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International audience; This study proposes a novel sepiolite-based Aquivion electrolyte membrane which could be operated at low relative humidity. In the study, it was discovered that the functionalization of sepiolite with fluorinated groups (i.e., -C7F15), named SEP-F, helps its dispersion in the composite membrane, compared with the use of natural sepiolite. The Aquivion/SEP-F composite membrane showed increased water uptake, thermo-mechanical and chemical stability as well as proton conductivity and decreased swelling compared with commercially available Nafion HP and pristine Aquivion. Their behavior in single cell MEA testing conditions was also assessed. Aquivion/SEP-F composite membrane can be an interesting alternative for low relative humidity operation of PEMFC (proton exchange membrane fuel cell).

This study introduces the relationship between physicochemical properties and blending time for two different clays, sepiolite nanofibers (SEP) and halloysite nanotubes (HNT) inside a Nafion matrix...

International audience; Aquivion membrane displays improved properties as compared to Nafion membrane, partly due to shorter side chains. However, some improvements are still necessary for proton exchange membrane fuel cell to operate at low relative humidity. To overcome this drawback, the addition of clay nanoparticle into the Aquivion matrix can be considered. In this study, different composite membranes have been prepared mixing short-side-chain PFSA (perfluorosulfonic acid) Aquivion and selectively modified halloysite nanotubes for PEMFC low relative humidity operation. Halloysites were grafted with fluorinated groups, sulfonated groups, or perfluoro-sulfonated groups on inner or outer surface of the tubes. The obtained composite membranes showed improved properties, especially higher water uptake associated with reduced swelling and better mechanical strength compared to pristine Aquivion membrane and commercially available Nafion HP used as reference. The best performance in this study was obtained with Aquivion loaded with 5 wt% of pretreated perfluoro-sulfonated halloysite. The composite membrane, referred to as Aq/pHNT-SF5, displayed the largest water uptake and proton conductivity among the panel of membranes tested. The chemical stability was not affected by the presence of halloysite in the Aquivion matrix.

International audience; A facile method to synthesize light weight thermally superinsulating composite aerogel is being presented here. A polymethylsilsesquioxane (PMSQ) – cellulose composite aerogel starting from a trifunctional precursor viz. methyltrimethoxysilane (MTMS) and kapok fibers have been synthesized. Kapok fibers have been employed as they have a homogeneous hollow structure and also possess intrinsic low density. A PMSQ-Kapok composite aerogel with a density as low as 0.053 gcm−3 with a thermal conductivity of 0.018 W m−1 K−1 in room conditions has been synthesized. Besides, a flexural strength (at maximum stress) of 108 kPa ± 21 has been obtained through three points bending test. All the composite aerogels are mesoporous as characterized by N2 sorption isotherms and hydrophobic as shown by sessile drop experiments. On comparison with the earlier reported works and with some of the commercially available aerogel composites, the current results seem promising. For demonstrating the real application purpose, thin composite aerogel sheets have also been synthesized which could be easily rolled/bended.

Aerogels are highly porous, ultra-light (density around 0.1 g/cm3) nanostructured materials. One of their most extraordinary properties is thermal super-insulation, i.e. thermal conductivity below that of the air: 0.015 vs 0.025 W/(m.K) in room conditions. However, classical silica aerogels are extremely fragile and organic/synthetic (resorcinol-formaldehyde) aerogels may include toxic components, which hinders their wide application. Bio-aerogels are a new generation of aerogels made from biomass-based polymers, mainly polysaccharides. We prepared aerogels from cellulose (“aerocellulose” /1, 2/) and pectin (“aeropectin” /3/) via polymer dissolution, coagulation and drying with super-critical CO2. Their density varies from 0.05 to 0.2 g/cm3 and specific surface area is around 200-300 m2/g. Bio-aerogels are mechanically strong, with Young’s moduli from 1-2 to 20-30 MPa and plastic deformation up to 60-70% strain before the pore walls collapse. Aeropectin thermal conductivity turned to be around 0.015 – 0.020 W/(m.K) making it the first reported thermal super-insulating fully biomass-based aerogel. The thermal conductivity of aerocellulose is rather “high”, around 0.030-0.035 W/(m.K), due to the presence of large macropores. We demonstrate that by using polysaccharide functionalization and making polymer-silica aerogel hybrids it is possible to vary specific surface area (increase to 800-900 m2/g) and decrease aerogel thermal conductivity below that of the air.

International audience; Silica aerogels are excellent thermal insulators, but their brittle nature has prevented widespread application. To overcome these mechanical limitations, silica–biopolymer hybrids are a promising alternative. A one-pot process to monolithic, superinsulating pectin–silica hybrid aerogels is presented. Their structural and physical properties can be tuned by adjusting the gelation pH and pectin concentration. Hybrid aerogels made at pH 1.5 exhibit minimal dust release and vastly improved mechanical properties while remaining excellent thermal insulators. The change in the mechanical properties is directly linked to the observed “neck-free” nanoscale network structure with thicker struts. Such a design is superior to “neck-limited”, classical inorganic aerogels. This new class of materials opens up new perspectives for novel silica–biopolymer nanocomposite aerogels.

International audience; Short man-made cellulose fibers (TENCEL® fibers) were used to mechanically reinforce thermal superinsulating silica aerogels. The aerogels were prepared via two drying techniques: ambient pressure drying and with supercritical CO2, in both cases resulting in monolithic non-brittle materials. The influence of fiber length and concentration on the thermal conductivity and flexural properties of both types of composite aerogels was evaluated. Thermal conductivity in room conditions varied from 0.015 to 0.018 W.K-1.m-1; it slightly increased with fiber concentration but remained in superinsulation domain. The importance of fiber percolation concentration for synthesizing monolithic ambient pressure dried composite aerogels was demonstrated. Contrary to neat silica aerogels, non-brittle behavior was observed for composite aerogels regardless of the drying method when reinforced with cellulose fibers. Macroscopic short cellulose based fibers are efficient and easy to use for preparing robust, monolithic, thermal superinsulating aerogel materials.

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International audience; Organic-inorganic composite aerogels were synthesized via a facile one-step impregnation of a wet hydrophobic cellulose II matrix with polyethoxydisiloxane solution followed by in situ NH4OH-catalysed formation of a nanostructured mesoporous silica network. The silica phase was subsequently hydrophobised with hexamethyldisilazane. By tuning synthesis conditions, organic-inorganic composite aerogels with various morphology (from meso- to macroporous), specific surface area (from 200 to 850 m2/g) and hydrophilic-hydrophobic balance (from fully hydrophilic to highly hydrophobic) were prepared after supercritical CO2 drying. The obtained organic-inorganic aerogels are monolithic, resistant to humidity and thermally superinsulating with the thermal conductivity 0.021–0.022 W/(m·K) in room conditions

International audience; Preface : In September 2016, the “Third International Seminar on Aerogels”, co-organized with the International Society for the Advancement of Supercritical Fluids (ISASF) and Technical University of Hamburg-Harburg (TUHH) was hosted by MINES ParisTech in Sophia Antipolis (Alpes-Maritimes, France). This Special Issue gathers 17 full original articles related to topics and presentations of this event.

International audience; Increasing the activity of a photocatalyst goes through the improvement of both its absorption (light) and adsorption (reactant) properties. For a given semiconducting material, the charge carrier separation is also a very important step. Properly combining chosen phases is one option to improve this separation (example of the commercial P25) and depositing platinum on the surface of the catalyst, another one. In some cases, coupling both may nevertheless lead to a decrease of photoactivity or at least limit the potentiality of the catalyst. A third option, consisting in modifying the morphology of the photoactive phase, has shown very promising results. In this study, we have elaborated, characterized and evaluated the hydrogen evolution potentiality (through methanol assisted water splitting) of different TiO2 morphologies: nanoparticles, nanotubes and aerogels. These materials have shown different behaviours depending on both their composition and morphology. Different types of separation processes have been claimed to account for the observed different photoactivities, with more or less pronounced synergetic effects, due to: the use of Pt as a co-catalyst, the mixture of different TiO2 phases (anatase and TiO2(B) or rutile) and the specific morphology of the samples (nanotubes or aerogels). Among all the tested samples, the TiO2 aerogel supported Pt one exhibited very promising performances, three times as active as P25 supported Pt, which is already much more active than pure P25 in our testing conditions.

A series of hybrid supported catalysts was prepared by sequentially grafting Cp 2 ZrCl 2 and ( n BuCp) 2 ZrCl 2 (1:3 ratio) onto synthesized xerogel, aerogel and commercial silicas. Supports and catalysts were characterized by Rutherford backscattering spectrometry, energy-dispersive X-ray scanning electron microscopy, atomic force microscopy and nitrogen adsorption. Grafted metal content laid between 0.15 and 0.5 wt.% Zr/SiO 2 . All the systems were shown to be active in ethylene polymerization with methylaluminoxane as the co-catalyst. Catalyst activity and molecular weight were shown to depend on the textural characteristic of the silicas, namely grain size and pore diameter. The highest activity in ethylene polymerization (ca. 5310 kg PE mol Zr −1 h −1 ) was obtained with the supported catalyst using commercial silicas with average particle size around 50 μm. Particles with sizes of 80–90 μm obtained less activity. Resulting polymers were characterized by gel permeation chromatography and differential scanning calorimetry.

International audience; Organic aerogels based on polyurethane were elaborated via sol-gel synthesis and dried with supercritical carbon dioxide (CO2). The influence of the catalyst concentration was investigated, first in order to decrease the reaction kinetics, then to study its impact on the obtained materials properties. It was shown that this parameter also influences the global shrinkage and the bulk density of the resulting materials. Its effect on the dry materials was studied in terms of morphological, textural and thermal properties in order to determine the main correlations thanks to scanning electron microscopy (SEM), nitrogen adsorption, non-intrusive mercury porosimetry and thermal conductivity measurements. Results allowed us to demonstrate a correlation between the bulk density, the texture and the thermal conductivity of this family of polyurethane aerogels and to determine an optimal density range for thermal performance associated with a fine internal mesoporous texture.

International audience; Low density, highly porous and hydrophobic cellulose-based new material, Xerocellulose, was prepared and characterised. First, tritylcellulose with different degrees of substitution (DS) was synthesised in homogeneous conditions. Xerocellulose was then prepared from tritylcellulose via dissolution–coagulation–drying route, similar to other polysaccharide-based aerogels, but drying was performed in ambient room conditions. The new material has a density between 0.1 and 0.2 g/cm3 and is highly hydrophobic with contact angle 140° for DS = 0.72. Compared with cellulose aerogel and pristine microcrystalline cellulose, Xerocellulose obtained from tritylcellulose with DS = 0.72 showed a drastically decreased water vapour uptake. The evolution of Xerocellulose density and morphology as a function of the DS is presented and discussed.

International audience; Aerogels based on interpenetrated cellulose–silica networks were prepared and characterised. Wet coagulated cellulose was impregnated with silica phase, polyethoxydisiloxane, using two methods: (i) molecular diffusion and (ii) forced flow induced by pressure difference. The latter allowed an enormous decrease in the impregnation times, by almost three orders of magnitude, for a sample with the same geometry. In both cases, nanostructured silica gel was in situ formed inside cellulose matrix. Nitrogen adsorption analysis revealed an almost threefold increase in pores specific surface area, from cellulose aerogel alone to organic-inorganic composite. Morphology, thermal conductivity and mechanical properties under uniaxial compression were investigated. Thermal conductivity of composite aerogels was lower than that of cellulose aerogel due to the formation of superinsulating mesoporous silica inside cellulose pores. Furthermore, composite aerogels were stiffer than each of reference aerogels.

A silyl-functionalized benzazole dye, fluorescent by excited state intramolecular proton transfer (ESIPT) mechanism, was synthesized by reacting 2-(2'-hydroxyphenyl)benzoxazole with 3-(triethoxysilyl)propyl isocyanate. The fluorescent silica gel was prepared by the addition of a solution of 2-propanol and the fluorescent dye after the gelation time. The monolithic aerogel was obtained via supercritical CO 2 drying of the fluorescent gel. The resulting aerogel is transparent in the visible light and fluorescent in the blue-green region under UV radiation.

Previously published results have shown that washing and aging treatments (respectively in water/ethanol and water/polyethoxydisiloxane solutions) of silica gels (synthesized from polyethoxydisiloxane precursors through HF catalysis) significantly enhance both the permeability and mechanical properties of the wet gels. Unfortunately, scaling-up the process induces severe cracks during supercritical drying. This phenomenon has been related to a pore-size distribution gradient between the surface and the bulk of the gel that appears after aging of thick gels. We report efforts to optimize the aging step. Aging in less-concentrated polyethoxydisiloxane solutions yields gels with a more homogeneous pore-size distribution and has enabled us to obtain large monolithic and crack-free aerogels that remain light and transparent.

International audience; 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. © 2003 Elsevier Science Ltd. All rights reserved.

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, ...

International audience; Large monolithic crack-free transparent silica aerogels were prepared by a patented sol-gel route, and then directly washed and dried with supercritical CO2. In order to characterise as precisely as possible the apparent thermal conductivity of such superinsulating materials, two methods were developed (so-called hot-band and micro-fluxmeter techniques) and the results obtained at room temperature and atmospheric pressure were compared with the standard hot-wire measurements under the same conditions. A reproducibility study showed no large deviations between the thermal conductivities obtained by these different techniques. This leads to the conclusion that aerogels prepared in this way present a good thermal and optical compromise (low thermal conductivity, high light transmission, and no significant radiative heat transport component at room temperature) and can be considered as transparent superinsulating materials.

International audience; Cellulose-silica composite aerogels were prepared via "one-pot" process: aqueous solutions of cellulose-8 wt% NaOH and sodium silicate were mixed, coagulated and dried with supercritical CO2. The system was studied both in the fluid and solid (dry) states. Cellulose and sodium silicate solutions were mixed at different temperatures and concentrations; mixture properties were monitored using dynamic rheology. The gelation time of the mixture was strongly reduced as compared to that of cellulose-NaOH solutions; we interpret this phenomenon as cellulose self-aggregation inducing partial coagulation due to competition for the solvent with sodium silicate. The gelled cellulose/sodium silicate samples were placed in aqueous acid solution which completed cellulose coagulation and led to in situ formation of sub-micronic silica particles trapped in a porous cellulose matrix. After drying with supercritical CO2, an organic-inorganic aerogel composite was formed. The densities obtained were in the range of 0.10-0.25 g/cm3 and the specific surface area was between 100 and 200 m2/g. The silica phase was shown to have a reinforcing effect on the cellulose aerogel, increasing its Young's modulus

6th International Symposium on Aerogels (ISA-6) Albuquerque, New Mexico, October 08-11, 2000; International audience; 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.

International audience; Nafion®-sepiolite composite membranes were prepared, characterized and integrated in Membrane-Electrodes Assembly to be tested in fuel cell operating conditions. The influence of the sepiolite content and its modification on the membrane properties was carefully analyzed. The performance of the different MEAs were compared at different operating temperatures and under various relative humidity. The sulfonation of pristine sepiolite has improved its ion exchange capacity as well as that of composites membranes. The introduction of sepiolite in Nafion® also allowed simultaneously to increase its water uptake and to improve its mechanical features. Much better performance were obtained at high temperature and low relative humidity with MEAs based upon Nafion®-sepiolite composite membranes compared to pure Nafion® membranes (50% more output power at 100 °C and 50%RH).

International audience; This chapter focuses on isocyanurate and cellulose-based aerogels. First, it presents the global sol-gel synthetic path by polycondensation. Then, it summarizes all the main results on these two families of organic aerogels. Finally, some of the recent advancements concerning their use for hybridization of silica aerogels are shortly presented. Through a brief description of the basics, together with a short overview of the main properties, this article highlights the huge potential of those two classes of urethane-based aerogels.

International audience; 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.

International audience; A series of nitrogen- and oxygen-functionalised carbon aerogels was produced from a carbon aerogel derived from cellulose acetate. Samples were oxidised by H2O2 or HNO3 and/or enriched in nitrogen by reaction with gaseous ammonia or co-heating of the carbon aerogel and melamine. Porosity variations and morphology were monitored using N2 adsorption and helium pycnometry. The surface chemistry was characterised by elemental analysis, FTIR and XPS spectroscopy, pH of the point of zero charge and acid/base titration. The prepared carbons are mainly mesoporous and show a moderate porosity development (SBET between 160 and 300 m2/g). The applied chemical methods allow producing a wide range of functionalised carbon aerogels differing in terms of oxygen and nitrogen groups, their distribution and basicity. Both oxidation methods introduce a similar amount of oxygen, while the produced carbons differ in term of their acid/base character. Treatment with ammonia produces the most basic materials, which is partly due to the introduction of basic nitrogen groups, but also to the reduction of the acidic oxygen functionalities.

International audience; New nanostructured carbons have been prepared from pyrolysis of recently developed highly porous cellulose, aerocellulose (AC). Aerocellulose is an ultra-light and highly porous pure cellulose material prepared from cellulose gels followed by drying in carbon dioxide supercritical conditions. The carbonized aerocellulose (CAC) materials were obtained after pyrolysis of the aerocellulose under nitrogen flow at 830 °C, and subsequently doped by platinum nanoparticles. The platinum insertion process consisted of (i) thermal activation at various temperatures in CO2 atmosphere, (ii) impregnation by PtCl62− and (iii) platinum salt chemical reduction. The aerocellulose materials and their carbonized counterparts were investigated by scanning and transmission electron microscopy (SEM and TEM), mercury porosimetry and thermogravimetric analysis. The morphology of the platinum particles deposited on the carbonized aerocellulose materials (Pt/CAC) was investigated by transmission electron microscopy (TEM) and X-ray diffraction (XRD): the Pt particles are of 4–5 nm size, mainly agglomerated, as a result of the complex surface chemistry of the CAC. Their electrocatalytic activity was investigated by quasi-steady-state voltammetry in the rotating disk electrode (RDE) setup, regarding the oxygen reduction reaction (ORR). The Pt/CAC materials exhibit ORR specific activities comparable with those of commercial Pt/Vulcan XC72R. Their mass activity is lower, as a result of the ca. 10 times smaller specific area of platinum as compared with the commercial electrocatalyst. We nevertheless believe that provided an appropriate pyrolysis temperature is chosen, such green carbonized aerocellulose could be a promising electrocatalyst support for PEM application.

International audience; A series of silica-based materials were employed as sorbents within solid-phase microextraction vials. The aim of the study was to evaluate the effect of an additional phase on the distribution of the volatile and less volatile analytes. The adsorption of six probe molecules, namely isoamyl acetate, ethyl hexanoate (ethyl caproate), phenylethyl alcohol, ethyl octanoate (ethyl caprilate), 2-phenylethyl acetate, and ethyl decanoate, was monitored by detecting the desorbed amount on a DVD-CAR-PDMS fiber from Pilsen beer. The microextraction process involved the presence of different silica-based phases produced via different methods: xerogel produced by hydrolytic and non-hydrolytic routes, aerogel, pyrogenic, and precipitated silica. The resulting data are discussed in correlation with sorbent texture properties (specific area and pore diameter). The modification of silica with alkyl groups also affects the preconcentrated amount of the target molecules in the headspace. The presence of sorbents was shown to affect the analyte signal more than the addition of NaCl or the use of ultrasound.

International audience; Two series of side chain liquid crystal (SCLC) polyacrylate-silica aerogel nanocomposites have been investigated by small-angle x-ray scattering (SAXS) and differential scanning calorimetry (DSC). The first series (ex-situ nanocomposite) was obtained by infiltration of a smectic SCLC polyacrylate prepared by polymerisation in solution into monolithic aerogel slabs. The second one (in-situ nanocomposite) was prepared by photopolymerisation of the monomer infiltrated in the aerogel. The results are compared with those obtained for bulk polyacrylates. It is shown that the smectic ordering is not destroyed by confinement in the aerogel. Spacing of the smectic layers and smectic correlation lengths were deduced from the fit of the SAXS profiles to a Lorentzian function with a quadratic correction. The principal results suggest that in-situ polymerisation enhances the degree of order and the stability of the smectic phase in the nanocomposite. © 2009 American Institute of Physics.

TiO 2 aerogel–10 mol% TiO 2 nanowire composite was prepared by a sol–gel technique with the addition of TiO 2 nanowires to TiO 2 sol, followed by supercritical drying in CO 2. TiO 2 nanowires (anatase with minor rutile phases) as dispersoid were prepared by a hydrothermal process followed by calcination in air at 600°C. The TiO 2 nanowires were dispersed in a 2-propanol/ H 2 O / HNO 3 solution, and the mixture was added drop by drop to a tetrabutyl orthotitanate [i.e. Ti (IV) n-butoxide] solution in 2-propanol. After gelation, the TiO 2 alcogel– TiO 2 nanowire composite was dried in supercritical CO 2 to obtain the final, TiO 2 aerogel– TiO 2 nanowire composite. TEM analysis revealed that a unique "nanowire network" structure was formed within the mesoporous aerogel matrix. The aerogel– TiO 2 nanowire composite had a relatively large surface area 427 m2/g, with mesopores ~ 16 nm in diameter and a pore of volume of 1.63 cm3/g.

International audience; New nanostructured carbons have been developed through pyrolysis of organic aerogels, based on supercritical drying of cellulose acetate gels. These cellulose acetate-based carbon aerogels (CA) are activated by CO2 at 800 °C and impregnated by PtCl62–; the platinum salt is then chemically or electrochemically reduced. The resulting platinized carbon aerogels (Pt/CA) are characterized with transmission electron microscopy (TEM) and electrochemistry. The active area of platinum is estimated from hydrogen adsorption/desorption or CO-stripping voltammetry: it is possible to deposit platinum nanoparticles onto the cellulose acetate-based carbon aerogel surface in significant proportions. The oxygen reduction reaction (ORR) kinetic parameters of the Pt/CA materials, determined from quasi-steady-state voltammetry, are comparable with that of Pt/Vulcan XC72R. These cellulose acetate-based carbon aerogels are thus promising electrocatalyst support for PEM application.

International audience; In order to enhance the mechanical properties of wet gels for aerogel production, aging studies by using three different routes was performed. The wet gels were prepared from a polyethoxydisiloxane precursor by using HF as a catalyst. The three different aging routes studied were i) aging in sealed mould, ii) aging in solvent and iii) aging in simulated pore liquid, i.e. a solvent with small amounts of water and HF resembling the mother liquor. All aging processes gave stronger and stiffer wet gels however, a maximum in strength and stiffness was observed after a certain aging time. The simulated pore liquids allowed short aging time in the range of 8 h to achieve the maximum mechanical strength, however the maximum in strength was lower than for the other two aging routes. From the wet gels, monolithic and transparent aerogels were obtained by supercritical drying at small-, mid- and large-scale. The aging strengthening process was successfully transferred to larger scales giving both lower density and higher transparency compared to small-scale.

International audience; 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.

New organic aerogels were prepared using cellulose derivatives as precursors. The elaboration process and the structural characterisations of these porous cellulose-based materials are described in the present study. Series of monolithic gels were synthesised in acetone by crosslinking cellulose acetate with a non-toxic isocyanate via sol–gel route, using tin-based catalyst. Gelation times (ranging from 15 to 150 min) were significantly dependent on reagents' nature and concentration. Low-density materials (from 0.25 cm 3 /g to 0.85 cm 3 /g) were obtained after supercritical carbon dioxide drying. These newly developed nanostructured materials were characterised using mercury porosimetry, nitrogen adsorption and scanning electron microscopy. All the prepared materials have shown both a nanostructured solid network (specific surface areas between 140 and 250 m 2 /g) and a nanoporous network (characteristic pore sizes between 13 and 25 nm) together with specific porous volumes as large as 3.30 cm 3 /g. Influence of sol–gel synthesis parameters as crosslinker content and cellulose degree of polymerisation or concentration was investigated. First empirical correlations between synthesis parameters and final material properties were obtained. A special attention was dedicated to the different shrinkages occurring during the elaboration process. In particular, the important shrinkage occurring during the supercritical drying step was studied in terms of affinity between the crosslinked polymeric network and carbon dioxide. In parallel, first thermo-mechanical properties were presented in terms of bulk modulus and effective thermal conductivity.

International audience; Previous work has demonstrated the feasibility of synthesizing low-density polyurethane- and polyisocyanurate-based aerogels that exhibit low effective thermal conductivity. On the basis of this literature, the present study synthesized and characterized nanostructured polyurethane aerogel-like materials processed via subcritical drying routes. Two families of polyurethane gels were studied. Wet gels were synthesized with two polyols of different functionality. The influence of the composition of the reaction media is discussed. Depending on the solubility of the precursors, macroporous foam-like or mesoporous aerogel-like materials can be obtained as observed by scanning electron microscopy coupled with mercury porosimetry. Prior to drying, specific washing steps were performed. Preliminary results obtained by evaporative and freeze-drying were then compared to reference aerogel materials dried through a direct supercritical route. Only a slight density increase was observed. Effective thermal conductivities were also measured and discussed.

International audience; Apparent thermal conductivity (λ) measurements were made on monolithic silica aerogels and divided low-density xerogels. The objective of the tests on monoliths was to determine the variation of λ with the solid content. Additional compressive tests were also made to follow the evolution of bulk modulus with apparent density, in order to further optimize the solid content of xerogel particles. Divided xerogels were characterized under the following conditions: at ambient conditions (T, P, and relative humidity of water vapor in air), under partial vacuum (from atmospheric pressure to 4 × 10−7 bar) and under different relative humidities. The effect of the divided xerogel granularity on λ has been studied by using different size range and distributions. The results of our studies indicate an optimum particle density at 0.15 g cm−3, and a decrease in λ for samples containing fine particles.

Issu de : 7th International symposium on Aerogels (ISA-7), Alexandria, Virginia, USA, 2-5 November, 2003; International audience; Studies were performed on silica gels synthesized in isopropanol to determine the parameters necessary to follow evaporative drying quantitatively, particularly during the last drying phase controlled by vapor diffusion in the gel nanostructure. As prepared and silanized gels were compared. Dynamic vapor sorption of isopropanol was used to determine adsorption isotherms and apparent diffusion coefficients in the silica network. A significant effect of surface modification (consisting in capping the silica surface with trimethylsilyl groups) has been observed. The diffusivity of solvent vapors was found to be very low, from 10-9 to 10-10 m2s-1, respectively, for treated and untreated samples, but in agreement with previously published works. Microcalorimetry confirmed a reduction in the affinity between the solid phase and solvent due to the silanization treatment. In parallel, evaporative drying experiments were conducted, recording the sample mass and taking photographs during the entire process of solvent evaporation. Comparison between untreated and treated samples during the last drying step showed how and when spring-back takes place for the latter.

International audience; Gels were prepared from a polyethoxydisiloxane precursor by using HF as a catalyst. During washing in water solution a significant increase in the permeability of the gels was observed, showing that dissolution-reprecipitation occurs. After washing, the gels were further soaked in a solution of polyethoxydisiloxane precursor to strengthen and stiffen the gel. As expected, a significant enhancement of the mechanical properties of the wet gels was observed. It is also interesting to note, however, that the permeability does not decrease below the value for the as-prepared gels. Hence, a promising process has been developed where both the stiffness and the strength have been increased as well as the permeability. The increase in permeability is of importance to facilitate the supercritical drying process. Reasonably successful scaling up of the supercritical drying of these gels to laboratory scale has been achieved, and monolithic and transparent gels are obtained. Optical properties have been measured on laboratory scale aerogels. The corresponding results have been correlated with structural characteristics measured by small-angle X-ray scattering (SAXS).

International audience; 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.

Five silyl-functionalized benzazole dyes, fluorescent by excited state intramolecular proton transfer (ESIPT) mechanism, were synthesized by reaction of amino benzazole derivatives with 3-(triethoxysilyl)propyl isocyanate. Fluorescent silica gels were prepared and monolithic aerogels (d ≈ 0.18 g cm−3) were obtained via supercritical CO2 drying of the fluorescent gel. The photophysical behaviour of the dyes and fluorescent silica aerogels was investigated by UV–vis and steady-state fluorescence spectroscopy.

International audience; Novel various aerogel-like materials were prepared from the biomass, cellulose and cellulose derivatives, either from their solutions or via physical or chemical gelation and drying in CO2 under supercritical conditions. Two classes of materials will be discussed: based on pure cellulose and on cellulose acetate. Pure cellulose highly porous material, Aerocellulose, was made from cellulose dissolved in green solvents, aqueous NaOH or ionic liquids. An overview of the main preparation steps is given, from cellulose dissolution and solution-state properties, to regeneration kinetics, solvent exchange and drying. The porosity of Aerocellulose is higher than 90% with pore sizes from a few tens of nanometers to a few tens of micrometers. The density ranges from 0.06 to 0.3 g/cm3. Aerogels based on cellulose acetate were synthesised by crosslinking cellulose acetate dissolved in acetone with a non-toxic isocyanate as a cross-linker, using different catalysts (dibutyltin dilaurate, a triethylamine and pyridine). Mesoporous nanostructured network was obtained after drying; pore sizes vary from 10 to 30 nm and density from 0.25 to 0.85 cm3/g. Two applications of these new materials are discussed: as thermal insulators and carbons in proton exchange membrane fuel cell.

This thesis introduces novel electrolyte membranes which can be operated at low relative humidity (below 50%) and intermediate temperature, i.e., 90℃. More specifically, the thesis takes benefit from hygroscopicity of microfibrous SEP (sepiolite) and tubular HNT (halloysite). Changes in Nafion membrane properties with blending time were studied. Moreover, these nanoclays are functionalized and pretreated to make them proton conductive and to improve their compatibility with short-side-chain PFSA (perfluorosulfonic acid) composite membranes based on Aquivion. To begin with, functionalized and pretreated clay nanoparticles are characterized prior to incorporation in polymer matrix: ATR-FTIR (attenuated total reflection-fourier transform infrared spectroscopy), Py-GC/MS (pyrolysis gas chromatography mass spectrometry), and TGA (thermogravimetric analysis). Composites membranes have them been prepared and characterized for proton conductivity, water uptake, swelling, thermo-mechanical strength and chemical stability. The dispersion state of SEP and HNT inside polymer phase was observed using SEM/EDS (field emission scanning electron microscopy/Energy dispersive X-ray spectroscopy). The properties of pretreated nanoclays are characterized using XRD (X-ray diffraction) and EDS. Chemical stability regarding radical attack against composite membranes is clarified using Ion meter through fluoride ion (F-) analysis. Proton conductivity of composite membranes is also measured under condition of different relative humidity and temperature. Following this, it is demonstrated by DMA (dynamic mechanical analysis) results that the particular elongated morphology of SEPs and HNTs participates to improving mechanical property of the composite membranes with decreased swelling ratio. MEAs (membrane electrode assembly) performance are evaluated to understand the advantage of the presence of nanoclays in the composite membranes regarding the relative humidity of the feeding gas, the operating temperature of the cell, and the hydrogen crossover. Detailed abstracts including main results were provided at the beginning of each chapter., Cette thèse introduit de nouvelles membranes électrolytiques pouvant fonctionner à faible humidité relative (inférieure à 50%) et à une température intermédiaire, c'est-à-dire 90°C voire au-delà. Plus spécifiquement, la thèse tire profit de l'hygroscopicité de la morphologie d’argiles naturelles, lasépiolite microfibreuse et l’halloysite tubulaire . Ces nanoargiles ont été intégrées à des suspensions de Nafion® ou Aquivion pour préparer des membranes composites. Elles ont été fonctionnalisées et prétraitées pour les rendre conductrices protoniques et améliorer leur compatibilité avec les matrices perfluorosulfoniques utilisés. Ces argiles ont d’abord été caractérisées avant leur incorporation dans la matrice polymère : ATR-FTIR (spectroscopie infrarouge à transformée de Fourier totale atténuée), Py-GC/MS (spectrométrie de masse par chromatographie en phase gazeuse à pyrolyse) et ATG (analyse thermogravimétrique). Les propriétés des nanoargiles prétraitées ont enfin été caractérisées par XRD (diffraction des rayons X) et EDS. Les membranes composites préparées ont ensuite été caractérisées pour la conductivité protonique, l'absorption d'eau, le gonflement, la résistance thermomécanique et la stabilité chimique. L'état de dispersion des argiles à l'intérieur de la phase de polymère a été observé par SEM/EDS (microscopie électronique à balayage à émission de champ / spectroscopie à rayons X à dispersion d'énergie). La stabilité chimique vis-à-vis de l'attaque radicale contre les membranes composites a été étudiée par mesure de la formation d’ions fluorure (F-). La conductivité protonique des membranes composites a également été calculée à partir des résistances mesurées dans dans une large gamme d'humidités relatives et de températures. Des mesures thermomécaniques par analyse mécanique dynamique ont montré que la morphologie allongée particulière des argiles choisies participe à l'amélioration des propriétés mécaniques des membranes composites tout en réduisant le taux de gonflement. Les performances en assemblage membrane électrodes ont été évaluées pour mettre en évidence l’avantage de la présence de ces nanoargiles dans les membranes composites en ce qui concerne l’humidité relative du gaz d’alimentation, la température de fonctionnement de la cellule et la perméation à l’hydrogène. Des résumés détaillés comprenant les principaux résultats ont été fournis au début de chaque chapitre.

Improving the thermal insulation of materials is a key challenge to lower global energy consumption and greenhouse effect gas emissions in the coming decades. This thesis focuses on the preparation and characterization of nanostructured composite materials combining the good mechanical properties of bio-aerogels with the excellent thermal insulation properties of silica aerogels. Two polysaccharides were used to make bio-aerogels: cellulose and pectin. Two strategies aiming to elaborate composite materials were investigated: “one-pot” process and impregnation of a porous “wet” polysaccharide matrix by polyethoxydisiloxane. Drying with supercritical CO2 yields the composite aerogels. While the one-pot method gave micron-sized silica particles embedded in a porous cellulose network, impregnation process allowed obtaining a nanostructured interpenetrated network of cellulose and silica. The specific surface area was 700-800 cm².g-1, the mechanical properties improved as compared to neat silica aerogels and thermal conductivity lower than that of cellulose aerogels. Using a chemically hydrophobized cellulose, tritylcellulose, as the impregnation matrix, hydrophobic composites were obtained showing a contact angle with water of 133° and thermal conductivities of 0.021 W.m-1.K-1. Aerogels from cross-linked pectin and their composites with silica show extremely low densities (around 0.05 g/cm3 for the neat pectin aerogels) and thermal conductivities in the 0.013-0.022 W.m-1.K-1 range.; L'amélioration des propriétés des matériaux pour l'isolation thermique est un défi clé pour la réduction de la consommation énergétique et de l'émission de gaz à effets de serre. Cette thèse a pour objectif l'élaboration de matériaux composites nanostructurés, combinant les bonnes propriétés mécaniques des bio-aérogels avec les excellentes propriétés d'isolation thermique des aérogels de silice. Deux polysaccharides ont été étudiés comme source de bio-aérogels : la cellulose et la pectine. Deux stratégies pour l'élaboration des composites ont été considérées : un procédé « one-pot »; et l'imprégnation d'une matrice polysaccharide poreuse. Les aérogels composites ont été obtenus par séchage au CO2 supercritique. Alors que la méthode « one-pot » génère des particules de silice micrométriques au sein d'un réseau poreux, le procédé d'imprégnation a permis d'obtenir un réseau nanostructuré interpénétré. La surface spécifique atteint 700-800 cm².g-1, les propriétés mécaniques sont améliorées par rapport aux aérogels de silice et la conductivité thermique est réduite comparée à l'Aerocellulose pure. Utiliser une cellulose hydrophobisée chimiquement, la tritylcellulose, comme matrice d'imprégnation, a permis d'obtenir des composites hydrophobes ayant un angle de contact avec l'eau de 133° et des conductivités thermiques de 0.021-0.022 W.m-1.K-1. Les aérogels à base de pectine réticulée et leurs composites avec la silice présentent des densités extrêmement basses (0.05 g.cm3) et des conductivités thermiques de 0.013-0.022 W.m-1.K-1.

Aerogels are light and mesoporous materials that appear very promising in terms of thermal insulation. They could play a great part in the future in the reduction of thermal losses through building and appliances envelopes. In this context, the goal of this thesis work is the development of a material combining a very low thermal conductivity and mechanical properties that are compatible with a use in this application sector, via an organic-inorganic hybridization process between a superinsulating - but fragile - silica aerogel, and a very insulating polyurethane aerogel with higher resistance to mechanical stress. In the first part, high performance silica and polyurethane aerogels have each been synthetized via a sol-gel route in mild conditions and in a reaction medium mainly consisting in acetonitrile, then dried in supercritical CO2, and characterized in terms of structure, morphology, texture, thermal conductivity and uniaxial compression. A work on the polyurethane aerogel formulation led to improved thermo-mechanical compromise (thermal conductivity 0.018 W.m-1.K-1, elastic modulus 7.8 MPa) . The mineral and organic matrixes were then coupled, at first in the form of composites (0.018 W.m-1.K-1, 6.1 MPa) based on a dry granular silica aerogel bead. An hybridization process was implemented between the silica and polyurethane phases, that included two successive steps of functionalization of the silica matrix (co-condensation followed by surface grafting and controlled growth of a secondary organic network). The obtained hybrid study was initiated via SEM and XPS analysis and showed a good cohesion at the interface between the polyurethane and the modified silica matrixes.; Les aérogels sont des matériaux légers et mésoporeux très prometteurs en termes d'isolation thermique. Il est vraisemblable qu'ils ont un rôle majeur à jouer dans la réduction des déperditions thermiques à travers les enveloppes de bâtiment et celles des équipements électrodomestiques. Dans ce contexte, l'objectif de ce travail de thèse est de développer un matériau conciliant une conductivité thermique très faible et des propriétés mécaniques compatibles avec une utilisation dans ce secteur applicatif, via un procédé d'hybridation organique-inorganique entre un aérogel de silice superisolant mais fragile et un aérogel de polyuréthane, également très isolant mais plus résistant aux sollicitations mécaniques. Dans une première partie, des aérogels de silice et de polyuréthane très performants ont été chacun élaboré par voie sol-gel en conditions douces dans un milieu réactionnel composé principalement d'acétonitrile, puis séchés au CO2 supercritique et caractérisés en termes de structure, morphologie, texture, conductivité thermique et compression uniaxiale. Un travail sur la formulation de l'aérogel de polyuréthane a permis d'atteindre un compromis thermo-mécanique très satisfaisant (conductivité thermique de 0,018 W.m-1.K-1, module élastique de 7,8 MPa) . Les matrices minérales et organiques ont ensuite été couplées, tout d'abord sous forme de composites (0,018 W.m-1.K-1, 6,1 MPa) à partir d'un lit granulaire de silice sèche. Un procédé d'hybridation a été mis en œuvre entre les matrices de silice et de polyuréthane, comportant deux étapes successives de fonctionnalisation de la matrice de silice (co-condensation puis greffage de surface et croissance contrôlée d'un réseau organique secondaire). L'étude de l'hybride obtenu par des observations MEB et XPS a été initiée et a montré l'existence d'une interface présentant une forte cohésion entre les deux matrices.

International audience; Les « bio-aérogels » sont des matériaux de type aérogels issus des polysaccharides, comme par exemple la cellulose ou la pectine et sont appelés Aérocellulose1,2 ou Aéropectine3. Ils sont préparés en plusieurs étapes : dissolution du polymère et gélification puis coagulation et séchage au CO2 supercritique. Ces bio-aérogels ont des propriétés structurales intéressantes comme une très grande porosité (> 90%), de faibles densités de 0.05-0.25 g/cm3 et de grandes surfaces spécifiques 200-300 m²/g ; mais également des propriétés mécaniques prometteuses avec un module d’Young de 10-30 MPa et une déformation plastique jusqu’à 60-70% avant la densification du réseau poreux. L’aérocellulose possède une large distribution de taille de pores, entre quelques dizaines de nanomètres jusqu’à plusieurs microns, ce qui ne permet pas d’atteindre des propriétés thermiques superisolantes (la conductivité thermique d’aerocellulose est autour de 0.030-0.035 W/(m.K)). Pour affiner la distribution de taille de pores, la cellulose dissoute dans le solvant 8%NaOH-eau a été réticulée chimiquement par l’épichlorhydrine. En jouant sur le degré de réticulation, les aérogels de cellulose obtenus sont plus finement nanostructurés et présentent des propriétés thermiques améliorées, avec une conductivité thermique autour de 0.025 W/(m.K). Les propriétés mécaniques de ces nouveaux matériaux sont également renforcées comparées à celles des aérocelluloses non-modifiées. La pectine est un polysaccharide naturel abondant et est un déchet de l’industrie agroalimentaire. L’aérogel de pectine correspondant, l’aéropectine, présente une plus faible conductivité thermique que l’aérocellulose (0.020 W/(m.K)) grâce à sa structure poreuse intrinsèquement plus nanostructurée.