Aluminum is probably one of the most familiar metals in the modern world thanks to its usage in everything from smartphones to aluminum foil. Aluminum is so commonly used because it has a low melting point, works well in alloys, and is relatively light compared with its overall strength. What if it could be even lighter, though? A team of international researchers has devised a new structural recipe for crystalline aluminum with extremely low density. It may even be able to float in water.
The scientists from Utah State University and Southern Federal University in Russia used computational design to work out the structure of the crystalline aluminum. They started from a known good crystal structure; in this case, diamond. A diamond lattice consists of repeating 8-atom groups wherein each atom is connected to four others. This is known as a face-centered cubic arrangement.
Making a diamond lattice into crystalline aluminum is deceptively complicated. All the team had to do was swap the carbon atoms in a diamond for aluminum atoms. Figuring out how to make that work chemically was an entirely different matter. Aluminum atoms are much larger than carbon and have different binding properties. You can’t just substitute aluminum for each carbon and call it a day. The team found that you could retain the cubic lattice arrangement of a diamond with aluminum if each carbon was replaced by a tetrahedral aluminum subunit. Thus, crystalline aluminum is composed of 4-atom blocks of aluminum, which are themselves connected to other tetrahedral aluminum subunits.
This material doesn’t exist in real life yet, and the team cautions it would be costly to manufacture. However, the simulations used to generate the structure predict that it would have extremely low density while retaining much of the strength of standard aluminum. Crystalline aluminum is expected to have a density of just 0.61 grams per cubic centimeter. That compares with 2.7 grams per cubic centimeter for normal aluminum. Steel is much heavier at 7.75 grams per cubic centimeter.
The structure of crystalline aluminum.
The determining factor for whether or not something is buoyant in water is whether it has higher or lower density than water. Crystalline aluminum is 0.61 grams per cubic centimeter, and water is 1 gram per cubic centimeter, so it should float quite well.
We know from past research that so-called supertetrahedral materials can be synthesized, but it’s not easy. If crystalline aluminum is produced, researchers can compare its properties to the predictions in this study. If it’s a match, the same computation approach could be used to design other exotic materials.
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