Manipulating the thermal conductivity of solids is important for practical applications. Due to the fact that phonons in thermoelectric materials have longer mean free paths (MFPs) than electrons, strengthening phonon scattering to reduce lattice thermal conductivity (κlat) becomes the most straightforward and effective approach to enhance the thermoelectric figure of merit, ZT, which determines the maximum device efficiency. Phonons have a wide range of MFPs in semiconductors, and different dimensions of lattice defects can be targeted to scatter particular phonons with distinct relaxation times. Designing hierarchical nano-microstructures, spanning from point defects to volume defects, would be beneficial to achieve low κlat via a full spectrum of phonon scattering. Herein, we review the formation and underlying mechanisms for lattice defects and highlight the role of all-scale hierarchical nano-microstructure on phonon engineering. Existing challenges in simulations are also discussed.

We study the temperature dependence of thermoelectric transport properties of four FeSb2 nanocomposite samples with different grain sizes. The comparison of the single crystals and nanocomposites of varying grain sizes indicates the presence of substantial phonon drag effects in this system contributing to a large Seebeck coefficient at low temperature. As the grain size decreases, the increased phonon scattering at the grain boundaries leads to a suppression of the phonon-drag effect, resulting in a much smaller peak value of the Seebeck coefficient in the nanostructured bulk materials. As a consequence, the ZT values are not improved significantly even though the thermal conductivity is drastically reduced.

Bismuth-Antimony alloys have been shown to have high ZT values below room temperature, especially for single crystals. For polycrystalline samples, impurity doping and magnetic field have proven to be powerful tools in the search for understanding and improving thermoelectric performance. Nanopolycrystalline Bi0.88Sb0.12 doped with 0.05, 0.5 and 3 % Ce were prepared by ball milling and dc hot pressing techniques. Electrical resistivity, Seebeck coefficient, thermal conductivity, carrier concentration, mobility, and magnetization are measured in a temperature range of 5-350 K and in magnetic fields up to 9 Tesla. The effects of Ce doping on the thermoelectric properties of Bi0.88Sb0.12 in zero magnetic field are discussed.

The Seebeck coefficient, electrical resistivity, thermal conductivity and Hall coefficient of FeSbx (x = 2.04, 2.00, and 1.96) nanocomposites hot pressed at 300 °C were measured. The power factor of FeSb1.96 was increased by 105% compared to FeSb2. Hall coefficient measurements revealed a decreased carrier concentration and increased mobility in FeSb1.96 with an overall enhancement in ZTof 45% in FeSb1.96 .

The thermoelectric properties of Bi-FeSb2 nanocomposites are reported. The electrical resistivity and the Seebeck coefficient measurements show a significant dependence on bismuth concentration. Our results reveal that the shifting of the Seebeck peak in FeSb2 nanocomposites is purely a grain size-effect. The thermal conductivity data indicates a presence of an electron-phonon interaction. Over all, our analysis of the the thermoelectric properties of Bi-FeSb2 nanocomposites provide additional evidence for phonon-drag in FeSb2.

Even though the crystal structure of lead telluride (PbTe) has been extensively studied for many years, we discovered that the structure has a strong tendency to form Pb-depleted disks on {001} planes. These disks are around 2–5 nm in diameter and less than 0.5 nm in thickness, with a volume density of around 9 × 1017 cm−3, resulting in lattice strain fields (3–20 nm) on both sides of the disks along their normal directions. Moreover, such disks were also observed in Pb-rich Pb1.3Te, Pb-deficient PbTe1.3, and thallium (Tl)-doped Tl0.01Pb0.99Te and Tl0.02Pb0.98Te crystals. Because of the effects of diffraction contrast imaging by transmission electron microscopy and orientations of the crystals, these native lattice strain fields were incorrectly recognized as precipitates or nanoinclusions in PbTe-based materials. This discovery provides new insight into the formation mechanism of the precipitates or nanoinclusions in PbTe-based materials.

The concept of using nanocomposite thermoelectric materials in bulk form for practical applications is presented. Laboratory studies have shown the possibilities of nanostructures to yield large reductions in the thermal conductivity while at the same time increasing the power factor. Theoretical studies have suggested that structural ordering in nano-systems is not necessary for the enhancement of ZT, leading to the idea of using nanocomposites as a practical scale-up technology for making bulk thermoelectric materials with enhanced ZT values. Specific examples are presented of nanocomposite thermoelectric materials developed by our group based on the familiar silicon germanium system, showing enhanced thermoelectric performance through nano-structuring.

The dimensionless thermoelectric figure-of-merit (ZT) in bulk materials has remained about 1 for many years. Here we show that a significant ZT improvement can be achieved in nanocrystalline bulk materials. These nanocrystalline bulk materials were made by hot-pressing nanopowders that are ball-milled from either crystalline ingots or elements. 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, the nanostructure approach has been demonstrated in a few thermoelectric material systems, proving its generosity. The approach can be easily scaled up to multiple tons. Thermal stability studies have shown that the nanostructures are stable at the application temperature for an extended period of time. It is expected that such enhanced materials will make the existing cooling and power generation systems more efficient.

The concept of using “self-assembled” and “force-engineered” nanostructures to enhance the thermoelectric figure of merit relative to bulk homogeneous and composite materials is presented in general terms. Specific application is made to the Si-Ge system for use in power generation at high temperature. The scientific advantages of the nanocomposite approach for the simultaneous increase in the power factor and decrease of the thermal conductivity are emphasized along with the practical advantages of having bulk samples for property measurements and a straightforward path to scale-up materials synthesis and integration of nanostructured materials into thermoelectric cooling and power generation devices.

Nanostructured thermoelectric materials have attracted lots of interest in recent years, due to their enhanced performance determined by their thermoelectric dimensionless figure of merit. However, because of equipment limitations, not much work has been done on combining simultaneous transport measurements and structural characterization on individual nanostructured thermoelectric materials. With an integrated TEM-STM system, we studied the structural behavior and electrical properties of bismuth (Bi) nanobelts and nanoparticles. Results showed that clean Bi nanostructures free of oxides can be produced by in-situ high temperature electro-migration and Joule annealing processes occurring within the electron microscope. Preliminary electrical measurements indicate a conductivity of two orders of magnitude lower for Bi nanoparticles than that for bulk Bi. Such in-situ studies are highly advantageous for studying the semimetal-semiconductor transition and how this transition could enhance thermoelectric properties.

The critical current density Jc of bulk ceramic superconductors is often several orders of magnitude lower than that of a single crystal or oriented fílm of the same material due to the weak links residing at the boundaries between single crystals within the ceramic sample. Assuming that the rate of electron transport within each single crystal is much faster than the average rate of electron transport between adjacent single crystals, a working hypothesis of Jc enhancement by granular reorientation is formulated which includes the“brickwall”model and melt-texturing as special cases with long-range granular alignment which usually leads to rigid and brittle bulk samples. Simple geometric considerations show that only short-range local granular alignment is necessary for high Jc. For mis reason, it should be possible to make flexible superconducting wires and tapes for commercial applications at or above 77K.