Mechanoluminescent Glass-Ceramic Could Provide Readout of Structural Strain

A team at Friedrich Schiller University Jena has developed a glass-ceramic material that emits light in response to mechanical stress. With further development, the mechanoluminescent material could provide a means to detect and monitor mechanical stress in buildings and other structures, as well as in artificial joints in the body, the team said.

Mechanoluminescence — light emission from an optically active material caused by mechanical stimulation — involves a mechanical stimulus that usually induces internal electric fields, for example, in a piezoelectric host material, triggering electronic transitions between dopant-associated energy levels that give rise to light emission. The current selection of materials with efficient mechanoluminescent emission is limited. Most of these materials are produced through conventional ceramic powder processing methods and are difficult to shape into various useful geometries.

The researchers turned to glass-ceramics, a relatively new material type that consists of crystals embedded in a glass matrix. The glass matrix allowed the material to be shaped using many of the same processes used for glass.

The glass-ceramic was composed of chromium-doped zinc gallate (ZGO) crystals embedded in a potassium germanate glass matrix. The crystals give the material its mechanoluminescent properties and avoid notably affecting the visual transparency of the glass due to their small size.

To create the material, the researchers developed a fast, stable crystallization process that allowed ZGO crystals to precipitate inside the glass after it has been shaped. Entropic phase separation and a self-limited crystal growth rate yielded a crystal number density above 10²³m⁻³. The residual glass matrix encapsulated the crystals in a dense, highly homogeneous material, and the microstructural stability and extended supercooling range of the glass enable glass-like processing.

“Most materials exhibiting mechanoluminescence have been made as powders, which aren’t very versatile,” said professor Lothar Wondraczek. He said the newly designed glass-ceramic allows glass-like processing approaches to be used to form virtually any shape, including fiber, beads, or microspheres that could be incorporated into various components and devices.

To demonstrate the ability of the glass-ceramic to emit light under mechanical stress, the researchers used the ball-drop test, a standard method for delivering a known impact force to a material.

The test showed that the mechanoluminescence response was reproducible and rechargeable, and that it exhibited a direct correlation with the impact energy, Wondraczek said.

The team now plans to adapt the glass composition so that the material can be made into sheet-like objects, optical fibers, and microscale spherical beads. Once the glass-ceramic has been formed into different shapes, the researchers will explore how it can be used in components and devices. The team plans to exploit the properties of glass-ceramics — for example, thermal, chemical, and mechanical stability — to obtain additional functionality from the material.

Friedrich Schiller University Jena researchers developed a glass-ceramic that emits light in response to mechanical stress. The highly transparent material is made from a potassium germanate glass matrix embedded with chromium-doped zinc gallate (ZGO) crystals that give the material its mechanoluminescent properties. Courtesy of Lothar Wondraczek/Friedrich Schiller University Jena.

As a straightforward way to visualize mechanical strain, mechanoluminescent materials could be useful for biophysical stress mapping and monitoring, optomechanical read-out of loading situations, information encryption and security labeling, or even photomechanical energy conversion.

“Our work could help mechanoluminescent materials find widespread use in a variety of applications, including light-emitting product labels and security codes,” Wondraczek said. “It also ties in well with the International Year of Glass, by demonstrating the wide versatility and unexpected properties of glassy materials.”

The research was published in Optical Materials Express (www.doi.org/10.1364/OME.459185).