•Monolithic SiO2/GO composite aerogel was prepared.•Thermal insulation performance of composite aerogels was improved.•The mechanical strength of composite aerogels was enhanced.In order to improve the thermal insulation and mechanical property of silica (SiO2) aerogels, the graphene oxide (GO), as nanofillers, was added into SiO2 matrix to prepare the SiO2/GO composite aerogel monoliths on the basis of sol-gel technology and submitted to supercritical drying. The results showed that the monoliths were maintained and GO was well-distributed in the aerogel, due to the interfacial interaction between GO nanosheets and SiO2 matrix. The thermal insulation property of composite aerogels was improved in contrast with that of pure aerogel. The experimental results suggested that the thermal conductivity was lowered from 0.0089 W/mK to 0.0072 W/mK. A possible mechanism was analyzed to explain this result in virtue of GO loadings. The mechanical strength of aerogels was enhanced. It was found that with the loading of GO from 0.0 wt% to 5.0 wt%, the compressive modulus was enhanced from 0.238 MPa to 0.394 MPa, which was a significant improvement for low-solid-content silica aerogels. Moreover, the composite aerogels exhibited some toughness compared to the fragility of pure aerogel.

A new strategy is proposed to make aerogels possessing both low thermal conductivity and high density by a multiple impregnation approach to the as-prepared silica aerogel monoliths using hydrolyzed sol. The result demonstrates that the thermal conductivity of aerogels after multiple impregnations does not increase compared with that of pristine aerogels and presents a minor decrease from 0.0090 to 0.0081 W/(mK) with the density increment from 0.074 to 0.218 g/cm3. A mechanism has been revealed that the decrease of macropores by multiple impregnation approach could be beneficial for improving the thermal insulation performance of silica aerogels in wide density range.Download high-res image (325KB)Download full-size image

•Thermal treatment was applied to lower the macropore fraction in aerogels.•Theoretical calculation was used to evaluate the experimental results.•This study provided a strategy to improve the thermal insulation performance of aerogels.In this work, thermal treatment to the as-prepared silica aerogels was explored as a strategy to reduce the macropores in aerogels, thus to improve the thermal insulation performance. The experimental results show that the pore size exhibits a slight shift to the value lower than the pore size of no-heat-treated silica aerogels and the macropore volume fraction experiences sharp decline from 63.05 to 24.82% after thermal treatment at 800 °C. The thermal conductivity doesn't increase with the enhancement of density but decreases from 0.0090 W/(mK) for the no-heat-treated sample to 0.0080 W/(mK) for the 400 °C- treated aerogels. The reason is that the reduction of macropores results in the restriction degree of gaseous thermal transfer being enhanced in aerogels, which is proved through theoretical calculation by two models. However, the thermal conductivity dramatically climbs to 0.030 W/(mK) after 800 °C owing to the pore structures being damaged and solid heat transfer gets significantly enhanced, which is confirmed by the theoretical calculation. This study suggests that appropriate high-temperature treatment to the as-prepared aerogels could improve the thermal insulation performance and the usability of silica aerogels at room temperature.

Silica aerogel is deemed as a kind of high-performance thermal insulation materials. However, the existence of macropores in the structure is always ignored in the research and application of aerogels. Thus the thermal insulation performance of silica aerogels could be further improved if the macropores are reduced. In this work, nano-sized Al2O3 powders are explored as nano fillers to reduce the macropore volume fraction in silica aerogels by filling the big voids among the silica aggregates, and further lower the thermal conductivity. The experimental results showed that the macropore volume fraction (VMAC) was dramatically reduced from 63.05% to 23.12% with the addition of Al2O3 powders ranging from 0.0 g to 1.0 g. This trend was also verified by the variation of (VT*-VBET) and (VBET/VT*). Accordingly, the thermal insulation performance was improved due to the reduction of macropores in aerogels. The lowest thermal conductivity of Al2O3-doped aerogels reached 7.41 mW/(m K) in contrast with that of pure silica aerogels (9.00 mW/(m K)), which was a significant decline for aerogel-based materials due to the gaseous heat transfer being further weakened. Moreover, the increment of thermal conductivity from 7.41 to 9.71 mW/(m K) with the Al2O3 powders increasing could be attributed to the enhancement of solid heat transfer in the system. The variation of experimental thermal conductivity was in good agreement with the result of theoretical calculation. This study proposed an innovative idea to improve the thermal insulation of aerogel under ambient conditions.

Low-density graphene/carbon composite aerogels were prepared by sol–gel polymerization, ambient pressure drying and carbonization in an inert gas atmosphere. The preparation conditions, including the initial pH, solid concentration of the precursor solution, and the GO loading content, were investigated in detail. The dispersed graphene nanosheets in the carbon aerogel (CA) matrix made significant contributions to the decreased densities (as low as 0.11 g cm−3) of the CAs. The resultant composite CAs exhibited high specific surface area (>400 m2 g−1), high compression strength (19.9 MPa at a density of 0.404 g cm−3), and extremely low thermal conductivity of 0.028 W m−1 K−1, equal to one fifth of the value of the pristine carbon aerogel.

In order to strengthen the nanostructure and suppress the collapse of nanopores of resorcinol–formaldehyde (RF) aerogels during the drying process, graphene oxide (GO) was incorporated into the RF matrix to prepare GO–RF composite aerogels by sol–gel polymerization. The influences of GO content on the sol–gel process, structure, and physical properties of RF aerogels were investigated. The morphologies of composite aerogels were characterized by scanning electron microscopy and transmission electron microscopy, and it was found that GO was well dispersed in the RF matrix. In addition, GO can obviously accelerate the gelation of the RF solution and reduce both the drying shrinkage and aerogel density. As the content of GO increased from 0 to 2 wt%, both the linear shrinkage and density of composite aerogels decreased progressively from 28.3% to 2.0% and 506 to 195 kg m−3, respectively, implying that GO is an effective additive for inhibiting the volume shrinkage of aerogels during the drying process.

Graphene nanosheets (GNSs) loading graphene-encapsulated iron microspheres (GEIMs) were fabricated by heat treatment of graphene oxide nanosheets (GONs) with ferric trichloride (FeCl3). The special pentagon-hexagonal graphene shells have been produced by precipitation of carbon from metal carbide solutions, thanks to the high reactivity of GONs and ferric nanoparticles dispersing homogeneously between graphene layers. The morphology, structure and elemental composition of GEIMs were investigated by scanning electron microscope, X-ray diffraction and electron energy disperse spectroscope, respectively. The formation mechanism of GEIMs was proposed. Hollow graphene microspheres (HGMs) on the GNSs were obtained with the removal of ferric species in GEIMs. When used as the anode materials for lithium-ion batteries, the almost graphitic HGMs exhibit stable voltage platform at ca. 0.2 V, excellent cycle capability and higher reversible capacity of about 440 mAh g−1 after 50 cycles and possess great potential application in lithium–ion batteries.

Graphene nanosheets (GNSs) loading graphene-encapsulated iron microspheres (GEIMs) were fabricated by heat treatment of graphene oxide nanosheets (GONs) with ferric trichloride (FeCl3). The special pentagon-hexagonal graphene shells have been produced by precipitation of carbon from metal carbide solutions, thanks to the high reactivity of GONs and ferric nanoparticles dispersing homogeneously between graphene layers. The morphology, structure and elemental composition of GEIMs were investigated by scanning electron microscope, X-ray diffraction and electron energy disperse spectroscope, respectively. The formation mechanism of GEIMs was proposed. Hollow graphene microspheres (HGMs) on the GNSs were obtained with the removal of ferric species in GEIMs. When used as the anode materials for lithium-ion batteries, the almost graphitic HGMs exhibit stable voltage platform at ca. 0.2 V, excellent cycle capability and higher reversible capacity of about 440 mAh g−1 after 50 cycles and possess great potential application in lithium–ion batteries.

In order to strengthen the nanostructure and suppress the collapse of nanopores of resorcinol–formaldehyde (RF) aerogels during the drying process, graphene oxide (GO) was incorporated into the RF matrix to prepare GO–RF composite aerogels by sol–gel polymerization. The influences of GO content on the sol–gel process, structure, and physical properties of RF aerogels were investigated. The morphologies of composite aerogels were characterized by scanning electron microscopy and transmission electron microscopy, and it was found that GO was well dispersed in the RF matrix. In addition, GO can obviously accelerate the gelation of the RF solution and reduce both the drying shrinkage and aerogel density. As the content of GO increased from 0 to 2 wt%, both the linear shrinkage and density of composite aerogels decreased progressively from 28.3% to 2.0% and 506 to 195 kg m−3, respectively, implying that GO is an effective additive for inhibiting the volume shrinkage of aerogels during the drying process.