Ulrich Heinemann

Optimization of monolithic silica aerogel insulants

Abstract In this work we systematically investigate thermal transport in opacified monolithic silica aerogels by changing their density and the concentration of infrared opacifier. The goal is to minimize the thermal conductivity of these highly porous inorganic materials. The lowest achieved thermal conductivities at 300 K are about 0.013 W m−1 K−1 for non-evacuated specimens, which have to be compared with values of about 0.020–0.025 W m−1 K−1 for CFC-blown insulating polyurethane foams and with 0.035 W m−1 K−1 for the best fiber insulations. Our investigations allow us to quantitatively determine the gaseous, solid and radiative conductivities λg, λs andλr, respectively. The derived variations with aerogel densityρ are: λg ∝ ρ−0.6, λs ∝ ρ1.5 and λr ∝ (ρ· e)−1 in the range 70

Permeation of Different Gases Through Foils used as Envelopes for Vacuum Insulation Panels

Vacuum insulation panels (VIPs) are distinguished by their outstandingly low thermal conductivity. In the evacuated state, the VIPs being examined in this study (which have fumed silica as a core material) have a thermal conductivity of 4 10 3 W/(m K). Gases (N2, O2, H2O,...), which penetrate the foil cover cause an increase in pressure and water content and hence, an increase in the thermal conductivity. To determine these increases, VIPs have been manufactured with laminated aluminum foils (AlF) and aluminum-coated multilayer foils (MFs). The pressure and mass increases are determined at various temperatures, humidity, and with various panel formats. Large differences in the rates of pressure increases (1 -70 mbar/yr) and in the rates of mass increases (0.02-4 mass%/yr) are recorded, depending on the foil type, climatic conditions, and panel formats. From these measurements, the air and vapor transmission rates of the foil covers and their dependence on temperature, relative humidity, and panel size are derived. Using these gas transmission rates, it is possible to estimate which pressure increases are to be expected for panel formats and climatic conditions occurring in building applications. With laminated Al foils and selected Al-coated multilayer foils, rates of pressure increases below 1-2 mbar/yr are achieved. The rates of mass increase for typical climatic conditions for laminated Al foils are significantly below 0.1 mass%/yr, while with Al-coated multilayer foils, depending on the foil quality, mass increases per time of up to 1 mass%/yr are recorded. Increases in gas pressure per time of 1 -2 mbar/yr lead to relatively small increases in thermal conductivity, allowing applications in the construction sector, where service lives of several decades are required. With respect to the humidity-related increase in thermal conductivity, one has to know the climatic conditions, which have a strong influence on the increase in mass, and, above all, the precise dependence of the thermal conductivity on the humidity in the VIP.

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.

Dependence of Thermal Conductivity on Water Content in Vacuum Insulation Panels with Fumed Silica Kernels

The influence of moisture in vacuum insulation panels (VIPs), with fumed silica kernels, on their thermal conductivity has been investigated. The VIPs are produced with different water contents. The thermal conductivities at different water contents are measured under stationary conditions in a hot-plate apparatus with an average temperature of 10°C (plate temperatures are 0 and 20°C). The increase in thermal conductivity is approximately proportional to the water content. The increase is ≈0.5 × 10 -3 W/(m K) per mass% of water. For typical middle European climate, a maximum moisture content of ≈6 mass% can be expected, which corresponds to a maximum increase of thermal conductivity of ≈3 × 10 -3 W/(m K) for VIPs with fumed silica kernels.

Prediction of Service Life for Vacuum Insulation Panels with Fumed Silica Kernel and Foil Cover

For vacuum insulation panels (VIPs) with fumed silica kernels and foils as cover, a calculation model is developed to predict the service life. It is defined as the period during which the thermal conductivity of the VIP has risen 50% due to infusion of air and moisture. Two panel sizes, 50 ×50 × 1 cm3 and 100 × 100 × 2 cm3 are considered. For VIPs with laminated aluminum foils, calculated service lives of many decades are determined. For VIPs with aluminum-coated multilayer foils, shorter service lives still above 20 are calculated. This is due to the higher water vapor transmission through the Al-coated multilayer foils (compared to laminated Al foil) and the humidity-related increase in thermal conductivity. Overall, our model predicts service lives, which are large enough for applications of VIPs in buildings. An open question that remains is the long-term stability of the foil cover.

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.

Thermal transport in organic and opacified silica monolithic aerogels

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 flat 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.

Predictions for the Increase in Pressure and Water Content of Vacuum Insulation Panels (VIPs) Integrated into Building Constructions using Model Calculations

The climatic conditions (temperature, relative humidity, and water vapor pressure) on both sides of vacuum insulation panels (VIPs) that were integrated into different building constructions are measured every hour. The influence of these conditions on the increase in air pressure and water content within the VIPs is estimated using a calculation model. The results of these model calculations are correlated with the pressure and mass measurements on VIPs, exposed to actual climate but removed for laboratory measurements. First, we find that upon use of the temperature-dependent air permeation rates for VIPs, the linear increase within the VIPs can be predicted reliably. Thus, it is sufficient to use annual average temperatures for these estimates. Second, the mass increase of VIPs due to infusion of water vapor through the barrier foil can be determined using the calculation model. The ‘driving’ force in this case is the difference in vapor pressure across the foil cover, which decreases with time, once the water vapor pressure within the VIP starts increasing. In effect, the water vapor pressure and the water content within the VIPs reach equilibrium. Depending on the climatic conditions, the maximum water content between 3 and 7 m% can be predicted.

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.

Thermal Transport in Straw Insulation

An evacuable guarded hot plate for thermal conductivity measurements between 200 and 800 C was used to investigate the heat transfer in barley straw. The different thermal transfer mechanisms (solid, gaseous conduction and infrared-radiative heat transfer) as well as coupling effects were separated. The measured thermal conductivities (λ = 0.041 W m−1 K−1) are similar to those of conventional insulation materials such as foams, glass or mineral fibres which are widely used as building insulation materials. Straw from barley or wheat, which is a low-cost, renewable material readily available world-wide, is therefore an interesting alternative to conventional insulation materials.