Thermal transport in organic and opacified silica monolithic aerogels

Journal of Non-Crystalline Solids 145 (1992) 207 210 North-Holland

NON-CRYiTIiL LiNi SOLIDS

Thermal transport in organic and opacified silica monolithic aerogels X. Lu, P. Wang, M.C. Arduini-Schuster, J. Kuhn, D. B i i t t n e r , O. N i l s s o n , U . H e i n e m a n n a n d J. F r i c k e Physikalisches Institut der Universitdt Wiirzburg, Am Hubland, W-8700 Wiirzburg, Germany

The thermal properties of monolithic aerogels were investigated by changing their density, the gas pressure and the temperature. Results show that opacified SiO2-aerogels and organic aerogels at room temperature in air have a total thermal conductivity as low as 0.013 W/m K and 0.012 W/m K, respectively, and show a fiat minimum on variation of density. Aerogels have a very low solid thermal conductivity due to their extremely high porosity. Nanosize pores are responsible for partial suppression of gaseous conduction. A low radiative conductivity caused mainly by absorption is observed for organic aerogels. In the case of SiO2-aerogels, the radiative transport can be reduced by integrating an opacifier such as carbon black in the SiO2-skeleton.

1. Introduction T h e r m a l transport in SiO2-aerogel monoliths was first studied by Kistler [1,2], who pointed out several i m p o r t a n t facts. A thermal conductivity of about 0.020 W m - 1 K - 1 at 300 K, suppression of gaseous conduction u p o n evacuation to about 50 mbar, and a thermal conductivity of about 0.010 W m i K - 1 or below for evacuated aerogel tiles at 300 K were reported. T h e specific extinction (absorption) coefficient of SiO2-aerogels is extremely small in the wavelength range from 3 to 5 Ixm, especially after removal of physisorbed water by heating. Thus, radiative heat transfer dramatically increases with increasing t e m p e r a t u r e . Complex coupling between conductive c o m p o n e n t s in the gel and the radiation field and a strong d e p e n d e n c e of the radiative transfer on the surface emissivity of the boundaries were r e p o r t e d at previous aerogel conferences [3,4]. Today, opacified SiO2-aerogel monoliths [5] are available, in which the specific extinction in the wavelength region m e n t i o n e d above is drastically increased. Such gels are m a d e by integration, e.g., of c a r b o n black in the gel. Also, organic

aerogels [6] have m u c h larger extinction coefficients than pure SiO2-aerogels. This has the imp o r t a n t c o n s e q u e n c e that such gels are optically thick in layers of 1 - 2 cm thickness. Thus, the radiative contribution to the thermal transport can be significantly r e d u c e d and the description of heat transfer b e c o m e s relatively simple. T h e total thermal conductivity, At, in this case can be considered a linear superposition of the solid conductivity, As, of the t e n u o u s skeleton, the gaseous conductivity, Ag, of the gas in the porous structure and the radiative conductivity, Ar [5,7]: At=As+Ag+A

r.

(1)

T h e solid conductivity due to the high porosity of aerogels can be a factor of 500 lower than for n o n - p o r o u s vitreous silica [8]. In a r o u g h approximation, AS can be assumed to be proportional to S , with a ~ 1.5 [5], in the density range between 70 and 300 kg m -3. T h e gaseous conductivity in aerogels is m u c h smaller than in free non-convecting air. This is due to the small pores, which are c o m p a r a b l e in size to the m e a n free path of the air molecules even at atmospheric pressure. As with increasing

0022-3093/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved

X. Lu et al. / Thermal transport in silica monolithic aerogels

208

density, where the pore sizes decrease, Ag ought to decrease, too. The radiative conductivity varies according to Ar

16 nZo-T; 3 ~ o c

T3

E'

(2)

where T is the absolute t e m p e r a t u r e and E the extinction coefficient. From the latter quantity, a mass specific extinction can be derived: (3)

e =e/p.

2. Experimental procedure Calorimetric m e a s u r e m e n t s were performed using a transient hot-wire technique [9]. A P t hot-wire is squeezed between a pair of identical aerogel blocks with the size of 2 x 3 x 10 cm 3. The thermal conductivity is determined by feeding a constant electric power into the wire and by observing its t e m p e r a t u r e increase with time. The relative error of the thermal conductivity is estimated to be less than 4% in air and within 6% for evacuated specimens. Silica, r e s o r c i n o l - f o r m a l d e h y d e (RF), and m e l a m i n e - f o r m a l d e h y d e (MF) aerogels of different densities were investigated under variation of gas pressure and temperature. The sample was

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Fig. 1. Spectral specific extinction of pure ( . . . . . . ) and opacified ( . . . . . . ) SiO2-aerogels as well as of organic ( R F ) aerogels ( ) [7].

enclosed in a heating cell, which was mounted in a vacuum chamber. The gas pressure is measured in the vacuum chamber, where the gas pressure can be varied between 10 -2 mbar and 1 bar. The t e m p e r a t u r e was varied for the silica aerogels between 30 and 200°C. For an RF-aerogel, measurements were performed in the t e m p e r a t u r e range from - 3 0 to 25°C. For this purpose, the vacuum chamber was enclosed in a refrigerator. In order to determine the specific spectral extinction of the specimens, the infrared optical transmission measurements were performed using an F T I R spectrometer. The measured wavelength range was from 2.3 to 45 ~xm. The errors are estimated to be about 15% of the measured values.

3. Results The spectral specific absorption of RF-aerogels as well as of pure and opacified SiO2-aerogels [10] are shown in fig. 1. The difference between the three types of materials are drastic in the near infrared. The t e m p e r a t u r e - d e p e n d e n t specific extinction, needed for eqs. (2) and (3), is derived by Rosseland averaging [11]. A specific extinction, e, of about 50 m 2 kg-1 resulted at 300 K for RF- and MF-aerogels; for pure SiO2-aerogels, e was about 20 m 2 kg -1 and for opacified SiO2-aerogels an e of 80 m 2 kg 1 was determined. With e = 5 0 m 2 kg -1 and p = 2 5 0 kg m -3, a radiative conductivity of 0.001 W m -1 K - 1 was calculated according to eq. (2). The variation of total thermal conductivity with gas pressure is depicted in fig. 2. The lowest total conductivity (A t = 0.012 W m -~ K -~) at ambient conditions was obtained for the RF-aerogel with a density, p, of 157 kg m -3. The lowest conductivity after evacuation (about 0.004 W m -1 K -1) was derived for a SiO2-aerogel with a density, p, of 75 kg m -3. The differences in conductivity between the non-evacuated and evacuated aerogels are in the range from 0.005 to 0.008 W m K-1, which is much smaller than the conductivity 0 . 0 2 6 W m - 1 K 1 of free air at 3 0 0 K . The full suppression of gaseous thermal conduction occurs at a pressure of about 10 mbar.

209

X. Lu et al. / Thermal transport in silica monolithic aerogels

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Fig. 2. Variation of thermal conductivity of various aerogels with air pressure.

Figure 3 shows the temperature dependence of the total thermal conductivity of various insulating systems. As can be seen, the monolithic aerogels on organic and opacified silica basis provide a thermal resistance about twice as high as that of commonly used CFC-blown insulating polyurethane foams. The opacified SiO2-aerogel powder [13], which is less expensive to produce and easier to handle than the monoliths, shows an intermediate value.

The variation of conductivity with density is shown in fig. 4 for organic and opacified SiO 2aerogels. Flat conductivity minima (A t = 0.012 and 0.013 W m 1 K 1) are obtained at a density of about 160 and 120 kg m -3, respectively. This is due to the fact that the gaseous conductivity, Ag, and radiative conductivity, AT, decrease with increasing density, p, whereas the solid conductivity, As, increases. The result confirms that the total thermal conductivity can be minimized by changing the density of the aerogels. Both RF- and MF-aerogels provide a large absorption in the infrared spectral range, although MF-aerogels are transparent within the visible spectrum [6]. Similar values of the total thermal conductivity at the same density were derived for both organic aerogels. This implies that MF-aerogels may be used as transparent thermal insulations.

5. Conclusions Measurements show that by optimizing the density, opacified silica aerogels and organic aerogels at room temperature in air achieve a

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for a CFC-blown polyurethane (PU) insulating foam ( • ) [12], an opacified SiO2 aerogel powder (n) [13], an opacified monolithic SiO2 aerogel (*) and a monolithic RF aerogel (×); the curves are a guide to the eye.

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Fig. 4. Variation of total thermal conductivitywith density for organic and opacified SiO2 aerogels; the curves are a guide to the eye.

210

x. Lu et al. / Thermal transport in silica monolithic aerogels

t o t a l t h e r m a l c o n d u c t i v i t y as low as 0 . 0 1 2 - 0 . 0 1 3 W / m K). T h u s , t h e s e n a n o p o r o u s m a t e r i a l s p r o v i d e t h e r m a l r e s i s t a n c e s a b o u t t w i c e as h i g h as the environmentally harmful CFC-blown insulating p o l y u r e t h a n e f o a m s . T h i s is an i m p o r t a n t a r g u m e n t to p r o m o t e t h e f u r t h e r d e v e l o p m e n t o f i n o r g a n i c a n d o r g a n i c a e r o g e l s f o r u s e as t h e r m a l insulations. T h e a u t h o r s a r e g r a t e f u l to R . W . P e k a l a f o r generously providing organic aerogels. This work was s u p p o r t e d by t h e G e r m a n Bundesminist e r i u m fiir F o r s c h u n g u n d T e c h n o l o g i e ( c o n t r a c t n u m b e r 0328654B).

[5]

[6] [7] [8] [9] [10]

[11]

References [12] [1] S.S. Kistler, J. Chem. 39 (1935) 79. [2] S.S. Kistler, J. Phys. Chem. 46 (1942) 19. [3] J. Fricke, ed., Aerogels, Springer Proceedings in Physics, Vol. 6 (Springer, Heidelberg, 1986). [4] R. Vacher, J. Phalippou, J. Pelous and T. Woignier, eds.,

[13]

Proc. 2nd Int. Symp. on Aerogels, Rev. Phys. Appl. 24, C4 (Les Editions de Physique, Les Ulis, 1989). J. Fricke, X. Lu, P. Wang, D. Biittner and U. Heinemann, Optimization of monolithic silica aerogels insulants, Report E21-0191- 8(1991), Phys. Inst. Univ. Wiirzburg, to be published in J. Heat Mass transfer (1992). R.W. Pekala, J. Mater. Sci. 24 (1989) 3221. X. Lu, M.C. Arduini-Schuster, J. Kuhn, O. Nilsson, J. Fricke and R.W. Pekala, Science 255 (1992) 971. P. Scheuerpflug, H.-J. Morper, G. Neubert and J. Fricke, J. Phys. D: Appl. Phys. 24 (1991) 1395. O. Nilsson, G. Riischenp6hler, J. Gross and J. Fricke, High Temp.-High Press. 21 (1989) 267. X. Lu, P. Wang, D. Biittner, U. Heinemann, O. Nilsson, J. Kuhn and J. Fricke, Thermal transport in opacified monolithic silica aerogels, in: Proc. 12th ECTP, Vienna, Sept. 1990, to be published in High Temp.-High Press. (1992). R. Caps and J. Fricke, in: Aerogels, Springer Proceedings in Physics, Vol. 6, ed. J. Fricke (Springer, Heidelberg, 1986) p. 110. J. Kuhn, H.-P. Ebert, M.C. Arduini-Schuster, D. Biittner and J. Fricke, Int. J. Heat Mass Transfer, in press. E. Hfimmer, Th. Rettelbach, X. Lu and J. Fricke, in: Proc. 11th Syrup. on Thermophysical Properties, Boulder, CO, USA, June, 1991, to be published in Thermochim. Acta (1992).