DIFFERENT INTERDIFFUSION CHARACTERISTICS BETWEEN AG/YBA2CU3O7-X AND AL/YBA2CU3O7-X CONTACT INTERFACES

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.

HIGH-TEMPERATURE SUPERCONDUCTOR, YBA2CU3O7-X AS ALL-SOLID-STATE LITHIUM CELL

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.

THE MODULATION PRINCIPLE OF VACUUM ON THE SUPERCONDUCTIVITY OF YBCO

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.

Chemical bond properties of REBa2Cu3O7 and its Mossbauer spectroscopy

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.

Y-Ba-Cu-O mixed oxides for production of diphenols in liquid-solid phase

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.

Investigation of Catalysis of Y-Ba-Cu-O Mixed Oxides in Phenol Hydroxylation

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.

COORDINATION CHEMISTRY AND SUPERCONDUCTIVITY

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.

Depth dependent elastic strain in ZnO epilayer: combined Rutherford backscattering/channeling and X-ray diffraction

A ZnO layer was grown by metalorganic chemical vapor deposition (MOCVD) on a sapphire (0 0 0 1) substrate. The perpendicular and parallel elastic strain of the ZnO epilayer, e(perpendicular to) = 0.19%, e(parallel to) = -0.29%, respectively, were derived by using the combination of Rutherford backscattering (RBS)/channeling and X-ray diffraction (XRD). The ratio vertical bar e(parallel to)/ e(perpendicular to)vertical bar = 1.5 indicates that ZnO layer is much stiffer in the a-axis direction than in the c-axis direction. By using RBS/C, the depth dependent elastic strain was deduced. The strain is higher at the depth close to the interface and decreases towards the surface. The negative tetragonal distortion was explained by considering the lattice mismatch and thermal mismatch in ZnO thin film. (c) 2004 Elsevier B.V. All rights reserved.

Depth dependence of the tetragonal distortion of a GaN layer on Si(111) studied by Rutherford backscattering/channeling

Rutherford backscattering and channeling have been used to characterize the structure of a GaN layer grown on a Si(111) substrate. The results show that a 1.26 mum GaN epitaxial layer with a rather abrupt interface and a good crystalline quality (chi(min)=3.4%) can be grown on a Si(111) substrate. Using the channeling angular scan around an off-normal <1 (2) over bar 13> axis in the {10 (1) over bar0} plane of the GaN layer, the tetragonal distortion e(T), which is caused by the elastic strain in the epilayer, can be determined. Moreover, the depth dependence of the e(T) can be obtained using this technique. A fully relaxed (e(T)=0) GaN layer for a thickness <2.8 mum is expected. (C) 2002 American Institute of Physics.

X-ray-diffraction study of quasipseudomorphic ErSi1.7 layers formed by channeled ion-beam synthesis

ErSi1.7 layers with high crystalline quality (chi(min) of Er is 1.5%) have been formed by 90 keV Er ion implantation to a dose of 1.6X10(17)/cm(2) at 450 degrees C using channeled implantation. The perpendicular and parallel elastic strain e(perpendicular to)=-0.94%+/-0.02% and e(parallel to)=1.24%+/-0.08% of the heteroepitaxial erbium silicide layers have been measured with symmetric and asymmetric x-ray reflections using a double-crystal x-ray diffractometer. The deduced tetragonal distortion e(T(XRD))=e(parallel to)-e(perpendicular to)=2.18%+/-0.10%, which is consistent with the value e(T(RBS))2.14+/-0.17% deduced from the Rutherford backscattering and channeling measurements. The quasipseudomorphic growth of the epilayer and the stiffness along a and c axes of the epilayer deduced from the x-ray diffraction are discussed.

Magnetic structure and orbital ordering in tetragonal and monoclinic KCrF3 from first-principles calculations

KCrF3 has been systematically investigated by using the full-potential linearized augmented plane wave plus local orbital method within the generalized gradient approximation and the local spin density approximation plus the on-site Coulomb repulsion approach. The total energies for ferromagnetic and three different antiferromagnetic configurations are calculated in the high-temperature tetragonal and low-temperature monoclinic phases, respectively.

Investigation of Pr valence and relationship between bond covalency and T-c in Y1-xPrxBa2Cu3O7 (x = 0-1)

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.

THE EFFECT OF DOPING WITH TANTALUM ON THE PROPERTIES OF YBA2CU3O7-Y

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).

Liquid phase epitaxy growth of AlxGayIn1-x-yPzAs1-zGaAs with direct band gap up to 2.0 eV

III-V pentenary semiconductor AlGaInPAs with a direct band gap of up to 2.0 eV has been grown successfully on GaAs substrates by liquid phase epitaxy;(LPE). With the introduction of the energy bowing parameters of quaternaries, the theoretical calculations agree well with the measured PL peak energy data from pentenary samples.