You can make liquid diamonds

High pressure experiment determines the melting point of diamond

In the high pressure experiment, physicists created a so-called triple point of carbon for the first time. In this state, solid diamond, liquid carbon and a previously only theoretically known form called bc8 occur together. As they report in "Science", this enables conclusions to be drawn about areas as diverse as nuclear fusion and the interior of the planet Neptune.

What does a projectile the size of a stick of chewing gum have to do with diamonds on Neptune or nuclear fusion? Not much at first glance. The projectile was part of an experiment with which physicists at Sandia National Laboratory in New Mexico succeeded for the first time in analyzing the behavior of carbon under enormous pressure ten times more precisely than before. They achieved several breakthroughs in the process.

Giant magnet hurls “flight plates” at diamonds

The team led by Marcus Knudson, Mike Desjarlais and Daniel Dolan initially used simulations to narrow down a range of speeds at which their projectiles, the so-called flight plates, generate sufficient pressure at the point of impact. The aim was to compress carbon in the form of diamond plates so strongly that it becomes liquid. In previous experiments, a laser was mostly used for this purpose, but this only allowed a very imprecise determination of the melting point.

The flight plates used in the experiment were 4 × 1.7 centimeters in size and only a few hundred micrometers thick. They were accelerated with the help of the extremely strong magnetic fields of the "Z Machine" of the Sandia Lab. With every “shot” the flight plate hit three diamond targets, each 1.9 carats. “These experiments are much more accurate than the previous ones that were done with laser beams,” explains Knudson. "Our flight plates hit several large diamond samples with precisely measured velocities, so that we can carry out very precise measurements of the shock wave velocity."

Exact melt pressure - and surprising triple point

After 15 runs, the researchers had what they needed: They were able to determine the exact pressure at which the diamond changes its state from solid to liquid - ten times more precisely than ever before. The values ​​determined are also in the middle of the limits set by the theory, so that the theory could also be experimentally confirmed for the first time.

But there was even a bonus: The physicists discovered the triple point, a point at which solid diamond, liquid carbon and a previously only theoretically predicted solid carbon state, known as bc8, existed together.

Liquid diamond on Neptune?

But what do melting and triple points have to do with Neptune and nuclear fusion? Most of the atmosphere in Neptune is methane. This carbon compound decomposes under high pressure, such as that found under the surface. One of the open questions is what form the carbon is in the interior of Neptune. When does diamond begin to form and is the pressure deep enough to liquefy it again?

If this were the case, this would have repercussions on the planet's magnetic field: "Liquid carbon is electrically conductive under such pressures, and this also influences the formation of magnetic fields," explains Desjarlais. "Therefore, the exact knowledge of the phases of the carbon inside the planets is crucial for the development of computer models of the properties of the celestial body."

Fusion in the diamond shell

In the case of nuclear fusion, the carbon in the form of diamond plays the role of a pressure chamber. At the Lawrence Livermoore National Laboratory in California, the deuterium and tritium nuclei to be fused are enclosed in it. The aim of the physicists is to use heat and pressure to fuse the atomic nuclei inside their capsule. For this form of completed fusion to work, the capsule must not deform irregularly or even melt. A transition to the triple state would also be very unfavorable because it is leaky. The new results from the Sandia researchers are now helping to avoid these critical pressures.

(Sandia Laboratory, February 19, 2009 - NPO)

February 19, 2009