Aerogels: Structure, properties and applications

Aerogels prepared via a sol-gel route and supercritical drying posses a unique microstructure with specifics like a 3D open porous network of nanosized particles, huge specific surface area up to a few thousand square meters per gram, extremely low thermal conductivity, extremely low density, such that applications in a variety of industrial sectors seemed obvious 30 years ago. The structure and properties are essentially independent of the chemical nature of the aerogels, which can be organic, inorganic, a combination of both or composites made from them. Although the large potential initiated intensive research it took almost twenty years before the still costly production process allows making aerogels finding their way into more and more industrial sectors. Aerogels are now utilized as super insolating materials in shoe soles or apparel, daylight illumination systems, pipeline isolation mats, medium temperature isolating materials, tennis rackets, drug delivery systems, foundry core and mould materials, building construction materials and many more are being developed in the last decade. The paper describes briefly typical aerogel structures, properties and then concentrates on a comprehensive presentation of industrial applications today and potential for the future

Materials Science Laboratory - Columnar-to-Equiaxed Transition in Solidification Processing and Microstructure Formation in Casting of Technical Alloys under Diffusive and Magnetically Controlled Convective Conditions

The Materials Science Laboratory - Columnar-to-Equiaxed Transition in Solidification Processing and Microstructure Formation in Casting of Technical Alloys under Diffusive and Magnetically Controlled Convective Conditions (MSL-CETSOL and MICAST) are two investigations which supports research into metallurgical solidification, semiconductor crystal growth (Bridgman and zone melting), and measurement of thermo-physical properties of materials. This is a cooperative investigation with the European Space Agency (ESA) and National Aeronautics and Space Administration (NASA) for accommodation and operation aboard the International Space Station (ISS). Research Summary: Materials Science Laboratory - Columnar-to-Equiaxed Transition in Solidification Processing (CETSOL) and Microstructure Formation in Casting of Technical Alloys under Diffusive and Magnetically Controlled Convective Conditions (MICAST) are two complementary investigations which will examine different growth patterns and evolution of microstructures during crystallization of metallic alloys in microgravity. The aim of these experiments is to deepen the quantitative understanding of the physical principles that govern solidification processes in cast alloys by directional solidification

Flow effects on the dendritic microstructure of AlSi-base alloys

Fluid flow changes heat and mass transport during solidification, thereby affecting the evolution of the microstructure. In order to quantify effects of convection, it is important that fluid flow can be modified experimentally. We performed directional solidification experiments with binary AlSi alloys of different compositions, using a microgravity environment for diffusive solidification and adding rotating magnetic fields to generate flow. Flow velocities up to 10 mm/s and various solidification velocities were realized while maintaining a constant temperature gradient at the solid-liquid interface. The microstructure observed in samples processed on earth and in space is characterized by primary and secondary dendrite arm spacing and the fractal dimension of the dendrites. It is found that fluid flow usually accelerates growth and coarsening of the dendritic structures and leads to new kinetic laws. The branching of dendritic networks, however, is hardly affected by flow

Brownian motion of droplets induced by thermal noise

Brownian motion (BM) is pivotal in natural science for the stochastic motion of microscopic droplets. In this study, we investigate BM driven by thermal composition noise at sub-micro scales, where inter-molecular diffusion and surface tension both are significant. To address BM of microscopic droplets, we develop two stochastic multi-phase-field models coupled with the full Navier-Stokes equation, namely Allen-Cahn-Navier-Stokes (ACNS) and Cahn-Hilliard-Navier-Stokes (CHNS). Both models are validated against capillary wave theory; the Einstein's relation for the Brownian coefficient at thermodynamic equilibrium is recovered. Moreover, by adjusting the co-action of the diffusion, Marangoni effect, and viscous friction, two non-equilibrium phenomena are observed. (I) The droplet motion transits from the Brownian to Ballistic with increasing Marangoni effect which is emanated from the energy dissipation mechanism distinct from the conventional fluctuation-dissipation theorem. (II) The deterministic droplet motion is triggered by the noise induced non-uniform velocity field which leads to a novel droplet coalescence mechanism associated with the thermal noise

Furnace Technology for Experiments on Sounding Rockets: Directional Solidification of Al-cast Alloys in ARTEX

Abstract The effect of controlled fluid flow on the microstructure formation in Al-cast alloys is studied experimentally: During three sounding rocket missions (TEXUS 39, TEXUS 41 and MAXUS 7) four samples of the composition Al-6wt.% Si were directionally solidified upwards under a medium temperature gradient (4K/mm) at constant solidification velocities (0.06 and 0.12mm/s). The application of rotating magnetic fields (RMF) during solidification offers the possibility to create defined flow conditions in solidification processing in microgravity to compare diffusive solidification conditions with convective ones. The paper shows that experiments under microgravity conditions yield other microstructures as experiments under earth conditions (with and without fluid flow): The secondary dendrite arm spacing was found to decrease, as the convection effects were reduced in microgravity. The forced fluid flow conditions result in pronounced macrosegregation effects. For high fluid flow velocity a change to pure eutectic solidification at the axis of the samples is observed

Impact of pearl-necklace-like skeleton on pore sizes and mechanical properties of porous materials: A theoretical view

The structural and mechanical properties of open-porous cellular materials are often described in terms of simple beam-based models. A common assumption in these models is that the pore walls have a constant cross section, which may be in agreement for a vast majority of such materials. However, for many of those materials that are characterized by a pearl-necklace-like network, this assumption seems too idealized. Aerogels are perfect examples of such materials. In this paper, we investigate the effect of such pore walls having a string of pearls-like morphology on the properties of such open-porous materials. First, the pore size is mathematically modeled. Three scenarios are described, where the pore sizes are calculated for cells in 2D, 3D, and 3D with overlapping particles. The dependency of the skeletal features on the resulting pore size is investigated. In the second part, pore walls with 3D overlapping spheres are modeled and subjected to axial stretching, bending, and buckling. The effect of the particle sizes and the amount of overlap between the particles on the mechanical features is simulated and illustrated. The results are also compared with models that assume a constant cross section of pore-walls. It can be observed that neglecting the corrugations arising from the pearl-necklace-like morphology in open-porous cellular materials can result in serious miscalculations of their mechanical behavior. The goal of this paper is not to quantify the bulk mechanical properties of the materials by accounting for the pearl-necklace-like morphology but rather to demonstrate the significant deviations that may arise when not accounted for

On the origin of power-scaling exponents in silica aerogels

The macroscopic properties of open-porous cellular materials hinge upon the microscopic skeletal architecture and features of the material. Typically, bulk material properties, viz. the elastic modulus, strength of the material, thermal conductivity, and acoustic velocity, of such porous materials are expressed in terms of power-scaling laws against their density. In particular, the relation between the elastic modulus and the density has been intensively investigated. While the Gibson and Ashby model predicts an exponent of 2 for ideally connected foam-like open-cellular solids, the exponent is found to lie between 3 and 4 for silica aerogels. In this paper, we investigate the origins of this scaling exponent. Particularly, the effect of the pearl-necklace-like skeletal features of the pore walls and that of the random spatial arrangement is extensively computationally studied. It is shown that the latter is the driving factor in dictating the scaling exponent and the rest of the features play a negligible or no role in quantifying the scaling exponent