In the present review, aerogels designate dried gels with a very high relative pore volume. These are versatile materials that are synthesized in a first step by low-temperature traditional sol-gel chemistry. However, while in the final step most wet gels are often dried by evaporation to produce so-called xerogels, aerogels are dried by other techniques, essentially supercritical drying. As a result, the dry samples keep the very unusual porous texture which they had in the wet stage. In general these dry solids have very low apparent densities, large specific surface areas, and in most cases they exhibit amorphous structures when examined by X-ray diffraction (XRD) methods. In addition, they are metastable from the point of view of their thermodynamic properties. Consequently, they often undertake a structural evolution by chemical transformation, when aged in a liquid medium and/or heat treated. As aerogels combine the properties of being highly divided solids with their metastable character, they can develop very attractive physical and chemical properties not achievable by other means of low temperature soft chemical synthesis. In other words, they form a new class of solids showing sophisticated potentialities for a range of applications. These applications as well as chemical and physical aspects of these materials were regularly detailed and discussed in a series of symposia on aerogels,1-5 the last of them being held in Albuquerque in 2000.6 Reviews were also regularly published, either on both xerogels and aerogels7 or more focused on the applications of aerogels.8-13 The particularly interesting properties of aerogels arise from the extraordinary flexibility of the solgel processing, coupled with original drying techniques. The wet chemistry is not basically different for making xerogels and aerogels. As this common basis has been extensively detailed in recent books,14 it does not need to be reviewed. Compared to traditional xerogels, the originality of aerogels comes from * To whom all correspondence should be addressed. † Institut de Recherches sur la Catalyse. ‡ Laboratoire d’Application de la Chimie à l’Environnement. 4243 Chem. Rev. 2002, 102, 4243−4265

https://is.muni.cz/el/1431/podzim2006/C7780/um/Read/2711172/aerogel_Pajonk_ChRev02_4243.pdf

Chemistry of Aerogels and Their Applications

Ultralight multiwalled carbon nanotube (MWCNT) aerogel is fabricated from a wet gel of well-dispersed pristine MWCNTs. On the basis of a theoretical prediction that increasing interaction potential between CNTs lowers their critical concentration to form an infinite percolation network, poly(3-(trimethoxysilyl) propyl methacrylate) (PTMSPMA) is used to disperse and functionalize MWCNTs where the subsequent hydrolysis and condensation of PTMSPMA introduces strong and permanent chemical bonding between MWCNTs. The interaction is both experimentally and theoretically proven to facilitate the formation of a MWCNT percolation network, which leads to the gelation of MWCNT dispersion at ultralow MWCNT concentration. After removing the liquid component from the MWCNT wet gel, the lightest ever free-standing MWCNT aerogel monolith with a density of 4 mg/cm(3) is obtained. The MWCNT aerogel has an ordered macroporous honeycomb structure with straight and parallel voids in 50-150 μm separated by less than 100 nm thick walls. The entangled MWCNTs generate mesoporous structures on the honeycomb walls, creating aerogels with a surface area of 580 m(2)/g which is much higher than that of pristine MWCNTs (241 m(2)/g). Despite the ultralow density, the MWCNT aerogels have an excellent compression recoverable property as demonstrated by the compression test. The aerogels have an electrical conductivity of 3.2 × 10(-2) S·cm(-1) that can be further increased to 0.67 S·cm(-1) by a high-current pulse method without degrading their structures. The excellent compression recoverable property, hierarchically porous structure with large surface area, and high conductivity grant the MWCNT aerogels exceptional pressure and chemical vapor sensing capabilities.

Ultralight multiwalled carbon nanotube aerogel.

Silica aerogels are among the lightest weight solid materials and exhibit other unique properties, in particular an outstanding insulation performance. However, these nanostructured highly porous materials show inherent brittleness that makes their processing and handling difficult and constrains their use in common daily applications. One of the most convincing and effective strategies to reinforce the silica aerogel structure is the manufacturing of aerogel composites with embedded fibres, which significantly contributes to widening their applications. This review encompasses a complete survey on scientific achievements related to silica aerogel composites reinforced with all types of fibres (natural, man-made, organic, inorganic and nanofibres), describing their synthesis approaches and properties. The influence of the embedment of different fibres on the final properties of the composites is thoroughly reported, considering their amount in the matrix and/or their specific characteristics, namely morphology, orientation and optical features. The applications linked to the fibre-reinforced silica aerogel composites are briefly addressed as well, evincing developments in typical functions of aerogels, as thermal and/or acoustic insulators and adsorbents, and also in emerging uses, for example as sensors or technical and protective apparel.

Silica aerogel composites with embedded fibres: a review on their preparation, properties and applications

Silicacellulose hybrid aerogels for thermal and acoustic insulation applications

Silica aerogels are popular in terms of production volume and real-world applications. Although the current market growth rate is driven exclusively by thermal insulation, aerogels may also be attractive for acoustic applications with the potential in aiding sound absorption/insulation. This paper is a summary of the acoustics related studies of silica aerogel-based products. It introduces silica aerogels, some acoustic characterization methods, and reviews systematically the available data on sound absorption/insulation of silica aerogels, polymer-silica aerogel composites, nonwoven-silica aerogel blankets, and aerogel renders/glazing. The work identifies areas where further research is required, including experimental and theoretical work on the physics of sound absorption in mesoporous materials, and more systematic and standardized evaluations of the acoustic properties of aerogel and aerogel-composites. Aside from this call to action, the opportunities and barriers for the commercialization of silica aerogel products for acoustic applications are presented.

A review on silica aerogel-based materials for acoustic applications

The longitudinal acoustic velocity in silica aerogel is presented as a function of the interstitial gas type and pressure. This was measured using air-coupled ultrasonic transducers configured for differential pulse transit time measurements. The results are interpreted in terms of the thermal relaxation of the acoustic pulse. The microscale temperature oscillations of the gas and solid phases of the aerogel due to the acoustic pulse are not identical if the rate of heat transfer between the two phases is slow compared to the period of the acoustic oscillation. The energy transferred from the gas to the solid phase is lost to the acoustic propagation and, thus, reduces the amplitude and velocity of the acoustic wave. The gas type and pressure may provide independent variables for probing these effects in aerogel.

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Microscale thermal relaxation during acoustic propagation in aerogel and other porous media

Many cell types regulate their volume in response to extracellular tonicity changes through a complex series of adaptive mechanisms. Several methods that are presently used to measure cell volume changes include Coulter counters, fluorescent techniques, electronic impedance, and video microscopy. Although these methods are widely used and accepted, there are limitations associated with each technique. This paper describes a new method to measure changes in cell volume based on the principle that fluid flow within a rigid system is well determined. For this study, cos-7 cells were plated to line the inner lumen of a glass capillary and stimulated to swell or shrink by altering the osmolarity of the perfusing solution. The cell capillary was connected in series with a blank reference capillary, and differential pressure changes across each tube were monitored. The advantages of this method include 1) ability to continuously monitor changes in volume during rapid solution changes, 2) independence from cell morphology, 3) presence of physiological conditions with cell surface contacts and cell-cell interactions, 4) no phototoxic effects such as those associated with fluorescent methods, and 5) ability to report from large populations of cells. With this method, we could detect the previously demonstrated enhanced volume regulation of cells overexpressing the membrane phosphoprotein phospholemman, which has been implicated in osmolyte transport.

Scott C. Davis

Papers

Microscale thermal relaxation during acoustic propagation in aerogel and other porous media

A novel method for measuring dynamic changes in cell volume

volumemethod for measuring dynamic changes in cell