Search

Polyimide (PI) aerogels possess a rather potential in thermal protection material for aircraft, owing to their low thermal conductivity, high thermal stability, and low bulk density. However, the large shrinkage of pure PI aerogels is still a challenge in the complete preparation process. Herein, a kind of PI hybrid aerogel composite was prepared successfully by introducing polyurethane fiber as the reinforcing agent. The aerogel composite could realize the enhancement for the pure PI aerogels from a micro/macroscopic perspective, ultimately ensuring the ultralow shrinkage rate of 3.52%. Due to the inhibition of the shrinkage characteristic, the aerogel composite obtained achieved excellent thermal insulation, especially retaining a low temperature of 40.7°C despite the hot surface temperature reached up to 150°C. Besides, the lightweight low‐density, exceptional flame resistance, and self‐extinguishing properties were reflected from PI hybrid aerogel composite constructed, including the outstanding hydrophobic feature (water contact angle of 120°). The PI hybrid aerogel composite as prepared may be used for the thermal management of aircraft in a great measure, illustrating the feasible strategy of fabricating PI‐based aerogel composites. [ABSTRACT FROM AUTHOR]

The inherent characteristics of cellulose-derived aerogels, such as their natural abundance and environmental friendliness, make them highly interesting. However, its significant shrinkage before and after the supercritical drying procedure and low mechanical strength limit its potential application. Here, we propose a strategy to prepare cellulose diacetate aerogels (CDAAs) with low drying shrinkage, exceptional thermal insulation, and superior mechanical strength. The low drying shrinkage (radial drying shrinkage of 1.4%) of CDAAs is attributed to their relative strong networking skeletons, which are greatly formed by tert-butanol solvent exchange in exerting the interaction of reducing the surface tension force. In this case, CDAAs are eventually endowed with the low bulk density of 0.069 g cm−3 as well. Additionally, as-prepared CDAAs possess an abundant three-dimensional networking structure whose pore size is concentrated in the diameter range of ~50 nm, and the result above is beneficial for improving the thermal insulation performance (thermal conductivity of 0.021 W m−1 K−1 at ambient environmental and pressure conditions). On the other hand, the optimal compressive stresses of CDAAs at 3% and 5% strain are 0.22 and 0.27 MPa respectively, indicating a mechanically well robustness. The above evidence demonstrates indeed the exceptional thermal insulation and superior compressive properties of CDAAs. This work may provide a new solution for developing a kind of high-performance cellulose-derived aerogel in the future. [ABSTRACT FROM AUTHOR]

Reusable nanoporous polyimide (PI) aerogels are rather promising for highly efficient thermal insulation of future thermal protection systems demanded in aerospace and domestic applications. However, the development of high-performance PI aerogels is limited by their unsatisfactory high-temperature resistance and poor reusability. Herein, an Al2O3-hybridization strategy toward PI aerogels is demonstrated, and the formed aerogels exhibit a nanoporous structure, unusual reusability, and exceptional high-temperature resistance. Furthermore, the aerogels retain their high-level properties such as the very small changing rates of mass (≤0.3%), shrinkage (≤2.53%), thermal conductivity (≤3%), and bulk density (near-zero difference) even after five cycles of high-temperature assessment (250 °C for 2400 s). The conventional huge heat-induced shrinkage of PI aerogels could be substantially restrained even bearing a high-temperature environment, realizing near-zero thermally insulating degradation, which has an outstanding practical application value. The high-temperature resistance of the aerogel is greatly improved by the Al–O network constructed inside the PI aerogels, and this resistance is retained even after multiple cycles under harsh conditions, ultimately affording superior reusability to PI aerogels for the first time. The high-temperature resistance and ultralow heat-induced shrinkage properties of aerogels, as well as their reusability, make them favorable candidates as thermal protection materials for the future development of aircraft and military. [ABSTRACT FROM AUTHOR]

Polyimide aerogels (PIAs) are thermally insulating materials that possess various advantages, such as exceptionally high temperature resistance and low thermal conductivity, which make them great potential candidate materials to be applied in the area of thermal protection. However, under harsh mechanical stresses, the macro- and microscopic structural stability of PIAs may be compromised due to shrinkage/collapse, leading to performance degradation. Herein, we propose a homogeneous organic/inorganic hybrid formation strategy for constructing nanoporous polyimide/silica aerogels (PIAs-A) with exceptional mechanical strength and ultralow thermal conductivity. The results obtained indicate that PIAs-A have an optimal three-dimensional nanoporous network structure with a pore size distribution mainly within the range of 10–20 nm. Monitoring the temperature evolution of the material's cold surface showed that it remained at a low temperature of 37.5 °C even when the material was placed against a hot surface of 150 °C. The residual mechanical strengths of the aerogels remained at a superior level (with strength degradation being less than 9%) after exposure to ultralow and high-temperature atmospheres. Furthermore, compressive stress values at 3% strain exceeded previously reported values for PIAs by 625 and 733% at −50 and 100 °C, respectively. Meanwhile, our aerogels displayed flame resistance even when exposed to a heat source of approximately 1200 °C, possessed favorable hydrophobic properties, and maintained dimensional stability up to 504 °C. The robust mechanical and thermal insulation properties of PIAs-A make them a promising substitute material for thermal superinsulation in aircraft. [ABSTRACT FROM AUTHOR]

Materials with high‐performance thermal insulation and excellent flame‐resistance are incredibly desirable for energy conservation and fire safety. In this study, a novel hybrid nanostructure network polybenzoxazine/silica (PBO/SiO2) aerogels were fabricated through facile in situ co‐polymerization sol‐gel methods with ambient pressure drying using benzoxazine (BO) monomers and SiO2 sol as reaction source, N, N‐dimethylformamide (DMF) as the solvent and hydrochloric acid (HCl) as the catalyst. The hybrid nanostructure network was retained by polymerization–induced nanoscale phase separation of PBO and SiO2. The resulting PBO/SiO2 aerogels exhibited finer microstructure, lower density (0.18 g/cm3), thermal conductivity (0.031 W/m·K), and better flame‐resistance in comparison with PBO aerogels. They demonstrated an excellent compressive strength of 0.81 to 1.12 MPa at 10% deformation. The remarkable improvement in thermal insulation and flame‐resistance of PBO/SiO2 aerogels could be attributed to the combined effects of finer microstructure and formation of SiO2 that was "in‐situ" interpenetrated and interacted with silanols (Si‐OH) in the PBO network during the combustion process. The successful synthesis of PBO/SiO2 aerogels highlights the possibility of fabricating a novel high‐performance thermal insulation and excellent flame‐resistance used for energy‐efficient buildings. [ABSTRACT FROM AUTHOR]

The quest for ambient pressure dried, cost‐effective, and environmentally friendly synthesis technology has been continuously pursued throughout the aerogel research process. Herein, oxalic acid catalyst is used to synthesize a series of polybenzoxazine (PBZ) aerogels through room temperature ring‐opening polymerization and ambient pressure drying technique based on phenol‐4,4′‐diaminodiphenylmethane benzoxazine (PH–mda) monomer. The resultant PBZ aerogels with a microstructure of 3D nanoporous network exhibit a low density of 0.13–0.36 g cm−3 and relatively low thermal conductivities of 0.0323 (105 Pa) and 0.0119 W m−1 K−1 (10 Pa), respectively. Remarkably, PBZ aerogels possess excellent compressibility (the maximum strain can be more than 80%), high compressive modulus, and specific modulus because of their rough skeleton structure. Taking advantage of the simple synthetic route, easily scaled‐up, lightweight, low thermal conductivity, and excellent mechanical property, such PBZ aerogels may have great potential to serve as a new material for energy‐efficient buildings and the other thermal insulating field. [ABSTRACT FROM AUTHOR]