Junzong Feng

Carbon aerogel composites prepared by ambient drying and using oxidized polyacrylonitrile fibers as reinforcements.

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.

Synthesis and characterization of ambient-dried microglass fibers/silica aerogel nanocomposites with low thermal conductivity

A new ambient-dried silica aerogel nanocomposites reinforced by smaller diameter microglass fiber mat were synthesized. Effects of gel treatment and drying temperature, molar ratio of modification agent and volume content of microglass fiber on the composites’ structure and properties were investigated. Increasing the gel treatment temperature with a gradient multi-segment drying process, the aerogel density and volume shrinkage decreased rapidly. Homogeneous and translucent bulk aerogel could be obtained with the density of 0.129 g/cm3, specific surface area of 731.76 m2/g and average pore size of 20 nm. Fewer cracks, more silica matrix and stronger fiber/silica interface, which significantly improves the mechanical performance of the nanocomposites with a high bending strength of 1.4 MPa. The thermal conductivity of the ambient-dried nanocomposites decreased and the bending strength increased with increasing fibers’ volume content. The retrieved nanocomposites is an excellent thermal insulation material with lower thermal conductivity (0.022 W/m K, 650 °C) and high mechanical performance.Graphical Abstract

Low-thermal-conductivity nitrogen-doped graphene aerogels for thermal insulation

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.

Preparation and anti-oxidation performance of Al2O3-containing TaSi2–MoSi2–borosilicate glass coating on porous SiCO ceramic composites for thermal protection

In order to improve the thermal oxidation resistance of carbon fiber-reinforced porous silicon oxycarbide (SiCO) ceramic composites, an Al2O3-containing TaSi2–MoSi2–borosilicate glass coating was formed on the surface of the composites via brushing and sintering. The anti-oxidation property of the coated composites at 1873 K was investigated. Microstructures and chemical compositions of the sample before and after anti-oxidation test were determined using XRD, SEM and EDS. After heating in air at 1873 K for 20 min, the Al2O3-containing TaSi2–MoSi2–borosilicate glass coating effectively protects the SiCO ceramic composites and the coated sample kept its appearance well without obvious defects on the surface. The cross-sectional SEM images show that the coating is covered by a film of oxidation products with a thickness of about 40 μm, which is dense and crack free. Inside the A-TMG coating, irregular-shaped silicides are surrounded by continuous borosilicate glass and no penetrating holes or visible cracks are found. Al2O3 increases the viscosity of the borosilicate glass, which improves oxidation resistance of the coated sample by enhancing gas-penetration resistance of the glass. In contrast, the sample without Al2O3 in the coating slurry is severely oxidized and exhibits lots of open pores on the surface after oxidation test.

Self-Sacrificial Salt Templating: Simple Auxiliary Control over the Nanoporous Structure of Porous Carbon Monoliths Prepared through the Solvothermal Route

The conventional sol-gel method for preparing porous carbons is tedious and high-cost to prepare porous carbons and the control over the nanoporous architecture by solvents and carbonization is restricted. A simple and novel self-sacrificial salt templating method was first presented to adjust the microporous structure of porous carbon monoliths synthesized via the solvothermal method. Apart from good monolithic appearance, the solvothermal route allowed for ambient drying because it made sure that the polymerization reaction was completed quickly and thoroughly. The intact and crack-free porous carbon monoliths were investigated by scanning electron microscopy (SEM), thermogravimetric differential scanning calorimetry (TG-DSC), Fourier transform infrared (FT-IR), energy dispersive spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and nitrogen sorption measurements. It was proven that the self-sacrificial salts NH4SCN had been removed during pyrolyzing and so, porous carbon monoliths could be directly obtained after carbonization without the need of washing removal of salts. Most importantly, the microporous specific surface area of the resultant porous carbon monoliths was dramatically increased up to 770 m2/g and the Brunauer–Emmett–Teller (BET) specific surface area was up to 1131 m2/g. That was because the salts NH4SCN as self-sacrificial templating helped to form more around 0.6 nm, 0.72 nm and 1.1 nm micropores. The self-sacrificial salt templating is also a suitable and feasible method for controlling the nanoporous structure of other porous materials.

High-pressure salt templating strategy toward intact isochoric hierarchically porous carbon monoliths from ionic liquids

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.