Anisotropic, lightweight, strong, and super thermally insulating nanowood with naturally aligned nanocellulose

Abstract

There has been a growing interest in thermal management materials due to the prevailing energy challenges and unfulfilled needs for thermal insulation applications. We demonstrate the exceptional thermal management capabilities of a large-scale, hierarchal alignment of cellulose nanofibrils directly fabricated from wood, hereafter referred to as nanowood. Nanowood exhibits anisotropic thermal properties with an extremely low thermal conductivity of 0.03 W/m·K in the transverse direction (perpendicular to the nanofibrils) and approximately two times higher thermal conductivity of 0.06 W/m·K in the axial direction due to the hierarchically aligned nanofibrils within the highly porous backbone. The anisotropy of the thermal conductivity enables efficient thermal dissipation along the axial direction, thereby preventing local overheating on the illuminated side while yielding improved thermal insulation along the backside that cannot be obtained with isotropic thermal insulators. The nanowood also shows a low emissivity of <5% over the solar spectrum with the ability to effectively reflect solar thermal energy. Moreover, the nanowood is lightweight yet strong, owing to the effective bonding between the aligned cellulose nanofibrils with a high compressive strength of 13 MPa in the axial direction and 20 MPa in the transverse direction at 75% strain, which exceeds other thermal insulation materials, such as silica and polymer aerogels, Styrofoam, and wool. The excellent thermal management, abundance, biodegradability, high mechanical strength, low mass density, and manufacturing scalability of the nanowood make this material highly attractive for practical thermal insulation applications.

Fig. 1. Completely derived from natural wood, nanowood with hierarchically aligned cellulose nanofibrils can be used as an anisotropic super thermal insulator.

(A) Schematics of the thermally insulating properties of the nanowood. (B) Digital photograph of the nanowood and the corresponding properties beneficial for building insulation applications.

Fig. 2. Structural characterization of nanowood.

(A) Schematics of the aligned cellulose nanofibrils in the nanowood before and after which the intermixed amorphous lignin and hemicellulose have been removed. (B) Concentration of lignin, hemicellulose, and cellulose in the natural wood and nanowood. (C) Photograph of a nanowood specimen that exhibits pure bight color and an aligned texture. (D) Nanowood exhibits a large porosity, a hierarchical structural alignment of fibril aggregates, and a maintained alignment of the fibril aggregates. (E) Side-view SEM image of the microsized porous and aligned channels inside the nanowood. (F) SEM image of the porous channel walls that composed of aligned nanofibrils. (G) Top-view SEM image of the nanowood channels with separated nanofibrils ends.

Fig. 3. Transverse and axial heat transport in nanowood.

(A) Schematic representation of heat conduction along the wood cell walls as axial heat transfer, whereas (B) heat conduction across the cell walls and hollow channels (that is, the lumen and the nanosized pores inside the fibril walls) is referred to as transverse heat transfer. (C) Measured thermal conductivity of the nanowood from room temperature to 65°C. (D) Measured thermal conductivity of the original wood from room temperature to 80°C. (E) Comparison of the thermal conductivity of the natural wood and nanowood at room temperature.

Fig. 4. Characterization of nanowood.

(A) Thermal conductivity comparison among existing thermally insulating materials. The nanowood exhibits a very low transverse thermal conductivity along with high anisotropy. (B) Mechanical properties of the nanowood in comparison to other materials with a thermal conductivity smaller than 0.05 W/m·K, as well as natural basswood. (C) Photographs of a bulk piece of a nanowood and a thin and rollable nanowood. The arrows indicate the alignment direction. (D) Reflectance of the nanowood. The nanowood exhibits a larger reflectance covering the spectrum of solar radiation (that is, a low solar-weighted emissivity compared with wood). The blue curve is air mass 1.5 solar spectrum. a.u., arbitrary units. (E) Infrared image of the natural wood and nanowood when illuminated by a laser with a wavelength at 820 nm. (F) Temperature profile for the samples in (E).

Fig. 5. Thermal insulation performance of nanowood in comparison with a silica aerogel, a Styrofoam, and a natural wood.

(A) Photograph of a 1-mm-thick specimen of a nanowood. (B) SEM side view of the nanowood channels composed of aligned cellulose nanofibrils. (C) Optical reflection, transmission, and absorption of the silica aerogel and nanowood illuminated by the standard solar simulator. (D) Schematic description of the nanowood being illuminated transversely (perpendicular to the nanofibrils). (E and F) Summary of results showing the stabilized backside temperatures of the thermal insulators when the top surface is in direct contact with a conductive heat source via thermal paste. (G) Schematic description of the measurement setup using radiative heat sources (solar simulator). (H and I) Summary of the results showing the stabilized backside temperatures of each thermal insulator, with the top surface receiving radiative energy from the solar simulator.