Many of the recent advances in enhancing the thermoelectric figure of merit are linked to nanoscale phenomena found both in bulk samples containing nanoscale constituents and in nanoscale samples themselves. Prior theoretical and experimental proof-of-principle studies on quantum-well superlattice and quantum-wire samples have now evolved into studies on bulk samples containing nanostructured constituents prepared by chemical or physical approaches. In this Review, nanostructural composites are shown to exhibit nanostructures and properties that show promise for thermoelectric applications, thus bringing together low-dimensional and bulk materials for thermoelectric applications. Particular emphasis is given in this Review to the ability to achieve 1) a simultaneous increase in the power factor and a decrease in the thermal conductivity in the same nanocomposite sample and for transport in the same direction and 2) lower values of the thermal conductivity in these nanocomposites as compared to alloy samples of the same chemical composition. The outlook for future research directions for nanocomposite thermoelectric materials is also discussed.

https://www.bc.edu/content/dam/files/schools/cas_sites/physics/pdf/Ren/AdvMater_19_1043_2007.pdf

New Directions for Low-Dimensional Thermoelectric Materials**

Thermoelectric materials have a large Peltier effect, making them attractive for solid-state cooling applications. Bismuth telluride (Bi2Te3)–based alloys have remained the state-of-the-art room-temperature materials for many decades. However, cost partially limited wider use of thermoelectric cooling devices because of the large amounts of expensive tellurium required. We report n-type magnesium bismuthide (Mg3Bi2)–based materials with a peak figure of merit (ZT) of ~0.9 at 350 kelvin, which is comparable to the commercial bismuth telluride selenide (Bi2Te3–xSex) but much cheaper. A cooling device made of our material and p-type bismuth antimony telluride (Bi0.5Sb1.5Te3) has produced a large temperature difference of ~91 kelvin at the hot-side temperature of 350 kelvin. n-type Mg3Bi2-based materials are promising for thermoelectric cooling applications.

/pdf/high-thermoelectric-cooling-performance-of-n-type-mg-3-bi-2-15ih2mx6j1.pdf

High thermoelectric cooling performance of n-type Mg 3 Bi 2 -based materials.

The absolute Seebeck coefficient, electrical resistivity, and thermal resistivity were simultaneously measured on pure bismuth single crystals of various orientations between approximately 80° and 300°K. Using an overlapping two‐band many‐valley model, numerical values for the temperature dependence and anisotropy (where appropriate) of the following parameters have been calculated: (1) the overlap energy and the Fermi energy of the electrons and of the holes, (2) the density of states effective mass of the electrons and of the holes, (3) the separate electronic and lattice thermal conductivities, (4) the actual index of thermo‐electric efficiency, and (5) the hypothetical ``optimum'' index of thermoelectric efficiency. The calculated electronic thermal conductivity includes a new term due to bipolar diffusion.

Transport Properties of Bismuth Single Crystals

The thermoelectric and thermomagnetic properties of undoped Bi-Sb alloys have been investigated with highly homogeneous single crystals throughout the entire alloy composition as functions of temperature (77–300°K), magnetic field (0–7·5 kilo-oersteds), and crystallographic direction (∥ and ⊥ to the trigonal axis). The undoped, n-type Bi85Sb15 alloy gave the largest magneto-thermoelectric figure of merit (11 × 10−3 deg−1) at 100°K and the same value at 80°K in transverse fields of 3 and 1·3 kilo-oersteds, respectively. Within the range of field intensity used, the largest thermomagnetic figure of merit (4·1 × 10−3 deg−1) was obtained at 100°K with undoped n-types Bi99Sb1 in a field of 7·5 kilo-oersteds. The possibility has been explored of obtaining p-type Bi-Sb alloys through additions of acceptor impurities and by applying transverse magnetic fields. The results showed that a Sn-doped Bi88Sb12 alloy yields a p-type figure of merit of 2·3 × 10−3 deg−1 at 85°K in 7·5 kilo-oersteds. The difference between the adiabatic and isothermal conditions was reviewed in some detail in connection with measurements of various transport coefficients that enter into the calculation of the figures of merit. Also, a comparison was given on the difference between the magneto-Peltier and the Ettingshausen cooling. With a hybrid cooling unit based on the combination of the Peltier and magneto-Peltier effects in telluride alloys and BiSb alloys, respectively, a continuous cooling as large as 172°K, from room temperature down to 128°K, has been achieved.

BiSb alloys for magneto-thermoelectric and thermomagnetic cooling

Electronic transport calculations were carried out for Bi1−xSbx nanowires (0⩽x⩽0.30) of diameters 10 nm⩽dW⩽100 nm at 77 K. A band structure phase diagram was generated, showing the dependence of the relative band edge positions on diameter and composition. Calculations of the thermoelectric figure-of-merit (ZT) predict that the performance of Bi1−xSbx nanowires is superior to that of Bi nanowires and to that of the bulk alloy. An exceptionally high value of ZT for p-type nanowires at 77 K was found for dW∼40 nm and x∼0.13, which is explained by the coalescence in energy of up to ten valence subband edges to maximize the density-of-states at the Fermi energy.

Anomalously high thermoelectric figure of merit in Bi1−xSbx nanowires by carrier pocket alignment

The electrical resistivity $\ensuremath{\rho}$ and Hall effect $R$ of zone-levelled single crystals of Bi-Sb alloys have been measured in the temperature range from 4.2\ifmmode^\circ\else\textdegree\fi{}K to 300\ifmmode^\circ\else\textdegree\fi{}K. $\mathrm{Log}(\frac{\ensuremath{\rho}}{{\ensuremath{\rho}}_{300}})$ vs $\frac{1}{T}$ curves suggest thermal activation of carriers in the concentration range from 5% to 40% Sb in the temperature range from 25\ifmmode^\circ\else\textdegree\fi{}K to 100\ifmmode^\circ\else\textdegree\fi{}K; approximate activation energies have been inferred from their slopes. The activation energy appears to have a maximum at a concentration near 12%. Some anomalies have been observed in the behavior of $\ensuremath{\rho}$ and $R$ on both sides of this concentration at low temperatures. Lattice parameters for these alloys have also been measured for the entire range of solid solubility. Both a maximum and minimum in the $c$-axis lattice parameter vs concentration occur near the concentration at which anomalies appear in transport properties. These phenomena are discussed in terms of a simple band model proposed by Blount and Cohen.

A. L. Jain

Papers

Temperature Dependence of the Electrical Properties of Bismuth-Antimony Alloys

Effect of europium on the structural and thermal properties of pristine iron phosphate glass