Kinetics of the oxidation of Ba 2 YCu 3 O x ceramics
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. INTRODUCTION Much confusion has accompanied the challenge of preparing BajYCujO^ ceramic with reproducible superconducting properties. Variations in Tc and in levitation strength are found in material made at different laboratories or within the same laboratory by different investigators, by the same investigator using different chemical sources, or by the same investigator using the same powder but different sintering/annealing/forming procedures. Such results have generated much speculation regarding the effects of slight changes in cation or oxygen stoichiometry, grain boundary precipitates, and order/disorder within the oxygen or cation lattices. For good superconducting ceramic it is necessary to achieve an orthorhombic crystal structure with the oxygen stoichiometry as close to x = 7 as possible.' During sintering the ceramic attains an oxygen stoichiometry that is determined by both the sintering temperature and the atmosphere. Figure 1 shows this dependence of oxygen stoichiometry on temperature and oxygen partial pressure for Ba2YCu3O;c ceramic that is single phase by microscopic examination.2 At 950 °C,x == 6.35 in 1 atm oxygen, while at 875 °C, x = 6.30 in air. When x < ^ 6 . 6 3 , the Ba2YCu3OJC ceramic has the tetragonal crystal structure.3 The temperature at which the transition from tetragonal to orthorhombic occurs depends strongly on the oxygen partial pressure, provided the specimen is free to change its composition. Even though the importance of the oxygen stoichiometry has been recognized, control and measurement are difficult. Direct chemical analysis requires reduction of the copper oxide and destruction of the sample either by H 2 reduction1 or an iodimetric titration. 45 Neutron diffraction offers a nondestructive alternative.6'7 Fortunately, there is good agreement between methods.8 Long anneals at low temperatures (400700 °C) are often used in anticipation of complete oxidation, but direct measurement of the resulting value of x is not usually made. If the kinetics of oxidation are rapid J. Mater. Res. 3 (4), Jul/Aug 1988
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at the annealing temperature, and if the equilibrium value of x is near 7, then the annealing will produce fully oxidized Ba 2 YCu 3 O x . If not, a possibly heterogeneous ceramic with a lower x value will result.9 This article reports data for the rate of oxidation for Ba2YCu3O;c samples with different densities and discusses the effects of temperature and oxygen stoichiometry on the kinetics. II. EXPERIMENTAL Samples of Ba 2 YCu 3 O x ceramic were prepared from a single powder batch that had been made from well-mixed Ba(OH) 2 , Y 2 O 3 , and CuO raw materials. This mixed powder was prereacted at 850 °C in oxygen for 6 h and then ball milled for 4 h using ZrO 2 media and acetone. Pellets (6 mm diam X 2 mm) were formed and sintered in oxygen at 925-975 °C for 5 h. For some samples the following special cooling environment was used in order to maintain the oxygen content near x = 6.3. At 875 °C the oxygen in the sintering atmosphere was repl
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