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  • 1
    Electronic Resource
    Electronic Resource
    Room Temperature Densification of Aerogel by Isostatic Compression (1998)
    Springer
    Journal of sol gel science and technology 13 (1998), S. 365-369 
    Details
    ISSN: 1573-4846
    Keywords: aerogel ; elastic properties ; plasticity
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract An alternative to the sintering process in densifying aerogels is the compaction by isostatic compression at room temperature. A porosimeter is used to compress the aerogel and to measure the amplitude of the shrinkage. We focus our study on the time dependent mechanisms such as densification kinetics and relaxation effects. These two mechanisms respectively increase or decrease the total densification. The densification kinetics is followed by the evolution of the volumetric shrinkage and stiffening with time. Hydroxylated and esterified aerogels show almost the same shrinkage evolution but the hydroxylated aerogels stiffen during compaction while the elastic bulk modulus of esterified aerogels decreases. Shrinkage is due to two opposing mechanisms: formation of new siloxane bonds that freeze the strained network, but also breakage of links between clusters which allows the restructuring of the solid phase. Relaxation is caused by the disentanglement of the clusters when the pressure is released (40% of the shrinkage can be recovered). However, if during the compression run a large quantity of siloxane bonds are formed, relaxation is not observed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1008648606382
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  • 2
    Electronic Resource
    Electronic Resource
    Mechanical Properties of Gel-Derived Materials (2000)
    Springer
    Journal of sol gel science and technology 19 (2000), S. 163-169 
    Details
    ISSN: 1573-4846
    Keywords: mechanical properties ; elastic properties ; aerogels
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract The mechanical behaviour of xerogels and aerogels is generally described in terms of brittle and elastic materials, like glasses or ceramics. The main difference compared to silica glass is the order of magnitude of the elastic and rupture moduli which are 104 times lower. However, if this analogy is pertinent when gels are under a tension stress (bending test) they exhibit a more complicated response when the structure is submitted to a compressive stress. The network is linearly elastic under small strains, then exhibits yield followed by densification and plastic hardening. As a consequence of the plastic shrinkage it is possible to densify and stiffen the gel at room temperature. These opposite behaviours (elastic and plastic) are surprisingly related to the same two kinds of gel features: the silanol content and the pore volume. Both elastic modulus and plastic shrinkage depend strongly on the volume fraction of pores and on the condensation reaction between silanols. On the mechanical point of view (rupture modulus and toughness), it is shown that pores and silanols play also an important role. Pores can be considered as flaws in the terms of fracture mechanics and the flaw size, calculated from rupture strength and toughness is related to the pore size distribution. Different kinds of gels structure (fractal or not fractal) have been synthesized by a control of the different steps of transformation such as sintering and plastic compaction. The relationships between structural and the elastic properties are discussed in terms of the percolation theory and fractal structure.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1008763616716
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  • 3
    Electronic Resource
    Electronic Resource
    Silica Dissolution-Redeposition in Gels and Aerogels (2000)
    Springer
    Journal of sol gel science and technology 19 (2000), S. 657-660 
    Details
    ISSN: 1573-4846
    Keywords: gels ; aerogels ; silica dissolution-redeposition
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract Silica dissolution-redeposition phenomenon is investigated on a microscopic scale using Small Angle Xray Scattering (SAXS) method. The changes occurring in the microtexture are different according to the procedure used to dry the gels. A xerogel, a CO2 supercritically dried aerogel and an alcohol supercritically dried aerogel were compared. Silica does not dissolve in CO2 supercritical. The transformation of heat treated gels into CO2 supercritically dried aerogels is demonstrated as having minor effect on the microtexture. SAXS data show oscillations around Porod law I(Q) ∝ Q −4 where Q is the wave vector. The oscillations are more pronounced for alcohol dried aerogel than for xerogel or CO2 dried aerogel. These oscillations are related to curvatures of the surface i.e. the surface roughness. It was found that according to silica dissolution extent, the solid network surface was continuously smoothened in alcohol supercritically dried aerogel. The average chord length of the solid phase is shifted to larger values while the distribution seems to be more narrow. This effect does not give rise to the formation of closed pores.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1008790002062
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  • 4
    Electronic Resource
    Electronic Resource
    Weibull analysis of the mechanical strength of silica gels (1994)
    Springer
    Journal of sol gel science and technology 2 (1994), S. 211-214 
    Details
    ISSN: 1573-4846
    Keywords: Weibull analysis ; mechanical properties ; toughness
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract Since gels are typically brittle materials, reliable analyses are essential to determine the strength distribution. The mechanical strength of silica gels has been measured by the three point bending technique. Based on the Weibull function, the statistical analysis allows determination of the Weibull modulus (m), which characterizes the strength distribution and the flaw size effect. Two kinds of gels, alcogels and aerogels, have been investigated with respect to their different mechanical behaviour owing to the transformations occurring during the supercritical drying (SD). The toughness of the two materials has been also measured by the SENB technique. It appears that the strength increases by a factor greater than two during SD, the KIC increases slightly. However, m is not greatly affected which would suggest that SD does not induce significant flaws or defects change in the network.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00486243
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  • 5
    Electronic Resource
    Electronic Resource
    The sintering of silica aerogels studied by thermoporometry (1994)
    Springer
    Journal of sol gel science and technology 2 (1994), S. 277-281 
    Details
    ISSN: 1573-4846
    Keywords: thermoporometry ; densification of aerogels ; PDA: Partially densified aerogels ; impregnation
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract The evolution of the texture of silica aerogels during sintering is studied by thermoporometry for both neutral and base catalysed materials. During the densification the macroporous volume drops and the analysis of the mesopore size distribution evolution shows that the collapse of the smallest mesopores is responsible for the macropore volume transformation. However, corrections of the measured volumes are necessary to characterize the most compliant materials.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00486256
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  • 6
    Electronic Resource
    Electronic Resource
    Raman study of structural defects in SiO2 aerogels (1995)
    Springer
    Journal of sol gel science and technology 5 (1995), S. 167-172 
    Details
    ISSN: 1573-4846
    Keywords: silica aerogels ; Raman spectroscopy ; structural defects ; 29Si NMR
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract The structure of the silica aerogels was studied by Raman spectroscopy. The spectra of the solid network resembles that of bulk silica with additional bands related to organic groups and a large amount of OH groups. The typical bands due to ring breathing also called defect bands D 1 and D 2 located at 490 and 610 cm−1 are present. However, the evolution of the D 2 band compared to that of OH band (980 cm−1) seems apparently, in contradiction with the results previously reported in the literature. During heat treatments between 25 and 300°C the D 2 and the OH bands increase simultaneously. Generally, in silica glass the defect band D 2 grows at the expense of the OH groups. This result is explained by the oxidation of the organic compounds which, in this temperature range, leads to the formation of the both species (OH) and those related to siloxane rings. 29Si MAS NMR results are in agreement with the Raman study.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00487013
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  • 7
    Electronic Resource
    Electronic Resource
    Plasticity in Aerogels (1997)
    Springer
    Journal of sol gel science and technology 8 (1997), S. 789-794 
    Details
    ISSN: 1573-4846
    Keywords: aerogels ; plasticity ; elasticity ; SAXS ; Hg porosimetry ; sintering
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: 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.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1018334107483
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  • 8
    Electronic Resource
    Electronic Resource
    Plasticity in aerogels (1997)
    Springer
    Journal of sol gel science and technology 8 (1997), S. 789-794 
    Details
    ISSN: 1573-4846
    Keywords: aerogels ; plasticity ; elasticity ; SAXS ; Hg porosimetry ; sintering
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: 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.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF02436939
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    Paper (German National Licenses)
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  • 9
    Electronic Resource
    Electronic Resource
    Association of OH groups with boron and silicon atoms in SiO2-B2O3 glasses by infrared spectroscopy (1989)
    Springer
    Journal of materials science 8 (1989), S. 1464-1467 
    Details
    ISSN: 1573-4811
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00720225
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  • 10
    Electronic Resource
    Electronic Resource
    Textural Properties of Densified Aerogels (1999)
    Springer
    Journal of sol gel science and technology 14 (1999), S. 249-256 
    Details
    ISSN: 1573-4846
    Keywords: aerogel ; densification ; texture and nitrogen adsorption-desorption
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract Base catalysed silica aerogels have been densified using both an isostatic pressure and a thermal treatment. The density range investigated corresponds to 0.186–0.65. Textural properties such as the specific surface area and the pore size distribution are analysed as a function of the sample's bulk density, using the N2 adsorption-desorption technique. A comparison between specific surface areas obtained previously by small angle X-ray scattering is done. Experiments show that the isostatic pressure leads to materials having a narrow pore size distribution while the specific surface area remains unchanged.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1008765428511
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