Synthetic diamonds grow in liquid metal at ambient pressure

The usual way of manufacturing synthetic diamonds involves applying huge pressures to carbon at high temperatures. Now, however, researchers at the Institute for Basic Science (IBS) in Korea have shown that while high temperatures are still a prerequisite, it is possible to make polycrystalline diamond film at standard pressures. The new technique could revolutionise diamond manufacturing, they say.

Grown at ambient pressure: A diamond sample in the solidified liquid metal. (Courtesy: Institute for Basic Science/Yan Gong)

Natural diamonds form over billions of years in the Earth’s upper mantle at temperatures of between 900 and 1 400°C and pressures of 5 to 6 gigapascals (GPa). For the most part, the manufacturing processes used to make most synthetic diamonds mimic these conditions. In the 1950s, for example, scientists at General Electric in the US developed a way to synthesise diamonds in the laboratory using molten iron sulphide at around 7GPa and 1 600°C. Although other researchers have since refined this technique (and developed an alternative known as chemical vapour deposition for making high-quality diamonds), diamond manufacturing largely still depends on liquid metals at high pressures and temperatures (HPHT).

A team led by Rodney Ruoff has now turned this convention on its head by making a polycrystalline diamond film using liquid metal at just 1 atmosphere of pressure and 1 025°C. When Ruoff and colleagues exposed a liquid alloy of gallium, iron, silicon and nickel to a mix of methane and hydrogen, they observed diamond growing in the subsurface of the liquid metal. The team attribute this effect to the catalytic activation of methane and the diffusion of carbons atoms in the subsurface region.

No seed particles
Unusually, the first diamond crystals in the IBS experiment began to form (or nucleate) without seed particles, which are prerequisites for conventional HPHT and chemical vapour deposition techniques. The individual crystals later merged into a film that is easy to detach and transfer to other substrates.