Pulsed electric field induced diamond synthesis from carbon nanotubes with solvent catalysts Faming Zhang a, , Martin Adam a , Furqan Ahmed b,c , Eileen Otterstein a , Eberhard Burkel a a Institute of Physics, University of Rostock, August Bebel Str. 55, Rostock 18055, Germany b Department of Materials Science and Engineering, University of Erlangen-Nürnberg, Martens Str. 5, Erlangen 91058, Germany c Department of Metallurgical and Materials Engineering, University of Engineering and Technology, Lahore 54890, Pakistan abstract article info Article history: Received 18 July 2010 Received in revised form 14 January 2011 Accepted 14 April 2011 Available online 19 April 2011 Keywords: Diamond crystal Carbon nanotubes Spark plasma sintering Synthetic diamond Catalytic processes Spark plasma sintering (SPS) was used to synthesize diamond from multiwalled carbon nanotubes (MWCNTs) with Fe35Ni powders as solvent catalysts. The MWCNTs/Fe35Ni mixtures were spark plasma sintered at various conditions. The microstructures and diamond phase were analyzed by using X-ray diffraction, Raman spectroscopy, scanning electron microscopy, and transmission electron microscope techniques. Experimental results showed that the diamond crystals can be synthesized from MWCNTs/Fe35Ni by using the SPS at lower temperature of 1200 °C under very low pressure of 70 MPa. Well-crystallized cubic diamonds consisted of mono-crystals and poly-crystals exhibiting particle sizes ranged from 10 to 40 μm. The Fe35Ni catalysts achieved an effective enhancement for diamond conversion from MWCNTs during the SPS. A model was also proposed to describe the diamond growth and revealed as a layer-by-layer growth mechanism. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Diamond particles and diamond films have now been successfully obtained by many methods including high-pressure and high- temperature (HPHT) [1], detonation [2], combustion flames [3] and chemical vapour deposition (CVD) with RF plasma [4] or microwave plasma [5] etc., where the HPHT method is still the most popular commercial method for the diamond synthesis. The HPHT synthesis of diamond from graphite, fullerenes, and carbon nanotubes (CNTs) has been studied [6–8]; generally, pressures above 5.0 GPa and high temperature above 1300 °C are needed. Additionally, incorporation of solvent catalysts such as Ni, Co, Fe, other transition metals and their alloys is a crucial point for aid of the diamond synthesis in the HPHT method. Spark plasma sintering (SPS), also defined as field assisted sintering technique (FAST) or pulsed electric current sintering (PECS), is an electric field assisted sintering process utilizing ON– OFF DC pulse energizing [9]. During the SPS treatment, pulsed DC current directly passes through the graphite die, as well as the powder compact, in case of conductive samples. When studying the thermal stability of multi-walled carbon nanotubes (MWCNTs) under the SPS, it was found that under SPS conditions of 1500 °C at pressure of 80 MPa CNTs are unstable and transform to diamonds without any catalysts being involved [10–12]. It is proposed that the spark plasmas play a key role to provide most of the energy required in this diamond transition. These studies indicate that the SPS has a potential to be used as an alternative method for diamond generation. But it needs further investigation to promote the SPS method to be used as a large- scale synthetic diamond production technique instead of the present hydrostatic HPHT method. 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 [13,14]. 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. It is indicated that the solvent catalysts may have the same effects to promote diamond growth from MWCNTs in the SPS method. 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 [15]. In order to increase the diamond transitional rate, the Fe35Ni alloy powders were chosen as catalysts for diamond synthesis from MWCNTs by the SPS method in this study. The MWCNTs/Fe35Ni mixtures were spark plasma sintered at various conditions. The microstructures and phases in the obtained carbon samples were analyzed by using X-ray diffraction (XRD), Raman spectroscopy (Raman), scanning electron microscopy (SEM), and transmission electron microscope (TEM) techniques. The growth model of the diamond crystals from the MWCNTs in the SPS process was proposed. Diamond & Related Materials 20 (2011) 853–858 Corresponding author. Tel.: + 49 381 4986864; fax: + 49 381 4986862. E-mail address: faming.zhang@uni-rostock.de (F. Zhang). 0925-9635/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2011.04.006 Contents lists available at ScienceDirect Diamond & Related Materials journal homepage: www.elsevier.com/locate/diamond