Silica Aerogel Composites 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.

Lightweight and hydrophobic silica aerogel/glass fiber composites with hierarchical networks for outstanding thermal and acoustic insulation

Silica aerogels are regarded as potential thermal and acoustic insulation materials with low density and porous structure. However, the hydrophilicity due to surface hydroxyl groups and the low mechanical intensity have limited its further application. Generally, it can be achieved by adjusting the process parameters of the sol-gel to refine the structure of silica aerogels and optimize performance. In this study, we adopted an appropriate modification strategy of incorporating glass fiber felts as the reinforcing phase and trimethylchlorosilane (TMCS) as the hydrophobic modifier. By adjusting the solvent exchange time, the micro-nano structure, i.e. hierarchical networks, was optimized. The TMCS-modified silica aerogel/glass fiber/hot-melt fiber composites (TSGMs) exhibited great sound insulation, and the maximum sound transmission loss (STL) was up to 34 dB at 6300 Hz. Compared to unmodified, STL improved by 8 dB in simulated practical applications. In addition, TSGMs also exhibited high hydrophobicity with a water contact angle of 147° and excellent thermal insulation with a thermal conductivity of 0.0345 W (m k)−1. The results could contribute to the refinement in the preparation process of silica aerogel composites and use in the fields of thermal and acoustic insulation.

Investigation of fibre-modified silica aerogel composites

Abstract This investigation evaluates the thermal and mechanical performance of fibre-reinforced silica aerogel composites by introducing an optimised fabrication process and vacuumization. The novelty of this investigation was the identification of the minimum amount of solvent being used to synthesise aerogel, as well as the introduction of temperature ageing (45 °C). These provide a valuable guide for lowering the fabricating cost with optimised properties of aerogel composites. The glass fibre (GF)–aerogel composites obtained from the optimised process showed excellent thermal conductivity (18.4 mW m−1 K−1 at a pressure load of 2 psi (13.8 kPa)). It is worth mentioning that vacuum sealing of the aerogel composites not only prevents dustiness during handling but also improves the thermal performance. The thermal conductivity could be further reduced to 13.8 mW m−1 K−1 by vacuum sealing the GF–aerogel composite. The compression and bending tests proved that the aerogel composites could endure considerable compressive and flexural strain without structural destruction. These outstanding characteristics indicate that GF–aerogel composites have great potential in the thermal insulation field, especially for a moderate temperature environment (i.e., less than 800 °C).

Effect of Titanium Dioxide Particles on the Thermal Stability of Silica Aerogels.

Silica aerogels exhibit a unique nanostructure with low thermal conductivity and low density, making them attractive materials for thermal isolation under extreme conditions. The TiO2 particle is one of the common industrial additives used to reduce the thermal radiation of aerogel composites under high-temperature environments, but its influence on thermal resistance is almost unknown. Herein, we report the effect of TiO2 nanoparticles with different crystal phases and different sizes on the thermal stability of silica aerogel composites. By adding TiO2 nanoparticles, the aerogel can significantly resist collapse at high temperatures (up to 1000 °C). And compared with the rutile phase TiO2, the anatase phase TiO2 shows much higher temperature resistance performance, with shrinkage of only one-sixth of the rutile phase after 800 °C treatment. Interestingly, energy-dispersive spectrometer mapping results show that after 800 °C treatment, silica nanoparticles (NPs) are squeezed out in between anatase TiO2 particles, which resists the coarsening of silica NPs and ultimately enhances the stability of aerogel composites. The optimal anatase phase TiO2-doped silica aerogel demonstrates the integrated properties of crack-free morphology (2.84% shrinkage), low thermal conductivity (29.30 mW/(m·K)) and low density (149.4 mg/cm3) after 800 °C treatment. This study may provide new insights for developing oxide-doped silica aerogels with both high-temperature resistance and low thermal radiation.

Cost-effective preparation of pre-oxygenated PAN fiber composite SiO2 aerogel felt with excellent mechanical and thermal insulation properties

Silica aerogel is an attractive thermal insulation material due to its low thermal conductivity, light weight and non-combustible inorganic properties, but its inherent brittleness and high production cost hinder its practical application on a large scale. Herein, the high-performance pre-oxidized polyacrylonitrile fiber composite silica aerogel felt (PPAF-SiO2) was successfully prepared by a cost-effective combustion drying technique for the first time. The results show that the PPAF-SiO2 has the feature of lightweight, good flexibility, and its heat resistance, hydrophobicity, and specific surface area are significantly improved. Due to the further polymerization of fiber structure molecules caused by combustion, the tensile strength of the PPAF-SiO2 is increased by 81.9 % to 1.71 MPa. More importantly, due to the high proportion of silica aerogel in felt, its thermal conductivity can be reduced to 0.02 W/(m·K), which is lower than most aerogel composite materials. Moreover, the thermal insulation performance of the PPAF-SiO2 was further confirmed by the infrared thermal imager from room temperature to 300 ℃. In addition, the innovative drying mechanism based on the combustion flame model was proposed. Finally, compared with traditional physical drying technologies in terms of drying time and drying conditions, the combustion drying technology shows great advantages in drying efficiency, energy consumption and equipment investment, which greatly reduces production costs. The combustion drying technology opens up a promisingly method for high efficiency, low energy consumption, and low cost preparation of silica aerogel composites.

Vibration isolation performance of silica aerogel blankets and boards, fumed silica, polymer aerogel and nanofoams

Buildings are often constructed on a continuous layer of vibration isolation to reduce vibrations and structure-borne noise. There are many vibration isolation products (rubber mats, textiles, polymer foams), but none fulfils all requirements in terms of load-bearing capacity, performance, and cost. In previous work, we have shown that silica aerogel particle beds display a low resonance frequency and correspondingly high vibration isolation performance. Loose silica aerogel granules are not a practical vibration isolation solution. In order to guide the future development of dedicated vibration isolation products based on aerogels, we determine the vibro-acoustic properties of a variety of existing commercial and R&D stage mesoporous composites, that have been developed for thermal insulation applications. The sample set includes monolithic aerogels (polyurethane), silica aerogel composites (aerogel – fibers, aerogel impregnated foams, aerogel bound with polymer foam), glued fumed silica-fiber boards, and nanoporous polymer foam composite boards. The silica aerogel and fumed silica composites display low resonance frequencies (down to 8 Hz for 40 mm, strongly dependent on composite type) and a low dynamic stiffness and loss factor over a wide range of static loads. The polymer aerogel, nanoporous foam and polymer-bound silica aerogel granule board all display higher resonance frequencies, consistent with their higher static stiffness. Although optimized for thermal insulation rather than vibration isolation, many of the tested silica aerogel and fumed silica composites are competitive with the best vibration isolation products on the market in terms of vibration isolation performance. The diversity of materials tested informs on which approaches, materials, and materials combinations are most promising to target vibration isolation.

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.

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.

High-temperature resistance performance of silica aerogel composites through fiber reinforcement

Aiming to meet the fireproof requirements for bridges, three fiber/silica aerogel composites were fabricated using basalt fiber, a composite of basalt fiber and high silica glass fiber, and high silica glass fiber as the matrix. The dimensional, structural, mechanical, and thermal insulation stability of these composites under high-temperature heating ranging from 800 °C to 1000 °C were systematically studied. Upon high-temperature heating, the fracture and deformation basalt fiber occurred, leading to the breakage of the fiber skeleton and the aerogel integrity. This resulted in significant densification and a consequent decrease in porosity in the heated basalt fiber/aerogel composite and basalt fiber-high silica glass fiber/silica aerogel composite. Crystallization of basalt fiber began after heating to 800 °C and fully completed at 1000 °C. Besides, part of the high silica glass fiber in the basalt fiber-high silica glass fiber/silica aerogel composite crystallized at 900 °C and 1000 °C. These variations in micro-macro structures and phase compositions led to a substantial degradation of mechanical property and thermal insulation performance in the composites containing basalt fiber. In sharp contrast, the fiber skeleton and aerogel integrity remained relatively stable under high-temperature heating, enabling a synergistic thermal insulation effect of fiber and aerogel. The integrated fiber skeleton reduced the radiative thermal conductivity of the aerogel, while the unbroken aerogel protected the fiber from crystallization. Consequently, the mechanical property and thermal insulation performance of the high silica glass fiber/silica aerogel composite remained stable after high-temperature heating. The ultimate tensile strength and thermal conductivity of the pristine and 1000 °C-heated high silica glass fiber/silica aerogel composite were 0.74 MPa and 0.53 MPa, 0.0213 W/(m·K), and 0.0279 W/(m·K), respectively. This novel composite, exhibiting low thermal conductivity, high strength, excellent thermal stability, is expected to be a desirable candidate for bridge fireproof applications.

Melamine Network as a Solution for Significant Enhancement of the Mechanical, Adsorptive, and Surface Properties in a Novel Carbon Nanomaterial-Silica Aerogel Composite.

Silica aerogels exhibit exceptional characteristics such as mesoporosity, light weight, high surface area, and pore volume. Nevertheless, their utilization in industrial settings remains constrained due to their brittleness, moisture sensitivity, and costly synthesis procedure. Several studies have proved that adding nanofillers, such as carbon nanotubes (CNT) or graphene nanoplatelets (GNP), can improve the mechanical strength of the aerogels. The incorporation of nanofillers is often accompanied by agglomeration and pore blockage, which, in turn, deteriorates the surface area, pore volume, and low density. Including flexible melamine foam (MF) as a scaffold for the silica aerogel and nanofiller composite can prevent the restacking of the nanofillers through π-π interaction, hence maintaining the incredible properties of aerogels and improving their mechanical properties. CNT, GNP, and the polymeric silica precursor, polyvinyltrimethoxysilane (PVTMS), were added to a MF, at varying concentrations, to fabricate the MF-aerogel nanocomposites. Surfactant and sonication were utilized to ensure a homogeneous dispersion of the nanofillers in the system. The presence of MF prevented the agglomeration of nanofillers, resulting in lower density and relatively higher surface properties (SBET up to 929 m2·g-1 and pore volume up to 4.34 cc·g-1). Moreover, the MF-supported samples could endure 80% strain without breakage and showed an outstanding compressive strength of up to ∼20 MPa. These aerogel nanocomposites also demonstrated an excellent volatile organic compound (∼2680 mg·g-1) and cationic dye adsorption (∼10 mg·g-1).

Diatom-inspired silicification process for development of green flexible silica composite aerogels

In this study, we have developed novel biomimetic silica composite aerogels and cryogels for the first time, drawing inspiration from the natural diatom’s silicification process. Our biomimetic approach involved the modification of tyrosinase-mediated oxidized silk fibroin (SFO) surfaces with polyethyleneimine (PEI). This modification introduced ample amine groups onto the SF polymer, which catalyzed the silicification of the SFO-PEI gel surface with silicic acid. This process emulates the catalytic function of long-chain polyamines and silaffin proteins found in diatoms, resulting in a silica network structure on the primary SFO-PEI network gel’s surface. The SFO-PEI gel matrix played a dual role in this process: (1) It provided numerous amine functional groups that directly catalyzed the silicification of silicic acid on the porous structure’s exterior surface, without encapsulating the created silica network in the gel. (2) It served as a flexible mechanical support facilitating the creation of the silica network. As a result, the final ceramic composite exhibits a mechanically flexible nature (e.g., cyclic compressibility up to 80% strain), distinguishing it from conventional composite aerogels. By mimicking the diatom’s silicification process, we were able to simplify the development of silica-polymer composite aerogels. It eliminates the need for surfactants, multi-step procedures involving solvent exchange, and gel washing. Instead, the reaction occurs under mild conditions, streamlining the composite aerogels fabrication process.

Flexible Silica Aerogel Composites for Thermal Insulation under High-Temperature and Thermal–Force Coupling Conditions

Flexible Silica Aerogel Composites for Thermal Insulation under High-Temperature and Thermal–Force Coupling Conditions

La doped silica aerogel as selective adsorbent for the desulfurization of model fuel

La doped silica aerogel composites (La2O3/SiO2-n) with different Si/La molar ratio were synthesized through sol-gel method and atmospheric drying technology, which were used to adsorb thiophenic compounds from model fuels. Compared with the pure SiO2 aerogel, the adsorption capacities of La2O3/SiO2-n obviously increased, owing to adding La species led to the appearance of the weak Lewis acid sites on the surface. The adsorption mechanism of La2O3/SiO2-n for thiophenics included the direct S-La bond and weak π-La acid-base interaction between La3+ and thiophenics, and the former was primary. Among La2O3/SiO2-n, the La2O3/SiO2-200 not only had the highest adsorption capacity for thiophene, up to 8.16 mg S/gads (i.e., 138.31 mg-S/mmol La), but also exhibited an excellent anti-aromatics competitive adsorption ability and a good regeneration performance. The adsorption kinetic and adsorption isotherms were well described by pseudo-second-order (PSO) and Langmuir models, respectively, indicating the adsorption of La2O3/SiO2-200 for TH was dominated by chemisorption, which was a spontaneous and endothermic process. The present of aromatics in model fuels had a slightly negative effect on the adsorption of La2O3/SiO2 for thiophene.

Experiments and numerical study on heat transfer of moist silica aerogel composites at high temperatures

Silica aerogel composites have promising applications in high-temperature heat storage insulation. However, the impact of high temperatures and moisture on their insulation performance remains unclear. To reveal the influences of high temperature and moisture absorption property on the heat transfer of silica aerogel composites, an experimental and numerical study was conducted to explore the micromorphology, thermophysical parameters, moisture absorption characteristics, and temperature response. The service temperature limit of the silica aerogel composite has been clarified. Measurements have conducted for the thermal conductivity, specific heat capacity, thermal diffusivity, specific surface area, density, porosity, and pore size distribution of the heated silica aerogel composite (at temperatures of 600, 800, 1,000, 1,100 and 1,200 °C). The moisture absorption characteristic curve at 20 °C has been obtained. Thermal testing of silica aerogel composites under varying heating temperatures and moisture content has been completed. Additionally, a numerical method has been developed to calculate the temperature curve of moist silica aerogel composites. The insulation performance of silica aerogel composite with varying moisture contents depends on the game between thermal conductivity and latent heat. Compared with the negative effect of the moisture content on insulation performance, the positive influence of moisture evaporation and heat absorption is dominant in situations involving temperatures higher than the phase transition temperature.

Study on the effect of semi-transparency on thermal insulation performance of silica aerogel composites

This paper centralizes its focus on elucidating the distinctive thermal insulation performance of silica aerogel composites, emphasizing their semitransparent attributes under diverse heating methodologies. Firstly, the theoretical prediction of conductive thermal conductivity is undertaken utilizing the spherical hollow cube model and Davies model. Subsequently, the radiative thermal conductivity is numerically derived employing the two-flux approximation model, integrating experimental measurements of optical radiation characteristic parameters. The prediction results under high-temperature are validated by the measured data via a Hot Disk instrument, utilizing the transient plane source method. Finally, the influence of semitransparent properties of the aerogel composites on thermal insulation performance is exhaustively investigated. This is conducted through a meticulous examination of temperature responses resulting from distinct heating modalities, namely contact heating, quartz lamp heating, and arc heated wind tunnel heating. The findings discernibly illustrate that these diverse heating mechanisms significantly impact the thermal insulation performance of aerogel composites endowed with semitransparent characteristics.

Design of hollow glass fiber/silica aerogel composites for high-temperature thermal insulation applications

Pure silica aerogels are not suitable for high-temperature (500 ∼ 2000 K) thermal insulation applications due to their transparency to infrared thermal radiation. The addition of solid glass/ceramic fibers, such as aluminum oxide and silicon oxide, into the silica aerogel matrix can remarkably suppress the radiative heat transfer. However, the introduced solid fibers increase the structural density and enhance heat conduction in silica aerogel composites. Herein, we propose adopting hollow fibers (HFs) to simultaneously minimize thermal radiation and heat conduction through silica aerogels in a high-temperature environment. The results show that increasing fiber hollowness improves the short-wavelength (<5 µm) extinction efficiency, indicating that using HFs is an effective way to lower the thermal conductivity (TC) at high temperatures of silica aerogels. To achieve minimal radiative TC, the fiber hollowness needs to increase with the external diameter of the fiber at a specific temperature, for example, at 1500 K. This study contributes to the understanding of heat transfer mechanisms in porous materials, particularly those with hollow structures, and offers valuable insights for the strategic design of thermal insulation structures effective at high temperatures.

Insulating materials based on silica aerogel composites: synthesis, properties and application.

The rapidly increasing growth in the world's population has created enormous environmental issues, such as the greenhouse effect, global warming, ozone layer depletion, acid rain, and excessive energy usage. The construction sector, in particular, accounts for a significant share of worldwide energy consumption, making it a major contributor to these challenges. Effective building insulation is crucial for decreasing energy demand, minimising heat loss, and lowering environmental impact. Aerogels have acquired popularity as insulation materials due to their outstanding qualities such as great thermal insulation, flame retardancy, lightweight construction, and environmental friendliness. Silica aerogels, in particular, are emerging as game changers in the construction insulation sector, accounting for a sizable proportion due to their low thermal conductivity and simplicity of manufacture. This paper gives a thorough overview of silica aerogel production processes, the key qualities necessary for aerogel-based composites, and the most recent advances in their use in building insulation. It also examines how to overcome inherent obstacles such as mechanical fragility and high manufacturing costs, providing technologies that improve mechanical strength and long-term durability. By incorporating silica aerogels into construction materials, the industry may significantly reduce energy consumption while also contributing to the creation of sustainable, energy-efficient buildings.

Preparing High-Quality Monolithic Carbon Nanotubes Reinforced Silica Aerogel Composites Based on a Vacuum Freeze-Drying Method

Preparing High-Quality Monolithic Carbon Nanotubes Reinforced Silica Aerogel Composites Based on a Vacuum Freeze-Drying Method

Fabrication and performance evaluation of aramid laminated fabrics incorporated with silica composite aerogel for protective clothing

In recent years, researchers have made efforts to promote the thermal protection capabilities of firefighting suits in response to frequent fire accidents. In this paper, a novel silica/aramid nonwoven fabric (SiO2/AF) aerogel composite was prepared by combining AF and SiO2 aerogel using the sol-gel method and dip coating technology. Polyimide (PI) and zeolitic imidazolate framework-L (ZIF-L) were used as insulation reinforcement materials to fabricate four types of laminated fabrics, namely, the outer layer + waterproof breathable layer + different insulation layers (AF, SiO2/AF, 0.8 % SiO2/AF/PI and 0.12 % SiO2/AF/ZIF-L, respectively). In addition, the fireman uniform was taken as the control group. The results showed that the insulation performance of the composite fabrics was better than that of the control group, and the outer layer + waterproof breathable layer+0.12 % SiO2/AF/ZIF-L laminated fabric exhibited the lowest thermal conductivity (0.038 W/(m·K)), best thermal radiation protective performance (RPP) (246.04 kW/m2) and excellent wear resistance (mass loss rate 0.46 %). This significant work will open up broad prospects for the applications of SiO2/AF composites in the area of heat-resistant clothing.

Fabrication and properties of polyethylene terephthalate (PET) aerogel composites from plastic bottle waste

To solve environmental problems of plastic waste, novel polyethylene terephthalate (PET) aerogel composites from recycled PET fibres of plastic bottles are developed successfully for the first time in this work. The PET honeycomb structures with various cell sizes are fabricated by layering and then bonding PET sheets with ethyl cyanoacrylate. The honeycomb cells are filled with the recycled PET fibres and a polyvinyl alcohol (PVA) cross-linker and then freeze-dried at −70 °C, 100 Pa for 48 h to obtain the PET aerogel composites (PACs). Effects of the various honeycomb cell sizes (1.0 – 2.0 cm), and PET-PVA ratios (5–1, 10–1, and 20–1), on the 3-dimensional (3D) mechanical and thermal properties of the PACs are investigated comprehensively. Due to the light-weight PET aerogel (PA) contribution, the PACs have the ultra-low densities of 0.042 – 0.072 g/cm3 and low thermal conductivities of 0.033 – 0.038 W/m·K. The honeycomb structure with the smallest cell size of 1.0 cm exhibits the highest compressive modulus of 15.30 MPa in the out-of-plane direction and 0.070 – 0.128 MPa in the other in-plane directions of the PAC. The compressive strength of the PACs in the out-of-plane direction is up to 730 times greater than those in the other in-plane directions and 2550 times greater than that of the PAs. The developed PACs perform better than aramid honeycomb-reinforced silica aerogel composites and common commercial products. They can be good alternatives for building insulation and cold chain applications.