Ai Du

A Special Material or a New State of Matter: A Review and Reconsideration of the Aerogel

The ultrahighly nanoporous aerogel is recognized as a state of matter rather than as a functional material, because of its qualitative differences in bulk properties, transitional density and enthalpy between liquid and gas, and diverse chemical compositions. In this review, the characteristics, classification, history and preparation of the aerogel were introduced. More attention was paid to the sol-gel method for preparing different kinds of aerogels, given its important role on bridging the synthetic parameters with the properties. At last, preparation of a novel single-component aerogel, design of a composite aerogel and industrial application of the aerogel were regarded as the research tendency of the aerogel state in the near future.

A versatile sol–gel route to monolithic oxidic gels via polyacrylic acid template

A versatile sol–gel route (dispersed inorganic sol–gel, DIS method) was developed to prepare diverse monolithic gels and, ultimately, monolithic aerogels by simply using the reagents of inorganic salt solution, polyacrylic acid (PAA), and propylene oxide (PO). The preliminary study on the gelation processes of diverse ions including Li+, Al3+, Ca2+, Ti4+, Cr3+, Mn2+, Fe3+, Co2+, Ni2+, Cu2+, Zn2+, Zr4+, Mo5+, Cd2+, and Ta5+via both DIS and epoxide addition method (EA method, without adding PAA) indicates that PAA facilitates the formation of monolithic gels, instead of precipitates or cracking gels. The formation mechanism of DIS method was studied by comparing the hierarchic microstructure and pore texture of the chromia gels via DIS and EA method. Conversion of DIS-derived and EA-derived chromia gels to the aerogels was accomplished by the supercritical fluid drying in order to maintain the nanoporous structure of the gels. At last, it is suggested that PAA acts as both dispersant and template in the sol–gel reaction, which not only provides steric hindrance, but also guides the gelation during the gelation processes of hydration, hydrolysis, nucleation & growth, and crosslinkage & formation.

One-pot synthesis, characterization and properties of acid-catalyzed resorcinol/formaldehyde cross-linked silica aerogels and their conversion to hierarchical porous carbon monoliths

A rapid and facile synthesis of resorcinol/formaldehyde cross-linked silica (RF/SiO2) aerogels was carried out in one pot based on an acid-catalyzed route, instead of the previously reported base-catalyzed ones. The gelation time was reduced to several hours at room temperature while it took several days even under heating conditions in the base-catalyzed ones. The interpenetrating network of RF/SiO2 aerogels showed similar porous structures with those of silica aerogels or RF aerogels. Their thermal conductivity was as low as that of the typical glass wool materials. The mechanical properties are characterized by dynamic mechanical analysis and compression testing. At room temperature, the results of compression testing show that the compressive Young’s modulus or ultimate failure strength of RF/SiO2 aerogel specimen is higher than that of native SiO2 aerogels with a similar density. The one-pot method improves the efficiency and reduces the cost of RF/SiO2 aerogels. The hierarchical porous carbon monoliths are also converted from carbonized RF/SiO2 aerogels by an additional HF treatment. Hence, they could be further explored as multifunctional candidate materials for thermal, mechanical, and electrochemical applications.

Aerogel: a potential three-dimensional nanoporous filler for resins

Considering its special microstructure and unique properties, silica aerogel was chosen as three-dimensional (3D) nanoporous filler for epoxy resin in this paper. Pure epoxy resin (0 wt%), 0.1 wt%, 1 wt%, 5 wt%, 10 wt%, and 100 wt% (pure silica aerogel) aerogel/epoxy composites were fabricated and then characterized by dynamic mechanical analyzer (DMA) and field emission scanning electron microscope. The results showed that small amount of filler efficiently increased the stiffness of the composites, but the stiffness decreased with the increase of the mass fraction of the aerogel in the composites (composites ratio); the glass transition temperature of the composites substantially increased, compared to pure epoxy resin. Also, the compressive modulus of the composites at glassy state, rubbery state, and hardening state were studied, respectively. At last, the effects which presumably affect the properties of aerogel/epoxy composites were discussed. Anchoring effect and interfacial effect were suggested t...Considering its special microstructure and unique properties, silica aerogel was chosen as three-dimensional (3D) nanoporous filler for epoxy resin in this paper. Pure epoxy resin (0 wt%), 0.1 wt%, 1 wt%, 5 wt%, 10 wt%, and 100 wt% (pure silica aerogel) aerogel/epoxy composites were fabricated and then characterized by dynamic mechanical analyzer (DMA) and field emission scanning electron microscope. The results showed that small amount of filler efficiently increased the stiffness of the composites, but the stiffness decreased with the increase of the mass fraction of the aerogel in the composites (composites ratio); the glass transition temperature of the composites substantially increased, compared to pure epoxy resin. Also, the compressive modulus of the composites at glassy state, rubbery state, and hardening state were studied, respectively. At last, the effects which presumably affect the properties of aerogel/epoxy composites were discussed. Anchoring effect and interfacial effect were suggested to explain the thermal—mechanical behaviors of the composites with different composite ratio.

Super Black Material from Low-Density Carbon Aerogels with Subwavelength Structures

Many scientists have devoted themselves to the study of the interaction between subwavelength structures and electromagnetic waves. These structures are commonly composed of regular arrays of subwavelength protuberances, which can be artificially designed. However, extending from 2D periodic patterns to 3D disordered subwavelength structures has not been studied yet. In this study, we studied the total diffuse reflectivity of carbon aerogels with various 3D networks of randomly oriented particle-like nanostructures by using normally incident visible light (430-675 nm). We observed that the different 3D network nanostructures of carbon aerogels, especially for the structures with the minimum size, reduced the reflectivity effectively. It was found that the key mechanism for the subwavelength-structure-induced ultralow reflectivity property is due to the decrease of the amplitude of electron vibration forced by the electromagnetic wave, which provides a simple method for designing perfect black materials.

Reaction-Induced Microsyneresis in Oxide-Based Gels: The Assembly of Hierarchical Microsphere Networks

Rigid and stable networks composed of litchi-shaped microspheres were formed via hierarchical self-assembly (SA) of oxide-based nanoparticles (NPs). The phenomenon of the apparent changes from NPs networks to microspheres networks after the gelation was similar to normal microsyneresis. However, in-situ composition evolution results indicate that the SA is driven by interparticle dehydration, but not affinity difference between the network for itself and for the solvent. In-situ small-angle X-ray scattering (SAXS), UV-vis-NIR, and electric conductivity were used to study the microsyneresis process. To further demonstrate the mechanism, extra complexant was added and successfully restrained the NPs-microsphere transition by inactivating the surface hydroxyl of the NPs. Considering the structural similarity, this work may provide a new approach to control the assemblies of diverse oxide-based NPs.

Microstructure control of the silica aerogels via pinhole drying

Pinhole drying process is a slowly dryness in the air caused by capillary force. It was found useful to improve the transparency of silica aerogel. So in this work, pinhole drying was considered as a modification on the microstructure. We studied the effect of pinhole drying on the light transmission of aerogel. The effect of pinhole drying on the microstructure of aerogel was also investigated. The spherical transmittance (τs) and directional transmittance (τd) were measured by ultraviolet–Visible–infrared spectrophotometer, the scattering light at the 90° direction of the gel was measured by a nephelometer, and the microstructure of the aerogels were analyzed applying small angle X-ray scattering, nitrogen adsorption porosimetry, and scanning electron microscope. The results showed that pinhole dried silica aerogels and non-pinhole dried aerogels exhibited different microstructure and optical properties. The directional transmittance of the pinhole dried aerogel and non-pinhole dried aerogel at 600 nm wavelength was 57.9 and 40.9%, respectively, an improvement of 17.0%. The transparency ratio (τd/τt) of pinhole dried aerogel and non-pinhole dried aerogel at 400 nm wavelength was 0.26 and 0.07, respectively, an improvement of 0.19. Small angle X-ray scattering measurements showed that these improvements can be attributed to the smaller mean cluster size of pinhole dried aerogels than that of non-pinhole dried aerogels. 90° scattering light experiments indicated that the clusters of the gel were becoming smaller during the drying process caused by capillary force according to Rayleigh scattering theory. It is the reason why pinhole dried aerogels have smaller mean cluster size and relatively transparent.Graphical Abstract

Template confined synthesis of Cu- or Cu2O-doped SiO2 aerogels from Cu(II)-containing composites by in situ alcohothermal reduction

Uniform and highly dispersed Cu- or Cu2O-doped SiO2 aerogels were synthesized via an in situ alcohothermal reduction strategy. The initial templates containing an accurately controllable doping fraction were prepared through a co-gelation method. After alcohothermal reduction and CO2 supercritical drying, a similar morphology of the resulting samples to the initial templates was retained. The entire reducing process was systematically studied through comparative experiments, separately using ethanol, ethylene glycol and glycerol as reducing agents, with the corresponding converted products obtained being cubic Cu2O- and Cu-contained silica composites. The specific surface area of the resulting products ranged from 500 m2 g−1 to 850 m2 g−1. The related microstructure evolution mechanism was comprehensively studied through analysis of the pore-size distribution. The high specific surface area and controlled doping amount make it suitable for possible use in specific applications, such as high efficiency photocatalysis and backlight targets.

Effects of monomer rigidity on the microstructures and properties of polyimide aerogels cross-linked with low cost aminosilane

Polyimide aerogels were formed from polyamide acid oligomers cross-linked by Si–O–Si network structures, which were derived from hydrolysis and condensation reactions of low cost bis(trimethoxysilylpropyl) amine (BTMSPA). To investigate the effects of the chemical structures on their properties and microstructures, polyimide aerogels produced using hybrid diamines with different rigidity are evaluated. It is found that polyimide aerogels with rigid building blocks (PI-RBs) have a density in the range of 0.245 to 0.300 g cm−3 depending on the uncontrollable shrinkage (27–36%), and high Youngs modulus of 50–76 MPa. While polyimide aerogels with flexible building blocks (PI-FBs) exhibit lower shrinkage of 8–15%, lower density ranging from 0.124 to 0.172 g cm−3, and a lower modulus of 28–34 MPa. Their various performances are closely related to the nanostructure difference between the particulate PI-FBs and the fibrous PI-RBs. The diverse morphology has been attributed to the different rigidities of their repeat units. Over all, the obtained polyimide aerogels are all excellent high-temperature thermal insulation materials with low thermal conductivity of 0.033–0.049 W (mK)−1 at room temperature and 5% weight loss temperature at about 550 °C in N2.

An investigation on the assembling of WO3 particles on the matrix of silica solution

A facile way to prepare modified WO3 structure by silica support through sol–gel method is reported. The WO3/SiO2 complex film was synthesized from a two steps process and dip-coating method. The films were characterized with laser particle analyzer, IR, Raman. The studies of gelation time and particle size distribution of WO3/SiO2 sol indicate that the silica addition could largely reduce the polycondensation of WO3 clusters. The reaction between WO3 and SiO2 were further systematically investigated using IR spectra, and an insight of this reaction was illuminated. Results reveal that Si–OH in SiO2 sols tended to crosslink with WO3 at the corner-sharing W–O sites, by which only edge-sharing WOx clusters could be detected. This modified the WO3 structure was also approved by the Raman spectra, TEM and AFM images. Moreover, gas sensing properties of the WO3/SiO2 films were tested. The assembled WO3 films exhibited more stable gas sensing stability than pure WO3 films.