Melt processed YBa2Cu3O7−x superconductors

The superconducting YBa₂Cu₃O_7−x samples were prepared by an Arc-Cast-Annealing (ACA) and Arc-Quench-Powder-Growth (AQPG) processes as modifications of QMG and MPMG techniques. Pe'lets of YBa₇Cu₃O_7−x were quenched by arc-casting in a water cooled copper mould and then the solidified rods were annealed at different temperatures and times to store the superconductivity. Annealed at an appropriate temperature the cast rods showed rising superconducting properties with increasing the annealing time. Some of the rods after solidification were crushed to give powder which was compacted and then subjected to a melt growth process. As a result of this processing, large grained textured YBCO superconductors with dispersed 211 inclusions in the superconducting grains were produced. The microstructure and physical properties of these ACA and AQPG samples were investigated when subject to various temperature cycles. It was found that the volume fraction and size distribution of the second phase inclusions were dependent upon the maximum temperature during the melt growth process. The critical current density (J_c) for ACA and AQPG samples was estimated from magnetization loops using Bean's critical state model. It was found that the value of J_c of AQPG sample was much higher than that of ACA sample.     

The effect of BaZrO3 addition on melt-processed YBa2Cu3O7–x superconductors

The effects of BaZrO₃ addition on the thermal behavior and microstructure of YBCO superconductors have been investigated. The differential thermal analysis indicates that the peritectic decomposition temperature of YBCO precursor powder is not change when BaZrO₃ is doped. The solidification temperature of YBCO changes with increasing of BaZrO₃ addition, thus the window of solidification temperature is affected. The results of scanning electron microscopy show that the BaZrO₃ particles are accumulated along the growth boundary, which result in the constitutional segregation of YBCO. The pushing–trapping theory is used to explain the phenomenon of segregation in the YBCO matrix.     

The equilibrium vortex melting transition in YBa2Cu3O7

The dynamic and thermodynamic experimental evidence supporting first order vortex melting in clean crystals of YBa₂Cu₃O₇ is reviewed.     

Fine structure and the mechanism of the low-temperature decomposition of the nonstoichiometric compounds YBa2Cu3O6.8 and YBa2Cu3O6.8 doped with Ce

The resistance of the nonstoichiometric compounds YBa₂Cu₃O_6.8 (single crystals, ceramics) and YBa₂Cu₃O_6.8 (Ce) (single crystal) to low-temperature decomposition (at 200°C) in air and an argon atmosphere is studied by X-ray diffraction and electron microscopy. At the first stage, the compounds are found to undergo oxygen separation into two phases, namely, oxygen-rich and oxygen-lean phases as compared to the initial state, that have different lattice parameters. After 20-to 35-h annealing, the disordering of the heavy Y, Ba, Ba, Y atoms along the c axis in ceramic pellets begins mainly because of the action of elastic stresses. This process occurs via the formation of the packets of numerous stacking faults on (001) planes. The disordering of the ceramic matrix ends in the formation of a CsCl-type cubic phase upon 100-h annealing, which is accompanied by a significant decrease in the diamagnetic response and an increase in T _c from 75 K in the initial state to 90 K. The retained superconductivity and the increase in T _c are caused by the presence of a large number of oxygenrich ortho-phase particles in a nonsuperconducting matrix; these particles are correlated with each other and form a multicoupled system of superconducting filaments. Original Russian Text ? S.V. Sudareva, E.P. Romanov, T.P. Krinitsina, E.I. Kuznetsova, Yu.V. Blinova, I.B. Bobylev, N.A. Zyuzeva, A.M. Burkhanov, 2008, published in Fizika Metallov i Metallovedenie, 2008, Vol. 106, No. 4, pp. 378–387.     

Bi2Sr2CaCu2Ox bulk superconductor with MgO particles embedded

MgO nano-size particles in various concentrations were added to the Bi-2212 superconductor precursor before partial melting. The resulting ceramic superconductors had a transition sharpness and volume fraction which increased with MgO addition up to 9 wt.%. Compositional variations in the superconducting matrix with varying MgO addition were found, with possible important consequences for the pinning mechanism, since compositional variations are expected to be accompanied by variations in T_c. By TEM it was directly documented that MgO particles could be embedded within the superconducting Bi-2212 grains. Hence wetting occurs. Thus, the requirements for MgO to play an effective role in pinning are satisfied. It is suggested that two distinct pinning mechanisms, both related to MgO addition, are likely to be present. Magneto-optic images clearly confirm the improvement of pinning, and screening, with increasing addition of MgO to Bi-2212.     

Structural and superconducting properties of YBa2Cu3−ξAlξO6+σ (0ξ0.20)

A series of YBa₂Cu_3−ξAl_ξO_6+σ (0ξ0.20) samples are prepared and their morphology and structural properties are investigated by scanning electron microscopy energy dispersive X-ray analysis and X-ray diffraction techniques. Superconducting parameters are obtained from the a.c. magnetic susceptibility and critical current density measurements. Transformation to a tetragonal phase occurs for ξ 0.2 and T_c. decreases strongly relative to the orthorhombic phase. A few degrees decrease of T_c upon tetragonal phase transformation is consistent with the importance of Cu(2) planes for the superconductivity in these oxides.     

On the extrusion of silver-sheathed superconducting billets fabricated by explosive compaction of YBa2Cu3O7 powder

Explosive compaction of YBa₂Cu₃O₇ superconducting powder and forward extrusion of the produced “green” billet were employed to fabricate high critical temperature, T_c, superconducting composite rods consisting of a superconducting YBa₂Cu₃O₇ core and a silver metallic sheath. Experimental observations regarding the “soundness” of the fabricated component with respect to the compacting and processing conditions used as well as to the properties of the superconducting powder are reported. Microstructural and stochiometric changes and defects occuring in the “green” compacted billet and the extruded rod are indicated and discussed.     

Effect of Nd doping on the grain-matrix contribution to ac losses in sintered Bi1.6Pb0.4Sr2(Ca1−xNdx)2Cu3Oδ superconductors

We have carried out measurements of complex ac susceptibility χ=χ′+χ″ as a function of temperature and ac field amplitude on rectangular bar-shaped high-temperature superconductors (HTS) with nominal composition of Bi_1.6Pb_0.4Sr₂(Ca_1−xNd_x)₂Cu₃O_δ superconducting samples prepared by the solid-state reaction method. The effect of Nd-substitution on the Bi–(Pb)–Sr–Ca–Cu–O system has been investigated in terms of ac susceptibility study. We estimated the effective volume fraction of the grains and the field dependence of the inter-granular critical current density comparing the maximum of the extracted matrix susceptibility and the corresponding calculated data which was obtained employing the power law critical state model.     

Subsolidus phase relation and crystal structure of the PrBa2−xSrxCu3O7±δ system

The PrBa_2−xSr_xCu₃O_7±δ solid solution was investigated by means of X-ray powder diffraction in combination with Rietveld analysis. The Sr-doped Pr123 single phase could be synthesized at 950 °C in air. The solubility of PrBa_2−xSr_xCu₃O_7±δ solid solution is 0.2≤x≤0.6. The structure of PrBa_2−xSr_xCu₃O_7±δ is orthorhombic for x=0.2. The structure transforms into tetragonal for 0.3≤x≤0.6. In the PrBa_2−xSr_xCu₃O_7±δ structure, Sr ions can replace Ba ions, the highest value is x=0.6 under our experimental condition. But Sr ions could not replace Pr ions. Furthermore Pr ions could not occupy the sites of Ba ions in the PrBa_2−xSr_xCu₃O_7±δ system. Both ionic radii and chemical properties play an important role in the mutual substitution of Pr, Ba and Sr ions in the Pr123 structure of the PrBa_2−xSr_xCu₃O_7±δ system.     

（TiB2＋TiAl3）／AlSi6Cu4复合材料的研究

利用K2TiF6 KBF4混合盐和AlSi6Cu4熔体反应可以制备TiB2 TiAl3颗粒增强AlSi6Cu4基复合材料。通过XRD和SEM等仪器对复合材料的凝固组织和力学性能进行了研究,随K2TiF6和KBF4混合盐加入量的增加,原位反应生成的颗粒尺寸减小,分布更均匀;所得到的AlSi6Cu4复合材料的强度比AlSi6Cu4基体明显提高。     

Effect of holmium additions on microstructure in YBa2Cu3O7−δ

The microstructure of Y–Ho–Ba–Cu–O samples with two different levels of Ho, Ho/Y = 0.5 and 1.0, processed by the trifluoroacetate metalorganic deposition process, have been studied by transmission electron microscopy (TEM) techniques, including high-resolution TEM, conventional TEM and fine-probe energy-dispersive spectroscopy. Oxide nanoparticles with varying levels of Ho have been characterized in detail, and related to other microstructural features. At a lower level of Ho, the (1 1 0) coarse twins with the boundary spacing of 30–60 nm form. Further increase in Ho content considerably produces fine microtwins, and increases the density of nanoparticles and microtwins. Ho additions increase the critical current for the magnetic field parallel to the c-axis, and decrease it for the field parallel to the a–b plane, resulting in a decreased anisotropy of critical current with field angle compared with undoped Y–Ba–Cu–O samples processed under similar conditions. These trends are discussed in terms of the detailed microstructural observations.     

Synthesis and properties of lithium ion conductors Li3−2x(Sc1−xZrx)2(PO4)3

Lithium ion conductors, Li_3−2x(Sc_1−xZr_x)₂(PO₄)₃ (0 x 0.3), were prepared by a solid-state reaction. TG–DTA analysis indicated no phase transition in the samples with x superior to 0.05. X-ray powder diffraction analysis of these samples clearly showed the stabilization of a superionic conduction phase at room temperature with an orthorhombic system Pbcn. The highest conductivity was observed for the sample with x=0.05, and ascribed to the stabilization of the superionic conduction phase and the introduction of vacancies on the Li⁺ sites by substituting Zr⁴⁺ for Sc³.     

High-field magnetization of YBa2Cu3−xMxO7 (M≠Pb, Sn) at 4.2 K

Magnetization measurements on lead- or tin-doped YBa₂Cu₃O₇ have been performed at 4.2 K in magnetic fields up to 29 T. The critical current densities are deduced from the irreversible magnetization and the pinning forces are evaluated. The effect of both lead and tin doping on the critical current density is quite appreciable. In particular, the critical current density of the sample doped with lead is more than twice that of the undoped sample. This effect is thought to be associated with the enhancement of the intergranular coupling of grains through the formation of BaPbO₃ phase at the interface of the 123 superconducting phase. The relationship between the critical current density and a micrograph of the tin-doped sample is also discussed.     

AC susceptibility study of Bi1.66Pb0.34Sr2Ca2−xMgxCu3Oy (x = 0, 0.2 and 0.4) superconductor systems

A systematic study of the intergranular properties of Bi_1.66Pb_0.34Sr₂Ca_2−xMg_xCu₃O_y (x = 0, 0.2 and 0.4) samples has been done, using the AC susceptibility technique. The samples were prepared by conventional solid state reaction method. It was found that Mg substitution in place of Ca reduces the intergranular coupling of Bi-2223 system. Analysis of the temperature dependence of the AC susceptibility near the transition temperature (T_c) has been done employing Bean's Critical State Model. The observed variation of intergranular critical current densities (J_c) with temperature indicates that the critical current density decreases by increasing the amount of Mg. The higher electronegativity of Mg in the unit cell promotes more intake of oxygen in the material, and the grain boundaries are in more over-doped regime. These over-doped regions reduce the intergranular coupling and increases weak link behavior of Mg doped samples.     

Synthesis and superconductivity of Y2Ba1Cu1O5 and Pt added Nd1Ba2Cu3O7?y bulk superconductor

Nd₁Ba₂Cu₃O_7−y (Ndl23) bulk superconductor with T_c>77K was prepared by a melt process under 1 atm pure oxygen and reduced oxygen partial pressures. The addition of Y₂Ba₁Cu₁O₅ (Y211) particles suppressed the generation of microcracks during oxygenation in resulting in the dispersion of Y211 fine particles. The addition of a small amount of Pt resulted in the reduction of the size of Y211 particles. The oxygen partial pressure (Po₂) of 0.01 atm was the optimum atmosphere for the synthesis of Ndl23 in which Y211 particles were dispersed finely and homogeneously. The addition of Y211 and Pt enhanced the flux pinning force in a melt-processed Ndl23 specimen because the presence of Y atoms in a melt phase suppressed, to some extent, the substitution of Nd on Ba site in an Ndl23 crystal prepared by a melt process.     

Ba4In2−x(CO3)1+xO6−2.5x and Ba4In0.8Cu1.6(CO3)0.6O6.2—new indium oxycarbonates closely related to n=3 Ruddlesden–Popper phases

Investigations of phase relations in the Ba-rich part of the In₂O₃–BaO(CO₂)–CuO pseudo-ternary system at 900 °C have revealed the existence of new indium–copper oxycarbonate – Ba₄In_0.8Cu_1.6(CO₃)_0.6O_6.2. Rietveld refinement of the X-ray powder diffraction data combined with infrared studies gives evidence that this phase is a oxycarbonate crystallising in the tetragonal structure (space group I4/mmm) with unit cell parameters: a=4.0349(1) Å and c=29.8408(15) Å. In the binary part of the In₂O₃–BaO(CO₂) system we have identified the occurrence of Ba₄In_2−x(CO₃)_1+xO_6−2.5x oxycarbonate solid solution showing a crystal structure also described by I4/mmm space group, but with the unit cell parameters: a=4.1669(1) Å and c=29.3841(11) Å for x=1. The existence range of this phase, −0.153<x<0.4, includes chemical compositions of earlier found phases: Ba₅In_2+xO_8+0.5x with 0≤x≤0.45 (known as the -solid solution), as well as the binary Ba₄In₂O₇ phase. The crystal structures of both new oxycarbonates are isomorphic and related to n=3 member of the Ruddlesden–Popper family.     

REBa2Cu3O7的化学键性质和Moessbauer谱研究

利用电介质的平均能带模型计算了REBa2Cu3O7(RE=Eu,Y)的化学键参数,得到Cu(1)-O键的平无共价性为0.41,Cu(2)-O键的平共价性为0.28,应用由共价性和极化率定义的化学环境因子计算了^57Fe和^119Sn在REBa2Cu3O7中的Moessbauer同质异能位移,确定了^57Fe在EuBa2Cu3O7和^119Sn在YBa2Cu3O7中的价态和占位情况。     

Phase Relations in the System CaO-Fe2O3-Y2O3 at 1273?K and Compatibility Between Y1?xCaxFeO3?0.5x and Stabilized Zirconia

Phase relations in the system CaO-Fe₂O₃-Y₂O₃ in air (( P_\textO2 P_{{{\text{O}}_{2} }} /P°) = 0.21) were explored by equilibrating samples representing eleven compositions in the ternary at 1273 K, followed by quenching to room temperature and phase identification using XRD. Limited mutual solubility was observed between YFeO₃ and Ca₂Fe₂O₅. No quaternary oxide was identified. An isothermal section of the phase diagram at 1273 K was constructed from the results. Five three-phase regions and four extended two-phase regions were observed. The extended two-phase regions arise from the limited solid solutions based on the ternary oxides YFeO₃ and Ca₂Fe₂O₅. Activities of CaO, Fe₂O₃ and Y₂O₃ in the three-phase fields were computed using recently measured thermodynamic data on the ternary oxides. The experimental phase diagram is consistent with thermodynamic data. The computed activities of CaO indicate that compositions of CaO-doped YFeO₃ exhibiting good electrical conductivity are not compatible with zirconia-based electrolytes; CaO will react with ZrO₂ to form CaZrO₃.