Xi Zhengping

BiPbSrCaCuO superconductor prepared by a three-step reaction process

A three-step reaction process was proposed to prepare the Bi-system superconductor based on the principles of physical chemistry. High-temperature chemical reactions in the preparation process of the superconductor, high-temperature behaviour of the superconducting phases, superconductivity of the material and the critical current density of Ag-sheathed composite tape were studied. High-temperature chemical reactions were found to obey the principles of metallurgical physical chemistry. Heat treatment temperature and composition were shown to have an important effect on superconductivity and the formation of the high-Tc phase, the 2212 and 2223 phases melted incongruently at 875 and 882 degrees C, respectively, and the liquid phase was found to consist of a 2201 type compound and Cu4O3. The best value of critical current density of Ag-sheathed composite tape obtained was over 1.3*104 A cm-1 (77 K, 0 T).

112–114 K superconductors Bi1.8Pb0.3Sr2Ca2Cu3Oy by means of a three-step reaction process

Abstract 112–114 K superconductors with nominal composition of Bi1.8Pb0.3Sr2Ca2Cu3Oy were prepared by a three-step reaction process. The process consists of (1) carbonate calcination and BiPbSrCaO matrix formation at 800°C–840°C, (2) flash heating matrix-CuO mixture at 900°C, and (3) sample annealing at 855°C and furnace cooling to 400°C. The samples possess a quite low resistivity before superconductive transition (ϱ-10 −1 -10 −2 mΩ.cm) and have a transition width of 2.5 K. Results from a.c. susceptibility measurement indicate there are two superconducting phases between 100 and 110 K.

The formation and enhancement of texture in a Bi-system superconductor

The powder-in-tube method was used to produce Ag-sheathed samples using two precursor methods to study the texture formation of the superconducting phases in Bi-system tapes. Experimental results showed that the texture of the 2223 phase results from that of the 2212 grains and the formation of the 2212 phase texture mainly originates with the oriented growth of grains due to residual stress produced during processing. Based on experimental results, a new, prepressing process was proposed to enhance the texture of the superconducting phases, and, therefore, improve the critical current densities of the composite tapes.

Effect of Bi/Pb ratio and annealing temperature onT c and formation of high-T c phase in Bi-Pb-Sr-Ca-Cu-O superconductor

We studied the effect of Bi/Pb ratio and annealing temperature onTc and formation of the high-Tc; phase in Bi-Pb-Sr-Ca-Cu-O superconductor by the three-step reaction process. The optimum Bi/Pb ratio is about 1.8∶0.3 and the optimum annealing temperature is about 845–855°C. It is found that a variate high-Tc phase existed at the higher annealing temperature. The zero-resistance temperature of the variate high-Tc phase decreased when the annealing temperature increased, although the phase is isostructural with the 110 K phase.

Standard Gibbs free energy of formation of the high-Tc superconducting compound YBa2Cu3O6.5+δ

Cells were constructed with a single-crystal CaF2 solid electrolyte and the cell EMF was measured. The standard change in free energy of formation of BaCuO2 (011 phase) and YBa2Cu3O6.5+δ (123 phase) from the constituent oxides was estimated. The oxygen non-stoichiometric fraction, δ, was taken into account in the estimation of ΔG123+δ0.

111 K superconductor Bi1.7Pb0.45Sr2Ca2Cu3Oy prepared by means of the three-step reaction process

Abstract Superconductor Bi 1.7 Pb 0.45 Sr 2 Ca 2 Cu 3 O y with zero-resistance temperature of 111 K was prepared by means of the three-step reaction process previously described. The short-time heating of the sample at 900° C is vital to achieving superconductivity at higher than 100 K. The optimal annealing temperature is 840° C when the calcination and matrix formation temperature is 800° C.

Effect of processing history on the properties of Ag-based tapes

The Bi-system tapes and coils were prepared by the powder-in-tube method. The effect of processing and heat treatment on the properties of the tapes and coils was studied. It is found that the shearing stress produced by the rolling process may destroy the 2223 phase, and the destroyed 2223 phase cannot be recovered by heat treatment, whereas the pressing stress produced by the uniaxial pressing process only makes the 2223 phase grains break, and the broken 2223 phase can be closed by the heat treatment. TheJc values of the rolling samples, pressing sample, and the coil with a diameter of 35 mm are over 1.3×104, 2.5×104, and 4×103 A/cm2 (77 K, 0 T), respectively.

Acid-base equilibrium―a thermodynamic study of formation and stability of the Bi-2223 phase

A general acid-base equilibrium theory was proposed to explain the formation and stability of the Bi-2223 phase based on the Lewis acid-base theory, and principle of metallurgical physical chemistry. The acid-base nature of oxide was defined according to the electrostatic force between cation and oxygen anion. A series of experimental facts were systematically explained based on the theory: substitution of Bi for Ca in. the Pb-free 2223 phase, and the effect of substitution of the high-valent cation for Bi3+; oxygen-pressure atmosphere, and the heat-schocking technique on the formation and stability of the 2223 phase.

Effects of Sr/Ca ratio and CuO content of the superconducting phases in the Bi-Pb-Sr-Ca-Cu-O system

The Bi-Pb-Sr-Ca-Cu-O superconductor was prepared by the three-step reaction process; the effects of Sr/Ca ratio and CuO content on the formation of the superconducting phases were studied. The Sr-short and Cu-rich nominal compositions are not advantageous for the formation of the 2223 phase but we are favored for the formation of the 2201 phase. The differences among the previous reports on the effect of composition on the formation of the superconducting phase is discussed. The best Jc value of the Ag-sheathed tape filled with the kind of powder is 13,000 A/cm2 (77 K, OT).

Highly oriented Bi system bulk samples prepared by a decomposition-crystallization process

A decomposition-crystallization method for preparing highly oriented Bi system bulk samples is reported. The effects of processing parameter, decomposition temperature, cooling rate, and post-treatment conditions on texture and superconductivity have been investigated. The method has been used to prepare highly textured Bi system bulk samples. High-temperature anealing does not destroy the growing texture, the cooling rate has some effect on texture and superconductivity, and annealing in N2-O2 atmosphere can improve the superconductivity of the textured sample.