**2. Major raw materials and methods**


phase within the onion cores (Shen, Zhang et al., 2006). The effect of the high temperatures and sparking plasmas is akin to the electron or ion beam irradiations or high pressures. As a result, diamond forms from the intermediate nano-onions when localized high energy conditions are satisfied (Shen, Zhang et al., 2006). Additionally, the low purity MWCNTs (60% purity) with a very cheap price also can be used as starting materials for the direct synthesis of diamond by the SPS technique (Zhang, Shen et al., 2006). It is postulated that the spark plasmas play a key role to provide most of the energy required in these diamond transitions. It is seen as an important evidence for the presence of spark plasmas during the SPS process. These studies indicate that the SPS has a potential to be used as an alternative method for diamond generation. But it needs further investigation to promote the SPS method to be used as a large-scale synthetic diamond production technique instead of the

This chapter will focus on the synthesis of diamond using the SPS technique. We aim to promote the SPS method to be used as a large-scale synthetic diamond production technique as an alternative to the present HPHT method. In the first part of this chapter, the thermal stability of carbon nanotubes, C60 and graphite under the pulsed DC field of SPS and AC field of conventional sintering will be studied. In the second part of this chapter, the application of catalysts in the diamond synthesis by the SPS will be investigated. In the last part of this chapter, the factors that influence the diamond growth in the SPS, including carbon modifications and atmospheres (Vacuum, Ar) will be studied in order to increase the

• The MWCNTs with purity above 95.0% were obtained from Shenzhen Nanotech Port, Ltd., China. The C60 powders with purity of 99.5% were obtained from SES Research, Huston, USA. The graphite powders with purity of 99.0% were purchased from Alfa

• The catalyst powders of Fe-35Ni, Ni etc. were purchased from Alfa Aesar, Germany.

• The SPS experiments were conducted using a Model HP D-125/5 FCT spark plasma sintering system (FCT systeme GmbH, Rauenstein, Germany) installed in our Tycho Sinter Lab at the University of Rostock, Germany. The powders were pressed into a ф20 mm graphite die for the SPS treatment to form disk-shaped samples with thickness of 5-

• The sintered samples were etched in a boiling solution of concentrated H2SO4 (90 vol.%) and HNO3 (10 vol.%) for 2 h. The etched samples were washed using deionized water

• The phase identification of the etched carbon samples was performed using highenergy X-ray diffraction with energies of 80-100 keV at beamline BW5 (DESY/HASYLAB Hamburger Synchrotron Laboratory). The carbon samples were also analyzed by a Renishaw-2000 Laser Raman spectroscopy system with a He–Ne laser

• Scanning electron microscope (SEM, Zeiss Supra 25, Germany) and transmission electron microscope (TEM, Zeiss-Libra120, Germany) operating at 120 keV, were

employed to characterize the products following the SPS treatment.

They were prepared by gas atomization method with 99.0% purity.

present hydrostatic HPHT method.

diamond sizes and transition rates.

Aesar, Germany.

8 mm.

**2. Major raw materials and methods** 

repeatedly, and dried in a vacuum oven.

excited at 512 nm to identify the diamond phase.

• The comparison investigation on the stability of the carbon materials was conducted by the in-situ high temperature X-ray diffraction at the MAX80/F2.1 high-pressure beamline of Helmholtz Centre Potsdam at HASYLAB/DESY Hamburg.
