Unusual Material Expands Dramatically Under Pressure

Unusual Material Expands
Dramatically Under Pressure
July 18, 2013 — If you squeeze a
normal object in all directions, it
shrinks in all directions. But a few
strange materials will actually grow
in one dimension when compressed.
A team of chemists has now
discovered a structure that takes this
property to a new level, expanding
more dramatically under pressure
than any other known material. The
finding could lead to new kinds of
pressure sensors and artificial
muscles.
Andrew Cairns, a graduate student at
the University of Oxford and a
member of the research team, will
discuss the new material and its
applications at the American
Crystallographic Association meeting
held July 20-24 in Honolulu.
Negative linear compression, or NLC,
has existed for millions of years; in
fact, biologists believe octopi and
squid use the phenomenon to make
their muscles contract. Only in recent
decades, however, have scientists
learned to design materials with this
property. Until a few years ago, none
of these humanmade structures had
been found to expand more than a
fraction of a percent under
compression, making them of limited
use in engineering. But researchers
are now learning how to design
materials that expand far more than
those previously known. The trick,
say the scientists presenting this
latest work, is to look for structures
that can respond to pressure by
rearranging their atoms in space
without collapsing.
The material the research team
discovered, zinc dicyanoaurate, does
just that. Its unique structure
combines a spring-like helical chain
of gold atoms embedded in a
honeycomb-like framework made of
gold, cyanide (carbon bonded to
nitrogen), and zinc. When the chain is
compressed, the honeycomb flexes
outward by as much as 10% --
several times what had been
achieved by any previous material.
The scientists call this large response
"giant negative linear
compressibility," and compare it to a
collapsible wine rack that folds up
horizontally by expanding
substantially in the vertical direction.
Andrew Goodwin of Oxford, leader of
the research team, says these wine
rack structures represent "a new
block in our Lego kit."
Zinc dicyanoaurate's unique
properties make it promising for
several applications. In the
immediate term, the material, which
is transparent, could be used as an
optical pressure sensor. Compression
causes the crystal spacing to narrow
in one direction and widen in
another, changing the path light
takes through the material in a way
that is sensitive to tiny variations in
pressure. A longer-term application is
artificial muscle design. Our muscles
contract in response to an electric
field, but new muscles could be
designed to contract when pressure is
applied, as biologists believe octopus
muscles do.
Goodwin's team is now working to
understand more fully the
mechanisms behind NLC. But even
without a complete picture of
nature's design principles, they feel
confident zinc dicyanoaurate is
already "pushing the limits" of how
far any material will be able to
expand under pressure. "We've got a
pretty good feel for what the limits
are," Goodwin says. "This material is
pretty special."