London - Saba:
Diamonds are known to be the hardest natural material on Earth, although harder synthetic forms have been developed - a feat that researchers have succeeded in achieving again, with a new approach to forming diamonds.
The team placed graphite (another very hard material) under intense pressure, before heating it to 1,800 Kelvin (1,527 degrees Celsius or 2,780 degrees Fahrenheit). The resulting diamonds have a hexagonal crystalline lattice structure, rather than the usual cubic structure.
Hexagonal diamonds (or lonsdaleite) first came to the attention of scientists more than 50 years ago, after they were discovered at the site of a meteorite impact. The new research is the first conclusive evidence that this internal structure enhances hardness.
“Natural and synthetic diamonds mostly have a cubic lattice, while the rare hexagonal structure – known as hexagonal diamond (HD) – has been largely unexplored due to the low purity and small size of most samples obtained,” the researchers write in their paper published in the journal Nature Materials. “…The composition of HD remains a challenge and even its existence remains controversial.”
The newly produced diamond has a hardness of 155 gigapascals (GPa), which is essentially a measure of pressure: the pressure a material can withstand. By comparison, natural diamond has a hardness of about 110 GPa.
The thermal stability is also impressive. The researchers report that the synthetic hexagonal diamond can remain intact up to temperatures of at least 1,100 degrees Celsius (2,012 degrees Fahrenheit), compared to 900 degrees Celsius (1,652 degrees Fahrenheit) for the nanodiamonds often used in industrial applications. Natural diamonds can withstand higher temperatures, but only in a vacuum.
In addition to overcoming Despite some of the limitations researchers have previously faced when making hexagonal diamonds, the team has identified ways in which the process could be scaled up in the future.
“We discovered that when graphite is compressed to much higher pressures—which has been rarely explored before—hexagonal diamonds form preferentially from post-graphitic phases when heating is applied under pressure,” the researchers wrote.
There’s a lot of work to be done before this type of diamond can be produced on a large scale, but the hardness and thermal stability readings from this first batch suggest that the material is promising for use in drilling, machinery, or data storage.
This isn’t the first time scientists have tried to create hexagonal lattice diamonds in the lab. In a previous project in 2016, scientists successfully created these diamonds from amorphous carbon, a material with no definite shape.
Now that another method of synthesis has been implemented in the lab—and has been shown to produce an extremely hard diamond—the search can continue into how to get the most out of this rare material.
“Our results provide valuable insights into the transformation of graphite into diamond under high pressure and temperature, providing opportunities for the fabrication and applications of this unique material,” the researchers wrote.
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