Three parallel algorithms for classical molecular dynamics are presented. The first assigns each processor a fixed subset of atoms; the second assigns each a fixed subset of inter-atomic forces to compute; the third assigns each a fixed spatial region. The algorithms are suitable for molecular dynamics models which can be difficult to parallelize efficiently—those with short-range forces where the neighbors of each atom change rapidly. They can be implemented on any distributed-memory parallel machine which allows for message-passing of data between independently executing processors. The algorithms are tested on a standard Lennard-Jones benchmark problem for system sizes ranging from 500 to 100,000,000 atoms on several parallel supercomputers--the nCUBE 2, Intel iPSC/860 and Paragon, and Cray T3D. Comparing the results to the fastest reported vectorized Cray Y-MP and C90 algorithm shows that the current generation of parallel machines is competitive with conventional vector supercomputers even for small problems. For large problems, the spatial algorithm achieves parallel efficiencies of 90% and a 1840-node Intel Paragon performs up to 165 faster than a single Cray C9O processor. Trade-offs between the three algorithms and guidelines for adapting them to more complex molecular dynamics simulations are also discussed.

Fast parallel algorithms for short-range molecular dynamics

Semiconductor-based photocatalysis is considered to be an attractive way for solving the worldwide energy shortage and environmental pollution issues. Since the pioneering work in 2009 on graphitic carbon nitride (g-C3N4) for visible-light photocatalytic water splitting, g-C3N4 -based photocatalysis has become a very hot research topic. This review summarizes the recent progress regarding the design and preparation of g-C3N4 -based photocatalysts, including the fabrication and nanostructure design of pristine g-C3N4 , bandgap engineering through atomic-level doping and molecular-level modification, and the preparation of g-C3N4 -based semiconductor composites. Also, the photo-catalytic applications of g-C3N4 -based photocatalysts in the fields of water splitting, CO2 reduction, pollutant degradation, organic syntheses, and bacterial disinfection are reviewed, with emphasis on photocatalysis promoted by carbon materials, non-noble-metal cocatalysts, and Z-scheme heterojunctions. Finally, the concluding remarks are presented and some perspectives regarding the future development of g-C3N4 -based photocatalysts are highlighted.

Polymeric Photocatalysts Based on Graphitic Carbon Nitride

Structure and nanostructure in ionic liquids.

In the laser treatment of a workpiece (9), e.g. for surface hardening, melting, alloying, cladding, welding or cutting, the adverse effect of reflectivity of the surface, when a pure, monochromatic laser (6) is used, is remedied by the simultaneous application of a relatively shorter wavelength beam (1). The two beams (1)(5) may be combined by a beam coupler (4) or may reach the workpiece (9) by separate optical paths (not shown). The shorter wavelength beam (1) improves the coupling efficiency of the higher- powered laser beam (5).

Laser Material Processing

The available literature on the crystal structure of the metastable alumina polymorphs and their associated transitions is critically reviewed and summarized. All the metastable alumina structures have been identified as ordered or partially ordered cation arrays on the interstitial sites of an approximately close-packed oxygen sublattice (either face-centered cubic or hexagonal close packed). The analysis of the symmetry relations between reported alumina polymorphs having an approximately face-centered cubic packing of the oxygen anions allows for an exact interpretation of all the complex domain structures that have been observed experimentally. Possible mechanisms for the phase transitions between the different alumina polymorphs also are discussed.

Metastable alumina polymorphs : Crystal structures and transition sequences

Preface to the Second Edition. Preface to the First Edition. 1. The Concept of Microstructure. 1.1. Microstructural Features. 1.2. Crystallography and Crystal Structure. 2. Diffraction Analysis of Crystal Structure. 2.1. Scattering of Radiation by Crystals. 2.2. Reciprocal Space. 2.3. X-ray Diffraction Methods. 2.4. Diffraction Analysis. 2.5. Electron Diffraction. 3. Optical Microscopy. 3.1. Geometrical Optics. 3.2. Construction of the Microscope. 3.3. Specimen Preparation. 3.4. Image contrast. 3.5. Working with Digital Images. 3.6. Resolution, contrast and Image Interpretation. 4. Transmission Electron Microscopy. 4.1. Basic Principles. 4.2. Specimen Preparation. 4.3. The Origin of Contrast. 4.4. Kinematic Interpretation of Diffraction Contrast. 4.5. Dynamic Diffraction and Absorption effects. 4.6. Lattice Imaging at High Resolution. 4.7. Scanning Transmission Electron Microscopy. 5. Scanning Electron Microscopy. 5.1. Components of The Scanning electron Microscope. 5.2. Electron Beam-Specimen Interactions. 5.3. Electron Excitation of X-Rays. 5.4. Backscattered Electrons. 5.5. Secondary Electron Emission. 5.6. Alternative Imaging Modes. 5.7. Specimen Preparation and Topology. 5.8. Focused Ion Beam Microscopy. 6. Microanalysis in Electron Microscopy. 6.1. X-Ray Microanalysis. 6.2. Electron Energy Loss Spectroscopy. 7. Scanning Probe Microscopy and Related Techniques. 7.1. Surface Forces and Surface Morphology. 7.2. Scanning Probe Microscopes. 7.3. Field-Ion Microscopy and Atom Probe tomography. 8. Chemical Analysis of Surface Composition. 8.1. X-ray Photoelectron Spectroscopy. 8.2. Auger Electron Spectroscopy. 8.3. Secondary-Ion Mass Spectrometry. 9. Quantitative and Tomographic Analysis of Microstructure. 9.1. Basic Stereological Concepts. 9.2. Accessible and Inaccessible Parameters. 9.3. Optimizing Accuracy. 9.4. Automated Image Analysis. 9.5. Tomography and Three-Dimensional Reconstruction. Appendices. Index.

Microstructural Characterization of Materials

Understanding the nature of solid-liquid interfaces is important for many processes of technological interest, such as solidification, liquid-phase epitaxial growth, wetting, liquid-phase joining, crystal growth, and lubrication. Recent studies have reported on indirect evidence of density fluctuations at solid-liquid interfaces on the basis of x-ray scattering methods that have been complemented by atomistic simulations. We provide evidence for ordering of liquid atoms adjacent to an interface with a crystal, based on real-time high-temperature observations of alumina-aluminum solid-liquid interfaces at the atomic-length scale. In addition, crystal growth of alumina into liquid aluminum, facilitated by interfacial transport of oxygen from the microscope column, was observed in situ with the use of high-resolution transmission electron microscopy.

https://science.sciencemag.org/content/sci/310/5748/661.full.pdf

Ordered Liquid Aluminum at the Interface with Sapphire

This paper reviews the fundamental concepts and the terminology of wetting. In particular, it focuses on high temperature wetting phenomena of primary interest to materials scientists. We have chosen to split this review into two sections: one related to macroscopic (continuum) definitions and the other to a microscopic (or atomistic) approach, where the role of chemistry and structure of interfaces and free surfaces on wetting phenomena are addressed. A great deal of attention has been placed on thermodynamics. This allows clarification of many important features, including the state of equilibrium between phases, the kinetics of equilibration, triple lines, hysteresis, adsorption (segregation) and the concept of complexions, intergranular films, prewetting, bulk phase transitions versus “interface transitions”, liquid versus solid wetting, and wetting versus dewetting.

/pdf/a-review-of-wetting-versus-adsorption-complexions-and-2rni10tbod.pdf

A review of wetting versus adsorption, complexions, and related phenomena: the rosetta stone of wetting

A comparative study of two different techniques for the application of wear-resistant coatings for contact surfaces of shroud shelves of gas turbine engine blades (GTE) has been conducted. Wear-resistant coatings were applied on In713 by laser cladding with direct injection of the cladding powder into the melt pool. Laser cladding was conducted with a TRUMPF-2500, CW-CO 2 laser. The laser cladding was compared with commercially available plasma cladding with wire. Both plasma and laser cladded zones were characterized by optical and scanning electron microscopy. It was found that the laser cladded zone has a higher microhardness value (650–820 HV) compared with that of the plasma treated material (420–440 HV). This is a result of the significant reduction in grain size in the case of laser cladding. Unlike the plasma cladded zones, the laser treated material is free of micropores and microcracks.

Laser cladding of turbine blades

In vapor-liquid-solid (VLS) growth, the liquid phase plays a pivotal role in mediating mass transport from the vapor source to the growth front of a nanowire. Such transport often takes place through the liquid phase. However, we observed by in situ transmission electron microscopy a different behavior for self-catalytic VLS growth of sapphire nanowires. The growth occurs in a layer-by-layer fashion and is accomplished by interfacial diffusion of oxygen through the ordered liquid aluminum atoms. Oscillatory growth and dissolution reactions at the top rim of the nanowires occur and supply the oxygen required to grow a new (0006) sapphire layer. A periodic modulation of the VLS triple-junction configuration accompanies these oscillatory reactions.

Wayne D. Kaplan

Papers

Microstructural Characterization of Materials

Ordered Liquid Aluminum at the Interface with Sapphire

A review of wetting versus adsorption, complexions, and related phenomena: the rosetta stone of wetting

Laser cladding of turbine blades

Oscillatory Mass Transport in Vapor-Liquid-Solid Growth of Sapphire Nanowires