Two reversible reactions are involved in YBa,Cu306 + , formation: a reaction between BaCO,, and CuO forming BaCuO, and CO,, and a reaction of BaCuO, with Y,03 and CuO forming YBa,Cu306 which undergoes phase transformation to YBa,Cu306 , upon cooling. In-situ isothermal time resolved HT-XRD of a thin film was used to quantib the effect of CO, 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 process with a very high activation eneqy. Noninstantaneous nucleation of the reaction products occurs at low CO, partialpressure (0.5-1%) and temperatures (700°C). The data fit a 2-0 difision-controlled mechanism with a zero nucleation rate for BaCO, decomposition and a second-order nucleation rate for YBa,Cu,06 formation. A comparison of the kinetics of a thin film (10 pm) as determined by HT-XRD with those of a thick sample (2 mm) determined by TG revealed that the transport of CO, within the sample pores and to the ambient gas sign$cantly affect the decomposition of BaC0,. For example, the formation of YBa,Cu306 in a thick precursor layer occurs in the 840 to 940°C range, exceeding by about 200°C that in which it is formed in thin films.formation from a spray roasted precursor
