September 25, 2012
Double-strength Glass May Be Within Reach With New Techniques
April Flowers for redOrbit.com - Your Universe Online
Glass is everywhere in our lives. From windows to phone screens to building materials, glass permeates our lives. Scientists who look at the structure of glass strictly by the numbers, however, think that some of the newest methods of microelectronics and nanotechnology could produce glass with twice the strength of what is currently available.A new study from Rice University, published in the Proceedings of the National Academy of Sciences, determined that a process called chemical vapor deposition, which is currently used to make thin films industrially, could yield a glass that withstands tremendous stress without breaking.
The research team's calculations are based on a modified version of a groundbreaking mathematical model first created to answer the decades old question of how glass forms. With the new modifications, their theory can now predict the ultimate strength of any glass, including the common silica varieties and more exotic polymer and metal types.
Glass has a unique molecular structure. It freezes into a rigid form when cooled, but unlike ice with regular crystalline patterns, the molecules in glass are suspended randomly as if they were in liquid form, with no particular pattern. The strong bonds that form between these randomly arrayed molecules are what holds the glass together and determines its ultimate strength.
The ability to handle great stresses before giving way, sometimes rather explosively, is shared by all glass types. Exactly how much strain a glass type can handle is determined by how much energy it can absorb before reaching the limitations of its intrinsic elastic quality. This limitation seems to be as much a quality of the processing and manufacturing as it is the material.
There has been a longstanding debate amongst materials scientists about what occurs when glass hardens and cools. This transition is one of the last great mysteries of the field. Though the cooling temperatures for particular kinds of glass are well defined by centuries of experience, Peter Wolynes of the Center for Theoretical Biological Physics at Rice´s BioScience Research Collaborative and his colleagues argue that it may be possible to use this information to improve upon glass's ultimate strength.
The configurational energy — the positive and negative forces between molecules — held in stasis by the “freezing” process and elastic properties of the finished product determine how close a glass gets to the theoretical ideal – the most stable glass possible.
“The usual impression of glass is that, relative to other materials in your life, it seems easy to break,” said Wolynes, Rice´s Bullard-Welch Foundation Professor of Science and a professor of chemistry. “The reality is that when it´s freshly made and not scratched, glass is very strong.”
Wolynes has an interest in glass that goes back over many years. His specialty is in how molecular systems move across microscopic “energy landscapes," particularly as they relate to protein folding in biology, and his random first-order transition theory of glasses helped set the stage for decades of debate over how glass forms. However, that theory did not take into account the strength of glass.
“You can come up with a theory of something and ignore one of the most practical implications because you just don´t think about it,” Wolynes explained.
“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 said.
People rarely think about glass until it breaks because it is such an everyday part of our lives.
“Even though we now have Gorilla Glass and other tempering developments, they´ve been developed in a somewhat Edisonian fashion,” he said, noting that such hardened glasses commonly used in cell phones have a self-healing surface treatment that protects the glass itself from scratching. “Our paper is about what determines the limits on the strength of the glass, if there is no surface problem.”
Materials strength has been studied since the 1920's when Russian scientist Yakov Frenkel “calculated how strong something could be if we just take into account the direct forces between atoms. He made a simple calculation: If you have a row of atoms and pull it over another row of atoms, when would it go from one way of aligning to the next?” This determines the material's elastic modulus, basically how springy the material is.
“The elastic modulus is related to the thermal vibrations in the material,” Wolynes said. “Basically, if you have a material that has a very high melting point, its elastic modulus is also very high. According to Frenkel, the strength should also be very high. That overall trend is true. That´s why fighter jets are made of titanium, one the highest-melting metals, and low-melting aluminum, which is not as strong but lighter, is used for other things.”
This theory seems counterintuitive for glass, however.
“In the early days, when people first measured the properties of glasses, they found they were easily breakable. Silica glass is very high-melting, so you´d expect it to be strong,” Wolynes said. “Then they did finally figure out this was because cracks at the surface were propagating in. If they could eliminate the cracks, they would get much higher strengths.”
Metallic glasses, for example Liquidmetal licensed by Apple for their consumer electronics, are at about a quarter of the Frenkel strength. The team wondered what limited the strength of such glasses.
"We ask whether the collective motions that go on in liquids as they´re becoming glasses are the same motions that are being catalyzed when we stress the material. 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 and the glass breaks," he said.
The theoretical results closely match experimental results for most materials.
“The good news is, according to this theory, if you could make a material that is much closer to ideal glass — the glass you would get if you could make it infinitely slowly — then you would be able to increase its strength.”
The team's research indicates this might not be possible for traditional cooling of silica, metal and polymer glasses as calculations indicate they are approaching their limits. But it might be possible through vapor deposition of atoms. This process would be akin to chemical vapor deposition processes used in microelectronics and nanotechnology.
“It would require tuning the deposition rate to the liquid/glass transition properties,” Wolynes said. "Our theory says the best you can do with this is get about halfway to ideal glass,” which he said some experimentalists have demonstrated. “It´s possible there´s some loophole we don´t yet see that will let us get even closer to the ideal. But at least, at this point, we can get halfway there. That means it would be possible, in principle, to get glass with at least twice the intrinsic strength of current glasses.”
This theory comes equipped with a caveat, however. Glass hardened in such a way can still be destroyed and probably in a much more dramatic way than traditional glasses.
“If you could have something infinitely strong, then you´d never need to worry about it,” he said. “But there´s a little bit of a problem if you make something that´s very strong but can eventually break. It contains a huge amount of energy, so when it breaks, it fails catastrophically.”