C. Bottino

The effect of MgAl2O4 on the formation kinetics of Al2TiO5 from Al2O3 and TiO2 fine powders

The formation of Al2(1−x)MgxTi(1+x)O5 solid solutions from Al2O3-TiO2-MgAl2O4 powder mixtures of ≈1 μm particle size and moderate purity has been studied at 1300°C for different final composition values: x=0 (“pure” Al2TiO5), 10−3, 10−2 and 10−1. Analysis of the kinetic data and microstructural observation indicates that MgAl2O4 affects the mechanism of Al2TiO5 formation by providing active nuclei for the growth of the new phase. These nuclei are probably constituted by Mg0.5AlTi1.5O5, i.e. the equimolar Al2TiO5-MgTi2O5 solid solution, and are formed by reaction between MgAl2O4 and TiO2 at temperatures above ≈ 1150 °C. As the value of x increases, the number of titanate particles per unit volume accordingly increases and the conversion of the original oxides is faster. At values of x⩽10−2, the prevailing mechanism is the nucleation and growth of Al2TiO5 nodules for fractional conversion up to ≈ 0.8. Further conversion of the residual Al2O3 and TiO2 particles dispersed into the titanate nodules is slower and controlled by solid-state diffusion through Al2TiO5. At x=0.1, a large number of nucleation sites is present, and solid-state diffusion through Al2TiO5 becomes important even in the initial stage of reaction, as the diffusion distances are strongly reduced. The study of Al2TiO5 formation under non-isothermal conditions in the temperature range 1250–1550°C shows that reaction proceeds between 1300 and 1350 °C for x=0.01 and between 1250 and 1300 °C for x=0.1. Densification of the titanate becomes important at temperatures above 1300°C for x=0.1, but only above 1450 °C for x=0.01.

Growth of ternary oxides in the Gd2O3-Fe2O3 system. A diffusion couple study

Abstract The simultaneous, diffusion-controlled growth of GdFeO 3 (perovskite) and Gd 3 Fe 5 O 12 (garnet) was studied at 1200–1400 °C in Gd 2 O 3 –Fe 2 O 3 diffusion couples. Both compounds were found to grow as parallel layers according to the parabolic rate law. The parabolic rate constants of the second kind for the exclusive growth of each compound ( k 1 II for GdFeO 3 , k 2 II for Gd 3 Fe 5 O 12 ) were calculated from the experimentally determined rate constants of the first kind assuming coupling between diffusion fluxes and chemical reactions at phase boundaries. In the case of GdFeO 3 , the calculated values of k 1 II are in good agreement with the experimental values measured on Gd 3 Fe 5 O 12 –Fe 2 O 3 couples, where exclusive growth of GdFeO 3 is observed. Growth of the perovskite phase, probably related to formation of gaseous Fe(OH) 2 , is also observed on Gd 2 O 3 –gas–Fe 2 O 3 couples. The values of k 2 II for of Gd 3 Fe 5 O 12 are very close to those found for the growth of the isostructural compound Y 3 Fe 5 O 12 . The most likely reaction mechanism is the coupled diffusion of Gd 3+ and O 2− for garnet growth and the coupled diffusion of Fe 3+ and O 2− for perovskite growth. The activation energy is ≈550 kJ mol −1 for Gd 3 Fe 5 O 12 and ≈400 kJ mol −1 for GdFeO 3 .