In situ X-ray diffraction study of diamond nanopowders spark plasma sintering under very high pressure

The sintering behavior and the thermal stability of different nanodiamond powders, produced either by detonation or by high-pressure high-temperature (HPHT) synthesis, is studied in situ during spark plasma sintering at 5 GPa. A portable high pressure spark plasma sintering device allowed the transformation of nanodiamonds during the sintering process to be followed by synchrotron radiation X-ray diffraction, in order to determine the optimal sintering conditions and investigate the impact of the sample environment. The powders were also characterized ex situ using advanced bulk and surface techniques to evaluate how the sp 2 carbon shell, agglomeration state, and surface chemistry affect sintering while preserving the diamond structure and particle size. A purified and deagglomerated detonated powder (5 nm) and a nanodiamond powder (<30 nm) produced by HPHT synthesis appeared to be the most stable. Sintered nanodiamonds were produced at 5 GPa and temperatures below 2000 °C without using any binders, retaining more than 95% of the diamond phase and limiting grain growth. This study opens up encouraging prospects for the production of binder-free sintered nanodiamonds under pressure and temperature conditions that are less extreme than those currently used.