Abstract
The pulsed DC field-induced thermal stability of nanodiamonds (NDs) and their transformation to carbon onions lack detailed understanding. In this study, a comprehensive study was conducted, analyzing the thermal stability of NDs and the optimum conditions required for the formation of carbon onions, using spark plasma sintering (SPS) utilizing ON–OFF DC pulse energizing. X-ray diffraction, Raman spectroscopy and electron microscopy were employed to monitor the phase transformation. Experimental results showed that NDs could almost remain stable until 950 °C under 60 MPa pressure. As the temperature was increased, amorphous carbon appeared on the surface of NDs, and then, graphitization began. At 1300 °C, lamellar graphite structures were formed and kept stable with increasing holding time, but no carbon onion was found. The optimum parameters for the synthesis of carbon onions from NDs via SPS are the temperature of 1400 °C and holding time of 15 minutes under a pressureless condition. The pressureless condition during the SPS processing creates a more favourable environment for the ND graphitization and curling into spherical carbon onions. The existence of pressure during the SPS processing can improve the thermal stability of NDs, delay the initial temperature for the graphitization transition of NDs and inhibit the graphite layer curling to form carbon onions.
Highlights
Nanodiamonds (NDs), diamonds with a nanoscale size, were discovered dozens of years ago,[1] and the investigation of their properties and applications has been ongoing ever since
It is obvious that two distinct diffraction peaks at 2q 1⁄4 43.8 and 2q 1⁄4 76.5 appeared in the X-ray diffraction (XRD) patterns of the NDs before and a er spark plasma sintering (SPS) at different temperatures up to 1250 C (Fig. 1a)
The experimental results demonstrate that the graphitization transitional temperature of ND can be delayed and the stability of NDs can be enhanced by appropriate pressure in the SPS
Summary
Nanodiamonds (NDs), diamonds with a nanoscale size, were discovered dozens of years ago,[1] and the investigation of their properties and applications has been ongoing ever since. Equipment.[22] To date, NDs with the size of 5 nm have been widely used as raw materials for the synthesis of carbon onions via thermal annealing, and the synthesized carbon onion has a characteristic size close to the size of the raw material and high conversion rate.[23] Due to their nano-scale size, unique cage structure and carbon atom hybridization (sp2) which is different from that of diamond (sp3), carbon onions have attracted signi cant interest for various applications. In order to verify the experimental results, the samples were characterized by X-ray diffraction (XRD), Raman spectroscopy, transmission electron microscopy (TEM), scanning electron microscopy (SEM), laser thermal conductivity (LTC), and thermal analysis (TA) These experiments provide a signi cant amount of information which can be helpful for understanding the thermal stability of NDs as well as the mechanism of the phase transformation from ND to carbon onion
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