Thermally Insulating Nanocellulose‐Based Materials

Abstract

Thermally insulating materials based on renewable nanomaterials such as nanocellulose could reduce the energy consumption and the environmental impact of the building sector. Recent reports of superinsulating cellulose nanomaterial (CNM)‐based aerogels and foams with significantly better heat transport properties than the commercially dominating materials, such as expanded polystyrene, polyurethane foams, and glass wool, have resulted in a rapidly increasing research activity. Herein, the fundamental basis of thermal conductivity of porous materials is described, and the anisotropic heat transfer properties of CNMs and films with aligned CNMs and the processing and structure of novel CNM‐based aerogels and foams with low thermal conductivities are presented and discussed. The extraordinarily low thermal conductivity of anisotropic porous architectures and multicomponent approaches are highlighted and related to the contributions of the Knudsen effect and phonon scattering.

Seaweed-Derived Alginate–Cellulose Nanofiber Aerogel for Insulation Applications

“…Another important emphasis for hygroscopic, bio-based aerogels is the influence of moisture on their properties, such as thermal conductivity. 16 Thus, there is a need to investigate the relative humidity dependence of the thermal conductivity of seaweed-derived aerogels in detail. Bio-based flame retardants represent a promising direction for designing next-generation insulation materials, and another key aspect in this development is to utilize the intrinsic properties that seaweed offers, as highlighted in this study.…”

“… 13 By utilizing the anisotropic properties of the fibrils in an aerogel structure, the thermal conductivity has been shown to be reduced, improving the insulation behavior in one direction. 14 − 16 However, flammability is one of the main drawbacks of cellulose-based aerogels, which hinders their use in most insulation applications. 17 , 18 By their in situ synthesis of magnesium hydroxide nanoparticles, Han et al 5 produced a flame-retardant cellulose aerogel that exhibited self-extinguishing behavior within 40 s but at the trade-off of increased thermal conductivity from 56 to 81 mW m –1 K –1 .…”

Recent advances in novel aerogels through the hybrid aggregation of inorganic nanomaterials and polymeric fibers for thermal insulation

“…The gas conduction is governed by the average pore size in aerogels and the mean free path of air molecules. [ 45 ] For aerogels, the mesoporous (2–50 nm) pore favors the Knudsen effect, which means the circulation of air molecules is confined inside pores (i.e., pore size smaller than the mean free path of air molecules), thus resulting in a very low gas conduction. Taken silica aerogel as example, the low thermal conductivity and high insulative capability can be attributed to the presence of nanosized pores full of air, as well as the small, scattered clusters of silica solid.…”

“…According to Fourier's law, the total heat transfer of porous materials (foams or aerogels) can be described as a summation of contributions from the solid conduction ( λ sc ), gas conduction ( λ gc ), heat radiation ( λ r ), and thermal convection ( λ c ) by gas flow. [ 45 ] The schematic diagram of the thermal conduction mechanism of an aerogel has been shown in Figure 13. Regarding solid conduction ( λ sc ), the heat transfer in solids is mainly carried by electrons and phonons.…”

Inherently Distinctive Potentialities and Uses of Nanocellulose Based on its Nanoarchitecture

“…The effective thermal conductivity κeff of an insulating material is expressed as the sum of the thermal conductivity values in multiple modes: the solid-state thermal conductivity κsolid, the gas-state thermal conductivity κgas, the radiation conductivity κrad, and the convection conductivity κconv. Among these four thermal conduction modes, the adiabatic function is significantly manifested by dividing air into small compartments and controlling κgas (Sakai et al 2016;Apostolopoulou-Kalkavoura et al 2020). The value of κgas is determined by the relationship between the mean free path of gas molecules colliding with each other and the pore size of the filled space.…”