R. Caps

Apparent thermal conductivity of evacuated SiO2-aerogel tiles under variation of radiative boundary conditions

Abstract The apparent thermal conductivity λ of evacuated SiO 2 -aerogel tiles was measured with our small guarded hot plate vacuum system LOLA II. In order to study the influence of the boundary emissivity on λ the plates (20 × 20 cm 2 ) were either used with their plasma-sprayed surfaces ( e ≈ 0.5) or with low-emissivity aluminum ( e ≈ 0.05) foils as covers. The difference in the apparent conductivity already showed at room temperature and rose to about 50% for radiative temperatures T r = 570 K . An important consequence is that superinsulating SiO 2 -aerogel systems should always be provided with low-emissivity boundaries around the aerogel. The calorimetric results for λ are compared with radiative conductivity values derived from spectral i.r. transmission measurements.

Radiation-conduction interaction: an investigation on silica aerogels

Thermal transport in low density silica aerogels was studied theoretically and experimentally over a wide range of optical thickness and ratio of radiative to conductive heat transfer. Measurements on the combined heat transfer were performed for aerogel densities between 5 and 220 kg m−3, for temperatures from 100 to 650 K, for internal gas pressures between 10−4 and 1000 hPa and two boundary emissivities of 0.04 and 0.77. A high precision numerical method for the calculation of the temperature profile and the total (combined) heat flux in these semi-transparent, non-scattering, non-grey media is presented.

Thermal conductivity of polyimide foams

Measurements of the thermal conductivity of polyimide foams have been performed in the temperature range 173–323K for different gas pressures and different gas types (CO2 and Ar). The extinction of thermal radiation has been determined by hemispherical transmission and reflection measurements in the infrared. With these data, a quantitative model has been established which predicts the thermal conductivity of polyimide foams as a function of density, gas pressure and temperature. In addition the influence of low emissivity foils integrated into low density polyimide foams on the thermal conductivity has been calculated using a three-flux model of combined radiation and solid/gas conduction.

Silica aerogel — a light-transmitting thermal superinsulator

Evacuated silica aerogel layers 15– 20 mm thick have thermal less coefficients of only about 0.5 W m−2 K−1 at ambient temperatures. These layers are either transparent (in the case of monolithic aerogels) or translucent (in the case of granular fillings) and provide a solar transmission of about 50 to 60 %. Aerogel layers could be used to reduce heat losses in all kinds of window systems. Furthermore, translucent thermal insulations are well suited for passive use of solar energy. The excellent thermal insulation of aerogel is due to the high porosity (up to 98 %) of the silica skeleton and to the effective attenuation (absorption) of arrbient thermal infrared radiation. Measurements with monolithic, granular, and segmented aerogel spacers are presented. The following parameters were investigated: surface emissivity, density, internal gas pressure, and external load on the spacers.

Spectral transmission and reflection properties of high temperature insulation materials

Abstract This paper describes two approaches for the determination of radiative flux in high, temperature insulations: (a) optical transmission and reflection studies; (b) application of rigorous Mie scattering theory and Monte Carlo simulations of both calorimetric and optical measurements. It is shown that the wavelength resolved extinction coefficient and the first-order term of the scattering phase function series expansion are indispensible parameters for an optimization of the insulation properties of anisotropically scattering insulation. The paper demonstrates the validity of the diffusion approximation for radiative transfer also for strong anisotropic scattering. A study of the extinction properties of different opacifiers is added.

Improved thermal radiation extinction in metal coated polypropylen microfibers

Abstract If polypropylen microflbers with 2–3 μm diameters are coated with aluminum the infrared extinction increases by a factor of 20 as compared to the non-coated fibers. This has been shown by Mie scattering calculations and infrared optical transmission and reflection measurements using an integrating sphere. Large infrared extinction is important for the reduction of thermal radiative heat transfer in low density insulations. Insulation systems with Al-coated polypropylen microfiber fleeces can be expected to provide the same excellent insulation performance as natural downs.

Radiative Heat Transfer in Silica Aerogel

At ambient temperatures evacuated aerogel exhibits a very low apparent thermal conductivity (λ ≅ 8…10 mW/(m K)). With rising temperatures, however, the heat transfer is increasing very rapidly. This is mainly due to the weak absorption region in the near infrared (ir) which transmits more and more of the thermal radiation when the spectrum is shifted towards shorter wavelengths. The dominance of the radiative transport over conduction and convection at high temperatures can also be observed in glass melts [1]. In addition to the radiative heat transport in evacuated aerogel the solid conduction via the fragile skeleton has to be considered. It is possible to determine the amount of solid conductivity λsc if the radiative contribution and the total heat flux are precisely known. This, however, implies a detailed experimental and theoretical investigation of the radiative heat transfer as well as the precise calorimetric measurement of the total thermal transport in aerogel.