| Autor: |
Kohav, T., Forster, K. M., Richardson, J. T., Luss, D. |
| Zdroj: |
AIChE Journal; April 1997, Vol. 43 Issue: 4 p1091-1099, 9p |
| Abstrakt: |
Two reversible reactions are involved in YBa2Cu3O6xformation: a reaction between BaCO3and CuO forming BaCuO2and CO2, and a reaction of BaCuO2with Y2O3and CuO forming YBa2Cu3O6which undergoes phase transformation to YBa2Cu3O6xupon cooling. Insitu isothermal time resolved HTXRD of a thin film was used to quantify the effect of CO2on the kinetics of the first reaction. Increased CO2partial pressure shifts the reactions to higher temperatures. At high CO2partial pressure >2 vol. , the rate of the first reaction becomes essentially a step process with a very high activation energy. Noninstantaneous nucleation of the reaction products occurs at low CO2partial pressure 0.5–1 and temperatures 700°C. The data fit a 2D diffusioncontrolled mechanism with a zero nucleation rate for BaCO3decomposition and a secondorder nucleation rate for YBa2Cu3O6formation. A comparison of the kinetics of a thin film 10 mm as determined by HTXRD with those of a thick sample 2 mm determined by TG revealed that the transport of CO2within the sample pores and to the ambient gas significantly affect the decomposition of BaCO3. For example, the formation of YBa2Cu3O6in 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. |
| Databáze: |
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