The high pressure behaviour of carbon has been of intense research focus for several decades. This interest can be attributed to carbon’s importance and abundance in planetary and astrophysical processes, and to the wide-spread use of carbon-based materials for many varied technological and tribological applications.
In this current work, the high pressure behaviour of glassy carbon is studied using diamond anvil cells. Glassy carbon is a synthetic form of carbon with a microstructure consisting of a tangle of graphite-like ribbons with no preferred structural orientation. The glassy carbon was subjected to pressures above 100 GPa and annealed at ~400°C. On pressure release the sample (of dimensions 50x50x10 µm) remained intact and was subjected to X-ray diffraction experiments using synchrotron radiation. In addition, several lamellae were prepared for electron microscopy and electron energy loss spectroscopy (EELS) using in-situ focused-ion-beam milling.
The high-resolution transmission-electron-microscopy results showed a distinct change in the microstructure, whereby the graphite-like ribbons appear to be broken and coarsely aligned compared to the unpressurised material. Electron and X-ray diffraction results highlight a preferred structural alignment. The measured d-spacings suggest a structure consistent with nano-crystalline hexagonal-diamond with cubic-diamond inclusions. Analysis of the electron diffraction data implies an average crystallite size in the order of 1 nm. Analysis of the low-loss EELS spectra show an apparent increase in density compared to the virgin material.