Electronic Thesis and Dissertation Repository

Thesis Format

Monograph

Degree

Master of Engineering Science

Program

Mechanical and Materials Engineering

Supervisor

Knopf, George K.

Abstract

A COMSOL Multiphysics simulation study on the relationship between ultrashort pulsed laser process parameters and the thermal modification of conductive carbon-based (e.g., graphite, graphene) thin films is presented in this thesis. The research objective is to utilize the theoretical models of heat transfer in thin films with finite element analysis (FEA) techniques to understand the impact of laser process parameters on the heat affected zone (HAZ) profile when an ultrashort pulsed laser beam strikes a thin conductive carbon film deposited on a polymer substrate. The goal is to be able to anneal or ablate the carbon-based films without causing thermal or structural damage to the underlying substrate. The laser process parameters include wavelength, pulse width, pulse energy, and beam diameters. In this study, the two-temperature model (TTM) and multi-temperature model (MTM) are examined in detail for different types of graphene layered films. The simulation experiments demonstrate that TTM is a suitable model for highly oriented pyrolytic graphite (HOPG) and reduced graphene oxide (rGO) films, whereas MTM is more appropriate for modeling single layer graphene. The impact of laser wavelength on surface annealing and ablation on graphite (HOPG) and graphene derivative (rGO) films on different substrates is explored in greater detail. The simulation results are compared with experimental observations reported in the published literature and found to be a realistic reflection of the physical processes. The FEA study provides the groundwork for using ultrashort laser pulses to anneal carbon-based conductive films and electrodes, micromachine printed films to remove excess material, and to reduce the thickness of functional layers. Furthermore, laser thermal processing represents an environmentally benign method of creating a wide variety of single-use disposable electrical and electrochemical sensors for healthcare, food safety inspection, intelligent packaging, environmental monitoring, and public security.

Summary for Lay Audience

Graphene oxide (GO) is an electrically insulating material that can be printed onto a polymer substrate, and using a pulsed laser, the film can be annealed to improve electrical conductivity producing reduced graphene oxide (rGO) or micromachined to reduce the thickness and remove excess material. The laser parameters must be precisely controlled to achieve the desired result and their influence on the temperature distribution and material removal in GO are not well understood. This thesis presents numerical simulations using the software COMSOL Multiphysics to investigate the impact of different laser parameters on the annealing and material removal of GO and rGO respectively. Preliminary simulations are performed on graphite (HOPG) and compared to physical experiments which are found to be in good agreement. Simulations are then performed on rGO for a range of laser parameters to determine the ideal parameters for material removal and annealing. An analysis of different substrates is also conducted to determine the amount of substrate damage experienced during laser micromachining. Overall, the models presented in this thesis can offer time and cost savings to realize processing parameters for carbon-based electronics and help to better understand laser-material interaction required for their fabrication.

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