Diamond is considered the hardest naturally occurring mineral, but even this exceptionally durable material can undergo dramatic changes when exposed to extreme pressures and temperatures. Scientists in the United States have now observed in unprecedented detail how diamond behaves under conditions resembling those found in extreme planetary environments, producing new data that could have important implications for the development of nuclear fusion.
Researchers at Lawrence Livermore National Laboratory (LLNL) studied diamond under pressures up to three times greater than those found at Earth’s core. Their findings were published in Nature Physics.
For the experiments, scientists used extremely small diamond samples and subjected them to sudden compression using powerful shock waves. The resulting conditions produced temperatures higher than those at the surface of the Sun and pressures far exceeding those found inside Uranus and Neptune.
Despite the extreme environment, the researchers were able to measure the material’s atomic structure, temperature and density, as well as how it interacted with light.
How does diamond melt?
The study addresses a question that has challenged scientists for roughly two decades. Earlier experiments suggested that when diamond melts under extremely high pressure, the resulting liquid carbon can be denser than solid diamond.
The behavior is unusual but has a familiar parallel in water: just as liquid water is denser than ice, liquid carbon can apparently become denser than diamond.
The challenge was that experimental measurements did not match theoretical predictions. Computer simulations had estimated a melting temperature roughly 20% different from the values recorded in laboratory experiments.
The new experiments appear to significantly narrow that gap. Researchers found that diamond’s crystal structure remains stable at pressures approaching one terapascal. Their measurements also indicate that under shock compression, diamond melts at temperatures of around 7,300 Kelvin, or approximately 7,000 degrees Celsius.
Another intriguing finding is that the melting temperature appears to decrease slightly as pressure increases.
Why it matters for nuclear fusion
The discovery could be particularly important for inertial confinement fusion. In this approach, extremely powerful laser beams are used to compress tiny fuel capsules until the extreme conditions required for nuclear fusion are achieved. Some of these fuel capsules are made from diamond.
A more precise understanding of how diamond behaves under extreme conditions could help scientists improve capsule designs and increase the efficiency of fusion experiments.
According to LLNL, a better understanding of the material’s properties could theoretically contribute to a substantial increase in energy output, with some estimates suggesting the potential for as much as a threefold improvement.
A window into the interiors of ice giants
The research could also help scientists better understand the environments deep inside Uranus and Neptune.
Both planets are subjected to enormous pressures and temperatures in their interiors. According to current models, carbon may become so compressed under these conditions that it forms diamonds. Those diamonds could then sink toward deeper layers of the planets in a spectacular phenomenon often described as “diamond rain.”
The new study therefore provides valuable insight not only into one of the most challenging areas of modern energy research, but also into the extreme environments found inside some of the Solar System’s most mysterious planets.