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

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

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

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 MWCNTs are found.

Synthesis of Diamond Using Spark Plasma Sintering 31

(a) (b) Fig. 2. SEM micrographs of the raw MWCNTs (a) and the spark plasma sintered MWCNTs

Figure 3(a) shows the synchrotron radiation-high energy X-ray diffraction patterns of raw C60 and the SPSed C60 at 1500 oC, 80 MPa for 20 min. The raw C60 exhibits diffraction peaks at d spacing of 5.01, 4.28, 4.11, 3.18, 2.9, 2.74 Å belonging to C60 (110), (112), (004), (114), (300), (006) planes (ICDD No. 47-0787), respectively. The C60 after SPS shows the cubic diamond diffraction peaks at d spacing of 2.06 and 1.23 Å and a broad graphite peak. The C60 diffraction peaks disappeared indicating the C60 has completely transformed into diamond and graphite phases after the SPS processing. Figure 3(b) shows the Raman spectra of the raw C60 and the SPSed C60. The raw C60 shows a sharp peak appearing at 1460 cm-1, and two weak, broad peaks centered at 1568 and 1515 cm-1. After SPS processing, it shows the cubic diamond peak at 1333 cm-1 and graphite peak at 1558 cm-1, but the C60 peak at 1460 cm-1 disappeared. It is consistent with X-ray diffraction results that the C60 has completely transformed into diamond and graphite phases after SPS at 1500 oC under 80

Figure 4 shows the SEM micrographs of the raw C60 and the spark plasma sintered C60 at 1500 oC under 80 MPa for 20 min. The raw C60 powders are nano-particle agglomerates and show bundles of C60 in the Figure 4(a). Some diamond crystals with sizes from 2 to 8 μm can be observed in the Figure 4(b). The structures of C60 are not noticeable in the

Figure 5 (a) shows the synchrotron radiation diffraction patterns of the raw graphite and the SPSed graphite at 1500 oC under 80 MPa for 20 min. The raw graphite sample presents Graphite-3R and Graphite-2H diffraction peaks those are centered at 3.348 Å [G-3R(003)], 1.674 Å [G-3R(006)], 1.228 Å [G-3R(110)] (ICDD No. 26-1079), and 2.138 Å [G-2H(100)] , 2.039 Å [G-2H(101)], 1.16 Å [G-2H(112)] (ICDD No. 41-1487). However, the diamond phase is not found in the graphite samples after the SPS processing. Only, an increased intensity in the

at 1500 oC, 80 MPa for 20 min showing one diamond crystal (b).

MPa for 20 min.

background of the diamond crystals.

Fig. 1. Synchrotron radiation-high energy X-ray diffraction patterns (a) and Raman spectra (b) of the raw MWCNTs and the spark plasma sintered MWCNTs at 1500 oC, 80 MPa for 20 min.

(a)

(b)

Fig. 1. Synchrotron radiation-high energy X-ray diffraction patterns (a) and Raman spectra (b) of the raw MWCNTs and the spark plasma sintered MWCNTs at 1500 oC, 80 MPa for 20

min.

Fig. 2. SEM micrographs of the raw MWCNTs (a) and the spark plasma sintered MWCNTs at 1500 oC, 80 MPa for 20 min showing one diamond crystal (b).

Figure 3(a) shows the synchrotron radiation-high energy X-ray diffraction patterns of raw C60 and the SPSed C60 at 1500 oC, 80 MPa for 20 min. The raw C60 exhibits diffraction peaks at d spacing of 5.01, 4.28, 4.11, 3.18, 2.9, 2.74 Å belonging to C60 (110), (112), (004), (114), (300), (006) planes (ICDD No. 47-0787), respectively. The C60 after SPS shows the cubic diamond diffraction peaks at d spacing of 2.06 and 1.23 Å and a broad graphite peak. The C60 diffraction peaks disappeared indicating the C60 has completely transformed into diamond and graphite phases after the SPS processing. Figure 3(b) shows the Raman spectra of the raw C60 and the SPSed C60. The raw C60 shows a sharp peak appearing at 1460 cm-1, and two weak, broad peaks centered at 1568 and 1515 cm-1. After SPS processing, it shows the cubic diamond peak at 1333 cm-1 and graphite peak at 1558 cm-1, but the C60 peak at 1460 cm-1 disappeared. It is consistent with X-ray diffraction results that the C60 has completely transformed into diamond and graphite phases after SPS at 1500 oC under 80 MPa for 20 min.

Figure 4 shows the SEM micrographs of the raw C60 and the spark plasma sintered C60 at 1500 oC under 80 MPa for 20 min. The raw C60 powders are nano-particle agglomerates and show bundles of C60 in the Figure 4(a). Some diamond crystals with sizes from 2 to 8 μm can be observed in the Figure 4(b). The structures of C60 are not noticeable in the background of the diamond crystals.

Figure 5 (a) shows the synchrotron radiation diffraction patterns of the raw graphite and the SPSed graphite at 1500 oC under 80 MPa for 20 min. The raw graphite sample presents Graphite-3R and Graphite-2H diffraction peaks those are centered at 3.348 Å [G-3R(003)], 1.674 Å [G-3R(006)], 1.228 Å [G-3R(110)] (ICDD No. 26-1079), and 2.138 Å [G-2H(100)] , 2.039 Å [G-2H(101)], 1.16 Å [G-2H(112)] (ICDD No. 41-1487). However, the diamond phase is not found in the graphite samples after the SPS processing. Only, an increased intensity in the

Synthesis of Diamond Using Spark Plasma Sintering 33

(a) (b)

Fig. 4. SEM micrographs of the raw C60 (a) and the spark plasma sintered C60 at 1500 oC, 80

graphite peaks indicating the improved crystallinity is visible. Figure 5(b) shows the Raman spectra of the raw graphite and the SPSed graphite. The raw graphite shows a sharp peak at 1579 cm-1, and a weak peak at 1350 cm-1. After SPS processing, the intensity of the peak at 1350 cm-1 has improved, but there is no diamond peak in the Raman spectra. The X-ray diffraction and Raman spectroscopy results confirmed that there is no diamond conversion

Fig. 6 shows the SEM micrographs of the raw graphite and the spark plasma sintered graphite at 1500 oC under 80 MPa for 20 min. The raw graphite shows the typical layered structure as shown in Figure 6(a). After the SPS of MWCNTs and C60 at the identical condition, there is no presence of diamond in the sample (Figure 6b). The sample shows the similar structure as the raw graphite. The SEM results agree well with the XRD and Raman results and confirmed that there is no diamond conversation from pure graphite after SPS at

Figure 7 (a) shows the synchrotron radiation-in situ X-ray diffraction patterns of the pure MWCNTs at 80 MPa under different temperatures. In the in-situ sintering furnace (AC filed) of the MAX80/F2.1 high-pressure beamline. The combining peak of MWCNT and graphite has shifted to lower energy values. It indicates the thermal expansion of the nanotubes and graphite planes with the increase of temperature. The boron nitride (BN) peaks are from the container of the powder sample during the in-situ high temperature X-ray experiments. However, there is no diamond formation at or below the temperature of 1500 oC under 80 MPa. This means that the MWCNTs are dynamically stable at this temperature 1500 oC

**3.2 Stability and phase transformation of carbon materials under AC field** 

from pure graphite after SPS at 1500 oC under 80 MPa for 20 min.

MPa for 20 min (b).

1500 oC under 80 MPa for 20 min.

Fig. 3. Synchrotron radiation-high energy X-ray diffraction patterns (a) and Raman spectra (b) of the raw C60 and the spark plasma sintered C60 at 1500 oC, 80 MPa for 20 min.

λ

(a)

(b)

Fig. 3. Synchrotron radiation-high energy X-ray diffraction patterns (a) and Raman spectra (b) of the raw C60 and the spark plasma sintered C60 at 1500 oC, 80 MPa for 20 min.

Fig. 4. SEM micrographs of the raw C60 (a) and the spark plasma sintered C60 at 1500 oC, 80 MPa for 20 min (b).

graphite peaks indicating the improved crystallinity is visible. Figure 5(b) shows the Raman spectra of the raw graphite and the SPSed graphite. The raw graphite shows a sharp peak at 1579 cm-1, and a weak peak at 1350 cm-1. After SPS processing, the intensity of the peak at 1350 cm-1 has improved, but there is no diamond peak in the Raman spectra. The X-ray diffraction and Raman spectroscopy results confirmed that there is no diamond conversion from pure graphite after SPS at 1500 oC under 80 MPa for 20 min.

Fig. 6 shows the SEM micrographs of the raw graphite and the spark plasma sintered graphite at 1500 oC under 80 MPa for 20 min. The raw graphite shows the typical layered structure as shown in Figure 6(a). After the SPS of MWCNTs and C60 at the identical condition, there is no presence of diamond in the sample (Figure 6b). The sample shows the similar structure as the raw graphite. The SEM results agree well with the XRD and Raman results and confirmed that there is no diamond conversation from pure graphite after SPS at 1500 oC under 80 MPa for 20 min.
