Synthesis of nano-polycrystalline diamond from glassy carbon at pressures up to 25 GPa

Nano-polycrystalline diamond (NPD) with various grain sizes has been synthesized from glassy carbon at pressures 15-25 GPa and temperatures 1700-2300 degrees C using multianvil apparatus. The minimum temperature for the synthesis of pure NPD, below which a small amount of compressed graphite was formed, significantly increased with pressure from similar to 1700 degrees C at 15 GPa to similar to 1900 degrees C at 25 GPa. The NPD having grain sizes less than similar to 50 nm was synthesized at temperatures below similar to 2000 degrees C at 15 GPa and similar to 2300 degrees C at 25 GPa, above which significant grain growth was observed. The grain size of NPD decreases with increasing pressure and decreasing temperature, and the pure NPD with grain sizes less than 10 nm is obtained in a limited temperature range around 1800-2000 degrees C, depending on pressure. The pure NPD from glassy carbon is highly transparent and exhibits a granular nano-texture, whose grain size is tunable by selecting adequate pressure and temperature conditions.