Humans have been making glass from silicon dioxide since prehistory, and industrialization has brought boron-based glasses, polymer glasses and metallic glasses. Now an international team of researchers has developed a new family of glass based on metals and organic compounds that measures up to the original silica in glass-forming ability.
The key to making glass is to melt the source materials and then cool them so that no crystals form. One way of doing this is by rapid cooling or quenching, a process which shortens the time available for crystals to form due to the rapid temperature drop.
Basic silica glass has a tetrahedral structure with silicon in the center and four oxygen atoms at the corners. The metal-organic glass produced substitutes zinc for silicon, but uses two similar but different organic compounds at the corners — imidazolate and benzimidazolate. These organic molecules randomly take the place of the oxygen atoms at the tetrahedron corners.
Silica glass demonstrated the best glass-forming ability of all glasses until the researchers synthesized zinc-based ZIF-62 metal-organic glass. The more benzimidazolate incorporated into the metal/organic framework, the better the glass-forming ability. The more viscous liquids with the bulkiest ligands tend to resist crystallization the best. 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.
Zinc-based organic glass is more difficult to produce than silica glass because the organic compounds imidazolate and benzimidazolate first need to be synthesized, mixed with hydrous zinc nitrate and a solvent. The mixture then melts at approximately 800 degrees F. The mixture must melt completely, but not reach approximately 980 degrees F, at which point the molten glass will vaporize.
Materials scientists from Wuhan University of Technology, China; University of Cambridge, U.K.; National Institute of Chemistry, Slovenia; Commonwealth Scientific and Industrial Research Organisation, Australia; Qilu University of Technology, China; The Pennsylvania State University; Aberystwyth University, U.K.; and Aalborg University, Denmark, participated in this research, which is published in Science Advances.