??Why silica aerogel is so fragile?
1. Structural Nature ?? Extremely Porous 3D Network with Ultra-high porosity (>90%)
Silica aerogel typically has a porosity between 90% and 99.8%, meaning:
路 Only 1??10% of its volume is solid material;
路 The rest is air-filled voids.
As a result:
路 The load-bearing cross-sectional area is extremely small;
路 Stress concentrates on the thin silica necks connecting nanoparticles;
路 Once one link breaks, a cascade collapse occurs through the network.
Analogy: It??s like a 3D scaffold made of glass needles ?? extremely light, but very easy to break.
2. Chemical Bond Nature ?? Strong but Brittle Si??O??Si Bonds
The silica skeleton is built from Si??O??Si covalent bonds, which are:
路 Strong (bond energy ?? 450 kJ/mol), but
路 Directional and non-ductile ?? they cannot deform plastically like metallic bonds.
Therefore, when external stress exceeds the critical point, the bonds fracture suddenly instead of yielding.
In other words: Once a Si??O??Si chain is overstressed, it snaps instantly rather than absorbing energy by deformation.
3. Microstructural Fragility ?? Stress Concentration at Nanoscales
路 Primary particle size: 10??50 nm
路 Particles are linked by thin ??neck-like?? bridges
路 Each node has a low coordination number, so load transfer paths are few
When force is applied:
路 Local bonds break first,
路 The network collapses locally,
路 Microcracks propagate into macroscopic fractures.
4. Drying Process Effects ?? Capillary Forces Create Microcracks
During the sol??gel process, the drying stage can introduce residual damage:
路 In non-supercritical drying, capillary pressure can crush the delicate gel network;
路 Even in supercritical drying, minor shrinkage or microcracks may remain;
路 These become stress initiation sites under later mechanical loading.
5. Macroscopic Mechanical Properties ?? Very Low Modulus and Fracture Toughness
Property | Silica Aerogel | Ordinary Glass | Aluminum Alloy |
Density (g/cm鲁) | 0.003??0.2 | 2.5 | 2.7 |
Young??s Modulus (GPa) | 0.001??0.1 | 70 | 69 |
Fracture Toughness (MPa路m鹿??虏) | 0.01??0.1 | 0.75 | 25??35 |
These values show: Silica aerogel??s fracture toughness is about 1/10 of glass and 1/1000 of metals,making it almost incapable of resisting bending, compression, or impact.
6. Methods to Improve Toughness
Researchers and manufacturers are developing several strengthening strategies:
(1)Fiber Reinforcement
路 Introduce glass, aramid, or polymer fibers;
路 Create aerogel blankets or composite panels;
路 Greatly improves flexural and compressive strength.
(2)Organic??Inorganic Hybridization
路 Graft flexible polymer chains (e.g., PDMS, PVA, PMMA) onto silica skeletons;
路 Increases elasticity and crack resistance.
(3) Nanostructure Optimization
路 Control particle size, neck thickness, and pore size distribution;
路 Build hierarchical porous networks to enhance energy absorption.
