The movement of atoms in complex ceramics is strongly dependent on the local structure. When ceramics are heated or irradiated, the atomic structure is damaged. In order to understand how defects in the atomic structure influence the motion of atoms, it is necessary to understand how the properties of the ceramic materials change and what can be done to restore the original atomic structure. These phenomena are the basis of material properties and lifetimes for radiation resistance during energy generation and in containers for storing nuclear wastes.
Diffusion of atoms in complex ceramic oxides is crucial to how atoms are transported and how the atomic structure of the ceramics evolve under the influence of radiation damages, sintering and aging. Individual atoms in ceramics carry electrical charges that determine atomic structure. Ions that carry a negative electrical charge are called “cations”. Ions that carry a positive electrical charge are called “anions”.
Pyrochlores are complex ceramic oxides which contain more than one type of cation. The diffusion of the cations through the material and the electrical conductivity of the material are strongly influenced by the structure of the crystal in terms of arrangement of cations. Diffusion and conductivity are very sensitive to cation disorder. The ability of these ceramics to maintain their crystallinity is dependent on cation disorder. This particular characteristic is the reason that pyrochlores are being investigated as a possible material to encapsulate nuclear waste. Radiation resistance and conductivity are increased by cation disorder, but it is not well understood exactly how this disorder influences cation transport.
Scientists at the U.S. Department of Energy have been researching the influence of defects on cation diffusion in gadolinium titanium oxide (Gd2Ti207) which is a pyrochlore. The defects are missing atoms in the ceramic crystalline structure. The researchers have been using standard and accelerated molecular dynamics simulations to track the movement of atoms in the crystal and increase their understanding of cation diffusion. These simulations are for cation behavior of microsecond (millionth of a second) duration. Typical atomic simulation track the behavior over nanosecond (billionth of a second) intervals due to the massive computer resources that are required. In the case of the ceramic simulations, new computational techniques have been used to simplify the dynamics of the individual atoms and reduce computational requirements.
The researchers found that cation diffusion through the ceramic crystals is slow when there is a low level of disorder. When the level of disorder reaches a specific value, the diffusion of the cations accelerates. One of the important aspects of this behavior is “anti-side defects.” This occurs when one cation of gallium occupies a position a titanium cation would normally occupy. When the threshold level of disorder is reached, the anti-site defects are so numerous that they are almost touching each other at the atomic level. This creates something called a “percolation network.” This network permits the cations to move rapidly through the crystal. The movement of these cations through the network permits the crystal structure to repair itself by destroying the anti-site defects which slows down the cation diffusion.
This self-healing is different than other behavior models for other complex ceramic oxides and disorder models. This new type of self-healing may be very important in extending the lifetime of complex ceramics that are used in extreme radiation environments.
gadolinium titanium oxide:
