Glass is strong but shows potential to be stronger, according to theoretical work by researchers at Rice University. |
Glass is strong enough for so much: windshields, buildings,
and many other things that need to handle high stress without breaking. But
scientists who look at the structure of glass strictly by the numbers believe
some of the latest methods from the microelectronics and nanotechnology
industry could produce glass that’s about twice as strong as the best available
today.
Rice University chemist Peter Wolynes is one of them.
Wolynes and Rice graduate student Apiwat Wisitsorasak determined in a new study
that a process called chemical vapor deposition, which is used industrially to
make thin films, could yield a glass that withstands tremendous stress without
breaking.
Wolynes, a senior scientist with the Center for Theoretical
Biological Physics at Rice’s BioScience Research Collaborative, and
Wisitsorasak reported their results in the Proceedings
of the National Academy of Sciences. Their calculations were based on a
modified version of a groundbreaking mathematical model that Wolynes first
created to answer a decades-old conundrum about how glass forms. With the
modifications, Wolynes’ theory can now predict the ultimate strength of any
glass, including the common varieties made from silica and more exotic types
made of polymers and metals.
If metal glass sounds odd, blame it on the molecules inside.
Glass is unique because of its molecular structure. It freezes into a rigid
form when cooled. But unlike ice, in which water molecules take on regular
crystalline patterns—think of snowflakes—the molecules in glass are suspended
randomly, just as they were as a liquid, with no particular pattern. The strong
bonds that form between these randomly-arrayed individual molecules are what
hold the glass together and ultimately determine its strength.
All glasses share the ability to handle a great deal of
strain before giving way, sometimes explosively. Exactly how much strain a
glass can handle is determined by how much energy it can absorb before its
intrinsic elastic qualities reach their limitations. And that seems to be as
much a property of the way the glass is manufactured as the material it’s made
of.
Materials scientists have long debated the physics of what
occurs when glass hardens and cools. In fact, the transition is one of their
last great puzzles of the field. Cooling temperatures for particular kinds of
glass are well defined by centuries of experience, but Wolynes argues it may be
possible to use this information to improve upon glass’s ultimate strength.
The elastic properties of the finished product and the
configurational energy (the positive and negative forces between the molecules)
held in stasis by the “freezing” process determine how close a glass gets to
the theoretical ideal—the most stable glass possible, he says.
“The usual impression of glass is that, relative to other
materials in your life, it seems easy to break,” says 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, who specializes in how molecular systems move
across microscopic “energy landscapes,” particularly as they relate to protein
folding in biology, has an interest in glass that goes back many years. His random
first-order transition theory of glasses, which quantifies the molecules’
kinetic properties as they cool, helped set the stage for decades of debate
among theorists over how glass actually forms.
But the theory, based on work by Wolynes and collaborators
that goes back to 1989, did not consider 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 explains.
A chance encounter with a metallurgist last year made
Wolynes think again about just how strong glass could be. “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.
Traditional glass is so ubiquitous that people rarely think
about it (until it breaks). “Even though we now have Gorilla Glass and other
tempering developments, they’ve been developed in a somewhat Edisonian
fashion,” he says, 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.”
Wolynes notes the strength of materials has been studied
since the 1920s, 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?” Wolynes says that determines a material’s elastic modulus—”how springy
the material is”—an easy concept to understand in metals that bend before they
break.
“The elastic modulus is related to the thermal vibrations in
the material,” he says. “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.”
The theory didn’t seem to relate to glasses, 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 says. “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.”
Current metallic glasses like the Liquidmetal famously licensed
by Apple for consumer electronics “come to be about a quarter of this
theoretical Frenkel strength,” Wolynes says. “So what is it that limits their
strength? 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 says.
He notes the theoretical results closely match experimental
ones 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.” That may not be possible through traditional cooling of silica,
metal and polymer glasses, which Wolynes’ and Wisitsorasak’s calculations
indicate are approaching their limits.
But it might be possible through vapor deposition of atoms,
akin to the chemical vapor deposition process used in microelectronics and
nanotechnology to make thin films. “It would require tuning the deposition rate
to the liquid/glass transition properties,” he says.
“Our theory says the best you can do with this is get about
halfway to ideal glass,” which he says 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,” Wolynes says. “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.”
Wolynes’ theory comes with a caveat, though. Glass hardened
even to the point of near indestructibility can still be destroyed, and with
dramatic effect. “If you could have something infinitely strong, then you’d
never need to worry about it,” he says. “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.”
Source: Rice University