Very lightweight glass-like materials, but extremely fragile At best: 1.5 mg/cc, Guinness World Records 99.8% porosity 1000 times less dense than glass about 40 times better thermal insulators than the best fiberglass JPL Website, Stardust Program Invented by S. S. Kistler (Stanford U.) in 1931 lengthy process, first major breakthrough: supercritical drying of wet gels retaining volume of the gel “Forgotten” for almost 30 years “Re-invented” in the 1960’s in France second major breakthrough: sol-gel process cutting Kistler’s method from weeks to hours Aerogels

Aerogels have been considered for: - thermal insulation (architectural, automotive industrial applications); - acoustic insulation (buildings, automobiles, aircraft); - dielectrics (for fast electronics); - supports for catalysts; and, - hosts of functional guests for chemical, electronic and optical applications. Silica aerogels have been actually used: - as Cerenkov radiation detectors - aboard spacecraft: o as collectors of cosmic particles (Stardust Program) o for thermal insulation (e.g., Sojourner Rover ) Commercialization has been slow, because silica aerogels are: - fragile; - hygroscopic; and, - require supercritical fluid (SCF) extraction Current and Projected Use for Aerogels

Crosslinking aerogels — Microscopically, nanocast conformal polymer coating on the silica nanoparticles conformal polymer coating Micropores are blocked thicker necks increase strength of material Leventis, et al, Nano Letters, 2002

Compressive Stress-Strain Curves for Templated Aerogels

conformal polymer coating Leventis (2007), Luo, Lu and Leventis (2003) Lightweight thermal insulation Acoustic Insulation Catalytic reformers and converters Dielectrics Ballistic materials Filtration membranes Membranes for fuel cells Optical sensors Aircraft structural components Cross-linked silica aerogel Pontential Applications for Crosslinked Aerogels

NativeCross-linked Section of Cross-linked Secondary particles Polymer coating Bending stresses at necks responsible for low failure strain leading to fragility in native silica aerogels Bending stress contours from FEM Increase in the cross-linked aerogel stiffness with the amount of polymer addition Simulation of Two-spheres Model for Two Secondary Particles Connected to Each Other

SEM of Crosslinked Templated Aerogels (X-MP4-T045)

Transmitted X-ray Image CCD  Incident X-ray Beam X-rays * Low depth of field, reject scattered light photons. CCD Optical Lens* Thin Single Crystal Scintillator Sample X-raysLight Nano-Computed Tomography (nano-CT)

3D discretized MPM simulation model 110 pixels along the length nano-CT structure for X-MP4-T045 Pixel size =480 nm/voxel; Average pore size 6−7μm Crosslinked Silica Aerogel – MPM Simulation

Flexural Modulus Estimation Weight ratio( Polymer: Silica)=70%: 30% Density of polymer = 1.2 g/cc Density of silica = 2.6 g/cc Modulus of polymer = 2 Gpa Modulus of silica =70 Gpa The volume ratio( Polymer: Silica) ≈5:1 The dimensionless radius of the model R=6 Based on the bending equation for the composite material The silica-aerogel modulus is about Gpa

Simulation Results  Microstructural evolution under compression Microstructure deformation characteristic (comparison with rohacell foam)  Dynamic equilibrium Dynamic stress equilibrium; velocity loading history  Compressive stress-strain curve Typical silica-aerogel material stress-strain relation  Effect of porosity on the material properties Gibson & Ashby beam structure analog for the honeycomb structure material; response for the different porosity silica- aerogel models

Microstructural Evolution

Cell buckling is not a primary deformation mechanism. Shear band Daphalapurkar et al, Mech. Adv. Mater. & Struc.,2008

Dynamic Stress Equilibrium Compressive stress-strain curves indicating the dynamic stress equilibrium. Time step in simulation is nanoseconds

Stress-Strain Curve Compressive Stress-strain curve for 3D model 2D simulation does not appear to be able to capture the initial elastic region accurately for an irregular porous structure.

Effect of Porosity 45% porosity, cutoff grayscale 53 50% porosity, cutoff grayscale 58 55% porosity, cutoff grayscale 64

Effect of Porosity Gibson & Ashby, Cellular Solids 1997

Effect of Porosity Gibson & Ashby, Cellular Solids 1997

Conclusion  MPM simulation indicates the dynamic stress equilibrium condition has been reached. The stress-strain relation agrees with the experimental results in the elastic region and yielding region.  The simulation shows the potential to simulate the nanostructure property relationship of the crosslinked templated aerogels.  The simulation can capture the elastic, compaction and densification behavior of the silica-aerogel.  The mechanical behavior of silica-aerogel follows a cubic power relation when the pores are not fully compacted. The relation does not hold when the pores are all closed.