Jun 5 2015
With nanotechnological advancements making defect-free materials possible, a research team from the University of Pennsylvania and Germany’s Max Planck Institute for Intelligent Systems has shown how defects initially form prior to failure.
A microscope image of the edge of a defect-free palladium nanowire. The array of dots represent single atomic columns.
The structure of crystalline materials is such that the atoms are ordered neatly in a repeating pattern. When crystalline materials break, the failure begins at a defect or at any position where there is a disruption of the pattern. It is, however, unclear how crystalline materials without any defects break.
This new study involved stretching defect-free palladium nanowires under strictly controlled conditions. The thickness of each nanowire was 1000 times less than that of human hair. In contrast to common belief, they discovered that it was impossible to predict the stretching force at which failure of the wires occurred, with the force occuring in a set of values that were highly impacted by the ambient temperature.
It can be inferred from this thermal uncertainty in the failure limit that the first point where a failure-inducing defect is seen is on the surface of the nanowire, where atoms behave in a more liquid-like manner. Since the atoms are highly mobile, there is a higher likelihood that they will reorder themselves to create a “line defect” across the nanowire, resulting in it breaking.
Graduate student Lisa Chen and associate professor Daniel Gianola of the Department of Materials Science and Engineering in Penn’s School of Engineering and Applied Science headed the study. Postdoctoral researcher Mo-Rigen He and graduate student Jungho Shin, and other members of Gianola’s lab were active contributors. They partnered with Gunther Richter of the Max Planck Institute for Intelligent Systems.
“Nanotechnology is not just about making things smaller,” Chen said, “it’s also about different properties that arise in materials at the nanoscale.”
“When you make these really small structures,” Gianola said, “they’re often grown from the bottom up, in an atom-by-atom, layer-by-layer process, and that can give you a much more pristine structure than if you were to take a big block of metal and whittle it down. In addition, the atoms on the surface comprise a much larger proportion of the total and can control the properties of the nanoscale material.”
Vapour deposition was used at high temperature to grow the palladium nanowires, providing each atom with the energy and time needed to traverse around until it found its desired spot in the crystalline structure of the metal.
The wires could be seen sprouting from a substrate similar to blades of grass. The team used a microscopic robotic manipulator to meticulously remove the wires and fix them to their testing platform inside an electron microscope.
This platform was designed in partnership with Sandia National Laboratory and behaves similar to an industrial testing machine at the nanoscale. By welding a nanowire to a grip fitted to several slanted bars that expand when subjected to heat by an electric current, the nanowire could be stretched by the researchers in a controlled manner. By increasing the voltage to a different maximum and then lowering it at the same rate repeatedly, it was possible for researchers to identify exactly when the first irreversible deformation occurred in the wire.
“Just pulling it until it fails doesn’t tell you exactly where and how that failure began,” Gianola said. “Our goal was to deduce the point where the first of the nanowire’s atoms begin to shift out of their original positions and form a mobile defect.”
According to computational studies, it would be possible to identify this point by analyzing the temperature dependence of failure. As in the past, defect-free nanowires were not available for physical experiments, it was inferred that the relationship between strength and temperature was deterministic. It was believed that by being aware of the temperature, it would be possible to estimate the failure limit of the nanowire.
The stretching experiments were conducted at a range of temperatures and researchers could plot these failure points. Strangely, they found that the strengths of the wires were scattered across a wide range of values, even when stretching was performed at the same temperature.
“We’ve been able to verify,” Chen said, “through experiment, and not just theory, that this process is thermally activated, and that there’s a large randomness to the process. Normally you can say a bulk material has certain strength at a certain temperature, but you have to take a different approach to specify the strength of the nanowire. Depending on the temperature you’re concerned with, even the distribution of strengths can vary drastically.”
As this distribution was found across a comparatively large range of values, it is implied that the amount of energy required to commence the nucleation of the first defect, which is the thermal activation barrier, was comparatively low. The researchers were able to understand what was driving this process by comparing the thermal activation barrier to other atomistic mechanisms.
“Diffusion of atoms on a surface,” Gianola said, “is the only mechanism that has this low thermal activation barrier. Surface diffusion is atoms hopping around, site to site, somewhat chaotically, almost like a fluid. A palladium atom sitting inside the bulk of the wire has 12 neighbors, and has to break most of those bonds to move around. But one on the surface might have only three or four to break.”
Device design can be performed in a more rational manner by understanding the origin of the distribution of strengths in nanostructures.
“Until recently,” Gianola said, “it’s been very difficult to make defect-free nanowires. But now that we can, there’s a reason to care about how they fail. Their strengths are nearly a thousand times what you would get from the bulk material with defects — in this experiment, we observed, to our knowledge, the highest strengths ever measured in that crystal structure of metal — so they’re going to be attractive to use in all sorts of devices.”
The National Science Foundation supported this research through a CAREER Award, DMR-1056293. This study was published in Nature Materials.