On the study of phase formation and critical current density in superconducting MgB2

Superconducting bulk MgB₂ samples have been synthesized by employing sintering technique without using any additional process steps, generally undertaken in view of the substantial loss of magnesium, during heat treatment. Starting with Mg rich powders having different atomic ratios of Mg : B, as against the nominally required Mg : B = 1:2 ratio, we have obtained superconducting MgB₂ samples of different characteristics. The effect of excess Mg in the starting mixture and processing temperature on the phase-formation, transition temperature (T _C) and critical current density (J _C) have been investigated by electrical transport and a.c. susceptibility measurements. The X-ray diffraction and X-ray photoelectron spectroscopic analyses of MgB₂ bulk samples have been carried out to understand the role of excess Mg and the effect of processing temperature. It is established that MgB₂ samples with high critical current density can be synthesized from a Mg rich powder having Mg : B in 2:2 ratio, at temperatures around 790°C. Critical current density has been found to vary systematically with processing temperature.     

Raman Investigations of C-, Li- and Mn-Doped MgB2

A systematic micro-Raman study of the atomic substitution effects has been carried out on single crystals of MgB₂, including C-doping for B and Li- or Mn-doping for Mg. The spectra of C (electron)-doped compounds exhibit remarkable differentiations with respect to pure MgB₂, and new bands of E_2g and A_1g/B_1g symmetry play the dominant role in the substituted spectra. We have observed a two-mode behavior apparently driven by the electron–phonon coupling strength and associated with an electronic topological transition of the σ bands. On the other hand, Li (hole) and Mn (magnetic impurity) doping do not seem to affect the well-defined broad band at 600 cm⁻¹. Combining Raman investigations and T _c -interdependence shows that this band is not related with the transition temperature, questioning its relationship to superconductivity in diborides.      

Surface Barrier and Bulk Pinning in MgB2 Superconductor

We present a modified method of preparation of the new superconductor MgB₂. The polycrystalline samples were characterized using X-ray and magnetic measurements. The surface barriers control the isothermal magnetization loops in powder samples. In bulk as prepared samples we always observed symmetric magnetization loops indicative of the presence of a bulk pinning mechanism. Magnetic relaxation measurements in the bulk sample reveal a crossover of surface barrier to bulk pinning.     

11B NMR Study of Pure and Lightly Carbon-Doped MgB2 Superconductors

We report a¹¹B NMR line shape and spin-lattice relaxation rate (1/(T ₁ T)) study of pure and lightly carbon-doped MgB_2−x C _x forx=0, 0.02, and 0.04, in the vortex state and in magnetic field of 23.5 kOe. We show that while pure MgB₂ exhibits the magnetic field distribution from superposition of the normal and the Abrikosov state, slight replacement of boron with carbon unveils the magnetic field distribution of the pure Abrikosov state. This indicates a considerable increase ofH _c ^2/c with carbon doping with respect to pure MgB₂. The spin-lattice relaxation rate 1/(T ₁ T) demonstrates clearly the presence of a coherence peak right belowT _c in pure MgB₂, followed by a typical BCS decrease on cooling. However, at temperatures lower than ≈10 K strong deviation from the BCS behavior is observed, probably from residual contribution of the vortex dynamics. In the carbon-doped systems both the coherence peak and the BCS temperature dependence of 1/(T ₁ T) weaken, an effect attributed to the gradual shrinking of the σ hole cylinders of the Fermi surface with electron doping.     

Investigation on MgB2 Wires Doped by ZrH2

ZrH₂-doped MgB₂ wires, with the component of Mg:ZrH₂:B= (1-x):x:2(x = 0, 5, 8, and 10%), were fabricated through thein situ powder-in-tube method by using low-carbon steel tube as the sheath material Samples were sintered at 750^{^}C for 1h in a flow of high-purity argon.The lattice parameters a and c of MgB₂exhibit a little decrease with the doping of ZrH₂.J_c(B)property of MgB₂wires are improved gradually with the increase of x, reaches the best whenx = 8%, but decreases significantlywhen x = 10%. The decrease of MgB₂grain size, the excellent grain connection, and the substitution of Zr⁴⁺for Mg²⁺which is caused by the doping of ZrH₂, are responsiblefor the enhancement of J_c(B)property forthe doped MgB₂wires. PACS: 74.70.Ad; 74.62.Dh; 74.62.Bf; 74.25.Sv     

Superconductivity in MgB2: Phonon modes and influence of carbon doping

Following a brief overview, results of our investigations on phonon modes in MgB₂, and superconducting transition in carbon doped MgB₂ are presented. The superconducting transition temperature in MgB_{2 x}C_x as obtained from susceptibility and resistivity measurements is observed to decrease systematically from 39-4 K forx = 0 to 26 K forx = 0.5. It is shown the changes in lattice volume, as obtained from x-ray diffraction measurements, can account only partially for the observed decrease inT _c . The observed variation ofT _c with carbon content is seen to correlate with the Debye temperatures, obtained from an analysis of the resistivity data. Investigation of the phonon modes in MgB₂, through infrared absorption measurements indicate three modes at 410,475 and 560 cm^-1. The former two are associated with the infrared active modes, and the third component is associated with the Raman mode, that gets activated due to disorder. A study of the temperature dependence of these modes indicates no changes across the superconducting transition. The mode at 560 cm^-1 shows a significant hardening and a corresponding decrease in linewidth, with the lowering of temperature, that can been accounted in terms of anharmonicity.     

Direct Numerical Simulations for Non-equilibrium Superconducting Dynamics at the Transition Edge: Simulation for MgB2 Neutron Detectors

We perform large-scale numerical simulations on the non-equilibrium superconducting dynamics after a neutron capture at the superconducting transition edge in MgB₂ by solving the time-dependent Ginzburg-Landau equation coupled with the Maxwell and the heat diffusion equations. The simulations are carried out under the current-biased condition in order to explain experimental results made in the JAEA reactor JRR-3, and the time scale of the obtained voltage signal is found to be almost consistent with the experiments. Moreover, the time evolution of the voltage signal is connected with that of the spatial profile of the superconducting order parameter.      

X-ray Diffraction Study of MgB2 at Low Temperatures

We have performed powder X-ray diffraction of MgB₂ superconductor between 10 and 300 K. Temperature dependence of integrated intensities of both (002) and (110) X-ray reflections shows a peak at around T_C, superconducting transition temperature. The integrated intensity of the X-ray reflection is related to the phonon frequency through a Debye--Waller factor. Lattice parameters a and c show negative thermal expansions at low temperatures. The negative thermal expansion might be due to an electronic origin and not directly related to the superconducting transition.     

Degradation of Superconducting Properties in MgB2 by Formation of the MgB4 Phase

Superconducting MgB₂ polycrystalline samples have been fabricated under two different conditions in order to determine the effect of MgB₄ phase. A series of samples was placed in an -alumina container closed with a cup and fired under high purity argon gas. The other series of samples was placed in an -alumina boot without any lid and fired under similar conditions. For the first series of samples, we have found pure MgB₂ phase formation and a narrow transition width at 0.4 K. For the second series of samples, significant amount of MgB₄ phase were formed and the T _zero was decreased to 27 K. For both the group of samples magnetization hysteresis loops obtained at various temperature range and applied field up to 2 T. The best J _cmag for the first series of samples was 1.9 × 10⁵ A/cm² at 10 K and 0 T, and for the second series of samples was 0.7 × 10⁴ A/cm² at 10 K and 0 T.     

A study of phase separation in ternary alloys

We have studied the evolution of microstructure when a disordered ternary alloy is quenched into a ternary miscibility gap. We have used computer simulations based on multicomponent Cahn-Hilliard (CH) equations for c _A and c_B, the compositions (in mole fraction) of A and B, respectively. In this work, we present our results on the effect of relative interfacial energies on the temporal evolution of morphologies during spinodal phase separation of an alloy with average composition, c_A = 1/4, c_B = 1/4 and c_c = 1/2. Interfacial energies between the ‘A’ rich, ‘B’ rich and ‘C’ rich phases are varied by changing the gradient energy coefficients. The phases associated with a higher interfacial energy are found to be more rounded than those with lower energy. Further, the kinetic paths (i.e. the history of A-rich, B-rich and C-rich regions in the microstructure) are also affected significantly by the relative interfacial energies of the three phases.     

Local Lattice Dynamics in the Mg0.5Al0.5B2 Superconductor

We have studied temperature evolution of the local as well as the average crystal structure of MgB₂ and Mg_0.5Al_0.5B₂ using real-space atomic pair distribution function (PDF) measured by high resolution neutron powder diffraction in a wide temperature range of T=10–600 K. The mean square relative displacements (MSRD) of atomic B–B, B–Mg (B–Al) pairs are compared with mean-square displacements (MSD) to calculate atomic correlations. In spite of the enhanced atomic disorder in Mg_0.5Al_0.5B₂, where the boron–boron, and boron–magnesium pair motions are found to be small, we find that the same atomic correlations in MgB₂ assume even slightly lower values and remain nearly constant in a wide temperature range of 0–600 K. This anomalous behavior and its physical interpretation provoke new questions on our understanding to the local lattice dynamics in this material.     

Stripe correlation of tilts and superconductivity in La2CuO4+Σ

In single crystal La₂CuO_4+Σ,Σ ≈ 0.015, c-axis interstitial oxygen ordering (staging) in the oxygen-rich phase of the phase-separated sample was observed through satellite peaks in the neutron scattering which first appear at 250 K. If the sample is cooled through the temperature region around 210 K over a period of several hours, these satellite peaks show splitting in the a*-direction. This first-order phase transition involves a periodic one-dimensional modulation of the in-plane order. Microscopic modeling leads to a picture where the excess oxygen, and therefore also the doped holes, order in stripes along the a-direction in addition to staging along the c-direction. This ordering in stripes can be regarded as a continuation of the phase separation but now on a local scale, leading to a local increase in oxygen and hole density. This increase can account for the increase of the superconductingT _c of 2.5 K observed in our sample and also for many previously reported annealing experiments.     

Microwave direct synthesis of MgB2 superconductor

Intermetallic compound superconductor MgB₂ was synthesized from spherical magnesium powder and lower purity amorphous boron powder by microwave direct heating. Powder X-ray diffraction (XRD) analysis indicates that the phases of the synthesis sample are MgB₂ (major phase) and a small amount of MgO. Scanning electron microscope (SEM) observation shows that the MgB₂ grain size is homogeneous and the particle size is about several hundreds of nanometers. The onset superconducting transition temperature of the MgB₂ sample measured by the temperature dependence of magnetization measurement is about 37.6 K. The critical current density J_c calculated according to the Bean model are about 2.0 × 10⁵ A/cm² at 20 K in self-field and 1.0 × 10⁵ A/cm² at 20 K in 1 T applied field.     

Preparation and characterization of Ag-added Bi1.84Pb0.4Sr2Ca2.2Cu3O10+x bulk tube conductors for cryogen free superconducting magnet

Bulk tube conductors of Bi _1.84 Pb _0.4 Sr ₂ Ca _2.2 Cu ₃ O _10+x with addition of silver varying from 0 to 25 wt% (not reported earlier) were systematically studied using X-ray diffraction (XRD), scanning electron microscopy (SEM), electrical transport and a.c. susceptibility techniques. The tube conductors formed by cold isostatic pressing (CIP) of the powders obtained from spray drying method have been made successfully. It was found that Ag addition has not only affected the formation of the desired Bi-2223 phase and the microstructure of these large bulk tube samples thereby influencing on the critical current ( I _c ),it also reduces the contact resistance to minimize the cryogen losses. These variations have been found to be Ag content dependent. An optimized value of 10 wt% Ag has been found to produce the best quality tubes showing reproducible I _c value > 120 Amp at 77 K which is in general a requirement to energies of the cryogen free conventional/HTSC superconducting magnets below 20 K.     

Weak Links and Phase Slip Centers in Superconducting MgB2 Wires

MgB₂ superconducting wires were produced by the Mg diffusion method. Scanning electron microscopy (SEM), optical microscopy, dispersive X-ray analysis (EDS), and XRD diffraction were used to study the physical structure and content of the wires. Magnetic properties (T _c ^m, H _c1, H _c2, J _c by the Bean model) were obtained with a SQUID magnetometer, and transport properties (T _c ^r , H _c2, resistivity and residual resistivity ratio) were measured using a standard four-lead configuration. The V-I characteristics of the wires close to the critical temperature showed a staircase response, which was attributed to the presence of weak links, creating phase slip centers. The origin of those weak links is discussed in relation to their formation and structure.     

Uses of α-Fe2O3 and fly ash as solid adsorbents

Solid adsorbents have shown great promise for control of particulate and non-particulate matter and as gas sensing devices in recent times. In the present study, adsorption of environmental toxic pollutant such as lead ions on solid adsorbents viz. α-Fe₂O₃ and fly ash, are reported. Considerable adsorption was observed on fly ash when compared to α-Fe₂O₃ surface. These studies are characterized by employing solid state and solution studies.     

Effect of Silver Admixture on Transport Properties in Polycrystalline MgB2

The normal-state and superconducting properties of polycrystalline MgB₂ samples including various amounts of Ag powder were investigated by measurements of electrical resistivity. The temperature-dependent electrical resistivity roughly obeys the power law (₀ + aT ⁿ with n > 1) in the normal state and becomes linear as we increase the Silver content. The critical superconducting temperatures remain almost unchanged. Silver, not reacting with MgB₂, reacts with excess magnesium to form intermetallic layers of AgMg that cluster at crystallite boundaries.     

Deposition and Properties of Superconducting MgB2 Thin Films

The recently discovered superconductor MgB₂ with T _c at 39 K has great potential in superconducting electronics. In this paper, we review the deposition techniques used for MgB₂ thin films in the light of a thermodynamic study of the Mg-B system with the calculation of phase diagrams (CALPHAD) modeling technique. This thermodynamic study identifies a growth window in the pressure–temperature phase diagram, in which the magnesium pressure is very high for likely in situ growth temperatures. A Hybrid Physical–Chemical Vapor Deposition (HPCVD) technique that successfully achieves such a high Mg pressure is shown to produce in situ epitaxial MgB₂ thin films with bulk superconducting properties.     

A new crystal structure for (BEDT-TTF)2SbF6 and some of its physical properties

A new crystal structure forbis (ethylenedithio)tetrathiafulvalene [(BEDT-TTF) ₂ SbF₆] was determined by single crystal X-ray diffraction. The crystal structure was refined in theP - 1 space group at room temperature. Crystal data for new structure are as follows: triclinic, a = 8.670 (2) Å,b = 8 664 (2) Å,c = 16.842 (5) Å,α = 89°.29 (2),β = 90°.71 (3),γ = 92°.67 (1),V = 1263.64 Å ³,Z = 2,D _x = 2.136 g cm ^-3, (Mo-Kα),λ = 0.7107 Å,R = 0.057 for a total of 5517 independent reflections. The donors form a trimerized column, and the band structure calculated by the tightbinding approximation shows band insulator properties. The temperature dependent of the d.c. resistivity shows a semiconducting behaviour with room temperature resistivity along the c-axis; ρ ₂₉₀ K = 5.6 ohm cm.