•Design and preparation of novel thermal insulation materials.•N-doped graphene aerogels composites reinforced by fibers were prepared.•The composites show ultralow density, low thermal conductivity and high strength.•The composites are useful in the aerospace field for thermal insulation.Lightening thermal insulation system is significantly crucial for the high-cost aerospace field. This paper prepared novel thermal insulation composites, N-doped graphene aerogels reinforced by ultrafine quartz fibers. The density is only 0.072 g/cm3, and the thermal conductivity measured by Hot disk apparatus is 0.0327 W/(m K). Meanwhile, with reinforced by ultrafine quartz fibers, the tensile strength of the composites reaches 3.61 MPa. The ultralow density, low thermal conductivity and high strength make the composites potentially useful thermal insulation materials in the aerospace field.

Through a facile and novel high-pressure salt templating approach, a crack-free hierarchically porous carbon monolith without visible volume changes was prepared using 1-ethyl-3-methyl-imidazolium dicyanamide (Emim-dca) as a carbon precursor. TG-DSC and FT-IR measurements revealed that Emim-dca pyrolysis, decomposition, crosslinking and carbonization reactions occurred in turn at temperatures of 234–350 °C, 350–520 °C and 520–1000 °C, respectively, which provides a guide for the preparation of an intact porous carbon monolith. The porous carbon monolith prepared at 4 MPa is amorphous and composed of small, uniform carbon particles with interconnected interstitial pores. Its bulk density is 0.072 g cm−3. Besides the advantages of well monolithic formability and integrity derived from high pressure strategy, interestingly, the obtained porous carbon monolith possesses a higher specific surface area compared to the porous carbon powders fabricated through ambient pressure salt templating. The meso and macro specific surface area of the resultant porous carbon monolith is nearly three times higher (314.0 versus 106.7 m2 g−1) with the content of 4 nm mesopore increasing dramatically compared to that of porous carbon powders prepared under ambient pressure, while the architecture of micropores keeps unchanged. These results might be explained as follows. The high pressure compresses gas molecules from the decomposition reaction into the carbon skeleton to form super-mesopores and macropores and simultaneously disperses salt clusters, generating small (∼4 nm) mesopores. As a solution to volume expansion in powders obtained via salt templating, the high pressure results in a porous carbon with an intact monolithic shape without volume expansion or shrinkage. Thus, fiber-reinforced porous carbon composites can be prepared for ultra-high-temperature insulation using the high-pressure salt templating method.

Aerogels with oriented sheets show better thermal insulation properties. However, it is difficult to prepare bulk oriented aerogels. In this paper, we prepared this kind of aerogels by using aligned fibers as the wall of galleries. This simple method makes the N-doped graphene sheets align along the in-plane orientation during impregnating. The as-prepared aerogel composites not only show much lower through-plane thermal conductivity (26.6–29.8 mW m−1 K−1) compared to their in-plane thermal conductivity (44.9–55.1 mW m−1 K−1), but also exhibit high tensile strength (higher than 3 MPa). After being heat treated at 300 °C, the through-plane thermal conductivity (23.3 mW m−1 K−1) is even lower than the thermal conductivity of pure aerogels. These properties significantly contribute to their applications in thermal insulation. Besides, the simple method is also useful in preparing other materials with oriented sheets and high strength.

•Free-standing chitosan gels could be formed by using ethanol as co-solvent.•Chitosan aerogels with large internal surface area were prepared in ethanol/water binary medium.•Chitosan aerogels enable to be utilized in effective removal of methyl orange.Acquiring high specific surface area (SSA) is a crucial reason why aerogels possess diverse unusual functionalities both in nanoscale structures and macroscopic properties. Although biomass aerogels can remedy the mechanical brittleness in contrast to conventional silica aerogels, regrettably causing a major decrease in SSA aspect. Here we present a generalizable synthetic means towards a family of chitosan aerogels (CAs) with large internal surface area, originating from chitosan sols in ethanol/water binary medium. The as-prepared CAs show high SSA of 973 m2 g−1, appropriate texture homogeneity and thermal stability, attributing to uniformly inter-associated structures. We found an emergent phenomenon that free-standing wet chitosan gel could be still produced despite greatly lowering its substance concentration, attesting the remarkable role of ethanol for promoting gelatum formation. This enhancement to gelation is due to the introduction of micro-dispersed active phases upon ethanol/water excitation, easily forming fine interconnected skeletons of CAs. The adsorption tests of CAs for abatement of methyl orange (MO) verify that dye-treated water almost recovers into pure one, well in line with the evidence of high SSA CAs. A convincing explanation for gelling and cross-linking is also analyzed. Our work facilitates to explain the generation mechanism of gels in binary solvents for the creation of CAs with high SSA.Download high-res image (207KB)Download full-size image

Polyimide aerogels for low density thermal insulation materials were produced by 4,4′-diaminodiphenyl ether and 3,3′,4,4′-biphenyltetracarboxylic dianhydride, cross-linked with 1,3,5-triaminophenoxybenzene. The densities of obtained polyimide aerogels are between 0.081 and 0.141 g cm–3, and the specific surface areas are between 288 and 322 m2 g–1. The thermal conductivities were measured by a Hot Disk thermal constant analyzer. The value of the measured thermal conductivity under carbon dioxide atmosphere is lower than that under nitrogen atmosphere. Under pressure of 5 Pa at −130 °C, the thermal conductivity is the lowest, which is 8.42 mW (m K)−1. The polyimide aerogels have lower conductivity [30.80 mW (m K)−1], compared to the value for other organic foams (polyurethane foam, phenolic foam, and polystyrene foam) with similar apparent densities under ambient pressure at 25 °C. The results indicate that polyimide aerogel is an ideal insulation material for aerospace and other applications.Keywords: cross-link; low temperature; polyimide aerogels; supercritical drying; thermal conductivity

Aerogels such as SiO2 aerogels, Al2O3 aerogels and carbon aerogels have been widely used in thermal insulation. However, graphene aerogels (or reduced graphene oxide aerogels), which have similar structures, have never been used in this field. In this paper, the concept of suppressing the thermal conductivities of graphene aerogels by introducing defects or doping atoms in graphene was introduced. Nitrogen-doped (N-doped) graphene aerogels with low thermal conductivity were prepared with paraphenylene diamine as a bridging and doping agent by CO2 supercritical drying. With the introduction of doping atoms and the bridging agent, the solid thermal conductivity is depressed. Also, with CO2 supercritical drying, the pore size is reduced, and the gaseous thermal conductivity is suppressed. The lowest thermal conductivity of N-doped graphene aerogels is 0.023 W (m−1 K−1), which is nearly 1/2 of the lowest reported value and even lower than that of static air. Meanwhile, the thermal insulation mechanisms were also studied. The low thermal conductivity and low bulk density make N-doped graphene aerogels a potentially useful thermal insulation material that may significantly lighten thermal insulation systems.

Carbon fiber-reinforced carbon aerogel composites (C/CAs) for thermal insulators were prepared by copyrolysis of resorcinol-formaldehyde (RF) aerogels reinforced by oxidized polyacrylonitrile (PAN) fiber felts. The RF aerogel composites were obtained by impregnating PAN fiber felts with RF sols, then aging, ethanol exchanging, and drying at ambient pressure. Upon carbonization, the PAN fibers shrink with the RF aerogels, thus reducing the difference of shrinkage rates between the fiber reinforcements and the aerogel matrices, and resulting in C/CAs without any obvious cracks. The three point bend strength of the C/CAs is 7.1 ± 1.7 MPa, and the thermal conductivity is 0.328 W m–1 K–1 at 300 °C in air. These composites can be used as high-temperature thermal insulators (in inert atmospheres or vacuum) or supports for phase change materials in thermal protection system.Keywords: ambient drying; bend strength; carbon aerogels; carbon−carbon composites; polyacrylonitrile fibers; residual stress; thermal conductivity;