Thermally conductive polymeric based nanocomposites

Polymers are thermal insulators by they own nature; therefore they are not generally used in the field of heat management. In this field usually metals have a dominant role due to their outstanding heat conduction. However, it could be very interesting the replacement of the commonly used metals with a polymeric based material due to several advantages polymers can provide (weight reduction, galvanic coupling avoidance, chemical resistance, easier and cheaper forming). Due to their intrinsic low values of thermal conductivity however polymers needs to be modified. The easiest way to improve thermal conductivity of polymers generally involves their mixing with highly thermally conductive fillers. This strategy however results in some issues and difficulties which are not easily overcome. First of all the thermal conduction of the filler used has to be taken into account since it is responsible for the thermal enhancement of the polymer matrix. Recently, a lot of attention has focused on carbon based fillers such as carbon fibers, carbon nanotubes, graphite and graphene due to their tremendous intrinsic specific thermal conductivity. The use of these fillers could in fact highly reduce the amount required to achieve a thermal conduction improvement. The reason of that is mostly related to their high aspect ratio, low density, micro-nanometric dimension and high dispersion and distribution degree achievable while mixed in polymers. All these peculiarities result in a high number of continuous thermally conductive paths inside the polymer insulating matrix and therefore to its better thermal performance. Despite these advantages, some drawbacks are present. Among them the continuity of the structure acts in general as the main driving force to the heat conduction improvement, since if the continuity is not guarantee the phonon damping matrix rules on the overall thermal conductivity. Another important factor is related to contact resistance between the filler particles. In order to reduce the contact resistance some strategies could be developed to limit the number of contacts required to cross the material, and among them the increase of the lateral dimension of the filler is one of the main ones. Also the preferential orientation of the filler has a positive effect relatively to the contacts resistance since it decreases the amount of contacts required to cross the material, improving the overall heat conduction efficiency. Relatively to the contacts in addition some effect is also related to their quality which can be developed and studied to improve the efficiency. During this work it was attempted a progressive improvement of the thermal transport of polymeric graphene/graphite based nanocomposites solving the above mentioned issues. A dispersion and distribution approach was done and the most common techniques investigated and progressively improved for different graphene and graphite fillers. Once the best dispersion technique was identified, a progressive refinement of the filler was done: fillers with high lateral dimensions and small packing densities were chosen and materials with higher filler loading prepared. Additional improvement was obtained after a purification of the fillers at high temperature to anneal defects, remove impurities and promote graphitization. Percolation issues were solved creating 3D structures accordingly to two strategies: wool cotton preform infiltration and in particular graphene aerogel creation. Relatively to the first case wool cotton was infiltrated with graphene oxide and a thermal annealing performed. In the second case both isotropic and anisotropic graphene aerogels were created to obtain a 3D self-standing continuous structure. After the polymeric infiltration it was discovered that the aerogel technique allows high thermal conductivity improvement at small filler loadings, minimizing the amount of filler to be inserted, dispersed, and distributed to achieve the desired result.