The phase diagram of carbon investigated by flash laser heating of glassy carbon

Understanding the phase diagram of carbon is challenging due to large energy barriers between phases and the presence of extensive metastability fields. We examine the phase composition and microstructure in the poorly understood region of the pressure-temperature diagram from the vicinity of the graphite-diamond-liquid triple point up to pressures of ∼35 GPa using flash laser heating of glassy carbon. Consistent with previous studies of the carbon phase diagram, above the triple point pressure (at ∼11 GPa) and temperatures below ∼2000 K, the graphite to diamond transformation was inhibited. Above this temperature and above the triple point pressure, a graphite/diamond nanocomposite was observed, arising from a partial transformation to diamond via solid state diffusion processes. As the pressure increased further to above ∼16 GPa, phase pure nanodiamonds were formed with crystal sizes down to ∼1 nm, depending on the temperature. At higher temperatures above the triple point pressure, clear evidence for melting was observed, resulting in the formation of larger diamond crystals, up to 0.5 μm in diameter consistent with growth from a liquid. Below the triple point pressure, glassy carbon gradually graphitizes up to the melting point of ∼5000 K, while above this temperature larger crystals with pillar-like morphology grow from the liquid phase. Our work demonstrates that the complex phase behaviour of carbon leads to a wide variety of microstructures in the vicinity of the graphite-diamond-liquid triple point including nanocomposites with crystallite sizes down to the nanoscale.