Sol-gel chemistry of transition metal oxides

Use of CeO2-based oxides in the three-way catalysis

This review article focuses on the sol-gel preparation of high temperature superconducting oxides wherein different classes of gel technologies were utilized. These involve: 1) the sol-gel route based upon hydrolysis-condensation of metal-alkoxides, 2) the gelation route based upon concentration of aqueous solutions involving metal-chelates, often called as “chelate gel” or “amorphous chelate” route, and 3) the organic polymeric gel route. This paper reviews the current status of these sol-gel processes, and illustrates the underlying chemistry involved in each sol-gel technology. It is demonstrated that the chemical homogeneity of the gel is often disturbed by the differences in the chemistries of the cations. Prior to gelation the starting precursor solution containing various forms of metal-complexes must be chemically modified to overcome this problem. Illustration of a variety of strategies for success in obtaining a homogeneous multicomponent gel with no precipitation is focal point of this review article.

Invited review “sol-gel” preparation of high temperature superconducting oxides

Mullite (3AI2O3. 2Si02) is becoming increasingly important in electronic, optical, and high-temperature structural applications. This paper reviews the current state of mullite-related research at a fundamental level, within the framework of phase equilibria, crystal structure, synthesis, processing, and properties. Phase equilibria are discussed in terms of the problems associated with the nucleation kinetics of mullite and the large variations observed in the solid-solution range. The incongruent melting behavior of mullite is now widely accepted. Large variations in the solid solubility from 58 to 76 mot% alumina are related to the ordering/disordering of oxygen vacancies and are strongly coupled with the method of synthesis used to form mullite. Similarly, reaction sequences which lead to the formation of mullite upon heating depend on the spatial scale at which the components are mixed. Mixing at the atomic level is useful for lowtemperature (<lOOO"C) synthesis of mullite but not for low-temperature sintering. In contrast, precursors that are segregated are better suited for lowtemperature (1250" to 1500°C) densification through viscous deformation. Flexural strength and creep resistS M Wiederhorn-contributing editor

Mullite for Structural, Electronic, and Optical Applications

Titanium alkoxides hydrolyse vigorously with water producing polycondensates whose equivalent oxide content varies from ∼ 70% to over 90%. This variation reflects the average molecular size and the nature of the terminal bonds. Hydrolysis conditions also affect the molecular and particle morphologies modifying the crystallization and sintering behaviour of oxide materials derived from polycondensates. It is also possible to form clear polymer solutions under excess water hydrolysis with the addition of certain acids. Investigations show that there is a window in the acid concentration which provides long-term solution stability. The gels formed from these clear solutions shrink in their liquor under certain conditions and the introduction of hydrogen peroxide into the liquor (surrounding the gel) causes vigorous gel shrinkage.

Hydrolysis of titanium alkoxide and effects of hydrolytic polycondensation parameters

This work shows that molecular structural variations can be introduced in oxide systems, including AI2O3, ZrO2, TiO2, and SiO2, by controlled hydrolysis and polymerization reactions during the formation of oxides from metal alkoxides. The chemical and stoichiometric makeup of hydroxides and oxides is not fixed and changes rapidly as a function of molecular size in the region where the size is extremely small. It was also observed that internal structural variations significantly alter the sintering behavior of oxide powders and affect their subsequent crystalline transformations.

Effect of Variations in Polymerized Oxides on Sintering and Crystalline Transformations

Starting powders containing 72 wt% Al/sub 2/O/sub 3/ and 28 wt% SiO/sub 2/ were prepared by sol-gel methods classified as colloidal and polymeric. Compacts fired at 1700/sup 0/C showed significant differences in microstructure. The specimens formed with the colloidal powder had mullite grains of prismatic shape and liquid phase; with polymeric powder, mullite grains were granular with no liquid phase present. It is shown that the mullite grains in the first case are higher in Al/sub 2/O/sub 3/ content, resulting in an excess of SiO/sub 2/ which is the base for the liquid phase. In the second case, the mullite grains have the same Al/sub 2/O/sub 3/ content as the starting powders. The presence of a liquid phase in the first case is considered to be metastable, resulting from the nature of the starting materials and processing conditions employed.

B. E. Yoldas

Papers

Effect of Variations in Polymerized Oxides on Sintering and Crystalline Transformations

Effect of Sol‐Gel Mixing on Mullite Microstructure and Phase Equilibria in the α‐Al2O3‐SiO2 System

Effect of Sol‐Gel Mixing on Mullite Microstructure and Phase Equilibria in the α‐Al2O3‐SiO2 System.