A. Young, C. Kowalczyk, A. Elman, M. Merwin, W. Sullivan, F.J. Burpo, E.A. Nagelli, Y. Sahoo
Flexcon,
United States
Keywords: aerogel, heat transfer, convection, surface design
Summary:
The heat transfer in an aerogel, like in any other material, will have conductive, convective and radiative components. However, their quantitative contributions will highly depend on the morphological changes induced in this highly deformable porous body. This study investigates behavior of thermal conductivity as a function of overall density of the polyurea aerogel brought about by progressive compression by hydraulic pressure. Polyurea aerogel is first produced as a low-density material at ~0.04 g/cm3 by carrying out a synthesis from a nucleophilic reaction between Hexamethylene diisocyanate and water followed by freeze drying. The aerogel blocks are compressed by systematically applying hydraulic pressure to asymptotically approach the bulk density of 1.2 g/cm3 while measuring the thermal conductivity at many points. From a different synthesis pathway, polyurea aerogel is prepared at different densities by tuning the synthesis method, namely by varying the concentrations of the reagents. The thermal conductivities of similar densities but by the two different routes, as explained, are compared and a clear difference noted. In the aerogel, the conductive component can be understood with the Debye model of phonon diffusion which predicts that the thermal conductivity should be largely linear in density of the aerogel. It was noted that the thermal conductivity is highly sub-linear with density when it was progressively increased by applying hydraulic pressure to the aerogel blocks. This study shows that the enclosed fluid mass (mostly air plus any solvent vapor) has a significant role in the overall heat transfer and as the density is increased through compression, the entire heat transfer within the aerogel is dominated by the convective mode. If the compression is made to result in the pore sizes so small as to kick in Knudsen effect i.e. the mean free path becomes smaller than pore sizes, then the convective heat transfer contribution will diminish perceptibly, thereby resulting in a different density dependence. A detailed understanding of this underlying mechanism helps design tailored surfaces for controlled and desired heat transfer for any insulation application.