Silica Nanofoam Research Articles

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.

Effects of porosity on dynamic indentation resistance of silica nanofoam

The dynamic indentation behaviors of monolithic silica nanofoams of various porosities are investigated. When the pore size is on the nm scale, as the porosity increases, despite the decrease in mass density, the resistance offered by silica nanofoam to dynamic indentation is maintained at a high level, higher than the resistance of solid silica or regular porous silica. This phenomenon is related to the fast collapse of nanocells, which produces a locally hardened region and significantly increases the volume of material involved in impact energy dissipation.

Fast-condensing nanofoams: Suppressing localization of intense stress waves

We investigated the propagation of intense stress waves across silica nanofoams, with the pore size ranging from ~50nm to ~1µm and the porosity of ~60%. The experimental results showed that if the pore size was relatively large, the stress wave remained localized and its energy was dissipated in narrow bands; if the pore size was below ~200nm, however, the stress wave was homogenized in a broad area and consequently, bulk distributed energy absorption was promoted and the maximum transmitted-wave pressure was significantly reduced. We attribute this phenomenon to the fast condensation of the smallest pores at the wave front. The ability of nanofoams to promote widespread energy absorption may enable efficient stress-wave mitigation techniques. The classic Grady model was modified to take account for the nanopore size effect.

High-temperature post-processing treatment of silica nanofoams of controlled pore sizes and porosities

The pore size and the porosity of a nanoporous silica are often highly correlated. In the current investigation, based on the phenomenon that calcination temperature has a much more pronounced influence on the porosity than the pore size, we developed the post-processing calcination (PPC) technique to decouple the pore size and the porosity. As PPC temperatures were precisely controlled for silica monoliths of different initial pore sizes, their porosities could be tailored to a similar level, while the changes in pore sizes were quite mild. Powder X-ray diffraction analysis showed that the PPC treatment did not affect the amorphous nature of the silica phase.

Silica Nanofoam (NF) Supported Single- and Dual-Site Catalysts for Ethylene Polymerization with Morphology Control and Tailored Bimodal Molar Mass Distributions

Highly active single- and dual-site catalysts supported on silica nanofoams (NF) enable the control of both polyethylene (PE) morphology and tailoring of bimodal PE molar mass distribution in catalytic ethylene polymerization. In a templating process, aqueous polystyrene (PS) nano suspensions are mineralized and calcinated at 600 °C, thus producing NF with specific surface area of 1200 m2/g and average pore diameter varying between 20 and 80 nm. Mineralization of the aqueous PS nano suspensions in a water-in-oil emulsion affords spherical NF with average diameter of 40 μm and control of pore size. The methylalumoxane- (MAO-) tethered NF immobilizes single-site catalysts such as metallocene (nBuZr), bisiminopyridine iron(II) (Fe) and constrained geometry chinolyl cyclopentadienyl chromium(III) complexes (Cr-1). Immobilization of binary blends of Fe/Cr-1 and nBuZr/Cr-1 affords NF-supported dual-site catalysts. The catalyst activity, PE particle size and molecular weight distribution varies as a function of the NF pore size. Typically, macroporous NF (pore size of 75 nm) are effective supports for Cr-1, producing ultrahigh molecular weight polyethylene (UHMWPE, Mw > 106 g/mol). Dual -site catalysts such as nBuZr/Cr-1 on mesoporous NF (pore size of 20 nm) enable tailoring of bimodal PE molar mass distribution with UHMWPE content increasing with increasing Cr-1/nBuZr molar ratio.

Density dependence of acoustic characteristics of silica nanofoam

In this study, we experimentally estimated the sound speed of silica nanofoam as a function of the density through acousto-optic measurements. He–Ne laser light at the wavelength of 632.8 nm was emitted through the sample in the direction perpendicular to the propagation of the ultrasound. The diffraction of the light wave by the ultrasonic longitudinal waves with frequency in the range of 500–2,000 kHz agreed with the Raman-Nath theory. The sound speed estimated from the Raman-Nath diffraction angle varied in the range of 55–178 m/s for the sample density of 100–300 kg/m3. The measured sound speed almost agreed with the sound speed calculated from the density, bulk Young’s modulus and Poisson’s ratio. This implies that the low sound speed might be attributed to the reduced bulk elastic constant owing to the structure and might not be affected much by air confined in the porous structure.

Polyhedral carbon nanofoams with minimum surface area partitions produced using silica nanofoams as templates

Polyhedral silica nanofoam (PNF-SiO 2) analogues of dry soap froths with minimal surface area were used as templates for making polyhedral carbon nanofoams (PNF-C). Furfuryl alcohol or triblock copolymers were used as carbon sources. The volume of carbon precursor relative to the internal pore volume of PNF-SiO 2’s was systematically varied between 50% and 100% in order to investigate the effect of filling fraction on internal structure of the corresponding PNF-C’s. Transmission electron microscopy, small-angle X-ray scattering and nitrogen physisorption were used to characterize the samples. To aid the interpretation of the experimental data, a model for X-ray scattering from spherical shells was used to approximate scattering from the polyhedral foam cells. PNF-C’s cast from the PNF-SiO 2’s, displayed the characteristic Plateau borders of minimal surface area foams defining interconnected, slit-like pore systems at all filling fractions. At relatively high filling fractions, inverse foam structures were obtained with the slit-like pores systems interpenetrating aggregated, close-packed, relatively low density polyhedral carbon nanoparticles co-joined by carbon struts. At relatively low filling fractions, polyhedral carbon nanofoams with relatively thin, fused double-wall structures and interconnected polyhedral pore systems were obtained.

Study of [Dy(η 6- p-xylene)(GaCl 4) 3]-incorporated polyhedral silica nanofoam

Preparation and characterization of organometallic dysprosium complex embedded polyhedral silica nanofoam (PNF-SiO 2) is reported. The nanoporous solid was obtained via a sol–gel process involving precipitation of polymer micelles, hydrolysis and condensation of silica by swelling of triblock copolymer template micelles of PEO–PPO–PEO at acidic pH using high oil concentration and fast stirring. Metal-complex incorporation was performed via wet impregnation (WI) at higher temperature, where the dysprosium complex is chemically bound to the silanol groups of the silica foam surface. Scanning electron microscopy (SEM), transmission electron microscopy (TEM) and small angle X-ray scattering (SAXS) revealed that the particles of the organometallic complex are evenly distributed on the walls of the polyhedral cells, while nitrogen adsorption/desorption measurements (BET) verified that the characteristic meso- and macrocell structure of PNF-SiO 2 composed of polyhedral cells of silica joined at mesosized windows are preserved also after incorporation.

2E-8 An in-plane velocimeter using low frequency acousto-optic modulator with silica nanofoam(Physical Acoustics & Nonlinear & High-power Ultrasound, Sonochemistry)

2E-8 An in-plane velocimeter using low frequency acousto-optic modulator with silica nanofoam(Physical Acoustics & Nonlinear & High-power Ultrasound, Sonochemistry)

Development of High-Sensitivity Ultrasonic Transducer in Air with Nanofoam Material

For a high-sensitivity ultrasonic transducer in air, it was considered that nanofoam be applied to its acoustic matching layer. Nanofoam is a porous material that is made by the sol-gel process. Since nanofoam has an extremely low acoustic impedance, it is effective for the acoustic matching layer of an ultrasonic transducer in air. In this study, we have fabricated an ultrasonic transducer that had two acoustic matching layers and estimated its acoustic properties. The two matching layers were made of silica nanofoam and porous ceramic. The sensitivity of the developed ultrasonic transducer was about twenty times higher than that of a conventional ultrasonic transducer in air.

Synthesis of Carbon Nanofoams and Nanospheres by Varying Ratio of Phenol–Formaldehyde Resin to Mesoporous Silica Foams

Abstract Carbon nanofoams and nanospheres of 25–30 nm were conveniently synthesized by using silica nanofoams as a template and commercial-grade phenol–formaldehyde resin as carbon source at different weight ratio of PF resin to silica template.