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Supplementary Materials for - Science ... Method for Scratch Hardness, ASTM G171-03(2009) with a linear reciprocating tribometer (Rtec Instruments Multi-function Tribometer). The measurement

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  • science.sciencemag.org/content/364/6442/760/suppl/DC1

    Supplementary Materials for

    A radiative cooling structural material

    Tian Li*, Yao Zhai*, Shuaiming He*, Wentao Gan, Zhiyuan Wei,

    Mohammad Heidarinejad, Daniel Dalgo, Ruiyu Mi, Xinpeng Zhao, Jianwei Song,

    Jiaqi Dai, Chaoji Chen, Ablimit Aili, Azhar Vellore, Ashlie Martini, Ronggui Yang,

    Jelena Srebric, Xiaobo Yin†, Liangbing Hu†

    *These authors contributed equally to this work.

    †Corresponding author. Email: [email protected] (L.H.); [email protected] (X.Y.)

    Published 24 May 2019, Science 364, 760 (2019)

    DOI: 10.1126/science.aau9101

    This PDF file includes:

    Materials and Methods

    Supplementary Text

    Figs. S1 to S30

    Tables S1 and S2

    References

  • 2

    Materials and Methods

    Fabrication of the cooling wood

    The natural wood block was first cut along the growth direction, which is

    compatible with industry cutting methods for making large dimension wood panels. The

    wood piece was then delignified with boiling H2O2 (30% solution, EMD Millipore

    Corporation) followed by subsequent washing in DI water. The solvent was then replaced

    by ethanol (190 proof, Pharmco-Aaper) before hot pressing.

    Material characterization

    The morphology of the wood samples was characterized by a scanning electron

    microscope (SEM, Hitachi SU-70). The FTIR spectrum was obtained by a

    ThermoNicolet NEXUS 670 FTIR spectrophotometer. The lignin content was measured

    using a standard method (Technical Association of Pulp and Paper Industry Standard

    Method T 222-om-83). The tensile strength of the wood was measured with a Tinius

    Olsen H5KT testing machine. The test was performed with the upper fixture moving

    downward at a constant velocity of 1 mm/min. The scratch hardness characterization

    experiments for wood samples were performed in accordance with the Standard Test

    Method for Scratch Hardness, ASTM G171-03(2009) with a linear reciprocating

    tribometer (Rtec Instruments Multi-function Tribometer). The measurement was carried

    out by applying a normal load and moving the specimens at a constant speed to generate

    a scratch on the surface. The scratch width was measured using a white light

    interferometer and the scratch hardness number (in Gpa) was calculated by kP/w 2 , where

    P is the applied normal force, w is the scratch width and k is the geometrical constant, k =

    24.98 when P is in grams-force and w is in µm. Each scratch hardness was calculated by

    the arithmetic mean value of three scratches at different locations. The Charpy impact test

    of the wood samples was performed on a Tinius Olsen pendulum impact tester. The

    dimensions of the samples were 60 mm × 5.5 mm × 2.7 mm. We measured the bending

    properties of the wood samples using an Instron 3367 tester. The dimensions for the

    bending samples were approximately 60 mm × 5.5 mm × 2.8 mm. Three-point bending

    tests were conducted for these samples, with a 35 mm span between the two bottom

    rollers and the top roller pressing down on the center at a speed of 1 mm min −1

    . We

    conducted compression tests on the samples using an Instron 3367 tester. The dimensions

    for the compressive samples were approximately 9.5 mm long, 9 mm wide, and 4.5 mm

    thick, and the samples were compressed along the thickness direction at a speed of 1 mm

    min −1

    .

    Optical characterization of the cooling wood

    The spectroscopic performance of the cooling wood was measured via an

    integrating-sphere-based characterization method. The polarization-dependent optical

    reflection spectra in the solar spectrum were tested in response to the incident

    polarization angle, whether parallel or perpendicular to the alignment direction of the

    cellulose nanofibers. A visible and near-IR linear polarizer was applied to polarize the

    incident light in the visible and near-IR region, respectively. The polarizer is placed in

    front of the sample compartment of the integrating sphere. θ is the angle between the

    directions of the electric field of the incident light and the aligned direction of the

  • 3

    cellulose nanofibers. The reflectivity spectrum was measured by collecting the spatial

    scattered light that is reflected from the sample surface and bounces within the inside

    wall of the integrating sphere. The emissivity spectrum was obtained by measuring

    reflectivity (R), which was calculated as 1-R where transmittance is negligible.

    Thermal conductivity measurement

    The thermal conductivities of wood samples were measured by laser flash method.

    Laser flash measurement is a widely used transient method to determine the thermal

    diffusivities of bulk materials, which employs noncontact and nondestructive temperature

    sensing (37). During the measurements, the instantaneous light is used as heat source to

    heat up the sample’s front side, and an infrared detector is adopted to record the

    temperature response of the rear side. A very thin graphite coating is applied on both

    faces of the samples to act as absorber on the front side and as emitter on the rear side.

    With the assumption that the heat transfer is one-dimensional, the thermal diffusivity α

    can be calculated by, 2

    2

    1/2

    1.38d

    t 

      (1)

    where is the thickness of the sample and is the time that takes for the sample

    to heat to one half of the maximum temperature on the rear surface. The thermal

    conductivity is then calculated by,

    pk c (2)

    where ρ is the density and cp is the heat capacity. In our measurements, the

    commercial Netzsch laser flash apparatus (LFA 457) was used for the thermal diffusivity

    measurement and Netzsch differential scanning calorimetry (DSC 204 F1 Phoenix) for

    heat capacity measurement, respectively.

    Supplementary Text

    Theoretical model of the radiative cooling performance of cooling wood

    In Fig. S1 (a) we schematically show the direct thermal measurement system, as

    tailored to the cooling wood specimens. When the cooling wood faces a clear sky in an

    open environment, its surface radiates heat to the sky while absorbing solar irradiance

    and downward thermal radiation emitted by atmosphere. At the same time, heat can be

    transferred from the ambient surroundings to the wood via conduction and convection

    because of the temperature difference between the cooling wood and ambient

    environment. This is referred to as non-radiative heat loss. The net cooling power is

    expressed as,

    net rad atm soalr conv leakP P P P P P     (3)

    here,

    Prad: the power density of thermal radiation emitted by the cooling wood;

    Patm: the power density of the downward thermal radiation from the

    atmosphere;

    Psolar: the heating power density resulting from the absorption of solar irradiance;

    Pconv: the convective and conductive power density from the top surface of the

    wood;

    https://www.netzsch-thermal-analysis.com/en/products-solutions/differential-scanning-calorimetry/dsc-404-f1-pegasus/

  • 4

    Pleak: the thermal leakage from the thermally isolated measurement box.

    Prad and Patm are determined by both the spectral data of the cooling wood and the

    emissivity spectrum of the atmosphere. The power density of the absorbed solar

    irradiance can be assessed by,

    solar wood solar 0

    P cos ( )I ( )d   

      λ, λ λ (4)

    where Isolar() is the solar spectral irradiance and film(,) is the wavelength and

    angle-dependent solar absorbance of the cooling wood. Angle φ is normal to the module

    and the solar irradiance. The no n-radiative heat exchange, Pconv, is contributed to by heat

    conduction and convection from the top surface of the wood. We applied a piece of 10-

    m-thick high-density polyethylene (HDPE) film on top of the thermal isolation box to

    reduce the conductive and convective heat exchange between the wood and the

    environment. Pleak is the thermal leakage from the thermal box made of polystyrene foam

    (4-in thick), which is much smaller than Pconv. The radiative cooling power, Prad, is the

    only outgoing heat flux that allows cooling of the wood. When the wood temperature is

    below ambient, all other power densities of Patm, Psolar, Pconv, and Pleak are inward and

    working against the cooling. A Kapton heater is also introduced into the box as an

    additional degree of freedom in evaluating the total radiative cooling power (see detailed

    experimental procedures described in (9).

    The heat exchange coefficient hconv is typically between 5 – 20 W/m 2 K, which is

    sensitive to weather conditions, such as wind speed. Pconv and Pleak can also be

    characterized experimentally. As shown in Fig. S1B, a heat flux is fed into the thermal

    box using the he