The shows the tetrahedral structure of ZIF-62. Image: Ang Qiao, Penn State.
The shows the tetrahedral structure of ZIF-62. Image: Ang Qiao, Penn State.

Lightning and volcanos both produce glass, and humans have been making glass from silicon dioxide since prehistory. Industrialization has brought boron-based glasses, polymer glasses and metallic glasses, but now an international team of researchers has developed a new family of glass based on metals and organic compounds that stacks up to the original silica in glass-forming ability.

Glass-forming ability refers to the ability of a liquid to avoid crystallization during cooling. "Glass is a liquid frozen into a solid-like material in non-crystalline form," explained John Mauro, professor of materials science and engineering at Penn State. "Mechanically it behaves as a solid but it is somewhere between a liquid and a solid."

The key to making glass is to melt the source materials and then somehow cool them so that no crystals form. One way of doing this is by rapid cooling or quenching, which shortens the time available for crystals to form due to the rapid temperature drop.

Basic silica glass has a tetrahedral structure – a triangular pyramid – with silicon in the center and four oxygen atoms at the corners. Each oxygen attaches to another silicon-centered tetrahedron. As they report in a paper in Science Advances, to produce their novel metal-organic glass, the researchers substitute zinc for silicon and substitute the oxygen atoms at the corners of the tetrahedron for two similar but different organic compounds– imidazolate and benzimidazolate.

Silica glass had the best glass-forming ability of all materials until the researchers synthesized their zinc-based metal-organic glass, termed ZIF-62. They noticed that the more benzimidazolate they incorporated into the metal/organic framework, the better the glass-forming ability. This is because more viscous liquids with the bulkiest ligands tend to be best at resisting crystallization.

ZIF-62 has the best glass-forming ability of 50 existing glasses, and while standard silica glass is brittle, the metal-organic glass is far more pliable, which may also add to its glass-forming ability. However, zinc-based organic glass is more difficult to produce than silica glass because the organic compounds imidazolate and benzimidazolate first need to be synthesized and then mixed with hydrous zinc nitrate and a solvent. Finally, this mixture needs to be melted at approximately 800°F.

Temperature is important because the mixture has to melt completely, but must not reach approximately 980°F, because at that point the molten glass will vaporize.

The researchers studied several material properties of the metal-organic glass using a variety of spectroscopic techniques and x-ray diffraction. They also measured the molten glass's mechanical properties and optical transmission.

"This family of glasses is so new that, while we have determined its glass-forming ability and a few other properties, we have not fully characterized all of its material properties," said Mauro. "There also needs to be research into how to scale-up this process for manufacturing."

The researchers would also like to identify other metallic-organic combinations to make different glasses. They next plan to investigate cobalt-based glass.

This story is adapted from material from Penn State, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.