A Tokyo Institute of Technology research team led by Professor Kenji Ohta has succeeded in measuring the electrical resistance of liquid iron at extremely high pressures and temperatures such as those found in the Earth's interior. Professor Ohta developed two new techniques that made the breakthrough possible.
Iron as an element
Iron is the most abundant element on Earth by mass. In its liquid state, it makes up most of the Earth's core. Although it is so abundant and well studied, it still puzzles scientists about its electrical and magnetic properties because measurements are largely based on solid iron. Predictions about the properties of liquid iron are difficult to verify experimentally. This is because it is difficult to maintain the shape and chemical composition of liquid iron samples using conventional high-pressure apparatus.
Newly developed research methods
The motion of iron in a planet's core is responsible for its magnetic field. Geophysicists use electrical resistivity measurements to study planetary magnetic fields and the evolution of planetary cores. The problem with measuring the electrical resistance of liquid metals, however, is that the material must maintain its shape during the measurements for researchers to get a true value. But because liquids tend not to do that, Ohta and his colleagues developed two new methods to measure the electrical resistivity of their samples.
Both techniques used a diamond anvil cell (DAC), which applies incredibly high pressure to a sample by squeezing it between the flat sides of two opposing diamonds. In the first technique, researchers used a sapphire capsule to hold the iron sample in the DAC while they heated it with a laser and electric current.
"The idea was to leave the geometry of the iron sample unchanged during melting and minimize the temperature differences within the sample," Dr. Ohta said.
The second technique took a radically different approach. Instead of preserving the shape of the sample during melting by encapsulation, the researchers used powerful lasers to melt the iron "instantaneously." The goal was to simultaneously measure the resistance, X-ray diffraction and temperature of the molten sample before it had enough time to change its geometry. Once the sample reached the target conditions, the team was able to make the measurements within the time window of a few milliseconds.
Research results
By combining these techniques, the team was able to determine the experimentally validated resistivity of liquid iron at pressures and temperatures up to 135 GPa and 6,680 K, respectively. This is 2 times higher than ever before achieved.
The measurements showed that the resistivity of liquid iron does not depend very much on temperature. Moreover, at higher pressures, it follows existing theoretical estimates quite well, including an anomalous decrease around 50 GPa that probably indicates a gradual magnetic transition. This is important because there are some discrepancies between the theoretical predictions and experimental data on the resistivity of liquid iron, especially at pressures below 50 GPa.
The results of this study will help clarify the origin of these discrepancies and help physicists develop more accurate models and theories about the behavior of iron. This, in turn, could lead to a more complete understanding of Earth's cores as well as related phenomena such as planetary magnetic fields.
The full study can be found here.
