Silica Aerogel Composites Research Articles

Investigation of thermal insulation performance of glass/carbon fiber-reinforced silica aerogel composites

Silica aerogels have been widely used as thermal insulators, but their fragility has hindered the potential applications. In this work, a novel way of improving aerogel skeletal structure was proposed where composites were reinforced by three functional layers of glass fiber (GF)/ carbon fiber (CF). Heat insulation performance of fiber-reinforced aerogel composites was also investigated in a simulated solar radiation system. Composites composed of three layers of fiber were prepared by a sol-gel process under ambient pressure drying. The results showed that, as three layers were all GF blankets, the composite heat conductivity increased with increasing glass fiber content. Flexural strength increased initially and reached a maximum at 20% glass fiber content before decreasing. To find a balance between promoting heat insulation performance and strengthening the composite structure, two layers of 5% glass fiber blankets were used as structure strengthening layers and one carding 5% carbon fiber as the heat insulation layer were recommended. Under these conditions, the composite showed extremely low thermal conductivity (0.031 W/m K), almost comparable to that of pure aerogel (0.036 W/m K) while maintaining high flexural strength (2.846 MPa), superior than previous studies. Composites with other ratios of glass/carbon fibers with sandwiched alignments also demonstrated better flexural strength and lower thermal conductivity than GF aerogel composites. This work provided an alternative way to prepare robust and sustainable heat insulation aerogel composites for practical uses.

Structure characteristics and hygrothermal performance of silica aerogel composites for building thermal insulation in humid areas

Building thermal insulation can enhance the thermal environment of buildings, thus decreasing energy consumption. Traditional inorganic insulation materials are easy to absorb moisture so that their insulation performance is reduced significantly when they are used in humid environment. It is of great significance to develop new building insulation materials with low moisture absorption and high thermal insulation. In this paper, sol-gel method and ambient pressure drying were used to prepare polyester fiber felt/silica aerogel composite thermal-insulation material. The hydrophobic properties and the microstructure of the aerogel composites were characterized. Moreover, the water-vapor transmission rate and the moisture absorption rate of the aerogel composites and the fiber felt in various humidity conditions were measured. Transient thermal plane method was applied to the measurement of the thermal conductivity. The thermal conductivity and moisture absorptivity of the polyester fiber felt increased significantly with the increase of ambient humidity. After being composited with silica aerogel, both moisture absorptivity and thermal conductivity decreased significantly and kept small value even in high humidity. The moisture absorption rate of the composites is approximately 1%, and the thermal conductivity is 0.0176–0.0187 W·m−1·K−1. Therefore, the prepared fiber felt/silica aerogel composites could be widely used for building insulation even in humid areas.

Thermal insulated and mechanical enhanced silica aerogel nanocomposite with in-situ growth of mullite whisker on the surface of aluminum silicate fiber

Fiber-reinforced silica aerogel composite has attracted increasing attention in thermal insulation area. Nevertheless, the suboptimal mechanical properties from the weak fiber-aerogel interface seriously restricted the development and application of the composite. In this work, mullite whisker was synthesized in-situ on the surface of aluminum silicate fiber to fabricate the fiber/whisker multiscale structure. The needle-like monocrystalline mullite whiskers were uniformly grown on the surface of fiber, with a length of about 15 μm and a diameter of 100–120 nm. Owing to the pinning effect of mullite whiskers, the fiber-aerogel interface was enhanced immensely. The aluminum silicate fiber/mullite whisker/silica aerogel demonstrated a remarkable high compression strength of 2.33 MPa, 9.32 times higher than that of the composite without whiskers, a low density of 0.26 g/cm3, and a low thermal conductivity of 37.3 mW/(m·K). These results indicate that the fiber/whisker/aerogel nanocomposite is a sort of promising thermal insulation material for heat preservation.

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

AbstractFor 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.

Synthesis and Characterization of Nickel Nitrate As Additive Into SiO <sub>2</sub> Matrix by Supercritical Drying Method

In current report, the investigation of synthesis of silica aerogel composite by reinforcing metal oxide particle nickel nitrate hexahydrate [Ni (NO3)2.6H2O)] as a filler into SiO2 matrix. In silica matrix with the help of tetraethoxysilane (TEOS),ethanol and oxalic acid catalyst ,using sol-gel technique followed by high temperature(N2 gas) solvent extraction with supercritical drying technique. The obtained aerogel doped with nickel nitrate display a well developed porous structure, excellent physical properties with less volume shrinkage, extremely high specific surface area (794.3 m²/g), average pore size of 240 nm. The aerogel composite possessed a BET specific surface area even after calcinations of 500oC for 4 hours by maintaining the pore shrinkage with the supporting skeletons nickel nitrate contents. The cross-linked aerogel composite, structural and physical properties were investigated by XRD (X-ray diffraction), field emission scanning electron microscopy (FE-SEM), Fourier- transform infrared-spectroscopy (FT-IR) and Brunauer, Emmett and Teller (BET) and BJH nitrogen gas adsorption-desorption.

Performance of oil sorbents based on reduced graphene oxide–silica composite aerogels

Clean-up of crude-oil spills ranks among the major environmental remediation issues and demands for sorbents with high and stable oleophilic and hydrophobic features. Here, aerogels with durable hydrophobicity are produced by incorporating reduced graphene oxide (rGO) in highly porous silica matrices. By a sol-gel protocol, high rGO loadings are achieved by exploiting highly dispersible hydrophilic graphene oxide as rGO precursor. Homogeneous rGO-silica composite monoliths are obtained with tunable loading up to 10 wt%, enabling a detailed analysis of the sorption behaviour towards oil spills. Composite aerogels demonstrate an excellent performance towards oil sorption with oil uptake from ∼7 to ∼10 times the aerogel mass. Although the sorption ability is lower than for plain hydrophobic silica, composites with a minimum loading of 5 wt% exhibit very high oil selectivity and show a durable and reliable behaviour over time, providing longer shelf-life than plain SiO2 aerogels. The synergetic effect of rGO towards long-term hydrophobicity and oil selectivity on silica composite aerogels allows for the implementation of such composite materials in real water remediation processes.

Facile synthesis of silica aerogel composites via ambient-pressure drying without surface modification or solvent exchange

Silica aerogels exhibit excellent properties, but their complex preparation procedure limits their broader applicability. Therefore, in the present study, mullite-fiber-reinforced silica aerogel composites (SACs) with low thermal conductivity were synthesized through ambient-pressure drying without surface modification or solvent exchange. The effect of the fiber volume content on the physical and mechanical properties and thermal conductivities of the composites was investigated. The results showed that the SACs with a fiber volume content of 5.7% exhibited the lowest thermal conductivity (0.101 and 0.0403 W m−1 K−1 at 1100 °C and room temperature, respectively), highest contact angle (158.2°) and compressive strength (0.1082 MPa at 10% strain), and lowest density (0.249 g cm−3) among the investigated SACs. Given the facile synthesis procedure and optimized performance of the SACs with a fiber volume content of 5.7%, this material is regarded as an effective and economical thermal insulation material.

Effect of heat treatment temperature on melamine sponge reinforced silica aerogel

A novel structure-controllable melamine sponge reinforcement silica aerogel composite (MS/aerogel) was successfully synthesized via the sol-gel and ambient pressure drying processes. MS/aerogel was heat treated under various temperature conditions to further study the basic properties. The results showed that the heat treatment temperature did have a great effect as the microstructure and inherited group changed. It turned out that MS/aerogel was converted from hydrophobic into hydrophilic. Besides, the thermal properties improved as the lowest thermal coefficient (0.0251 W · m−1 · K−1) was obtained after the sample being heated under 300 °C. As to the compressive performance, it was improved to a high level, and the heat treatment temperature had altered compressive responses significantly. A model of the aerogel particle transformation process was proposed at last to describe the strengthening mechanism briefly. This work could provide some instructional suggestions for the sustainable application of the MS/aerogel composite.

Mechanical and thermal-insulating performance of silica aerogel enhanced jointly with glass fiber and fumed silica by a facile compressing technique

We presented a facile compressing approach to prepare glass fiber and fumed silica jointly enhanced silica aerogel composite. The results show that as-prepared composite has a high flexural modulus of 2.65 MPa with low thermal conductivity of 0.0226 W/m·K. The reason is that the silica aerogel particles merged into the space of the fumed silica strengthening the porous texture of the silica aerogel, and they adhered to the glass fiber (GF) surface tightly resulting in the strong connection of GF with the silica aerogel. This simple method can produce the silica aerogel composite cost-efficiently and enable its wide practical application.

Synthesis of hydrophobic silica aerogel and its composite using functional precursor

Hydrophobic silica aerogel (HSA) was synthesized by a facile sol–gel method and ambient pressure drying with methyltrimethoxysilane as precursor. The effect of precursor concentration on the sol–gel behavior, structure, and thermal conductivity of HSA was investigated. HSA with apparent density of 0.088 g/cm3 showed a thermal conductivity of 0.018 W/(m·K), a specific surface area of 502 m2/g and a pore volume of 1.66 cm3/g. The hydrophobic groups in HSA had a good thermal stability up to 450 °C. Polypropylene felt was used to synthesize hydrophobic silica aerogel composite (HSAC). The effect of sol loading on the structure and thermal conductivity of HSAC was investigated. HSAC with equivalent sol loading exhibited a low shrinkage of 2.5% and a low thermal conductivity of 0.020 W/(m·K). Finally, the preparation method of the HSA is facile and environmentally friendly, and the resulting composite is a promising candidate for thermal insulation under humid environments.

Microstructure characterization and thermal performance of reticulated SiC skeleton reinforced silica aerogel composites

The silica aerogel composite reinforced by reticulated SiC skeleton (SiC/CF-Aerogel) was manufactured successfully via the chemical vapor deposition of the SiC coating and sol-gel process of silica aerogel in this paper. The results indicated that the SiC coating was deposited on the carbon skeleton with the induction of silica aerogel in the pores. The mechanical properties were enhanced due to the 3-dimensional reinforcement, which possessed a higher compressive response compared with the pure silica aerogel. Besides, SiC/CF-Aerogel possesses fine adiabatic performance with a thermal conductivity of 0.161 W m−1 K−1 at 700 °C. A physical heat model was proposed to illustrate the heat transfer to better understand the excellent thermal properties. It had a big practical potential for future applications.

Design and thermal insulation performance analysis of endothermic opacifiers doped silica aerogels

In order to reduce the thermal radiation and further deter the heat transfer in silica aerogel thermal insulation materials, the endothermic opacifier is designed with the shell of SiC and the core of phase change material (PCM) in this paper. Then the transient heat transfer characteristics of the endothermic opacifiers doped silica aerogel (EOSA) are numerically investigated and its short-term thermal insulation performance is estimated with a modified lattice Boltzmann method (LBM) model. The modified LBM scheme is established with a new source term to remedy the effect of the variation in the volumetric heat capacity of the specimen, and this scheme is fully validated based on the analytical results and available experimental data. The results demonstrate that adding the endothermic opacifier to silica aerogels results in a significant heat transfer delay and an improvement in short-term thermal insulation performance by suppressing radiation heat transfer and solid-liquid phase change heat absorption. With the same doping amount, the endothermic opacifiers doped silica aerogel exhibits more excellent short-term thermal insulation performance and a lower density than that of SiC opacifiers doped silica aerogels (SOSA). It indicates that the EOSA possesses great potential to be a candidate as the lightweight and efficient thermal insulation material at high temperature. Besides, exhaustively parametric studies based on the EOSA, including doping amount, latent heat of PCM, phase change temperature and endothermic opacifier particle size are conducted, and several design rules are established. This analysis could serve as a framework to develop and design silica aerogel composites with low-density and excellent short-term thermal insulation performance.

Mechanical and thermal properties of reticulated SiC aerogel composite prepared by template method

A novel structure-controllable reticulated silicon carbide (SiC) skeleton-reinforced silica aerogel composites (SiC/aerogel) were fabricated successfully by template method. Three-dimensional SiC skeletons prepared by different deposition time were prepared via the chemical vapor deposition technology, and then the silica aerogel was induced by the sol–gel process. The test results showed that the mechanical properties increased and thermal conductivities decreased remarkably after impregnating reticulated SiC skeleton with silica aerogel. The SiC/aerogel-24 possessed the highest compressive strength of 0.82 MPa with the thermal conductivity of 0.1597 W/(m·K) at 600℃, while the SiC/aerogel-12 exhibited the lowest thermal conductivity of 0.1244 W/(m·K) and its compressive strength was 0.64 MPa. The present work reported a novel method to manufacture the structure-controllable reticulated SiC aerogel composite which could be used as a high-temperature super-thermal insulation material for the potential applications.

Novel Insulation Material Based on Basalt Fibres and Silica Aerogels

Basalt fibre reinforced silica aerogel composites with two different fibre lengths and two different fibre volume contents were successfully produced and analysed. After aging and drying of the fibre reinforced aerogel specimens, the geometrical data as well as the weight of each specimen were registered. The microstructure of selected specimens was recorded by tomography. Three-point bending tests were carried out in order to evaluate the effects of the fibrous reinforcement. The shrinkage of the samples was reduced with increasing fibre volume fractions. Specimen with a fibre length of 3.2 mm and a fibre volume fraction of 1.5 % showed the most homogeneous fibre distribution. Specimens with a fibre length of 12.7 mm fibre and a fibre volume fraction of 1.5 % demonstrated the best performance during the three-point bending test, achieving a fivefold increase in the maximum bending stress compared to pure silica aerogel samples with up to 33 kPa.

Microstructure and Thermal Insulation Property of Silica Composite Aerogel.

Tetraethyl orthosilicate was selected as a matrix of heat insulating materials among three silanes, and an anti-infrared radiation fiber was chosen as a reinforcement for silica aerogel insulation composite. The silica aerogel was combined well and evenly distributed in the anti-infrared radiation fiber. The heat insulation effect was improved with the increase in thickness of the aerogel insulation material, as determined by the self-made aerospace insulation material insulation performance test equipment. The 15 mm and 30 mm thick thermal insulation material heated at 250 °C for 3 h, the temperatures at the cold surface were about 80 °C and 60 °C, respectively, and the temperatures at 150 mm above the cold surface were less than 60 °C and 50 °C, respectively. The silica aerogel composites with various thicknesses showed good thermal insulation stability. The silica insulation composite with a thickness of 15 mm exhibited good heat insulation performance, meets the thermal insulation requirements of general equipment compartments under low-temperature and long-term environmental conditions. The thermal conductivity of prepared silica aerogel composite was 0.0191 W·m−1·k−1 at room temperature and 0.0489 W·m−1·k−1 at 500 °C.

Fast microwave dried mechanical robust melamine sponge/methyltrimethoxysilane-based silica aerogel composites with bendability and superhydrophobicity

In this work, melamine sponge (MS) reinforced methyltrimethoxysilane (MTMS)-based silica aerogels were fabricated via a sol–gel method followed by fast microwave drying. The aerogel composites with large size (20 × 20 × 2 cm) can be dried within 30 min. The MS/MTMS-based silica aerogel composites exhibit macro-pore structure, low density (95–178 kg m−3), excellent integrity and bending flexibility, together with high mechanical strength (Young’s modulus up to 4.787 MPa). Moreover, the superhydrophobicity with a contact angle as high as 164° and a low thermal conductivity of 0.033 W m−1 K−1 are demonstrated. Our work offers an effective method to fabricate high-quality aerogel composites as promising candidates for energy-saving applications.

Silica aerogel composites with embedded fibres: a review on their preparation, properties and applications

Silica aerogels are among the lightest solid materials but they are also very fragile. Fibres embedment is the most versatile and effective method to preserve a monolithic shape during drying, even at large scale, thus widening their applications.

Effect of aging on thermal conductivity of fiber-reinforced aerogel composites: An X-ray tomography study

Abstract Silica aerogels display an ultra-low thermal conductivity (λ) and are used as thermal superinsulators. Here, we study the influence of aging and drying processes on the microstructure and thermal conductivity of fiber-reinforced silica aerogel composites. Glass wool-silica gel composites were aged for variable times, hydrophobized, and dried either at ambient pressure or from supercritical CO2 (scCO2). The X-ray micro-tomographic data display three distinct phases: silica aerogel, glass fibers, and macroscopic pores and cracks. The silica aerogel appears as a continuous medium in the tomograms because the spatial resolution (6–11 μm) is insufficient to resolve the aerogel mesopores (∼0.02–0.10 μm). For the composites prepared by ambient pressure drying, insufficient aging led to prominent drying shrinkage and cracking, and a high macro-porosity, as quantified by 3D image analysis. Insufficient aging also led to an increase in λ from 15.7 to 21.5 mW m−1 K−1. On the contrary, composites that were nearly free of cracks and displayed a constant λ of 16.3 ± 0.8 mW m−1 K−1 could be prepared by scCO2, independent of aging time. The thermal conductivity was reproduced from the macro-porosity to within 0.7 mW m−1 K−1 using simple thermal transport models consisting of thermal elements connected in series or parallel. Our results illustrate the usefulness of X-ray micro-tomography to quantify the 3D microstructure and its effects on the bulk composite properties and the data highlight the importance of aging for the production of low λ aerogel-fiber composites by ambient pressure drying.

Mechanical reinforced fiber needle felt/silica aerogel composite with its flammability

Fiber needle felt–silica aerogel composite was successfully prepared by via sol–gel process based on water glass. The thermal conductivity show V-type variation tendency with the increase of water to Si. Thermogravimetric analysis-differential scanning calorimetry analysis revealed that the thermal stability was up to approximately 390.58 °C. It has been found that the fire hazard of the composites decreased with the increased ratio of water to Si according to the cone calorimeter test, which can be characterized by peak heat release rate, fire performance index, and fire growth rate index. The fiber needle felt/aerogels present greatly improved compressive and flexural strength (elastic modulus: 0.1–0.97 MPa; flexural modulus: 0.33–0.66 MPa) while keeping inherent properties of pure silica aerogel: low bulk density (0.166 g/cm3), low thermal conductivity of 0.0236 W/m·K, and high specific surface area (1091.62 m2/g). As a result, the as-prepared composite shows a great potential to be applied in the thermal insulation field.

Design and optimization of core/shell structures as highly efficient opacifiers for silica aerogels as high-temperature thermal insulation

Opacifiers are usually doped in the silica aerogels to reduce the radiative heat transfer at high temperature. However, the doped opacifiers will enhance the heat conduction in the solid phase and increase the density of silica aerogels dramatically. For developing lightweight and efficient opacifiers, in this paper, different types of core/shell opacifiers are designed, and their extinction performance is investigated, theoretically. Then the optimal temperature-dependent particle size and doping amount of these opacifiers are obtained by maximizing the Rosseland mean extinction coefficient and minimizing the total thermal conductivity of silica aerogel composite. The results show that the hollow carbon black opacifiers might greatly reduce the densities of the opacifier-doped silica aerogels without deteriorating appreciably their insulating capability, and the carbon/SiC, carbon/TiO2 and carbon/Al2O3 core/shell opacifiers exhibit excellent extinction performance and would be strong candidates for high temperature due to their high extinction performance, low density and high-temperature stability. Finally, a core/shell opacifier-gradient-doped silica aerogel based on the optimal opacifier types and doping amount is designed, which significantly improves the insulating performance and reduces the density of silica aerogel composite. The results of this paper present important references for the process design and improvement of comprehensive performance of opacifier-doped silica aerogels.