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

다중혈관 관상동맥 환자에서 y-문합을 이용하여 양쪽 내흉동맥만을 사용한 우회술의 조기 성적

Deposits of clastic carbonate-dominated (calciclastic) sedimentary slope systems in the rock record have been identified mostly as linearly-consistent carbonate apron deposits, even though most ancient clastic carbonate slope deposits fit the submarine fan systems better. Calciclastic submarine fans are consequently rarely described and are poorly understood. Subsequently, very little is known especially in mud-dominated calciclastic submarine fan systems. Presented in this study are a sedimentological core and petrographic characterisation of samples from eleven boreholes from the Lower Carboniferous of Bowland Basin (Northwest England) that reveals a >250 m thick calciturbidite complex deposited in a calciclastic submarine fan setting. Seven facies are recognised from core and thin section characterisation and are grouped into three carbonate turbidite sequences. They include: 1) Calciturbidites, comprising mostly of highto low-density, wavy-laminated bioclast-rich facies; 2) low-density densite mudstones which are characterised by planar laminated and unlaminated muddominated facies; and 3) Calcidebrites which are muddy or hyper-concentrated debrisflow deposits occurring as poorly-sorted, chaotic, mud-supported floatstones. These

Phd by thesis

The physical and chemical properties of nanophase materials rely on their crystal and surface structures. Transmission electron microscopy (TEM) is a powerful and unique technique for structure characterization. The most important application of TEM is the atomic-resolution real-space imaging of nanoparticles. This article introduces the fundamentals of TEM and its applications in structural determination of shape-controlled nanocrystals and their assemblies. By forming a nanometer size electron probe, TEM is unique in identifying and quantifying the chemical and electronic structure of individual nanocrystals. Electron energy-loss spectroscopy analysis of the solid-state effects and mapping the valence states are even more attractive. In situ TEM is demonstrated for characterizing and measuring the thermodynamic, electric, and mechanical properties of individual nanostructures, from which the structure−property relationship can be registered with a specific nanoparticle/structure.

Transmission Electron Microscopy of Shape-Controlled Nanocrystals and Their Assemblies

Alloying has long been used to confer desirable properties to materials. Typically, it involves the addition of relatively small amounts of secondary elements to a primary element. For the past decade and a half, however, a new alloying strategy that involves the combination of multiple principal elements in high concentrations to create new materials called high-entropy alloys has been in vogue. The multi-dimensional compositional space that can be tackled with this approach is practically limitless, and only tiny regions have been investigated so far. Nevertheless, a few high-entropy alloys have already been shown to possess exceptional properties, exceeding those of conventional alloys, and other outstanding high-entropy alloys are likely to be discovered in the future. Here, we review recent progress in understanding the salient features of high-entropy alloys. Model alloys whose behaviour has been carefully investigated are highlighted and their fundamental properties and underlying elementary mechanisms discussed. We also address the vast compositional space that remains to be explored and outline fruitful ways to identify regions within this space where high-entropy alloys with potentially interesting properties may be lurking. High-entropy alloys have greatly expanded the compositional space for alloy design. In this Review, the authors discuss model high-entropy alloys with interesting properties, the physical mechanisms responsible for their behaviour and fruitful ways to probe and discover new materials in the vast compositional space that remains to be explored.

High-entropy alloys

Graphite paradox in Baikal geyserite paleovalley, Russia

Climate change and scarcity of natural resources demand sustainable solutions to supply our worldwide growing need for electric energy. One possible contribution to satisfy this need is fuel cells, as they offer all qualifications required of a suitable replacement for currently applied energy sources: they are fed by easily accessible fuels like hydrogen or methanol, emit no climate-changing gases and the produced energy is retrieved on a demand based level. This PhD work contributes to this promising technology by studying high-temperature polymer-electrolyte-membrane fuel cells (HTPEMFC) that can supply households with energy and heat. The commonly used electrode material of HT-PEMFCs, high-surface-area carbon decorated with catalytic platinum (Pt) nanoparticles, suffers degradation during fuel cell operation. In this work, carbon is replaced by tungsten suboxide (WO3-x) and Pt by a Pt/ruthenium (Pt/Ru) alloy. The individual components are studied by electron microscopy in the as-prepared state and partly after various HT-PEMFC operation times. This study found that the growth behavior and distribution of Pt on WO3-x grains differ markedly from Pt on high-surface-area carbon. Instead of nanoparticles, the catalyst forms micrometer sized, highly porous, octahedral-shaped networks on the WO3-x support. These networks are developed during the applied 2-step synthesis route. Their growth mechanism was revealed by in-depth transmission electron microscopy (TEM) studies, including elemental distribution maps and electron tomography. The performance and degradation of the Pt/WO3-x system as a catalyst layer for the anode side of HT-PEMFCs was studied in detail in an earlier work. The second investigated material system is a high-surface-area carbon supported Pt/Ru catalyst alloy that shows an improved carbon monoxide tolerance compared to Pt. Crystallographic analysis revealed that the Pt/Ru alloy forms a face centered cubic lattice (like Pt) rather than a hexagonal close packed lattice (like Ru) despite having locally high Ru contents exceeding the solubility limit. The degradation mechanisms during the dynamic fuel cell operation mode were investigated using cyclic voltammetry measurements combined with identical location scanning TEM (IL-STEM) analysis and electron tomography, where the same area was studied after several thousands of potential cycles. Dissolution was identified as the main degradation mechanism and the occurrence of agglomeration could also be demonstrated, while Ostwald ripening has minor effects. Finally, the degradation mechanisms occurring in real HT-PEMFC stacks with the Pt/Ru loaded highsurface-area carbon as anode were studied. The long-term performance and the degradation behavior of this system were tested under constant current densities at an operation temperature of 160°C. Detailed TEM analysis revealed an operation related redistribution of a small portion of both the anodic Pt/Ru and the cathodic Pt catalyst. Nanoparticles emerged in the whole membrane area after fuel cell operation, and they differed in size and elemental composition, depending on the distance to the electrodes. Additionally, a band of nanoparticles was present in the membrane close to the cathode side, whose position is related to the crossover rates of hydrogen and oxygen. This thesis demonstrates that electron microscopy is a useful method to get insights in the microstructure and composition of new fuel cell materials and to discover microand nanometer changes and degradation mechanisms. Knowledge of these are prerequisite to continue with targeted optimization of fuel cell components.

Investigation of alternative catalyst and support materials and their effect on degradation in high-temperature polymer-electrolyte-membrane fuel cells

Microstructural analysis of germanium modified tin-copper brazing filler metals for transient liquid phase bonding of aluminium

Stainless steels like X210Cr12 consist of metastable austenite that transforms into martensite when a critical temperature or critical strain is reached. However, the mechanism of martensite formation and the shape and appearance of martensite can differ significantly depending on the used mechanical or thermal pathway. This necessitates a systematic study on the martensitic transformation and twinning of metastable austenite in X210Cr12 after liquid nitrogen cooling, deep rolling and their sequential combinations. The thereby obtained surface modifications were characterized by hardness penetration measurements, electron backscatter diffraction (EBSD), transmission electron microscopy (TEM) and orientation and phase mapping using automated scanning electron nano-diffraction (SEND). X-Ray diffraction (XRD) was performed to measure residual stresses in the surface near regions of the samples. The here obtained results show that liquid nitrogen cooling yields formation of coarse grained martensite with grain sizes between 1 and 10 μm and deep rolling causes formation of a nanosized microstructure containing martensite and twins. Moreover, the here obtained results suggest that compressive residual stresses of up to −750 MPa together with the martensite/twin nanostructure stabilizes metastable austenite and prevents the formation of large martensite grains in the surface zone during subsequent liquid nitrogen cooling. A substantial hardness increase ranging from 355 HV0.1 up to about 850 HV0.1 was measured for all treatments. The highest value of about 950 HV0.1 was measured after martensite transformation induced by liquid nitrogen cooling.

Controlled twinning and martensitic transformation in metastable AISI D3 (X210Cr12) steel by sequential deep rolling and liquid nitrogen cooling

This paper investigates the manipulation of chemical composition of a laser weld by dissimilar filler material and its effect on residual stress. The aim is to minimize residual stresses in the weld seam. In order to negate residual stresses, dissimilar combinations of low-carbon manganese steel (S235JR) base material with high-alloyed solid filler wires (G19 9 and G25 20), as well as similar combinations with low-alloyed solid filler wire G3Si1 are analyzed. The goal of the paper is to show that the so-called low-transformation-temperature effect can be used to induce residual compressive stresses in a weld without the use of specially manufactured filler wires. Chemical compositions are generated within a laser-beam-welding process by means of dilution, proving that the concept of in situ alloying is usable in order to affect the martensite formation on a weld. Dilatometry measurements show that a varying Cr and Ni content in a weld reduces the phase-transformation temperature and increases dilatation. EBSD analysis indicates that a fully martensitic weld with a negligible amount of retained austenite is created while the base material preserves its ferritic-pearlitic microstructure. Residual stress measurements with the hole-drilling method demonstrate a reduction in longitudinal tensile residual stresses, whereby the magnitude of the induced residual compressive stresses depend on the Ms temperature. As a result of this research, it was proven that a reduction in tensile residual stress by means of targeted alloying with conventional materials in low-carbon manganese steel is possible. Under the experimental conditions, residual stress in the weld seam could be reduced to 0 MPa. In some cases, even compressive residual stress in the weld could be achieved.

/pdf/residual-stress-reduction-with-the-ltt-effect-in-low-carbon-1jg6006o.pdf

Residual Stress Reduction with the LTT Effect in Low Carbon Manganese-Steel through Chemical Composition Manipulation Using Dissimilar Filler Material in Laser Beam Welding

Joachim Mayer

Papers

Graphite paradox in Baikal geyserite paleovalley, Russia

Investigation of alternative catalyst and support materials and their effect on degradation in high-temperature polymer-electrolyte-membrane fuel cells

Microstructural analysis of germanium modified tin-copper brazing filler metals for transient liquid phase bonding of aluminium

Controlled twinning and martensitic transformation in metastable AISI D3 (X210Cr12) steel by sequential deep rolling and liquid nitrogen cooling

Residual Stress Reduction with the LTT Effect in Low Carbon Manganese-Steel through Chemical Composition Manipulation Using Dissimilar Filler Material in Laser Beam Welding