CHEMICAL SCIENCE

Lightweight ultra-flexible, high thermal stability and high temperature ultra-insulated ceramic aerogel


On June 29, 2022, the team of Professor Xu Xiang and Professor Li Hui of Harbin Institute of Technology and the team of Professor Duan Jifeng of the University of California, Los Angeles published the research results entitled “Hypocrystalline ceramic aerogels for thermal insulation at extreme conditions” online in the international top academic journal Nature.

After three years of continuous efforts, through multi-scale superstructure design, the use of semicrystalline (hypocrystalline) ceramic material design combined with zig-zag macroscopic structure design, giving ceramic aerogel near-zero Poisson ratio (3.3×10-4) and near-zero thermal expansion (1.2×10-7/°C) “double zero” unconventional physical properties, so as to obtain lightweight ultra-flexible, high thermal stability and high temperature superheating and other characteristics. At the same time, the research team innovatively proposed a method of “gas turbulence” to assist the direct preparation of three-dimensional nanofiber ceramic aerogel by electrospinning, which expanded the binding of traditional electrospinning preparation of two-dimensional membrane materials, and provided new ideas and methods for the realization of multi-scale superstructure design, high performance, large-scale and low-cost preparation of materials.

The corresponding authors of the paper are Professor Xu Xiang, Professor Li Hui, professor Duan Zhifeng; The first authors are Dr. Guo Jingran, Dr. Fu Shubin, and Dr. Deng Yuanpeng; Harbin Institute of Technology is the first unit and communication unit.

Thermal control under extreme conditions, such as complex mechanical loads and severe temperature changes in environments such as deep space and deep ground, requires excellent thermodynamic properties and thermal insulation properties of thermal insulation materials. Traditional ceramic aerogel superhediastomers have the bottleneck problem of “force-heat mutual exclusion” that has plagued them for nearly a hundred years, such as high-temperature crystallization and powdering while amorphous deformation of ceramics is toughened, low thermal expansion effect is trapped in structural geometry and mechanical properties, thermal synergy is enhanced while sacrificing thermal insulation performance, and low-density reduces phonon heat transfer while not effectively blocking high-temperature thermal radiation, etc., which is difficult to meet the actual extreme environmental thermal control needs.

The ceramic aerogel material is composed of semicrystalline ceramic nanofiber mesh pore structure, combined with zig-zag macroscopic assembly design, imparting the material near zero Poisson ratio (3.3×10-4) and near zero thermal expansion (1.2×10-7/°C) anti-conventional physical properties, so that the elasticity of the material can be restored compression strain up to 95%, both excellent tensile (fracture strain > 40%) and bending (bending strain > 90%); 10,000 high-frequency violent thermal shocks (about 200 °C/s) and long-term high temperature (>1000 °C) aerobic exposure to strength loss and volume shrinkage is almost zero; In addition, semicrystalline ceramics show stronger coating ability to carbon, improve the high-temperature oxidation resistance of carbon materials, thereby effectively blocking high-temperature thermal radiation, and achieving the current minimum high-temperature thermal conductivity of “low-density” ceramic aerogels (less than 100 mW/m K at 1000 °C), making up for the shortcomings of lightweight aerogel materials in the field of high-temperature insulation. The material also has capacitive self-sensing characteristics, which can monitor the structural damage of the thermal insulation material in real time, further enhancing the safety and reliability of the thermal control system.

Figure 1: Multiscale hyperstructure design of semicrystalline nanofiber ceramic aerogel

The study first designed semicrystalline ZAGs with zig-zag macroscopic structures at multiple scales. Prevent the sliding of nanocrystal domains by embedding nanocrystals into amorphous matrix, with amorphous matrix as the grain boundary; At the same time, nanocrystalline domains are used to limit the migration of amorphous matrix at high temperatures, and excellent thermomechanical properties are achieved by mutual nailing of crystals and amorphous. Under this semicrystalline material design, the deformation of ceramic fibers under mechanical and thermal stress excitation will be carried out in a higher-order buckling mode, providing additional deformation freedom to excite the high-order deformation mode, thereby reducing the material’s Poissonite ratio and coefficient of thermal expansion, bringing it closer to zero. In order to extend this “double zero” structural unit to the whole, the zig-zag macrostructure fiber aerogel is assembled with triangles, squares and pentagons as units, so that the double zero characteristics can be expressed at the macroscopic scale.

Figure 2: Gas turbulence assisted electrospinning preparation process and material element and structure characterization

In order to experimentally prepare this double-zero superconjunctive aerogel material, the research team introduced a coaxial pneumatic device in the conventional electrospinning device, and the high-speed air blown out of the external shaft aperture first formed a jet, which was deformed into turbulence after the electrospinning Taylor cone, thus forming a complex 3D turbulent flow field. This turbulent field efficiently causes the nanofibers to move in complex trajectories and wind around each other, eventually forming a randomly wound fiber gas condensation structure. Subsequently, a semicrystalline nanofiber ceramic aerogel material was prepared by further using an automatic mechanical folding process to form a zig-zag macroscopic structure in the initially generated fiber aerogel, combined with a simple air 1,100 °C thermal annealing treatment. This turbulent-assisted electrospinning preparation method expands the constraint that traditional electrospinning can only prepare 2D membrane materials, which can achieve high-performance, large-scale and low-cost commercial production of materials.

Figure 3: Study of the mechanical properties of materials

To study the mechanical properties of ZAGs, the study performed uniaxial quasi-static compression, tensile, bending and torsion tests on ZAGs. In compression testing, samples can be compressed from 10 mm to 0.5 mm with a strain of 95% (the maximum known to date) and fully return to their original state after pressure release; Under 50% strain, the repetitive cycle compresses 1,000 times, with almost no stress degradation (less than 7%), with excellent fatigue resistance; ZAGs also exhibit excellent flexibility, with tensile fracture strains up to 40%, bending strains up to 90%, and torsional angles up to 360°.

Figure 4: Study of the thermal properties of materials

For the study of the thermal properties of materials, the study focused on the thermal expansion effect of materials, taking into account the inherent high oxidation resistance and thermal stability of semicrystalline zircon. The results show that the coefficient of thermal expansion of ZAGs is 1.2×10-7/°C below 200 °C, and the temperature rise to 400°C is still only 1.6×10-7/°C. The coefficient of thermal expansion of the material near zero can greatly reduce the thermal strain mismatch between the fibers and prevent the dissociation of the crosslinked fibers. At the same time, the strength loss and volume shrinkage of the sample under 10,000 high-frequency violent thermal shocks (about 200 °C/s) and long-term high temperature (>1000 °C) aerobic exposure were almost zero, showing excellent structural stability. In addition, semicrystalline ceramics show stronger coating ability to carbon, improve the high-temperature oxidation resistance of carbon materials, thereby effectively blocking high-temperature thermal radiation, and achieving the current minimum high-temperature thermal conductivity of “low-density” ceramic aerogels (less than 100 mW/m K at 1000 °C), making up for the shortcomings of lightweight aerogel materials in the field of high-temperature insulation. Through the aero-engine fuel pipe thermal control test, the excellent fireproof and thermal insulation performance of the material is verified, and the results are of great scientific significance and practical value for meeting the heat insulation, weight reduction and capacity increase, energy saving and consumption reduction, system safety and performance stability in China’s aerospace and other fields under extreme and complex service conditions. (Source: Science Network)

Related paper information:https://doi.org/10.1038/s41586-022-04784-0



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