Aerogel characterization methods

Comment. Gas sorption analysis is a well-established tool for the characterization of open porous solids. For aerogels the method provides reliable information on the surface area. However, care has to be taken in case of microporous aerogels here a well-equilibrated isotherm in combinatiOTi with the right choice of the evaluation range will still yield reliable values for the microporosity and the specific surface area. For detailed analysis of microporosity measurements with CO2 at 273 K are recommended. 

In this chapter we review the principles of monolith fabrication, routes for febrication of monoUths such as xerogel monoliths, nanocomposite monoliths, ORMOSIL monohths, hybrid monoliths, and aerogel monohths. In addition, we will present several characterization methods of monohths, encapsulation or doping in monohths, and examples of apphcations of monohths. 

O. B. Koper, I. Lagadic, A. Volodin, and K. J. Klabunde, Alkaline-earth oxide nanoparticles obtained by aerogel methods. Characterization and rational for unexpectedly high surface chemical reactivities, Chem. Mater. 9, 2468-2480 (1997). 

Sol-gel and dip-coating procednres were apphed to prepare copper nanomaterials. Alnmina-snpported copper nanoparticles from Cn(acac)2 and Al(OPr-i)3 were synthesized by Kantam and coworkers by the aerogel procednre. The Cn-Al203 nanoparticles were characterized by several analytical methods and were snccessfuUy nsed as catalysts for the preparation of 1,2,3-triazoles by the reaction of terminal alkynes, NaN3 and alkyl/allyl halides. 

The application of electrochemical investigations is an efficient method to characterize the microstructure of porous electrically conducting materials. Modifications in the chemical constitution of the skeleton which do not change the BET values can be identified by the electrochemically derived surface capacitance, as demonstrated in the case of anodically oxidized carbon aerogels. 

The dependence of E of RF aerogels on water content has to be investigated further. The described investigations are not only a method to characterize the material and the parameters of the inner surface (chemistry, surface morphology, pores etc.) but might also be the first step towards RF aerogels as sensor for humidity or environmental pollution. 

To reveal the thermal properties of aerogels, stationary hot-plate measurements are usually employed [45]. In such a measurement two equal aerogel specimens are sandwiched between a hot plate and two cold plates. If the electrical power fed into the hot plate and the temperature difference between the hot and the cold plates, as well as the thickness of the specimens, are known, the thermal conductivity can be derived. For the thermal characterization of opacified aerogels, the faster nonstationary hot-wire method can also be used. In this case a thin platinum wire is embedded into the aerogel specimen and a constant power is delivered into the wire, which also serves as a temperature sensor. From the temperature increase in the wire as a function of time, the thermal conductivity of the aerogel specimen can be determined [49]. 

In this chapter, we focus on the importance of nanoarchitecture in determining the electrochemical properties of transition metal oxide aerogels. After a brief introduction to aerogels and their synthesis methods we give an overview of the electrochemical characterization techniques employed when dealing with nanostructures. This chapter concludes with a case study on how aerogel nanostructures have improved the electrochemical properties of vanadium pentoxide. 

El Rassy H, Buisson P, BouaU B, Perraid A, Pierre AC (2(X)3) Surface Characterization of Silica aerogels with Different Proportions of Hydrophobic Groups, dried by the CO2 Supercritical Method Langmuir, 19 358-363 Harreld JH, Ebina T, Tsubo N, Stucky G (2(X)2) Manipulation of pore size distributions in silica and ormosil gels dried under ambient pressure crmditions. J Non-Cryst Solids 298 241 251... 

El Rassy, H, Buisson, P, Bouali, B, Perrard, A, Pierre, A C (2003) Surface characterization of silica aerogels with different proportions of hydrophobic groups, dried by the CO2 supercritical method. Langmuir 19(2) ... 

Mesoporous aerogels based rai sidfated zirconia doped with chromium have been synthesized by sol-gel method and dried in supercritical conditions of the solvent by Ghorbel et al. [45]. The characterization results revealed that Zr02 tetragonal phase is stabilized by both sulfate groups and metal at high temperature. The physisorption of N2 showed that the sol-gel procedure coupled to the drying under supercritical conditions of the... 

Pietron J J, Rolison D R (2004) Improving the efficiency of titania aerogel-based photovoltaic electrodes by electrochemically grafting isopropyl moieties on the titania surface. J Non-Cryst Solids 350 107-112 Tursiloadi S, Imai H, Hirashima H (2004) Preparation and characterization of mesoporous titania-alumina ceramic by modified sol-gel method. J Non-Cryst Solids 350 271-276... 

In between, additional methods have been developed or adjusted for the structural characterization of aerogels. Particularly interesting is here the beam bending that allows determining mechanical and structural characteristics already in the gel state. 

For the characterization of the sample with optical and X-ray scattering methods a 1 nun thick slice was cut from the aerogel with a diamond saw. The adsorption was performed by placing the cuvette for the optical measurement holding the sample next to a reservoir of 2-propanol for several hours. The amount of 2-propanol adsorbed was determined by weighing. After each sorption step the sample was sealed in the cuvette. 

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