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

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

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

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.

Synthesis of Diamond Using Spark Plasma Sintering 49

(a) (b)

(c) (d) Fig. 15. SEM micrographs of spark plasma sintered C60 samples at 1150 oC(a), 1200 oC (b),

The particle size of the diamond crystals made from C60 is up to 250 μm. It is a very large size for such conversion without any catalyst being involved in the process. The carbon atoms in C60 are sp2 hybridized with a high fraction of sp3 hybridized structure due to angular strain. It is difficult to transform the planar sp2 structure to the diamond sp3 network. The C60 can be considered as a folded graphite sheet with the predominant hybridization sp3 in the pentagons. This makes the transformation of C60 into diamond easier. A dense assembly of C60 spheroids, where 48 out of 60 carbon atoms have quasitetrahedral coordination, is sterically fairly close to that of the diamond (Regueiro et al., 1992). It implies that a small rearrangement of the atoms of C60 can result in the change of

1300 oC (c), and 1500 oC (d) after etching, showing the diamond formation.

It is obvious that the diamond crystal sizes do not increase with the increase in temperature; however, well-defined diamond crystals are created. A processing temperature of 1300 oC is the best for the phase transformation of C60 directly to diamond, according to this study.

Fig. 14. Raman spectra (a) and Synchrotron radiation-high energy X-ray diffraction patterns (b) of the raw C60 and the spark plasma sintered C60 at different temperatures under 50 MPa pressure.

It is obvious that the diamond crystal sizes do not increase with the increase in temperature; however, well-defined diamond crystals are created. A processing temperature of 1300 oC is the best for the phase transformation of C60 directly to diamond, according to this study.

(a)

(b)

Fig. 14. Raman spectra (a) and Synchrotron radiation-high energy X-ray diffraction patterns (b) of the raw C60 and the spark plasma sintered C60 at different temperatures under

50 MPa pressure.

Fig. 15. SEM micrographs of spark plasma sintered C60 samples at 1150 oC(a), 1200 oC (b), 1300 oC (c), and 1500 oC (d) after etching, showing the diamond formation.

The particle size of the diamond crystals made from C60 is up to 250 μm. It is a very large size for such conversion without any catalyst being involved in the process. The carbon atoms in C60 are sp2 hybridized with a high fraction of sp3 hybridized structure due to angular strain. It is difficult to transform the planar sp2 structure to the diamond sp3 network. The C60 can be considered as a folded graphite sheet with the predominant hybridization sp3 in the pentagons. This makes the transformation of C60 into diamond easier. A dense assembly of C60 spheroids, where 48 out of 60 carbon atoms have quasitetrahedral coordination, is sterically fairly close to that of the diamond (Regueiro et al., 1992). It implies that a small rearrangement of the atoms of C60 can result in the change of

Synthesis of Diamond Using Spark Plasma Sintering 51

{ }

( ) <sup>3</sup> 2 3sin sin

−+ + <sup>Δ</sup> Δ = +Δ − +

The above equation shows the that decrease of R leads to the decrease of △G. The value of R is the inner radius of CNTs. It means the decrease of the diameter of CNTs could decrease the energy for the diamond nucleation △G. The lower △G leads to higher transition rates and larger particle sizes of diamond. This was validated by the following experiments.

The MWCNTs with diameters from 10-100nm were mixed with Fe35Ni catalyst at a weight ratio of 1:1 by ball milling, and pressed into a graphite die for SPS treatment (1200 °C, 50 MPa, 20 min) to form disk-shaped samples. The raw materials used in this experiment was shown in Table 1 with internal diameters of 10-20 nm, 20-40 nm, 40-60 nm and 60-100 nm.

Length 5-15 μm 5-15 μm 5-15 μm 5-15 μm Purity >95% >95% >95% >95% Ash <0.2 wt% <0.2 wt% <0.2 wt% <0.2 wt%

Amorphous carbon <3% <3% <3% <3%

The SPSed samples after etching were subjected to the SEM analysis. Figure 17 shows the SEM micrographs of the diamond crystals obtained from different MWCNTs. It really shows various diamond sizes. The 10-20 nm MWCNTs generated 15-30 μm diamond crystals. The 20-40 nm MWCNTs generated 10-20 μm diamond. The 40-60 nm MWCNTs produced 7-10 μm diamonds, and the 60-100 nm MWCNTs formed 4-10 μm diamonds. The results are summarized in Table 2. It indicates that smaller diameters MWCNTs could

size 15-30 μm 10-20 μm 7-15 μm 4-10 μ<sup>m</sup>

*R*

2 2

*r R*

+ − +−

*E*

*V*

*<sup>r</sup> <sup>g</sup> G g*

2 (1 cos ) (1 sin )

θ ( ) ( )

θ

θ

10-20 nm 20-40 nm 40-60 nm 60-100 nm

m2/g 40-300 m2/g 40-300 m2/g 40-300 m2/g

10-20 nm 20-40 nm 40-60 nm 60-100 nm

 θ

 θ

θ

3 3 3 3

2 3cos cos

Nuclear powder of diamond from carbon onion △G,

α β

πσ

Main range of diameter of MWCNTs

Specific surface area 40-300

produce larger diamond particles.

Main range of diameter of MWCNTs

Diamond particle

Table 1. The raw materials those used in this experiments.

Table 2. The diamond particle size from various MWCNTs.

π

its structure. However, it still needs solely superhigh pressure or high pressure and high temperatures for the phase transition from C60 to diamond (Zhang, Ahmed et al., 2011). We got diamond from C60 by the SPS at only 50 MPa and temperatures above 1150 oC. Wellcrystallized diamonds with particle sizes up to 250 μm are obtained at 1300 oC and no further growth in particle size is seen beyond this temperature.

#### **5.2 Effects of the CNTs diameters on the diamond sizes by the SPS**

In the previous research, it was proposed that the CNTs transformed to carbon nano-onions, and the nucleation and growth of the diamond phase within the onion cores (Shen, Zhang et al., 2006). A model for the diamond nucleation at the internal surface of carbon onion was established, as shown in Fig.16. Based on this model, the energy need for the nucleation of diamond at the internal surface of carbon onion can be formulized (Zhang, 2005):

$$
\Delta G = \Delta G\_V + \Delta G\_S + \Delta G\_E
$$

Where △G — Difference of Free energy for the nucleation of diamond;

△Gv— Difference of Volume Free energy for the nucleation of diamond; △GS— Difference of Surface Free energy for the nucleation of diamond;

△GE— Difference of Elastic strain energy for the nucleation of diamond.

Fig. 16. Model for diamond (*β*) nucleation at the internal surface of carbon onion (*α*).

$$
\Delta G\_{V} = \frac{\pi \Delta \mathbf{g}\_{V}}{3} \left[ r^{3} \left( 2 - 3 \cos \theta + \cos^{3} \theta \right) + R^{3} \left( 2 - 3 \sin \theta + \sin^{3} \theta \right) \right]
$$

$$
\Delta G\_{S} = 2 \pi \sigma\_{\alpha \beta} \left[ r^{2} \left( 1 - \cos \theta \right) + R^{2} \left( 1 - \sin \theta \right) \right]
$$

$$
\Delta G\_{E} = V \Delta \mathbf{g}\_{E} = \Delta \mathbf{g}\_{E} \left\{ \frac{\pi r^{3}}{3} \left( 2 - 3 \cos \theta + \cos^{3} \theta \right) + \frac{\pi R^{3}}{3} \left( 2 - 3 \sin \theta + \sin^{3} \theta \right) \right\}
$$

its structure. However, it still needs solely superhigh pressure or high pressure and high temperatures for the phase transition from C60 to diamond (Zhang, Ahmed et al., 2011). We got diamond from C60 by the SPS at only 50 MPa and temperatures above 1150 oC. Wellcrystallized diamonds with particle sizes up to 250 μm are obtained at 1300 oC and no

In the previous research, it was proposed that the CNTs transformed to carbon nano-onions, and the nucleation and growth of the diamond phase within the onion cores (Shen, Zhang et al., 2006). A model for the diamond nucleation at the internal surface of carbon onion was established, as shown in Fig.16. Based on this model, the energy need for the nucleation of

Δ =Δ +Δ +Δ *GG G G VSE*

further growth in particle size is seen beyond this temperature.

**5.2 Effects of the CNTs diameters on the diamond sizes by the SPS** 

*E*

Where △G — Difference of Free energy for the nucleation of diamond; △Gv— Difference of Volume Free energy for the nucleation of diamond; △GS— Difference of Surface Free energy for the nucleation of diamond; △GE— Difference of Elastic strain energy for the nucleation of diamond.

diamond at the internal surface of carbon onion can be formulized (Zhang, 2005):

Fig. 16. Model for diamond (*β*) nucleation at the internal surface of carbon onion (*α*).

θ

<sup>Δ</sup> Δ= − + + − +

{ } 2 2 2 (1 cos ) (1 sin ) Δ= − + − *Gr R <sup>S</sup>*

θ

Δ = Δ =Δ − + + − +

*<sup>g</sup> G r <sup>R</sup>*

πσ αβ

3 3 *E EE r R G Vg g* π

3 *V*

π

*V*

{ ( ) ( )} <sup>3</sup> 3 3 <sup>3</sup> 2 3cos cos 2 3sin sin

( ) ( ) 3 3 3 3 2 3cos cos 2 3sin sin

 π  θθ

θ

θθ

 θ

θ

> θ

Nuclear powder of diamond from carbon onion △G,

$$\begin{aligned} \Delta G &= (\frac{\pi \Delta \mathbf{g}\_{\boldsymbol{V}}}{3} + \Delta \mathbf{g}\_{\boldsymbol{E}}) \begin{Bmatrix} r^3 \left(2 - 3 \cos \theta + \cos^3 \theta \right) + \\ R^3 \left(2 - 3 \sin \theta + \sin^3 \theta \right) \\ r^2 \left(1 - \cos \theta \right) + R^2 \left(1 - \sin \theta \right) \end{Bmatrix} \end{aligned}$$

The above equation shows the that decrease of R leads to the decrease of △G. The value of R is the inner radius of CNTs. It means the decrease of the diameter of CNTs could decrease the energy for the diamond nucleation △G. The lower △G leads to higher transition rates and larger particle sizes of diamond. This was validated by the following experiments.

The MWCNTs with diameters from 10-100nm were mixed with Fe35Ni catalyst at a weight ratio of 1:1 by ball milling, and pressed into a graphite die for SPS treatment (1200 °C, 50 MPa, 20 min) to form disk-shaped samples. The raw materials used in this experiment was shown in Table 1 with internal diameters of 10-20 nm, 20-40 nm, 40-60 nm and 60-100 nm.



The SPSed samples after etching were subjected to the SEM analysis. Figure 17 shows the SEM micrographs of the diamond crystals obtained from different MWCNTs. It really shows various diamond sizes. The 10-20 nm MWCNTs generated 15-30 μm diamond crystals. The 20-40 nm MWCNTs generated 10-20 μm diamond. The 40-60 nm MWCNTs produced 7-10 μm diamonds, and the 60-100 nm MWCNTs formed 4-10 μm diamonds. The results are summarized in Table 2. It indicates that smaller diameters MWCNTs could produce larger diamond particles.


Table 2. The diamond particle size from various MWCNTs.

Synthesis of Diamond Using Spark Plasma Sintering 53

In the SPS process, we usually use the vacuum atmosphere. We proposed that the plasma plays the key role for the diamond transition from various carbon modifications. Based on the theory of plasma physics, the gases like H2, Ar, O2, and so on can be used as plasma generating gases which can enhance the plasmas generation (Zheng et al., 2009). Therefore, in this study, the effect of the atmosphere on the diamond growth in the SPS was studied. The MWCNTs/FeNi mixture powders were spark plasma sintered in vacuum and Ar gas atmospheres at 1200 oC under 10 MPa which is the minimum pressure of the Model HP D-5 FCT SPS system. The sintered samples were etched and examined by the SEM and Raman

(a) (b)

(e)

(c) (d) Fig. 18. SEM micrographs of the MWCNTs/FeNi samples SPSed at 1200 oC under 10 MPa in

**5.3 Effect of atmospheres on the diamond growth in the SPS** 

spectroscopy.

vacuum (a, b) and Ar gas (c-e).

The Raman results also confirmed the higher transition rate in the 10-20 nm MWCNTs. Therefore, the experiments validated the theoretical assumption. Consequently, the MWCNTs have been transformed into diamond under SPS conditions in presence of a FeNi catalyst at pressures of 50 MPa and temperatures of 1200 °C. The diamond particle size depends on the diameter of the MWCNTs. The MWCNTs with diameters from 60 to 100nm produced diamond particle from 4 to 10 μm, while in the sample of MWCNTs with diameters from 10 to 20 nm generated diamond particle sizes from 15 to 30 μm.

Fig. 17. SEM micrographs of the SPSed MWCNT/FeNi samples with various internal nanotube diameters of 10-20 nm (a), 20-40 nm (b), 40-60 nm (c) and 60-100 nm (d) showing the different diamond sizes.

The Raman results also confirmed the higher transition rate in the 10-20 nm MWCNTs. Therefore, the experiments validated the theoretical assumption. Consequently, the MWCNTs have been transformed into diamond under SPS conditions in presence of a FeNi catalyst at pressures of 50 MPa and temperatures of 1200 °C. The diamond particle size depends on the diameter of the MWCNTs. The MWCNTs with diameters from 60 to 100nm produced diamond particle from 4 to 10 μm, while in the sample of MWCNTs with

(a) (b)

(c) (d)

Fig. 17. SEM micrographs of the SPSed MWCNT/FeNi samples with various internal nanotube diameters of 10-20 nm (a), 20-40 nm (b), 40-60 nm (c) and 60-100 nm (d) showing

the different diamond sizes.

diameters from 10 to 20 nm generated diamond particle sizes from 15 to 30 μm.

## **5.3 Effect of atmospheres on the diamond growth in the SPS**

In the SPS process, we usually use the vacuum atmosphere. We proposed that the plasma plays the key role for the diamond transition from various carbon modifications. Based on the theory of plasma physics, the gases like H2, Ar, O2, and so on can be used as plasma generating gases which can enhance the plasmas generation (Zheng et al., 2009). Therefore, in this study, the effect of the atmosphere on the diamond growth in the SPS was studied. The MWCNTs/FeNi mixture powders were spark plasma sintered in vacuum and Ar gas atmospheres at 1200 oC under 10 MPa which is the minimum pressure of the Model HP D-5 FCT SPS system. The sintered samples were etched and examined by the SEM and Raman spectroscopy.

Fig. 18. SEM micrographs of the MWCNTs/FeNi samples SPSed at 1200 oC under 10 MPa in vacuum (a, b) and Ar gas (c-e).

Synthesis of Diamond Using Spark Plasma Sintering 55

In this part, we investigated the factors affecting the diamond growth in the SPS. It is found that the C60 is able to increase the diamond size in the SPS process. Due to the high price of the C60, it is suggested that the C60 can be used as a doping catalyst material to promote the diamond transition. The diamond particle size depends on the internal diameter of the MWCNTs. Smaller diameter MWCNTs generated larger diamond particles. It is suggested to choose the smaller diameter MWCNTs when the MWCNTs are used as carbon source for the diamond synthesis in SPS. The Ar gas atmosphere enhanced plasma generation and promoted the diamond transition. It indicates that we select the Ar gas atmosphere to

In this chapter, the diamond synthesis using the SPS was investigated and discussed. The diamond synthesis from pure carbon nanotubes has been covered by China Patent ZL 200410044157.0. (Shen, Zhang, 2004). The diamond synthesis from all the carbon modifications with catalysts as well as pure C60, graphene to diamond has been protected by the Deutsches Patent P162-11 (Zhang, Burkel et al. 2011). The SPS is a new technique for the diamond synthesis. It still needs further investigations to promote the SPS method to be

The thermal stability of MWCNTs, C60 and graphite has been investigated under the pulsed DC field in the SPS furnace. Cubic diamond and n-diamond have been converted from pure MWCNTs; cubic diamond has been converted from pure C60 without catalysts being involved by the SPS at conditions of 1500 oC and 80 MPa for 20 min. There was no notice of diamond formation in the case of pure graphite sample processed by SPS at this condition. The graphite is the most stable crystalline modification of carbon among the MWCNTs, C60 and graphite allotropes under the SPS. The parallel investigations by using the synchrotron radiation in-situ high temperature X-ray diffraction show that there is no diamond formation in the MWCNTs and C60 samples at the same pressure (80 MPa) and temperature (1500 oC). Their phase transition mechanisms from MWCNTs and C60 to diamond indicated the high localized temperatures between particles due to the presence of momentary plasmas during the SPS process. The thermal dynamic analysis reveals that the plasmas have increased the entropy of the SPS system resulting in milder conditions for the diamond

Catalysts were involved in the SPS diamond synthesis with carbon modifications of carbon nanotubes and graphite. A Fe35Ni solvent catalyst has been incorporated to synthesize diamond from MWCNTs by using the SPS. Cubic diamond crystals were synthesized from the MWCNTs/Fe35Ni mixtures at lower SPS temperature of 1200 oC under pressure of 70 MPa. Well-crystallized diamond mono-crystals and poly-crystals with particle sizes ranging around 10-40 μm are synthesized. 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 the 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. The Ni, MnNi, MnNiFe and AlCuFe quasicrystal powder were used as the catalysts for the diamond synthesis from graphite by the SPS. Diamond crystals have been

enhance the plasma effect in the SPS.

**6. Conclusions and outlook** 

formation.

used as a large-scale synthetic diamond production technique.

Figure 18 shows the SEM micrographs of the MWCNTs/FeNi samples SPSed at 1200 oC under 10 MPa in vacuum and Ar gas. There are only a few poor quality diamond crystals created in the vacuum atmosphere of the SPS (Figure 18 a, b). In the case of the SPS treatment in Ar gas atmosphere, some flower-like structured carbon are observable (Figure 18c). This indicates that sparking plasmas may have happened and generated such flowerlike structured carbon. It is exciting that some high quality diamond crystals with hexahedron structures are found in the sample (Figure 18 d). A high magnification SEM micrograph of one perfect diamond crystal is shown in Figure 18 (e).

Figure 19 shows the Raman spectra of the MWCNTs/FeNi samples SPSed at 1200 oC under 10 MPa in vacuum and Ar gas. Both of them show the cubic diamond peak at 1332 cm-1 Raman shift. The vacuum atmosphere SPSed sample shows a very weak diamond peak; however, the Ar gas atmosphere SPSed sample exhibits a strong diamond peak. Both of the G bands centered at the same Raman shift. But it is much sharper in the vacuum atmosphere SPSed sample than that in the Ar gas. The Raman results are accordingly to the SEM results that the Ar gas atmosphere in the SPS promotes the diamond formation. The SPS pressure that we used is very small (10 MPa), so that the pressure effect is negligible for the diamond conversion. The diamonds have been generated by the SPS from such low pressure in vacuum and Ar gas atmospheres. It means that the plasma played the key role for the diamond formation. It provided another evidence for the existence of plasma during the SPS that such a low pressure diamond formation. The Ar gas atmosphere enhanced plasma generation and promoted the diamond transition.

Fig. 19. Raman spectra of the MWCNTs/NiFe samples SPSed at 1200 oC in vacuum and Ar gas.

Figure 18 shows the SEM micrographs of the MWCNTs/FeNi samples SPSed at 1200 oC under 10 MPa in vacuum and Ar gas. There are only a few poor quality diamond crystals created in the vacuum atmosphere of the SPS (Figure 18 a, b). In the case of the SPS treatment in Ar gas atmosphere, some flower-like structured carbon are observable (Figure 18c). This indicates that sparking plasmas may have happened and generated such flowerlike structured carbon. It is exciting that some high quality diamond crystals with hexahedron structures are found in the sample (Figure 18 d). A high magnification SEM

Figure 19 shows the Raman spectra of the MWCNTs/FeNi samples SPSed at 1200 oC under 10 MPa in vacuum and Ar gas. Both of them show the cubic diamond peak at 1332 cm-1 Raman shift. The vacuum atmosphere SPSed sample shows a very weak diamond peak; however, the Ar gas atmosphere SPSed sample exhibits a strong diamond peak. Both of the G bands centered at the same Raman shift. But it is much sharper in the vacuum atmosphere SPSed sample than that in the Ar gas. The Raman results are accordingly to the SEM results that the Ar gas atmosphere in the SPS promotes the diamond formation. The SPS pressure that we used is very small (10 MPa), so that the pressure effect is negligible for the diamond conversion. The diamonds have been generated by the SPS from such low pressure in vacuum and Ar gas atmospheres. It means that the plasma played the key role for the diamond formation. It provided another evidence for the existence of plasma during the SPS that such a low pressure diamond formation. The Ar gas atmosphere enhanced plasma

Fig. 19. Raman spectra of the MWCNTs/NiFe samples SPSed at 1200 oC in vacuum and

micrograph of one perfect diamond crystal is shown in Figure 18 (e).

generation and promoted the diamond transition.

Ar gas.

In this part, we investigated the factors affecting the diamond growth in the SPS. It is found that the C60 is able to increase the diamond size in the SPS process. Due to the high price of the C60, it is suggested that the C60 can be used as a doping catalyst material to promote the diamond transition. The diamond particle size depends on the internal diameter of the MWCNTs. Smaller diameter MWCNTs generated larger diamond particles. It is suggested to choose the smaller diameter MWCNTs when the MWCNTs are used as carbon source for the diamond synthesis in SPS. The Ar gas atmosphere enhanced plasma generation and promoted the diamond transition. It indicates that we select the Ar gas atmosphere to enhance the plasma effect in the SPS.

In this chapter, the diamond synthesis using the SPS was investigated and discussed. The diamond synthesis from pure carbon nanotubes has been covered by China Patent ZL 200410044157.0. (Shen, Zhang, 2004). The diamond synthesis from all the carbon modifications with catalysts as well as pure C60, graphene to diamond has been protected by the Deutsches Patent P162-11 (Zhang, Burkel et al. 2011). The SPS is a new technique for the diamond synthesis. It still needs further investigations to promote the SPS method to be used as a large-scale synthetic diamond production technique.
