Sintering mechanism of Ag-doped MgB<Subscript>2</Subscript> superconductor from low temperature to high temperature

Combined with thermal analysis and phase identification, the sintering process of Ag-doped MgB₂ superconductor was investigated. It is found that the Ag doping could form Mg–Ag liquid through the eutectic reaction at low temperature (about 470 °C) and then obviously accelerated the formation of MgB₂ phase. Moreover, a sintering model is also proposed to illustrate the liquid activated sintering mechanism present in the sintering process of Ag-doped MgB₂ samples. The sintering model is supposed to provide theoretic guidance for optimizing the sintering condition in the synthesis of doped MgB₂ superconductors.     

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.     

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.     

Role of Aniline Addition in Structural and Superconducting Properties of MgB<Subscript>2</Subscript> Bulk Superconductor

In this study, the structural and superconducting properties of aniline-added MgB₂ superconductors were investigated by X-ray diffraction (XRD), thermal analysis techniques, and ac susceptibility measurements. The amount of aniline was changed from 0 to 1 mol%. Phase analysis and lattice parameters were determined from XRD measurements. X-ray diffraction analysis indicates that the main phase is MgB₂ and that there is a small amount of Mg as the secondary phase in aniline-added samples. According to the determination of lattice parameters, it is seen that the addition of aniline does not give a proper distribution with the contribution amount of a and c lattice parameters. From DSC curves, two exothermic peaks and one endothermic peak were observed in all samples. Pure and aniline-added samples were found to be dependent on the magnetic field in the ac susceptibility measurements, and the superconducting transition temperature (T _c ) was found to decrease to lower temperatures due to an increase in the amount of aniline. It has been determined that changes in the in-phase (χ ^′) and out-of-phase (χ ^″) components of the ac susceptibility by increasing the aniline amount have weakened the MgB₂ phase structure and thus cause changes in the pinning mechanism. In addition, ac losses of all the samples were calculated under external fields ranging from 160 to 1280 A/m and at 25 K.     

Comparative Study on Transport and Magnetic <Emphasis Type="Italic">J</Emphasis><Subscript>c</Subscript> of Textured MgB<Subscript>2</Subscript> Thin Film

Critical current densities of c-axis-oriented MgB₂ thin film were measured comparatively by transport and magnetization measurements. Experimental results manifest that transport critical current densities J _ct are quite different from magnetic critical current densities J _cm in both quantity and field dependence. The scaling behaviors of the flux pinning force are also quite different. To explain the inequality of J _cm and J _ct, a microstructure examination has been performed. Based on our results, we believe that J _cm and J _ct correspond to different current paths and different pinning centers. More than two kinds of pinning center are effective in MgB₂ superconductors.     

Fabrication and magnetoresistivit of ex-situ processed MgB<Subscript>2</Subscript>/Fe monofilament tapes without any intermediate annealing

We have fabricated MgB₂/Fe monofilament wires and tapes by a powder-in tube (PIT) technique, using an ex-situ process without any intermediate annealing. MgB₂/Fe monofilament tapes were annealed at 650–1,050°C for 60 min and 950°C for 30–240 min. We have investigated the effect of annealing temperatures and times on the formation of MgB₂ phase, activation energy, temperature dependence of irreversibility field H _irr(T) and upper critical field H _c2(T), transition temperature (T _c), lattice parameters (a and c), full width at half maximum, crystallinity, resistivity, residual resistivity ratio, active cross-sectional area fraction and critical current densities. We observed that the activation energies of the MgB₂/Fe monofilament samples increased with increasing annealing temperature up to 950°C and with increasing annealing time up to 60 min while it decreased with increasing magnetic field. For the MgB₂/Fe monofilament tape, the slope of the H _c2–T and H _irr–T curves decreased with increasing annealing temperature from 850 to 950°C as well as with increasing annealing time from 30 to 60 min. The transport and microstructure investigations show that T _c, J _c and microstructure properties are remarkably enhanced with increasing annealing temperature. The highest value of critical current density is obtained for the sample annealed at 950°C for 60 min. The J _c and T _c^offset values of the sample annealed at 950°C for 60 min were found to be 260.43 A/cm² at 20 and 38.1 K, respectively.     

Vortex Motion and Quasiparticle Resistivity in MgB<Subscript>2</Subscript> at Microwave Frequencies

The electrical transport in superconductors at nonzero frequencies is affected by the normal and superfiuid fractions, as well as moving vortices, resulting in intricate expressions for the complex resistivity. In MgB₂, additional complications arise from the two-band nature of this material. We present an accurate study of microwave resistivity data measured in MgB₂ thin films by means of the Corbino disk broadband technique between 2 and 20 GHz. We show that a two-fluid model applies well in a relatively large region of the H–T phase diagram. Excellent agreement is found between the derived superfluid parameters (superfluid density, upper critical field) and theoretical predictions. In the same H–T region we isolate and discuss the vortex motion complex resistivity. To this end, we make use of the expressions given by the model by Coffey and Clem (CC). We show that the frequency dependence of the complex vortex resistivity recovers the CC model. However, the temperature and field dependence of the obtained parameters are at odds with the assumptions of the model. We discuss possible explanations of these oddities by considering collective pinning of vortices.     

Enhancement of Critical Current Density of MgB<Subscript>2</Subscript> 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.     

Ac Susceptibility Measurements and Mechanical Performance of Bulk MgB<Subscript>2</Subscript>

The effects of Ar ambient pressure (vacuum and 0, 10, and 20 B) and annealing times (0.5 and 1 h) on microstructural, superconducting, and mechanical properties of bulk superconducting MgB₂ are investigated. The samples are produced using the solid-state reaction method. X-ray diffraction (XRD) and scanning electron microscopy (SEM) measurements are performed for determination of the crystal structure and surface morphology of MgB₂ samples, respectively. The superconducting properties are studied by AC magnetic susceptibility measurements. Microhardness analyses are made using the Vickers microhardness test for determination of mechanical properties of all samples. Increasing the Ar pressure decreases the lattice parameters and hence the average grain size. Increasing the annealing time results in larger lattice parameters and larger grain formation. The susceptibility measurements revealed a two-step transition which is reminiscent of granular superconductors. The intra-grain transition temperature is determined to be 38.4 K for all samples. The inter-grain transition temperature of 37.2 K is obtained for samples produced under Ar ambient pressure. The samples produced under Ar ambient pressure have better superconducting properties than the ones produced in vacuum. Increasing the annealing time under vacuum further decreases the superconducting properties probably due to Mg loss.     

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.     

Comparative Experimental and Density Functional Theory (<Emphasis Type="Italic">DFT</Emphasis>) Study of the Physical Properties of MgB<Subscript>2</Subscript> and AlB<Subscript>2</Subscript>

In the present study, we report an intercomparison of various physical and electronic properties of MgB₂ and AlB₂. In particular, the results of phase formation, resistivity ρ(T), thermoelectric power S(T), magnetization M(T), heat capacity (C _P ), and electronic band structure are reported. The original stretched hexagonal lattice with a=3.083 Å, and c=3.524 Å of MgB₂ shrinks in c-direction for AlB₂ with a=3.006 Å, and c=3.254 Å. The resistivity ρ(T), thermoelectric power S(T) and magnetization M(T) measurements exhibited superconductivity at 39 K for MgB₂. Superconductivity is not observed for AlB₂. Interestingly, the sign of S(T) is +ve for MgB₂ the same is ?ve for AlB₂. This is consistent with our band structure plots. We fitted the experimental specific heat of MgB₂ to Debye–Einstein model and estimated the value of Debye temperature (Θ _D) and Sommerfeld constant (γ) for electronic specific heat. Further, from γ, the electronic density of states (DOS) at Fermi level N(E _F) is calculated. From the ratio of experimental N(E _F) and the one being calculated from DFT, we obtained value of λ to be 1.84, thus placing MgB₂ in the strong coupling BCS category. The electronic specific heat of MgB₂ is also fitted below T _c using α-model and found that it is a two gap superconductor. The calculated values of two gaps are in good agreement with earlier reports. Our results clearly demonstrate that the superconductivity of MgB₂ is due to very large phonon contribution from its stretched lattice. The same two effects are obviously missing in AlB₂, and hence it is not superconducting. DFT calculations demonstrated that for MgB₂, the majority of states come from σ and π 2p states of boron on the other hand σ band at Fermi level for AlB₂ is absent. This leads to a weak electron phonon coupling and also to hole deficiency as π bands are known to be of electron type, and hence obviously the AlB₂ is not superconducting. The DFT calculations are consistent with the measured physical properties of the studied borides, i.e., MgB₂ and AlB₂.     

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.     

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.     

Properties of MgB2 superconductors with regard to space applications

Thin MgB₂ conductors with diameters down to 50 μm have been developed for application as current leads on a satellite and as LH₂ level sensor. The high transition temperature of 39 K makes applications of MgB₂ at temperatures around 20 K possible, significantly above the temperature margin of low temperature superconductors like NbTi or Nb₃Sn. The absence of weak link behaviour and the fact that there is no need to texture MgB₂ to obtain high critical current densities is a crucial advantage compared to high temperature superconductors. We give an overview of the current status of MgB₂ conductor development and show the potential of these wires for space applications.     

Studies on the Composite System of Bi-2212 Glass Ceramic and MgB<Subscript>2</Subscript> 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.     

Progress in Critical Current Density (<Emphasis Type="Italic">J</Emphasis><Subscript><Emphasis Type="Italic">c</Emphasis></Subscript>) in Sintered MgB<Subscript>2</Subscript> 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 T_c,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 (J_c) 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 J_c at around 500 and 350 kA/cm² in the self-field and 1 T, respectively, which makes it suitable for several industrial applications.     

Thermodynamic and Magnetic Properties of Superconductors with Anisotropic Energy Spectrum,MgB<Subscript>2</Subscript>

The behavior of the superconducting transition temperature T _c and that of the jump of electron heat capacity (C _S −C _N )/C _N of the compound MgB₂ at T=T _c with substitution of boron and magnesium atoms by other atoms from the periodic table of the elements, corresponding to introduction of additional electrons or holes in this compound are researched. The microscopic superconductivity theory in MgB₂ systems in the magnetic field parallel to the crystallographic axis (H ‖ c) is built. The magnitude of the upper critical field H _c2 is determined and its temperature dependence in two-band systems with different and identical topologies of Fermi surface cavities of the corresponding bands is studied. The obtained results and their comparisons with the experimental data demonstrate that all kinds of anomalies of the physical properties of the compound MgB₂ are effectively described by the two-band model.