Effect of density, phonon scattering and nanoporosity on the thermal conductivity of anisotropic cellulose nanocrystal foams

Varvara Apostolopoulou-Kalkavoura,Pierre Munier,Lennart Bergström,Veit-Lorenz Heuthe,Lukasz Dlugozima

doi:10.1038/s41598-021-98048-y

Varvara Apostolopoulou-Kalkavoura, Pierre Munier + Show 3 more

Open Access

https://doi.org/10.1038/s41598-021-98048-y

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Journal: Scientific Reports	Publication Date: Sep 21, 2021
Citations: 8	License type: open-access

Affiliation: Stockholm University

Abstract

Anisotropic cellulose nanocrystal (CNC) foams with densities between 25 and 130 kg m−3 (CNC25 –CNC130) were prepared by directional ice-templating of aqueous dispersions. Estimates of the solid and gas conduction contributions to the thermal conductivity of the foams using a parallel resistor model showed that the relatively small increase of the radial thermal conductivity with increasing foam density can be attributed to interfacial phonon scattering. The foam wall nanoporosity and, to a lesser extent, the orientation of the CNC particles and alignment of the columnar macropores, also influence the insulation performance of the foams. The insight on the importance of phonon scattering for the thermal insulation properties of nanocellulose foams provides useful guidelines for tailoring nanofibrillar foams for super-insulating applications.

Highlights

Anisotropic cellulose nanocrystal (CNC) foams with densities between 25 and 130 kg m −3 (CNC25 – CNC130) were prepared by directional ice-templating of aqueous dispersions
Recent work on ice-templated anisotropic cellulose nanofibril (CNF)-based foams has shown that the thermal conductivity perpendicular to aligned CNFs depends on both the diameter of the particles and the moisture-controlled swelling of the foam walls[16]
The relative importance of foam density, CNC particle orientation, foam wall nanoporosity and macropore alignment are discussed, and phonon scattering at the interfaces between aligned CNCs is identified as the main reason for the low thermal conductivity of the CNC foams perpendicular to the freezing direction

Summary

Introduction

Anisotropic cellulose nanocrystal (CNC) foams with densities between 25 and 130 kg m −3 (CNC25 – CNC130) were prepared by directional ice-templating of aqueous dispersions. Recent work on ice-templated anisotropic cellulose nanofibril (CNF)-based foams has shown that the thermal conductivity perpendicular to aligned CNFs depends on both the diameter of the particles and the moisture-controlled swelling of the foam walls[16]. Studies on CNM-based isotropic foams and aerogels revealed a complex and non-linear relationship between density and thermal conductivity[12,14,17,18], which suggests that other parameters related to the structure and morphology of the foams and aerogels, and the nanostructure of the nanofibrillar pore/cell walls, probably have a strong impact on the thermal conductivity. The relative importance of foam density, CNC particle orientation, foam wall nanoporosity and macropore alignment are discussed, and phonon scattering at the interfaces between aligned CNCs is identified as the main reason for the low thermal conductivity of the CNC foams perpendicular to the freezing direction

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