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

Ice templated anisotropic foams based on cellulose nanocrystals (CNC) with densities ranging between 25 to 130 kg.m-3 were prepared from aqueous CNC dispersions. The thermal conductivities perpendicular to the columnar macropores direction increased in a non-monotonous way with the increasing CNC foam density while the thermal conductivity reached a minimum value for the CNC foam with the highest nanoporosity. Theoretical calculations including the solid and gas conduction within the foams as well as the thermal conductivity of water showed that phonon scattering at the solid-solid interfaces is responsible for reaching low thermal conductivity values. The foam wall nanoporosity, the particle alignment, the macropores orientation and the foam wall thickness seem to have a minimal effect on the thermal conductivity but can explain the deviations between the theoretical estimates and the experimental data. To this end the identification of the important influencing factors and the great importance of phonon scattering paves the way to design new fossil-free nanofibrillar foams with superinsulating properties.

冰模板法制备的基于纤维素纳米晶体（CNC）的各向异性泡沫，其密度范围为25至130 kg·m⁻³，由CNC水分散液制得。垂直于柱状大孔方向的热导率随CNC泡沫密度增加呈非单调上升趋势，而具有最高纳米孔隙率的CNC泡沫的热导率达到最小值。包含泡沫内部固相与气相传导以及水的热导率的理论计算表明，固-固界面处的声子散射是实现低热导率值的关键原因。泡沫壁的纳米孔隙率、颗粒排列、大孔取向及泡沫壁厚度对热导率的影响似乎极小，但可解释理论估算值与实验数据之间的偏差。为此，明确关键影响因素及声子散射的重要性，为设计具有超绝热性能的新型无化石基纳米纤维泡沫奠定了基础。