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

Formation of a porous transparent Al2O3 from aluminium alkoxides has been previously reported. During the process, alkoxides are hydrolyzed and the resultant hydroxide is peptized to a clear sol. The sol then must be gelled and pyrolyzed to 500° C to obtain the aluminium oxide. This paper discusses the gel state and the requirements for the system to retain its integrity during the drying and pyrolysis. Influence of electrolytes on the sol-gel transformation shows that there is a critical electrolyte concentration at which the gelling volume goes through a pronounced minimum. Deviation in either direction of this electrolyte concentration causes a sharp increase in the relative gelling volume and detrimentally effects the capability of the gel to retain its integrity. The sols that gel at concentrations less than ∼4 g equivalent oxide per 100 ml do not retain their integrity during pyrolysis.

Alumina gels that form porous transparent Al2O3

The invention relates to synthetic, non-fused, aluminum oxide-based abrasive mineral having a microcrystalline structure of randomly oriented crystallites comprising a dominant continuous phase of α-alumina and a secondary phase, to a method of making the same employing chemical ceramic technology, and to abrasive articles made with the abrasive mineral.

Non-fused aluminum oxide-based abrasive mineral

Alumina aerogels were prepared through the addition of propylene oxide to aqueous or ethanolic solutions of hydrated aluminum salts, AlCl3·6H2O or Al(NO3)3·9H2O, followed by drying with supercritical CO2. This technique affords low-density (60−130 kg/m3), high-surface-area (600−700 m2/g) alumina aerogel monoliths without the use of alkoxide precursors. The dried alumina aerogels were characterized using elemental analysis, high-resolution transmission electron microscopy, powder X-ray diffraction, solid-state NMR, acoustic measurements, and nitrogen adsorption/desorption analysis. Powder X-ray diffraction and TEM analysis indicated that the aerogel prepared from hydrated AlCl3 in water or ethanol possessed microstructures containing highly reticulated networks of pseudoboehmite fibers, 2−5 nm in diameter and of varying lengths, whereas the aerogels prepared from hydrated Al(NO3)3 in ethanol were amorphous with microstructures comprised of interconnected spherical particles with diameters in the 5−15 nm ra...

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Synthesis of High-Surface-Area Alumina Aerogels without the Use of Alkoxide Precursors

Aerogels: production, characterization, and applications

A study of hydrolysis of aluminium alkoxides as a function of water temperature, and structural transformation of the resultant hydroxides was conducted. Hot water hydrolysis of aluminium alkoxides produces boehmite, whereas cold water hydrolysis results in formation of amorphous monohydroxide. This amorphous phase contains OR groups whose removal by water during aging causes the material to convert to bayerite or boehmite.

Bayerite conversion takes place below 80 °C by a dissolution-recrystallisation process in which the gradual liberation of OR groups by water appears to play an important role. Boehmite conversion takes place above 80 °C where vigorous liberation of OR groups occurs and the slow stoichiometric bayerite conversion diminishes. Neither conversion takes place in pure alcohol.

Hydrolysis of aluminium alkoxides and bayerite conversion

The α-AI2O3 transformation of a monolithic active alumina has been increased from 1200 to ∼1380‡ C through structural incorporation of silica. This shift is significant since α-Al2O3 transformation determines the limits of the usefulness of these materials as catalysts and catalyst carriers. The thermal stabilization effect is optimized at around 6% silica doping. At elevated temperatures, the material containing no silica rapidly loses surface area, primarily by α-Al2O3 transformation, whereas the material containing excess silica loses surface area by classical sintering.

Thermal stabilization of an active alumina and effect of dopants on the surface area

A transparent, activated, nonparticulate alumina with a total porosity of about 63% which consists of a unique pore morphology and size distribution and is thermally stable up to about 1,200°C. at which temperature it can be nondestructively converted to alpha alumina is disclosed as well as a method of preparing said alumina by hydrolyzing aluminum alkoxides to form a particular sol which is essentially clear to the naked eye and the gel of which retains its integrity during drying and pyrolysis. The alumina thus produced is useful as a catalyst, absorbent and desiccant. In addition, a method of preparing the intermediate clear colloidal sol or gel consisting essentially of aluminum monohydrate is disclosed. This intermediate can be used to coat various substrates.

Transparent activated nonparticulate alumina and method of preparing same

Impregnated refractory structures, such as, for example, bricks which have increased resistance to the destructive action of molten glass are disclosed. The structures are produced by an impregnation and densification process in which conventional, prefabricated, solid, porous, refractory structures are first treated with a solution of a polysilicon compound which is thermally decomposable to silica, and the solution-treated structure is then heat treated, to cause a deposition of amorphous silica in the brick pores. Because of the silica deposition and pore impregnation, a densification and decrease in porosity is effected which greatly decreased molten glass penetration and internal destruction of the silica impregnated refractory structure. The use of these impregnated structures also produce a glass with substantially fewer seeds. A particularly suitable polysilicon compound which may be employed is a hydrolyzed and condensed ethyl silicate.

Bulent E. Yoldas

Papers

Alumina gels that form porous transparent Al2O3

Hydrolysis of aluminium alkoxides and bayerite conversion

Thermal stabilization of an active alumina and effect of dopants on the surface area

Transparent activated nonparticulate alumina and method of preparing same

Refractory article and method for producing same