Fullerene C60 monomer crystals were compressed to a face-centered cubic (fcc) phase with a lattice parameter of a = 11.93(5) A and a micro-Vickers hardness of 4500 kg/mm2 using high-pressure and high-temperature conditions of 15 GPa at 500-600 degrees C. The hardness is compatible with that of cubic boron nitride (c-BN), suggesting the formation of a 3D C60 polymer. The single-crystal X-ray structural analysis revealed that each C60 molecule in the polymer was linked to the 12 nearest neighbors by [2+2] cycloaddition between the common pentagon-hexagon (56) edges. However, ab initio geometry optimization and molecular dynamics calculations suggested that the 3D polymer should have a rhombohedral structure with the space group of R containing [3+3] cycloaddition between the pentagons of C60 molecules within the plane perpendicular to the 3-fold axis. The higher apparent symmetry of fcc was observed as an averaged structure of different orientations of the rhombohedral structure. The R structure can be derived by only a slight rotation of each C60 unit in the (111) plane of the fcc structure. The band-structure calculation suggested that the 3D polymer (R) was a semiconductor; the activation energy for the electrical conductivity was experimentally determined to be 0.25 eV at 550 K.

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Obtaining the distribution of the voltage and temperature of the punch-die-compact systems in the spark sintering process is useful for generalization of its process control. The temperatures of these systems using two kinds of powders, titanium and alumina, of a conductor and an insulator, respectively, are measured by thermocouples in the constant current discharging process. The steady state showing the constant values of temperature and relative density is achieved in this process, and the measured temperature distribution in the systems is changed depending on the kind of compacts. The voltage and temperature of the systems at the steady state are also calculated on the basis of Ohm's and Fourier's laws. The calculated voltage distribution or current flow in the punch and die for the titanium compact system is similar to that for the alumina compact system, although there is the minor difference of the voltage distribution between titanium and alumina compacts because of the difference in the specific resistivity of compacts. The calculated temperature distribution is changed in these systems depending on the kind of compacts, because of the difference in the amount of Joule's heat of compacts, which agrees with the temperature distribution measured by thermocouples. It is found from the result obtained in this study that distributions in voltage and temperature can be estimated by this calculation method, regardless of a kind and shape of the punches, dies and compacts.