Heat transfer in microcellular polystyrene/multi-walled carbon nanotube nanocomposite foams

We report the heat-transfer characteristics of polystyrene (PS)/multi-walled carbon nanotube (MWCNT) nanocomposite foams with a large expansion ratio (18-fold) and a microcellular cell size (5 μm) that have never been achieved before. These PS/MWCNT foams exhibited excellent thermal insulation performance even without insulation gas. The 5 μm-sized cells in these PS/MWCNT foams are small enough to induce Knudsen effect, and also lead to a distinct radiation behavior that has never been reported; that is, because of the unique synergy of the small cells and the MWCNTs, the short wavelength radiation below the cell size is 100% blocked while the long wavelength radiation over the cell size is strongly attenuated. We made an in-depth analysis of the heat transfer through the PS/MWCNT foams using models. Adding 2 wt% MWCNTs into the PS matrix, 86% of the radiative thermal conductivity was effectively blocked, and the radiative contribution was reduced to 3.5% of the total thermal conductivity. However, the MWCNTs increased the solid conductivity of the PS/MWCNT foams due to their inherently high thermal conductivity. So, a compromised content of 1 wt% MWCNTs was added to optimize solid conduction and radiation, and thereby to minimize the total thermal conductivity to 32.8 mW/m K.