Nanoscale phonon thermal conductivity via molecular dynamics

Jonathan M Dunn, Purdue University

Abstract

Molecular dynamics (MD) simulations provide a useful and simple means of calculating the nanoscale thermal properties of materials, which requires special analysis since the thermal properties of materials change when their dimensions reach the nanoscale. In this research, MD is used to investigate the nanoscale phonon thermal transport of materials that are attracting much interest in the areas of materials science and nuclear physics. In order to evaluate two distinct methods of calculating the thermal conductivity of materials using MD, the simulation methods are first applied to Si. Once an understanding of each simulation method is established, they are then used to analyze the thermal conductivity of MoTe2 and MoTe2Cu, which are lesser-researched two-dimensional materials with promising applications in nanotechnology. Lastly, the simulations methods are applied to calculate the thermal conductivity of nuclear matter, which is formed within the extreme conditions of neutron stars. The high temperatures of neutron stars cause protons and neutrons to break apart from their usual nucleic form and instead bond together to form much larger structures. Research into these materials will advance the development of nanotechnology as well as contribute to the ongoing research to better understanding the thermal processes that occur within neutron stars. The two methods of thermal conductivity simulations used in this research are the thermostat method and the Müller-Plathe method. The thermostat method is analyzed using two thermostats: the Nosé-Hoover thermostat and the Langevin thermostat. When evaluating these simulation methods using Si, it is found that the Müller-Plathe method and the Nosé-Hoover thermostat method give very similar results that align with published values, whereas the Langevin results differ considerably, giving larger values than the other methods. The reason of this difference is that the Langevin thermostat affects the phonons of the system, which changes the phonon transport properties of the system and causes an increase in the thermal conductivity. This result demonstrates that the means of energy input into a system can affect its thermal transport properties, which provides an additional means of controlling nanoscale thermal transport properties for nanotechnology applications. Because no values for the thermal conductivity of MoTe 2 have yet been published, this research provides an initial description of it thermal transport properties. Using a recently developed reactive force field, the thermal conductivity of MoTe2 is calculated to range from 1–3 W/mK for channel lengths of 10–150 nm, which fits well with the thermal properties of other materials in the same class. Intercalating Cu into the MoTe2 modifies slightly its thermal properties and can be used as a means of engineering a precise thermal conductivity value for nanoscale devices. The investigation into nuclear matter is to contribute to research endeavors into the thermal properties of neutron stars. Because of the high temperature of neutron stars, temperature is measured in MeV rather than K. Instead of analyzing size-effects on thermal conductivity, the goal is to measure the changes in thermal conductivity as the nuclear matter undergoes phase-transitions at various temperatures. The results indicate that the thermal conductivity of nuclear matter decreases from around 5 W/mMeV to 2.5 W/mMeV as the temperature is increased from 0.48–0.86 MeV, with phase transition occurring at around 0.6 MeV and 0.8 MeV.

Degree

M.S.M.S.E.

Advisors

Strachan, Purdue University.

Subject Area

Nanotechnology|Nuclear physics|Materials science

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