Designed and fabricated anisotropic thermal conductivity polymer composite with high RF transmittance
Conventional graphite polymer composites blend the graphite platelets and polymer matrix together. This approach, however, would have several disadvantages in our context. For one, blending the graphite platelets into the polymer matrix would negatively affect our ability to align the polymer chains and raise the intrinsic thermal conductivity of the polymer matrix. In the case of stretched fibers, the presence of platelets would cause the fibers to break at lower stress thresholds, thus limiting the final draw ratio and thermal conductivity. Secondly, dispersing graphite platelets inside the polymer matrix maximizes the effect of the thermal interface resistance between the platelets and matrix. The thermal interface resistance in some situations can dominate the final composite thermal conductivity by essentially decreasing the effective thermal conductivity of the filler when embedded in the matrix.
Our main innovation is in the development of a manufacturing process that will circumvent these issues. First we will increase the thermal conductivity of the polymer itself > 10 W m-1 K-1, by mechanically stretching high density polyethylene (HDPE) fibers to achieve better polymer chain alignment. The fibers will be melt spun and stretched through a heated bath to maintain a well-controlled processing environment and reach higher draw ratios. After drawing, the fiber quality will be checked by tensile tests to determine the modulus, since for stretched polymers, it has been well established that an increase in modulus is accompanied by a corresponding increase in thermal conductivity. Fibers of sufficiently high quality will then be wound onto a mandrel and graphite platelets will be sprayed during the winding process as a high thermal conductivity filler material. It is in this way that we decouple the problems associated with polymer stretching when graphite has already been infused. The composite monolith will then be pressure compacted and heated to slightly sinter the fibers to the graphite regions, resulting in a dense bulk material that is machinable into desired shapes. Bulk material samples will be characterized by measuring their thermal conductivity with a heater and infrared camera setup. An iterative process of refinement will be used to engineer the appropriate processing conditions for maximum thermal conductivity. Molecular dynamics simulations will also be used to guide the process optimization, by providing fundamental insight into the matrix/filler thermal interface resistance and thermal conductivity predictions for alternative polymer matrix materials, such as polyacetylene (PA) or polyvinyl chloride (PVC).
- Multi-scale Model:
(Finite Element Method (FEM) & Effective Medium Theory (EMT) & Molecular Dynamics (MD))
2. Automated scale-up manufacture set-up: