Physico-Chemical nanomaterials science

Melting mechanism of a nanocrystal
Yu. F. Zabashta, M.M. Lazarenko, О.М. Alekseev, M. V. Ushcats, K.I. Hnatiuk, L. Yu. Vergun, L.A. Bulavin
1 Taras Shevchenko National University of Kyiv, Physical Faculty, Volodymyrs’ka St. 64/13, Kyiv 01601, Ukraine.
E-mail: 20gnatk@gmail.com
2 Admiral Makarov National University of Shipbuilding, Mykolayiv, Ukraine.
3 State research institute of building constructions, 5/2 Preobrazhenska str., Kyiv-03037, Ukraine.

The melting mechanism of a crystal has been studied for a long time. To describe the melting of crystals, the classical theory of phase transformations was used. This theory is also traditionally used to describe confined space melting.
In our work, the classical theory of phase transformations is shown to be inapplicable to describe the melting of nanocrystals. The mechanism of their melting is different from the mechanism of melting of bulk crystallites. For this purpose, the theory of non-classical vacancies is used. When a nanocrystal is heated to the critical temperature, its lattice becomes defect-free. When this temperature is exceeded, thermal expansion leads to the appearance of vacancies in the lattice. These vacancies are conventionally called non-classical, since their appearance is caused by lattice deformations, in contrast to classical vacancies, the appearance of which is caused by thermal fluctuations. The vacancy concentration linearly increases with increasing thermal expansion deformation. In this case, the bulk elastic modulus of the nanocrystal decreases, and the ability of the lattice to resist stretching caused by thermal expansion decreases. In this work, the energy of a nanocrystal containing defects of the indicated type is calculated. It was found that at a certain temperature TF the lattice of such a nanocrystal loses its mechanical stability. This process is identified as the melting, and the TF temperature is considered as the melting temperature of the nanocrystal. The smaller nanocrystal size, the higher melting temperature. This model is shown to be used to describe the experimental dependences of the melting temperature of nanocrystals on their size.