Research Papers: Experimental Techniques

Thermal Properties of Silica Aerogel Formula

[+] Author and Article Information
Ellann Cohen

Building Technology Program,
Department of Architecture and
Department of Mechanical Engineering,
Massachusetts Institute of Technology,
Cambridge, MA 02139
e-mail: ellann@alum.mit.edu

Leon Glicksman

Building Technology Program,
Department of Architecture and
Department of Mechanical Engineering,
Massachusetts Institute of Technology,
Cambridge, MA 02139
e-mail: glicks@mit.edu

Contributed by the Heat Transfer Division of ASME for publication in the JOURNAL OF HEAT TRANSFER. Manuscript received June 28, 2013; final manuscript received October 3, 2014; published online April 21, 2015. Assoc. Editor: Patrick E. Phelan.

J. Heat Transfer 137(8), 081601 (Aug 01, 2015) (8 pages) Paper No: HT-13-1325; doi: 10.1115/1.4028901 History: Received June 28, 2013; Revised October 03, 2014; Online April 21, 2015

The thermal conductivity of silica aerogel developed in this research program was measured using the transient hot-wire technique. The thermal conductivity of monolithic samples drops significantly from 9.3 mW/m · K to 3.2 mW/m · K with modest pressure reduction from 1 atm to 0.1 atm. The same aerogel in granular form has a thermal conductivity of 15.0 mW/m · K at ambient gas pressure with a modest compression applied to compact the granules and reduce the air void sizes. Radiation heat transfer in the hot-wire test may not be representative of its contribution in large scale applications. Measurements of the monochromatic extinction coefficient over the wavelengths of interest resulted in a Rosseland mean extinction coefficient of 2400 m−1 at 300 K. The small thermal penetration distance during the hot-wire measurements suggest that in actual use radiation could contribute approximately 2.5 mW/m · K with a possible upper limit of 3.0 mW/m · K to the effective thermal conductivity over that measured using the transient hot-wire method.

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Fig. 1

Survey of literature silica aerogel thermal conductivities across pressure

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Fig. 2

Various radiation intensities during an FTIR test

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Fig. 3

Measured transmissivity from FTIR machine of four sample thicknesses of MIT aerogel (16 E) and corrected transmissivity for the same samples (thicknesses: sample A—0.23 mm, sample B—0.39 mm, sample C—0.60 mm, and sample D—0.75 mm)

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Fig. 4

Extinction coefficient of MIT aerogel (16 E) at each wavelength in the mid to far infrared regions

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Fig. 5

Predicted temperature distributions for transient tests, temperature versus distance from the wire surface (m)

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Fig. 6

Thermal conductivity of aerogel (16 E) measured with two hot-wires: blackened (high emissivity) and silver-like/polished (low emissivity)

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Fig. 7

Thermal conductivities of aerogels across different pressures including Cabot Corp's granules

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Fig. 8

Thermal conductivity of our aerogel (16 E) at reduced pressures

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Fig. 9

Thermal conductivity of our aerogel (16 E) compared to aerogels in the literature

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Fig. 10

Pore size distribution current material [17], left, and previously reported [21], right

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Fig. 11

Granular aerogel (16 E) with ruler

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Fig. 12

Thermal conductivity of granular aerogel (16 E) versus external pressure

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Fig. 13

Thermal conductivity comparison of all formula aerogels across pressures. The standard silver-like/polished wire had a higher emissivity than the blackened wire.



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