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

[신간의 별자리x] 우리/미술, 그리고 ‘슬픔의 박물관’

Statistical method for testing the neutral mutation hypothesis by DNA polymorphism.

BACKGROUND Materials by design is an appealing idea that is very hard to realize in practice. Combining the best of different ingredients in one ultimate material is a task for which we currently have no general solution. However, we do have some successful examples to draw upon: Composite materials and III-V heterostructures have revolutionized many aspects of our lives. Still, we need a general strategy to solve the problem of mixing and matching crystals with different properties, creating combinations with predetermined attributes and functionalities. ADVANCES Two-dimensional (2D) materials offer a platform that allows creation of heterostructures with a variety of properties. One-atom-thick crystals now comprise a large family of these materials, collectively covering a very broad range of properties. The first material to be included was graphene, a zero-overlap semimetal. The family of 2D crystals has grown to includes metals (e.g., NbSe 2 ), semiconductors (e.g., MoS 2 ), and insulators [e.g., hexagonal boron nitride (hBN)]. Many of these materials are stable at ambient conditions, and we have come up with strategies for handling those that are not. Surprisingly, the properties of such 2D materials are often very different from those of their 3D counterparts. Furthermore, even the study of familiar phenomena (like superconductivity or ferromagnetism) in the 2D case, where there is no long-range order, raises many thought-provoking questions. A plethora of opportunities appear when we start to combine several 2D crystals in one vertical stack. Held together by van der Waals forces (the same forces that hold layered materials together), such heterostructures allow a far greater number of combinations than any traditional growth method. As the family of 2D crystals is expanding day by day, so too is the complexity of the heterostructures that could be created with atomic precision. When stacking different crystals together, the synergetic effects become very important. In the first-order approximation, charge redistribution might occur between the neighboring (and even more distant) crystals in the stack. Neighboring crystals can also induce structural changes in each other. Furthermore, such changes can be controlled by adjusting the relative orientation between the individual elements. Such heterostructures have already led to the observation of numerous exciting physical phenomena. Thus, spectrum reconstruction in graphene interacting with hBN allowed several groups to study the Hofstadter butterfly effect and topological currents in such a system. The possibility of positioning crystals in very close (but controlled) proximity to one another allows for the study of tunneling and drag effects. The use of semiconducting monolayers leads to the creation of optically active heterostructures. The extended range of functionalities of such heterostructures yields a range of possible applications. Now the highest-mobility graphene transistors are achieved by encapsulating graphene with hBN. Photovoltaic and light-emitting devices have been demonstrated by combining optically active semiconducting layers and graphene as transparent electrodes. OUTLOOK Currently, most 2D heterostructures are composed by direct stacking of individual monolayer flakes of different materials. Although this method allows ultimate flexibility, it is slow and cumbersome. Thus, techniques involving transfer of large-area crystals grown by chemical vapor deposition (CVD), direct growth of heterostructures by CVD or physical epitaxy, or one-step growth in solution are being developed. Currently, we are at the same level as we were with graphene 10 years ago: plenty of interesting science and unclear prospects for mass production. Given the fast progress of graphene technology over the past few years, we can expect similar advances in the production of the heterostructures, making the science and applications more achievable.

2D materials and van der Waals heterostructures

The invention provides a mounting and removing device for a core of a turbo expander. The core is mounted in a shell; the mounting and removing device comprises a supporting base fixed on the shell, a threaded cover fixed on the core, a protection barrel body arranged on the supporting base, and a screw rod which penetrates through the protection barrel body and is fixedly supported by the threaded cover, wherein under the action of the screw rod, the core is pulled into the protection barrel body from the shell or the core is pushed into the shell from the protection barrel body. According to the mounting and removing device, the radial mounting precision of the core of the turbo expander can be improved, and a standby core can be stored properly. The invention further provides a removing method and a mounting method for the core of the turbo expander.

Mounting and removing device and method for core of turbo expander

Liquid air energy storage (LAES) is a promising large-scale energy storage technology. A packed bed cryogenic regenerator was investigated for cold energy storage in the LAES system. As the thermophysical properties of the filling material directly affects the performance of the regenerator, sensitivity analyses of the specific heat capacity and thermal conductivity were carried out. Results show that the thickness of the thermocline in the packed bed can be reduced by increasing the specific heat capacity and decreasing the thermal conductivity. Furthermore, the synergy effect of the specific heat capacity and thermal conductivity on the system was considered. In the case of simultaneous changes of the two parameters, the selection method of the solid material was proposed innovatively, and the feasibility of the method was verified by theoretical calculation of various materials.

https://doi.org/10.1088/1757-899x/1240/1/012105

Study on the selection method of solid cold energy storage medium for liquid air energy storage

High heterogeneous compatibility of HfB2-SiC ceramic and Zr-4 alloy with in-situ assembled interface

The intensification of carbon emissions and the energy crisis have made the safe and reliable application of renewable energy in the energy supply system significantly urgent. Liquid air energy storage (LAES) system is an effective means to solve the time and space mismatch between energy supply and demand. The LAES has the advantages of no geographical restrictions, high energy storage density and flexible operation, making it available to integrate with other industrial processes. In traditional LAES, the heat generated in the compression process is used to supplement the heat in the expansion process, and the excess heat is dissipated. In this study, the compressor inlet air is cooled by absorption refrigeration cycle to avoid excess compression heat, which reduces the power consumption of the compressor and improves the efficiency of the system. The sensitivity analysis of the ambient temperature and the compressor stages on the system efficiency is performed. Furthermore, a comparative analysis between the absorption refrigeration cycle driven by the waste heat and the compression refrigeration cycle driven by electricity for air precooling of the compressor inlet was carried out, and the system round-trip efficiency, exergy efficiency and initial investment cost was selected as indicators for system evaluation.

Technical and economic evaluation of a liquid air energy storage system with air precooling for compressor inlet

A n-GaAsSb single waveguide layer semiconductor laser with an InP/In0.55Ga0.45As/AlGaAs asymmetrical barrier is designed in order to improve output power, which not only reduces optical loss in the p-region but also effectively suppresses carrier leakage. The results show that a GaAsSb single waveguide structure almost completely shifts the optical field to the n-region, which reduces the absorption of photons by holes. When the injected current is 1 A, the device’s optical loss decreases from 15.60 to 13.20 cm−1. Ensuring that carrier leakage and internal quantum efficiency are almost unaffected, the InP/In0.55Ga0.45As/AlGaAs asymmetric barrier makes optical loss further reduce. The power of the new-structure device is 0.74 W, and its wall-plug efficiency reaches 70.84%. This structure design will provide both experimental data and theoretical support for the growth of the epitaxial structure of InP-based 1550 nm semiconductor lasers.

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Wei Ji

Papers

Mounting and removing device and method for core of turbo expander

Study on the selection method of solid cold energy storage medium for liquid air energy storage

High heterogeneous compatibility of HfB2-SiC ceramic and Zr-4 alloy with in-situ assembled interface

Technical and economic evaluation of a liquid air energy storage system with air precooling for compressor inlet

InP/InGaAs/AlGaAs quantum-well semiconductor laser with an InP based 1550 nm n-GaAsSb single waveguide structure