An Extraordinary Metal Alloy Flips From Solid to Liquid and Back
Oddball gallium, which melts below body temperature, stars in a new microscope study.
A mixture of gallium and copper showed spontaneous flip-flopping from liquid to solid.
The flip-flopping extended up to 8-10% into the heated and observed sample morsels.
In newly published research, a team of engineers in Australia have observed a special metal alloy that goes back and forth from liquid to solid—quickly, and to a depth of several nanometers. That’s practically an Olympic diving well in materials science. After all, this kind of phase-phasing was believed to be unusual, but it may be happening every time this type of metal crystallizes into a solid. And these researchers’ special setup could allow more scientists to study the crystallization, or solidifying process, of alloys in a lot more detail than before.
Humans have been using metals for many thousands of years, and the quest to turn other things into gold was one of the driving forces for the emergence of scientific study among people. Even so, we still don’t have a great understanding of what happens to metals when they solidify from molten form. “This is largely due to the opaque nature of molten metals to most light-based characterization like UV–vis, Raman, and infrared spectroscopy,” the researchers explain in their paper, which appears now in the journal Advanced Science.
The techniques that do work well involve “hard X-rays”—the most energetic form of the same radiation that shows us our broken bones and bowel obstructions. But these must be averaged over an extended period, and don’t show the crystallizing in real time. It’s like seeing one photo of a liquid puddle and one photo of a frozen puddle instead of a video showing ice crystals forming in the water. There must be a better way for scientists to observe and record the reaction.
For this experiment, the scientists chose gallium—an oddball metal that melts at just under human body temperature. They mixed it with copper in order to create a binary alloy, where speckles of solid copper (with a much higher melting point) are distributed evenly throughout the liquefied gallium. What results is something called an intermetallic colloid, which is a complete suspension of one metal in another.
Intermetallics form neat, crystal structures when they solidify, often becoming quite strong and brittle compared with some softer and more malleable other metals and compounds. But with two very different melting points, turning this mixture from “liquid metal mother liquor” into a solid does not happen uniformly. That’s one reason it was a great candidate for this experiment.
The researchers used a transmission electron microscope (TEM), which uses a tiny beam of electrons as a kind of nano-scale X-ray to look through thin samples of chemical and biological materials. To prepare a sample for observation, they heated solvent salts and then added the metals, letting them agitate using a sonic device. Then, they let the resulting solid sample cool and dissolved it into smaller particles for observation.
This dissolving rinse left intermetallic particles, or tiny samples that could be very gently heated under the TEM and watched at the nano scale. And this is where the magic was revealed.
“[T]he outer layers of the crystal spontaneously become liquid and then recrystallize at room temperature,” the authors wrote in their paper. “[T]his behavior is persistent, remaining several days and at temperatures far below the formal melting point. Furthermore, this effect can extend deep into the solid lattice, causing up to ≈50 atom layer to spontaneously transition temporarily into a liquid state.”
In other words, if room temperature is 72 Fahrenheit, the compound of gallium and copper began melting at over 10 Fahrenheit below gallium’s modest melting temperature, and those layers turned from solid to liquid and back again in fits and starts. It seems to work a bit like trying to measure a half life. The half life doesn’t mean a material completely transforms at that time—it refers instead to an ongoing likelihood that any random atom or particle could change at any time.
These sample particles were just 100-120 nanometers wide, and the observed flip-flop zone was up to 10 nanometers deep. The findings also seem to suggest that this behavior is not caused by the way that heat was applied, because non-uniform heat would create a hot spot. Even if individual atoms of copper were shielding more interior gallium atoms from heat, that would create a cool spot. Instead, this is a back-and-forth flip-flop spot.
With this research in mind, who knows what’s really happening on the very edge—where the solid meets the liquid? Only time and the transmission electron microscope will tell.
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