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 research involves material synthesis and characterization of thermal properties. Simultaneously we use first-principles atomistic simulations 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 thermal conductivity of polymer-graphene nanocomposites by aligning the high k planar surface of graphene nanoplatelets with the direction of heat transfer. 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 have been published in the journal of Nanoscale, DOI: 10.1039/C7NR04686C. [pdf]
Edge-bonding scheme with superior interface thermal conductance
As a second effect we are exploring the potential of edge-bonding in enabling superior thermal interaction between polymer and graphene. Enhancing interface thermal conductance between polymer and graphene is key to achieving high composite k. Interface thermal conductance between polymer and graphene is higher at a junction on the edge relative to on the basal plane of graphene. We demonstrated this effect using state-of-the-art nonequilibrium Green’s function method. Interatomic force interactions needed for the analysis are derived ab initio allowing a complete first-principles prediction of interface thermal conductance. Initial results indicate edge bonding to enable almost two-fold higher interfacial thermal conductance through efficient coupling of polymer phonons with the in-plane vibrations of graphene. These results were recently published in Applied Physics Letters, 110, 093112 (2017)[pdf].
Near-field radiative heat transfer
When the separation of two surfaces approaches sub-nanometre scale, the boundary between the two most fundamental heat transfer modes, heat conduction by phonons and radiation by photons, is blurred. We have developed an atomistic framework based on microscopic Maxwell’s equations and lattice dynamics to describe the convergence of these heat transfer modes and the transition from one to the other. The formalism uses non-equilibrium Green’s function method to compute energy transmission across the gap. Short range force interactions are modeled using empirical nearest neighbor potential. Long range Coulomb forces are computed using the electric fields generated by oscillating ions. For gaps>1 nm, heat transfer occurs through resonant coupling of electric fields with high-frequency optical phonons, that is phonon-polaritons. At these gaps, the predicted conductance is in excellent agreement with Rytov’s continuum theory of fluctuating electrodynamics. However, for sub-nanometre gaps we find the conductance to be enhanced up to four times compared with continuum theory. This enhancement is found to be due to tunneling of low frequency acoustic phonons through the vacuum gap assisted by coupling with evanescent electric fields, providing additional channels for energy transfer. Our results were recently published in Nature Communications 6, 6755 (2015) [pdf] and hold importance for accurate prediction of heat transfer across small gaps in technologies such as heat assisted magnetic recording and nanoporous materials.
First-principles study of thermal transport in semiconductor materials
We are using state-of-the art first-principles methods to gain fundamental understanding of thermal transport processes in wide array of semiconductor materials including bulk, nanostructured and disordered semiconductors. Dominant heat carriers in these materials are lattice vibrations called phonons. We use highly accurate harmonic and anharmonic interatomic force interactions derived from density-functional theory along with an exact solution of phonon Boltzmann transport equation to achieve unprecedented accuracy in the prediction of phononic thermal conductivity in these materials. This is seen by the excellent agreement between measured and predicted k values for natural GaN [pdf], GaAs [pdf] and PbTe [pdf]. We have also used these methods to accurately predict thermal conductivity of alloys, recently presented in Physical Review Letters [pdf], and to demonstrate coherent transport of phonons in superlattices, presented in the journal of Science [pdf].
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 energy conversion efficiency of solid state thermionic devices based on superlattice single-barrier heterostructures. We are using first-principles driven coupled electron-phonon Monte Carlo simulations for predicting the performance as a function of device parameters. Effective Seebeck coefficient is predicted as a function of barrier width. Superlattices also offer the advantage of ultra-low thermal conductivity, further enhancing efficiency. Finally role of sub-band formation in superlattices in suppressing joule heating is being studied.