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Tadatomo Suga Department of Precision Engineering, The University of Tokyo, Tokyo 113-8656, Japan

Hisao Kanda and Minoru Akaishi Advanced Materials Laboratory, National Institute for Materials Science, Tsukuba 305-0044, Japan

Kazunari Yamaura and Eiji Muromachi Superconducting Materials Center, National Institute for Materials Science, Tsukuba 305-0044, Japan

Hideki Kakisawa Ecomaterials Center, National Institute for Materials Science, Tsukuba 305-0047, Japan (Received 10 September 2004; accepted 15 December 2004)

C60 nanowhiskers fabricated by liquid–liquid interfacial precipitation method and pristine C60 powder were sintered at 800 °C under 5.5 GPa for 2 h. The specimens showed high micro-Vickers hardness about 1100 kg/mm2 and electrical resistivity as low as several ⍀m. Structural characterization by x-ray diffraction, high-resolution transmission electron microscopy, Fourier transform infrared, and Raman spectroscopy revealed that most of C60 molecules were broken to form turbostratic graphite by high-pressure and high-temperature treatment. Comparison with the pristine C60 powder sintered under the same conditions suggests that the structural change depends microscopically on the C60 molecules, but the size and the morphology of the starting material also affect the macroscopic sintering process.

I. INTRODUCTION

After Krätschmer et al. found a simple method to manufacture bulk fullerenes in macroscopic quantities,1 fullerenes became potential material as a precursor for new forms of carbon because their spherical shape with distorted sp2 bonds implied different reactivity from those of graphite or diamond. An effective way to react C60 molecules is to press C60 crystals under high temperatures. By heating at moderate pressures, additional bonds are formed between the molecules, i.e., polymerization of C60 takes place.2,3 Many high-pressure and high-temperature experiments revealed various structures of solid C60 such as orthorhombic, tetragonal, rhombohedral, face-centered cubic (fcc), and bodycentered cubic system with different polymerization states of C60 molecules.2–4 Under extremely highpressure and high-temperature conditions, C60 molecules are not stable any more; they are expected to react into either diamond or various grades of graphitic material depending on the pressure-temperature condition in the

a)

Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2005.0095 742

http://journals.cambridge.org

J. Mater. Res., Vol. 20, No. 3, Mar 2005 Downloaded: 14 Mar 2015

phase diagram of atomic carbon.4–6 Blank et al. pointed out in their review the difference in hardness of the samples between the studies was due mainly to the detailed experimental conditions such as pressure, temperature, and duration, especially at the condition range where C60 molecules start to break down.4 However, two or more phases with different properties are sometimes distinguishable even at the optical microscope level within a high-pressure sintered C60 specimen.5 This suggests t