Ice in Nanoseconds from Sandia's Z Machine
May 1, 2007
Water was turned to ice in mere nanoseconds during an experiment using the Z machine equipment owned by the National Nuclear Security Administration’s Sandia laboratory in Albuquerque, N.M. However, commercialized applications aren’t expected any time soon: the ice is hotter than the boiling point of water.
“The three phases of water as we know them - cold ice, room temperature liquid and hot vapor - are actually only a small part of water’s repertory of states,” says Daniel Dolan, Sandia researcher. “Compressing water customarily heats it, but under extreme compression, it is easier for dense water to enter its solid phase [ice] than maintain the more energetic liquid phase [water].”
In the experiment, researchers abruptly and discontinuously shrank the volume of water, consistent with the formation of almost every known form of ice except the ordinary kind, which expands, in an effort to understand material at extreme conditions.
The work was undertaken partly because phase diagrams that predict water’s state at different temperatures and pressures are not always correct. Accurate knowledge of water’s behavior is important for Sandia because the 20-million-ampere electrical pulses the Z machine’s accelerator sends through the water compress that liquid. Ordinarily, the water acts as an insulator and as a switch. But because the machine is being refurbished with modern and more powerful components, questions about water’s behavior at extreme conditions are of increasing interest. The multi-purpose Z machine mainly is used to produce data to improve the safety and reliability of the U.S. nuclear deterrent.
One unforeseen result of Dolan’s test was the rapid speed at which the water froze. The freezing process typically requires many seconds. In this experiment, Dolan thinks the very fast compression caused very fast freezing. At Z and also at Sandia’s nearby Shock Thermodynamic Applied Research gas gun facility, thin water samples were compressed to pressures of 50,000 to 120,000 atmospheres in less than 100 nanoseconds. The compressed water appeared to solidify into ice within a few nanoseconds.
As pressure increased, Sandia instruments observed the water becoming rapidly opaque, a sign of ice formation in which water and ice coexist. At the 70,000 atmosphere mark and after, 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,” says Dolan.
For the 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" in cross-section. This created a shockless, but rapidly increasing, compression across a 25 micron deep packet of water.