Nanoscale Structures and Fracture Processes in Advanced Ceramics: Million-Atom MD Simulations on Parallel Architectures.

Properties and processes in silicon nitride and graphite are investigated using molecular-dynamics (MD) simulations. Scalable and portable multiresolution algorithms are developed and implemented on parallel architectures to simulate systems containing 10\sp6 atoms interacting via realistic potentials. Structural correlations, mechanical properties, and thermal transport are studied in microporous silicon nitride as a function of density. The formation of pores is observed when the density is reduced to 2.6 g/cc, and the percolation occurs at a density of 2.0 g/cc. The density variation of the thermal conductivity and the Young\u27s modulus are well described by power laws with scaling exponents of 1.5 and 3.6, respectively. Dynamic fracture in a single graphite sheet is investigated. For certain crystalline orientations, the crack becomes unstable with respect to branching at a critical speed of $\sim$60% of the Rayleigh velocity. The origin of the branching instability is investigated by calculating local-stress distributions. The branched fracture profile is characterized by a roughness exponent, $\alpha\sim0.7,$ above a crossover length of 50A. For smaller length scales and within the same branch, $\alpha\sim0.4.$. Crack propagation is studied in nanophase silicon nitride prepared by sintering nanoclusters of size 60A. The system consists of crystalline cluster interiors, amorphous intercluster regions, and isolated pores. These microstructures cause crack branching and meandering, and the clusters undergo significant rearrangement due to plastic deformation of interfacial regions. As a result, the system can withstand enormous deformation (30%). In contrast, a crystalline sample in the same geometry cleaves under an applied strain of only 3%

Quasi-static cracks and minimal energy surfaces

We compare the roughness of minimal energy(ME) surfaces and scalar ``quasi-static'' fracture surfaces(SQF). Two dimensional ME and SQF surfaces have the same roughness scaling, w sim L^zeta (L is system size) with zeta = 2/3. The 3-d ME and SQF results at strong disorder are consistent with the random-bond Ising exponent zeta (d >= 3) approx 0.21(5-d) (d is bulk dimension). However 3-d SQF surfaces are rougher than ME ones due to a larger prefactor. ME surfaces undergo a ``weakly rough'' to ``algebraically rough'' transition in 3-d, suggesting a similar behavior in fracture.Comment: 7 pages, aps.sty-latex, 7 figure

Phase-Field Model of Mode III Dynamic Fracture

We introduce a phenomenological continuum model for mode III dynamic fracture that is based on the phase-field methodology used extensively to model interfacial pattern formation. We couple a scalar field, which distinguishes between ``broken'' and ``unbroken'' states of the system, to the displacement field in a way that consistently includes both macroscopic elasticity and a simple rotationally invariant short scale description of breaking. We report two-dimensional simulations that yield steady-state crack motion in a strip geometry above the Griffith threshold.Comment: submitted to PR

Atomistic Simulations of Nanotube Fracture

The fracture of carbon nanotubes is studied by atomistic simulations. The fracture behavior is found to be almost independent of the separation energy and to depend primarily on the inflection point in the interatomic potential. The rangle of fracture strians compares well with experimental results, but predicted range of fracture stresses is marketly higher than observed. Various plausible small-scale defects do not suffice to bring the failure stresses into agreement with available experimental results. As in the experiments, the fracture of carbon nanotubes is predicted to be brittle. The results show moderate dependence of fracture strength on chirality.Comment: 12 pages, PDF, submitted to Phy. Rev.

Molecular Dynamics for Low Temperature Plasma-Surface Interaction Studies

The mechanisms of physical and chemical interactions of low temperature plasmas with surfaces can be fruitfully explored using molecular dynamics (MD) simulations. MD simulations follow the detailed motion of sets of interacting atoms through integration of atomic equations of motion, using inter-atomic potentials that can account for bond breaking and formation that result when energetic species from the plasma impact surfaces. This article summarizes the current status of the technique for various applications of low temperature plasmas to material processing technologies. The method is reviewed, and commonly used inter-atomic potentials are described. Special attention is paid to the use of MD in understanding various representative applications, including tetrahedral amorphous carbon film deposition from energetic carbon ions; the interactions of radical species with amorphous hydrogenated silicon films; silicon nano-particles in plasmas; and plasma etching.Comment: Manuscript #271801, Accepted in J. Phys. D, November 10th, 200

Structure, mechanical properties, and thermal transport in microporous silicon nitride—molecular-dynamics simulations on a parallel machine

Equilibrium and non-equilibrium molecular-dynamics simulations are performed to determine the structure, mechanical properties, and thermal conductivity of amorphous silicon nitride under uniform dilation. For mass densities below $\rho = 2.6\;\rm g/cc$, we observe a significant growth of pores. Near 2.0 g/cc the average pore volume diverges with an exponent of 1.95. The thermal conductivity and Young's modulus are found to scale as $\rho^{1.5}$ and $\rho^{3.6}$, respectively

Early stages of sintering of silicon nitride nanoclusters: a molecular-dynamics study on parallel machines

Sintering of $\rm Si_3N_4$nanoclusters is investigated with the molecular-dynamics approach. At 2000 K thermally rough nanocrystals develop an asymmetric neck in 100 picoseconds. The neck contains more fourfold than threefold coordinated Si atoms. Amorphous nanoclusters develop a symmetric neck which has nearly equal number of threefold and fourfold coordinated Si atoms. In both cases, sintering is driven by surface diffusion of Si and N atoms. The diffusion is much more rapid in the neck joining amorphous nanoclusters than in the neck region of nanocrystals

IMMUNOBIOLOGICAL ACTIVITY OF REGULATORY PEPTIDE FRACTIONS SYNTHESIZED BY NEUTROPHILS, AS TESTED IN A MACROPHAGE MODEL

The article presents experimental data on regulatory effect of neutrophilokine helper fractions on the macrophage (Mph) functional activity in the course of antiplague immunity formation. It has revealed that these fractions content biologically active, low-molecular weight peptides. They stimulate Mph killing activity by increasing phagosome-lysosome fusion, thus boosting transformation of monocytes to Mph, and causing redistribution of macrophage subpopulations in the total cellular pool. The helper effect of neutrophilokine fractions upon functional activity of MPh is more pronounced during secondary immune response