Molecular simulations for determination of transport properties of nano-composites

Sanket S Mahajan, Purdue University

Abstract

In several recent applications, including those aimed at developing novel thermal interface materials, nano-particulate systems have been proposed to improve the effective behavior of the system. One critical challenge in using nano-particulate systems is the lack of knowledge regarding their thermal conductivity. In this thesis, techniques based on Molecular Dynamics (MD) simulations are developed to determine transport properties of various types of homogeneous and inhomogeneous systems. In particular, the thermal conductivity values of bulk silica, silica nano-wire and nano-particle are determined using MD simulations. The equilibrium MD simulations of nano-particles using Green-Kubo relations are demonstrated to be computationally very expensive and unsuitable for nano-scale systems. The reverse non-equilibrium MD method of imposing heat flux is shown to be efficient and more accurate. The method is first demonstrated on bulk amorphous silica and silica nano-wires. The mean thermal conductivity values for bulk silica and silica nano-wire are estimated to be 1.221 W/mK and 1.430 W/mK, respectively. To model nano-particles, a novel methodology inspired by the imposition of heat flux technique, is developed by dividing the nano-particle into concentric shells so as to capture the naturally radial mode of heat transfer. The mean thermal conductivity value of a 600-atom silica nano-particle obtained using this approach is 0.589 W/mK. This value is ∼50-60% lower than those of bulk silica and silica nano-wire. The above developed technique for estimating the thermal conductivity of nano-structured homogeneous systems is naturally extended to determine the Kapitza resistance between solid-solid interfaces. The systems considered are interfaces between Si-SiO2 and Si-HfO2 thin films. For the Si-SiO2 interface, the average Kapitza resistance for ∼8 Å thick oxide layer system is 0.503 × 10-9 m2K/W and for the ∼11.5 Å thick oxide layer system is 0.518 × 10-9 m 2K/W. For the Si-HfO2 interface, the average value of the Kapitza resistance obtained using four independent sets of simulations involving different atoms (sequence of exchanges between Si-Hf and/or Si-O) in the exchange process is 0.446 × 10-6 m 2K/W. The Kapitza resistance for the Si-HfO 2 interface is observed to be three orders of magnitude higher than the Kapitza resistance for the Si-SiO2 interface.

Degree

Ph.D.

Advisors

Subbarayan, Purdue University.

Subject Area

Mechanical engineering|Molecular physics|Condensed matter physics

COinS