EXECUTIVE SUMMARY
The formation mechanisms of uniform, submicron, inorganic particles were explored in this three year study. This project was initiated after a detailed literature review demonstrated that insufficient data was available to provide the basis on which to test various growth models. A research program was developed to explore growth mechanisms of a variety of systems such that common themes resulting in narrow size distributions could be extracted. This program sought to determine if uniformity is the result of details of bond breaking and making during the chemical reaction or if there is an underlying physical chemistry which must be satisfied for uniform precipitates to be formed. Following a general summary and summaries from each material studied, a detailed discussion is presented.
1. In conjunction with US Department of Energy supported research, three experimental systems were investigated; i) silica from hydrolysis and condensation of tetraethylortho- silicate, ii) titania from hydrolysis and condensation of tetraethylortho-titanite and iii) gold from the reduction of auric acid with sodium citrate.
2. In each system, particle growth rates, rates of loss of solution phase metal containing species and colloidal properties of the growing particles were characterized. Particular attention was paid to developing methods of distinguishing between growth by molecular . addition and growth by aggregation of smaller particles. Kinetic models were developed which linked particle growth with rates of reaction in solution and the effects of variables such as precursor concentration; ionic strength and pH.
3. This research program demonstrated that the LaMer model for the formation of uniform particles is rarely, if ever, followed. Indeed, developing reactor schemes based on the LaMer model can result in focusing on the wrong control variables.
4. The central result of this research program is that uniformity is a consequence of particle interaction potentials which act to generate a constant number density of colloidally stable particles early in the precipitation reaction. This result suggests that in the development of reactor conditions resulting in uniform precipitates, the correct control variable set must include factors which influence colloidal stability. ,
5. This research program demonstrates the importance of understanding size dependency of heterocoagulation and the interaction potentials of sub-10 nm particles. In particular, the ability of small adsorbed molecules to provide a steak barrier which prevents aggregation into, the primary van der Waals minimum has been demonstrated: a result of tremendous technological significance.