Alex V. Hamza

Surface-chemistry-driven actuation in nanoporous gold

Although actuation in biological systems is exclusively powered by chemical energy, this concept has not been realized in man-made actuator technologies, as these rely on generating heat or electricity first. Here, we demonstrate that surface-chemistry-driven actuation can be realized in high-surface-area materials such as nanoporous gold. For example, we achieve reversible strain amplitudes of the order of a few tenths of a per cent by alternating exposure of nanoporous Au to ozone and carbon monoxide. The effect can be explained by adsorbate-induced changes of the surface stress, and can be used to convert chemical energy directly into a mechanical response, thus opening the door to surface-chemistry-driven actuator and sensor technologies.

Point design targets, specifications, and requirements for the 2010 ignition campaign on the National Ignition Facility

Point design targets have been specified for the initial ignition campaign on the National Ignition Facility [G. H. Miller, E. I. Moses, and C. R. Wuest, Opt. Eng. 443, 2841 (2004)]. The targets contain D-T fusion fuel in an ablator of either CH with Ge doping, or Be with Cu. These shells are imploded in a U or Au hohlraum with a peak radiation temperature set between 270 and 300 eV. Considerations determining the point design include laser-plasma interactions, hydrodynamic instabilities, laser operations, and target fabrication. Simulations were used to evaluate choices, and to define requirements and specifications. Simulation techniques and their experimental validation are summarized. Simulations were used to estimate the sensitivity of target performance to uncertainties and variations in experimental conditions. A formalism is described that evaluates margin for ignition, summarized in a parameter the Ignition Threshold Factor (ITF). Uncertainty and shot-to-shot variability in ITF are evaluated, and...

Nanoporous Au: A high yield strength material

The plastic deformation of nanoporous Au under compressive stress was studied by depth-sensing nanoindentation combined with scanning electron microscope characterization. The nanoporous Au investigated in the current study exhibits a relative density of 42%, and a spongelike morphology of interconnecting ligaments on a length scale of ∼100nm. The material is polycrystalline with a grain size on the order of 10–60nm. Microstructural characterization of residual indentation impressions reveals a localized densification via ductile (plastic) deformation under compressive stress and demonstrates the ductile behavior of Au ligaments. A mean hardness of 145(±11)MPa and a Young’s modulus of 11.1(±0.9)GPa was obtained from the analysis of the load-displacement curves. The hardness of investigated np‐Au is ∼10 times higher than the hardness predicted by scaling laws of open-cell foams thus potentially opening a door to a class of high yield strength—low-density materials.

Ultralow Loading Pt Nanocatalysts Prepared by Atomic Layer Deposition on Carbon Aerogels

Using atomic layer deposition (ALD), we show that Pt nanoparticles can be deposited on the inner surfaces of carbon aerogels (CA). The resultant Pt-loaded materials exhibit high catalytic activity for the oxidation of CO even at loading levels as low as approximately 0.05 mg Pt/cm2. We observe a conversion efficiency of nearly 100% in the 150-250 degrees C temperatures range, and the total conversion rate seems to be limited only by the thermal stability of the CA support in ambient oxygen. The ALD approach described here is universal in nature, and can be applied to the design of new catalytic materials for a variety of applications, including fuel cells, hydrogen storage, pollution control, green chemistry, and liquid fuel production.

Mechanically robust and electrically conductive carbon nanotube foams

We describe the fabrication of ultralow-density carbon nanotube (CNT) foams that simultaneously exhibit high electrical conductivities and robust mechanical properties. Our approach utilizes carbon nanoparticles as a binder to crosslink randomly oriented bundles of single-walled CNTs. The resulting CNT foams are the stiffest low-density nanoporous solids reported and exhibit elastic behavior up to strains as large as ∼80%. The use of the carbon binder also allows bulk electrical conductivity to be maintained at low densities.

Symmetric Inertial Confinement Fusion Implosions at Ultra-High Laser Energies

Ignition Set to Go One aim of the National Ignition Facility is to implode a capsule containing a deuterium-tritium fuel mix and initiate a fusion reaction. With 192 intense laser beams focused into a centimeter-scale cavity, a major challenge has been to create a symmetric implosion and the necessary temperatures within the cavity for ignition to be realized (see the Perspective by Norreys). Glenzer et al. (p. 1228, published online 28 January) now show that these conditions can be met, paving the way for the next step of igniting a fuel-filled capsule. Furthermore, Li et al. (p. 1231, published online 28 January) show how charged particles can be used to characterize and measure the conditions within the imploding capsule. The high energies and temperature realized can also be used to model astrophysical and other extreme energy processes in a laboratory settings. Laser-driven temperatures and implosion symmetry are close to the requirements for inertial-fusion ignition. Indirect-drive hohlraum experiments at the National Ignition Facility have demonstrated symmetric capsule implosions at unprecedented laser drive energies of 0.7 megajoule. One hundred and ninety-two simultaneously fired laser beams heat ignition-emulate hohlraums to radiation temperatures of 3.3 million kelvin, compressing 1.8-millimeter-diameter capsules by the soft x-rays produced by the hohlraum. Self-generated plasma optics gratings on either end of the hohlraum tune the laser power distribution in the hohlraum, which produces a symmetric x-ray drive as inferred from the shape of the capsule self-emission. These experiments indicate that the conditions are suitable for compressing deuterium-tritium–filled capsules, with the goal of achieving burning fusion plasmas and energy gain in the laboratory.

Surface-enhanced Raman scattering on nanoporous Au

Colloidal solutions of metal nanoparticles are currently among the most studied substrates for sensors based on surface-enhanced Raman scattering (SERS). However, such substrates often suffer from not being cost-effective, reusable, or stable. Here, we develop nanoporous Au as a highly active, tunable, stable, biocompatible, and reusable SERS substrate. Nanoporous Au is prepared by a facile process of free corrosion of AgAu alloys followed by annealing. Results show that nanofoams with average pore widths of ∼250nm exhibit the largest SERS signal for 632.8nm excitation. This is attributed to the electromagnetic SERS enhancement mechanism with additional field localization within pores.

ALD Functionalized Nanoporous Gold: Thermal Stability, Mechanical Properties, and Catalytic Activity

Nanoporous metals have many technologically promising applications, but their tendency to coarsen limits their long-term stability and excludes high temperature applications. Here, we demonstrate that atomic layer deposition (ALD) can be used to stabilize and functionalize nanoporous metals. Specifically, we studied the effect of nanometer-thick alumina and titania ALD films on thermal stability, mechanical properties, and catalytic activity of nanoporous gold (np-Au). Our results demonstrate that even only 1 nm thick oxide films can stabilize the nanoscale morphology of np-Au up to 1,000°C, while simultaneously making the material stronger and stiffer. The catalytic activity of np-Au can be drastically increased by TiO(2) ALD coatings. Our results open the door to high-temperature sensor, actuator, and catalysis applications and functionalized electrodes for energy storage and harvesting applications.

Interaction of slow, very highly charged ions with surfaces

The present article reviews recent advances in the studies of the interaction of slow (v < vBohr), very highly charged ions (such as Xe 52+ and Au 69+ ) with surfaces of metals, semiconductors, and insulators (including biological materials). After a brief summary of past developments, we describe key experimental techniques for studies of secondary particle emission and the de-excitation dynamics of the highly charged ions. Recent progress in measurement and determination of the mechanisms leading to secondary electron yields, secondary ion yields and total sputtering yields will be discussed. The deexcitation dynamics are addressed in experiments on projectile neutralization and energy loss in thin films of material. We review the theoretical concepts briefly and introduce theoretical models in the discussion of experimental results. Following the presentation of fundamental studies we will address emerging applications of slow, very highly charged ions in surface analysis and surface modification. # 1999 Published by Elsevier Science Ltd. All rights reserved.

The experimental plan for cryogenic layered target implosions on the National Ignition Facility—The inertial confinement approach to fusion

Ignition requires precisely controlled, high convergence implosions to assemble a dense shell of deuterium-tritium (DT) fuel with ρR>∼1 g/cm2 surrounding a 10 keV hot spot with ρR ∼ 0.3 g/cm2. A working definition of ignition has been a yield of ∼1 MJ. At this yield the α-particle energy deposited in the fuel would have been ∼200 kJ, which is already ∼10 × more than the kinetic energy of a typical implosion. The National Ignition Campaign includes low yield implosions with dudded fuel layers to study and optimize the hydrodynamic assembly of the fuel in a diagnostics rich environment. The fuel is a mixture of tritium-hydrogen-deuterium (THD) with a density equivalent to DT. The fraction of D can be adjusted to control the neutron yield. Yields of ∼1014−15 14 MeV (primary) neutrons are adequate to diagnose the hot spot as well as the dense fuel properties via down scattering of the primary neutrons. X-ray imaging diagnostics can function in this low yield environment providing additional information about ...