Graphene/Amorphous Boron Nitride Airgel
Thermal superinsulating materials, which have low thermal conductivity, are required for protection and thermal insulation in adverse conditions. These materials are urgently needed in fields such as aerospace, mechanics, deep space exploration, and thermal power engineering, where exceptional reliability and stability are required.
(a) Illustration of the a-BNGA fabrication process. (b) SEM images of the a-BNGA framework. (c,d) Cross-sectional TEM images of a-BNGA cell wall with multi-layer nanostructures. (e) Optical photo of the a-BNGA with the spacesuit and the basic shape of the moon. Image credit: Science China Press
Inorganic aerogels have shown several excellent features such as high deformability, very light weight, low thermal conductivity, and excellent fire/corrosion resistance, making them potential as thermal insulators. Nevertheless, inorganic aerogels are still affected by the compromise between their thermal and mechanical characteristics, which presents a major roadblock for further examining their functionality.
While the improvement of thermal or mechanical characteristics has been well examined in inorganic aerogels, there is still a lack of effective synergistic approaches to solve these characteristic exchanges.
In a new study article published in National Science Review, scientists at Harbin Institute of Technology and Southeast University presented the chemically bonded multi-nanolayer design and synthesis of graphene/amorphous boron nitride aerogel (a-BNGA) to improve the thermal and mechanical properties simultaneously. Unlike in previous work, the graphene framework is uniformly deposited by a-BN nanolayers on both sides, thus creating a chemically bonded multi-nanolayer structure.
It was determined that the chemically bonded interface firmly anchored the uniform a-BN jacket to the graphene framework, serving through a tendon-like mechanism, guaranteeing synergistic load transfer and deformation within the framework. The a-BN nano coating can increase the elastic firmness of the cell wall. This provides the necessary distribution of bending moments, which realizes a toughness effect that combines to increase structural strength.
As a result a-BNGA exhibits very low density with very high flexibility (up to 90% elastic bending strain, up to 99% elastic compressive strain) and outstanding thermal stability (almost no sharp thermal shock strength degradation). Scientists establish flexible deformability by the process of opening and folding airgel flowers in human hands.
Remarkably, the a-BN nanolayers in the aerogel, exceeding 20% by volume, are mechanically critical but thermally inactive—perfect conditions for a thermal insulation material. Solid conduction and radiation contribution together contribute to the material’s apparent thermal conductivity in a vacuum. Taking advantage of the disadvantages of low-density effective conduction pathways and extra phonon scattering by the interface, dense conduction can be successfully ordered.
In addition, graphene can be used as an infrared absorber to minimize radiant heat transfer. Scientists experimentally demonstrated these aerogels with the lowest thermal conductivity in a vacuum of any free-standing solid material so far. In addition, they developed a model of a lunar base operating in high vacuum to establish the thermal superinsulation capability of airgels in space exploration applications.
We achieved an extraordinary combination of mechanical and thermal properties from inorganic aerogels and defined a robust material system for thermal superinsulation under extreme conditions, such as lunar and Martian bases, satellites and spacecraft. Such materials and structural designs may also provide opportunities for inorganic aerogels to provide other unique functions.
Prof. Xiang Xu, Harbin Institute of Technology and Southeast University
