Computer Simulation of Laser-Induced Ultrarapid Phase Changes
- PDF / 353,447 Bytes
- 6 Pages / 420.48 x 639 pts Page_size
- 68 Downloads / 244 Views
COMPUTER SIMULATION OF LASER-INDUCED ULTRARAPID PHASE CHANGES MYRON T. MACLIN, MICHAEL J. SABOCHICK AND JAMES P. MILLS Air Force Institute of Technology, Department of Engineering Physics Wright-Patterson Air Force Base, OH 45433
ABSTRACT The results of a molecular dynamics study of phase change in a system of condensed Lennard-Jones atoms is presented. These molecular dynamics results are compared to the rates of phase change predicted by two analytical phase change models. Both analytical models are supported by the molecular dynamics results.
INTRODUCTION Ultrarapid phase change has been the subject of a great deal of study in recent years. Of particular interest have been phase changes induced by ultrashort laser pulses. Such pulses can melt a very thin surface layer of a material that subsequently refreezes at extremely fast rates as heat diffuses into the solid phase. This phenomenon has been investigated using analytic models [1-8], atomistic simulation[8-11] and experiments [3,12-15]. In general, the analytic models are often difficult to evaluate by comparison with experiment, since the phenomenon is characterized by small length scales on the order of tens of nanometers and short time scales on the order of tens of picoseconds. In this work, we have studied melting and freezing using the atomistic simulation method of molecular dynamics (MD), which allows direct control over experimental parameters, and provides detailed microscopic knowledge of the state of the system. The melting and freezing phase changes are first order processes in which the two coexisting phases are separated by an interface or phase front. Therefore, the rate of phase change can be discussed in terms of an interface velocity for which various analytic models exist. These models usually relate the interface velocity to the change in free energy or chemical potential associated with the phase change. One frequently cited model of this type was used by Broughton [8] to study the solidification of a Lennard-Jones (LJ) fluid. Broughton found that the solidification interface velocities observed in his MD simulations fitted an equation of the form v(Ti) = fd [3kTi/M]1/ 2 {1 - exp(-Apl/kTi)}
(1)
where Ti is the interface temperature, k is Boltzmann's constant, M is the atomic mass, and Ap is the difference in chemical potential between the liquid and solid phases. The factors in this equation can be physically interpreted as follows. The quantify f is the fraction of interactions with the interface that result in an atom occupying a lattice site; d is a dimensionless quantity which accounts for the distance the interface moves for each such successful interaction; and [3kT1/M]1 / 2 is the thermal velocity of atoms in the system. The factor in braces is the probability that an atom, once occupying a lattice site at the interface, will not reenter the liquid. As described here, this equation applies only to solidification. A difficulty can occur with (1) if it is applied to melting. In this case, Att is negative and velocities greater t
Data Loading...
