Publication
Transparent polycrystalline alumina has many possible promising areas of application from jewelry and the watch industry to wave guides, energy economical lamp envelopes, and optical windows. Ultrahigh density, submicron sized grains and/or oriented microstructures have been identified as the key requirements to synthesize transparent alumina. The highest real inline transmittance (RIT) aluminas reported in the literature are still not good enough to be used for transparent applications. The goal of the present thesis was to use atomistic modeling to understand the basic mechanisms of the physical/chemical phenomena involved in the various issues pertaining the processing of transparent alumina. The three main issues which were addressed in the present work are: segregation of cation-dopants/anion-impurities to the alumina interfaces, solid state oxygen diffusion in alumina, and adsorption of polymers on alumina surfaces. Doping of alumina with transition elements (e.g. Y, Mg, La) has been used in the literature for grain growth reduction and creep enhancement. Codoping with a combination of dopants (e.g. Mg-La) has been reported to be more effective. However the atomistic level effects of codoping on alumina microstructure and hence on properties are not very well understood. The energy minimization method was used to calculate the segregation energies and the relaxed atomistic structures of as many as 9 codoped (Y-La, Mg-La, Mg-Y) surfaces and twin grain boundaries (GBs). Only codoping with a combination of bivalent-trivalent (Mg-La and Mg-Y) dopants was found to be energetically more favorable than single doping. Disparity in the ionic sizes was identified as the key reason for the favorable codoping with Mg. Effects of the dopants type and concentration on the GB atomistic structures have been discussed in the light of the GB complexion transitions and GB packing. Coordination number calculations were made to analyze the GB chemical environment. The existence of anion impurities such as chlorides and sulphates in industrial alumina powder synthesis is well known. But its effects on the processing of alumina ceramics have been grossly neglected. Energy minimization calculations showed that the segregation of Cl is 4-6 times stronger than the cation dopants. Cl-Al coordination number analysis suggests strong adhesion of Cl on the powder surface, making the removal of Cl ions difficult at low temperatures. Atomistic Modeling of Transparent Alumina Oxygen diffusion plays an important role in grain growth and densification during the sintering of alumina ceramics and governs high temperature processes such as creep. The atomistic mechanism for oxygen diffusion in alumina is however still debated. The calculations are usually performed for perfectly pure crystals, whereas virtually every experimental alumina sample contains a significant fraction of impurity/dopants ions. In the present study atomistic defect cluster and nudged elastic band calcula