Surface properties, using molecular simulation

Study Surface Properties Using Molecular Simulation 96... 

Finally, a relatively new area in the computer simulation of confined polymers is the simulation of nonequilibrium phenomena [72,79-87]. An example is the behavior of fluids undergoing shear flow, which is studied by moving the confining surfaces parallel to each other. There have been some controversies regarding the use of thermostats and other technical issues in the simulations. If only the walls are maintained at a constant temperature and the fluid is allowed to heat up under shear [79-82], the results from these simulations can be analyzed using continuum mechanics, and excellent results can be obtained for the transport properties from molecular simulations of confined liquids. This avenue of research is interesting and could prove to be important in the future. 

This approach was successfully used in modeling the CVD of silicon nitride (Si3N4) films [18, 19, 22, 23]. Alternatively, molecular dynamics (MD) simulations can be used instead of or in combination with the MC approach to simulate kinetic steps of film evolution during the growth process (see, for example, a study of Zr02 deposition on the Si(100) surface [24]). Finally, the results of these simulations (overall reaction constants and film characteristics) can be used in the subsequent reactor modeling and the detailed calculations of film structure and properties, including defects and impurities. 

The diffusion of individual Pb atoms on the Cu(llO) surface has been investigated by molecular dynamics simulations (section 3.4). In this case the interaction potential is derived from a phenomenological model similar to that used in the tight binding method [44, 45]. This potential satisfactorily describes bulk and surface properties of noble and transition metals except for the surface energies. In the Pb/Cu(l 10) studies the tight binding functional form is used to describe the Pb-Pb and Pb-Cu interactions. Parameters for both the pure metal and cross interaction potentials are obtained from fits to experimental values. 

Electronic structure calculations of the type described above, provide the energy and related properties of the system at the absolute zero of temperature and do not account for any time-dependent effect. In some cases, temperature and/or time scale effects may be important and must be included. The appropriate theoretical approach is then molecular dynamics (MD) either in the classical or ab initio implementations. In the first approach, Newton s motion equations are solved in the field of a potential provided externally, which constitutes the main limitation of this approach. To overcome this problem, ab initio Molecular Dynamics (AIMD)94,95 solves Newton s motion equations using the ab initio potential energy surface or propagating nuclei and electrons simultaneously as in the Car-Parrinello simulation.96 The use of AIMD simulations will increase considerably in the future. In a way they furnish all the information as in classical MD, but there are no assumptions in the way the system interacts since the potential energy surface is obtained in a rather crude manner. 

Fig. 1 Molecular simulation of a microporous hypercrossUnked polydichloroxylene network (a-c) and simulation of hydrogen sorption within the micropores (d) [31]. This model simulates properties such as pore volume, density, and average pore size quite well. Hydrogen sorption is overestimated by the simulation shown in (d) because a Connolly surface, rather than a solvent accessible surface [30], is used to calculate the uptake...

It is clear from the above discussion that surface properties are extremely important in microscale systems and their importance grows as the characteristic channel dimension decreases. However, there is no straightforward way to take these effects into account, with the models developed to describe this phenomenon being problem specific. Generally they are based on a combination of classical solutions of the Navier-Stokes equations, coupled with ad hoc models of molecular slip flow. Therefore, in the simulation of microchannel flows, it is important to keep in mind that the use of the no-slip boundary condition may not be appropriate and that additional physics may need to be included in the modeling to capture the correct behavior. 

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