The conditions (temperature, pressure and time) for attaining equilibrium with oxygen for
different YBCO materials are reported. The equilibrium oxygen content was used in order
to determine more exact values for the enthalpies and entropies of interaction of
YBa2Cu3Oy
with oxygen. Internal equilibrium in YBCO is proposed and is able
to explain the effects of ageing and low temperature annealing on
Tc. Homogeneity and transitions to superconductivity of ceramics having equilibrium and
non-equilibrium oxygen contents are discussed.
We have developed a method d determining oxygen stoichiometry (y) of small masses of cuprate oxides, based on iodometric micrdiration. It comprises double microtitration. The first microtitration was carried w t on a Cuzt solution of accurately known mncentration. A small mass (-500pg) was accurately determined by an absolute calibration d a microbalance to within f N 0.8 pg. We obtained y = 6.960 f 0.009 from ten microtitrations on small masses extracted from a large ceramic sample of YBazCu&. In this sample y = 6.958 5 0.005 was determined by conventional macrotitration. This method can be used to determine oxygen stoichiometry in single wystals and in thin films of cuprate superconductors.
Work with ceramic samples of (Ca x La 1−x )(La u Ba 1−u ) 2 Cu 3 O y (these compounds have previously been denoted as CLBLCO, CLBCO or CaLaBaCuO) has shown that ordinary cooling rates used after annealing at high oxygen pressures may result in samples having higher (overestimated) T c values when compared to the fast cooled ones having the same y. Thus, exposure of a sample of x = 0.1 and u = 0.175 to temperatures lower than 200 • C, which occur during cooling, leads (without oxygen absorption) to an overestimation of the T c value. Increase of the cooling rate by dropping the pellet into the cold part of the autoclave suppresses this effect. Another effect is oxygen absorption during cooling when it is performed after high temperatures of annealing. The products of this absorption have an inhomogeneous distribution of oxygen ions in the ceramic grain. Their T c values are also overestimated.
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