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

To use solar irradiation or interior lighting efficiently, we sought a photocatalyst with high reactivity under visible light. Films and powders of TiO 2- x N x have revealed an improvement over titanium dioxide (TiO 2 ) under visible light (wavelength 2 has proven to be indispensable for band-gap narrowing and photocatalytic activity, as assessed by first-principles calculations and x-ray photoemission spectroscopy.

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Visible-Light Photocatalysis in Nitrogen-Doped Titanium Oxides

Titanium Dioxide Nanomaterials: Synthesis, Properties, Modifications, and Applications

This critical review shows the basis of photocatalytic water splitting and experimental points, and surveys heterogeneous photocatalyst materials for water splitting into H2 and O2, and H2 or O2 evolution from an aqueous solution containing a sacrificial reagent Many oxides consisting of metal cations with d0 and d10 configurations, metal (oxy)sulfide and metal (oxy)nitride photocatalysts have been reported, especially during the latest decade The fruitful photocatalyst library gives important information on factors affecting photocatalytic performances and design of new materials Photocatalytic water splitting and H2 evolution using abundant compounds as electron donors are expected to contribute to construction of a clean and simple system for solar hydrogen production, and a solution of global energy and environmental issues in the future (361 references)

Heterogeneous photocatalyst materials for water splitting

2.3. Evaluation of Photocatalytic Water Splitting 6507 2.3.1. Photocatalytic Activity 6507 2.3.2. Photocatalytic Stability 6507 3. UV-Active Photocatalysts for Water Splitting 6507 3.1. d0 Metal Oxide Photocatalyts 6507 3.1.1. Ti-, Zr-Based Oxides 6507 3.1.2. Nb-, Ta-Based Oxides 6514 3.1.3. W-, Mo-Based Oxides 6517 3.1.4. Other d0 Metal Oxides 6518 3.2. d10 Metal Oxide Photocatalyts 6518 3.3. f0 Metal Oxide Photocatalysts 6518 3.4. Nonoxide Photocatalysts 6518 4. Approaches to Modifying the Electronic Band Structure for Visible-Light Harvesting 6519

Semiconductor-based Photocatalytic Hydrogen Generation

The n-type thermoelectric material has a composition represented by (A a B b C c D t )Co 4-y Fe y Sb 12 . In the composition, 0≦a≦0.5, 0≦b≦0.7, 0 0; Element A is Mg, Ca, Sr and/or Ba; Element B is Y, Sc and/or La to Lu; Element C is Al, Ga and/or In; and Element D is Zn and/or Ti. The A a B b C c D t (=R x ) satisfies R x =[Ba d A′ 1-d ] a [Yb e B′ 1-e ] b [In f C′ 1-f ] c D t . In the formula, 0 0; Element A′ is the element A other than Ba; Element B′ is the element B other than Yb; and Element C′ is the element C other than In. The n-type thermoelectric material contains five or more kinds in total of the element A to the element D.

N-type thermoelectric material

We have performed the first-principles calculations on (Ca/sub 2/CoO/sub 3/)/sub 4/(CoO/sub 2/)/sub 6/ to understand electronic structures of the misfit-layered calcium cobaltite, (Ca/sub 2/CoO/sub 3/)/sub x/CoO/sub 2/, within the generalized gradient approximation. The optimized structure, consisting of a triple rock-salt-type Ca/sub 2/CoO/sub 3/ subsystem and a CdI/sub 2/-type CoO/sub 2/ subsystem in which their respective octahedra are significantly distorted, gives good agreement with recent experiment. The calculated electronic structures show two-dimensionally dispersive e/sub g/ bands across the Fermi energy, which yield a p-type conductivity in the rock-salt subsystem, while the Fermi energy lies in the crystal field gap of the d states in the CoO/sub 2/ subsystem. The calculated density of states is consistent with features of the measured X-ray photoemission spectra. The spin-polarized calculations show that ferrimagnetic ordering with a small net magnetic moment within the rock-salt subsystem is found to be the ground state, which gives a reasonable explanation to low-temperature behavior of the resistivity and the susceptibility observed in experiment.

Electronic structure of misfit-layered calcium cobaltite

We applied a pulsed laser deposition (PLD) technique to fabricate nanocomposite half-Heusler thermoelectrics by employing two different methods: a dry process and a wet process. First, we tried to obtain nanosized thermoelectric particles by using PLD in a liquid solvent. Nanosized (<100 nm) spherical and crystalline half-Heusler particles containing Ti, Zr, Hf, Ni, and Sn elements were obtained by this method, showing good controllability of stoichiometry. The key is to select a solvent that prevents oxidation. Second, the dry PLD process was employed to coat the thermoelectric powder with metal oxides. To this end, we developed a PLD coating apparatus. After sintering the coated powder using the spark plasma sintering (SPS) technique, we confirmed that a nanosized layer of the metal oxides was uniformly formed at the grain boundaries of the half-Heusler matrix. With these two examples, the capability of the PLD techniques to fabricate well-controlled nanocomposite thermoelectric materials is demonstrated.

Fabrication of Nanocomposite Thermoelectric Materials by a Pulsed Laser Deposition Method

Crystallographic orientation and phase identification on textured Ca-doped (ZnO)mIn2O3 (m=3 and 4, denoted as ZmIO) ceramics were investigated for better understanding and further improvement of the high-efficiency n-type thermoelectric oxide materials. Textured ceramics were prepared by the reactive-templated grain growth (RTGG) method, in which plate-like reactive seeds were exploited. A combination of electron backscatter diffraction pattern (EBSP) and energy dispersive X-ray spectrometry (EDS) techniques was utilized for analyzing the textured specimen parallel and perpendicular to the original sheet plane in detail. We revealed that in the specimen plate-like grains were aligned parallel to the sheet plane with the thickness direction parallel to the c-axis of each grain. Parallel aligned trigonal Z3IO domains with a 60° misorientation angle were observed in a single grain. Coexisting trigonal Z3IO and hexagonal Z4IO phases were also observed in a single grain. These axis-sharing crystallites must be formed by the in situ topotactic conversion from a common hexagonal template crystal.

Analysis of Crystallographic Orientation on Textured Ca-doped (ZnO)mIn2O3 Ceramics by Electron Back Scattering Diffraction Pattern

We propose a solar thermal cogeneration system using a cylindrical thermoelectric module for efficient solar energy convergence. Numerical simulations are presented to evaluate the system efficiency compared with a conventional pillar-type thermoelectric cogeneration system. We consider the effects of thermal radiation, contact resistances, and heat flux in the connecting wire, which significantly affect the system efficiency. Compared with the pillar-type device, the cylindrical device can achieve a higher heat flux and lower thermal radiation loss from the sides. In particular, the thermal radiation loss from the sides becomes negligible in a scaled-up cylindrical device. When the areas of the light-absorbing layer are the same in both devices, the power efficiencies, which are defined as power extracted from the module over input heat to the module, are comparable, but the system efficiency, which is defined as extracted heat from the module over input heat to the module, of the cylindrical device is higher than that of the pillar-type device. In the case of the unileg cylindrical device, where the hot side is connected to the cold side by the Cu wire, the system efficiency increased but the power efficiency decreased owing to the heat flux through the Cu wire. On the other hand, the p-n couple cylindrical device can overcome the trade-off and achieve system efficiency as high as 91.6%, including 8.1% power efficiency.

Ryoji Asahi

Papers

N-type thermoelectric material

Electronic structure of misfit-layered calcium cobaltite

Fabrication of Nanocomposite Thermoelectric Materials by a Pulsed Laser Deposition Method

Analysis of Crystallographic Orientation on Textured Ca-doped (ZnO)mIn2O3 Ceramics by Electron Back Scattering Diffraction Pattern

Solar Thermal Cogeneration System Using a Cylindrical Thermoelectric Module