Building metals like a 3D puzzle - nanoparticles join together to form very hard materials

Humans have been making and using various metals for thousands of years and our alloys are definitely one of the cornerstones of our civilization. However, we are still improving our methods.

Now scientists at Brown University have created metals that are 4 times harder than the naturally occurring metal structures.

All metal objects are made from tiny crystals called grains. In very simple terms, the smaller those crystals are, the harder metal is going to be. We have means to break and arrange those crystals using mechanical power. We beat them with hammers, roll and twist them. We can also break and arrange those crystals by quickly changing the temperature of the metal – by dunking a red-hot metal into cold water or oil. However, none of these methods create a particularly uniform hard structure.

The problem is that mechanical power is poorly transmitted through the entire thickness of the metal piece. This means that some crystals remain large, others – quite small. Controlling the size of the crystals is very difficult and so adjusting the properties of a particular metal object is very hard. But now scientists have created a different approach – instead of taking something with big crystals and breaking them, we can take tiny crystals and join them together.

Those tiny crystals are called metal nanoclusters. Basically, these are just very very small metal pieces that can be joined together by applying some pressure. Typically metal nanoparticles are covered with organic molecules called ligands, which generally prevent the formation of metal-metal bonds between particles. Scientists stripped them off using a chemical process, which allowed nanoclusters to join  together with just a bit of pressure.

Scientists made some coins using this method from  gold, silver, palladium and other metals. Gold was particularly interesting because nanoparticles of this metal are completely different color. Ou Chen, corresponding author of this new research, explained: Because of what’s known as the plasmonic effect, gold nanoparticles are actually purplish-black in color. But when we applied pressure, we see these purplish clusters suddenly turn to a bright gold color. That’s one of the ways we knew we had actually formed bulk gold”.

This method can be introduced in industry fairly easily and would help making very hard metals that feature very specific characteristics. The method itself is similar to sintering, but involves way smaller particles and less energy. Hopefully this technology can be developed and commercialized soon.

Source: Brown University