Qiming Wang and Xuanhe Zhao are researchers at Duke Univ. Credit: Duke Univ. Photography.
Univ. team has seen for
the first time how soft polymers, such as wire insulation, can break down under
exposure to electrical current.
Researchers have known for decades that polymers,
such those insulating wires, may break down due to deformation of the polymers.
However, the process had never been seen.
In a series of experiments, Duke Univ.
engineers have documented at the microscopic level how plastic deforms to
breakdown as it is subjected to ever-increasing electric voltage.
The findings by the Duke engineers could help in
developing new materials to improve the durability and efficiency of any
polymer that must come into contact with electrical currents, as well as in the
emerging field of energy harvesting.
“We have long known that these polymers will
eventually break down, or fail, when subjected to an increasing electrical
voltage,” said Xuanhe Zhao, assistant professor of mechanical engineering
and materials science at Duke’s Pratt School of Engineering. He is the senior
scientist in the series of experiments performed by a graduate student Qiming
Wang. “Now we can actually watch the process as it happens in real
The work is published online in the Physical Review Letters.
This photo shows the creasing to cratering in soft polymer. Credit: Qiming Wang.
By attaching a soft polymer to another rigid
polymer layer, or protective substrate, the Duke team was enabled to observe
the deformation process without incurring the breakdown. They then subjected
the polymer-substrate unit to various electrical voltages and observed the
effects under a microscope.
“As bread dough rises in a bowl, the top
surface of the dough may fold in upon itself to form creases due to compressive
stresses developing in the dough,” Zhao said, “Surprisingly, this
phenomenon may be related to failures of electrical polymers that are widely
used in energy-related applications.”
“When the voltage reached a critical point,
the compressive stress induced a pattern of creases, or folds, on the
polymer,” Zhao. “If the voltage is increased further, the creases
evolved into craters or divots in the polymer as the electrical stress pulls
the creases open. Polymers usually break down electrically immediately after
the creasing, which can cause failures of insulating cables and organic
The substrate the researchers developed for the experiments not only allowed
for the visualization of the creasing-to-cratering phenomenon, it could also be
the foundation of a new approach to improving the ability of wires to carry