Nanofibers reinforced silica aerogel composites having flexibility and ultra-low thermal conductivity

For the sake of enhancing the performance of flexible silica aerogel in practical applications, flexible SiO₂/SnO₂ nanofibers (SSNF) reinforced flexible silica aerogel composites (abbreviated as SiO₂-SSNF) were successfully prepared. Firstly, the SiO₂/SnO₂ nanofibers with fine diameter (~320 nm) and excellent flexibility were prepared by electrospinning technology. Then the aerogel composites were synthesized by adding the flexible SSNF to the silica solution and through the sol-gel method and ethanol supercritical drying technology. The effects of different content of the nanofibers on thermal conductivity and Yong's modulus of SiO₂-SSNF aerogel composites were investigated. The SiO₂/SnO₂ nanofibers were randomly dispersed in the flexible silica aerogel and the great integrity of the material result in smaller linear shrinkage, better thermal protection, and mechanical properties compared with those pure SiO₂ aerogels. The final SiO₂-SSNF aerogel composites possess excellent thermal conductivity (0.025-0.029 W/(m∙K)) and higher Yong's modulus (70 kPa), which was twice than that of the pure silica aerogel. This prepared SiO₂-SSNF aerogel composites can be better used in thermal insulation due to its excellent flexible and thermal insulation property.     

Facile preparation for gelatin/hydroxyethyl cellulose-SiO2 composite aerogel with good mechanical strength,heat insulation,and water resistance

The advanced thermal insulation materials with low cost and high mechanical properties play an important role in transport packaging and thermal protection fields. An inorganic/organic composite aerogel was prepared through hydrogen bonds and chemical crosslinking among silica aerogel particles, gelatin (GA), and hydroxyethyl cellulose (HEC). The as-prepared GA/HEC-SiO₂ composite aerogels were characterized by compression tests, scanning electron microscopy, Fourier transform infrared, thermogravimetric analyzer, and contact angle tests to investigate the chemical composition and physical structure. The GA/HEC-SiO₂ composite aerogels exhibited a strong mechanical strength (0.53–4.01 MPa), a high compression modulus (1.33–11.52 MPa), a lower volume density (0.035–0.081 g/cm³), thermal conductivity as low as 0.035 W/[m K]), a porosity of more than 93%, and hydrophobic angle as high as 150.01° after hydrophobic modification. These results indicate that biopolymer composite aerogels embedded with SiO₂ aerogel particles display a bright future in thermal insulation.     

Opacified graphene-doped silica aerogels with controllable thermal conductivity

In this work, we developed a new type of thermal insulation materials by combining the silica aerogel (SiO₂) and graphene (G) followed by aging and supercritical drying. The effects of different G/SiO₂ mass ratios on the microstructures and properties of opacified G/SiO₂-x composite aerogels were investigated. The results showed that the graphene was well-distributed in the SiO₂ matrix. Meanwhile, the opacified composite aerogels showed high-specific surface area (~?1000 m²/g). Due to the unique bandgap feature and conjugated large π bond of graphene, the thermal insulation property of G/SiO₂-x composite aerogels was enhanced in contrast with the pure SiO₂ aerogel. Moreover, a possible mechanism of heat transfer was discussed to interpret the result.     

Novel high-temperature-resistant Y2SiO5 aerogel with ultralow thermal conductivity

A novel Y₂SiO₅ ternary aerogel was prepared from tetraethoxysilane and yttrium chloride hexahydrate via the sol-gel method followed by high-temperature calcination. The effects of different calcination temperatures on the microstructure, mechanical and thermal stability of the Y₂SiO₅ aerogels were investigated. The aerogels exhibited low densities of 0.33-0.62 g/cm³, low thermal conductivities of 0.029-0.05 W/(m·K), and a relatively high strength of 0.16-56.47 MPa. Moreover, compared with the Al₂O₃–SiO₂ aerogel, the Y₂SiO₅ aerogel has higher thermal stability and more excellent high-temperature insulation, which has potential applications as a thermal protection material in hypersonic vehicles.     

Super-insulating,ultralight and high-strength mullite-based nanofiber composite aerogels

Ceramic nanofiber aerogel is one of the most attractive insulation materials in recent years. However, its practical application ability is limited at high temperature due to high radiation heat transfer. Herein, we constructed a novel closed-cell/nanowire structured mullite-based nanofiber composite aerogel via electrospinning technology and solvothermal synthesis method. Hollow TiO₂ spheres were used as pore-making material and infrared opacifier to reduce fiber solid heat conduction and high temperature radiation heat transfer simultaneously. In addition, TiO₂ nanowires grown in-situ on the fiber surface further decrease the radiation heat transfer of aerogel and improve the mechanical properties. The unique structure endows the aerogel with high mechanical robustness (0.32–0.35 MPa, 10% strain), low density (39.2–47.5 mg/cm³) and ultralow thermal conductivity (~0.017 W m^?1 K^?1 at 25 ℃ and ~0.056 W m^?1 K^?1 at 1000 ℃). This work offers a novel strategy for the development of ceramic nanofiber aerogel at high temperature.     

Synthesis of monolithic SiC aerogels with high mechanical strength and low thermal conductivity

The monolithic silicon carbide (SiC) aerogels were converted from catechol-formaldehyde/silicon composite (CF/SiO₂) aerogels through carbothermal reduction and calcination. In the process of preparing the CF/SiO₂ aerogel, a new method was proposed to produce more silicon carbide and enhanced the mechanical properties of the SiC aerogel. This method was realized by adding an alkaline silica sol as supplemental silicon source. The principle process of CF/SiO₂ aerogels converting to SiC aerogels was discussed based on experiment and results analysis, while the microstructure, mechanical properties, and thermal properties of the prepared SiC aerogels were investigated. The results show that the as-synthesized SiC aerogels consist of β-SiC and a small amount of α-SiC nanocrystalline. It possessed a mesoporous structure and a low thermal conductivity 0.049 W/(m∙K), a relatively high compressive strength 1.32 MPa, and a relatively high specific surface area 162 m²/g. Due to their outstanding thermal and mechanical properties, the prepared SiC aerogels present potential applications in thermal insulation field, such as space shuttles and aerospace carrier thermal protection materials.     

The unusually formation of porous silica nano-stalactite structure by high temperature heat treatment of SiO2 aerogel synthesized from rice hull

The SiO₂ aerogel has attracted the attention of many researchers in recent years in terms of energy saving due to its properties such as very low density, high porosity, and superior insulation. The primary purpose of this study was to synthesize SiO₂ aerogel. For this purpose, the sodium silicate obtained from rice hull ash was used as pre-initiator and dried at ambient pressure and SiO₂ aerogel was synthesized. BET surface area of the obtained SiO₂ aerogel was found as 241 m²/g. The difference of this study is the structures obtained after this stage. After obtaining SiO₂ aerogel, it was subjected to heat treatment at a high temperature to increase the BET area of aerogel and after examining under SEM, it was noticed that some uncommon structures that have not seen before formed. After observing nano-stalactite type structures in the sample, the study was expanded on the causes of the formation of these structures. It was seen that this structure forming after heat treatment was a stalactite composed of SiO₂ nanoparticles having a crystal structure. This study examined the formation mechanism and some properties of this structure which was not encountered in previous studies. Brunauer-Emmett-Teller (BET), Transmission electron microscopy (TEM), Scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) analysis were conducted in order to determine the physical and chemical properties of both synthesized SiO₂ aerogel and nano-stalactite type structure.     

Preparation and high-temperature service performance of hierarchically pore-structured BN fiber aerogels

Multifunctional aerogels with high porosity and good thermal insulation have attracted much attention in the field of energy and aerospace engineering. In this work, a three-dimensional BN fiber aerogel with hierarchically porous structure was prepared through a freeze-drying combined with in-situ carbothermal reduction nitridation route. The synthesized BN fiber aerogel exhibits a specific surface area of 154.3 m²/g, a high porosity of 96.8% and hydrophobicity. Moreover, the BN fiber aerogel shows a low thermal conductivity of 0.051 W/(m·K) and excellent thermal insulation properties owing to its hierarchical porous structure. Particularly, the BN fiber aerogel still maintains its low thermal conductivity and a rather high mechanical strength after re-heated at 1473 K for 3 h in Ar atmosphere, suggesting excellent high-temperature service performance. The successfully developed multifunctional BN fiber aerogel holds promising potential in high-temperature thermal insulation fields.     

High-performance heterocyclic para-aramid aerogels for selective dye adsorption and thermal insulation applications

The high-performance polymer para-aramid (PPTA) is discovered to gel too soon during the polymerization process, resulting in poor processing performance. In this work, a homogeneous polymer solution containing heterocyclic para-aramid (HPPTA) was successfully synthesized by introducing 2,4-aminophenyl-5-aminobenzimidazole groups into the molecular chains of PPTA, and then HPPTA aerogel was prepared using a supercritical drying technique that took advantage of the HPPTA solution's excellent property of slow gelation. When the HPPTA polymer mass fraction was 1 wt%, the aerogel had the lowest density of 0.086 g cm⁻³ with a BET specific surface area of 376.59 m² g⁻¹. The HPPTA-2 aerogel had better adsorption performance for anionic dye methyl orange, with a maximum adsorption capacity of 319.47 mol g⁻¹; however, its adsorption capacity for cationic dye methylene blue and neutral dye dimethyl yellow was very low, at only 19.68 and 0 mol g⁻¹, respectively. The selective adsorption ability of HPPTA aerogel made it a simple and scalable platform for removing anionic dyes from water solutions. Furthermore, the HPPTA aerogel has outstanding thermal properties for thermal insulation applications in severe environments due to the synergistic effect of the 3D porous structure inside the aerogel and the exceptional thermal stability of the HPPTA.     

Influence of dielectric barrier effect and thermally-conductive network on thermal and electrical properties of epoxy under different frequencies and temperatures

Epoxy resin (EPR) insulations play a vital role in the insulation of modern power electronic equipment owing to their excellent dielectric properties. However, due to the high-power density and miniaturization of power equipment which causes high heat fluxes under high voltage and high-frequency stresses, EPR with good thermal and insulation properties is urgently needed. In this study, the polydopamine functionalized micro-BN and core-shell nano TiO₂–SiO₂ particles are dispersed in EPR to simultaneously improve thermal and dielectric insulation properties. It is revealed that the addition of micro-nano particles significantly improves the thermal and dielectric performance. Particularly, the high thermal conductivity of micro-BN and the dielectric barrier effect due to the core-shell structure of nano TiO₂–SiO₂ are the main reasons for improved thermal and dielectric insulation performance, respectively. The EPR composite containing 3 wt% of micro-BN and 1 wt% of nano TiO₂–SiO₂ exhibits the optimal performance with 0.49 W/mK thermal conductivity and the highest dielectric strength among all the samples, that is, 60.61 kV/mm even at 10 kHz and 90°C. This study found that the crucial factors are the surface encapsulation, weight percent, and homogeneous dispersion of particles in EPR, the dielectric barrier effect, thermal conductivity, and the mismatch between the dielectric constant of EPR and particles. This study proposes the optimal weight percent of suitable micro-nano particles for EPR to produce suitable composites for high-frequency and high-temperature applications.     

Novel self-reinforcing ZrO2–SiO2 aerogels with high mechanical strength and ultralow thermal conductivity

Novel self-reinforcing ZrO₂–SiO₂ aerogels with high mechanical strength and ultralow thermal conductivity are fabricated by impregnating hydrolyzed ZrO₂–SiO₂ sol into wet gel matrix and drying. The ZrO₂–SiO₂ sol fills the macropores and defects of ZrO₂–SiO₂ aerogel matrix generating during the gelation process, which contributes to the improvement of the mechanical properties of the ZrO₂–SiO₂ aerogel matrix. The mechanical and thermal properties of the as-prepared ZrO₂–SiO₂ aerogel are investigated and discussed. The results show that the mechanical strength of the self-reinforcing aerogels obviously increases from 0.51 to 3.11?MPa with the increase of impregnation times, while the thermal conductivity of the aerogels slightly increases from 0.0235 to 0.0306?W?m^?1 K^?1. The novel self-reinforcing ZrO₂–SiO₂ aerogel could have interesting applications in aerospace and energy because of its outstanding mechanical and thermal properties.     

Novel monolithic yttria-alumina aerogel with low thermal conductivity made from inorganic precursors

A novel monolithic yttria-alumina aerogel was successfully prepared using inorganic precursors through the sol-gel method. The yttria-alumina aerogel possesses a three-dimensional network structure composed of numerous nanoscale flake like particles and nanoscale pores. In addition, the aerogel is mainly made up of nano Y₂O₃ grains, and Al atoms are dissolved in the Y₂O₃ grains to form the solid solution. The aerogel with a low density of 0.203 g/cm³ exhibits low thermal conductivity of 0.016 W/(m·K). Therefore, the yttria-alumina aerogel shows promise for application as a thermal insulation material.     

Novel Al<Subscript>2</Subscript>O<Subscript>3</Subscript>–SiO<Subscript>2</Subscript> aerogel/porous zirconia composite with ultra-low thermal conductivity

Highly porous zirconia fibers networks with a quasi-layered microstructure were successfully fabricated using vacuum squeeze moulding. The effects of inorganic binder content on the microstructure, room-temperature thermal and mechanical properties of fibrous porous zirconia ceramics were systematically investigated. Al₂O₃–SiO₂ aerogel was impregnated into fibrous porous ceramics, and the microstructures, thermal and mechanical properties of Al₂O₃–SiO₂ aerogel/porous zirconia composites were also studied. Results show that the Al₂O₃–SiO₂ aerogel/porous zirconia composites exhibited higher compressive strength (i.e., 1.22 MPa in the z direction) and lower thermal conductivity [i.e., 0.049 W/(m/K)]. This method provides an efficient way to prepare high-temperature thermal insulation materials.     

Preparation of amine-modified SiO<Subscript>2</Subscript> aerogel from rice husk ash for CO<Subscript>2</Subscript> adsorption

Amine-modified SiO₂ aerogel was prepared using 3-(aminopropyl)triethoxysilane (APTES) as the modification agent and rice husk ash as silicon source, its CO₂ adsorption performance was investigated. The amine-modified SiO₂ aerogel remains porous, the specific surface area is 654.24 m²/g, the pore volume is 2.72 cm³/g and the pore diameter is 12.38 nm. The amine-modified aerogel, whose N content is up to 3.02 mmol/g, can stay stable below the temperature of 300 °C. In the static adsorption experiment, amine-modified SiO₂ aerogel (AMSA) showed the highest CO₂ adsorption capacity of 52.40 cm³/g. A simulation was promoted to distinguish the adsorption between the physical process and chemical process. It is observed that the chemical adsorption mainly occurs at the beginning, while the physical adsorption affects the entire adsorption process. Meanwhile, AMSA also exhibits excellent CO₂ adsorption–desorption performance. The CO₂ adsorption capacity dropped less than 10 % after ten times of adsorption–desorption cycles. As a result, AMSA with rice husk ash as raw material is a promising CO₂ sorbent with high adsorption capacity and stable recycle performance and will have a broad application prospect for exhaust emission in higher temperature.     

Lightweight,robust, porous heterocyclic para-aramid aerogel hollow fibers for multifunctional applications

In nature, many fibers with warmth-retention properties, such as the hair of polar bears and rabbits, both have a hollow cross-section structure. The static air in fiber cavities can effectively inhibit heat conduction and serve as an effective thermal insulator. In this work, the high-performance heterocyclic para-aramid polymer was selected as the spinning solution, and aerogel hollow fiber was prepared by coaxial wet spinning and freeze-drying techniques. The effects of spinning solution concentration and lyophilized solvent on the micromorphology, mechanical properties, and specific surface area of heterocyclic para-aramid aerogel hollow fiber (HPAAHF) were systematically studied. The produced HPAAHF possessed excellent mechanical properties (tensible strength ~3.85 MPa), high specific surface area (~ 260.90 m² g⁻¹), and lightweight advantages. The thermal conductivity of HPAAHF was only 0.0278 W m⁻¹ K⁻¹, indicating its excellent thermal insulation properties. The aerogel fabric exhibited outstanding flame retardancy properties, with a total heat release of only 0.7 MJ m⁻² in the cone calorimetric experiment, making it a self-extinguishing fabric. In addition, phase change material was injected into the hollow structure to obtain aerogel-phase change material composite fibers, which exhibited great energy storage prospects. As a result, the high-performance heterocyclic para-aramid polymer-based aerogel hollow fiber was successfully prepared and had multifunctional applications in thermal insulation, flame retardancy, and heat energy storage fields.     

A simple and efficient method for the preparation of SiO2/PI/AF aerogel composite fabrics and their thermal insulation performance

As a new type of insulation material, aerogels are characterized by a high specific surface area, high porosity, low density and low thermal conductivity, which makes them a new alternative to the use of traditional insulation materials. In this paper, a simple method for preparing aerogel insulation materials is proposed. Specifically, SiO₂/PI/AF (aramid fiber) aerogel composite fabrics were successfully obtained by combining coating technology and finishing processes to use tetraethoxysilane (TEOS) as the precursor, polyimide (PI) powder as the reinforcing agent, and nonwoven AF as the substrate. These composite fabrics were characterized using field-emission scanning electron microscopy (FESEM), tensile testing with an Instron 5967, Fourier transform infrared spectroscopy (FT-IR) and thermal infrared imaging. The results show that the composite fabrics exhibited excellent performance and could effectively block heat transfer. Moreover, the thermal conductivity of the front decreased from 4.08 to 3.91 (W/cm·°C) × 10^-4. This work provides a novel method for the structural design of thermal insulation clothing.     

Effects of poly (vinyl alcohol) (PVA) content on preparation of novel thiol-functionalized mesoporous PVA/SiO2 composite nanofiber membranes and their application for adsorption of heavy metal ions from aqueous solution

Thiol-functionalized mesoporous poly (vinyl alcohol)/SiO₂ composite nanofiber membranes and pure PVA nanofiber membranes were synthesized by electrospinning. The results of Fourier transform infrared (FTIR) indicated that the PVA/SiO₂ composite nanofibers were functionalized by mercapto groups via the hydrolysis polycondensation. The surface areas of the PVA/SiO₂ composite nanofiber membranes were >290 m²/g. The surface areas, pore diameters and pore volumes of PVA/SiO₂ composite nanofibers decreased as the PVA content increased. The adsorption capacities of the thiol-functionalized mesoporous PVA/SiO₂ composite nanofiber membranes were greater than the pure PVA nanofiber membranes. The largest adsorption capacity was 489.12 mg/g at 303 K. The mesoporous PVA/SiO₂ composite nanofiber membranes exhibited higher Cu²⁺ ion adsorption capacity than other reported nanofiber membranes. Furthermore, the adsorption capacity of the PVA/SiO₂ composite nanofiber membranes was maintained through six recycling processes. Consequently, these membranes can be promising materials for removing, and recovering, heavy metal ions in water.     

Solution-blow spun PLA/SiO2 nanofiber membranes toward high efficiency oil/water separation

With the ever frequent of industrial organic solvent emissions and oil spillages, the development of high efficiency oil/water separation materials has attracted extensive attention. Here, PLA-based nanofiber membranes modified with metal oxides (SiO₂, TiO₂, Al₂O₃, and CeO₂) are fabricated through blow spinning the mixed solution of polylactic acid (PLA) and metal oxide nanoparticles (NPs). Results shows that the addition of SiO₂ NPs significantly increases the hydrophobicity of the membranes, while maintaining the excellent superoleophilicity. The PLA/SiO₂ nanofiber membranes demonstrate a higher separation performance than pure PLA, PLA/TiO₂, PLA/Al₂O₃, and PLA/CeO₂ nanofiber membranes with high separation efficiency (~100%) and permeation flux (17,800 L m⁻² h⁻¹ for n-heptane), as well as prominent oil adsorption capacity (19.9 g/g for n-hexane). The successful fabrication of metal oxides modified PLA nanofiber membranes with high separation and adsorption ability, and excellent durability hold great application potential in the field of oily wastewater treatment.     

Preparation of monolith SiC aerogel with high surface area and large pore volume and the structural evolution during the preparation

Resorcinol–formaldehyde/silica composite (RF/SiO₂) aerogel was synthesized by sol–gel process followed by supercritical drying (SCD). Monolithic SiC aerogel was obtained from RF/SiO₂ aerogel after carbothermal reduction. The evolution of physical property, crystal structure, morphology and pore structure from RF/SiO₂ to SiC aerogel was investigated by different methods, such as X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM) and N₂ adsorption/desorption. The as-synthesized SiC aerogel presented typical mesoporous structure and possessed high porosity (91.8%), high surface area (328 m²/g) and large pore volume (2.28 cm³/g). Carbothermal reduction mechanism was also discussed based on the experiment and characterization results.     

Improvement of thermal stability of ZrO<Subscript>2</Subscript>–SiO<Subscript>2</Subscript> aerogels by an inorganic–organic synergetic surface modification

The thermal stability of ZrO₂–SiO₂ aerogels was significantly improved by inorganic–organic synergetic surface modifications: inorganic ions [Fe(III)] surface modification and hexamethyldisilazane gas phase modification. The replacement of Hs from surface hydroxyl groups on the aerogel by Fe(III) ions and silyl groups played a critical role in isolating the hydrous particles of ZrO₂–SiO₂ aerogels. So the particle growth caused by the condensation of hydroxyl groups upon firing was inhibited. Meanwhile, the decomposition of the silyl groups upon heat treatment produced SiO₂ particles, which could serve as pining particle to inhibit the crystallization of ZrO₂. Hence, the porous microstructure of the modified aerogels was still well preserved up to 1000 °C, with a high specific surface area of 203.5 m²/g, and a considerable pore volume of 0.721 cc/g. These characteristics of the modified aerogels suggest that it has great potential on ultrahigh-temperature applications in the fields of thermal insulation, catalysis, and catalyst support, etc.