Nanoshell Collapse

To investigate segregation effects, we have used the nanoshells of an ideal alloy with n = 4a and re = 8a at low temperature (e ) with different ratios of frequencies K (10, 50, 100) and concentrations Cg (0.25, 0.50, 0.75,1.00). We may conclude that the larger the K and/or Cb, the more intensive is the segregation of the more mobile component near the internal surface (Figure 7.26) and the slower is the process of the nanoshell collapse (Figure 7.27). 

Here y is the surface tension of a phase, rm and Vgo are the internal and external radii ofa nanoshell right after the reaction, and rj is the ultimate radius of the nanosphere after the nanoshell s collapse. The mechanism of such a collapse is stipulated by the vacancy flux from the internal surface to the outer one and corresponding atomic... 

In fact, it is apparent that the collapse rate is controlled by a slower component (A in our case), which increases the nanoshells lifetime. 

We propose some models (both phenomenological and computer ones), which allow one to analyze the influence of a nanoshell s size and structure on the process of its shrinkage into a solid nanoparticle and to estimate the stability of nanoshells. Shrinkage kinetics and collapse time are generally determined by a slowly diffusing component, which makes nanoshells more stable than expected. 

Figure 7.5 Dependence of dimensionless collapse time tcollapse = Dgtcollapse/y6 on the ratio of diffusivities Dg/Dl for Models 2 and 3 at different initial nanoshell sizes rio/p (for example, 11 (M3) calculated in Model 3 for r,o/p = 11).

There are 76662 atoms of B initially to form a nanoshell with fee lattice. Initially, the internal radius of a shell is n = 7o (o is the lattice parameter) and the external radius is re = 17a, point defects being absent. Thereafter, the faceting of both internal and outer surfaces of the shell becomes observable, while the bulk becomes saturated with vacancies that migrate from the internal pore outside. As a result of the applied RTA-algorithm [13], the whole shell collapses after 3650 Monte Carlo steps (MCS). 

Hollow Nanoshell Formation and Collapse in One Run Model for a Solid Solution... 

In this section, we suggest phenomenological models for both steps of the process - formation of a hollow nanoshell from a core-shell stmcture with full solubihty during interdilfusion and shrinkage of this just-formed nanoshell with a transformation into a compact particle. The description of the shrinkage looks simpler. Therefore, we start with the model of collapse, and then modify this model to describe the formation stage. 

Figure 7.22 Time evolution of an initially tiny void first, a fast formation of a hollow nanoshell and then its collapse (slow and then faster and faster). The inserts (a), (b), and (c) show the concentration profiles of the faster component at the formation, crossover, and collapse stages respectively.

Alternatively to the offered phenomenological models, we developed a 3D MC model of nanoshell formation and collapse for the fee structure (lattice parameter... 

In Section 7.4.5.1, we simulate the formation of a nanoshell for the case of a nonideal solution. In Section 7.4.5.2, we study the crossover from formation to collapse, as well for nonideal solutions but for another average concentration, to obtain formation and collapse in one run at a reasonable computation time. In Section 7.4.5.3, we investigate mainly the segregation caused (kinetic origin) by the inverse Kirkendall effect for this reason, we treat an ideal solution in this subsection. 

Figure 7.24 Nanoshell formation and collapse In one run atomistic simulation and phenomenology. MC (solid line) and phenomenological (dashed line) plots are superimposed with respective time rescaling, 1 MCS = 62 ms. MC-modeling at a high temperature e ( j), K = 10, and a...

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