Lead chalcogenides have long been used for space-based and thermoelectric remote power generation applications, but recent discoveries have revealed a much greater potential for these materials. This renaissance of interest combined with the need for increased energy efficiency has led to active consideration of thermoelectrics for practical waste heat recovery systems—such as the conversion of car exhaust heat into electricity. The simple high symmetry NaCl-type cubic structure, leads to several properties desirable for thermoelectricity, such as high valley degeneracy for high electrical conductivity and phonon anharmonicity for low thermal conductivity. The rich capabilities for both band structure and microstructure engineering enable a variety of approaches for achieving high thermoelectric performance in lead chalcogenides. This Review focuses on manipulation of the electronic and atomic structural features which makes up the thermoelectric quality factor. While these strategies are well demonstrated in lead chalcogenides, the principles used are equally applicable to most good thermoelectric materials that could enable improvement of thermoelectric devices from niche applications into the mainstream of energy technologies.

Band Engineering of Thermoelectric Materials

Mg(2)Si and Mg(2)Sn are indirect band gap semiconductors with two low-lying conduction bands (the lower mass and higher mass bands) that have their respective band edges reversed in the two compounds. Consequently, for some composition x, Mg(2)Si(1-x)Sn(x) solid solutions must display a convergence in energy of the two conduction bands. Since Mg(2)Si(1-x)Sn(x) solid solutions are among the most prospective of the novel thermoelectric materials, we aim on exploring the influence of such a band convergence (valley degeneracy) on the Seebeck coefficient and thermoelectric properties in a series of Mg(2)Si(1-x)Sn(x) solid solutions uniformly doped with Sb. Transport measurements carried out from 4 to 800 K reveal a progressively increasing Seebeck coefficient that peaks at x=0.7. At this concentration the thermoelectric figure of merit ZT reaches exceptionally large values of 1.3 near 700 K. Our first principles calculations confirm that at the Sn content x≈0.7 the two conduction bands coincide in energy. We explain the high Seebeck coefficient and ZT values as originating from an enhanced density-of-states effective mass brought about by the increased valley degeneracy as the two conduction bands cross over. We corroborate the increase in the density-of-states effective mass by measurements of the low temperature specific heat. The research suggests that striving to achieve band degeneracy by means of compositional variations is an effective strategy for enhancing the thermoelectric properties of these materials.

https://link.aps.org/accepted/10.1103/PhysRevLett.108.166601

Convergence of conduction bands as a means of enhancing thermoelectric performance of n-type Mg2Si(1-x)Sn(x) solid solutions.

This review discusses recent developments and current research in high performance bulk thermoelectric materials, comprising nanostructuring, mesostructuring, band alignment, band engineering and synergistically defining key strategies for boosting the thermoelectric performance. To date, the dramatic enhancements in the figure of merit achieved in bulk thermoelectric materials have come either from the reduction in lattice thermal conductivity or improvement in power factors, or both of them. Here, we summarize these relationships between very large reduction of the lattice thermal conductivity with all-scale hierarchical architecturing, large enhanced Seebeck coefficients with intra-matrix electronic structure engineering, and control of the carrier mobility with matrix/inclusion band alignment, which enhance the power factor and reduce the lattice thermal conductivity. The new concept of hierarchical compositionally alloyed nanostructures to achieve these effects is presented. Systems based on PbTe, PbSe and PbS in which spectacular advances have been demonstrated are given particular emphasis. A discussion of future possible strategies is aimed at enhancing the thermoelectric figure of merit of these materials.

The panoscopic approach to high performance thermoelectrics

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

From an environmental perspective, lead-free SnTe would be preferable for solid-state waste heat recovery if its thermoelectric figure-of-merit could be brought close to that of the lead-containing chalcogenides. In this work, we studied the thermoelectric properties of nanostructured SnTe with different dopants, and found indium-doped SnTe showed extraordinarily large Seebeck coefficients that cannot be explained properly by the conventional two-valence band model. We attributed this enhancement of Seebeck coefficients to resonant levels created by the indium impurities inside the valence band, supported by the first-principles simulations. This, together with the lower thermal conductivity resulting from the decreased grain size by ball milling and hot pressing, improved both the peak and average nondimensional figure-of-merit (ZT) significantly. A peak ZT of ∼1.1 was obtained in 0.25 atom % In-doped SnTe at about 873 K.

https://www.pnas.org/content/pnas/110/33/13261.full.pdf

High thermoelectric performance by resonant dopant indium in nanostructured SnTe

Mg2(Si,Sn) compounds have shown great promise for thermoelectric applications due to good thermoelectric properties, nontoxicity, and abundantly available constituent elements Herein we report on the thermoelectric properties and microstructure of high performance Mg2Si04−xSn06Sbx (0⩽x⩽0015) alloys The state-of-the-art ZT value of ∼11 has been attained in the samples with x=00075 due to the relatively low thermal conductivity In light of the simple cubic structure and mostly light constituent elements, the reduction in lattice thermal conductivity has been discussed in connection with a fairly large amount of in situ formed nanostructures in these samples

High figures of merit and natural nanostructures in Mg2Si0.4Sn0.6 based thermoelectric materials

It has been shown previously that the thermoelectric properties of the Zintl phase compound, YbZn2Sb2 can be finely tuned via substitution at the cationic Yb-site Here we report the results of the investigation of isoelectronic substitution of Zn by Mn in the anionic (Zn2Sb2)2− framework The p-type YbZn2−xMnxSb2 (00⩽x⩽04) samples have been synthesized via a solid-state reaction followed by suitable cooling, annealing, grounding, and hot-pressing densification processes In samples with x=00, 005, 010, 015, 02, 03, and 04, the electrical conductivity, Seebeck coefficient, and thermal conductivity measurements have been performed as a function of temperature from 300to750K It is found that the Mn substitution effectively lowers the thermal conductivity for all samples, while it significantly increases the power factor for x⩽015 As a result, a dimensionless figure of merit ZT of ∼061–065 has been attained at 726K for x=005–015 as compared to the ZT of ∼048 in the unsubstituted YbZn2Sb2

Improved thermoelectric performance in the Zintl phase compounds YbZn2−xMnxSb2 via isoelectronic substitution in the anionic framework

Bulk amorphous chalcogenide samples of Ge 20 Te 80− x Se x ( x  = 0, 1, 2, 8) have been prepared using a melting-quench method, and characterized by the differential scanning calorimetry, X-ray powder diffraction, high-resolution transmission electron microscopy, specific heat and thermal conductivity measurements. The low temperature specific heat measurements identified some localized low-frequency oscillation modes (Einstein modes) in conjunction with a Debye-like behavior. It was found that with increasing Se concentration the characteristic Debye temperature did not change whereas the Einstein temperature slightly decreased. The lattice thermal conductivity of all Ge 20 Te 80− x Se x samples exhibited typical amorphous heat conduction behavior, which has been discussed in connection with the phonon mean free path and in the context of a phenomenological model of heat conduction for highly disordered system.

Thermal conductivity and specific heat of bulk amorphous chalcogenides Ge20Te80−xSex (x = 0,1,2,8)

The thermoelectric properties of the Zintl compound YbZn2Sb2 with isoelectronic substitution of Zn by Mn in the anionic (Zn2Sb2)2− framework have been studied. The p-type YbZn2−x
 Mn
 x
 Sb2 (0.0 ≤ x ≤ 0.4) samples were prepared via melting followed by annealing and hot-pressing. Thermoelectric property measurement showed that the Mn substitution effectively lowered the thermal conductivity for all the samples, while it significantly increased the Seebeck coefficient for x < 0.2. As a result, a dimensionless figure of merit ZT of approximately 0.61 to 0.65 was attained at 726 K for x = 0.05 to 0.15, compared with the ZT of ~0.48 in the unsubstituted YbZn2Sb2.

Thermoelectric Properties of Zintl Compound YbZn2Sb2 with Mn Substitution in Anionic Framework

${\text{Cd}}_{2}{\text{Re}}_{2}{\text{O}}_{7}$ single crystals exhibit an amorphouslike lattice thermal conductivity, ${\ensuremath{\kappa}}_{\text{latt}}$. At temperatures about twice the Debye temperature, the phonon mean-free path approaches the value of the average Cd-Cd spacing while the specific heat is at values notably lower than the classical Dulong-Petit limit. We show that a model that treats Cd ions as Einstein oscillators embedded in a Debye host with strong optical-acoustic phonon coupling and some very high-frequency optical phonon modes in the system, can account for these features. Despite no structural change, ${\ensuremath{\kappa}}_{\text{latt}}$ undergoes a smooth slope change at $\ensuremath{\sim}340\text{ }\text{K}$, above which the resistivity adopts a weak power-law behavior. The combined results of resistivity, thermopower, specific-heat, thermal-conductivity, and Hall coefficient measurements corroborate that a phonon-mediated electron-hole scattering mechanism sets in above room temperature.

S. N. Zhang

Papers

High figures of merit and natural nanostructures in Mg2Si0.4Sn0.6 based thermoelectric materials

Improved thermoelectric performance in the Zintl phase compounds YbZn2−xMnxSb2 via isoelectronic substitution in the anionic framework

Thermal conductivity and specific heat of bulk amorphous chalcogenides Ge20Te80−xSex (x = 0,1,2,8)

Thermoelectric Properties of Zintl Compound YbZn2Sb2 with Mn Substitution in Anionic Framework

Probing lattice dynamics of Cd 2 Re 2 O 7 pyrochlore: Thermal transport and thermodynamics study