Sol–gel coatings on metals for corrosion protection

The preparation of glass and ceramic systems from metal-organic compounds permits the mixing of the constituents at the molecular level. These mixes form clear glasses or sinter to dense bodies at temperatures considerably lower than the equivalent compositions prepared by classical methods. A significant recent development in this field has been the preparation of monolithic glass and ceramic material without the need of melting or high temperature sintering. Glass-forming reactions, previously achievable only by thermal means, can take place by chemical polymerization at room temperatures. Structural studies indicate that these materials are indeed amorphous or glassy in naturE. Thus, the often used description of glass as a “supercooled liquid” is, in the literal sense, inapplicable for these materials.

Preparation of glasses and ceramics from metal-organic compounds

The use of solution mixing followed by gelation to make ultrahomogeneous glasses and ceramics of various oxide compositions for laboratory experimentation, with first organic and later principally colloidal inorganic, precursors was developed over 30 years ago. Major technologies that use the solution-sol-gel route have been developed to make nuclear fuel pellets, ceramic fibers, thin coatings, and abrasive grain. This article reviews the early history and summarizes present research in this field, particularly new processes aimed at achieving xerogel precursors of maximum heterogeneity with respect either to composition or to structure. Such nanocomposites provide major advantages in lowering sintering temperatures, refining microstructure, and controlling morphology and final phase composition.

Ceramics by the solution-sol-gel route

Two series of xerogels (desiccated hydrogels), one consisting of single phases and the other of two phases, were prepared from organic and inorganic precursors in the system Al2o3-Sio2. The effect of these nanostructural differences in the xerogel powders on the energy content and behavior on heating were studied using DTA, powder X-ray diffraction, and ultrafast heating on a strip furnace. The results demonstrate that diphasic xerogels are substantially different materials from single-phase xerogels of the same composition.

Diphasic xerogels, a new class of materials: phases in the system al2o3-sio2

Highly dense translucent polycrystalline bodies of stoichiometric aluminum silicate (mullite) were obtained by vacuum-hot-pressing of high-purity submicron mixed oxide powders. The powders were prepared by the hydrolytic decomposition of mixed metal alkoxides. X-ray diffraction and electron microscopy indicated that the initially amorphous needlelike fine particulates transform into highly crystalline orthorhombic mullite at ∼1200°C. Optimum hot-pressing conditions were 5 kpsi and 1500°C for 30 min. Densities within experimental error of the theoretical value of 3.19 g/cm3 were obtained. A typical microstructure consisted of fine interlocking needlelike grains arranged in an overall mosaic or “jigsaw” pattern. Microprobe traverses across the samples indicated homogeneous dispersion of SiO2 in the AlO3 matrix. Room-temperature mechanical properties were measured and correlated with the microstructure and crystal structure of the ceramic compact. A slightly higher melting temperature than has been previously reported was observed for the 3Al2O3·2SiO2 studied. All specimens exhibited a smooth surface finish and excellent thermal-shock resistance from 1200°C to room temperature.

Synthesis and Mechanical Properties of Stoichiometric Aluminum Silicate (Mullite)

Various methods for the preparation of mixtures for phase equilibrium studies are described. Especial attention has been paid to compositions which cannot be preserved in a glass structure

Aids in hydrothermal experimentation. II: Methods of making mixtures for both dry and wet phase equilibrium studies. Editorial comment

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Aids in hydrothermal experimentation. II: Methods of making mixtures for both dry and wet phase equilibrium studies. Editorial comment