Optical Ceramics

As the grain size of sintered ceramics the move into the nanosized domain many traditional ceramic materials have shown interesting optical properties. One example is alumina which is known to become transparent if the grain size gets sufficiently small. Control of the grain size is often established by the use of dopants and high pressure sintering techniques. However, as the grain size decreases the relative fraction of grain boundary volume to bulk material volume increases rapidly. This raises the question of how grain boundaries and their composition with respect to the bulk crystal influence the optical properties of the material. This is especially important for materials such as alumina that have an anisotropic crystal structure which leads to birefringence. To what degree this birefringence is exacerbated by grain boundary segregation which influences its structure, density and atomistic composition is of great practical and scientific interest.

Goals
Our goal in this project is to model the atomic structure of grain boundaries in materials such as alumina. We want to use these models to extract the optical properties of grain boundaries and to predict the influence of dopants. Aspects to be studied are the segregation behaviour, theoretical doping limits, effect grain boundary orientations or type and eventually to gain some knowledge on the diffusivity of the constituent atoms or ions in grain boundaries versus the bulk.

Results
In collaboration with Professor Steve Parker at the University of Bath (UK), we have simulated the segregation of Mg and Y to 7 different surfaces and a few simple grain boundaries in alumina.
It was found that Y forms pattern on both alumina surfaces and at the grain boundaries, which may decrease diffusion and thus creep. For magnesium many configurations with similar energies exist and oxygen vacancies are created in the structure. This is likely to enhance diffusion and to result in better sintering behaviour.

Ongoing work
We want to study more complex grain boundaries and the effect of dopants in the grain boundary on both the optical and mechanical properties.

Publications
Aschauer U., Bowen P., Parker S.C., Oxygen vacancy diffusion in alumina: New atomistic simulation methods applied to an old problem, Acta Materialia, 57(16), (2009), 4765-72, (Link to article)
Galmarini S., Aschauer U., Bowen P., Parker S.C., Atomistic Simulation of Y-Doped α-Alumina Interfaces, Journal of the American Ceramic Society, 91(11), (2008), 3643-51, (Link to article)