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Abstract: Recently, efforts to optimize catalytic performance have included the utilization of catalytic nanoparticles with careful control over their shape and size, chemical composition, surface functionalization, and structural architecture. Here, direct measurements are reported detailing the impact of applied external bias on the exothermic catalytic formation of water for a Pt/TiO2 nanocomposite under exposure to gaseous oxyhydrogen environments. As external voltages of opposite polarity are applied to the system, the surface temperature kinetics, total pressure kinetics within the analytical chamber, and water turnover frequency transition in a reproducible manner between two well‐defined states which result in 1) decreased catalytic activity, 2) decreased initial molecular adsorption onto the Pt catalyst, and 3) lower rates of water production under negative voltage. These findings pave the road toward the realization of a catalytic switch that can be activated by applying a voltage to Schottky barrier systems to enhance or mitigate catalytic surface reactions. [ABSTRACT FROM AUTHOR]

This work provides a discussion of bistability conditions, switching autowave properties and emergence of dissipative structures in semiconducting fibers with anomalous positive dependence of electrical resistivity on temperature of sigmoid type, (1 + e −T )−1. An open system thermodynamics approach is utilized for the analysis of this dissipative solid-state system. The approach aims to represent the structure of the solution space of its governing equation in the form of physical phase diagrams, known as non-equilibrium phase diagrams, and two specific binary diagrams have been obtained here. One of the diagrams, where the electrical power density and ambient temperature represent external parameters, shows a wide region with dissipative structures as non-uniform steady-state temperature profiles on the fiber. The possibility of efficient external control over the dissipative structure geometry is also demonstrated. [ABSTRACT FROM PUBLISHER]

Coupled multiscale approaches for the analysis and simulation of complex multiphysics phenomena in solids have been of great interest during the past decade. These include concurrent MD/FEM models, atomistic-to-continua homogenization techniques and deterministic multiple time scale approaches. In this article we discuss a generic approach to concurrently couple the Monte-Carlo master equation of microscopic kinetic processes, not accessible by the direct particle dynamics, with the continuum mechanics formulation. This approach can be adequate for the modeling and validation of advanced contemporary materials with dynamic internal structure, as well as evolutionary and degradation processes in materials. The approach is illustrated in the application to the models of interstitial and vacancy diffusion in fuel cell catalysts and binary material systems. [ABSTRACT FROM AUTHOR]

In this paper, we present all necessary generalisations to extend the bridging scale, a finite-temperature multiple scale method which couples molecular dynamics (MD) and finite element (FE) simulations, to two dimensions. The crucial development is a numerical treatment of the boundary condition acting upon the reduced atomistic system, as such boundary conditions are analytically intractable beyond simple one-dimension systems. The approach presented in this paper offers distinct advantages compared to previous works, specifically the compact size of the resulting time history kernel, and the fact that the time history kernel can be calculated using an automated numerical procedure for arbitrary multi-dimensional lattice structures and interatomic potentials. We demonstrate the truly two-way nature of the coupled FE and reduced MD equations of motion via two example problems, wave propagation and dynamic crack propagation. Finally, we compare both problems to benchmark full MD simulations to validate the accuracy and efficiency of the proposed method. [ABSTRACT FROM AUTHOR]

We present a method for deriving molecular dynamics boundary conditions for use in multiple scale simulations that can be applied at a planar boundary for any solid that has a periodically repeating crystal lattice. The method is based on a linearization in the vicinity of the boundary, and utilizes a Fourier and Laplace transforms in space and time to eliminate the degrees of freedom associated with atoms outside the boundary. This method is straightforward to implement numerically, and thus can be automated for a general crystal lattice. We show that this method reproduces the known kernel for a 1D linear chain, and apply the approach to obtain the damping kernel matrices for two real crystal lattices: the graphene and diamond structures of carbon. [Copyright &y& Elsevier]

We present a simple method for calculating a continuum temperature field directly from a molecular dynamics (MD) simulation. Using the idea of a projection matrix previously developed for use in the bridging scale, we derive a continuum temperature equation which only requires information that is readily available from MD simulations, namely the MD velocity, atomic masses and Boltzmann constant. As a result, the equation is valid for usage in any coupled finite element (FE)/MD simulation. In order to solve the temperature equation in the continuum where an MD solution is generally unavailable, a method is utilized in which the MD velocities are found at arbitrary coarse scale points by means of an evolution function. The evolution function is derived in closed form for a 1D lattice, and effectively describes the temporal and spatial evolution of the atomic lattice dynamics. It provides an accurate atomistic description of the kinetic energy dissipation in simulations, and its behavior depends solely on the atomic lattice geometry and the form of the MD potential. After validating the accuracy of the evolution function to calculate the MD variables in the coarse scale, two 1D examples are shown, and the temperature equation is shown to give good agreement to MD simulations. [Copyright &y& Elsevier]

Negative thermal expansion is an interesting and appealing phenomenon for various scientific and engineering applications, while rarely occurring in natural materials. Here, using a universal antichiral metamaterial model with bimetal beams or strips, a generic theory has been developed to predict magnitude of the negative thermal expansion effect from model parameters. Thermal expansivity of the metamaterial is written as an explicit function of temperature and only three design parameters: relative node size, chirality angle, and a bimetal constant. Experimental measurements follow theoretical predictions well, where thermal expansivity in the range of negative 0.0006–0.0041 °C−1 has been seen. [ABSTRACT FROM AUTHOR]

This paper discusses an x-braced metamaterial lattice with the unusual property of exhibiting bandgaps in their deformation decay spectrum, and, hence, the capacity for reprogramming deformation patterns. The design of polarizing non-local lattice arising from the scenario of repeated zero eigenvalues of a system transfer matrix is also introduced. We develop a single mode fundamental solution for lattices with multiple degrees of freedom per node in the form of static Raleigh waves. These waves can be blocked at the material boundary when the solution is constructed with the polarization vectors of the bandgap. This single mode solution is used as a basis to build analytical displacement solutions for any applied essential and natural boundary condition. Subsequently, we address the bandgap design, leading to a comprehensive approach for predicting deformation pattern behavior within the interior of an x-braced plane lattice. Overall, we show that the stiffness parameter and unit-cell aspect ratio of the x-braced lattice can be tuned to completely block or filter static boundary deformations, and to reverse the dependence of deformation or strain energy decay parameter on the Raleigh wavenumber, a behavior known as the reverse Saint Venant's edge effect (RSV). These findings could guide future research in engineering smart materials and structures with interesting functionalities, such as load pattern recognition, strain energy redistribution, and stress alleviation. [ABSTRACT FROM AUTHOR]

A negative extensibility material structure pulls back and contracts when the external tensile load reaches a certain critical level. In this paper, we reveal basic mathematical features of the nonlinear strain energy function responsible for this unusual mechanical property. A systematic discussion leads to a comprehensive phase diagram in terms of design parameters for a simple unit cell structure that provides a panoramic view of all possible nonlinear mechanical behaviors. A negative extensibility region clearly is identified in the diagram. The sought property is seen to be rare, occurring only for a very narrow range of the design parameters. Nonetheless, due to the simplicity of the studied structure we suggest that the negative extensibility should be a more common phenomenon than previously thought. It can appear in simple bistable cells made of only several linearly elastic links, although at some peculiar combinations of their properties. These bistable unit cells can be used to design periodic mechanical metamaterials whose examples are shown as well as innovative architectural metastructures. [ABSTRACT FROM AUTHOR]

Studies of chemically induced hot electron flow over Schottky barriers in catalytic planar nanostructures provide a direct insight into underlying charge transfer processes involved in chemical energy dissipation at solid surfaces. A systematic approach is described here to separate the hot electron and thermal current contributions to the total generated current based on in-situ resistive heating of cathode nanolayer of the Schottky structure. The method is applicable at high pressures in the gas phase. Analysis of the current induced by H2 oxidation to H2O on Pt/n-GaP nanostructure is performed for surface temperatures in the range of 453-513 K, and 120 Torr oxyhydrogen environment with 15 Torr H2. All the current components grow monotonously with temperature, while relative fraction of the hot electron current decreases with temperature from 85 to 52%. [ABSTRACT FROM AUTHOR]

The possible origins and the magnitude of an electric current arising in nanofilm metal-semiconductor heterostructures when the metal surface is used to catalyze an exothermic chemical reaction are discussed. Two key mechanisms are considered that are responsible for the current generation: electron motion due to a temperature drop across the metal-semiconductor interface (thermionic emission mechanism) and the Seebeck effect in the two layers (thermoelectric mechanism). It is predicted that (i) current up to 10-3 A·cm-2 can arise, (ii) thermoelectric mechanism due to the Seebeck effect in the semiconductor layer plays a dominant role for the current generated under stationary chemical reaction conditions, and (iii) thermoelectric current strongly depends on the temperature. The carrier transport through the metal-semiconductor interface is described by the thermionic emission theory. The obtained results are discussed in view of the experimental data reported earlier for the chemical reaction-induced currents in metal/n-Si structures. [ABSTRACT FROM AUTHOR]

Abstract: Inspired by the pioneering work of Professor T.J.R. Hughes on the variational multi-scale method, this document summarizes recent developments in multiple-scale modeling using a newly developed technique called the bridging scale. The bridging scale consists of a two-scale decomposition in which the coarse scale is simulated using continuum methods, while the fine scale is simulated using atomistic approaches. The bridging scale offers unique advantages in that the coarse and fine scales evolve on separate time scales, while the high frequency waves emitted from the fine scale are eliminated using lattice impedance techniques. Recent advances in extending the bridging scale to quantum mechanical/continuum coupling are briefly described. The method capabilities are demonstrated via quasistatic nanotube bending, dynamic crack propagation and dynamic shear banding. [Copyright &y& Elsevier]

Here we report on direct evidence of a correlation between hydrogen-to-water oxidation on mesoporous Pt/TiO 2 nanocomposites at room temperature and the conversion of surface-released chemical energy into a stationary electrical current. The Pt phase of this heterojunction device is an electrically continuous 15 nm thick mesh deposited onto a mesoporous TiO 2 substrate fabricated with a plasma electrolytic oxidation process. The H 2 O turnover frequency approaches an asymptotic value associated with the saturation of the Pt/TiO 2 interface as the concentration of hydrogen gas is increased. In situ measurements of the reaction-induced current concurrently with mass spectrometry measurements illuminate the polarity switch of the reaction current (from thermionic emission to a reverse steady-state flow) simultaneously with the production of water. Furthermore, a concentration-dependent value of 5 min is measured as the time constant for the adsorption of the initial addition of H 2 and H 2 O formation and desorption.

There have been many works analyzing thermionic currents and chemicurrents generated on various electrolyte-free metal/semiconductor nanostructures. More recently, the chemicurrent phenomenon was reported for mesoporous Pt/semiconductor systems adept at converting surface-released chemical energy into a stationary electrical signal at room-temperature conditions. The present work points out the existence of an entire class of such surface-driven functional nanosystems. Here, the reaction current generation of Pt/ZrO2 systems was studied at room temperature under exposure to oxyhydrogen environments for mesoporous zirconia; this nanostructure was capable of the continuous oxidation of hydrogen, producing a long-standing stationary current. Synthesis parameters during the anodization process were manipulated to control sample pore density and the average pore diameter. Deposited via wide-angle PVD sputtering, the Pt phase forms an electrically continuous topographical nanomesh layer, and thus the Pt/ZrO2/gas interface is regulated through the manipulation of zirconia porosity. We observed reaction current enhancements with increasing porosity due to the lengthening of the Pt/ZrO2 interface. The most porous sample was significantly more sensitive to initial hydrogen additions, pointing toward the spillover of positive ionic charge across the Pt/ZrO2 interface as the origin of the observed electromotive force.