Densification behaviour of silica aerogels upon isothermal sintering

Emmerling, A.; Lenhard, Wolfgang; Fricke, J.; Vorst, G.A.L. van de

doi:10.1007/bf02436947

Cited by 9 publications

(5 citation statements)

References 12 publications

(13 reference statements)

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“…At lower temperatures (1200 and 1300 °C) F-20-30 samples shrink more (43−48%) than their more carbon-rich F-20-45 counterparts (30−40%) and that difference is reflected directly into higher bulk densities (∼0.60 g cm −3 versus ∼0.45 g cm −3 , respectively). To explain these shrinkage and density differences it is suggested that although all carbothermal reduction temperatures used are well above the typical sintering temperatures of sol−gel silica (∼1050 °C), , nevertheless the thicker C-coating of the F-20−45 samples prevents that process more effectively, in agreement with Koc’s report on reduced agglomeration of carbon-coated Carbosil, as summarized in the Introduction . The tendency of F-20-30 samples to shrink more and contain slightly more unreacted silica, as well as the relative ease by which more C-rich F-20-45 samples are converted to pure SiC (their ρ s values become within error equal to that of pure SiC even at 1300 °C for 72 h of processing), directed our further attention to the latter.…”

Section: Resultssupporting

confidence: 71%

“…After pyrolysis at 1300 °C the relative amount of SiC increases (the SiC:SiO 2 mol ratio reaches 6.6), and after pyrolysis at 1600 °C polycrystalline SiC (mostly 3C with a small amount of 6H) is the only detectable Si-phase. Importantly, using literature density values for silicon carbide (3.20 g cm −3 ) and sintered amorphous silica (2.1 g cm −3 ), , the SiC:SiO 2 mol ratios determined by 29 Si NMR can be converted to the skeletal densities expected of the final C-clean samples. Thus, it is calculated from the data of spectra B and C in Figure that the expected skeletal densities (ρ s ) of terminal free-of-carbon F-20-30 samples should be 2.969 and 3.054 g cm −3 for samples pyrolyzed at 1200 and 1300 °C, respectively.…”

Section: Resultsmentioning

confidence: 99%

“…The carbothermal reduction of eq should start either as a solid−solid or even perhaps as a liquid−solid reaction between SiO 2 and C . (It is noted that although quartz melts at about 1650 °C, nanoparticulate silica in base-catalyzed silica aerogels sinters completely at 1050 °C, , implying surface melting at much lower temperatures.) Hence, the reaction between SiO 2 and C, at least initially, depends critically on the contact area between the two reactants; from that perspective, the starting material of this study has been designed specifically to turn the entire mesoporous surface of silica into a contact area with C. There, the initial elementary process between SiO 2 and C should follow eq SiO 2 false( normals false) + normalC false( normals false) → SiO false( normalg false) + CO false( normalg false) Once SiO(g) is formed, synthesis of SiC could follow via a gas−solid reaction with carbon: SiO false( normalg false) + 2 normalC false( normals false) → SiC false( normals false) + CO false( normalg false) CO(g) from reactions and reacts with silica via eq , producing more SiO(g) SiO 2 false( normals false) + CO false( normalg false) → SiO false( normalg false) + CO 2 false( normalg false) whereasCO 2 comproportionates with C according to the Boudouard reaction − CO 2 false( normalg false) + normalC false( normals false) → 2 CO false( normalg f...…”

Section: Discussionmentioning

confidence: 99%

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Click Synthesis of Monolithic Silicon Carbide Aerogels from Polyacrylonitrile-Coated 3D Silica Networks

et al. 2010

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SiC retains high mechanical strength and oxidation stability at temperatures above 1500 °C, representing a viable alternative to silica, alumina, and carbon, which have been in use as catalyst supports for more than 60 years. Preparation of monolithic porous SiC is usually elaborate and porosities around 30% v/v are typically considered high. This report describes the synthesis of monolithic highly porous (70% v/v) SiC by carbothermal reduction (1200−1600 °C) of 3D sol−gel silica nanostructures (aerogels) conformally coated and cross-linked with polyacrylonitrile (PAN). Synthesis of PAN-cross-linked silica aerogels is carried out in one pot by simple mixing of the monomers, whereas conversion to SiC is carried out in a tube reactor by programmed heating. Intermediates after aromatization (225 °C in air) and carbonization (800 °C under Ar) were isolated and characterized for their chemical composition and materials properties. Data are interpreted mechanistically and were used iteratively for process optimization. Solids 29Si NMR validates use of skeletal densities (by He pycnometry) for the quantification of the conversion of silica to SiC. Consistent with the topology of the carbothermal process, data support complete conversion of SiO2 to SiC requiring a C:SiO2 ratio higher than the stoichiometric one (=3). The morphology of the SiC network is invariant of the processing temperature between 1300 and 1600 °C, and hence it is advantageous to carry out the carbothemal process at higher temperatures where reactions run faster. Those samples are macroporous and consist of pure polycrystalline β-SiC (skeletal density: 3.20 g cm−3) with surface areas in the range reported previously for biomorphic SiC (∼20 m2 g−1). Although the micromorphology remains constant, the crystallite size of SiC increases with processing temperature (from 7.1 nm at 1300 °C to 16.5 nm at 1600 °C). Samples processed at 1200 °C are mesoporous and amorphous (by XRD), even though they consist of ∼75% mol/mol SiC. The change in the morphology of SiC in the 1200−1300 °C range has been explained by a melting mechanism. This comprises the first report of using a polymer cross-linked aerogel for the synthesis of another porous material.

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Section: Resultssupporting

confidence: 71%

Section: Resultsmentioning

confidence: 99%

Section: Discussionmentioning

confidence: 99%

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Click Synthesis of Monolithic Silicon Carbide Aerogels from Polyacrylonitrile-Coated 3D Silica Networks

et al. 2010

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“…In the near future we also hope to say something about the applicability of the proposed mathematical approach in the case of sintering aerogels (cf. Emmerling et al [31]). In that paper experimental data will be validated using the rounded-off square pore unit cell of subsection 3.1.…”

Section: Resultsmentioning

confidence: 99%

Numerical Simulation of Viscous Sintering by a Periodic Lattice of a Representative Unit Cell

Vorst

1998

Journal of the American Ceramic Society

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In this paper numerical simulations of the viscous sintering phenomenon are presented, i.e. of the process that occurs (for example) during the densification of a porous glass heated to such a high temperature that it becomes a viscous fluid. The numerical approach consists of simulating the shrinkage of a two-dimensional unit cell which is in some sense representative for the porous glass. Hence it is assumed that the microstructure of the glass can be described by a periodic continuation in two directions of this unit cell. In this way it is possible to obtain quite a few theoretical insights of the viscous sintering process with respect to both pore size and pore distribution of the material. In particular this model is able to examine the consequences of microstructures on the evolution of the pore size distribution. It appears that only for higher densities the numerical densification rate of one cylindrical pore in a square unit cell is in agreement with the widely used closed pores model of Mackenzie and Shuttleworth. However, the major finding is that the pores vanish in order of size one afte~another; so the smallest pores first, followed by the larger ones. Moreover it is shown that pores with concave boundary parts may initially grow before they start shrinking at a later time stage.

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“…Surface areas and mechanical properties can be reconciled with those of mildly densied silica aerogels, as our processing temperatures were close to the densication temperature of silica ($900 C). 20 Only the crust of Mg-PAN aerogels exhibited signicant changes with a hardness of 27.7 MPa, a modulus of 374 MPa and a surface area of 169.23 m 2 g À1 . XRD of the core of all samples yielded a broad diffraction peak characteristic of poorly ordered silica (Fig.…”

Section: B Materials Characterizationmentioning

confidence: 98%

Regioselective cross-linking of silica aerogels with magnesium silicate ceramics

et al. 2013

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Nanoporous monoliths with a typical aerogel core and a mechanically robust exterior ceramic layer were synthesized from silica aerogels cross-linked with polyacrylonitrile (X-PAN aerogels). Following supercritical drying, X-PAN aerogels were wrapped in Mg and heated to 850 C in inert atmosphere to generate Mg-PAN monoliths. Mg penetrated 2-3 mm inside the monolith, reacted with the silica framework and formed a mechanically strong crust with a hardness of 28 MPa, a reduced Young modulus of 374 MPa and a surface area of 170 m 2 g À1 . The core of the monoliths maintained chemical and physical characteristics close to those of native silica aerogels with a hardness of 0.81 MPa, a modulus of 7.42 MPa and a surface area of 370 m 2 g À1 . The cross-linking crust was composed of forsterite (Mg 2 SiO 4 ), enstatite (MgSiO 3 ), cristobalite and magnesium oxide (MgO). SEM and TEM showed that the Mg silicates bridged between silica aggregates, thus realizing the first known example of ceramic crosslinking of aerogels. Control experiments showed that carbon from PAN was responsible for the crust formation. A thin, mechanically weak crust formed in all other samples and was composed of MgO and Mg 2 SiO 4 . EELS showed the presence of disordered, graphite-type carbon in Mg-PAN aerogels and traces of amorphous carbon in all other control samples. The role of PAN was confirmed by using masking techniques, whereas acrylonitrile was photopolymerized in selected regions of aerogel monoliths. The crust formed only in these exposed regions. The role of carbon in crust formation is discussed, as well as potential applications of our technique for the fabrication of anisotropic ceramics.

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Densification behaviour of silica aerogels upon isothermal sintering

Cited by 9 publications

References 12 publications

Click Synthesis of Monolithic Silicon Carbide Aerogels from Polyacrylonitrile-Coated 3D Silica Networks

Click Synthesis of Monolithic Silicon Carbide Aerogels from Polyacrylonitrile-Coated 3D Silica Networks

Numerical Simulation of Viscous Sintering by a Periodic Lattice of a Representative Unit Cell

Regioselective cross-linking of silica aerogels with magnesium silicate ceramics

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