If you dissolve sugar in hot water and then cool it down, you’ll see pure sugar crystals form while impurities stay in the liquid. You can even watch the beautiful sugar crystals slowly grow in the water.
You can do the same thing with metals, though probably not in your kitchen.
At high temperatures, one molten metal can dissolve another. As the mixture cools, the dissolved metal begins to crystallise inside the melt, just like sugar forming crystals from water.
In new research published in Nature Communications, we have observed this process in more detail than ever before – and used the information we gained to produce crystals that are perfect for harvesting hydrogen from water, in the first of countless possible applications.
Liquid metals are different from water
There are important differences between water and liquid metals.
For one thing, we cannot see inside a liquid metal, so we cannot watch metallic crystals forming and growing. This is because liquid metals block electromagnetic waves, including visible light, so nothing can pass through for us to observe the process.
A second difference is how easily we can separate crystals from the liquid. With sugar and water, we can simply pour the mixture through a sieve and collect the crystals.
But liquid metals have very high surface tension, so the liquid behaves like it has a tight, stretched skin. This makes it difficult for the metal to pass through a sieve, and both the liquid metal and the crystals stay on top instead of separating.
The growth of crystals inside liquid metal is not a new idea. However, because people could not see inside the liquid metal, they did not focus on watching or controlling the formation of small crystals. Instead, they mainly grew large crystals and they could not see the dynamics of the crystal growth.
Peering inside liquid metals
However, we now have advanced tools that let us see inside liquid metals. We use high-energy X-rays, which can penetrate liquid metals, using a technique called X-ray micro-computed tomography (micro-CT).
Micro-CT uses X-rays to take many cross-section images of an object, and then builds a virtual 3D model of the object without damaging the original sample. The system we used could produce images with micrometre-sized pixels, and newer technologies are even reaching the nanometre range.
Moonika Widjajana
Using micro-CT was extremely important for our work. By adjusting conditions (such as how fast we cooled the liquid metal, or the type of solvent we used) we could actually see how the crystals formed and how their shapes changed. These are new observations that past researchers have not reported before.
Taking one 3D image of a sample with crystals inside liquid metal takes several hours. We captured many such images over several days to watch how the crystals grew over time.
Sifting crystals and putting them to work
Another helpful step was applying an electric voltage to the surface of the liquid metal after the completion of the growth. When we do this, the surface tension of liquid metals drops almost to nothing.
With no surface tension, the liquid metal can easily flow through a sieve and leave the crystals behind.
Observing crystals inside metals has many practical applications. By understanding how crystals grow, we can learn to control their size and shape to produce the best crystals for particular purposes.
To test our work, we created optimal crystals of special metals for producing hydrogen from water. That’s not all this technique will be good for: it can be used to make materials that are just right to catalyse chemical reactions, store energy in batteries, build advanced electronics and countless other things.