Objectives

Montecarlo Simulation of Two Component Aerosol Processes Online Course
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Section: Introduction
Lesson: Objectives
Montecarlo Simulation of Two Component Aerosol Processes.
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Nanophase materials
More than 30 years ago, Nobel Prize–winning physicist Richard P. Feynman mused that in the future, when scientists have learned how to control the arrangement of matter on a very small scale, they would see materials take on an enormously richer variety of properties. In recent years, considerable attention was focused on the production of these materials, so-called nanophase materials.These materials, also referred to as nanostructured, nanocrystalline, or nanometer-sized crystalline solids, are single-phase or multiphase polycrystals, with dimensions of the order of 1–100 nm. They can be classified according to the number of dimensions of the grain in the nanometer range as shown in Table 1. A general review of nanophase materials is found in References 2 and 3.
Classification of nanophase materials.
Compared to conventional materials, nanophase materials possess unique advantages with respect to properties and processing. As grain size decreases down to the nanometer-size range, nanophase metals generally get stronger and harder, while nanophase ceramics show ductility, even superplasticity, at lower temperatures than conventional brittle ceramics. For example, 1000% increase in fracture stress, 2000% increase in magnetic susceptibility, 25% decrease in density, and 165% improvement in critical temperature for superconductivity have been observed in nanophase metals. As an example, shows the stress–strain curve for a) nanophase (14 nm) Pd sample compared with that for a coarse-grained (50 μm) Pd sample and b) nanophase (25 nm) Cu sample compared with that for a coarse-grained (50 μm) Cu sample. It shows that the yield stress for nanophase materials for these two materials is higher than the conventional material.

Stress–strain curve for a) nanophase (14 nm) Pd sample compared with that for a coarse-grained (50 μm) Pd sample, and b) nanophase (25 nm) Cu sample compared with that for a coarse-grained (50 μm) Cu sample.

These unique properties arise because of the dramatic grain refinement and the novel characteristic of grain boundaries. Nanophase materials contain a high concentration of grain boundaries with random orientation, and consequently a substantial fraction of atoms lies in the interfaces. For example, when grain size is about 5 nm, there are approximately 1019 boundaries/cm3 with random orientation and 50% volume fraction of atoms at the interfaces. The volume fraction decreases to about 30% for 10 nm grains and to about 3% for 100 nm grains. is a schematic representation of a nanophase metal.
The increase in strength of nanophase materials stems from the smaller grain sizes, which cause a change in the fundamental mechanism of plastic deformation. In conventional metals, the fundamental mechanism is dislocation migration, where rows of missing atoms within the lattice (dislocations) migrate in the direction opposite of an applied stress. The result of this migration is that dislocations pile up on one side of the grain, and thus the grains themselves are deformed. The ease of migration of these dislocation lines is related to the length of the dislocation line; longer lines migrate more readily than shorter lines.

Schematic representation of an equiaxed nanocrystalline metal distinguishing between the atoms associated with the individual crystal grains (filled circles) and those constituting the boundary network (open circles).

In conventional metals where grain sizes are large, these dislocation lines are very long and consequently can migrate readily. In nanocrystalline materials where grain sizes are very small, the dislocation lines are very short, and do not migrate readily. Dislocation migration is then effectively shut down as a mechanism of plastic deformation in nanocrystalline materials. Grain boundary sliding bec
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