Patrick Achard

Aerogel-based thermal superinsulation: an overview

This review is focused on describing the intimate link which exists between aerogels and thermal superinsulation. For long, this applied field has been considered as the most promising potential market for these nanomaterials. Today, there are several indicators suggesting that this old vision is likely to become reality in the near future. Based on recent developments in the field, we are confident that aerogels still offer the greatest potential for non-evacuated superinsulation systems and consequently must be considered as an amazing opportunity for sustainable development. The practical realization of such products however is time-consuming and a significant amount of R&D activities are still necessary to yield improved aerogel-based insulation products for mass markets.

Ammonia as a feedstock for a hydrogen fuel cell; reformer and fuel cell behaviour

In this paper, an indirect ammonia fuel cell system is proposed which circumvents the problem of hydrogen storage. System analysis shows that the specific energy density (kWh/kg or kWh/m3) of such a system is very attractive compared with that of a direct hydrogen fuel cell system. The fuel cell stocks have an important impact on these figures. However, both the indirect ammonia and the direct hydrogen systems use the same type of fuel cells, and improvement in fuel cell characteristics will improve the specific power and energy densities of both systems. Earlier work also showed that ammonia is a more attractive fuel than methanol for an indirect fuel cell. Ammonia therefore merits more attention as a fuel cell feedstock.

Drying of Silica Gels to Obtain Aerogels:Phenomenology and Basic Techniques

Silica gels form a large family of materials, obtained from more or less complex processes; their elaboration involves several steps that basically include a sol gel transition or a precipitation, followed by drying. This latter operation generally leads to dry materials with a structure significantly different from that in the wet state. If this structure is preserved after drying, a new variety of silica gels is obtained, known as aerogels. For the last 30 years, aerogels have been the subject of an increasing number of research works, because of their extraordinary

Physical properties of silica gels and aerogels prepared with new polymeric precursors

Abstract Silica aerogels were made by sol-gel techniques using industrial silicon derivatives (polyethoxydisiloxanes, PEDS-Px) obtained by the reaction of tetraethoxysilane (TEOS) and water (water to TEOS molar fraction, n, ranging between 0.8 and 1.8) in the presence of sulfuric acid. For each type of precursor, sol-gel reactions were performed under acidic, neutral or basic conditions, in alcohols (methanol or ethanol) or acetone. The gels were dried under hypercritical conditions with respect to alcohol or CO2 leading to monolithic organic-dried aerogels and CO2-dried aerogels. Gels were characterized by the gelling time, tg, and the shrinkage during aging, τs, and the aerogels by their apparent density, da, their global shrinkage, τg, and their specific surface area, SBET. It was shown that these values are strongly correlated. In most cases, tg decreases whereas τs increases when n varies between 0.8 and 1.8. This study shows that the use of these new precursors allows synthesis of a very large variety of monolithic transparent aerogels.

Characterization of hyperporous polyurethane-based gels by non-intrusive mercury porosimetry

Evaporative drying of polyurethane-based gels produces xerogels. Supercritical drying after replacement of interstitial liquid by supercritical CO2 produces aerogels. SEM micrographs show that both materials are made up of small size particles gathered up in filament-shaped, strongly cross-linked aggregates. Density measurements show that they both have a large pore volume. When submitted to mercury porosimetry, the behavior of these materials is similar to that of inorganic aerogels, as previously observed. Mercury does not penetrate the pore network, but the whole material is densified. The usual Washburn equation cannot be used to analyze the mercury porosimetry. A well-suited equation based on a buckling model of filament-shaped aggregates has been developed in order to determine the pore volume distribution of mineral dried gels. This equation is also valid for analyzing the texture of organic hyperporous materials like polyurethane dried nanoporous gel.

DLS and SAXS investigations of organic gels and aerogels

Abstract Recent investigations have shown that the structure of organic aerogels can be significantly modified by changing the precursors, the solvent and the nature of the catalyst involved in the sol–gel reaction. It is therefore highly desirable to investigate the sol–gel mechanism. For this purpose, dynamic light scattering (DLS) measurements have been performed at different stages of the reaction for base- or acid-catalyzed gelation of resorcinol–formaldehyde (RF) using water or acetone as solvents. The structure of aged gels was investigated by small-angle X-ray scattering (SAXS) and compared to that of the aerogels obtained after exchange of solvent by supercritical CO2 and drying of the aged gels. It is shown that acid-catalyzed gelation of RF in acetone can be described by percolation, which explains that this series of aerogels consists of mass fractal aggregates (Dm=2.5). The partial collapse of this polymeric gel yielding colloidal particles in the aerogel can be attributed to deswelling in supercritical CO2. DLS indicates that gelation of RF with a base catalyst yields a colloidal gel whose structure remains practically unchanged in the aerogel, as shown by SAXS.

Diffusion During the Supercritical Drying of Silica Gels

Drying is the most critical elaboration step of large monolithic and crack-free silica aerogel plates. In the present work, we are studying the supercritical CO2 drying and more precisely the first step, here called the supercritical washing step. This phase consists of replacing the liquid phase contained in the nanopores with supercritical CO2. Within this study, this step is governed by molecular diffusion through the gels. These phenomena were investigated experimentally in order to estimate the duration of the washing step. The experimental results were then fitted with an analytical mass transfer model to identify the effective diffusion coefficient.

Aerogels for Superinsulation: A Synoptic View

The present chapter is focused on describing the intimate link which exists between aerogels and thermal superinsulation. For long, this applied field has been considered as the most promising potential market for these nanostructured materials. Most likely this old vision will become reality in the near future.Following a short presentation of the global need for superinsulation together with a closer look at the specific situation in the building sector, we propose within this synopsis a brief analysis of (1) the world’s insulation markets, (2) superinsulating aerogel materials and their alternatives, (3) commercial aerogel insulation products available today, and (4) our estimation of their most likely applications worldwide in the future. We conclude this chapter with some first considerations on health, toxicity, and environmental aspects.Based on recent developments in the field, it can be stated that aerogels still offer the greatest potential for nonevacuated superinsulation systems and consequently must be considered as an amazing opportunity for sustainable development. This chapter of the handbook bridges the gap between those dealing with thermal insulation properties of aerogel materials in general (Chap. 21) and the various commercial products described in Part XV.

Investigation of the multi-scale structure of silica aerogels by SAXS

An investigation is described into polyethoxydisiloxane (PEDS-Px) silica precursors of aerogels prepared in ethylacetoacetate (etac) under HF conditions and dried under CO2 supercritical conditions. The influence upon the internal nanostructure of aerogels of the number of water molecules (n*) used to synthesize the precursors is studied, as well as that of their volume fraction (xPrec) in solution during the second catalytic step. Correlation between structure and optical transmission (%TR) is necessary to improve optical behavior of monolithic silica aerogels for double-window applications. For this reason, small angle X-ray scattering (SAXS) was performed to characterize the nanostructural silica skeleton, and especially to investigate particle and cluster properties. Optical transmission was also measured to estimate visible optical quality of the samples (%TR). Among other results, it is shown in this study that increasing n* and xPrec contributes to the decrease of the particle and cluster sizes, which indirectly improves %TR in the visible range. These observations confirm and help to explain previously published results.

Structural investigation in monolithic silica aerogels and thermal properties

The influence, of the number of water molecules (n*) used to synthesize polyethoxydisiloxane (PEDS-Px) silica precursors, on the internal structure of aerogels made with those precursors in ethylacetoacetate (etac) under HF conditions and dried under CO2 supercritical conditions was studied. Gas adsorption, water thermoporometry and mercury porosimetry were used to characterize specific area and pore size distribution. The results were used to interpret the evolution of the apparent thermal conductivity of those aerogels with n*. Optical transmission was also measured to estimate optical quality of the aerogels prepared.