Composites of metals and ceramics have many desirable and tailorable mechanical, electrical, and optical properties. But controlling the microstructures of these inorganic composite materials is extraordinarily difficult. Researchers have now developed a technique to make eutectic composite materials with well-ordered, periodic three-dimensional microstructures called Archimedean lattices.

Eutectics are homogeneous mixtures of elements that melt or solidify at a single temperature (eutectic point on the phase diagram) and have a lower melting point than the individual elements. Lead-tin alloys used for soldering, for example, can be fused at a temperature lower than each metal’s melting point. As a eutectic liquid solidifies, the components separate and self-assemble into a microscopic pattern, typically alternating layers.

However, says Paul Braun, materials science and engineering professor at the University of Illinois in Urbana-Champaign, “unlike fiber composites where you have a carbon fiber in a polymer matrix and everything is well organized so you can optimize properties, a typical solidified inorganic material has a random microstructure.”

Braun, Katsuo Thornton at the University of Michigan, and their colleagues combined templating and self-assembly to control the structure of a eutectic mix of silver chloride and potassium chloride. Directing self-assembling materials to grow in a template can give unique structures not formed when using either technique (self-assembly or template growth) alone. But “the use of a template to control the microstructure of a eutectic is really an unexplored area and is our unique contribution,” Braun says.

The AgCl–KCl eutectic forms a layered microstructure with alternating layers of each material as it cools and solidifies on its own. The researchers decided to see what would happen if it were allowed to self-assemble inside a template, in the present study a hexagonal lattice of nickel pillars that were 4–6 μm tall and 500–620 nm in diameter. They filled the molten eutectic into the template and then cooled it at different rates along the length of the pillars.

Outside the pillars, the eutectic still formed the usual layered structure. But as the layers solidified in the array of pillars, they fanned out in different ways depending on how fast the material was cooled down. As a result, inside the pillar array, the eutectic took on different periodic square, triangular, and honeycomb-shaped Archimedean lattices.

The team chose AgCl–KCl because it is a well-studied eutectic, but the principle is universal and “simulations show that this can be applied to any system,” says doctoral candidate Ashish Kulkarni, a co-author on their article published in a recent issue of Nature. The team is now looking into creating microstructured magnetic alloys using this method. Using other material combinations and templates could lead to structured metamaterials that can be used to control the flow of heat and light.

“This is a pretty creative approach to controlling self-assembly that allows new pattern behaviors of microstructures that were non-intuitive,” says Elizabeth Dickey, professor of materials science and engineering at North Carolina State University. The novel technique uses physical constraints to modify the kinetics of mass and thermal transport, she says, which allows control of the symmetry of the microstructures. “Symmetry is important for the meta property of any composite type structure,” she says. 

Read the abstract in Nature.