Brian Donovan’s paper, “Thermal boundary conductance across metal-gallium nitride (GaN) interfaces from 80 – 450 K,” has been published in Applied Physics Letters (Appl. Phys. Lett. 105, 203502 (2014)).    In this work, we show that the thermal boundary conductance across metal/GaN interfaces can impose a thermal resistance similar to that of GaN/substrate interfaces. We also show that these thermal resistances decrease with increasing operating temperature and can be greatly affected by inclusion of a thin adhesion layers.  Congrats Brian!!!

Abstract

Brian Foley’s paper, “Glass-like thermal conductivity of (010)-textured lanthanum-doped strontium niobate synthesized with wet chemical deposition,” has been published in the Journal of the American Ceramic Society (DOI: 10.1111/jace.13318).    In this work, we have demonstrated that solution-based chemistry can produce highly textured, single crystalline oxide-films of strontium niobate with ultra-low thermal conductivities.  the low thermal conductivities originate due to the variable and layered bonding environments in this perovskite structure.  Congrats Brian!!!

Abstract

Brian’s paper, “Spectral phonon scattering effects on the thermal conductivity of nano-grained barium titanate,” has been published in Applied Physics Letters (Appl. Phys. Lett. 105, 082907 (2014)).  In this work, we show that grain sizes as large as 50 nm can still lead to reductions in the thermal conductivity of BaTiO3 thin films.  For films with grain sizes of 36 nm (on average), the thermal conductivity of the BaTiO3 thin film can reach as low as 1.0 W/m/K.  We attribute this reduction in thermal conductivity to the spectral nature of phonon mean free paths in BaTiO3, with some phonons having mean free paths larger than the grain sizes (which, in our samples, range from 36-63 nm).  This is in contrast to the typically assumed view of phonon transport in ABO3 perovskite ferroelectrics in which gray phonon mean free paths are assumed with length scales less than 5 nm.  Congrats Brian!!!

Abstract

Ash’s paper, “Spectral analysis of thermal boundary conductance across solid/classical liquid interfaces: a molecular dynamics study,” has been published in Applied Physics Letters (Appl. Phys. Lett. 105, 033106 (2014)).  In this work, we use molecular dynamics simulations to show that the adhesion between a solid and a liquid (macroscopically referred to as the degree of wetting) has a pronounced spectral effect on phonon transmission, drastically affecting the transmission of low frequency modes across solid liquid interfaces.  This quantifies, from an atomistic perspective, how wetting affects thermal boundary conductance across solid/liquid interfaces.  Congrats Ash!!!

Abstract

Caroline’s paper, “Ion irradiation of the native oxide/silicon surface increases the thermal boundary conductance across aluminum/silicon interfaces,” has been published in Physical Review B (Phys. Rev. B 90, 024301 (2014)).  In this work, we demonstrate that mixing and damage resulting from ion irradiation can INCREASE the thermal boundary conductance across Al/native oxide/Si interfaces.  This counter intuitive result shows that “less perfect” interfaces can be better conductors (i.e., have less resistance) than compositionally abrupt, more “perfect” interfaces.  We wish Caroline all the best in her PhD pursuits with Prof. Alan McGaughey at CMU.  Congrats Caroline!!

Abstract

Patrick has been selected by the Blavatnik Family Foundation as a 2014 National Award Finalist for the Blavatnik Award for Young Scientists in the area of Physical Sciences and Engineering.  This is the inaugural year for the National Award, which will honor three scientists under the age of 42 from leading research universities and academic medical centers in chemistry, life sciences, and physical sciences and engineering with the $250,000 prize.  The national faculty competition of the Blavatnik Awards for Young Scientists recognizes the country’s most promising young faculty-rank scientists and engineers in the disciplinary categories of Life Sciences, Physical Sciences & Engineering, and Chemistry.  More information can be found here: http://blavatnikawards.org/news/items/blavatnik-national-awards-announce-30-finalists/

Ash’s paper, “Influence of hot electron scattering and electron-phonon interactions on thermal boundary conductance at metal/non-metal interfaces,” has been accepted for publication into the Journal of Heat Transfer.  In this work, we present a coupled thermodynamic and quantum mechanical derivation of electron-phonon scattering at free electron metal/non-metal substrate interfaces.  A simplified approach to the Fermi’s Golden Rule with electron energy transitions between only three energy levels is adopted to derive an electron-phonon diffuse mismatch model, that account for the electron-phonon thermal boundary conductance at metal/insulator interfaces increases with electron temperature.  Our approach demonstrates that the metal-electron/non-metal phonon conductance at interfaces can be an order of magnitude larger than purely phonon driven processes when the electrons are driven out of equilibrium with the phonons, consistent with recent experimental observations.

This work was supported by the Air Force Office of Scientific Research Young Investigator Program.

Brian was recently awarded a Graduate STEM Research Fellowship for 2014-2015 by the Virginia Space Grant Consortium (VSGC).  The title of his research proposal was the “Experimental investigation of thermal hot-spot generation in high-frequency transistors due to non-equilibrium electron-phonon energy transfer”, in which he will study the interactions between electrons moving in a semiconductor and the atoms that comprise the semiconductor while under operation.  This transfer of energy between the moving electrons and the semiconductor lattice results in an increased crystal temperature within the semiconductor, which can ultimately lead to the catastrophic failure of the device.  Brian’s goal is to better understand the fundamental mechanisms of these energy transfer processes on the nanoscale to help device engineers take a more “thermal-first” approach in future designs.  Congrats Brian!

We have reported the first measurements of the thermal conductivity of films of water-insoluble solid protein films.  These measurements allow us to evaluate the heat transfer mechanisms in proteins, and evaluate models and hypotheses based on how thermal vibrations transport energy in fractal geometries.  Our work was recently published in Journal of Physical Chemistry Letters (“Protein thermal conductivity measured in the solid state reveals anharmonic interactions of vibrations in a fractal structure,” Journal of Physical Chemistry Letters DOI: 10.1021/jz500174x).  Congratulations to Brian Foley who is the first author on this work!!! This work summarizes collaborations with Brian Kaehr at Sandia National Laboratories and University of New Mexico along with Prof. Costel Constantin at James Madison University.  We all appreciate the generous support from the Office of Naval Research (YIP), U.S. Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences (BES), Division of Materials Sciences and Engineering, the Commonwealth Research Commercialization Fund (CRCF) of Virginia and the 4-VA mini-grant for university collaboration in the Commonwealth of Virginia.

Abstract

Brian Foley and Brian Donovan were awarded first and second place for student presentations at the Electronic Materials and Applications Conference this year.  Brian Foley presented work on the glass-like thermal conductivity of fully-crystalline Strontium Niobate thin films.  Through his measurements made using time-domain thermoreflectance experiments, he was able to show that the interfaces between adjacent slabs of octahedra in the naturally-layered structure are the dominant phonon scattering mechanism.  Brian Donovan presented work on grain size effects on thermal conductivity of Barium Titanate.  Using time-domain thermoreflectance he showed that thermal conductivity was limited by grain size and film thickness of the material system.  Congrats to the Brians!

Brian Foley receiving 1st place award at EMA 2014 banquet.

Brian Donovan receiving 2nd place award at EMA 2014 banquet.

Brian Donovan is quite proud after just finishing his first oral presentation as a U.Va. grad student.