Study on Flame Retardancy of Aerogel

Aerogel, often poetically described as "frozen smoke," represents one of materials science's most intriguing triumphs—a porous solid where air comprises up to 99% of its volume, yet it possesses remarkable strength against fire and heat. First synthesized by Samuel Kistler in 1931 using a sol-gel process, silica aerogel (SiO₂ aerogel) has long been prized for its intrinsic flame-retardant qualities. Its Si–O–Si framework, when exposed to flames, transforms into a dense silica barrier that shields underlying structures from thermal assault. However, early formulations suffered from brittleness, moisture sensitivity, and incomplete flame suppression, limiting their real-world utility. Over the past century, relentless research has addressed these flaws, evolving aerogel into a versatile powerhouse. Today, with optimized preparation techniques and smart modifications, it not only withstands temperatures exceeding 1,200°C but also integrates seamlessly into composites for enhanced performance. This exploration uncovers the latest advancements in flame retardancy, dissects the underlying thermal mechanisms, and envisions aerogel's role in safeguarding critical applications, from spacecraft re-entry to battery fire prevention.

Research and Application Progress on Modification of Silica Aerogel

The journey of silica aerogel begins with its foundational structure: a delicate web of silica nanoparticles linked in chains, forming mesopores that endow it with unparalleled lightness and insulation. Yet, its hydrophilic nature—stemming from surface hydroxyl groups—leads to moisture absorption, structural weakening, and reduced longevity. Moreover, residual organic modifiers from synthesis can volatilize at 200–400°C, generating flammable gases that undermine fire resistance. To counter these, scientists have pioneered hydrophobic treatments, grafting methyl or phenyl groups via vapor-phase silanization, elevating contact angles above 150° and rendering the material virtually impervious to water. This not only preserves porosity but also stabilizes the network against environmental degradation.

A pivotal leap came from refining preparation methods. Supercritical drying (SA_sd), which avoids capillary collapse by transitioning solvents to gas without surface tension, yields aerogels with 36.45% lower total heat release (THR = 1.02 MJ/m²) than ambient-dried counterparts, due to minimized organic residues and a more uniform lattice. Ambient-pressure alternatives, enhanced by phosphoric acid (PA) catalysis as demonstrated by Li's team, introduce phosphorus chemistry: at 300°C+, PA decomposes into phosphoric acid, catalyzing dehydration and char formation. This results in a robust carbon layer with 42% residue yield, slashing THR by 49.3% and peak heat release rate (PHRR) to 18.7 kW/m²—achieving UL-94 V-1 certification. Such phosphorus-nitrogen synergies create gas-phase radical quenchers and condensed-phase barriers, effectively starving flames of oxygen and fuel.

As fillers in composites, silica aerogels amplify host materials' retardancy. Deng's PVA/SA/GF ternary system, crafted via freeze-drying, forms hierarchical pores; 20 wt% aerogel boosts char residue to 75% and cuts HRR 61.2%. Lee's PDMS infusion with 15 vol% aerogel, using pore-repair techniques, elevates the oxygen index from 25.2% to safer levels, eliminating drips. These hybrids exemplify "structural barrier + chemical synergy," paving the way for V-0 rated materials that retain superhydrophobicity (>160° contact angle). Future research eyes organic-inorganic hybrids for even greater resilience, targeting aerospace re-entry shields (withstanding 1,600°C plasma) and energy storage separators (preventing thermal runaway in lithium batteries at 500°C+).

Thermal Insulation Mechanisms of Aerogels

Aerogel's thermal wizardry—conductivity as low as 0.012–0.024 W/(m·K)—stems from a masterful multi-scale blockade of heat's three pathways, outperforming traditional insulators by orders of magnitude and enabling featherlight designs in demanding arenas.

Gas Confinement Effect (Convection Suppression): With pores (2–50 nm) smaller than air's mean free path (~70 nm), molecules collide with walls rather than each other, invoking the Knudsen regime. This confines gas motion, reducing convective transfer to negligible levels—imagine air "frozen" in place, unable to circulate heat.

Radiation Scattering Effect (Thermal Radiation Shielding): The fractal network (~10¹⁸ pore walls per cm³) acts as a hall of mirrors for infrared waves. Multi-level scattering diffuses radiation, dropping emissivity to 0.05 and slashing radiative flux by two orders—crucial for high-temp scenarios like rocket nozzles, where aerogels prevent thermal breakthrough.

Phonon Barrier Effect (Solid-State Thermal Conductivity Suppression): The sparse SiO₂ skeleton elongates phonon paths 10–20 times the material's thickness, while quantum confinement at nanoscale junctions scatters vibrations. Solid conductivity plummets to 1/100th of bulk silica, ensuring minimal heat creep even in vacuum-adjacent conditions.

This triad has propelled aerogels into elite roles: Long March rockets rely on them for thermal cloaks during launch, maintaining electronics at ambient amid 1,000°C exhaust. In energy storage, they insulate batteries against runaway chains, potentially averting disasters. As composites evolve, aerogels promise even broader horizons, blending retardancy with adaptability for a fire-safe future.