Hydrophobic Silica Aerogels Research Articles

FUNCTIONAL OPTICAL MATERIALS BASED ON AEROGELS

Sol-chemistry is an efficient physico-chemical method for the preparation of porous glassy or nanocrystalline materials with tailored electrical, thermal or optical properties. This paper focuses on the dependence preparation–structure-properties of hydrophobic silica aerogel granules, powders and composites with a potential application as optical materials. Using the technique of subcritical preparation of silica aerogel powders, multicolour emitting composites containing Eu(phen)2(NO3 )3 or Tb(phen)2(NO3 )3 nanocrystals are obtained and characterized with luminescence spectroscopy, quantum yield determination and texture measurements. A dependence of the luminescence quantum yield on the degree of hydrophobicity α of the silica aerogel powders is described.

Experimental Investigation of the Combustion Characteristics and Thermal Hazards of Methylsilyl-Modified Silica Aerogels

The thermal safety of hydrophobic silica aerogels (SAs) is essential to thermal insulation applications. Herein, trimethylchlorosilane (TMCS), dimethyldichlorosilane (DMDCS), and methyltrichlorosilane (MTCS) were employed as surface modifiers to prepare three different methylsilyl-modified SAs (i.e., TSA, DSA, and MSA) and their combustion characteristics and thermal hazards were experimentally studied in detail. The cone calorimeter test found that the three SAs have similar combustion processes and the variations in ignition time and fire spread rate with the heat flux obey simple logarithmic and linear relationships, respectively. It further found that TSA has the most methylsilyl groups on silica skeletons and thus has the largest heat release, followed by DSA and MSA in turn, implying that TSA has the greatest fire hazard among the three SAs. These results further demonstrate that the type and quantity of methylsilyl groups on the skeletons of SAs significantly affect the thermal hazard of methylsilyl-modified SAs. In addition, the combustion mechanism of the methylsilyl-modified SAs is discussed. In total, this work experimentally studies the combustion characteristics of methylsilyl-modified SAs and compares their thermal hazards, clarifying the potential fire risk of methylsilyl-modified SAs in practical thermal insulation applications.

One-pot synthesis of rationally-designed flexible, robust, and hydrophobic ambient-dried molecularly-bridged silica aerogels with efficient and versatile oil/water separation applications

One-pot synthesis of rationally-designed flexible, robust, and hydrophobic ambient-dried molecularly-bridged silica aerogels with efficient and versatile oil/water separation applications

Effectiveness of using hydrophobic silica aerogel and grape seed extract in creating a sunscreen formula

Abstract Wearing sunscreen products protects human skin from the damaging effects of ultraviolet radiation from the sun. An experimental study was conducted to evaluate the effectiveness of utilizing hydrophobic silica aerogel and grape seed extract as additional raw materials in creating a sunscreen formula. This study aims to evaluate the acceptability of the sunscreen formulation in terms of determining its sun protection factor using UV-vis spectrophotometry, ability to remain on the surface using water resistance testing, and shelf life using a stability chamber. The methodology begins with obtaining a modified hydrophobic silica aerogel classified as cosmetic grade. The sunscreen formulation has a combination of 2 wt. % of hydrophobic silica aerogel, 3 wt.% of grape seed oil, and other ingredients using thermal procedures. The results demonstrated that the addition of hydrophobic silica aerogel and grape seed extract in sunscreen formulation exhibits satisfactory UV protection attaining an SPF value of 28.17073 which indicates a medium sun protection factor according to the standardized category of SPF values. On the contrary, the sunscreen without the active ingredients has an SPF value of 4.762. Additionally, both sunscreen’s water resistance was assessed by testing a total of 60 minutes in both tap water and salt water. The findings showed that both samples of sunscreens in tap water were more resistant compared to saltwater. Furthermore, the sunscreen with hydrophobic silica aerogel and grape seed extract stayed intact and did not dissolve in tap water after a 40-minute exposure time, but it did gradually disintegrate after 20 minutes in salt water since salt water has a higher density than tap water. In stability testing of both sunscreen formulations, the results revealed that the moisture content of the sunscreen formulation with hydrophobic silica aerogel and grape seed extract does not exceed 10% which indicates a low presence of oil. Hence, it proves its stability under the controlled conditions of 40°C and 75% relative humidity examined over one (1) month since it had a high moisture content of 78.98% obtained using the gravimetric method. For the sunscreen without these ingredients, the total moisture content under the same conditions and method is 29.07%. Thus, it indicates a high presence of oil and does not attain the standard moisture content for sunscreens. Overall, the evaluated performance of adding the hydrophobic silica aerogel and grape seed extract to the sunscreen formulation ensures its efficacy regarding its SPF, water resistance, and shelf life.

Facile Fabrication of Hydrophobic and Elastic Silica Aerogels with Enhanced Anti/De-Icing Performance

The anti/de-icing methods are mainly divided into two types: active method and passive method. The passive method generally adopts three ideas: reducing water contact, inhibiting water icing and reducing ice adhesion. Silica aerogels, which have adjustable surface wettability, thermal conductivity and porosity, would be designed to satisfy the requirement for passive anti/de-icing methods. In this work, the organic–inorganic hybrid aerogels based on organosilicon were facilely prepared in a sol–gel procedure, which showed hydrophobic and elastic properties. It could be seen that the thermal insulation effect originating from aerogel-like porous network endowed the efficient anti-icing property. Moreover, the lowest ice adhesion strengths of the above silica aerogels and the related oil-based lubricant-filled aerogels were 7.1[Formula: see text]kPa and 5[Formula: see text]kPa, respectively, which belonged to the icephobic surface. Besides, the ice adhesion strength only increased to 15.6[Formula: see text]kPa after 30 icing–deicing cycles, showing that hydrophobic and elastic silica aerogels had efficient deicing durability.

Enhancing Mesopore Volume and Thermal Insulation of Silica Aerogel via Ambient Pressure Drying-Assisted Foaming Method.

Ambient pressure drying (APD) of silica aerogels has emerged as an attractive method adapting to large-scale production. Spring-back is a unique phenomenon during APD of silica aerogels with volume expansion after its shrinkage under capillary force. We attribute the intense spring-back at elevated drying temperatures to a dense structure formed on the surface and the formation of positive internal pressure. Furthermore, an APD-assisted foaming method with an in situ introduction of NH4HCO3 was proposed. NH4HCO3 decomposing at drying temperatures hastened the emergence of positive pressure, thereby increasing the expansion volume. Compared to the previous method, the porosity of silica aerogel increased from 82.2% to 92.6%, and mesopore volume from 1.79 cm3 g-1 to 4.54 cm3 g-1. By adjusting the amount of the silicon source, silica aerogels prepared by the APD-assisted foaming method generated higher volume expansion and lower thermal conductivity. After calcination to remove undecomposed ammonium salts, the hydrophobic silica aerogel with a density of 0.112 g cm-3 reached a mesopore volume of 5.07 cm3 g-1 and a thermal conductivity of 18.9 mW m-1·K-1. This strategy not only improves the thermal insulation properties, but also offers a significant advancement in tailoring silica aerogels with specific porosity and mesopore volume for various applications.

Low phosphorus-containing hydrophobic silica aerogels with high thermal stability and outstanding flame retardancy

To enhance the thermal safety of hydrophobic silica aerogel (SA), we synthesized a high thermal safety SA utilizing phytic acid (PA) as a catalyst. Tests confirmed that the SA/PA maintain low density (0.09 g/cm3) and low thermal conductivity (23.2 mW/m/K). The thermal stability and combustion characteristics of SA/PA were evaluated by thermogravimetric analysis (TG), gross calorific value (GCV) and cone calorimeter. The TG data clearly show that the addition of PA can greatly increase the decomposition temperature of silylmethyl by 208.1 °C and the residual char content to 92.6%. The GCV results show that a small amount of PA could greatly reduce the GCV of SA, with a maximum decrease of 51.6 %. Information from cone calorimeter data affirmed that PA can decrease the burning factors by decreasing the total heat release and peak heat release rate by 63.6% and 28.7%, respectively. The flame retardant mechanism of PA against SA was scientifically examined by thermogravimetric-infrared-mass spectrometry analysis and residual char analysis. In the gas phase, PA by capturing radicals produced during the combustion process of the material, the combustion chain reaction is quenched. In the condensed phase, PA promotes the rapid dehydration and carbonization of SA to establish a physical barrier and achieve flame retardancy. Our findings highlight the beneficial impact of PA on SA. The aerogel exhibited enhanced thermal stability and effective flame retardancy after PA treatment.

Highly efficient 3D-screen printed interfacial solar water desalination system

Interfacial solar steam generation (ISSG) using floating devices is one of the most efficient and simple methods of purifying saline water. In this work, multi-layered solar steam generation systems are fabricated using 3D-screen printing. The printed devices with a uniform layer-by-layer structure contain light-harvesting, thermal insulation, thermal conduction, and water-transfer layers. The graphite (Gr) photothermal layer as an absorber layer is screen printed on the felt substrate with three different 3D patterns, which is isolated by transparent hydrophobic silica aerogel (SA) as a light scattering agent to provide efficient photon capturing and heat localization. Furthermore, to transfer the generated heat to the water proficiently, a copper (Cu) layer is coated on the downside of the absorber layer. The devices with the best performances are introduced by evaluating the effect of the fabricated layers thickness and different configurations. The multi-layered device containing a photothermal layer with SP1-3rd pattern and different thicknesses of SA 32 μm (under Cu), Cu 16 μm, Gr 32 μm, and SA 32 μm (on top of the Gr) shows a steam generation efficiency of 97 %, and water evaporation flux of 1.43 kg.m−2.h−1, which are 86 % higher than the performance of the reference system.

A polyimide separator inlaid by methylsilyl-modified silica aerogels for high thermal safety and low polarization lithium-ion battery

The separator is a crucial component of lithium-ion batteries, playing a vital role in ion transfer and preventing polarization of the battery. To solve the problem of poor thermal safety of separators in lithium-ion batteries, in this work, three types of hydrophobic silica aerogels are compared. A suitable separator is fabricated by adding hydrophobic silica aerogels with monomethylsilyl as a modifier to pure polyimide. The aerogel composite separator displays a highly cross-linked three-dimensional nanoporous network, a low thermal shrinkage rate (<4 % at 200 °C), and high thermal stability (peak temperature of 670 °C) and oxygen index (>45 %). In addition, owing to the introduced hydrophobic functional groups of Si–CH3, the prepared separator decreases the polarization of the lithium-ion batteries and leads to improved power performance and cycle stability (its discharge voltage difference is 0.42 V higher than that of Celgard 2325 at 5 C). This work provides a new method of improving the safety and simultaneously reducing the polarization of lithium-ion batteries, which has promising application potential in power batteries.

Synthesis, characterization, and application of silica aerogel thin films prepared via surface derivatization: A comprehensive study on micro-silica nanofilms for enhanced photoelectric conversion efficiency in photovoltaic solar cells

Silica aerogels are nanoporous materials with exceptional optical and physical properties, making them promising candidates to enhance solar cell efficiency as antireflective coatings. This study synthesized hydrophobic silica aerogel thin films under ambient conditions and characterized their porous structure, surface morphology, and optical performance. The films were deposited on monocrystalline silicon solar cells to assess their impact on photovoltaic properties. A two-step acid/base catalyzed sol-gel process was utilized, followed by solvent exchange and surface modification with trimethylchlorosilane. Structural analysis via SEM revealed successful deposition of crack-free films when aging occurred in an ethanol environment. The aerogel displayed considerable specific surface area (115 m2/g), porosity (77.92%) and surface roughness (55-78%) along with a low refractive index (1.05), benefiting light harvesting. Preliminary solar testing showed increased output voltage with a 0.2 mm aerogel coating versus a bare cell. Further IV measurements demonstrated enhanced charge transport and conversion efficiency for the treated cell. The aerogels antireflective and light-trapping effects appear to improve photon absorption. This initial research validates the potential of ambient pressure-synthesized hydrophobic silica aerogels to increase the performance of silicon photovoltaics cost-effectively. Further optimization of film thickness and morphology could realize higher efficiency gains.

A novel additive for enhancing biomass energy production from agro-industrial wastes: synthesis of hydrophobic nanoporous silica aerogel and its effect on methane production

AbstractAnaerobic digestion (AD) is one of the most preferred processes for the treatment of organic waste. However, additional processes such as co-digestion, pretreatment, and additive addition continue to be explored to remove the limits on the applicability of AD. This study investigated the effects of hydrophobic nanoporous silica aerogel (NpSA) synthesized from waste rice husks on the anaerobic co-digestion (AnCD) of the mixture consisting of sewage sludge and fruit processing industry wastes. All bioreactors containing NpSA-free, 0.1 g, 0.2 g, 0.5 g, and 1 g NpSA (0.03–0.3 gNpSA/gVSadded) were operated in a mesophilic-batch process. Biogas and methane yields increased from 346 mL/gVS (NpSA-free) to 387 mL/gVS and from 231 mL/gVS (NpSA-free) to 288 mL/gVS, respectively, with 0.1 g NpSA addition. NpSA additive increased biogas production in all bioreactors compared to the blank. However, biogas production rate and methane content increased faster at lower doses of NpSA. Maximum soluble chemical oxygen demand (sCOD), protein, carbohydrate, and volatile solid (VS) reductions were between 45–71%, 35–54%, 44–65%, and 34–91% for NpSA added mixtures, respectively. The hydrophobic NpSA additive was effective in improving the AnCD performance and biogas/methane production. Experimental results fit the kinetic models frequently preferred in such AD processes. In addition, the possible energy and financial potential of the produced methane were also discussed, and it was determined that the direct sale of methane gas produced by the addition of NpSA in the global market could provide 1.4 $/Lmixture more financial gain than the mixture NpSA-free. Graphical Abstract

Enhancing thermal safety of hydrophobic silica aerogels by incorporating sodium dodecyl sulfate intercalated layered double hydroxides

Enhancing thermal safety of hydrophobic silica aerogels by incorporating sodium dodecyl sulfate intercalated layered double hydroxides

Silica aerogel-PVA dough: A high internal phase composite with superior thermal insulation and gas barrier

Silica aerogel (SA) has shown great promise in constructing thermal insulation composites in various forms. However, it is still challenging to disperse a huge amount of SA particles into other matrices for fabricating superior thermal insulation materials or highly filled masterbatch, owing to its high specific surface area and ultra-low density. Herein, inspired by high internal phase emulsion (HIPE), we successfully prepare processable SA/polyvinyl alcohol (PVA) composites (SA-PVA dough) with over 74 vol% dispersed SA, using a PVA aqueous solution as a binder. This is achieved by gradually mixing a large amount of hydrophobic SA filler into a small amount of PVA aqueous solution. The resulting SA-PVA dough exhibits an internal structure resembling that of HIPEs, with hydrophobic SA particles stabilized in the water phase by amphiphilic PVA chains, tightly packed within the thin continuous PVA aqueous phase. Benefiting from such a HIPE structure, the SA-PVA dough can be easily shaped into various forms and exhibits good applicability in further processing. The dried dough inherits the excellent properties of SA, including ultra-low density, superhydrophobicity (water contact angle: 153°), thermal insulation (thermal conductivity: 0.03 W m−1 K−1), and flame resistance. Moreover, the continuous PVA network structure, combined with the nanoscale pore structure of SA, provides the dough with excellent gas barrier properties, greatly expanding the application scope of SA-based composites.

Cost-Effective Preparation of Hydrophobic and Thermal-Insulating Silica Aerogels.

The aim of this study is to reduce the manufacturing cost of a hydrophobic and heat-insulating silica aerogel and promote its industrial application in the field of thermal insulation. Silica aerogels with hydrophobicity and thermal-insulation capabilities were synthesized by using water-glass as the silicon source and supercritical drying. The effectiveness of acid and alkali catalysis is compared in the formation of the sol. The introduction of sodium methyl silicate for the copolymerization enhances the hydrophobicity of the aerogel. The resultant silica aerogel has high hydrophobicity and a mesoporous structure with a pore volume exceeding 4.0 cm3·g-1 and a specific surface area exceeding 950 m2·g-1. The obtained silica aerogel/fiber-glass-mat composite has high thermal insulation, with a thermal conductivity of less than 0.020 W·m-1·K-1. The cost-effective process is promising for applications in the industrial preparation of silica aerogel thermal-insulating material.

Reversible thermochromic fibers with excellent elasticity and hydrophobicity for wearable temperature sensors.

Color-changing fibers, which can intuitively convey information to the human eye, can be used to facilely add functionality to various types of clothing. However, they are often expensive and complex, and can suffer from low durability. Therefore, in this study, we developed highly elastic and hydrophobic thermochromic fibers as wearable temperature sensors using a simple method that does not require an electric current. A thermochromic pigment was embedded inside and outside hydrophobic silica aerogel particles, following which the thermochromic aerogel was fixed to highly elastic spandex fibers using polydimethylsiloxane as a flexible binder. In particular, multi-strand spandex fibers were used instead of single strands, resulting in the thermochromic aerogels penetrating the inside of the strands upon their expansion by solvent swelling. During drying, the thermochromic aerogel adhered more tightly to the fibers by compressing the strands. The thermochromic fiber was purple at room temperature (25 °C), but exhibited a two-stage color change to blue and then white as the temperature increased to 37 °C. In addition, even after 100 cycles of tension-contraction at 200%, the thermochromic aerogel did not detach and was strongly attached to the fiber. Additionally, it was confirmed that color change due to temperature was stable even after exposure to 1 wt% NaCl (artificial sweat) and 0.1 wt% detergent solutions. The developed thermochromic fiber therefore exhibited excellent elasticity and hydrophobicity, and is expected to be widely utilized as an economical wearable temperature sensor as it does not require electrical devices.

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.

Surface modification of miscanthus fiber with hydrophobic silica aerogel for high performance bio-lightweight concrete

Bio-fiber reinforced cementitious material is now attracted widespread attention thanks to its low carbon footprint and its advantages in energy conservation in buildings. However, the main concerns of using large volume of bio-fibers in concrete is its high water absorption and the leaching of organic matter that is incompatible with cement. This study investigates a surface modification method of a bio-fiber (miscanthus) by using hydrophobic silica aerogel and further evaluates its mechanical and insulating performance in lightweight concrete and increase its compatibility with cement. X-ray diffraction, thermal gravimetry and calorimeter test were carried out to investigate the effect of hydrophobic miscanthus fibers on the microstructure of cement matrix. The results show silica aerogel modified miscanthus fiber enhances both the compressive and flexural strength, meanwhile significantly improving the insulating performance of concrete. The use of superhydrophobic aerogel particles reduces the sugar leached from miscanthus and the hydration heat peak is only slightly delayed with the incorporation of aerogel coated miscanthus fibers. The results of this study shed light on surface modifications of bio-fibers for applications in high performance construction materials.

Tuning thermal stability and fire hazards of hydrophobic silica aerogels via doping reduced graphene oxide

This study synthesizes composites of reduced graphene oxide doped silica aerogel (SA/rGO) and assesses their thermal safety. The introduction of rGO minimally affected SA polymerization, as confirmed by FTIR and XRD. Even with rGO (0.05-0.45 %), SA/rGO maintains low density (∼ 0.10 g/cm³), low thermal conductivity (∼ 23 mW/m/K), and large surface area (863.4–913.1 m²/g). TG-DSC results show enhanced thermal stability in SA/rGO, with onset and peak decomposition temperatures rising by 118 °C and 94 °C, respectively. Additionally, gross calorific values and total heat release rates decrease by 12.4 % and 25.4 %. Thus, this work establishes that the incorporation of rGO enhances thermal stability and flame resistance of hydrophobic SAs without impairing their properties, which provides a new insight for developing thermally safer hydrophobic SA materials.

Preparation and characterization of silica aerogel foam concrete: Effects of particle size and content

In order to study the effect pattern of hydrophobic silica aerogel on aerogel foam concrete (AFC), this work successfully prepared silica aerogel foam concrete (AFC) and investigated the influence of the silica aerogel (SA) particle size and content on the properties of AFCs in detail. The microstructure shows that the SAs are inlaid in the foam concrete matrix, forming a physical combination between the two components. Due to the hydrophobic property of SA, its incorporation into AFCs significantly alters the physical and chemical properties of the material. Notably, the primary impact lies in the remarkable antifoaming effect imparted by hydrophobic SA, a phenomenon regulated by both the content and particle size of SA. Furthermore, the introduction of SA imparts hydrophobic characteristics to the prepared AFCs, with enhanced hydrophobicity observed at higher SA content and smaller particle sizes. The study outcomes reveal that the density, compressive strength, and thermal conductivity of AFCs are influenced by the defoaming effect of SA. Specifically, a decrease in particle size of SA correlates with increased density and compressive strength in AFCs, primarily attributed to the defoaming effect of SA influencing the pore structure of the material. Simultaneously, the thermal conductivity of AFCs falls within the range of 59.0–83.5 mW/m/K, a metric affected by both the SA content and the concurrent defoaming effect. By investigating the influences of silica aerogel particle size and content, this work contributes some novel insights into optimizing the comprehensive performance of AFCs, which is conducive to the development of aerogel composites.

Insights into the thermal safety of ambient pressure dried hydrophobic montmorillonite/silica aerogel composites

To improve the thermal safety of hydrophobic silica aerogels (SAs), montmorillonite (MMT) was used as a dopant to prepare MMT/silica aerogel composites (MMT/SAs) and their basic physicochemical properties and thermal safety were investigated in detail. It finds that a physical combination is formed between MMT and SAs, and the introduction of MMT does not change the three-dimensional mesoporous structures of the MMT/SAs. With the MMT content increasing, the MMT/SAs maintain a low density (0.12–0.19 g/cm3), high porosity (91.1–93.5 %), low thermal conductivity (0.023–0.025 mW/(m∙K)) and high hydrophobicity (>140°). Compared with pure SAs, the thermal stability of the MMT/SAs was significantly improved, with the onset and peak temperatures of thermal decomposition increasing by 147° and 117°, respectively. The gross calorific value, peak heat release rate (PHRR) and total heat release (THR) decrease by 50.8 %, 29.6 % and 34.7 % respectively, while the time to ignition (TTI) increases, all of these verify the greatly reduced flammability and fire risk. This work confirms that introducing MMT into hydrophobic SAs is feasible to improve their thermal safety without impairing their excellent properties too much, thus contributing to the expansion of their thermal insulation applications.