Highly efficient preparation and carboxylation of nanocelluloses were achieved simultaneously using innocuous and recyclable citric acid with the assistance of ultrasonication.

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Biomass aerogels have received extensive attention due to their unique natural characteristics. However, biomass-based chitosan aerogels are often confronted with the traditional issue concerning a weak skeleton structure, namely, the corresponding huge shrinkage for chitosan aerogels in the stage from the final gel to the aerogel. Herein, we put forward a new approach to enhance chitosan aerogels by introducing natural biomaterial cellulose nanocrystal (CNC). CNC is applied to connect/cross-link chitosan chains to form its networking construction through supramolecular interaction/physical entanglement, eventually realizing the enhancement of the chitosan aerogel network structure. Chitosan aerogels modified with CNC exhibit a high specific surface area of 578.43 cm2 g–1, and the pore size distribution is in the range of 20–60 nm, which is smaller than the mean free path of gas molecules (69 nm), triggering a “no convection” effect. Hence, the gaseous heat transfer of chitosan aerogel is effectively suppressed. Chitosan aerogels with the addition of CNC show an excellent thermal insulation property (0.0272 W m-1 K-1 at ambient condition) and an enhanced compressive strength (0.13 MPa at a strain of 3%). This improvement method of chitosan aerogel in enhancing the skeleton structure aspect provides a new kind of idea for strengthening the nanoscale morphology structure of biomass aerogels.

Phenolic-based aerogel composites possess particular characteristics, such as low thermal conductivity, light weight, and superior ablative properties, in near-space vehicles. However, their machinability and flammability require further improvement, hampering their application for high-performance thermal protection systems in the aerospace field. Here, we prepare nanoporous polybenzoxazine (PBO) aerogels with outstanding thermally insulating and self-extinguishing properties by an effective-cost ambient pressure drying (APD) approach rather than a conventional high-pressure supercritical drying pattern. The PBO aerogels are prepared by adopting different exchange solvents of tert-butyl alcohol, ethanol, and n-hexane. The results obtained indicate that PBO aerogels (with n-hexane as the exchange solvent) by APD have an optimal three-dimensional nanoporous network structure, a pore size distribution in the range 20−140 nm, an obvious self-extinguishing property (extinguished itself instantly despite exposure to a 1200 °C flame), superior thermal insulation (the cold surface temperature of the sample reached only 36.4 °C after 400 s despite its opposite surface existing at a 65.8 °C atmosphere), and excellent machinable properties (easy-to-form special-shaped sample). PBO aerogels are a great potential candidate material system for thermal protection systems in aerospace applications in the future.

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