2 bulks fabricated by HIP and infiltration growth

MgB2 in bulk form shows great promise as trapped field magnets (TFMs) as an alternative to bulk (RE)BCO materials to replace permanent magnets in applications such as rotating machines, magnetic bearings and magnetic separation, and the relative ease of fabrication of MgB2 materials has enabled a number of different processing techniques to be developed. In this paper, a comparison is made between bulk MgB2 samples fabricated by the hot isostatic pressing (HIP), with and without Ti-doping, and infiltration growth (IG) methods and the highest trapped field in an IG-processed bulk MgB2 sample, Bz = 2.12 at 5 K and 1.66 T at 15 K, is reported. Since bulk MgB2 has a more homogeneous Jc distribution than (RE)BCO bulks, studies on such systems are made somewhat easier because simplified assumptions regarding the geometry and Jc distribution can be made, and a numerical simulation technique based on the 2D axisymmetric H-formulation is introduced to model the complete process of field cooling (FC) magnetization. As input data for the model, the measured Jc(B,T) characteristics of a single, small specimen taken from each bulk sample are used, in addition to measured specific heat and thermal conductivity data for the materials. The results of the simulation reproduce the experimental results extremely well: (1) indicating the samples have excellent homogeneity, and (2) validating the numerical model as a fast, accurate and powerful tool to investigate the trapped field profile of bulk MgB2 discs of any size accurately, under any specific operating conditions. Finally, the paper is concluded with a numerical analysis of the influence of the dimensions of the bulk sample on the trapped field.

Partial dissolution of MgO and the effect on critical current density in urea-doped MgB2 bulks

Bulk MgB2 samples containing xwt.% urea (x=0, 1.5, 2, 2.5, and 6) have been prepared by conventional solid state sintering at 800°C. The effects of urea on the microstructure, in particular the MgO existence form, and the superconducting properties were investigated. With the help of the decomposition product of urea, the supposed large MgO agglomeration partially dissolved to be nano-sized particles (∼10nm), which served as effective pinning centers rather than current barrier, and increased the upper critical field. As a result, the critical current density of the 2wt.% urea-doped MgB2 sample was enhanced (4×103Acm−2, 20K and 3T) in contrast with the un-doped sample when the field is larger than 2.5T. However, the decomposition product inevitably dissolved the MgB2 particles as the side effect, deteriorating the grain connectivity and therefore leading to a yet acceptable decrease of low-field critical current density. Taken all these into accounts, urea doping is still a more straightforward way to reduce MgO than pre-treating the powder with extra acid.

Effect of Er doping on the superconducting properties of porous MgB 2

MgB2 bulk sample with porous structure was produced by using the in-situ solid-state reaction method under argon (Ar) atmosphere of 10 bar. Elemental Er in powder form was doped into MgB2 with different compositions (Mg 1−x Er x )B2, where x=0.00, 0.03 and 0.05, in order to investigate the effect of rare-earth (RE) element Er on the structural and electromagnetic properties of porous MgB2. The Er-doped samples result in small grain size structure compared to the undoped one. The lattice constants a and c of the doped samples, determined from X-ray diffraction (XRD) analysis, increase with the increasing Er content, and consequently the superconducting transition temperature (Tconset) of MgB2, determined from resistivity measurements, is slightly suppressed. Also, the upper critical field (B c2), the irreversibility field (B irr) and the critical current density (J c) values are significantly enhanced in the doped samples. For the best sample (x=0.03), at 15 K under a magnetic field of 4 T, the J c value reaches 2.4 ×104 A cm−2, which is higher than that of the porous sample by an order of 103, and the B irr value at 20 K reaches 9.7 T. These results imply that the RE element Er fills the pores, enhances the density and the grain connectivity. Hence, the superconducting properties of the porous MgB2 sample improve by Er doping.