Pure MgB2 Research Articles

Effect of combined addition of graphene oxide and citric acid on superconducting properties of MgB2

In the present work, polycrystalline samples with compositions MgB2+3wt% GO+xwt% C6H8O7 (x=0, 5 and 10) have been synthesized to study the effect of combined addition of graphene oxide (GO) and citric acid (C6H8O7) on superconducting properties of MgB2. X-ray diffraction studies show the formation of hexagonal crystal structure of MgB2 with space group P6/mmm in all synthesized samples. We observe that the addition of GO in the sample improves the grain connectivity and consequently enhances the critical current density significantly with no substantial change in Tc. However for this sample, there is no significant improvement in Hc2 and Hirr. With the combined addition of GO and citric acid, the JC(H), Hc2 and Hirr are observed to improve substantially as compared to the pristine MgB2 and GO added MgB2 samples. For example JC(10K, 5 T) of sample x=10 has improved by a factor of ∼15 as compared to pure MgB2 sample and by a factor of ∼5.5 as compared to the x=0 sample. Furthermore, Hc2(0) for x=10 sample has enhanced by 13 T as compared to pure MgB2 while it has increased by ∼10 T in comparison to x=0 sample. Enhanced flux pinning has been observed with the combined addition of GO and citric acid.

Effect of field cooling heights on the levitation force of pure and starch/polystyrene/MWCNT added bulk MgB2 superconductors

A series of MgB2 pellets with and without addition of carbon from different sources (viz. starch, polystyrene and carbon nanotubes) have been synthesized by solid state reaction under argon atmosphere. XRD analysis indicates a decrease in lattice parameters of MgB2 with addition of starch, polystyrene (PS) and MWCNT and confirms substitution of carbon in boron sites. The presence of nanosized carbon inclusions between the grain boundaries in the present set of samples is evident in TEM photographs. Resistivity data confirms a decrease in superconducting transition temperature (Tc0) for MgB2 doped with starch/PS/MWCNT. The effect of different field cooling heights (HIFC) at 20K on maximum levitation force (FMLF) and maximum attractive force (FMAF) of pure MgB2 and MgB2 doped with starch/PS/MWCNT have been investigated. Except for MWCNT, doping of starch and PS in MgB2 is found to improve FMLF and FMAF and the best result is obtained for MgB2 doped with 1wt.% PS. Levitation force measured as a function of decreasing initial field cooling height indicates exponential dependence of both maximum levitation force (FMLF) and maximum attractive force (FMAF). However, the gap distance between PM and the sample (H0AF and HMAF) corresponding to maximum attractive force (FMAF) and zero attractive force (F0AF) varies linearly and their difference remains constant. This constancy in (HMAF−H0AF) is understood in terms of constant reduction rate of magnetic flux density between H0AF and HMAF.

Microstructure of MgB2 superconducting wire prepared by internal magnesium diffusion process

We have analyzed the microstructure of dense pore-free undoped MgB2 single filament wire made by internal magnesium diffusion process (IMD). The superconducting layer consists of nearly pure MgB2 matrix with small volume fraction of nano-sized Mg and MgO precipitates and some of amorphous or nanocrystalline B-rich grains. Content of oxygen in IMD wire was significantly suppressed in comparison to PIT technology. Thin Mg layers were observed on plate-shaped MgB2 grain boundaries. Electrical measurements have shown high critical current densities in MgB2 layer, which are promising for further improvements and also for possible applications.

Effect of (Bi, Pb)-2223 addition on thermal transport of superconducting MgB2 pellets

Polycrystalline MgB2 pellets containing 0, 3, 7, and 10wt.% of (Bi, Pb)-2223 {Bi1.8Pb0.26Sr2Ca2Cu3O10+x} superconducting powder has been synthesized by solid state reaction. Micro-structural observations confirm a well developed MgB2 phase with randomly oriented hexagonal grains. XRD data shows absence of substitution effect and presence of unreacted 2223 as isolated grains or, clusters is indicated in FESEM. In addition, transition temperature (Tc) of MgB2 remains practically unaffected with 2223 addition in MgB2. Temperature dependence of thermal conductivity of the present set of samples does not show any anomalous features at/below Tc. However, the magnitude of thermal conductivity decreases substantially with addition of 2223. Ideal Wiedemann–Franz’s law fails to provide acceptable estimation of the electronic part of the total thermal conductivity. Two band analysis of the electronic thermal conductivity in superconducting state reveals dominance of π band contribution. Analysis also confirms that addition of 2223 in MgB2 introduce extra defects/disorder that cause a substantial decrease in π band gap, as well as, intraband relaxation time of pure MgB2. On the other hand, σ band gap of MgB2 is not much affected due to addition of 2223. Lattice thermal conductivity analyzed using Callaway’s expression indicates that it is mainly governed by phonon scattering from boundary, point defects and sheet-like faults. However, dominant role is played by phonon scattering from sheet like faults, which decides the lattice thermal conductivity of both pure and 2223 added MgB2.

The role of starch doping on the superconducting properties of MgB2

The effect of different amount of starch addition on structural and superconducting properties of superconducting MgB2 has been discussed. The samples are synthesized by conventional solid reaction method. XRD analysis confirms that carbon, which is produced during synthesis due to decomposition of starch, is substituted in the B sites. High resolution transmission electron microscope (HRTEM) picture infers the presence of large number of nanosized precipitates (size ∼10–20nm) in starch doped MgB2 pellets. Superconducting transition temperature (Tc0) of MgB2 (∼38K) decreases due to the addition of starch. The critical current density (Jc) of starch added MgB2 samples, however, shows significant improvement in whole field range, especially at high magnetic fields. MgB2 added with 2wt.% of starch gives the best performance amongst the investigated samples and at 20K displays ∼42 times enhancement in Jc under 4T field compared to that for pure MgB2. The field dependence of the critical current density (Jc) of starch added MgB2 is explained fairly well in terms of collective pinning theory. An excellent scaling of the reduced critical current density, Jn(=Jc/Jc(0)) and the reduced field hn (=H/H0) is observed for both pure and starch added MgB2 polycrystalline pellets at different temperatures, where Jc(0) and H0 are the fitting parameters obtained from collective pinning model. However, the normalized pinning force density (FP/Fp(max)) of starch added samples does not display any scaling, but shows an excellent correspondence with modified Dew–Hughes expression. The presence of anisotropy and grain orientation is thought to be responsible for the absence of scaling of normalized pinning force density in starch added MgB2 pellets.

Role of MgO impurity on the superconducting properties of MgB2

We address the effect of MgO impurity on the superconducting properties of MgB2. The synthesis of MgB2 is very crucial because of sensitivity of Mg to oxidation which may lead to MgO as a secondary phase. Rietveld refinement was performed to determine the quantitative volume fraction of MgO in the samples synthesized by two different techniques. Both the samples were subjected to magnetization measurements under dc and a.c. applied magnetic fields and the observed results were compared as a function of temperature. Paramagnetic Meissner effect has been observed in a sample of MgB2 having more amount of MgO (with Tc=37.1K) whereas the pure sample MgB2 having minor quantity of MgO shows diamagnetic Meissner effect with Tc=38.8K. M–H measurements at 10K reveal a slight difference in irreversibility field which is due to MgO impurity along with wide transition observed from ac magnetic susceptibility measurements. The magnetotransport measurements ρ(T) using ρN=90%, 50% and 10% criterion on pure sample of MgB2 has been used to determine the upper critical field whereas the sample having large quantity of MgO does not allow these measurements due to its high resistance.

Thermal Conductivity of n-Aluminium Nitride-Added MgB2 Superconductor in Normal and Superconducting State

Thermal conductivity (k) of 0, 5, 10, and 15 wt.% aluminium nitride (n-AlN)-added polycrystalline MgB 2 superconductors, synthesized by solid reaction is discussed both in the normal and superconducting states between 20 and 300 K. The prepared samples are characterized using X-ray diffraction (XRD) and field electron gun scanning electron microscope (FEG–SEM). Resistivity measurement confirms a decrease in superconducting transition temperature of MgB 2 (T c=38.5 K) with n-AlN addition and decreases to ∼35 K in case of 15 wt.% n-AlN-added MgB 2 sample. Thermal conductivity of both MgB 2 and n-AlN-added MgB 2 pellets does not show any hump around T c, and the absolute values of k decrease with increasing n-AlN in MgB 2. Temperature dependence of the thermal conductivity of MgB 2 and n-AlN-added MgB 2 has been analyzed, assuming the role of both electrons and phonons. The Wiedemann–Franz law does not work well for the present samples, which indicates inelastic scattering (L eff < L 0). Thermal conductivity of MgB 2 and n-AlN-added MgB 2 pellets is explained by assuming effective Lorentz number, L eff= 0.1 L 0. Electronic thermal conductivity in superconducting state ( $k_{\text {el}}^{\mathrm {s}} )$ follows “two-gap model” and has been used to estimate the values of band gaps, relative contribution of each band in thermal transport, and intraband scattering relaxation time. The estimated values are fairly consistent with the previously reported results for MgB 2. We further confirm that n-AlN addition in MgB 2 introduces disorders in π bands, which reduce the π band gapsand intraband relaxation time ( $\tau _{\pi }^{\text {im}})$ . The lattice contribution of thermal conductivity in both normal and superconducting states is analyzed in the terms of Callaway’s model, assuming various phonon scatterings. Our analysis indicates that the lattice thermal conductivity of MgB 2 is dominated by phonon-sheet-like fault scattering. Addition of n-AlN in MgB 2 enhances the phonon scattering from sheet-like faults, and dislocations induced strain field scattering by >7 times compared to that for pure MgB 2 pellets.

Improvement of Current-Carrying Capacity and In Situ Control of the Superconducting Fraction of MgB2-MgO Composites

Superconducting magnesium diboride-magnesium oxide (MgB 2-MgO) composite attracts our attention due to its relatively high superconducting transition temperature close to the one of a pure MgB 2 bulk and metallic transport behavior with low resistivity in the normal state even at a high fraction of insulating MgO. In this paper, we report that the current-carrying capacities of MgB 2-MgO composites were enhanced by improving the replacement reaction process, and the superconducting MgB 2 fraction can be in situ controlled by using magnesium (Mg), boron trioxide (B 2O3), and boron (B) as the raw materials. The composites with mass fractions of superconducting MgB 2 from 27 to 80 % show that the zero-resistance transition temperatures are above 36.7 K, and the self-field critical current densities are from 0.2 ×106 to 4 ×106 A/cm 2 at 10 K. The high JcS of the superconducting composites under the applied fields prove that there is no appreciable difference between a perfect MgB 2 sample and one with MgO doping, but the critical current density and flux pinning are improved. These results are meaningful for studying further the flux-pinning mechanism in MgB 2.

Determination of the electron-phonon coupling constants from the experimental temperature dependences of superconducting gaps in MgB2

Experimental temperature dependences Δσ, π(T) of the energy of superconducting gaps for MgB2 samples with the critical temperatures 22 K < T c < 41 K have been fitted by selecting the renormalized electron-phonon coupling constants λ ij with the use of the Moskalenko-Shul system of equations, the expression for the frequency of collective plasma oscillations obtained by Leggett for two-gap superconductors, and two fitting parameters. We previously obtained the dependences Δσ, π(T) by the multiple Andreev reflection spectroscopy of superconductor-constriction-superconductor junctions based on MgB2 with various degree of disorder of the crystal structure. It has been shown that the intraband pairing constants are decisive for the superconductivity mechanism in MgB2; in this case, √V σσ V ππ/V σπ = 8–22 and the ratio of the interband constants α can range from 3 to 11. The set of the Eliashberg coupling constants λ 0 has been qualitatively determined for relatively pure MgB2 with maximum values T c ≈ 40 K. The leading constant is 0.7 < λ σσ 0 ≈ λ eff 0 < 0.9 and depends on the choice of the upper integration limit in the Bardeen-Cooper-Schrieffer (BCS) model and the effective Coulomb repulsion μ iff * . The characteristic ratio for the gap in the σ band is 2Δσ/k B T c = 5.0–6.5.

Critical current density of MgB2 superconductor with (Bi,Pb)-2223 addition

Polycrystalline MgB2 pellets with and without addition of superconducting Bi1.8Pb0.26Sr2Ca2Cu3O10+x powder has been synthesized by solid reaction. The effect of (Bi,Pb)-2223 addition on structural and critical current density of MgB2 superconductor has been investigated. XRD data reveals no substitution for Mg or B. The superconducting transition temperature (Tc0) of MgB2 (∼37.97K) remains almost invariant with 2223 addition. Temperature dependence of the normal state electrical resistivity of 2223 added MgB2 is well explained by modified Rowell’s model and it suggests that 2223 addition in MgB2 marginally influences the percentage connectivity and intragrain residual resistivity. Critical current density (Jc) has been estimated at 4, 10, 20 and 30K from M–H curves using Bean’s model. MgB2 sample with 1wt.% of 2223 addition gives the best performance and display nearly two times enhancement in Jc compared to that of pure MgB2 in the field range between 0 and ±6T. Our analysis confirms that the observed magnetic field dependence of Jc of the present system is explained well in terms of the collective pinning model. Using the best fitted parameters (Jc (0) and H0) of collective pinning model, an excellent scaling is established between the reduced critical current density, Jn (=Jc/Jc (0)) and the reduced field hn (=H/H0) at different temperatures for both pure and 2223 added MgB2 polycrystalline pellets. It is also confirmed that δTc type pinning dominates in pure MgB2 and for 2223 added MgB2 major contribution originates from δl type pinning at lower temperatures.

Effect of Co addition on the critical current density of MgB2 superconductor

In this study, the (MgB2)1−xCox samples, where x=0.0, 0.1, 0.3, 0.6 and 0.8, were prepared by using the solid-state reaction technique. Effect of the Co-addition on the structural, magnetic, critical current density, Jc, and flux pinning properties of the superconducting MgB2 system was investigated. A magnetic transition was observed with increasing the Co-concentration in MgB2. For the pure MgB2, Jc was obtained to be 2.78×104Acm−2 at 5K and 1.7kOe, but Jc was suppressed by addition of Co to the system. It was found that a large amount of Co in MgB2 did not act as pinning centers which improves Jc. The magnetic behavior observed by the Co-addition originates from the magnetic properties of cobalt itself.

Enhancement of Critical Current Density in MgB2 Bulk with CNT-coated Al Addition

Bulk MgB2 sample with carbon nanotube (CNT)-coated Al addition was prepared by conventional solid-state reaction at 900 °C for 30 min. We investigated the effects of Al and C co-doping on the lattice parameter, the microstructure, and the critical current density of MgB2. The substitution of Al and C atoms for the sites of Mg and B in the MgB2 lattice resulted in dislocations in the MgB2 grains, which makes great contributions, along with the nanoscale oxide particles, to the enhancement of critical current density at high field (103 A cm−2, 7 T, 5 K) in the co-doped sample. These results contrasted significantly with the measured values of the pure MgB2 and Al- and C-doped samples. Co-doping introduced more electrons into MgB2 and decreased both the parameters c and a of MgB2 lattice, and the used coating technique delayed and shortened the oxidation process of Mg and Al, leading to the decrease in the size and the content of the oxide. These advantages should be responsible for the enhancement of the critical current density as well.

Effect of Nano-AlN Addition on the Critical Current Density of MgB2 Superconductors

A series of MgB2 polycrystalline pellets with and without aluminum nitride (nano)addition have been synthesized by a solid-state reaction. No detectable shift in the XRD peak position of MgB2 has been observed with AlN nanoparticles addition indicating no substitution for Mg or B. Surface morphology displays randomly oriented well-defined grains. The presence of AlN nanoparticles between the grain boundaries and also on the grain surface is observed in AlN-added MgB2. Addition of AlN nanoparticles decreases the superconducting transition temperatures from 38.5 to 37 K. Resistivity data confirm an improved connectivity of the grains in MgB2 pellets for low-level addition of AlN nanoparticles. Magnetization data measured at various temperatures between ±6 T are used to estimate the critical current density (Jc) of undoped and AlN nanoparticles doped MgB2 pellets. Jc enhances significantly up to 4 T at 20 K for MgB2 containing 0.5 wt.% AlN. Our analysis confirms excellent correspondence of the measured field dependence of critical current density (Jc) in terms of collective pinning model. A new universal scaling behavior is established between the reduced critical current density, Jn(=Jc/Jc(0)) and the reduced field hn(=H/H0) for both undoped and AlN-doped MgB2 pellets at different temperatures, where Jc(0) and H0 are the best fit parameters obtained from the collective pinning model. Our analysis further confirms the dominance of δTc-type pinning for pure MgB2, and for n-AlN-added MgB2 pellets at low temperatures the major contribution comes from δl-type pinning.

Magnetic hysteresis behavior and magnetic pinning in a d ferromagnet/superconductor nanostructure

We investigate the interaction between superconductivity and defect-induced d0 ferromagnetism using a composite consisting of MgB2 and MgO nanocrystals. The composite exhibits a ferromagnetic hysteresis behavior in the temperature region from 40 to 300 K. Defective MgO nanocrystals (∼20 nm) embedded in the composite are considered to be responsible for the observed ferromagnetism. The zero field cool and field cool magnetization curves show that the superconducting transition occurs at Tc = 38.6 K, in agreement with Tc of pure MgB2. In the temperature region from Tc to 0.9Tc (∼35 K), the magnetization hysteresis curves show a superposition of ferromagnetic (F) and superconducting (S) signals. When the temperature of the system is decreased below 0.65Tc (∼25 K), the S signals dominate over the F signals. The resulting magnetic hysteresis loops are highly asymmetric and the descending filed branch is nearly flat, as predicted in the case of surface pinning. At temperatures below 0.5Tc (∼20 K), a sharp peak is developed near zero field in the magnetization hysteresis curves, implying an enhancement of superconducting vortex pinning. The observed pinning enhancement most likely results from magnetic pinning due to randomly distributed magnetic MgO grains, which yield the magnetic inhomogeneity and the related pinning potential in a length scale of ∼100 nm. Thus, the present ferromagnetic/superconducting composite provides an ideal model system that demonstrates the availability of d0 ferromagnetism as a source of magnetic potential for effective vortex pinning.

Correlation between the Critical-current Anisotropy and Microstructure of ex-situ Processed MgB2 Tapes Using Carbon-substitution Treatment

Correlation between the critical current anisotropy and the textured microstructure of the ex-situ processed MgB2 tapes with the Fe sheath was studied to obtain basic data related to the commercial application of the ex-situ processed MgB2. Ex situ processed MgB2 tapes were fabricated using newly developed technique to substitute carbon for boron in the MgB2 compound through the ex situ route. These MgB2-xCx tapes showed comparable critical current anisotropy to that of pure MgB2 tapes, but in higher magnetic fields because of their higher irreversible critical field than the pure MgB2 tapes. The x-ray pole figure data show that the tape core texturing of the MgB2-xCx tapes was moderate, but it was a slightly higher degree than that of the pure MgB2 tapes. This result was consistent with results of the critical current anisotropy.

An innovative technique to synthesize C-doped MgB2 by using chitosan as the carbon source

Here, we report a new technique to synthesize carbon-doped MgB2 powder. Chitosan was innovatively used as the carbon source during the synthesis of boron from boron oxide. This allowed the introduction of local defects, which later on served as pinning centers in MgB2, in the boron lattice itself, avoiding the traditional and time consuming ways of ex situ MgB2 doping (e.g. ball milling). Two volume percentages of C-doping have been tried and its effect on the superconducting properties, evaluated by magnetic and transport measurements, are discussed here. Morphological analysis by scanning electron microscopy revealed nano-metric grains’ distribution in the boron and MgB2 powder. Mono-filamentary MgB2 wires have been fabricated by an ex situ powder-in-tube technique by using the thus prepared carbon-doped MgB2 and pure MgB2 powders. Transport property measurements on these wires were made and compared with MgB2 wire produced using commercial boron.

Pinning behavior of glycine-doped MgB2 bulks with excellent critical current density by Cu-activated low-temperature sintering

Abstract Mg–B system with single glycine addition was composed of MgO phase rather than MgB 2 when sintered at 600 °C for 2 to 5 h. Cu was further introduced into Mg–B–glycine system to active the low-temperature sintering efficiency, and more than 80% MgB 2 superconducting phase was successfully obtained in Cu and glycine co-doped samples sintered at 600 °C for 8 h, 12 h, and 15 h, respectively. The sample sintered for 8 h showed uncompetitive critical current density ( J c ) because of the uncompleted MgB 2 phase and the unreacted Mg phase, while the 15-h sample showed the best J c of 1.5 × 10 4 A cm −2 at 3 T and 20 K. The pinning behavior of MgO (∼20 nm) and MgCu 2 (∼10 nm), which should be responsible for the exciting current-carrying capability that the J c performance was improved over the entire field in contrast with pure MgB 2 , was systematically investigated. According to the phase identification and the microstructure observation, both the pre-generated MgO involved in the MgB 2 grains and the MgCu 2 precipitated along the steps of the screw dislocation growth mechanism could serve as effective pinning centers. Finally, we illustrated the whole sintering process of the sample sintered at 600 °C for 15 h.

Effects of MgO evolution on the critical current density in bulk MgB2 containing histidine

Glycine, an amino acid with the simplest composition, was introduced into MgB2 systems, making great contribution to the enhancement of critical current density in our previous study. Aiming at investigating the effects of histidine, another amino acid with more complicated structure and lower decomposition temperature than glycine, on the superconducting properties of MgB2, samples of MgB2+xwt.% histidine (with x=0, 2, 3, 5, and 8) were sintered at 800°C after mechanical mixing. The best critical current density was obtained in the sample with 2wt.% histidine addition, owing to the small-sized MgO and C substitution. The sample showed a significant increase in critical current density under high field compared with pure MgB2, and this property maintained at a relatively high level under low field as well. However, the growth and agglomeration of MgO with the increasing amount of histidine should be responsible for the tendency of the decrease in the connectivity and critical current density versus doping content. Finally, the conditions that the used amino acid should meet were investigated as a guide for effective amino acid doping.

A new scaling relation for n-AlN doped superconducting MgB2

The scaling behavior of nano-aluminum nitride added polycrystalline MgB2 superconductor is discussed. A series of polycrystalline MgB2 samples with different amounts of nanosized AlN addition are synthesized by solid reaction. All the synthesized pellets are subjected to x-ray diffraction, field emission gun scanning electron microscopy (FEG-SEM), and transmission electron microscopy (TEM) to examine their micro-structural features. A marginal decrease in lattice parameters of pure MgB2 with AlN nanoparticles addition is observed. Surface morphology reveals randomly oriented hexagonal MgB2 grains decorated with AlN nanoparticles between the grain boundaries and also scattered on the grain surface. For higher concentration, n-AlN agglomerates are visible. Resistivity data confirm a decrease in superconducting transition temperature (Tc) from 38.5 to 37 K and increase in transition width (ΔTc) with increased loading of n-AlN in MgB2. The critical current density (Jc) of the pellets at 4, 10, 20, and 30 K is evaluated from the magnetization data between ±6 T and is explained well in the framework of collective pinning model. The normalized pinning force density of n-AlN doped MgB2 at various temperatures indicates an excellent scaling with respect to Hn (the field corresponding to which Fp drops to half of its maximum value) as the scaling field. A new scaling expression derived, using the expression of field dependence of Jc proposed by “collective pinning model” in small bundle regime, demonstrates an excellent agreement with the measured normalized pinning force density (viz., Fp/Fpmax vs. hn) of the AlN nanoparticles doped MgB2 superconductors.

Vortex flux pinning mechanism and enhancement of in-field Jc in succinic acid doped MgB2

The field dependence of the resistivity and the critical current density, Jc(B), of MgB2 doped with 10wt% wet and dry succinic acid have been investigated by magnetic measurements. The dry succinic acid significantly enhanced the upper critical field, the irreversibility field, and the Jc(B) compared to the wet succinic acid doped MgB2 and the pure MgB2. The field dependence of Jc(B) was analyzed within the collective pinning model. The observed temperature dependence of the crossover field, Bsb(T), from the single vortex to the small vortex bundle pinning regime shows that flux pinning arising from variation in the critical temperature, δTc, is the dominant mechanism for the wet sample over the whole studied temperature range, while there is a competition between δTc pinning and the pinning from variation in the mean free path, δl, for the dry sample.