Low-temperature mechanical properties of fullerites: structure, elasticity, plasticity, strength (Review Article)
The results of long-term investigations of physical-mechanical properties of С60 and С70 molecular crystals (the fullerites) were systematized and described. These materials are the new allotropic form of carbon in which fullerenes (the stable molecules consisting of 60 and 70 atoms) represent the elementary base units. The molecules are combined into the crystal structures substantially by dispersive forces (van der Waals interaction) with a small part of covalent bond. The peculiarities of the crystal structures of the fullerites and the features of the phase transitions, occurring in them, which are caused by dispersive forces, orientation ordering and dynamics of rotational degrees of freedom of the molecules, are discussed. The most interesting transformations of the lattice structures and orientation state of the fullerites are observed in the temperature range of 77 K ≲ Т ≲ 350 K. The majority of the experimental investigations were carried out at these temperatures. The experiments were concentrated on the study of the effect of lattice-orientation phase transitions on the mechanical properties of single crystals, polycrystals and compacts. Acoustical spectroscopy at low and high frequencies of oscillations, micro- and nanoindentation and macrodeformation methods were used in the experimental research. The crystallogeometric aspects and dislocation mechanisms of plastic glide in the fullerites, the methods of the observation of dislocations and the investigation of their mobility are described in detail. Also the influence of different external factors, namely, illumination (photoplastic effect), pulsed magnetic field (magnetoplastic effect), pressure at the sample compaction (baropolymerization effect), saturation of the samples with different morphology by impurities of hydrogen, oxygen and inert gases on the mechanical properties of the fullerites was considered. The discussion of the experimental results is attended with the concise theoretical interpretation on the basis of the analysis of the interaction of elastic and plastic deformations in the fullerite lattice with the processes of the orientation ordering, the rotation and libration oscillations of the molecules.