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Research Papers: Radiative Heat Transfer

Determination of Scattering and Absorption Coefficients of Porous Silica Aerogel Composites

[+] Author and Article Information
Tairan Fu

Key Laboratory for Thermal Science and
Power Engineering of Ministry of Education,
Beijing Key Laboratory of CO2 Utilization
and Reduction Technology,
Department of Thermal Engineering,
Tsinghua University,
Beijing 100084, China
e-mail: trfu@mail.tsinghua.edu.cn

Jiaqi Tang

Key Laboratory for Thermal Science and
Power Engineering of Ministry of Education,
Beijing Key Laboratory of CO2 Utilization
and Reduction Technology,
Department of Thermal Engineering,
Tsinghua University, Beijing 100084, China

Kai Chen, Fan Zhang

Beijing Aerospace Technology Institute,
Beijing 100074, China

1Corresponding author.

Contributed by the Heat Transfer Division of ASME for publication in the JOURNAL OF HEAT TRANSFER. Manuscript received March 26, 2015; final manuscript received September 18, 2015; published online November 3, 2015. Assoc. Editor: Zhuomin Zhang.

J. Heat Transfer 138(3), 032702 (Nov 03, 2015) (7 pages) Paper No: HT-15-1228; doi: 10.1115/1.4031734 History: Received March 26, 2015; Revised September 18, 2015

Silica aerogels are porous ultralight materials with exceptional physical properties that are promising materials for thermal insulation applications. This paper theoretically and experimentally investigates the spectral scattering and absorption coefficients of a porous silica aerogel. Silica aerogel samples were prepared with the same compositions and various thicknesses using the sol-gel technique and supercritical drying. The spectral normal-hemispherical transmittances and reflectances of the silica aerogel samples with various thicknesses were measured for wavelengths of 0.38–15 μm. The reflectance and transmittance are higher at short wavelengths than in the infrared region due to the strong scattering and weak absorption at short wavelengths. The thicker samples strongly attenuate the spectral normal-hemispherical transmittance, but have little effect on the spectral normal-hemispherical reflectance. A modified two-flux radiative transfer model was used to analyze the radiation propagation in the silica aerogel with a rough surface morphology and millimeter thicknesses to develop theoretical expressions for the spectral directional-hemispherical reflectance and transmittance. Then, the optical constants, including the absorption coefficient and the scattering coefficient, were determined for wavelengths of 0.38–15 μm based on experimental data by the least-squares algorithm. The results show that when considering the radiation propagation inside the sample, the surface reflection at the air–aerogel interface can be neglected for aerogel thicker than 1.1 mm when the absorbing and scattering effects inside the sample are quite important. The analysis provides valuable data for the optical properties for silica aerogel applications.

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References

Figures

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Fig. 1

SEM image of the silica aerogel sample surface

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Fig. 2

SEM image of the silica aerogel sample cross section

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Fig. 3

Measurement method for the normal-hemispherical reflectance and transmittance

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Fig. 4

Normal-hemispherical transmittances for samples with different thicknesses for wavelengths of 0.38–15 μm: sample A, 1.10 mm; sample B, 2.04 mm; and sample C, 3.17 mm

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Fig. 5

Normal-hemispherical reflectances for samples with different thicknesses for wavelengths of 0.38–15 μm: sample A, 1.10 mm; sample B, 2.04 mm; and sample C, 3.17 mm

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Fig. 6

Normal emittances for samples with different thicknesses for wavelengths of 0.38–15 μm: sample A, 1.10 mm; sample B, 2.04 mm; and sample C, 3.17 mm

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Fig. 7

Normal transmittances of a fused quartz sample for wavelengths of 0.38–15 μm (1.10 mm thickness and 2200 kg/m3 density)

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Fig. 8

Radiation propagation in the silica aerogel at room temperature

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Fig. 9

Spectral absorption coefficient and scattering coefficient for the silica aerogel sample

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