Opacified aerogel

To reveal the thermal properties of aerogels, stationary hot-plate measurements are usually employed [45]. In such a measurement two equal aerogel specimens are sandwiched between a hot plate and two cold plates. If the electrical power fed into the hot plate and the temperature difference between the hot and the cold plates, as well as the thickness of the specimens, are known, the thermal conductivity can be derived. For the thermal characterization of opacified aerogels, the faster nonstationary hot-wire method can also be used. In this case a thin platinum wire is embedded into the aerogel specimen and a constant power is delivered into the wire, which also serves as a temperature sensor. From the temperature increase in the wire as a function of time, the thermal conductivity of the aerogel specimen can be determined [49]. 

Beds of aerogel granulate or powder are used for technical thermal insulation (Part 15), because they offer specific advantages in comparison to standard insulation materials, e.g., they can be poured into complicated shaped cavities. Opacified aerogel powders and granulates are used as thermal insulation in space applications [55], automotive applications... 

Figure 4 Spectral specific extinction (absorption) of pure silica aerogel (dashed), silica aerogel doped with 5% carbon black (dotted), and RF aerogel (solid). Note that the transmission window in pure silica aerogel between 3 and 5 j4m leads to a dramatic increase in thermal radiative transport and renders pure Si02 aerogels ineffective as thermal insulators above 100°C. The integration of an opacifier markedly improves the thermal resistance of Si02 aerogels.

Figure 9 Thermal conductivity X as a function of temperature for a CFC-blown PU foam (triangles), an opacified SiOa aerogel powder (squares), an opacified monolithic Si02 aerogel (circles), and a monolithic RF aerogel (exes). The solid lines are guides to the eye.

Aerogels are particularly well suited for insulation applications because of their exceptionally low density, thermal stability, and high transparency. In fact, they can have a thermal conductivity only one-third that of polyurethane or polystyrene foam, and with recent process improvements that reduce the cost of manufacmre by an order of magnitude their practical use in certain construction applications is now feasible [31]. The insulating properties can be enhanced through the addition of IR opacifiers [32]. The high transparency of aerogels makes them suitable as insulation in windows or translucent panels. 

Lee D, Stevens PC, Zeng SQ, Hunt AJ (1995) Thermal characterization of carbon-opacified silica aerogels. 

For optically thick aerogels, e.g., for most organic, opacified or carbon aerogels, radiative heat transfer is described by the diffusion of photons. The photons interact within short distances in comparison to the macroscopic dimension of the aerogel with its solid backbone. A corresponding solution to the diffusion equation for photons can be derived in analogy to the diffusion of phonons by way of ... 

Figure 23.7. Effective specific extinction of a nonopacified and opacified silica aerogel, resorcinol-formaldehyde (RE) aerogel and carbon aerogel as a function of the wavelength. Additionally, the normalized Rosseland weighting function is depicted, which shows the maximum thermal radiation for 300 K at a wavelength of 8 pm [24].

Figure 23.8. Rosseland-averaged total effective specific extinction e (T) using the spectral values (Figure 23.7) as a function of temperature of an opacified silica aerogel, resorcinol—formaldehyde (RF) aerogel and cathtm aerogeL...

Figure 23.9. Thermal crmductivity values of evacuated silica aerogels (p < 0.01 Pa) as a function of temperature. Black symbols total effective thermal conductivity of a silica aerogel (p = 153 kg m ) opacified with 2.8% carbon black and its correspondingly derived soUd thermal conductivity (blue line), radiative ctmductivity (red line), and the superposition of both (black line) [30]. Also depicted are the predicted effective total thermal conductivity values given by Zeng et aL for an optimal loading level of carbon black [28] and experimental data provided by Lee et al. [29].

Figure 23.10. Total effective thermal conductivity of an evacuated opacified silica aerogel (p = 153 kg m , opacifier 2.8% carbon black) as function of T (gas pressure Pg < 0.01 Pa). A linear increase in this representation can be observed with increasing temperatures due to the diffusive radiative heat transfer according to (23.12). Solid line linear regression curve, dashed lines corresponding 95% coincidence interval.

Figure 23.24. Effective total thermal conductivity of evacuated opacified silica aerogel powders (gas pressure Pg < 0.01 Pa) with different weight percents of opacifier (0/5/10% carbon black). The specimens were loaded with an external pressure of 0.1 MPa [60, 61].

Lu, X., Wang, P., Buttner, D., Heinemaim, U., Nilsson, O., Kuhn, J. and Fricke, J., Thermal transport in opacified monolithic silica aerogels. High Temperatures - High Pressures, 1991. 23(4) p. 431-436. 

Rettelbach, T., Saeuberlich, J., Korder, S. and Fricke, J., Thermal conductivity of IR-opacified silica aerogel powders between 10 K and 275 K. Journal of Hiysics D Applied Physics, 1995. 28(3) p. 581-587. Rettelbach, T., Der Warmetransport in evakuierten Pulvem bei Temperaturen zwischen 10 K und 275 K, Dissertation, University of Wiirzburg / Germany, 1996... 

Figure 32.11. Silica aerogel (density of 20 mg/cc) opacified with graphite was cut and attached to the walls. The aerogel is seen as the bluish-gray materials under the gold Kapton.

Figure 32.15. Thermal conductivity of siUca aerogel opacified with Ti02 powder in varying concentrations. The aerogel density was 50 mg/cc.

Thermal transport in organic and opacified silica monolithic aerogels. Proceedings of 3rd International Symposium on Aerogels, J. Non-Cryst. Solids 145 ... 

---