2002 年 43 巻 6 号 p. 1306-1311
The mechanism of nucleation of crystals is discussed from a view point of atom clusters with the following results: (1) The nucleus size of crystals is larger than those expected up to date, which is at least larger than Φ 3 nm in metals and alloys. (2) Atomistic and electronic structures of atom clusters are very sensitive to their size, so that both the local volume free energy difference and the local surface energy of them widely change until the crystal nucleation is finished. (3) When the size approaches the nucleus size, a hybrid orbital corresponding to the final crystal structure is formed in the center region of each atom cluster. This stage corresponds to formation of the embryo, after which an additional energy is induced by the atomic misfit between the hybridized region and the matrix. (4) The additional energy is very sensitive to the thermal energy, and increases with increasing size of the hybridized region within each atom cluster. As a result, a part of the additional energy is generally relieved during nucleation of crystals. (5) Nucleation of crystals is finished when each atom cluster is fully hybridized, and the nucleus size is determined by a condition under which the total volume free energy difference is always larger than a sum of the total surface energy and the additional energy. The nucleation process is discussed on both isolated and embedded atom clusters.
