Peritektische Zersetzung von YBa2Cu3O7−x

During the heating of YBCO a peritectic reaction takes place at 1020C, which can be described by: 2YBa₂Cu₃O_7–xY₂BaCuO₅+L+(1-2x)/2O₂ (1) whereL = 3BaCuO₂ +2CuO is a fluid with limited amount of yttrium.It has been reported, that many parameters can influence the reaction. From one side not only the starting size of grains but also the heating rate have an influence on the resulting Y₂BaCuO₅-phase. From the other side, there is a change of the peritectical temperature caused by changing of the partial pressure of O₂ and the presence of parasitic phase.From general kinetic consideration one can draw the conclusion, that different mechanisms (nucleation, phase-boundary reaction and diffusion) can control the reaction.Using DTA/TG measurements, the peritectic reaction has been examined. Classical kinetic methods (Kissinger and Friedman) has been used. The Friedman method has given the dependence of the activation energy from reaction degree. This suggests many steps reactions. The dependence of the DTA-peaks from the heat rate suggest a parallel steps of reaction. This assumption can be motivated by evaluation of free O₂, one solid and liquid phase formation. Amount of this phases depends on the heating rate. Additionally X-ray and microscopic methods has been used. In this way was shown, that the perovskit structure is stable up to peritectical temperature and than is dramatically destroyed. From microscopic observations has been got information about shape and size of solid phase and it's creation as a function of temperature, time and starting grain size.

Die Arbeit wurde BMBF gefördert und durch Land NRW unterschtützt.     

X-ray photoelectron spectroscopy of the YBa2Cu3O7−x superconductor system

X-ray photoelectron spectroscopy (XPS) or electron spectroscopy for chemical analysis (ESCA) has been employed to investigate the chemical nature and the electronic structure of the YBa₂Cu₃O_7−x system at different stages of preparation. The binding energy measurements showed that O 1s, Ba 3d and Y 3d core levels undergo appreciable changes during the firing and subsequent heat treatment. The chemical shift in O 1s is believed to be due to a higher concentration of holes in the superconductor phase. No indication for the existence of a monovalent Cu(I) species was found at any stage of preparation.     

Phase relations in the vicinity of the compound YBa2Cu3O7 − δ at constant oxygen pressure

The phase relations in part of the BaOY₂O₃CuO system were investigated by differential thermal analysis coupled with thermographimetric measurements, X-ray and microprobe analysis, microstructural examinations in a light microscope and in a scanning electron microscope and inductive measurements of the superconducting transition temperatures. The superconducting compound YBa₂Cu₃O_{7 − δ} shows no homogeneity range with respect to the metallic constituents. Solidus temperature for the pseudobinary BaCuO₂CuO and formation temperatures for the compounds in the copper-rich part of the phase diagram are given.     

Positron annihilation studies in YBa2Cu3O x (6<x≤7)

Positron lifetime and Doppler broadening of the annihilation line measurements, as a function of temperature between 14 and 300 K, have been performed on four YBa₂Cu₃O _x (6<x7) samples with different oxygen deficiency. It was found that the positron lifetime and theS parameter values increase as the oxygen deficiency increases in the YBa₂Cu₃O _x (6<x7) samples. It was also observed that the positron annihilation parameers show different temperature dependence in the YBa₂Cu₃O _x (6<x7) samples associated with their different oxygen deficiency.     

Effect of substrate materials on the superconducting properties of electrophoretically deposited YBa2Cu3O7−x coatings

Coatings of the high-temperature superconductor YBa₂Cu₃O_7–x (x = 0.1–0.2) (YBCO), were deposited on commercial Ni,Ti and stainless steel substrates, by an electrophoretic deposition technique. Sintering and annealing procedures were followed in order to get strongly adherent superconducting films. The suitability of the used substrates was tested by measuring the stoichiometry and the superconducting properties of the deposited coatings by X-ray diffraction analysis (XRD), Raman spectroscopy and magnetization measurements with a superconducting quantum interference device (SQUID).     

Cathodic reduction of superconducting oxocuprates YBa2Cu3O7− x at high current densities

The cathodic reduction of YBa₂Cu₃O₇ in aqueous electrolytes at ambient temperature turns out to be strongly dependent upon current density with respect to the reaction mechanism. At low current density, topotactic electron/proton transfer is the dominant process, while at higher current densities, two competing reactions appear, i.e. the topotactic conversion and an irreversible reaction leading to products amorphous in terms of X-ray diffraction. Received: 21 April 1999 / Accepted: 7 June 1999     

The oxygen potentials corresponding to the YBa2Cu3O6+x phase in the YBaCuO system

Phase relations in the vicinity of the YBa₂Cu₃O_6+x in the CuO-rich part of the YBaCuO system were studied by the equilibration and quenching technique. At 950°C, the system is characterized by the large number of solid four-phase combinations. The oxygen potential as a function of temperature for two four-phase combinations, i.e., YBa₂Cu₃O_6+x + BaCuO₂ + BaCu₂O₂ + Y₂BaCuO₅ and YBa₂Cu₃O_6+x + BaCu₂O₂ + Y₂BaCuO₅ + Cu₂O, were studied by means of solid state EMF measurements. At higher oxygen potentials the pair of YBa₂Cu₃O_6+x and Y₂BaCuO₅ is stable in contact with BaCuO₂. At lower oxygen pressures this pair coexists in equilibrium with BaCu₂O₂ or Cu₂O.     

High temperature characterization of the YBa2Cu3O9−y phase

We report the high temperature behaviour of single-phase YBa₂Cu₃O_{9 − y}. This compound reveals superconductivity above 90 K. We performed high temperature (20–960 °C) resistivity measurements, differential thermal analysis and thermogravimetric analysis investigations.It was found that substantial oxygen depletion occurred above 420°C. The oxygen content changed reversibly with temperature at ambient pressure. The second-order-like transition around 650 °C, observed by differential thermal analysis, is an orthorhombic-tetragonal rearrangement of the lattice. These results are important for optimizing the processing conditions of this new class of high T_c superconducting compounds.     

Enhancement of critical current density in superconducting YBa2Cu3O7−x films by nanostructure development of substrate surface using sol–gel method

In this paper, a simple sol–gel route has been adopted in developing substrate surface for the first time. Only by adjusting the concentration of precursor solution, LaAlO₃ (LAO) nanodot arrays were directly obtained on LAO single crystal substrate, without incorporating additional processing steps. Subsequently, YBa₂Cu₃O_7?x (YBCO) superconducting films were prepared on the nanodot arrays using a low-fluorine solution process. In contrast, J _c of YBCO films grown on the substrate developed with nanodot arrays is almost three times as large as that of YBCO films grown on undeveloped substrate in an applied magnetic field with the strength of 3 Tesla. The pinning force density (F _p ) of YBCO film on the nanodot arrays developed substrate is 2.7 GN/m³ which exceeds that (1.8 GN/m³) of YBCO films on undeveloped substrate. The results of cross-section transmission electron microscope (TEM) images show that the extended defects along the a–b planes of YBCO, induced by the developed substrate with nanodot arrays, should be the origin of the J _c enhancement.     

Quantitative analysis of thin films of RBa2Cu3O7−x (R-rare earth elements) and ZrO2 (Y2O3) buffer layers by SNMS

It is shown that RBa₂Cu₃O_7-X single crystals may be used as reference samples for the quantitative analysis of RBa₂Cu₃O_7-X thin films by SNMS. RSF-values for Y and Cu (relative to Ba) determined for ceramic RBa₂Cu₃O_7-X samples are higher than those for single crystals. This difference may be caused by Ba segregation on grain boundaries. The depth profile analysis of YSZ/Al₂O₃ samples was performed by DBM using a Ni grid to prevent sample charging. The reproducibility of analysis was better than 10%.     

YBa2Cu3O7—x复合超导带材透射电镜研究

本文通过透射电镜对具有不同临界电流密度(J_c)的YBa_2Cu_3O_(7-x)超导复合带的微观组织结构进行了分析,并详细地讨论了复合带显微组织与J_c之间的关系。研究结果表明:随着带材中晶界细化、孪晶密度提高和晶界析出物的减少,带材的J_c显著提高。但是,在目前所制备的超导复合带的晶界上,少量的非超导相析出物几乎是不可避免的。所以改善超导体内的微观组织结构是提高复合带J_c的关键因素之一。     

Existence of the homologous series of Y n Ba m Cu m + n O y (m = 2, 3, 5; n = 1, 2) oxides with the tetragonal and orthorhombic structures of YBa2Cu3O6 + δ

The phase composition of Y _x Ba_1?x CuO _y (x = 0.29?0.40) samples annealed in air (at 930?C990°C) and in an oxygen atmosphere (450?C800°C, P(O₂) = 101 kPa) was studied by X-ray powder diffraction, chemical analysis, electron diffraction, and elemental analysis in a transmission electron microscope. A considerable cation nonstoichiometry was discovered in particles having the tetragonal and orthorhombic structures of YBa₂Cu₃O_{6 + ??}. The variation range of particle compositions comprises matrix oxides of the Ba _m Cu _{m + n} O _y series with (Ba: Cu) 3: 5, 5: 8, 2: 3, and 5: 7, which in the presence of yttrium form the Y _n Ba _m Cu _{m + n} O _y series. Tetragonal oxides Y₂Ba₃Cu₅O _y (235), Y₃Ba₅Cu₈O _y (358), YBa₂Cu₃O _y (123), and Y₂Ba₅Cu₇O _y (257) are formed at the primary synthesis step in air and are preserved in an orthorhombic structure during short-term (1 h) oxygen annealing. Most particles of the 3: 5 and 5: 8 oxides are undersaturated with yttrium relative to the stoichiometry of the Y _n Ba _m Cu _{m + n} O _y series, those of the 2: 3 oxide correspond to this stoichiometry, and those of the 5: 7 oxide are supersaturated with yttrium over the stoichiometry. A trend is observed for the fractions of these oxides to change during long-term (5?C51 h) annealing in an oxygen atmosphere at 450°C and to the alternation of the dominant role of one of the four phases with the superconducting transition temperature T _c = 82, 85, 86, and 91 K. Each orthorhombic oxide undergoes structural transformations during oxygen annealing with a change in T _c. The coexistence of these oxides in the form of nanometer-sized domains does not allow their individual superstructures to be recognized.     

On melt-processed RBa2Cu3O7−x (R ≡ Y,Eu, Ho) superconducting oxides

We have determined the peritectic decomposition temperatures for YBa₂Cu₃O_7−x, EuBa₂Cu₃O_7−x and HoBa₂Cu₃O_7−x: they are 1030, 1060 and 980 °C respectively at P = 1 atm O₂. We have also determined the pressure-temperature behaviour of the orthorhombic-tetragonal transition in HoBa₂Cu₃O_7−x. Evidence of the possibility of improving the texture of these samples by partial melting and treatment in thermal gradient is also pointed out.     

Effect of oxygen deficit on the state of iron atoms and57Fe Mössbauer spectral parameters in YBa2(Cu1−xFex)3O6.0 by the results ofX α -DV calculations

The electronic structure of the clusters [Fe^mO_n]^m–2n, which model local structure defects arising in doping the tetragonal phase of yttrium-barium ceramics with iron atoms, has been calculated using the X-discrete variation (DV) method. According to the results of calculations of the hyperfine magnetic field and the quadrupole splitting of⁵⁷Fe Mössbauer spectra in YBa₂(Cu_1-xFe_x)₃O_6.0, the gamma resonance (GR) doublet with quadrupole splitting ||2 mm/s is most likely attributable to the Fe atoms localized in the Cu(1) position and having the valence 2+ and a dumbbell configuration of the nearest environment cluster.Institute of Solid State Chemistry, Ural Branch, Russian Academy of Sciences. Translated fromZhurnal Strukturnoi Khimii, Vol. 34, No. 4, pp. 71–76, July–August, 1993.Translated by A. Sergeeva     

Sintering, microstructures and dielectric properties of Ba1−xPbx(Ti1−xLix)O3−3xF3x ferroelectric ceramics

Various compositions (1−x)BaTiO₃ + xPbF2 + xLiF were prepared, shaped to pellets then sintered at 900°C for 2 h in gold sealed tubes. The purity and the symmetry of the obtained samples were checked by X-ray diffraction. A new solid solution with Ba_1−x Pb _x (Ti_1−x Li _x ) O_3−3x F_3x formula occurs in the composition range 0 ≤ x ≤ 0.20. SEM observations were performed on polished and fractured ceramics. The complex permittivity was measured as a function of temperature (−120°C ≤ T ≤ 250°C) and frequency (50 Hz ≤ f ≤ 4 × 10⁷ Hz). The dielectric performances are the best for ceramic Ba_0.97Pb_0.03(Ti_0.97Li_0.03)O_2.91F_0.09. The real component ε′, exhibits a maximum of approximately 7500 at the ferroelectric Curie temperature T _C ≈ -18°C, the dielectric losses tan δ value being 0.012. At room temperature, the relaxation frequency f _r is around 40 MHz for this ceramic. This novel ferroelectric oxifluoride is a promising material for applications, in particular in the field of Z5U multilayer capacitors.      

Polymeric (NO)3(N2O) n , (NO)3(N2O) n + , and (NO)3(N2O) n − : an interpretation of experimental observations

Semi-empirical and ab initio calculations are reported which provide a possible explanation for reported experimental results on 2-photon ionization of NO containing a few percent of N₂O, which found (NO)₃(N₂O) _n ^+or? clusters to be significantly more abundant than other (NO) _m (N₂O) _n products. It is found that the observed abundances of (NO)₃(N₂O) _n ionic clusters may be accounted for by the existence of covalent cyclic trimers of nitric oxide attached to oligomers of nitrous oxide. The extra stability of NO trimers in the observed clusters appears to arise from (NO) ₃ ⁺ rather than (NO)₃. Attachment of an (N₂O) _n side chain to (NO) ₃ ⁺ occurs exothermically. It is suggested that the addition of N₂O to cyclic-(NO) ₃ ⁺ might provide a means of making a polymer of nitrous oxide, which could have useful properties.     

Structure of the X-ray emission CuLα-line in YBa2Cu3O7−δ and CuO based on the SCFXα-SW calculation

Using the SCF X-SW method we have calculated the final states of the X-ray L-transition of the CuO ₄ ^–6 cluster modeling the nearest environment of the copper atom (one of two types) in YBa₂Cu₃O_7- and the nearest environment of copper in CuO. The energies of transitions forming the L-spectrum (with or without many-electron excitations) have been calculated. It has been shown that for interpretation of the main singularities of the X-ray emission L-spectrum, not only should the Coster-Kronig transitions be considered but also the shake-up excitation 2b_1g3b_1g processes and the multiple ionization satellites 3p^–1. An experimental schame has been suggested that allows separation of the effects of different many-electron processes on the L-spectrum.Institute of Inorganic Chemistry, Siberian Branch, Russian Academy of Sciences. Translated fromZhurnal Strukturnoi Khimii, Vol. 34, No. 4, pp. 3–9, July–August, 1993.Translated by L. Smolina     

YBa2Cu3O7-x涂层导体的研究进展

以YBa2Cu3O7-2（YBCO）超导涂层为导体的第二代高温超导带材由于其优异的本征性能，吸引着科学家不断探索和研究其实用成材技术。近年来，百米级长带的研制成功，使其成为当前国际上超导研究的热点。综述了近年来二代超导带材的发展状况，并对在镍和镍合金基带上制备涂层导体的工艺技术以及其基本结构特征做了较为系统的介绍，包括基带的制备技术、隔离层的生长、化学法和物理气相沉积法生长YBCO等。