New Techniques for Building Super-Hard Metals by Breaking Small Nanoparticles

The gold “coin” was made of nanoparticle building blocks, thanks to a new technique developed by Brown University researchers. Making large metals in this way allows for precise microstructure of metals, which enhances their mechanical properties. Credit: Chen Lab / Brown University

Brown University researchers have demonstrated how to make large metals by breaking down small metal nanoparticles, which allows for special grain structures and improved mechanical properties and so on.

Metallurgy has all sorts of ways to make a set of metals more difficult. They can bend, twist, run them between two rolls or pounds with a hammer. This method works by breaking down the structure of metal grains – microscopic crystal domains that form large pieces of metal. Smaller grains for harder metals.

Now, a group of Brown University researchers have found a way to change the structure of metal grain from bottom to top. In the papers are published in the journal Chem, researchers suggest methods for slamming individual metal nanoclusters to form macro-scale hunks of solid metals. Mechanical tests of metal made using the technique show that it is up to four times more difficult than natural metal structures.

“Hammering and other hardening methods are all up-and-down ways to change the grain structure, and it is very difficult to control the size of the grain you end up with,” said Ou Chen, assistant professor of chemistry at Brown and co-author of the new study. “What we have done is create nanoparticle building blocks that unite when you squeeze. This way we can have a uniform grain size that can be adjusted for enhanced properties.”

Mass Metal From Nanoparticle Building Blocks

Researchers from Brown University have shown how to make large metals from nanoparticle-shaped blocks. For the new study, the team made “coin” metals from gold, silver, palladium and other metals nanoparticles. Credit: Chen lab / Brown University

For this study, researchers made centimeter-scale “coins” using nanoparticles of gold, silver, palladium and other metals. Goods of this size can be used to make high performance coating materials, electrodes or thermoelectric generators (devices that convert heat flux into electricity). But researchers think the process could easily be scaled up to make super-hard metal coatings or larger industrial components.

The key to the process, Chen said, is the chemical treatments given to nanoparticle-shaped blocks. Metal nanoparticles are typically covered by organic molecules called ligands, which generally prevent the formation of metal-metal bonds between particles. Chen and his team found a way to isolate the ligand away chemically, allowing the clusters to merge with just a little bit of pressure.

Metal coins made using this technique are better than standard metals, research shows. Gold coins, for example, are two to four times harder than normal. Other properties such as electrical and light conduction reflect almost the same standard metal, the researchers found.

The optical properties of gold coins are attractive, Chen said, because there is a dramatic change in color when nanoparticles are compressed into mass metals.

“Because of what is known as the plasmonic effect, gold nanoparticles are actually black and black,” Chen said. “But when we get under pressure, we see this purplish cluster suddenly becomes a bright golden color. That’s one of the ways we know that we really build big gold.”

In theory, Chen states, the technique can be used to make any type of metal. In fact, Chen and his team show that they can make exotic metal shapes known as metal glasses. Amorphous metal glasses, meaning they do not have a crystal structure that is normal for normal metals. They give rise to incredible properties. Metal glasses are easier to form than traditional metals, can be stronger and more durable, and show superconductivity at lower temperatures.

“Making metal glasses from one famous component is difficult to do, so most metal glasses are mixed,” said Chen. “But we can start with amorphous palladium nanoparticles and use our technique to make palladium metal glasses.”

Chen said that he hopes that the technique will one day work for commercial products. The chemical treatments used in nanoclusters are relatively simple, and the pressures used to press them are within the range of standard industrial equipment. Chen has patented the technique and hopes to continue learning.

“We think there is a lot of potential here, for the industry and the scientific research community,” Chen said.

References: “Mass Grain-Boundary Materials from Nanocrystals” by Yasutaka Nagaoka, Masayuki Suda, Insun Yoon, Na Chen, Hanjun Yang, Yuzi Liu, Brendan A. Anzures, Stephen W. Parman, Zhongwu Wang, Michael Grünwald, Hiroshi M. Yamamoto and Ou Chen, January 22, 2021, Chem.
DOI: 10.1016 / j.chempr.2020.12.026

Coauthor Chen on paper is Yasutaka Nagaoka, Masayuki Suda, Insun Yoon, Na Chen, Hanjun Yang, Yuzi Liu, Brendan A. Anzures, Stephen W. Parman, Zhongwu Wang, Michael Grünwald and Hiroshi M. Yamamoto. This research is supported by the National Science Foundation (CMMI-1934314, DMR-1332208, DMR-1848499) and the U.S. Department of Energy (DE-AC02-06CH11357).

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