Analysis of the Thermal Insulation Performance of Aramid Fiber-Reinforced Aerogel Composites and Their Application Fields

1. Introduction
   Aerogels are low-density, predominantly mesoporous solids with exceptional properties, including low density, high specific surface area, low dielectric constant, and ultra-low thermal conductivity. Examples include graphene or carbon nanotube aerogels, polyurethane and polyimide aerogels, biopolymers such as cellulose, chitosan, and protein aerogels, as well as their composites and hybrids. Especially in the past decade, the number of scientific papers and patents describing new aerogel materials, production processes, and applications has seen a literal explosion, covering fields such as thermal insulation, transportation, environmental remediation, catalysis, and acoustics.

Despite the growing importance of the aerogel field, or perhaps because of it, the definition of "aerogel" remains争议. Early definitions often focused on the drying technique used in production: such as aerogels for supercritical drying, cryogels for freeze-drying, and xerogels for evaporative drying. However, newer definitions are more based on material properties, especially a high proportion of mesoporosity. Ultimately, the broadest definition of an aerogel is any material derived from a gel by replacing the pore fluid with air, without restrictions on pore size or other characteristics. This broader definition primarily encompasses macroporous materials that do not possess the mesoporosity, high surface area, or ultra-low thermal conductivity typically associated with aerogels, such as freeze-dried cellulose foams.

Silica aerogels, produced through the sol-gel process, have seen multiple variations proposed to enhance resource and cost efficiency. However, most processes still follow the same basic steps. The gelation of silica sols is usually triggered by the addition of an acid or base to reduce the charge stability of nanoparticles. After gelation, the mechanical stability of the gel is enhanced through dissolution-precipitation reactions of silica, strengthening the interparticle forces. The industrial success of silica aerogels is almost entirely due to their performance in thermal insulation applications. Their thermal conductivity can be as low as 0.012 W/(m·K), primarily attributed to the high porosity and tortuosity of the particle network, which limits solid thermal conduction. Simultaneously, due to the Knudsen effect, small pore sizes below the mean free path length of gas molecules reduce gas-phase thermal conduction. This ultra-low thermal conductivity (half that of still air and conventional insulating materials) has spawned a rapidly growing multi-billion-dollar market. Total thermal conductivity is closely related to material density, as shown in Figure 1. For conventional insulating materials, radiative contributions are significant, especially with large pore sizes where air convection is also not negligible. As density increases, radiative thermal conduction decreases, while solid thermal conduction increases. Due to these competing effects, thermal conductivity exhibits a U-shaped dependence on density. For aerogel materials, these effects are also present, but because the pore sizes within the aerogel are smaller than the mean free path of air, gas-phase conduction is drastically reduced, decreasing the frequency of air molecule collisions and thus reducing gas heat transfer, shifting the minimum total thermal conductivity to higher densities and (multi) low-conductivity regions.

Figure 1. A) Thermal conductivity of conventional insulating materials B) Thermal conductivity of aerogel materials

Silica aerogel nanoparticles form a multi-grid structure through interconnection, but the weak connections between particles result in poor mechanical properties, low strength, and high brittleness of pure silica aerogels. To address these issues, researchers have explored various reinforcement strategies. Aramid fibers, with their low density, low thermal conductivity, and high mechanical strength, are an ideal choice for reinforcing silica aerogels. Aramid fibers have a decomposition temperature in air of around 450°C, making them particularly suitable for high-temperature thermal insulation applications. In 2016, aramid fiber-reinforced silica aerogel composites (AF/aerogel) were successfully prepared, followed by the introduction of composites with glycidoxypropyltrimethoxysilane (GPTMS) grafted aramid fibers and polytetrafluoroethylene-coated aramid fiber aerogels. These composites significantly improve compressive and flexural strengths while maintaining low density and low thermal conductivity.

Further research has shown that the thermal and mechanical properties of aramid fibers make them excel in ballistic applications. Compared to aramid fabrics, aerogel-integrated ballistic test samples showed a 72% reduction in fabric perforation. In 2021, Almeida et al. compared the reinforcing effects of silica aerogels with aramid fibers and felts, finding that composites using slender fibers had lower bulk densities and better flexibility, suitable for shape adaptation and vibration applications.

Figure 2. Preparation flowchart of aramid-reinforced aerogel composites

The combination of aramid and aerogel achieves complementary and enhanced material properties. As the reinforcing phase, aramid fibers provide strong mechanical support to the aerogel, improving its mechanical properties, while the aerogel leverages its thermal insulation and sound absorption characteristics, complementing the aramid fibers. For example, aramid/aerogel composites prepared through the wet papermaking process not only maintain the performance of aramid paper but also exhibit better heat resistance. These composites have promising application prospects in the field of thermal insulation, providing new ideas and possibilities for the development of materials science.

2. Thermal Insulation Performance: A High-Temperature "Shield"
   The thermal insulation performance of aramid aerogel composites is exceptional, primarily thanks to their unique microstructure. Aerogels have a large number of nanoscale pores that are interconnected, forming a three-dimensional porous network structure. When heat is transferred, gas molecules conduct heat within the pores, and the high porosity of the aerogel increases the mean free path of gas molecules, greatly reducing the efficiency of gas thermal conduction. Simultaneously, the solid skeleton of the aerogel is composed of nanoscale particles or fibers with a large surface area, greatly impeding heat conduction within the solid skeleton. Furthermore, the low emissivity of the aerogel effectively blocks the transfer of thermal radiation.

Aramid aerogel composites exhibit excellent thermal insulation performance at different temperatures. Compared to traditional insulating materials such as rock wool and glass wool, they have a lower thermal conductivity. The thermal conductivity of rock wool is typically between 0.04 and 0.06 W/(m·K), while that of glass wool is about 0.03 to 0.05 W/(m·K). In contrast, the thermal conductivity of aramid aerogel composites can be as low as 0.01 to 0.03 W/(m·K), meaning they more effectively prevent heat transfer. In high-temperature environments, the performance of traditional insulating materials may be affected, such as rock wool shrinking and brittleness at high temperatures, leading to reduced thermal insulation performance. However, aramid aerogel composites have good thermal stability and can maintain stable thermal insulation performance in high-temperature environments. In industrial high-temperature equipment, such as boilers and furnaces, aramid aerogel composites can be used to make thermal insulation linings and insulation jackets, effectively reducing heat loss and improving energy utilization efficiency. In the field of building exterior wall insulation, using aramid aerogel composites as insulation materials can significantly reduce building energy consumption and improve indoor comfort. In extremely cold regions, this material can block cold air from entering the interior, keeping it warm; in hot summers, it can prevent outdoor heat from entering, reducing the energy consumption of air conditioning and other cooling equipment.

3. Application Fields
   3.1 Aerospace
   In the aerospace field, aramid aerogel composites shine. In terms of aircraft structural components, they have become ideal materials for manufacturing airplane wings, fuselages, tail sections, and other parts due to their excellent mechanical properties and low density. Taking Boeing aircraft as an example, some models use aramid aerogel composites in their structural components, effectively reducing the aircraft's weight and improving flight performance. Compared to traditional metallic materials, the density of aramid aerogel composites is significantly lower, which reduces fuel consumption during flight and significantly lowers operating costs. Simultaneously, their high strength and high modulus ensure structural stability during high-speed flight and in complex meteorological conditions, providing solid safety for flight.

In terms of thermal insulation components, the thermal insulation performance of aramid aerogel composites makes them a "thermal shield" in the aerospace field. When spacecraft re-enter the atmosphere, they experience extremely high temperatures due to intense friction with the air. At this time, thermal insulation components made of aramid aerogel composites can effectively block heat transfer, protecting the equipment and personnel inside the spacecraft. On the International Space Station, aramid aerogel composites are used to make thermal insulation materials, ensuring the station can operate normally in extreme temperature environments. Their application can also reduce heat loss in aerospace engines, improve engine thermal efficiency, and extend engine life. It can be said that the emergence of aramid aerogel composites has provided strong technical support for the development of the aerospace industry, helping humanity reach higher and farther skies.

Figure 3. The cockpit of the "Jaguar" fighter jet uses aerogel insulation material

3.2 Construction Industry
In the construction industry, the application of aramid aerogel composites provides strong support for creating green, comfortable, and safe homes. In terms of building insulation materials, their excellent thermal insulation performance can effectively prevent heat transfer, reducing building energy consumption. Whether in cold northern regions or hot southern regions, using aramid aerogel composites as insulation materials can significantly improve building energy efficiency. In some newly built green buildings, using aramid aerogel composite insulation boards can reduce building energy consumption by 20% to 30% compared to traditional insulation materials, greatly reducing dependence on energy and achieving energy conservation and emission reduction goals.

In terms of fire-resistant materials, aramid aer