Growth of Epitaxial Graphite Crystal – Formation of Austenite Halo at the Periphery of Graphite Flakes and Continuous Branching of Graphite
Epitaxial graphite (EG) is an extreme form of highly ordered, three-dimensional crystalline material. It is found mainly in tight-packed granular graphite (GPG), but can also be found in laminar graphite and other graphite products. Since GPG is the main form of graphite in industrial production, EG is one of the most important materials used in the production of batteries, fuel cells and hydrogen storage materials.
EG is often grown as an epitaxial film on a suitable substrate. One typical example is the EG growth on a single crystal sapphire substrate, which is often formed by depositing a thin film of platinum on sapphire and then annealing at temperatures in the range of 700 ~ 1200°C. During the annealing process, a diffusion interface forms between the sapphire and the platinum which melts and forms graphite layers on the sapphire surface. The resultant EG on the sapphire surface has an orderly, periodic array of graphite layers, and the layers are highly ordered.
One interesting feature of EG is the formation of an austenite halo at the periphery of graphite flakes which could explain the continuous branching of graphite into multiple sub-grains. As heated graphite melts, the liquid graphite migrates over the crystal edges and coats the grain boundaries. Once the graphite solidifies and cools, the layer acts as a protective barrier which is difficult to remove. The presence of the protective layer at the grain or sub-grain boundaries keeps the branch-like structures intact and is known as the austenite halo.
The austenite halo at the peripheral of graphite flakes thus provides a physical strength to EG which increases the brittleness of EG. The formation of new grains in EG is possible through the halo, but their size is limited so that the austenite halo provides a protection against fragmentation. The austenite halo also protects the remaining graphite layers from further damage, thus maintaining its unique periodic arrangement.
In addition, the branch-like structure of EG is also influenced by the nature and extent of the impressed fields in graphite. It has been observed that subjected to a higher impressed field, the branches would become finer and their layering patterns would become more evident.
Finally, the continuous branching of graphite into many sub-grains can be explained by the diffusion of dissolved graphite ions from regions at the center of the film towards areas at the periphery. Upon contact with the austenite halo, the dissolved graphite ions deposit on the halo and form a new branch-like structure. These newly formed branches can then fuse together to form larger and larger sub-grains as the diffusion continues.
In conclusion, the continuous branching of graphite into multiple sub-grains is largely due to the formation of an austenite halo at the periphery of graphite flakes which acts as a protective barrier against further damage. Furthermore, the size and shape of the branches can be finely controlled by adjusting the nature and extent of the applied impressed fields. Lastly, the diffusion of dissolved graphite ions from the center of the film towards the periphery plays an important role in the formation of larger and larger sub-grains.
