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

Mechanical properties of nanocrystalline materials

N G van Kampen 1981 Amsterdam: North-Holland xiv + 419 pp price Dfl 180 This is a book which, at a lower price, could be expected to become an essential part of the library of every physical scientist concerned with problems involving fluctuations and stochastic processes, as well as those who just enjoy a beautifully written book. It provides an extensive graduate-level introduction which is clear, cautious, interesting and readable.

https://iopscience.iop.org/article/10.1088/0031-9112/34/4/040/pdf

Stochastic Processes in Physics and Chemistry

This paper reviews the past, present, and future of the hydroxyapatite (HAp)-based biomaterials from the point of view of preparation of hard tissue replacement implants. Properties of the hard tissues are also described. The mechanical reliability of the pure HAp ceramics is low, therefore it cannot be used as artificial teeth or bones. For these reasons, various HAp-based composites have been fabricated, but only the HAp-coated titanium alloys have found wide application. Among the others, the microstructurally controlled HAp ceramics such as fibers/whiskers-reinforced HAp, fibrous HAp-reinforced polymers, or biomimetically fabricated HAp/collagen composites seem to be the most suitable ceramic materials for the future hard tissue replacement implants.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0884291400042527

Processing and properties of hydroxyapatite-based biomaterials for use as hard tissue replacement implants

During the past five years, the authors have developed in their laboratory a new type of solar cell that is based on a photoelectrochemical process. The light absorption is performed by a monolayer of dye (i.e., a Ruthenium complex) that is adsorbed chemically at the surface of a semiconductor (i.e., titanium oxide (TiO{sub 2})). When excited by a photon, the dye has the ability to transfer an electron to the semiconductor. The electric field that is inside the material allows extraction of the electron, and the positive charge is transferred from the dye to a redox mediator that is present in solution. A respectable photovoltaic efficiency (i.e., 10%) is obtained by the use of mesoporous, nanostructured films of anatase particles. The authors show how the TiO{sub 2} electrode microstructure influences the photovoltaic response of the cell. More specifically, they focus on how processing parameters such as precursor chemistry, temperature for hydrothermal growth, binder addition, and sintering conditions influence the film porosity, pore-size distribution, light scattering, and electron percolation and consequently affect the solar-cell efficiency.

Nanocrystalline titanium oxide electrodes for photovoltaic applications

The uniaxial tensile deformation behavior of 3 molpercent Y/sub 2/O/sub 3/-tetragonal ZrO/sub 2/ polycrystals was studied at temperatures up to 1500 C in ambient atmosphere. The tensile specimens which were elongated at constant displacement rates from 0.00011/s to 0.00055/s at 1450 C exhibited unusually large deformation. The superplastically deformed specimens retained a bending strength of 976 MPa at room temperature. Steady creep rates measured at a constant load in the range of relatively small strains were described by a stress component, n, of 2 and by an activation energy of 586 + or - 40 kJ/mol. Large deformations during the accelerating creep were accompanied by extensive formation of cavities. 19 references.

Superplasticity of yttria-stabilized tetragonal ZrO2 polycrystals

SUPERPLASTICITY is defined phenomenologically as the ability of a material to exhibit exceptionally large elongations during tensile deformation1. It is a property of some poly crystalline solids, and is well established for metals and alloys2. Superplasticity has also been observed in some ionic crystals, such as Y2O3-stabilized tetragonal ZrO2 polycrystals3,4, but has not been found previously for covalent crystals. Here we report superplastic elongation (by more than 150%) of a covalent crystal composite, Si3N4/SiC. The superplasticity is probably related to the presence of an inter-granular liquid phase. Combined with its hardness, this property suggests several useful applications for the novel material: for example, to form engine components—superplasticity will make it readily mouldable at high temperatures.

A superplastic covalent crystal composite

Three-dimensional microstructural evolution in ideal grain growth—general statistics

The superplastic behavior of the composite Y-TZP/Al{sub 2}O{sub 3} (20 wt%) in uniaxial tension has been evaluated. Large elongation (>200%) indicated superplasticity. The stress exponent and activation energy of thee composite were found to be the same order with those of Y-TZP. The flow behavior of the composite containing Al{sub 2}O{sub 3} grains could be described by a rheological model as a non-Newtonian flow modified by the second-phase grains. The cavitation damage and the creep crack growth could be reduced by keeping the strain rate low enough that a specimen elongated 100% at elevated temperature maintained a strength of 1,800 MPa at room temperature.

Superplasticity of TZP/Al sub 2 O sub 3 composite

Dense and translucent hydroxyapatite polycrystals (Ca10(PO4)6(OH)2 with a grain size of 0.64 μMm) were obtained by hot isostatic pressing at 203 MPa and 1000°C for 2 h in argon. The material exhibited superplastic elongation (>150%) in a tension test at temperatures from 1000° to 1100°C and at strain rates from 7.2×10−5 to 3.6 × 10−4 s−1. Extensive strain hardening was observed. The stress exponent of the yield stress was larger than 3.

Fumihiro Wakai

Papers

Superplasticity of yttria-stabilized tetragonal ZrO2 polycrystals

A superplastic covalent crystal composite

Three-dimensional microstructural evolution in ideal grain growth—general statistics

Superplasticity of TZP/Al sub 2 O sub 3 composite

Superplasticity of Hot Isostatically Pressed Hydroxyapatite