Unstable Plane in Bi-Sr-Ca-Cu-O System by TGA
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UNSTABLE PLANE IN Bi-Sr-Ca-Cu-O SYSTEM BY TGA H.Zhang, Q.R. Feng, S. Q. Feng, X.Zhu and Z. Z. Gan Department of Physics, Peking University, Beijing 100871, China Abstract By thermogravimetry analysis (TGA), x-ray diffraction and R(T) relationship determination, the BiSrCaCuO system superconductors with composition of 2201, 2212 and 2223 were studied systematically. The experiments discovered that there is an unstable plane in 2223 sample, but not in 2201 and 2212. Comparing this result with the structure and property of 123 phase in YBCO-system, analyzing carefully, we found that only those superconductors which have the unstable plane are of higher Tc value. The role of the unstable plane in a superconductor is discussed. It is possible that 110K superconductivity in Bi- system has a close relationship with the existence of this unstable plane. Introduction Since the discovery of high temperature superconductors, much work has been done to discover the unique characteristics of the crystallographic structure of these materials, then to understand the origin of superconductivity from the structure. After the discovery of the Bi-containing superconductors, the transition temperature has empirically been found to increase with increasing number of Cu-O perovskite layers (up to 3) in the unit cell[ 1 ,2 1. For example, in the one Cu-O layer containing phase 2201, Tc is about 20K; two in 2212 phase, Tc is 80K; three in 2223 phase, Tc is I10K. Some researchers[ 3 ,4] have given theoretical explanations for this interesting result. However, the superconductivity can not be explained clearly by considering the number of the Cu-O layers only. For example, the 2212 and Y-containing 123 phases both have two Cu-O layers, but the Tc of the former is 80K, latter 90K. Recently, Kambe et all 5] found that when the sample of 2212 was annealed in absence of oxygen, the Tc could be increased to 98.5K.Thc present paper introduces some experimental evidence which has a strong correlation with superconductivity. We found that in Bi-containing superconductors, there is a unstable plane in 2223 phase, but not in 2201 and 2212 phases. Comparing this result with the structure and property of Y-containing 123 phase, we found that those samples with unstable planes have higher Tc values than that without unstable planes. The authors consider that the unstable plane may have strong correlation with 110K superconductivity. Mat. Res. Soc. Symp. Proc. Vol. 209. @1991 Materials Research Society
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Experimental The samples of Bi- 2201,2212 and 2223 phases were prepared by solid-state reaction method under appropriate conditions which have been reported in reference[ 6]. The structure of the samples were determined with a RIGAKU x-ray diffractometer. The relationship between the resistance and temperature were measured by a standard dc four-probe technique on regularly shaped samples; the resolution of the voltage measurement was l×10- 8 V. A LCT thermal analyzer was used to carry the TG (thermogravimetry) analysis. The weight of the samples me
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