Parametric Model to Analyze the Components of the Thermal Conductivity of a Cellulose-Nanofibril Aerogel

Cellulose-nanofibril aerogels have been found to be highly effective thermal insulators, where some reports state that they have lower thermal conductivity than air. To further enhance the performance of the material, it is important to understand the contribution of the three heat-transfer components: solid-heat conduction by the fibrils, gas-heat conduction by the Knudsen gas in pores, and thermal radiation between the fibrils. The overall effective thermal conductivity is measured by an in-house steady-state setup under atmospheric and vacuum conditions. Contributions from each heat transfer component are quantified by constructing a simple open-cell model and fitting it to the experimental measurements, which vary based on the solid-volume fraction. The thermal conductivity values of a single cellulose-nanofibril filament that constitutes the struts of the open-cell structure are well within the range of previous studies, which confirms the validity of the analysis results. The analysis model can also be used to reveal target dimensions when fabricating aerogels with minimum thermal conductivity. All in all, the simple analysis method can be further applied to improve other porous thermal-insulation materials.