Diamonds Lead to Ultra-High Pressure Situations

Trying to find the perfect diamond has always been stressful, especially in a high-pressure environment. However, recent experimental results take the relationship between diamonds and pressure to a whole new level. An ultra-high level, in fact, that could expose new secrets of matter.

An image of the pressure chamber in a diamond anvil cell, taken under an optical microscope through the diamond window.
Image Credit: Dubrovinskaia, et al. Sci. Adv. 2016; 2 : e1600341

Exploring what happens to materials under extreme conditions is a revealing way to study what goes on in harsh locations—like the center of a planet or the depths of the oceans—and to learn more about the everyday stuff that makes up our surroundings. One of these extreme conditions is high pressure.

Under extremely high pressure, the structure of a material can fundamentally change. Depending on the type of material and strength of the pressure, materials can do things like turn superconducting, change states (e.g. liquid to solid), become metallic, change color, and more.

In order to study these transitions and unlock leaps in knowledge and technology that could accompany them, we need scientific equipment that can create—and remain stable under—incredibly high pressures. New work published Wednesday in the journal Science Advances enables scientists to run experiments on materials at higher pressures than ever before. The research was carried out by Natalia Dubrovinskaia and Leonid Dubrovinsky from the University of Bayreuth (Germany) and colleagues from Germany, Belgium, France, the United States, and Russia.

The stars of their work are microscopic balls of nanodiamond. A diamond can be a large, single crystal (like most of the diamonds in jewelry stores), or they can be made of many smaller grains. Nanodiamond is made from tiny nanoparticles, all less than 50 nanometers across, and is one of the strongest known materials on Earth. The team used a high temperature, high pressure technique to create tiny balls of nanodiamond for their research.

Nanocrystalline diamond balls as seen under a microscope: a transparent ball (left) and translucent one. 20 µm = 0.02 cm.
Image Credit: Dubrovinskaia, et al. Sci. Adv. 2016; 2 : e1600341.

These balls have turned out to have extraordinary properties. Tests indicate that they are ultra-hard, can withstand extremely high pressures, and many are transparent to visible light. At one point researchers tested their compressibility by placing transparent balls in a diamond anvil cell, which is kind of like a vice grip with very hard diamond tips, and balls actually dented the diamond tips! These properties are the result of their unique microscopic structure.

These features indicated that scientists might be able to use the tiny balls to put materials under enormous amounts of pressure—and that’s just what the researchers tried next. Most high pressure experiments take place in diamond anvil cells. In this case, the researchers used a modified version called a double stage diamond anvil cell (ds-DAC), which can reach even higher pressures.
A ds-DAC consists of two opposing diamond tips. On the end of each tip is a tiny hemisphere of nanodiamond material. The material being tested goes between the two hemispheres, and then the squeezing begins.

An image of the ds-DAC assembly (not to scale) for experiments at ultrahigh pressures (above 1 terapascal). The dark central piece is a gasket. One diamond tip comes down from the top and one comes up from the bottom. Two transparent nanodiamond hemispheres are used as secondary anvils with a sample placed between them (red). The diameter of the semiballs is about 20 μm (0.002 cm), and the initial size of the sample is about 3 μm (0.0003 cm) in diameter and about 1 μm (.0001 cm) in thickness.
Image Credit: Dubrovinskaia et al. Sci. Adv. 2016; 2 : e1600341

In this case, the researchers cut in half some of the transparent nanodiamond balls and used them as the secondary anvils, the tiny hemispheres. They placed a gold sample between the hemispheres and turned up the pressure. The pressure they research was greater than one terapascal – over 9,800,000 times the standard atmospheric pressure. Scientists have never been able to experimentally test the behavior of materials under a static pressure this high before.

The team didn’t stop there. They also devised a way to test samples that were initially liquid and gas in the ds-DAC, and they explored the optical properties of the transparent balls. Their results suggest that the balls could be used as a kind of lens for controlling x-rays.

They may not be as sparkly and romantic as the diamonds in jewelry cases, but since nanodiamond balls could enable us to discover new materials, test theories that have never been testable before, and model the cores of giant planets, I’d say they shine pretty bright.

Kendra Redmond

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