Scanning electron microscopy images of template-directed eutectics grown at different rates of solidification. At slow solidification rates, a three-spoke trefoil pattern is obtained. By increasing the solidification rate, the number of spokes can be increased until a six-spoke hexafoil pattern is achieved. Here, the AgCl-KCl eutectic was directionally solidified within an alumina-coated nickel pillar template. The images are partially false-colored to depict AgCl in yellow, KCl in blue, and pillars in gray. [Image credit: Ashish Kulkarni.]
Scanning electron microscopy images of template-directed eutectics grown at different rates of solidification. At slow solidification rates, a three-spoke trefoil pattern is obtained. By increasing the solidification rate, the number of spokes can be increased until a six-spoke hexafoil pattern is achieved. Here, the AgCl-KCl eutectic was directionally solidified within an alumina-coated nickel pillar template. The images are partially false-colored to depict AgCl in yellow, KCl in blue, and pillars in gray. [Image credit: Ashish Kulkarni.]
A model showing how a simple layered material, depicted in orange and blue, transforms into a complex Archimedean-structured composite material when it freezes around a template, depicted in gray.
A model showing how a simple layered material, depicted in orange and blue, transforms into a complex Archimedean-structured composite material when it freezes around a template, depicted in gray.
Paul Braun and his coworkers have developed a new templating system to help control the quality and unique properties of a special class of inorganic composite materials.
Paul Braun and his coworkers have developed a new templating system to help control the quality and unique properties of a special class of inorganic composite materials.
University of Michigan engineers professor Katsuyo Thornton, left, and graduate student Erik Hanson, right, carried out computer simulations of the composite structures.
University of Michigan engineers professor Katsuyo Thornton, left, and graduate student Erik Hanson, right, carried out computer simulations of the composite structures.

Researchers have applied a well-known organic synthesis growth technique to a unique class of inorganic composites called eutectics, introducing a new level of control over their physical structure [Kulkarni et al., Nature 577 (2020) 355-358, https://doi.org/10.1038/s41586-019-1893-9].

The use of templates to direct the self-assembly of organic materials has emerged in recent years, but the approach has not been applied to eutectic composites. This type of inorganic composite is composed of elements and compounds with different melting and solidification temperatures. The resulting eutectic material, however, has a single melting and freezing temperature and self-assembles naturally into layers of its individual components as it solidifies. This unique property is highly useful in many applications from high-performance turbine blades to solder alloys.

“Having a single melting point has advantages in composite materials processing,” explains Paul Braun, professor of materials science and engineering and director of the Materials Research Lab at the University of Illinois at Urbana-Champaign, who led the research with colleagues at the University of Michigan. “Instead of depositing layers of material individually, we start with a liquid that self-assembles as it solidifies. This can speed up production and allows us to make larger volumes at one time.”

The template, which the researchers created, consists of an array of nickel pillars around 500 nm in diameter arranged in a hexagonal lattice pattern. The well-studied eutectic AgCl-KCl was melted and introduced into the template, where it was allowed to solidify under different conditions. Instead of layers of the eutectic’s component parts, the researchers found that the material inside the template takes up a pattern of alternating three, four, five or six ‘spokes’ of KCl and AgCl arranged around the template pillars.

“We can vary the cooling rate to make the layers thicker or thinner, but the pattern stays the same,” points out Katsuyo Thornton, professor of materials science and engineering at Michigan who, along with Erik Hanson, carried out computer simulations of the eutectic.

The findings indicate that the template interferes with the usual layer formation so that the molten AgCl-KCl solidifies around the pillars in the characteristic spoke-like formation. Depending on the spacing between the pillars, different structures can be produced.

“The repeating nature of these templates and newly formed structures reduces the chances for defects to form,” adds Braun. “So, not only did we form exciting new microstructures, but we also reduced the number of defects in the resulting composite material.”

The usual structures that would not form under normal conditions could prove useful in the future.

“We’re still a long way from real applications,” cautions Braun, “but the possibilities are abundant… and there is a lot of potential in the area of photonic crystals.”

The research was supported by the Air Force Office of Scientific Research.