G. Amaral-Labat

Electromagnetic shielding efficiency in Ka-band: carbon foam versus epoxy/carbon nanotube composites

Abstract. The wide application of microwaves stimulates searching for new materials with high electrical conductivity and electromagnetic (EM) interference shielding effectiveness (SE). We conducted a comparative study of EM SE in Ka-band demonstrated by ultra-light micro-structural porous carbon solids (carbon foams) of different bulk densities, 0.042 to 0.150u2009u2009g/cm3, and conventional flexible epoxy resin filled with carbon nanotubes (CNTs) in small concentrations, 1.5 wt.%. Microwave probing of carbon foams showed that the transmission through a 2 mm-thick layer strongly decreases with decreasing the pore size up to the level of 0.6%, due to a rise of reflectance ability. At the same time, 1 mm thick epoxy/CNT composites showed EM attenuation on the level of only 66% to 37%. Calculating the high-frequency axial CNTs’ polarizability on the basis of the idea of using CNT as transmission lines, we devised a strategy to improve the EM SE of CNT-based composites: because of the high EM screening of inner shells of multi-walled CNTs in the GHz range, it is effective to use either single-walled CNT or multi-walled CNTs with a relatively small number of walls (up to 15, i.e., those taking part in the EM interaction, if the CNT length is 20 μm).

Highly mesoporous organic aerogels derived from soy and tannin

The first organic aerogels natural at the 91% level, based on soy and tannin, are reported. Such materials were prepared from denatured soy protein which was formylated, crosslinked with flavonoid tannin at different pHs, gelled and cured at 85 °C, and finally dried with supercritical CO2. The resultant aerogels presented both low bulk density and high mesopore volumes. Morphology, pore texture, chemical structure and thermal performances have been investigated by SEM, envelope and helium pycnometry, nitrogen adsorption at 77 K, FTIR, elemental analysis, XPS, NMR and hot disk transient plane source methods, respectively. We show that soy–tannin aerogels are among the “greenest” organic aerogels combining low cost, renewable character, highly developed mesopore texture and good thermal performances.

Systematic studies of tannin–formaldehyde aerogels: preparation and properties

Abstract Gelation of tannin–formaldehyde (TF) solutions was systematically investigated by changing pH and concentration of TF resin in water. In this way we constructed the TF phase diagram, from which chemical hydrogels could be described, and also synthesized thermoreversible tannin-based hydrogels. Conditions of non-gelation were also determined. Hydrogels were dried in supercritical CO2, leading to a broad range of TF aerogels. The latter were investigated for volume shrinkage, total porosity, micro-, meso- and macropore volumes, Brunauer–Emmett–Teller (BET) surface area, microscopic texture, mechanical and thermal properties. All these properties are discussed in relation to each other, leading to an accurate and self-consistent description of these bioresource-based highly porous materials. The conditions for obtaining the highest BET surface area or mesopore volume were determined and explained in relation to the preparation conditions. The highest BET surface area, 880 m2 g−1, is remarkably high for organic aerogels derived from a natural resource.

Bimodal activated carbons derived from resorcinol-formaldehyde cryogels

Abstract Resorcinol-formaldehyde cryogels prepared at different dilution ratios have been activated with phosphoric acid at 450 °C and compared with their carbonaceous counterparts obtained by pyrolysis at 900 °C. Whereas the latter were, as expected, highly mesoporous carbons, the former cryogels had very different pore textures. Highly diluted cryogels allowed preparation of microporous materials with high surface areas, but activation of initially dense cryogels led to almost non-porous carbons, with much lower surface areas than those obtained by pyrolysis. The optimal acid concentration for activation, corresponding to stoichiometry between molecules of acid and hydroxyl groups, was 2 M l−1, and the acid–cryogel contact time also had an optimal value. Such optimization allowed us to achieve surface areas and micropore volumes among the highest ever obtained by activation with H3PO4, close to 2200 m2 g−1 and 0.7 cm3 g−1, respectively. Activation of diluted cryogels with a lower acid concentration of 1.2 M l−1 led to authentic bimodal activated carbons, having a surface area as high as 1780 m2 g−1 and 0.6 cm3 g−1 of microporous volume easily accessible through a widely developed macroporosity.

Adsorption by Carbon Gels

The present chapter deals with adsorption by carbon gels. In the introduction, we recall what carbon gels are and how they are prepared, with a special focus on those derived from phenolic resins. Details about drying process and resultant porous structure and bulk density are given. Adsorption as a characterization tool is considered in section 2 , especially nitrogen adsorption at 77xa0K. The main kinds of isotherms encountered with carbon gels are reviewed and discussed in relation to their most frequent porous structures. A critical comparison of porosity assessment methods is provided. In section 3 , we consider adsorption in the gas phase: adsorption of permanent gases with applications such as storage or separation, and adsorption of condensable vapors with the aim of abating or detecting them. Finally, adsorption in the liquid phase is discussed in section 4 : organics, halides, metals, as well as adsorption in the framework of energy storage.

Biosourced, highly porous, carbon xerogel microspheres

The first tannin-based carbon xerogel microspheres were prepared and characterised. The materials were synthesised by inverse emulsion polymerisation in sunflower oil, based on the same formulation but using two main independent variables: stirring speed and surfactant amount. The resultant sol–gel spheres were then washed, dried in air, and pyrolysed. The effect of stirring speed and surfactant amount on carbon microsphere size distribution and porous texture was investigated in detail. Depending on the cases, ultramicroporous carbon microspheres with extremely narrow pore size distributions centred at 0.4–0.5 nm, zero mesoporosity, negligible macroporosity and median diameters close to 40 μm, could be obtained. These characteristics are typical of expensive commercial carbon molecular sieves, whereas the present materials were prepared with cheap and renewable precursors using a very simple method.

New families of carbon gels based on natural resources

Carbon gels are versatile materials which can be used for many applications. They are extremely expensive, because generally prepared from resorcinol – formaldehyde (RF) resins first gelled and next dried with supercritical carbon dioxide. In the present work, resorcinol has been substituted partly or completely by tannins, a family of molecules extracted from mimosa tree barks. Tannins are natural, non-toxic products, typically thirty times cheaper than resorcinol. Their chemical resemblance with the latter makes them be often called natural resorcinol. Using tannins not only substantially decreases the cost but also allows preparing materials in a much wider range of pHs than that usually employed for RF gels. Consequently the main pore size and the fraction of given families of pores, controlling the carbon gels properties, are tuned in an easier way, and a much wider range of pore structures is obtained. Finally, two alternative ways of drying are suggested for further decreasing the cost: freeze-drying and supercritical drying in acetone. Both are shown to lead, in some conditions described below, to materials having similar characteristics to those of expensive RF carbon aerogels previously dried in supercritical CO2.