Why nanoprojectiles work differently than macroimpactors: The role of plastic flow

Atomistic simulation data on crater formation due to the hypervelocity impact of nanoprojectiles of up to 55 nm diameter and with targets containing up to 10^10 atoms are compared to available experimental data on micron-, mm-, and cm-sized projectiles. We show that previous scaling laws do not hold in the nanoregime and outline the reasons: within our simulations we observe that the cratering mechanism changes, going from the smallest to the largest simulated scales, from an evaporative regime to a regime where melt and plastic flow dominate, as is expected in larger microscale experiments. The importance of the strain-rate dependence of strength and of dislocation production and motion are discussed.Fil: Anders, Christian. Universitat Kaiserslautern; AlemaniaFil: Bringa, Eduardo Marcial. Universidad Nacional de Cuyo. Facultad de Ciencias Exactas y Naturales; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mendoza; ArgentinaFil: Ziegenhain, Gerolf. Universitat Kaiserslautern; AlemaniaFil: Graham, Giles A.. Natural History Museum; Reino UnidoFil: Hansen, J. Freddy. Lawrence Livermore National Laboratory; Estados UnidosFil: Park, Nigel. No especifíca;Fil: Teslich, Nick E.. Lawrence Livermore National Laboratory; Estados UnidosFil: Urbassek, Herbert M.. Universitat Kaiserslautern; Alemani

Observation of Fundamental Mechanisms in Compression-Induced Phase Transformations Using Ultrafast X-ray Diffraction

As theoretically hypothesized for several decades in group IV transition metals, we have discovered a dynamically stabilized body-centered cubic (bcc) intermediate state in Zr under uniaxial loading at sub-nanosecond timescales. Under ultrafast shock wave compression, rather than the transformation from alpha-Zr to the more disordered hex-3 equilibrium omega-Zr phase, in its place we find the formation of a previously unobserved nonequilibrium bcc metastable intermediate. We probe the compression-induced phase transition pathway in zirconium using time-resolved sub-picosecond x-ray diffraction analysis at the Linac Coherent Light Source. We also present molecular dynamics simulations using a potential derived from first-principles methods which independently predict this intermediate phase under ultrafast shock conditions. In contrast with experiments on longer timescale (> 10 ns) where the phase diagram alone is an adequate predictor of the crystalline structure of a material, our recent study highlights the importance of metastability and time dependence in the kinetics of phase transformations

Bonding Low-density Nanoporous Metal Foams Using Sputtered Solder Bonding Low-density Nanoporous Metal Foams Using Sputtered Solder

INTRODUCTION A method has been developed for bonding low-density nanoporous metal foam components to a substrate using solder that is sputtered onto the surfaces. Metal foams have unusual properties that make them excellent choices for many applications, and as technologies for processing these materials are evolving, their use in industry is increasing dramatically. Metal foams are lightweight and have advantageous dynamic properties, which make them excellent choices for many structural applications. There are many applications that require micro-scale metal foams precisely bonded to substrates. This study was motivated by a physics experiment that used a laser to drive a shock wave through an aluminum foil and into a copper foam, in order to determine the speed of the shock in the copper foam. To avoid disturbing the shock, the interface between the copper foam and the aluminum substrate had to be as thin as possible. There are many other applications that could benefit from the bonding technology developed in this study, such as small-scale lightweight structural members, high-strength thermal insulating layers for electronics, and micro-scale mechanical dampers, to name but a few. Each of these applications requires one or more small metal foam components precisely bonded to a substrate. Several methods for bonding metal foam components have been developed by previous researchers. [5] Macroscopic metal foam parts have been successfully bonded by laser welding to create T-sections and butt joints. [6] Ultrasonic welding has been used to join aluminum sheet metal to aluminum foam for structural applications

Onset of a Two-Dimensional Superconducting Phase in a Topological-Insulator–Normal-Metal Bi1-xSbx/Pt Junction Fabricated by Ion-Beam Techniques

Inducing superconductivity in a topological insulator can lead to novel quantum effects. However, experimental approaches to turn a topological insulator into a superconductor are limited. Here, we report on superconductivity in topological insulator Bi_{0.91}Sb_{0.09} induced via focused ion-beam deposition of a Pt thin film. The superconducting phase exhibits a Berezinski-Kosterlitz-Thouless transition, demonstrative of its two-dimensional character. From the in-plane upper critical field measurements, we estimate the superconducting thickness to be ∼17  nm for a 5.5-μm-thick sample. Our results provide evidence that the interface superconductivity could originate from the surface states of Bi_{0.91}Sb_{0.09}

Photoacoustically Measured Speeds of Sound of Liquid HBO₂: Semi-Empirical Modeling of Boron-Containing Explosives

Elucidation of geodynamic, geochemical, and shock-induced processes is often limited by challenges to determine accurately molecular fluid equations of state (EOS). High-pressure liquid-state reactions of carbon species underlie physicochemical mechanisms such as differentiation of planetary interiors, deep carbon sequestration, propellant deflagration, and shock chemistry. Here we introduce a versatile photoacoustic technique developed to measure precise adiabatic speeds of sound of high-pressure molecular fluids and fluid mixtures. Metaboric acid, HBO₂ speeds of sound are measured up to 0.5 GPa along the 277 °C isotherm. A polarized Exponential-6 interatomic potential form, parametrized using our data, enables EOS determinations and corresponding semiempirical evaluations of >2000 °C thermodynamic states including energy release from bororganic formulations. Our thermochemical model predictions of boronated hydrocarbon shock Hugoniot states differ from experiment by <3%

The Role of Pore Geometry in Single Nanoparticle Detection

We observe single nanoparticle translocation events <i>via</i> resistive pulse sensing using silicon nitride pores described by a range of lengths and diameters. Pores are prepared by focused ion beam milling in 50 nm-, 100 nm-, and 500 nm-thick silicon nitride membranes with diameters fabricated to accommodate spherical silica nanoparticles with sizes chosen to mimic that of virus particles. In this manner, we are able to characterize the role of pore geometry in three key components of the detection scheme, namely, event magnitude, event duration, and event frequency. We find that the electric field created by the applied voltage and the pore’s geometry is a critical factor. We develop approximations to describe this field, which are verified with computer simulations, and interactions between particles and this field. In so doing, we formulate what we believe to be the first approximation for the magnitude of ionic current blockage that explicitly addresses the invariance of access resistance of solid-state pores during particle translocation. These approximations also provide a suitable foundation for estimating the zeta potential of the particles and/or pore surface when studied in conjunction with event durations. We also verify that translocation achieved by electro-osmostic transport is an effective means of slowing translocation velocities of highly charged particles without compromising particle capture rate as compared to more traditional approaches based on electrophoretic transport