MA Xiaoliang ^{1, 2}, WANG Guigen ¹, ZHANG Mengyu ³, ZHAO Baojun ^{2, 3},

ZHONG Yeshen ³, SHI Liping ³, HE Xiaodong ³

(1. School of Materials Science and Engineering, Harbin Institute of Technology (Shenzhen), Shenzhen 518055,

Guangdong, China; 2. China State Construction Hailong Technology Co., Ltd., Shenzhen 518110, Guangdong, China;

3. National Key Laboratory of Science and Technology on Advanced Composite in Special Environments,

Harbin Institute of Technology, Harbin 150001, Heilongjiang, China)

Extended abstract:[Background and purposes] Silica (SiO₂) aerogels are renowned for their exceptional thermal insulation, low density, high porosity and large specific surface area, for promising applications in aerospace, construction and energy-saving. However, their inherent brittleness, hygroscopic nature and poor interfacial adhesion with traditional ceramic fiber reinforcements severely limit practical utilization. Previous studies attempted to enhance mechanical properties by incorporating ceramic fibers, but the absence of chemical bonding between fibers and aerogels led to interfacial debonding. Mullite whiskers (MW) have shown potential as secondary structures to anchor aerogels onto fibers, improving both mechanical and thermal performance. However, conventional methods relying on toxic fluoride-based catalysts (e.g., AlF₃) pose environmental and structural risks. This study was aimed to develop a safer high-performance aluminum silicate fiber/mullite whisker- reinforced silica aerogel composite (ASF/MW-SiO₂) using boron-based catalysts. The objectives include optimizing in-situ MW growth conditions, enhancing interfacial adhesion and achieving balanced mechanical-thermal properties while avoiding harmful fluorides.[Methods] ASF/MW reinforcements were synthesized through in-situ growth of MW on aluminum silicate fibers (ASF) using borax as the catalyst. Optimal parameters (900–950 ℃ heat treatment, 2–4 h dwell time, 0.17–0.18 mol·L⁻¹ catalyst concentration) were selected based on previous results. The SiO₂ wet gel was prepared via sol-gel method by mixing tetraethyl orthosilicate (TEOS), ethanol, acetic acid and deionized water. The gel was impregnated into ASF/MW preforms, aged and solvent-exchanged with ethanol/hexane to minimize shrinkage. Ambient pressure drying at 50 ℃ yielded the final composite. X-ray diffraction (XRD) and Fourier-transform infrared spectroscopy (FT-IR) were used to confirm phase composition and chemical bonding. Scanning electron microscopy (SEM) was employed to analyze microstructural evolution and interfacial integrity. Mechanical properties were evaluated by using compressive tests (60% strain, 10 mm·min⁻¹ loading rate). Thermal conductivity was measured using a transient plane source method, while pore structure (BET/BJH) and thermal stability (infrared imaging at 900 ℃) were assessed. Comparative analyses with literature data validated the improvements in performance.[Results] The ASF/MW-SiO₂ composite exhibited a maximum compressive strain of 60%, with a compressive strength of 1.745 MPa at 60% strain, which is 3.4 times higher than pure ASF/SiO₂ aerogel (0.518 MPa). Samples treated at 900 ℃ (e.g., S1) outperformed those at 950 ℃, where excessive whisker growth degraded fiber integrity (e.g., S8 fragmented at 45% strain). SEM results revealed that the MW was effectively anchored SiO₂ aerogel to fibers, preventing debonding during compression. Cracks propagated around fibers rather than along interfaces, confirming enhanced adhesion. Higher treatment temperatures (950 ℃) induced fiber damage and whisker overgrowth, reducing mechanical stability. The composite maintained low density (0.24–0.25 g·cm⁻³), high specific surface area (106.21 m2·g⁻¹) and ultra-low thermal conductivity (0.038 W m⁻¹·K⁻¹ for S1). Infrared imaging demonstrated superior thermal resistance: after 3 min at 900 ℃, S1's surface temperature reached only 206°C, compared with 390°C for pure ASF/SiO₂. The introduction of MW reduced average pore size from 10.4 nm (pure aerogel) to 7.8 nm (S1), enhancing interfacial connectivity and lowering gaseous heat transfer. BET analysis confirmed increased specific surface area and reduced pore volume. The composite surpassed literature-reported MW-reinforced aerogels in compressive strain (60% vs. 25–50%), density (0.246 g·cm⁻³ vs. 0.26–0.27 g·cm⁻³) and balanced thermal conductivity (0.038 W·m⁻¹·K⁻¹ vs. 0.027–0.037 W·m⁻¹·K⁻¹).[Conclusions] An environmentally friendly ASF/MW-SiO₂ composite with enhanced mechanical strength and thermal stability was successfully developed. In-situ MW growth at 900 ℃ led to improvement in compressive strength by 23–240% and enabled 60% strain tolerance, addressing the brittleness of conventional aerogels. The composite retained ultra-low thermal conductivity (0.038 W·m⁻¹·K⁻¹) and exceptional high-temperature resistance, demonstrating viability for applications in extreme environments. Borax-catalyzed MW synthesis avoided toxic byproducts and preserved fiber integrity, while pore structure refinement enhanced interfacial bonding. The composite’s balanced properties, including low density, high resilience and thermal insulation, positioned it as a candidate for advanced insulation systems in aerospace and energy sectors. Future work may be focused in scalable production and long-term stability under cyclic thermal-mechanical loads. This achievement provides a sustainable pathway to overcome the limitations of traditional aerogel composites, emphasizing the synergy between microstructure design and eco-friendly processing.

Key words: mullite whisker; in situ growth; aerogel composite material; mechanical properties; thermophysical property