Density Of Aerogels Research Articles

“Tuning density and morphology of organic-inorganic hybrid-silica aerogels through precursor dilution for lightweight applications”

“Tuning density and morphology of organic-inorganic hybrid-silica aerogels through precursor dilution for lightweight applications”

A Building Information Modeling driven approach for improving building envelope energy efficiency in hot arid climates

As a response to growing energy demand and greenhouse gas emissions, enhancing the energy performance of buildings has become a global focus. Studies have shown that building openings are responsible for 40–50% of their energy loss, and these percentages are growing steadily. Silica aerogel represents a potential energy-saving technology for buildings. It has demonstrated its effectiveness as an insulating material in various climates; especially cold climates, however its success in hot arid climates has not been examined yet. Therefore, this paper aims to assess the thermal properties of silica aerogel to determine the dominant parameters that impact its thermal performance in a hot, arid climate. A numerical model of the silica aerogel is conducted using Autodesk Ecotect Analysis. The impact of different factors (thermal conductivity, density, specific heat, and thickness of silica aerogel) on the energy efficiency of the glass window was investigated. Results show that the thickness of silica aerogel has the highest impact on the thermal behavior of silica aerogel, followed by thermal conductivity. Meanwhile, density and specific heat showed a negligible effect on the energy performance of the material. Results showed that a 5 mm-thick aerogel layer and a thermal conductivity of 0.014 W/(mK) exploit the energy performance of the proposed material in hot, arid climates and could provide energy savings of around 63% and 15% in heating and cooling demands, respectively.

Modelling of hollow-fiber doping in silica aerogel composites for radiative and conductive insulation under high temperatures

Doping opacifier and fiber has been developed to improve the thermal insulation performance of silica aerogels at high tempearture. However, this will inevitably increase the density and enhance heat conduction in practical applications. In the present work, to explore a way of not only avoiding the thermal insulation compromise but also reducing the density, hollow fibers as dopants are investigated. Their radiative properties and the overall thermal insulation performance of hollow fiber-doped silica aerogel composites are numerically evaluated. In addition, the dependence of the optimal hollow ratio on the fiber diameter at different temperatures is analyzed, and the temperature-dependent optimal diameter of fibers for different hollow ratios is obtained. It is shown that the hollow-fiber-doped aerogel composite with a hollow ratio of 0.6 can present not only a low effective thermal conductivity (compared with the conventional solid fiber doped ones), but also a decrease in density by up to 11.45%, between 300 and 1300 K. The present work can illustrate the potential of hollow fiber doping towards an optimal trade-off between the density and thermal insulations of silica aerogels for high temperature circumstances and can inspire more explorations of hollow structures in silica aerogels.

Preparation and electromagnetic wave absorption of compressible graphene aerogels

In this study, a developed easy-operation method was adopted to generate ultralight and compressible graphene aerogels by using graphene oxide and ammonia. By changing the process parameters, such as reduction temperature, reducing agent content and ammonia concentration, the variation laws of the density and pore size of aerogels were obtained, which aided in realizing the controllable preparation of aerogel structures. The prepared graphene aerogel has good compressive performance, and its density can reach 5.26 mg/cm3. Although it was repeatedly compressed 200 times under a load that was 4000 times as large as its own weight, it maintained its structural integrity and mechanical properties. An ideal model of three-dimensional graphene aerogel was constructed, and the electromagnetic wave absorption performance was simulated using the Computer Simulation Technology (CST) Microwave Studio software. The results showed that when the thickness, pore size and height of the sheet were 1.4, 5 and 14 mm, respectively, an optimal electromagnetic wave absorption effect of −31.08 dB could be obtained. Furthermore, the effects of thickness, pore size and height of the sheet on the electromagnetic wave absorption performance are revealed, which provide a reference for the structural design of aerogels with both compressibility and electromagnetic wave absorption performance.

Synthesis and Surface Strengthening Modification of Silica Aerogel from Fly Ash.

This study focuses on using activated fly ash to preparate silica aerogel by the acid solution-alkali leaching method and ambient pressure drying. Additionally, to improve the performance of silica aerogel, C6H16O3Si (KH-570) and CH3Si(CH3O)3 (MTMS) modifiers were used. Finally, this paper investigated the factors affecting the desilication rate of fly ash and analyzed the structure and performance of silica aerogel. The experimental results show that: (1) The factors affecting the desilication rate are ranked as follows: hydrochloric acid concentration > solid-liquid ratio > reaction temperature > reaction time. (2) KH-570 showed the best performance, and when the volume ratio of the silica solution to it was 10:1, the density of silica aerogel reached a minimum of 183 mg/cm3. (3) The optimal process conditions are a hydrochloric acid concentration of 20 wt%, a solid-liquid ratio of 1:4, a reaction time of two hours, and a reaction temperature of 100 °C. (4) The optimal performance parameters of silica aerogel were the thermal conductivity, specific surface area, pore volume, average pore size, and contact angle values, with 0.0421 W·(m·K)-1, 487.9 m2·g-1, 1.107 cm3·g-1, 9.075 nm, and 123°, respectively. This study not only achieves the high-value utilization of fly ash, but also facilitates the effective recovery and utilization of industrial waste.

Low shrinkage polyimide aerogel cross-linked with amino-functionalized hollow glass microbeads for thermal and acoustic insulation

As a nanoporous material, polyimide (PI) aerogels have excellent thermal and acoustic properties. However, their high air flow resistance is detrimental to the development of their acoustic properties. To enhance the thermal and acoustic insulation properties of PI aerogels, we have selected hollow glass microbeads (HGM) with a low thermal conductivity coefficient to be combined with the PI aerogel, leading to the fabrication of PI/HGM aerogel composite materials. In this study, there are two preprocessing methods for HGM: untreated and aminized (HGM-NH2). These were then individually combined with PI aerogel to fabricate two distinct PI/HGM composite aerogels: a physically blended variant (PIHGM) and a chemically bonded one (PHN). The thermal insulation and acoustic properties of these two composite aerogels were compared. The results show that HGM-NH2 not only functions as a cross-linking agent but also plays a key role in forming the three-dimensional network structure, which effectively reduces the shrinkage rate and density of the composite aerogel. Moreover, the thermal conductivity of PHN reached a low of 27.92 mW m⁻1 K⁻1, which is a reduction compared to PIHGM's thermal conductivity of 29.83 mW m⁻1 K⁻1. Moreover, the PHN composite aerogel with 10% HGM addition exhibited an average sound insulation level that was 120% higher than that of PIHGM. The method of incorporating pore structures with different scales also offers valuable insights for enhancing the sound insulation performance of other materials.

Carbon aerogel materials synthesized from recycle paper and their VOC adsorbtion capacity

This study presented the process of synthesizing carbon aerogel materials from waste paper for the purpose of absorbing volatile organic compounds (VOCs). The fabricated materials showed suitable properties as adsorbent materials: low density, high porosity and large surface area. The carbonization temperature and density of carbon aerogel were studied and showed a clear influence on the adsorption capacity of the materials. The adsorption capacity of the material has been tested and it is shown that it can reach 400 mg/g for toluene and acetone.

Ionothermal synthesis of fiber-reinforced carbon aerogel composites from fructose with high mechanical and thermal insulation properties

In this work, carbon fiber reinforced biomass carbon aerogel composites (CF/CA) were prepared by ionothermal synthesis using fructose as the raw material and oxidized polyacrylonitrile fiber as the reinforcing agent. During the preparation process, we have mixed fructose with oxidized polyacrylonitrile fibers and subjected them to a carbonization process under the conditions of hot ionic liquids. By controlling the reaction conditions, we successfully prepared fiber-reinforced carbon aerogel composites with excellent properties. The CF/CA has a high compressive strength (3.306 MPa). At the same time, the original low thermal conductivity (0.106 W·m−1·K−1) and low density (0.2976 g/cm2) of the carbon aerogel are maintained. In this work, the mechanical properties of carbon aerogel are improved to 230 % without changing the thermal insulation properties of CA, which makes a certain progress compared with the similar research. These excellent properties make CF/CA a promising engineering material. Moreover, we found that about 95 % of the ionic liquid can be recovered by evaporating under reduced pressure, and the recovered ionic liquid is still usable. This discovery provides a possibility for the green and sustainable development of the preparation process of carbon aerogels. This study provides new ideas and methods to further optimize the properties and applications of biomass carbon aerogels and their composites.

Construction of three-dimensional porous network Fe-rGO aerogels with monocrystal magnetic Fe3O4@C core-shell structure nanospheres for enhanced microwave absorption

Single dielectric loss material has defects such as strong dielectric properties and impedance mismatch. Hence, developing electromagnetic wave (EMW) absorption materials with both outstanding EMW absorption and favourable impedance matching performances is still a great challenge. A large number of literature have reported that combining dielectric materials with magnetic materials to enhance microwave absorption. However, there is little literature on the effect of crystal structure on the absorbing properties of materials. Owing to the virtues of light weight, low density and thin thickness of aerogel, which has been widely utilized in EMW absorption materials. Herein, Fe-based metal organic framework-reduced graphene oxide (Fe-rGO) aerogel was synthesized via simple hydrothermal method and freeze-drying. Besides, Polyvinyl Pyrrolidone (PVP) was used as a surfactant to endow Fe3O4 nanospheres with uniform size and smooth morphology. Moreover, Fe3O4 nanospheres with different crystal phase (monocrystal and polycrystal) structure were obtained under different carbonization temperatures (600 and 800 °C). Through comparison, the monocrystal Fe3O4 nanospheres show better microwave absorption and impedance matching performance than that of polycrystal Fe3O4 nanospheres. In addition, the electromagnetic parameters can be adjusted by regulating the mass ratio of Fe-based metal organic framework (Fe-MOF) to GO. As a consequence, the prepared 600Fe-rGO 2:1 aerogel achieves the minimum reflection loss (RLmin) of −64.89 dB and a broad effective absorption bandwidth (EAB) of 6.96 GHz with a low filling content of 5 wt%, showing excellent EMW absorption and impedance matching property. In short, the structural design of single crystal magnetic Fe3O4 nanospheres provides inspiration and guidance for future research of enhanced microwave absorption materials.

A study on preparation and properties of fly ash-based SiO2 aerogel material

Using fly ash acid leaching residue as the silicon source, sodium silicate solution was produced by alkali leaching and the wet gel was obtained by the sol-gel method. Hexamethyldisiloxane (HMDSO) was used to modify the surface of the wet gel, thereby reducing the capillary tension during the atmospheric pressure drying process, and finally preparing a silica aerogel material with hydrophobic properties. The effects of different alkali leaching conditions, different sol pH, HMDSO dosage on the aerogel density and the changes of aerogel material properties and composition during the preparation process were investigated, and the reaction mechanism of silica aerogel preparation by atmospheric pressure drying was described. The results showed that the optimum conditions for alkali leaching were 20 wt% NaOH concentration, 2 h reaction time and 100 °C, and the desilication rate was 81.72%; the lowest Silica aerogel density of 108 mg/cm3 at pH 5 to 6; the lowest density of silica aerogel was 110 mg/cm3 at a modifier to wet gel volume ratio of 1:1.4.

The Effect of Fe3O4 Addition on the Density and Porosity of Cellulose Nanofiber Aerogel Extracted by Oil Palm Trunk

This study investigates the influence of Fe3O4 addition on cellulose nanofiber aerogels' density and porosity characteristics. The cellulose nanofiber aerogels were synthesized with varying concentrations of Fe3O4: 0%, 0.25%, 0.5%, 0.75%, and 1%. The characterization of the cellulose nanofiber aerogels included physical tests to determine density and porosity and Fourier transform infrared (FTIR) analysis for functional group analysis. The results reveal a progressive increase in density from the lowest to the highest Fe3O4 concentrations: 0.115 g/cm3, 0.135 g/cm3, 0.162 g/cm3, 0.163 g/cm3, and 0.241 g/cm3 for Fe3O4 concentrations of 0%, 0.25%, 0.5%, 0.75%, and 1%, respectively. Similarly, the porosity of the cellulose nanofiber aerogels exhibited a trend of decreasing values from the lowest to the highest Fe3O4 concentrations: 90.808%, 89.499%, 88.064%, 87.764%, and82.844% for Fe3O4 concentrations of 0%, 0.25%, 0.5%, 0.75%, and 1%, respectively. Furthermore, FTIR analysis indicated that the structural integrity of the cellulose aerogels remained unchanged even after the incorporation of Fe3O4 . While no new functional groups emerged, a discernible shift in wave numbers suggests the formation of bonds between the polymer network and Fe3O4 . So, adding Fe3O4 to cellulose nanofiber aerogels led to notable alterations in density and porosity, while FTIR analysis confirmed the establishment of bonds between the polymer network and Fe3O4 without causing significant structural changes.

Thermal stability of MxOy‐doped zirconia aerogels (M = Y, Yb, Gd, Ce, Ca) studied through 1200°C

AbstractThe high porosities and low densities of ceramic aerogels offer outstanding insulative performance in applications where weight is a critical factor. The high surface area‐to‐volume ratios and specific surface areas provide extremely low thermal conductivity, but also contribute to rapid densification of the pore structure at elevated temperatures. This densification diminishes their favorable properties and inhibits use of aerogels in high‐temperature applications. This work contributes to a design framework for thermally stable aerogels via the study of dopant chemistry (Y, Yb, Gd, Ca, Ce) in zirconia aerogels. The structural evolution was studied through 1200°C using nitrogen physisorption, scanning electron microscopy, and X‐ray diffraction. The role of dopant identity and concentration in thermal stability was elucidated. In the context of the design framework, dopant chemistry is an aggregate for many closely related material properties, each of which may contribute to aerogel structural evolution. To develop a truly predictive design framework for ceramic‐based aerogels, systematic and comprehensive evaluation of thermodynamic and kinetic properties must be performed in conjunction with studies on structural evolution.

An analytical model for the energy storage potential of phase change materials supported by polymeric colloidal aerogels

Effective thermal energy storage has become one of the well-recognized aspects of modern energy management. Phase change materials (PCMs) shape-stabilized in the porous structure of porous support materials are one of the candidates to reach stable and effective thermal energy storage. This study presents an analytical model for the prediction of thermal energy storage density and thermal conductivity of colloidal aerogels impregnated by a PCM (TCAP model). The TCAP model is a series-parallel model that combines the thermal conductivity of the structure of the supporting aerogel and the thermal characteristics of the impregnated PCM. To validate the model, a mesoporous resorcinol-formaldehyde aerogel with an average pore diameter of 5.2 nm is synthesized, and two types of polyethylene glycol PCMs (PEG600 and PEG2000) are impregnated into the porous structure of the supporting material. The results confirm that the TCAP model can accurately predict the thermal energy storage characteristics of the system in terms of energy storage rate and density. Based on parametric studies, the highest heat capacity that can be achieved in a resorcinol-formaldehyde aerogel/polyethylene glycol is 35 kJ/kg.k with a ΔT of 40 K. On the other hand, it can be predicted that when ΔT increases from 10 K to 20 K, the energy storage of the system will change in the range of 52 % to 74 %. Moreover, The TCAP model developed in this study can be used to design/optimize energy storage materials for various applications such as smart buildings, solar energy storage devices, environmental thermal management systems, wearable thermal management devices, and temperature control in electronics.

FABRICATION OF TERNARY SILICA-CALCIUM-MAGNESIUM AEROGELS: EFFECT OF FEEDING RATE AND MOLAR RATIO ON PROPERTIES

The silica-calcium-magnesium ternary aerogels were prepared by a solvent exchange method and a subsequent ambient pressure drying process. The effect of process parameters such as feeding rate (9-70 mL.min-1) and molar ratio (Si/(Ca:Mg) = 1:1 - 3:1) on the material characteristics including density, elemental content, surface area, pore size, pore volume, and morphology of powders were investigated. Aerogels were characterized by Fourier transform infrared spectroscopy (FTIR), inductively coupled plasma optical emission spectroscopy (ICP-OES), scanning electron microscopy (SEM), Brunauer–Emmett–Teller (BET), Barrett–Joiner–Halenda (BJH) and tapping density analysis. It was found that the molar ratio of Si/(Ca:Mg) could remarkably affect the surface area and density of aerogels, while the feeding rate had slight effect. The resultant aerogels exhibited high specific surface areas. The results showed that the aerogel has a Si/(Ca:3Mg) molar composition obtained with 9 mL.min-1 had the highest surface area (524 m2.g−1). The increase of Ca to Mg molar ratio caused a decrease in the surface area and density of samples. The resultant aerogels are promising candidates as adsorbents to remove various contaminants.

Ultra-light-weight, anti-flammable and water-proof cellulosic aerogels for thermal insulation applications

Cellulosic aerogels (CNF) are considered naturally available thermal insulating materials as substitutes for conventional polymeric aerogels owing to their extensive sources, low density, low thermal conductivity, sustainability and biodegradability. However, cellulosic aerogels suffer from high flammability and hygroscopicity. In this work, a novel P/N-containing flame retardant (TPMPAT) was synthesized to modify cellulosic aerogels to improve their anti-flammability. TPMPAT/CNF aerogels were further modified by polydimethylsiloxane (PDMS) to enhance the water-proof characteristics. Although the addition of TPMPAT and/or PDMS slightly increased the density and thermal conductivity of the composite aerogels, those values were still comparable to the commercial polymeric aerogels. Compared with pure CNF aerogel, the cellulose aerogel modified by TPMPAT and/or PDMS had higher T-10%, T-50% and Tmax, which indicated that the modified cellulose aerogels have better thermal stability. TPMPAT modification made CNF aerogels highly hydrophilic, while TPMPAT/CNF aerogel modified by PDMS became a highly hydrophobic material with a water contact angle (WCA) of 142°. Pure CNF aerogel burned rapidly after ignition, showing a low limiting oxygen index (LOI) of 23.0% and no UL-94 grade. In contrast, both TPMPAT/CNF-30% and PDMS-TPMPAT/CNF-30% showed self-extinction behaviors with a UL-94 V-0 grade, implying high fire resistance. Combined with high anti-flammability and hydrophobicity, the ultra-light-weight cellulosic aerogels show great potential for thermal insulation applications.

Non-supercritical drying synthesis of hydrophobic, low-density, and high surface area silica aerogel using a sonication technique

Replacing the solvent in silica aerogel production with air is critical in getting the desired physical properties. Even though drying by evaporation under ambient pressure is thought to be the simplest way, it shrinks and collapses the gel network. In this research, uniform sol was prepared by sonication at room temperature. The surface of the developed wet gel was modified with trimethylchlorosilane (TMCS) after solvent exchange. The prepared aerogels underwent various characterization techniques to evaluate their functional, structural, morphological, surface, and thermal properties. The textural and physical characteristics of prepared silica aerogel were examined in relation to the precursor concentration, catalysts that affect the density of silica aerogel, volumetric shrinkage, and gelation time. The silica aerogel was found thermally stable up to 800 °C while hydrophobicity retained up to 350 °C. The contact angle of prepared aerogels confirms their hydrophobic nature. Field emission scanning electron microscopy (FE-SEM) and X-Ray Diffraction (XRD) results demonstrate the porous nature of silica aerogel. The Brunauer-Emmett-Teller (BET) surface area analysis revealed that the surface area and the pore radius were 784 m2/g and 36 Å, respectively.

Construction of Ultrasensitive Surface‐Enhanced Raman Scattering Substates Based on TiO2 Aerogels

AbstractRecent advances in surface‐enhanced Raman scattering (SERS) on semiconductor substrates offer this technology improved selectivity on top of other advantages, such as cost efficiency. However, the enhancement factor (EF) based on the semiconductors is still low compared with the noble metal substrates. Here, a new strategy of developing the semiconductor substrates based on aerogels is proposed for the first time. According to the modified Herzberg–Teller coupling rule, TiO2 aerogels are selected as the control object because of their large tunability. The surface area, amorphousness, and surface oxygen vacancy densities of TiO2 aerogels are regulated synergically. Due to the tuning of band structure, including band gap and defect band, multiresonant interband charge transfer (CT) pathways are generated and enhanced CT efficiency. A strong, intrinsically activated SERS effect is generated. Amorphous TiO2 aerogel with the highest surface oxygen vacancies shows a significant EF of 2.42 × 107, and TiO2 aerogels afford the large surface area and more active sites, which is conducive to promoting the adsorption of molecules. The aerogel‐based SERS is demonstrated to have wide applicability for ultrasensitive detection of explosives and organic dyes. The aerogel nanomaterials demonstrated here open a way for the construction of low‐cost and high‐sensitivity SERS substrate materials.

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.

Enhanced flame resistance of cellulose aerogel by ammonium polyphosphate for heat insulation

Cellulose aerogels are a potential candidate for heat insulation, but one of their drawbacks is high flammability hindering their applications in practice. This study synthesized cellulose aerogels from microfibrillated cellulose fibers (MFC) extracted from discarded pineapple leaves. The procedure started with simply mixing the extracted fibers with polyamide amine-epichlorohydrin (PAE) as a chemical crosslinker and ammonium polyphosphate (APP) (10-20%) as a green and effectively flame-resistant additive, followed by freeze-drying. The produced aerogels are characterized in terms of their morphology, thermal stability and conductivity, and flame resistance via advanced and standardized methodologies including Scanning Electron Microscopy (SEM), Thermogravimetric Analysis (TGA), thermal conductivity measurement by a heat flow meter, and UL94 horizontal burning test. The flame-resistant cellulose aerogels exhibit ultra-low density (25.5-26.8 mg/cm3), high porosity (98.0-98.2%), excellent heat insulation (35.9-36.7 mW/m۰K), and are completely flame-resistant. In addition, the varied APP content (10-20%) shows little effect on the density, heat conductivity, and thermal stability of the flame-resistant cellulose aerogels in comparison with that of the neat cellulose aerogel. Based on the findings, the synthesized flame-resistant cellulose aerogels are considered a promising bio-based heat insulation material.

Fabrication and property of hydrophobic polyvinyl alcohol /clay aerogel via irradiation-crosslinking and ambient-drying

Preparation of inexpensive aerogel via ambient-drying is an important goal in aerogel field. In this study, the sol-gel transformation of polyvinyl alcohol (PVOH)/clay was achieved by γ-ray irradiation, followed by solvent exchanging, 1H, 1H, 2H, 2H-Perfluorodecyltrimethoxysilane (FAS) grafting and ambient-drying to obtain a hydrophobic aerogel. Compared with the freeze-drying method, the specific surface area and pore size distribution of the aerogels prepared by ambient-drying obviously increased, which is more favorable for the grafting of FAS. The water contact angle of the resulting aerogel increased to 139.5° after grafting, while the flammability slightly decreased. The compressive modulus, density and specific modulus of aerogels also increased gradually with the increase of irradiation. When the absorbed dose arrived 100 kGy, the compressive modulus, density and specific modulus reached 25.8 MPa, 0.213 g/cm3 and 118 MPa·cm3/g, respectively. This novel method offers an easy and inexpensive way for fabricating hydrophobic aerogel using water soluble raw materials.