Kinetics of solid state formation of the YBa2Cu3O6.5+x phase

Solid state formation of the YBa2Cu3O7 phase (1237 for short) was thoroughly studied with differential scanning calorimetry (DSC), thermal gravitometric analysis (DTG), on the mixtures of Y2O3, BaCO3 and CuO in cation stoichiometry of 1:2:3, 1:3:2 and 2:1:1; and x-ray diffractometry (XRD) on the interruptedly quenched specimens. The activation energy of the formation reaction was calculated with the Kissinger's method. The 1237 phase was found to form in two consecutive and overlapping steps during continuous heating up. The first has a peak at 965 degrees C (at 2 degrees C/min heating rate), and is proposed to be the formation of the 1237 phase from the reaction between the previously formed BaCuO2 and Y2O3/CuO, with an activation energy of 120 kcal/mole. The second has a peak at 980 degrees C, and is the reaction between the previously formed Ba2CuO3 and Y2O3/CuO to form the 1237 phase with an activation energy of 180 kcal/mole. The effect of particle size on the formation of the 1237 phase was also studied. The optimum isothermal temperature for forming the 1237 phase is lowered from 927 to 890 degrees C, due to the lowering of the activation energy by a quarter, when the average particle size is reduced from 8 to 0.5 microns