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

[신간의 별자리x] 우리/미술, 그리고 ‘슬픔의 박물관’

Soil amendment with biochar is evaluated globally as a means to improve soil fertility and to mitigate climate change. However, the effects of biochar on soil biota have received much less attention than its effects on soil chemical properties. A review of the literature reveals a significant number of early studies on biochar-type materials as soil amendments either for managing pathogens, as inoculant carriers or for manipulative experiments to sorb signaling compounds or toxins. However, no studies exist in the soil biologyliterature that recognize the observed largevariations ofbiochar physico-chemical properties. This shortcoming has hampered insight into mechanisms by which biochar influences soil microorganisms, fauna and plant roots. Additional factors limiting meaningful interpretation of many datasets are the clearly demonstrated sorption properties that interfere with standard extraction procedures for soil microbial biomass or enzyme assays, and the confounding effects of varying amounts of minerals. In most studies, microbial biomass has been found to increase as a result of biochar additions, with significant changes in microbial community composition and enzyme activities that may explain biogeochemical effects of biochar on element cycles, plant pathogens, and crop growth. Yet, very little is known about the mechanisms through which biochar affects microbial abundance and community composition. The effects of biochar on soil fauna are even less understood than its effects on microorganisms, apart from several notable studies on earthworms. It is clear, however, that sorption phenomena, pH and physical properties of biochars such as pore structure, surface area and mineral matter play important roles in determining how different biochars affect soil biota. Observations on microbial dynamics lead to the conclusion of a possible improved resource use due to co-location of various resources in and around biochars. Sorption and therebyinactivation of growth-inhibiting substances likelyplaysa rolefor increased abundance of soil biota. No evidence exists so far for direct negative effects of biochars on plant roots. Occasionally observed decreases in abundance of mycorrhizal fungi are likely caused by concomitant increases in nutrient availability,reducing theneedfor symbionts.Inthe shortterm,therelease ofavarietyoforganic molecules from fresh biochar may in some cases be responsible for increases or decreases in abundance and activity of soil biota. A road map for future biochar research must include a systematic appreciation of different biochar-types and basic manipulative experiments that unambiguously identify the interactions between biochar and soil biota.

http://www.css.cornell.edu/faculty/lehmann/pictures/publ/SoilBiolBiochem%2043,%201812–1836,%202011%20Lehmann.pdf

Biochar effects on soil biota – A review

Biochar as a sorbent for contaminant management in soil and water: a review.

Core/Shell Nanoparticles: Classes, Properties, Synthesis Mechanisms, Characterization, and Applications

TiSiN coatings were synthesised onto AISI M42 tool steel substrates via closed field unbalanced magnetron sputtering ion plating, using bias voltages of −40 and −50 V. The aim of this study is to investigate the underlying deformation mechanisms of TiSiN coatings, prepared at two different substrate bias voltages, following scratching, wear and indentation tests. A graded columnar microstructure evolved in these coatings. A hardness value of ~30.2 GPa was determined in the coating prepared at the lower bias voltage (i.e., −40 V), which was correlated to the fine nanocomposite structure and the presence of a high compressive residual stress. Of note, for the coating deposited at −40 V enhanced scratch adhesion strength, i.e., higher critical loads (Lc1 and Lc2) against cohesive and adhesive failure were determined with the higher H/Er and H3/Er2 values. The improved scratch resistance was ascribed to the hierarchical structure that hindered crack propagation in the TiSiN coatings during progressive loading. An approximately 21% decrease in wear rate was obtained at the lower bias voltage, which was attributed to the slightly lower Si concentration (~8.3 at.%) and, in turn, higher hardness. Deformation behaviour under indentation loading was dominated by shear sliding along the columnar grain boundaries.

Influence of substrate bias on the scratch, wear and indentation response of TiSiN nanocomposite coatings

The wear of as-cast eutectic Al–Si was studied using pin-on-disk tribotests in two different environments, air and dry argon. The counterface in all tests was yttria-stabilized zirconia. It was found that wear of the Al–Si was reduced by about 60% by the removal of oxygen from the test environment. The zirconia counterfaces showed measurable wear after tests performed in air, while there was very little wear of the zirconia for tests conducted under argon. The near-surface regions of the Al–Si pins were examined using a transmission electron microscope (TEM), using specimens produced by focussed ion beam milling. The specimens that had been worn in air were characterized by a near-surface mechanically mixed layer containing a considerable amount of both aluminum oxide and zirconium oxide—the aluminum oxide particles had evidently acted as abrasive agents to remove material from the zirconia counterface. In contrast, TEM analysis of the Al–Si tested in argon showed little zirconium oxide in the near-surface regions.

Dry sliding wear of eutectic Al-Si

Growth of NiO nanorods, SiC nanowires and monolayer graphene via a CVD method

Uniting Superhardness And Damage-tolerance In A Nanosandwich-structured Ti-B-N Coating

Medium entropy alloys (MEAs), such as CoCrNi, have been demonstrated to combine high hardness and excellent ductility, thereby outperforming many high entropy alloys reported to date. In this study, a multilayered CoCrNi/Ti coating was deposited onto a M2 steel substrate using a DC magnetron sputtering system. Columnar grains can be observed in both the CoCrNi and Ti layers. A high density of periodic twin boundaries, aligned in a direction normal to the growth direction, was also observed within the columnar CoCrNi grains. Moreover, different crystal structures were identified for different CoCrNi layers. The outermost CoCrNi layer exhibited a FCC structure, whilst in contrast, both the middle and bottom CoCrNi layers exhibited a BCC structure. It was assumed that Shockley partial dislocations were responsible for the FCC to BCC transition occurring in both the bottom and middle CoCrNi layers. A high hardness of ~7.6 GPa and elastic modulus of ~233 GPa were determined for this multilayered coating by nanoindentation testing. Further, extraordinary damage tolerance was found in the multilayered CoCrNi/Ti coating under indentation loading. The steady shear banding behaviour during deformation may benefit energy dissipation and promote structural plasticity.

Paul Munroe

Papers

Influence of substrate bias on the scratch, wear and indentation response of TiSiN nanocomposite coatings

Dry sliding wear of eutectic Al-Si

Growth of NiO nanorods, SiC nanowires and monolayer graphene via a CVD method

Uniting Superhardness And Damage-tolerance In A Nanosandwich-structured Ti-B-N Coating

Microstructure and mechanical properties of a multilayered CoCrNi/Ti coating with varying crystal structure