Properties Of Silica Aerogels Research Articles

Data‐driven inverse design and optimisation of silica aerogel model networks

AbstractSilica aerogels are highly porous ultralight materials with extremely low density and thermal conductivity. These exceptional properties of silica aerogels are often accounted to microstructure morphology, thus making them of keen research interest for analysing their structure‐property relationships. The classical approach for this involved the microstructure modelling of the silica aerogels with aggregation‐based modelling algorithm viz., diffusion‐limited cluster‐cluster aggregation (DLCA) and then performing finite element method (FEM) on the generated representative volume element (RVEs). However, the process often requires large computation time and resources.The objective of this work was thus to introduce an artificial intelligence approach based on neural networks and reinforcement learning to eliminate the necessity of generating and simulating 3D silica aerogel models for predicting their structural and mechanical properties. To this end for the forward prediction of the elastic modulus and fractal dimension of the silica aerogels from DLCA parameters, an artificial neural network was developed. Furthermore, to reverse engineer the material and perform inverse material design, a reinforcement learning framework was developed, that is shown to have learned to determine appropriate DLCA model parameters as actions for a desired fractal dimension and elastic modulus.

Effects of several additives on silica aerogel properties and adsorption performance for n-hexane, carbon tetrachloride and toluene

In this work, hydrophobic and monolithic silica aerogels were prepared at ambient pressure by using β-cyclodextrin, cetyltrimethylammonium bromide and ethylene glycol as additive. The aerogels were characterized and used for adsorption of n-hexane, carbon tetrachloride and toluene. These additives have different effects on the morphology and specific surface area of silica aerogels. Compared to cetyltrimethylammonium bromide and ethylene glycol, introducing β-cyclodextrin into silica aerogel could increase adsorption capacity for of n-hexane, carbon tetrachloride and toluene more effectively.

Effect of residual Na+ on the properties of aerogel prepared with sodium silicate via APD

The removal of Na+ in the process of preparing silica aerogel with sodium silicate as the precursor is a very important and costly step. In this study, effects of residual Na+ content (0∼3.0 wt.%) in the precursor on the properties of silica aerogel were investigated. The thermal conductivity, bulk density, porosity and hydrophobic of the aerogel were tested. And the action mechanism of Na+ was discussed, based on the test results of pore structure, microscopic morphology, and chemical structure composition characterized by N2 adsorption-desorption, SEM and FT-IR. The results showed that as the content of residual Na+ increased, the thermal conductivity and density of the aerogel increased, and the porosity and hydrophobicity decreased. It may be the alkaline dissolution effect of Na+ and the agglomerations of Na+ block part of mesopores, but did not change the skeleton composition of the silica aerogels.

Influence of 1D and 2D carbon nanostructures in silica-based aerogels

Carbon nanostructures-silica aerogel composites were synthesized and characterized to assess the effect of carbon nanotubes (CNTs) or graphene oxide (GO) on the silica aerogel properties. The sol-gel chemistry was based on methyltrimethoxysilane (MTMS) and 3-aminopropyltrimethoxysilane (APTMS) as silica precursor system, with varying amounts of APTMS (0–20% mol of Si). APTMS significantly impacted the materials' physical properties. The chemistry and microstructure were investigated by FTIR, NMR, TEM, SEM, SAXS and BET. The addition of CNTs induced the growth of the silica matrix around them; thus, an elongated shape was observed in the silica structural units. APTMS and CNTs have a synergistic effect on the mechanical properties, increasing the Young's modulus up to 14 MPa. Small amounts of carbon materials (∼1 wt%) in the MTMS-matrix improved its thermal insulation property, particularly for temperatures above 50 °C. In terms of electrochemical properties, the carbon nanostructures lead to higher specific capacitances and a reduction in resistance. The characterizations here performed allowed a better understanding of the interactions between the silica and carbon phases. The possibility to obtain materials with tailored properties demonstrates their application potential in several areas, such as thermal insulation and energy storage.

Eco-friendly approach for preparation of water-based superhydrophobic silica aerogels via ambient pressure drying

In this work, superhydrophobic methyltrimethoxysilane (MTMS)-based silica aerogels were fabricated via water-based sol–gel reaction by ambient pressure drying (APD) method in the presence of surfactant. The structure, morphology, and hydrophobic properties of the obtained silica aerogel were characterized by scanning electron microscopy (SEM), Brunauer–Emmett–Teller (BET) methods, X-ray diffractometer (XRD), and water contact angle measurement. The effects of the concentration of MTMS, the pH value of the solution, and the amount of surfactant cetyltrimethylammonium bromide (CTAB) on properties of silica aerogels were investigated, respectively. Increasing the concentration of MTMS to a great extent resulted in higher density and lower porosity of silica aerogel. Alkali catalyst was an extremely crucial factor for preparing silica aerogels. With the increase of the pH value from 7.5 to 10 the gelation time of the prepared aerogels was observed to decrease from 40 min to 2 min, and the shrinkage of aerogel sample decreased from 15.5% to 3.5%. The skeleton structure of the prepared silica aerogel gradually transformed from coarsened spherical to continuous irregularity finer structures with the increased concentration of CTAB. In addition, the prepared silica aerogel showed outstanding anti-adhesion property and superhydrophobic ability with a water contact angle (WCA) of 160.6 ± 1.3°.

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.

Synthesis and physicochemical characterization of silica aerogels by rapid seed growth method

Thanks to the high optical transparency and ultra-low thermal conductivity, silica aerogels are ideal materials for energy-saving windows. However, their preparation is commonly based on either one-step base-catalyzed method, or two-step acid-base catalyzed method, which is difficult to inhibit the aggregation of clusters while keeping the size of clusters as small as possible and thus degrading silica aerogel's properties. Here, a new idea for synthesizing silica aerogels is presented from the viewpoint of controlling the growth and aggregation of silica clusters. A certain amount of Tetramethoxysilane (TMOS) used as seed precursor is firstly added into the mixture of methanol and distilled water for hydrolysis. A certain time later, the additional TMOS and a defined amount of ammonia are added to the obtained sol for promoting the rapid formation of the gel in several minutes. The silica aerogels prepared by this method have higher optical transparency and lower thermal conductivity than those prepared by the other two methods. This approach may also shed substantial light on controlling the microstructure of other materials prepared by sol-gel process.

Theoretical investigation of heat transfer in structurally graded silica aerogels with pores diameter changing

In graded structure aerogels, change of pores diameter through the thickness affects the effective thermal conductivity as the most important parameter. As the diameter of the pores is reversely correlated to the density, the effective thermal conductivity ($$\lambda_{\text{eff}}$$) of aerogel is often normalized to the apparent density ($$\rho$$) and it is expressed as the B ($$\frac{{\lambda_{\text{eff}} }}{\rho }$$) parameter. Lower values of B would be the optimum conditions for the insulation performance of resulting aerogel. The objective of this work is to simulate the heat transfer of the optimum structure and to compare it with functionally graded structures that pore diameter varies through the thickness. For this purpose, the structural characteristics and properties of silica aerogel along with the effect of coupling thermal conductivity have to be taken into consideration. The heat transfer and time–temperature history diagram were modeled for an optimum structure (OPT) having a minimum value of the B parameter. The results were compared to the structurally graded aerogels in which the density was varied in two fashions, from higher to lower values (HtL) density and from lower to higher values (LtH) density. The change of temperature with time was tracked for all the cases. Results indicated that the minimum value of heat transfer was obtained for the structurally graded aerogel of the type of LtH (2% increase in efficiency for LtH compared to OPT). Therefore, this structure introduces as the best candidate for producing a thermal insulator.

Effect of different types of surfactants on the microstructure of methyltrimethoxysilane-derived silica aerogels: A combined experimental and computational approach

HypothesisSurfactants interfere with sol-gel particle/pore growth, influencing the structure and properties of silica aerogels. Their ability to induce microscopic changes in the aerogel’s structure may be useful to improve/control the thermal insulation performance of aerogels. ExperimentsThe influence of different types of surfactants (anionic, cationic and non-ionic) on the microstructural arrangement and macroscopic properties of methyltrimethoxysilane (MTMS)-based aerogels was evaluated for the first time, using an experimental and computational comparative approach. Molecular dynamics simulations were performed based on two representative silica molecular structures derived from MTMS, while the experimentally-obtained silica aerogels were characterized in terms of chemical/structural/mechanical/thermal insulation properties. FindingsThe use of both hexadecyltrimethylammonium bromide (CTAB) and sodium dodecylsulfate (SDS) led to a decrease in bulk density, thermal conductivity and average pore size of the aerogels, with notorious increase of their flexibility. The observed changes were due to microstructural arrangements, as evidenced by scanning electron microscopy (SEM). However, the non-ionic surfactant, Pluronic F-127, did not have a positive impact on the desired properties. Globally, the simulation results support the experimental findings, suggesting differentiated microstructural changes induced by the use of cationic or anionic surfactants. The addition of CTAB and SDS generally resulted in smaller or larger silica aggregates, respectively.

Tuned optical transmittance in single-step-derived silica aerogels through pH-controlled microstructure

We have systematically studied the relationship between synthesis pH and morphological and optical properties of silica aerogels. We have determined through SEM and BET that there is a systematic correlation between the pH of the initial silica solution and the surface area, porosity, and pore size of the resulting aerogel. We find that optical transmittance, particularly its wavelength dependence (dispersion), is strongly governed by the microstructure and, therefore, synthesis pH. We have determined that the microstructure of the aerogels fall into three broad categories: monostructural, which is characterized by repeating elongated microstructural features; fractal, which shows a distinctive structure that is emulated on multiple length scales; and isotropic, which is characterized by having no distinct features in its microstructure. Simply by controlling the pH of the synthesis environment, we can tune the optical properties of silica aerogels through pH controlled the microstructural modification. We have found that pH = 1–5 gives high dispersion, pH = 6–7 results in low transmittance and low dispersion, pH = 8 shows the highest transmittance with the lowest dispersion, and pH = 9–10 transitions back to lower transmittance and higher dispersion.

High temperature annealing for structural optimization of silica aerogels in solar thermal applications

Optically transparent, thermally insulating monolithic silica aerogel, with its high solar transmittance and low thermal conductivity, is well-suited for solar thermal applications, particularly concentrated solar power systems. The properties of silica aerogel are directly determined by the structure of the highly porous, interconnected silica network. By using high temperature annealing to control this structure post-synthesis, we were able to optimize the material to increase solar transmittance with minimal effect on the effective thermal conductivity using an easy and scalable method. Samples made from two silica aerogel chemistries and two annealing temperatures (400°C and 600°C) were investigated as a function of annealing time. The annealed samples showed a maximum increase in solar spectral transmittance of over 3% while the effective thermal conductivity (including radiative and conductive contributions) was shown to increase by as much as 40%, indicating a need to optimize the annealing time for maximum performance. The properties of the characterized aerogels were used to demonstrate aerogel annealing optimization using a concentrated solar power receiver model operating at 400°C. The model predicted a maximum receiver efficiency gain of 1% by annealing for 24h at 400°C, representing a significant gain in overall system efficiency.

Effect of aging on silica aerogel properties

Silica aerogels’ unique physical and chemical properties make them fascinating materials for a wide variety of applications. In addition to hydrophobization by silylation, aging is very important in the synthesis of silica aerogel by ambient pressure drying. Here we systematically study the effect of aging on the physico-chemical properties of silica aerogel with emphasis on ambient dried materials. Silica gels were aged for different times and at different temperatures in their gelation liquid (without solvent exchange), hydrophobized in hexamethyldisiloxane and subsequently dried either at ambient pressure or from supercritical CO2. Dynamic oscillatory rheological measurements demonstrate that aging reinforces the alcogels, particularly at high strain. The specific surface area decreases with increasing aging time and temperature as a consequence of Ostwald ripening processes during aging. With increasing aging time and temperature, the linear shrinkage and bulk density decrease and the pore size and pore volume increase for the ambient dried gels, but remain nearly constant for supercritically dried gels. Small-Angle X-ray scattering does not detect significant structural changes at length scales smaller than about hundred nanometers, but hints at systematic variations at larger length scales. The findings of this study highlight the importance of aging to increase the ability of the gel particle network to withstand irreversible pore collapse during ambient pressure drying.

Surface Chemistry of Hydrophobic Silica Aerogels

The properties of silica aerogels, and the ability to prepare them by ambient pressure drying, hinge on their chemical surface modification. Until now, quantitative analysis of the surface chemistry has not been possible by state-of-the-art analytical methods. Here, we determine the surface chemistry of six archetypal, ambient pressure dried, hydrophobic silica aerogels and open-porous foams with quantitative 1H, 13C, and 29Si and 1H–29Si heteronuclear solid-state NMR spectroscopy. The quality of the external calibration, the validation by elemental analysis, and the consistency among the 1H, 13C, and 29Si MAS NMR data enable us to robustly quantify the surface chemistry of these classical silica materials. Four aerogels were derived from tetraethoxysilane (TEOS), polyethoxydisiloxane (PEDS), or waterglass precursors and hydrophobized with trimethylsilyl (TMS) groups through immersion in hexamethyldisilazane (HMDZ) in heptane, trimethylchlorosilane (TMCS) in heptane, or hexamethyldisiloxane (HMDSO) in eth...

Fabrication and Characterization of Silica Aerogel @Polystyrene Composite Beads by Suspension Polymerization

In this study, the powder of hydrophobic silica aerogel remaining nanopore by ambient pressure drying was successfully introduced into polystyrene beads during suspension polymerization. nanosilica powder without nanopore was also used. Pure polystyrene, silica aerogel/polystyrene (PS) and nanosilica/polystyrene beads were fabricated, respectively. The structure and properties of silica aerogel @PS composite beads were characterized by SEM, EDS, TG-DSC and FT-IR. The results of TG-DSC, FT-IR and EDS show silica aerogel is located in the PS beads, but it is difficult to be observed by SEM because of its low content. The introduction of silica and silica aerogel both decrease the transparency of PS beads. Silica aerogel loosen the microstructure of PS beads.

Hydrophobic Flexible Silica Aerogels Felts Fabricated by Ambient Pressure Drying

Hydrophobic flexible silica aerogels felts were fabricated successfully by two-steps sol-gel process via ambient pressure drying. First of all, the sol with various pH values was obtained from tetraethylorthosilicate (TEOS) as silicon source, hydrochloric acid/aqueous ammonia ethanol solution as catalysts. Then glass fiber felts was incorporated into the sol to increase the mechanical properties of silica aerogel. After the completion of solvent exchange and surface modification by using trimethylchlorosilane (TMCS)/n-hexane solution, the gel felts were dried under ambient pressure. The samples show excellent hydrophobic properties.

Effect of surface modification of SiO<SUB align="right">2 nanoparticles on the mechanical properties of ambient pressure dried silica aerogels

In this study, we synthesised silica aerogel composites using SiO2 nanoparticles and studied the effects of surface modification of silica nanoparticles on the properties of silica aerogel composites. The surface condition of the silica nanoparticles was controlled to be either hydrophobic or hydrophilic. Accordingly, the effect of the surface chemical state of silica nanoparticles on the network structure of hybridised silica aerogels and finally on their mechanical properties could be investigated. The microstructural and mechanical properties of silica aerogels were strongly dependent on the nature of the surface of hybridised silica nanoparticles.

Extraordinary Adsorption Properties of Nano Porous Hydrophobic Silica Aerogels

The adsorption properties of hydrophobic silica aerogels were studied. Polyethoxy- disiloxanes (E-40), ethanol (EtOH), hydrogen fluoride (HF) were used as silican precursor, solvent and catalyst, respectively, followed by solvent substitution and surface modification to prepare silica aerogels. Scanning electronic microscopy, nitrogen adsorption analyzer, contact angle measurement and Fourier transform infrared spectroscopy were used to characterize the structure and properties of silica aerogels. The conclusion is that the silica aerogels are with good hydrophobicity and the gas adsorption capacities is excellent for toxic gases such as benzene and carbon tetrachloride, which is 2~3 times higher than that of activated carbon fiber (ACF) or granule of activated carbon (GAC). Moreover, the adsorption capacity for organic solvent is 20-30 times of its own weight, which is much larger than that of GAC or Poly vinyl alcohol (PVA). In addition, the adsorption capacity of silica aerogels remains almost the same value after two times of adsorption- desorption processes, which means that the process is recyclable, low-cost and environmental friendly.

Tableting properties of silica aerogel and other silicates

Context: In solid oral dosage forms silicates are commonly used as glidants in low concentration. However, due to their large specific surface area, silicates may also be used as carrier materials for drugs. Moreover, silicates allow amorphisation of drugs by co-grinding or processing with supercritical fluids. Objective: The aim of this study was to investigate the physical and the tableting properties of Silica Aerogel (special type of silica with an extremely large specific surface area), Neusilin® US2 (magnesium aluminometasilicate), Florite® (calcium silicate) and Aerosil® 200 (colloidal silica). Materials and methods: Powder blends of Avicel® PH102 (microcrystalline cellulose) and different amounts of the respective silicate were compacted and analyzed for their tabletability (tensile strength vs. compaction pressure) as well as their Heckel plot. Results and discussion: With Neusilin® the tabletability appeared to be independent of the silicate concentration, whereas with Florite® an increasing silicate concentration led to a higher tensile strength. In contrast, the addition of Silica Aerogel and Aerosil® resulted in a decrease of the tensile strength. With Aerosil® a maximum tolerable concentration of 20% [w/w] was determined. Plastic deformation of all powder blends decreased with increasing silicate concentration. This effect was most pronounced with Aerosil® and least with Florite®. Conclusion: Tablets with acceptable tensile strength were obtained with all plain silicates except for Aerosil®. Therefore, these silicates may be used in tablet formulations, e.g. as carrier materials for liquid or amorphous drugs.

Microstructural Characterization and Properties of Silica Aerogels Doped with Nano-ITO Powder

Silica-based aerogels doped with nano-ITO powder were synthesized using sol–gel process followed by supercritical drying. The main factors were studied including dosage of compling agent KH570 and ITO. The result shows that thermal radiation conductivity is 0.02659 W/(m•K) with ITO of 44.44% after 10 g ITO powder is modified with 1 mL KH570. The microstructure and physical characteristic of the SiO2–ITO aerogels were characterized by FTIR, SEM, TG-DSC and BET. Their specific surface was in the range of 513–775m2/g and their average pore size was mainly 20 nm. Nano-ITO powders are physically embedded by SiO2 aerogels.

Preparation and Properties of Hydrophobic Silica Aerogels by Ambient Pressure Drying Method

Silica alcogels were prepared by hydrolysis with hydrochloric acid and condensation with NH4OH of ethanol diluted tetraethylorthosilicate (TEOS) precursor and trimethylchlorosilane and hexane as surface modifying agent. The physical properties such as density, appearance, hydrophobicity, surface area, pore size distribution and thermal stability were measured. It was found that the physical and hydrophobic properties of the silica aerogels depend on the TMCS/hexane (V) volume ratio. The density decreased with increase ofV, and the aerogels are more hydrophobic asV=3%. The aerogels were thermally stable up to a temperature of 350 °C, and the aerogel prepared has a high surface area and large pore volume.