AbstractsEngineering

Nano-composite materials exhibit unique electrical, magnetic, and mechanical properties. It is desirable to make these materials by colloidal processing since it provides better control of the microstructure. However, nanometer-sized particles form colloidal gels at very low density and it is difficult to achieve high-density consolidation. To solve this problem, the structures of colloidal aggregates of nanometer-sized particles were studied at both the micrometer and nanometer scale. In addition, various aspects of the viscoelastic properties of the colloidal gels were studied, and these properties were correlated to the structures. The studies described in this dissertation have enhanced the understanding of colloidal systems of nanoparticles and helped to improve colloidal processing of such systems. As a result, it is not only possible to achieve high-density consolidation with nanometer-sized particles ($>$55% for silica particles), it is also possible to tailor the degree of homogeneity of composite materials on a nanometer scale by controlling the interaction energies between the particles.The structures and properties of colloidal systems of nanometer-sized particles were studied by transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), laser light scattering, and dynamic rheometry. The application of HRTEM provides direct observation of the interfaces between particles, which are critical to the properties of the systems. In the first part of the thesis, structures of colloidal aggregates were studied. It was found that the high binding energy associated with the solid bonding at the particle interfaces prevents the aggregates from relaxing to higher density states. Surfactants were applied to coat the particle surfaces so that the particles were kept separated by a distance determined by the length of the surfactant. The particle-particle interaction was determined by the concentration of the surfactant. It has been shown that the fractal dimension is directly related to the particle-particle interaction. Compared to other studies in which only two kinds of aggregates have been observed, the structures varied from ramified clusters to dense clusters to compact objects, depending upon the binding energy between the particles. The work done in one-component systems was then extended to two-component systems, where the distribution of the two species were studied by Monte Carlo simulation and the Cluster Variation Method. In the second part of the thesis, various aspects of viscoelastic properties of colloidal gels, such as elasticity at high concentrations, nonlinear behavior after breakdown, and loss modulus were studied. These properties are very important to colloidal processing and, as yet, have not been addressed in the literature. The experimental results are explained in terms of the structures of the gels observed by TEM and light scattering.