Scientific Method / Science & Exploration

Simulating hypersonic nanoparticle collisions

New molecular dynamics simulation work has examined what happens when silicon …

What happens when you fire a silicon nanoparticle at a silicon surface at 900 m/s? What about at 2,000 m/s? As it turns out, two completely different things occur; bouncing or sticking, and the transition between the two responses occurs at a speed between 1,250 and 1,550 m/s.

Why would anyone care about this? As scientists and engineers seek to create new materials by exploiting nanoscale features and the different physical and chemical effects that dominate at these small scales, control of the entire process becomes key. Control over nanoparticle formation in the gas phase, and deposition onto a surface are key for "manufacturing novel nanostructured surfaces and thin films," according to, M. Suri and T. Dumitrica, a pair of mechanical engineers at the University of Minnesota, and the authors of an upcoming paper in Physical Review B.

The authors look at the example of coating a surface with silicon spheres, which has been problematic. The difficulty is due to the different physics that play a dominant role at such short length and time scales. On the macroscale, inelastic impacts have their energy released due to the formation of dislocations within the material, but according to the authors there is not enough time or space for this mechanism to occur within a nanoparticle. Secondly, silicon nanospheres have been found to be extremely hard; over four times the bulk yield stress was required to generate yield in these tiny balls. Finally the system is complicated by the chemistry of the surface of the particle and surface material. If they are both bare silicon, then the nanoparticle will easily become chemically bound by a substrate. If the surfaces are passivated with hydrogen, then bonds do not form as easily.

To gain a deeper understanding of the dynamics of a collision between a nanoparticle and substrate the duo performed a series of molecular dynamics simulations of a five nanometer silicon sphere slamming into a silicon surface at speeds between 900 and 2000 m/s. It should be noted that while these speeds are large, the total energy per atom in the nanoparticle is less than about 1 eV.  Over all the runs the researchers found that the particle did not fracture, and the surface did not crater, yet at the highest speeds a few surface hydrogen atoms did get knocked free.

Image Credit: American Physical Society

What they did find was that for entry velocities below 1000 m/s, the nanoparticle was reflected off the surface, regardless of the chemical makeup of the substrate. These collisions were nearly elastic; the receding velocities were up to 80 percent of the approach velocities. However, at greater velocities and entirely different response was computed. Regardless of the chemical makeup of the surface, at velocities greater than 1,550 m/s the nanoparticle was captured by the surface, with no oscillations.

In both impact types, the nanoparticle under goes significant distortion. In the case where the particles are captured the energy has to go somewhere, with no rebound some form of stress relieving transformation must occur. By examining the radial distribution function—a statistical measure of the likelihood of finding an atom some distance from any given atom—of the silicon atoms in the nanoparticle, the researchers found something surprising. While the nanoparticle started with the silicon atoms in a simple tetrahedral arrangement, each atoms was connected to four neighbors, they found that after the impact, the system transformed to a higher density phase of silicon, where each atom was connected to six neighbors. This transformation was the result of the stress imparted from the impact.

This new phase was not thermodynamically stable. By the end of the simulation it was found that a percentage of the silicon had transformed from a crystalline arrangement to an amorphous silicon. The authors point out that this is not thermodynamically favorable, but it accommodates both the crystalline regions far away from the impact, and the remainder of the high coordination region near the center of the impact. The authors conclude that it is indeed possible to get nanoparticles to stick to surface, even when the sticking is chemically unfavorable. The sticking can be accomplished by firing the particle at hypersonic speeds. It is now up to other engineers to design ways to accomplish this in the laboratory or manufacturing center.

Physical Review B, upcoming.

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