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Comparison between sintered and compressed aerogels
Abstract
Aerogels can be densified either by thermal sintering, or at room temperature by isostatic compression. We report here a comparative analysis of silica aerogel densified by these two methods. To better follow their structural evolutions we use SAXS measurements performed on aerogels exhibiting a fractal geometry. This fractal geometry specially gives information about the way the solid network is firstly established and how it evolves with densification. The structural features such as particle and cluster sizes are observed to change differently according to the densification method. While the specific surface area of sintered aerogels decreases with densification, it does not change when densification is performed under isostatic compression. Furthermore the pore size distribution analysis evidences that the pressure induces the collapse of the largest pores while sintering acts on all pores. A microscopic model is proposed. It allows to explain the structural changes observed both by isostatic compression and by thermal sintering. Moreover, it agrees well with the evolution of elastic constant and internal friction with densification.
... Silica aerogels can be modified by physical or chemical processing to exhibit enhanced properties. Phalippou et al. 28 showed that sintering improves the mechanical properties of aerogel and that Y ∝ ρ m with m ≈ 3.3. Miner et al. 29 showed that the Young's modulus of hygroscopic aerogels increases with relative humidity. ...
... These data are shown in Figure 2. These exponents are comparable with those found experimentally, 8,15,18,20,28 and agree within uncertainties with those found for the bulk modulus in previous work. 38 (5) a d is the primary particle diameter, α gel is the volume fraction of the gels, ρ a is the gel density, and Y HMC and Y MD are the Young's modulus calculated by HMC simulations and by MD simulations (with ν = 0.20), respectively. ...
... The moduli of the model materials displays a power-law dependence on aerogel density with an exponent of approximately 3.0, nearly independent of constituent particle size. This exponent is within the range reported in the literature 8,15,18,20,28 and very similar to that previously found for the bulk modulus in the same models. 38 Measurements of modulus and Poisson ratio along different axes indicated that these properties are isotropic to within the uncertainty of the data, though there is rather more anisotropy for the low-density materials than for the high-density ones. ...
Simulations of a flexible coarse-grained model are used to study silica aerogels. This model, introduced in a previous study (J. Phys. Chem. C 111 [2007] 15792), consists of spherical particles which interact through weak nonbonded forces and strong interparticle bonds that may form and break during the simulations. Small-deformation simulations are used to determine the elastic moduli of a wide range of material models, and large-deformation simulations are used to probe structural evolution and plastic deformation. Uniaxial deformation at constant transverse pressure is simulated using two methods: a hybrid Monte Carlo approach combining molecular dynamics for the motion of individual particles and stochastic moves for transverse stress equilibration; and isothermal molecular dynamics simulations at fixed Poisson ratio. Reasonable agreement on elastic moduli is obtained except at very low densities. The model aerogels exhibit Poisson ratios between 0.17 and 0.24, with higher-density gels clustered around 0.20, and Young's moduli that vary with aerogel density according to a power-law dependence with exponent near 3.0. These results are in agreement with reported experimental values. The models are shown to satisfy the expected homogeneous isotropic linear-elastic relationship between bulk and Young's moduli at higher densities, but there are systematic deviations at the lowest densities. Simulations of large compressive and tensile strains indicate that these materials display a ductile-to-brittle transition as the density is increased, and that the tensile strength varies according to a power law with density, with exponent in reasonable agreement with experiment. Auxetic behavior is observed at large tensile strains in some models. Finally, at maximum tensile stress very few broken bonds are found in the materials, in accord with the theory that only a small fraction of the material structure is actually load-bearing.
... Wei et al [7], Cubic array of Standard equivalent model Lu et al [8], nano-spherical structure circuit method Zeng et al [15] Numerical model Zhao et al [10], Von Koch snowflake Finite volume method based on Spagnol et al [21,22] fractal structure or the mesh division random DLCA structure Analytical Present 3D random DLCA Equivalent circuit method and model study structure improved parallel-series model conductivities, which are even lower than that of air under ambient conditions [21][22][23][24][25][26][27][28]. Silica aerogels have great potential for use in space vehicles, nuclear reactors, and building and freezer superthermal insulations [1][2][3][4][5][6][7][29][30][31][32][33][34][35][36][37][38][39][40]. Thermal conductivity models for silica aerogels are needed to understand and predict the relationship between the superthermal insulation properties and the microstructures. ...
... The typical silica aerogel microstructural characteristics with intersecting nanoparticles and refined open-cell nanopores can be retained well with no significant shrinkage for T < 1300 K. However, figures 1 and 2 show that the typical nanoporous aerogel microstructure tends to vanish with significant volume shrinkage and particle sintering due to the viscous flow for T = 1300-1500 K [37]. The amorphous SiO 2 rapidly turns into the crystalline SiO 2 for T > 1300 K [37][38][39][40]. ...
... However, figures 1 and 2 show that the typical nanoporous aerogel microstructure tends to vanish with significant volume shrinkage and particle sintering due to the viscous flow for T = 1300-1500 K [37]. The amorphous SiO 2 rapidly turns into the crystalline SiO 2 for T > 1300 K [37][38][39][40]. Thus, the crystallization and densification of silica aerogels for T > 1300 K dramatically increase the aerogel density (figure 2(a)) and the average diameter of the secondary nanoparticles (figure 2(c)), significantly reducing the specific surface area (figure 2(b)) to nearly zero. ...
An analytical heat transfer model based on scanning electron microscopy, Brunauer–Emmett–Teller and pycnometry measurements and a 3D random diffusion-limited cluster–cluster aggregation structure is proposed to calculate the temperature-dependent microstructural parameters and thermal conductivities of silica aerogels. This model is a pure prediction model, which does not need experimentally fitted empirical parameters and only needs four measured structural parameters as input parameters. This model can provide high-temperature microstructural and thermophysical properties as well as theoretical guidelines for material designs with optimum parameters. The results show that three stages occur during the thermal evolution processes of the aerogel structure with increasing temperature from 300 to 1500 K. The current analytical model is fully validated by experimental data. The constant structure assumptions used in previous heat transfer models are found to cause significant errors at higher temperatures as the temperature-dependent structure deformation significantly increases the aerogel thermal conductivity. The conductive and total thermal conductivities of silica aerogels after high-temperature heat treatments are much larger than those with no heat treatment.
... Both densification methods resulted in a shift of the pore distribution to smaller pores. Phalippou et al. [25] also studied the effect of thermal sintering and isostatic compression on silica aerogels. They attribute densification in sintering to viscous flow, which Fig. 3 Photographs of (a) grayscale variable shadow pattern used to etch aerogels at a speed of 100% for power levels of (b) 40% (20 W), (c) 55% (27.5 W), (d) 75% (37.5 W) and at speed levels of (e) 60% (125 cm/s) and (f) 90% (187 cm/ s) for a laser cutter power of 55% (27.5 W). ...
... In addition, the silica aerogels used in the present study are less dense (0.09 g/cm 3 vs. 0.27 g/cm 3 [24] and 0.19 g/cm 3 [25]). ...
Silica aerogels are unique materials with characteristics that allow them to be used in a wide variety of applications. They are nanoporous, with low density, large surface area, low thermal conductivity, and they are relatively translucent. Recently, the use of a CO2 engraving and cutting system to etch a variety of patterns, including text and photographs, onto the surface of silica aerogel with minimal damage to the bulk aerogel was demonstrated; however, the mechanism by which the aerogel surface is altered was not understood and the extent of the damage not quantified. In this paper we present results on the effect of etching on, and cutting through, silica aerogel material with a CO2 laser engraving and cutting system, using scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR) and gas adsorption analysis. SEM analyses of the etched aerogel material show evidence of melting. Surface areas of the etched portion of aerogel monoliths are lower compared that the unetched material while pore diameters are larger. FTIR shows that little structural change occurs on the molecular level during etching of the silica aerogel.
Silica aerogel can be etched using a CO2 laser engraver and cutting system: (a) a photograph of aerogel etched with a geometric pattern shows no damage to the bulk material; (b) an SEM micrograph of a single laser pulse indicates that some of the material is removed via vaporization; and (c) a higher magnification SEM micrograph of an etched section of the aerogel shows evidence of melting/sintering due to interaction of the laser pulse with the surface.
... Many studies have employed theoretical calculations and experimental models to investigate the sintering process of aerogels. Scherer [40][41][42] proposed cell models for the viscous sintering of silica aerogels; Rosa-Fox et al. [43] proposed an aggregation model to study the aggregation process of silica aerogels during sintering; Sempéré et al. [44] proposed a simple sintering model for connected fractal aggregate materials; and Phalippou [45] proposed a structural evolution model to explain the sintering and compressing process. In these previous studies, the structure and properties of silica aerogels have been systematically analyzed after the heat treatment process, which has spanned a range of heating temperatures, along with a determination of the maximum operating temperature for silica aerogels and suggestions for improving the temperature resistance of aerogels. ...
... The surface free energy is minimized when the temperature is increased by reducing the specific surface area of the aerogels, which allows the aerogels to achieve a stable state. The shrinkage of SiO 2 aerogels is primarily caused by the shrinkage of primary particles, and a sintering model is proposed to describe this process [45]. Necking occurs in the contact points of the primary particles during the heat treatment process. ...
The direct in situ TEM imaging method is adopted to investigate the sintering behavior of SiO2 aerogels during the rapid heating process. The structural evolution of SiO2 aerogels and composites at different times during the heat treatment process are further investigated via SEM, FT-IR, BET and XRD. The results indicate that the shrinkage of the SiO2 aerogels and composites primarily occurs during the initial stage of the heating process (within 20 min) with the shrinkage primarily linked to the fusion of small aerogel particles at high temperatures. The aerogel structure then stabilizes with no further shrinkage observed as the heating process continues. The heat treatment process only promotes the space rearrangement and fusion of small aerogel particles with no observed changes to the amorphous structure of the aerogels, and the small-sized particles fusion was the main causes for the structural evolution of SiO2 aerogels and composites under rapid heating condition.
... 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 compressed (compression test). The network is linearly elastic under small strains, then exhibits yield, followed by densification and plastic hardening (Pirard et al. 1995;Scherer et al. 1995;Duffours et al. 1995;Perrin et al. 2004;Phalippou et al. 2004). As a consequence of the plastic shrinkage, it is possible to eliminate the pores and stiffen the gel at room temperature. ...
... Sintering Aerogels are easily transformed into dense silica glass by oxidation and sintering (Woignier et al. 1990(Woignier et al. , 2011. During these treatments, the structure of the aerogel is modified and the mechanical properties are improved (Phalippou et al. 2004). Figure 9a, b shows the evolution on a log-log plot, of Young's modulus (E), the fracture strength (σ), and the toughness, K IC , as a function of the density, produced Fig. 9 Evolution of the elastic and mechanical properties E, σ, and K IC as a function of the bulk density (a) for the neutral and base-catalyzed gels and (b) for sintered gels by different kinds of catalyst (Fig. 9a) and by sintering (Fig. 9b). ...
... The original silica aerogel consists of nano-clusters sticking to each other to form an low-density network. Cold compression of aerogel leads to densification due to the breaking and re-bonding of the ridges between clusters and interpenetration of clusters (e.g., Phalippou et al. 2004). The bulk porosity is reduced but not the specific surface area, and thus not the primary size scale of clusters and pores (Perin et al. 2003). ...
... The microstructure is not strongly modified since the material undergoes mainly brittle compaction. Thermal sintering is a way to produce dense glass from aerogel (e.g., Phalippou et al. 2004), but it is a kinetically dependant process. For instance, it requires several hours at 1050 °C to complete (Scherer et al. 1998; Perin et al. 2003). ...
We report the results of high-resolution, analytical and scanning transmission electron microscopy (STEM), including intensive element mapping, of severely thermally modified dust from comet 81P/Wild 2 caught in the silica aerogel capture cells of the Stardust mission. Thermal interactions during capture caused widespread melting of cometary silicates, Fe-Ni-S phases, and the aerogel. The characteristic assemblage of thermally modified material consists of a vesicular, silica-rich glass matrix with abundant Fe-Ni-S droplets, the latter of which exhibit a distinct core-mantle structure with a metallic Fe,Ni core and a iron-sulfide rim. Within the glassy matrix, the elemental distribution is highly heterogeneous. Localized amorphous "dust-rich" patches contain Mg, Al, and Ca in higher abundances and suggest incomplete mixing of silicate progenitors with molten aerogel. In some cases, the element distribution within these patches seems to depict the outlines of ghost mineral assemblages, allowing the reconstruction of the original mineralogy. A few crystalline silicates survived with alteration limited to the grain rims. The Fe- and CI-normalized bulk composition derived from several sections show CI-chondrite relative abundances for Mg, Al, S, Ca, Cr, Mn, Fe, and Ni. The data indicate a 5 to 15% admixture of fine-grained chondritic comet dust with the silica glass matrix. These strongly thermally modified samples could have originated from a finegrained primitive material, loosely bound Wild 2 dust aggregates, which were heated and melted more efficiently than the relatively coarse-grained material of the crystalline particles found elsewhere in many of the same Stardust aerogel tracks (Zolensky et al. 2006).
... Solid phase merger and pore structure collapse during SiO 2 aerogel sintering are detrimental for the thermal insulation performance. In the sintering process of SiO 2 aerogel reported by Jean et al., 26 solid phase merger and pore collapse occurred sequentially. The SiO 2 aerogel high surface energy leads to facile sintering in high temperature environment, resulting in the aerogel pore-structure-collapse and heat insulation performance shrinkage. ...
Insights into the micro-texture, micro-morphology, and pore structure of Si3N4/SiO2 aerogel composites at high temperatures are presented. At high heat treatment temperatures, the silica aerogel inside the composite material gradually crystallised, and the fusion of micropores caused the decrease of pores and the increase of pore size. Compared with the pure SiO2 aerogel, Si3N4 particles embedded in the nano-network structure provided effective support and hindered the aerogel crystallisation at high temperatures. To reduce the radiative thermal conductivity, Si3N4/silica aerogel composites were doped with the opacifier TiO2. At higher TiO2 content, the thermal diffusivity and thermal conductivity of the composites decreased more slowly below 800 °C, and substantially above 1000 °C. For TiO2 20 wt%, the measured dielectric constant was 2.85, and the thermal conductivity of the composite decreased by approximately 35% (at 1300 °C). The results show that an appropriate TiO2 content improved the thermal insulation performance of the composite, but damaged the wave permeability, whereas high contents were unfavourable. This study provides theoretical and technical support for the preparation and application of high temperature wave permeable insulation materials.
... An overview on structural properties and characterisation methods is given in the Aerogels Handbook by Reichenauer (2011). In addition characterisation is done by Pollanen et al. (2008), Gonçalves et al. (2016), Phalippou et al. (2004) or Roiban et al. (2016). ...
chapter 3: Advanced porous material for superinsulation, description of properties, characterization methods.
Chapter 3 : Advanced Porous Materials - with contributions from: Bernard Yrieix, EDF, France Ulrich Heinemann, ZAE Bayern, Germany Christoph Sprengard, FIW Munich, Germany Michael O'Conner, ASPEN Aerogel, USA Bettina Gerharz-Kalte, Gabriele Gärtner and Matthias Geisler, Evonik, Germany Miltiadi Vlachos and Georg Gärtner, Cabot, Germany/USA Pierre-André Marchal and Brice Fiorentino, Enersens, France - now online at https://research.chalmers.se/en/publication/515133 (and other parts at https://research.chalmers.se/en/project/6771 and https://www.iea-ebc.org/projects/project?AnnexID=65 )
... An overview on structural properties and characterisation methods is given in the Aerogels Handbook by Reichenauer (2011). In addition characterisation is done by Pollanen et al. (2008), Gonçalves et al. (2016), Phalippou et al. (2004) or Roiban et al. (2016). ...
Chapter 3 : Advanced Porous Materials
- with contributions from:
Bernard Yrieix, EDF, France Ulrich Heinemann, ZAE Bayern, Germany Christoph Sprengard, FIW Munich, Germany
Michael O'Conner, ASPEN Aerogel, USA Bettina Gerharz-Kalte, Gabriele Gärtner and Matthias Geisler, Evonik, Germany Miltiadi Vlachos and Georg Gärtner, Cabot, Germany/USA Pierre-André Marchal and Brice Fiorentino, Enersens, France
- now online at https://research.chalmers.se/en/publication/515133
(and other parts at https://research.chalmers.se/en/project/6771)
... Thermal sintering was due to viscous flow. The particle size increased since the surface energy minimization acted to increase the curvature radius of particles [27,28]. Particle surfaces had positive curvature radius, while neck regions between particles which were contacting with each other had negative curvature radius. ...
Ceramic fiber reinforced silica aerogel composites are novel insulation materials in the thermal protection field for hypersonic vehicles. Before the aerogel composites are applied in load-bearing structures, it is necessary to investigate their mechanical properties including load-bearing and deformation recovery capabilities. High temperature from service conditions will have important effects on the mechanical properties of thermal protection materials. In this paper, compression tests including loading and unloading stages were conducted for ceramic fiber reinforced silica aerogel composites at room temperature and elevated temperatures (300°C, 600°C and 900°C). Influences of thermal exposure to high temperature and high temperature service environment on the compression property and deformation recovery were both investigated. Scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FT-IR) and X-ray diffraction (XRD) were applied to help understand the mechanisms of mechanical property variations. The experimental results show that the compression modulus and strength both increase with the increasing thermal exposure temperature and testing temperature, but the deformation recovery capability decreases. The microstructure changes caused by thermal sintering are considered as the main reason for the property variations. Viscous flow and matter transport due to high temperature resulted in the fusion of aerogel particles. This made the particle skeleton thicker and stronger, which led to higher stiffness and strength of the composites. However, matrix cracks induced by the formation and fracture of larger pores made unrecoverable deformation more serious. In the tests at elevated temperatures, the aggregation of aerogel particles in a fused state got more severe because of the addition of mechanical load. As a result, the degradation of deformation recovery capability became more significant.
... It can be seen that when the temperature below 600 ℃, aerogels own network structures which made up of nanoscale clusters. The aerogels are fused when heat-treated at 800 ℃-1000 ℃ and the aerogel particle size increases and fused together [27](with a symbol of dashed cycle in Fig.8 ). Fig. 9 exhibits the XRD patterns for ST3 treated at different temperatures for 2 h. ...
Monolithic SiO2-TiO2 aerogels were prepared via supercritical drying using tetraethoxysilane and tetrabutyltitanate as precursors and ethanol as solvent. Influence of the heat-treatment on the microstructure and properties of SiO2-TiO2 aerogels were investigated in detail. The results showed that the as-prepared SiO2-TiO2 aerogels had low densities, high specific surface areas, small average pore diameters, and three-dimensional nanoporous structures. The anatase TiO2 phase of SiO2-TiO2 aerogels could form during supercritical drying process, and the transition to rutile TiO2 phase occurred after experiencing 1200°C for 2 h. SiO2-TiO2 aerogels containing 30 wt% TiO2 (ST3) still presented relatively high specific surface area of 451 m2/g even they undergo the treatment of 1000°C for 2 h. And the SEM images indicated that the agglomerated particles derived from ST3 appeared gradually to some extent. The glassy luster of ST3 heat-treated at 1200°C for 2 h illuminates SiO2 started to vitrify. Besides, the thermal conductivity of ST3 at room temperature is up to 0.03257 W·m-1·K-1.
... TEM images of the aerogel in these sections show a reduction of microporosity and an increase in mesoparticle size (densification) from particle capture (Fig. 1E). The observed aerogel texture indicates a combination of densification through both compression and thermal sintering (Phalippou et al. 2004). The remaining pore space is filled by organic matter, obscuring the edges of pores in TEM images. ...
Carbonaceous matter in Stardust samples returned from comet 81P/Wild 2 is observed to contain a wide variety of organic functional chemistry. However, some of this chemical variety may be due to contamination or alteration during particle capture in aerogel. We investigated six carbonaceous Stardust samples that had been previously analyzed and six new samples from Stardust Track 80 using correlated transmission electron microscopy (TEM), X-ray absorption near-edge structure spectroscopy (XANES), and secondary ion mass spectroscopy (SIMS). TEM revealed that samples from Track 35 containing abundant aliphatic XANES signatures were predominantly composed of cometary organic matter infilling densified silica aerogel. Aliphatic organic matter from Track 16 was also observed to be soluble in the epoxy embedding medium. The nitrogen-rich samples in this study (from Track 22 and Track 80) both contained metal oxide nanoparticles, and are likely contaminants. Only two types of cometary organic matter appear to be relatively unaltered during particle capture. These are (1) polyaromatic carbonyl-containing organic matter, similar to that observed in insoluble organic matter (IOM) from primitive meteorites, interplanetary dust particles (IDPs), and in other carbonaceous Stardust samples, and (2) highly aromatic refractory organic matter, which primarily constitutes nanoglobule-like features. Anomalous isotopic compositions in some of these samples also confirm their cometary heritage. There also appears to be a significant labile aliphatic component of Wild 2 organic matter, but this material could not be clearly distinguished from carbonaceous contaminants known to be present in the Stardust aerogel collector.
... TEM images of the aerogel in these sections show a reduction of microporosity and an increase in mesoparticle size (densification) from particle capture (Fig. 1E). The observed aerogel texture indicates a combination of densification through both compression and thermal sintering (Phalippou et al. 2004). The remaining pore space is filled by organic matter, obscuring the edges of pores in TEM images. ...
Abstract– Carbonaceous matter in Stardust samples returned from comet 81P/Wild 2 is observed to contain a wide variety of organic functional chemistry. However, some of this chemical variety may be due to contamination or alteration during particle capture in aerogel. We investigated six carbonaceous Stardust samples that had been previously analyzed and six new samples from Stardust Track 80 using correlated transmission electron microscopy (TEM), X-ray absorption near-edge structure spectroscopy (XANES), and secondary ion mass spectroscopy (SIMS). TEM revealed that samples from Track 35 containing abundant aliphatic XANES signatures were predominantly composed of cometary organic matter infilling densified silica aerogel. Aliphatic organic matter from Track 16 was also observed to be soluble in the epoxy embedding medium. The nitrogen-rich samples in this study (from Track 22 and Track 80) both contained metal oxide nanoparticles, and are likely contaminants. Only two types of cometary organic matter appear to be relatively unaltered during particle capture. These are (1) polyaromatic carbonyl-containing organic matter, similar to that observed in insoluble organic matter (IOM) from primitive meteorites, interplanetary dust particles (IDPs), and in other carbonaceous Stardust samples, and (2) highly aromatic refractory organic matter, which primarily constitutes nanoglobule-like features. Anomalous isotopic compositions in some of these samples also confirm their cometary heritage. There also appears to be a significant labile aliphatic component of Wild 2 organic matter, but this material could not be clearly distinguished from carbonaceous contaminants known to be present in the Stardust aerogel collector.
Flexible temperature sensors have been extensively investigated due to their prospect of wide application in various flexible electronic products. However, most of the current flexible temperature sensors only work well in a narrow temperature range, with their application at high or low temperatures still being a big challenge. This work proposes a flexible thermocouple temperature sensor based on aerogel blanket substrate, the temperature-sensitive layer of which uses the screen-printing technology to prepare indium oxide and indium tin oxide. It has good temperature sensitivity, with the test sensitivity reaching 226.7 μ V °C ⁻¹ . Most importantly, it can work in a wide temperature range, from extremely low temperatures down to liquid nitrogen temperature to high temperatures up to 1200 °C, which is difficult to be achieved by other existing flexible temperature sensors. This temperature sensor has huge application potential in biomedicine, aerospace and other fields.
Comprehensive characterization mechanical properties of aerogels and their composites are important for engineering design. In particular, some aerogel composites were reported to have varied tension and compression moduli. But conducting tension tests is difficult for the reason that low strength and brittleness will lead to unexpected failure in the non-test area. A method is presented for measuring both the tension and compression moduli of a ceramic-fiber reinforced SiO 2 aerogel composite by bending via digital image correlation. First, the relationship between bending behavior and the tension/compression moduli was introduced for bimodular materials. Then a bending test was conducted to predict tension and the compression moduli of the ceramicfiber- reinforced SiO 2 aerogel composite via digital image correlation. In addition, uniaxial tension and compression tests of the aerogel composites were carried out, respectively for measuring tension and compression moduli. The tension and compression moduli measured were numerically similar to results obtained from uniaxial tests with a difference of less than 14 %.
Silica aerogel is a typical nanoporous material exhibiting excellent thermal insulation property, and its nanostructure change at high temperature has attracted extensive attention. However, there is a lack of real-time research on the nanostructure evolution of silica aerogel under rapid heating. Here, silica aerogel was rapidly heated and imaged via an in-situ heating TEM to characterize the nanostructure evolution of from 600 °C to 1300 °C. After rapid heating at different temperatures, the silica aerogels were characterized by SEM, BET, TG-DSC, XRD and FT-IR. The nanostructure evolution of the aerogels can be divided into three temperature stages and experimental strategies for inhibiting aerogel shrinkage at high temperature can be provided. In stage Ⅰ (600 °C–1000 °C), the shrinkage of the silica aerogels occurred during the initial time of the heating process (within 2 h), and then the structure of silica aerogel became stable. The shrinkage of silica aerogels is primarily caused by the fusion of secondary particles, and increasing the secondary silica aerogel particle size may contribute to reducing the aerogel shrinkage. In stage Ⅱ (1100 °C), the aerogel shrinkage can be observed all the time with the heating process continues, and finally closed to the dense state. The shrinkage of silica aerogels is caused by the fusion of secondary particles and the macropore collapse, increasing the particle size and reducing the macropore content may slow down aerogel shrinkage. In stage Ⅲ (1200 °C–1300 °C), the aerogel will shrink rapidly and reach a dense state in a very short time (within 0.5 h). The shrinkage of silica aerogels is caused by the rapid pore collapse and the rapid fusion and shrinkage of the secondary particles of the aerogel, and pure silica aerogels cannot be used at this temperature.
Herein, crosslinked poly (4‐vinylpyridine)/SiO2 (P4VP/SiO2) composite aerogel, an organic‐inorganic interpenetrating network, was successfully prepared by means of the sol‐gel process, acid‐base interaction method, and supercritical CO2 drying process. In this novel composite aerogel, SiO2 was used as an inorganic material, and nitrogen‐containing heterocyclic polymer was used as the active component. The effects of aging time, monomer ratios, solvent usage, and cross‐linker ratios were analyzed to obtain the optimal conditions for the P4VP/SiO2 aerogel formation. The as‐prepared aerogel was characterized by using FT‐IR, XRD, Brunauer‐Emmett‐Teller (BET) analysis, and SEM analysis. Results showed that the aging time of 48 h, tetraethoxysilane (TEOS)/4‐vinylpyridine (4‐VP) mass ratio of 2:1, and diethylene glycol dimethacrylate (DEGDMA) content lower than 20 wt% were required to ensure the best porous amorphous structure of the product. These optimal conditions endowed the product with a relative high BET specific area (314 m²/g) and a low density (0.12 g/cm³). The adsorption performance of P4VP/SiO2 was investigated via its capacity to remove Cu(II) from wastewater. Results showed that the adsorption capacity of up to 85 mg/g was attained under neutral conditions and the adsorption process highly follows the pseudo‐second‐order kinetics model.
Different kinds of structure in alcogels and aerogels (fractal or not fractal) can be synthesized by a control of the chemical parameters and also by different steps in the preparation such as sintering and plastic compaction. The porosity of the gels is affected either by the adjustment of the gelifying concentration, by a precise control of the viscous flow sintering process, or by an isostatic pressure deformation. The different kinds of gels cover the whole range of porosity between 99% and 0%, and their mechanical properties (elastic modulus, strength, toughness) are strongly dependent on the porosity but also on their structure. We follow the mechanical properties of the over the whole process alcogel - aerogel - glass. They vary by five orders of magnitude as a function of the density, and for the same relative density, the elastic modulus and strength can increase by one order of magnitude due to a change in connectivity. The influence of the sintering process compared to isostatic pressure on the mechanical properties is explained by the associated structural changes. © Springer International Publishing AG, part of Springer Nature 2018.
This chapter examines high-temperature mechanical properties of the mullite fiber-reinforced silica aerogel composites through an experiment. The composites were treated at varied temperatures in a chamber. The Z direction shrinkage was found to be 12.40% after treatment at 1000°C. The compressive stress and Young's modulus decreased with increase in the temperature. SEM images showed that the aerogel particles collapsed and sintered in the compression tests. The results showed that with increasing the temperature, the plane direction almost has no shrinkage, but the thickness shrinkage in the Z direction increases. The compressive stress of composites at 1000°C showed different characteristic history with increasing strain.
Different kinds of structure in alcogels and aerogels (fractal or not fractal) can be synthesized by a control of the chemical parameters and also by different steps in the preparation such as sintering and plastic compaction. The porosity of the gels is affected either by the adjustment of the gelifying concentration, by a precise control of the viscous flow sintering process, or by an isostatic pressure deformation. The different kinds of gels cover the whole range of porosity between 99 % and 0 %, and their mechanical properties (elastic modulus, strength, toughness) are strongly dependent on the porosity but also on their structure. We follow the mechanical properties of the over the whole process alcogel – aerogel – glass. They vary by five orders of magnitude as a function of the density, and for the same relative density, the elastic modulus and strength can increase by one order of magnitude due to a change in connectivity. The influence of the sintering process compared to isostatic pressure on the mechanical properties is explained by the associated structural changes.
The new-typed MLI comprised of alumina silicate fiber-papers, high temperature adhesive, hydrophobic silica aerogels granules and potassium hexatitanate (K2Ti6O13) whisker was prepared by felting process. The two samples by adding aerogels and K2Ti 6O13 whisker respectively were heat treatment for 100 h and 100 times at intervals of 1 h at 800 °C and 1000 °C. The microstructure, phase transfer and thermal conductivity of the two kinds of samples were characterized by SEM, XRD and calorimeter determination. These MLI samples took on less mass loss. The serious sinter of adhesive layers and decomposition of whiskers resulted in thermal conductivity increase.
Low-density xSiO2-(1-x) Al2O3 xerogels with x = 0.9, 0.8, 0.7 and 0.6 (mole fractions) were prepared by sol-gel and non-supercritical drying. Silica alkogels, which were the network supports of binary composite materials, formed from TEOS by hydrolytic condensation with a molar ratio of TEOS:H2O:alcohol:HCl:ammonia = 1:4:10:7.5 × 10-4:0.0375. Aluminum hydroxide derived from Al (NO3)3 and NH4OH acting in the alcohol solution under the condition of catalysts. The structural changes and crystallization of the binary xerogels were investigated after heating at 300, 600, 900, 1200°C for 2h by the means of nitrogen adsorption experiments, X-ray diffraction, FT-IR spectroscopy, SEM and TEM. The resulting mixed xerogels possess of nano-mesoporous structure which is characteristic of the amorphous framework and crystal Al2O3, high specific surface area and a relatively narrow pore distribution. Al2O3 introduces into the SiO2 phase and some of Si-O-Al bonds form, the amount of which increases with the aluminum content increasing. The alumina-containing xerogels before calcinations are well-crystallized γ-AlOOH that converts to γ-alumina or α-alumina by calcination. The binary xerogels improve the thermal resistance in comparison with the pure SiO2 xerogels because of the presence of Al2O3 in the network.
Silica aerogel is hopeful used as thermal insulation material in thermal protection system due to its extremely low thermal conductivity and low density. However, the weakness in mechanical properties has limited its application. By adding ceramic fibers, the strength and toughness of silica aerogel are improved obviously without sacrificing much of its thermal conductivities. Basic mechanical experiments including tension, compression and shear tests were carried out at room temperature for ceramic fiber reinforced silica aerogel composites. The compression tests along in-plane direction of the composite fiber layer at 300 °C, 600 °C and 900 °C were also conducted, the microstructure of the samples tested at elevated temperature was analyzed by using SEM. The experimental results reveal that the mechanical properties of the ceramic fiber reinforced silica aerogel composites are transverse isotropic. Both the elastic modulus and the ultimate strength of in-plane samples are about over 28 times higher than those of the out-of-plane samples. The composites show asymmetric elastic modulus for tension and compression. The ratios of tensile modulus to corresponding compression modulus along each direction are 1.60, 1.83 and 0.56 for X, Y and Z directions, respectively. The composite keeps shrinking along thickness direction as temperature increases; the largest shrinkage can be 10.8% at 900 °C. The compression properties of the composite layer along in-plane direction at elevated temperature get enhanced with increasing temperature.
Low density silica xerogels were obtained by reducing the capillary forces and thus the shrinkage upon ambient pressure drying. This was achieved by increasing the particle and thus also the pore size at a given target density. To increase the particle size beyond the values characteristic for classical silica aerogels, particle growth and formation of the interconnected network were decoupled. Uniform silica nanoparticles of 20 to 30 nm in diameter synthesized by a Stöber process were subsequently cross-linked. To strengthen the gel network additional treatments were applied including an organic coating of the gel structure. For that purpose 3-Aminopropyltriethoxysilane (APTES) was used as a linker between the inner surface of the silica backbone and the organic component. Simultaneously, APTES serves as a catalyst for the gelation. Since the components for the organic coating of the silica backbone do not interfere with the network formation, a one-pot synthesis is feasible. Several bifunctional epoxides were investigated to obtain highly homogeneous low density xerogels. A further reduction in density was accomplished by the oxidative decomposition of the organic components in the xerogel state thus providing xerogels with a density of about 180 kg/m3 and a pore size of roughly 100 nm.
The dynamic compressive properties of resorcinol–formaldehyde (RF) aerogel were investigated using a spilt Hopkinson pressure bar. The effects of strain rate, water absorption and sample basal area on the dynamic behaviors of RF aerogel were investigated through a series of dynamic experiments. Morphological changes of RF aerogel under compression were studied by SEM, TEM, BET and BJH methods. Results show that the compressive behaviors of RF aerogel display a remarkable strain rate strengthening effect. The water-saturated RF aerogel shows stiffened behavior at high strain rates in comparison with the dry RF aerogel, but the dynamic failure strain is small. The dynamic compressive behaviors of RF aerogel display a remarkable size effect. The stress increases with the sample basal area at the same strain. At high strain rates, the pores shrink rapidly; RF particles fuse together to form larger particles and surface area reduces rapidly. It is the fusing of gel particles that allows RF aerogel to be much more ductile than silica aerogels and not to break into fragments at high strain rates.
We have investigated, theoretically, the physical properties of cake layers formed from aggregates to obtain a better understanding of membrane systems used in conjunction with coagulation/flocculation pretreatment. We developed a model based on fractal theory and incorporated a cake collapse effect to predict the porosity and permeability of the cake layers. The floc size, fractal dimension, and transmembrane pressure were main parameters that we used in these model calculations. We performed experiments using a batch cell device and a confocal laser-scanning microscope to verify the predicted specific cake resistances and porosities under various conditions. Based on the results of the model, the reduction in inter-aggregate porosity is more important than that in intra-aggregate porosity during the cake collapsing process. The specific cake resistance decreases upon increasing the aggregate size and decreasing the fractal dimensions. The modeled porosities and specific cake resistances of the collapsed cake layer agreed reasonably well with those obtained experimentally.
The method of obtaining monolithic dry gels by hypercritical solvent evacuation is presented. The influence of various parameters on the possibility of obtaining crack-free pieces is studied in some detail. The influence of these parameters on the final properties of the gel is also discussed.
Using small-angle x-ray scattering, we show that porous silica aerogel has a fractal backbone structure. The observed structure is traced to the underlying chemical (polymerization) and physical (colloid aggregation) growth processes. Comparison of scattering curves for aerogel with silica aggregates confirms this interpretation.
Elastic properties of densified aerogels were investigated by Brillouin light scattering in silica aerogels densified by viscous sintering and isostatic compression. Sound velocity and acoustic attenuation characterize the evolution of the elastic properties vs. the bulk density for the two sets of samples. Elastic modulus increases strongly during sintering while the attenuation α decreases, which is coherent with a larger connectivity in the solid network. Viscous flow sintering creates new siloxane bonds, eliminates pores and as expected, the aerogel stiffens. Compressed aerogel has a completely different behavior. In the low pressure range, elastic modulus decreases and α rises. This is attributed to breakage of links between clusters during compression. Weakening of the aerogels is the consequence of large strain of the solid network. For higher pressure, the density increase is accompanied by stiffening, suggesting that condensation occurs more than link breakage.
A method is developed for analyzing the outer part of the small-angle x-ray or neutron scattering curve for porous scatterers in which the pore boundaries can be described by fractals. When the results are applied to the scattering data from a lignite coal, the fractal dimension of the boundary surface of the pores in this coal is found to be 2.56 ±0.03.
Silica aerogels were densified by isostatic compression or by thermal sintering. Small angle X-ray scattering experiments show that both types of aerogels exhibit oscillations around Porod behavior at high q values. Oscillations were modified by sintering but remain unchanged by compression. According to the Babinet's principle, it is not usually possible to associate intensity change with a peculiar phase of the porous solid. However, the fact that the oscillations remain unchanged with compression, which only acts on the porous part, indicates that SAXS intensity experiments can be directly linked to the solid part. Using the Porod law [G. Porod, Kolloid Z. 124 (1951) 83; P. Debye, A.M. Bueche, J. Appl. Phys. 20 (1949) 518], the specific surface area has been estimated. Surface area of the compressed aerogels does not change with density. In contrast, the specific surface area of the sintered samples decreases markedly. The absence of shape changes of SAXS curves indicates that the mean size of the solid phase remains constant during the compression densification.
The elastic modulus, modulus of rupture (MOR), and viscosity of wet silica gels were measured as functions of age of the gels, using a beam-bending method. The gels were prepared by acid-catalyzed hydrolysis of tetraethyl orthosilicate (TEOS) with a water/TEOS ratio of 16/1. The gels were aged and their properties were measured while immersed in the pore liquid. The modulus increased by about two decades and the MOR and viscosity increased by about one decade over a two-week period. Measurements of stress relaxation showed a strong dependence on sample size that is attributed to the flow of pore liquid out of the samples. The gels were found to be linearly viscoelastic.
An isostatic pressure provided by a Hg porosimeter is applied on monolithic silica aerogels. Due to the small pore size, the mercury does no intrude the aerogel and induces a compression. The bulk modulus is measured for an aerogel series. Above a certain pressure, the aerogel does not recover its initial dimension and, consequently, it may be densified using this method. The densification is more important for oxidized aerogel. This irreversible compaction is explained by an interpenetration of clusters constituting the aerogel. Such an interpenetration increases the contact number between reactive species, borne by the arms of the clusters. The reactive species for silica aerogels are the silanol groups. They polycondense to form siloxane bonds; hence, the aerogel network is more reticulated. The reaction is deduced from the structural change observed by infrared spectroscopy.
The mechanical properties of silica alcogels and aerogels were measured by the three-point bending technique. Elastic moduli and the fracture strength were investigated as a function of concentration of silicon compound, catalysis conditions, and aging time. The evolution of modulus and strength was followed during gel to glass transformation. Toughness and fracture energy results of aerogels are presented.
Proper equilibration upon nitrogen sorption at 77.4 K is the key for a reliable characterization of porous materials with respect to their mesopores. In the past, aerogels were often insufficiently equilibrated, thus yielding misleading isotherms and highly erroneous pore size distributions (PSDs). To allow for the extraction of the mean mesopore size with an accuracy better than 10% total run times up to 150 h are required for 2 mm sized samples with porosities above 90%. The total run time needed to measure a well equilibrated isotherm is increasing with mesoporosity, sample size and compliancy of the sample. If highly porous aerogels are well equilibrated and their deformation due to capillary forces upon sorption is taken into account, both, adsorption and desorption branches of the isotherm yield the same mean pore size when calculated via the Kelvin equation for cylindrical pores.
