Silica-based Aerogels Research Articles

Preparation of novel silica-clay mineral aerogel nanocomposites with outstanding thermal insulation performance

Monolithic silica-based aerogels, produced with tetraethoxysilane (TEOS), reinforced with aramid fibers and incorporating aluminosilicates were prepared for the first time. The nanocomposites were obtained by varying the amount and type of aramid fibers and replacing a part of the synthetic Si (up to 35 wt% of Si) of TEOS by natural aluminosilicates, kaolin and palygorskite. The effects of the type and amount of both fiber and mineral in the materials key properties were investigated. Most samples displayed very low volumetric shrinkages during ambient drying (<5%), high porosity (∼90%), and low bulk densities, down to 120 kg m−3. Composites with both minerals exhibited superinsulating thermal performance, with thermal conductivity reaching values as low as 22 mW m−1 K−1, while also showing excellent thermal stability. The mechanical properties of the designed clay mineral-silica aerogels can be tailored, since the addition of short length fibers (Kevlar pulp) produces stiffer composites, while the longer fibers (Teijinconex) lead to more flexible ones.

A Review of High-Temperature Aerogels: Composition, Mechanisms, and Properties.

High-temperature aerogels have garnered significant attention as promising insulation materials in various industries such as aerospace, automotive manufacturing, and beyond, owing to their remarkable thermal insulation properties coupled with low density. With advancements in manufacturing techniques, the thermal resilience of aerogels has considerable improvements. Notably, polyimide-based aerogels can endure temperatures up to 1000 °C, zirconia-based aerogels up to 1300 °C, silica-based aerogels up to 1500 °C, alumina-based aerogels up to 1800 °C, and carbon-based aerogels can withstand up to 2500 °C. This paper systematically discusses recent advancements in the thermal insulation performance of these five materials. It elaborates on the temperature resistance of aerogels and elucidates their thermal insulation mechanisms. Furthermore, it examines the impact of doping elements on the thermal conductivity of aerogels and consolidates various preparation methods aimed at producing aerogels capable of withstanding temperatures. In conclusion, by employing judicious composition design strategies, it is anticipated that the maximum tolerance temperature of aerogels can surpass 2500 °C, thus opening up new avenues for their application in extreme thermal environments.

Catalytically Active SiO2 Aerogels Comprising Chelate Complexes of Palladium.

A series of silica-based aerogels comprising novel bifunctional chelating ligands was prepared. To produce target aerogels, two aminosilanes, namely (3-aminopropyl)trimethoxysilane (APTMS) and N-(2-aminoethyl)-3-aminopropyltrimethoxysilane (AEAPTMS), were acylated by natural amino acids ((S)-(+)-2-phenylglycine or L-phenylalanine), followed by gelation and supercritical drying (SCD). Lithium tetrachloropalladate was used as the metal ion source to prepare strong complexes of Pd2+ with amino acids covalently bonded to a silica matrix. Aerogels bearing chelate complexes retain the Pd2+ oxidation state after supercritical drying in CO2, but the Pd ion is reduced to Pd metal after SCD in isopropanol. Depending on the structure of amino complexes, Pd-containing aerogels showed catalytic activity and selectivity in the hydrogenation reactions of C=C, C≡C and C=O bonds.

Hydrosilylation Adducts to Produce Wide-Temperature Flexible Polysiloxane Aerogel under Ambient Temperature and Pressure Drying.

Despite incorporation of organic groups into silica-based aerogels to enhance their mechanical flexibility, the wide temperature reliability of the modified silicone aerogel is inevitably degraded. Therefore, facile synthesis of soft silicone aerogels with wide-temperature stability remains challenging. Herein, novel silicone aerogels containing a high content of Siare reported by using polydimethylvinylsiloxane (PDMVS), a hydrosilylation adduct with water-repellent groups, as a "flexible chain segment" embedded within the aerogel network. The poly(2-dimethoxymethylsilyl)ethylmethylvinylsiloxane (PDEMSEMVS) aerogelis fabricated through a cost-effective ambient temperature/pressure drying process. The optimizedaerogel exhibits exceptional performance, such as ultra-low density (50mg cm-3), wide-temperature mechanical flexibility, and super-hydrophobicity, in comparison to the previous polysiloxane aerogels. A significant reduction in the density of these aerogels is achieved while maintaining a high crosslinking density by synthesizing gel networks with well-defined macromolecules through hydrolytic polycondensation crosslinking of PDEMSEMVS. Notably, the pore/nanoparticle size of aerogels can be fine-tuned by optimizing the gel solvent type. The as-prepared silicone aerogels demonstrate selective absorption, efficient oil-water separation, and excellent thermal insulation properties, showing promising applications in oil/water separation and thermal protection.

Biomedical applications of silica-based aerogels: a comprehensive review

Biomedical applications of silica-based aerogels: a comprehensive review

Hydrophobic and elastic silica-based aerogels with organic and inorganic in-situ hybrid structure for cryogenic insulation

Silica aerogels have potential to be used as alternatives in the cryogenic applications, but are still limited by the complex preparation process and high cost. In this work, a simple self-catalyzed gelling process and ambient drying method were utilized to prepare hydrophobic and low-density silica-based gels (HSAs) with organic and inorganic in-situ hybrid structure using methyltrimethoxysilane and bis[3-(triethoxysilyl)propyl]amine (BTPA) as precursors. The self-catalyzed gelling strategy is ascribed to the amine groups from BTPA. HSAs show a reversible deformation of 14 % and high failure strength up to 1.6 MPa even at the low temperature of −50 °C. Benefiting from the low density and high specific surface area, the thermal conductivity of the silica-based aerogels at −50 °C is as low as 21.8 mW·m−1·K−1. The HSAs are potential as ideal cryogenic insulation materials in a low-temperature and humid environment.

Resilient silica-based aerogels with organic and inorganic molecular hybrid structure prepared by a novel self-catalyzed gelling strategy for efficient heat insulation and CO2 adsorption

Silica aerogels have been attractive candidates for thermal insulation in many scenes, but they still suffer from high costs and especially brittle nature. In this work, a novel self-catalyzed gelling strategy was proposed to prepare resilient hybrid aerogels (HAs) with an organic–inorganic in-situ hybrid structure at the molecular level. Bis(trimethoxysilylpropy)amine is chosen as the single silicon source because it has alkalinity from the amine groups, high six functionality and molecular organic–inorganic hybrid nature, which help to achieve self-catalyzed gelling, high strength, and great resilience, respectively. For the as-prepared HAs, a 20 % fully recoverable strain, high Young’s modulus up to 10.40 MPa, high ultimate stress as high as 2.41 MPa and large ultimate deformation of 52 % are achieved together. Interestingly, HAs exhibit an improved modulus and strength after multiple compression, and this can increase the servicing safety in practical application. Benefiting from the synthetic effects of low density, high specific surface area and small pore size, the thermal conductivity of our aerogels is down to 18.8 mW·m−1·K−1 at ambient condition. Furthermore, the hydrophobic aerogels prepared through chlorotrimethylsilane modification also display a low thermal conductivity of 18.9 mW·m−1·K−1 at −50 °C and desirable mechanical properties at low temperatures. Unexpectedly, the rich amine groups and large specific surface area endow the fabricated aerogels with a remarkably high CO2 adsorption capacity up to 5.21 mmol/g at 273 K and excellent cyclic reusability. Considering these excellent functionalities, HAs have great prospects to serve as ideal thermal insulators, cold insulators and CO2 sorbents. The self-catalyzed gelling strategy provides an alternative route for the simple preparation of silica-based aerogels.

Enhanced CO2 adsorption property of amine in-situ hybrid SiO2 aerogels by the incorporation of micropores

Silica aerogels are promising CO2 adsorbents, but they still suffer from serious limitations including high cost and low CO2 adsorption capacity. According to the knowledge of micropores promoting faster and more uptake of CO2, amine in-situ hybrid SiO2 aerogels (AIHSAs) with hierarchically porous structure were prepared. Due to the steric hindrance effect of long organic chain from bis[3-(triethoxysilyl)propyl]amine as silicon precursor, the micropores were constructed. The obtained AIHSAs possess the high content of amine groups, high specific surface area (as high as 579.63 m2/g) and high pore volume. Therefore, a fascinating CO2 capture ability up to 6.57 mmol/g at 273 K was achieved, which is significantly higher than the values of other silica-based aerogels. The as-prepared AIHSAs have great application prospects in the field of CO2 capture to promote carbon neutrality.

Using small angle x-ray scattering to examine the aggregation mechanism in silica nanoparticle-based ambigels for improved optical clarity.

Silica-based aerogels are a promising low-cost solution for improving the insulation efficiency of single-pane windows and reducing the energy consumption required for space heating and cooling. Two key material properties required are high porosity and small pore sizes, which lead to low thermal conductivity and high optical transparency, respectively. However, porosity and pore size are generally directly linked, where high porosity materials also have large pore sizes. This is unfavorable as large pores scatter light, resulting in reduced transmittance in the visible regime. In this work, we utilized preformed silica colloids to explore methods for reducing pore size while maintaining high porosity. The use of preformed colloids allows us to isolate the effect of solution conditions on porous gel network formation by eliminating simultaneous nanoparticle growth and aggregation found when using typical sol-gel molecular-based silica precursors. Specifically, we used in situ synchrotron-based small-angle x-ray scattering during gel formation to better understand how pH, concentration, and colloid size affect particle aggregation and pore structure. Ex situ characterization of dried gels demonstrates that peak pore widths can be reduced from 15 to 13nm, accompanied by a narrowing of the overall pore size distribution, while maintaining porosities of 70%-80%. Optical transparency is found to increase with decreasing pore sizes while low thermal conductivities ranging from 95 +/- 13 mW/m K are maintained. Mechanical performance was found to depend primarily on effective density and did not show a significant dependence on solution conditions. Overall, our results provide insights into methods to preserve high porosity in nanoparticle-based aerogels while improving optical transparency.

Self-catalyzed Gelling Synthesis of Aerogels with Inorganic and Organic Nanocomponents for Thermal Insulation and CO2 Capture

Silica aerogels have already been recognized as potential candidates in the fields of thermal insulation, sorption, and separation, but the intrinsic brittleness and cumbersome preparation process limit their broad applications. In this work, a simple self-catalyzed gelling strategy was applied to prepare organic–inorganic in situ hybrid aerogels (OIHAs) derived from bis[3-(triethoxysilyl)propyl]amine (BTPA) and methyltrimethoxysilane (MTES). The Si–CH3 groups from MTES make it possible for OIHAs to be prepared through ambient drying and exhibit good hydrophobicity. Owing to the alkalinity of the amine groups from BTPA, self-catalyed hydrolyzation and condensation reactions occur between two silicon precursors to build a uniform pearl-chain nanoporous structure. The high specific surface area and amine groups existing on the nanoskeleton surface of the obtained OIHAs endow them with fascinating CO2 capture ability up to 5.89 mmol/g at 298 K and 100 kPa, which are significantly higher than the values of other aerogels reported under the similar conditions. Furthermore, ultralow thermal conductivity (as low as 20.1 mW/(m·K) at 25 °C), high strength (up to 2.38 MPa), and great elasticity (fully recoverable elastic strain of 14%) are achieved together for the OIHAs due to the organic–inorganic in situ cross-linked architecture. The as-prepared OIHAs with versatile properties have great application prospects in the fields of building thermal insulation and CO2 capture. The self-catalyzed gelling strategy provides an alternative routine for the simple preparation of silica-based aerogels.

Polyurethane foam scaffold for silica aerogels: effect of cell size on the mechanical properties and thermal insulation

Silica-based aerogels have been successfully reinforced by means of reticulated polymeric polyurethane (PU) foams with different cell sizes. The resultant silica aerogel-PU foam composites (Sil-PU composites) were fully characterized (density, shrinkage, aerogel percentage, and porous structure), and the mechanical properties and thermal conductivities were analyzed. Moreover, the effect of the application of a surface modification was assessed. A clear influence of the foam pore size on the final properties was found, and the mechanical properties of the aerogels have been notably improved reaching higher elastic modulus (from 130 to 307 kPa), excellent recovery ratios (above 95%), and significant deformations (more than 70%) without breaking. Therefore, the synthesized composites showed a great elasticity (high recovery ratios), tenacity, resilience, and stiffness in comparison with the non-reinforced aerogels. The obtained samples also showed excellent insulating capacities, reaching values between 14.0 and 12.3 mW/(m·K) for the surface-modified composites that were dried under supercritical conditions. Thus, using reticulated PU foams as a skeleton for aerogels is a promising strategy for a broad spectrum of applications in which silica aerogels are suitable candidates.

Novel aminophosphonate ligand for the preparation of catalytically active silica aerogels with finely dispersed palladium

Silica-based aerogels bearing a novel aminophosphonate-containing substituent were prepared. Pd2+ and Cu2+ ions were introduced into aerogel matrix to form chelated complexes with aminophosphonate moiety. Both complexes securely retain Me2+ after supercritical drying (SCD) in CO2. In silica matrix, Pd complex is reduced to metallic Pd after treatment by gaseous H2 at 120 °C. In lyogel matrix, Pd complex can also be reduced to metallic Pd upon SCD in isopropanol. Pd-containing aerogel showed excellent catalytic activity in the reaction of C=C and C=Cbonds hydrogenation.

Biological Thermal Performance of Organic and Inorganic Aerogels as Patches for Photothermal Therapy.

Aerogels are materials with unique properties, among which are low density and thermal conductivity. They are also known for their exquisite biocompatibility and biodegradability. All these features make them attractive for biomedical applications, such as their potential use in photothermal therapy (PTT). This technique is, yet, still associated with undesirable effects on surrounding tissues which emphasizes the need to minimize the exposure of healthy regions. One way to do so relies on the use of materials able to block the radiation and the heat generated. Aerogels might be potentially useful for this purpose by acting as insulators. Silica- and pectin-based aerogels are reported as the best inorganic and organic thermal insulators, respectively; thus, the aim of this work relies on assessing the possibility of using these materials as light and thermal insulators and delimiters for PTT. Silica- and pectin-based aerogels were prepared and fully characterized. The thermal protection efficacy of the aerogels when irradiated with a near-infrared laser was assessed using phantoms and ex vivo grafts. Lastly, safety was assessed in human volunteers. Both types presented good textural properties and safe profiles. Moreover, thermal activation unveils the better performance of silica-based aerogels, confirming the potential of this material for PTT.

Aesthetic Aerogel Window Design for Sustainable Buildings

Transport of heat through windows accounts for more than 25% of heating and cooling losses in residential buildings. Silica-based aerogels are translucent with extremely low thermal conductivity, which make them attractive for incorporation into the interspaces of glazing units. Widespread incorporation of monolithic-silica-aerogel-based windows could result in significant energy savings associated with the heating and cooling of buildings. However, monolithic silica aerogels do not have the optical clarity of vision glass, due to light scattering by the solid matrix, and often have surface imperfections, both of which render these materials less appealing for glazing applications. Here, we demonstrate a variety of approaches to preparing aesthetically pleasing monolithic silica aerogel by a rapid supercritical extraction method for incorporation into glazing units, including: (1) process improvements that result in monoliths with higher visible light transmission; (2) innovative mold design for the preparation of uniform aerogel monoliths; (3) glazing designs that use thinner monoliths; and (4) the incorporation of artistic effects using dyes and laser etching to prepare glazing units with mosaic- or stained-glass-like patterns in which surface imperfections are perceived as features of the design rather than flaws.

Research progress and applications of silica-based aerogels - a bibliometric analysis.

Silica aerogels are three-dimensional porous materials that were initially produced in 1931. During the past nearly 90 years, silica aerogels have been applied extensively in many fields. In order to grasp the progress of silica-based aerogels, we utilize bibliometrics and visualization methods to analyze the research hotspots and the application of this important field. Firstly, we collect all the publications on silica-based aerogels and then analyze their research trends and performances by a bibliometric method regarding publication year/citation, country/institute, journals, and keywords. Following this, the major research hotspots of this area with a focus on synthesis, mechanical property regulation, and the applications for thermal insulation, adsorption, and Cherenkov detector radiators are identified and reviewed. Finally, current challenges and directions in the future regarding silica-based aerogels are also proposed.

Porous Aerogels and Adsorption of Pollutants from Water and Air: A Review.

Aerogels are open, three-dimensional, porous materials characterized by outstanding properties, such as low density, high porosity, and high surface area. They have been used in various fields as adsorbents, catalysts, materials for thermal insulation, or matrices for drug delivery. Aerogels have been successfully used for environmental applications to eliminate toxic and harmful substances—such as metal ions or organic dyes—contained in wastewater, and pollutants—including aromatic or oxygenated volatile organic compounds (VOCs)—contained in the air. This updated review on the use of different aerogels—for instance, graphene oxide-, cellulose-, chitosan-, and silica-based aerogels—provides information on their various applications in removing pollutants, the results obtained, and potential future developments.

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.

Silica-Based Aerogels with Tunable Properties: The Highly Efficient BF3-Catalyzed Preparation and Look inside Their Structure

This work presents a solution to one of the most fundamentally and practically important challenges in the production of silica-based aerogels, time consumption and expensiveness of these processes...

Advanced Fabrication of Lightweight Aerogels from Fly Ash for Thermal Insulation

For the first time, an environmentally friendly and effective procedure to produce high-value engineering aerogels from fly ash (FA) has been developed by dispersing FA particles into a mixture of biodegradable polyvinyl alcohol (PVA) and carboxymethyl cellulose (CMC), followed by freezedrying. The effect of FA content on the physical properties, morphology, mechanical strength, and thermal conductivity of FA aerogels is also studied comprehensively. The lightweight FA aerogels show a low density of 0.072 – 0.093 g/cm3 with high porosity of 94.94 – 95.78%. The morphology of aerogels shows the uniform distribution of FA particles in PVA-CMC matrixes that creates a porous structure with a pore size of 2-5 mm. Therefore, the FA aerogels exhibit good heat insulation with extremely low thermal conductivity of 0.040 – 0.047 W/m.K at ambient temperature and pressure that is comparable to some commercial insulation materials such as mineral wool, fiberglass, expanded polystyrene, and other silica-based aerogels from waste. Moreover, the compressive modulus of FA aerogels is about 67.73 – 254.75 kPa indicating their excellent mechanical properties under 1 kN vertical compression. The experimental results indicate the significant better durability of FA aerogels than that of previous aerogels from other wastes such as sugarcane bagasse (88 kPa), pineapple leaf fibers (1.64 – 5.34 kPa), recycled polyethylene terephthalate (1.16 – 2.87 kPa), spent coffee grounds (5.41 – 15.62 kPa), and silica – cellulose (86 – 169 kPa). It is concluded that FA aerogels are a promising candidate as a lightweight thermal insulating material.

Fiber engineering of silica-based aerogels with surface specificity and regenerability for continuous removal of dye pollutants from wastewaters

Aerogels are attractive candidates as advanced adsorbents in water-treatment given their high specific surface area and reticulated porous structure. We present here hollow silica aerogel fibers (SAFs) engineered based on a wet spinning approach for dye removal from wastewaters. The surface characteristics of SAFs can be flexibly adjusted via a simple post-modification by silane coupling agents, delivering not only high dye removal efficiency but also great feasibility for various dyes. By the means of incorporating commercially used photocatalysis active nanoparticles (P25) into SAFs (denoted as SAF-P25), a photo-regenerable SAF-P25 adsorbent can be successfully prepared. SAF-P25 can recover its initial adsorption properties via a simple UV or natural light irradiation treatment. Consequently, the novel SAF-P25 is able to continuously adsorb and degrade dyes in wastewaters with a long-term dye removal performance. In addition to high adsorption efficiency and photo-renewability, the SAF-based adsorbent also preserves the merits of low cost, good scalability, wide applicability, and operation simplicity; all these make it become a promising dye removal system for future wide usage in various wastewater purifications. • Silica aerogel fibers (SAFs) were manufactured by wet spinning. • Specific surface modification endowed SAFs with adsorption selectivity. • After the introduction of photocatalyst, SAFs showed renewability and maintained excellent adsorption performance. • SAF-P25 can continuously and efficiently remove dyes under UV or natural light.