Reaction kinetics in a coarse equimolar powder mixture were slow enough to allow for the different stages to be identified, notably in the lower and higher temperature ranges, respectively. In the former (T≤ 1600 K), Al2TiO5 nucleation was hindered by the strain energy contribution to the overall driving force. The setting up of metastable layer sequences Al2TiO5/TiO2/Al2O3 was found to occur generally during subsequent growth. The high Al mobility in the TiO2 provided a rapid aluminum transport from the metastable Al2O3/TiO2 interface to the TiO2/Al2TiO5 front. At temperatures above ∼1700 K the Al2O3/TiO2 interface was very rapidly sealed off by Al2TiO5 formation. Reactant transport across the Al2TiO5 was slow because of the low mobilities in the product phase. Therefore, much lower product growth velocities were observed at higher temperatures than at lower temperatures.

Aluminum Titanate Formation by Solid-State Reaction of Coarse Al2O3 and TiO2 Powders

Reaction diffusion in the Y2O3-Fe2O3 system

We synthesized spinel ZnAl2O4 film on α-Al2O3 substrate using a solid-phase reaction between the pulsed-laser-deposited ZnO film and α-Al2O3 substrate. Auger electron spectroscopy showed that the atomic distribution in the spinel ZnAl2O4 was inhomogeneous, which indicated that the reaction was diffusion controlled. Based on X-ray fluorescence measurements, the apparent growth activation energy of ZnAl2O4 was determined as 504 kJ/mol. X-ray diffractometry spectra showed that, as the growth temperature increased, the ZnAl2O4 film became disoriented from the single (111) orientation. The ZnAl2O4 (333) diffraction peak shifted toward a small angle, and its full-width at half-maximum decreased from 1.30° to 0.37°. At the growth temperature of 1100°C, the morphology of the ZnAl2O4 was initially transformed from islands to stick structures, then to bulgy-line structures with increased growth time. X-ray diffractometry spectra showed that these transformations were correlated with changes of ZnAl2O4 orientation.

Synthesis of Zinc Aluminate Spinel Film through the Solid‐Phase Reaction between Zinc Oxide Film and α‐Alumina Substrate

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

Alkaline-earth titanates, MTiO 3 (M = Mg, Ca, Sr and Ba), were prepared by solid-state reactions in air and water vapor atmospheres to investigate the effects of water vapor on the formation of MTiO 3 . The formation of MTiO 3 was accelerated more or less by water vapor. Acceleration effect by water vapor increased in the following order: MgTiO 3 and CaTiO 3  ≪ SrTiO 3 3 . The formation of SrTiO 3 and BaTiO 3 was drastically accelerated by water vapor even though the coarse particles of SrCO 3 and BaCO 3 remained in the starting mixtures. The difference in its effect might be mainly attributed to the vapor pressure of M(OH) 2 . Gas-phase transport of M(OH) 2 would become important for the formation of MTiO 3 by solid-state reactions in water vapor atmosphere.

Preparation of alkaline-earth titanates by accelerated solid-state reaction in water vapor atmosphere

The rate of ZnA12O4 formation for binary powder mixtures of ZnO and α-Al2O3 (dense coarse particles and weak agglomerates of fine powder) fired in air or O2 atmospheres was measured and the microstructures of those systems were observed by scanning electron microscopy. With dispersed dense particles of α-Al2O3, the Al2O3 surfaces were covered with ZnO and the spinel grew into the particles maintaining essentially a constant reaction interface area. Calculations based on geometric measurements and use of Jander's equation gave a similar high activation energy, 354 kJ/mol, which corresponds to the activation energy of volume diffusion of Zn2+ in ZnAl2O4. An oxygen atmosphere had no effect. With a matrix of fine α-Al2O3 powder and dispersed granules of ZnO, a higher reaction rate occurred because of an increase in reaction interface area due to penetration of the powder compact matrix by ZnO vapor, which was enhanced by an O2 atmosphere. The reaction layer grew into the alumina matrix adjoining the ZnO granules with a parabolic rate law. Apparent activation energies below ∼200 kJ/mol were calculated.

H. Kawakami

Papers

Effect of Physical Nature of Powders and Firing Atmosphere on ZnAI2O4 Formation