Role of intermediate solid phase in the process of magnesium and lanthanum aluminates formation in sub- and supercritical water

Abstract

The synthesis of inorganic complicated powder oxides is of significant interest owing to wide application of these materials in modem engineering. In this paper the mechanism of fine crystalline magnesium and lanthanum aluminates formation in sub- and supercritical water was investigated. Mechanical mixtures of aluminium hydroxide and magnesia as well as lanthana were used as precursors. The synthesis was carried out in water steam atmosphere at P ≈ 20 MPa and temperature of 300-400° C in the presence of small amounts of Cr(VI) ions. It was found, that the mechanism of this process cannot be represented in the frameworks of the traditional approach dissolution of precursors in supercritical fluid and precipitation of the products. This approach assumes a complete homogenisation of reacting system at the intermediate stage of the process. By means of X-ray analysis of products of the intermediate stage of synthesis, the presence of a crystalline phase was revealed by us. This phase will be derived in an outcome dehydration of precursors, which are flowing past in quasi equilibrium conditions. In the case of MgAl₂0₄ the intermediate phase is boehmite, and in the case of LaAlO₃ -lanthanum hydroxide. The structure and properties of these phases exert a decisive influence- on the synthesis process and characteristics of formed product.

Key Words:

• Magnesium
• Lanthanum aluminates
• Intermediate phase
• Sub- and supercritical water