Critical Current Densities and Vortex Dynamics in Ba(Fe<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Co<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>)<Subscript>2</Subscript>As<Subscript>2</Subscript> Single Crystals

Superconducting properties are evaluated for high-quality single crystals of Ba(Fe_1−x Co _x )₂As₂ in a wide range of doping levels. The critical current density, J _c , in an optimally-doped crystal (T _c ∼24 K) shows a fishtail effect with its value over 10⁵ A/cm² even at 5 T below 10 K. Magneto-optical imaging has clarified rather homogeneous supercurrent flow in the crystal, in spite of a large amount of impurities. In the heavy-ion irradiated sample, the presence of columnar defects are confirmed and J _c has been enhanced by a factor of five at low temperatures, reaching 6×10⁶ A/cm² at 2 K under zero field. Flux creep rate in the heavy-ion irradiated sample has been reduced in accordance with the enhancement of J _c .     

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.     

Synthesis,microstructure and microwave dielectric properties of Ca<Subscript>4-x</Subscript>Mg<Subscript>x</Subscript>La<Subscript>2</Subscript>Ti<Subscript>5</Subscript>O<Subscript>17</Subscript> ceramics

Ca_4-xMg_xLa₂Ti₅O₁₇ ceramics were prepared by a solid state ceramic route for x = 0, 0.5, 1, 2, 3 and 4. The structure and microstructure of the ceramics were investigated using X-ray diffraction, scanning electron microscopy and energy dispersive X-ray spectroscopy. X-ray diffraction results show that the Ca_4-x Mg _x La₂Ti₅O₁₇ adopts an orthorhombic crystal structure with no secondary phase observed for x from 0 to 0.5. Secondary phase, MgTiO₃ occurs with further increasing doping level (1 ≤ x ≤ 3). When x = 4, mixture phases La_0.66TiO_2.993, MgTiO₃ and a trace of unknown phase coexist. Ca₄La₂Ti₅O₁₇ ceramic exhibits a relative permittivity (ε_r) ~ 65, quality factor (Q × f) ~13,338 GHz (at ~4.75 GHz), and temperature coefficient of resonant frequency (τ _f ) ~ 165 ppm/°C. The sintering temperature was distinctly reduced from 1,580 °C for x = 0 to 1,350 °C for x = 4. With increasing Mg content, ε_r and τ_f obviously decrease, while Q × f value initially decreases and then increases. The ceramic for x = 2 shows ε_r ~ 50, Q × f ~ 9,451 and τ _f  ~ 62.5 ppm/°C. By the complete replacement of Ca with Mg, Mg₄La₂Ti₅O₁₇ ceramic sintered at 1,350 °C for 4 h combines a high dielectric permittivity (ε _r  = 31), high quality factor (Q × f ~ 15,021) and near-zero temperature coefficient of resonant frequency (τ _f  ~ 4.0 ppm/°C). The materials are suitable for microwave applications.     

Effect of Mg doping on the properties of combustion synthesized LiCoO<Subscript>2</Subscript> powders

The LiCo_1-xMg_xO₂ (x = 0–0.1) cathode materials for rechargeable lithium ion batteries were synthesized by starch assisted combustion route method. The structural characterization was carried out by X-ray powder diffraction and Laser Raman Spectroscopy. The sample exhibited a well-defined rhombohedral structure and the lattice parameters varied with the increasing magnesium contents. Surface morphology of the synthesized materials was determined by Scanning electron microscope. The cathode materials consist of highly-ordered single crystalline particles with spherical shape. The electrical resistivities of the samples were studied by Hall Effect. Electrical resistivities decrease with increase in magnesium content. Electrochemical properties were characterized by the assembled test cells using Galvanostatic discharge studies that were carried out at a current rate of 0.1 C. Magnesium doped LiCo_0.95Mg_0.05O₂ show improved structural stability, high reversible capacity and excellent electrochemical performance.     

Improving the Physical Properties of (Bi,Pb)-2223 Phase by SnO<Subscript>2</Subscript> Nano-particles Addition

In this work, the effect of SnO₂ nano-particles (40 nm) addition to the physical properties of Bi_1.6Pb_0.4Sr₂Ca₂Cu₃O_10+δ superconducting phase was studied. (Bi, Pb)-2223 superconductor phase added by SnO₂ nano-particles was prepared by a conventional solid-state reaction technique. The SnO₂ nano-particles concentrations x varied from 0.0 to 2.0 wt% of the sample’s total mass. The prepared samples were investigated by X-ray powder diffraction (XRD), scanning electron microscope (SEM) and electron dispersive spectroscopy (EDS) for analyzing phase formation and microstructure. Also, the electrical resistivity and transport critical current density, for investigated samples, were measured by standard dc four-probe method. Phase examination by XRD indicated that SnO₂ nano-particles enhanced the (Bi, Pb)-2223 phase formation up to x=0.4 wt%. On the other hand, the high concentrations of SnO₂ nano-particles retarded the phase formation. Granular investigation, from scanning electron microscope, showed that both number and size of voids decreased as x increased from 0.0 to 0.4 wt%. The superconducting transition temperature and transport critical current density were found to have optimal values at x=0.4 wt%. The enhancement rates in T _c and J _c were 12 and 58%, respectively, which had a maximum enhancement in both J _c and T _c for all investigated nano-particles.     

Wetting of grain boundaries in Al by the solid Al<Subscript>3</Subscript>Mg<Subscript>2</Subscript> phase

The microstructure of binary Al_100−x –Mg _x (x = 10, 15, 18 and 25 wt%) alloys after long anneals (600–4000 h) was studied between 210 and 440 °C. The transition from incomplete to complete wetting of Al/Al grain boundaries (GBs) by the second solid phase Al₃Mg₂ has been observed. The portion of completely wetted GBs increases with increasing temperature beginning from T _wsmin = 220 °C. Above T _wsmax = 410 °C all Al/Al GBs are completely wetted by the Al₃Mg₂ phase.     

Synthesis and sintering of nano-sized BaSnO<Subscript>3</Subscript> powders containing BaGeO<Subscript>3</Subscript>

The formation of solid solutions of the type [Ba(HOC₂H₄OH)₄][Sn_1−x Ge _x (OC₂H₄O)₃] as BaSn_1−x /Ge _x O₃ precursor and the phase evolution during its thermal decomposition are described in this paper. The 1,2-ethanediolato complexes can be decomposed to nano-sized BaSn_1−x /Ge _x O₃ preceramic powders. Samples with x = 0.05 consist of only a Ba(Sn,Ge)O₃ phase, whereas powders with x = 0.15 and 0.25 show diffraction patterns of both the Ba(Sn,Ge)O₃ and BaGeO₃ phase. The sintering behaviour was investigated on powders with a BaGeO₃ content of 5 and 15 mol%. These powders show a specific surface area of 15.4–15.9 m²/g and were obtained from calcination above 800 °C. The addition of BaGeO₃ reduced the sintering temperature of the ceramics drastically. BaSn_0.95Ge_0.05O₃ ceramics with a relative density of at least 90% can be obtained by sintering at 1150 °C for 1 h. The ceramic bodies reveal a fine microstructure with cubical-shaped grains between 0.25 and 0.6 μm. For dense ceramics, the sintering temperature could be reduced down to 1090 °C, when the soaking time was extended up to 10 h.     

Electrochemical performance of Sn-doped δ-MnO<Subscript>2</Subscript> hollow nanoparticles for supercapacitors

Sn-doped δ-MnO₂ (Sn-MnO₂) hollow nanoparticles have been synthesized via chemical process at room temperature. Many characterizations have been carried out to fully identify the intrinsic information of the as-prepared samples and investigate their electrochemical properties. The results indicate that the morphologies of the samples can be adjusted by changing the concentration of Sn while the capacitance of Sn-MnO₂ nanoparticles increased corresponded with that of the undoped δ-MnO₂ nanoparticles. The specific capacitance of Sn(1 at.%)-MnO₂ is up to 258.2 F g^??1 at a current density of 0.1 A g^??1. What’s more, over 90% of the initial specific capacitance still remains after 1000 cycles at a current density of 2.0 A g^??1, displaying excellent cycling stability.     

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.     

Enhancement of Critical Current Density in YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7−<Emphasis Type="Italic">y</Emphasis></Subscript> HTS Through Boron Addition

The effect of B₂O₃ addition on the superconducting transition and grain boundary critical current density of the boron free (control) and boron doped HTS ceramics with nominal composition YBa₂Cu₃B _x O_7−y (x=0, 0.025, 0.05, 0.075, 0.1 and 0.15) has been investigated. For the lowest-level boron doping (x=0.025) an increase by nearly 1.5 times was observed in the critical current density J _c compared to the control sample. The small additives of boron in YBa₂Cu₃B _x O_7−y (x=0.025 and 0.05) do not essentially affect the critical temperature T _c =92.5 K of nominally pure Y123. Higher-level boron added compounds revealed a decrease in both T _c and J _c values. The data obtained indicate the possibility of boron dopant being inserted either into interstitial or into substitutional sites of the lattice.     

Preparation and Properties of MPMG YBCO Bulk with Y<Subscript>2</Subscript>O<Subscript>3</Subscript> Layer

In this study, four kinds of melt-processed YBCO samples were fabricated with the MPMG procedure. The compacted powders were located on a crucible with a buffer layer of Y₂O₃ to avoid liquid to spread on the furnace plate. Their microstructures were defined by XRD analysis and polarized light optical microscopy. The microstructure investigations indicated that the 123 grains were very big and fine and dispersed 211 particles remained in the samples. Resistivities of the samples were measured by a standard continuous dc four-probe method. Magnetization measurements were made and flux jumps were observed at a relatively higher temperature for Y1060. The critical current density, J _c , values of the samples, measured by VSM in 5 T magnetic field, exceeded 0.6×10³ A⋅cm⁻² at 77 K and 4 T.     

Improvement of the High-Magnetic-Field Critical Current Density of the <Emphasis Type="Italic">Ex-Situ</Emphasis> Annealed MgB<Subscript>2</Subscript> Thick Films by Oxygen Doping

This paper presents a very simple way to synthesis MgB₂ thick films with high critical current density in a magnetic field by ex-situ annealing precursor B films in air with excessive Mg in a sealed quartz tube. The films show a significant improvement of critical current density in a magnetic field compared to the high purity films annealed in vacuum, while its zero-resistance transition temperature T _c ^zero and normal state resistivity still maintain about 38 K and 17 μΩcm. The results demonstrate MgB₂ thick films have great potential applications in superconducting coated conductors.      

Enhanced Superconducting Properties of Air-Processed GdBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7-δ</Subscript> Single Domains with BaCO<Subscript>3</Subscript>/BaCuO<Subscript>2−<Emphasis Type="Italic">x</Emphasis></Subscript> Addition

Single domain GdBa₂Cu_7-δ (Gd123) bulk superconductors were fabricated in air by top-seeding melt-texture growth. Performance of the air-processed Gd123 was successfully enhanced by addition of both BaCO₃ and BaCuO_2−x , which suppress the formation of Gd_1+x Ba_2−x Cu₃O_7-δ solid solutions. The optimum doping amount ranges from 0.05 to 0.15, M BaCO₃ and 0.05 to 0.1, M BaCuO_2−x per molar Gd123. The distribution of the second phase particles was observed by scanning electron microscopy. A narrow band formed by Gd₂BaCuO₅ particle concentration appeared around the seeding zone in both a–b plane and c-growth sector in Gd123 single grain. Trapped magnetic field density reached 0.67, T for sample with 24 mm in diameter and 8, mm in thickness and a high critical current density J _c up to 91,200, A/cm² was achieved at 77, K under self-field.     

Facile microwave synthesis of Sn-doped WO3 for pseudocapacitor applications

In the present work, we have successfully synthesized pure tungsten oxide (WO₃) and Sn (3 and 5 wt%)-doped WO₃ nanoparticles using facile microwave irradiation method and studied about the electrochemical performances for supercapacitor electrode material. Structural and morphological studies of the prepared nanomaterials were investigated systematically. The powder XRD analysis reveals that pure WO₃ and Sn-doped WO₃ have monoclinic crystal structure and also crystallite size of the material decreases from 38 to 30 nm with increasing dopant concentration. Micro-Raman analysis confirms the formation of monoclinic phase with υ(O–W–O) stretching and δ(O–W–O) bending mode of vibration. SEM and micrographs show the elongation of the plate-like nanostructure of WO₃ for the doping of Sn. High-resolution transmission electron microscope images depict the morphological change and increased porosity in doped samples. The supercapacitive performance and the electrochemical conductivity of the samples were analysed using cyclic voltammetry, chronopotentiometry and electrochemical impedance spectroscopy measurements. The results demonstrate that the 5 wt% Sn-doped WO₃ electrode has the enhanced electrochemical performance in 1 M KOH with a maximum specific capacitance of 418 F g^?1 at low current density of 1 A g^?1. Also, it shows the increase in energy density from 4.88 to 11.77 Wh kg^?1 with respect to the Sn concentration at the power density of 225 W kg^?1_.

     

Study of defect states in a-Se<Subscript>85</Subscript>Te<Subscript>15−<Emphasis Type="Italic">x</Emphasis></Subscript>Pb<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> thin films by space charge limited conduction mechanism

Current–Voltage (I–V) characteristics have been studied at various temperatures in vacuum evaporated thin films of a-Se₈₅Te_15−x Pb _x (x = 0, 2, 4, 6) alloys. These characteristics show that, at low electric fields, an ohmic behaviour is observed. However, at high electric fields (E ∼ 10⁴ V/cm), the current becomes superohmic. At high fields, in case of samples having 0 and 2 at% of Pb, the experimental data fits well with the theory of space charge limited conduction (SCLC) in case of uniform distribution of localized states in the mobility gap. Such type of behaviour is not observed at higher concentration of Pb in the present glassy system due to high conductivity. In these samples, joule heating due to large currents may prohibit the measurement of SCLC. Using the theory of SCLC for the uniform distribution of the traps, the density of localized defect states near Fermi level is calculated for these compositions. The results indicate that the density of defect states near Fermi level increases on addition of Pb to binary Se₈₅Te₁₅alloy. This is explained in terms of electronegativity of Pb as compared to host elements.     

Crystal-Melt Partition Coefficients of Impurities in Forsterite,Mg<Subscript>2</Subscript>SiO<Subscript>4</Subscript>: Experimental Determination,Crystal-Chemical Analysis,and Thermodynamic Evaluation

The forsterite-melt partition coefficients K are determined experimentally for a large number of mono-, di-, tri-, and tetravalent impurities. The energies of native defects and impurities (E _d) and the solution energies (E _s) of impurities in forsterite are evaluated using computer simulation. The defect energy is shown to vary linearly with the difference in ionic radius between the host and substituent atoms (Δr) and with the impurity cation charge, while the partition coefficient and solution energy of impurities are quadratic functions of these parameters. The plots of lnK versus (Δr)² and E _s versus (Δr)² for isovalent substitutions and Me _Mg ^x and Me _Si ^x ) pass close to the origin, in contrast to the plots for heterovalent substitutions (Me _Mg ^′ and Me _Si ^⋅ ). The significant y intercept of the latter plots is interpreted as evidence for the formation of extra defects maintaining electroneutrality. The y intercept of the plot of E _s versus (Δr)² is 2 eV, which is about half the formation energy of Frenkel defects in forsterite. The best fit equations representing the correlation between the partition coefficients and solution energies of impurities demonstrate that heterovalent substitutions increase the entropy contribution to the free energy of solution of impurities.__________Translated from Neorganicheskie Materialy, Vol. 41, No. 6, 2005, pp. 720–732.Original Russian Text Copyright © 2005 by Dudnikova, Urusov, Zharikov.     

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.     

Some physical properties and Vickers hardness measurements of Fe diffusion-doped Bi<Subscript>1.8</Subscript>Pb<Subscript>0.35</Subscript>Sr<Subscript>1.9</Subscript>Ca<Subscript>2.1</Subscript>Cu<Subscript>3</Subscript>O<Subscript>y</Subscript> superconductors

In this study we have investigated the influence of iron diffusion and diffusion-annealing time on the mechanical and the superconducting properties of bulk Bi_1.8Pb_0.35Sr_1.9Ca_2.1Cu₃O_y superconductors by performing X-ray diffraction (XRD), scanning electron microscopy (SEM), Vickers hardness, dc resistivity (ρ-T) and critical current density (J_c) measurements. The samples are prepared by the conventional solid-state reaction method. Doping of Bi-2223 was carried out by means of iron diffusion during sintering from an evaporated iron film on pellets. Then, the Fe layered superconducting samples were annealed at 830 °C for 10, 30 and 60 h. The mechanical properties of the compounds have been investigated by measuring the Vickers hardness (H_v). The mechanical properties of the samples were found to be load dependent. The load independent Vickers hardness (H₀), Young’s modulus (E), yield strength (Y), and fracture toughness (K_IC) values of the samples are calculated. These all measurements showed that the values of the Vickers hardness, critical current density, and critical transition temperature and lattice parameter c increased with increasing Fe doping and diffusion-annealing time.     

Influence of Zr doping on the dielectric properties of CaCu<Subscript>3</Subscript>Ti<Subscript>4</Subscript>O<Subscript>12</Subscript> ceramics

Pure and Zr-substituted CaCu₃(Ti_1−x Zr _x )₄O₁₂ (x = 0, 0.01, 0.02, 0.03) ceramics were prepared by the Pechini method. X-ray powder diffraction analysis indicated the formation of single-phase compound, and all the diffraction peaks were completely indexed by the body-centered cubic perovskite-related structure. The effects of Zr⁴⁺ ion substituting partially Ti⁴⁺ ion on the dielectric properties were investigated in frequency range between 100 Hz and 1 GHz. The low frequency (f ≤ 10⁵ Hz) dielectric constant decreases with Zr substitution and the high frequency (f ≥ 10⁷ Hz) dielectric constant is unchanged. Interestingly, a low-frequency relaxation was observed at room temperature through Zr substitution. The observed dielectric properties in Zr-substituted samples were discussed using the internal barrier layer capacitor model. A corresponding equivalent circuit was adopted to explain the dielectric dispersion. The characteristic frequency of low-frequency relaxation rises due to the decrease of the resistivity of grain boundary with Zr substitution, which is likely responsible for the large low-frequency response at room temperature.     

Thermally activated dissipation mechanism in Sn doped CuTl-1223 superconductors

We have carried out magneto-resistance measurements of Sn-doped Cu_0.5Tl_0.5Ba₂Ca₂Cu_3−y S _y O_10−δ (y = 0.5, 1.0, 1.5) superconductors and from there studied the effect of Sn doping on the thermally activated dissipation mechanism. A systematic decrease in T _c(R = 0) and shift of T _c (onset) towards lower temperature is typical feature of Sn-doped samples with the increased strength of external magnetic field. In these samples pronounced broadening of resistive transitions has been observed on the application of external magnetic field. The activation energy of carriers obtained from the Arrhenius plots of log (ρ) versus 1000/T have been found to decrease with the increase in the applied external field as well as increased Sn doping concentration. From log (ρ) versus U _o/T plots we have found that for lower concentration of Sn the thermally activated dissipation can be explained in terms of flux creep. On the other hand for higher concentration of Sn, flux flow mechanism of energy dissipation seems to be dominant.