Patrick’s recent review paper on thermal transport across solid interfaces with nanoscale imperfections was recently published in ISRN Mechanical Engineering: Hopkins, P.E., “Thermal transport across solid interfaces with nanoscale imperfections: Effects of roughness, disorder, dislocations and bonding on thermal boundary conductance,” ISRN Mechanical Engineering, 2013, 682586 (2013).

Abstract

The efficiency in modern technologies and green energy solutions has boiled down to a thermal engineering problem on the nanoscale. Due to the magnitudes of the thermal mean free paths approaching or overpassing typical length scales in nanomaterials (i.e., materials with length scales less than one micrometer), the thermal transport across interfaces can dictate the overall thermal resistance in nanosystems. However, the fundamental mechanisms driving these electron and phonon interactions at nanoscale interfaces are difficult to predict and control since the thermal boundary conductance across interfaces is intimately related to the characteristics of the interface (structure, bonding, geometry, etc.) in addition to the fundamental atomistic properties of the materials comprising the interface itself. In this paper, I review the recent experimental progress in understanding the interplay between interfacial properties on the atomic scale and thermal transport across solid interfaces. I focus this discussion specifically on the role of interfacial nanoscale “imperfections,” such as surface roughness, compositional disorder, atomic dislocations, or interfacial bonding. Each type of interfacial imperfection leads to different scattering mechanisms that can be used to control the thermal boundary conductance. This offers a unique avenue for controlling scattering and thermal transport in nanotechnology.

This work was funded by the National Science Foundation (CBET Grant no. 1134311) and Sandia National Laboratories through the LDRD program office.

Patrick has been selected to receive an award from the Young Investigator Program of the Air Force Office of Scientific Research.  The objective of this program is to foster creative basic research in science and engineering, enhance early career development of outstanding young investigators, and increase opportunities for the young investigators to recognize the Air Force mission and the related challenges in science and engineering.  Patrick’s proposed research is focused on the fundamental interactions between femtosecond laser pulses and solids, exploring the regimes of ballistic transport, electron distribution, and electron, phonon, and interfacial scattering mechanisms.

The title of the awarded project is “Electron Dynamics During High-Power, Short-Pulsed Laser Interactions with Solids and Interfaces.”

The official announcement can be found here:  http://www.eurekalert.org/pub_releases/2013-01/afoo-aag010913.php#

Our paper, Duda et al. “Exceptionally low thermal conductivities of films of the fullerene derivative PCBM” – was recently published in Physical Review Letters (Phys. Rev. Lett. 110, 015902 (2013)).  In this paper, we report on the thermal conductivities the fullerene derivative PCBM ([6,6]-phenyl C61-butyric acid methyl ester).  The fullerenes that arrange in a FCC structure in PCBM weakly interact in the mirocrystal causing this material to transport heat via independent, random Einstein oscillations.  The functional tail reduce the thermal conductivity even further, leading PCBM to exhibit the lowest thermal conductivity of any fully dense solid.

Abstract

We report on the thermal conductivities of microcrystalline [6,6]-phenyl C₆₁-butyric acid methyl ester (PCBM) thin films from 135 to 387 K as measured by time domain thermoreflectance. Thermal conductivities are independent of temperature above 180 K and less than 0.030±0.003  W m^-1 K^-1 at room temperature. The longitudinal sound speed is determined via picosecond acoustics and is found to be 30% lower than that in C₆₀/C₇₀ fullerite compacts. Using Einstein’s model of thermal conductivity, we find the Einstein characteristic frequency of microcrystalline PCBM is 2.88×10¹²  rad s^-1. By comparing our data to previous reports on C₆₀/C₇₀ fullerite compacts, we argue that the molecular tails on the fullerene moieties in our PCBM films are responsible for lowering both the apparent sound speeds and characteristic vibrational frequencies below those of fullerene films, thus yielding the exceptionally low observed thermal conductivities.

This work was funded by NSF (CBET Award #1134311) and Sandia National Laboratories through the LDRD Program Office.

Our paper, Cheaito et al. “Experimental investigation of size effects on the thermal conductivity of silicon-germanium alloy thin films” – was recently published in Physical Review Letters (Phys. Rev. Lett. 109, 195901 (2012)).  In this paper, we used TDTR to measure the thermal conductivity of a range of Si_1-xGe_x films of varying composition and thicknesses.  We find that the heat transport in alloy films are substantially limited by the size effects.  We also find that the thermal conductivities of Si_1-xGe_xsuperlattices are ultimately limited by finite size effects and sample size rather than periodicity or alloying.  Therefore, if a comparison is to be made between the thermal conductivities of superlattices and alloys, the total sample thicknesses of each must be considered.

Abstract

We experimentally investigate the role of size effects and boundary scattering on the thermal conductivity of silicon-germanium alloys. The thermal conductivities of a series of epitaxially grown Si_1-xGe_x thin films with varying thicknesses and compositions were measured with time-domain thermoreflectance. The resulting conductivities are found to be 3 to 5 times less than bulk values and vary strongly with film thickness. By examining these measured thermal conductivities in the context of a previously established model, it is shown that long wavelength phonons, known to be the dominant heat carriers in alloy films, are strongly scattered by the film boundaries, thereby inducing the observed reductions in heat transport. These results are then generalized to silicon-germanium systems of various thicknesses and compositions; we find that the thermal conductivities of Si_1-xGe_xsuperlattices are ultimately limited by finite size effects and sample size rather than periodicity or alloying. This demonstrates the strong influence of sample size in alloyed nanosystems. Therefore, if a comparison is to be made between the thermal conductivities of superlattices and alloys, the total sample thicknesses of each must be considered.

This work was funded by NSF (CBET Award #1134311) and Sandia National Laboratories through the LDRD Program Office.

Our paper – Foley et al., “Thermal conductivity of nano-grained SrTiO3 thin films” – was recently published in Applied Physics Letters (Appl. Phys. Lett. 101, 231908 (2012)). In this work, we measured the thermal conductivity of a series of SrTiO3 thin films with varying nanoscale grain sizes.  We found that the thermal conductivities of these films are well described by a model that accounts for the spectral, dispersive nature of phonon transport and the interplay between anharmonic Umklapp scattering and grain-boundary scattering.

Abstract

We measure the thermal conductivities of nano-grained strontium titanate (ng-SrTiO₃) films deposited on sapphire substrates via time-domain thermoreflectance. The 170 nm thick oxide films of varying grain-size were prepared from a chemical solution deposition process. We find that the thermal conductivity of ng-SrTiO₃ decreases with decreasing average grain size and attribute this to increased phonon scattering at grain boundaries. Our data are well described by a model that accounts for the spectral nature of anharmonic Umklapp scattering along with grain boundary scattering and scattering due to the film thickness.

This work was funded by NSF (CBET Award #1134311) and Sandia National Laboratories through the LDRD Program Office.