Tunable electrical and thermal transport in ice-templated multilayer graphene nanocomposites through freezing rate control.

We demonstrate tunable electrical and thermal conductivities through freezing rate control in solution-based nanocomposites. For a prototypical suspension of 1 vol % multilayer graphene suspended in hexadecane, the solid-liquid electrical conductivity contrast ratio can be tuned from 1 to 4.5 orders of magnitude for freezing rates between 10(2) and 10(-3) °C/min. We hypothesize that this dramatic variation stems from ice-templating, whereby crystal growth drives nanoparticles into concentrated intercrystal regions, increasing the percolation pathways and reducing the internanoparticle electrical resistance. Optical microscopy supports the ice-templating hypothesis, as these dramatic property changes coincide with changing crystal size. Under the same range of freezing rates, the nanocomposite solid-liquid thermal conductivity contrast ratio varies between 2.3 and 3.0, while pure hexadecane's varies between 2.1 and 2.6. The nanocomposite's thermal conductivity contrast ratios and solid phase enhancements are greater than effective medium theory predictions. We suggest this is due to ice-templating, consistent with our electrical measurements, as well as nanoparticle-induced molecular alignment of alkanes.