1.Influence of Felt Type on the Microstructure Morphology of Flexible SiO₂ Aerogel Composite Felts
The mechanical and chemical properties of different fiber felts significantly impact the preparation of flexible aerogel-fiber composite felts, determining the composite material’s performance.
Extensive literature review indicates that glass fiber felt, ceramic fiber felt, and basalt fiber felt can all serve as substrates for aerogel composites.
Aerogel-Glass Fiber Composite Felt:
The aerogel bonds well with the glass fibers, and the inter-fiber voids are largely filled with aerogel, which contributes to its further reduced thermal conductivity.
Additionally, the glass fibers provide substantial mechanical support to the aerogel, enhancing the mechanical properties of the aerogel-fiber composite felt.
Aerogel-Ceramic Fiber Composite Felt:
Ceramic fibers have a smaller diameter, which leads to higher volume shrinkage. While the aerogel bonds well with the ceramic fibers, there is less aerogel filling between the fibers.
Aerogel-Basalt Fiber Composite Felt:
Basalt fibers have the largest diameter, but the degree of aerogel composite formation on the fibers is relatively poor. However, the aerogel can uniformly adhere to the fibers.
From the above, it is evident that in basalt fiber composite felt, the integration between the fibers and the aerogel is less optimal.
In ceramic fiber felt, although the aerogel can adhere extensively to the fibers due to their fine diameter, there is insufficient filling between the fibers.
In contrast, glass fiber composite felt shows excellent adhesion between the fibers and the aerogel, with substantial aerogel filling the inter-fiber spaces, resulting in a uniform overall composite.
This provides better space for the growth of the aerogel’s skeletal structure. The tight integration of the two components endows the aerogel-fiber composite felt with good mechanical properties, effectively leveraging the advantages of both aerogel and fiber combination.
The thickness of the fibers themselves influences aerogel adhesion and further affects the cross-linking of the composite skeleton.
2. Influence of Felt Type on the Thermal Conductivity of Flexible SiO₂ Aerogel Composite Felts
After aerogel compositing, direct solid heat transfer between fibers is interrupted by the aerogel, giving the aerogel-fiber composite felts superior heat transfer characteristics. Using felt as the substrate effectively reduces the probability of aerogel shrinkage and structural collapse during drying, leading to a more uniform distribution of the aerogel’s own microstructure, which further reduces the gaseous phase conduction in the composite felt.
The aerogel-glass fiber composite felt exhibits the lowest thermal conductivity, 0.0294 W/(m·K) at room temperature.
The composite felts prepared with ceramic fiber felt and basalt fiber felt also show low thermal conductivity, measuring 0.0311 W/(m·K) and 0.0303 W/(m·K) respectively at 25°C.
All three types of aerogel-fiber composite felts demonstrate excellent thermal insulation performance across different temperatures. Among them, the insulation performance ranks as follows: aerogel-glass fiber composite felt > aerogel-basalt fiber composite felt > aerogel-ceramic fiber composite felt.
3. Influence of Felt Type on the Hydrophobicity of Flexible SiO₂ Aerogel Composite Felts
All three fiber felts exhibit a certain degree of hydrophobicity after being composited with aerogel using MTES as the silicon source. The hydrophobicity of aerogel significantly affects its service life and insulation performance.
The thermal conductivity of water is 24 times that of air. Water adsorption increases the thermal conductivity of traditional insulation materials, damages their insulation structure, and is a major issue for traditional cold insulation materials.
Testing results show the static water contact angles for the glass fiber, ceramic fiber, and basalt aerogel felts are 149.9°, 147.7°, and 136.5°, respectively.
These angles indicate that all three composite felts possess strong hydrophobicity, which is a crucial property for ensuring service life and insulation performance.
Among them, the aerogel-glass fiber composite felt has the best hydrophobicity.
4. Influence of Felt Type on the Mechanical Properties of Flexible SiO₂ Aerogel Composite Felts
The test results for the mechanical properties of the fiber felts and the aerogel-fiber composite felts. From Table 2, it can be seen that the glass fiber felt has the highest tensile strength, followed by basalt fiber felt, which is superior to ceramic fiber felt.
Regarding compressive strength at 20% and 30% compression, the basalt fiber felt shows the highest values, with glass fiber felt outperforming ceramic fiber felt.
The aerogel, filling the voids between fibers in the composite, increases the resistance to external forces under both tension and compression, leading to improved mechanical strength.
The tensile strengths of the glass fiber, ceramic fiber, and basalt fiber composite felts are 1.27 MPa, 0.58 MPa, and 0.81 MPa, representing increases of 30.93%, 38.1%, and 28.57%, respectively.
5. Influence of Felt Type on the Cold Insulation Performance of Flexible SiO₂ Aerogel Composite Felts
In low-temperature environments, traditional insulation materials can suffer from deformation and shrinkage.
Therefore, the low-temperature resistance of the three composite felts was investigated. A small piece was cut from each composite felt sample and immersed in liquid nitrogen for 24 hours to observe any apparent changes before and after immersion.
The aerogel-fiber composite felts after 24-hour immersion are shown in the figure below.
From the figure above, it can be observed that after 24 hours of liquid nitrogen treatment, the three aerogel-fiber composite felts show no significant changes in appearance.
Initial frosting occurred, but after returning to room temperature, the overall materials exhibited no shrinkage or deformation. Subsequent testing confirmed they still maintained good mechanical and thermal insulation properties.
All three materials demonstrate excellent low-temperature resistance, proving the application potential of aerogel-fiber composite felts in the field of low-temperature cold insulation.
6. Conclusion
Using the same process, flexible SiO₂ aerogel composite felts were prepared with glass fiber felt, ceramic fiber felt, and basalt fiber felt as substrates. The study found:
1) The comprehensive performance of the aerogel-glass fiber felt is superior to that of the ceramic and basalt fiber felts.
After compositing, the glass fiber felt had a volume shrinkage rate of 9.4%, a density of 0.158 g/cm³, thermal conductivities of 0.0294 W/(m·K), 0.0264 W/(m·K), and 0.0241 W/(m·K) at 25°C, 0°C, and -10°C respectively, a water contact angle of 149.9°, a tensile strength of 1.27 MPa, and compressive strengths of 65 N and 97 N at 20% and 30% compression.
2) Aerogel-ceramic fiber felt is easier to process but has lower mechanical strength and a shorter service life, making it suitable for use as vacuum insulation panel core material.
3) Aerogel-basalt fiber felt has high mechanical strength and excellent compressive performance, with compressive strengths of 91 N and 267 N at 20% and 30% compression.