Micro/Nano Scale Heat Transfer Lab
Aerospace and Mechanical Engineering
865 Asp Avenue, Room 217
University of Oklahoma
Norman, OK 73019
Ph: 405 325 5232
Ph.D Massachusetts Institute of Technology, 2011
Our group focuses on design of advanced composite materials for thermal management and energy conversion applications. Experimental work involves material synthesis and characterization of thermal properties. Simultaneously we use first-principles atomistic methods to elucidate the mechanisms of thermal transport processes in a wide range of materials including bulk semiconductors, nanostructured materials such as superlattices, disordered materials like alloys, and polymer composites.
High thermal conductivity (k) aligned polymer-graphene nanocomposites
The goal of this research is to enhance k of polymers by two orders of magnitude through the ideas of simultaneous alignment of polymer chains and graphene nanoplatelets. Unstrained polymeric systems contain ordered-crystalline regions called lamellae which can have thermal conductivities exceeding those of metals. Random orientation of polymer lamellae, however, leads to an overall low thermal conductivity. Mechanical strain aligns these high thermal conductivity regions along one direction, enhancing polymer k. Similarly, graphene nanoplatelets (GnPs) have very high in-plane k (~1000-2000 W/mK) but low out-of-plane k (~10-20 W/mK). Aligning the planar surface of GnPs along the direction of heat transfer realizes full advantage of their high in-plane k. Through simultaneous alignment of polymer lamellae and dispersed GnPs, we recently achieved 1100% enhancement in composite k using just 6 volume% GnPs. These promising results were recently published in the journal of Nanoscale, DOI: 10.1039/C7NR04686C.[pdf]
Covalent bonding schemes with superior interface thermal conductance
We are investigating thermal transport at the interface between different polymers and graphene. The goal is to tune the interface structure to achieve the highest interface thermal conductance between polymer and graphene. Two different interfaces- at the edge and basal plane of graphene are studied. Atomistic Green’s function method is used for this study. Second order interatomic force interactions needed for the analysis are derived from DFT allowing a first-principles analysis. Initial results indicate edge bonding to be more effective in enhancing interfacial conductance through efficient coupling with in-plane vibrations in graphene. These results were recently published in Applied Physics Letters, 110, 093112 (2017).
Near-field radiative heat transfer
At extremely small gaps of less than a nanometer new mechanisms of heat transfer come in to play involving tunneling of acoustic waves across these small gaps. This holds potential to enhance heat transfer beyond that predicted by continuum theories based on coupling of evanescent waves. We are using atomistic models to understand heat transfer at these length scales. Our results were recently published in Nature Communications 6, 6755 (2015).
First-principles study of thermal transport in materials
We are using state-of-the art first-principles methods to gain fundamental understanding of thermal transport processes in wide array of materials including nanostructured semiconductors, polymers and 2D materials including graphene. Dominant heat carriers in these materials are lattice vibrations called phonons. Accurate description of phonons requires the knowledge of second (harmonic) and third (anharmonic) order interatomic force interactions. First-principles approach involves deriving these force interactions from density-functional theory which has been shown to yield highly accurate values, leading to unprecedented accuracy in the prediction of thermal conductivity of materials. We have used these methods to accurately predict thermal conductivity of alloys, recently presented in Physical Review Letters, to demonstrate coherent transport of phonons in superlattices, presented in the journal of Science, and to show weakening of three-phonon scattering in materials with a large phononic band-gap, paving way for developing high thermal conductivity materials, Nano Letters. We are further using such a first-principles approach to study thermal transport in functionalized graphene and in polymers.
Barrier heterostructures for higher thermoelectric efficiency
The goal of this research is to better understand the role of highly coupled non-equilibrium transport of both phonons and electrons in enhancing cooling efficiency of solid state thermionic coolers based on single-barrier heterostructures through use of first-principles driven coupled electron-phonon Monte Carlo simulations.