Secrets of glass unlocked by studying crystals

LONDON - We use glass in our daily lives in many ways - as crockery, mirrors or to see what the weather is like before going outside.

What’s not known to many is that “glass transitions,” where changes in structure of a substance accompanying temperature change get “frozen in,” can show up during cooling of most any material, liquids through metals.

This produces “glassy states,” of that material - exotic states that can be unfrozen and refrozen by merely changing the temperature a little up and down around the transition temperature.

“For liquids, it’s fairly simple, glasses form when crystals don’t,” said C. Austen Angell, an Arizona State University Regents professor.

Beyond this, matters get slightly more complicated.

The new study uses the unusual behavior of a non-liquid substance to help unlock the secrets. It is a metallic alloy consisting of equal parts of cobalt and iron.

Angell and his colleagues - Shuai Wei, Isabella Gallino and Ralf Busch, all of Saarland University, Germany - describe the behaviour of iron-cobalt (Fe50Co50) superlattice material as it cools down from its randomly ordered high temperature state.

Heat capacity is the amount of energy it takes to heat a sample by one degree Kelvin.

The iron-cobalt alloy heat capacity showed two features - a sharp spike at 1000 K (1340 F) called a lambda transition (which is quite common in metal alloys as the two types of atoms order themselves onto two individual interpenetrating lattices) - and near 750K (890 F) another feature which is very unusual for a metallic crystal, a glass-like transition, where the state of order gets “frozen in” during cooling.

Most glassy forms of matter experience a gradual increase in heat capacity as they are heated until this special transition point is reached. At this point (called the glass temperature) the materials suddenly jump to a new, higher heat capacity zone, often 100 percent higher, and change from a solid material to a very viscous liquid.

The study shows that the disordering of the superlattice has the kinetic characteristic of strong liquids. But because the alloy lambda transition is well understood, researchers know that a property called the “correlation length” is decreasing as the temperature decreases from the lambda spike towards the (glass) transition temperature. This is the opposite behavior from what has been thought to be characteristic of liquids as they cool towards their transition temperature.

“We now argue that strong and fragile extremes are not really extremes, so much as they are opposites. This shows that static correlation length changes do not, by themselves, account for the liquid turning solid at the glass transition,” Angell said.

“So now we see strong liquids and fragile liquids as occupying opposite flanks of some generalized ‘order-disorder’ transition,” Angell explained.

He pointed out that if optical fiber glasses, being silica or silica-like, have shorter range organization at lower temperatures, then fibers that have been annealed at lower temperatures than their fiber-drawing temperature (more than 2000 K) should be less scattering of light, hence better for communications purposes. Thus this new information can mean better performing materials in the future.

“Patent literature suggests that the fiber optics scientists already learned the benefits of annealing (a heat treatment that alters the microstructure of a material causing changes in properties such as strength, hardness and ductility). Now we would know exactly why this is so, and we could actually design that property into the material forming process,” Angell said.

The study appears in the Nov. 28, 2010, issue of Nature Physics. (ANI)