2:30 PM - MS01.11.04
The Transformation of Glassy Carbon to Diamond at High Pressure Using Pulse Laser Heating
Brenton Cook1,Thomas Shiell2,Sherman Wong1,David McKenzie3,Matthew Field1,Bianca Haberl4,Reinhard Boehler4,Jodie Bradby5,Dougal McCulloch1
RMIT University1,Carnegie Institution of Washington2,The University of Sydney3,Oak Ridge National Laboratory4,The Australian National University5
The synthesis pathway to diamond from graphitic precursors using high pressures and temperatures has been studied and applied for many decade . Industrial manufacturing methods require temperatures up to 1500 K, pressures between 5-6 GPa, and the use metallic catalysts . Without these catalysts the pressure and temperature conditions required are far more extreme to overcome kinetic energy barriers. Recently, there has been considerable interest in the synthesis of pure, catalyst-free diamond and other novel forms of carbon from non-crystalline precursors, with the hope that the energy barrier impeding transformation will be reduced. For example, nanocrystalline diamond has been formed from carbon nanotubes following compression to 17 GPa at 2500 K . The size of the nanocrystals were found to be similar to the diameter of the nanotubes, demonstrating that the type of precursor can influence the microstructure of the resulting diamond. Sumiya et al. observed the formation of nanocrystalline diamond (<10 nm in diameter) from a glassy carbon (GC) precursor when compressed for 10s at pressures of ~20 GPa and temperatures of ~3000 K . When compressed for longer times, the nanocrystalline diamond evolved into larger crystals then exhibited a lamellar structure. Recently, it has been reported that it is possible to create an amorphous form of diamond has been synthesisd from GC following compression to 50 GPa and laser heating to ~1800 K . However, this previous work has only investigated the compression of non-crystalline graphitic precursors within a limited temperature range. There is a need for a more thorough experimental study at temperatures above 3000 K, in the region of the PT diagram encompassing the liquid-diamond phase boundary.
In this study, GC samples were loaded into diamond anvil cells with an Ar pressure medium and compressed to 16 GPa. A 1070 nm pulse laser was then used to heat the samples to temperatures ranging from 1900-4500 K. A total of 15 samples were made for ex-situ analysis using raman spectroscopy, scanning electron microscopy and transmission electron microscopy. At low temperatures (1900-2200 K), the GC was found to have transformed into an oriented graphitic material in which its graphene layers are preferentially aligned perpendicular to the compression axis. Nanodiamonds (~10-200 nm) begin to form near the surface of the GC at temperatures of ~2200 K. These nanodiamonds increase in size and density as the temperature increases up to 4500 K. Interestingly, above ~3500 K voids were observed in the microstructure, some of which contained Ar, which appears to have an epitaxial relationship with the surrounding diamond. This observation supports the proposition that at these high temperatures, the GC may have entered a liquid state prior to the formation of diamond crystallites.
The authors acknowledge the Australian Research Council for financial support (Discovery Project #DP170102087).
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