Mechanical and Acoustic Properties of Silica Aerogel

Gronauer, M.; Kadur, A.; Fricke, J.

doi:10.1007/978-3-642-93313-4_22

“…This power law evolution has been observed whatever the kind of catalyst used [4--6] but also in the case of sintered aerogels [4]. The exponent value has been discussed in terms of percolation theory [4,7] and also fractal structures [8,9]. A simple non fractal model of disordered networks of elementary units like rods, plates and plates of rods, has also been proposed to account for the exponent [10].…”

Section: Introductionmentioning

confidence: 96%

Plasticity in aerogels

Woignier

¹

,

Duffours

²

,

Beurroies

³

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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“…A variety of technical applications are currently being investigated and optimized with respect to transparent superinsulating systems for solar architecture (Heinemann, Hfimmer, Bfittner, Caps & Fricke, 1986), substitutes for CFC blown polyurethane foams (Fricke, Arduini-Schuster, Bfittner, Ebert, Heinemann, Hetfleisch, Hfimmer, Kuhn & Lu, 1989) and acoustic impedance-matching layers (Gronauer & Fricke, 1986).…”

Section: Introductionmentioning

confidence: 99%

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

Wang¹,

Emmerling²,

Tappert³

et al. 1991

Self Cite

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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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“…The particular network structure of aerogels results in impressive acoustic properties which, as mentioned above, correlate with the elastic properties [236]. The sound velocities in SiO 2 aerogels of 100 -300 m/s are among the lowest for inorganic solids (which is noteworthy when one considers that sound velocities of 5000 m/s were measured in quartz glass) [237], [238].…”

Section: Methodsmentioning

confidence: 99%

Aerogels

Hüsing

¹

,

Schubert

²

2006

Ullmann's Encyclopedia of Industrial Chemistry

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The article contains sections titled: 1. Introduction 2. Synthesis 2.1. Network Formation and Structure 2.1.1. Inorganic Aerogels 2.1.1.1. SiO 2 Aerogels 2.1.1.2. Metal Oxide Aerogels 2.1.2. Inorganic/Organic Hybrid Aerogels 2.1.3. Organic Aerogels 2.2. Drying Methods 2.2.1. Supercritical Drying 2.2.1.1. Drying in Organic Solvents 2.2.1.2. Supercritical Drying with CO 2 2.2.2. Freeze Drying 2.2.3. Ambient Pressure Drying 2.3. Modification of Aerogels after Drying 3. Properties 3.1. Structural Properties 3.2. Physical and Chemical Properties 3.2.1. Optical Properties 3.2.2. Mechanical and Acoustic Properties 3.2.3. Thermal Conductivity 3.2.4. Hydrophobicity 4. Applications 4.1. Çerenkov Detectors 4.2. Thermal Insulation 4.3. Catalysis 4.4. Application as Storage Media 4.5. Electrical and Electronic Applications 4.6. Acoustic and Mechanical Applications 4.7. Optical Applications 4.8. Other Applications 5. Future Perspectives Aerogels are unique among solid materials. They have extremely low densities (up to 95 % of their volume is air), large open pores, and a high inner surface area. This combination of properties results in interesting physical properties, e.g., for silica aerogels extremely low thermal conductivities and low sound velocities are combined with optical transparency. Aerogels are typically prepared via wet chemical processes, such as sol – gel processing, and thus their pore system is initially filled with liquid. Only special drying techniques allow for exchange of the pore liquid against air while maintaining the filigrane solid framework. The most common drying technique is supercritical extraction, and sometimes chemical modification of the inner surface can be used to facilitate drying. The structure of the gel network, and thus the physical properties of aerogels, decisively depends on the choice of the precursors and the chemical reaction parameters for preparing the gels. Therefore, the later materials properties can be deliberately designed in the starting reaction solution.

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Mechanical and Acoustic Properties of Silica Aerogel

Cited by 54 publications

References 7 publications

Plasticity in aerogels

Plasticity in aerogels

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

Aerogels

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