Effect of vibration frequency and sample composition on acoustic properties of Y?Ba?Cu?O high-Tc superconductors

Temperature dependences of absorption and velocity of longitudinal ultrasound in high-T_c ceramics Y Ba Cu O with different Y and Ba content at frequencies 100 kHz and 7.5 MHz are studied in the temperature range 6 to 300 K. For 100 kHz three peaks at 60, 140, and 230 K are observed in the temperature dependences of decrement. Temperature dependences of sound velocity show a broad hysteresis for thermocycling. The temperature boundaries of hysteresis and peak locations are almost independent of sample composition. Sizes of hysteresis and peak heights are very different in samples of various composition. For 7.5 MHz, a peak was found at 210 K in the temperature dependences of ultrasound absorption increment. It is shown that the acoustic anomalies are due to a structural phase transition of martensitic type. Thermoactivation parameters of the main microscopic mechanism responsible for the phase transition are estimated.     

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.     

Effect of the relative content of Y,Ba, and Cu on the superconducting transition characteristics of the Y?Ba?Cu?O System

By varying the content ratio of the initial products, nine compounds of the Y Ba Cu O system have been produced. These were studied to find their T_c values, critical currents j_c at 77 K and effects of compression stress on the U(j)-curves. Mechanical loading has been found to result in a shift of the U(j)-curve towards a larger current. A correlation between the critical current value, the slope of the U(j)-curve, and the stress induced shift is shown to exist. Samples with equal absolute deviations from the stoichiometric composition reveal close properties, the better the smaller is the deviation.     

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.     

Positron lifetime study in BaTiO3 ceramics with different Ba/Ti ratios

The positron lifetime spectra in 0.25 at.% La-doped BaTiO₃ with different Ba/Ti ratios have been measured at room temperature. It was found from the experiments that with deviating from exact stoichiometry the positron lifetime τ₂ decreases and the intensity I₂ increases for Ba/Ti ratio smaller than stoichiometric and they have more significantly similar changes for Ba/Ti ratio larger than stoichiometric. The non-monotonic dependence of positron lifetime parameters on Ba/Ti ratio is regarded as that the τ₂ component is contributed together by positrons annihilating at the grain boundaries and Ba vacancies and/or Ti vacancies, and small deviations from exact stoichiometry would be expected to increase the concentration of defects such as Ba vacancies for Ba/Ti < and Ti vacancies for Ba/Ti > 1.     

Resistance and Current Density Studies on Plasma Sprayed Coatings of Bi(Pb)?Ca?Sr?Cu?O Superconducting Materials

Plasma spraying is a potential technique for forming flexible tapes from brittle high T_c oxides. It is possible to obtain superconducting Bi(Pb) Ca Sr Cu O coating by suitable heat treatment schedule after spraying. In an effort to get maximum transport current densities (J_c) of the coating, the content of lead and sintering time have been optimised. A J_c value of 200 Amp/cm² is obtained in Bi_1.4Pb_0.6 · Ca₂Sr_1.9Cu₃O_y specimen coated on silver sprayed Fe[(Ag)/Fe] substrate. Remarkable improvement in J_c values up to 694 Amp/cm² is obtained in the same specimen coated on Ca₂Sr_1.9Cu₃O_y sprayed Fe[Ca₂Sr_1.9Cu₃O_y)/Fe] substrate. The observed decrease in J_c(B) curves with increase in magnetic field shows the presence of weak coupling between the grains.     

NMR Studies of Silicon Clathrate Compounds

Abstract

NMR spectra of ²⁹Si and ¹³⁷Ba in silicon clathrate compounds, Na_xBa_ySi₄₆, Na_xSi₁₃₆ and Ba₈ T _xSi₄₆ (where T = Au, Ag, and Cu) are reported. We found that the local density of states at the Fermi level increases with rising superconducting transition temperature. The silicon Sl site was suggested to hybridize with Ba-orbital in the superconducting clathrate and has a large local density of states.     

Strontium-based Mercury Cuprates with Perovskite-like Crystal Structures

Conditions of synthesis, structure, and temperatures of superconducting transitions of strontium-based mercury cuprates Hg—Sr—Me—Cu—O (Me = Ca, La, Ce, Sm, Ho) with perovskite-like crystal structure are described. The samples belong to two homological series: HgSr₂ (Ln, Ce)_n–1CunO_2n + _{2 + δ} with the Hg—O bond directed along <110> and (Hg, Cu) Sr₂Ln_n–1Cu_nO_{2n + 2 + δ} with the (Hg, Cu)—O bond directed along <100>. The variation of composition leads to rotation of the Hg—O bond along the hk0 direction and consequently to the increase of T_c from 28 K to 81 K. An anomaly of the resistivity was observed around 200 K for most of the investigated samples, independent of their content and phase composition.     

Thermal expansion in CuAlTe2 compound

The lattice parameters a and c as well as the axial thermal expansion coefficients α ⟂ and α ∥ in the CuAlTe₂ chalcopyrite-type compound are determined as a function of temperature in the range from 80 to 650 K by a X-ray diffractometry technique. The data obtained are used to evaluate the axial ratio c/a, the tetragonal distortion δ = 2 — c/a, the interatomic distances for Cu Te and Al Te bonds and their temperature coefficients. It is found that the thermal expansion behaviour of CuAlTe₂ is similar to that of other CuB^IIIC^VI₂ compounds in having a relatively small expansivity along the c-axis and a large one in the perpendicular direction. When comparing the results for a series of the CuB^IIIC^VI₂ compounds (B Al, Ga, In; C S, Se, Te) it is shown that the correlations between the thermal expansion coefficients α ⟂, α ∥, α_m, dδ/dT and the tetragonal distortion δ, as well as the molar mass of the compound take place.     

Positron Lifetime and Defects in Y-doped (Ba0.6Sr0.4)TiO3 Ceramics

The positron lifetime spectra in Y-doped (Ba_0.6Sr_0.4)TiO₃ ceramics have been measured at room temperature for Y contents of 0–0.8 at. %. It was found from the experiments that with increasing Y content, the variation of the positron lifetime parameters is nonmonotonic, and the Y content corresponding to the knee is about 0.7 at. %, which is much larger than the La content corresponding to the knee in the La-doped BaTiO₂ (TANG 1994a). The experimental results show that the Y ions substitute predominantly for Sr ions, and this substitution induces a large number of Sr vacancies; when the Y content increases to about 0.7 at. %, the Y ions begin to substitute for Ba ions, and then the associated defects (2 Y_BaV“_SrSr) and (2 Y_SrV”_BaBa) form gradually.     

Synthesis and X-ray Crystal Structure Analysis of CU2(CH3CH2COO)4(OP)2

The complex of [CU(CH₃CHCOO)₂(OPph)]₂ has been synthesized and its X-ray crystal structure determined at room temperature, M = 975.96, tetragonal, space group: 14_1/a(#88), Dx = 1.39 g/ cm³, The final R is 0.067 for 2087 independent observed reflections with 1 > 3α(I). The molecule has an inversion center on the middle of the Cu Cu axis. The bond-length of the Cu Cu is 2.61/(2) Å. The coordinate polyhedron of Cu corresponds to a tetragonal bipyramid. The angle of Cu O P is significantly smaller than that of its adducts [Cu(O₂CR)₂L]₂.     

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.     

A Novel Coordination Structure of Dichromates Bonding to Copper (II): [Cu(bipy)2(Cr2O7)] · 2 H2O

Dark-green multimetal compound crystal [Cu(bipy)₂(Cr₂O₇)]· 2 H₂O was obtained from aqueous solution of Cu(NO₃)₂, K₂Cr₂O₇ and bipyridine. The crystal structure was determined by X-ray crystallography: triclinic P1 , a = 7.716(3) Å, b = 9.656(3) Å, c = 15.517(5)Å, α = 77.41(3)°, β = 81.04(3)°, γ = 82.33(3)°, Z = 2. In this compound, two chromium atoms and a copper atom are linked by two oxo bridges (Cu(II) O Cr(VI) O Cr(VI)). The copper coordination polyhedron corresponds to a five-coordinated distorted trigonal bipyramid.     

The influences of O/Zn ratio and growth temperature on carbon impurity incorporation in ZnO grown by metal-organic chemical vapor deposition

The strong correlations between the O/Zn ratio and carbon impurity incorporation have been observed on the ZnO films grown using N₂O or O₂ as oxygen source in metal-organic chemical vapor deposition (MOCVD). From in-situ mass spectrometric measurements, the O/Zn ratio in the MOCVD reactor is found to decrease to a minimum value as the growth temperature increased till a critical growth temperature T_c, and then increased above T_c due to different dissociation rates of the oxygen and Zn sources. The strongest D and G modes, which are ascribed to carbon clusters sp² related modes, have been observed in Raman scattering spectroscopy for the ZnO samples grown at T_c, indicating the highest incorporation rate of carbon impurity in the samples grown at T_c. Compared with O₂, N₂O has a low dissociation rate and that leads to a lower value of O/Zn ratio, resulting in much stronger D and G modes and higher incorporation rate of carbon impurities in the samples grown at T_c. It is interesting to note that the lowest specific resistances from Hall effect measurements were also obtained on the samples grown at T_c, indicating possible electrical contributions from the formation of carbon clusters, which should be highly conductive regions in ZnO. Furthermore, ionization or addition of H₂ in the case of N₂O can significantly enhance the dissociation of N₂O, with film quality improved significantly. This study shows that a high O/Zn ratio is critical to suppress carbon impurity incorporation and to grow high quality ZnO by MOCVD, especially at low growth temperature.     

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)     

Erholung und Rekristallisation des stabilen Austenits bei der Verformung durch Walzen

For the explanation of the processes which take place during a high-temperature thermomechanical treatment (HTTT) in the stable austenite hardness, grain structure and dislocation structure of a Fe 24Ni 0,5C alloy are examined in dependence of the degree and temperature of rolling deformation (ϵ = 13 to 52%; T_v = 830 and 1150°C). The observed structures can be represented by a recovery-recrystallization-diagram, that describes the influence of technological parameters on the recovery and the recrystallization of stable austenite.     

Diffusion in bulk-metallic glass-forming Pd–Cu–Ni–P alloys: From the glass to the equilibrium melt

Since the discovery of bulk-metallic glasses there has been considerable research effort on these systems, in particular with respect to mass transport. Now the undercooled melt between the melting temperature and the caloric glass transition temperature, which has not been accessible before due to the rapid onset of crystallization, can be investigated and theories can be tested. Here we report on radiotracer diffusion measurements in metallic bulk-glass-forming Pd–Cu–Ni–P alloys. Serial sectioning was performed by grinding and ion-beam sputtering. The time, temperature as well as the mass dependence, expressed in terms of the isotope effect E, of Co-diffusion were investigated. In the glassy state as well as in the deeply supercooled state below the critical temperature T_c, where the mode coupling theory predicts a freezing-in of liquid-like motion, the experimentally determined very small isotope effects indicate a highly collective hopping mechanism involving some ten atoms. Below T_c the temperature dependence shows Arrhenius-type behavior with an effective activation enthalpy of 3.2 eV. Above T_c the onset of liquid-like motion is evidenced by a gradual drop of the effective activation energy and by the validity of the Stokes–Einstein equation, which is found to break down below T_c. This strongly supports the mode coupling scenario. The Stokes–Einstein equation is presently tested for other constituents of the alloy. The Co isotope effect measurements, which have never been carried out near T_c in any material, show atomic transport up to the equilibrium melt to be far away from the hydrodynamic regime of uncorrelated binary collisions.     

Optical Reflection Spectra of Silicon Clathrate Compounds Ba8AgxSi46−x

Abstract

Optical reflection spectra are measured for Ba₈Ag_xSi_46?x (0 ≤ x ≤ 6) at room temperature. A systematic decrease in the plasmon energy is found with increasing x, indicating that the carrier concentration decreases with increasing x. When x increases, the superconducting transition temperature T_c decreases. An origin of the decrease in T_c is assigned to the decrease in the density of states at the Fermi level due to the decrease in the carrier concentration.     

Zu dem Transportsystem V-O-Te-Cl und dem Transportverhalten der Vanadinoxide mit TeCl4(II). Der chemische Transport der Magnéliphasen VnO2n–1, des V6O13 und des V2O5

It was reported on the transport system V O Te Cl and on the transport behaviour of VO₂ in a previous paper. This paper deals with the chemical transport of Magnéli phases V_nO_2n–1, and of V₆O₁₃, and of V₂O₅. Resulting from the composition of the gas phase over the solid phases in the presence of TeCl₄ the chronological sequence of the transported phases as well as the direction and rate of transport are calculated. These results are compared with experiments. All of the oxides are obtained without deposition of any other phases. Their phase boundaries are determined. The dependence on temperature is investigated for the upper phase boundary of V₂O₃.     

Growth,structure, and superconducting properties of Bi2Sr2Ca2Cu3O10 and (Bi,Pb)2Sr2Ca2Cu3O10‐y crystals

Large and high‐quality single crystals of both Pb‐free and Pb‐doped high temperature superconducting compounds (Bi_1‐xPb_x)₂Sr₂Ca₂Cu₃O_10‐y (x = 0 and 0.3) were grown by means of a newly developed “Vapour‐Assisted Travelling Floating Zone” technique (VA‐TSFZ). This modified zone‐melting technique was realised in an image furnace and allowed for the first time to grow Pb‐doped crystals by compensating for the Pb losses occurring at high temperature. Crystals up to 3×2×0.1 mm³ were successfully grown. Post‐annealing under high pressure of O₂ (up to 10 MPa at T = 500°C) was undertaken to enhance T_c and improve the homogeneity of the crystals. Structural characterisation was performed by single‐crystal X‐ray diffraction (XRD) and the structure of the 3‐layer Bi‐based superconducting compound was refined for the first time. Structure refinement showed an incommensurate superlattice in the Pb‐free crystals. The space group is orthorhombic, A2aa, with cell parameters a = 27.105(4) Å, b = 5.4133(6) Å and c = 37.009(7) Å. Superconducting studies were carried out by A.C. and D.C. magnetic measurements. Very sharp superconducting transitions were obtained in both kinds of crystals (ΔT_c ≤ 1 K). In optimally doped Pb‐free crystals, critical temperatures up to 111 K were measured. Magnetic critical current densities of 2�10⁵ A/cm² were measured at T = 30 K and μ₀H = 0 T. A weak second peak in the magnetisation loops was observed in the temperature range 40‐50 K above which the vortex lattice becomes entangled. We have measured a portion of the irreversibility line (0.1‐5 Tesla) and fitted the expression for the melting of a vortex glass in a 2D fluctuation regime to the experimental data. Measurements of the lower critical field allowed to obtain the dependence of the penetration depth on temperature: the linear dependence of λ(T) for T < 30 K is consistent with d‐wave superconductivity in Bi‐2223. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)