MgB2 Bulk Superconductors Research Articles

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.

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.

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.

Microstructures and superconducting properties of MgB2 bulk samples processed by ultra-high pressure-assisted sintering

Nanocrystalline MgB2 bulk superconductors have been fabricated by ultra-high pressure-assisted sintering (∼ 5 GPa) over a range of temperatures (700 °C-1100 °C). Phase evolution and morphology, grain size and lattice defects were systematically investigated. The superconducting performance was measured using magnetization methods and linked to the corresponding microstructural features. A sample processed at 900 °C and 5 GPa achieved a Jc value of 4.5 × 107 A/m2 at 4.2 K, 6 T, 30 times and more than 100 times higher than those prepared at 800 and 1100 °C respectively, under similar pressures. Its superior superconducting properties arise from the combination of limited grain growth, retained crystal defects and complete densification achieved in a rapid process by the application of ultra-high pressure. This study reveals the importance of microstructure in controlling the superconducting behavior in sintered MgB2, especially the sample homogeneity that can affect the length scales over which the supercurrents flow.

High Critical Current Density of Nanostructured MgB2 Bulk Superconductor Densified by Spark Plasma Sintering.

In situ MgB2 superconducting samples were prepared by using the spark plasma sintering method. The density of the obtained bulks was up to 95% of the theoretical value predicted for the material. The structural and microstructural characterizations of the samples were investigated using X-ray diffraction and SEM and correlated to their superconducting properties, in particular their critical current densities, Jc, which was measured at 20 K. Extremely high critical current densities of up to 6.75 × 105 A/cm2 in the self-field and above 104 A/cm2 at 4 T were measured at 20 K, indicating that vortex pinning is very strong. This property is mainly attributed to the sample density and MgB2 nanograins in connection to the presence of MgO precipitates and areas rich in boron.

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

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

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

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

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

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

Improving the connectivity of MgB2 bulk superconductors by a novel liquid phase sintering process

This work investigates a new processing method developed to improve the connectivity of ex-situ MgB2 bulks at low sintering temperatures. Mg additions (1–10 wt.%) were mixed to pre-synthesised MgB2 to make composite powders that were sintered at 900 ∘C by the field assisted sintering technique. Addition of 10 wt.% Mg resulted in a substantial increase in density from 68% to 79% and a dramatic reduction in MgB4 from 11 to ∼0 wt.%. Pressure and dilatometry data recorded in-situ during the sintering process revealed that Mg additions led to different sintering mechanisms depending on the Mg fraction. For large Mg fractions (6 and 10 wt.%) a Mg liquid phase was formed and led to significant density improvements, and all pre-existing MgB4 was transformed into MgB2. A small amount of residual Mg remained in the bulks after the sintering process. Connectivity was improved with Mg additions, increasing four fold in the 10 wt.% Mg-MgB2 sample compared to unmodified MgB2. J c values at low field were also significantly improved by Mg additions, in particular the 6 and 10 wt.% Mg-MgB2 specimens showed J c (20 K, 0 T) values 4–5 times higher than for unmodified MgB2.

Synthesis of Dense MgB2 Superconductor via In Situ and Ex Situ Spark Plasma Sintering Method.

In this study, high-density magnesium diboride (MgB2) bulk superconductors were synthesized by spark plasma sintering (SPS) under pressure to improve the field dependence of the critical current density (Jc-B) in MgB2 bulk superconductors. We investigated the relationship between sintering conditions (temperature and time) and Jc-B using two methods, ex situ (sintering MgB2 synthesized powder) and in situ (reaction sintering of Mg and B powder), respectively. As a result, we found that higher density with suppressed particle growth and suppression of the formation of coarse particles of MgB4 and MgO were found to be effective in improving the Jc-B characteristics. In the ex situ method, the degradation of MgB2 due to pyrolysis was more severe at temperatures higher than 850 °C. The sample that underwent SPS treatment for a short time at 850 °C showed higher density and less impurity phase in the bulk, which improved the Jc-B properties. In addition, the in situ method showed very minimal impurity with a corresponding improvement in density and Jc-B characteristics for the sample optimized at 750 °C. Microstructural characterization and flux pinning (fP) analysis revealed the possibility of refined MgO inclusions and MgB4 phase as new pinning centers, which greatly contributed to the Jc-B properties. The contributions of the sintering conditions on fP for both synthesis methods were analyzed.

Levitation force of Graphene added bulk MgB2 superconductor

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

Improved pulsed field magnetisation in MgB2 trapped-field magnets

Bulk superconductors can act as trapped-field magnets with the potential to be used for many applications such as portable medical magnet systems and rotating machines. Maximising the trapped field, particularly for practical magnetisation techniques such as pulsed field magnetisation (PFM), still remains a challenge. PFM is a dynamic process in which the magnetic field is driven into a superconducting bulk over milliseconds. This flux motion causes heating and a complex interplay between the magnetic and thermal properties. In this work, the local flux density during PFM in a MgB2 bulk superconductor has been studied. We find that improving the cooling architecture increases the flux trapping capabilities and alters the flux motion during PFM. These improvements lead to the largest trapped field (0.95 T) for a single MgB2 bulk sample magnetised by a solenoidal pulsed field magnet. The findings illustrate the fundamental role bulk cooling plays during PFM.

The discrepancies in different facets of MgB2 bulk superconductors prepared under various sintering durations by spark plasma sintering

Seven MgB2 bulk superconductors with good superconductivity and crystallinity were prepared under the pressure of 50 MPa at 950 °C and different sintering durations (15 ∼ 240 min) by spark plasma sintering (SPS), with the onset critical temperature (T c,onset) around 37.5 K. The superconducting properties and microstructure measured from the facets perpendicular (PeF) and parallel (PaF) to the compression direction of the SPSed MgB2 bulk samples were also analyzed to investigate the effect of the sintering duration on the MgB2 bulks and the discrepancy between the PeF and the PaF. The optimum performance was obtained from the sample prepared for 45 min for both facets, and the critical current density (J c) measured from the PeF and the PaF are 364 kA cm−2 and 344 kA cm−2 in self field at 20 K, respectively. The PeF exhibits better Jc performance than the PaF at low fields and the PaF exhibits better J c performance than the PeF at high fields. It can be concluded that the sintering duration affects the properties of the PeF mainly by changing their crystallite size. While for the PaF, the sintering duration mainly affects the microstructural defects, such as cracks, pores and secondary phase particles, thus further affecting their J c performances. In terms of the flux pinning mechanism of the SPSed MgB2 bulk samples, the PaF is less susceptible to sintering duration than the PeF, and the better J c performance of the PaF at high fields is due to the stronger grain boundary pinning. It can be also found that the point pinning being stronger at higher temperature is the result of the decreasing anisotropy of MgB2 bulk samples with the temperature.

Enhanced critical current density of MgB2 superconductor using a milled MgB4 precursor

The synthesis of bulk MgB2 superconductors from Mg and MgB4 precursors instead of the conventional in situ process increases the packing density, but further enhancement of the critical current density is required. Herein, to improve the critical current density of bulk MgB2, the MgB4 precursor was subjected to planetary milling in toluene. The MgB4 powder was then mixed with Mg and heat-treated at 650 °C for 1 h to synthesize bulk MgB2. The microstructure, phases, and superconductivity of bulk MgB2 were investigated to determine the effect of MgB4 milling. MgB4 milling decreased the MgB2 grain size, and carbon incorporation from the milling solvent was observed, which caused the deterioration of the MgB2 crystallinity. These changes caused a reduction of the critical temperature; however, the critical current density was improved due to the increased magnetic flux pinning sites such as grain boundary and lattice distortion.

Superconducting and levitation force characterisation of pyrene added MgB2 bulk superconductors

This research presents the influence of pyrene (C16H10) addition (0, 2, 4, 6, 8, 10 wt%) on the superconducting properties of bulk MgB2. For C16H10 added MgB2 samples, a shrinkage in a-lattice parameters was determined, whereas c-lattice parameters remained practically unchanged. The (110) peak observed at 2θ ≈ 59.9◦ slightly shifted to higher angles, the lattice strain increased and the crystallite size reduced after C16H10 addition. By virtue of induced stronger pinning force, the in-field critical current density (Jc) increased after C16H10 addition. The Jc value at 20 K and 4 T was obtained about two times higher (8.54 × 103 A/cm2) for 4 wt% C16H10 added MgB2 sample than that of the pure one. In addition, the variation of levitation force characteristic of MgB2 with increasing C16H10 content was determined at 20, 25 and 30 K. The best vertical levitation force performance in zero-field-cooled regime was obtained as 7.09 N/g at 20 K for 4 wt% C16H10 added MgB2 sample with 13 mm in diameter.

Enhancement of Bc2 and Birr in bulk MgB2 superconductors with SnO2 Additions

Three (MgB2)1-x(SnO2)x samples with x ranging from 0 to 5 wt% were prepared by the in situ route to study the effect of tin dioxide additions on the superconducting properties of MgB2 bulk materials. All of the reacted samples were slightly Mg deficient although the starting Mg:B precursor powder ratio was 1:2. A heat treatment (HT) temperature of 700 °C with a dwell time of 30 min was used. XRD results showed evidence of peak shifts for MgB2 phases with SnO2 addition. The magnitude of the a-axis lattice constant change (0.361 ± 0.075 %) calculated for the 3 wt% doped samples is comparable in magnitude to that seen previously for the C-doped MgB2 bulks which exhibited enhanced BC2. The upper critical fields (BC2) and the irreversibility fields (Birr) were measured resistively in fields up to 14 T at 5 K to Tc. The best BC2 value at 20 K (15.2 T based on extrapolation) was seen for sample IS3 (x =3 wt%), and was comparable to the best BC2 values (≈ 15 T at 20 K) seen for C-doped MgB2 bulks. IS3 had a corresponding Birr = 10.8 T (20 K). The superconducting transition temperature (Tc) appeared to increase slightly with doping, although within the range of error bars (37.4 K to 37.6 K for 1.6 T BC2 increase at 20 K), in contrast to C doping which is accompanied by a significant decrease in Tc (39 K to 36 K for 3.8 % C doped MgB2 bulk). We attribute the observed increase in both BC2 and Birr for SnO2-additions to lattice strain caused by the introduction of precipitates within the grains.

Improvement of critical current density of MgB2 bulk superconductor processed by Spark Plasma Sintering

AbstractSpark Plasma Sintering (SPS) is a promising rapid consolidation technique that allows a better understanding and optimization of the sintering kinetics and therefore makes it possible to obtain MgB2 bulk superconductor with tailored microstructures consisting of grains with either spherical or elongated morphology. In this contribution, the role of the precursor powders on the superconducting properties of MgB2 is investigated. Three sets of bulk MgB2 material were processed from: (i) a commercially available MgB2 powder; (ii) a mixture of Mg metal and amorphous B using a single‐step solid‐state reaction process and (iii) a mixture of amorphous boron coated with carbon and Mg metal. The samples were prepared in the same SPS processing conditions. The microstructure of the samples was investigated by X‐ray diffraction, SEM and TEM and correlated to their superconducting properties. The critical current density of the best sample at 20K was Jc = 500 kA/cm2 in self‐field, which is one of the highest critical current density reported for MgB2 bulk superconductors.

Investigation of flux jumps during pulsed field magnetization in graphene-added MgB2 bulks

MgB2 bulk superconductors are expected to be utilized as rare-earth-free and lightweight trapped field magnets. However, the flux jumps frequently happen during the magnetizing processes, and heavily degrade the field-trapping performances. We have investigated the effect of additives to the MgB2 bulk samples prepared by spark plasma sintering process, and observed various flux jumps during the pulsed field magnetizing processes, which were carried out at 14 K which was obtained by the 2-stage GM cryocooler. The authors classified the flux motions as three categories as “no flux flow”, “fast flux flow”, and “flux jump” regions, and investigated the conditions where the flux jumps happen. We observed some drastic flux jumps in the pristine and clarified the effect of graphene addition to the flux jumps. The experimental results showed us a possible expansion of no flux jump region, and suggested us the improvement of field trapping capability.