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A D V A N C E D
M A T E R I A L S
&
P R O C E S S E S | J U N E
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WOODEN WINDOWS SEE
BRIGHT FUTURE
Windows and solar panels could
one day be made from one of the best—
and least expensive—construction
materials known: wood. Researchers
at KTH Royal Institute of Technology,
Sweden, developed a new transparent
wood material suitable for mass pro-
duction. “Transparent wood is a good
material for solar cells, because it’s a
low-cost, readily available, and renew-
able resource,” says Professor Lars
Berglund. “This becomes particularly
important in covering large surfaces with
solar cells.” The optically transparent
wood is a type of wood veneer in which
lignin—a component of the cell walls—is
chemically eliminated. “When lignin is
removed, the wood becomes beautifully
white. But because wood is not naturally
transparent, some nanoscale tailoring
is required,” he says. The white porous
veneer substrate is impregnated with
a transparent polymer, and the optical
properties of the two are then matched.
SELF-HEALING MATERIAL ACTS
AS ARTIFICIAL MUSCLE
A new, extremely stretchable poly-
mer film created by researchers at Stan-
ford University, Calif., can repair itself
when punctured, an important feature
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OUTRAGEOUS MATERIALS GOODNESS
Transparent wood is made by
removing the lignin in the wood
veneer. Courtesy of Peter Larsson.
Jolting a new polymer material with an
electrical field causes it to twitch in a
muscle-like fashion. It can also stretch to
100 times its original length and repair
itself if punctured.
Flexible electrocaloric fabric of nanowire
array can provide personal cooling. Cour-
tesy of Qing Wang/Penn State.
in a material that could act as artificial
muscle. Damaged polymers typically
require a solvent or heat treatment to
restore their properties, but this new
material has a remarkable ability to
heal itself at room temperature, even
if damaged pieces are aged for days.
Researchers even found that it could
self-repair at temperatures as low as
-4°F (-20°C), about as cold as a commer-
cial walk-in freezer.
The team attributes the extreme
stretching and self-healing ability of
its new material to critical improve-
ments to a chemical bonding process
known as crosslinking. This process,
which involves connecting linear chains
of linked molecules in a fishnet pat-
tern, has previously yielded a tenfold
stretch in polymers. For more informa-
tion: Zhenan Bao, ,
.
NANOWIRE ARRAY
COOLS CLOTHING
Firefighters entering burning
buildings, athletes competing in the
broiling sun, and workers in foundries
may eventually be able to carry per-
sonal lightweight cooling units, thanks
to a nanowire array that cools. “Most
electrocaloric ceramic materials con-
tain lead,” says Qing Wang, professor
of materials science and engineering
at Pennsylvania State University, State
College. “We try not to use lead. Con-
ventional cooling systems use coolants
that can be environmentally problem-
atic as well. Our nanowire array can
cool without these problems.”
The vertically aligned ferroelec-
tric barium strontium titanate nano­
wire array can cool about 5.5°F using
36 V—an electric field level safe for
humans. A 500 gram battery pack
about the size of an iPad could power
the material for roughly two hours. The
material is grown in two stages. First,
titanium dioxide nanowires are grown
on fluorine doped, tin oxide coated
glass. Researchers use a template so
all the nanowires grow perpendicular
to the glass surface and to the same
height. Then, barium and strontium
ions are infused into the titanium diox-
ide nanowires. A nanosheet of silver is
applied to the array to serve as an elec-
trode. This nanowire forest can then be
moved from the glass substrate to any
substrate—including clothing fabric—
using a sticky tape. .