PERITECTIC DECOMPOSITION OF YBA2CU3O7-X

During the heating of YBCO a peritectic reaction takes place at 1020 d egrees C, which can be described by: 2YBa(2)Cu(3)O(7-x) reversible arr ow Y2BaCuO5 + L + (1-2x)/2O(2) where L = 3BaCuO(2) +2CuO is a fluid wi th limited amount of yttrium. It has been reported, that many paramete rs can influence the reaction. From one side not only the starting siz e of grains but also the heating rate have an influence on the resulti ng Y2BaCuO5-phase. From the other side, there is a change of the perit ectical temperature caused by changing of the partial pressure of O-2 and the presence of parasitic phase. From general kinetic consideratio n one can draw the conclusion, that different mechanisms (nucleation, phase-boundary reaction and diffusion) can control the reaction. Using DTA/TG measurements, the peritectic reaction has been examined. Class ical kinetic methods (Kissinger and Friedman) has been used. The Fried man method has given the dependence of the activation energy from reac tion degree. This suggests many steps reactions. The dependence of the DTA-peaks from the heat rate suggest a parallel steps of reaction. Th is assumption can be motivated by evaluation of free O-2, one solid an d liquid phase formation. Amount of this phases depends on the heating rate. Additionally X-ray and microscopic methods has been used. In th is way was shown, that the perovskit structure is stable up to peritec tical temperature and than is dramatically destroyed. From microscopic observations has been got information about shape and size of solid p hase and it's creation as a function of temperature, time and starting grain size.