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In the Z experiment, the volume of water shrank
abruptly and discontinuously, consistent with the formation of almost
every known form of ice except the ordinary kind, which expands. (One
might wonder why this ice shrank instead of expanding, given the
common experience of frozen water expanding to wreck garden hoses left
out over winter. The answer is that only "ordinary" ice expands when
water freezes. There are at least 11 other known forms of ice
occurring at a variety of temperatures and pressures.)
"This work," says Dolan, "is a basic science study
that helps us understand materials at extreme conditions."
But it has potential practical value. The work,
which appears online March 11 (2007) in Nature Physics, was undertaken
partly because phase diagrams that predict water’s state at different
temperatures and pressures are not always correct - a fact worrisome
to experimentalists working at extreme conditions, as well as those
having to work at distances where direct measurement is impractical.
For example, work reported some months ago at Z demonstrated that
astronomers’ ideas about the state of water on the planet Neptune were
probably incorrect.
Closer at hand, water in a glass could be cooled
below freezing and remain water, in what is called a supercooled state.
Accurate knowledge of water’s behavior is
potentially important for Z because the 20-million-ampere electrical
pulses the accelerator sends through water compress that liquid.
Ordinarily, the water acts as an insulator and as a switch. But
because the machine is being refurbished with more modern and thus
more powerful equipment, questions about water’s behavior at extreme
conditions are of increasing interest to help avoid equipment failure
for the machine or its more powerful successors, should those be built.
One unforeseen result of Dolan’s test was that the
water froze so rapidly. The freezing process as it is customarily
observed requires many seconds at the very least.
The answer, says Dolan, seems to be that very fast
compression causes very fast freezing. At Z and also at Sandia’s
nearby STAR (Shock Thermodynamic Applied Research) gas gun facility,
thin water samples were compressed to pressures of 50,000-120,000
atmospheres in less than 100 nanoseconds. Under such pressures, water
appears to transform to ice VII, a phase of water first discovered by
Nobel laureate Percy Bridgman in the 1930s. The compressed water
appeared to solidify into ice within a few nanoseconds.
Ice VII has nothing to do with ice-nine, an
entirely fictional creation of author Kurt Vonnegut in his 1963 novel
Cat’s Cradle. There, a few molecules of the invented substance acts as
a precipitating seed to cause an extended chemical reaction that
freezes almost all of Earth’s water. Ice VII, on the other hand, only
stays frozen as long as it is under enormous pressure. The pressure
relenting, the ice changes back to ordinary water.
Nucleating agents, of course, are often used to
hasten sluggish chemical processes, such as when clouds are "seeded"
with silver iodide to induce rain. Dolan already had demonstrated, as
a graduate physics student at Washington State University, that water
can freeze on nanosecond time scales in the presence of a nucleating
agent.
However, the behavior of pure water under high
pressure remained a mystery.
Sandia instruments observed the unnucleated water
becoming rapidly opaque - a sign of ice formation in which water and
ice coexist - as pressure increased. At the 70,000 atmosphere mark and
thereafter, the water became clear, a sign that the container now held
entirely ice.
"Apparently it’s virtually impossible to keep water
from freezing at pressures beyond 70,000 atmospheres," Dolan says.
For these tests, Z created the proper conditions by
magnetic compression. Twenty million amperes of electricity passed
through a small aluminum chamber, creating a magnetic field that
isentropically compressed aluminum plates roughly 5.5 by 2 inches in
cross section. This created a shockless but rapidly increasing
compression across a 25-micron-deep packet of water.
The multipurpose Z machine, whose main use is to
produce data to improve the safety and reliability of the US nuclear
deterrent, has compressed spherical capsules of hydrogen isotopes to
release neutrons - the prerequisite for controlled nuclear fusion and
essentially unlimited energy for humanity. |
