EFFECT OF CO2 CONCENTRATION ON YBA2CU3O6+X FORMATION

Two reversible reactions are involved in YBa2Cu3O6+x formation: a reac tion between BaCO3 and CuO forming BaCuO2 and CO2, and a reaction of B aCuO2 with Y2O3 and CuO forming YBa2Cu3O6 which undergoes phase transf ormation to YBa2Cu3O6+x. upon cooling. In-situ isothermal time resolve d HT-XRD of a thin film was used to quantify the effect of CO2 on the kinetics of the first reaction. Increased CO, partial pressure shifts the reactions to higher temperatures. At high CO, partial pressure (>2 vol. %), the rate of the first reaction becomes essentially a step pr ocess with a very high activation energy. Noninstantaneous nucleation of the reaction products occurs at low CO2 partial pressure (0.5-1%) a nd temperatures (700 degrees C). The data fit a 2-D diffusion-controll ed mechanism with a zero nucleation rate for BaCO3 decomposition and a second-order nucleation rate for YBa2Cu3O6 formation. A comparison of the kinetics of a thin film (10 mu m) as determined by HT-XRD with th ose of a thick sample (2 mm) determined by TG revealed that the transp ort of CO, within the sample pores and to the ambient gas significantl y affect the decomposition of BaCO,. Far example, the formation of YBa 2Cu3O6 in a thick precursor layer occurs in the 840 to 940 degrees C r ange, exceeding by about 200 degrees C that in which it is formed in t hin films.