Research ArticleMATERIALS SCIENCE

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

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Science Advances  09 Mar 2018:
Vol. 4, no. 3, eaar3724
DOI: 10.1126/sciadv.aar3724
  • 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.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/4/3/eaar3724/DC1

    fig. S1. Nanowood is composed of hierarchical aligned nanofibrillar cellulose arrays derived from natural wood.

    fig. S2. The lignin content and appearance between chemical processes.

    fig. S3. Nanowood drying process.

    fig. S4. SEM images of natural wood.

    fig. S5. SEM images of nanowood.

    fig. S6. Molecular level alignment in the hierarchal alignment of nanowood.

    fig. S7. Nanowood samples can be fabricated within a wide size distribution.

    fig. S8. Compressive stress test of nanowood along the axial and radial direction.

    fig. S9. Tensile strength of the nanowood and original wood.

    fig. S10. Comparison between commercially available silica aerogel and nanowood.

    fig. S11. Temperature plots of isotropic and anisotropic thermal insulators from a point heat source.

    fig. S12. The two levels of porosity (microsized and nanosized pores) in nanowood.

    fig. S13. Thermogravimetric analysis.

    fig. S14. Digital images of the delignified wood piece after >1 year under ambient environment.

    fig. S15. Air permeability test of nanowood.

    fig. S16. Industry compatible wood board cutting method.

    fig. S17. Nanowood is composed of aligned cellulose nanofibers with mesoporous structure.

    fig. S18. Reflectance comparison between vertically cut plane and horizontally cut plane of nanowood.

    fig. S19. Thermal conductivity in transverse and axial direction under humidity of 20% and 80%, respectively.

    fig. S20. The tensile strength of nanowood under 20 and 80% humidity.

    table S1. Materials cost for nanowood production.

    table S2. Comparison between nanowood, paper, and honeycomb paper wraps.

    discussion S1. Mechanical property analysis of nanowood

    discussion S2. Numerical simulations of isotropic and anisotropic thermal insulators

    discussion S3. Thermal conductivity estimation

    discussion S4. Thermal stability of nanowood

    discussion S5. Permeability of nanowood

    discussion S6. Scalable manufacturing

    discussion S7. Comparison with a stack of paper and honeycomb wrapping paper

    discussion S8. The effect of humidity

    References (6470)

  • Supplementary Materials

    This PDF file includes:

    • fig. S1. Nanowood is composed of hierarchical aligned nanofibrillar cellulose arrays derived from natural wood.
    • fig. S2. The lignin content and appearance between chemical processes.
    • fig. S3. Nanowood drying process.
    • fig. S4. SEM images of natural wood.
    • fig. S5. SEM images of nanowood.
    • fig. S6. Molecular level alignment in the hierarchal alignment of nanowood.
    • fig. S7. Nanowood samples can be fabricated within a wide size distribution.
    • fig. S8. Compressive stress test of nanowood along the axial and radial direction.
    • fig. S9. Tensile strength of the nanowood and original wood.
    • fig. S10. Comparison between commercially available silica aerogel and nanowood.
    • fig. S11. Temperature plots of isotropic and anisotropic thermal insulators from a point heat source.
    • fig. S12. The two levels of porosity (microsized and nanosized pores) in nanowood.
    • fig. S13. Thermogravimetric analysis.
    • fig. S14. Digital images of the delignified wood piece after >1 year under ambient environment.
    • fig. S15. Air permeability test of nanowood.
    • fig. S16. Industry compatible wood board cutting method.
    • fig. S17. Nanowood is composed of aligned cellulose nanofibers with mesoporous structure.
    • fig. S18. Reflectance comparison between vertically cut plane and horizontally cut plane of nanowood.
    • fig. S19. Thermal conductivity in transverse and axial direction under humidity of 20% and 80%, respectively.
    • fig. S20. The tensile strength of nanowood under 20 and 80% humidity.
    • table S1. Materials cost for nanowood production.
    • table S2. Comparison between nanowood, paper, and honeycomb paper wraps.
    • discussion S1. Mechanical property analysis of nanowood
    • discussion S2. Numerical simulations of isotropic and anisotropic thermal insulators
    • discussion S3. Thermal conductivity estimation
    • discussion S4. Thermal stability of nanowood
    • discussion S5. Permeability of nanowood
    • discussion S6. Scalable manufacturing
    • discussion S7. Comparison with a stack of paper and honeycomb wrapping paper
    • discussion S8. The effect of humidity
    • References (64–70)

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