High porosity solids, such as plastic foams and silica aerogels, are thermally insulating. Their insulation performance strongly depends on their pore structure, which dictates the heat transfer process in the material. Therefore, it is important to determine the relationships between pore structures and their heat transfer properties. For a direct comparison of the heat transfer modes in various pores, it is essential to prepare high porosity solids with different pore sizes and shapes to be similar in solid fraction and chemical composition. However, a solid composition that can meet these requirements has not been reported until recently, and the heat transfer properties of foams and aerogels have been separately investigated. In the present study, we have directly compared the heat transfer properties of foams and aerogels with very low solid volume fractions of 0.3-2.7%. These two porous structures were solely composed of crystalline cellulose nanofibers (CNFs). CNF is a skeletal component of trees, or representative porous structures in nature. The foams featured rather closed, microscale pores (1-100 μm) formed with a thin film-like solid phase of CNF aggregates (thickness 0.1 μm), whereas the aerogels featured nanoscale open pores (1-100 nm) formed with a network-like solid skeleton of individualized CNFs (diameter 3-15 nm). Unlike the aerogel samples, the thermal diffusivity α (m2 s-1) of the foam decreased considerably with a slight increase in the solid fraction. The decrease in α of the foam is explained by the interfacial thermal resistance of air against the film-like solid phase within the foam, whereas that for the aerogel is based on the blocking of the free path of air molecules (~70 nm) by the network-like solid skeleton within the aerogel. In fact, the present results indicate that for suppressing the thermal diffusion of air (a gaseous fluid), creating microscale spaces with distinct partitions is more effective than directly blocking collisions between air molecules at the nanoscale. We also found that α of these high porosity solids are described in common as an exponential function of the solid fraction, where the exponent reflects their pore structure.

Thermal diffusivity in ultrahigh porosity solids of nanocellulose

251st National Meeting (2016)

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