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.

Our paper – Duda et al., “Influence of crystallographic orientation and anisotropy on Kapitza conductance via classical molecular dynamics simulations” – was recently published in Journal of Applied Physics (J. Appl. Phys. 112, 093515 (2012)). In this work, we studied the influence of crystallographic orientation on thermal boundary conductance between two solids with molecular dynamics simulations and wave packet simulations.  We found that anisotropy in a solid has a greater effect on thermal boundary conductance than on thermal conductivity.  We also conclude that the Debye temperatures of two materials comprising an interface does not serve an accurate gauge of the efficiency of interfacial thermal transport when those materials have different crystal structures.

Abstract

We investigate the influence of crystallographic orientation and anisotropy on local phonon density of states, phonon transmissivity, and Kapitza conductance at interfaces between Lennard-Jones solids via classical molecular dynamics simulations. In agreement with prior works, we find that the Kapitza conductance at an interface between two face-centered cubic materials is independent of crystallographic orientation. On the other hand, at an interface between a face-centered cubic material and a tetragonal material, the Kapitza conductance is strongly dependent on the relative orientation of the tetragonal material, albeit this dependence is subject to the overlap in vibrational spectra of the cubic and tetragonal materials. Furthermore, we show that interactions between acoustic phonons in the cubic material and optical phonons in the tetragonal material can lead to the interface exhibiting greater “thermal anisotropy” as compared to that of the constituent materials. Finally, it is noted that the relative match or mismatch between the Debye temperatures of two materials comprising an interface does not serve an accurate gauge of the efficiency of interfacial thermal transport when those materials have different crystal structures.

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

In collaboration with Prof. Joshua Zide’s group at University of Delaware, our paper “Enhanced room temperature electronic and thermoelectric properties of the dilute bismuthide InGaBiAs” was recently published in Journal of Applied Physics (J. Appl. Phys. 112, 093710 (2012)).  In this work, the ExSiTE lab measured the thermal conductivity of Si doped InGaBiAs films with varying Bi concentration to support the electrical resistivity and Seebeck coefficients measurements conducted at the University of Delaware.  We found that the bismuth scattering suppressed the thermal conductivity of these bismuthides, that, along with enhanced electrical conductivity and carrier concentrations, could make these bismuthides an exceptional thermoelectric material.

Abstract

We report room temperature electronic and thermoelectric properties of Si-doped In0.52Ga0.48 BiyAs1-y with varying Bi concentrations. These films were grown epitaxially on a semi-insulating InP substrate by molecular beam epitaxy. We show that low Bi concentrations are optimal in improving the conductivity, Seebeck coefficient, and thermoelectric power factor, possibly due to the surfactant effects of bismuth. We observed a reduction in thermal conductivity with increasing Bi concentration, which is expected because of alloy scattering. We report a peak ZT of 0.23 at 300 K.

Our paper – Duda et al., “Bidirectionally tuning Kaptiza conductance through the inclusion of substitutional impurities” – was recently published in Journal of Applied Physics (J. Appl. Phys. 112, 073519 (2012)). In this work, we explored methods of tuning the thermal boundary conductance between two solids with molecular dynamics simulations.  We found that the presence of impurities at solid interfaces can lead to an increase in thermal boundary conductance if the masses of the impurities are in between the masses of the atoms on either side of the interface.  This counter-intuitive finding contradicts typical impurity scattering theory in a homogeneous material where impurity scattering will always lead to a decrease in thermal transport, the magnitude of which is governed by the difference in masses.  The increase in Kaptiza conductance due to interfacial impurities is due to a bridging in vibrational states on either side of the interface due to the impurity masses.

Abstract

We investigate the influence of substitutional impurities on Kapitza conductance at coherent interfaces via non-equilibrium molecular dynamics simulations. The reference interface is comprised of two mass-mismatched Lennard-Jones solids with atomic masses of 40 and 120 amu. Substitutional impurity atoms with varying characteristics, e.g., mass or bond, are arranged about the interface in Gaussian distributions. When the masses of impurities fall outside the atomic masses of the reference materials, substitutional impurities impede interfacial thermal transport; on the other hand, when the impurity masses fall within this range, impurities enhance transport. Local phonon density of states calculations indicate that this observed enhancement can be attributed to a spatial grading of vibrational properties near the interface. Finally, for the range of parameters investigated, we find that the mass of the impurity atoms plays a dominant role as compared to the impurity bond characteristics.

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

Our paper, “Minimum thermal conductivity considerations in aerogel thin films,” was recently published in the Journal of Applied Physics (J. Appl. Phys. 111, 113532 (2012)).  In this work, we show that time domain thermoreflectance can be used to measure the thermal conductivity of the solid network of highly porous aerogel films.  Following in depth analysis of the measurement theory of TDTR applied to porous films, we measure the thermal conductivity of thin aerogel films with ~10 and 15% porosity.  We show that at room temperature, the thermal conductivity of aerogel films scales with porosity as predicted by differential effective medium theory.  We develop a modification to the minimum limit to thermal conductivity that accounts for porosity and agrees with our aerogel data, along with other data on fully dense and porous silica structures well.  This was was performed in collaboration with Professors Bryan Kaehr and Jeffrey Brinker at University of New Mexico and Sandia National Laboratories along with Ed Piekos at Sandia National Laboratories.

Abstract

We demonstrate the use time domain thermoreflectance (TDTR) to measure the thermal conductivity of the solid silica network of aerogel thin-films. TDTR presents a unique experimental capability for measuring the thermal conductivity of porous media due to the nanosecond time domain aspect of the measurement. In short, TDTR is capable of explicitly measuring the change in temperature with time of the solid portion of porous media independently from the pores or effective media. This makes TDTR ideal for determining the thermal transport through the solid network of the aerogel film. We measure the thermal conductivity of the solid silica networks of an aerogel film that is 10% solid, and the thermal conductivity of the same type of film that has been calcined to remove the terminating methyl groups. We find that for similar densities, the thermal conductivity through the silica in the aerogel thin films is similar to that of bulk aerogels. We theoretically describe the thermal transport in the aerogel films with a modified minimum limit to thermal conductivity that accounts for porosity through a reduction in phonon velocity. Our porous minimum limit agrees well with a wide range of experimental data in addition to sound agreement with differential effective medium theory. This porous minimum limit therefore demonstrates an approach to predict the thermal conductivity of porous disordered materials with no a priori knowledge of the corresponding bulk phase, unlike differential effective medium theory.

This work was funded by NSF (CBET Award #1134311)

Our paper, “Contributions of electron and phonon transport to the thermal conductivity of gdfeco and tbfeco amorphous rare-earth transition-metal alloys,” was recently published in the Journal of Applied Physics (J. Appl. Phys. 111, 103533 (2012)).  In this work, we quantify the electron and phonon contributions to thermal conductivity of amorphous rare-earth transition-metal alloys, a class of material systems that is widely used in magneto-optical recording media.  We measure the thermal conductivity of thin films of GdFeCo and TbFeCo with time domain thermoreflectance.  Thought electrical resistivity measurements, we separate the contribution of electron and phonon transport to thermal conductivity in these materials.  We show that the phonon contribution in these amorphous metals is substantial, and accounts for the majority of the thermal conduction in these materials from 80 – 400 K. This work was performed in collaboration with Professor Joe Poon in the Physics Department at U.Va.

Abstract

We experimentally investigate the electron and phonon contributions to the thermal conductivity of amorphous GdFeCo and TbFeCo thin films. These amorphous rare-earth transition-metal (RE-TM) alloys exhibit thermal conductivities that increase nearly linearly with temperature from 90 to 375 K. Electrical resistivity measurements show that this trend is due to an increase in the electron thermal conductivity over this temperature range and a relatively constant phonon contribution to thermal conductivity. We find that at low temperatures (∼90 K), the phonon systems in these amorphous RE-TM alloys contribute ∼70% to thermal conduction with a decreasing contribution as temperature is increased.

This work was funded by NSF (CBET Award #1134311)

Our paper, “Strategies for tuning phonon transport in multilayered structures using a mismatch-based particle model,” was recently published in the Journal of Applied Physics (J. Appl. Phys. 111, 084310 (2012)). In this work we developed a thermal mismatch model (TMM) based on thermal impedance of material to predict the phonon transmission across solid interfaces.  Using a particle based interference model, this TMM was extended to multilayer systems showing excellent agreement with more-computationally-expensive wave-packet simulations.  This work was in collaboration with Pam Norris’ group and Thomas Beechem at Sandia National Laboratories.

Abstract

The performance of many micro- and nanoscale devices depends on the ability to control interfacial thermal transport, which is predominantly mediated by phonons in semiconductor systems. The phonon transmissivity at an interface is therefore a quantity of interest. In this work, an empirical model, termed the thermal mismatch model, is developed to predict transmissivity at ideal interfaces between semiconductor materials, producing an excellent agreement with molecular dynamics simulations of wave packets. To investigate propagation through multilayered structures, this thermal mismatch model is then incorporated into a simulation scheme that represents wave packets as particles, showing a good agreement with a similar scheme that used molecular dynamics simulations as input [P. K. Schelling and S. R. Phillpot, J. Appl. Phys. 93, 5377 (2003)]. With these techniques validated for both single interfaces and superlattices, they are further used to identify ways to tune the transmissivity of multilayered structures. It is shown that by introducing intermediate layers of certain atomic masses, the total transmissivity can either be systematically enhanced or reduced compared to that of a single interface. Thus, this model can serve as a computationally inexpensive means of developing strategies to control phonon transmissivity in applications that may benefit from either enhancement (e.g., microelectronics) or reduction (e.g., thermoelectrics) in thermal transport.

The Hopkins Lab was supported by a LDRD initiative through Sandia National Labs for this work.

Our paper, ” Systematically controlling Kaptiza conductance via chemical etching,” was recently published in Applied Physics Letters (Appl. Phys. Lett. 100, 111602 (2012)).  In this work, we investigated the relationship between surface roughness and Kaptiza conductance at an Al/Si interface and showed that with low cost, chemical etching, precise control over this thermal conductance can be realized.  The temperature dependence of the Kaptiza conductance lends insight into the various phonon frequencies that are contributed to interfacial heat flow.   We see similar temperature trends as we previously observed with quantum dot patterning, indicting that the phonon scattering mechanisms at rough interfaces are based on the geometry and degree of roughness, not the means of roughening.

Abstract

We measure the thermal interface conductance between thin aluminum films and silicon substrates via time-domain thermoreflectance from 100 to 300 K. The substrates are chemically etched prior to aluminum deposition, thereby offering a means of controlling interface roughness. We find that conductance can be systematically varied by manipulating roughness. In addition, transmission electron microscopy confirms the presence of a conformal oxide for all roughnesses, which is then taken into account via a thermal resistor network. This etching process provides a robust technique for tuning the efficiency of thermal transport while alleviating the need for laborious materials growth and/or processing.

This work was funded by NSF (CBET Award #1134311)