Jochen Fricke

Thermal Conductivity of Monolithic Organic Aerogels

The total thermal conductivity λ of resorcinol-formaldehyde aerogel monoliths has been measured as a function of density ρ in the range from ρ = 80 to 300 kilograms per cubic meter. A record-low conductivity value in air at 300 K of λ ≈ 0.012 watt per meter per kelvin was found for ρ ≈ 157 kilograms per cubic meter. Caloric measurements under variation of gas pressure as well as spectral infrared transmission measurements allowed the determination of solid conductivity, gaseous conductivity, and radiative conductivity as a function of density. The development of such low conductivity materials is of great interest with respect to the substitution of environmentally harmful insulating foams made from chlorofluorocarbons.

Correlation between structure and thermal conductivity of organic aerogels

Abstract Organic aerogels are derived from the sol-gel polymerization of resorcinol with formaldehyde. While these materials are usually produced as monoliths, this paper describes a new method for the production of organic aerogel microsphere powders. Supercritical drying provides highly porous aerogels which have an open-cell structure consisting of interconnected solid particles with typical diameters of 10 nm. The structure is controlled by the sol-gel polymerization conditions. This paper addresses the correlation between structure and thermal conductivity of these novel materials. Thermal conductivity measurements have been performed on both monoliths and powders using a hot-wire device. The measurements under variation of gas pressure as well as spectral infrared transmission measurements allow the determination of the solid, gaseous and radiative thermal conductivity as a function of density and catalyst concentration. The results show that the thermal conductivity components are clearly correlated with the aerogel structure: porosity and connectivity between the particles determine the solid conductivity, while the pore size influences the gaseous conductivity and radiative transport depends on the mass specific infrared absorption of the building units.

Thermal transport in polystyrene and polyurethane foam insulations

Abstract The thermal transport in polystyrene (PS) and polyurethane (PU) foam insulations is described, with special emphasis on the radiative transfer. Calorimetric measurements of the total conductivity are performed in a guarded hot-plate device. The radiative properties of the foams are derived from infraredoptical investigations with an FTIR spectrometer. In the case of the PS foam, radiative properties are also extracted from calculations using the Mie scattering theory for infinite cylinders and platelets, representing the struts and walls of the foam cells, respectively. The total thermal conductivity of PU foams, including condensation effects of the blowing agent R 11, is modelled.

New organic aerogels based upon a phenolic-furfural reaction☆

The aqueous polycondensation of (1) resorcinol with formaldehyde and (2) melamine with formaldehyde are two proven synthetic routes for the formation of organic aerogels. Recently, we have discovered a new type of organic aerogel based upon a phenolic-furfural (PF) reaction. This sol-gel polymerization has a major advantage over past approaches since it can be conducted in alcohol (e.g., 1-propanol), thereby eliminating the need for a solvent exchange step prior to supercritical drying from carbon dioxide. The resultant aerogels are dark brown in color and can be converted to a carbonized version upon pyrolysis in an inert atmosphere. BET surface areas of 350--600 m{sup 2}/g have been measured, and transmission electron microscopy reveals an interconnected structure of irregularly-shaped particles or platelets with {approximately}10 nm dimensions. Thermal conductivities as low as 0.015 W/m-K have been recorded for PF aerogels under ambient conditions. This paper describes the chemistry-structure-property relationships of these new materials in detail.

Carbon Aerogels for Electrochemical Double Layer Capacitors

Carbon aerogels are prepared here via pyrolysis of resorcinol-formaldehyde aerogels. Their open porous and electrically conductive structure renders carbon aerogels suitable for the application in supercapacitors. Different types of electrodes can be derived from the sol-gel-precursors of carbon aerogels: Monolithic fibre-reinforced electrodes and polymer-carbon compounds. Both carbon fibre reinforced and polymeric bound aerogel electrodes based on polytetrafluoroethylene (PTFE) have been investigated in this work with respect to their electrical conductivity, surface area and capacitive performance. The capacitance of both electrode types is above 65 F/cm3 in aqueous electrolytes and this meets the demands of supercapacitor electrodes.

Carbon aerogels from dilute catalysis of resorcinol with formaldehyde

Abstract Aqueous polycondensation of resorcinol with formaldehyde leads to organic gels the structure of which can be controlled by the reaction parameters. The amount of catalyst controls the size of the particles constituting the gel network. We could show that for very low catalyst concentrations in a high dilution of reactants, the particle growth can be further enhanced if the reaction temperature is kept low, and the gel time is prolonged. The resultant structure is largely affected by this treatment, producing particles with sizes of about 2 μm and average pore sizes of up to 7 μm. Due to these coarse structures it is possible to dry these RF gels subcritically with very little shrinkage. The structure has been studied by nitrogen sorption, small angle X-ray scattering and acoustic sound propagation. The change of the elastic modulus caused by pyrolysis at around 1000°C has been investigated. Mechanical properties of carbon aerogels are correlated with their electrical properties. The derived carbon aerogels have large specific surface areas, very little mesopore volume however, micron-sized macropores.

Relationship between optical transparency and nanostructural features of silica aerogels

Silica aerogels are considered to be of great promise for use in transparent thermal insulation systems in solar architecture. The optical transparency of these highly porous materials is influenced by the reaction parameters upon preparation and the precursor used. Previously it was shown that the specific extinction due to bulk scattering decreases both with increasing macroscopic density and increasing pH-value of the sol-gel starting solution. Recently, it was also found that within the measurement accuracy the light scattering intensity of the aerogel bulk equals the extrapolated small-angle X-ray scattering intensity towards scattering angle zero if both types of measurement are performed with respect to an absolute scale. In the meantime, ultra small-angle X-ray scattering measurements have been performed in order to close the gap in momentum space between light and conventional small-angle X-ray scattering. As a result it can be stated that the nearly isotropic (Rayleigh) scattering is caused by the same nanostructural inhomogeneities of the aerogel network which lead to the characteristic small-angle scattering pattern. As a consequence, the amount of isotropically scattered light and thus the optical extinction can be directly related to a quantity called the correlation volume. For a variety of silica aerogels, it is shown how the latter depends on the nanostructural features of the gel network, such as average particle size, interparticle arrangement, pore diameter and an ordering parameter, which accounts for concentration effects.

Thermal diffusivity of semitransparent materials determined by the laser-flash method applying a new analytical model

We have developed an analytical model to determine the thermal diffusivity of nonscattering materials from samples with low optical thickness and opaque boundaries with arbitrary emissivities. The paper outlines the new analytical model and describes measurements on two samples: a microscope slide glass and a high-grade fused quartz plate. Results show that the new model applied to measurements on gold- or graphite-coated samples leads to the same results as if a conventional model is used on gold-coated samples.

Carbon Aerogels as Electrode Material in Supercapacitors

Due to their large specific surface area and their high electrical conductivity carbon aerogels are promising materials for electrodes in electrochemical double-layer capacitors (“supercapacitor”). The carbon aerogels were made via pyrolysis of resorcinol formaldehyde aerogels. The latter were prepared by supercritical and subcritical drying as well. The important findings of our investigation were, that the highest capacities of 46 F/cm3 were measured for samples with a density of about 800 kg/m3 pyrolyzed at 800°C. Also it was shown that RF-gels with molar resorcinol/catalyst ratios ≥1000 or higher can be dried subcritically without cracking or significant shrinkage. Carbon aerogels derived from these RF-aerogels have a small mesopore surface area, however an especially large micropore area. They provide electrical capacities which are most suitable for their use in supercapacitors.

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