Effect of flourine doping on BiPbSrCaCuO Superconductor

Physica C 185- 189 (1991) 477-478 North-Holland

Effect of Fluorine Doping on Bi - Pb - Sr - Ca - Cu - O Superconductor Soo Young Lee*, Shigeru Suehara and Shigeo Horiuchi National Institute for Research in Inorganic Materials, Namiki 1-1,Tsukuba, Ibaraki,305, Japan. A Pb-doped Bi-based superconductor, prepared by the additional doping of F, shows the critical temperature of 122 K, which is the highest temperature among those so far reported. The specimens are sintered in two sintedng stages. The effect of F doping is to get a considerable amount of Ca2PbO4 as an intermediate phase after the first sintering, which eventually converts to Ca2CuO3 and then enhances the formation of the 2223 phase. The process may restrain the volatility of Pb as well as F and promote the incorporation of both elements in the structure of the 2223 phase. Since the Bi-based superconductor has been found, many researches have been conducted to produce the high Tc phase(2223 phase) effectively. In this study, the role of fluorine doping in the enhancement of the growth of the 2223 phase has been investigated. Starting materials of Bi203, SrCO3, CaCO3, CaF2, PbO, and CuO were mixed with the nominal composition of Bi:Pb:Sr:Ca:Cu = 0.8:0.2:1:1:1.6 (CaF2 / CaCO3 = 1/9) 1 . The powder mixtures were calcined at 740 °C for 15 hours. The calcined powder were pulverized and pressed into pellets (1.6 mm dia. x 1.5 mm). These pellets were sintered two times in air. The specimens after sintedng were examined by using X-ray diffractometer ( Rigaku D - 9C), SEM (Akashi DS130), EDX (Phillips 9900). The electrical resistivity of the specimens was measured by the conventional four-probe method. The enhanced growth of the 2223 phase was noticed after the second sintering at 838 °C for 150 hours. The resistivity curves of the specimens are shown in Fig. 1. The first sintering was done at 820 °C for the specimen A and at 835 °C for the specimen B for 60 hours. The specimen A shows Tc = 122 K ('l'c(on) = 128 K and Tc(0) = 116 K) with a very small resistivity tail.

The XRD result of the

specimen A indicates a strongly enhanced 2223 phase (Fig.2(a)). If the second sintering time is prolonged longer than 150 hours, the resistivity tail can be removed because this specimen has still contained an small amount of the 2212 phase and the intermediate phase even after sintering for 150 hours. As the first sintering temloerature increased to 835 °C, The electrical resistivity of the specimen B was significantly degraded. As seen in the XRD result of the specimen B (Fig. 2(b)), the intensities of the 2212 phase and 2201 phase are much higher than those for the specimen A.

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* On leave from Korea Institute of Machinery & Metals, Changwon, Kyungnam, Korea 0921-4534/91/$03.50 © 1991 -Elsevicr Science Publishcrs B.V. All rights rcscn ed.

S.Y. Lee et aL / Effect o~fluorine doping on Bi-Pb-Sr-Ca-Cu-O superconductor

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The doping of CaF2 in this experiment enhanced the formation of Ca2PbO4 which decomposed to produce Pb-rich Ca2CuO3 during the second sintering. This lead-rich intermediate phase may accelerate the diffusion of Ca and Cu and provide effective nucleation sites for the 2223 phase 3 in addition to lowering the sintering temperature below than 840 °C. This may restrain the volatilization of Pb as well as F and enhance the incorporation of both elements in the 2223 phase during sintering.

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It was necessary for the enhancement of the 2223 phase during the second sintering that the starting materials transform into the 2212 phase and the intermediate phase such as Ca2PbO4 and Ca2CuO3 as much as possible during the first sintering. Then, these intermediate phases will accelerate the diffusion the extra calcium and copper into the 2212 phase , resulting in the enhancement of the 2223 phase 2. The optimum temperature for the first sintering really depends on the composition of the starting materials. However, the amount of the 2212 phase seems to be maximized at 820 C in this study. Fig.3 shows the SEM micrograph of the specimen A. In spite of the extensive large grain growth, the specimen reveals a very dense layered structure, resulting in the good intergrain superconducting coupling.

Figure 3 The SEM micrograph of the specimen A REFERENCES 1. S. Horiuchi, K. Shoda and Y. Matsui, J. Ceram. Soc. Jpn. Inter.Ed. 97 (1989) 977. 2. D. Shi, M.S. Boley, J.G. Chen, M. Wu, K. Vandervoort, Y.X. Lioa and A. Zangvil, Appl. Phys. Lett. 55(7) (1989) 699. 3. F.H. Chert, H.S. Koo and T.Y. Tseng, Appl. Phys. Lett. 58(6) (1991) 637.