MgB2 Bulk Superconductors Research Articles

Determination of magnetic levitation force properties of bulk MgB2 for different permanent magnetic guideways in different cooling heights

Abstract In our study as different from the literature, the magnetic levitation force between a bulk MgB2 and two different permanent magnetic guideway (PMG) arrangements, which are Halbach and Conventional PMGs, were investigated in different cooling heights (CHs) and Field-Cooled (FC) condition at the temperatures of 37 K and 33 K. The cylindrical bulk MgB2 superconductor was fabricated by in-situ solid state reaction process with the diameter of 18 mm and the height of 5 mm. The XRD data indicates well developed MgB2 phase and the Jc value was obtained as 68 kA/cm2 at 30 K in the self-field. Experimental results show that MgB2 bulk above the Halbach PMG can exhibit better load capability at all the cooling heights between the bulk MgB2 and the PMG due to a more suitable magnetic field distribution. The maximum levitation force for Halbach PMG corresponds to 16.86 N whereas the conventional PMG shows 9.02 N at 37 K in cooling height of 77 mm. Additionally, the maximum levitation force increases while the CH increases because flux exclusion is more effective for larger CHs. It is considered that the experimental results obtained in study are very useful for future Maglev applications, because there are limited number of studies on magnetic levitation force of MgB2 bulk for different MgB2-PMG arrangements.

Superconducting properties of saccharin-added bulk MgB2 superconductors

In this paper, saccharin was used as the carbon source for addition in bulk MgB2 superconductor. MgB2 samples were successfully prepared with saccharin and ethanol-processed boron using a wet-mixing method. Actual carbon substitution level in the MgB2 lattice increased from 0 to 3.58% with the increasing saccharin content up to 1% and slightly decreased at higher saccharin content. While the 0.1% saccharin-added sample showed the highest critical current density of 2.17 × 105 A/cm2 in the self field at 20 K, the undoped sample showed a lower critical current density of 1.19 × 105 A/cm2. In addition to this, the critical current densities in 4 T field at 10 K for 0.1% and 0.5% saccharin-added samples were calculated to be 1.5 × 104 and 1 × 104 A/cm2, respectively. It was obtained that the addition of low-rate saccharin content did not affect the superconducting transition temperature of the MgB2 sample. These results show that MgB2 fabrication with small amount of saccharin is beneficial to improve critical current density.

Processing and Characterization of Charcoal Added Bulk MgB2 Superconductor

In this study, we focus on methods to further improve the flux pinning and critical current density (Jc) of disk shaped MgB2 bulk superconductors by a comparative study with locally obtained charcoal at optimized processing conditions. The doping of charcoal was varied at 0, 0.2, 0.4 and 0.6 wt%. Bulk MgB2 samples were produced by in-situ solid state reaction in Ar gas and ambient pressures at 775 °C, employing high purity of Mg metal and nano B powder mixed in a fixed ratio of Mg:B = 1:2 and optimal conditions. The phase has been investigated by X-ray diffractometer (XRD). The effects of the dopants on superconducting properties especially the field dependence of Jc have been determined by superconducting quantum interference device (SQUID) and critical temperature (Tc). Measurements of the magnetic susceptibility showed sharp superconducting transitions (ΔTc ∼ 1 K). Jc at self-field and 20 K was determined to be ~280 and 467 kA/cm2 for the 0 and 0.4 wt% charcoal added samples respectively. The structural and superconducting results were correlated with the addition of charcoal in the final product.

Effect of coumarin addition on the superconducting properties of bulk MgB2 superconductors

In the present study, undoped and coumarin doped bulk MgB2 superconductors were successfully fabricated by using the wet-mixing method and conventional solid-state reaction method. It was found that grain size of the MgB2 samples decreases from 24.3 to 20.5 nm with increasing coumarin content. Actual carbon substitution level in the MgB2 lattice is calculated to be from 0.70% to 1.75% for different coumarin contents. The critical current density and flux pinning force of MgB2 were enhanced by coumarin doping. Among the all MgB2 samples, the highest critical current density value was achieved to be 1 × 105 A cm−2 in the self field at 5 K. It was obtained that coumarin additive did not reduce superconducting transition temperatures of the MgB2 samples. The results show that coumarin assisted production of MgB2 is a promising way to improve its superconducting properties.

Optimization of Mg Precursor Concentration to Obtain High <inline-formula> <tex-math notation="LaTeX">$J_{c}$</tex-math> </inline-formula> in MgB2 Synthesized With Ag Addition and Carbon Encapsulated Boron

In this work, we report an optimum amount of Mg precursor required for obtaining high critical current density (J c ) of bulk MgB 2 superconductor synthesized via solid state sintering. We add a slight excess of Mg precursor to balance its losses while formation of pinning phases and impurities such as Ag-Mg phases and MgO, respectively. Several amounts of Mg had been varied such as x = 1.05, 1.075, 1.1, 1.125, and 1.15 in MgxB 2 . To obtain the best critical current density values, we used carbon encapsulated boron (1.5% carbon) and added 4 wt% Ag. The x-ray diffraction analysis showed the presence of AgMg and AgMg 3 phases and a minute amount of MgO. The SQUID measurements indicated that highest J c of 350 kA/cm 2 at 20 K and self-field in sample with x = 1.075. All results analyzed explain the improved critical current performance based on nanometer-sized Ag-Mg particles in the final product.

Effects of a compaction method for powder compacts on the critical current density of MgB2 bulk superconductors

In this study, the effects of the compaction method for (Mg+2B) powders on the apparent density and superconducting properties of MgB2 bulk superconductor were investigated. The raw powders used in this study were nano-sized boron (B) and spherical magnesium (Mg). A batch of a powder mixture of (Mg+2B) was put in a steel mold and uniaxially pressed at 1 ton or 3 tons into pellets. Another batch of the powder mixture was uniaxially pressed at 1 ton and then pressed isostatically at 1800 kg/cm2 in the water chamber. All pellets were heat-treated at 650℃ for 1 h in flowing argon gas for the formation of MgB2. The apparent density of powder compacts pressed at 3 ton was higher than that at 1 ton. The cold isostatic pressing (CIP) in a water chamber allowed further increase of the apparent density of powder compacts, which influenced the pellet density of the final products (MgB2). The compaction methods (uniaxial pressing and CIP) did not affect the formation of MgB2 and superconducting critical temperature (Tc) of MgB2, but affected the critical current density (Jc) of MgB2 significantly. The sample with the high apparent density showed high Jc at 5 K and 20 K at applied magnetic fields (0-5 T).

Magnetic Characterization of Bulk C-Added MgB2

To find a cheap and efficient way to produce high-performance bulk MgB2 superconductors, a series of samples produced with different contents of carbon-coated, nanometer-sized amorphous boron was prepared to improve the magnetic properties. A single-step solid-state reaction at the optimal reaction temperature of 805 °C for 3 h in pure argon atmosphere was used. Small pieces cut from the bulk were characterized by magnetic hysteresis loop measurements in magnetic fields up to ±7 T at temperatures ranging from 5 to 35 K using a physical property measurement system. Critical current densities and flux pinning forces were calculated from the magnetization data. The pinning force data of all samples were compared using the pinning force scaling approach by Eisterer. The peak positions h 0 were found at h 0 ∼ 0.33, indicating a dominant flux pinning at the grain boundaries for samples with 50% and 30% carbon-coated boron, while the scaling reveals a peak position of 0.44 for the 10% sample that indicates pinning at point defects.

Study of Superconducting, Structural, and Thermal Properties of SnO2 Added MgB2 Bulks.

A series of SnO2 added MgB2 bulk superconductors were prepared by in situ route to study the effect of oxygen doping on superconducting and structural properties of MgB2. Several (MgB2)1-x (SnO2) x samples were fabricated with x ranging from 0, 3 wt%, 4 wt%, and 6 wt%. Upper critical field (B C2) and irreversible field (B irr) were measured by physical property measurement system. Thermal analysis was performed on the as-received SnO2 powder. Critical current densities (J cm ) were obtained at 4.2 K using magnetic measurement. X-ray diffraction results showed evidence of full SnO2 decomposition in all the doped bulk samples and a shift of a-axis in MgB2 lattice was seen. Oxygen was successfully released during heat treatment, yet no enhancement of B C2 or B irr was seen, indicating that oxygen atoms did not end up in the host lattice. Further exploration of different processing procedures is still needed in order to get oxygen substitution on the host lattice sites.

Evaluation of Superconducting Properties and Diffusion Behavior of Ex Situ and In Situ Bulk MgB2 Materials with Ni Coating

Ex situ and in situ bulk MgB2 superconducting materials have successfully been produced by the simple amorphous boron and nano-amorphous boron powders with the processes of ball milling, pressing, and annealing. The superconducting properties and diffusion behavior of MgB2 samples after nickel (Ni) coating process have been characterized and compared to the microstructure and performance of uncoated (bare) MgB2 bulk sample. One surface of MgB2 superconductor sample was coated with a thin Ni layer of about 50 − 60μm thickness using vapor deposition techniques in versatile high vacuum coater, then every sample was annealed at temperatures between 923 and 1123 K for 1 h. The role of annealing temperature on physical, electrical, superconducting, and structural characterizations of bare and Ni-coated ex situ/in situ MgB2 bulk superconductors has been studied using X-ray diffraction and dc electrical resistivity versus temperature measurements. Finally, the Ni diffusion coefficients (DNi = D0exp(E/kBT)) are calculated in the temperature range to calculate the required minimum activation energy value for the Ni atoms/ions into the ex situ MgB2 crystal structure for the first time.

High Trapped Fields in C-doped MgB2 Bulk Superconductors Fabricated by Infiltration and Growth Process

The grain boundaries in superconducting MgB2 are known to form effective magnetic flux pinning sites and, consequently, bulk MgB2 containing a fine-grain microstructure fabricated from nanoscale Mg and B precursor powders exhibits good magnetic field-trapping performance below 20 K. We report here that the trapped field of MgB2 bulk superconductors fabricated by an infiltration and growth process to yield a dense, pore-free microstructure, can be enhanced significantly by carbon-doping, which increases intra-band scattering within the superconducting grains. A maximum trapped field of 4.15 T has been measured at 7.5 K at the centre of a five-sample stack of Mg(B1−xiCxi)2 bulk superconductors processed by infiltration and growth, which not only represents a ~40% increase in trapped field observed compared to undoped bulk MgB2, but also is the highest trapped field reported to date in MgB2 samples processed under ambient pressure. The trapped field is observed to decay at a rate of <2%/day at 10 K, which suggests that bulk MgB2 superconductors fabricated using the infiltration and growth technique can be used potentially to generate stable, high magnetic fields for a variety of engineering applications.

Influence of coronene addition on some superconducting properties of bulk MgB2

This study reports the effect of coronene (C24H12) addition on some superconducting properties such as critical temperature (Tc), critical current density (Jc), flux pinning force density (Fp), irreversibility field (Hirr), upper critical magnetic field (Hc2), and activation energy (U0), of bulk MgB2 superconductor by means of magnetisation and magnetoresistivity measurements. Disk-shaped polycrystalline MgB2 samples with varying C24H12 contents of 0, 2, 4, 6, 8, 10 wt%, were produced at 850 °C in Ar atmosphere. The obtained results show an increase in field-Jc values at 10 and 20 K resulting from the strengthened flux pinning, and a decrease in critical temperature (Tc) because of C substitution into MgB2 lattice, with increasing amount of C24H12 powder. The Hc2(0) and Hirr(0) values are respectively found as 144, 181, 172 kOe, and 128, 161, 145 kOe for pure, 4 wt% and 10 wt% C24H12 added samples. The U0 depending on the magnetic field curves were plotted using thermally activated flux flow model. The maximum U0 values are respectively obtained as 0.20, 0.23 and 0.12 eV at 30 kOe for pure, 4 wt% and 10 wt% C24H12 added samples. As a result, the superconducting properties of bulk MgB2 at high fields was improved using C24H12, active carbon source addition, because of the presence of uniformly dispersed C particles with nanometer order of magnitude, and acting as effective pinning centres in MgB2 structure.

Effects of SPS pressure on the mechanical properties of high packing ratio bulk MgB2 superconductor

In order to investigate the effects of SPS pressure on the mechanical properties of MgB2 bulk, MgB2 bulk samples were processed by SPS under different pressures. Mechanical properties of these bulk samples were evaluated at room temperature through the bending tests for specimens cut from the bulk samples. Stress-strain behaviours of these bulk samples were almost linear until the fracture. No significant difference was observed for the Young’s modulus values among the bulk samples. The average bending strength values of these bulk samples were also similar to each other. However, the bending strength data of the samples processed under higher pressure scattered widely in comparison with those of the sample processed under lower pressure. The bending strength values were related with the pore sizes observed on the tensile side fracture surfaces.

Flux Pinning and Superconducting Properties of MgB2-Diamond Nanocomposites

The present investigation focuses on a further performance improvement of disk-shaped bulk MgB2 superconductors by means of a nanoscopic diamond powder, using a single-step solid-state reaction process. MgB2 bulks were produced by in-situ solid state reaction in Ar gas using high purity commercial powders of Mg metal and amorphous B mixed in a fixed ratio of Mg:B = 1:2. Further, 0, 0.4, 0.8, and 1.2 wt% of nanoscopic diamond powder was added to improve flux pinning performance of the bulk MgB2 material. All samples were sintered at 775 °C for 3 hours in Ar atmosphere. X-ray diffraction confirmed that the sample consisted mainly of MgB2 phase and a small quantity of MgO phase. DC magnetization measurements showed a sharp superconducting transition with onset Tc at around 39.25 K and 37.42 K for the pure and 0.8 wt.% of nanoscopic diamond powder added samples, respectively. The highest self-field critical current density (Jc) around 300 kA/cm2 and 105 kA/cm2 were recorded at 20 K, in self-field and 1.5 T for the sample with 0.8 wt.% of nanoscopic diamond. The present results show that nanoscopic diamond powder is an effective pinning medium for bulk MgB2 and might thus be an alternative to further improve the superconducting performance of the bulk MgB2 material.

Enhanced Magnetic Levitation and Guidance Force in MgB2 Bulks by Synthetic Engine Oil Immersion

We have used ‘Synthetic engine oil’ as a rich easily accessible carbon source and investigated the effect of engine oil on the microstructure, critical current density (Jc), vertical levitation force (Fz) and guidance force (Fx) properties of MgB2 superconductor. The polycrystalline disk-shaped MgB2 bulk samples have been prepared using two-step solid-state reaction process. After the first heating process, MgB2 bulk samples immersed in the engine oil for different durations (30, 120, 300 and 1440 min). XRD analysis indicated the decreasing of lattice parameters of samples and confirmed C substitution in boron sites. Vertical levitation and guidance force measurements were carried out with both Field-cooling (FC) and zero-field-cooling (ZFC) regimes at different temperatures of 20, 24 and 28 K. It was found that engine oil addition slightly increases the levitation and guidance force properties. The $F_{z}$ values were obtained as 11.17 and 11.58 N for pure and MgB2 sample immersed for 30 min at 20 K in ZFC regime. In addition, the MgB2 sample immersed for 300 min in engine oil showed the highest $J_{c}$ value of $5.3\times 10^{4}$ A cm− 2 at 15 K and self-field and showed the highest guidance force value of 4.14 N at 20 K. These finding suggest that synthetic engine oil is a cost-effective and promising method to improve the levitation and guidance force performance as well as the critical current density of MgB2 bulk superconductor.

Progress in Critical Current Density (Jc) in Sintered MgB2 Bulks

The present paper focuses on methods of further improving the flux pinning and critical current density of disk-shaped MgB2 bulk superconductors by adding excess Mg metal in combination with an optimum silver content and optimized processing conditions. Bulk MgB2 samples were produced by in situ solid-state reaction in Ar gas ambient using high purity commercial powders of Mg metal and 1.5 wt% carbon-coated amorphous B powders mixed in a fixed ratio of Mg/B = 1.1:2. Further, 4 wt% silver was added to improve flux pinning as well as mechanical performance of the bulk MgB2 material. The magnetization measurements confirmed a sharp superconducting transition with Tc,onset at around 37 K, which is only by 1 K lower than in bulk MgB2 material produced without carbon-coated amorphous boron. The critical current density (Jc) values significantly improved in the MgB2 material with 4 wt% of silver and 1.5 wt% of carbon-coated amorphous boron, sintered at 775 ∘C for 3 h. At 20 K, this sample showed Jc at around 500 and 350 kA/cm2 in the self-field and 1 T, respectively, which makes it suitable for several industrial applications.

Mechanical Properties of Bulk MgB2 Superconductors Processed by Spark Plasma Sintering at Various Temperatures

In order to improve the mechanical properties of bulk MgB2 superconductor, high packing ratio bulk samples were processed by spark plasma sintering (SPS) at various temperatures. Effects of the SPS temperature on the mechanical properties of the bulk MgB2 superconductor are investigated through bending tests of specimens cut from the bulk samples. Both Young's modulus and bending strength were improved by the increase in the SPS temperature. The improvements of the mechanical properties are mainly due to the improvement in the packing ratio. The relationships between the bending strength and packing ratio of the spark plasma sintered bulk samples are similar to those of other bulk MgB2 pieces processed by capsule method and hot isostatic pressing.

Improvement in levitation force performance of bulk MgB2 superconductors through coronene powder adding

The effect of coronene (C24H12) addition on the levitation force properties of MgB2 superconductor has been investigated for the first time in this study. The polycrystalline disk-shaped MgB2 + y wt % C24H12 samples (y = 0, 2, 4, 6, 8, 10), were synthesized by a pellet/closed tube method at 850 °C under Ar atmosphere, after hot pressing at 200 °C. XRD analysis indicates a decrease in lattice parameters of coronene added samples and confirms substitution of carbon in boron sites. An increase in lattice strain and a decrease in grain size are observed due to the carbon substitution effect. Vertical and lateral levitation force measurements under zero-field-cooled and field-cooled regimes were carried out at different temperatures of 20, 25 and 30 K. It was found that the coronene addition significantly increases the high-field critical current density of MgB2. The Jc values were obtained as 4.6 × 103 Acm−2 and 1.3 × 104 Acm−2 for pure and 4 wt % coronene added samples at 20 K and 4 T. In addition, the levitation force measurements show that 4 wt % coronene adding is very effective in increasing both the vertical and lateral levitation force performances at 20 K. The maximum levitation force for 4 wt % coronene added sample corresponds to 7.58 N/g whereas the reference sample shows 6.73 N/g at 20 K in ZFC regime. The results point out that the hydrocarbon of C24H12 is an effective carbon-containing additive for MgB2 and can be useful for optimizing the levitation performance of MgB2 superconductors for potential applications.

Enhancement of Critical Current Density of MgB2 by Glutaric Acid Doping: a Simultaneous Improvement on the Intrinsic and Extrinsic Properties

In this study, we report an enhancement of critical current density of bulk MgB2 superconductors by glutaric acid (C5H8O4) doping. The effects of glutaric acid doping on MgB2 lattice resulted in a record self-field J c of the order of 106 A/cm2. A simultaneous improvement in the connectivity, pinning force, and H c2 is the major factor that determined excellent J c performance. X-ray diffraction analysis showed that samples were single-phase MgB2 with a minor trace of impurities. A dramatic change in grain morphology and homogeneity in grain distribution was found in the SEM images of doped samples. We observed that homogeneity in grain distribution played a crucial role in the connectivity and the upper critical field (H c2) of the doped samples. We were able to introduce a new dopant through a two-step mixing approach which is suitable to overcome the degradation of low field and self-field J c reported for carbon-doped MgB2 superconductor samples.

The investigation on the regional nanoparticle Ag doping into MgTi0.06B2 bulk for improvement the magnetic levitation force and the bulk critical current

Fabrication methods are important way to improve structural and superconducting properties of MgB2 such as critical current, magnetic levitation force (MLF) and magnetic field trapping capability. Although the graded fabrication technique has been used for single-grain bulk YBCO superconductor to improve critical current and bulk superconducting properties, similar technique as regional doping has not been used for bulk MgB2, until now. In this study, nanoparticle silver doping was carried out in to the bulk MgTi0.06B2 superconductor by using in-situ solid state reaction and partial graded (regional) doping method together, to improve the radius independent uniform bulk current density and the magnetic levitation force as well as the structural properties of the MgB2 bulk superconductors. Both the Jc(0) self-field critical current and Fp (μ0H) pinning force density values enhanced in comparison with the inner region values, when the nanoparticle Ag doping is carried out in to the outer section of the sample. Addition to the enhancement of the structural and the micro electromagnetic properties as Jc(0) and Fp (μ0H), our study also focused on the improved of the bulk Jc and the radius of shielding current loop r, to improve bulk electromagnetic properties as the levitation force. It is seen that the structural properties enhanced and both the vertical levitation force and the lateral guidance force value increased with Ag doping to the outer section of MgTi0.06B2 sample. On the other hand, the increasing ratio of the lateral guidance force of 19.7% and the vertical levitation force of 10.8% of the sample with 3 wt% Ag-doped than the undoped one points out that the regional doping method to the outer section is very suitable for guidance force applications, which is important in the magnetic bearing application such as Maglev and magnetic energy storage systems.

Solution-processed nanometers thick amorphous carbon-coated boron as an efficient precursor for high-field performance of MgB2

We report the fabrication of high-performance MgB2 bulk superconductors doped with amorphous carbon (aC), derived from PMMA polymer, a safe and cost effective carbon source as compared to explosive gaseous and expensive C-containing nanoadditives. The commonly used C-containing nanoadditives and boron (B) precursor nanopowders are usually prone to agglomeration in solid state mixing which causes poor reactivity and non-uniform distribution that leads to deterioration of critical current properties of MgB2. We have overcome this problem by achieving uniform 3–4 nm thin coating of aC on B nanopowders by pyrolysis of PMMA through solution process. Compared to undoped MgB2, significantly high current-carrying capability (Jc (5 K, 8 T) ∼3.1 × 104 A/cm2 and Jc (20 K, 6 T) > 103 A/cm2) was obtained for aC-doped MgB2, which is comparable to the record high in-field Jc reported for SiC-doped MgB2 and much better than those reported in literature with other C sources. Furthermore, significant improvement in Hirr and Hc2 were also observed for aC-doped MgB2. The improved high-field properties in PMMA-derived aC-doped MgB2 bulk suggest that the solution process could be a powerful technique to obtain uniform mixture of C-coated B to develop high-performance Mg(B1−xCx)2 wires for practical applications.