Researchers from the Technion and George Mason University have set a new record for the strength of metallic materials. Potential applications include ultra-strong light-metal wires that could hypothetically be considered for testing the “space elevator” idea as a new space transportation system of the future. The team’s metallic materials have the potential to meet the specific strength requirements for the cable material (alongside the carbon and Boron-Nitride nanotubes and diamond nanothreads) necessary for such a system.

The researchers investigated the mechanical properties of nickel nanoparticles under compression. The maximal strength of these particles, 34 Giga Pascals, is 3 times higher than the strength of micrometer-thin iron fibers which held the previous record of metal strength. For comparison, the ultimate strength of the strongest steels available on the market is in the range from 3 to 5 Giga Pascals. Their particles are the metal structures with the highest-ever measured strength. This achievement is based on unprecedented control of the configuration of the particle facets, and on the presence of thin surface layer of nickel oxide. The team’s findings were published recently in Nature Communications.

Principal investigators Professor Eugen Rabkin from the Department of Materials Science and Engineering at the Technion and Professor Yuri Mishin from George Mason University see this work as setting the path for investigation of the behavior of metallic materials under ultra-high mechanical loads.

“The strength of a material is an important property, which determines the range of its potential applications,” explained Professor Rabkin. “On an atomic level, the strength is determined by the type of chemical bonds between atoms. For example, a diamond is a very strong material due to the strong covalent (i.e. directional) bonds between the constituent carbon atoms. Aluminum, on the other hand, is relatively weak and ductile because the metallic bonds between its atoms are weak and isotropic.”

Strong materials are obviously a central goal of science and technology research, which has brought many groups to pursue their development. Materials have a theoretical strength limit — much like the speed of light which constitutes the upper limit of the speed of matter and energy. According to Professor Rabkin, “we are aiming to approach the upper limit of the strength of a material, but, in actuality, we were very far from it due to defects in the atomic structure of the crystal. Therefore, for many years already, we are investigating the possibility of improving the quality of the crystal, to consequently increase its strength.”

In 2011, Professor Rabkin and Professor Dan Mordehai, Professor Bill Nix and Professor David Srolovitz, his colleagues at the Technion, Stanford and Princeton, respectively, synthesized gold nanoparticles exhibiting the strength approaching its theoretical limit. This was due to the unique structure of the particles they synthesized. However, the theoretical strength of gold is relatively low in comparison with other metals and therefore, the researchers attempted to apply the new technique to nickel.

The current research, which combined experimental and computational tools, resulted in nickel particles that exhibited much higher compression strength than their gold counterparts. “We were particularly surprised,” said Professor Rabkin, “because our particles were also stronger than expected from the previous computational estimates of the theoretical strength of nickel particles. It is like exceeding the speed of light in a vacuum. We carefully checked the calibration of our instruments, but the results were fully reproducible.”

To understand these findings, the researchers carefully characterized the nickel particles and confirmed their assumptions: blunt-edged particles exhibit higher strength than their sharp-edged counterparts. This discovery served as a motivation for a unique computational algorithm they then developed to simulate in computer the nanoparticles that vary in the character of their edges. Indeed, the computer simulations confirmed that the particles with rounded edges exhibit higher strength than the polyhedral particles of a similar size with sharp edges. Moreover, the computer simulations indicated that the thin oxide layer on the nickel particles serves as a “soft cushion” resulting in more homogeneous stress distribution within the particles, which further increases their strength.

As said, this study paves the way toward understanding the physical origins of materials strength, and generating stronger particles from different types of metals and alloys.

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