Strong yet flexible ceramic aerogel

Developing thermal insulators that are simultaneously strong and flexible under compressive, tensile, and bending deformations is important for heat preservation and thermal protection in abyssal sea and aerospace vehicles. Ceramic aerogels are attractive thermal insulation materials due to their low density, low thermal conductivity, and good thermal stability. However, conventional ceramic aerogels composed of oxide nanoparticles usually exhibit low strength and brittleness due to weak intergranular necking and the brittle nature of ceramics. Polymer-crosslinked and fiber-reinforced ceramic aerogels have been fabricated to improve strength and deformability, but issues such as thermal instability and dust release impede practical applications. Flexible ceramic nanostructure-based aerogels provide another approach, with reversible compressibility and even recoverable bendability and stretch; 3D printing has also been used to tune mechanical properties. Nevertheless, strengths and moduli typically remain in the several-to-tens of kPa range, insufficient for load-bearing applications (e.g., thermal sealing materials for hypersonic aircraft). Increasing density can improve modulus and strength but often reduces deformability and increases thermal conductivity via higher solid conduction. Thus, achieving ceramic aerogels that combine mechanical robustness, adequate deformability, and good thermal insulation remains challenging. Inspired by natural materials such as silkworm cocoons that are lightweight, strong, tough, and thermally insulating due to high-strength, flexible silk and laminated microstructures, the authors propose a laminated microstructure design in SiC–SiOx nanowire aerogels to resolve these conflicts.

Prior strategies to strengthen and toughen ceramic aerogels include polymer crosslinking and fiber reinforcement, which improve strength and deformability but can introduce thermal instability and dust release. Flexible ceramic nanostructure-based aerogels and 3D-printed ceramic aerogels offer reversible compressibility, bendability, and stretchability, yet typically exhibit strengths and moduli of only several to tens of kPa, inadequate for load-bearing. Increasing aerogel density raises modulus and strength but compromises deformability and increases thermal conductivity due to enhanced solid conduction. Nature-inspired laminated microstructures (e.g., silkworm cocoons) suggest a path to simultaneous strength, toughness, and thermal insulation. The work builds on earlier SiC–SiOx nanowire aerogels with random networks and demonstrates that structural anisotropy and interlayer connections can elevate mechanical performance without severely degrading thermal insulation.

Synthesis of raw SiC–SiOx nanowire aerogel paper: A siloxane xerogel was synthesized using methyltrimethoxysilane (MTMS, 99%) and dimethyldimethoxysilane (DMDMS, 99%) at a weight ratio of 1:4. Ethanol (≥99.7%) served as solvent, deionized water as hydrolytic reagent, and nitric acid (65.0–68.0%) as catalyst. The mixture was stirred and allowed to gel, then dried at 100 °C for 2 h to form a siloxane xerogel. Decomposition was carried out in a gas-pressure furnace: the xerogel in a graphite crucible was heated in argon at 0.25 MPa and 1150 °C for 3 h. Decomposed SiO and CO gases reacted to grow and self-assemble SiC–SiOx nanowires, yielding a paper-like nanowire aerogel on the crucible’s inner surface.

Formation of laminated aerogel: The nanowire aerogel paper was cut into pieces, stacked into a bulk, immersed in ethanol to fully infiltrate, then removed and dried naturally, producing a laminated SiC–SiOx nanowire aerogel. Microstructure evolution during ethanol evaporation was observed using an Olympus BX51 optical microscope: a piece of aerogel on a 50 °C glass slide was wetted with ethanol. Capillary flow along the radial direction and vertical shrinkage during drying reoriented nanowires, forming wavy, layered, laminated structures with interweaving nanowires and bundles. Interlayer “Velcro-like” connections arose as nanowires from adjacent pieces embedded into pores and were fastened during shrinkage under capillary forces.

Characterization: Microstructure was characterized by SEM and TEM (showing amorphous SiOx bonding between SiC sub-nanowires). Density (~50 mg cm−3) and porosity (~98%) were measured. N2 sorption at 77 K and BJH analysis provided specific surface area and mesopore distribution; mercury intrusion porosimetry and total pore calculations were combined to assess macroporosity and overall pore size distribution. Interlayer adhesion was measured by tensile loading perpendicular to layers. Mechanical testing included compression (0.5 mm min−1) with cyclic fatigue, tension (tensile stress–strain up to fracture), three-point bending with multiple loading–unloading cycles, and two-point buckling. Thermal stability was assessed by bendability at −196 °C (liquid nitrogen) and under a butane blow torch (~1200 °C). Thermal insulation was evaluated using a butane blow torch as a heat source and an infrared camera to monitor backside temperatures of a 10 mm thick sample. Anisotropic heat transport was studied using a 2 mm diameter cylindrical heater and IR thermography to track 25 °C isothermal lines in laminated versus isotropic aerogels; an anisotropy factor (horizontal/vertical length of the 25 °C isotherm) was calculated. Room-temperature thermal conductivity perpendicular to layers was measured (Supplementary Table 1).

• Laminated hierarchical microstructure: Capillary flow-induced self-assembly and vertical shrinkage during ethanol drying produced wavy sub-micrometer nanowire layers and lamination at tens-of-micrometers scale. Interweaving nanowires and bundles are interconnected via amorphous SiOx, with Velcro-like interlayer connections.
• Density and porosity: Density ≈ 50 mg cm−3; porosity ≈ 98%.
• Surface area and pores: Specific surface area decreased from 29.1 m2 g−1 (raw aerogel) to 15.4 m2 g−1 (laminated). Average mesopore sizes: 14.3 nm (raw) vs 11.8 nm (laminated). Micropore+mesopore volumes ≈ 0.15% of total; most pores are macropores <355 µm; average pore size across all pores ≈ 5.1 µm.
• Interlayer adhesion: Maximum adhesion stress ≈ 2.3 kPa under tensile loading perpendicular to layers; bridging nanowires at interfaces provide Velcro-like connections.
• Compression: Reversible compressibility up to 40% strain; at 60% and 80% strain, small permanent deformations (~5% and ~6%). Compressive modulus 222 ± 32.7 kPa (initial linear stage). Maximum compressive stress at 80% strain 1255 ± 116.3 kPa, 5–10+ times higher than many resilient ceramic aerogels.
• Compression fatigue (40% strain, 100 cycles): Permanent strain ~6% at 20 cycles, ~10% at 100 cycles. Max stress at 40% strain stabilizes after ~20 cycles, retaining ~85% of initial value. Energy loss coefficient: 0.62 (first cycle), ~0.30 after 20 cycles.
• Tension: Four-stage behavior; tensile modulus 4855 ± 111.0 kPa (≤1.2% strain). Transition region 1.2–6.0% strain; rapid stress increase beyond 6% with large-strain ductile deformation. Fracture strength 399 ± 83.4 kPa at 20% strain; stress decays to ~0 by ~22% strain, indicating nonbrittle fracture with crack deflection and layer-by-layer failure; sample retains integrity across break. Strength and modulus exceed prior stretchable ceramic aerogels by ~5–20x.
• Bending: Recoverable bendability through multiple loading–unloading cycles; bending strength 261 ± 11.4 kPa at 9 mm displacement; large deflections (to <45°) without visible damage and partial recovery to ~130° upon unloading.
• Buckling: Two-point buckling up to 80% strain with recovery to near-original shape (maximum buckling stress not specified in provided text).
• Thermal stability: Reversible bendability maintained at −196 °C (liquid nitrogen) and under ~1200 °C butane blow torch.
• Thermal insulation: Under butane blow torch heating, the backside of a 10 mm-thick sample stabilized at ~135 °C, indicating strong insulation. Room-temperature thermal conductivity (perpendicular to layers): 39.3 ± 0.4 mW m−1 K−1, comparable to silk cocoons.
• Thermal transport mechanisms: High porosity and tortuous nanowire network reduce solid conduction; amorphous SiOx and SiC stacking faults introduce phonon barriers. Despite ~9× higher density than raw random aerogel (5.7 mg cm−3; 28.4 mW m−1 K−1), thermal conductivity increases only by ~1.37×. Reduced nanoscale pore width lowers gas conduction via Knudsen effect (though nanoscale pores constitute only ~0.15% of volume). Laminated structure induces anisotropic heat flow; 25 °C isothermal line anisotropy factor stabilizes at ~1.3 in laminated vs ~1.0 in isotropic aerogel, directing heat along layers and reducing perpendicular transport.
• Overall: The laminated SiC–SiOx nanowire aerogel achieves simultaneous high strength/stiffness and flexibility under compression, tension, bending, and buckling, without significant compromise in thermal insulation, outperforming many prior ceramic aerogels by up to an order of magnitude in mechanical metrics.

The study addresses the central challenge of combining mechanical robustness with flexibility and thermal insulation in ceramic aerogels. In random nanowire aerogels, flexibility arises from nanowire bending/buckling within large pores, but weak constraints limit strength and modulus. The laminated architecture imposes planar constraints on nanowire deformation, increasing bending and buckling resistance and improving load transfer. Denser nanowire packing enhances inter-nanowire interactions, while Velcro-like interlayer connections permit relative movement and energy dissipation under bending and buckling, enabling elastoplastic and recoverable deformations without brittle failure. Mechanically, this yields compressive modulus and strength up to an order of magnitude above typical resilient ceramic aerogels, alongside high tensile modulus and fracture strength with nonbrittle, layer-by-layer crack propagation and deflection, preserving macroscopic integrity. Thermally, despite higher density (which would typically increase solid conduction), the aerogel maintains low thermal conductivity due to high porosity, tortuous pathways, and phonon scattering at amorphous SiOx and stacking-fault interfaces. Reduced nanoscale pore widths lessen gas conduction via the Knudsen effect. Crucially, the laminated microstructure induces anisotropic heat transport that channels heat along layers and impedes perpendicular flow, as evidenced by anisotropic isothermal contours (anisotropy factor ~1.3). Thus, the design reconciles the trade-offs among strength, deformability, and thermal insulation, expanding the applicability of ceramic aerogels in demanding environments.

A laminated microstructure in SiC–SiOx nanowire aerogels resolves conflicts between mechanical robustness, deformability, and thermal insulation. Capillary flow-induced self-assembly and vertical shrinkage create hierarchical, interwoven nanowire layers with strong interconnections, delivering high compressive and tensile strength and modulus, reversible compressibility, recoverable buckling, and flexible, partially recoverable bending. Thermal conductivity remains low (39.3 ± 0.4 mW m−1 K−1) and heat transport is anisotropic, reducing perpendicular heat flow. The aerogel maintains flexibility from −196 °C to ~1200 °C, making it a promising strong yet flexible thermal insulator for dynamic thermal sealing in aerospace vehicles and for thermal protection in lunar rovers and deep-sea bathyscaphes. Future work could further optimize layer architecture and interlayer adhesion to tailor anisotropy and mechanical energy dissipation, and expand precise mechanical and thermal characterization under extreme service conditions.

The authors note that more precise mechanical characterization at extreme temperatures (e.g., −196 °C and ~1200 °C) is needed to fully assess performance under such conditions. Additionally, pore structure analysis via N2 sorption and BJH methods primarily captures micro- and mesopores; macroporosity required complementary mercury intrusion and total pore analyses, highlighting measurement limitations of standard gas sorption for the full pore size spectrum.