The drive towards increased energy efficiency and reduced air pollution has led to accelerated worldwide development of fuel cells. As the performance and cost of fuel cells have improved, the materials comprising them have become increasingly sophisticated, both in composition and microstructure. In particular, state-of-the-art fuel-cell electrodes typically have a complex micro/nano-structure involving interconnected electronically and ionically conducting phases, gas-phase porosity, and catalytically active surfaces. Determining this microstructure is a critical, yet usually missing, link between materials properties/processing and electrode performance. Current methods of microstructural analysis, such as scanning electron microscopy, only provide two-dimensional anecdotes of the microstructure, and thus limited information about how regions are interconnected in three-dimensional space. Here we demonstrate the use of dual-beam focused ion beam-scanning electron microscopy to make a complete three-dimensional reconstruction of a solid-oxide fuel-cell electrode. We use this data to calculate critical microstructural features such as volume fractions and surface areas of specific phases, three-phase boundary length, and the connectivity and tortuosity of specific subphases.
The constructive interference of surface plasmon polaritons (SPP) launched by nanometric holes allows us to focus SPP into a spot of high near-field intensity having subwavelength width. Near-field scanning optical microscopy is used to map the local SPP intensity. The resulting SPP patterns and their polarization dependence are accurately described in model calculations based on a dipolar model for the SPP emission at each hole. Furthermore, we show that the high SPP intensity in the focal spot can be launched and propagated on a Ag strip guide with a 250 x 50 nm2 cross section, thus overcoming the diffraction limit of conventional optics. The combination of focusing arrays and nano-waveguides may serve as a basic element in planar nano-photonic circuits.
A fibrous herringbone-modified helicoidal architecture is identified within the exocuticle of an impact-resistant crustacean appendage. This previously unreported composite microstructure, which features highly textured apatite mineral templated by an alpha-chitin matrix, provides enhanced stress redistribution and energy absorption over the traditional helicoidal design under compressive loading. Nanoscale toughening mechanisms are also identified using high load nanoindentation and in-situ TEM picoindentation. A Sinusoidally-Architected Helicoidal BiocompositeBy Nicholas A. Yaraghi, Nicolás Guarín-Zapata, Lessa K. Grunenfelder, Eric Hintsala, Sanjit Bhowmick, Jon M. Hiller, Mark Betts, Edward L. Principe, Jae-Young Jung, Leigh Sheppard, Richard Wuhrer, Joanna McKittrick, Pablo D. Zavattieri Keywords: (Composites, Toughness, Impact, Biomineral, Ultrastructure) Submitted to 3 Biologically mineralized composites offer inspiration for the design of next generation structural materials due to their low density, high strength and toughness currently unmatched by engineering technologies. [1][2][3][4][5][6][7][8][9] Such properties are based on the ability for the organism to utilize structural organics and acidic proteins to guide and control the mineralization process to yield hierarchical architectures with well-defined compositional gradients.One notable example is the highly developed raptorial appendage, or dactyl, of the stomatopods, a group of aggressive marine crustaceans that use these structures for feeding upon hard-shelled and soft-bodied prey. [10][11][12][13][14] The dactyls of the "smashers", those that feed primarily on hard-shelled prey, (see Figure 1A) takes the form of a bulbous club ( Figure 1B), which is used to smash through mollusk shells, crab exoskeletons, and other tough mineralized structures with tremendous force and speed. [11][12][13][14][15][16] Achieving accelerations over 10,000g and reaching speeds of 23 m/s from rest, the dactyl strike is recognized as one of the fastest and most powerful impacting events observed in Nature. [11,12] The club is capable of delivering and subsequently enduring repetitive impact forces up to 1500 N and cavitation stresses without catastrophically failing, demonstrating its utility as an exceptionally damage-tolerant natural material.The origins of such a mechanical response lie in the structural design. Previous work identified the club as a multi-regional composite material containing an organic matrix composed of alpha-chitin fibers mineralized by amorphous forms of calcium carbonate and calcium phosphate as well as crystalline apatite. [17,18] These investigations revealed mechanisms responsible for providing damage-tolerance and impact-resistance to the club, which were largely attributed to the interior of the club (periodic region), identified as the primary energy-absorbing layer. [17,18] The combination of soft polymeric nanofibers and stiffer mineral provides a periodic modulus mismatch leading to crack deflection, which in co...
Porous alumina membranes are commercially available and have been widely used in recent nanoscale research, for example, as templates in nanowire fabrication through electrodeposition. In this report, we present a new use for porous alumina membranes in the fabrication of alumina nanotubes/nanowires desired in electrochemical devices and catalytic applications. A high yield of alumina nanotubes/nanowires is obtained by etching porous alumina membranes in an aqueous sodium hydroxide solution. We studied the effects of etching time and solution concentration and characterized the alumina nanotubes/nanowires using a scanning electron microscope (SEM). A discussion of the possible mechanism for the formation of nanotubes/nanowires is given. Our results also imply that in nanowire fabrication through the template approach where alumina membranes are removed with sodium hydroxide solution to release the nanowires special attention is needed in characterizing the nanowires with the SEM because alumina nanotubes/nanowires can be easily mistaken for electrodeposited nanowires.
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