Kimberlee C. Collins

Thermal conductance and phonon transmissivity of metal–graphite interfaces

The thermal boundary conductances between c-axis oriented highly ordered pyrolytic graphite and several metals have been measured in the temperature range 87–300 K and are found to be similar to those of metal–diamond interfaces. The values obtained are indicative of the thermal interface conductance between metals and the sidewalls of multiwall carbon nanotubes (CNTs) and, therefore, have relevance for the accurate characterization of the thermal properties of CNTs, graphene, and the design and performance of composite materials and electronic devices based on these structures. A modified diffuse mismatch model is used to interpret the data and extract the phonon transmissivity at the interface. The results indicate that metal–graphite adhesion forces and interfacial mixing effects play important roles in determining the boundary conductance.

Effects of surface chemistry on thermal conductance at aluminum-diamond interfaces

Synthetic diamond has potential as a heat spreading material in small-scale devices. Here, we report thermal conductance values at interfaces between aluminum and diamond with various surface terminations over a range of temperatures from 88 to 300 K. We find that conductance at oxygenated diamond interfaces is roughly four times higher than at hydrogen-treated diamond interfaces. Furthermore, we find that Al grain structure formation is not strongly dependent on diamond surface chemistry, which suggests that interfacial bonding influences thermal conductance. The results reported here will be useful for device design and for advancing models of interfacial heat flow.

Measuring Phonon Mean Free Path Distributions by Probing Quasiballistic Phonon Transport in Grating Nanostructures

Heat conduction in semiconductors and dielectrics depends upon their phonon mean free paths that describe the average travelling distance between two consecutive phonon scattering events. Nondiffusive phonon transport is being exploited to extract phonon mean free path distributions. Here, we describe an implementation of a nanoscale thermal conductivity spectroscopy technique that allows for the study of mean free path distributions in optically absorbing materials with relatively simple fabrication and a straightforward analysis scheme. We pattern 1D metallic grating of various line widths but fixed gap size on sample surfaces. The metal lines serve as both heaters and thermometers in time-domain thermoreflectance measurements and simultaneously act as wire-grid polarizers that protect the underlying substrate from direct optical excitation and heating. We demonstrate the viability of this technique by studying length-dependent thermal conductivities of silicon at various temperatures. The thermal conductivities measured with different metal line widths are analyzed using suppression functions calculated from the Boltzmann transport equation to extract the phonon mean free path distributions with no calibration required. This table-top ultrafast thermal transport spectroscopy technique enables the study of mean free path spectra in a wide range of technologically important materials.

Non-diffusive relaxation of a transient thermal grating analyzed with the Boltzmann transport equation

The relaxation of an one-dimensional transient thermal grating (TTG) in a medium with phonon-mediated thermal transport is analyzed within the framework of the Boltzmann transport equation (BTE), with the goal of extracting phonon mean free path (MFP) information from TTG measurements of non-diffusive phonon transport. Both gray-medium (constant MFP) and spectrally dependent MFP models are considered. In the gray-medium approximation, an analytical solution is derived. For large TTG periods compared to the MFP, the model yields an exponential decay of grating amplitude with time in agreement with Fouriers heat diffusion equation, and at shorter periods, phonon transport transitions to the ballistic regime, with the decay becoming strongly non-exponential. Spectral solutions are obtained for Si and PbSe at 300 K using phonon dispersion and lifetime data from density functional theory calculations. The spectral decay behaviors are compared to several approximate models: a single MFP solution, a frequency-i...

Examining thermal transport through a frequency-domain representation of time-domain thermoreflectance data

Laser-based time-domain thermoreflectance (TDTR) and frequency-domain thermoreflectance (FDTR) techniques are widely used for investigating thermal transport at micro- and nano-scales. We demonstrate that data obtained in TDTR measurements can be represented in a frequency-domain form equivalent to FDTR, i.e., in the form of a surface temperature amplitude and phase response to time-harmonic heating. Such a representation is made possible by using a large TDTR delay time window covering the entire pulse repetition interval. We demonstrate the extraction of frequency-domain data up to 1 GHz from TDTR measurements on a sapphire sample coated with a thin layer of aluminum, and show that the frequency dependencies of both the amplitude and phase responses agree well with theory. The proposed method not only allows a direct comparison of TDTR and FDTR data, but also enables measurements at high frequencies currently not accessible to FDTR. The frequency-domain representation helps uncover aspects of the measurement physics which remain obscured in a traditional TDTR measurement, such as the importance of modeling the details of the heat transport in the metal transducer film for analyzing high frequency responses.

Experimental Investigation of Metal-Diamond Thermal Interface Conductance With Different Diamond Surface Terminations

Synthetic diamond has potential as a heat spreading material due to its uniquely high thermal conductivity. In small-scale devices, interfaces can dominate the resistance to heat transport, and thus play an important role in determining device performance. Here we use transient thermoreflectance techniques to measure the thermal interface conductance at metal-diamond interfaces. We study single crystal diamond samples with various surface terminations. We measure thermal interface conductance values over a range of temperatures from 88 K to 300 K, and find roughly 60 percent higher thermal interface conductance between Al and oxygenated diamond samples as compared to hydrogen terminated samples. The results reported here will be useful for device design and for advancing models of interfacial heat transport.Copyright