Fabrication and analysis of highly conducting graphite flake composites

Graphite has high in-plane thermal conductivity and is a potential candidate to combat the thermal management problems in high density electronic devices. However, the properties of graphite are not fully exploited and this study was carried out with the aim of fabricating and
analysing high thermally conducting graphite based composites.

Model composites were prepared with three different average flake sizes: 180 μm, 300 μm and 600 μm. The constituents( flakes and binders) were characterised in terms of density,
carbon yield and crystallographic perfection using combination of techniques. The electrical resistivity of a single flake was found to be 0.6 μS2 m.

A fabrication route was developed using 75% starting volume fraction (67% - 72%) estimated volume fraction in the composite) of small flake graphite. A high volume fraction
was used so as to obtain composites with properties dominated by the flakes. The selection of the optimum route of fabrication was based on achieving the lowest electrical resistivity of the composite after carbonisation (1000 °C).

An extensive study was carried out on composites prepared using 75% (starting volume fraction) flakes. The mis-alignment of the 002 plane was found to decrease with an increase in the flake size. The optical texture of small flake composites showed that the binder was inhomogeneously
distributed and was observed to shrink away from the flakes whereas in the case of composites with large flakes, the binder was found to wet the flakes. The layers of
the binder were also found to align along the basal planes of the flakes in the vicinity of the flakes. The composites with small flakes were found to be `brittle' whereas the large flake composites showed a more `ductile' behaviour. The Young's modulus and work-of-fracture were estimated from load versus extension curves. Raman studies showed an increase in the a-direction coherence length in the binder with an increase in heat-treatment temperature and the electrical resistivity of the composites was found to decrease with increase in flake size and heat-treatment
temperature. The thermal conductivities were determined
and samples were imaged in a thermal microscope. Comparison between quantitative thermal conductivity and that predicted using Lavin's relationship showed that Lavin's relationship is not applicable to these composites. A thermal conductivity of 655 W/m K (160% of that of
copper) was achieved in graphitised large flake composites.

The volume fraction of graphite was varied (estimated volume fraction 45%-75% in the composite) and graphitised large flake composites were studied. The density of the
composites increased with an increase in the volume fraction whereas the mis-alignment of the 002 plane was found to decrease with an increase in the estimated volume fraction of graphite up to -70%. When the estimated volume fraction of graphite was further increased (-75%), the mis-alignment was found to increase. This was attributed to the microstructure which showed high packing density giving rise to twisting and distortion of flakes in the composite. Existing composite-type models failed to correctly predict the transport properties and hence, a first approach to developing a model specifically for these composites was attempted. However, due to their complex nature and insufficient information to define the modelling parameters its validity is uncertain.

A thermal conductivity of 750 W/m K (190% that of copper) and density -1.8 g/cm3 (nearly one fourth of that of copper) and all this at relatively lower cost was achieved in graphitised large flake composites with -75% estimated volume fraction of graphite. This material is particularly attractive to combat thermal management problems.