Reaction diffusion in the Y2O3-Fe2O3 system

Solid state reactions in the Y2O3-Fe2O3 system were studied by means of different types of diffusion couples. Simultaneous growth of the YFeO3 and Y3Fe5O12 ternary phases was observed on Y2O3-Fe2O3 couples, whereas only the YFeO3 phase grows on Y2O3-Y3Fe5O12 and Y2O3-gas-Fe2O3 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 Y3Fe5O12 growth, the intrinsic rate constants were compared with those predicted by Wagner's theory. The probable mechanism is the equidirectional coupled diffusion of O2- and Y3+ ions. Gas phase transport of iron, in the form of Fe(OH)2, becomes important above 1300°C.