MgB2 Bulk Samples Research Articles

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

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.

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.

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.

Effect of polysilane addition on spark plasma sintering and superconducting properties of MgB2 bulks

Polycrystalline MgB2 bulk samples were produced by ex-situ spark plasma sintering (SPS) using the oxygen-free preceramic polymer additive poly(dimethylsilane) as a source for carbon doping and as a sintering aid. Two major effects were identified. One is the significant enhancement of the densification kinetics during sintering for all tested compositions. The second one is the improvement of the high field critical current density for a certain level of the poly(dimethylsilane) addition.

Investigation of ball milling effect on superconducting properties of oleic acid added bulk MgB2 superconductors produced by two different methods

The effect of ball milling on the microstructure and some superconducting parameters such as flux pinning force (Fp) and critical current density (Jc) of the oleic acid (C18H34O2) added MgB2 bulk samples produced by two different methods, was analysed in this article. In the first method, ball milling was applied to the samples produced by using boron (B) powders coated with carbon (C) released from oleic acid. In the second method, oleic acid was mixed with magnesium (Mg) and B powders at the same time and then the same ball milling process used in the first method was applied to the powder mixture. The structural, magnetic and electrical properties of the produced samples were analysed. The results showed that ball milling process enhances the homogeneity of the structure, decreases the grain size and improves the grain connectivity of MgB2. Also, C substitution into MgB2 lattice resulting in an increase in electron scattering and disorders, enhances after ball milling process. It supplies a significant increase in Jc at high fields and causes a slightly decrease in transition temperature (Tc), especially for the samples produced by using the first method. Because the first method supports the ball milling effect on the homogenous dispersion of oleic acid addition in the MgB2 structure and the C entrance to MgB2 lattice, acting as pinning centres, the best Jc and Fp values at high fields was obtained for the produced samples with second method.

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.

The study of superconducting and magnetic properties of bis oxalate (C22H22O6) added bulk MgB2

The effect of bis oxalate (C22H22O6) addition on critical current density (Jc), lattice parameters, microstructure, superconducting temperature (Tc), magnetic levitation force (Fz) and lateral force (Fx) properties of bulk MgB2 has been studied for the first time in this study. Bulk MgB2 samples containing × wt% C22H22O6 (x ranging from 0, 1.5, 3, 4.5, 6 and 9 wt%) were fabricated with by in-situ solid state sintering technique at 775 °C. Fz and Fx measurements at 25 K and 30 K were made under zero-field-cooled (ZFC) and field- cooled (FC) regimes. The analysis of XRD and SEM points out a drop in lattice parameters and grain size of bis oxalate added samples and endorses C replacement in B sites in MgB2 lattice. It was observed that the bis oxalate addition has a progressive effect on magnetic levitation performance of MgB2. Whereas the maximum Fz and Fx values of undoped sample show 16.91 N and 7.10 N at 25 K, the maximum force values of the 3 wt% (BO) added sample correspond to 18.62 N and 8.49 N at 25 K. Furthermore, the (BO) addition meaningfully rises the critical current density (Jc) values of MgB2. The Jc values observed as 57 kA/cm2 and 70 kA/cm2 in self-field at 25 K for pure and 3 wt% (BO) doped MgB2 sample. The experimental results reveal that C22H22O6, as an example of carbohydrate, is an effective C-containing additive for the enhancement of the superconducting and magnetic levitation capacities of MgB2.

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.

Increase in the infield critical current density of MgB2 thin films by high-temperature post-annealing

We propose a novel fabrication technique based on the formation of a Nb protective layer on a MgB2 thin film and high-temperature post-annealing to increase the critical current density (J c) of MgB2 films under an external magnetic field. Analyses of the crystal structure and the composition of the processed MgB2 films confirmed the suppression of the evaporation and oxidation of Mg by high-temperature annealing above 550 °C. The MgB2 film annealed at 650 °C exhibited a J c of 1.62 MA cm−2 under 5 T, which is the highest reported value for MgB2 films, wires, and bulk samples to date.

High-field superconductivity in C-doped MgB2 bulk samples prepared by a rapid synthesis route

The upper critical field sets the thermodynamic limit to superconductivity. A big gap is present between the upper-critical-field values measured in MgB2 polycrystalline bulk superconductors and those of thin films, where values as high as ~ 50 T have been achieved at 4.2 K. Filling this gap would unlock the potential of MgB2 for magnet applications. This work presents the results of an extensive experimental campaign on MgB2 bulk samples, which has been guided by a Design of Experiment. We modeled the dependence of the upper critical field on the main synthesis parameters and established a new record (~ 35 T at 4.2 K) preparing C-doped bulk samples by a non-conventional rapid-synthesis route. This value appears to be an upper boundary for the upper critical field in bulk samples. Structural disorder in films seems to act selectively on one of the two bands where superconductivity in MgB2 takes place: this enhances the upper critical field while reducing the critical temperature only by few Kelvins. On the other hand, the critical temperature in bulk samples decreases monotonically when structural disorder increases, and this imposes a limit to the maximum achievable upper critical field.

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.

Enhanced magnetic field dependent critical current density of MWCNT doped magnesium diboride superconductor

Abstract MgB2 bulk samples added with multiwall carbon nanotube (MWCNT) were synthesized by powder–in–sealed tube (PIST) method. The effect of MWCNT doping on the structural and superconducting properties of MgB2 superconductor were investigated. The XRD analysis reveals the hexagonal structure of MgB2 in all the samples. FESEM, TEM and Raman analysis indicate the microstructural changes due to MWCNT doping. Raman spectra show the shifting of the E 2 g peak is more for lightly doped samples. DC magnetization measurement shows a sharp superconducting transition in all the samples and a slight reduction in critical temperature ( T c ) for the doped samples. The J c (B) characteristics of doped samples show a significant enhancement of J c compared with pristine MgB2 at 10 and 20 K. MgB2 doped with 1 and 2 wt% MWCNT show maximum enhancement in J c i.e., 1.3 × 10 5 and 1.1 × 10 5 A/cm2 at 5 T and 10 K, respectively, while heavily doped (5 wt%) sample shows lower J c (B) characteristics. The flux–pinning properties of MWCNT doped MgB2 have been enhanced due to the formation of point pinning centers.

Effects of silver doping on magnetic levitation performance of MgB2 superconductors

MgB2 bulk samples with the silver contents of 0, 1.5, 3.0, 4.5, 6.0, 9.0 and 12.0 wt% were fabricated at 775 °C for 3 h in Ar atmosphere by solid-state reaction method. The structural and electromagnetic properties of MgB2 samples were studied using X-ray diffraction (XRD), scanning electronic microscope (SEM), energy dispersive spectrometry (EDS), magnetic hysteresis (M-H and M-T), magnetic levitation force (Fz) and lateral force (Fx) measurements. The effect of silver addition on the levitation force performance of MgB2 has been studied for the first time in this work. All the samples indicate the onset of superconducting critical temperature (Tc,onset) in the temperature range 37.97–38.11 K. The Jc values of the 3 wt% of silver doped MgB2 sample at 15 and 25 K were 98 and 49 kA/cm2 at 0.5 T as the highest remnant Jc value was around 110 and 65 kA/cm2 at 15 and 25 K, respectively. The repulsive and attractive levitation force at 25 K increases with rising x content up to 3 wt% silver and then decreases with increasing silver addition up to 12 wt%. The maximum attractive (repulsive) levitation force values at 25 and 30 K were observed for 3 wt% silver addition as −7.09 (+19.03) and −6.03 (+15.22), respectively. These results reveal that the silver addition is the effective metallic dopant for MgB2 and can be beneficial for improving the magnetic levitation achievement of MgB2 superconductors for high field engineering applications.

Increased flux pinning force and critical current density in MgB2 by nano-La2O3 doping

MgB2 superconducting wires and bulks with nano-La2O3 addition have been studied. A series of MgB2 superconducting bulk samples with nano-La2O3 addition levels of 0, 5, 7, 18wt% were prepared. AC resistivity data showed slight increases of Bc2 and unchanged Birr for the bulk samples with doping levels lower than 7 wt% and decreased critical fields for the heavily doped (18 wt%) bulk. X-ray diffraction (XRD) showed the presence of LaB6 in the nano-La2O3 doped MgB2 bulk samples and decreased MgB2 grain size in nano-La2O3 doped bulks. Monocore powder-in-tube (PIT) MgB2 wires without and with 5 wt% nano-La2O3 addition (P-05) were prepared for transport property measurement. 2mol%C-doped Specialty Materials Inc. (SMI) boron powder was used for wire P-05 and previously prepared control wires (control wires were made without the addition of nano-La2O3 powder, W-00 and P2). Low field magnetic properties were obtained from magnetization loop (M–H), transport critical current density (Jc) was measured at 4.2 K for the nano-La2O3 doped PIT wire (P-05) and the control samples (P2 and W-00). The transport critical current density Jc (B) of P-05 at 4.2 K and 8 T (6.0 ×104 A/cm2) was twice that of the control wire. The critical magnetic fields (Bc2 and Birr ) of P-05 and the control sample P2 were compared. The critical fields of P-05 were slightly less than those of P2. Kramer-Dew-Hughes plots indicated a change from surface pinning to a mixture of volume pinning and surface pinning. It is shown that enhancement of P-05’s transport properties is due to additional flux pinning by the fine-size rare-earth borides rather than enhanced Bc2 or Birr.

On the origin of the sharp, low-field pinning force peaks in MgB2 superconductors

Various MgB2 thin films and single crystals were found in the literature to exhibit a sharp, narrow peak at low fields in the volume pinning force, Fp(H)-diagrams. The origin of this peak is associated with a steep drop of the current density when applying external magnetic fields and is ascribed to sample purity. We show here that bulk MgB2 prepared by spark-plasma sintering also shows the sharp, narrow peak in Fp. The peak is also seen in the volume pinning force scaling, Fp/Fp,max vs h = H/Hirr. Furthermore, polycrystalline bulk MgB2 samples prepared close to the optimum reaction temperature reveal this peak effect as well, but other samples of the series show a regular scaling behavior. The combination of magnetization data with data from electric transport measurements on the same samples demonstrates the origin of this peak effect. On increasing preparation temperature, the pinning force scaling changes from grain boundary pinning to point pinning and the grain connectivity gets worse. Hence, the sharp, low-field peak in Fp vanishes. Therefore, the occurrence of the peak effect in Fp gives important information on the grain coupling in the MgB2 samples.