Implementing an Air Suction Effect Induction Strategy to Create Super Thermally Insulating and Superelastic SiC Aerogels.

Silicon carbide (SiC) aerogels are promising thermal insulators that are lightweight and possess high thermal stability. However, their application is hindered by their brittleness. Herein, an air suction effect induction (ASEI) strategy is proposed to fabricate a super thermally insulating SiC aerogel (STISA). The ASEI strategy exploits the air suction effect to subtly regulate the directional flow of the SiO gas, which can induce directional growth and assembly of SiC nanowires to form a directional lamellar structure. The sintering time is significantly reduced by >90%. Significant improvements in the compression and elasticity performance of the STISA are achieved upon the formation of a directional lamellar structure through the ASEI strategy. Moreover, the lamellar structure endows the STISA with an ultralow thermal conductivity of 0.019 W m-1 K-1 . The ASEI strategy paves the way for structural design of advanced ceramic aerogels for super thermal insulation.

Improvement of the Thermal Insulation Performance of Silica Aerogel by Proper Heat Treatment: Microporous Structures Changes and Pyrolysis Mechanism.

A simple heat treatment method was used to optimize the three-dimensional network structure of the hydrophobic aerogel, and during the heat treatment process at 200-1000 °C, the thermal conductivity of the aerogel reached the lowest to 0.02240 W/m·K between 250 °C and 300 °C, which was mainly due to the optimization of microstructure and pyrolysis of surface groups. Further Fluent heat-transfer simulation also confirmed the above results. Synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS) was used to finely measure the pyrolysis process of aerogels, and the pyrolysis process of aerogel was divided into four stages. (I) Until 419 °C, as the temperature continued to rise, surface methyl groups were oxidized to form hydroxyl. (II) As the temperature reached to 232 °C, the oxidation proceeded. In addition, inside the aerogel, because of lacking oxygen, the reaction produced CH4 and C-Si bonds would form. (III) After 283 °C, Si-OH groups began to condense to form Si-O-Si, which optimized the three-dimensional network structures to be beneficial to improve the thermal insulation performance of silica aerogel. (IV) When it reached 547 °C, the chemical reaction was terminated, and all the primary particles gradually fused into secondary particles and sintered to form clusters.

Mechanically Strong and Tailorable Polyimide Aerogels Prepared with Novel Silicone Polymer Crosslinkers.

Polyimide (PI) aerogels were prepared using self-designed silicone polymer cross-linkers with multi-amino from low-cost silane coupling agents to replace conventional small-molecule cross-linkers. The long-chain structure of silicone polymers provides more crosslinking points than small-molecule cross-linkers, thus improving the mechanical properties of polyimide. To investigate the effects of amino content and degree of polymerization on the properties of silicone polymers, the different silicone polymers and their cross-linked PI aerogels were prepared. The obtained PI aerogels exhibit densities as low as 0.106 g/cm3 and specific surface areas as high as 314 m2/g, and the maximum Young's modulus of aerogel is up to 20.9 MPa when using (T-20) as cross-linkers. The cross-linkers were an alternative to expensive small molecule cross-linkers, which can improve the mechanical properties and reduce the cost of PI aerogels.

"Robust-Soft" Anisotropic Nanofibrillated Cellulose Aerogels with Superior Mechanical, Flame-Retardant, and Thermal Insulating Properties.

Advanced thermal insulation materials with low thermal conductivity and robustness derived from regenerative resources are badly needed for building energy conservation. Among them, nanofibrillated cellulose aerogels have huge application potential in the field of thermal insulation materials, but it is still a challenge to prepare cellulose aerogels of excellent comprehensive properties in a simple way. Herein, we demonstrate a unidirectional freeze-drying strategy to develop a novel "robust-soft" anisotropic nanofibrillated cellulose aerogel (NFC-Si-T) by integrating nanofibrillated cellulose (NFC) and Si-O-Si bonding networks under the catalytic dehydration of p-toluenesulfonic acid (TsOH). The anisotropic structure endows the NFC-Si-T with high flexibility that can be easily bent or even tied with a knot, and in addition, it possesses high Young's modulus (1-3.66 MPa) that can resist the compression weight of 10,000 times of its own weight without deformation. Furthermore, the NFC-Si-T aerogels exhibit anisotropic thermal insulation performances with a low average thermal conductivity (0.028-0.049 W m-1 K-1). More importantly, the limited oxygen index of the NFC-Si-T reaches up to 42.6-51%, showing excellent flame-retardant performance. Therefore, the "robust-soft" anisotropic NFC-Si-T aerogels can be used as an advanced thermal insulation material for building thermal insulation applications.