**4. Diamond synthesis with catalysts by the SPS**

In the HPHT method, the involved solvent catalysts could decrease the energy barrier and affect the rate of a kinetics reaction for diamond nucleation and contribute to the formation of diamond from graphite. Besides being able to reduce the transforming temperature and pressure from graphite to diamond, they can also affect the quality and crystal form of the diamond (Zhang, Adam et al., 2011). It is indicated that the solvent catalysts may have the same effects to promote diamond growth from MWCNTs and graphite in the SPS method In this part, the catalysts were involved in the SPS diamond synthesis with carbon modifications of MWCNTs and graphite and the effects of catalysts were investigated.

#### **4.1 Carbon nanotubes with FeNi catalyst**

Currently preferred metal catalyst materials are Fe-Ni alloys, such as Fe-35Ni, Fe-31Ni-5Co, Fe-30Ni, and other INVAR alloys, where Fe-35Ni being the most preferred and more readily available (Zhang, Adam et al., 2011). In order to increase the transitional rate of diamond, the Fe35Ni alloy powders were chosen as catalysts for diamond synthesis from MWCNTs by the SPS method here. The starting MWCNTs materials have an external and internal diameter of ca. 40 nm and 20 nm, respectively. The MWCNTs/FeNi samples were sintered in the SPS furnace at various temperatures under 70 MPa for 20 min.

Figure 8 (a) shows the Raman spectra of the starting MWCNTs and the spark plasma sintered MWCNTs/Fe35Ni samples at 1100-1500 oC before etching. The diamond band (D band) of the starting MWCNTs appeared at 1345 cm-1. After spark plasma sintering at 1100 oC, the D band has shifted to 1342 cm-1. It was found that the characteristic Raman shift of the cubic diamond phase appeared at 1333 cm-1 in the 1200-1500 oC sintered samples. The D band shifted from the starting 1345 cm-1 to 1333 cm-1 indicating the diamond formation above temperatures of 1200 oC. The broad peak of Raman spectra at 1333 cm-1 in the 1200- 1500 oC sintered samples is due to the existence of un-reacted MWCNTs. The G band is due to the E2g mode of graphite band (G band), relating to the sp2 bonded carbon vibrations in a 2-dimensional graphitic hexagonal lattice. The G bands appeared at 1570 cm-1 in the starting MWCNTs and 1100 oC sintered sample, has shifted to 1574 cm-1 (1200 oC), 1572 cm-1 (1300 oC), 1576 cm-1 (1400 oC), 1561 cm-1 (1500 oC), which implied the vibrations of sp2 bonded carbon during the SPS. The Raman results indicated that diamonds are converted from the MWCNTs/Fe35Ni at temperatures of 1200-1500 oC.

Additionally, all the samples were etched in boiling acid to remove the FeNi catalysts and the un-reacted MWCNTs. Figure 8(b) shows the X-ray diffraction patterns of the starting MWCNTs, Fe35Ni catalyst, and the spark plasma sintered MWCNTs/Fe35Ni samples at 1100-1500 oC after etching with a CuKα radiation Bruker-XRD. The starting MWCNTs show the (002) plane at 2θ of 25.86 degree without Ni and La catalysts peaks. The Fe35Ni catalysts show diffraction peaks at 2θ of 43.60 and 50.79 degree. After etching of the obtained MWCNTs/Fe35Ni samples, no obvious Fe35Ni diffraction peaks were detected in the 1100- 1500 oC sintered samples. It indicated that the FeNi catalysts have been completely removed from the carbon samples by the boiling acid treatment. The peak at 2θ of 42.90 degree in the raw CNTs has shifted to 43.37 degree in the 1100 oC sintered sample.

Synthesis of Diamond Using Spark Plasma Sintering 41

It is noted that this sample is in the transitional stage from CNTs to diamond. It is identified that the cubic diamond peak at 2θ of 43.95 with d spacing of 0.26 nm in the samples of 1200- 1500 oC. There is still a broad CNTs (002) peak indicating there are some un-reacted and unremoved CNTs in the samples. Additionally, haxonite (Fe,Ni)23C6 peaks are found in the XRD results of 1200-1500 oC. With temperature increase from 1200 to 1500 oC, the haxonite peaks get stronger and stronger. This is due to the reaction between the FeNi catalysts and the MWCNTs at high temperatures. The Raman and XRD results confirmed the diamond

The SEM micrographs of the spark plasma sintered MWCNTs/FeNi samples at 1100 oC and 1200 oC after etching are shown in Figure 9. Compared with the starting CNTs, the MWCNTs in the 1100 oC sample were almost melted and adhered together, but the tubular structure of the CNTs is noticeable in the growing diamond (Figure 9a). After sintered at 1200 oC, diamond crystals with sizes of 10-40 μm are observed in the samples (Figure 9b). These diamond crystals are in shape of hexahedron. Some flake-like carbons are noticed in the sample, as indicated by circles. The higher magnification SEM micrograph shows that the diamond crystals without residual CNTs left on their surface (Figure 9c). The high magnification micrograph in Figure 9 (d) indicated the layer-by-layer texture on the diamond crystals. The SEM micrographs of the spark plasma sintered MWCNTs/FeNi samples from 1300 oC to 1500 oC after etching were also studied (Zhang, Adam et al., 2011).. The particle size of the diamond crystals did not increase with the increase of sintering temperatures. A 1500 oC synthesized diamond with crystal size about 20 μm also showed clear layer-by-layer textures. On the matrix of this diamond crystal, there are many flakelike carbons. Such flake-like carbons were found in all these samples from 1300 to 1500 oC. The carbon flakes in these higher temperature sintered samples are similar to those in the 1200 oC sintered one. These flakes also showed layer-by-layer microstructures. The SEM

Figure 10 shows the TEM micrographs and selected area diffraction patterns of the spark plasma sintered MWCNTs/Fe35Ni sample at 1200 oC after etching. There are some monocrystal and poly-crystal diamonds in the samples (Figure 10a-d). The monocrystalline diamond also shows the layer-by-layer structures (Figure 10a), which is consistent with the results of the SEM. The selected area diffraction pattern of the crystal in the bottom of the Figure 10 (a) confirmed the diamond is mono-crystal along [110] direction (Figure 10b). The poly-crystal diamond is in size of tens of micrometers (Figure 10c). The selected area diffraction pattern with diffraction rings were calculated and confirmed the diamonds are cubic poly-crystals (Figure 10d). The Raman, XRD, SEM, and TEM identification results have confirmed the diamond formation in MWCNTs with Fe35Ni as catalysts at a lower

The Raman, XRD, SEM and TEM results confirmed that monocrystalline and polycrystalline diamonds were synthesized in the MWCNTs/Fe35Ni sample after SPS at temperatures above 1200 oC. The SEM results showed better diamond crystal shapes in the 1200 oC sintered samples. The TEM with selected area diffraction patterns showed the existence of diamond mono-crystals and poly-crystals in the 1200 oC sintered samples. Higher temperatures (1300-1500 oC) did not lead to larger diamond crystals as seen in the SEM images. The temperature 1200 oC is the optimal SPS temperature for the MWCNTs/Fe35Ni samples. This temperature 1200 oC for the diamond synthesis is much lower than that of the MWCNTs without catalyst (1500 oC) in our previous research (Zhang, Shen et al., 2005).

formation in the 1200-1500 oC sintered samples.

observations agree well with the Raman and XRD results.

temperature of 1200 oC.

Fig. 8. Raman spectra of the starting MWCNTs and the spark plasma sintered MWCNTs/Fe35Ni samples at 1100-1500 oC before etching (a), and X-ray diffraction patterns of the starting MWCNTs, Fe35Ni catalyst, and the sintered samples after etching.

(a)

(b)

MWCNTs/Fe35Ni samples at 1100-1500 oC before etching (a), and X-ray diffraction patterns

Fig. 8. Raman spectra of the starting MWCNTs and the spark plasma sintered

of the starting MWCNTs, Fe35Ni catalyst, and the sintered samples after etching.

It is noted that this sample is in the transitional stage from CNTs to diamond. It is identified that the cubic diamond peak at 2θ of 43.95 with d spacing of 0.26 nm in the samples of 1200- 1500 oC. There is still a broad CNTs (002) peak indicating there are some un-reacted and unremoved CNTs in the samples. Additionally, haxonite (Fe,Ni)23C6 peaks are found in the XRD results of 1200-1500 oC. With temperature increase from 1200 to 1500 oC, the haxonite peaks get stronger and stronger. This is due to the reaction between the FeNi catalysts and the MWCNTs at high temperatures. The Raman and XRD results confirmed the diamond formation in the 1200-1500 oC sintered samples.

The SEM micrographs of the spark plasma sintered MWCNTs/FeNi samples at 1100 oC and 1200 oC after etching are shown in Figure 9. Compared with the starting CNTs, the MWCNTs in the 1100 oC sample were almost melted and adhered together, but the tubular structure of the CNTs is noticeable in the growing diamond (Figure 9a). After sintered at 1200 oC, diamond crystals with sizes of 10-40 μm are observed in the samples (Figure 9b). These diamond crystals are in shape of hexahedron. Some flake-like carbons are noticed in the sample, as indicated by circles. The higher magnification SEM micrograph shows that the diamond crystals without residual CNTs left on their surface (Figure 9c). The high magnification micrograph in Figure 9 (d) indicated the layer-by-layer texture on the diamond crystals. The SEM micrographs of the spark plasma sintered MWCNTs/FeNi samples from 1300 oC to 1500 oC after etching were also studied (Zhang, Adam et al., 2011).. The particle size of the diamond crystals did not increase with the increase of sintering temperatures. A 1500 oC synthesized diamond with crystal size about 20 μm also showed clear layer-by-layer textures. On the matrix of this diamond crystal, there are many flakelike carbons. Such flake-like carbons were found in all these samples from 1300 to 1500 oC. The carbon flakes in these higher temperature sintered samples are similar to those in the 1200 oC sintered one. These flakes also showed layer-by-layer microstructures. The SEM observations agree well with the Raman and XRD results.

Figure 10 shows the TEM micrographs and selected area diffraction patterns of the spark plasma sintered MWCNTs/Fe35Ni sample at 1200 oC after etching. There are some monocrystal and poly-crystal diamonds in the samples (Figure 10a-d). The monocrystalline diamond also shows the layer-by-layer structures (Figure 10a), which is consistent with the results of the SEM. The selected area diffraction pattern of the crystal in the bottom of the Figure 10 (a) confirmed the diamond is mono-crystal along [110] direction (Figure 10b). The poly-crystal diamond is in size of tens of micrometers (Figure 10c). The selected area diffraction pattern with diffraction rings were calculated and confirmed the diamonds are cubic poly-crystals (Figure 10d). The Raman, XRD, SEM, and TEM identification results have confirmed the diamond formation in MWCNTs with Fe35Ni as catalysts at a lower temperature of 1200 oC.

The Raman, XRD, SEM and TEM results confirmed that monocrystalline and polycrystalline diamonds were synthesized in the MWCNTs/Fe35Ni sample after SPS at temperatures above 1200 oC. The SEM results showed better diamond crystal shapes in the 1200 oC sintered samples. The TEM with selected area diffraction patterns showed the existence of diamond mono-crystals and poly-crystals in the 1200 oC sintered samples. Higher temperatures (1300-1500 oC) did not lead to larger diamond crystals as seen in the SEM images. The temperature 1200 oC is the optimal SPS temperature for the MWCNTs/Fe35Ni samples. This temperature 1200 oC for the diamond synthesis is much lower than that of the MWCNTs without catalyst (1500 oC) in our previous research (Zhang, Shen et al., 2005).

Synthesis of Diamond Using Spark Plasma Sintering 43

always below 5 oC (Vanmeensel et al., 2005). The catalysts of Fe35Ni alloy powders were melted at SPS temperature of 1200 oC, which was noticed during the SPS of the MWCNTs/Fe35Ni sample ; so that, it reduced the SPS temperature to 1200 oC and the pressure to 70 MPa for the diamond synthesis as well as increased the diamond transition rate using the MWCNTs as carbon sources. In general, milder conditions were realized for the diamond synthesis by using the Fe35Ni catalysts in this study. In the HPHT method, the carbon-carbon diagram for the diamond synthesis is crucial (Novikov, 1999). In this SPS method, there will be a new carbon-carbon diagram for the diamond synthesis, which can predict the optimal temperature, pressure regions for the diamond synthesis in this SPS

Fig. 10. TEM micrographs with selected area diffraction patterns (SADP) of the 1200 oC spark plasma sintered MWCNTs/Fe35Ni sample after etching, showing the monocrystalline

diamond (a, b) and polycrystalline diamond (c, d).

method.

Fig. 9. SEM micrographs of the spark plasma sintered MWCNTs/Fe35Ni samples at 1100 oC (a), 1200 oC (b-d) after etching, exhibiting the growing process of diamond.

It indicates that the FeNi catalysts are effective to enhance the diamond conversion from MWCNTs in the SPS technique. The melting point of Fe35Ni alloy is about 1460 oC as measured in its phase diagram. During the SPS processing, the melting point of this Fe35Ni powder has been decreased due to the pulsed current induced powder activation, and the applied pressures. There is usually some temperature difference between the mold surface and the actual temperature in the SPS sample. The temperature measurement design in the FCT spark plasma sintering system allowed a very accurate temperature control since the temperature difference between the centre of the sample and the controlling pyrometer was

(a) (b)

(c) (d)

Fig. 9. SEM micrographs of the spark plasma sintered MWCNTs/Fe35Ni samples at 1100 oC

It indicates that the FeNi catalysts are effective to enhance the diamond conversion from MWCNTs in the SPS technique. The melting point of Fe35Ni alloy is about 1460 oC as measured in its phase diagram. During the SPS processing, the melting point of this Fe35Ni powder has been decreased due to the pulsed current induced powder activation, and the applied pressures. There is usually some temperature difference between the mold surface and the actual temperature in the SPS sample. The temperature measurement design in the FCT spark plasma sintering system allowed a very accurate temperature control since the temperature difference between the centre of the sample and the controlling pyrometer was

(a), 1200 oC (b-d) after etching, exhibiting the growing process of diamond.

always below 5 oC (Vanmeensel et al., 2005). The catalysts of Fe35Ni alloy powders were melted at SPS temperature of 1200 oC, which was noticed during the SPS of the MWCNTs/Fe35Ni sample ; so that, it reduced the SPS temperature to 1200 oC and the pressure to 70 MPa for the diamond synthesis as well as increased the diamond transition rate using the MWCNTs as carbon sources. In general, milder conditions were realized for the diamond synthesis by using the Fe35Ni catalysts in this study. In the HPHT method, the carbon-carbon diagram for the diamond synthesis is crucial (Novikov, 1999). In this SPS method, there will be a new carbon-carbon diagram for the diamond synthesis, which can predict the optimal temperature, pressure regions for the diamond synthesis in this SPS method.

Fig. 10. TEM micrographs with selected area diffraction patterns (SADP) of the 1200 oC spark plasma sintered MWCNTs/Fe35Ni sample after etching, showing the monocrystalline diamond (a, b) and polycrystalline diamond (c, d).

Synthesis of Diamond Using Spark Plasma Sintering 45

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.

(a) (b)

(c) (d)

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.

**4.2 Graphite with various catalysts** 

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 the large diamond crystals synthesis by using the SPS technique.

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 mechanism.
