The effect of Sm substitution on properties of Bi1.6Pb0.4Sr2Ca2? x Sm x Cu3O y superconductors

The effect of the partial substitution of Ca by Sm in the Bi-2223 superconducting samples have been investigated in terms of X-ray diffraction (XRD), EDXRF (Energy Dispersive X-ray Fluorescent), magnetoresistivity, critical temperature, transport critical current density, and ac susceptibility measurements. The samples were prepared by the conventional solid-state reaction method. XRD patterns are used to calculate lattice parameters and phase ratio of the Bi-2223 samples. The volume fraction was determined from the intensities of Bi-2223 and Bi-2212 peaks. The room temperature XRD patterns of the samples showed the presence of Bi-2223 phase decreases with increasing the Sm content. We estimated the transition temperature of the samples from the resistivity versus temperature measurements in dc magnetic fields up to 0.6 T. We observed that transition temperature, T _c , and transport critical current density, , depend on the Sm substitution. They both decrease with increasing the Sm substitution. We extracted the peak temperature, T _p , and the pinning force density from our previous ac susceptibility measurements. The pinning force density decreased with increasing the Sm content. The possible reasons for the observed decreases in critical temperature and critical current density due to Sm substitution were discussed.     

Improvement of the Irreversibility Line of the 1212 Compound Tl0.5Pb0.5Sr2CaCu2O7?δ

The superconducting properties and the irreversibility line of the Tl_0.5Pb_0.5Sr₂CaCu₂O_7– were studied by ac susceptibility on both ceramic and powder samples prepared in sealed quartz tube at 960°C. In parallel, carefull investigations by DTA/Tg, X rays diffraction and plasma emission spectroscopy were performed on each sample after each thermal treatment. It is shown that the superconducting properties are strongly dependent on further heat treatments and that the irreversibility line may be optimized. Nevertheless, the results obtained suggest that further improvement of the irreversibility line might be expected.     

Effect of Dissipation on Quantum Tunneling of Vortices in Tl2Ba2Ca2Cu3O10+δSuperconductors

The temperature and magnetic field dependence of the magnetic relaxation rate has been investigated at low temperatures (1.8 < T < 10 K) on two Tl₂Ba₂Ca₂Cu₃O₁₀₊ samples (epitaxial thin film and sintered pellet). The temperature dependence gives evidence of a crossover in the mechanism of vortex motion, from classical thermal activation to quantum tunneling as temperature decreases. The field dependence of the relaxation rate indicates a crossover in the dimensionality of vortices, from three-dimensional flux lines to two-dimensional pancake vortices as field increases. For the thin film, the temperature dependence of the rate has been fitted to the theoretically predicted expressions for finite-temperature enhancement of the quantum rate in different regimes of dissipation. In spite of the similarity of the fits, the estimate of the ratio of Hall to viscous drag terms in the equation of motion indicates that quantum tunneling in this system occurs in an intermediate dissipative regime, where both terms contribute to the motion of vortices.     

Anisotropic Ultrafast Dynamics of Quasiparticles on CuO2 Planes of Y0.7Ca0.3Ba2Cu3O7?δ

The orientation-resolved femtosecond spectroscopy, combined with the well-textured (110)- and (001)-Y_0.7Ca_0.3Ba₂Cu₃O_7−δ thin films, serves as an effective probe to quasiparticle relaxation dynamics on the ab planes and along the diagonal orientations. The significant divergences in the temperature-dependent relaxation time (τ) associated with the opening of superconducting gap were observed along the nodal directions and on the CuO₂ planes which are dominated by the a axis and b axis in the overdoped region. Moreover, the divergence in the temperature-dependent τ along the nodal direction disappears around optimal doped region. This implies that the superconducting gap evolves from the dominant s-wave symmetry in overdoped region into the dominant d-wave symmetry in optimal doped region.     

Interpretation of Resistivity of Nd1.85Ce0.15CuO4: Electron–Phonon Mechanism

The temperature-dependent normal state resistivity of single crystal Nd_1.85Ce_0.15CuO_{4 –} is theoretically analyzed within the framework of classical electron–phonon i.e., Bloch-Gruneisen model of resistivity. For the reason of inherent acoustic (low frequency) phonons (_ac) as well as high-frequency optical phonons (_op), the contributions to the resistivity were first derived. The optical phonons of the oxygen breathing mode yields a relatively larger contribution to the resistivity compared to the contribution of acoustic phonons. Estimated contribution to in-plane resistivity by considering both phonons, i.e., _ac and _op, along with the zero-temperature-limited resistivity, when subtracted from single crystal data infers a quadratic temperature dependence over most of the temperature range [25 T 300]. Quadratic temperature dependence of _diff. = [_exp. – {₀ + _e–ph (=_ac + _op)}] is understood in terms of 3D electron–electron inelastic scattering. The comparison of single crystal experimental data appears favorable with the present analysis.     

Highlighting of structural units of B2O3–Li2O–P2O5 system under heat treatment

As technology evolves towards the design of small size – high efficiency devices there is a necessity for the development of solid, stable electrolytes that can be fabricated in various shapes. Accordingly, a glass system of xB₂O₃·0.4Li₂O·(0.6 − x)P₂O₅ with 0 ≤ x ≤ 0.6 mol%, was prepared by melting the raw materials at 1200 °C and rapidly cooling the melts at room temperature. The samples were afterwards heat treated to develop crystalline structures, for better identification of the units that build up the network.     

Effect of Nano-Sized ZnO on the Physical Properties of (Cu0.5Tl0.25Pb0.25)Ba2Ca2Cu3O10?δ

High-temperature superconductor phase of (Cu_0.5Tl_0.25Pb_0.25)-1223 was synthesized by solid-state reaction technique and characterized using X-ray powder diffraction (XRD). XRD analysis revealed that the prepared sample was nearly monophase and exhibited tetragonal structure with space group P4/mmm. Nano-zinc-oxide, prepared by Co-precipitation method, was added to the sample. ZnO-concentrations y varied from 0.0 to 2.0 wt.% of the sample’s mass. The prepared samples were investigated through XRD, scanning electron microscope (SEM), energy dispersive X-ray (EDX), particle size analyzer (PSA), differential scanning calorimeter (DSC), electrical resistivity and transport critical current density measurement. X-ray data analysis showed that the nano-Zn addition does not affect the tetragonal structure of (Cu_0.5Tl_0.25Pb_0.25)-1223 phase, whereas the lattice parameters showed an insignificant variation. The results of the superconducting transition temperature, the transport critical current density and melting point of the prepared samples were found to depend on nano-ZnO concentrations.     

Effects of copper and Al2O3 particles on characteristics of Cu–Al2O3 composites

High-energy milling was used for production of Cu–Al₂O₃ composites. The inert gas-atomized prealloyed copper powder containing 2 wt.%Al and the mixture of the different sized electrolytic copper powders with 4 wt.% commercial Al₂O₃ powders served as starting materials. Milling of prealloyed copper powders promotes formation of nano-sized Al₂O₃ particles by internal oxidation with oxygen from air. Hot-pressed compacts of composites obtained from 5 and 20 h milled powders were additionally subjected to the high-temperature exposure in argon at 800 °C for 1 and 5 h. Characterization of processed material was performed by optical and scanning electron microscopy (SEM), X-ray diffraction analysis (XRD), microhardness, as well as density and electrical conductivity measurements. Due to nano-sized Al₂O₃ particles microhardness and thermal stability of composite processed from milled prealloyed powders are higher than corresponding properties of composites processed from the milled powder mixtures. The results were discussed in terms of the effects of different size of starting copper powders and Al₂O₃ particles on the structure, strengthening of copper matrix, thermal stability and electrical conductivity of Cu–Al₂O₃ composites.     

Percolation Behavior of the Normal-State Resistivity and Superconductivity of the YBa2Cu3O7?δ-Ba2GdNbO6 Composite System

The percolation behavior of the normal-state resistivity and superconductivity of the YBa₂Cu₃O_7–Ba₂GdNbO₆ composite system were studied by X-ray diffraction and temperature-resistivity measurements. No detectable chemical reaction was observed between the YBa₂Cu₃O_7– superconductor and the ceramic insulator Ba₂GdNbO₆, even after severe heat treatment above 950°C. The normal-state and superconducting percolation threshold values were found to be 17 vol.% and 30 vol.% of YBa₂Cu₃O_7– respectively in the YBa₂Cu₃O_7–-Ba₂GdNbO₆ composite system. The values obtained for the critical exponents describing the normal-state pecolation behavior of the system matched fairly well with the theoretically expected values for an ideal metal-insulator composite system.     

Obtainment of the Fe0.70?xCrxAl0.3/Al2O3 nanocomposite by the method of SHS of mechanoactivated Cr2O3 + Fe + Al mixtures

Applying M?ssbauer spectroscopy methods, we have studied the structure of nanocomposites obtained by a technique combining the preliminary mechanical activation of an 8.1 wt % Cr₂O₃ + 65.9 wt % Fe + 25 wt % Al mixture and self-propagating high-temperature synthesis (SHS). It has been found that, at the stage of mechanical activation, an Fe/Al/Cr₂O₃ composite with a low impurity of the Fe₂Al₅ intermetallide is formed. At the stage of SHS, the interaction between activated components of the mixture leads to the formation of the Fe_{0.7 − x} Cr _x Al_0.3 (x = 0 − 0.2)/Al₂O₃ composite. Original Russian Text ? T.Yu. Kiseleva, A.A. Novakova, T.L. Talako, T.F. Grigor’eva, A.N. Falkova, 2009, published in Neorganicheskie Materialy, 2009, Vol. 45, No. 7, pp. 892–896.     

Influence of conditions on the potential of the couple of nonoxygenated U5+/U4+ ions bound in complexes with the P2W17O 6110? and SiW11O 398? anions

The formal potential of the couple of nonoxygenated U⁵⁺/U⁴⁺ ions bound in complexes with unsaturated heteropoly anions (HPAs) P₂W₁₇O₆₁¹⁰⁻ (I) and SiW_{11 O39}⁸⁻ (II) in 0–1 M NaNO₃ and 1 M (NaCl + HCl) in the range of pH 0.7–4.7 was measured. In 1 M NaNO₃ solutions at pH 4.7–3.0 for I and 4.3–3.9 for II, the formal potentials are constant: 0.820 and 0.730 V, respectively. They preserve approximately the same value with a decrease in pH to 0.7 in 1 M (NaCl + HCl). The potential noticeably decreases with a decrease in the NaNO₃ or NaCl concentration from 1 M to 0 (pH 4.1–4.7): to 0.09 and 0.05–0.06 V for I and II, respectively. Approximate constancy of the potential of the U⁵⁺/U⁴⁺ couple with a decrease in pH to 1 and lower distinguishes this couple from the M⁴⁺/M³⁺ couples (M = Ce, Am, Bk) whose potential appreciably grows with increasing acidity. This is due to the fact that the U⁵⁺ and M⁴⁺ ions in acid solutions remain in the form of complexes with the ratio M: HPA = 1: 2, whereas the M³⁺ ions pass into the form of 1: 1 complexes. Thus, variation of the formal potentials of all the M ^{n + 1}/M ⁿ⁺ couples in the presence of H⁺ and Na⁺ ions is associated with variation of the stability constants of the complexes M(HPA)₂, which, in turn, is caused by interaction of single-charged ions with HPA. However, the H⁺ and Na⁺ ions interact with HPA by different mechanisms and therefore affect the potential of the U⁵⁺/U⁴⁺ couple differently. Original Russian Text ? V.P. Shilov, A.B. Yusov, A.M. Fedoseev, Ph. Moisy, 2008, published in Radiokhimiya, 2008, Vol. 50, No. 5, pp. 393–396.     

A Study of the Magnetic Properties of?Bi1.64?xPb0.36CdxSr2Ca2Cu3Oy Superconductor

The flux pinning energy and magnetic properties of Bi_1.64−x Pb_0.36Cd _x Sr₂Ca₂Cu₃O _y (BPCSCCO) with x=0.0, 0.02, 0.04 and 0.06 were studied. A series of Bi-2223 superconductor samples with a nominal composition of BPCSCCO was synthesized and the effect of Cd substitution for Bi was investigated. As a result, Cd addition has been found to improve the superconducting properties of the Bi-Pb-Sr-Ca-Cu-O system. The effects of the annealing time and the amount of Cd doping on the structure, AC magnetic susceptibility, ρ–T curves and flux pinning energy were investigated. Also, for all samples the relation between the current and voltage in the mixed state was found to follow the model relationship V=α I ^β . The maximum value of β is 22.30, which is obtained for the sample with an annealing time of 270 h and a Cd content of 0.04.     

Structure study of MoO3-ZnO-B2O3 glasses by Raman spectroscopy and formation of α-ZnMoO4 nanocrystals

Molybdenum oxide (MoO₃)-containing glasses of xMoO₃-50ZnO-(50−x)B₂O₃ (x = 10, 20, and 30) are prepared using a conventional melt quenching method, and the glass structure and crystallization behaviour are clarified. It is found that the thermal stability against crystallization of the glasses decreases drastically with increasing MoO₃ content. The main valence of Mo ions in the glasses is found to be Mo⁶⁺ from X-ray photoelectron spectroscopy measurements. The Raman bands observed at ∼860 cm⁻¹ and 950 cm⁻¹ suggest that the coordination state of Mo⁶⁺ ions in the glasses is mainly (MoO₄)²⁻ tetrahedral units. All glasses examined in this study give the formation of α-ZnMoO₄ as the initial crystalline phase. In particular, 30MoO₃-50ZnO-20B₂O₃ glass shows the bulk crystallization of α-ZnMoO₄ nanocrystals with a diameter of ∼5 nm. The crystallized glasses consisting of Eu³⁺-doped ZnMoO₄ crystals are synthesized, and enhanced photoluminescence emissions (i.e., the quantum yield is 9%) due to the 4f transitions ⁵D₀ → ⁷F_J (J = 0-4) of Eu³⁺ ions is observed.     

Effect of piezoelectric grain size on magnetoelectric coefficient of Pb(Zr0.52Ti0.48)O3–Ni0.8Zn0.2Fe2O4 particulate composites

This study investigates the variation of magnetoelectric (ME) coefficient as a function of the piezoelectric grain size in the composite system of 0.8 Pb(Zr_0.52Ti_0.48)O₃–0.2 Ni_0.8Zn_0.2Fe₂O₄. It was found that as the piezoelectric-phase grain size increases the overall resistivity, piezoelectric, dielectric, and ferroelectric property of the composite increases and saturates above 600 nm. Below 200 nm average grain size, piezoelectric and dielectric properties decrease rapidly. The ferroelectric Curie temperature was found to decrease from 377 to 356 °C as the average grain size decreases from 830 to 111 nm. ME coefficient of the composite showed a rapid change below grain size of 200 nm and was found to saturate above 600 nm to a value of 155 mV/cm.Oe.     

Effect of the doped nitrogen on the optical properties of β-Ga2O3 nanowires

In this study, optical properties of the nitrogen-doped β-Ga₂O₃ nanowires (N-doped β-Ga₂O₃ NWs) were synthesized by exposing β-Ga₂O₃ NWs under high input power nitrogen plasma (2 kW), using a microwave plasma enhanced chemical vapor deposition (MPECVD) system. The nitrogen contents in the NWs were as-prepared about 7.4, 8.9, 9.7, 13.9, 19.3, and 26.6 at.%, respectively. Low temperature (10 K) cathodoluminescence (CL) spectra exhibit significantly different optical properties for the different nitrogen contents. The CL result of the N-doped β-Ga₂O₃ NWs (210 s N₂ plasma treatment) exhibited four distinct emission peaks at 378, 516, 759, and 970 nm. The possible light emission mechanism including the effect of the nitrogen dopant was discussed.     

Enhancement of the Current Density J C for Bi2Sr2CaCu2O8 by?Means of Carbon and NbSe2 Nanotubes

Three polycrystalline Bi₂Sr₂CaCu₂O₈, Bi₂Sr₂CaCu₂O₈ with carbon nanotubes, Bi₂Sr₂CaCu₂O₈ with NbSe₂ nanotubes were synthesized by solid state reaction method and studied by scanning electron microscopy, X-ray diffraction, magnetization measurements, and high resolution transmission electron microscopy. The critical temperature T _C for the three compounds was about 85 K. There is an enhancement in the critical current density, J _C for samples with carbon and NbSe₂ nanotubes as compared with pure Bi₂Sr₂CaCu₂O₈. The enhancement provides evidence that wetting exists for the two doped samples investigated.      

Effect of gradient composite structure in cofired bilayer composites of Pb(Zr0.56Ti0.44)O3–Ni0.6Zn0.2Cu0.2Fe2O4 system on magnetoelectric coefficient

This study investigates the ferroelectric, ferromagnetic, and magnetoelectric properties of the cofired bilayer composites consisting of piezoelectric phase with formulation 0.9 Pb(Zr_0.56Ti_0.44)O₃–0.1 Pb[(Zn_0.8/3Ni_0.2/3)Nb_2/3] + 2 (mol%) MnO₂ and 40 mol% ferrite phase with formulation Ni_0.6Zn_0.2Cu_0.2Fe₂O₄ (NCZF). A bulk composite of the same composition was also synthesized for comparison. Scanning electron microscope (SEM) investigation using quadrant back scattering detector (QBSD) shows migration of ferrite phases through the interface and energy dispersive X-ray spectroscopy (EDX) analysis with X-ray mapping clarifying these as Cu-rich phases. Improved piezoelectric (d ₃₃ ~ 80 pC/N), ferroelectric (polarization of 60 μC/cm² and 0.1% strain), higher magnetization (25 emu/g) and lower coercive field (2.8 Oe) were recorded for bilayer composite. The results indicate that the gradient bilayer composites with tailored composition such that the fraction of the secondary phase is higher may lead to better magnetoelectric material.     

Thermo-mechanical properties of La2NiO4+δ

Elastic and fracture behavior of La₂NiO_4+δ have been assessed. Fracture stress and elastic modulus of porous La₂NiO_4+δ were evaluated from room temperature (RT) up to 900 °C on the basis of 4-point bending tests. Both parameters increase slightly from RT to 700 °C. However, at higher temperatures the elastic modulus decreases, whereas the fracture stress increases. In addition, elastic modulus and damping/internal friction of dense specimens were measured by a resonance method. A strong change of elastic modulus and internal friction between RT and 100 °C appears to be related to an orthorhombic-tetragonal phase transition. No indications of phase transition can be observed at higher temperatures. Although thermogravimetric measurements suggest that oxygen was continuously released from the lattice up to 1000 °C with increasing temperature, the thermal expansion coefficient showed a rather stable value from RT up to 1000 °C.     

Characterization of epitaxial Bi2Sr2CaCu2O8+δ thin films

We report a dc sputtering method for the fullin situ preparation of Bi₂Sr₂CaCu₂O₈₊ thin films on SrTiO₃ and LaAlO₃.T _c values of more than 90 K can be achieved by oxidizing annealing below the melting point, followed by a reducing anneal at 500°C. The structural properties of the films are revealed by X-ray diffraction in Bragg-Brentano geometry (strongc-axis orientation with FWHM (0 0 10)=0.3) and also by scans (epitaxy within the substrate plane). Rutherford backscattering and channeling confirmed the correct composition of the cations while the minimum yield, _min, is 23%. Depth profiles by SNMS show a very homogeneous distribution of the cations with no detectable loss of bismuth near the surface. The surface morphology of the films was studied by SEM and by STM. Patterning of the films in lateral geometry can be performed by photolithographic techniques without degradation ofT _c .     

Microwave properties of highly oriented YBa2Cu3O7-δ superconducting thin films

We have performed millimeter-wave frequency (94 GHz) measurements on high-quality YBa₂Cu₃O₇- superconducting films on yttrium-stabilized (100) ZrO₂ and MgO substrates. The 0.2m thin films fabricated by magnetron sputteringin situ with the YBa₂Cu₃O₇- powders as target exhibit superconducting transition temperatures up to 88 K. The critical current density of 6×10⁵ A/cm² at 77 K and the X-ray diffraction spectrum as well as scanning electron microscope photographs indicate these thin films are fullyc-axis oriented, extremely high in density, and universally homogeneous. Millimeter-wave surface resistances have been measured on a hemisphere open resonator in the temperature range of 20 K toT _c and beyond. The surface resistance at 94 GHz and 77 K for these films is found to be about 30 m, nearly 1/4 that for copper, and a drop of two orders in the surface resistance within 4 K is observed, which indicates that these films are good materials for applications in the millimeter-wave range, especially for fabricating microwave devices. We observed such low surface resistance in these thin films due to the near absence of grain and phase boundaries coupled with a high degree of crystalline orientation.