Conversion of carbon nanotubes to diamond by spark plasma sintering

The diamond phase has been converted directly from carbon nanotubes by spark plasma sintering (SPS), at 1500 °C under 80 MPa pressure, without any catalyst being involved. Well-crystallized diamond crystals, with particle sizes ranging from 300 nm to 10 μm were obtained. After sintering at 1200 °C, the tips of the carbon nanotubes were found to be open and the conversion from carbon nanotubes to diamond started. The mechanism for carbon nanotube to diamond conversion in SPS may be described as that from carbon nanotubes to an intermediate phase of carbon nano-onion, and then to diamond. It is believed that the plasmas generated by the low-voltage, vacuum spark, via a pulsed DC in the SPS process, played a critical role in the low pressure diamond formation. This SPS process provides an alternative approach to diamond synthesis.