Porous Silica Aerogel Research Articles

Revealing nanoscale sorption mechanisms of gases in a highly porous silica aerogel.

Geological formations provide a promising environment for the long-term and short-term storage of gases, including carbon dioxide, hydrogen and hydro-carbons, controlled by the rock-specific small-scale pore structure. This study investigates the nanoscale structure and gas uptake in a highly porous silica aerogel (a synthetic proxy for natural rocks) using transmission electron microscopy, X-ray diffraction, and small-angle and ultra-small-angle neutron scattering with a tracer of deuterated methane (CD4) at pressures up to 1000 bar. The results show that the adsorption of CD4 in the porous silica matrix is scale dependent. The pore space of the silica aerogel is fully accessible to the invading gas, which quickly equilibrates with the external pressure and shows no condensation on the sub-nanometre scale. In the 2.5-50 nm pore size region a classical two-phase adsorption behaviour is observed. The structure of the aerogel returns to its original state after the CD4 pressure has been released.

Dual role of two-dimensional graphene in silica aerogel composite: Thermal resistance and heat node

Incorporating two-dimensional graphene sheets, known for their high in-plane thermal conductivity, into silica aerogel enhances both the thermal insulation and mechanical properties of the composite. This work delves into the impact of graphene doping on the solid thermal conductivity of silica composites and their mechanical characteristics, employing molecular dynamics simulations that encompass both amorphous silica bulk and porous silica aerogel structures. Our findings reveal that graphene–silica interface thermal resistance notably restricts heat conduction in graphene doped silica aerogel, with the most pronounced effect occurring when graphene aligns vertically with the heat flux. Within porous silica aerogel, graphene serves as a “heat node”, bridging and connecting adjacent pores. A notable “trade-off” emerges: high weight percentages of graphene sheets augment heat transfer through the “heat node effect”, whereas the interfacial thermal resistance between graphene and aerogel reduces solid heat transfer, leading to decreased thermal conductivity. Additionally, while the inclusion of graphene sheets offers limited mechanical property enhancements, their two-dimensional structure can cause detachment and slipping from the silica matrix, leading to localized improvements in mechanical strength. This work sets the stage for advancing graphene-based aerogel composites characterized by low thermal conductivity and enhanced mechanical properties.

Magnetic Aerogels for Room-Temperature Catalytic Production of Bis(indolyl)methane Derivatives.

The potential of aerogels as catalysts for the synthesis of a relevant class of bis-heterocyclic compounds such as bis(indolyl)methanes was investigated. In particular, the studied catalyst was a nanocomposite aerogel based on nanocrystalline nickel ferrite (NiFe2O4) dispersed on amorphous porous silica aerogel obtained by two-step sol-gel synthesis followed by gel drying under supercritical conditions and calcination treatments. It was found that the NiFe2O4/SiO2 aerogel is an active catalyst for the selected reaction, enabling high conversions at room temperature, and it proved to be active for three repeated runs. The catalytic activity can be ascribed to both the textural and acidic features of the silica matrix and of the nanocrystalline ferrite. In addition, ferrite nanocrystals provide functionality for magnetic recovery of the catalyst from the crude mixture, enabling time-effective separation from the reaction environment. Evidence of the retention of species involved in the reaction into the catalyst is also pointed out, likely due to the porosity of the aerogel together with the affinity of some species towards the silica matrix. Our work contributes to the study of aerogels as catalysts for organic reactions by demonstrating their potential as well as limitations for the room-temperature synthesis of bis(indolyl)methanes.

A novel pearls-like hierarchical porous silica aerogel monolith for efficient oil/water separation

A novel pearls-like hierarchical porous silica aerogel monolith for efficient oil/water separation

Fiber-based hydrophobic and stretchable UV-photochromic textile

Wearable sensors composed of fabric have attracted considerable research interest as they can detect various environmental factors that affect the human body, such as ultraviolet (UV) radiation, temperature, and humidity. Among these sensors, color-conversion sensors are especially valuable as they can provide a visual representation of data without the need for additional electrical devices. For a sensor to be used on the human body, it must maintain its sensing capabilities even when exposed to solutions such as rainwater, sweat, and various cleansing liquids or when subjected to mechanical deformation. Therefore, this study was aimed at fabricating a UV-photochromic fiber that can maintain its color-change reaction to UV light even when exposed to chemical and physical stressors. To this end, UV-photochromic pigment was infused into porous and hydrophobic silica aerogels to create UV-photochromic aerogels, which were then placed between the inner strands of spandex fibers. During the drying process, the UV-photochromic aerogels adhered firmly to the spandex strands and were not separated even when the fibers were stretched and contracted up to 100 times under 200% tensile conditions. The resulting UV-photochromic fibers initially appeared white but transformed to magenta when exposed to UV light. This color change persisted through seven repeated on–off cycles of UV light. Moreover, the hydrophobic nature of the fibers ensured that the color-change reactivity remained stable even under exposure to water, artificial sweat, and detergent solutions. Overall, the proposed UV-photochromic fibers, with enhanced reversibility, stability, and reliability, can promote the commercialization of wearable-based colorimetric sensors.

Theoretical study of heat transfer model of silica aerogel based on the porous structure of secondary particles

In this paper, the secondary particles of silica aerogels are equivalently modeled using the spherical hollow cube model, and the heat transfer of porous silica aerogels are linked to their internal structural parameters, thus establishing a theoretical model of the heat transmission process of porous silica aerogels formed by the interconnected accumulation of secondary particles. The model analyzes and calculates the pertinent parameters as well as the overall thermal conductivity while accounting for the secondary particles' porous structure. It then compares its predictions with experimental results and predictions from other models for various pressures and temperatures. The results demonstrate good agreement between the constructed model and the experimental data, the deviation is controlled within 12% and the minimum deviation can reach 1.6%. The model is beneficial for enhancing the thermal insulation performance of nanoporous aerogels because it is effective in predicting the heat transmission process within the porous medium silica aerogel.

A multi-scale model for use in hydrodynamic simulations of impacts and hypervelocity penetrations into porous silica aerogel

Low-density silica aerogel is an ideal material for capturing hypervelocity interplanetary dust. To investigate the behavior of hypervelocity shock waves at different particle velocities using silica aerogel densities ranging from 80 kg/m3 to 320 kg/m3, a multi-scale Smooth Particle Hydrodynamics (SPH) approach is employed. The simulation results demonstrate a good agreement with the macroscopic experimental findings across a wide range of particle velocities, from 0.5 km/s to 20 km/s, while considering the shock wave velocities of the porous material and their linear variation. In addition, simulations of pressure-specific volume curves were conducted using the microscopic model. According to the simulation results, we present a novel compression model of silica aerogel materials regarding the classical form of the P−V relationship for porous materials. Building upon the microscopic model, we propose a macroscopic equivalent model for dynamic compression of low-density aerogel, which is subsequently incorporated into the hydrodynamic calculation of interplanetary dust hypervelocity penetration into aerogel. The results of the simulation of hypervelocity penetration indicate that the multi-scale model presented in this work can be used to simulate the macroscopic mechanical behavior of aerogels in a non-uniform field.

Using Ex Situ and In Situ HERFD-XANES to Reveal the Superior Oxidation and Reduction Cycling of Ceria Nanocubes Dispersed in Silica Aerogel.

The oxygen storage capacity of ceria-based catalytic materials is influenced by their size, morphology, and surface structure, which can be tuned using surfactant-mediated synthesis. In particular, the cuboidal morphology exposes the most reactive surfaces; however, when the capping agent is removed, the nanocubes can agglomerate and limit the available reactive surface. Here, we study ceria nanocubes, lanthanum-doped ceria nanocubes, and ceria nanocubes embedded inside a highly porous silica aerogel by high-energy resolution fluorescence detection-X-ray absorption near edge spectroscopy at the Ce L3 edge. In situ measurements showed an increased reversibility of redox cycles in ceria nanocubes when embedded in the aerogel, demonstrating enhanced reactivity due to the retention of reactive surfaces. These aerogel nanocomposites show greater improvement in the redox capacity and increased thermal stability of this catalytic material compared to the surfactant-capped nanocubes. Ex situ measurements were also performed to study the effect of lanthanum doping on the cerium oxidation state in the nanocubes, indicating a higher proportion of Ce4+ compared to that of the undoped ceria nanocubes.

Three-Dimensional-Network-Structured Bismuth-Based Silica Aerogel Fiber Felt for Highly Efficient Immobilization of Iodine.

The effective capture and deposition of radioactive iodine in the spent fuel reprocessing process is of great importance for nuclear safety and environmental protection. Three-dimensional (3D) fiber felt with structural diversity and tunability is applied as an efficient adsorbent with easy separation for iodine capture. Here, a bismuth-based silica aerogel fiber felt (Bi@SNF) was synthesized using a facile hydrothermal method. Abundant and homogeneous Bi nanoparticles greatly enhanced the adsorption and immobilization of iodine. Notably, Bi@SNF demonstrated a high capture capacity of 982.9 mg/g by forming stable BiI3 and Bi5O7I phases, which was about 14 times higher than that of the unloaded material. Fast uptake kinetics and excellent resistance to nitric acid and radiation were exhibited as a result of the 3D porous interconnected network and silica aerogel fiber substrate. Adjustable size and easy separation and recovery give the material potential as a radioactive iodine gas capture material.

Microscopic revelation of the solid–gas coupling and Knudsen effect on the thermal conductivity of silica aerogel with inter-connected pores

As a star insulation material, aerogel plays a significant role in saving energy and meeting temperature requirements in industry due to its extremely low thermal conductivity. The prediction of aerogel’s thermal conductivity is of great interest in both research and industry, particularly because of the difficulty in measuring the separated gas conductivities directly by experiment. Hence, the proportions of separated gas conduction and solid–gas coupling conduction are debatable. In this work, molecular dynamics simulations were performed on porous silica aerogel systems to determine their thermal conductivities directly. The pore size achieved in the present study was improved significantly, making it possible to include the gas phase in the investigation of aerogel thermal conductivity. The separated solid conductivity {lambda }_{s} and the separated gas thermal conductivity {lambda }_{g} as well as the effective solid conductivity {lambda }_{s}^{e} and the effective gas conductivity {lambda }_{g}^{e} were calculated. The results suggest that the solid–gas coupling effect is negligible in rarefied gas because the enhancement of thermal conduction due to the short cut bridging effect by gas between gaps in the solid is limited. The gas pressure is the most significant factor that affects the solid–gas coupling effect. The large differential between the prediction and the actual value of the thermal conductivity is mainly from the underestimate of {lambda }_{g}, and not because of ignoring the coupling effect. As a conclusion, the solid–gas coupling effect can be neglected in the prediction of silica aerogel’s thermal conductivity at low and moderate gas pressure, i.e., decreasing the gas pressure is the most efficient way to suppress the coupling effect. The findings could be used in multi-scale simulations and be beneficial for improving the accuracy of predictions of aerogel thermal conductivity.

A computational micromechanics model to predict mechanical properties of porous silica aerogels

Porous silica aerogel is an ultra-low-density material with nanostructures that leads to its excellent physio-chemical properties. Considering the characteristics of the material, a micromechanical model to assess the microstructure–property relations is highly demanded. In this study, a cuboctahedron unit cell is proposed as a representative volume element of the silica aerogel to correlate with its density and compressive stress–strain curves. The backbone lattice-structure combined with the dead ends is established to represent the periodic configuration of the aerogel. It is implemented in both the numerical scheme with the finite element method and the analytical model by modifying the Gibson–Ashby model. Furthermore, the crushing behaviors of the material under large deformation are discussed in the numerical study. Due to the features of load-bearing skeletons and non-load-bearing short pillars, the compression process of the silica aerogel exhibits strong nonlinear behaviors. Overall, this computational micromechanics model is capable of accurately simulating the stress–strain curves of silica aerogels with different densities under different loading levels. This work provides a general framework to quantify the microstructure–property relations of porous silica aerogels and also other porous materials.

Ultralow dielectric cross-linked silica aerogel nanocomposite films for interconnect technology

An increase in resistance and capacitance (RC) delay has been a critical issue in reducing integrated circuit scales. The delay is successfully reduced using low dielectric constant (κ) interconnects and porous low-κ materials adopted in real devices. However, an increase in porosity could lower κ values with a dramatic decrease in elastic modulus, which would not sustain the stress during the packaging process. Highly porous cross-linked silica aerogel thin film with an ultralow-κ value of 1.7 applied to nanodevice interconnect technology has been suggested. Cross-linking using an epoxide ring-opening reaction produced a high elastic modulus (> 5.1 GPa), stable enough to sustain the chemical mechanical polishing (CMP) and the plasma dry etching process. The thickness of aerogel films is approximately 700 nm with a hydrophobic surface. Closed-surface pores created by selective solvent evaporation can prevent metal penetration. Our cross-linked aerogel films are the first among the low-dielectric aerogel films validated as interlayer low-dielectric materials and robust candidates for ultralow -κ dielectric materials.

Core shell Fe3O4@TiO2/silica aerogel nanocomposite; synthesis and study of structural, magnetic and photocatalytic properties

In this work, magnetic Fe3O4@TiO2 nanospheres supported by a mesoporous silica aerogel substrate were synthesized using simple combining procedure of sol-gel and ultrasound assisted reflux technique for photocatalytic activity. The characteristics study of the as-synthesized ternary nanocomposite showed highly dispersion and covering of the Fe3O4@TiO2 nanoparticles on the silica aerogel. The hydrophobic porous silica aerogel was prepared using industrial diatomite earth (DE) as a silica source by a sol-gel procedure under ambient pressure drying. The specifications results of the resulting silica aerogel revealed a fine particulate morphology with an average size of 26 nm, a low density of about 0.85 g/cm3 and a good degree of porosity with a BET surface area of 572 m2/g. In addition, the structural and morphological studies of the prepared Fe3O4@TiO2 also represented the formation of a pure Fe3O4@TiO2 phase with a core shell spherical morphology and an average particle size of 22 nm. The introduction of the Fe3O4@TiO2 on the silica aerogel nanoparticles led to increase the particle size to 38 nm and decrease of BET surface area to 347 m2/g confirming the formation of Fe3O4@TiO2/silica aerogel nanocomposite. The photocatalytic activity of the prepared ternary nanocomposite was also studied for the removal of tetracycline (TC) as an organic pollutant from water under visible light in details.

A predictive multiphase model of silica aerogels for building envelope insulations

This work develops a systematic uncertainty quantification framework to assess the reliability of prediction delivered by physics-based material models in the presence of incomplete measurement data and modeling error. The framework consists of global sensitivity analysis, Bayesian inference, and forward propagation of uncertainty through the computational model. The implementation of this framework on a new multiphase model of novel porous silica aerogel materials is demonstrated to predict the thermomechanical performances of a building envelope insulation component. The uncertainty analyses rely on sampling methods, including Markov-chain Monte Carlo and a mixed finite element solution of the multiphase model. Notable features of this work are investigating a new noise model within the Bayesian inversion to prevent biased estimations and characterizing various sources of uncertainty, such as measurements variabilities, model inadequacy in capturing microstructural randomness, and modeling errors incurred by the theoretical model and numerical solutions.

Enhanced phosphate removal by nano-lanthanum hydroxide embedded silica aerogel composites: Superior performance and insights into specific adsorption mechanism

Removal of phosphate from wastewater is a global crucial goal and grand challenge for resource recovery and eutrophication control. However, it represents a major application bottleneck in the phosphate-selective sorption system to enhance the selectivity towards phosphate and the stability of adsorbents. In this study, nano-lanthanum hydroxide was embedded inside the porous silica aerogel (SA) to synthesize a novel aerogel composite (LSA) for highly efficient and selective phosphate removal. La(OH)3 is expected to offer the specific binding sites toward phosphate, while the porous SA host serves as a “shield” with a protective network structure and supplies abundant non-specific sites. LSA possessed enhanced phosphate uptake even when competing anions coexisted at higher levels. Furthermore, LSA exhibited pH-independency due to its high pHpzc. LSA also performed the characteristics of high adsorption capacity, La utilization efficiency, and reusability. Specifically, the maximum Langmuir adsorption capacity of LSA-1/10 reached up to 153.8 mg P/g at pH 6.0. The exhausted LSA-1/10 could be effectively regenerated after 8 adsorption–desorption cycles. The underlying mechanism for such satisfactory performances of LSAs was revealed by XRD, STEM-EDS and XPS analysis. The predominant pathway for selective phosphate removal was confirmed to be the inner-sphere complexation between P and La, intriguingly, the formation of LaPO4·xH2O was detected. Moreover, LSAs were proved to be stable and exhibit no potential adverse effects on the ecosystem or human health. Overall, results proved LSA to be a promising sorbent for efficient phosphate capture in complex water remediation with the core features of preference and selectivity.

Thermal Gelation for Synthesis of Surface-Modified Silica Aerogel Powders.

A spherical silica aerogel powder with hydrophobic surfaces displaying a water contact angle of 147° was synthesized from a water glass-in-hexane emulsion through ambient pressure drying. Water glass droplets containing acetic acid and ethyl alcohol were stabilized in n-hexane with a surfactant. Gelation was performed by heating the droplets, followed by solvent exchange and surface modification using a hexamethyldisilazane (HMDS)/n-hexane solution. The pH of the silicic acid solution was crucial in obtaining a highly porous silica aerogel powder with a spherical morphology. The thermal conductivity, tapped density, pore volume, and BET surface area of the silica aerogel powder were 22.4 mW·m−1K−1, 0.07 g·cm−3, 4.64 cm3·g−1, and 989 m2·g−1, respectively. Fourier transform infrared (FT–IR) spectroscopy analysis showed that the silica granule surface was modified by Si-CH3 groups, producing a hydrophobic aerogel.

Preparation and Characterization of Lithium Ion Conductive Organic/Inorganic Composite Solid Electrolyte.

The purpose of this study was to improve the ionic conductivity and physical properties of a polymer electrolyte by complexing it with ceramic particles. First, a polymer/ceramic composite solid electrolyte was prepared by synthesizing a super porous silica aerogel powder and adding it to a slurry containing a polyethylene oxide (PEO)-based polymer electrolyte; then, the electrochemical properties of electrolyte of different compositions were confirmed. PEO and polymethyl methacrylate (PMMA) were copolymerized, the optimum ratio of lithium salt, plasticizer, and silica aerogel was determined, and ion conductivity of the composite electrolyte was improved. When the EO:Li ratio was 10:1, the glass transition temperature was the lowest, and the room-temperature ion conductivity was improved to 3.0 × 10-5 S/cm. As a result of XRD and thermal analysis, it was confirmed that the stability and durability of the electrolyte interface can also be improved by complexation of the polymer electrolyte with ceramic particles.

A Facile Synthesis of Highly Porous Silica Aerogel Hybrid Materials for BTX Adsorption

Abstract Benzene, toluene, and m-xylenes are among the pollutants in the environment that may harm human health. These fugitive volatile organic compound (VOC) emissions from refineries and petrochemical industries are perennial, although at low concentrations in ppm levels. On similar grounds, the separation of benzene, toluene, and m-xylene compounds (BTX) from the feed gas of the sulfur recovery unit in natural gas processing industries is critical, as it is known to severely poison the catalyst in the Claus process. In this connection, a new hybrid material was synthesized using a precursor (metal-organic framework (MOF)) and silica aerogels (SA). The precursor (ZIF-8) proportion was varied to understand its effect on the structural and adsorption characteristics. Various advanced analytical characterizations were performed to understand the physicochemical characteristics of the synthesized material. Additionally, the synthesized materials were subjected to gas-phase adsorption of BTX to generate the adsorption isotherm at 25 °C. The hybrid material SA-ZIF-8 (20%) having a ZIF-8 proportion of 20% were found to have better adsorption capacity than the virgin ZIF-8 and silica aerogel adsorbents. The maximum adsorption capacity near the 90% saturation vapor pressure corresponds to 337 mg/g, 227 mg/g, and 263 mg/g at 25 °C for BTX, respectively.

Influence of Silica-Aerogel on Mechanical Characteristics of Polyurethane-Based Composites: Thermal Conductivity and Strength.

Polyurethane foam (PUF) has generally been used in liquefied natural gas (LNG) carrier cargo containment systems (CCSs) owing to its excellent mechanical and thermal properties over a wide range of temperatures. An LNG CCS must be designed to withstand extreme environmental conditions. However, as the insulation material for LNGC CCSs, PUF has two major limitations: its strength and thermal conductivity. In the present study, PUFs were synthesized with various weight percentages of porous silica aerogel to reinforce the characteristics of PUF used in LNG carrier insulation systems. To evaluate the mechanical strength of the PUF-silica aerogel composites considering LNG loading/unloading environmental conditions, compressive tests were conducted at room temperature (20 °C) and a cryogenic temperature (−163 °C). In addition, the thermal insulation performance and cellular structure were identified to analyze the effects of silica aerogels on cell morphology. The cell morphology of PUF-silica aerogel composites was relatively homogeneous, and the cell shape remained closed at 1 wt.% in comparison to the other concentrations. As a result, the mechanical and thermal properties were significantly improved by the addition of 1 wt.% silica aerogel to the PUF. The mechanical properties were reduced by increasing the silica aerogel content to 3 wt.% and 5 wt.%, mainly because of the pores generated on the surface of the composites.

Micro-cellular polymer foam supported silica aerogel: Eco-friendly tool for petroleum oil spill cleanup

Flexible micro-cellular open porous 3D polymer foam silica aerogel composites with exceptional oil sorption characteristics were prepared by implanting silica aerogel in the interstices of foam substrates. Silica aerogels were engineered from silane precursors namely tetraethoxysilane (TEOS) and methyltrimethoxysilane (MTMS) adopting economical, energy efficient and scalable ambient pressure drying technique. These flexible composites exhibited outstanding structural ruggedness. The foam-aerogel composites deliver superior hydrophobic characteristics in terms of contact angle data. The aerogel composites recorded an oil uptake of 31.3 g per g of the composite for engine oil and the uptake was very swift. Mechanical squeezing enables near complete oil removal from the composites. The foam aerogel composites displayed superior recyclability features, as first time absorption is retained even after 10 cycles of repeated squeezing and re-absorption operations.