(402f) Delamination of Graphite Sheets in Stirred Media Mills

Graphite is a well-known allotropic form of carbon. Carbon atoms in graphite planes are covalently bonded to three neighbours in plane with atoms placed in corners of hexagon. One plane of those carbon atoms is described as graphene. Whereas the bonding forces in a graphene layer are strong, the forces in the direction perpendicular to the layers are very weak, assumed that they are the van der Waals forces. Therefore graphite shows a highly anisotropic behavoir, especially in its mechanical, thermal and electrical properties.

In this paper we will present results of different grinding experiments in the stirred media mill. For this purpose the initial graphite particles are dispersed in water and afterwards stressed under different milling conditions (rotational speed of the stirrer, milling bead size). Beside the breakage of the particles between the milling beads, a simultaneous stabilization of the new created surfaces against agglomeration is necessary to advance the grinding progress. Especially in the submicron size range large interparticle interaction and high collision rates of the particles can lead to an agglomeration or in the case of a multi-layered material to a reformation of the single sheets. Therefore different stabilization mechanisms have been investigated. A steric stabilization with polymers as well as an electrostatic stabilization with ionic surfactants were used in the milling experiments. By means of X-ray diffraction analysis, SEM and AFM measurements we found out that a delamination the graphite layers occurs in the shear field of the mill. We observe nearly no breakage inside a layer, but a good grinding progress perpendicular to the sheets. An optimization of the stabilization mechanism and the milling process, i.e. the use of smaller milling beads and lower rotational speeds, enables the production of extremely thin graphite layers with a thickness down to 5 nm but concurrent broad layers with a mean diameter of about 1 µm. Under the mild milling conditions (small milling beads, low rotational speed of the stirrer) the crystalline microstructure in a layer will barely be affected. Therefore the specific properties for example the electrical conductivity along a graphene layer is still available after stressing. In addition we found out that those thin layers seem to be very flexible and nearly transparent. This opens totally new applications in many areas of industry.

In general we showed for graphite particles as an example for a multi-layered material that it is possible to reach a selective and orientation depending comminution which can be controlled by the stabilization mechanism as well as the milling process parameters.