Highly compressible and anisotropic lamellar ceramic sponges with superior thermal insulation and acoustic absorption performances

Ceramic sponge materials are attractive for thermal insulation, water treatment, catalyst support, energy absorption, and high-temperature air filtration due to their lightweight, high surface area, low thermal conductivity, and chemical/thermal stability. Yet, conventional ceramic sponges made from oxide nanoparticles or lattices are brittle and their fabrication often involves complex, multi-step processes that impede scale-up. Existing approaches such as electrospinning-based assemblies yield disordered, non-woven structures with poor compressive resistance and irregular shapes; CVD/ALD routes demand stringent conditions, templates, and post-removal steps. The research question is how to develop a simple, scalable, and low-cost method to fabricate flexible ceramic sponges that combine high and temperature-invariant compressibility with robust thermal insulation and acoustic absorption. This study proposes a solution blow spinning plus calcination route to create anisotropic lamellar SiO2–Al2O3 composite sponges addressing these needs.

Recent advances include carbon nanotube, graphene, biomass-derived, ceramic and carbon nanofiber aerogels exhibiting superelasticity under large deformation owing to porous 3D networks. For ceramics, flexible sponges have been built by directly assembling electrospun ceramic nanofibers, but resulting mats are typically random, poorly compression-resistant, and hard to shape into true 3D forms. CVD-produced ceramic aerogels (e.g., hexagonal BN aerogels with auxetic behavior and ultralow thermal conductivity) show robust properties but require template-assisted deposition under controlled atmospheres and involve multiple steps. Other assembly routes (hydrogen bonding, ALD, melt spinning) also face cost, complexity, or structural limitations. There remains a gap for a facile, scalable process delivering flexible, highly compressible ceramic sponges with stable performance from cryogenic to high temperatures and with low thermal conductivity and acoustic absorption capability.

Fabrication of SAC sponges (SiO2–Al2O3 composite):

• Spinning solution preparation: Dissolve 1.125 g PVA (Mw ≈ 205,000) in 10.8 g deionized water at 90 °C for 1 h. Add 3.9 g TEOS and 0.0183 g H3PO4, then different masses of AlCl3·6H2O to achieve Al:Si molar ratios of 0.1:1, 0.2:1, 0.4:1, or 0.8:1; stir ~2 h at room temperature until transparent (AlCl3 catalyzes TEOS hydrolysis and accelerates solution clarification). PVA serves as tackifier/template for microfiber formation during blow spinning.
• Blow spinning: Load solution into a 1 mL syringe with a coaxial needle (ID 0.21 mm, OD 0.4 mm). Feed at 5 mL h−1 using an injection pump; use compressed air at ~50 kPa; collect PSAC (PVA–SiO2–Al2O3 composite) microfibers on an air-permeable cage placed ~50 cm from the nozzle at ambient conditions. The as-spun PSAC sponges exhibit lamellar stacking of microfiber layers.
• Calcination: Heat PSAC sponges in air (muffle furnace) at 1000–1300 °C for 1 h with 5 °C min−1 ramp to remove organics, yielding SAC sponges. Unless noted, samples used Al:Si = 0.8:1. Control SiO2-only sponges were prepared without AlCl3.

Structural and chemical characterization:

• SEM/FESEM for morphology (overhead and cross-sectional), microfiber diameters (PSAC ≈ 4.8 μm; post-calcination SAC ≈ 2.7 μm), and lamellar architecture. EDS for elemental mapping (Si, Al, O). XPS to confirm Al-doped silica and Al2O3 presence. XRD to assess phase evolution after heat treatments (amorphous SiO2 up to 1200 °C; β-quartz crystallization at 1300 °C); SEM corroborates fiber fusion/adhesion at 1300 °C due to crystallite growth. TGA/DTA in air up to 1400 °C for thermal stability.

Mechanical testing:

• Quasi-static compression: Universal testing machine (50 N load cell) with loading/unloading rate 5 mm min−1 for stress–strain curves; cyclic fatigue at 50% max strain with 100 mm min−1 for up to 600 cycles. Evaluate energy loss coefficient ΔU/U from hysteresis, maximum stress retention, and Poisson’s ratio versus strain (in situ video/SEM to observe fiber/layer deformation and lap joints). Long-term storage stability tested after 2 months at room temperature and after cryogenic/high-temperature treatments.
• Molecular dynamics (MD) simulations: Construct 10×10×20 nm models of SiO2 and SAC fibers; uniaxial tensile simulations along z to extract Young’s modulus and tensile strength/strain, informing scaling law estimates for cellular moduli.

Thermal properties:

• Thermal conductivity: Hot Disk TPS (transient plane source) per ISO 22007-2:2015 on 50×50 mm2 samples. Measure along directions vertical and parallel to fiber layers across temperatures (20–120 °C), and versus density (≈10–40 mg cm−3). Demonstration of thermal insulation via flower-on-sponge over alcohol lamp.
• Fire/high-temperature resilience: In situ compression while heating by alcohol lamp (~600 °C) and butane blowlamp (~1200 °C). Additional anneal at 1000 °C for 24 h followed by compression tests to assess retention.

Low-temperature performance:

• Compression in liquid nitrogen (−196 °C), including 24 h soaking, then cyclic compression (50% strain) and stress retention assessment.

Acoustic absorption:

• Impedance tube measurements per ISO 10534-2:1998. Cylindrical specimens (diameters 100 mm for 63–1600 Hz, 30 mm for 1000–6300 Hz) with varying thicknesses (e.g., up to 29 mm) and density ~20 mg cm−3. Determine absorption coefficient versus frequency and compute noise reduction coefficient (NRC).

• Scalable fabrication: Simple solution blow spinning plus calcination yields large-area anisotropic lamellar SAC sponges with stacked microfiber layers; single layers can be peeled, and blocks can be cut/stacked into arbitrary shapes.
• Ultra-low density: As low as 10 mg cm−3; sponge supported by a flower stamen indicates lightweight nature.
• High compressibility and anisotropy: Up to 80% compressive strain with full shape recovery along the z-direction (perpendicular to layers); along x (parallel to layers) recovery is incomplete, reflecting anisotropic mechanics.
• Mechanical robustness: Three-stage compressive response with elastic, plateau (to ~65% strain), and densification regimes; 80% strain reached under ~13.5 kPa. Density-dependent strength consistent with scaling law E/E_s ~ (ρ/ρ_s)^3; predicted elastic moduli ~8–130 kPa match measured tangential moduli.
• Fatigue resistance: Withstand 600 cycles at 50% strain (100 mm min−1) with minimal degradation; energy loss coefficient ΔU/U decreases from 0.33 (cycle 1) to ~0.30 (steady); retains 88.3% of initial maximum stress after 600 cycles. After 2 months storage, retains 96.9% after 100 cycles at 50% strain.
• Zero Poisson’s ratio: Poisson’s ratio ~0 and strain-independent during loading–unloading, attributed to lamellar architecture.
• MD insights: SiO2 fiber E ≈ 142.0 GPa; SAC fiber E ≈ 168.6 GPa. SiO2 tensile strength ≈ 13.8 GPa at strain up to 16.8%; SAC ≈ 11.6 GPa at 12.1% strain, indicating comparable ductility with higher stiffness for SAC.
• Thermal insulation: Thermal conductivity as low as 0.034 W m−1 K−1 at 20 °C (density 13 mg cm−3) vertical to layers; increases slightly to 0.038 W m−1 K−1 at 40 mg cm−3. Anisotropic conductivity: for 16 mg cm−3, vertical direction rises from 0.035 to 0.046 W m−1 K−1 from 20 to 120 °C; parallel direction higher due to structural anisotropy.
• High-temperature resilience: Maintains structure and elasticity during in situ compression at ~600 °C and ~1200 °C. After 1000 °C for 24 h, mechanical properties show no significant deterioration; full recovery upon compression maintained.
• Low-temperature resilience: At −196 °C (liquid N2), sponges compress to ~80% strain and recover without fracture; after 24 h at −196 °C, retain 91.4% of initial maximum stress after 100 cycles at 50% strain.
• Structural/phase stability: Amorphous SiO2 up to 1200 °C; β-quartz crystallization and fiber fusion at 1300 °C for 1 h. Al2O3 presence inhibits SiO2 crystallization (raises onset from 1200 to 1300 °C). TGA shows no obvious mass change up to 1400 °C in air.
• Acoustic absorption: For density ~20 mg cm−3 and thickness 29 mm, NRC ≈ 0.77; absorption increases with frequency and thickness due to longer propagation paths through the lamellae.
• Additional: Water uptake imparts a gel-like state with storage modulus > loss modulus over 0.5–200 rad s−1, indicating solid-like behavior when wet.

The study addresses the need for flexible, highly compressible, and thermally stable ceramic sponges by introducing a simple, scalable solution blow spinning process that creates an anisotropic lamellar architecture. The lamellar stacking and inter-fiber lap joints enable large, recoverable compressive strains with near-zero Poisson’s ratio, while the Al2O3–SiO2 composite fibers provide enhanced stiffness (per MD) and thermal stability, supporting temperature-invariant superelasticity from −196 to at least 1000 °C and resistance to flame (~1200 °C). The combination of ultralow density and trapped air within/between layers suppresses thermal transport, yielding thermal conductivities competitive with leading insulators. The anisotropic structure further enhances sound absorption, delivering high NRC at modest thickness and density. Collectively, these outcomes meet the targeted multifunctionality and demonstrate clear advantages over more complex fabrication routes (electrospinning reconstructions, CVD/ALD), positioning SAC sponges as promising materials for thermal insulation, fireproofing, acoustic energy absorption, and catalytic support under extreme temperatures.

This work presents a facile, scalable solution blow spinning plus calcination method to fabricate anisotropic lamellar SiO2–Al2O3 ceramic sponges with ultralow density, high and temperature-invariant compressibility, near-zero Poisson’s ratio, low thermal conductivity, and excellent acoustic absorption. The lamellar microarchitecture and composite fiber chemistry underpin the superior mechanical resilience from cryogenic to high temperatures and robust thermal/acoustic performance. The approach overcomes limitations of existing ceramic sponge fabrication techniques and offers a cost-effective pathway to large, shapeable components. Future work could optimize microstructure (layer spacing, fiber diameter, orientation) and composition for tailored anisotropy, broaden the material palette (other oxides), and integrate functional additives for application-specific roles (e.g., catalytic activity, EM shielding) while preserving mechanical and thermal performance.

• Anisotropic mechanical recovery: Full elastic recovery occurs primarily along the z-direction (perpendicular to layers); compression parallel to the layers (x-direction) shows incomplete recovery, which may limit isotropic load-bearing applications.
• High-temperature crystallization: At 1300 °C, SiO2 transitions to β-quartz and microfibers fuse/adhere, potentially degrading mechanical resilience; although Al2O3 raises the crystallization onset relative to pure SiO2, this sets an upper processing/use temperature for maintaining optimal microstructure.
• Structure–property dependence on density: Mechanical strength and thermal conductivity increase with density, implying trade-offs between insulation and mechanical robustness that must be balanced for specific applications.