Hyper- and Hypobaric Processing of T1-Ca-Ba-Cu-O Superconductors
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HYPER- AND HYPOBARIC PROCESSING OF Tl-Ca-Ba-Cu-O SUPERCONDUCTORS
K. C. GORETTA. J. L. ROUTBORT. DONGLU SHI, J. G. CHEN, MING XU, AND M. C. HASH Argonne National Laboratory, Argonne, IL 60439.
ABSTRACT Tl-based superconductors of initial composition Tl:Ca:Ba:Cu equal to 2:2:2:3 and 1:3:1:3 were heated in oxygen at pressures of 104 to 6 x 105 Pa. The 2:2:2:3 composition formed primarily the 2-layer superconductor with zero resistance from 77 to 104 K. The 1:3:1:3 composition formed nearly phase pure 3-layer superconductor with a maximum zero resistance temperature of 120 K. Application of hyperbaric pressure influenced phase purities and transition temperatures slightly; phase purities decreased significantly with application of hypobaric pressures. INTRODUCTION Superconducting properties of Tl-based high-temperature superconductors are known to be influenced by oxygen. Annealing in oxygen after sintering raises zero-resistance temperatures (TO) significantly, with final To values ranging from about 112 to 120 K [1-3]. Sintering of Tl-based superconductors is also dependent on oxygen pressure, but the effects are not well established [2,4]. To date, however, phase pure bulk materials have yet to be obtained. Low oxygen pressures have proved effective in synthesizing YBa2Cu30x and Bi2Sr2CaCu20x [5-7]. This work was undertaken to establish whether control of oxygen pressure during synthesis of Tl-based superconductors can result in materials of higher phase purity. EXPERIMENTAL METHODS The compositions examined were nominally Tl 2 Ca2 Ba2 Cu30x (2223) and TlCa 3 BaCu 3 0x (1313) [8]. Appropriate amounts of T1 2 0 3 . Ca2CuO3 . Ba2Cu 3 0 5 or Ba2CuO 3 . and CuO [4] were mixed by mortar and pestle. The mixed powders were seived and then pressed at a pressure of 140 MPa into 1-gram. 12.7-mm-diameter pellets. The pellets were then tightly wrapped in 25-pm-thick Au foil and heated in pure. stagnant oxygen. with pressure ranging from 104 to 6 x 105 MPa. Sintering temperatures were from 860 to 940'C, with higher temperatures used with higher pressures. All heat treatments employed 2-h holds at the maximum temperature and 3*C/min heating and cooling rates.
Mat. Res. Soc. Symp. Proc. Vol. 169. ©1990 Materials Research Society
366
Resistance as a function of temperature was measured for all specimens. For most of the specimens, X-ray diffraction patterns were obtained and chemical analysis was performed by atomic emission spectroscopy. RESULTS AND DISCUSSION Although the vast majority of literature on Tl-based superconductors refers to a specific composition of Tl2Ca2Ba 2Cu 3Ox, the actual composition of the 3-layer phase is Tl deficient owing to Ca substitution on Tl sites [1,9]. A 2223 composition tends to form the 2-layer, nominally Tl 2 CaBa 2Cu2O, phase [8]. Compositions rich in Ca and Cu lead to nearly pure 3-layer phase [4,8]. The results of this study indicate that hyperbaric oxygen pressures have only a slight influence on phase equilibria. For the 2223 initial composition, processing at atmospheric pressure yield
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