Scientists from ANU and RMIT Melbourne Create Diamonds at Room Temperature Using Only High Pressure

The popular Australian website https://theconversation.com/au published a news item on November 19, 2020, headlined, “We created diamonds in mere minutes, without heat – by mimicking the force of an asteroid collision” which created waves of excitement and disbelief around the world, as a team of Australian Scientists from ANU and RMIT Melbourne claimed that they created two different types of diamond, at room temperature and in a matter of minutes, the first time diamonds were produced successfully in a lab, without added heat. The findings of the Australian Scientists were published in the Journal “Small.”

Natural diamonds found in various regions of the world, were actually produced 1 billion to 3.5 billion years ago, deep inside the earth’s mantle, at depths between 150 – 250 km, at conditions of extremely high pressure and temperature, which were subsequently and more recently, ranging from tens to hundreds of millions of years ago, carried to the surface of the earth by volcanic eruptions and deposited in igneous rocks known as kimberlites and lamproites.

Diamonds were first synthesized in the Laboratory by Tracy Hall of General Electric on December 16, using a “Belt Press“ capable of producing pressures above 10 Gpa and temperatures above 2,000°C, having a pyrophyllite container in which graphite was dissolved within molten nickel, cobalt or iron, which acts as a solvent catalyst which not only dissolved carbon but also accelerated its conversion into diamond. The first diamonds synthesized were extremely small, just 0.15 mm across, usable only as industrial abrasives.

Synthetic gem-quality diamond crystals were first produced by General Electric only in 1970, using a pyrophyllite tube seeded at each end with thin pieces of diamond, and the graphite feed material placed at the centre, with the Nicklel metal solvent between the graphite and the seeds. The tube was heated to a very high temperature, while the pressure was raised to about 5.5 Gpa. The crystals grew as the graphite flowed from the center to the ends of the tube, and by extending the length of the process, keeping all conditions as stable as possible, larger crystals were produced; a week-long growth process producing gem-quality stones of around 5 mm in diameter and weighing around 1 carat or 0.2 g. Such diamonds were known as HPHT diamonds, because of the High Pressure and High Temperature employed in their synrhesis. Diamond Anvil Cells are used in creating the high pressures and temperatures needed in the HPHT method of synthesizing diamonds,

Diamond Anvil cell

The second method, using chemical vapor deposition (CVD), creates a carbon plasma over a substrate onto which the carbon atoms deposit to form diamond. The advantages of CVD that makes it the preferred choice for growth of large diamonds are that it can be carried out at low pressures of under 27 kPa, at a relatively low temperature of around 800 °C; the ability to grow diamond over large areas and on various substrates, and the fine control over the chemical impurities and thus properties of the diamond produced. The gases used for CVD include a Carbon source, usually Methane, which is mixed with Hydrogen gas in the ratio 1:99. The gases are ionized into chemically active radicals in the growth chamber using one of the following alternate ways auch as microwave power, an arc discharge, a laser, electron beam etc. Any non-diamond substrate within the chamber is given a coating of diamond poweder to prevent any material being etched off by the plasma that can be incorporated as an impurity in the growing diamond.

Carbon atoms bond in different ways to form the various crystalline allotrpoes of carbon, such as graphite, fullerenes and diamonds. Among the amorphous allotropes of carbon are charcoal, coal and coke.

In graphite, each carbon atom uses only 3 of its 4 outer energy level electrons in covalently bonding to three other carbon atoms in a plane. Each carbon atom contributes one electron to a delocalized system of electrons that is also a part of the chemical bonding. The delocalized electrons are free to move throughout the plane. For this reason, graphite conducts electricity along the planes of carbon atoms. Other forms of carbon with graphite-like bonding include graphene, graphenylene, AA-graphite and diamane.

In diamond all four outer energy level electrons of each carbon atom are used in covalent bonding with four neighbouring carbon atoms. In normal diamond, atoms are arranged in a cubic crystalline structure.

However, sometimes the carbon atoms in diamond can also be arranged in a hexagonal crystal structure as found in Lonsdaleite, named after the Irish crystallographer Kathleen Lonsdale, who studied the structure of different allotropes of carbon using X-rays. Lonsdaleite is believed to form from graphite present in meteorites upon their impact on the Earth. The great heat and stress of the impact transforms the graphite into diamond, but retains graphite’s hexagonal crystal lattice. Hexagonal diamond has also been synthesized in the laboratory, by compressing and heating graphite either in a static press or using explosives.

Lonsdaleite is said to be 58% harder than regular diamond, which further enhances the status of diamond as the hardest naturally occuring material on Earth. It was first discovered in nature, at the site of the Canyon Diablo Meteorite Crater in Arizona. However, tiny amounts of Lonsdaleite have since been synthesized in the Lab, by heating and compressing graphite, using either a high-pressure press or explosives.

Apart from violent impact of meteorites on earth that produces diamonds, high speed asteroid collisions in the solar system also creates diamonds, that are known as “Extraterrestrial Diamonds.” In studying how these impact or extraterrestrial diamonds are formed, scientists believe that in addition to high temperatures and pressures, an additional force, known as “sliding or shear” forces, also play an important part in their formation.

Using this important information, scientists at the Australian National University (ANU) and RMIT University designed an experiment in which a small chip of graphite-like carbon was placed in a Diamond Anvil Cell and subjected to both extreme shear forces and high pressures of around 80 Gpa, maintaining the Anvil at room temperature. without supplying any external heat. The resulting sample was studied under an advanced electron microscope and was found to contain both regular diamond and Lonsdaleite, within bands referrred to as “rivers” of diamond.

Electron Microscope image showing a river of diamond in a sea of lonsdaleite
Electron Microscope image showing a river of diamond in a sea of lonsdaleite

“Our pictures showed that the regular diamonds only form in the middle of these Lonsdaleite veins, under this new method developed by our cross-institutional team,” says RMIT’s Professor Dougal McCulloch. “Seeing these little “rivers” of Lonsdaleite and regular diamond for the first time was just amazing and really helps us understand how they might form”

“The twist in the story is how we apply the pressure,” says ANU Professor Jodie Bradby. “As well as very high pressures, we allow the carbon to also experience something called ‘shear’ – which is like a twisting or sliding force. We think this allows the carbon atoms to move into place and form Lonsdaleite and regular diamond.”

The structure’s arrangement is reminiscent of “shear banding” observed in other materials, wherein a narrow area experiences intense, localised strain. This suggest shear forces were key to the formation of these diamonds at room temperature.

The ability to make diamonds at room temperature, in a matter of minutes, opens up numerous manufacturing possibilities.

Specifically, making the “harder than diamond” Lonsdaleite this way is exciting news for industries where extremely hard materials are needed. For example, diamond is used to coat drill bits and blades to extend these tools’ service life.

The next challenge for us is to lower the pressure required to form the diamonds.

In our research, the lowest pressure at room temperature where diamonds were observed to have formed was 80 gigapascals. This is the equivalent of 640 African elephants on the tip of one ballet shoe!

If both diamond and Lonsdaleite could be made at lower pressures, we could make more of it, quicker and cheaper.

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