Air
is a great insulator—except when it becomes a
conductor. Under the right conditions, miniature
lightning bolts of electricity will “arc” through
the air between two electrically conducting points.
Engineers can accurately predict how this happens
on the macroscopic scale—when electricity from a
downed power line, for example, jumps to a nearby
metal object—but less is known about the process
at the micro scale.
To help quantify the process, a new device and technique
have been developed at the National Institute of
Standards and Technology (NIST) for measuring “breakdown” voltage—the
voltage required to produce electrical arcs when
electrodes are 400 nanometers to 45 micrometers apart.
The advance could be useful in microelectronics,
such as in the design of microelectro-mechanical
systems (MEMS), in which arcing could cause device
failure. The NIST work may also be useful in the
design of larger scale electrical systems for automobiles.
The device (a novel capacitor design), described
in the March issue of Review of Scientific Instruments
,* includes two horseshoe-shaped circuits that each
end in two smooth rectangular electrodes. The distance
between electrodes of different circuits can be varied
in micrometer increments. To determine the breakdown
voltage for each distance, the researchers changed
the amount of voltage applied and measured the resulting
current flowing between the two electrodes. For example,
NIST researchers found that, at an electrode separation
of 7 micrometers, the breakdown voltage is 185 volts.
Conventional wisdom for electrical behavior at the
macroscale says that at least 360 volts are needed.
Air insulates at low electric fields because electrons
are scattered by nitrogen and oxygen atoms, thereby
slowing them down. But at higher fields, the electrons
gain enough energy to knock additional electrons
out of the molecules in the air, and a current can
form. Electrode shape is also a factor; sharp points
produce electric fields that vary with position,
and are therefore more likely to cause problems than
blunt shapes. The NIST capacitor enables very accurate
measurements because it produces nearly parallel
electric fields and has very smooth electrodes made
of thin films of gold only 70 nm thick (even slight
surface roughness amplifies electric fields).
*E. Hourdakis, B.J. Simonds, and N.M. Zimmerman.
2006. Sub-micron gap capacitor for measurement of
breakdown voltage in air. Review of Scientific Instruments
. March.
Media Contact:
Laura Ost, laura.ost@nist.gov , (301)
975-4034
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