Highly Compressible, Anisotropic Aerogel with Aligned Cellulose Nanofibers

Abstract

Aerogels can be used in a broad range of applications such as bioscaffolds, energy storage devices, sensors, pollutant treatment, and thermal insulating materials due to their excellent properties including large surface area, low density, low thermal conductivity, and high porosity. Here we report a facile and effective top-down approach to fabricate an anisotropic wood aerogel directly from natural wood by a simple chemical treatment. The wood aerogel has a layered structure with anisotropic structural properties due to the destruction of cell walls by the removal of lignin and hemicellulose. The layered structure results in the anisotropic wood aerogel having good mechanical compressibility and fragility resistance, demonstrated by a high reversible compression of 60% and stress retention of ∼90% after 10 000 compression cycles. Moreover, the anisotropic structure of the wood aerogel with curved layers stacking layer-by-layer and aligned cellulose nanofibers inside each individual layer enables the wood aerogel to have an anisotropic thermal conductivity with an anisotropy factor of ∼4.3. An extremely low thermal conductivity of 0.028 W/m·K perpendicular to the cellulose alignment direction and a thermal conductivity of 0.12 W/m·K along the cellulose alignment direction can be achieved. The thermal conductivity is not only much lower than that of the natural wood material (by ∼3.6 times) but also lower than most of the commercial thermal insulation materials. The top-down approach is low-cost, scalable, simple, yet effective, representing a promising direction for the fabrication of high-quality aerogel materials.

Fire‐Resistant Structural Material Enabled by an Anisotropic Thermally Conductive Hexagonal Boron Nitride Coating

“…The resulting temperature distributions of the top and cross‐section of the densified wood surface were monitored and recorded by an infrared (IR) camera. Figure b shows the temperature contour of the top surface, which features a maximum value of 74 °C in the densified wood after exposure to the laser for 5 min, suggesting obvious heat concentration in densified wood surface caused by the poor thermal conductivity of the wood material . Due to the rapid heat accumulation on the surface, the densified wood with a relative low ignition temperature will easily ignite.…”

Wood Derived Composites for High Sensitivity and Wide Linear‐Range Pressure Sensing

“…Fourier transform infrared spectroscopy was conducted to confirm the removal of lignin, as shown in Figure S1 (Supporting Information). [10a] To identify the optimal carbonization condition, we varied the temperature from 600 to 1000 °C, and tested the resistivity of the carbonized wood sample in both the along‐the‐channel and across‐the‐channel directions. As shown in Figure a, the resistivity in both directions decreases drastically when the carbonization temperature is raised from 600 to 800 °C and then remains almost unchanged with further increased temperature.…”

“…First, the wood trunks were cut into cubes with the same volume, and immersed in the mixed solution of NaOH (2.5 mol L −1 in deionized (DI) water, Sigma‐Aldrich) and Na 2 SO 3 (0.4 mol L −1 in DI water, Unichem Laboratories LTD.) to remove most of the lignin. [10a,12] The solution was heated up on the hot plate, and kept boiling for 12 h. After that, the wood cubes were transferred into the boiling water to remove the chemicals in the former step. Then the wood cubes were kept in the boiling solution of H 2 O 2 (2.5 mol L −1 in DI water, International Laboratory USA) for 3 h to further remove the lignin.…”

Breathable Nanowood Biofilms as Guiding Layer for Green On‐Skin Electronics

“…Trees transport the water, salts, and other nutrients for photosynthesis through their anisotropic tubular structures, where most of the CNs are “naturally well‐aligned” along the wood growth direction in the cell wall matrix of hemicellulose and lignin . Recently, a large‐scale, hierarchal alignment of CNs directly fabricated from wood is referred to as “nanowood,” top‐down methods have been designed to take advantage of the naturally well‐aligned CNs to make advanced nanowood biofilms capable of highly in‐plane anisotropic water transport, optical transparency, and mechanical properties (but permit low cross‐plane breathability due to extremely densified structure) and bulk nanowoods for anisotropic heat‐insulating and microfluidic applications. But the method for tailoring the structure of nanowood biofilm to achieve strong heat‐ and sweat‐guiding performance and simultaneously guarantee adequate cross‐plane breathability has yet to be explored.…”