Understanding 'metallic water'
The electrically conducting
structure of metallic water
occurs at a more accessible
part of the water phase
diagram than formerly
thought. Here, a snapshot
from a computer simulation
demonstrates the atomic
disorder. Red spheres are
hydrogen atoms, white
spheres are oxygen atoms,
and the electron density
from a partially occupied
electron state responsible
for the conductivity is
shown in gold.
A new computational model developed
at Sandia alters significantly the theoretical
diagram used to understand water at extreme
temperatures and pressures and expands the
known range of water’s electrical conductivity,
according to Labs’ researchers Mike Desjarlais
and Thomas Mattsson.
The Sandia work shows that phase boundaries
for “metallic water” — water with its electrons
able to migrate like a metal’s — should be lowered
from 7,000 to 4,000 kelvin and from 250 to
100 gigapascals of pressure. A phase boundary
describes conditions at which materials change
state. An example would be water changing to
steam or ice. In this instance, water in its pure
state — an electrical insulator — becomes a
conductor.
One ramification of the lowered boundary
is revision of astronomy calculations of the
strength of the magnetic cores of gas-giant
planets like Neptune. Because the planet’s temperatures
and pressures lie partly in the revised
sector, electrically conducting
water probably
contributes to Neptune’s
magnetic field.
This view of Neptune, from
NASA’s Voyager spacecraft,
shows its blue-green
atmosphere.
The recent calculations
— published in
Physical Review Letters
last summer and presented
at the 12th International
Workshop on
the Physics of Non-Ideal
Plasmas, held in Darmstadt,
Germany — agree
with experimental
measurements in research
led by Peter Celliers of
Lawrence Livermore National Laboratory.
The electrically conducting
structure of metallic water
occurs at a more accessible
part of the water phase
diagram than formerly
thought. Here, a snapshot
from a computer simulation
demonstrates the atomic
disorder. Red spheres are
hydrogen atoms, white
spheres are oxygen atoms,
and the electron density
from a partially occupied
electron state responsible
for the conductivity is
shown in gold.
The effort began with a look at a specific
problem. “We were trying to understand
conditions at [a powerful Sandia accelerator
known as] Z,” says Mattsson, a theoretical
physicist, “but the problems
are so advanced that they linked to
other branches of science.”
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