Themed issue on the chemistry of thermoelectric materials

Umut Aydemir

Umut Aydemir is a postdoctoral research associate in the Department of Materials Science and Engineering at Northwestern University. His research interests are synthesis, chemical and physical characterization of novel functional materials focused on energy technologies. He received his BSc in Chemistry and Physics and MSc in Materials Science and Engineering at Koç University in Istanbul, Turkey. He conducted his doctoral work at the Max Planck Institute for Chemical Physics of Solids in Dresden, Germany studying synthesis and characterization of intermetallic clathrates with Prof. Yuri Grin. Before joining Northwestern University, Dr Aydemir was a postdoctoral scholar at the California Institute of Technology investigating novel thermoelectric materials with Prof. G. Jeffrey Snyder. His work has resulted in the publication of over 30 research papers.

G. Jeffrey Snyder

G. Jeffrey Snyder is a Professor of Materials Science and Engineering at Northwestern University in Evanston, Illinois. His interests are focused on the materials physics and chemistry for thermoelectric engineering, such as band engineering, design of complex Zintl compounds and use of nanostructured composites. His interdisciplinary approach to thermoelectrics stems from studies of Solid State Chemistry at Cornell University and the Max Planck Institute for Solid State Research, Applied Physics at Stanford University and thermoelectric materials and device engineering at NASA/Jet Propulsion Laboratory (JPL) and California Institute of Technology (Caltech). He started working in thermoelectrics in 1997 after joining the Thermoelectrics group at JPL. In 2006 he started the Caltech thermoelectrics group. In 2002 he served as technical program chair for the ICT in Long Beach California. He has served on the board of ITS since 2007 and as treasurer since 2010. He has over 300 publications in thermoelectrics and mentored several students and postdocs in the field including three Goldsmid and two ITS Young Investigator Award winners.

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Introduction

Thermoelectrics as silent, reliable and sustainable energy materials are gaining increasing interest due to their potential applications for power generation and cooling technologies. Many novel thermoelectric (TE) materials including but not limited to chalcogenides, half-Heuslers, Zintls, silicides – stannides, clathrates, tetrahedrites, and oxides have been developed by manipulating the doping, electronic structure, phonon dispersion and scattering as well as microstructure. Chemistry affects all of these enabling, for example, the tuning and engineering of the electron band structure or phonon scattering. This themed issue broadly covers the chemistry aspects of thermoelectric materials including their synthesis and processing, chemical and microstructure characterization, transport measurements and theoretical modeling of their physical properties.

Chalcogenides

PbTe-based bulk materials show one of the best thermoelectric performances in the intermediate temperature range. Ohta et al. (DOI: 10.1039/c5tc01652e) synthesized n-type PbTe_1−xI_x–yMgTe (x = 0.0012–0.006; y = 0 and 1%) and investigated their microstructures. Apparently TE properties of n-type PbTe can benefit from MgTe addition having pseudo-spherical and disk shaped nanoprecipitates, which reduce the lattice thermal conductivity. Introducing MgTe also leads to a slow rate of increase in electrical resistivity through increased solubility of I and enhances the thermoelectric power factor. Bux et al. (DOI: 10.1039/c5tc01648g) introduce mechanochemically synthesized La_3−xCa_xTe₄ which is an n-type highly efficient TE material. Fine control over the carrier concentration by the substitution of trivalent La with divalent Ca was reported to modify the density of states to increase the power factor and to reach zT_max ∼ 1.2 at 1273 K for La_2.2Ca_0.78Te₄. Nolas et al. (DOI: 10.1039/c5tc01606a) highlight the recent progress in quaternary tetrahedral chalcogenides. They report the synthesis, crystal structure, and high temperature transport properties of Cu_2.1Fe_0.9SnSe₄, Cu_2.2Fe_0.8SnSe₄ and Cu_2.2Zn_0.2Fe_0.6SnSe₄ for which a zT value of 0.45 was obtained at 750 K for Cu_2.2Fe_0.8SnSe₄. Gascoin et al. (DOI: 10.1039/c5tc01766a) report on the crystal structure of single crystalline Tl(V_1−xCr_x)₅Se₈ (x = 0–1 and Δx = 0.2) with slight Tl deficiency, leading to the formation of holes as free charge carriers. Two sign changes were observed for TlV₅Se₈ suggesting simultaneous electron and hole conduction. In addition to lowering the thermal conductivity, the charge carrier concentration should be effectively tuned to maximize the power factor (σS²). Biswas et al. (DOI: 10.1039/c5tc01429h) succeed at this through introducing a Sb deficiency in p-type AgSbSe₂. In this way, the power factor values increased to ∼6.94 mW cm⁻¹ K⁻² and a zT of ∼1 at 610 K was achieved for AgSb_1−xSe₂.

As an alternative to conventional bulk thermoelectric materials, thin film nanocomposites may display a significant enhancement in TE efficiency due to quantum-confinements effects providing favourable electronic transport. Additionally, interfaces in the microstructure of the films can scatter phonons, leading to low thermal conductivity. Johnson et al. (DOI: 10.1039/c5tc01570g) designed (PbSe)_1+δ(TiSe₂)_n (1 ≤ n ≤ 18) thin-film intergrowths, in which the charge donation from PbSe to the TiSe₂ constituent becomes limited across more layers as n increases, providing a systematic increase in the Seebeck coefficient. These films display very low thermal conductivities due to structural anisotropy and misregistration between intergrowth constituents. Altering the nanoarchitecture seems to be a promising approach to achieve high TE efficiency for this class of materials.

Micro- and nanostructured composites have become one of the major fields in the thermoelectric materials research due to their low thermal conductivity and tunable electronic transport properties. Oeckler et al. (DOI: 10.1039/c5tc01509j) investigated heterostructures of germanium antimony tellurides with the nominal composition [CoSb₂(GeTe)_0.5]_x(GeTe)_10.5Sb₂Te₃ (x = 0–2) containing nano- to microscale skutterudite-type precipitates. Apparently, heterostructures with nanoscale skutterudite type precipitates decrease the thermal conductivity, which is attributed to the effective scattering of phonons at the domain boundaries without disrupting electronic transport and enhance the TE properties. Ren et al. (DOI: 10.1039/c5tc01560j) studied the effect of nickel doping on electron and phonon transport in the n-type nanostructured thermoelectric material CoSbS. They observed a two fold increase in the figure-of-merit due mainly to optimized carrier concentration, high effective mass and strong electron–phonon scattering upon Ni doping. The latter was verified by the quantitative calculation of the various phonon scattering mechanisms according to the Callaway model. Wu et al. (DOI: 10.1039/c5tc01364j) prepared Te-doped [010]-oriented Sb₂Se₃ whiskers by a self-assisted vapour–solid method displaying a layered structure stacked up by nano-sized sheets. The power factor of the Sb₂Se₃ whiskers reached ∼2 μW m⁻¹ K⁻² at room temperature, which is four orders of magnitude higher than that of the bulk material. Hence, this specially ordered material may have great potential for thermoelectric cooling device applications. Schierning et al. (DOI: 10.1039/c5tc01248a) outline a strategy based on a nanobulk fabrication process that allows decoupling of electronic and phononic transport properties in Sb₂Te₃. While electronic transport properties were tuned by the choice of the molecular precursor as well as the nanoparticle fabrication process (a single-source precursor decomposed in an ionic liquid by microwave heating), the thermal conductivity was optimized by introducing porosity by cold pressing followed by annealing in Sb₂Te₃ nanopowders. The Sb₂Te₃ nanomaterial designed in this way was reported to have dramatically enhanced zT values of up to 1.5 at 500 K. The effect of texture and concentration control on Sb₂Te₃/Ag₂Te (ST/TA) bulk composites as Pb-free p-type thermoelectric materials was investigated by Rhyee et al. (DOI: 10.1039/c5tc01623a). The low temperature wet milling process used in this research dispersed Ag₂Te particles effectively in the Sb₂Te₃ matrix. They revealed that the electrical resistivity and thermal conductivity decreased with increasing AgTe dispersion concentration. Compared to related Pb-free chalcogenides, the composite with the ST : AT ratio of 1 : 1 displays a significantly enhanced zT value of 1.5 at 700 K.

Half-Heuslers

Half-Heusler phases have attracted great interest due to their high TE efficiency. Bos et al. (DOI: 10.1039/c5tc02025e) investigated the XNiSn (X = Ti, Zr, Hf) half-Heusler alloys focusing on the accurate determination of the experimental compositions. The amount of Ni was found to have a significant impact on the thermoelectric properties. They revealed that all samples containing Ti have 2–3% excess Ni, whereas the samples with X = Zr, Hf are almost stoichiometric. In addition, they present a detailed thermoelectric analysis of degenerately and non-degenerately doped XNiSn and X_0.5X′_0.5NiSn compositions. Reducing the lattice thermal conductivity is an effective way of increasing the thermoelectric efficiency, which can be achieved by e.g., having complex crystal structures or nanostructuring process. Balke et al. (DOI: 10.1039/c5tc01196e) introduce an alternative approach by an intrinsic micrometer-scale phase separation that increases the phonon scattering and reduces the lattice thermal conductivity of the (Ti/Zr/Hf)CoSb_1−xSn_x system.

Zintl antimonides

14-1-11 Zintl phases (e.g., Yb₁₄MnSb₁₁) are one of the best high temperature p-type thermoelectric materials with tunable electronic transport behavior, along with low thermal conductivities due to their complex crystal structures. These chemically rich bulk thermoelectric materials were further investigated by Bobev et al. (DOI: 10.1039/c5tc01605c). They reported a detailed crystal structure analysis on three new compounds, Eu₁₄CdAs₁₁, Sr₁₄Cd_1.06(1)As₁₁, and Eu₁₄Cd_1.27(1)Sb₁₁ which are isostructural, but not isoelectronic with Yb₁₄MnSb₁₁. Kauzlarich et al. (DOI: 10.1039/c5tc02326b) investigated the effect of cationic substitution on the TE properties of Yb_14−xRE_xMnSb₁₁ (RE = Pr and Sm, 0 < x < 0.55). Apparently, at higher substitution levels, (Yb,RE)Sb and (Yb,RE)₁₁Sb₁₀ formed as secondary phases together with the main Zintl phase. Yb_13.82Pr_0.18Mn_1.01Sb_10.99 was reported to have a peak zT value of ∼1.2 at 1275 K. Aydemir et al. (DOI: 10.1039/c5tc01645b) present the TE properties of Zn-doped Eu₅In_2−xZn_xSb₆ (x = 0, 0.025, 0.05, 0.1, 0.2) Zintl compounds. A possible two band behavior was suggested for the parent compound by an observed increase in the band mass as a function of the Zn content. In addition, the p-type Hall mobility of Eu₅In₂Sb₆ was found to be much larger than that of the alkaline earth containing A₅In₂Sb₆ phases (A = Sr, Ca), in agreement with the reduced hole effective mass. TE materials in the Zn–Sb system are attractive because they contain cheap, earth abundant and non-toxic elements. Besides Zn₄Sb₃, ZnSb is another promising TE material which is hard to produce as a single phase due to the high vapor pressure of Zn. Iversen et al. (DOI: 10.1039/c5tc01611h) outline a direct large scale, one step spark plasma sintering synthesis of ZnSb pellets from Zn and Sb powders. Besides producing phase pure pellets in this method, they revealed that ZnSb is not a line phase as commonly reported in the binary Zn–Sb phase diagrams.

Silicides – stannides

One of the important aspects of thermoelectric materials is their thermal stability. Tang et al. (DOI: 10.1039/c5tc01434d) tested Sb-doped Mg₂Si_0.3Sn_0.7 solid solutions under various heat treatment conditions, e.g., annealing temperature, annealing time, annealing atmosphere and preventive boron nitride (BN) coatings. They revealed that annealing samples in vacuum and in air results in Mg loss and oxidation, respectively, which can be effectively prevented with BN-coating. Boor et al. (DOI: 10.1039/c5tc01535a) prepared an optimized Sb doped solid solution of magnesium silicide and stannide, leading to a zT value of ∼0.95 at 740 K. This material is composed of several phases and exhibits intrinsic nanostructuring, demonstrating a drastically reduced thermal conductivity and an increased effective mass, compared to Mg₂Si. Shi et al. (DOI: 10.1039/c5tc01536g) synthesized Re-substituted higher manganese silicides (HMS), comprising relatively small-sized MnSi platelets. They reported that the reduced lateral size of MnSi in the quenched sample resulted in an increased carrier concentration without degrading the carrier mobility and increasing the thermal conductivity. The thermoelectric properties of an intermetallic compound Al_6−xRe_4.7Si_4+x (x = 0–0.9) were reported by Takagiwa et al. (DOI: 10.1039/c5tc01608h). A crossover from p- to n-type conduction was observed at a very high Si content (x = 0.9), which was attributed to the shift of the Fermi level to the conduction band. This phase suffers from a simple crystal structure leading to high thermal conductivity.

Clathrates

Clathrates with phonon-glass electron-crystal properties are promising TE materials. Keiber et al. (DOI: 10.1039/c5tc01641j) applied an extended X-ray fine structure (EXAFS) analysis of the type-I clathrates Ba₈Ga₁₆X₃₀ (X = Si, Sn) and compared the results with other studies on X = Ge. As reported previously, they verified the tendency for the avoidance of a Ga–Ga bond in the crystal structure. Additionally, a substantial buckling of the cages was identified, particularly for Ba₈Ga₁₆Sn₃₀ along with much distorted Ba environments, leading to a higher resistivity and a lower zT than Ba₈Ga₁₆Ge₃₀.

Tetrahedrites

Tetrahedrites, a class of natural minerals, have attracted attention due to their low cost and high thermoelectric efficiency. Bouyrie et al. (DOI: 10.1039/c5tc01636c) synthesized Te substituted tetrahedrite phase Cu₁₂Sb_4−xTe_xS₁₃ (0.5 ≤ x ≤ 2.0) by two synthetic routes, from precursors and from the direct melting of the elements, with off-stoichiometry in the Sb site. A transition from p-type metallic to semiconducting behavior was observed depending on the Te content. An optimized power factor with Te substitution along with low thermal conductivities (κ = 0.5 W m⁻¹ K⁻¹) result in a maximum value of zT ∼ 0.8 at 623 K for this quaternary tetrahedrite.

Oxides

TE oxides comprised of less toxic elements have generated renewed interest as promising thermoelectric materials. Although their thermoelectric efficiencies are inferior compared to other state of the art materials, they are very durable under ambient operational conditions. Terasaki et al. (DOI: 10.1039/c5tc01619c) present the unconventional thermoelectric properties of polycrystalline Li₂Ru_1−xIr_xO₃ and Li₂Ru_1−xTi_xO₃. This research paper proposes that phase transition control by impurity substitution (Ir and Ti substitution in Ru site) is beneficial for good thermoelectric properties in strongly correlated electron systems.

Phonon engineering

Strategies for suppressing the phonon thermal conductivity in thermoelectric materials are discussed in the review article by Kim (DOI: 10.1039/c5tc01670c) by considering each component of phonon thermal transport, such as specific heat, phonon group velocity and mean free path.

High throughput screening

The Materials Project (http://www.materialsproject.org) became a community resource for density functional theory electronic band structures, phase diagram prediction and other applications. Zhu et al. (DOI: 10.1039/c5tc01440a) introduce a new group of thermoelectric materials, trigonal and tetragonal XYZ₂ (X, Y: rare earth or transition metals, Z: group VI elements) by means of high-throughput computational screening. This family of thermoelectric materials show extremely low thermal conductivity (e.g., 0.2–0.3 W m⁻¹ K⁻¹ for T > 600 K for trigonal TmAgTe₂). Due to X_Y antisite defects which act as hole killers, low hole carrier concentration was observed which limits the TE efficiencies.

Applications

Powering wearable devices without a battery is an important application area of thermoelectric generators, harvesting energy from the human body. In the Application article by Bahk et al. (DOI: 10.1039/c5tc01644d), a comprehensive review is given on the recent advances in the development of flexible thermoelectric materials (such as polymer-based materials and screen-printed paste-type inorganic materials) and devices for wearable body-heat energy harvesting applications. Additionally, the feasibility of the scalable and cost-effective manufacturing of thermoelectric energy harvesting devices is discussed with desired dimensions. Kao et al. (DOI: 10.1039/c5tc01662b) investigated reliable bonding materials for PbTe-based thermoelectric modules durable for long-term operations at high temperatures. The results show that neither Cu nor Ag are good bonding materials as they both react vigorously (Cu diffuses faster than Ag) with Pb_0.6Sn_0.4Te. To prevent Cu diffusion into PbTe, the use of a diffusion barrier is proposed.

We would like to express our sincere thanks and appreciation to all the coauthors of the invited articles for their invaluable contributions to this themed issue and in general to the progressive research on thermoelectric materials and applications. We also thank the editorial and production staff of J. Mater. Chem. C for their outstanding assistance.

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