Abstract
Solid state reactions in the Y 2O 3-Fe 2O 3 system were studied by means of different types of diffusion couples. Simultaneous growth of the YFeO 3 and Y 3Fe 5O 12 ternary phases was observed on Y 2O 3-Fe 2O 3 couples, whereas only the YFeO 3 phase grows on Y 2O 3-Y 3Fe 5O 12 and Y 2O 3-gas-Fe 2O 3 couples. The growth rates were determined by layer thickness measurements on couples reacted at 1220, 1300 and 1352°C in air and at 1403°C in oxygen for times ranging from 8 to 256 h. The growth kinetics is parabolic in all cases. Based on a model for growth of multiple layers, the intrinsic parabolic growth constants were calculated from the apparent experimental rate constants. In the case of Y 3Fe 5O 12 growth, the intrinsic rate constants were compared with those predicted by Wagner's theory. The probable mechanism is the equidirectional coupled diffusion of O 2− and Y 3+ ions. Gas phase transport of iron, in the form of Fe(OH) 2, becomes important above 1300°C.
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