Thermal superinsulation materials play a vital role in thermal management and emergency protection in extreme environments. Ceramic aerogels are particularly attractive due to their low density, low thermal conductivity, excellent chemical and thermal stability, and thus show great potential for application in the field of thermal insulation. Specific geometries with small pore sizes, high porosity, and nanoparticle interconnect frames impart limited heat transfer of ceramic aerogels through solid pathways and gas molecules. However, unlike polymers or carbon, ceramics are inherently brittle and rigid. As a result, most existing ceramic porous materials, in the absence of polymers or carbon matrix as maintenance, usually experience mechanical strength degradation, structural collapse and volumetric shrinkage under external mechanical loads, and these materials may not exist in air above 600 ° C. Therefore, we continue to manufacture ceramic aerogel materials with strong mechanical properties, high temperature resistance (1100 °C) and good thermal insulation properties.
In view of this, Academician Yu Jianyong, Professor Ding Bin and Researcher Si Yang of Donghua University proposed a simple strategy to prepare nanofiber-particle binary synergistic ceramic aerogel with elastic and robust mechanical properties and excellent thermal insulation properties. The method includes crosslinked silica particle aerogel (SGA) and ZrO2-SiO2 nanofiber layers to form a layered multi-arched honeycomb 3D network. The nanoparticle interconnect network of silica particle aerogels explains the low thermal conductivity of composite aerogels, while multi-arched sheets and flexible nanofibers ensure excellent mechanical properties. The resulting composite ceramic aerogel has lightweight properties (23 mg cm–3), superelasticity with recoverable compressive strain up to 80%, excellent fatigue resistance with plastic deformation of 1.2% after 1000 cycles of compression, low thermal conductivity (0.024 W m–1K–1) and good high temperature superinsulation properties. In addition, due to the high temperature resistance and structural thermal stability of the ceramic material, the aerogel maintains elasticity at ultra-low (-196 °C) and ultra-high (1100 °C) temperatures. This simple technique for manufacturing nanofiber-particle composite ceramic aerogels proposed in this study has great potential for large-scale application in practice. The work was published in ACS Nano, a top international journal, under the title "All-Ceramic and Elastic Aerogels with Nanofibrous-Granular Binary Synergistic Structure for Thermal Superinsulation."
Preparation and structural characteristics of ZrO2-SiO2 nanofiber-particle composite aerogels (ZNGAs).
The authors prepared a flexible ZrO2-SiO2 nanofiber membrane using the sol-gel method and electrospinning method, and then manufactured ZNGA by ultrasonic-assisted cryoforming process, as shown in Figure 1. The SEM microstructure cross-section of the nanofiber-particle composite ceramic aerogel shows a layered cellular structure, including lamellar arched cells, nanofiber cell walls with SGA, and bond domains between fibers and SGA. Thanks to good compatibility, ZNGAs can be customized into any shape, such as squares, circles, pentagrams and triangles, without additional or pre-processing, easily meeting the requirements of different appearances in the industry. ZNGAs stand freely on the sparse needle of dogtail grass and provide a lightweight, low-density (23 mg cm–3) composite with a porosity of ~99.58%, which is significantly better than the porosity of composite aerogels. In addition, the all-ceramic nature of ZNGAs and the strong interface interaction of silica sols between nanofibers and SGA allow them to resist strong butane flame jets (~1100 °C) without any visual damage or volume shrinkage, exhibiting favorable high temperature resistance of composite aerogels.
Figure 1 Preparation and structural characterization of ZNGAs
Mechanical properties and hyperelastic mechanism
The synergy of the flexible honeycomb structure and the strong bonding network of the silica sol matrix can effectively improve the mechanical properties of ceramic aerogels. ZNGAs are able to withstand large compressive strains (80%) without breaking and quickly recovering their original structure. At the same time, the corresponding stress (102 kPa) is more than 60 000 times the weight of the specimen (Fig. 2). The specimen underwent 1000 compression cycles and exhibited a plastic deformation of 1.2% at the 1000th cycle, indicating that it has a strong and durable resilience. ZNGA can withstand loads of 8500 times its own weight and last up to 24 hours without any visual collapse, meaning ZNGA has impressive structural durability. To further explore the evolutionary mechanism of long-lasting elasticity of ZNGAs, the authors also observed SEM during compression recovery until the strain reached 80%. The results show that the layered multi-arch honeycomb structure combines the strong interfacial interaction between the brittle SAG and the flexible nanofiber frame, which together promote robust mechanical properties.
Figure 2 Mechanical properties of ZNGAs
Thermal stability, mechanical stability and thermal insulation properties
To further evaluate the thermal resistance and mechanical stability of ZNGAs, several dynamic mechanical analysis measurements were performed over a wide temperature range of -100 to 500 °C (Figure 3). Small changes in modulus and damping ratio indicate constant temperature mechanical stability of ZNGAs. ZNGAs were treated in liquid nitrogen (~-196 °C) for 12 hours and in muffle furnaces (1100 °C) for 1000 cycles after 6 hours of treatment. The ZNGAs treated at extreme temperatures maintained elasticity and maintained reversible deformation levels well, with plastic deformations of 10.4% and 9.2% after treatment at -196 and 1100°C, respectively. In addition, the addition of nanopores introduced to ZNGAs allows the thermal conductivity of ZNGAs with optimized composition to be reduced from 0.028 W m–1K–1 (unopoted ZNGAs) to 0.024 W m–1K–1 (SGA concentration of 0.75 wt%). The excellent thermal insulation properties of ZNGAs are attributed to the low thermal conductivity of the nanopores and the lamellar honeycomb structure (Figure 4).
Figure 3 Thermal resistance and mechanical stability of ZNGAs
Figure 4 Heat transfer characteristics of ZNGAs
Summary: The authors designed and prepared ceramic nanofiber-particle composite aerogels with a layered multi-arch honeycomb structure and a leaf-shaped fiber-particle binary network. The proposed ZNGAs architecture reflects its excellent comprehensive properties, including lightweight properties, hyperelasticity, good mechanical strength and good fatigue resistance. After 1000 compression cycles, 2% plastic deformation and mechanical properties that do not change with temperature are also obtained. Thanks to the thermal properties of SGA and the fire resistance of ceramic components, composite aerogels exhibit a low thermal conductivity of 0.024 W m–1K–1 as well as resistance to high temperatures. The strategy of this research offers hope for the fabrication and engineering of nanofiber-particle composite ceramic aerogels.
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https://pubs.acs.org/doi/10.1021/acsnano.1c09668
Source: Frontiers of Polymer Science