Diamond may be famous as a gemstone, but scientists are interested in it for much bigger reasons.
The extremely hard form of carbon plays an important role in fusion-energy experiments and may also exist deep inside planets such as Neptune and Uranus.
Now, scientists at Lawrence Livermore National Laboratory (LLNL) have discovered new details about how diamond melts under enormous pressure.
The findings, published in Nature Physics, could help improve fusion experiments and provide a clearer picture of what happens inside ice-giant planets.
The researchers compressed tiny diamond samples to pressures about three times greater than those at Earth’s core.
The samples also reached temperatures hotter than the surface of the sun. Despite these extreme conditions, the team was able to measure the diamond’s atomic structure, temperature, density and other properties.
Scientists have been trying to understand diamond under extreme pressure for decades. About 20 years ago, LLNL researchers found something unusual: when diamond melts at very high pressure, the resulting liquid carbon becomes denser. This means solid diamond could actually float on liquid carbon, much as ice floats on water.
However, those early experiments created a puzzle. The measured melting temperature of diamond differed from computer predictions by about 20%. Even highly advanced simulations based on quantum mechanics could not explain the difference.
Other experiments raised another question. Research at Sandia National Laboratories suggested that diamond might transform into a different crystal structure before finally melting.
To investigate, the LLNL team carried out new experiments using the Omega Laser Facility at the University of Rochester. Powerful lasers vaporized the outer layer of tiny samples, producing shock waves that rapidly compressed the diamond.
The extreme conditions lasted only about a billionth of a second. During that tiny window, researchers used X-ray diffraction to observe how the atoms were arranged.
The new measurements produced a melting temperature that closely matched computer simulations, resolving the disagreement that had lasted roughly two decades. They also showed that under a single shock wave, diamond remains diamond right up until it melts. No intermediate crystal structure appeared.
The discovery could have important consequences for fusion energy. At LLNL’s National Ignition Facility, powerful lasers crush tiny diamond capsules containing fusion fuel. Scientists need the diamond to melt smoothly so that the fuel can be compressed evenly.
The new results suggest researchers may be able to use a slightly weaker initial shock while still completely melting the diamond.
A gentler shock could make the fusion fuel easier to compress, potentially allowing substantially more energy to be produced from the same amount of laser energy. Models suggest the approach could potentially triple fusion energy gain if other sources of energy loss can also be controlled.
The findings may even help explain distant planets. Scientists believe carbon could crystallize deep inside Neptune and Uranus and fall toward their centers as “diamond rain.”
Because the new experiments reached pressures beyond those found inside these ice giants, the results could improve models of their mysterious interiors, formation and evolution.
