Aggregation of colloidal particles with a finite attraction energy was investigated with computer simulations and with gold particles coated with a surfactant. Computer simulations were carried out with the Shih-Aksay-Kikuchi (SAK) model, which incorporates a finite nearest-neighbor attraction energy - E into the diffusion-limited-cluster-aggregation (DLCA) model. Both the computer simulations and the experiments showed that (i) with a finite interparticle attraction energy, aggregates can still remain fractal, and (ii) the fractal dimension remains unchanged at large interparticle attraction energies and increases when the interparticle attraction energy is smaller than 4k(B)T, where T is the temperature and k(B) is the Boltzmann constant. The agreement between the simulations and the experimental results suggests that the reversible aggregation process in a colloidal system can be represented by the SAK model.
Ceramic precursor mixt. comprising a metal cation capable of being converted to a metal oxide by thermal energy, a carbohydrate, and an anion capable of participating in an anionic oxidn.-redn. reaction with the carbohydrate is converted to ceramic powder by forming droplets of the precursor mixt., removing all the solvent from the droplets, thermally initiating an anionic oxidn.-redn. reaction between the anion and carbohydrate to form a multiphase ceramic material consisting of carbonates, hydroxides, and oxides of metal cation, and heating the particles to convert the multiphase ceramic into a single-phase ceramic. This process is used for the manuf. of superconductive and nonsuperconductive ceramic powders.
During the last decade, significant advances have been made in the processing of ceramics by a combination of techniques utilizing molecular precursors and colloids for powder consolidation. Powder consolidation methods have mainly dealt with the formation of unagglomerated powders in the size range of 0.1-1-mu-m, the preparation of colloidal suspensions that are suitable for the formation of high density compacts by filtration and/or plastic forming techniques, the removal of the processing aids, and the role of consolidation methods on microstructural evolution. In contrast, the molecular and/or sol-gel techniques dealt with processing at a finer dimensional scale of 10-1000 angstrom with either molecularly homogeneous precursors or nanometer-sized particulates that are used in the preparation of gels that display linear viscoelastic behavior. Similar to green compacts of micron-sized powders, these gels are then converted to dense ceramics by heat treatment. This review summarizes the concepts that are common to both of these regimes and points to the synergistic benefits of coupling molecular precursors with colloids in a process path. The emphasis is on the control of the structure of a final product at scale lengths ranging from molecular to micro- and macroscopic dimensions.
Mullite (3Al2O3.2SiO2) 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 mol% 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 low-temperature (< 1000-degrees-C) synthesis of mullite but not for low-temperature sintering. In contrast, precursors that are segregated are better suited for low-temperature (1250-degrees to 1500-degrees-C) densification through viscous deformation. Flexural strength and creep resistance at elevated temperatures are significantly affected by the presence of glassy boundary inclusions; in the absence of glassy inclusions, polycrystalline mullite retains > 90% of its room-temperature strength to 1500-degrees-C and displays very high creep resistance. Because of its low dielectric constant, mullite has now emerged as a substrate material in high-performance packaging applications. Interest in optical applications mainly centers on its applicability as a window material within the mid-infrared range.
Recent studies have shown that the mullitization of diphasic aluminosilicate matrices comprising transitional alumina and amorphous silica occurs via a nucleation and growth process. Nucleation is preceded by a temperature-dependent incubation period. Following this incubation period, rapid nucleation of mullite occurs, producing about 1.8 x 10(11) nuclei/cm3, which remains constant throughout the rest of the transformation. Both incubation and mullite growth are thermally activated processes with apparent activation energies of 987 +/- 63 and 1070 +/- 200 kJ/mol, respectively. The growth rate of mullite grains under isothermal conditions is time dependent. An interpretation of these results is proposed on the basis of the nucleation and growth concepts of LaMer and Dinegar which supports the concept that the growth rate of mullite grains is controlled by the dissolution of transitional alumina into the amorphous matrix.