Aldo F. Combariza, Ethan Sullivan, Scott M. Auerbach
and Cristian Blanco, "Simulating the Relaxation Dynamics
of Microwave Driven Zeolites," J. Phys. Chem. B
109, 18439-18444 (2005).
We have performed equilibrium and non-equilibrium molecular dynamics
simulations to study how microwave (MW) heated zeolite systems relax
to thermal equilibrium. We have simulated the relaxation of both ionic
and dipolar phases in FAU-type zeolites, finding bi-exponential relaxation
in all cases studied. Fast-decay times are uniformly below 1 ps,
while slow-decay times were found to be as long as 14 ps.
Fast-decay times increase with an increase in the
initial temperature difference between MW-heated ions/dipoles and
the equilibrium system. Slow-decay times were found to be relatively
insensitive to the details of the MW-heated non-equilibrium state.
Velocity, force and orientational correlation functions, calculated
at equilibrium to explore the natural dynamics of energy transfer,
decay well before 1 ps and show little evidence of bi-exponential decay.
In contrast, kinetic energy correlation functions show strong bi-exponential
behavior with slow-decay times as long as 14 ps.
We suggest a two-step mechanism involving initial,
efficient energy transfer mediated by strongly anharmonic zeolite-guest forces,
followed by a slower process mediated
by weakly anharmonic couplings among normal modes of the zeolite framework.
In addition to elucidating relaxation
from MW-heated states, we expect that these studies will shed light on energy
transfer in other contexts, such as adsorption and reaction in zeolites,
which often involve significant heat release.
Prof SM Auerbach
24 October 2005