Series Of Silica Aerogels Research Articles

Tailoring structure and properties of silica aerogels by varying the content of the Tetramethoxysilane added in batches

Abstract Silica aerogels with high optical transparency and ultra-low thermal conductivity are ideal candidates for energy-saving windows. Often such applications require rather exquisite control of microstructure (i.e., cluster size and cluster packing geometry). In this research, a series of silica aerogels with the same composition and density but different microstructures and properties are prepared by varying the content of the Tetramethoxysilane (TMOS) added in batches. It was found that the mean pore size and thermal conductivity of the silica aerogels initially decrease with the decrease in the content of the pre-added TMOS before bottoming out and eventually increasing again, while the specific surface area, pore volume and light transmittance show the reverse change trend. Compared with the conventionally initial addition of total TMOS, the specific surface area and transparency ratio of the silica aerogels prepared under the optimum level of the pre-added TMOS increase from 845 to 1060 m2/g and from 71% (550 nm) to 88% (550 nm), respectively. Meanwhile, the thermal conductivity of the silica aerogels decreases from 24.6 to 20.2 mW/(m∙K) with the mean pore size decreases from 20.7 to 16.6 nm. This method provides a new approach to design structure and properties of silica aerogels.

Zeolite/silica aerogel composite monoliths and microspheres

A series of silica aerogels and composite materials based on Fe-silicalite-1 nanocrystals embedded into silica aerogel matrix in the form of monoliths and microspheres have been synthesized by drying with supercritical CO2. Using equivolume water/ethanol mixture as a medium for the aging of monoliths enabled to produce monoliths of silica aerogel and composite without cracks. The synthesis of the composite and silica aerogel microspheres by the emulsion/gelation technique has been designed. The different effects of the stirring rate during the emulsification on the particle size and texture of composites and silica aerogels has been observed. The synthesized samples were characterized by X-ray diffraction, scanning electron microscopy and nitrogen adsorption measurements.

Analysis of the effect of drying conditions on the structural and surface heterogeneity of silica aerogels and xerogel by using cryogenic nitrogen adsorption characterization

Monolithic silica aerogels were prepared by means of sol–gel method followed by carbon dioxide (CO 2) supercritical drying (SCD). By changing the pressure of liquid CO 2 used to replace ethanol in the autoclave before CO 2-SCD from 8.5 MPa to 4.5 MPa in steps of 1 MPa, a series of silica aerogels with different ratio of micropore, mesopore and macropore volume, having a BET surface area of 678–949 m 2 g −1, a pore volume of 1.63–5.16 cm 3 g −1 and a average pore width of 9.61–21.76 nm were obtained. Silica aerogel and xerogel were also prepared by ethanol-SCD and ambient pressure drying respectively for comparison. N 2 adsorption isotherm was used to characterize the porous texture of the silica aerogels as prepared. Surface energy distributions were calculated by Density Functional Theory (DFT) method deduced from N 2 adsorption data. Infrared spectroscopy (IR) was used to determine the surface groups of these samples. All samples exhibited a multimodal pore size distribution with maxima in the micropore and meso-/macropore regions. The pore size distribution of CO 2-SCD silica aerogels was narrowed by increasing the pressure of liquid CO 2, and the position of maximum also shifted from large pore width region to small pore width region. Maximum in pore volume was attained at 6.5 MPa for mesopores with little decrease of surface area. Surface energy distribution depended not only on the pore size but also on the pore size distributions and surface properties. Different surface groups lead to significant change of surface energy distribution between ethanol supercritical drying and CO 2 supercritical drying silica aerogels. The relationship between surface energy distribution and pore size was introduced.

Positive curvature effects and interparticle capillary condensation during nitrogen adsorption in particulate porous materials

The adsorption of nitrogen in a collection of spheres that touch or merge in a sintering-like manner is modeled using a Derjaguin–Broeckhof–de Boer approach. The proposed model accounts for both positive curvature effects and for capillary condensation at the contact between two spheres. A methodology is proposed to fit the P / P 0 > 0.4 adsorption region with the coordination number of the spheres as the only adjustable parameter. The use of the model is illustrated on a series of silica aerogels. The suitability of various standard isotherms needed for the modeling is also discussed.

Structural Efficiency and Microstructural Modeling of Wet Gels and Aerogels

We present a microstructural model of aerogels that includes the effect of particle necks, tortuosity and dangling ends on the scaling of elastic moduli with density. Relative neck radii can be determined for sintering series of silica aerogels and for Resorcinol Formaldehyde (RF) aerogels produced with different catalyst concentrations. The density of elastically ineffective dangling ends and the tortuosity can be estimated using information from thermal conductivity and elastic modulus measurements in silica aerogels. Typical values for the load bearing mass range from >50% for high density and heat treated aerogels to <10% of the total mass for low density wet-gels.

Dynamics of weakly connected solids: Silica aerogels.

Apply neutron spin-echo spectroscopy, we investigate low-energy vibrational excitations in a series of silica aerogels of varying connectivity. Connectivity is adjusted both by precursor solution chemistry and by heat treatment. Polymeric aerogels show fractonlike densities of states, whereas colloidal materials exhibit a peak in the \ensuremath{\mu}ev regime. On sintering, the low-frequency excitations disappear. We conclude that neutron spin-echo spectroscopy provides a measure of the topological aspects of disordered materials.

Experiments on the structure and vibrations of fractal solids

Measurements performed on carefully prepared series of silica aerogels are reported. These materials were studied with small-angle neutron scattering, revealing a fractal structure that can extend up to at least two orders of magnitude in length. The corresponding collective vibrations were investigated by Brillouin, Raman, and inelastic neutron scattering. All results are consistently explained in terms of fractons.

Fractal structure of base catalyzed and densified silica aerogels

Small angle neutron scattering results obtained on several series of silica aerogels are presented. It is shown that basic catalysis can produce materials that are fractal over at least two orders of magnitude in length. These have bulk densities lower than ∼ 0.02 g/cm3, corresponding to porosities of 99% or larger. In this case, the constituent particles, whose gyration radii are larger than 10 Å, have a fractal surface that cannot be smoothened by oxidation. Densification at elevated temperatures mostly increases the particle size. The fractal structure is destroyed when the particle dimensions equal the correlation length at the gel-glass transformation.