Oxygen-enrichment of YBa2Cu3 YBa2Cu3O7-δ using the fluidization techniqueusing the fluidization technique

The oxygen-deficient phase of the highT _c superconductor, YBa₂Cu₃O₇, was oxygen-enriched using the fluidization technique to give good superconducting properties. The normal method of oxygen treatment at 900°C for 24 h and at 600°C for 24 h has been reduced to just one treatment at 600°C for 12 h by the fluidization technique to achieve almost the same strength of superconducting signal for the YBa₂Cu₃O₇ powder, which establishes the attractiveness of the latter route for the large-scale preparation of superconducting material. The particle sizes were in the range 0–90, 90–180 and 180–420 μm. The fluidized particles were crystalline with orthorhombic distortion.T _c ^onset , estimated using the a.c. magnetic susceptibility method, was 91·3 K. The volume fraction of superconducting material in the product was 83·7–85·3%, one of the highest values reported so far for YBa₂Cu₃O₇.     

Effect of oxide additives on the properties of high temperature superconductor,YBa2Cu3O7

The effect of oxide additives-CuO, SiO₂, Y₂O₃, Bi₂O₃ and ZnO in 1–10 mol% on the sintering and superconducting properties of YBa₂Cu₃O₇ was studied. SEM studies indicated improvement of grain size and interconnectivity due to the additives, the best results being obtained with Bi₂O₃, SiO₂ and Y₂O₃. The superconducting transition temperature is unaffected (92±2 K) even with 10 mol % of the additives. ZnO, however, decreases theT _c as expected.     

YBa2Cu3O7 superconductor as a mild,deep oxidation catalyst

Several Y-Ba-Cu-O compositions including the superconducting YBa₂Cu₃O₇ are scanned for their oxygen uptake using low temperature oxygen chemisorption (LTOC) technique as the principal probe at 195 K. BET surface area and pore size distribution were also determined. The results indicate their potential for use as mild, deep oxidation catalysts even under ambient and sub-ambient conditions.     

Chemical stability of the high-T csuperconductor YBa2Cu3O 7−δ +

YBa₂Cu₃ O_7−δ samples stored in different environments have been studied over a period of 30 days employing TGA, electrical resistivity and X-ray diffraction measurements. Oxygen content andT _cdo not change significantly in samples stored in the normal laboratory environment, in a sealed tube or in a vacuum desiccator. Superconductivity is lost only in samples stored in a very humid atmosphere around 320 K. Contribution No. 508 from the Solid State and Structural Chemistry Unit.     

Effect of substitution at the barium site in YBa2Cu3O7-δ

The oxygen content in YBa₂Cu₃O_7-δ has been varied by substitution at Ba site with Sr or K and its influence on the structural and superconducting properties has been studied. These properties are compared with that of unsubstituted YBa₂Cu₃O_7-δ prepared by quenching. The increase inδ value of YBa₂Cu₃O_7-δ decreases theT _c drastically, whereas in YBa_2-χK_χCu₃O_7-δ system increase in theδ value (as high as 0·92) does not affect the superconducting transition temperature, which remains above 77 K. In the YBa_2-χSr_χCu₃O_7-δ system, as Sr content increases there is marginal decrease in the oxygen content as well as in the superconducting transition temperature.     

Thermal and magnetic measurements on YBa2Cu3O7−y

Specific heat measurements in zero and7T magnetic fields from 0·4 to 100 K, and magnetic susceptibility measurements aboveT _c on a series of polycrystalline samples of YBa₂Cu₃O_7−y (YBCO) reveal a number of regularities. The size of ΔC(T _c )/T _c for the samples [ΔC(T _c ) is the jump in the specific heat atT _c ] appears to vary linearly with the low-temperature value of the Debye Θ, with the entropy change between ≈ 85 K andT _c induced by the application of a 7 ^Г magnetic field, and with the variation of the observed low temperatureγ with magnetic field, dγ/dH. On the other hand the temperature-independent part of the magnetic susceptibility aboveT _c appears to be essentially independent of ΔC(T _c )/T _c . These results are consistent with the idea that samples of YBCO belowT _c are a mixture of superconducting and normal phases. Supported by the Director, Office of Energy Research, Office of Basic Energy Sciences, Materials Sciences Division of the U.S. Dept. of Energy under Contract DE-AC03-76SF00098, and by an EXXON Education Grant from the Research Corporation.     

Effect of oxygen content on the electrical properties of YBa2Cu3O7−x single crystals

The effect of oxygen content in the single crystals of high-temperature superconductor YBa₂Cu₃O_7−x on the electrical resistivity, the Hall effect in the plane perpendicular to thec axis and the energy gap Δ, measured with tunnelling electron microscope, has been studied. The distribution of the gap along the surface of the crystal was also studied. The results of the study on the relationship between the magnitude of the energy gap Δ and the superconducting transition temperatureT _c of single crystals with various oxygen contents are approximated by the linear dependence 2Δ_av=4·4kT _c .     

Intrinsic Pinning in the Layered Superconductor YBa2Cu3O7 Studied by the Conversion of Ultrasonic to Electromagnetic Waves

Intrinsic pinning in a single-crystal YBa₂Cu₃O₇ superconductor has been investigated through the conversion of ultrasonic to electromagnetic waves. The interaction with a longitudinal ultrasonic wave was dramatically enhanced as a result of the intrinsic pinning of the vortex so as to form a sharp peak when a dc magnetic field was applied nearly parallel to the superconducting plane. Not only the height but also the angle width of the peak decreased with the temperature increase because of the weakening two-dimensionality. The crossover between the intrinsic pinning and the point-defect pinning is discussed by taking into account the pinning strength.     

Local lattice anomalies in underdoped YBa2Cu3O7-Σ probed by X-ray absorption fine structure

Temperature-dependent local lattice anomalies in underdoped YBa₂Cu₃O_7-Σ are studied by extended X-ray absorption fine structure (EXAFS) in a fluorescence mode. The in-plane polarized Cu A^-EXAFS is measured on underdoped epitaxial YBa₂Cu₃O_7-Σ films (100-120 nm) prepared onto SrTiO₃ substrates using a pulsed laser ablation technique. The Fourier transform (FT) results for a sample withT _c = 55K (△T = 4K) shows an in-plane lattice anomaly atT* = 83 K (~ 1.5T _c ) below which the in-plane oxygen distribution consists of the two Cu-O peaks separated by 0.2-0.3 A. The contribution of the elongated Cu-O bond increases on loweringT, indicating either the in-plane rhombic distortion of the square-planar CuO₄ or the tilting of the CuO₅ pyramid. Contrary to the anomalies in spin susceptibility and transport measurements indicating the presence of a spin gap, the onset temperature of the in-plane lattice anomaly (T*) shifts to the lowerT _c on decreasing the carrier density. The results suggest that the in-plane local lattice anomaly is anextrinsic lattice effect which is not directly related to the spin-phonon interaction.     

Growth and characterization of laser-deposited Ag-doped YBa2Cu3O7−x thin films on bare sapphire

Microstructural and superconducting properties of YBa₂Cu₃O_7−x thin films grownin situ on bare sapphire by pulsed laser deposition using YBa₂Cu₃O_7−x targets doped with 7 and 10 wt% Ag have been studied. Ag-doped films grown at 730°C on sapphire have shown very significant improvement over the undoped YBa₂Cu₃O_7−x films grown under identical condition. A zero resistance temperature of 90 K and a critical current density of 1·2×10⁶ A/cm² at 77 K have been achieved on bare sapphire for the first time. Improved connectivity among grains and reduced reaction rate between the substrate and the film caused due to Ag in the film are suggested to be responsible for this greatly improved transport properties.     

Structural Changes of YBa2Cu3O7-δSuperconductor with Co Substitution

YBa²(Cu^1-yCo^y)³O^7-δ superconductors with y = 0.00 ∼0.06 are synthesized. The content of oxygen increases with increase in cobalt content, while the oxidation of copper is almost independent of cobalt concentration. The orthorhombic-to-tetragonal transition occurs in the range of 0.028< y< 0.038 according to neutron powder diffraction studies on samples with y = 0.03. Structural refinements for X-ray and neutron diffraction data show that there is an orthorhombic phase with y≤0.02 and a tetragonal phase with y>0.04. There is an orthorhombic and a tetragonal phase coexisting in a sample withY = 0.03. The content of cobalt in the tetragonal phase (0.038) is larger than that in the orthorhombic one (0.028). It seems that the structure of YBa₂Cu_|1-y Co_{yO x} is unstable near the point of phase transition.     

Evidence for polarons and itinerant carriers in superconductivity from time-resolved optical measurements on YBa2Cu3O7-△

Time-resoived optical relaxation measurements on time scales from 10^-13 to 10^-8 s as a function of energy (0.8–3 eV) and temperature give evidence for the existence of two different carrier relaxation processes, indicating the presence of two distinct electronic subsystems in the normal state in high-temperature superconductors, one exhibiting band-like relaxation properties and the other those of localized, polaronic states. Both components of the electrodynamic response exhibit significant changes atT _c , indicating that both carrier types may be involved in superconductivity. The experiments also show that both polarons and band-like carriers exist predominantly in the CuO planes, with an out-of-plane component of the order of 10–20% which is close to the experimentally observed occupancy ratio of holes of O ions of the CuO5 pyramids. Spectrally, the superconducting condensate is also found to be coupled to two distinct components in frequency (for both carriers): one below 0.1 eV and one in the range 1.5-2 eV, but with different anisotropies. A natural explanation of these datas is that the two types of carriers correspond to the two phases suggested by the structural data and as such present the first direct evidence of two-component superconductivity. The underdoped sample data can be interpreted as evidence of pre-formed pairs aboveT _c .     

Preparation of epitaxial YBa2Cu3O7−x films by magnetron sputtering

Thin films of highT _c superconductor YBa₂Cu₃O_7−x were obtained by magnetron sputtering. MgO, YSZ, YSH and Al₂O₃ single crystals were used as substrates. Epitaxial films with tetragonal structure havingT _c 55–60 K grow at substrate temperaturesT _s between 930 K and 980 K. Orientation of the films in thisT _s range was (100) and (001) for (100) MgO substrate, (111) and (001) for (1012) Al₂O₃ and (111) YSH and (113) or (103) on (110) YSZ and (111) YSH. Single crystalline films with orthorhombic structure and (001) orientation were grown on all the substrates whenT _s exceeded 980 K. They haveT _c>80 K.     

Superconducting YBa2Cu3O7-δ thick films on Ba2RETaO6 (RE = Pr5 Nd, Eu, and Dy) substrates

Thick films of YBa₂Cu₃O_7-δ fabricated on polycrystalline Ba₂RETaO₆ (where RE = Pr, Nd. Eu, and Dy) substrates by dip-coating and partial melting techniques are textured and oaxis oriented, showing predominantly (00/) orientation. All the thick films show a superconducting zero resistance transition of 90 K. SEM studies clearly indicate platelike and needlelike grain growth over a wide area of the thick films. The values of the critical current density for these thick films are ∼10⁴ A/cm² at 77 K as determined by the nonresonant R.F. absorption method. Various processing conditions that affect the critical current density of thick films are also discussed.     

溶胶-凝胶法制备YBa2Cu3O7-x超导薄膜的研究

传统的TFA-MOD法制备YBa2Cu3O7-x薄膜,采用Y、Ba、Cu 3种金属的三氟醋酸盐(TFA)为先驱体,在热分解时,会产生大量的HF气体,即使通过将近20h的缓慢升温过程来对薄膜进行热分解,也难以获得较为光洁的表面,从而无法实现厚膜的制备.本文提出了一种新的含氟溶胶-凝胶工艺,通过减少溶胶中F的含量,并利用二乙醇胺做修饰剂,缩短了热分解时间,提高了薄膜的表面光洁度.利用该方法在(010)LaAlO3衬底上制备了具有良好c轴取向,临界转变温度为89K的YBCO超导薄膜.     

Oxygen inhomogeneities in YBa2Cu3O7−x single crystals withx>0.5 (oxygen inhomogeneities in YBa2Cu3O7−x …)

The microstructure of HTS YBa₂Cu₃O_7−x single crystals withx⩾0.5 has been investigated by TEM and selected area diffraction. An inhomogeneous oxygen distribution over the crystal was observed. Large differences between the bulk crystal structure and the surface have been established. The bulk structure was presented by orthorhombic blocks (≈100×10×20 nm in size) embedded in tetragonal matrix withx≈1. The bulk structure was not of a superconductive type. The crystal surface was enriched by oxygen and had the usual orthorhombic twinned structure. The superconductive properties of the whole specimen were determined by surface layer about 5 μm thick.     

YBa2 Cu3 O7-x单晶的制备

介绍了制备高温超导体YBa2Cu3O7-x单晶的主要方法,分析了在制备过程中存在的问题,指出液相的性质是影响生长速率的重要因素.在此基础上,着重探讨了通过控制氧压和元素替代等方法来改变液相的性质,从而达到提高YBCO单晶生长速率的目的.     

Oxygen ordering and chain formation in YBa2Cu3Oz

A distinctive aspect of the thermodynamic properties of YBa₂Cu₃O_z is the basal-plane oxygen ordering which leads to the formation of Cu-O chains and the development of orthorhombic phases. Theoretically, this phenomenon may be explained in terms of an Ising Hamiltonian with two types of second-neighbor interactions (ASYNNNI model). This symmetry breaking and ground-state degeneracy lead to kinetically arrested multi-domain structures. This paper reports new simulation results for these mesoscopic features, with specific emphasis on the effects of dopants, elastic deformation, and possible structural phase separation.     

Mössbauer study of 3d-ion-substituted YBa2(Cu,Fe, M)3O7−δ; M=V,Mn

⁵⁷Fe Mössbauer investigations at 300K have been carried out on V,Mn substituted YBa₂Cu₃O_7?δ system. Preliminary studies show that for small concentration of V (4%), Fe occupies preferentially the Cu(1) site whereas for the same concentration of Mn, Fe prefers the Cu(2) site. It is suggested that for low concentrations V has a preference for Cu(2) site whereas Mn prefers Cu(1) site in the 1-2-3 lattice.     

MOCVD-grown homoepitaxial YBa2Cu3Ox thin films and its multilayers

YBa₂Cu₃O _x (YBCO) films, Zn-doped YBCO (YBCO : Zn) films, and their bilayers have been epitaxially grown on SrTiO₃(100) and single-crystal YBCO(001) substrates by metalorganic chemical vapour deposition. The YBCO(001) films homoepitaxially grown on YBCO(001) substrates have flat surfaces on an atomic scale, and interfaces free from crystalline defects. We can systematically reduce the superconducting transition temperature (T _c) of YBCO : Zn films from 90 K to 37 K by increasing Zn concentration. The bilayers have a sharp distribution of Zn as evaluated fromT _c measurements of the upper YBCO films and depth profiles of secondary ion mass spectrometer, suggesting the possibility to form the homoepitaxial SNS (S, superconductor; N, normal metal) junction operatable between 40 K and 90 K.