**6. Conclusions and outlook**

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 formation.

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

Synthesis of Diamond Using Spark Plasma Sintering 57

of diamond/ceramics or diamond/metals composites from CNTs/ceramics or

The authors acknowledge the financial support from the DFG-Deutschen Forschungsgemeinschaft (German Research Foundation) with grant No. BU 547/10-1 and the HASYLAB/DESY project with grant No. II-20090264. We also thank Dr. Christian Lathe, Dr. Martin von Zimmermann, Dr. Jozef Bednarcik at HASYLAB/DESY, Hamburg for their supports in the F2.1 and BW5 experiments, and Dr. Furqan Ahmed at the University of

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Shen, J.; Zhang, F.; Sun J. F.; Wang, G. (2004). Low pressure synthesis of diamonds from carbon nanotubes by a new process. *China Patent.* ZL 200410044157.0 Shen, J.; Zhang, F.; Sun, J. F.; Zhu, Y. Q.; McCartney, D. G. (2006). Spark plasma sintering

Vanmeensel, K.; Laptev, A.; Hennicke, J.; Vleugels, J.; Van der Biest, O. (2005). Modelling of

Vereschagin, A. L.; Sakovich, G. V.; Komarov, V. F.; Petrov, E. A. (1994). Properties of

Watanabe, I.; Matsushita, T.; Sasahara, K. (1992). Low-temperature synthesis of diamond

current and temperature distributions. *Mater. Sci. Eng. A.* 394: 139-148 Giardini, A. A.; Tydings, J. E.; Levin, S. B. (1960). A very high pressure-high temperature

Combustion Flames. *Journal of Applied Physics*. 68(12):6401-6415

`spark plasma sintering'. *J. Appl. Phys.* 104(3): 033305-7

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*Related Materials.* 8(8-9):1427-1432

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microwave plasma chemical-vapor deposition-morphology and growth of

assisted diamond formation from carbon nanotubes at very low pressure.

the temperature distribution during field assisted sintering. *Acta Materialia*. 53:

ultrafine diamond clusters from detonation synthesis. *Diamond and Related* 

films in thermoassisted Rf plasma chemical vapor-deposition. *Japanese Journal of* 

CNTs/metals by the SPS is also a very interesting subject for the future researches.

Erlangen-Nürnberg for his support in the Raman spectroscopy experiments.

**7. Acknowledgements** 

**8. References** 

217-221

converted from the graphite at the SPS condition of 1300 oC and 50 MPa for 20 min. Diamond nano- and micro-rods (80 nm-2 μm) have been obtained with the Ni catalysts from the graphite by the SPS. Diamond crystals with good diamond shapes from 1 to 3 μm have been converted from the graphite with the AlCuFe catalyst.

The factors affecting the diamond growth including carbon modifications and atmospheres have been studied in order to increase the diamond crystal size and transition rate. Wellcrystallized diamonds with particle sizes up to 250 μm are obtained at 1300 oC by using the fullerene C60 as the carbon source. The mechanism analysis indicates that the high sp3 hybrid fraction in the C60 leads to its transformation to diamond at lower pressures and temperature during the SPS application. It is suggested that the C60 can be used as doping catalyst materials to promote the diamond transition. A model for the diamond nucleation at the internal surface of carbon onion has been established. Based on the model, the energy need for the nucleation of diamond at the internal surface of carbon onion has been formulized. It is postulated that the decrease of diameter of CNTs could decrease the energy for the diamond nucleation. The results show that 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. The transition rate has been increased in the 10-20 nm MWCNTs. The experiments validated the theoretical assumption. 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 the minimum pressure of 10 MPa. The diamonds have been generated by the SPS from such a low pressure in vacuum and Ar gas atmospheres. High quality diamond crystals with hexahedron structures are created in the Ar gas atmosphere of the SPS. It provided another evidence for the existence of plasma during the SPS because of such a low pressure diamond formation. The Ar gas atmosphere enhanced the plasma generation and promoted the diamond transition.

What makes the SPS so interesting for the diamond synthesis is the fact that only relatively low pressures of 10-80 MPa are needed. The SPS possesses a wide range of sintering temperatures from a few hundreds up to 2000 ◦C, controllable heating rates which can be set to several hundred degrees per minute for extremely rapid processing, as well as the capacity to process large samples up to 100 mm in diameter and 20 mm thick. As we know, the HPHT technique only can prepare very small samples in order to achieve the several GPa level high pressures. Therefore, the SPS is a highly efficient and energy saving technique for diamond synthesis. The investigation in this chapter indicate that the SPS, a pulsed electric field processing technique, has great potential to be used as an alternative and novel method for high-efficiency diamond generation. The diamond generation at such low pressures also provided some indirect evidences for the presence of plasmas during the SPS operation. However, it still needs further investigations to promote the SPS method to be used as a large-scale synthetic diamond production technique. The future highlights will be the development of diamond purification methods to get high purity diamond crystals from the SPSed carbon compacts and the synthesis of millimetre sized diamond crystals and achieving higher diamond transitional rate by using the SPS technique. The mechanical properties (hardness, Elastic modulus) and functional properties including electrical, thermal, optical, magnetic properties etc. of the SPSed diamond particles and the SPSed carbon samples with in-situ formed diamonds need to be investigated. The in-situ synthesis of diamond/ceramics or diamond/metals composites from CNTs/ceramics or CNTs/metals by the SPS is also a very interesting subject for the future researches.

#### **7. Acknowledgements**

56 Sintering of Ceramics – New Emerging Techniques

converted from the graphite at the SPS condition of 1300 oC and 50 MPa for 20 min. Diamond nano- and micro-rods (80 nm-2 μm) have been obtained with the Ni catalysts from the graphite by the SPS. Diamond crystals with good diamond shapes from 1 to 3 μm have

The factors affecting the diamond growth including carbon modifications and atmospheres have been studied in order to increase the diamond crystal size and transition rate. Wellcrystallized diamonds with particle sizes up to 250 μm are obtained at 1300 oC by using the fullerene C60 as the carbon source. The mechanism analysis indicates that the high sp3 hybrid fraction in the C60 leads to its transformation to diamond at lower pressures and temperature during the SPS application. It is suggested that the C60 can be used as doping catalyst materials to promote the diamond transition. A model for the diamond nucleation at the internal surface of carbon onion has been established. Based on the model, the energy need for the nucleation of diamond at the internal surface of carbon onion has been formulized. It is postulated that the decrease of diameter of CNTs could decrease the energy for the diamond nucleation. The results show that 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. The transition rate has been increased in the 10-20 nm MWCNTs. The experiments validated the theoretical assumption. 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 the minimum pressure of 10 MPa. The diamonds have been generated by the SPS from such a low pressure in vacuum and Ar gas atmospheres. High quality diamond crystals with hexahedron structures are created in the Ar gas atmosphere of the SPS. It provided another evidence for the existence of plasma during the SPS because of such a low pressure diamond formation. The Ar gas atmosphere enhanced

What makes the SPS so interesting for the diamond synthesis is the fact that only relatively low pressures of 10-80 MPa are needed. The SPS possesses a wide range of sintering temperatures from a few hundreds up to 2000 ◦C, controllable heating rates which can be set to several hundred degrees per minute for extremely rapid processing, as well as the capacity to process large samples up to 100 mm in diameter and 20 mm thick. As we know, the HPHT technique only can prepare very small samples in order to achieve the several GPa level high pressures. Therefore, the SPS is a highly efficient and energy saving technique for diamond synthesis. The investigation in this chapter indicate that the SPS, a pulsed electric field processing technique, has great potential to be used as an alternative and novel method for high-efficiency diamond generation. The diamond generation at such low pressures also provided some indirect evidences for the presence of plasmas during the SPS operation. However, it still needs further investigations to promote the SPS method to be used as a large-scale synthetic diamond production technique. The future highlights will be the development of diamond purification methods to get high purity diamond crystals from the SPSed carbon compacts and the synthesis of millimetre sized diamond crystals and achieving higher diamond transitional rate by using the SPS technique. The mechanical properties (hardness, Elastic modulus) and functional properties including electrical, thermal, optical, magnetic properties etc. of the SPSed diamond particles and the SPSed carbon samples with in-situ formed diamonds need to be investigated. The in-situ synthesis

been converted from the graphite with the AlCuFe catalyst.

the plasma generation and promoted the diamond transition.

The authors acknowledge the financial support from the DFG-Deutschen Forschungsgemeinschaft (German Research Foundation) with grant No. BU 547/10-1 and the HASYLAB/DESY project with grant No. II-20090264. We also thank Dr. Christian Lathe, Dr. Martin von Zimmermann, Dr. Jozef Bednarcik at HASYLAB/DESY, Hamburg for their supports in the F2.1 and BW5 experiments, and Dr. Furqan Ahmed at the University of Erlangen-Nürnberg for his support in the Raman spectroscopy experiments.

#### **8. References**


**3** 

Rosidah Alias

*Malaysia* 

**The Effects of Sintering Temperature** 

The successful development and commercialization of high performance ceramic materials has attracted much attention especially for multilayer substrates using the Low Temperature Co-fired Ceramic (LTCC) technology. This technology has become a popular technology for automobiles and wireless communications due to the advantages of the excellent combination of electrical, thermal, mechanical and chemical stability for a wide range of applications, thus allowing preparation of 3-dimensional circuits incorporating passive components within a multilayer construction (Matters-Kammerer et al., 2006; Zhou et al., 2008). This approach also allows the presence a number of interfaces and thus reduction of the overall substrate size and cost can be realized (Lo and Duh, 2002; Chen et al., 2004 and Zhu et al., 2007). The circuits are capable of withstanding sintering during processing temperatures up to 1000 °C. For telecommunication applications the usage of ceramic is implemented in Telecom control station and power supply circuits for the capability to dissipate excess heat and maintain dimensional control stability of the ceramic package. This is important where back-up power is required to maintain operation during primary power outages when cooling is restricted (Barlow and Elshabini, 2007). Another important parameter for wireless communication devices is the requirement to have low dielectric loss (tan δ ∼ 10-3 or less) for higher processing speed, higher dielectric constant (ε'>10) for miniaturization of the devices and higher integration density (≥ 3 g/cm3) (Kume et al., 2007; Long et al., 2009). For this reason, it is important to prepare high quality LTCC substrate/package whose properties are strongly dependent on microstructure, phase purity and sintering temperature (Xiang et al., 2002). Therefore the microstructure must be carefully controlled to get dense and fine grained ceramics in order to improve their

The starting point of the LTCC technology is the development of LTCC tape materials containing a glass-ceramic system that usually needs to show good compatibility with the paste system which acts as a conductive track for RF signal transportation from one location on the circuit to another. It should also have low energy loss in microwave applications to make sure high circuit performance can be achieved (Wang et al., 2009). One of the most important processes in LTCC manufacturing of multilayer LTCC substrates involves co-

properties and reliability in many applications (Hsu et al., 2003).

**1. Introduction** 

**Variations on Microstructure** 

**Changes of LTCC Substrate** 

*TM Research & Development Sdn. Bhd.* 

