Pure MgB2 Research Articles

Comparison of carbon-doped MgB2 bulks fabricated from pre-synthesized Mg/CNT and Mg/amorphous carbon composites

MgB2 samples containing 2.5 wt% multi-walled carbon nanotubes (CNTs) and amorphous carbon (nano-C) were sintered at 800 °C. The doped samples were prepared from Mg/CNT and Mg/nano-C composites, which were previously synthesized from coarse Mg and fine Mg by chemical vapor deposition, respectively. We made comparisons between the two samples on the C-doping level, the MgO contents, and the effects of the carbon source on critical current density (Jc) by investigating the phase composition, microstructure, and magnetic properties. Compared with pure MgB2 (coarse Mg), the Jc performance at high field was improved in the CNT-doped sample (coarse Mg) due to the high C substitution level induced lattice defects as pinning centers. However, Jc values of the amorphous C-doped sample with sufficient C doping decreased over the entire field in contrast with the pure sample (fine Mg), which exhibited the highest Jc performance among the samples. This is attributed to the much more MgO impurities, mostly generated during the preparation of Mg/C composites, than the CNT-doped sample.

Effect of SWCNT Dilution on the Resistivity of MgB2

AbstractWe report an increase in superconducting temperature of magnesium diboride (MgB2) by minute single-wall carbon nanotubes (SWCNT) inclusions. The SWCNTs concentration was varied from 0.1wt% to 1.0wt%. The temperature dependence resistivity of sintered MgB2- SWCNTs composites containing 0.1wt%, 0.5wt% and 1.0wt% were measured and compared with that of the pure MgB2. The superconducting critical temperature (Tc) of the MgB2 increased from 40 K to as high as 42.4 K for the MgB2 containing 0.5wt% of SWCNTs. The room temperature resistivity ratio (RRR) shows dependence on the sample composition. The temperature width (ΔT) decreases with increasing SWCNT content from 0.1wt% to 1.0wt%. The normal state resistivity data were fitted with the generalized Block-Grüneisen function obtaining a Debye temperature of ∼ 900K.

The avalanche process in gold covered MgB2 films

Below a certain threshold temperature, the critical state of superconducting thin films can become unstable, which results in large magnetic flux jumps, called flux avalanches. The phenomenon of magnetic flux avalanches is strongly connected to the thermal and electrical conductivity in the material. Enhancing both by metallic cover layers on the superconducting film can suppress the magnetic avalanches. It was found that it is feasible to subdivide the avalanche process into a formation and a propagation step. In this work we want to address the question regarding the stage at which the metallic cover layer influences the avalanche mechanism. The investigations are carried out on thin MgB2 films with an inhomogeneous current density distribution, where both the formation and propagation of magnetic flux avalanches are highly supported. Evaporating a gold cover layer on top of the inhomogeneous MgB2 films leads to a suppression of the instabilities in this case. Magnetization measurements and magneto-optical imaging are used to observe the instabilities in pure MgB2 films and MgB2 films covered with gold layers. We investigate how the two steps of the avalanche process, nucleation and propagation, are influenced by the additional gold layer. We find a particular influence on the initial phase of the avalanche formation.

In situ synthesis and current-carrying characteristics of superconducting MgB2–B4C composites with MgB2 fractions ranging from 18% to 85%

Carbon is known to be the most effective dopant to enhance the high-field current-carrying capacity for high-power applications of MgB2 superconductors. Using B4C instead of B to synthesize MgB2 will be expected to produce superconducting MgB2 together with in situ carbon doping. By using just two reaction substances, B4C and Mg, superconducting MgB2–B4C composites with MgB2 fractions ranging from 18 to 85% were fabricated successfully by solid-state reaction. The influence of the grain size of the B4C raw material on the critical current density (Jc), the fraction of MgB2, and the critical transition temperature (Tc) of MgB2–B4C composites were investigated. Tc, Jc, and the fraction of MgB2 were increased with decrease in the grain size of the B4C raw material. Compared with a pure MgB2 superconductor, the critical transition temperature of the MgB2 composites fabricated with small B4C grain size was suppressed slightly, but the in-field current-carrying capacity was improved significantly. By using B4C raw material with a grain size of 2.5 μm, the resultant superconducting composite showed a critical transition temperature of 33.0 K, a high self-field Jc of 0.7 × 106 A cm−2, and a remarkable in-field current density value of 1 × 104 A cm−2 under 6 T at 10 K. These results indicate that the composites can be applied directly in high-power applications of superconductors. The enhancement of flux pinning in the superconducting MgB2–B4C composite is discussed and considered to be mainly due to the increment in the effective surface area of grain boundaries per unit volume.

Use of preceramic polymers for magnesium diboride composites

We used preceramic polysiloxane polymers to fabricate superconducting MgB2 composites that are doped with carbon and nanosized inclusions to improve the pinning properties. The polysiloxanes were prepared by atom transfer radical polymerization and the composites were fabricated by the short time spark plasma sintering method. We found that the superconducting critical temperatures were higher than expected from the carbon content found from the X-ray diffraction analysis of the (110) peak of MgB2. To explain this finding we propose that the grains are unevenly doped, with a core–shell distribution. We also found that both, the upper critical fields and the critical current densities are higher in the preceramic-doped samples than in pure MgB2, in agreement with the carbon doping level. When ferrocene-grafted polysiloxane is used, the upper critical field is the largest, while the critical current density is the lowest. We attribute this fact to the fact that the polymer pyrolysis results in iron-based nanostructures which have a pair breaking effect.

Artificial Pinning Centers on MgB2 Superconducting Thin Films Coated by FeO Nanoparticles

MgB2 thin films were fabricated on MgO (100) single crystal substrates. First, deposition of boron was performed by rf magnetron sputtering on MgO substrates and followed by a post deposition annealing at 850 °C in magnesium vapor. In order to investigate the effect of FeO nanoparticles on magnetic properties of MgB2 thin films, the films were coated with different concentrations of FeO nanoparticles by spin coating process. The magnetic field dependence of the critical current density \(J_{\mathrm{c}}\) was calculated from the M–H loops and also magnetic field dependence of the pinning force density \(f_{\mathrm{p}}(b)\) was determined for the films containing different concentrations of FeO nanoparticles. The values of the critical current density \(J_{\mathrm{c}}\) in zero field at 5 K was found to be around 1×106 A/cm2 for pure MgB2 film, 1.4×106 for MgB2 film coated with 25 %, 7.2×105 for MgB2 film coated with 33 %, 9.1×105 for MgB2 film coated with 50 % and 1.1×106 A/cm2 for MgB2 film coated with 100 %. It was found that the film coated with 25 % FeO nanoparticles has slightly enhanced critical current density and it can be noted that especially the film coated with 25 % FeO became stronger in the magnetic field. The films coated with FeO were successfully produced and they indicated the presence of artificial pinning centers created by FeO nanoparticles. The superconducting transition temperature of the film coated with 25 % FeO nanoparticles was determined by moment–temperature (M–T) measurement to be 34 K which is 4 K higher than that of the pure film.

Elastic Properties of MgB2 and SiC-Doped MgB2 Superconductors

We investigated the elastic properties of MgB2 and 5 wt.% SiC-doped MgB2 superconductors utilizing the pulse-echo overlap technique. Longitudinal and shear ultrasound velocities were measured for each sample at 80 K and 300 K. The measured velocities at 80 K were used to calculate the various elastic moduli, i.e. longitudinal (CL), shear (G), bulk (B) and Young’s (Y) modulus, and the Debye temperature D, for both samples. The high D at 694 K and 706 K obtained for MgB2 and SiC-doped MgB2, respectively, provide strong evidences via direct acoustic measurements to support numerous theoretical and non-acoustic data-based calculations. At 300 K, the higher longitudinal velocity of the pure MgB2 compared to the doped MgB2 but converging towards the same value of longitudinal modulus at 80 K, seems to suggest that the SiC-doped MgB2 undergoes greater elastic stiffening through temperature range of 300 K down to 80 K. Using the D values, the electron-phonon coupling constant  was calculated within the BCS framework and the two dimensional van Hove scenario. The results led us to conclude that MgB2 is a moderately-strong coupled superconductor.

Influence of Carbon Doping on MgB2 Superconducting Properties

Abstract Superconducting properties of MgB2 wires with and without carbon doping up to 10 at.% have been investigated. Carbon particles consisted of 10 - 40 nm nano-tubes or 10 – 20 nm size SiC. Wires were made by “in-situ” technique. The regimes of ultrasound homogenization of the mixture of nano- and micro-particles were elaborated. Mixture homogeneity was determined by SEM analysis. The influence of carbon on MgB2 synthesis con ditions (temperature and anneal time), lattice parameters was studied by metallographic and X-ray analysis. Superconducting properties (Ic, Tc) of MgB2 wires were also studied. Tc value dropped from 39 K for pure MgB2 to 35 K in the case of carbontubes doping and to 32 K in the case of SiC doping. Critical current density values in magnetic fields 2 – 12 T (4.2 K) were compared for wires with pure and carbon-doped MgB2. Ic significant improvement was fixed in the fields higher than 4 T.

Transport properties of superconducting MgB2 composites with carbon-encapsulated Fe nanospheres

We present the magnetic and transport properties of superconducting composites fabricated by admixing carbon-encapsulated Fe nanospheres and MgB2 powder. The addition of nanoparticles is expected to enhance the critical current density by carbon-doping the MgB2 matrix and by providing artificial pinning sites. Three samples with estimated amounts of 0.35, 0.6, and 1.0 wt. % metallic Fe were prepared using the spark plasma sintering technique. The average size of these nanoparticles is comparable to the superconducting coherence length of MgB2 at approximately 5 nm. We found that the additions do not significantly alter the critical temperature which is very high, close to that of the pure MgB2 samples. We have also observed improved current densities, as high as 1100 kA/cm2 for the samples with 0.35 wt. % metallic Fe at 5 K and 1 T. A core-shell model for explaining the transport data is presented. The field and temperature dependence of the reduced pinning force is described in terms of pinning on grain boundaries and/or on point defects.

Influence of Bi, Se and Te additions on the formation temperature ofMgB2

The formation of the MgB2 superconducting compound from a mixture of Mg and amorphous B powderswith various low melting point metals (Bi, Se and Te) was studied in situ bymeans of high-energy (synchrotron) x-ray diffraction in wires with a compositeCu/Nb sheath. In comparison with an undoped sample, it was found that theaddition of Bi results in a clear lowering of the formation temperature ofMgB2, whereas Se and Te have no significant influence.Tc is slightly higher in the Bi-doped sample than in the others but thejc in this case is lowerthan in the pure MgB2 sample, probably due to the presence of remainingMg3Bi2 particles that formed as an intermediate compound during reaction. Likewise,in the Se-and Te-doped samples, MgSe and MgTe respectively form below450 °C. Whereasjc is also depressed in the Se-doped sample, the MgTe particles do not appear to affect theperformance of the Te-doped wire.

Structure and properties of solid solutions in the Mg-Al-B system

Abstract Compounds with the general formula Mg1−x AlxB2 were obtained by two-step ceramic synthesis. All compounds were characterized by X-ray diffraction, NMR spectroscopy, and by four point probe resistivity measurements in various magnetic fields method. The diborides unit cell parameters were determined as a function of the Al mole fraction. With the vaues of x up to 0.40 (where x is the composition of the stock prepared for sintering), the unit cell parameters of Mg1−x AlxB2 are similar to those of pure MgB2 and the superconducting transition temperature was lowered. For stock compositions of 0:25 ≤ x ≤ 0:60, the products contain a superstructure, also superconducting phase, which becomes the only product at x = 0:50, and at x > 0:60 this phase is replaced by AlB2-based solid solutions.

Comparison of the critical current density of a polycrystalline MgB2 superconductor by ac-susceptibility and Bean's model

In this paper, the critical current density measured from the real part of ac-susceptibility is in agreement with that calculated from Bean's formula at constant frequency (f=98 Hz) in the temperature range 30–38 K for a pure MgB2 sample. The temperature dependence of the imaginary part of ac-susceptibility (χ″–T) shows the shifts in absorption peaks towards the lower temperature when the applied field is increased. Scanning electron microscopy images confirmed the synthesis of a good sample and agglomerates with fine grains.

MgB 2 fabrication by diffusion-controlled three layered (B–Mg–B) technique

MgB 2 was successfully fabricated through diffusion-controlled three-layered (B–Mg–B) technique under high pressure. Due to melting temperature of Mg, the material was pre-heat treated at 600 °C between 1 and 48 h. Optimum pre-heat treatment condition was found to be 600 °C for 48 h. Then, the compacted material was grinded and pelletized under pressure of 2 ton. The pellets were heat treated at 600–900 °C for 1–48 h. Optimum heat treatment condition was determined to be 800 °C for 1 h for formation of almost pure MgB 2. Diffusion coefficient was determined with Fick's law and EDX data. Diffusion coefficient value for B in Mg matrix and Mg in B matrix was determined to be 1.66×10 −7 and 3.14×10 −8 cm 2/sn, respectively. Best T c value (39.4 K) was obtained for material heat treated at 800 °C for 1 h. A symmetric hysteresis was obtained for the best MgB 2 material and magnetization decreased with increase in the temperature and the applied magnetic field.

Analytical solutions of the microscopic two-band theory for the temperature dependence of the upper critical fields of pure MgB2 compared with experimental data

Major theoretical results from the microscopic two-band theory for the temperature dependence of the upper critical fields Hc2(ab) and Hc2(c) in pure two-band systems such as MgB2 are presented. Analytical solutions for the upper critical fields near the superconducting transition temperature and near zero temperature are transformed for direct comparison with experimental data. The temperature variations of Hc2(ab) and Hc2(c) for textured MgB2 films near the superconducting transition temperature are measured and compared with the theory. The results of this theoretical approach are also compared with previously published experimental data. The theory and experimental data are in good agreement.

Effect of thermal strain on Jc and Tc in high density nano-SiC doped MgB2

The influences of lattice strain on the superconducting critical current density Jc and critical transition temperature Tc in pure MgB2 and a SiC-MgB2 composite made by the diffusion process are explored, based on the thermal expansion coefficients and the low temperature effects on Raman scattering. The strong thermal strain provides a strong flux pinning force for the supercurrents at the interfaces between SiC and MgB2. The high Tc of SiC-MgB2 is also discussed according to the expanded lattice and Raman characteristics.

A Novel Approach for Efficient Ni Nanoparticle Doping of MgB$_{\bf 2}$ by Liquid-Assisted Sintering

Ni nanoparticles were successfully introduced for preparing polycrystalline MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> samples by a novel reduction method. According to transmission electron microscopy, the obtained Ni nanoparticles with average grain size of 5 nm are distributed in the B matrix without agglomeration. The missing shift in X-ray diffraction peaks indicates that Ni nanoparticles could not substitute Mg sites in the lattice, but rather form the ternary compound MgNi <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2.5</sub> B <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> above 600°C. The low-melting eutectic liquid formed by Mg-Ni at 506°C is responsible for the formation of plate-like MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> grains and the fast fabrication of MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> at low temperature. The sample doped with nano-Ni has better grain connectivity and the critical current density (J <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</i> ) with respect to the commercial Ni-doped sample, but there is no improvement compare with the pure MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> sample. A sintering model was accounted for understanding the reaction between Mg and B with the assistance of Ni nanoparticles.

Superconducting transition width under magnetic field in MgB2 polycrystalline samples

A systematic study on the superconducting transition width as a function of the applied magnetic field was performed in polycrystalline MgB2. A quantitative, yet universal relation between the two parameters was observed in all of the ceramics. It was found that the width decreases linearly with decreasing field in pure MgB2 samples. Whereas, samples with boron and/or Mg atoms partially replaced by other elements show this linear relation in the temperature range below 0.7–0.8 of the superconducting transition temperature (corresponding to a field of about 2 T), at temperatures higher than this range, an abnormal upturn in the width was found. This upturn is ascribed to multiple superconducting transitions. A core-shell model is proposed to describe the multiple transitions.

Nanoscale disorder in pure and dopedMgB2 thin films

MgB2 thin films have superior superconducting properties compared to bulkMgB2 and demonstrate the potential for further improving the performances ofMgB2 wiresand tapes. Using transmission electron microscopy, we have characterized the microstructure of pure andC-doped MgB2 using various carbon sources grown by hybrid physical–chemical vapor deposition (HPCVD),and cold-grown–annealed film deposited by molecular beam epitaxy (MBE). TheMgB2 HPCVD films increase in crystal quality in the order(MeCp)2Mg-sourced films,CH4-sourced films,B(CH3)3-sourced films, purefilms, while the Hc2 values of these films follow the opposite order. The cold-grown–annealedMgB2 MBE film containsnon-epitaxial ≤ 10 nm MgB2 grains and MgO nanoparticles. The microstructural origins of electron scatteringand flux pinning in both films are discussed. We also show the structure andchemistry of the degraded phase in HPCVD films and its effects on superconductingproperties.

Intergrain connectivity of MgB2 ceramics studied by impedance analysis

First, by using of the conventional Rowell analysis, we demonstrated that the addition of nano BN particles can effectively eliminate MgO and pores in MgB2 resulting in a very high density and good connectivity of BN-doped MgB2. Then, another method—low-frequency dielectric impedance analysis—was introduced to characterize the properties of the grain boundaries of MgB2. A comparative impedance study was performed in the frequency range from 100 Hz to 100 MHz on pure and nano BN-doped MgB2. The study revealed some following interesting results: (1) a dielectric resonance around frequency of 108 in both samples was observed, which was argued to be related to an inductance-capacitance and (2) the pure sample has two dielectric relaxations originating from intergrains, while the doped sample has only one intergranular contribution. This convinces that the electric connectivity of the doped sample is really improved by the addition of nano BN particles. Our results indicate that dielectric technique may be a useful tool to characterize the grain boundary properties and grain boundary-related properties of MgB2.

Electrical transport characteristics of superconducting MgB2–MgO composite near continuous percolation

Superconducting MgB2–MgO composite with about 75% mole concentration of MgO was synthesized in situ by a single-replacement reaction. The resistance versus temperature curve shows that the composite has a high superconducting transition temperature to 38.0 K and metallic transport behavior with low resistivity and highly residual resistance ratio comparable to a pure MgB2 sample. These characteristics are explained by using the statistical percolation model and a conductivity expression with temperature for the metal–insulator MgB2 composite is given in this paper. The results indicate that MgB2 superconductor can tolerance a high content of insulating contamination and this metal–insulator MgB2–MgO composite can be utilized for the superconducting fault current limiter.