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1.
The effects of Pb doping, oxygen doping (δ= 0.1,0.2,0.4 and 0.5) and high pressate (4, 8, 15 and 20GPa) on the electronic structure of Hg0.8Pb0.2Ba2Ca2Cu3O8+δ have been examined by the recursion method. Our calculations show that Pb doping only decreases the hole concentration slightly and oxygen doping increases the hole concentration monotonically and significantly as δ varies from 0 to 0.5, with each excess oxygen atom contributing about 1.7 holes to the CuO2 layers. The optimal δ is estimated to be around 0.4, The hole conceatration increases initially with pres-sure but decreases as P > 8GPa (i.e., dn/dP does change sign at ~ 8GPa). The suppressed Tc(P) by Pb substitution has to be described in the modified Neumeier's model.  相似文献   

2.
We have investigated the structure, optical and magnetic properties of ferroelectric KNb1-xFexO3-δ (X=0, 0.01, 0.03, 0.05, 0.10, 0.15, 0.20, 0.25) synthesized by a traditional solid-state reaction method. According to the X-ray diffraction and the results of Rietveld refinement, all the samples maintain orthorhombic distorted perovskite structures with Amm2 space group without any secondary phase, suggesting the well incorporation of Fe ions into the KNbO3 matrix. With the increase of Fe concentration, the band gap of each sample is decreased gradually, which is much smaller than the 3.18 eV band gap of pure KNbO3. Through X-ray photoelectron spectrum analysis, the increased density of oxygen vacancy and Fe ions may be responsible for the observed decrease in band gap. Compared with the pure KNbO3, Fe doped samples exhibit room-temperature weak ferromagnetism. The ferromagnetism in KNb1-xFexO3-δ with low-concentration dopants (X=0.01-0.10) can be attributed to the bound magnetic polaron mediated exchange. The enhancement of magnetism for the high-concentration (X=0.10-0.20) doped samples may arise from the further increase of magnetic Fe ions.  相似文献   

3.
A series of Y123 single phase samples with various Ba vacancy concentration was prepared by making their Ba contents deviate from the stoichiometric composition. The measurements of their structure, superconductivity and flux pinning behaviour were systematically carried out. It is found that, compared with YBa2Cu3O6.96 sample, the strength of the flux pinning in YBaxCu3O7-δ(1.8≤x<2.0) samples is increased, and that there is an optimum value of Ba vacancy concentration for the maximum flux pinning force density. The possible origin of the flux pinning centers to determine the flux pinning behavior at higher field is discussed in detail. We suggest that the flux pinning effect at lower field may stein from the interaction between the vortex and the surfaces of grains, and that the flux pinning mechanism at higher field belongs to the core interaction.  相似文献   

4.
Ni-based catalysts supported on di erent supports (α-Al2O3,γ-Al2O3, SiO2, TiO2, and ZrO2) were prepared by impregnation. Effects of supports on catalytic performance were tested using hydrodeoxygenation reaction (HDO) of anisole as model reaction. Ni/α-Al2O3 was found to be the highest active catalyst for HDO of anisole. Under the optimal conditions, the anisole conversion is 93.25% and the hydrocarbon yield is 90.47%. Catalyst characteriza-tion using H2-TPD method demonstrates that Ni/α-Al2O3 catalyst possesses more amount of active metal Ni than those of other investigated catalysts, which can enhance the cat-alytic activity for hydrogenation. Furthermore, it is found that the Ni/α-Al2O3 catalyst has excellent repeatability, and the carbon deposited on the surface of catalyst is negligible.  相似文献   

5.
王秩伟  贾丽  杨槐馨  田焕芳  施洪龙  王臻  李建奇 《中国物理 B》2011,20(11):117402-117402
Superconductivities and structural properties of Ti-Zr-Ta ternary alloys are extensively investigated. The TiZrTa sample has a cubic structure (β -phase) and shows a sharp superconducting transition at a critical temperature (Tc) of about 7.3 K. In addition, two series of Ti-Zr-Ta alloys, with nominal compositions of Ti65-xZr35Tax and TixZr65-xTa35 respectively, are prepared, and their superconductivities and crystal structures change regularly with the chemical composition. Our experimental study also indicates that the annealing processing of this kind of material can cause the transition temperature to increase and the highest Tc is observed to be about 8.3 K in annealed samples.  相似文献   

6.
We have investigated, by X-ray diffraction, a series of single crystals of Bi-based oxides with the nominal composition Bi2Sr2(Cu1-zFez)O6+δ(0≤z≤0.55). In this system we observed two structural phase transitions with the increase of the doping content. The first transition, from an incommensurate monoclinic phase to an incommensurate orthorhombic phase, occurs at a doping content of iron zFe=0.027. The second one corresponds to a phase transition from an incommensurate orthorhombic phase to a commensurate orthorhombic phase at zFe=0.34. The comparison of these results with those for more limited substitutions of Zn and Ni indicates the significant role of the insertion of the extra oxygen in the (Bi-O) double layers.  相似文献   

7.
Undoped and WO3-doped Y-Ba-Cu-O ceramics have been slowly cooled in flowing oxygen or quenched in air from 900℃ to room temperature, after sintering at 900℃ for 3Oh in flowing oxygen. The ac susceptibility measurements show that the Tc of the quenched WO3-doped Y-Ba-Gu-O superconductor can reach 88K, while the undoped Y-Ba-Cu-O ceramic quenched in air is a non-superconductor. X-ray diffraction data show that WO3 does not enter into the YBa2Cu3O7-x lattice but forms all impurity phase. The WO3-doped YBCO specimens quenched in air consist of orthorhombic YBa2Cu3O7-x phase and an impurity phase which increases with increasing WO3 content. Raman spectra chow that oxygen contents are different for undoped and WO3-doped samples. It la suggested that suitable doping with WO3 can change the oxygen content and reduce the effect of thermal treatments on the superconductivity of Y-Ba-Cu-O system.  相似文献   

8.
The present work establishes a systematic approach based on the application of in-situ Fourier transform infrared spectroscopy (FTIR) for the investigation of the crystal structure, thermal stability, redox behavior (temperature-programmed reduction/temperatureprogrammed re-oxidation) as well as the catalytic properties of Co3O4 thin films. The syntheses of Co3O4 were achieved by chemical vapor deposition in the temperature range of 400-500℃. The structure analysis of the as-prepared material revealed the presence of two prominent IR bands peaking at 544 cm-1 (υ1) and 650 cm-1 (υ2) respectively, which originate from the stretching vibrations of the Co-O bond, characteristic of the Co3O4 spinel. The lattice stability limit of Co3O4 was estimated to be above 650℃. The redox properties of the spinel structure were determined by integrating the area under the emission bands υ1 and υ2 as a function of the temperature. Moreover, Co3O4 has been successfully tested as a catalyst towards complete oxidation of dimethyl ether below 340 ℃. The exhaust gas analysis during the catalytic process by in situ absorption FTIR revealed that only CO2 and H2O were detected as the final products in the catalytic reaction. The redox behavior suggests that the oxidation of dimethyl ether over Co3O4 follows a Mars-van Krevelen type mechanism. The comprehensive application of in situ FTIR provides a novel diagnostic tool in characterization and performance test of catalysts.  相似文献   

9.
Photoemission measurements have been carried out for Bi2Sr2CaCu2-xSnxO8+δ system with conventional x-ray photoemission spectroscopy for core-level spectra and synchrotron radiation photoemission spectroscopy for valence band. With Sn doping, all core levels shift differently in binding energy, and the intensity near fermi energy becomes smaller in valence hand. From the experiment, we can deduce that the shifts of all core levels and valence hands may involve some other mechanisms, such ms electrostatic effects, in addition to binding energy referencing effects. We argue that the chemical environment plays a crucial role in the electronic structure of high-temperature superconductors.  相似文献   

10.
曹效文 《中国物理》1994,3(8):583-588
Superconducting transition temperature Tc and normal-state electrical resistivities ρ of the Tm1-xPrxBa2Cu3O7-δ system have been measured. The results indicate that Tc remains constant for x = 0 to 0.08 before it drops steadily with higher x. Following the Abrikosov-Gor'kov model, a total suppression of superconductivity occurs at a critical Pr concentration xcr≈0.56. It is found for the first time, as far as we know, that the logarithmic resistivity at 270 K increases linearly with increasing x, yielding ρ= ρ0eax. Judging from the relation between dρ/dT and x, a metal to insulator transition occurs just beyond xcr.  相似文献   

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