Researchers Create Glass Within A Glass; Mobile Silver Ions Appear Key To New Materials

In a paper published in the April 26 issue of the journal Nature, Professor Punit Boolchand and Research Associate Wayne Bresser demonstrate that the glass transition will occur at precisely the same temperature whenever silver iodide is placed in what they call a "special matrix" or "base glass."

"If you try to make silver iodide into a glass by going through the traditional melt/quench steps, you can't make it glassy. The solution was to put it in a matrix."

The UC researchers looked for special matrices that form glasses, but which are also "self-organized" which makes them relatively stress-free in molecular terms. "When we added silver iodide to these 'special matrices, we succeeded in making silver iodide glassy."

The new materials are so light sensitive and prone to tarnish that they must be made "fresh" periodically and stored in the dark. In a bit of economical genius, the reseachers discovered an empty Ovaltine jar works perfectly to perfect their samples on the 6th floor of the UC Engineering Research Center labs.

It took something much more complex and expensive to run the actual measurements. Boolchand uses a state-of-the-art Modulated Differential Scanning Calorimeter (MDSC) to precisely monitor when materials change phase. This equipment was important in measuring the key glass transition temperature. No matter which base glass or special matrix was used, the glass transition temperature remained the same: 75 degrees Celsius.

Boolchand says the unusual glass forming properties of silver iodide relate to the mobility of silver ions and the ability of silver iodide to serve as a solid electrolyte. "This was actually discovered by a German scientist back in 1910:

Today, it means the new materials not only demonstrate an "unprecedented increase in conductivity," they have potential applications in batteries for pacemakers and other high-demand devices, new sensors, and displays."

The University of Cincinnati researchers also saw the unusual glass transition in silver selenide, and they are now extending their work on both the experimental and theoretical fronts. Their calculations can predict where the glass transition will occur, and they are now testing new combinations of solid electrolytes and base glasses, including glasses which normally are insulators rather than semiconductors.

So, far they recognize three distinct classes of glasses:

* Class one where there is complete segregation of silver iodide into a distinct glass.

* Class two where some silver additive mixes with the base glass, while the remainder becomes glassy a different transition temperature

* Class three where all the silver additive mixes with the base glass, and only one glass transition temperature is seen.

The research is funded by the National Science Foundation.