Atomistic Modeling of Processing Issues for Transparent Polycrystalline Alumina

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 calculations have been used to model the effect of Mg impurities/dopants on defect binding energies and migration barriers. It was found that oxygen vacancies can form energetically favorable clusters with Mg, which reduces the number of mobile species. Moreover diffusive jumps leading away from Mg have migration energies up to twice the value in pure alumina, whereas those approaching Mg are lowered by up to a factor of four, which will slow down the kinetics of diffusion. Other effects of Mg such as vacancy destabilization and the vacancy-vacancy interactions have also been discussed in detail. Majority of the computational segregation studies are done on the highly symmetrical twin grain boundaries. However, the fraction of special twin grain boundaries found in sintered alumina samples is reported to be very small. Therefore, to fulfill the ultimate goal of the simulations, i.e. linking the simulations with the experiments, experimentally characterized general GBs were simulated using near coincidence GB approach and the energy minimization method. Although the segregation of Y was found to be energetically favorable, dopants were found to be occupying only 25% cation sites on the GB. GB complexion phases, which are less favorable to grain growth reduction, were found to be more probable on the general GBs in contrast to the twin GBs. Controlling the agglomeration of ultrafine powders is a big challenge in the processing of nano scaled ceramics. Understanding of the conformation of adsorbed dispersants and the interplay of the adsorption with powder surface characteristics is still limited and requires further work on a rather fundamental level. The present thesis could not address this issue in detail due to the unavailability of an adequate force field. The preliminary results on the development of such a force field as well as the progress made so far are discussed in the last chapter of the thesis. The present thesis helps understand basic fundamental issues pertaining to the processing-microstructure-property relationship in transparent alumina which should help overcome the major roadblocks in the progress of the field of transparent alumina ceramics. The work is generic and the methods can be successfully applied to other ceramic systems.