Ph.D. defense of Mr. Harish Veeravenkata on Ab-initio Modelling of Materials for Energy Applications

Abstract: 
The aim of this thesis is to examine thermal transport in semiconductors. In crystalline semiconductors, phonons (atomic vibrations) play a crucial role in thermal transport, whereas in metals, electrons are the primary carriers of both heat and charge.

To predict the properties of phonon and electron transport, we employ first-principles calculations. This involves considering various scattering mechanisms such as phonon-phonon, phonon-boundary, and phonon-isotope interactions, and utilizing the Boltzmann transport equation. The scattering rates associated with phonon-phonon interactions are determined by conducting calculations based on both harmonic and anharmonic lattice dynamics. In order to perform these calculations, we obtain harmonic and cubic force constants, which are necessary inputs, through density functional theory and density functional perturbation theory calculations.
The thermal transport properties of biphenylene network (BPN), a novel sp2-hybridized two-dimensional allotrope of carbon atoms recently realized in experiments, are studied using the density functional theory-driven solution of the Boltzmann transport equation. The thermal transport in BPN is anisotropic and the obtained thermal conductivities are more than an or-
der of magnitude lower than that in graphene, despite similar sp2-hybridized planar-structure of both allotropes. The lower thermal conductivity in BPN is found to originate from enhanced anharmonicity which in turn is a result of reduced crystal symmetry of BPN.

The thermal transport properties of the recently synthesized high-pressure phase of BNC2 are investigated using the iterative solution of the Boltzmann transport equation with inputs from density functional theory calculations. The thermal conductivity of BNC2 is found to be extremely sensitive to pressure, and the thermal conductivity at room temperature under 20 GPa pressure is over 1400 W/m-K which is more than 40% higher than the cor- responding value at 0 GPa. Similar to diamond, the origin of this extremely high thermal conductivity is rooted at large phonon group velocities, which gives rise to fluid-like hydrodynamic thermal transport in BNC2. However, unlike diamond, the large isotope disorder of boron atoms in BNC2 results in a large phonon-isotope scattering, which renders hydrodynamic flow prevalent only in isotopically pure samples at length scales of up to 65 ?m at 100 K.
 

Date: 1 Feb 2024
Time: 4 PM
Venue: ME Auditorium 

External Examiner: Prof. Abhishek Singh, IISc
Internal Examiner: Prof. Amit Singh
Chairperson: Prof. Aftab Alam, Physics 

Supervisor: Prof. Ankit Jain