Christopher B. Saltonstall

On the Assumption of Detailed Balance in Prediction of Diffusive Transmission Probability During Interfacial Transport

Models intended to predict interfacial transport often rely on the principle of detailed balance when formulating the interfacial carrier transmission probability. However, assumptions invoked significantly impact predictions. Here, we present six derivations of the transmission probability, each subject to a different set of preliminary assumptions regarding the type of scattering at the interface. Application of each case to phonon flux and thermal boundary conductance allows for a final quantitative comparison. Depending on the preliminary assumptions, predictions for thermal boundary conductance span over two orders of magnitude, demonstrating the need for transparency when assessing the accuracy of any predictive model.

Effect of interface adhesion and impurity mass on phonon transport at atomic junctions

With the characteristic lengths of electronic and thermal devices approaching the mean free paths of the pertinent energy carriers, thermal transport across these devices must be characterized and understood, especially across interfaces. Thermal interface conductance can be strongly affected by the strength of the bond between the solids comprising the interface and the presence of an impurity mass between them. In this work, we investigate the effects of impurity masses and mechanical adhesion at molecular junctions on phonon transmission via non-equilibrium Greens functions (NEGF) formalisms. Using NEGF, we derived closed form solutions to the phonon transmission across an interface with an impurity mass and variable bonding. We find that the interface spring constant that yields the maximum transmission for all frequencies is the harmonic mean of the spring constants on either side of the interface, while for a mass impurity, the arithmetic average of the masses on either side of the interface yields...

Assessment and prediction of thermal transport at solid–self-assembled monolayer junctions

Self-assembled monolayers (SAMs) have recently garnered much interest due to their unique electrical, chemical, and thermal properties. Several studies have focused on thermal transport across solid-SAM junctions, demonstrating that interface conductance is largely insensitive to changes in SAM length. In the present study, we have investigated the vibrational spectra of alkanedithiol-based SAMs as a function of the number of methylene groups forming the molecular backbone via Hartree-Fock methods. In the case of Au-alkanedithiol junctions, it is found that despite the addition of nine new vibrational modes per added methylene group, only one of these modes falls below the maximum phonon frequency of Au. In addition, the alkanedithiol one-dimensional density of normal modes (modes per unit energy per unit length) is nearly constant regardless of chain length, explaining the observed insensitivity. Furthermore, we developed a diffusive transport model intended to predict interface conductance at solid-SAM junctions. It is shown that this predictive model is in an excellent agreement with prior experimental data available in the literature.

Single element Raman thermometry

Despite a larger sensitivity to temperature as compared to other microscale thermometry methods, Raman based measurements typically have greater uncertainty. In response, a new implementation of Raman thermometry is presented having lower uncertainty while also reducing the time and hardware needed to perform the experiment. Using a modulated laser to excite the Raman response, the intensity of only a portion of the total Raman signal is leveraged as the thermometer by using a single element detector monitored with a lock-in amplifier. Implementation of the lock-in amplifier removes many sources of noise that are present in traditional Raman thermometry where the use of cameras preclude a modulated approach. To demonstrate, the portion of the Raman spectrum that is most advantageous for thermometry is first identified by highlighting, via both numerical prediction and experiment, those spectral windows having the largest linear dependence on temperature. Using such windows, the new technique, termed single element Raman thermometry (SERT), is utilized to measure the thermal profile of an operating microelectromechanical systems (MEMS) device and compared to results obtained with a traditional Raman approach. The SERT method is shown to reduce temperature measurement uncertainty by greater than a factor of 2 while enabling 3 times as many data points to be taken in an equal amount of time as compared to traditional Raman thermometry.

Impedance Matching of Atomic Thermal Interfaces Using Primitive Block Decomposition

We explore the physics of thermal impedance matching at the interface between two dissimilar materials by controlling the properties of a single atomic mass or bond. The maximum thermal current is transmitted between the materials when we are able to decompose the entire heterostructure solely in terms of primitive building blocks of the individual materials. Using this approach, we show that the minimum interfacial thermal resistance arises when the interfacial atomic mass is the arithmetic mean, whereas the interfacial spring constant is the harmonic mean of its neighbors. The contact-induced broadening matrix for the local vibronic spectrum, obtained from the self-energy matrices, generalizes the concept of acoustic impedance to the nonlinear phonon dispersion or the short-wavelength (atomic) limit.

Thermal Conductivity of Turbostratic Carbon Nanofiber Networks

Composite material systems composed of a matrix of nano materials can achieve combinations of mechanical and thermophysical properties outside the range of traditional systems. While many reports have studied the intrinsic thermal properties of individual carbon fibers, to be useful in applications in which thermal stability is critical, an understanding of heat transport in composite materials is required. In this work, air/ carbon nano fiber networks are studied to elucidate the system parameters influencing thermal transport. Sample thermal properties are measured with varying initial carbon fiber fill fraction, environment pressure, loading pressure, and heat treatment temperature through a bidirectional modification of the 3ω technique. The nanostructures of the individual fibers are characterized with small angle x-ray scattering and Raman spectroscopy providing insight to individual fiber thermal conductivity. Measured thermal conductivity varied from 0.010 W/(m K) to 0.070 W/(m K). An understanding of the intrinsic properties of the individual fibers and the interactions of the two phase composite is used to reconcile low measured thermal conductivities with predictive modeling. This methodology can be more generally applied to a wide range of fiber composite materials and their applications.

Assesment of Vibrational Coupling at Solid-SAM Junctions

Self-assembled monolayers (SAMs) have recently garnered much interest due to their unique electrical and chemical properties. The limited literature detailing SAM thermal properties has suggested that thermal boundary conductance (TBC) at solid-SAM junctions is not only low, but also insensitive to changes in SAM length as the number of methylene groups (-CH2 -) along alkanedithiol chains is varied from 8 to 10. The present study investigates the vibrational spectra of alkanedithiol SAMs as a function of the number of methylene groups forming the molecule backbone via Hartree-Fock methods and the subsequent effects on TBC calculated using a diffuse scattering model. In particular, the vibrational overlap between the alkanedithiol and Au is studied. It is found that despite the addition of 9 new vibrational modes per added methylene group, only one of those modes is elastically accessible to Au. It is believed that this “vibrational inaccessibility” is the cause of the insensitivity of thermal conductance to molecule length.Copyright