Rapid and inexpensive synthesis of liter-scale SiC aerogels

Aerogels combine low density, high specific surface area, low thermal conductivity, and high-temperature stability, making them attractive for insulation and protection. Damage-tolerant ceramic aerogels with cyclic elasticity and unusual thermo-mechanical properties (e.g., near-zero or negative Poisson’s ratio and thermal expansion) are increasingly important, but oxide aerogels like SiO2 and Al2O3 are typically limited to ~800–900 °C due to coarsening and pore collapse at higher temperatures. Non-oxide aerogels such as SiC, with strong covalent bonds and high melting point, can extend service temperatures. SiC aerogels built from 3D-connected nanowires provide enhanced elasticity, flexibility, damage tolerance, low thermal conductivity, high specific modulus, and a wide stability window (from liquid nitrogen to ~1100 °C). However, existing synthesis routes (sol–gel, carbothermal reduction, pulsed laser deposition, chemical vapor deposition, freeze forming, and 3D printing) are costly, slow, complex, and difficult to scale, hindering industrial deployment. There is an urgent need for an inexpensive, rapid, and scalable synthesis route for SiC aerogels to accelerate their practical adoption.

Combustion synthesis (self-propagating high-temperature synthesis), pioneered by Merzhanov and co-workers, is rapid, low-cost, and scalable to ton-scale batches, and has been widely used for ceramic powders including SiC. Optimization for powders typically targets crystallinity, purity, size distribution, and sinterability. Prior studies reported that certain promoters (e.g., PTFE) can occasionally yield 1D SiC nanostructures (nanowires, fibers) during combustion of silicon-containing mixtures or Si with PTFE, including in calorimetric bomb setups. These routes, however, generally produced loose powders or wools, not monolithic 3D bulk aerogels. Reports also describe combustion processes that generate high-porosity products via gas-driven expansion. Building on this body of work, the present study leverages PTFE-promoted Si–C combustion and controlled gas evolution to directly synthesize 3D-connected SiC nanowire aerogels at bulk, liter-scale dimensions in a one-step process.

Materials and reactant preparation: High-purity Si powders (≥99.99%, d50=3.0 µm) and high–molecular-weight PTFE powders ((C2F4)n, ≥99.99%, M≈9.27×10^7, d50=10 µm) were mixed at a molar ratio Si:PTFE = 3:1. The mixture was ball-milled in a 500 mL nylon jar with 3 mm zirconia balls for 2 h using anhydrous ethanol as grinding medium (powder:ethanol mass ratio 1:1), ball-to-mixture ratio 5:1, and rotation speed 360 rpm. The slurry was dried and sieved. Reaction chamber and ignition: The mixed powders were placed in a stainless-steel reaction boat and loaded into a 30 L stainless-steel pressure vessel rated to 8 MPa. The chamber was evacuated to −0.09 MPa then backfilled with Ar to 1 MPa. Ignition was initiated by resistively heating a tungsten wire placed at one end of the powder bed, locally heating ~1 cm^3 to start the reaction. No further external energy was supplied after ignition. Process monitoring: Chamber temperature was measured using C-type W–Re thermocouples (φ 0.5 mm, length 80 mm; accuracy ±1% t) connected to a four-channel data acquisition system (≥25 ms sampling). The hot junction was positioned near the center of the reactant bed. Pressure was recorded at 10 Hz by the reactor’s pressure display. Combustion front propagation speed was determined from high-speed video frames and time offsets between thermocouples of known spacing. Synthesis characteristics: Using fine Si and high–molecular-weight PTFE, the Si–PTFE reaction exhibited highly exothermic behavior and fast wave propagation. In-situ records showed a peak pressure >57 atm within 2 s, peak temperature >1800 °C within 4 s, and a volumetric expansion of 1076% upon cooling, yielding monolithic SiC aerogels conformal to the chamber volume. The high-temperature duration above 1600 °C was ~4.69 s, and the aerogel expansion rate was ~1.56 L s−1. The process is self-sustaining after ignition. Estimated synthesis time per batch is ~0.5 h, and cost ~0.7 USD L−1 (~7 USD kg−1). Characterization: Phases were identified by XRD (Bruker D8 Focus, Cu Kα). Microstructures were examined by SEM (Hitachi S-4800) and TEM/HRTEM (JEOL JEM-2100F). Densities were computed from mass and geometry; porosity was inferred accordingly. Thermal analyses employed DSC/TGA (PerkinElmer STA 6000). Chemical bonding and surface composition were probed by FTIR (Varian Excalibur 3100) and XPS (Thermo ESCALAB 250Xi). Oxygen content was measured by O/N analyzer (HORIBA EMGA-820). N2 adsorption/desorption was conducted at 77 K (Quantachrome Quadrasorb SI-MP). Mechanical testing: Poisson’s ratio was determined from geometric changes under compression up to 25% strain (three repeats to obtain mean and SD). The linear thermal expansion coefficient was measured on a dilatometer (Linses L75VD1600) under N2 with L=3 mm, room temperature to 800 °C, 10 K min−1 heating rate, 50 mN contact force, 0.03 nm resolution. Dynamic mechanical analysis (DMA) was conducted on TA Q800. Quasi-static compression tests recorded stress–strain responses up to 20–80% strain at room temperature and −196 °C (liquid nitrogen). Cyclic compression up to 40% strain was performed to assess durability and hysteresis; high-temperature stability was evaluated by repeated heating to 1200 °C in air followed by room-temperature mechanical tests; in-situ compression at 1100 °C was also demonstrated. Thermal transport and insulation tests: Thermal conductivity was measured by Hot Disk TPS2500S (transient mode, ISO 22007-2:2015) using a Kapton-insulated nickel sensor (7577 F1, radius 2.0 mm), 4 mW heating power, 5 s measurement time, on aerogel specimens (4 cm × 3 cm × 5 mm). Triplicate measurements provided mean and SD. Surface temperatures were monitored by IR thermography (InfraTec VarioCAM HD). Macroscale insulation demonstrations included hotplate tests with various aerogel thicknesses (4–14 mm) and direct flame exposure (~1100 °C butane burner) on 20 mm thick samples. Safety and waste handling: Due to rapid pressure rise and toxic fluorosilicon gas (e.g., SiF4) generation, initial trials should use small batches, robust pressure-rated vessels, and appropriate gas scrubbing (lye/base solution) prior to venting.

• One-step, self-sustaining combustion synthesis produced liter-scale SiC aerogels within seconds, with demonstrated throughput ~16 L min−1 in the lab, volumetric expansion 1076%, peak temperature >1800 °C (in ~4 s), and peak pressure >57 atm (in ~2 s).
• Production cost is extremely low (~0.7 USD L−1; ~7 USD kg−1) and synthesis time per batch ~0.5 h, over an order of magnitude faster and more than two orders of magnitude cheaper than typical methods.
• Structure: β-SiC nanowire aerogels (confirmed by XRD), exhibiting a laminated mesoscale morphology and 3D-connected nanowire networks with load-bearing nodes; nanowire diameters 20–300 nm, lengths of microns to tens of microns; ~5 nm amorphous SiOx surface layer (verified by TEM, FTIR, XPS). Bulk density ~12 mg cm−3 (porosity ~99.6%).
• Mechanical and metamaterial properties: Negative Poisson’s ratio −0.09 ± 0.02; near-zero to negative linear thermal expansion (e.g., −1.01 × 10−6 K−1 at 800 °C). Room-temperature compression shows an initial elastic regime (modulus ~0.95 kPa) followed by nonlinear hardening; maximum stress 8.24 kPa at 80% strain. High compressibility and recoverability maintained at −196 °C and after repeated heating to 1200 °C in air (stiffness initially increases, then slightly decreases with cycling). After 100 cycles between 0–40% strain, compressive strength degraded <10% and thickness decreased ~8.5%.
• Thermal transport: Room-temperature thermal conductivity as low as 0.023 ± 0.004 W m−1 K−1 (density 6.4 mg cm−3), 0.025 ± 0.001 W m−1 K−1 (8.7 mg cm−3), and 0.027 ± 0.002 W m−1 K−1 (12 mg cm−3). Thermal conductivity increases modestly with temperature for ~12 mg cm−3 aerogels: 0.061 ± 0.008 W m−1 K−1 at 200 °C, 0.088 ± 0.012 W m−1 K−1 at 400 °C, 0.125 ± 0.007 W m−1 K−1 at 600 °C.
• Insulation performance: Strong thickness dependence observed on hotplate tests; under ~1100 °C butane flame for 5 min, a 20 mm-thick aerogel maintained a ~900 °C temperature difference between flame-side and opposite face, with no observable damage or shape change.
• Thermal stability: No phase transition up to 1100 °C in air and up to 1700 °C in Ar (0.5 h holds); minimal microstructure changes and volume shrinkage up to 1000 °C in air.
• Scalability and shape control: Product volume conforms to reactor chamber; macroscopic size/shape highly controllable. Process highlights potential for high-throughput, large-scale aerogel manufacturing.

The work addresses the critical bottlenecks of cost, time, and scalability in producing non-oxide ceramic aerogels by repurposing combustion synthesis from powder formation to direct monolithic aerogel fabrication. Using fine Si and high–molecular-weight PTFE both activates the weakly exothermic Si–C system and generates substantial gaseous products that drive rapid expansion. The solid products (C and SiC) form and inherit morphology consistent with polymer templating, assembling into 3D-connected nanowire networks. The observed laminated mesoscale architecture arises from constraint during expansion within the reactor. The resulting aerogels exhibit metamaterial behaviors—negative Poisson’s ratio and near-zero/negative thermal expansion—explained by hinged, concave pore-wall networks and thermally induced stress release/buckling in the nanowire framework. These structural features, combined with ultrahigh porosity, enable high compressibility, recoverability across a broad temperature range (−196 to ≥1100 °C), and excellent insulation with low thermal conductivity retained at elevated temperatures. Overall, the process demonstrates a practical, energy-minimal pathway for scalable production of high-performance SiC aerogels and encourages rethinking combustion synthesis as a one-step route to bulk porous architectures ready for applications.

A flash, self-sustaining combustion synthesis was invented to produce large, monolithic SiC nanowire aerogels rapidly and inexpensively. The method achieves liter-scale outputs within seconds, at ~0.7 USD L−1 production cost, yielding aerogels with ultralow density, low thermal conductivity, wide service temperature range, and metamaterial properties (negative Poisson’s ratio and near-zero/negative thermal expansion). The approach significantly reduces time, energy, and capital requirements relative to conventional routes and offers controllable size and shape. Future directions include: scaling equipment and throughput toward industrial production; extending the combustion-aerogel concept to other ceramics (nitrides, carbonitrides, oxides, MAX phases, and composites), with preliminary evidence for TiC nanowire formation; and exploring multifunctional applications such as EM wave absorption, self-cleaning, and wastewater treatment. Strict safety and gas-scrubbing protocols are essential for handling combustion byproducts during scale-up.

• The synthesis requires a robust high-pressure vessel capable of withstanding rapid pressure rises (>57 atm) and high temperatures, posing engineering and safety challenges.
• Toxic gaseous byproducts (e.g., SiF4) are generated and must be properly scrubbed (e.g., with base solution) before venting, adding operational complexity.
• Product size and lamellar mesoscale structure are influenced by the reactor chamber geometry and constraints during expansion, which may limit certain architectures without redesigned equipment.
• A thin amorphous SiOx surface layer (~5 nm) forms due to residual oxygen in materials and reactor, which may influence certain surface-sensitive applications.
• Although stability up to 1100 °C in air and 1700 °C in Ar is demonstrated for 0.5 h holds, comprehensive long-term lifetime data under diverse service environments will be needed for specific applications.