Solid State Formation of Superconducting Phases in the T1-Ca-Ba-Cu-O System
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SOLID STATE FORMATION OF SUPERCONDUCTING PHASES IN THE Tl-Ca-Ba-Cu-O SYSTEM T.W. Huang*, M.P. Hung*, T.S. Chin#, H.C. Ku##, S.C. Yang**, and S.E. Hsu** *Department of Materials Engineering, National Cheng Kung University, Tainan, 70100, Taiwan, Rep. of China #Department of Materials Sci. & Eng., and ##Department of Physics National Tsing Hua University, Hsinchu, 30043, Taiwan, R.O.C. "-Materials R & D Center, Chung Shan Inst. of Sci. & Tech., Lung-Tang, 32526, Taiwan, Rep. of China ABSTRACT Differential thermal analysis (DTA) and thermal gravitometry analysis (TG A) under flowing oxygen show that the formation temperatures of the high Tc TICa2 Ba Cu 0 (2223) and the low Tc T1 CaBa 2 Cu O (2212) phases lie
at around 916 • and 840
C, respectively.
Rhe activation energies are as
low as 50 kcal/mole for the 2223 phase and 35 kcal/mole for the 2122 phase. Thus the time for the formation of these phases can be within 30 minutes. Two preparation methods were tried, and the one with Ca3 BaCu 0 as a precursor was easier to attain pure high Tc 2223 phase than tNa prepared directly from component oxides and carbonates. INTRODUCTION Oxide superconductors based on thallium have by far the highest proved Tc of 125 OK [1 - 4]. That makes possible real applications at liquid nitrogen temperature. Much work has been done to increase the Tc of the T1 system by various additives and/or treatments [1-6], without a full understanding of what is going on in the basic reactions. In this paper, we studied the solid state formation by thermal and XRD techniques. EXPERIMENTAL PROCEDURES The experimental setup was constructed to be a closed system, as a positive pressure was always maintained. The outlet gas was passed through alkali, glycerine and acid liquids in sequence to absorb the poisonous Tl fume. The same setup was used in the thermal analyses. Double quartz tubes were used inside the furnace for safety reason. The ratio of cations was TI : Ba : Ca : Cu = 2 : I : 3 : 3, since TI is lost more easily, and the T1 ratio was increased from 1 to 2. The content of Ca and Cu increased from the stoichiometry TI 2Ca Ba2Cu3 010 (in short as 2223 t Ca BaCu3010 (in short as 2313 ), for tie purpose of fluxing to facilitate Re homogeneity of reaction and to lower the optimal temperature of reaction. The original chemical reagents were Tl 0., BaCO3 , CaCO3 and CuO, with purity better than 99.9%. The precursor oi 2a 3 BaCu 3 0 7 was also prepared by calcining the raw mixture at 9700C for 16 hours. The rai mixture was mixed by grinding in an agate mortar, pressing under 15 kg/mm to a diameter of 7 mm, thickness of 0.5 1 mm, weighing about 0.2 gm and wrapped with gold foil. The atmosphere during the whole thermal cycle was 100% oxygen. Samples for solid state reaction study were heated at a rate of 10 °C/min and interrupt-quenched from 270, 350, 460, 812, 840, and 900 0C, respectively in air.
Mat. Res. Soc. Symp. Proc. Vol. 169. ©1990 Materials Research Society
354
DTA and TGA analyses were done with a DU POND 1910B, heating rate of DTA was 10
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