Specific Surface Area Of Aerogels Research Articles

Preparation of monolithic Al2O3–SiO2 aerogels with high-temperature resistance using boehmite nanorods

In this study, monolithic Al2O3–SiO2 aerogels with excellent thermal stability were synthesized by ambient pressure drying (APD) or supercritical drying (SCD) using tetraethyl orthosilicate and boehmite nanorods (L = 20–30 nm, D = 3–5 nm) as precursors. The high specific surface area of aerogels prepared by APD or SCD was maintained after calcination at 800 °C (585 m2/g or 664 m2/g), 1000 °C (415 m2/g or 487 m2/g), and 1100 °C (175 m2/g or 231 m2/g). The anti-sintering mechanism of the aerogels prepared with boehmite nanorods as the aluminum source was discussed in detail. Successful preparation by APD offers the possibility of low-cost and large-scale application of monolithic Al2O3–SiO2 aerogels. Al2O3–SiO2 aerogels with excellent thermal stability have the potential to be used as high-temperature thermal insulating materials in aerospace applications.

Reusable, high specific surface areas, and excellent thermal stability Al2O3–SiO2 aerogel composites as high-temperature thermal insulators for radome applications

In this study, monolithic Al2O3–SiO2 aerogels with excellent thermal stability and high specific surface area were synthesized using boehmite nanorods (L=20–30 nm, D=3–5 nm). The high specific surface area of aerogels was maintained after calcination at 800 °C (663.54 m2/g), 1000 °C (486.91 m2/g), and 1100 °C (230.76 m2/g). The anti-sintering mechanism of the Al2O3-SiO2 aerogels was discussed. Thanks to the composite of Al2O3-SiO2 aerogel and mullite fibers, the disadvantages of brittleness and low strength of aerogel are overcome. The thermal conductivity of the composite is extremely low at 0.034 W/(m·K)-0.089 W/(m·K) (25 °C-1000 °C). Five muffle cycle back-temperature tests at 1000 °C and a butane blowtorch back-temperature test at 1350 °C also proved that the composites have excellent thermal insulation properties. Composites with excellent thermal insulation properties combined with lightweight, thinness, machinability, and good dielectric properties meet the need for high-temperature thermal insulating-wave transmitting integrated materials for hypersonic missile radomes.

Green and single-step simultaneous composite starch aerogel formation-high bioavailability curcumin particle formation

The high porosity and specific surface area of aerogels offer an ideal platform for loading bioactive molecules. In the present study, the microstructure of the bio-based starch aerogels was modulated by the incorporation of chitosan. The starch hydrogel precursors were prepared from high amylose corn starch in the presence of 0, 0.50, and 0.75 wt% chitosan. Afterward, a green single-step simultaneous aerogel formation-curcumin deposition method was applied to impregnate curcumin into the aerogels through supercritical carbon dioxide (SC-CO2) drying technology. Composite starch/chitosan aerogels showed a more open porous structure and lighter weight than the neat starch counterpart. Confocal microscopy and fluorescence spectroscopy analysis confirmed curcumin molecules' attachment to the aerogels' hydrophobic cavities. The impregnation capacity was 24–27 mg curcumin per gram of aerogel depending on the composition of the aerogels. The loading of curcumin in the aerogels significantly enhanced the bioaccessibility of curcumin in the simulated gastrointestinal fluid by almost 30-fold when compared to the unloaded curcumin. Furthermore, the bioaccessibility of the curcumin loaded in starch-chitosan composite aerogels was higher than that in neat starch aerogels. While unloaded curcumin showed an undetectable intestinal Caco-2 cell transportation, curcumin-loaded aerogels revealed a cumulative curcumin passing of 0.15–0.23 μg/mL.

Hyaluronic Acid Aerogels Made Via Freeze-Thaw-Induced Gelation.

The biodegradability, biocompatibility, and bioactivity of hyaluronic acid (HA), a natural polysaccharide, combined with the low density, high porosity, and high specific surface area of aerogels attract interest for biomedical applications such as wound dressings. In this work, physically cross-linked HA aerogels were prepared via the freeze-thaw (FT) induced gelation method, solvent exchange, and drying with supercritical CO2. The morphology and properties of HA aerogels (volume shrinkage, density, and specific surface area) were investigated as a function of several process parameters: HA concentration, solution pH, number of FT cycles, and type of nonsolvent used during solvent exchange. We demonstrate that the HA solution pH plays a key role in the aerogel formation, as not all conditions result in materials with high specific surface area. HA aerogels were of low density (<0.2 g/cm3), high specific surface area (up to 600 m2/g), and high porosity (≥90%). Scanning electron microscopy pictures revealed that HA aerogels present a porous structure with meso- and small macropores. The results show that HA aerogels are promising biomaterials with tunable properties and internal structure that offer high potential as, e.g., wound dressings.

Iron-nickel aerogels anchored on GO nanosheets as efficient oxygen evolution reaction catalysts under industrial conditions

The oxygen evolution reaction (OER) has been acknowledged as a bottleneck in electrocatalytic water splitting due to its sluggish kinetic, so it is eager to search for non-precious metal catalysts with high activity and stability. Herein, we demonstrate a facile strategy to obtain a nickel-iron-aerogels and graphene oxide composite, grown on nickel foam (NiFe@GO/NF) via a mild sol-gel method. The aerogels with the three-dimensional cross-linked networks are successfully anchored to graphene oxide (GO) nanosheets, which greatly increases the specific surface area of aerogels (242.4 m2 g−1) and effectively avoids the agglomeration of the cross-linked networks. Benefited from the special composite morphology and the synergistic effect of NiFe-bimetal and GO, the NiFe@GO/NF driving a 10 mA cm−2 only requires a low overpotential of 233 mV in 1 M KOH. Surprisingly, the NiFe@GO/NF shows considerable OER performance in industrial operating environments (6 M KOH and 50 °C). Thus, the NiFe@GO/NF could be a very promising electrocatalyst for water oxidation.

Versatile protonic acid mediated preparation of partially deacetylated chitin nanofibers/nanowhiskers and their assembling of nano-structured hydro- and aero-gels

The partially deacetylated α-chitin nanofiber/nanowhisker (DEChN) aqueous dispersions were prepared by alkali deacetylation. The following mechanical treatment under acidic conditions was mediated by five different organic protonic acids, including acetic acid, gluconic acid, itaconic acid, citric acid and ascorbic acid. Thereafter, these acidic DEChN dispersions were transformed into hydrogels under an alkali gas phase coagulation bath. After solvent exchange and freeze drying, the DEChN-based aerogels were prepared. Previous research showed that the kinds of acid used for the adjustment of the pH value during the fibrillation process influenced the nanofibrillation efficiency. In this study, it was discovered that the types of protonic acid also influenced the properties of the DEChN hydrogels and aerogels under the same mass concentration (0.6%) of DEChNs, including the appearance, strength of hydrogels, the pore size and specific surface area of aerogels, as well. Nevertheless, all of the hydrogels showed good mechanical strength and all of the aerogels possessed a porous nanostructure sustained by nanofibrillar networks. The highly porous chitin aerogels showed a broad size mainly distribution from 2 to 100 nm, and the specific surface area of chitin aerogels ranged from 90 to 170 m2g−1. Although it was not clear how the types of protonic acids influenced the properties of the hydrogel and aerogel, we were able to obtain DEChN dispersions and gels by mechanical treatment of partially deacetylated chitin at acidic conditions mediated by versatile acids. In the meantime, the observed differences among the gels provided the possibility to use the acid medium to select the gels with certain mechanical strength, porosity or specific surface area for the prospective desired applications.

Structural, mechanical and thermal behaviors of novolac/graphene oxide nanocomposite aerogels

Main microstructural properties of organic aerogels dictate the thermal degradation of these materials. Incorporating of nanomaterials into the structure of organic aerogels can result in controlling and engineering the microstructural characteristics and as a result, the thermal degradation of organic aerogels. Here, the role of graphene oxide (GO) presence on the microstructure and morphology of novolac/GO nanocomposite aerogels is investigated. Although, the presence of GO nanosheets results in an enhancement of the density, but the specific surface area of aerogels increased with the contents of loaded-nanosheets, due to the high available surface area of GO nanosheets. The sol-gel polymerization of novolac resin in the presence of GO nanosheets results in formation of colloidal nanoparticles on the surface of GO nanosheets. Chemical characterizations confirm that the reason behind this phenomenon is the formation of covalent interactions between polymer chains and GO surface. Moreover, the thermal degradation and thermal conductivity behaviors of novolac/GO nanocomposite aerogels are studied here, to investigate the influences of GO presence on the heat-transfer mechanism through the aerogel. Thermal studies approve that the heat transfer depends strongly on the reduction of GO nanosheets, as well as on the contents of loaded-GO. As the contents of loaded-GO reach a percolation threshold, the structural characteristics, and consequently, the mechanism of thermal degradation throughout aerogel structure, change.

Biomass-Derived Carbon Fiber Aerogel as a Binder-Free Electrode for High-Rate Supercapacitors

A flexible carbon fiber aerogel with a very high surface area for supercapacitor application is reported by carbonization and chemical activation of low-cost natural cotton with KOH. The carbon fibers in the aerogel present as a twisted and tubular structure. Depending on the amount of KOH used in the activation process, the specific surface area of aerogels ranges from 1536 to 2436 m2 g–1, while their electrical conductivity remains ∼860 S m–1. In spite of pore size in the range of 1.0–4.0 nm and pore volume mainly contributed by micropores, the carbon aerogel exhibits a high specific capacitance of 283 F g–1 (1 A g–1) in 6 M KOH aqueous electrolyte and retains a high capacitance retention of 224 F g–1 at current density up to 100 A g–1. Importantly, a symmetric capacitor built with the aerogel electrodes exhibits a rather small time constant (0.56 s). The superior capacitive performance of a CF electrode is closely related to its distinct structural advantage. The tubular carbon fibers that are several ...

Microstructure of Al&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;3&lt;/sub&gt; Xerogels and Aerogels from Aluminum-tri-&lt;i&gt;sec&lt;/i&gt;-Butoxide Modified with Ethylacetoacetate

Microstrucutre of Al2O3 xerogels and aerogels prepared from aluminum-tri-sec-butoxide (Al (O-sec-Bu)3) modified with ethylacetoacetate (EAcAc) was investigated for controlling the microstructure of Al2O3. The specific surface area and the pore volume of heat-treated xerogels decreased with an increase in the EAcAc/Al (O-sec-Bu)3 molar ratio, whereas the size of pores was not affected by the ratio. The specific surface area of aerogels decreased monotonously with an increase in the heat-treatment temperature up to 1000°C and was larger than that of xerogels in the whole temperature range of heat treatment, while that of xerogels decreased with increasing temperature due to sintering and drastically decreased at 1000°C.

Critical concentrations of polymers in solutions according to measurements of the viscosity and specific surface area of aerogels resulting after sublimation of the solvent

Critical concentrations of polystyrene and cyclolinear polyphenylsiloxane solutions corresponding to rising of structural networks have been determined by viscosity measurements and by measurements of specific surface of aerogels obtained by solvent sublimation. Theoretical and experimental values of critical concentrations coincide. Conditions for preparation of aerogels with developed surface by sublimation drying have been determined. Dependence of aerogel specific surface on polymer concentration in solutions shows distinct maximum at critical concentration found by viscosimetric method.

Critical concentrations of polymers in solutions according to measurements of the viscosity and specific surface area of aerogels resulting after sublimation of the solvent

Critical concentrations of polystyrene and cyclolinear polyphenylsiloxane solutions corresponding to rising of structural networks have been determined by viscosity measurements and by measurements of specific surface of aerogels obtained by solvent sublimation. Theoretical and experimental values of critical concentrations coincide. Conditions for preparation of aerogels with developed surface by sublimation drying have been determined. Dependence of aerogel specific surface on polymer concentration in solutions shows distinct maximum at critical concentration found by viscosimetric method.