**1. Introduction**

26 Sintering of Ceramics – New Emerging Techniques

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Carbon is a complex system rich in polymorphs due to its ability to form *sp1*-, *sp*2-, and *sp*3 hybridized C-C bonds. One of the most outstanding achievements in carbon science was the synthesis of diamond from graphite under high-pressure and high-temperature (HPHT) conditions (Giardini et al., 1960). Since diamond particles and films have now been obtained by many other methods including detonation (Vereschagin et al., 1994), combustion flames (Hirose et al., 1990) and chemical vapour deposition (CVD) with RF plasma (Watanabe et al., 1992) or microwave plasma (Kobashi et al., 1988) etc., the HPHT method is still the most popular commercial method for the diamond synthesis. Nowadays, man-made diamond plays an indispensable role in modern industry for abrasives, tool coatings, microelectronics, optics and other applications.

Spark plasma sintering (SPS), commonly also defined as field assisted sintering (FAST) or pulsed electric current sintering (PECS) is a novel pressure assisted pulsed electric current sintering process utilizing ON-OFF DC pulse energizing. Due to the repeated application of an ON-OFF DC pulse voltage and current in powder materials, the spark discharge point and the Joule heating point (local high temperature-state) are transferred and dispersed to the overall specimen (Zhang & Burkel, 2010). The SPS process is based on the electrical spark discharge phenomenon: a high energetic, low voltage spark pulse current momentarily generates high localized temperatures, from several to ten thousand degrees between the particles resulting in high thermal and electrolytic diffusion. During SPS treatment, powders contained in a die can be processed for diverse novel bulk material applications, for example nanostructured materials, functional gradated materials, hard alloys, biomaterials, porous ceramics and alloys etc (Zhang & Burkel, 2010). The SPS has been used to prepare diverse advanced materials; nevertheless, it is still a new technique for the diamond synthesis.

In the year 2004, during the study of the thermal stability of multi-walled carbon nanotubes (MWCNTs) under various SPS conditions, Zhang et al first found that under SPS conditions of 1500 oC at very low pressure (80 MPa) carbon nanotubes were unstable and transformed into diamonds without any catalysts being involved (Zhang, Shen et al., 2005). The transformation mechanism involves the breakage of some C–C bonds, the formation of carbon nano-onions (multilayer fullerenes), and the nucleation and growth of the diamond

Synthesis of Diamond Using Spark Plasma Sintering 29

• 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

The transformation of carbon nanotubes to diamond at very low pressure under SPS has been observed for the first time by Zhang, Shen et al. (2004). Recently, Inam et al reported that multiwall carbon nanotubes were not preserved for ceramic matrices that require high sintering temperatures (>1600°C) and longer processing times (>13 min) in the SPS (Inam et al., 2010). Zhang et al proposed that the spark plasmas may play a key role to provide most of the energy required in this diamond transition, and it provided an indirect way to validate the existence of the plasmas during the SPS. Due to the still on-going arguments about whether the spark plasmas actually occur during the SPS process (Anselmi-Tamburini et al., 2005, Hulbert et al., 2008), it needs further investigations on this point. In this part, we used such an indirect way to prove the presence of plasmas during the SPS. The thermal stability and phase transitional behaviour of carbon nanotubes, C60 and graphite were investigated under the SPS (pulsed DC field). For a comparison study, these carbon materials were also studied using the

beamline of Helmholtz Centre Potsdam at HASYLAB/DESY Hamburg.

in-situ high temperature (AC field) synchrotron radiation X-ray diffraction.

the MWCNTs sample after SPS at 1500 oC under 80 MPa for 20 min.

MWCNTs are found.

**3.1 Stability and phase transformation of carbon materials under pulsed DC field** 

The pure MWCNTs were SPSed at 1500 oC under pressure of 80 MPa for a holding time of 20 min. Figure 1(a) shows the synchrotron radiation-high energy X-ray diffraction patterns of the raw MWCNTs and the spark plasma sintered (SPSed) MWCNTs. The raw MWNCTs show a main diffraction peak at 3.43 Å corresponding to the CNTs (002) plane spacing, and weak peaks at 2.10 and 1.70 Å corresponding to the CNTs (100) and (004) plane spacing, respectively. After SPS processing, the MWCNTs diffraction peaks are still present in the sintered MWCNTs compacts, but the peaks of the CNTs (002), (100) and (004) are stronger than those in the raw MWCNTs. It indicates that the SPS process improved the crystallinity of the MWCNTs (Zhang, Mihoc et al., 2011). Additionally, new peaks were detected in the sample centered at 2.05, 1.23, 1.06 and 1.76 Å corresponding to the cubic diamond (ICDD No. 65-537) (111), (220), (311) and n-diamond (ICDD No. 43-1104) (200) plane spacing, respectively. Figure 1(b) shows the Raman spectra of the raw MWCNTs and the SPSed MWCNTs. The result of the raw MWCNTs show that their D band appeared at 1344 cm-1 and G band appeared at 1569 cm-1. After SPS processing, the D peak shifted to 1333 cm-1 corresponding to the cubic diamond but there was still a weak peak at 1344 cm-1 belonging to the un-reacted MWCNTs, the G band shifted to 1566 cm-1 relating to the sp2 bonded carbon vibrations. The results of the X-ray diffraction and Raman spectroscopy confirmed the diamond formation in

Figure 2 shows the SEM micrographs of the raw MWCNTs and the spark plasma sintered MWCNTs at 1500 oC under 80 MPa for 20 min. The fibrous structures of the raw MWCNTs can be observed in the Figure 2(a). But these structures have disappeared, and some diamond crystals are found in the samples after SPS. Figure 2 (b) shows one diamond crystal with particle size of 35 μm around. In the background of this diamond crystal, no

**3. Stability of carbon nanotubes, C60 and graphite under various fields** 

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 present hydrostatic HPHT method.

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 diamond sizes and transition rates.
