Thermodynamic study at high temperature of the superconducting system YBa 2 Cu 3 O z with 6 <z<7
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I. INTRODUCTION
II. EXPERIMENTAL
Recently, great interest has focused on the possibility of superconductivity at high temperature in a new class of material having a perovskite-related orthorhombic structure.1'2 Several authors noted the influence of synthesis procedures on the superconducting properties. Transition temperatures are observed above liquid nitrogen temperature after an appropriate annealing treatment in oxygen.3'4 Recent structure determinations have shown the presence of oxygen vacancies: a transition from a tetragonal phase structure at low oxygen content to an orthorhombic structure at high concentration is observed. The present article deals with other new phase transitions observed by the oxygenation or deoxygenation of the material directly formed at high temperature, 950 °C and 105 Pa, by the solidstate reaction between BaCO3, CuO, and Y 2 O 3 . The phase transitions are observed through thermogravimetric measurements carried out in various oxygen pressures at constant temperature or at variable temperature in a fixed oxygen pressure. For each step the kinetics of the reaction have been carefully analyzed in order to ensure equilibrium conditions. A special procedure has been used in order to reach the absolute value z of the oxygen content; the samples were formed directly from the oxide into the crucible of the thermobalance. At the highest available oxygen partial pressure POi = 105 Pa, the oxygen content is found to be a constant, Z = 7 + 0.005, at a temperature below 250 °C. Deviations from stoichiometry have been analyzed in this orthorhombic phase.
The experimental apparatus has been previously described.5 It consists of a Ugine Eyraud B60 thermobalance with a working sensibility of 0.015 mg corresponding to dz varying from 10" 3 -5 X 10 ~ 4 for a mass sample ranging from 600-1200 mg. A Chevenard furnace and Wostoff pumps were used to adjust both the temperature and the oxygen partial pressure in the experimental setup. The accuracy of the experimental measurements performed with this apparatus has been previously described.5 The YBa2Cu3Oz was prepared from the appropriate mixture of BaCO3 (purity 99.9%), Y 2 O 3 (99.99%) and CuO (99.99%). The components were carefully weighed in order to give a standard for the evaluation of the absolute value of the oxygen content. The mixed power materials were pressed into pellets and heated directly in the thermobalance. The samples were placed in a recrystallized alumina crucible. The mixture was then heated in a dry oxygen flow and the weight variations were monitored all along the solidstate reaction performed at 950 °C and P o , = 105 Pa. The high temperature of the reaction was chosen in order to ensure a complete decomposition of the BaCO3. The reaction was performed over one week in several steps separated by grindings of the pellets in order to obtain a homogeneous single phase. After oxygenation, x-ray diffraction studies indicate that the compound was single phase with an orthorhombic structure. Because of the complete decomposition of t
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