Variation of the partial thermodynamic
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INTRODUCTION
T H E R M O D Y N A M I C properties of YBa2Cu~O7_~ are important for answering questions related to its stability under different conditions and for statistical mechanical investigation of defects and order-disorder transition in this oxide. Thermodynamic stability of the superconducting phase has to be assessed with respect to (a) other phases in the quaternary system Y-Ba-Cu-O, (b) H20, CO~, and SOx present in the ambient atmosphere, and (c) oxygen content within the homogeneous solid solution. Several investigators have measured the oxygen nonstoichiometric parameter (6) and the oxygen content corresponding to the orthorhombic to tetragonal transition as a function of the partial pressure of oxygen and temperature. IH6] The results are summarized in Table I. Most investigators IHu have used the thermogravimetric technique to measure the variation of the nonstoichiometric parameter (6) with partial pressure of oxygen and temperature. In most cases, the oxygen partial pressure was controlled by mixing oxygen with an inert gas. Typically, oxygen pressure varied from 10 to 105 Pa. Verweij and Bruggink t")l have reported studies down to an oxygen partial pressure of 10 ~- Pa. The accuracy of oxygen partial pressure control decreases with its partial pressure. The thermodynamic capacity of oxygen, (Ono:/OtZo,)r.e.,j, in argon-oxygen mixtures decreases dramatically with oxygen partial pressure, i~TI A change in the oxygen potential of the gas accompanying T O M M A T H E W S , Graduate Student, and K.T. JACOB, Professor and Chairman, are with the Department of Metallurgy, Indian Institute of Science, Bangalore 560 012, India. Manuscript submitted December 3, 1990. METALLURGICAL TRANSACTIONS A
a finite oxygen exchange with the solid will be large at low pressures. Also, a large volume of the gas is required to effect a finite change in the oxygen content of the condensed phase at low pressure. Significant errors are expected in the control of oxygen pressure below 10 Pa. Thermogravimetry measures only the relative changes in composition. To obtain absolute values, chemical analysis is necessary at least for selected compositions. Both hydrogen reduction and iodometric titration have been used extensively. The products of hydrogen reduction are BaO, Y203, and Cu. The mass loss on reduction can be used to calculate oxygen content. The literature[3.6,8,~J,~81 indicates that the temperature of reduction should be higher than 1200 K. Some water retention is encountered at lower temperatures. Marucco el al. 17} have deduced the composition from mass change during synthesis using BaCO3, Y203, and CuO as starting materials. This method assumes that BaCO3 completely decomposes. Recent studies 1~gl show that there is a significant solubility of CO2 in YBa2Cu307-~. Yamaguchi et al. tSI have measured the volume of the gas evolved by reduction of the compound at 2973 K. Strobel e t a / . 141 and Gerdanian et al. Ij51 have assumed ยข5 = O for samples annealed at Po, =- 105 Pa and a specified temperature and then slow
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