Critical Temperature of Smart Meta-superconducting MgB<Subscript>2</Subscript>

Enhancing the critical temperature (T _C ) is important not only to widen the practical applications but also to expand the theories of superconductivity. Inspired by the meta-material structure, we designed a smart meta-superconductor consisting of MgB₂ microparticles and Y₂O₃/Eu³⁺ nanorods. In the local electric field, Y₂O₃/Eu³⁺ nanorods generate an electroluminescence (EL) that can excite MgB₂ particles, thereby improving the T _C by strengthening the electron–phonon interaction. An MgB₂-based superconductor doped with one of four dopants of different EL intensities was prepared by an ex situ process. Results showed that the T _C of MgB₂ doped with 2 wt% Y₂O₃, which is not an EL material, is 33.1 K. However, replacing Y₂O₃ with Y₂O₃/Eu³⁺II, which displays a strong EL intensity, can improve the T _C by 2.8 to 35.9 K, which is even higher than that of pure MgB₂. The significant increment in T _C results from the EL exciting effect. Apart from EL intensity, the micromorphology and degree of dispersion of the dopants also affected the T _C . This smart meta-superconductor provides a new method to increase T _C .     

Improving the Critical Temperature of MgB<Subscript>2</Subscript> Superconducting Metamaterials Induced by Electroluminescence

The MgB₂ superconductor was doped with electroluminescent Y₂O₃:Eu, to synthesise a superconducting metamaterial. The temperature dependence of the resistivity of the superconductor indicates that the critical temperature (T _C) of samples decreases when increasing the amount of doped Y ₂ O ₃ nanorods, due to impurity (Y ₂ O ₃, MgO and YB ₄). However, the T _C of the samples increase with increasing amount of doped Y ₂ O ₃:Eu ³⁺ nanorods, which are opposite to doped Y ₂ O ₃ nanorods. Moreover, the transition temperature of the sample doped with 8 wt % Y ₂ O ₃:Eu ³⁺nanorods is higher than those of doped and pure MgB ₂. The T _C of the sample doped with 8 wt % Y ₂ O ₃:Eu ³⁺ nanorods is 1.15 K higher than that of the sample doped with 8 wt % Y ₂ O ₃. The T _C of sample doped with 8 wt% Y ₂ O ₃:Eu ³⁺ is 0.4 K higher than that of pure MgB ₂. Results indicate that doping electroluminescent materials into MgB ₂ increases the transition temperature; this novel strategy may also be applicable to other superconductors.     

Phase formation of superconducting MgB<Subscript>2</Subscript> at ambient pressure

MgB₂ superconductor has been synthesized using a simple technique at ambient pressure. The synthesis was carried out in helium atmosphere over a wide range of temperatures. Magnesium was employed in excess to the stoichiometry to prevent the decomposition of MgB₂. Samples of MgB₂ thus prepared have been almost free from MgO as compared to other methods. Resistivities of the samples are quite low with residual resistivity ratio (RRR) of around 3.T _c (R = 0) is 38.2-38.5 K with ΔT _C of 0.6–1.0 K. Comparative studies of various methods of low pressure synthesis have been presented.     

Effect of nano-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> doping on formation and superconductivity of bulk MgB<Subscript>2</Subscript>

Bulk materials of MgB₂ have been prepared with the stoichiometry of MgB₂(Al₂O₃) _x (x = 0, 2, 5, 10 and 20% nano-Al₂O₃ powders), by using solid-state reaction route. All samples were sintered at 750 °C for 30 min in a calorimeter to monitor the sintering reaction process. It is found that the onset temperatures of reaction between Mg and B powders increase significantly with increasing the amount of Al₂O₃. However, the reaction time is shortened for the nano-Al₂O₃ powders can effectively activate the reaction as a catalyst. The critical transition temperature decreases from 38.5 to 31.6 K, and the corresponding temperature window becomes narrow (less than 2.6 K). Furthermore, the amount of MgO impurity was found to increase with the increase of Al₂O₃, which probably indicates that partial Mg was replaced by Al.     

Al-Doping Effects on the Structural Change of MgB<Subscript>2</Subscript>

A series of polycrystalline Al doped Mg_1−x Al _x B₂ (x=0.00, 0.01, 0.03, 0.06, 0.10, 0.15) samples were prepared using the solid-state reaction route. Phase analysis showed that Al is alloyed into the MgB₂ lattice and there were some Al₂O₃, MgAlB₄ particles present in bulk samples of MgB₂. It is shown that the suppression of T _c by doping originates largely from structural changes and the structure properties play an important role in influencing the normal-superconductor transport. The introduction of defects into the Mg layers and other aluminum compound (Al₂O₃, MgAlB₄) impurity phases both influence the polycrystalline structure.     

Phase formation sequences and mechanism of MgB<Subscript>2</Subscript> superconductor with minor Sn doping

Combined with thermal analysis and phase identification, the phase formation of Sn-doped MgB₂ superconductor during the sintering process were systematically investigated. As compared to the sintering of MgB₂, the first exothermal peak occurs at a lower temperature, which suggests the accelerated formation of MgB₂ after minor Sn doping. The sintering process of minor Sn-doped MgB₂ orderly underwent the melting of Sn, the reaction between Mg and Sn, the eutectic Mg–Sn reaction, the solid–solid Mg–B reaction, the melting of Mg, the liquid–solid Mg–B reaction and the Sn precipitation. Based on the phase formation mechanism, MgB₂ bulks was successfully synthesized by Sn-activated sintering at 600 °C for only 5 h, exhibiting a dramatic decrease in the sintering time compared to the sintering of undoped MgB₂.     

Fluctuation induced conductivity of polycrystalline MgB<Subscript>2</Subscript> superconductor

We report fluctuation-induced conductivity (FIC) of the polycrystalline MgB₂ superconductor in the presence of magnetic field. The results are described in terms of the temperature derivative of the resistivity, dρ/dT. The dρ/dT peak temperature observed for H = 0 Tesla at 39 K remains very distinct under applied fields of 6 Tesla and 8 Tesla at 22 and 20 K respectively. Aslamazov and Larkin (AL) equations are used to explain the anisotropic nature of the polycrystalline MgB₂. The effective coherence length, ξ _p (0) determined experimentally is 55.17 Å, which roughly matches with previously reported experimental work.     

Liquid-Phase Shock-Assisted Consolidation of Superconducting MgB<Subscript>2</Subscript> Composites

An original two-stage liquid-phase hot explosive compaction (HEC) procedure of Mg-B precursors above 900 ^°C provides the formation of superconductivity MgB₂ phase in the whole volume of billets with maximal T _c = 38.5 K without any further sintering. The liquid-phase HEC strongly increases the solid-state reaction rate similar to photostimulation, but in this case, due to the high penetrating capability of shock waves in a whole volume of cylindrical billets and consolidation of MgB₂ precursors near to theoretical density allows one to produce bulk, long-body cylindrical samples important for a number practical applications.     

Effect of Sn-doping on the sintering process of MgB<Subscript>2</Subscript> and its superconductive properties

Bulk (Mg_1.02B₂)_1−x Sn _x samples (x = 0.0, 0.01, 0.03 and 0.05) were synthesized by in situ sintering at 850 °C for one hour. Based on the phase identification and microstructure observation, the Mg₂Sn and Sn impurities are found as the main impurities in Sn-doped samples. According to the magnetization measurements, the low doping level of Sn was observed to have small influence on the grain connectivity, and thus a high critical current density was maintained at low field. However, the values of the critical current density at high field in the Sn-doped samples show a little decrease.     

Nano Fe<Subscript>3</Subscript>O<Subscript>4</Subscript> Induced Fluxoid Jumps and Low Field Enhanced Critical Current Density in MgB<Subscript>2</Subscript> Superconductor

Nano particle of Fe₃O₄ (nFe₃O₄) up to 6 at% were doped in the superconducting MgB₂ samples. Despite the strong ferromagnetic nature of Fe₃O₄, both the ac susceptibility and the resistivity measurements show that up to 4 at% of Fe₃O₄, T _c =38 K is not changed, whereas for 6% T _c decreases by 6 K. This indicates that a low concentration of Fe does not substitute either the Mg or B sites and probably occupies the intergrain spaces. For 0.5% doped Fe₃O₄, an increase in J _c with respect to the pure MgB₂ samples is observed in the lower field and temperature regions (H<2 T and 20 K) indicating an enhanced flux pinning and the magnetic activation, i.e., the interaction between the magnetic dipole of Fe ion and the vortices is weak in comparison to the effective pinning potential. Whereas, at H>2 T, J _c of the doped samples is always less than that of MgB₂, and the activation is dominant in comparison with the effective pinning potential provided by the doping. Flux jumps are observed in lower T and H regions for the samples doped up to 1% nFe₃O₄ only. Magnetization plots of higher Fe content samples exhibited clear paramagnetic background. Mossbauer measurements for the higher (4, 6 at%) nFe₃O₄ doped MgB₂ samples show that at RT, the hyperfine field for both samples is ∼100 kOe and ∼120 kOe at 90 K. This means that the nFe₃O₄ particles decompose and form possibly an intermetallic Fe-B phase in the matrix.     

Band Modification Effect on the Normal and Superconducting State Properties of Bulk MgB<Subscript>2</Subscript>

Modification of σ and π bands was studied in MgB₂ by doping 3, 6 and 9 wt% of C and Fe, respectively. The samples synthesized by a solid-state route were characterized by XRD, and magnetization (M) and resistivity (ρ) measurements were in the temperature range (T) 4.2–300 K and magnetic field range (B) 0–12 T, respectively. The decrease (increase) of the lattice parameter a with C (Fe) doping, consistent with B (Mg) site substitution, confirms the expected changes in σ (π) bands. This is supported by the fact that normal-state ρ(T) of all the samples can be fitted by a two-band model and the scattering rates in both the bands are found to be dependent on the dopant. The influence of C and Fe doping on various superconducting properties of the host MgB₂ is also found to be significantly different. For instance, in the presence of magnetic field, Fe doping shows a much larger broadening of the superconducting transition when compared to C doping. The critical current density (J _C(B)) at 4.2 K vanishes for C (Fe) doping at around T～12 (～3). It is shown that the band modification and the superconducting properties are correlated.     

MgB<Subscript>2</Subscript> with addition of Sb<Subscript>2</Subscript>O<Subscript>3</Subscript> obtained by spark plasma sintering technique

Superconducting bulks of MgB₂ with addition of Sb₂O₃ and Sb with different stoichiometric compositions ((MgB₂) + (Sb₂O₃) _x , x = 0.0025, 0.005, 0.015, and (MgB₂) + (Sb)_y, y = 0.01) were obtained by the Spark Plasma Sintering (SPS) technique. All added samples have high density, above 95% and critical temperature, T _c, of 38.1–38.6 K. This result and XRD data suggest that Sb does not enter the lattice of MgB₂. Impurity phases are Mg₃Sb₂, MgO, and MgB₄. The optimum addition is Sb₂O₃ for x = 0.005. This sample shows the critical current density, J _c(5 K, 0 T) = 4 × 10⁵ A/cm² and J _c(5 K, 7 T) = 6 × 10² A/cm², while the irreversibility field, H _irr (5 K, 100 A/cm²) = 8.23 T. Indicated values of J _c and H _irr are higher than for the pristine sample. The mechanism of J _c and H _irr increase in the Sb₂O₃ added samples is complex and composed of opposite effects most probably involving morphology elements, the presence of nano metric MgB₄ and the indirect influence of oxygen or oxygen and Sb. Crystallite size of MgB₂ is decreasing when Sb-based additions are introduced and the effect is stronger for the Sb-metal addition. The sample with Sb-metal addition does not improve J _c and H _irr when compared with pristine sample.     

Effect of Ag Addition on the Surface Topography and the Vibrational Dynamics of MgB<Subscript>2</Subscript>

We fabricated MgB₂ samples with Ag additions using in situ solid-state reaction via a single-step sintering to study the effect of Ag on the structural, vibration, and superconducting properties of MgB₂ samples. Ag addition to MgB₂ resulted in a significant improvement in J _c although no appreciable effect was observed in the lattice parameters and the superconducting transition temperature T _c. Dramatic increase in the grain size was observed with Ag addition and topographic measurements with atomic force microscopy revealed the formation of Ag–Mg nanoparticles 5–20 nm in size at 2 and 4 wt% Ag additions. The fact that these samples showed high J _c values suggests that the nanoparticles formed as a result of Ag addition are responsible for enhanced flux pinning. Raman spectroscopy measurements showed that Ag additions also increased disorder in the system and thereby affected the line width of the Raman active E _2g mode.     

Addition of Sb<Subscript>2</Subscript>O<Subscript>5</Subscript> into MgB<Subscript>2</Subscript> Superconductor Obtained by Spark Plasma Sintering

High-density (92–98 % of the theoretical density) MgB ₂ samples added with Sb ₂ O ₅ ((MgB ₂)+ (Sb ₂ O ₅) _x ,x= 0, 0.0025, 0.005, 0.015) were obtained by Spark Plasma Sintering. A higher amount of additive decreases density. In added samples, grains of secondary phases are located at MgB ₂ grain boundaries and they are of large size. Hence, Sb ₂ O ₅ does not promote effective flux pinning, connectivity is lower, and this suppresses the critical current density and the irreversibility field. Pinning force-related parameters indicate that added samples are close to the point pinning region and they show a higher grain boundary pinning contribution when compared with pristine MgB ₂ sample and when temperature is lower. It is speculated that for fixed processing conditions and Sb-oxide phases, a lower stability of the additive, reflected by a lower melting temperature, may promote reactive processes to start earlier leading to coarsening of the grains belonging to secondary phases.     

Nano-Ni addition to MgB<Subscript>2</Subscript>: effects on the superconducting properties

Samples of MgB₂ pure phase, with Ni nanoparticles addition, were prepared using a solid diffusion reaction method. Clearly, the Ni nanoparticles act as effective pinning centers and enhance the critical current density values, especially for a sample with 0.5%Ni. A negligible amount of Ni diffuses inside the MgB₂ grains, thus having a small effect on the transition temperature, which remains around 37.5 K.     

Kinetics analysis for the sintering of bulk MgB<Subscript>2</Subscript> superconductor

The pertaining kinetic characteristics during the sintering of bulk polycrystalline MgB₂ superconductors is essentially important for the improvement of properties. Here Differential Thermal Analysis was adopted to record the heat effect during the preparation of bulk MgB₂ samples. The reaction between Mg and B powders starts before the melting point of pure Mg and the evolution for the fractions of MgB₂ were determined as a function of sintering temperatures. After fitting with different kinetic mechanism functions assumed, the sintering process of bulk MgB₂ superconductors was attributed to a solid-state interface-reaction controlled mechanism with an apparent activation energy of 4.54 × 10⁵ J mol⁻¹. Combined with microstructural observations by scanning electron microscopy and phase identification by X-ray diffraction, the formation process of MgB₂ phase was classified into two different stages: (i) solid-solid reaction stage, in which Mg and B powder starts to react and the growth of MgB₂ grain is restricted by the pinning effects of pores; (ii) solid–liquid reaction stage, in which the molten Mg melt promotes the reaction process and the regular hexagon bulk MgB₂ grain forms in a solution-reprecipitation and growth mode.     

Enhanced Critical Current Properties of MgB<Subscript>2</Subscript> Bulks by Doping Amorphous Carbon Containing Magnetic Impurity

The doping effect of amorphous carbon (C) containing magnetic impurity in MgB₂ bulk has been studied. Structural characterization by means of X-ray diffraction and the superconducting transition temperature, T _c , measurement indicate that little C effectively enters the MgB₂ structure. This should be due to the lower sintering temperature. The upper critical field, H _c2, and irreversibility field, H _irr, of samples show no systematic evolution with C doping. However, critical current density J _c (H) performance is greatly improved with C doping at 4, 15, and 28 K, respectively. Corresponding to this case, scanning electron microscope (SEM) image indicates that the grain size in samples becomes very small and grain boundary is developing roundness with the increasing of C content. This should be intimately related with the increase of magnetic impurity along with C doping. The result is discussed.      

Effect of Rare-Earth Oxides Doping on the Superconductivity and Flux Pinning of MgB<Subscript>2</Subscript> Superconductor

A series of rare-earth-oxide-doped MgB₂ bulks are prepared by in situ solid-state reaction with Pr₆O₁₁, CeO₂, Lu₂O₃, and Ho₂O₃ as the dopants. The superconducting properties are investigated and compared with the nano-Fe₃O₄ doped MgB₂. It is found that different from doping ferromagnetic nano-Fe₃O₄ which drastically suppresses superconductivity of MgB₂, doping the rare-earth-oxides has little effect on superconductivity of MgB₂ although most rare-earth elements have strong magnetic moment. In addition, some boride impurities formed during the reaction between rare-earth oxides and boron can work as effective pinning centers and significantly improve J _c and H _irr of MgB₂ when these fine nanoboride precipitates (<20 nm) are embedded into the MgB₂ intragrains.     

Time-Dependent Diffusion Coefficient of Fe in MgB<Subscript>2</Subscript> Superconductors

The iron (Fe) diffusion in superconducting MgB₂ bulk samples has been studied for sintering time durations of 15 min, 30 min, 1 h, 2 h, and 4 h at 900^°C. Fe coating bulk polycrstalline superconducting MgB₂ samples for Fe coating were prepared by pelletizing and used in the diffusion experiments with initial sintering at 800^°C for 1 h. A thin layer of Fe was coated on MgB₂ pellets by evaporation in vacuum. Effects of Fe diffusion on the structural, electrical, and superconducting properties of MgB₂ have been investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), infrared spectroscopy (IR), energy-dispersive X-ray spectroscopy (EDS), and resistivity measurements. Fe diffused samples have slightly increased critical transition temperatures and have larger lattice parameter c values, in comparison with bare samples. Fe diffusion coefficients were calculated from depth profiles of c parameter and room temperature resistivity values. Depth profiles were obtained by successive removal of thin layers from Fe diffused surfaces of the samples. Our results have shown that the Fe diffusion coefficient decreases with increasing sintering time and resistivity measurements can be utilized for determination of diffusion coefficient.