Dynamic thermal transport in graphene-based compressible foams
Thermal management of many electronic devices is challenged by their strict optimal operating temperature range, various operation modes, and extreme environmental conditions. The design of thermal switches and thermal regulators provides possible solution. A thermal switch with high switching ratio can be an efficient conductor that dissipates heat to avoid overheating and an insulator that protects the device from degradation and damage due to low environment temperature. A thermal regulator offers continuous tuning between maximum and minimum conductance/resistance. Graphene-based compressible foams are studied in this work as a potential material in all-solid-state thermal switch/regulator with dynamic tuning. Molecular dynamics simulations are performed on graphene nanofoams to provide insight on thermal behavior under compressive loading. The switching ratio in the cross-plane thermal conductance reaches 4.6 from 25% to 60% strain. We find that, unexpectedly, the thermal conductivity decreases as the strain increases (i.e., density increases) in low strain region. This is explained using a 1D spring model, whose thermal conductance remains constant with increasing strain if no internal contact is made in the spring and therefore, the cross-plane thermal conductivity decreases. The compressible foam structure is further studied in a continuum model, again confirming the behavior of the spring model. The strain-tuned thermal properties of macroscopic graphene/PDMS composite foam are then measured using infrared microscope. It is observed that such properties are more sensitive to strain at high compression level which confirms the MD simulation results.