The compressive behavior of isocyanate-crosslinked silica aerogel at high strain rates

Aerogels are low-density, highly nano-porous materials. Their engineering applications are limited due to their brittleness and hydrophilicity. Recently, a strong lightweight crosslinked silica aerogel has been developed by encapsulating the skeletal framework of amine-modified silica aerogels with polyureas derived by isocyanate. The mesoporous structure of the underlying silica framework is preserved through conformal polymer coating, and the thermal conductivity remains low. Characterization has been conducted on the thermal, physical properties and the mechanical properties under quasi-static loading conditions. In this paper, we present results on the dynamic compressive behavior of the crosslinked silica aerogel (CSA) using a split Hopkinson pressure bar (SHPB). A new tubing pulse shaper was employed to help reach the dynamic stress equilibrium and constant strain rate. The stress-strain relationship was determined at high strain rates within 114–4386 s⁻¹. The effects of strain rate, density, specimen thickness and water absorption on the dynamic behavior of the CSA were investigated through a series of dynamic experiments. The Young’s moduli (or 0.2% offset compressive yield strengths) at a strain rate ∼350 s⁻¹ were determined as 10.96/2.08, 159.5/6.75, 192.2/7.68, 304.6/11.46, 407.0/20.91 and 640.5/30.47 MPa for CSA with densities 0.205, 0.454, 0.492, 0.551, 0.628 and 0.731 g cm⁻³, respectively. The deformation and failure behaviors of a native silica aerogel with density (0.472 g cm⁻³), approximately the same as a typical CSA sample were observed with a high speed digital camera. Digital image correlation technique was used to determine the surface strains through a series of images acquired using high speed photography. The relative uniform axial deformation indicated that localized compaction did not occur at a compressive strain level of ∼17%, suggesting most likely failure mechanism at high strain rate to be different from that under quasi-static loading condition. The Poisson’s ratio was determined to be 0.162 in nonlinear regime under high strain rates. CSA samples failed generally by splitting, but were much more ductile than native silica aerogels.     

Synthesis of centimeter-scale monolithic SiC nanofoams and pore size effect on mechanical properties

By pressure infiltrating pre-ceramic polymer polycarbosilane (PCS) into thermally and mechanically stable silica nanofoam, followed by PCS pyrolysis and silica template removal, synthesis of large-scale monolithic SiC nanofoams has been accomplished. Tailoring of the porosity and cell size of the SiC nanofoam has been realized by dissociating the porosity and pore size of the silica nanofoam. Because of the surface hardening and increased surface volume ratio of deformable nanopores, with the same porosity, the decrease of nanopore size has led to an increase in the quasi-static and dynamic indentation resistance for SiC nanofoams.     

Effect of Ag–Au composition and acid concentration on dealloying front velocity and cracking during nanoporous gold formation

Nanoporous gold has many potential applications in various fields, including energy storage, catalysis, sensing and actuating. Dealloying of Ag–Au alloys under free corrosion conditions is a simple method to fabricate nanoporous gold. Here, we systematically investigate the dealloying rate of Ag–xAu alloy for a range of alloy compositions (x = 20–40 at.%) and nitric acid concentration (7.3–14.9 M) using in situ transmission X-ray microscopy. High-resolution in situ X-ray projections and ex situ tomographic reconstructions allow imaging of the dealloying front position during dealloying. The dealloying front velocity is constant with time, and depends exponentially on the alloy Ag/Au atomic ratio and the acid molar concentration. Only the leanest alloy, Ag–20 Au, shows a large macroscopic shrinkage in sample diameter (～38%) after dealloying, which leads to crack nucleation and growth observed in real time during dealloying. Finite element modeling is used to estimate dealloying-induced stresses and strains, and sheds light on the cracks created by the diameter shrinkage.