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

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

Synthesis of Diamond Using Spark Plasma Sintering 35

(a) (b) Fig. 6. SEM micrographs of the raw graphite (a) and the spark plasma sintered graphite at

under 80 MPa in a non-oxygen atmosphere during the AC sintering. Figure 7 (b) shows the synchrotron radiation-in situ X-ray diffraction patterns of the pure C60 at 80 MPa under different temperatures. There is no diamond formation in the C60 sample. It shows that the C60 is stable below the temperature of 900 oC. However, the C60 is unstable above that temperature point. The C60 (110) peak disappeared above temperature of 900 oC and C60 (112) peak disappeared above temperature of 990 oC. It is found that the graphite is very stable in the in-situ high temperature X-ray experiments at or below

Synchrotron radiation-high energy X-ray diffraction was used to identify the diamond phase in the carbon samples after SPS. In order to confirm the diamond formation, Raman spectroscopy was also used to identify the formation of sp3 bonded diamonds. By using the high energy X-ray diffraction and Raman spectroscopy, the cubic diamond phases were identified and confirmed in the SPSed MWCNTs and C60 samples. The n-diamond was also found in the SPSed MWCNTs sample. The n-diamond is a new kind of carbon allotrope, which is a metallic form of carbon with face-centred cubic structure. It is a metastable and intermediate phase, can decompose slowly at room temperature, and has been synthesized accidentally by various processes (Zhang, Mihoc et al., 2011). It is noted that the n-diamond can also be synthesized by the SPS process. The standard d spacings of the cubic diamond (111), (220) and (311) planes are centered at 2.059 Å, 1.261 Å and 1.075 Å (ICDD No. 65-537). The cubic diamond in the SPSed MWCNTs centered at 2.05, 1.23 and 1.06 Å, and in the SPSed C60 appeared at 2.06 and 1.23 Å spacing. The diffraction peaks of the synthesized diamond from MWCNTs and C60 are very close to the standard diamond diffraction data, but there is a little shift. The diamond peak shifts are due to the existence of residual stress

1500 oC, 80 MPa for 20 min (b).

1500 oC under 80 MPa.

**3.3 Mechanisms** 

Fig. 5. Synchrotron radiation-high energy X-ray diffraction patterns (a) and Raman spectra (b) of the raw graphite and the spark plasma sintered graphite at 1500 oC, 80 MPa for 20 min.

λ

(a)

(b)

Fig. 5. Synchrotron radiation-high energy X-ray diffraction patterns (a) and Raman spectra (b) of the raw graphite and the spark plasma sintered graphite at 1500 oC, 80 MPa for 20 min.

Fig. 6. SEM micrographs of the raw graphite (a) and the spark plasma sintered graphite at 1500 oC, 80 MPa for 20 min (b).

under 80 MPa in a non-oxygen atmosphere during the AC sintering. Figure 7 (b) shows the synchrotron radiation-in situ X-ray diffraction patterns of the pure C60 at 80 MPa under different temperatures. There is no diamond formation in the C60 sample. It shows that the C60 is stable below the temperature of 900 oC. However, the C60 is unstable above that temperature point. The C60 (110) peak disappeared above temperature of 900 oC and C60 (112) peak disappeared above temperature of 990 oC. It is found that the graphite is very stable in the in-situ high temperature X-ray experiments at or below 1500 oC under 80 MPa.

#### **3.3 Mechanisms**

Synchrotron radiation-high energy X-ray diffraction was used to identify the diamond phase in the carbon samples after SPS. In order to confirm the diamond formation, Raman spectroscopy was also used to identify the formation of sp3 bonded diamonds. By using the high energy X-ray diffraction and Raman spectroscopy, the cubic diamond phases were identified and confirmed in the SPSed MWCNTs and C60 samples. The n-diamond was also found in the SPSed MWCNTs sample. The n-diamond is a new kind of carbon allotrope, which is a metallic form of carbon with face-centred cubic structure. It is a metastable and intermediate phase, can decompose slowly at room temperature, and has been synthesized accidentally by various processes (Zhang, Mihoc et al., 2011). It is noted that the n-diamond can also be synthesized by the SPS process. The standard d spacings of the cubic diamond (111), (220) and (311) planes are centered at 2.059 Å, 1.261 Å and 1.075 Å (ICDD No. 65-537). The cubic diamond in the SPSed MWCNTs centered at 2.05, 1.23 and 1.06 Å, and in the SPSed C60 appeared at 2.06 and 1.23 Å spacing. The diffraction peaks of the synthesized diamond from MWCNTs and C60 are very close to the standard diamond diffraction data, but there is a little shift. The diamond peak shifts are due to the existence of residual stress

Synthesis of Diamond Using Spark Plasma Sintering 37

gradients during the operation of the SPS. Therefore, the diamond peaks in the SPSed MWCNTs and C60 have shifted a little. Combining the results of the Raman spectroscopy, the formation of diamond phases in these MWCNTs and C60 samples is confirmed. It is found that there are no C60 peaks in the X-ray diffraction and Raman results of the SPSed C60 sample, but there are strong unreacted MWCNTs peaks in the SPSed MWCNTs sample, and there are no diamond phases in the SPSed graphite sample. There exists a high activation barrier from the graphite, MWCNTs and C60 to diamond, the exact height of which is unknown. The results this study indicate that the activation barrier between the C60 and diamond is lower than that of the MWCNTs with diamond, and the barrier between MWCNTs and diamond is lower than that of the graphite with diamond. The graphite is the most stable crystalline modification of carbon among the MWCNTs, C60 and

The SPS is a remarkable technique to synthesize and consolidate a large variety of materials. The process typically uses moderate uniaxial pressures usually below 100 MPa in combination with a pulsing on-off DC current during its operation. There are many mechanisms proposed to account for the enhanced sintering abilities of the SPS process; for example, field assisted diffusion, spark impact pressure, plasma cleaning of particle surfaces, Joule's heating, local melting and evaporation especially in metallic systems, surface activation on particles and electron wind force (Zhang, Mihoc et al., 2011). The one that draws the most controversy of these mechanisms involves the presence of momentary plasma. In this study, the diamond converted from the MWCNTs and C60 without any catalysts being involved in the SPS. However, 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 in the AC sintering at the same pressure (80 MPa) and temperature (1500 oC). What is the phase transition mechanism from MWCNTs and C60 to diamond in the SPS? Such a clear, significant difference in the products is due to the special sintering principle of SPS. It is a field activated sintering technique based on an DC electric spark discharge phenomenon, i.e. a high energy and low voltage spark pulse DC momentarily generates sparking plasma between particles, which causes localized high temperatures. It is an electric AC heating in the in-situ high temperature experiments. Without plasma effect, it would need 8000-10000 oC at pressure of 80 MPa to get diamond from the MWCNTs and C60, as we calculated. Therefore, super-high pressures (5-10 GPa) are required for the diamond formation in the hydrostatic HPHT technique. Since the SPS only needs MPa level pressure, it is believed that the plasma plays the key role for the diamond transformation from the MWCNTs and C60. The high current, low voltage, momentary pulsed plasma discharge have generated highly localized Joule's heating up to a few thousand degrees Celsius between particles in few minutes. The current density in the SPS is typically on the order of 102 A/cm2 and is highly concentrated at the inter-granular contact or interface (Yang et al., 2010). The momentary pulsed plasma provided energy equivalent to thousand degrees to help the nano-carbon across their activation barriers to the diamond phase. It leads to the transformation of mainly sp2 bonded MWCNTs and C60 to sp3 bonded diamonds. Despite the on-going argument about whether the spark plasmas actually occur during the SPS process, our present study, regarding generating diamond under such a low pressure, suggests that such spark plasmas indeed take place during SPS of these nano-carbon materials with excellent electrical conductivities and high surface areas. The plasmas generated very high localized temperatures up to about 8000-10000 oC and dramatically reduced the pressures required

graphite allotropes under the SPS processing.

Fig. 7. Synchrotron radiation-in situ X-ray diffraction patterns of the pure MWCNTs (a) and C60 (b) at 80 MPa under different temperatures.

in the synthesized diamonds from MWCNTs and C60 by using the SPS. The residual stress of the diamond is because of the stress that remains after the original cause of the stresses (uniaxial forces, heat gradient) has been removed after the SPS processing. In this study, an uniaxial force of 80 MPa was applied and there generally existed some temperature

(a)

(b)

Fig. 7. Synchrotron radiation-in situ X-ray diffraction patterns of the pure MWCNTs (a) and

in the synthesized diamonds from MWCNTs and C60 by using the SPS. The residual stress of the diamond is because of the stress that remains after the original cause of the stresses (uniaxial forces, heat gradient) has been removed after the SPS processing. In this study, an uniaxial force of 80 MPa was applied and there generally existed some temperature

C60 (b) at 80 MPa under different temperatures.

gradients during the operation of the SPS. Therefore, the diamond peaks in the SPSed MWCNTs and C60 have shifted a little. Combining the results of the Raman spectroscopy, the formation of diamond phases in these MWCNTs and C60 samples is confirmed. It is found that there are no C60 peaks in the X-ray diffraction and Raman results of the SPSed C60 sample, but there are strong unreacted MWCNTs peaks in the SPSed MWCNTs sample, and there are no diamond phases in the SPSed graphite sample. There exists a high activation barrier from the graphite, MWCNTs and C60 to diamond, the exact height of which is unknown. The results this study indicate that the activation barrier between the C60 and diamond is lower than that of the MWCNTs with diamond, and the barrier between MWCNTs and diamond is lower than that of the graphite with diamond. The graphite is the most stable crystalline modification of carbon among the MWCNTs, C60 and graphite allotropes under the SPS processing.

The SPS is a remarkable technique to synthesize and consolidate a large variety of materials. The process typically uses moderate uniaxial pressures usually below 100 MPa in combination with a pulsing on-off DC current during its operation. There are many mechanisms proposed to account for the enhanced sintering abilities of the SPS process; for example, field assisted diffusion, spark impact pressure, plasma cleaning of particle surfaces, Joule's heating, local melting and evaporation especially in metallic systems, surface activation on particles and electron wind force (Zhang, Mihoc et al., 2011). The one that draws the most controversy of these mechanisms involves the presence of momentary plasma. In this study, the diamond converted from the MWCNTs and C60 without any catalysts being involved in the SPS. However, 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 in the AC sintering at the same pressure (80 MPa) and temperature (1500 oC). What is the phase transition mechanism from MWCNTs and C60 to diamond in the SPS? Such a clear, significant difference in the products is due to the special sintering principle of SPS. It is a field activated sintering technique based on an DC electric spark discharge phenomenon, i.e. a high energy and low voltage spark pulse DC momentarily generates sparking plasma between particles, which causes localized high temperatures. It is an electric AC heating in the in-situ high temperature experiments. Without plasma effect, it would need 8000-10000 oC at pressure of 80 MPa to get diamond from the MWCNTs and C60, as we calculated. Therefore, super-high pressures (5-10 GPa) are required for the diamond formation in the hydrostatic HPHT technique. Since the SPS only needs MPa level pressure, it is believed that the plasma plays the key role for the diamond transformation from the MWCNTs and C60. The high current, low voltage, momentary pulsed plasma discharge have generated highly localized Joule's heating up to a few thousand degrees Celsius between particles in few minutes. The current density in the SPS is typically on the order of 102 A/cm2 and is highly concentrated at the inter-granular contact or interface (Yang et al., 2010). The momentary pulsed plasma provided energy equivalent to thousand degrees to help the nano-carbon across their activation barriers to the diamond phase. It leads to the transformation of mainly sp2 bonded MWCNTs and C60 to sp3 bonded diamonds. Despite the on-going argument about whether the spark plasmas actually occur during the SPS process, our present study, regarding generating diamond under such a low pressure, suggests that such spark plasmas indeed take place during SPS of these nano-carbon materials with excellent electrical conductivities and high surface areas. The plasmas generated very high localized temperatures up to about 8000-10000 oC and dramatically reduced the pressures required

Synthesis of Diamond Using Spark Plasma Sintering 39

momentary plasmas during the SPS process. The plasmas have increased the entropy of the

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.

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

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

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

SPS system resulted in milder conditions for the diamond formation.

in the SPS furnace at various temperatures under 70 MPa for 20 min.

raw CNTs has shifted to 43.37 degree in the 1100 oC sintered sample.

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

**4.1 Carbon nanotubes with FeNi catalyst** 

MWCNTs/Fe35Ni at temperatures of 1200-1500 oC.

for diamond formation from the GPa to the MPa level. Eventually, this research provided some new indirect evidences for the presence of plasmas during the SPS operation. Therefore, we take plasma into consideration in the thermodynamic analysis (Zhang, 2005; Zhang, Mihoc et al., 2010). The total energy for the diamond formation:

$$Q = \Delta H\_T + Q\_P + \Delta H\_{M\text{-}V}$$

where *Q* is Total Energy, △*HT* is the Energy due to temperature difference, *Qp* is the Energy due to pressure difference, △*H M* is the Energy due to plasma effect. The enthalpy of plasma:

$$H = H\_E + H\_K + H\_D + H\_{I'} \ '$$

where *H* is the plasma contribution, *HE* is the kinetic contribution, *HK* is the excitation contribution, *HD* is the dissolution contribution, *HI* is the electrolytic contribution. Then,

$$dS = \frac{\delta \mathbb{Q}}{T} \,\prime$$

$$
\Delta Q(T) - \Delta Q(T\_0) = \Delta S \left(T\_0 - T\right) \cdot \frac{1}{2}
$$

Where *T* is the temperature, *T0* is the starting temperature, △*Q* is the difference of mol free energy, △*S is* the difference of mol entropy. Only when △*Q(T)*<0,MWCNTs and C60 can be transformed into diamond. So, we can get an equation:

$$T > T\_0 + \frac{\Delta Q(T\_{\text{0}})}{\Delta S}$$

The effect of the plasmas in the SPS has increased the entropy △*S* of the whole SPS system resulting in a lower sintering temperature *T* for the diamond formation. Diamond were converted from MWCNTs and C60 at 1500 oC under very low pressure of 80 MPa. The SPS is a marvelous process to prepare a wide range of advanced materials. The technique significantly uses uniaxial pressures normally 30-100 MPa to integrate a on-off DC current while its running. In this study, the diamond conversion in the SPSed MWCNTs and C60 samples without any catalysts being involved has validated the high localized temperatures between particles. This is due to the presence of momentary plasmas during SPS of these electrically conductive and high surface area nano-carbon materials. The plasmas have increased the entropy of the whole SPS system resulting in milder conditions for the diamond formation.

In summary, the thermal stability of MWCNTs, C60 and graphite has been investigated under the pulsed DC field in a SPS furnace. Cubic diamond and n-diamond have been converted from pure MWCNTs; cubic diamond has been converted from pure C60, both without catalysts being involved by the SPS at conditions of 1500 oC, 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 (AC field) 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 transitional mechanism from MWCNTs and C60 to diamond indicated the high localized temperatures between particles due to the presence of

for diamond formation from the GPa to the MPa level. Eventually, this research provided some new indirect evidences for the presence of plasmas during the SPS operation. Therefore, we take plasma into consideration in the thermodynamic analysis (Zhang, 2005;

*Q HQ H* =Δ + +Δ *TP M* ,

*HH H H H* =+++ *EKDI* , where *H* is the plasma contribution, *HE* is the kinetic contribution, *HK* is the excitation contribution, *HD* is the dissolution contribution, *HI* is the electrolytic contribution. Then,

> *<sup>Q</sup> dS T* δ

Δ −Δ =Δ − *QT QT S T T* () ( ) 0 0 ( ) ,

Δ > +

resulting in a lower sintering temperature *T* for the diamond formation. Diamond were converted from MWCNTs and C60 at 1500 oC under very low pressure of 80 MPa. The SPS is a marvelous process to prepare a wide range of advanced materials. The technique significantly uses uniaxial pressures normally 30-100 MPa to integrate a on-off DC current while its running. In this study, the diamond conversion in the SPSed MWCNTs and C60 samples without any catalysts being involved has validated the high localized temperatures between particles. This is due to the presence of momentary plasmas during SPS of these electrically conductive and high surface area nano-carbon materials. The plasmas have increased the entropy of the whole SPS system resulting in milder conditions for the

In summary, the thermal stability of MWCNTs, C60 and graphite has been investigated under the pulsed DC field in a SPS furnace. Cubic diamond and n-diamond have been converted from pure MWCNTs; cubic diamond has been converted from pure C60, both without catalysts being involved by the SPS at conditions of 1500 oC, 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 (AC field) 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 transitional mechanism from MWCNTs and C60 to diamond indicated the high localized temperatures between particles due to the presence of

0 *Q T*( ) *T T*

= ,

*HT* is the Energy due to temperature difference, *Qp* is the Energy

*H M* is the Energy due to plasma effect. The enthalpy of plasma:

△

△

△

0

*S*

Δ

*Q* is the difference of mol free

*S* of the whole SPS system

*Q(T)*<0,MWCNTs and C60 can

Zhang, Mihoc et al., 2010). The total energy for the diamond formation:

△

△

Where *T* is the temperature, *T0* is the starting temperature,

be transformed into diamond. So, we can get an equation:

*S is* the difference of mol entropy. Only when

The effect of the plasmas in the SPS has increased the entropy

where *Q* is Total Energy,

energy,

△

diamond formation.

due to pressure difference,

momentary plasmas during the SPS process. The plasmas have increased the entropy of the SPS system resulted in milder conditions for the diamond formation.
