Superconductivity of Annealed and Consolidated Amorphous YNi2B2C Powders

Amorphous YNi₂B₂C powders have been produced by mechanical attrition. Superconductivity is absent in the amorphous state but recovers upon crystallization. However, the superconducting transition temperature TC is lower than that of the melted analogue. TC can be increased by increasing the annealing temperature. High temperature compaction leads to dense bulk superconducting samples. TC of these bulk samples can be optimised by annealing. Variations in TC are attributed to deviations from stoichiometry. Contamination of the powders with WC wear debris upon annealing may deteriorate the superconducting properties.     

Influence of annealing conditions on structural properties of Y?Ba?Cu oxide superconductors

The chemical composition, the topography, and the defect structure of high-temperature superconducting Y Ba Cu oxide samples have been studied in dependence on the annealing conditions by scanning electron microscopy, energy dispersive X-ray analysis, Rutherford backscattering, nuclear reaction analysis, and transmission electron microscopy. The topography is characterized by smooth and rough grains. The chemical composition of samples varies in dependence on the annealing time and temperature. The microstructure of the superconducting material is dominated by a lamellar twin structure.     

Anomalous Up-and-Down Variation of Positron Lifetime Near Tc in High-temperature Superconductors

The temperature dependence of the positron lifetimes in the polycrystalline high T_c superconducting Y Ba Cu O and Bi Sr Ca Cu O and the non-superconducting Y Ba Cu O have been investigated by using the position lifetime technique. It is found from the experiments that there is an evident temperature dependence for the two kinds of superconducting samples but no temperature dependence for non-superconducting sample. It is important that the anomalous up-and-down variation of positron lifetime near T_c has been observed in the measured superconducting samples. We suggest that this strange temperature dependence of the positron lifetimes near T_C may result from some changes induced by the phase transition.     

Relationship between superconducting transition temperature and crystal-structure parameters of ceramics Tl2Ba2CuO6±x prepared by high-pressure cold pressing

The relationship between the superconducting transition temperature T _c and the crystal-structure parameters of high-temperature superconducting ceramic samples of the Tl2201 phase, which were prepared using high-pressure cold pressing, has been determined. The cold pressing under high pressure has been shown to lead to elongation of the Ba-Ba distances and a decrease in the hole concentration in thallium-containing high-temperature superconducting samples. The planar Cu-O1_plan distances and the structural distortion of the CuO₂ layers were found to have the greatest effect on the T _c value.     

The Particularities of Infinite‐Layer Compounds Sr1–xLnxCuO2–δ (Ln=La,Pr, Nd,Eu)

The superconducting phases with general formula Sr_1–xLn_xCuO_2–δ with Ln=La (Tc=24…40K); Pr (Tc=30…44 K); Nd (Tc=8K…40 K); Eu (Tc…40 K) and non‐superconducting phases of CaCuO₂– type structure were prepared by high‐pressure synthesis. The products were examined by X‐ray powder diffraction. For the determination of composition Rietveld method was used. Structure investigations have shown that non‐superconducting phases have oxygen vacancies. With in the Cu formal charge range from 1.81 to 1.93 the infinite layer cuprates exhibit superconductivity. Coexistence of both superconducting and non‐superconducting phases of the CaCuO₂–type structure in samples with Ln=Nd and Eu was found.     

Correlation between the residual resistivity ratio and the power‐law of the normal‐state resistivity in MgB2

MgB₂ polycrystalline superconducting specimens were irradiated with several doses of γ‐rays up to 100 MR. An increase in the normal state resistivity and a broadening of the resistive transition to the superconducting state were observed with increasing γ‐irradiation dose. Although very small changes to the superconducting transition temperature were obtained after γ‐irradiation, different temperature dependence of normal‐state resistivity and different residual resistivity ratios, RRR were obtained for different doses. We have found a correlation between RRR and the power law dependence of resistivity, n as the irradiation dose increases. This correlation may be an indication that the electron‐phonon interaction is important in these samples. These results are attributed to the disorder caused by γ‐rays. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Precision structural diagnostics of layered superconductor/ferromagnet nanosystems V/Fe by reflectometry and diffuse scattering of synchrotron radiation

Layered superconducting ferromagnetic nanosystems Cu(32 nm)/V(40–80 nm)/Fe(0.5–4 nm)/MgO(001) have been investigated by reflectometry and the diffuse scattering of synchrotron radiation. The data obtained make it possible to determine the important characteristics of samples such as the layer thickness and the rms heights and lateral correlation lengths of roughness at the interfaces.     

Superconducting properties of an NbN–SiO2 disordered system obtained by ammonolysis of NbN–SiO2 sol–gel derived coatings

Studies in superconducting properties of NbN–SiO₂ films are reported. The films were obtained through nitridation of sol–gel derived Nb₂O₅–SiO₂ coatings at 1200 °C, a process leading to the formation of disordered structures with NbN metallic grains dispersed in the insulating SiO₂ matrix. Electrical resistivity was measured with the conventional four-terminal method in the temperature range from 5 to 280 K. The samples’ superconducting properties, examined with magnetically modulated microwave absorption (MMMA), depend on the NbN/SiO₂ molar ratio and the film’s thickness.     

The influence of small deviations from stoichiometry on the properties of Y?Ba?Cu?O superconductor ceramics

According to a central composite design samples with deviations from stoichiometry YBa₂Cu₃O_7−x of ±2.5 wt% in the ingredients were prepared. The influence of these deviations on phase content, superconducting properties and microstructure of the ceramics was studied.     

On the synthesis and characterization of La doped MgB2 superconductor

We report the synthesis of La doped MgB₂ superconductors with nominal compositions Mg_1‐xLa_xB₂ (with x = 0.01, 0.03, 0.05, 0.07) by solid state reaction at ambient pressure. A special encapsulation technique has been used by us to prepare high quality superconducting MgB₂ samples. The bulk polycrystalline samples possess superconducting transition temperature T_c(R=0) ranging between 36‐39 K. It has been found that transport critical current density J_c of the samples change significantly with the doping level of La. A high transport (J_c) value ∼1.9 x 10³ A/cm² at 15 K has been achieved for Mg_0.97La_0.03B₂ sample. The XRD and TEM investigations indicate that the samples prepared by encapsulation method are devoid of MgO, which is generally found when synthesis of MgB₂ is done through sintering of Mg and B powders. The detailed microstructural investigations of Mg_0.97La_0.03B₂ specimens by transmission electron microscopy (TEM) reveal the presence of partial dislocation network, moiré fringes and superlattice structure in the as synthesized samples. The higher transport critical current density observed in Mg_0.97La_0.03B₂ superconductor has been attributed to the partial dislocations which are capable of providing pinning centres. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Enhancing the pinning strengths in polycrystalline MgB2

The dependence of the critical current density, J_c, on the gamma irradiation dose in MgB₂ polycrystalline superconducting specimens has been studied. The MgB₂ samples were irradiated with 10 MR and 20 MR doses of γ‐rays. The critical current density, J_c was found to be enhanced almost at all temperatures after irradiation. The enhancement of J_c was found to be linear with the irradiation dose at all temperatures. We also report on the correlation between the superconducting transition temperature and the residual resistivity ratio. Most of our results agree with the Testardi and Mooij rules which suggest that the dominant pairing mechanism of MgB₂ is the phonon‐mediated interaction. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Structure and electrical properties of nitrided NbN–TiN sol–gel derived films

The results of investigations of electrical conductivity and the structure of NbN–TiN thin films in a different NbN/TiN molar ratio are presented in this work. Sol–gel derived xNb₂O₅?(100?x)TiO₂ coatings (where x = 100, 90, 80, 70, 60, 50, 40, 0 mol%) were nitrided at 1200 °C to obtain NbN–TiN films. The structural transformations occurring in the films as a result of ammonolysis were studied using X-ray diffraction (XRD), atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). The electrical conductivity was measured with a conventional four-terminal method in the temperature range of 5–280 K. The NbN–TiN samples exhibited a negative temperature coefficient of resistivity. The positive temperature coefficient of resistivity was observed only for the x = 0 sample. The results of conductivity versus temperature may be described on the grounds of a model proposed for a weakly disordered system. The film thickness effect on the superconducting properties was studied for x = 80 and x = 100 samples. The superconducting transition was not observed in all samples, the exception was x = 80 sample, 1050 nm in thickness. It is not clear, why all x = 100 samples do not exhibit superconducting transition in resistivity measurements. It seems to be possible, that the Josephson junction formation between NbN grains could be blocked by non-superconducting phases present in these samples.     

Magnetic properties of TiCx/C nanocomposites

The magnetic properties of nanocrystalline titanium carbide dispersed in a carbon matrix (TiC_x/C) prepared by the non-hydrolytic sol–gel process have been studied by dc magnetization measurements. The superconducting phase of titanium carbide has been observed at low temperatures with the onset of the superconducting transition temperature T_c at about 3.5 K, superimposed on a ferromagnetic component. At T > T_c the magnetic response of TiC_x/C is determined by the interplay of the ferromagnetic contribution with the paramagnetic/diamagnetic signal of the metallic system and the contribution of exchange coupled paramagnetic ions. Moreover, significant differences are observed in the magnetic response for samples of the same preparation batch, indicative of the magnetic/electronic inhomogeneity of nanocrystalline titanium carbide which is important for its practical applications.     

Role of Silver on Superconducting Properties of BPSCCO (2223) System

The role of silver for the superconducting properties of the Bi Pb Sr Ca Cu O (2223) system have been investigated systematically. Samples with various concentrations of Ag were prepared by the matrix method. D.C. resistance results showed that T_c(0) varied between 100–109 K for various compositions of Ag. J_c measured at LNT from I–V data shows an increase with silver addition. J_c decreased from 90 Amp/cm² at zero field to 16 Amp/cm² at 100 Oe. The T_c(0) obtained form the susceptibility data agreed with those obtained from resistance measurements. X-ray diffraction results showed that the dominant phase in BPSCCO is the high-temperature (2223) phase and the same is improved in silver added BPSCCO samples. The scanning electron micrographs indicated that silver is precipated along the grain boundaries, separating the superconducting grains. These results have showed that silver addition does not destroy the superconductivity and at the same time enhances the critical current density.     

Nucleation of an MTG YBa2Cu3Oy

The kinetics of the appearance of the superconducting 123 phase in partially melted at 1200 °C YBCO samples has been studied in the temperature range of low undercoolings from 1040 °C to 920 °C. An EDS analysis of the phases formed at different stages of the cooling of the melt has been performed and the experimental conditions for crystallization of 123 phase with finely dispersed 211 phase in it have been established. The results have been interpreted in terms of the classical theory of nucleation and they have been applied to the MTG process for formation of highly textured YBa₂Cu₃O_y materials.     

Magnetic Levitation Force of Bulk YBaCuO Pieces as a Function of Texture

Very well textured YBaCuO forms can be produced by means of seed crystals implanted in an YBaCuO melt. Narrow distributions of the directions (FWHM < 6°) of the c‐axes of the crystalline grains as well as the a‐ and b‐axes, respectively, are required for this purpose. Small angles between the c‐axes of adjacent grains, and small angles between the a‐ and b‐axes, respectively, of the neighbors, constitute but small obstacles in the path of the superconducting current induced. If the entire volume of the melt‐textured forms, in this case YBaCuO cylinders, is well textured, and if no random volume fractions remain, a maximum inducible superconducting current in the superconductor can be achieved and, thus, a maximum magnetic repellent force against the inducing permanent magnet, can be attained. This ensures maximum magnetic levitation force for a superconducting self‐stabilizing magnetic bearing.     

Electron Energy-Loss Spectroscopy Study of the Change in the Free-Electron Density in Thin Superconducting NbN Films under Ion-Beam Irradiation

The change in the free-electron density in ultrathin (5 nm) superconducting NbN films in the initial state and after irradiation by O⁺ ions to doses of (0.1–0.9) × 10¹⁷ cm^–2 has been studied by electron energy-loss spectroscopy (EELS). The analysis has been performed on cross section samples prepared by the focused ion beam method, using plasmon oscillations with energies up to 50 eV. The radiation-induced replacement of nitrogen atoms with oxygen atoms in niobium nitride is found to change the electrical properties of the material, which leads to a decrease in the free-electron density with an increase in the irradiation dose.     

Superconducting Properties in Metallic Phase of NbS3

Metallic phase of NbS₃ was synthesized by heating the crystal of semiconducting NbS₃. Metallic NbS₃ undergoes superconducting transition around 2 K. The analysis of the angular dependence of the upper critical magnetic field suggests that the effect of filmy or fibrous morphology plays an important role in the superconducting properties.     

实用化Bi系超导带材的制备工艺研究进展

超导材料具备优异的电学和磁学性能,具有很大的发展前景。目前应用最多的是NbTi和Nb₃Sn两种低温超导材料,但是其需要在4.2 K超低温下(液氦制冷)使用,成本高昂。Bi-2223的临界超导温度高达110 K,液氮制冷就可以使用,展现出良好的实用价值。Bi-2223是一种层状结构的化合物,常被制备成带材,使层面方向平行于带材表面,以发挥其最佳超导性能。但是将其实用化过程中还存在着Bi-2223相纯度不高、致密度不够、晶粒连接性不佳、抗拉伸强度和延伸率不足等问题。为了能够实用化,必须兼顾电学性能、机械性能、制冷技术等因素。本文综述了实用化Bi系超导带材的制备工艺与研究进展,分别对Bi-2223前驱粉制备工艺、Bi-2223带材的制备工艺及研究进展、Bi-2223超导带材实用化进展等进行介绍,为其制备工艺和实用化的进一步深入研究提供参考。     

TEM Investigation of Stacking Faults in the Superconducting Compound YNi2B2C

To evaluate the influence of composition and high-temperature heat treatment on the occurrence of stacking faults and intergrowths in the layered superconducting compound YNi₂B₂C a series of polycrystalline samples and a single crystal were investigated by TEM. Stacking faults were found in the most inhomogeneous sample only, which was annealed at 1100°C. Homogenisation at temperatures of 1450°C and higher results in a recovery of stacking faults. The observed faults should develop during crystallisation by formation of extra planes of YC or Ni₂B, resulting in a local deviation from the stoichiometric composition. In highly faulted areas the distance of stacking faults was about 50nm. These faults have apparently no influence on the superconduction transition temperature T_c.