Bulk MgB2 Research Articles

MgB2 thick films on three-dimensional structures fabricated by HPCVD

Magnetic shielding has been a key factor in the measurement of ultra-weak magnetic fields, especially for shielding from low frequency electromagnetic noise. With the recent development of superconducting quantum interference devices, superconducting magnetic shielding has become an important area of research. MgB2 has shown great potential in magnetic shielding for its remarkable superconducting properties, the feasibility of its use in this capacity having been demonstrated by MgB2 bulk samples. However, the potential for application of such bulk samples is limited. In this work, we have investigated the possibility of the fabrication of MgB2 films on three-dimensional (3D) structures using a hybrid physical−chemical vapor deposition system. MgB2 films 10 μm thick have been fabricated on the outer surface of a polycrystalline Al2O3 cylinder. The deposited film showed a transition temperature (TC) of 39 K and JC of 5.1 × 105 A · cm−2, which are comparable to those of planar MgB2 films. This work shows the feasibility of depositing MgB2 films onto a 3D structure, and sheds light on the potential use of MgB2 films in superconducting magnetic shielding.

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.

Critical current density improvements in MgB2 superconducting bulk samples by K2CO3 additions

MgB2 bulk samples with potassium carbonate doping were made by means of reaction of elemental Mg and B powders mixed with various amounts of K2CO3. The Tc of the superconducting phase as well as its a-axis parameter were decreased as a result of carbon doping. Potassium escaped the samples during reaction. The critical current density of MgB2 was improved both in self field and under applied magnetic field for T ≤ 30 K, with optimum results for 1 mol% K2CO3 addition. The normalized flux pinning force (f(b)) shows that the flux pinning mechanism at low field is similar for all samples, following the predictions of the point pinning model. In contrast the behavior of f(b) is significantly altered at reduced fields (b) larger than unity by K2CO3 additions, tending towards surface pinning. Besides providing carbon, another effect of K2CO3 may originate from the presence of a transient liquid phase that appears to improve the crystallinity and thus the critical current density at low field.

The effect of graphene coated Si, Ti and Nb additions on the superconducting properties of MgB2 bulks

The effect of graphene (G) coated Si, Ti and Nb additions on the superconducting properties of MgB2 was systematically studied. The additives were prepared by a coating process and all introduced powders were scattered and combined with graphene. The coating process was useful for the uniform distribution of additives in MgB2 samples. The additives could effectively make the refinement of MgB2 grain size, which achieved an enhancement of surface pinning. Meanwhile, the densities of all the doped samples were larger than those of pure samples, which ensured a better intergrain connection for supercurrent. Therefore, the critical current density of MgB2 with 8 wt% G coated Nb addition got a significant enhancement due to the improvement of grain connectivity and flux pinning properties.

Studies on the Composite System of Bi-2212 Glass Ceramic and MgB2 Superconductors

The composites of glass ceramic Bi-2212 and MgB2 superconductors were prepared at ambient conditions. The transmission electron microscopy images of the composite samples illustrate the presence of glass ceramic inclusions in bulk MgB2. Temperature-dependent magnetization of the composite samples shows two superconducting transitions: one at 80 K corresponding to the Bi-2212 phase and a second one at 39 K corresponding to the MgB2 phase, suggesting that the two superconducting phases are separated with clear boundaries. The critical current density (J c) and pinning force values are increased in composite systems by an order of magnitude compared to that of individual samples. The pinning mechanism in the composite sample is the same as in the matrix phase. Reduced field maxima (h max) are observed at 0.15 for composite samples. A low value of h max for composite samples indicates the random orientation of grain boundaries and repulsive pinning force in the composite samples.

Influence of oxygen doping on critical fields in MgB2 bulk superconductors

In this work we studied the influence of SnO2 doping on the critical fields and temperatures of MgB2 bulk samples. Bulk samples were made by mixing ex-situ MgB2 powder with 5 wt% SnO2 powder and then pressing the mixed powders into pellets using a pressure of 2000 psi. The bulk pellets were sintered at 900°C in a furnace under flowing Ar. The samples were quenched to room temperature after dwelling at 900°C for 5h. XRD, SEM, and magnetic measurements were made on doped and control samples. XRD showed a decomposition of the SnO2 and very slight reductions in the a-axis and c-axis lattice parameters of the MgB2 phase. M-T (Magnetization-Temperature) curves showed a decrease in Bc2 of approximately 1 T in the temperature range of 24 K - 39 K with SnO2 additions as compared to the control samples.

A bulk superconducting MgB[formula omitted] cylinder for holding transversely polarized targets

An innovative solution is being pursued for the challenging magnetic problem of producing an internal transverse field around a polarized target, while shielding out an external longitudinal field from a detector. A hollow bulk superconductor can trap a transverse field that is present when cooled through its transition temperature, and also shield its interior from any subsequent field changes. A feasibility study with a prototype bulk MgB2 superconducting cylinder is described. Promising measurements taken of the interior field retention and exterior field exclusion, together with the corresponding long-term stability performance, are reported. In the context of an electron scattering experiment, such a solution minimizes beam deflection and the energy loss of reaction products, while also eliminating the heat load to the target cryostat from current leads that would be used with conventional electromagnets.

Evolution of multigap superconductivity in the atomically thin limit: Strain-enhanced three-gap superconductivity in monolayer MgB2

Starting from first principles, we show the formation and evolution of superconducting gaps in MgB$_2$ at its ultrathin limit. Atomically thin MgB$_2$ is distinctly different from bulk MgB$_2$ in that surface states become comparable in electronic density to the bulk-like $\sigma$- and $\pi$-bands. Combining the ab initio electron-phonon coupling with the anisotropic Eliashberg equations, we show that monolayer MgB$_2$ develops three distinct superconducting gaps, on completely separate parts of the Fermi surface due to the emergent surface contribution. These gaps hybridize nontrivially with every extra monolayer added to the film, owing to the opening of additional coupling channels. Furthermore, we reveal that the three-gap superconductivity in monolayer MgB$_2$ is robust over the entire temperature range that stretches up to a considerably high critical temperature of 20 K. The latter can be boosted to $>$50 K under biaxial tensile strain of $\sim$ 4\%, which is an enhancement stronger than in any other graphene-related superconductor known to date.

Mechanism of the formation of structure during high-temperature annealing of MgB2 bulk samples deformed under pressure

MgB2 superconductors subjected to deformation under pressure on Bridgeman anvils and subsequent annealings at 800 and 950°C were investigated by X-ray diffraction, scanning electron microscopy, and microanalysis. It was shown that at these temperatures crystals of MgB2 phase along the residual boron dissolute in the residual liquid magnesium with the formation of both dense and loose areas of MgB2 phase. The latter has a negative influence on critical current. At the same time, after these treatments, the samples become more uniform by the phase and chemical composition.

Publisher’s Note: “Enhanced electron-phonon coupling and critical current density in rapid thermally quenched MgB2 bulk samples” [AIP Advances 7, 085014 (2017)

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Improved Superconducting Properties in Graphene-Doped MgB2 Prepared by Coating Method

The effects of graphene doping on the phase formation and superconductivity of MgB2 bulks synthesized with different process have been studied systemically. Considering the scattering structure of graphene, coating method was applied to enhance the uniformity of graphene doping. The graphene coated B addition was expected to improve the critical current density of MgB2 bulks. In our study, several experiments were performed to find out the suitable way for graphene doping. The coating method could enhance the critical current density of MgB2 from 1.9 × 105 to 2.5 × 105 A/cm2 at 20 K and 0 T, compared with that of the undoped sample. And the superconductivity of MgB2 prepared by coating method got obvious improvement at high field compared with that of pure graphene doping bulk. It can be concluded that the coating method could ensure the uniformity of graphene doping in MgB2 and refined the grain crystalline effectively.

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 grain connectivity and critical current density in the ex-situ sintered MgB2 superconductors by doping minor Cu

The influence of Cu addition on the microstructure and superconducting performance of ex-situ sintered MgB2 is systemically studied. It is found that the critical current density (Jc) of the Cu-doped sample is improved compared to the un-doped samples. In particular, the Jc of the Cu-doped sample sintered at 900 °C for only 10 min is the best value at low fields. The reason is that Cu addition can obviously promote the decomposition of MgB2: MgB2↔MgB4+Mg, which produces more Mg that can then react with Cu, forming local Mg-Cu liquid at high temperature. The presence of this local Mg-Cu liquid can significantly enhance the migration and self-sintering of MgB2, leading to the significant improvement in the grain connectivity and Jc. Our results indicate that Cu addition is a promising method to fabricate high-performance ex-situ MgB2 bulks and wires.

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.

Optimization of vortex pinning at grain boundaries on ex-situ MgB2 bulks synthesized by spark plasma sintering

Grain boundaries are well known to be the predominant pinning centers in MgB2 superconductors. To study the effects of grain boundaries on the trapped field properties of MgB2 bulk, we prepared MgB2 bulks by a spark plasma sintering method using a ball-milled starting powder. The trapped field was maximized for the bulk made from the ball-milled powder with crystallite size, τ, of 27 nm; the highest trapped field, , of 2.3 T achieved at 19.3 K was 1.2 times larger than that of the bulk made from the non ball-milled powder (τ = 50 nm). The degradation of the trapped field for the bulk from finer powder (τ = 6 nm) originated mainly from the lowered . The critical current density, , and the pinning force density, , were also maximized for the bulk from τ = 27 nm. The competition between the increase of the numerical density of grain boundaries and the degradation of superconductivity determined the vortex pinning properties for the MgB2 bulks with mechanically refined grains. The scaling analysis for the pinning force density suggested that the change in the dimension of the dominant pinning source from 2D (surface) to 0D (point) was induced by grain refining. Although the nanometric impurity particles such as MgB4, MgO and Mg-B-O were created in the bulk during both ball-milling and spark plasma sintering processes, we considered the point-contact between the refined grains was the predominant point pinning source.

Enhanced electron-phonon coupling and critical current density in rapid thermally quenched MgB2 bulk samples

We report Rapid Thermal Quenching (RTQ) studies on MgB2 samples from optimized sintering temperature of 800 °C down to liquid nitrogen temperature with different sintering duration. Superior electron-phonon coupling strength (λe−E2g), critical current density (Jc) and irreversibility fields (Hirr) compared to doped MgB2 were observed without compromising transition temperature Tc. Structural studies showed a contraction of the unit cell due to thermal stress induced by RTQ. Enhanced λe−E2g evaluated from line width, and phonon frequency of Raman spectra using Allen equation was consistent with structural and magnetic studies. Microstructural analysis showed a decrease in grain size resulting in increased Jc and Hirr.

Improved Superconducting Properties by Graphene Doping of Bulk MgB2 Prepared by the Diffusion Method

A study of the effects of graphene doping on the superconducting properties of MgB2 bulks prepared by the diffusion method has been conducted over a range of doping levels of 0–30 wt%. The increase in a large quantity of a C-containing nonsuperconducting phase together with the effects of C-substitution on the phase itself results in a change of superconducting properties. While the transition temperature Tc monotonically decreases with increasing graphene addition, the critical current density Jc is increased for small graphene additions due to the larger volume fraction of the superconducting phase. Larger additions resulted in larger quantities of the nonsuperconducting phase and consequently these samples have a lower Jc .

Magnesium diboride: An effective light-to-heat conversion material in solid-state

We report herein a giant light-to-heat transduction observed in bulk magnesium diboride (MgB2) on exposure to incoherent, continuous wave, broadband light-sources. The heat-flow from the sample was accurately determined using photocalorimetry, to register an ultrahigh value of 45 W/g upon irradiation with a low-pressure mercury vapour lamp (λ ∼ 250–450 nm; irradiance ∼800 mW/cm2) with a photo-to-thermal conversion efficiency of about 83% (with an error of ±1.86%). As a practical application, light-induced heat was used to generate electrical power using a thermoelectric generator (open voltage ∼125 mV, when illuminated by a solar simulator). The bulk MgB2 powder was found to be photo-stable under different irradiation conditions, and the photothermal effect was found to be highly reproducible.

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.