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September 25th, 2012

Understanding ideal glass structure could be key to doubling glass strength

Published on September 25th, 2012 | By: Eileen De Guire

No more broken glass? Theoretical work by researchers at Rice University shows the potential for doubling the strength of glass. Credit: Rice University.

Glass is one of the intrinsically strongest of manmade materials, however, only a fraction of that strength is retained. Glass is arguably one of the most useful manmade materials, too, and if more if its intrinsic strength could be captured, it could be infinitely more useful.

The importance of the issue is evident in several recent initiatives. In the May 2012 issue of the Bulletin, the cover story article describes the work of a coalition of manufacturers and glass researchers to establish the “Usable Glass Strength Coalition,” whose mission is to support fundamental research into the sources of flaws and cracks in glass.

The (oft-mentioned by me) Grand Challenges in Ceramics and Glasses workshop last spring identified this issue, too, referring to it the “Harnessing order within disorder” challenge. The idea is that if the short- and intermediate-range order in glasses and its effects on glass properties were better understood, something could be done about controlling and engineering glass structure at and near the surface, and hence, its properties, especially strength.

The German Research Foundation (analogous to the US National Science Foundation) last fall made improving glass strength a priority when it established a six-year priority program called “Topological Engineering of Ultra-strong Glasses.” The program goal is described as “dedicated to the exploration of the mechanical properties of disordered materials on a molecular or atomic level, bridging the fields of metallic glasses and inorganic oxide glasses. It considers the topological origin of elasticity, plasticity and fracture in these two classes of materials with the objective of setting the path towards the design of glasses with superior toughness and/or stiffness.”

How much strength are we talking about? The draft report from the Grand Challenges workshop (full report to be published in the December issue of JACerS) says that tensile strengths of up to 14 GPa have been demonstrated for silica fibers drawn in vacuum. However, the practical strength of glass is only about 50 MPa, a loss of three orders of magnitude!

The draft report also points out that disordered solids like glass are “far from their equilibrium states.” Thus, the glassmaking process matters. The report explains, “One consequence is that the structure and properties can be especially dependant on the synthesis route, particularly on the thermal or energetic history of the synthesis process.” The group calls for better computational methods and in-situ characterization and measurement tools, to generate better and more accurate theories of the structure-property relationship in glass.

A research team at Rice University just published a paper in the Proceedings of the National Academy of Sciences that may have taken a big step in understanding the structure-strength relationship in glasses.

Peter Wolynes and his students, a university press release says, modified a mathematical model for glass formation that Wolynes had developed decades earlier. The original model—a random first-order transition theory of glasses—described the “molecule’s kinetic properties as they cool” and subsequently has led a robust scientific debate over how glass forms.

However, the model only addressed how the glassy state forms and did not address the issue of glass strength. The news release relays that Wolynes got to thinking about the strength issue after a discussion he had when he unexpectedly crossed paths with a metallurgist. “We had never worked on that kind of property, and the problem struck me as intriguing—and relatively simple in the framework of the theory we already had. We just hadn’t thought to calculate it,” he says.

The new paper considers what the strength of glass would be if “there were no surface problem.” The Wolynes model builds on previous work by Yakov Frenkel (of Frenkel defect fame) that modeled the strength of materials based just on the strength of the bonds between atoms. The team, wondering how strength is limited, looked at whether the “collective motions that go on in liquids as they’re becoming glasses” are the same motions that happen when the materials is under stress.

“Basically, we applied our theory for what determines how the liquid rearranges as it’s becoming glass. Add to that the extra driving force when you apply stress, and see what that predicts for the limit of how much it can be pushed before the atoms roll over each other,” i.e., breaking the glass, he says in the press release.

The team’s model shows that strength is related to the material’s elastic modulus, but also on the “configurational energy” that is frozen into the glass. A high elastic modulus generally corresponds to a high melting point and a high strength. Wolynes cites the example of titanium. The reason fighter jets are built of is that titanium alloys are strong and light, but titanium is also high-melting (compared, for example, to lower melting, less strong aluminum alloys).

The configurational energy is the entropy associated with the position of atoms independent of their motion (velocity, momentum, etc.). In the press release, Wolynes explains that the theory says that the closer the structure is to an ideal glass, the closer the strength will be to the ultimate strength. Cooling a glass infinitely slowly would do that, but (obviously) is not practical. The team postulates, however, that ideal glassy structures might be fabricated, for example, via chemical vapor deposition.

Wolynes admits that the theory says the “best you can do with this is get about halfway to ideal glass.” But, in his opinion, it is a doorway to doubling the practical strength of glass. That may be enough to interest the Usable Glass Strength Coalition, and it may be a good start on addressing the issues raised by the research community here and abroad.

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