[Image above] Panes of glass. Credit: fabi42; Flickr CC BY-NC-SA 2.0
Glass is a mysterious thing.
Humankind has been making glass since about 5,000 B.C and has been using obsidian—naturally-formed glass—even longer. However, we still don’t understand well the fundamental structure of glass, which determines properties.
Scientists at the University of California, Davis are providing new structural insights, however, with a new publication in Science that describes the first-ever glimpse of a borosilicate glass structure transition under elevated pressures.
Boron atoms in borosilicate glass are arranged within flat triangles of three oxygen atoms (BO3), but that configuration changes in response to pressure and temperature changes. Those changes cause a transformation to tetrahedral structures of four oxygen atoms encasing a boron atom (BO4), according to the UC Davis press release.
Previously, these structures could only be studied in one coordination state or the other—but the new research provides a rare glimpse into the structural transition at pressures up to 2.5 GPa.
Corning scientist Randall Youngman, of the company’s Science and Technology Division, sums up the work nicely in a Perspective article from the same Science issue:
“Using a novel nuclear magnetic resonance (NMR) methodology, they measured the boron-11 NMR spectrum of a borosilicate glass as a function of pressure in situ, in contrast to the more common postcompression analyses of glass. Their findings, augmented with ab initio calculations, demonstrate deformation of planar BO3 triangles, a key feature in boron-containing glasses, that leads to their eventual conversion to fourfold coordinated boron.”
The study showed that applying isotropic pressure to ambient-temperature glass leads to an anisotropic response in the structure. The pressure pushes the boron atom out of the flat triangular plane to form pyramid-like structure units. The authors suggest this intermediate step allows the boron to contact another oxygen atom and form a tetrahedral structure around the single boron.
This rare glimpse is key because the structure of glasses determines their properties—and a more thorough understanding of that structure can help scientists tweak and manipulate glasses with specific characteristics. Lack of a high-pressure NMR probe prevented such studies from being possible in the past. The group succeeded in designing a high-pressure NMR probe that can maintain hydrostatic pressures up to 2.5 GPa.
“This is an unexpected finding that may have far-reaching implications for understanding a wide range of stress-induced phenomena in amorphous materials,” says Sabyasachi Sen, senior author and UC Davis materials science professor.
The paper is “Observation of the transition state for pressure-induced BO3→ BO4 conversion in glass” (DOI: 10.1126/science.1256224).