Formation mechanism of critical nucleus during nucleation process of liquid metal sodium

Abstract

To deeply understand the formation mechanism of a critical nucleus during the nucleation process of liquid metal sodium, a system consisting of 10 000 Na atoms has been simulated by using molecular dynamics method. The evolutions of nuclei are traced directly, adopting the cluster-type index method. It is found that the energies of clusters and their geometrical constraints interplay to form the favorable microstructures during the nucleation process. The nucleus can be formed through many different pathways, and the critical size of the nucleus would be different for each pathway. It is also found that the critical nucleus is nonspherical and may include some metastable structures. Furthermore, the size of the cluster and its internal structure both play a crucial role in determining whether it is a critical nucleus, and this is in agreement with the simulations by computing the free energy of the Lennard-Jones system [D. Moroni, P. R. ten Wolde, and P. G. Bolhuis, Phys. Rev. Lett. 94, 235703 (2005)].

Kinetic details of nucleation in supercooled liquid Na: A simulation tracing study

“…However, the HA bond-type index method is incapable of characterizing atomic cluster structures, especially the larger or nano-clusters. Based on the HA bond-type index, we have proposed a new cluster structure description method of CTIM [15,16] Based on the CTIM, larger clusters combined by different types and numbers of basic clusters can be clearly described. If the central atoms of two basic clusters are neighbors, we define that they belong to the same larger cluster.…”

“…Then the damped force method [21][22][23] (also called the Gaussian thermostat) is adopted to decrease the temperature of system to 173 K at a cooling rate of 1 Â 10 11 K s À1 . As the crystalline temperature T c is about 243 K at this cooling rate which has determined by our previous work [15], the supercooled temperature scale is in the temperature region of 371-243 K. Then the crystallization of supercooled liquid Na is performed by isothermally running 100 000 time steps (i.e. 500 ps) at 253 K. Finally, the time dependences of the dynamic properties during the isothermal relaxation process are detected by using the time-correlation functions of mean-square displacement (MSD) and non-Gaussian parameter (NGP).…”

“…To determine the initial nucleation time and reveal the kinetic details of nucleation in supercooled liquid, the evolutions of atomic clusters in the supercooled liquid Na are traced based on the cluster structure description method of cluster-type index method (CTIM) proposed by us previously [15,16].…”

Formation and evolution of nano-clusters in a large-scale system of Cu–Zr alloy during rapid solidification process

“…of 1541, 1431 and 1321 bond-types, based on our previous works [7,8,22,23,[33][34][35][36], a new cluster-type index method (CTIM-3) has been proposed. In CTIM-3, a basic cluster can be expressed by nine-integer index of (N, n 1 , n 2 , n 3 , n 4 , n 5 , n 6 , n 7 , n 8 ), where N is the number of nearest-neighbour atoms of the central atom, namely, the coordination number (CN), and n 1 , n 2 , n 3 , n 4 , n 5 , n 6 , n 7 , n 8 in turn denote the numbers of 1441, 1551, 1661, 1421, 1422, 1541, 1431 and 1321bond-types, respectively.…”

“…Adopting a new cluster-type index method (CTIM-3) to analyse the local atomic clusters, it is found that the dominant microstructural configurations in the supercooled liquid and amorphous are the icosahedrons and their combinations, especially the intercross-sharing (IS) clusters linked of two icosahedrons (IS-ICO-clusters) can be used to build some larger IS-ICO nanoclusters with different sizes. Highly interesting, in the system the number of atoms in various IS-ICO nano-clusters demonstrates a magic number sequence of 19,23,25,27,29,31,33,35,37,41,43, and so on. The magic number clusters can be classified into three main types: chain-like, triangle-tailed and quadrilateral-tailed.…”