Single crystals of metal

An amazingly simple method converts polycrystalline films into single crystals

Annealing in the Spzial oven turns a polycrystalline metal foil into a single crystal © Institute for Basic Science
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A single-crystal film: Researchers have developed an amazingly simple way to turn ordinary polycrystalline metal foils into valuable single crystals. The vertically suspended films are heated to temperatures just below the melting point - this dissolves the lattice boundaries, as the researchers report in the journal "Science". In the experiment, they produced such highly ordered copper, nickel or platinum of up to 30 centimeters in size.

Most solids are crystals: their atoms or molecules form regular lattice structures and thus characterize a large part of the material properties. However, the crystal lattices are not always consistent: Especially with metals, so-called grain boundaries often form. Because the material does not crystallize uniformly, areas with differently oriented crystal lattice arise. The metal piece becomes polycrystalline.

Losses due to grain boundaries

The problem is that "polycrystalline metals have many grain boundaries, which affects their electrical and mechanical properties, " explain Sunghwan Jin of the Korean Institute for Fundamental Research and his colleagues. For example, a copper single crystal has better conductivity than polycrystalline copper. For steel, the crystal structure plays a crucial role in the strength and flexibility.

However, producing single-crystal pieces of metal or foils has been very time-consuming: They must be grown from tiny seed crystals or applied as a thin film to monocrystalline substrates. "These methods, however, only lead to small and very expensive metal monocrystals, " the researchers say.

It is important to have a tension-free and stress-free suspension of the slides © Institute for Basic Science

Transform instead of new breeding

Jin and his colleagues have therefore chosen a different approach: they eliminate the grain boundaries with their method by forcing rearrangements of the lattice regions in the crystal lattice. For this, a thin, normal polycrystalline metal foil is suspended vertically and then heated to just below the melting point of the metal. The steady pull of gravity and the heat then cause the grain boundaries to disappear. display

In the experiment, the researchers tested this method with films of copper, nickel, cobalt, platinum and palladium. In the case of copper, for example, the film was suspended on quartz stators and heated in a furnace with hydrogen-argon atmosphere for several hours at 1050 degrees. By comparison, the melting point of copper is 1, 085 degrees.

Embers unify the crystal lattice

The result: The previously polycrystalline copper foils became single crystals of up to 32 square centimeters in size. In the case of nickel and cobalt, the conversion of polycrystalline to monocrystalline films was achieved in this way. As the researchers discovered, during conversion the individual grid areas gradually grew, until finally a uniform lattice type prevailed throughout the film.

In the case of platinum and palladium, the scientists used electric power as the heating method instead of the furnace, because these metals have a higher melting point. But even with that they successfully produced single crystals. "As long as the mechanical deformation during annealing is minimized, this method should also work for films of other metals, " say Jin and his colleagues. "Our results also argue that the production of single-crystal metal foils on an industrial scale is also possible."

Many applications

After all, such large and inexpensive monocrystal films would be in many areas, as the researchers emphasize. In electronics, for example, this would allow more conductive components. But these films would also be suitable for the production of high-tech materials such as graphene, boron nitride or diamonds. Because on them as a base defect-free layers and crystals of these substances can breed.

"Now that such cheap single-crystal metal foils are available, we are excited about how they will be used in the science and engineering community, " says Jins colleague Rodney Ruoff. (Science, 2018; doi: 10.1126 / science.aao3373)

(Institute for Basic Science, 22.10.2018 - NPO)