Board of Examiners:

1. Chairman: Prof. Bhaskaran Muralidharan, Electrical Engineering, IIT Bombay
2. External Examiner: Prof. Abhishek Dhar, International centre for theoretical sciences, TIFR, Bangalore
3. Internal Examiner: Prof. Titas Dasgupta, Metallurgical Engg & Materials Science, IIT Bombay
4. Supervisor: Prof. Amit Singh,  Mechanical Engineering, IIT Bombay

Abstract:

Understanding how heat flows in two-dimensional (2D) materials like graphene is crucial for designing future nanoelectronic and thermal management systems. We present a detailed study of thermal transport properties and the thermal conductivity (TC) in pristine, layered, twisted, strained and polycrystalline graphene systems as well as Fermi Pasta Ulam based  model to study the thermal transport behaviour in anharmonic solids using molecular dynamics (MD) simulations and normal mode decomposition (NMD) analysis. As part of our first work, we use nonequilibrium MD and spectral energy density (SED)-based NMD methods to calculate size-dependent TC of single-layer graphene (SLG), AB-bilayer graphene (AB-BLG), and twisted bilayer graphene (tBLG). Our results explain the inconsistency in TC reported in earlier studies by analyzing phonon lifetimes, group velocities, and mode contributions, and report that twisting alters group velocity significantly, which leads to reduced TC. We also point out a widely used incorrect direct method of calculating heat flux in literature through rigorous imposition of the first law of thermodynamics and more accurate Irving-Kirkwood based heat flux calculation.In the second part of our work, we study heat flow in polycrystalline graphene with various grain orientations. Using NMD, we show that TC strongly depends on grain tilt angle and provide how phonon properties like lifetime and velocity distribute across frequencies. Next we explore the impact of strain on SLG and show how phonon dispersion changes resulting in reduced group velocities but nearly unchanged phonon lifetimes. Lastly, we study heat transport in a simplified 2D anharmonic solid using the Fermi-Pasta-Ulam (FPU)-β model. Using both the Green-Kubo (GK) method and NMD, we find that TC increases logarithmically with system size due to the scaling of phonon lifetimes and group velocities, where rms of group velocities asymptotically saturate with increase in system size but the mean value of phonon lifetimes almost scale logarithmically for high values of system size.  Thus our findings try to overcome the reported inconsistencies of the previous work in the area of thermal study of different graphene systems and lay the foundation for better understanding and design of 2D materials with tailored thermal properties.