Isothermal sintering of SiO2-aerogels

Emmerling, A.; Groß, Joachim; Gerlach, Robert W; Goswin, R.; Reichenauer, Gudrun; Fricke, J.; Haubold, H.‐G.

doi:10.1016/0022-3093(90)90853-e

Cited by 106 publications

(51 citation statements)

References 19 publications

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“…These results are clearly different from those obtained on a set of aerogels densified by sintering for which the cluster size [13] but also the particle size and the compactness is modified (a and D increase, decreases) [14]. The interpretation of the SAXS data suggests that during the densification by compression, the main restructuring is due to a new spatial arrangement of the clusters.…”

Section: E (Mp )contrasting

confidence: 74%

Plasticity in aerogels

Woignier

Duffours

Beurroies

et al. 1997

J Sol-Gel Sci Technol

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Abstract. When gently stressed, aerogels show an elastic response. However it was found that under isostatic pressure aerogels display an irreversible shrinkage which may be attributed to plastic behaviour. As a consequence of this plastic shrinkage it is possible to densify and modify the elastic properties of aerogels at room temperature.The structural evolution is followed by Small Angle X ray Scattering and the increase of the connectivity is revealed by the evolution of the elastic properties of the material.The SAXS data show that the densification mechanism is different from that obtained by sintering at high temperature. The densification mechanism induces a textural change at the periphery of the constitutive clusters but not inside, conversely to a sintering effect. We also show that the elasticity of the material is strongly influenced by this structural transformation. The power law evolution of the elastic modulus as a function of the density, usually observed on as-prepared and sintered aerogels, is not valid for compressed material.

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Section: E (Mp )contrasting

confidence: 74%

Plasticity in aerogels

Woignier

Duffours

Beurroies

et al. 1997

J Sol-Gel Sci Technol

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“…1c) [5] which is expected because the networked solid of the aerogel and ambigel inhibits particle growth. [25] Obtaining single-phase La 9.33 Si 6 O 26 using sol-gel chemistry, rather than a silica/lanthanum oxide nanocomposite, is important for the use of this class of silicates in SOFC electrolytes. Because we were unable to create gels by adding either TMOS to methanolic solutions of LaCl 3 [26] We propose a three-stage mechanism leading to the formation of lanthanum silicate gels: i) epoxide-promoted hydrolysis [15] to form La(OH) Our sol-gel protocol can be readily adapted to prepare composite materials or additional mixed metal oxides with complex formulations.…”

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confidence: 99%

Synthesis of La_9.33Si₆O₂₆ Pore–Solid Nanoarchitectures via Epoxide‐Driven Sol–Gel Chemistry

Célérier¹,

Laberty-Robert²,

Long³

et al. 2006

Advanced Materials

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Silicate-based oxides are of interest as electrolytes and electrode materials in intermediate-temperature solid-oxide fuel cells (SOFCs). [1][2][3][4][5][6][7] We have developed a sol-gel-based strategy for the production of mesoporous, nanostructured, singlephase La 9.33 Si 6 O 26 apatite in an aerogel-type framework. Silicon alkoxides react with lanthanum(III) aqueous ions in alcohol solutions driven by a proton-scavenging agent, propylene oxide. Varying the means by which pore fluid is removed from the resultant lanthanum silicate gels yields three distinct pore-solid monolithic nanoarchitectures: aerogels, ambigels, and xerogels. The ambigel and aerogel nanoarchitectures exhibit high specific surface areas (from 285 to 408 m 2 g -1), aperiodic through-connected networks of mesopores, and covalently bonded networks of non-agglomerated nanoparticles. Calcination at relatively mild temperatures for this oxide (800°C) converts the amorphous gels to nanocrystalline apatite La 9.33 Si 6 O 26 . Although densified during calcination, the aerogel and ambigel nanoarchitectures retain porosity, high surface area, and limited particle agglomeration. Aerogels and related highly porous architectures are defined by a bonded three-dimensional network of nanoparticles, commingled with interconnected porosity. [8][9][10] The inherent structural characteristics of aerogels are highly beneficial for electrochemical applications, [9] where the high interfacial surface area enhances the kinetics of electrochemical reactions and the continuous mesoporous network facilitates the transport of molecules, ions, and nanoscale objects to the active electrode surface while the solid network of covalently linked nanoparticles promotes charge transport of electrons and ions. It is only recently that aerogel-like materials have been synthesized in electrically conductive, SOFC-relevant compositions and then characterized at operating temperatures. [11,12] An earlier sol-gel route to mixed La-Si oxides modified an acid-catalyzed silica sol-gel method by including lanthanum salts. [13,14] Investigation of a range of hydrolysis rates showed that only very narrow synthesis conditions led to apatite La 9.33 Si 6 O 26 upon calcination (800°C), [1] and impurity phases were common. Subsequent attempts to produce aerogels or ambigels (requiring washing steps with ethanol) led to amorphous oxides after calcination (1000°C), suggesting that this protocol generates a silica network with the lanthanum salt dispersed on its surface.[1]Our approach to synthesize lanthanum silicate nanoarchitectures is based on the initial formation of oligomers of La-O-Si by using epoxide-and alkoxide-driven hydrolysis and condensation reactions. This synthetic protocol is a modification of innovative methods reported by Gash et al. for the preparation of transition metal oxide aerogels. [15][16][17] This route was recently extended to mixed transition metal oxide aerogels [11,18] and nanocomposites of silica and metal oxide. [19,20] We obtain gels by mixing tetramethoxysi...

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“…Elasticity and thus acoustical properties, for instance, depend on the interparticle arrangement and connectivity of the gel particles. The microstructural details can, in principle, be controlled during the gelation and aging periods of the sol-gel process (Brinker & Scherer, 1990), during densification via isothermal viscous sintering (Emmerling, Gross, Gerlach, Goswin, Reichenauer, Haubold & Fricke, 1990) and -as we report during the supercritical drying process.…”

Section: Introductionmentioning

confidence: 99%

High-temperature and low-temperature supercritical drying of aerogels – structural investigations with SAXS

Wang¹,

Emmerling²,

Tappert³

et al. 1991

J Appl Cryst

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Base-catalyzed silica aerogels are composed of particles with a mean size of about 5 nm, which form a chain-like porous network. Up to now it has been assumed that supercritical drying (SCD) of highly porous gels in autoclaves leaves the structure nearly unchanged. Small-angle X-ray scattering (SAXS) measurements provide evidence that this is only true for the low-temperature CO2 drying process with a critical temperature /',=304.2 K and a critical pressure p,--73.9 × 105 Pa. In the high-temperature methanol process (7", = 512.5 K and p, = 80.9 × 105 Pa) with remaining water and catalyst, however, structural changes are introduced. The SAXS measurements can be explained by a narrowing of the particle-size distribution during the heating period of the autoclave process. In a double-logarithmic representation of the scattered intensity I vs the scattering vector q, intermediate slopes smaller than -4 in the Porod region as well as oscillations in an lq 4 VS q plot appear. On the contrary, SCD hardly affects the particle structure of acid-catalyzed gels and, if the basic solvent is exchanged for pure methanol, of base-catalyzed gels.

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Isothermal sintering of SiO2-aerogels

Cited by 106 publications

References 19 publications

Plasticity in aerogels

Plasticity in aerogels

Synthesis of La_9.33Si₆O₂₆ Pore–Solid Nanoarchitectures via Epoxide‐Driven Sol–Gel Chemistry

High-temperature and low-temperature supercritical drying of aerogels – structural investigations with SAXS

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Isothermal sintering of SiO2-aerogels

Cited by 106 publications

References 19 publications

Plasticity in aerogels

Plasticity in aerogels

Synthesis of La9.33Si6O26 Pore–Solid Nanoarchitectures via Epoxide‐Driven Sol–Gel Chemistry

High-temperature and low-temperature supercritical drying of aerogels – structural investigations with SAXS

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Synthesis of La_9.33Si₆O₂₆ Pore–Solid Nanoarchitectures via Epoxide‐Driven Sol–Gel Chemistry