Crystal Chemistry and Phase Equilibrium Studies of the Bao(baco 3 )-r 2 o 3 -cuo Systems. V. Melting Relations in ba 2 (
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CRYSTAL CHEMISTRY AND PHASE EQUILIBRIUM STUDIES OF THE BaO(BaCO3 )R2 0 3 -CuO SYSTEMS. V. MELTING RELATIONS IN Ba 2 (Y,Nd,Eu)Cu 3 O6+x WINNIE WONG-NG, LAWRENCE P. COOK, MICHAEL D. HILL, BORIS PARETZKIN AND E.R. FULLER, JR., Ceramics Division, National Institute of Standards and Technology, Gaithersburg, Md 20899.
ABSTRACT The influence of the ionic size of the lanthanides R on melting relations of Ba 2 RCu 3 O6 +x, where R=Y, Eu and Nd, was studied and compared with that of a high Tc superconductor mixed-lanthanide phase Ba 2 (Y. 7 5 Eu. 1 2 5 Nd. 1 2 5 )Cu 3O 6 +x. These materials have been characterized by a variety of methods including differential thermogravimetric analysis (DTA), scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDX) and X-ray powder diffraction. Single phase samples of Ba 2 (Y. 7 5 Eu.1 2 5 Nd.1 2 5 )Cu 3 O 6 +x were annealed at 1004, 1040, 1052, 1060, 1078, 1107 and I160°C and quenched into a helium gas container cooled by liquid nitrogen. The SEM micrographs of these samples showed the progressive chnages in features of the microstructures from sintering and grain growth through melting and then recrystallization from the melt. The addition of the SEM technique in conjunction with X-ray diffraction has been helpful in the study of phase equilibria in this system.
INTRODUCTION Knowledge of the phase compatibilities and melting relations of compounds formed in the Ba-R-Cu-O systems, where R=lanthanides or yttrium, is essential for understanding material properties and controlling processing parameters for the high Tc superconductor phases Ba 2 RCu 3 O 6 +x. Phase equilibrium studies of the systems BaO-Y 2 0 3 -CuO, BaO-La 2 O3 -CuO and other lanthanide systems have been conducted by Roth et al. [1], Kilbabow et al. [2] and Wong-Ng et al. [3,4], respectively. To date, the most complete phase diagram studies including information pertaining to the subsolidus phase compatibility as well as the melting relations, have been done for the Y-system. Figure 1 shows the BaO-Y 2 0 3 -CuO system prepared in air at 950°C with suggested primary phase fields of the "green phase" BaY2 CuO5 (1:2:1) and of the superconductor phase Ba2YCu3O6+x(2:l:3). It has been found by Roth et al. [1] that the superconducting phase Ba 2 YCu 3 O6+x melts incongruently at about 10020C to the "green phase", BaY2 CuO5 , plus liquid. The outline of the primary phase field of the superconducting phase Ba2YCu3O6+x is extremely narrow, indicating that the compositional range for crystal growth of the high Tc phase is very small. The discovery of the fact that a variety of lanthanide substitutions also exhibit an onset Tc of -90K not only provides us materials with similar structures, but also allows us to prepare mixed-lanthanide superconductor materials to investigate possible increases of the primary phase field. By mixing lanthanide elements of appropriate size and amount, single phase mixed-lanthanide high Tc materilas could be obtained. The mixed lanthanide system which we have chosen for these 3 i
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