Diamonds are renowned for their brilliance, but their value extends far beyond jewelry. This exceptionally hard form of carbon is used to manufacture tiny capsules for inertial confinement fusion experiments. Scientists also believe that diamonds may form and fall as “diamond rain” deep inside icy giant planets such as Neptune and Uranus.
In both settings, diamonds experience pressures far greater than those found on Earth’s surface. However, researchers have struggled for decades to determine exactly how diamond behaves under these extreme conditions because laboratory experiments and computer simulations have produced conflicting results.
New research published in Nature Physics may finally resolve the mystery. Scientists at Lawrence Livermore National Laboratory (LLNL) measured how diamond melts at pressures more than three times greater than those at Earth’s core.
“We were able to take a small diamond sample and shock-compress it to temperatures higher than the surface of the Sun and pressures greater than those at the centers of Neptune and Uranus. We were still able to measure its atomic structure, temperature, density, and light reflectance,” said LLNL scientist Marius Millot.
The findings resolve two long-standing questions in diamond physics and bring experimental measurements into close agreement with quantum mechanics-based simulations. The results could also have important practical applications. In inertial confinement fusion, the improved understanding of diamond melting may help researchers increase energy gain by as much as three times. The findings may also improve models of planetary interiors.
The 20-year mystery of diamond melting
LLNL researchers have studied diamond under extreme conditions for decades. Approximately 20 years ago, experimental physicist John Eggert and his colleagues conducted pioneering experiments on diamond melting at high pressure. Their work produced the unusual observation that diamond becomes denser when it changes from a solid to a liquid.
“This is quite unusual among most materials, but we all know examples of such behavior,” said LLNL scientist Marius Millot. “Ice cubes float because liquid water is denser than ice. John’s discovery means diamonds could float in high-pressure liquid carbon.”
Although the experiment represented an important advance, it raised a major scientific puzzle. The melting temperatures measured in the laboratory differed by approximately 20% from those predicted by theoretical models.
“No matter what the theorists did, even with the most advanced computer simulation techniques, they could not reproduce the experiment,” Millot said.
Experiments at Sandia National Laboratories raised another unanswered question. Using the powerful magnetic field of the Z machine, researchers shock-compressed tiny diamond samples. Their measurements produced a signal suggesting that diamond might pass through several different crystal structures before melting completely into liquid carbon.
Computer simulations supported this interpretation. However, the researchers could not directly observe the atomic structure of the compressed material, and the proposed intermediate phase, or mesophase, remained unidentified.
Laser experiments reveal how diamond melts
To investigate both mysteries, the LLNL team conducted laser-driven dynamic compression experiments at the University of Rochester’s Laboratory for Laser Energetics (LLE).
At the Omega Laser Facility, researchers used intense laser energy to vaporize the outer layer of a tiny sample. This process generated a powerful compression shock wave that traveled through the diamond.
Collecting accurate measurements was extremely challenging because the high-pressure conditions lasted for only about one billionth of a second. During that brief interval, researchers had to record several properties of the material, including X-ray diffraction data that revealed its atomic arrangement.
“This was the first time that shock-compressed diamond was examined using X-ray diffraction all the way to the point of melting,” Millot said. “These measurements are very difficult because carbon is a small, light atom. Carbon scatters very few X-rays, so the signal we needed to measure was extremely weak.”
LLE researchers helped develop and maintain improved diagnostic equipment that made the measurements possible. With this technology, the team obtained updated diamond melting temperatures that closely matched computer simulations, resolving a scientific discrepancy that had persisted for approximately 20 years.
“While we were frustrated to discover that our original temperature readings were off by more than 1,000 degrees, we are pleased that the new diagnostics have improved data quality so dramatically,” Eggert said. “Even better, our initial speculation about melting was directly confirmed by X-ray diffraction.”
Diamond remains a diamond until it melts
Although the new experiments resolved the disagreement over diamond’s melting temperature, they produced a different answer to the questions raised by the Sandia experiments.
The carbon retained its diamond structure until it became liquid instead of changing into another crystalline phase before melting. The researchers found no evidence of a mesophase during the experiment.
“This is likely because the sample does not have time to change after a single impact; it remains ‘trapped’ within the diamond structure,” Millot said.
This discovery could influence future experiments and simulations involving materials exposed to extremely high energy densities. It suggests that the way a shock is applied can affect how a material responds. Pressure and temperature alone may not determine the structure a material ultimately adopts.
Diamond physics could improve fusion energy
The close agreement between theory and experiment has direct implications for inertial confinement fusion research.
In these experiments, powerful lasers create shock waves that compress and heat a tiny diamond capsule containing fusion fuel. As the capsule implodes, the fuel reaches the extraordinary pressures and temperatures needed to initiate a fusion reaction.
One of the most important goals during the initial impact is to melt the diamond capsule into a smooth, uniform fluid. Any irregularities during the implosion can interfere with compression and reduce the strength of the fusion reaction.
To ensure that the diamond capsule melts completely, scientists at LLNL’s National Ignition Facility (NIF) typically use a relatively strong initial shock.
The new measurements indicate that the initial shock may not need to be as powerful as previously believed.
“Our study shows that complete melting of diamond can be achieved in a NIF implosion even when using a slightly slower initial impact,” Millot said. “This is important because slower shocks increase the compressibility of the fusion fuel, which can raise the maximum energy yield produced with the same amount of laser energy.”
Models suggest that slower initial shocks could potentially increase fusion energy gain by as much as three times, provided researchers can control other processes that limit performance.
New clues about diamond rain inside icy giant planets
The discovery may also help scientists understand the deep interiors of Neptune and Uranus.
Because researchers cannot directly observe the interiors of these distant ice giants, they rely on laboratory experiments and computer models to recreate the extreme pressures and temperatures found beneath their surfaces.
Some studies suggest that carbon may crystallize into diamonds deep inside these planets. As the diamonds sink toward the interior, they could create a phenomenon known as “diamond rain.”
Because the new experiment examined diamond at pressures even higher than those expected inside icy giant planets, the improved melting data provide planetary scientists with a stronger foundation for modeling how Neptune and Uranus formed and how their interiors have evolved.
Pushing diamond research to new limits
The LLNL team plans to use the capabilities of the National Ignition Facility to study diamond under conditions that are even more difficult to reproduce in the laboratory.
Future research will examine how diamond capsules behave during the later stages of fusion implosions. Scientists also plan to study how long diamond’s crystal structure can remain stable when the material is subjected to a series of multiple shock waves.
Additional LLNL authors include Federica Coppalli, Amy Radzicki, Youngjae Kim, Otto Landen, Vladimir Smaluk, and Peter Seljes. LLNL target manufacturing experts Renee Posadas and Eric Folsom of the HED Science Center Technical Facility also contributed to the research. The work was supported by LLNL’s Institute-Directed Research and Development Program.
Source: www.sciencedaily.com