Our knowledge of glass is steadily improving and it appears
manufacture using a vapor deposition method could be used to double
current strength specifications. This takes it to the half way mark
toward the theoretical limit.
All good stuff and adds to the persistent improvements we have
witnessed in the technology.
It is also nice to know that we have plenty of room for going much
further. A doubling of strength takes it way beyond most
specifications and plausibly allows for items that are truly
unbreakable in terms of real application.
Glass half full:
Double-strength glass may be within reach
MIKE WILLIAMS
SEPTEMBER 20, 2012
Rice University study
suggests possible method for increasing the strength of glass
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 this week 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 said.
“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, 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
explained.
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 said.
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 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.”
Wolynes noted 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 said 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 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.”
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 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.”
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 said. “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 said.
He noted 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 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,” Wolynes said. “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 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.”
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