2, Department of Forest Biomaterials, College of Natural Resources, North Carolina State University, Raleigh, North Carolina, United States
3, Department of Mechanical Engineering, The University of Tokyo, Tokyo, , Japan
Efficient thermal insulation using materials with a small environmental footprint is essential for sustainable thermal management in buildings. Nanocellulose produced from wood cellulose fibers is a nanosized, renewable, light weight cellulose particle of high aspect ratio which exhibits tunable surface properties and low thermal conductivity1. We have previously shown that anisotropic nanocellulose foams prepared by directional freezing exhibit lower thermal conductivity than air (=25 mW/mK) perpendicular to the fibers direction at room temperature2.
Here, we will present the heat transfer of anisotropic nanocellulose foams as a function of relative humidity (RH) and temperature (T). Hence, we measure the axial and radial thermal conductivities of anisotropic freeze-casted nanocellulose foams by using a customized hot disk thermal constant analyzer at controlled RH and T. Thermal conductivity measurements have been combined with X-ray diffraction and modelling. The strong dependence of the thermal conductivity on the RH will be related to the interaction of the hygroscopic nanocellulose fibrils with water and humidity-dependence of the directional interfacial thermal resistance. We will also address the effect of the crystallinity and alignment of nanocellulose on the thermal conductivity of anisotropic nanocellulose foams.
1. Lavoine, N. & Bergström, L. Nanocellulose-based foams and aerogels: processing, properties, and applications. J. Mater. Chem. A 5, 16105–16117 (2017).
2. Wicklein, B. et al. Thermally insulating and fire-retardant lightweight anisotropic foams based on nanocellulose and graphene oxide. Nat. Nanotechnol. 10, 277–283 (2014).