Year-end Sale % OFF 
Abstract
Summary Cellulose nanomaterial (CNM)-based foams and aerogels with thermal conductivities substantially below the value for air attract significant interest as super-insulating materials in energy-efficient green buildings. However, the moisture dependence of the thermal conductivity of hygroscopic CNM-based materials is poorly understood, and the importance of phonon scattering in nanofibrillar foams remains unexplored. Here, we show that the thermal conductivity perpendicular to the aligned nanofibrils in super-insulating ice-templated nanocellulose foams is lower for thinner fibrils and depends strongly on relative humidity (RH), with the lowest thermal conductivity (14 mW m−1 K−1) attained at 35% RH. Molecular simulations show that the thermal boundary conductance is reduced by the moisture-uptake-controlled increase of the fibril-fibril separation distance and increased by the replacement of air with water in the foam walls. Controlling the heat transport of hygroscopic super-insulating nanofibrillar foams by moisture uptake and release is of potential interest in packaging and building applications.
Highlights
Insulation materials frequently used in buildings and in packaging, such as gas-filled polyurethane foams or expanded polystyrene (EPS), are derived from fossil sources and use hazardous precursors.[1]
X-ray diffraction (XRD) and atomic force microcopy (AFM) image analysis (Figure S1), together with conductometric titration and sedimentation measurements[38] of non-oxidized cellulose nanofibrils (CNFs) with an average diameter of 19 nm (CNF19), medium-charge 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO)oxidized CNFs with an average diameter of 4.4 nm (CNF4.4), and high-charge TEMPO-oxidized CNFs with an average diameter of 2.3 nm (CNF2.3) showed that more intensive oxidation led to smaller fibril diameter, higher aspect ratio (L/d; Equation S1 and S2), and lower crystallinity index (Equation S3) (Table 1)
Directional growth of the ice crystals during freeze-casting resulted in strongly anisotropic foams with the CNF particles aligned in the growth direction of the ice crystals (Figure 1B)
Summary
Insulation materials frequently used in buildings and in packaging, such as gas-filled polyurethane foams or expanded polystyrene (EPS), are derived from fossil sources and use hazardous precursors.[1] Biobased insulation materials with thermal conductivities below those of polyurethane or EPS (20–40 mW mÀ1 KÀ1) could both reduce the carbon footprint of thermal insulation materials and the energy needed for heating or cooling. The relative humidity dependence of the thermal conductivity of superinsulating nanocellulose foams is controlled by moisture-induced phonon scattering and the replacement of air with water. Humidity-dependent phonon engineering could be used to tailor the heat transfer properties of biobased nanofibrillar materials for packaging and thermal management in buildings
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
