Atomic layer deposition for the conformal coating of nanoporous materials

Atomic layer deposition (ALD) is ideal for applying precise and conformal coatings over nanoporous materials. We have recently used ALD to coat two nanoporous solids: anodic aluminum oxide (AAO) and silica aerogels. AAO possesses hexagonally ordered pores with diameters d ∼ 40 nm and pore length L ∼ 70 microns. The AAO membranes were coated by ALD to fabricate catalytic membranes that demonstrate remarkable selectivity in the oxidative dehydrogenation of cyclohexane. Additional AAO membranes coated with ALD Pd films show promise as hydrogen sensors. Silica aerogels have the lowest density and highest surface area of any solid material. Consequently, these materials serve as an excellent substrate to fabricate novel catalytic materials and gas sensors by ALD

Aerogels And Sol–Gel Composites As Nanostructured Energetic Materials

In the last 20 years, there have been a significant number of investigations of the application of aerogels and sol–gel-derived materials and methods to the field of energetic materials (e.g., explosives, propellants, thermites, and pyrotechnics) specifically through the synthesis and characterization of nanostructured energetic composites. Aerogels have unique density, composition, porosity, and particle sizes as well as low temperature and benign chemical synthetic methods all of which make them attractive for energetic nanomaterials candidates. The application of these materials and methods to this technology area has resulted in three general types of sol–gel energetic materials: (1) sol–gel inorganic oxidizer/metal fuel thermite-like composites; (2) sol–gel-derived porous pyrophoric metal powders, films, and monoliths; and (3) sol–gel metal or organic fuel/inorganic oxidizer nanocomposites (propellant, explosive, thermite, and pyrotechnic composites). This chapter summarizes results from synthesis and characterization research in all three areas. General trends are detailed, analyzed, and discussed. In general, all sol–gel nanostructured energetic material behaviors are highly dependent on several factors including the nanomorphology of the network, its surface area, the degree of mixing and contact area between phases, the type of mixing (sol–gel, physical mixing, interpenetration), solids loading, and the presence of impurities. Sol–gel methods are attractive to the area of nanostructured energetics because they offer a great deal of many processing options such as monoliths, powders, and films and have broad compositional versatility. These attributes coupled with strong synthetic control of the microstructural properties of the sol–gel matrix enable the preparation of energetic nanocomposites with tunable performance characteristics. Various aspects of the present literature work are reviewed and future challenges for this technological area are presented and discussed

Synthesis Of Metal Oxide Aerogels Via Epoxide-Assisted Gelation Of Metal Salts

Over the past two decades, the diversity of metal and metalloid oxide materials prepared using sol–gel techniques has increased significantly. This transformation can be attributed in part to the development of the technique known as epoxide-assisted gelation. The process utilizes organic epoxides as co-reactants for the sol–gel polymerization of simple inorganic metal salts in aqueous or alcoholic media. In this approach, the epoxide acts as a proton scavenger, which drives hydrolysis and condensation of hydrated metal species in the sol–gel reaction. This process is generalizable and applicable to the synthesis of a wide range of metal and metalloid oxide aerogels, xerogels, and nanocomposites. In addition, modification of synthetic parameters allows for control over the structure and properties of the sol–gel product. The method is particularly amenable to the synthesis of multicomponent and nanocomposite sol–gel systems with intimately mixed nanostructures. This chapter describes both the reaction mechanisms associated with epoxide-assisted gelation and an overview of materials that have been prepared using this technique

Iron-Doped Carbon Aerogels: Novel Porous Substrates for Direct Growth of Carbon Nanotubes

We present the synthesis and characterization of Fe-doped carbon aerogels (CAs) and demonstrate the ability to grow carbon nanotubes directly on monoliths of these materials to afford novel carbon aerogel-carbon nanotube composites. Preparation of the Fe-doped CAs begins with the sol-gel polymerization of the potassium salt of 2,4-dihydroxybenzoic acid with formaldehyde, affording K + -doped gels that can then be converted to Fe 2+ -or Fe 3+ -doped gels through an ion exchange process, dried with supercritical CO 2 , and subsequently carbonized under an inert atmosphere. Analysis of the Fe-doped CAs by TEM, XRD, and XPS revealed that the doped iron species are reduced during carbonization to form metallic iron and iron carbide nanoparticles. The sizes and chemical composition of the reduced Fe species were related to pyrolysis temperature as well as the type of iron salt used in the ion exchange process. Raman spectroscopy and XRD analysis further reveal that, despite the presence of the Fe species, the CA framework is not significantly graphitized during pyrolysis. The Fe-doped CAs were subsequently placed in a thermal CVD reactor and exposed to a mixture of CH 4 (1000 sccm), H 2 (500 sccm), and C 2 H 4 (20 sccm) at temperatures ranging from 600 to 800°C for 10 min, resulting in direct growth of carbon nanotubes on the aerogel monoliths. Carbon nanotubes grown by this method appear to be multiwalled (∼25 nm in diameter and up to 4 µm long) and grow through a tip-growth mechanism that pushes catalytic iron particles out of the aerogel framework. The highest yield of CNTs was grown on Fe-doped CAs pyrolyzed at 800°C treated at CVD temperatures of 700°C

Parallel trapping of multiwalled carbon nanotubes with optoelectronic tweezers

Here we report the use of optoelectronic tweezers and dynamic virtual electrodes to address multiwalled carbon nanotubes (MWCNTs) with trap stiffness values of approximately 50 fN∕μm. Both high-speed translation (>200 μm∕s) of individual-MWCNTs and two-dimensional trapping of MWCNT ensembles are achieved using 100,000 times less optical power density than single beam laser tweezers. Modulating the virtual electrode’s intensity enables tuning of the MWCNT ensemble’s number density by an order of magnitude on the time scale of seconds promising a broad range of applications in MWCNT science and technology