Equilibrium phase diagram for the system PbO-CaO-CuO
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In order to obtain essential information on the formation process of the high-Jc phase in the Bi, Pb-Sr-Ca-Cu-O system, phase equilibria in the system PbO-CaO-CuO have been studied, mainly by x-ray diffraction analysis and thermal analysis. Temperature versus composition diagrams were established for the systems PbO-CuO(Cu2O) and PbO(PbO2)-CaO in air. Three invariant points were detected in these systems: a eutectic reaction (PbO + Ca2PbO4 = L) at 847 ± 6 CC, a peritectic reaction (Ca2PbO4 = L + CaO) at 980 ± 2 °C, and a eutectic reaction (PbO + CuO = L) at 789 ± 3 °C. For the system PbO(PbO2)-CaO-CuO, subsolidus phase equilibrium in air was established. The liquidus was also examined at 780, 800, and 820 °C, and a ternary (PbO-Ca2PbO4-CuO) eutectic point was detected at 772 ± 6 °C.
I. INTRODUCTION
In previous papers, we have reported that a partial substitution of lead for bismuth in the Bi-Sr-Ca-Cu-O system facilitates the formation of the high-Tc phase and that the lead substitution modifies the structural modulation mode.1"5 Much effort has been expended to understand the formation process in the lead-added system, and participation of a molten phase has been reported by various research groups.6"8 According to our systematic study, Ca2PbO4, which is formed in the early stage of reaction, seems to play an indispensable role in the subsequent sintering process, especially in the formation of the molten phase. However, the reaction process has not been explained completely, because basic knowledge of the phases9"17 and their reactions in this complex system is lacking. In this paper, we report the equilibrium phase diagrams of three lead-containing systems included in the parent Bi, Pb-Sr-Ca-Cu-O system: the PbO(PbO2)-CaO, PbOCuO(Cu2O), and PbO(PbO2)-CaO-CuO systems. A related study of the lead-free Bi-Sr-Ca-Cu-O system has been reported elsewhere.1819 Furthermore, studies of the Pb-Sr-R-O (R = Ca and Cu) and Bi-Pb-R-O (R = Ca, Sr, and Cu) systems are now in progress20'21 and will be reported soon. II. EXPERIMENTAL
PbO (purity = 99%), CuO (purity = 99.9%), and CaCO3 (purity = 99%), all supplied from Nakarai Tesque Co., Ltd., were used as the starting reagents. Samples of various atomic ratios were prepared by a conventional ceramic method. After mixing and molding, specimens were heated at 700 °C-850 CC for various periods in air with intermittent homogenizing processes. Calcium-rich specimens were preheated at 800 °C in air for several hours in order to decarbonate. J. Mater. Res., Vol. 5, No. 5, May 1990
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After each heating, the specimens were rapidly cooled to room temperature and subjected to powder x-ray diffraction analysis (XRD, CuK« radiation, Rigaku RAD-1A or RU-200) to identify the solid phases present. Liquidus or solidus points were determined from the results of thermogravimetric and differential thermal analyses (TG-DTA, Rigaku TAS-100 & 8112BRH, heating rate = 10 °C/min, gas flow: O 2/N2 = 0.21/0.79, 20 ml/min) and visual sample observation
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