04-16 flexible glass 2

[Image above] Glasses in the binary sulfur–selenium system are likely the only examples of inorganic glass that are flexible in bulk form. Credit: Sabyasachi Sen

When it comes to electronics, people like them opposite to how they like their ketchup—the thinner (not thicker), the better.

The desire for slim, sleek designs in laptops, tablets, and mobile phones is one reason why materials that are small and thin (think: graphene) receive so much press coverage—because they allow realization of these idealized designs. Additionally, materials that are thin usually allow for another property that smartphone manufacturers are pushing big-time this year: flexibility.

When a material bends, areas farther away from the bend axis experience more stress than areas near the bend axis. Imagine a bend like the pinwheel maneuver performed at horse shows—horses at the edge of the pinwheel move a lot more (experience more stress) than horses at the center (near the axis).

For a material like glass that is brittle in bulk form, creating it in thin sheets can allow more flexibility because no one area is far enough removed from the bend axis to experience breaking-point stress. Corning used this knowledge when it created Willow Glass—a flexible glass between 100–200 µm thick—and a new ultrathin bendable glass, which is still in development.

From an electronics standpoint, this necessity that glass be made thin in order to bend aligns with preferred device design. But from a materials standpoint, wouldn’t it be cool if we could create a glass that bends in the bulk form, even if applications of such a material are not yet known?

That wonder for discovery—can such a glass be created?—is why ACerS member Sabyasachi Sen, professor of materials science and engineering at the University of California, Davis, finds glasses in the sulfur–selenium (S–Se) system so fascinating. According to Sen, these chalcogenide glasses are the only examples of inorganic glass that could be flexible in bulk form.

“To the best of our knowledge, this type of glasses are the only examples of inorganic glass (non-polymer) being flexible in the bulk form (not in 100 micron-thick sheets as in Corning’s Willow glass),” Sen says in an email. He adds that also unlike Willow Glass, the S–Se glasses are transparent in the infrared rather than visible spectrum and so would be used in different application areas.

A cylinder of glass composition S75Se25, 7 mm in diameter. UC Davis professor Sabyasachi Sen says glasses in the S–Se system are so far the only examples of inorganic glasses that are flexible in bulk form. Credit: Sabyasachi Sen

Sen says they discovered the glasses’ unique flexibility while preparing samples for a structural study last year (a paper on that study was published in The Journal of Physical Chemistry B).

In the structural study, they and researchers from the National High Magnetic Field Laboratory and Corning Inc. found that tendencies evident in individual elements—amorphous sulfur is predominantly composed of S8 molecular rings, while amorphous selenium consists almost exclusively of polymeric [Se]n chains—are mostly preserved in binary S–Se glasses, though there is significant mixing between sulfur and selenium in both chains and rings.

Following the structural study, Sen and his UC Davis colleagues made S–Se samples of varying compositions that were a couple of inches long for mechanical measurements. They have not yet completed these measurements, but “just being able to bend them between fingers showed how uniquely flexible they are!” Sen adds.

One downside to S–Se binary glasses is that they are not very stable at room temperature because these glasses—particularly those with high sulfur concentration—have glass transition temperatures (Tg) a few degrees below room temperature. When placed in environments above their Tg for extended periods, these glasses tend to crystallize. Sen says investigating this challenge (in addition to mechanical measurements) will be the next step in their research.

“We have plans to extend this study to investigate if we could add a small amount of ‘something’ (such as a cross-linker) to stabilize these glasses at ambient [temperature] and still keep their flexibility,” Sen says.

Credit: Sabyasachi Sen

Even though it is not yet clear what practical applications of these glasses will be, I know one application in which this glass would excel—as an awesome science demo for students!

The paper on these glasses’ structure, published in The Journal of Physical Chemistry B, is “Structure and chemical order in S–Se binary glasses” (DOI: 10.1021/acs.jpcb.8b10052).