London, March 7 : Using a computer simulation, scientists at the University of Michigan have cast some light on how quasi-crystals form.
Unlike ordinary crystals, quasi-crystals are solids whose atoms are not arranged in a repeating pattern. However, they still form intricate patterns that are technologically useful.
Sharon Glotzer, a professor in the Department of Chemical Engineering, says that quasi-crystals incorporate clusters of atoms as they are, without rearranging them as regular crystals do.
When a normal crystal grows upon the freezing of liquids into solids, a crystallite nucleus develops first. The atoms in the liquid attach one-by-one to the crystallite, as though following a template.
If the atoms have already formed a cluster on their own, they must rearrange in order to fit the template, and this is how a repeating pattern forms.
However, when quasi-crystals develop, atoms that have already formed stable shapes away from the crystallite can still bind to it, and they do not have to make adjustments.
"In our simulations of quasi-crystals, we observed that the atoms attach to the crystallite in large groups. These groups have already formed locally stable arrangements, and the growing quasi-crystal assimilates them with minimal rearrangement," Nature magazine quoted chemical engineering doctoral student Aaron Keys as saying.
The fact that quasi-crystals are not as regimented as regular crystals also allows the former to grow more easily than regular crystals. This is so because the solid can reach a "structural compromise", where liquid-like molecular arrangements are retained in the solid state.
Certain metal alloys, which tend to resist wear and corrosion, comprise of quasi-crystals. Such materials are known to have high tensile strength, i.e. high forces are required to stretch them to their breaking point.
"Learning how they grow will help us figure out to how engineer quasi-crystalline structures from new building blocks, which could lead to a slew of new materials," Glotzer said.
A paper describing Glotzer and Keys' research has been published in Physical Review Letters.