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.

Bulk nanostructured thermoelectric materials: current research and future prospects

A dimensionless thermoelectric figure-of-merit (ZT) of 0.95 in p-type nanostructured bulk silicon germanium (SiGe) alloys is achieved, which is about 90% higher than what is currently used in space flight missions, and 50% higher than the reported record in p-type SiGe alloys. These nanostructured bulk materials were made by using a direct current-induced hot press of mechanically alloyed nanopowders that were initially synthesized by ball milling of commercial grade Si and Ge chunks with boron powder. The enhancement of ZT is due to a large reduction of thermal conductivity caused by the increased phonon scattering at the grain boundaries of the nanostructures combined with an increased power factor at high temperatures.

https://www.bc.edu/content/dam/files/schools/cas_sites/physics/pdf/Ren/NanoLetters%208_4670_2008.pdf

Enhanced thermoelectric figure-of-merit in nanostructured p-type silicon germanium bulk alloys.

Recently a significant figure-of-merit (ZT) improvement in the most-studied existing thermoelectric materials has been achieved by creating nanograins and nanostructures in the grains using the combination of high-energy ball milling and a direct-current-induced hot-press process. Thermoelectric transport measurements, coupled with microstructure studies and theoretical modeling, show that the ZT improvement is the result of low lattice thermal conductivity due to the increased phonon scattering by grain boundaries and structural defects. In this article, the synthesis process and the relationship between the microstructures and the thermoelectric properties of the nanostructured thermoelectric bulk materials with an enhanced ZT value are reviewed. It is expected that the nanostructured materials described here will be useful for a variety of applications such as waste heat recovery, solar energy conversion, and environmentally friendly refrigeration.

Enhancement of Thermoelectric Figure-of-Merit by a Bulk Nanostructuring Approach

The dimensionless thermoelectric figure of merit (ZT) of the n-type silicon germanium (SiGe) bulk alloy at high temperature has remained at about one for a few decades. Here we report that by using a nanostructure approach, a peak ZT of about 1.3 at 900 °C in an n-type nanostructured SiGe bulk alloy has been achieved. The enhancement of ZT comes mainly from a significant reduction in the thermal conductivity caused by the enhanced phonon scattering off the increased density of nanograin boundaries. The enhanced ZT will make such materials attractive in many applications such as solar, thermal, and waste heat conversion into electricity.

Enhanced thermoelectric figure of merit in nanostructured n-type silicon germanium bulk alloy

An enhancement in the dimensionless thermoelectric fi gure-of-merit ( ZT ) of an n-type half-Heusler material is reported using a nanocomposite approach. A peak ZT value of 1.0 was achieved at 600 ° C‐700 ° C, which is about 25% higher than the previously reported highest value. The samples were made by ball-milling ingots of composition Hf 0.75 Zr 0.25 NiSn 0.99 Sb 0.01 into nanopowders and hot-pressing the powders into dense bulk samples. The ingots were formed by arc-melting the elements. The ZT enhancement mainly comes from reduction of thermal conductivity due to increased phonon scattering at grain boundaries and crystal defects, and optimization of antimony doping.

https://www.bc.edu/content/dam/files/schools/cas_sites/physics/pdf/Ren/240%20Adv%20Energy%20Mater%201%20643-647%202011.pdf

Enhancement in Thermoelectric Figure‐Of‐Merit of an N‐Type Half‐Heusler Compound by the Nanocomposite Approach

The recent results of C. B. Vining, W. Laskow, J. O. Hanson, R. R. Van der Beck, and P. D. Gorsuch [J. Appl. Phys. 69, 4333 (1991)] regarding the effect of grain size on the thermoelectric figure of merit of heavily doped p‐type silicon germanium alloys are compared to earlier results on similar materials. The data confirm that the room‐temperature figure‐of‐merit is significantly increased in materials with a small grain size.

D. M. Rowe

Papers

Comments on the thermoelectric properties of pressure‐sintered Si0.8Ge0.2 thermoelectric alloys