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In daily life, maintaining body temperature is essential for human survival, especially for people exposed to cold and windy environments for a long time, such as soldiers and workers in high altitude areas, wilderness and construction sites. Various materials, such as fibers, metals, aerogels, foams, etc., have been used for thermal insulation, all of which can partially prevent heat loss from the human body. Aerogels, in particular, have a thermal conductivity as low as 15 mW m-1 K-1, even lower than static air (24 mW m-1 K-1), showing superior thermal insulation capabilities. This super-insulating performance can be attributed to the high porosity (>90%), interconnected porous nanostructures, and pore sizes below the mean free path of gas molecules (~66 nm for ambient air). These inherent properties enable aerogels to effectively prevent heat transfer. However, zero-dimensional aerogel powders have inherent disadvantages of brittleness and hygroscopicity, which limit their wearable applications. In contrast, fibrous materials composed of natural and synthetic fibers are widely used for warmth preservation due to their ideal wearability, availability and affordability. However, commercial fiber materials have large pore sizes (typically bbb100 μm) and limited porosity (typically <50%), which prevents them from inhibiting air heat conduction by restricting the movement of gas molecules. These inherent bottlenecks severely hinder their thermal insulation effectiveness (thermal conductivity > 40 mW m-1 K-1), limiting their ability to maintain human body temperature in extremely cold and windy environments. Reducing fiber diameter is believed to be beneficial for achieving high porosity and small pore size, thereby restricting more heat transfer by reducing convection diffusion.
The main point of this paper
Technical advantages:
Electrospinning technology can produce nanofibers with small diameter, high porosity and adjustable pore structure, which show potential in the manufacture of high-performance thermal insulation materials
Thermal insulation performance challenges:
Currently, electrospun fibers have problems such as insufficient pore size, insufficient porosity and completely open cell structure, which limit the dissipation of thermal energy, especially under windy conditions
Modified electrospun membranes:
Some modified electrospun membranes for thermal insulation have been prepared by combining coating or doping methods. Although convection suppression is enhanced, the thermal resistance capacity is limited because of the short heat transfer path, uncontrollable structure and limited porosity
Nanopores and closed cell structures Structure:
The three-dimensional micro/nanofibre sponge developed by the research team effectively extends the heat transfer path by freeze drying or direct electrospinning combined with high porosity and volume structure, showing enhanced thermal insulation performance
Wind resistance and gas insulation:
The fiber sponge structure is monotonous and fully open, resulting in poor wind resistance and low gas insulation efficiency, and cannot effectively inhibit the heat transfer of gas molecules
Cross-scale pore formation strategy:
A cross-scale pore formation strategy is demonstrated to create a hierarchical cell-structured aerogel micro/nanofibre membrane (CAMM) through a strategy combining heterogeneous electrospinning and moisture-induced solution casting to achieve comfortable wind resistance and warmth
Technological breakthrough:
Through a strategy combining electrospinning and solution casting, an aerogel micro/nanofiber membrane (CAMM) with a hierarchical cell structure was developed to achieve comfortable windproof and warm insulation.
Material properties:
The material has interconnected nanopores (30-60 nm), ultrafine fiber diameter and high porosity, making the aerogel micro/nanofiber membrane lightweight, ultrathin (about 0.5 mm) and excellent thermal insulation performance.
Thermal insulation performance:
The material exhibits ultra-low thermal conductivity (14.01 mW m-1 K-1), and the customized semi-closed wall membrane has significant wind resistance and satisfactory thermal and hygroscopic comfort, which is 3.4°C warmer than the most advanced thermal underwear.
Material innovation:
The synergy of nanopores and well-controlled closed-cell structure provides excellent thermal insulation performance while maintaining the material's moisture permeability and hydrophobicity.
Challenges overcome:
The trade-off between thickness and thermal resistance of existing cold-proof materials is solved, and personal cold-proof performance is improved while maintaining thermal and hygroscopic comfort.
In summary, we developed CAMMs by tailoring hierarchical porous structures through the Knudsen effect and convection suppression principles. The fabricated membranes exhibit high porosity (91.2%), lightweight performance (92.3 mg cm-3), good wear resistance, hydrophobicity, and excellent thermal resistance (ultra-low thermal conductivity 14.01 mW m-1 K-1). Moreover, the tailored closed-cell structure enables effective suppression of thermal convection and multifunctional multi-interface reflectivity, contributing to windproof heating performance (25 times the CLO value of conventional textiles under high wind speed conditions) without blocking water vapor transport. We envision that such nanoporous micro/nanofiber membranes with tailored cellular architectures will provide a promising strategy for high-performance thermal insulation materials such as military operations, firefighting, outdoor equipment, space exploration, and industrial insulation.
