Fan Chenggao

Preparation and superconductivity of the high-Tc phase of Pb-doped Bi-Sr-Ca-Cu-O superconductor

Abstract With the aim of preparing a single high-Tc phase in the BiPbSrCaCuO system, optimization of the composition and heat treatment have been studied. It has been found that the temperature range at which the high-Tc phase forms is very narrow (850–880°C). The high-Tc single phase Bi2.5Pb0.5Sr2Ca2Cu3Oy has been prepared. The zero-resistance temperature (Tce) is as high as 105K. There are no steps in the resistivity and the susceptibility. The structure is tetragonal with a=5.414 A and c=37.106 A . It is suggested that the phase with c=37.106 A is responsible for the high temperature (110K) superconductor.

Superconductivity in the (Bi,Pb)CaSrBaCuO system

Abstract In the (Bi,Pb)CaSrBaCuO system, a rather large orthorhombic cell with a=5.4A, b=26.9A and c=30.7A have been found. The ac susceptibility measurement shows that there exists only one superconducting transition at 110K, the zero-resistance temperature is as high as 106K.

The role of oxygen content in the (Bi,Pb)2Sr2Ca2Cu3Oy system

Abstract The 110K single phase sample (Bi,Pb)2Sr2Ca2Cu3Oy was prepared by solid state reaction method. The samples were characterized using X-ray powder diffraction, the measurements of resistivity and dc. susceptibility, and the X-ray photoemission spectrum and specific heat measurement and the electron diffraction analysis. The results show that the superconductivity of (Bi,Pb)SrCaCuO system is sensitive to oxygen content like the 1-2-3 phase material. We suppose that different oxygen content leads to the changes of modulated structure and electronic structure and the ratio of Cu3+, and so to great difference in the superconductivity.

The substructure of quasicrystalline Al-Mn alloy observed by TEM

The morphology of the icosahedral phase in Al86Mn14 alloy has been observed as flower‐like and of granular shape by TEM. These two kinds of morphologies may be regarded as the different sections of a three‐dimensional flower‐like grain. Inside these grains, a fine substructure of a few nm in size has been found. A dendritic morphology with a diffraction pattern showing five‐fold symmetry have also been observed in the Al86Mn14 alloy. On the basis of these observations, a model based on nucleation, growth and aggregation has been proposed to explain the formation process of the quasicrystalline state.

Study of the superconductivity of the 132 K Sb-doped Bi-system oxide superconductor

According to powder X-ray diffraction analyses and electron diffraction for the 132 K Sb-doped Bi-system oxide superconductor it can be found that a new phase exists in this system; the new phase belongs to the monoclinic system with cell dimensions a=22.10 AA, b=5.90 AA, c=19.91 AA and beta =99.6 degrees and is not superconducting itself. Furthermore, there exists intergrowth between the monoclinic phase and 2:2:2:3 (=Bi:Sr:Ca:Cu) phase, and for this reason crystals of the 2:2:2:3 phase are distorted. The superconductivity at 132 K may be caused by the distorted 2:2:2:3 phase. After thermal cycles, this kind of intergrowth phenomenon disappears so that only superconductivity at 110 K occurs as usual. All this can explain why the 132 K phase is not stable. In addition, how and why the measurement current influences Tc is also discussed.

Structure and properties of the La-doped Bi-2201 system

In this article, the influence of La substitution for Sr on structure and physical properties of the 2201 phase is studied. First, the crystal microstructure of the Bi2.1Sr1.9-xLaxCuOy (0 ≤ x ≤ 1.0) is characterized by means of x-ray diffraction and electron diffraction analyses, it is discovered that in the La-doped 2201 system with increasing La content the 2201 phase undergoes tetragnoal-orthorhombic-monoclinic structural transition. With this phase transition, the modulation vector of the 2201 phase transforms from incommensurate to commensurate, and the period of the modulation wave decreases. Secondly, the superconductivity of this system is analyzed systematically. The results of the resistivity measurement show that the appropriate amount of La doping can raise the superconducting transition temperature (Tc) of the 2201 phase, with x=0.2, Tc reaches as high as 25 K. However the excessive doping of La (x ≥ 0.3) leads the 2201 phase to transit from metal to insulator and the superconductivity disappears. The measurement of thermoelectric power (TEP) of this system shows that with increasing La content the TEP value also increases gradually. But the interesting point is that the transition from positive to negative in TEP occurs for x=0.2 sample. In addition, the electronic structure and oxygen content of this system are also studied. Based on these, we analyze the relation between the microstructure, electronic structure of the La-doped 2201 phase and its superconductivity, indicating that the type of element substitution with higher valence, together with the microstructural distortion induced by the substitution affects the electronic states of Cu 3d seriously, that is, changes the degree of coulomb correlation of Cu 3d electrons, thus resulting in the transition of the type of carrier of this system and the decrease of hole carrier concentration, as well as the corresponding variation of the superconductivity of La-doped 2201 phase.

Coexistence of superconductivity with paramagnetism in magnetic field for RBa2Cu3Oy system (R = Sm, Gd, Er, Ho, Dy)

Abstract The variations of magnetization for RBa2Cu3Oy (where R represents rare-earth element: Sm, Gd, Er, Ho and Dy) were studied in magnetic fields up to 2T at 77K. It was found that the paramagnetism appeared, and the effective magnetic moment tended to its theoretical value when field was higher than 1.5T, but the system was in superconducting state. The experimental results indicate that the paramagnetism co-exists with the superconductivity in magnetic field.

STUDY OF CRYSTAL STRUCTURE AND PHYSICAL PROPERTIES OF THE COMPOUND Bi-Pb-Sr-Ca-Ba-Cu-O

With the aid of transmission electron microscopy, the phenomenon of additional superstructure along

Preparation of Planview TEM Samples of Yba 2 Cu 3 0 7−x Films Grown on BaF 2 (001)

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Intercalation Reaction of Bi2(Sr,Nd)2CuOy Bi2Sr2(Ca,Gd)Cu2Oy and Related Compounds with Iodine

direction was observed for a sample of Bi1.7Pb0.3Sr2Ca1.8Ba0.2Cu3Oy, and two kinds of modulation mode along b axis was found to coexist in this sample. The images of SEM showed that the addition of Ba ions improved the property of two-dimensional growth. The results of XRD indicated that the substitution of Ba2+ for Ca2+ increased the volume fraction of the high-Tc phase (Tc>100 K). From the initial part of dc magnetization curve, it is estimated that about 36 percent of the sample volume is in the 110 K superconducting phase at 77 K. Its frozen magnetic flux density is about 8.6 G, which is 5–6 G higher than that of general Pb-doped sample, and its zero-resistance temperature can reach as high as 111 K. These changes of physical properties may be related to the adjustment of the microstructure.