**4.2 Graphite with various catalysts**

44 Sintering of Ceramics – New Emerging Techniques

A layer-by-layer structure of diamond crystals was found in the SEM and TEM micrographs. Our previous research revealed the initial diamond growth mechanism from MWCNTs to diamond in SPS without catalysts, that is from CNTs to intermediate phase carbon onions and finally to diamond. The diamond crystals in the samples without FeNi catalysts also show the similar layer-by-layer structures. Many flake-carbons with layers structures were found in the samples of 1200-1500 oC. These indicate that the diamonds were grown up from these carbon flakes. Based on the above analysis, a model for the growth of diamond crystals during the SPS is proposed in Figure 11. The direction of pressure during the SPS is in two axial directions, but not in six directions as the HPHT sixanvil press. Therefore, the diamonds were easier to growth in the direction without pressure. As a result, the MWCNTs were grown to layered diamond flakes vertically to the direction of pressure. Finally, several diamond flakes reacted together and formed a threedimensional diamond crystal. The growth mechanism of diamond from MWCNTs is a layer-by-layer growth model in the SPS method. This model is available for the MWCNTs to diamond with and without catalysts. This mechanism will be constructive and helpful for

Fig. 11. Schematic illustration of the growth model of diamond crystals from MWCNTs in SPS.

In summary, a Fe35Ni solvent catalyst has been involved to synthesize diamond from MWCNTs by using the SPS technique. Cubic diamond crystals were synthesized from the MWCNTs/Fe35Ni mixtures at lower SPS temperature of 1200 oC under pressure of 70 MPa. In the sample, well-crystallized diamond mono-crystals and poly-crystals consisted particle sizes ranged 10-40 μm. The Fe35Ni catalysts achieved an effective reduction of the SPS temperature to 1200 oC and the SPS pressure to 70 MPa for the diamond synthesis, as well as an increment of diamond transition rate from MWCNTs in the SPS. A model was also proposed to describe the diamond growth and revealed as a layer-by-layer growth

the large diamond crystals synthesis by using the SPS technique.

mechanism.

Graphite has been used as the main carbon source for the diamond synthesis in the HPHT technique. In this study, the Ni, MnNi, MnNiFe and AlCuFe quasicrystal powder were tested as the catalysts for the diamond synthesis from the graphite by the SPS technique. Each catalyst was weighted and mixed with the graphite powder in the mass ratio of 4:6. The mixture powders were mixed homogeneously by high energy ball milling for 5 hours.

Fig. 12. SEM micrographs of the spark plasma sintered graphite samples at 1300 oC under 50 MPa with Ni (a), AlCuFe quasicrystal (b), MnNi (c), and MnNiFe powder catalysts (d) exhibiting the different diamonds.

Synthesis of Diamond Using Spark Plasma Sintering 47

the strong diamond peaks at d-spacing of 2.06 and 1.26 Å. The Graphite/MnNi and Graphite/MnNiFe samples showed weak diamond peaks at d-spacing of 2.06, 1.26 and 1.07 Å. The XRD results agree well with the SEM results that good crystalline diamonds have

In this part, the factors that could affect the diamond growth will be studied in order to increase the diamond crystal size and transition rate. The factors including carbon

For the carbon modification selection, we will use the C60 as the carbon source for the diamond generation in the SPS. Our previous work has shown that C60 can be converted into diamond under the same SPS conditions as carbon nanotubes are converted to diamond (1500 oC, 80 MPa). Since the C60 has a higher sp3 hybridization fraction than carbon nanotubes, it makes the transformation of C60 into diamond easier. Therefore, the C60 may be able to increase the diamond size in the SPS process. In this study, the diamond synthesis

The C60 powders were spark plasma sintered (SPSed) at different temperatures under a pressure of 50 MPa. Figure 14 (a) shows the Raman spectra of the raw C60 and the SPSed C60 samples after etching. The raw C60 shows a sharp peak appeared at 1460 cm-1 and two weak broad peaks centered at 1568 and 1413 cm-1. The cubic diamond peaks can also be detected at 1333 cm-1 in the Raman spectra taken for the samples processed in the temperature range from 1150 oC to 1500 oC under 50 MPa. However, the diamond band of the sample sintered at 1150 oC is very broad having the lowest height. Its graphite band at 1568 cm-1 is at the same value as that of the raw C60. It indicates that there is only a small fraction of diamond in this 1150 oC SPSed sample. With an increase in temperature to 1200 oC, 1300 oC and 1500 oC, the diamond band at 1333 cm-1 gets sharper and sharper, as well as the graphite band is shifted to a higher value of 1576 cm-1. The result of the 1300 oC SPSed C60 shows the Raman spectra similar to the 1500 oC SPSed sample. Figure 14 (b) shows the XRD results of the raw C60 and the SPSed C60 samples after etching. In the 1150 oC sintered C60 sample, we found very weak diamond peaks. The C60 after SPS at temperatures above 1200 oC show the cubic diamond diffraction peaks at d spacing of 2.06 and 1.26 Å and a broad graphite peak. The C60 diffraction peaks disappeared indicate that the C60 has completely transformed into diamond and graphite

The SEM micrographs of the C60 samples SPSed from 1150 to 1500 oC after etching are shown in Figure 15. There are few fine diamond crystals in the 1150 oC SPSed sample (Figure 15a). Some diamond crystals with hexagonal, tetragonal or triangular shapes are found in the micrograph of the 1200 oC SPSed sample (Figure 15b). The particle sizes of the diamond crystals are from tens of micrometers up to 200 μm. The diamond crystals with perfect hexahedron shapes are clearly observed in the 1300 oC sintered sample (Figure 15c). The diamond sizes range from 100 to 250 μm, they are larger than those of the sample sintered at 1200 oC. The SEM micrographs of the 1500 oC sintered C60 samples show that the big diamond crystals are almost melted. There are many small diamond crystals below 4 μm on big crystals.

been successfully synthesized from the graphite with catalysts of Ni and AlCuFe.

modifications and atmospheres (Vacuum, Ar) will be studied and discussed.

**5. Factors affecting the diamond growth in the SPS** 

**5.1 Large diamond crystals from the C60 by the SPS** 

from the C60 was studied in the SPS (Zhang, Ahmed et al., 2011).

phases after the SPS processing at temperatures from 1200-1500 oC.

Then, the mixtures were subjected to the SPS machine. The sintered samples were tested at SPS temperatures of 1200-1500 oC and under pressures of 10-80 MPa. The results show that diamond crystals can be converted from graphite at the SPS temperature of 1300 oC for holding time of 20 min under the pressure of 50 MPa.

Figure 12 shows the SEM micrographs of the spark plasma sintered graphite samples at 1300 oC for 20 min under 50 MPa with Ni, AlCuFe quasicrystal, MnNi, and MnNiFe powder catalysts after etching. It is interesting that we got diamond nano- and micro-rods with the Ni catalysts from the graphite (Figure 12a) by the SPS. The diameter of the diamond rods are from 80 nm to 2 μm. It is noted that the SPS also can be used as a new method to synthesize diamond nano- and micro-rods. The AlCuFe quasicrystal was introduced as catalyst for the graphite to diamond in the SPS. The diamond crystals with good diamond shapes from 1 to 3 μm were converted from the graphite with the AlCuFe catalyst. It indicates that the AlCuFe quasicrystal powder also can be used as catalyst for the conversion from graphite to diamond in the SPS. With MnNi catalysts, some short rod like diamond crystals are found in the sample (Figure 12c). However, in the sample of graphite/MnNiFe, there are no good diamond crystals visible (Figure 12d).

Additionally, the diamond phase was identified by the X-ray diffraction. Figure 13 shows the synchrotron radiation-high energy X-ray diffraction patterns of the spark plasma sintered graphite samples at 1300 oC under 50 MPa with Ni, AlCuFe quasicrystal , MnNi, and MnNiFe powder catalysts after etching. The Graphite/Ni sample shows strong cubic diamond peaks at d-spacing of 2.06, 1.26 and 1.07 Å. The Graphite/AlCuFe samples show

Fig. 13. Synchrotron radiation-high energy X-ray diffraction patterns of the spark plasma sintered graphite samples at 1300 oC under 50 MPa with Ni (a), AlCuFe quasicrystal (b), MnNi (c), and MnNiFe powder catalysts (d).

Then, the mixtures were subjected to the SPS machine. The sintered samples were tested at SPS temperatures of 1200-1500 oC and under pressures of 10-80 MPa. The results show that diamond crystals can be converted from graphite at the SPS temperature of 1300 oC for

Figure 12 shows the SEM micrographs of the spark plasma sintered graphite samples at 1300 oC for 20 min under 50 MPa with Ni, AlCuFe quasicrystal, MnNi, and MnNiFe powder catalysts after etching. It is interesting that we got diamond nano- and micro-rods with the Ni catalysts from the graphite (Figure 12a) by the SPS. The diameter of the diamond rods are from 80 nm to 2 μm. It is noted that the SPS also can be used as a new method to synthesize diamond nano- and micro-rods. The AlCuFe quasicrystal was introduced as catalyst for the graphite to diamond in the SPS. The diamond crystals with good diamond shapes from 1 to 3 μm were converted from the graphite with the AlCuFe catalyst. It indicates that the AlCuFe quasicrystal powder also can be used as catalyst for the conversion from graphite to diamond in the SPS. With MnNi catalysts, some short rod like diamond crystals are found in the sample (Figure 12c). However, in the sample of graphite/MnNiFe, there are no good

Additionally, the diamond phase was identified by the X-ray diffraction. Figure 13 shows the synchrotron radiation-high energy X-ray diffraction patterns of the spark plasma sintered graphite samples at 1300 oC under 50 MPa with Ni, AlCuFe quasicrystal , MnNi, and MnNiFe powder catalysts after etching. The Graphite/Ni sample shows strong cubic diamond peaks at d-spacing of 2.06, 1.26 and 1.07 Å. The Graphite/AlCuFe samples show

Fig. 13. Synchrotron radiation-high energy X-ray diffraction patterns of the spark plasma sintered graphite samples at 1300 oC under 50 MPa with Ni (a), AlCuFe quasicrystal (b),

holding time of 20 min under the pressure of 50 MPa.

diamond crystals visible (Figure 12d).

λ

MnNi (c), and MnNiFe powder catalysts (d).

the strong diamond peaks at d-spacing of 2.06 and 1.26 Å. The Graphite/MnNi and Graphite/MnNiFe samples showed weak diamond peaks at d-spacing of 2.06, 1.26 and 1.07 Å. The XRD results agree well with the SEM results that good crystalline diamonds have been successfully synthesized from the graphite with catalysts of Ni and AlCuFe.
