Austin J. Minnich

High-thermoelectric performance of nanostructured bismuth antimony telluride bulk alloys.

The dimensionless thermoelectric figure of merit (ZT) in bismuth antimony telluride (BiSbTe) bulk alloys has remained around 1 for more than 50 years. We show that a peak ZT of 1.4 at 100°C can be achieved in a p-type nanocrystalline BiSbTe bulk alloy. These nanocrystalline bulk materials were made by hot pressing nanopowders that were ball-milled from crystalline ingots under inert conditions. Electrical transport measurements, coupled with microstructure studies and modeling, show that the ZT improvement is the result of low thermal conductivity caused by the increased phonon scattering by grain boundaries and defects. More importantly, ZT is about 1.2 at room temperature and 0.8 at 250°C, which makes these materials useful for cooling and power generation. Cooling devices that use these materials have produced high-temperature differences of 86°, 106°, and 119°C with hot-side temperatures set at 50°, 100°, and 150°C, respectively. This discovery sets the stage for use of a new nanocomposite approach in developing high-performance low-cost bulk thermoelectric materials.

Bulk nanostructured thermoelectric materials: current research and future prospects

Thermoelectrics have long been recognized as a potentially transformative energy conversion technology due to their ability to convert heat directly into electricity. Despite this potential, thermoelectric devices are not in common use because of their low efficiency, and today they are only used in niche markets where reliability and simplicity are more important than performance. However, the ability to create nanostructured thermoelectric materials has led to remarkable progress in enhancing thermoelectric properties, making it plausible that thermoelectrics could start being used in new settings in the near future. Of the various types of nanostructured materials, bulk nanostructured materials have shown the most promise for commercial use because, unlike many other nanostructured materials, they can be fabricated in large quantities and in a form that is compatible with existing thermoelectric device configurations. The first generation of these materials is currently being developed for commercialization, but creating the second generation will require a fundamental understanding of carrier transport in these complex materials which is presently lacking. In this review we introduce the principles and present status of bulk nanostructured materials, then describe some of the unanswered questions about carrier transport and how current research is addressing these questions. Finally, we discuss several research directions which could lead to the next generation of bulk nanostructured materials.

Power Factor Enhancement by Modulation Doping in Bulk Nanocomposites

We introduce the concept of modulation doping in three-dimensional nanostructured bulk materials to increase the thermoelectric figure of merit. Modulation-doped samples are made of two types of nanograins (a two-phase composite), where dopants are incorporated only into one type. By band engineering, charge carriers could be separated from their parent grains and moved into undoped grains, which would result in enhanced mobility of the carriers in comparison to uniform doping due to a reduction of ionized impurity scattering. The electrical conductivity of the two-phase composite can exceed that of the individual components, leading to a higher power factor. We here demonstrate the concept via experiment using composites made of doped silicon nanograins and intrinsic silicon germanium grains.

Coherent Phonon Heat Conduction in Superlattices

Coherent Heat Flow Typically, heat in solids is transported incoherently because phonons scatter at interfaces and defects. Luckyanova et al. (p. 936) grew super-lattice films made from one to nine repeats of layers of GaAs and AlAs, each 12-nm thick. Thermal conductivity through this sandwich structure increased linearly with the number of superlattice repeats, which is consistent with theoretical simulations of coherent heat transport. Coherent phonon transport is evidenced by linear increases of thermal conductivity with total superlattice thickness. The control of heat conduction through the manipulation of phonons as coherent waves in solids is of fundamental interest and could also be exploited in applications, but coherent heat conduction has not been experimentally confirmed. We report the experimental observation of coherent heat conduction through the use of finite-thickness superlattices with varying numbers of periods. The measured thermal conductivity increased linearly with increasing total superlattice thickness over a temperature range from 30 to 150 kelvin, which is consistent with a coherent phonon heat conduction process. First-principles and Green’s function–based simulations further support this coherent transport model. Accessing the coherent heat conduction regime opens a new venue for phonon engineering for an array of applications.

Modified effective medium formulation for the thermal conductivity of nanocomposites

This letter introduces a modified effective medium formulation for composites where the characteristic length of the inclusion is on the order of or smaller than the phonon mean free path. The formulation takes into account the increased interface scattering in the different phases of the nanocomposite and the thermal boundary resistance between the phases. The interface density of inclusions is introduced and is found to be a primary factor in determining the thermal conductivity. The predictions are in good agreement with results from Monte Carlo simulations and solutions to the Boltzmann equation.

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.

Spectral mapping of thermal conductivity through nanoscale ballistic transport

Controlling thermal properties is central to many applications, such as thermoelectric energy conversion and the thermal management of integrated circuits. Progress has been made over the past decade by structuring materials at different length scales, but a clear relationship between structure size and thermal properties remains to be established. The main challenge comes from the unknown intrinsic spectral distribution of energy among heat carriers. Here, we experimentally measure this spectral distribution by probing quasi-ballistic transport near nanostructured heaters down to 30 nm using ultrafast optical spectroscopy. Our approach allows us to quantify up to 95% of the total spectral contribution to thermal conductivity from all phonon modes. The measurement agrees well with multiscale and first-principles-based simulations. We further demonstrate the direct construction of mean free path distributions. Our results provide a new fundamental understanding of thermal transport and will enable materials design in a rational way to achieve high performance.

Advances in the measurement and computation of thermal phonon transport properties

Heat conduction by phonons is a ubiquitous process that incorporates a wide range of physics and plays an essential role in applications ranging from space power generation to LED lighting. Heat conduction has been studied for over two hundred years, yet many of the microscopic details have remained unknown in most crystalline solids, including which phonon-phonon interactions are primarily responsible for thermal resistance and how heat is distributed among the broad thermal spectrum. This lack of knowledge was the result of limitations on the available tools to study heat conduction. However, recent advances in both computation and experiment are enabling an unprecedented microscopic view of thermal transport by phonons in both bulk and nanostructured crystals, from the level of atomic bonding to mesoscopic transport in complex devices. In this topical review, we examine these techniques and the microscopic insights gained into the science and engineering of heat conduction.

Coherent and Incoherent Thermal Transport in Nanomeshes

Coherent thermal transport in nanopatterned structures is a topic of considerable interest, but whether it occurs in certain structures remains unclear due to a poor understanding of which phonons conduct heat. Here, we perform fully three-dimensional, frequency-dependent simulations of thermal transport in nanomeshes by solving the Boltzmann transport equation with an efficient Monte Carlo method. From the spectral information in our simulations, we show that thermal transport in nanostructures that can be created with available lithographic techniques is dominated by incoherent boundary scattering at room temperature. Our result provides important insights into the conditions required for coherent thermal transport to occur in artificial structures.

Theoretical studies on the thermoelectric figure of merit of nanograined bulk silicon

In this paper, we investigate the phonon transport in silicon nanocomposites using Monte Carlo simulations considering frequency-dependent phonon mean free paths, and combine the phonon modeling with electron modeling to predict the thermoelectric figure of merit (ZT) of silicon nanocomposites. The model shows that while grain interface scattering of phonons is negligible for large grain sizes around 200 nm, ZT can reach 1.0 at 1173 K if the grain size can be reduced to 10 nm. Our results show the potential of obtaining a high ZT in bulk silicon by the nanocomposite approach.