15 resultados para YBa2Cu3O7

em Chinese Academy of Sciences Institutional Repositories Grid Portal


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The differences between the interdiffusion characteristics of Ag/YBa2Cu3O7-x and Al/YBa2Cu3O7-x contact interfaces have been revealed by secondary ion mass spectrometry (SIMS). The different electrical properties of Ag/YBa2Cu3O7-x and YBa2Cu3O7-x films after high temperature treatment are well understood by the SIMS results.

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YBa2Cu3-xTaxO7-y (x = 0, 0.1, 0.2, 0.3, 0.4, 0.5 and 0.6) superconductors have been prepared, X-ray diffraction shows that the system remains orthorhombic for all compositions studied but. for x > 0.2, Ta2O5 was detected as an impurity phase. Substitution of Ta5+ for Cu2+ occurs in the Cu(2) sites on the Cu(2)O planes. The introduction of the high-valence element tantalum produces extra free electrons. These electrons recombine with the positive carrier of the system, which causes the mobility and the Hall number of YBa2Cu3-xTaxO7-y to decrease and also results in a depression in T(c).

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The utility of the high-temperature superconductor, YBa2Cu3O7-x as the cathode material for an all-solid-state lithium cell has been examined. The capacity of YBa2Cu3O7-x is 223 mA h g-1 and the discharge efficiency is > 92%. Measurements of a.c. impedance show that the charge-transfer resistance at the interface of the electrolyte/cathode is very low and increases with the depth-of-discharge of the battery. Studies using X-ray photoelectron spectroscopy (XPS) reveal that the cathode becomes doped with Li+ ions as the cell discharges.

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A Raman-forbidden phonon mode at about 840 cm(-1) is observed popularly on the surface of pun and La-doped Bi2Sr2-xLaxCuO6+y (0 less than or equal to x less than or equal to 0.8) single crystals annealed in oxygen. A remarkable excitation dependence of this additional line is found. Based on the properties of the structure of the Bi-O layer with excess oxygen atoms and the similarity in the appearance of the Raman-forbidden modes between RBa2Cu3Ox (R = Y, Nd, Gd, Pr) and Bi2Sr2-xLaxCuO6+y systems, we attribute the manifestation of this additional line to the ordering of the interstitial oxygen in the Bi-O layers. Our results provide Raman evidences for confirming that the ordering of the movable oxygen may exist universally in high-temperature superconductors.

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By neutron diffraction and other experiments, we have found that oxygen ions in YBCO can diffuse out of the sample in vacuo at room and low temperature, while the T(c) decreases greatly. We have also found that if the vacuum-deoxidation process lasts for several days there will be a damping oscillation of T(c) with time (t), and higher vacuum corresponds to a greater amplitude and a shorter period. We tentatively think that T(c) should satisfy the following function: T(c0) is-proportional-to T(c)e(-betat)cos (omegat + phi); it may be due to the diffusion of oxygen and the saturation of the valence state.

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An estimation method of thermal expansion coefficient in term of lattice energy which was developed earlier for simple materials is extended to a complex material of Bi2Sr2CaCu2O8 (Bi-2212). The calculation of the chemical bond property and thermal expansion coefficient of Bi-2212 has been carried out and the theoretical values were in good agreement with the corresponding experimental results. The dependence of the thermal expansion coefficient on the different structures and on the flexible oxidation states of Bi and Cu are investigated. The results indicate that the thermal expansion coefficients of Bi-2212 are insensitive to the low lattice distortion of the average structure and the changes of formal valences of Bi and Cu ions.

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By using the average bond-gap model, the chemical bond properties of REBa2Cu3O7 were calculated. The calculated covalencies for Cu(1)-O and Cu(2)-O bonds in REBa2Cu3O7 compounds are 0.41 and 0.28 respectively. Mossbauer isomer shifts of Fe-57 doped in EuBa2Cu3O7-x and Sn-119 doped in YBa2Cu3O7-x were calculated by using the chemical surrounding factor, h, defined by covalency and electronic polarizability. The valence states and sites of Fe-57 in EuBa2Cu3O7 and Sn-119 in YBa2Cu3O7-x were identified.

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The valence of Pr and relationship between bond covalency and T-c in Y1-xPrxBa2Cu3O7 (x = 0-1) have been studied using complex chemical bond theory. The results indicate that the depression of superconductivity in Y1-xPrxBa2Cu3O7 can be reasonably explained by bond covalency difference for the bonds between CuO2 plane and CuO chain. T-c decreases with the decreasing of bond covalency difference and reaches zero when bond covalency difference is zero (or bond covalency in CuO2 exceeds that in CuO chain) at Pr concentration 0.55 and valence +3.30. These are in good agreement with the experiments and meanwhile suggest that the valence of Pr is + 3.30 in Y1-xPrxBa2Cu3O7. The results also indicate that for Pr valence less than +3.15, superconductivity always exists for whatever Pr concentration, whereas for Pr with a valence of +4.0, superconductivity disappears as soon as Pr concentration exceeds 0.19. This supports with the viewpoint that higher valence Pr will contribute more electrons to CuO2 plane, filling the mobile holes responsible for conduction. For PrBa2Cu3O7 with no Ba-site Pr, our calculation suggests that it will be a superconductor if the average valence of Pr is less than +3.15. (C) 1998 Published by Elsevier Science B.V. All rights reserved.

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The valences of Cu in Y1-xGaxBa2Cu3O6+y have been investigated by using complex chemical bond theory The results for the valences of Cu(1) and Cu(2) in the calculation suggest that the holes introduced by calcium substitution only reside in CuO2 planes, and there is a competition mechanism for the hole density in CuO2 planes between Ca doping and oxygen depletion. These conclusions are satisfactorily in agreement with experiments.

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The mixed oxides, including YBa2Cu3O7, LaBa2Cu3O7, LaBaCu2O5, La2BaCu3O7, La4BaCu5O12 with perovskite structure, were prepared. The catalysts were characterized by means of chemical analysis, XRD, TPD and TPR method. It was found that they were the active catalysts for the NO decomposition and NO reduction by CO. The existance of Cu3+ is an important factor to give the catalysts a high activity for the NO reduction by CO.

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Superconductor mixed oxides were often used as catalysts at higher temperature in gas phase oxidations, and considered not suitable for lower temperature reactions in the liquid-solid phase; here the catalysis of YBa2Cu3O7+/-x and Y2BaCuO5+/-x in the phenol hydroxylation at lower temperature with H2O2 as oxygen donor was studied, and found that the superconductor YBa2Cu3O7+/-x, has no catalytic activity for phenol hydroxylation, but Y2BaCuO5+/-x does, even has better catalytic activity and stability than most previously reported ones. With the studies of catalysis of other simple metal oxides and perovskite-like mixed oxides, a radical substitution mechanism is proposed and the experimental facts are explained clearly, and draw a conclusion that the perovskite-like mixed oxides with (AO)(ABO(3)) and (AO)2(ABO(3)) structure have better catalytic activity than the simple perovskite oxides with (ABO(3))(3) structure alone, and (AO) structure unit is the key for the mixed oxides to have the phenol hydroxylation activity. No pollution of this process is very important for its further industrial application.

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Superconductor Y-Ba-Cu-O mixed oxides were synthesized and their catalysis in phenol hydroxylation was studied too. Results show that, Y2BaCuO5 has better activity than that of YBa2Cu3O7-x, With the catalysis study of another mixed oxide La2CuO4 a conclusion that AO structure unit is the key for mixed oxides to have high activity in phyenol hydroxylation was drawn. Meanwhile, the effects of reaction temperature, medium and medium (water) pH on phenol hydroxylation catalyzed by Y2BaCuO5 and the stability of the mixed oxides were also studied.

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Analysing the coordination state of copper ions in cuprate superconductors, it is found that the larger the energy splitting between d(x2-y2) and d(z2) orbitals of Cu or the higher the energy of the d(x2-y2) orbital, the higher the Tc. Thus, appropriate coordination structures and strong-field ligands must be chosen for expanding the energy splitting and increasing the energy of the d(x2-y2) orbital when searching for new high-Tc superconductors. Summarizing the experimental results of ESR and XPS, it is considered that the [Cu2+ - O open-square-box 2- - Cu3+] resonance exists in cuprate superconductors and the electron field breathing mode is present. Analysing the mechanism and the relationship between the coordination state of Cu and Tc, we consider that the two dimensional Cu-O planes are responsible for the superconductivity of YBa2Cu3O7-y.