David Song

Measurements of nanofluid viscosity and its implications for thermal applications

Experimental results on the viscosity of alumina-based nanofluids are reported for various shear rates, temperature, nanoparticle diameter, and nanoparticle volume fraction. From the data it seems that the increase in the nanofluid viscosity is higher than the enhancement in the thermal conductivity as reported in the literature. It is shown, however, that the viscosity has to be increased by more than a factor of 4—relative to the increase in thermal conductivity—to make the nanofluid thermal performance worse than that of the base fluid.

Modeling the Thermal Conductivity and Phonon Transport in Nanoparticle Composites Using Monte Carlo Simulation

This paper presents a Monte Carlo simulation scheme to study the phonon transport and the thermal conductivity of nanocomposites. Special attention has been paid to the implementation of periodic boundary condition in Monte Carlo simulation. The scheme is applied to study the thermal conductivity of silicon germanium (Si-Ge) nanocomposites, which are of great interest for high-efficiency thermoelectric material development. The Monte Carlo simulation was first validated by successfully reproducing the results of (two-dimensional) nanowire composites using the deterministic solution of the phonon Boltzmann transport equation reported earlier and the experimental thermal conductivity of bulk germanium, and then the validated simulation method was used to study (three-dimensional) nanoparticle composites, where Si nanoparticles are embedded in Ge host. The size effects of phonon transport in nanoparticle composites were studied, and the results show that the thermal conductivity of nanoparticle composites can be lower than that of the minimum alloy value, which is of great interest to thermoelectric energy conversion. It was also found that randomly distributed nanopaticles in nanocomposites rendered the thermal conductivity values close to that of periodic aligned patterns. We show that interfacial area per unit volume is a useful parameter to correlate the size effect of thermal conductivity in nanocomposites. The key for the thermal conductivity reduction is to have a high interface density where nanoparticle composites can have a much higher interface density than the simple ID stacks, such as superlattices. Thus, nanocomposites further benefit the enhancement of thermoelectric performance in terms of thermal conductivity reduction. The thermal conductivity values calculated by this work qualitatively agrees with a recent experimental measurement of Si-Ge nanocomposites.

Thermal conductivity of periodic microporous silicon films

This letter reports the experimental in-plane thermal conductivity of microfabricated, free-standing, single-crystal silicon thin films with periodically arranged through-film micropores. The experimental data suggest that strong size effects exist even in micro-sized porous silicon structures, particularly at low temperatures. Even at room temperature, all porous membranes show thermal conductivity values lower than expected by porosity and bulk phonon mean free path of silicon, and small-pore membranes have smaller thermal conductivity compared to the large-pore ones despite that they have close porosity values.

Thermal conductivity of nanoporous bismuth thin films

The thermal conductivity of nanoporous Bi thin films has been experimentally determined. Samples are fabricated by a liquid phase deposition, and their thermal conductivities are measured by a differential 3ω method. Nanoporous Bi thin films exhibit an order-of-magnitude reduction in thermal conductivity compared to that of solid films, most likely the result of a reduction in phonon mean free path. When porous Bi films are exposed to a hydrogen plasma, thermal conductivity measurements reveal no variation with extent of porosity, while electrical conductivity is much more sensitive to porosity, suggesting the possibility of independent control of these two intrinsic properties.

Phonon engineering in nanostructures for solid-state energy conversion

Solid-state energy conversion technologies such as thermoelectric and thermionic refrigeration and power generation require materials with low thermal conductivity but good electrical conductivity, which are difficult to realize in bulk semiconductors. Nanostructures such as quantum wires and quantum wells provide alternative approaches to improve the solid-state energy conversion efficiency through size effects on the electron and phonon transport. In this paper, we discuss the possibility of engineering the phonon transport in nanostructures, with emphases on the thermal conductivity of superlattices. Following a general discussion on the directions for reducing the lattice thermal conductivity in nanostructures, specific modeling results on the phonon transport in superlattices will be presented and compared with recent experimental studies to illustrate the potential approaches and remaining questions.

Thermal conductivity of skutterudite thin films and superlattices

Experimental results on the temperature-dependent cross-plane thermal conductivity of skutterudite thin films are presented. The films examined include IrSb3, CoSb3, and Ir0.5Co0.5Sb3 single layers, and IrSb3/CoSb3 superlattices that are grown by pulsed-laser deposition. A differential 3ω method is used to measure the cross-plane thermal conductivity of these films from 80 to 300 K. The experimental results show a significant reduction in their thermal conductivity values compared to those of their corresponding bulk samples reported in literature. Possible mechanisms contributing to the thermal conductivity reduction are discussed.

Thermal conductivity of AlAs0.07Sb0.93 and Al0.9Ga0.1As0.07Sb0.93 alloys and (AlAs)1/(AlSb)11 digital-alloy superlattices

A differential 3ω technique is employed to determine the thermal conductivity of the AlAs0.07Sb0.93 ternary alloy, the Al0.9Ga0.1As0.07Sb0.93 quaternary alloy, and an (AlAs)1/(AlSb)11 digital-alloy superlattice. Between 80 and 300 K, the thermal conductivities for all three samples are relatively insensitive to temperature. The thermal conductivity of the (AlAs)1/(AlSb)11 superlattice is smaller than that of the AlAs0.07Sb0.93 ternary alloy, but much larger than the predictions of a model for phonon transport across the superlattice interfaces.

Thermal conductivity reduction mechanisms in superlattices

The large thermal conductivity reduction observed in superlattices has led to the reports of significantly increased thermoelectric figure-of-merit by several groups. The mechanisms of thermal conductivity reduction in superlattices were scrutinized from different angles over the last decade. This paper summarizes our current understanding on the heat conduction mechanisms in superlattices. Among several potential mechanisms, the interface scattering, particularly diffuse interface scattering, plays the most important role. Conclusions drawn from the studies on superlattices have implications to other nanostructured thermoelectric materials.

Thermoelectric property characterization of low-dimensional structures

Thermoelectric property characterization of low-dimensional structures is a very challenging task. We will discuss issues encountered in the characterization of thermoelectric properties of low-dimensional structures, particularly thin film structures and their possible solutions. Emphasis is placed on measuring the thermoelectric properties in the same direction, i.e., either parallel or perpendicular to the thin-film plane. Our focus has been on Si/Ge superlattices that are grown by molecular beam epitaxy (MBE). These samples are typically grown on graded buffers, due to the lattice constant mismatch between Si and Ge, which significantly complicate the measurements, since the properties of the buffers are also unknown.

Thermal conductivity of Si/Ge superlattices

We report in this paper the thermal conductivity measurement of Si/Ge superlattices as a function of the temperature and the period thickness. The symmetrized Si/Ge superlattices are grown by MBE on Si substrates with a graded buffer layer. A comparative 3/spl omega/ method is used to measure the thermal conductivity of the buffer and the superlattices between 80K-300K. The thermal conductivity is carried out in conjunction with X-ray and TEM sample characterization. The measured thermal conductivity values are lower than that of their corresponding alloys and show a decreasing trend with increasing period thickness which are corroborated with the TEM characterization of the dislocation density.