Silica Aerogel Composites Research Articles

Novel GO/silica composite aerogels with enhanced mechanical and thermal insulation properties prepared at ambient pressure

ABSTRACTNovel graphene oxide (GO)/silica composite aerogels were fabricated to strengthen the nanostructure and overcome inherent fragility of silica aerogels by a sol-gel method using tetraethyl orthosilicate (TEOS) and low concentration of graphene oxide solution as the main materials, followed by an environmentally friendly ambient pressure drying (APD). The interaction between GO nanosheets and silica matrix was investigated by using FTIR and SEM. Meanwhile, the mechanical properties and thermal conductivity of aerogels were also studied. The results showed that GO were uniformly dispersed in the silica aerogel, and there is a good physico-chemical combination between GO and silica matrix. The addition of GO can enhance the mechanical of the composite aerogels, and when the GO content is increased from 0 to 3wt%, the mechanical strength is improved from 0.04 MPa to 0.65 MPa. Meanwhile, the thermal conductivity of composite aerogels first decreases and then increases with increasing GO content, and the optimal value is 0.018 W/m K. The composites with outstanding properties prepared by APD are potential for insulation applications.

Facile Fabrication of Flexible Bacterial Cellulose/Silica Composite Aerogel for Oil/Water Separation

Abstract Methylene diphenyl diisocyanate (MDI)-modified bacterial cellulose (BC)/silica composite aerogel was prepared by a facile sol-gel process followed by freeze-drying. The obtained composite aerogel showed excellent hydrophobicity and flexibility, making it a promising oil sorbent for oil/water separation.

Lithium-ion doped NiFe2O4/SiO2 nanocomposite aerogel for advanced energy storage devices

Acid and base mediated sol–gel process followed by supercritical extraction of ethanol used for the synthesis of lithium-ion doped nickel ferrite loaded silica composite aerogels using a high-temperature high-pressure autoclave. The physical-chemical properties of synthesized materials are characterized using powder X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), Field emission scanning electron microscopy (FE-SEM) and DC-conductivity measurements. The conductivity results suggest that maximum electrical conductivity of 8.63 × 10−9 Scm−1 is obtained for 4% Li2O doped 96%{10% NiFe2O4 + 90% SiO2} composite aerogel due to Li+ migration between grain-grain interface and electron hopping between Fe2+ and Fe3+. Further, the XRD result also confirms the formation of amorphous SiO2, Li2O with crystalline NiFe2O4 composite aerogel. FTIR results confirm the bond formation through molecular vibration between Si-O-Si and Ni2+/Fe3+-O. FESEM micrographs reflect the three-dimensional, highly porous interconnected network formation of 4LNS composite aerogel. Aqueous supercapacitor performance of Li2O doped NiFe2O4–SiO2 composite aerogels were examined in 1 molar CH3COONa, Na2SO4 and NaOH electrolytes on respective potential regions of 0.5 to −1 V, 0.4 to −0.8 V and 0 to −1.0 V using three electrode systems. The Li2O addition improves the conductivity and hence improve the specific capacitance of SiO2 and NiFe2O4–SiO2 composite aerogels synergically within the potential window.

A thermally stable and hydrophobic composite aerogel made from cellulose nanofibril aerogel impregnated with silica particles

A thermally stable and hydrophobic cellulose nanofibril (CNF)–silica composite aerogel was prepared by simply immersing the CNF aerogel into the silica sol with different tetraethyl orthosilicate (TEOS) concentration and shaping it by means of low-risk ambient pressure drying. After the introduction of the mesoporous silica particles into the cellulose network structure, the BET surface area was found to have sharply increased from 11.3 to 497.8 m2 g−1. All composite aerogels displayed good thermal stability and super-hydrophobicity compared with pure cellulose aerogel. The onset temperature of pyrolysis rose from 317 to 348 °C, and the contact angle reached 152.1°. The TEOS concentration was found to have a great influence on the silica content and the dispersion of silica particles in the cellulose scaffold. Good chemical compatibility at the nanoscale level was present, which indicates that a continuous and homogeneous CNF–silica interface would yield great improvement in thermal properties and water resistance. The results show that a composite aerogel prepared at 2.5 mol L−1 TEOS concentration has better comprehensive performance with only a slightly decrease in mechanical properties compared to CNF aerogel. Thus, this study details a new direction for the synthesis of a cellulose–silica composite aerogel with tailored properties achieved by controlling the silica content and silica dispersion in the cellulose scaffold. Thermally stable, water-resistant, and environmentally friendly cellulose–silica composite aerogels may provide a promising development for designing new functional aerogels that can be applied in various fields.

Enhancement Studies on Manufacturing and Properties of Novel Silica Aerogel Composites.

Silica Aerogel composites are ultra-low density, highly porous foam-like materials that exhibit excellent thermal insulation and high strain recovery characteristics. In the present work, environment-friendly silica aerogel composites are fabricated using silica aerogel granules with bio based porcine-gelatin as the binding agent dissolved in water and by further drying the mix at sub-zero condition. This article focuses on improvement studies carried on the mold design and the manufacturing process to achieve better geometric compliance for the silica aerogel composites. It also presents contact angle measurements, compressive behavior under different cycles of loading, time dependent behavior and flexural response of the composites. The influence of additives, such as fumed silica and carbon nanotubes on mechanical properties of the composites is also deliberated. Water droplet contact angle experiments confirmed the ultra-hydrophobic nature of the composites. The mechanical properties were characterized under cyclic loading-unloading compression and three-point flexure tests. On successive compression in three consecutive load cycles, the strain and thickness recovery were found to decrease by around 30%. The flexural properties of the aerogel composites were investigated using it as the core covered by thin carbon composite face sheets. It was found that the flexural strength and the failure strain of this aerogel sandwich composites is approximately half of the conventional nomex honeycomb sandwich equivalent.

Advances in carbon nanostructure–silica aerogel composites: a review

Synthesis & applications of advanced silica aerogel composites incorporating carbon nanostructures and their valuable properties for thermal insulation, adsorption or electrodes.

Methyltrimethoxysilane silica aerogel composite with carboxyl-functionalised multi-wall carbon nanotubes

To complement the low mechanical strength of pristine methyltrimethoxysilane (MTMS) silica aerogels, a composite silica aerogel with structurally strong multi-wall carbon nanotubes (MWCNTs) was synthesised by supercritical drying. The surface structure of stable MWCNTs was oxidised and defected by acid treatment to attach carboxyl groups, which result in good dispersibility of the MWCNTs. By a sol-gel process, functionalised MWCNTs formed a network structure with the MTMS silica sol. Through the compositisation of the MTMS silica aerogel with functionalised MWCNTs, the mechanical strength of the composite aerogel became higher than that of the pristine MTMS silica aerogel; changes in the surface characteristics, chemical bonding state, and microstructure were also observed. The aerogels exhibit hydrophobicity except at the functionalised MWCNTs, which impart hydrophilicity due to polar functional groups. Chemical bonding, including carbonyl and carboxyl bonds, was also observed following functionalisation. Some fibre structures of MWCNTs and pore characteristics in composites were observed.

Mechanical performance and thermal stability of glass fiber reinforced silica aerogel composites based on co-precursor method by freeze drying

In order to maintain the integrity, glass fiber (GF) reinforced silica aerogel composites were synthesized using methltrimethoxysilane (MTMS) and water glass co-precursor by freeze drying method. The composites were characterized by scanning electron microscopy, Brunauer-Emmett-Teller analysis, uniaxial compressive test, three-point bending test, thermal conductivity analysis, contact angle test, TG-DSC analysis. It was found that the molar ratio of MTMS/water glass could significantly affect the properties of composites. The bulk density and thermal conductivity first decreased and then increased with the increasing molar ratio. The composites showed remarkable mechanical strength and flexibility compared with pure silica aerogel. Moreover, when the molar ratio is 1.8, the composites showed high specific surface area (870.9 m2/g), high contact angle (150°), great thermal stability (560 °C) and low thermal conductivity (0.0248 W/m·K). These outstanding properties indicate that GF/aerogels have broad prospects in the field of thermal insulation.

Synthesis and characterization of a xonotlite fibers–silica aerogel composite by ambient pressure drying

Xonotlite fibers (XFs) reinforced silica aerogel composites were prepared by a sol–gel method under ambient pressure drying. XFs were synthesized through a dynamic hydrothermal route and had a noodle-like structure with length of 5–10 μm and average diameter of 150–200 nm. The microstructure analysis showed that XFs were inlaid in silica aerogel matrix by physical combination which contributed to restrict the volume shrinkage of alcogels and maintain the integrality aerogels during drying process. The physical, naonporous and thermal properties of the as prepared aerogel composites were investigated and discussed in detail. The new aerogel composites possessed porous nanostructure, which exhibited typical properties of 0.126 g/cm3 density, 4.132 cm3/g pore volume, and thermal conductivity of 0.0285 W/(m K). The results indicated that the introduced XFs didn’t significantly alter the porosity, hydrophobicity or thermal conductivity of aerogel matrix. It was also found that the aerogel composites had much more outstanding porosity than that of pure aerogel upon calcinations at 800 °C. This study fabricated XFs–silica aerogel composites and explored a new way for silica aerogels to endure and remain monolithic under ambient pressure drying.

Polypropylene/hydrophobic-silica-aerogel-composite separator induced enhanced safety and low polarization for lithium-ion batteries

Separator as an important part of lithium-ion batteries, allowing the ion to transfer and preventing the direct contact of anode with cathode, determines the safety of the batteries. In this work, a kind of polypropylene/hydrophobic silica-aerogel-composite (SAC) separator is fabricated through combining hydrophobic silica aerogel and polypropylene (PP) separator. The rationally designed SAC effectively increases the thermal stability of the separator with slightly growing weight (the area retention rate is 30% higher than that of the PP separator after being heated for 30 min at 160 °C). In addition, the hydrophobic silica aerogel layer in SAC significantly improves the wettability of PP separator to electrolyte owning to the introduced hydrophobic functional groups of -Si(CH3)3 and porous structure, and the contact angles of SAC separator to several common organic electrolytes (EC/DMC, DMC/DOL, Diglyme) are close to 0°. Electrochemical tests show that the prepared SAC separator can decrease the polarization of Li-ion batteries and leads to improved power performance and cycle stability. And the SAC separator is firm with neglectable abscission after folding 200 times. This work provides a new way to improve the safety and simultaneously reduce the polarization of the batteries, implying promising application potential in power batteries.

Transparent Cellulose–Silica Composite Aerogels with Excellent Flame Retardancy via an in Situ Sol–Gel Process

Cellulose–silica composite aerogels were prepared by in situ formation of silica nanoparticles via a two-step sol–gel process in cellulose gel, which was prepared by dissolving cotton pulp in 1-allyl-methylimidazole chloride ionic liquid (AmimCl) and then regenerating from AmimCl/water bath. Related properties were investigated with ultraviolet–visible spectrometer, scanning electron microscopy, compression tests, thermogravimetric analysis, and ignition tests as well as a microscale combustion calorimeter. The results showed that the composite aerogels displayed increasing transparency, compressive properties, and thermal and thermal-oxidative stability with increasing of silica content. The incorporation of silica nanoparticles also improved the mesoporous characteristics of aerogels including specific surface area and mesopore volume and significantly delayed the decomposition of cellulose, suppressing the heat release during combustion. The composite aerogels with high silica content (33.6% or more) e...

Measuring radiative properties of silica aerogel composite from FTIR transmittance test using KBr as diluents

Radiative properties such as spectral extinction coefficient, Rosseland mean extinction coefficient and radiative thermal conductivity are the key features of porous insulation materials. For silica aerogel composites with fiber and opacifier, it is difficult to determine the spectral extinction coefficient and radiative thermal conductivity via experiment accurately. The spectral extinction coefficient of such composites is usually determined by a transmittance test using KBr as a diluent. The traditional model has several assumptions on determining the extinction coefficient from the transmittance measurement: the diluents are assumed as a pure transparent medium, the pressed pellet has no reflection, and the composite powder is in series with the KBr when calculating the effective thickness of the composite. Thus, the extinction coefficient determined from the traditional model with these assumptions will result in some uncertainty on predicting the radiative thermal conductivity. To overcome the drawbacks, improved models are proposed by considering the absorption of KBr diluents, sample reflection and random distribution of sample powder in the diluents. KBr diluted silica aerogel composite pellets with the same constituents and concentrations but different thicknesses are prepared using the pressed pellets method. The transmittances of the pressed pellets are measured by Fourier transform infrared spectroscopy (FTIR). The spectral extinction coefficients are calculated from the measured transmittances with proposed models. The radiative thermal conductivity of silica aerogel composite is calculated with the measured Rosseland mean extinction coefficient determined from different models. This study found that the developed models could eliminate the assumptions of the traditional model.

Effect of extrusion and compression moulding on the thermal properties of nylon-6/silica aerogel composites

The article presents the effect of a lower extrusion speed and compression moulding processes on the thermal properties of polyamide 6 (PA-6)/aerogel composite. Scanning electron and optical microscope images showed that although most of the aerogel was destroyed during extrusion at 65 r/min, extrusion at 5 r/min showed a better retention of the aerogel structure. However, when subjected to moulding in a compression press, both composites extruded at different speeds suffered significant damage. Nevertheless, the extruded samples did show a lower thermal conductivity compared to the virgin polymer. Further, it was observed that the sample extruded at 5 r/min had a lower damage coefficient value with an overall loss of around 33% to the aerogel structure when compared to the material extruded at 65 r/min, which endured a structural loss of 41% to the aerogel in it.

Effect of high temperature on compression property and deformation recovery of ceramic fiber reinforced silica aerogel composites

Ceramic fiber reinforced silica aerogel composites are novel insulation materials in the thermal protection field for hypersonic vehicles. Before the aerogel composites are applied in load-bearing structures, it is necessary to investigate their mechanical properties including load-bearing and deformation recovery capabilities. High temperature from service conditions will have important effects on the mechanical properties of thermal protection materials. In this paper, compression tests including loading and unloading stages were conducted for ceramic fiber reinforced silica aerogel composites at room temperature and elevated temperatures (300°C, 600°C and 900°C). Influences of thermal exposure to high temperature and high temperature service environment on the compression property and deformation recovery were both investigated. Scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FT-IR) and X-ray diffraction (XRD) were applied to help understand the mechanisms of mechanical property variations. The experimental results show that the compression modulus and strength both increase with the increasing thermal exposure temperature and testing temperature, but the deformation recovery capability decreases. The microstructure changes caused by thermal sintering are considered as the main reason for the property variations. Viscous flow and matter transport due to high temperature resulted in the fusion of aerogel particles. This made the particle skeleton thicker and stronger, which led to higher stiffness and strength of the composites. However, matrix cracks induced by the formation and fracture of larger pores made unrecoverable deformation more serious. In the tests at elevated temperatures, the aggregation of aerogel particles in a fused state got more severe because of the addition of mechanical load. As a result, the degradation of deformation recovery capability became more significant.

A theoretical model for the effective thermal conductivity of silica aerogel composites

In the present paper, a complete engineering calculation procedure for fast evaluations of the effective thermal conductivity of the silica aerogel composites is developed as follows. First, the spherical hollow model is adopted to calculate the conductive thermal conductivity of pure silica aerogels. Then, a mixing model is used to consider the conduction enhancement effects of doped opacifiers and fibers. Next, the Mie theory is adopted to calculate the temperature-dependent Rosseland mean extinction coefficient of the aerogel composites, and the radiative thermal conductivity is obtained according to the Rosseland equation. Finally, the superposition of the conductive thermal conductivity and radiative thermal conductivity gives the total effective thermal conductivity of aerogel composites. To validate the accuracy of the present model, some corresponding experimental measurements are conducted based on the Hot Disk thermal constant analyzer, and the experiment data agree well with the theoretical calculation values. The maximum deviation is about ±10%. The influences of the temperature, pressure and the structure parameters on the prediction of the effective thermal conductivity are then investigated. To determine the dominant factor of the effective thermal conductivity, the contributions of gas, solid and radiation to the total effective thermal conductivity are investigated individually and some significant results are obtained.

Experiment and identification of thermal conductivity and extinction coefficient of silica aerogel composite

Highly porous silica aerogel composites have shown excellent potential as insulation materials for various insulation systems, the knowledge of their conductive and radiative properties is of essential importance. In this study, several experiments were performed to measure the transient heat transfer behavior of silica aerogel composite at temperature between 290 and 1090 K with various gas pressures ranging from 0.01 Pa to 100 kPa. The temperature dependent conductive part of thermal conductivity kc(T), and global extinction coefficient of the silica aerogel composite were simultaneously determined from the experimental temperature data by solving an inverse problem employing the genetic algorithm as optimization method. The retrieved conductive thermal conductivity fell within 0.013–0.032 W/(m·K), increased nonlinear with increasing gas pressure, and showed an approximate linear increasing trend as temperature increases. The global extinction coefficients were independent of gas pressure and showed relatively good agreement with those predicted from Mie theory. These properties could be used to predict the effective thermal conductivity keff(T) for evaluating thermal performance of the insulation systems, and to separate contributions of conductive and radiative heat transfer in silica aerogel composite at various temperatures and gas pressures.

Thermal conductivity of fiber and opacifier loaded silica aerogel composite

Due to pure silica aerogel is fragile and nearly transparent to 3–8μm infrared beam, reinforced fiber and opacifier are composited with silica aerogel to optimize its performance. The suppression of radiative heat transfer is related to opacifier type, size and content. However, the addition of opacifier will enhance solid conduction inevitably. In addition, the influence of doping opacifier will also be affected by temperature since thermal radiation is temperature and wavelength dependent. Therefore thermal conductivities of 10 fiber and opacifier doped silica aerogel composites are studied experimentally and theoretically. Transmittances of silica aerogel doped with opacifier of different type, size and content are measured within 1.4–25μm by FTIR. The spectral extinction coefficient of fiber/opacifier loaded silica aerogel composite is obtained by considering the practical absorption of KBr diluents. The thermal conductivity of silica aerogel composite is measured by transient plane source method at different temperature and gas pressure. Effective model with considering heat conduction via gaseous molecules, matrix skeleton, fiber, opacifier and thermal radiation via skeleton, fiber and opacifier is developed for fast predicting the thermal conductivity of fiber/opacifier doped silica aerogel composite. It is found that the silica aerogels doped with SiC or TiO2 particles has lower radiative thermal conductivity than that doped with ZrO2 particles with volume fraction of 3.75% and diameter of 3.5μm. Silica aerogel doped with 3.75%, 3.5μm SiC particles has the lowest thermal conductivity at temperature less than 1000K.

Silica nanowires reinforced self-hydrophobic silica aerogel derived from crosslinking of propyltriethoxysilane and tetraethoxysilane

Silica aerogels are utilized in the industry and scientific investigations. However, most silica aerogels are brittle, which hinders their development and application. In this paper, we reported the design and synthesis of a silica aerogel composite reinforced with silica nanowires by using two silicon sources: propyltriethoxysilane and tetraethoxysilane. The effects of varying silica nanowires contents on the properties of the silica aerogel composites were studied and compared with those of neat silica aerogel under identical conditions. The composites were obtained by mixing silica nanowires with hybrid silica sol without surface modification and dried in ambient atmosphere. The samples showed low density (0.172 g cm−3), high porosity (over 90%), and super hydrophobicity (the water contact angle reached 153°). The cross-linked silica aerogel composites showed excellent mechanical behavior and can be compressed by as high as 70% of their initial length before they were broken. The maximum completely recoverable compressive strain reached 50%. The elastic modulus of the optimal silica nanowires/silica aerogel composite increased by 67% (from 0.21 to 0.35 MPa) compared with that of the neat silica aerogel. The cross-linked propyltriethoxysilane and tetraethoxysilane can be used to synthesize new types of elastic silica aerogels, and the excellent performance of the silica nanowires/silica aerogels composite will enable its application in various fields.

Hydrophobic silica composite aerogels using poly(methyl methacrylate) by rapid supercritical extraction process

A poly(methyl methacrylate) solution was mixed in an optimized tetraethoxysilane-based silica sol and a silica aerogel was obtained by a rapid supercritical extraction process. The hydrophobicity was enhanced by an addition of poly(methyl methacrylate), the silica aerogel prepared with 6 wt% poly(methyl methacrylate) has the contact angle of 156° with low density (0.067 g/cm3), high surface area (829 m2/g), and low thermal conductivity (0.072 W/m·K). The thermogravimetric-differential thermal analysis also showed that the aerogels were hydrophobic up to a temperature of 393 °C. An extra hydrophobicity could be expected for silica aerogel using poly(methyl methacrylate) through the surface modification of silica aerogel with generated –OCH3 and –CH3 radicals by the thermal decomposition of poly(methyl methacrylate) excluding poly(methyl methacrylate) itself. This study provided a simple and cost effective method that used an inexpensive polymer additive without using an expensive surface modification agent nor hydrophobic silica precursor.

A facile method to prepare cellulose whiskers–silica aerogel composites

Highly porous and hydrophobic cellulose whiskers–silica aerogel composites were successfully fabricated by a novel co-precursor method based on sol–gel process under ambient pressure drying. Cellulose whiskers were dispersed in the initial sol and were finally inlaid in silica aerogel skeleton by physical combination, which retained the integrality of the aerogel matrix. With the increase of cellulose whiskers content from 0 to 15%, the volume shrinkage of alcogels during drying process decreased from 25.5 to 17.6%, while the porous nano-structure of aerogels were not significantly altered. The potential impact of cellulose whiskers on the thermal conductivity and thermal stability of the prepared composites was investigated. The new composite was intact and white, which exhibited typical properties of 0.137 g cm−3 density, 3.525 cm3 g−1 pore volume, and 139.6° contact angle, respectively. This work explained how the addition of an organic filler into the silica aerogels influenced their properties and provided a technique for silica aerogels to endure and remain monolithic under ambient pressure drying.