Atomistic simulation in powder technology : from growth control and dispersion stabilization to segregation at doped interfaces

Despite many years of intensive research there still remain many unresolved questions in powder and ceramic technology. A majority of these issues are linked to interfacial phenomena of atomic scale origin, which makes their experimental investigation very difficult due to limitations in the spatial resolution of the available analysis techniques. Computer simulations at the atomic scale provide with the advent of more and more advanced methods and increasing computer power an ever more powerful tool for the investigation of these phenomena. The understanding of experimental systems gained at a fundamental theoretical level will help target key experiments in a knowledge based fashion for the rapid advance in top-performance materials development, saving time and resources with simulations showing the most promising routes and key parameters to be explored in experiments. The present thesis investigates several key steps in the production cycle of a ceramic material using atomic scale computer simulation techniques, leading to advances in the understanding of the underlying atomic scale phenomena in each case. Instead of treating a single material throughout the ceramic production process, a series of different systems of experimental interest are looked at in order to show the generic nature of the approach. Very often ceramic powders are produced by precipitation from solution, where many powders properties can be modified, amongst them the particle size, morphology and state of agglomeration, all of which have an important effect on the quality of the final ceramic. Growth can be modified by the reactive environment (temperature, pH) or extrinsic species such as ions or molecular additives. Growth modification of hematite (α-Fe2O3) by phosphonic acid molecules was simulated by energy minimization and experimentally validated, making the method useful as a predictive tool for other phosphonic acid molecules. Significant changes in morphology were predicted and experimentally observed, the main reason for preferential adsorption being the surface geometry and topology, major distortions of the molecule leading to unfavorably high adsorption energies on some faces. Since experiments showed calcite (CaCO3) particles grown in presence of polyaspartic acid (p-ASP) to have a higher specific surface area than with polyacrylic acid (PAA) despite the same functional groups on both additives, molecular dynamics simulations were used to investigate this difference. The presence of charged surface defects at steps was found to be the key requirement for adsorption, the molecules' approach being hindered by the highly coordinated water at the surface and the additive even desorbing without the attractive electrostatic force. PAA was found to form more complexes with counterions in solution and a more negative enthalpy of adsorption was found for p-ASP, both of which will lead to more marked growth suppression by binding of p-ASP to steps, which are expected