Development of novel thermoelectric materials by reduction of lattice thermal conductivity

Abstract

Thermal conductivity is one of the key parameters in the figure of merit of thermoelectric materials. Over the past decade, most progress in thermoelectric materials has been made by reducing their thermal conductivity while preserving their electrical properties. The phonon scattering mechanisms involved in these strategies are reviewed here and divided into three groups, including (i) disorder or distortion of unit cells, (ii) resonant scattering by localized rattling atoms and (iii) interface scattering. In addition, we propose construction of a ‘natural superlattice’ in thermoelectric materials by intercalating an MX layer into the van der Waals gap of a layered TX₂ structure which has a general formula of (MX)_1+x(TX₂)_n (M=Pb, Bi, Sn, Sb or a rare earth element; T=Ti, V, Cr, Nb or Ta; X=S or Se and n=1, 2, 3). We demonstrate that one of the intercalation compounds (SnS)_1.2(TiS₂)₂ has better thermoelectric properties compared with pure TiS₂ in the direction parallel to the layers, as the electron mobility is maintained while the phonon transport is significantly suppressed owing to the reduction in the transverse phonon velocities.     

Electrical conduction in some (CnH2n) (NH 3 + )2 FeCl4 compounds

The d.c. conductivity, a.c. conductivity and thermoelectric power of the compounds H₃N₊(CH₂)₊ NH₃FeCl₄, wheren=2, 3, 7 and 10, have been studied over a temperature range of 150–500 K. The conductivity results confirm the presence of more than one structural phase transition for each compound investigated. The thermoelectric power measurements showed that electrons are the main charge carriers in all crystal phases. The conductivity results were explained on the basis of an electron hopping mechanism over the whole temperature range.     

Metallic n-Type Mg3Sb2 Single Crystals Demonstrate the Absence of Ionized Impurity Scattering and Enhanced Thermoelectric Performance

Mg₃(Sb,Bi)₂ alloys have recently been discovered as a competitive alternative to the state-of-the-art n-type Bi₂(Te,Se)₃ thermoelectric alloys. Previous theoretical studies predict that single crystals Mg₃(Sb,Bi)₂ can exhibit higher thermoelectric performance near room temperature by eliminating grain boundary resistance. However, the intrinsic Mg defect chemistry makes it challenging to grow n-type Mg₃(Sb,Bi)₂ single crystals. Here, the first thermoelectric properties of n-type Te-doped Mg₃Sb₂ single crystals, synthesized by a combination of Sb-flux method and Mg-vapor annealing, is reported. The electrical conductivity and carrier mobility of single crystals exhibit a metallic behavior with a typical T^−1.5 dependence, indicating that phonon scattering dominates the charge carrier transport. The absence of any evidence of ionized impurity scattering in Te-doped Mg₃Sb₂ single crystals proves that the thermally activated mobility previously observed in polycrystalline materials is caused by grain boundary resistance. Eliminating this grain boundary resistance in the single crystals results in a large enhancement of the weighted mobility and figure of merit zT by more than 100% near room temperature. This work experimentally demonstrates the accurate understanding of charge-carrier scattering is crucial for developing high-performance thermoelectric materials and indicates that single-crystalline Mg₃(Sb,Bi)₂ solid solutions can exhibit higher zT compared to polycrystalline samples.     

The effects of bismuth intercalation on structure and thermal conductivity of TiS2

The structure and thermal conductivity of the bismuth (Bi) intercalated compounds Bi_xTiS₂ (0 ≤ x ≤ 0.25) were investigated by using X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) and thermal conductivity measurements. The results indicated that besides lattice expansion and distortion, bismuth intercalation caused structural transition of Bi_xTiS₂ from stage-1 to stage-2 as x ≥ ∼0.1, which led to the appearance of D₄ and A₂ modes in Raman spectra. The enhancement of relative intensities of D₄ and A₂ peaks with increasing Bi content reflected increase of the concentration of stage-2 phase in the samples. The red shift of mode E_g as well as D₄ and A₂ would reflect weakening of intra-layer bonds, while the blue shift of A_1g after intercalation suggested the enhancement of chemical binding in the van der Waals gaps due to charge transfer. In addition, the weakening of A_1g intensity can be explained by the lattice distortion produced by bismuth intercalation. Remarkable reduction in (lattice) thermal conductivity of titanium disulfide (TiS₂) through Bi intercalation was realized, which could be attributed to the phonon scattering by “rattling” of the intercalated bismuth atoms in the van der Waals gaps of TiS₂.     

Revisiting some chalcogenides for thermoelectricity

Abstract

Thermoelectric materials that are efficient well above ambient temperature are needed to convert waste-heat into electricity. Many thermoelectric oxides were investigated for this purpose, but their power factor (PF) values were too small (～10^?4 W m^?1 K^?2) to yield a satisfactory figure of merit zT. Changing the anions from O^2? to S^2? and then to Se^2? is a way to increase the covalency. In this review, some examples of sulfides (binary Cr–S or derived from layered TiS₂) and an example of selenides, AgCrSe₂, have been selected to illustrate the characteristic features of their physical properties. The comparison of the only two semiconducting binary chromium sulfides and of a layered AgCrSe₂ selenide shows that the PF values are also in the same order of magnitude as those of transition metal oxides. In contrast, the PF values of the layered sulfides TiS₂ and Cu_0.1TiS₂ are higher, reaching ～10^?3 W m^?1 K^?2. Apparently the magnetism related to the Cr–S network is detrimental for the PF when compared to the d⁰ character of the Ti⁴⁺ based sulfides. Finally, the very low PF in AgCrSe₂ (PF = 2.25 × 10^?4 W m¹ K^?2 at 700 K) is compensated by a very low thermal conductivity (κ = 0.2 W m^?1 K^?1 from the measured C_p) leading to the highest zT value among the reviewed compounds (zT_700K = 0.8). The existence of a glassy-like state for the Ag⁺ cations above 475 K is believed to be responsible for this result. This result demonstrates that the phonon engineering in open frameworks is a very interesting way to generate efficient thermoelectric materials.     

Effects of Se substitution on the thermoelectric performance of n-type Co4Sb11.3Te0.7−xSex skutterudites

A series of double-substituted Co₄Sb_11.3Te_0.7?xSe_x skutterudites have been fabricated by combining the solid state reaction and the spark plasma sintering method, and the effects of Se substitution on the thermoelectric properties are characterized by measurements of the electrical conductivity, the Seebeck coefficient and the thermal conductivity in the temperature range of 300–800 K. Doping Se into the Co₄Sb_11.3Te_0.7?xSe_x matrix suppresses the carrier concentration, and the electrical conductivity actually decreases with the Se content. However, moderate Se doping is effective in enhancing the thermoelectric performance of the n-type Co₄Sb_11.3Te_0.7?xSe_x, because of the resulted dramatically decreased thermal conductivity. Analyses indicate that the heightened point-defect scattering induced by Se doping together with the electron–phonon scattering induced by Te doping is responsible for the reduction of lattice thermal conductivity of these compounds.     

Crystal growth and thermoelectric properties of tetradymite-like layered chalcogenides and (Bi2Te3)1−x−y (Sb2Te3) x (Sb2Se3) y solid solutions

Thermoelectric materials for segmented n-and p-legs of thermoelectric generators have been prepared by Czochralski growth with melt supply through a floating crucible. Two-segment ingots have been obtained using melt compositions corresponding to ternary layered compounds in the PbTe-Bi₂Te₃ and PbTe-Sb₂Te₃ systems, with (Bi₂Te₃)_1?x?y (Sb₂Te₃) _x (Sb₂Se₃) _y solid solutions as seed materials. Seeded growth of the ternary compounds makes it possible to fabricate legs without joining segments by soldering. Using scanning hot point microprobe measurements, we have studied the thermoelectric power distribution across the seed-crystal interface. The results attest to a steep thermoelectric power gradient across the seed-crystal interface in a narrow region. Quantitative analysis of the distribution of the number of measurements with respect to thermoelectric power has revealed peaks corresponding to individual segments.     

Thermoelectric Properties of Layered Compounds and Multicomponent Solid Solutions in the Pseudoternary System Sb<Subscript>2</Subscript>Te<Subscript>3</Subscript>–PbTe–Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript>

The thermoelectric properties of layered compounds and solid solutions in the Sb₂Te₃–PbTe–Bi₂Te₃ system have been studied in a wide temperature range. The quaternary compounds and multicomponent solid solutions in this system have been shown to have a very low lattice thermal conductivity. The electrical conductivity of all the materials decreases with increasing temperature. All of the materials have been shown to be n-type. The layered compounds and multicomponent solid solutions have high thermoelectric efficiency, which makes them promising n-type thermoelectric materials.     

Fabrication of ultrafine powder using processing control agent,and investigation of its effect on microstructure and thermoelectric properties of p-type (Bi,Sb)2Te3 alloys

Low-dimensional materials can significantly enhance the efficiency of thermoelectric devices for power generation and cooling applications. In the present work, ultra-fine powders of p-type (Bi, Sb)₂Te₃ alloys are fabricated through high energy ball milling using stearic acid as a process control agent (PCA). The influence of the PCA addition on powder characteristics, microstructure and thermoelectric transport properties are studied. Further, the ultra-fine powder is subjected to calcination (Cal-PCA) and subsequently consolidated all powders using spark plasma sintering (SPS). The PCA, Cal-PCA, and non-PCA powder morphological effects on the microstructure and thermoelectric properties are systematically investigated and elucidated thoroughly. The electron beam scattering diffraction (EBSD) results confirmed that the PCA sample exhibited very fine grains (average grain size of ～800 nm) compared to the non-PCA (average grain size of about 2.6 µm), while the grains were distributed randomly for all samples. Formation of fine grains and partial existence of the stearic acid (carbon and oxygen phases) in the matrix were strongly inhibiting the transport of the carriers that severely decreased the carrier mobility, reflecting the severe reduction in electrical conductivity for PCA sample compared to Cal-PCA and non-PCA. The lowest thermal conductivity (κ) of 0.745 W/mK was achieved for the PCA sample, which is 19%, 12% lower than that of non-PCA, and Cal-PCA samples. The strong reduction in κ was mainly attributed to the dramatic decrease in the phonon thermal conductivity owing to phonon scattering at numerous grain boundaries and oxide phases. The obtained high electrical conductivity with balanced thermal conductivity in Cal-PCA sample is attributed to the significant improvement in ZT of 1.1, which is 27%, and 47% higher than that of the Non-PCA sample at room temperature, and 350 K, respectively.     

Synthesis and properties of Na<Subscript> <Emphasis Type="Italic">x</Emphasis> </Subscript>CoO<Subscript>2</Subscript> (<Emphasis Type="Italic">x</Emphasis> = 0.55, 0.89) oxide thermoelectrics

Na_xCoO₂ (x = 0.55, 0.89) sodium cobaltites have been prepared by solid-state reactions; their structural parameters have been determined; their microstructure has been studied; and their thermal (thermal expansion, thermal diffusivity, and thermal conductivity), electrical (electrical conductivity and thermoelectric power), and functional (power factor, thermoelectric figure of merit, and self-compatibility factor) properties have been investigated in air at temperatures from 300 to 1100 K. The results demonstrate that, with increasing sodium content, the electrical conductivity and thermoelectric power of the materials increase and their thermal conductivity decreases. As a result, the power factor and thermoelectric figure of merit of the Na_0.89CoO₂ ceramic at a temperature of 1100 K reach 0.829 mW/(m K²) and 1.57, respectively. The electron and phonon (lattice) contributions to the thermal conductivity of the ceramics have been separately assessed, and their linear thermal expansion coefficients have been evaluated.     

Charge transport and mechanisms of electron scattering in (HgSe)3(In2Se3) crystals doped with 3d transition metals

We have studied the transport properties of (HgSe)₃(In₂Se₃), (HgSe)₃(In₂Se₃)〈Mn〉, and (HgSe)₃(In₂Se₃)〈Fe〉 crystals and identified the predominant mechanisms of electron scattering in them. The transport properties of the (HgSe)₃(In₂Se₃) crystals, undoped and doped with 3d transition metals, have been studied by the four-probe technique. The results demonstrate that the Hall coefficient of the crystals is temperature-independent, their electrical conductivity shows metallic behavior and is an almost linear function of temperature, and their thermoelectric power increases with increasing temperature.     

The enhanced thermoelectric properties of Ba0.25Pb0.05Co4Sb11.5Te0.5 alloys prepared by HPHT at different pressure

Skutterudite Ba_0.25Pb_0.05Co₄Sb_11.5Te_0.5 compounds with bcc crystal structure were prepared by high-pressure and high-temperature (HTHP) method. We have studied the structure and thermoelectric properties of Te-substituted partially filled Ba_0.25Pb_0.05Co₄Sb_11.5Te_0.5 skutterudites which were synthesized at different pressure. The electrical resistivity, Seebeck coefficient and thermal conductivity of those samples were measured in the temperature range of 320-720 K. The combination of Ba and Pb fillers inside the voids of the skutterudite structure provided a broad range of resonant phonon scattering, so that a strong suppression in the lattice thermal conductivity was observed. A dimensionless thermoelectric figure of merit of 1.0 at 720 K was achieved for n-type Ba_0.25Pb_0.05Co₄Sb_11.5Te_0.5 at last.     

Decoupled phononic-electronic transport in multi-phase n-type half-Heusler nanocomposites enabling efficient high temperature power generation

Strongly coupled electronic and thermal transport behavior in thermoelectric (TE) materials has limited their figure of merit (zT). Here we provide breakthrough in decoupling TE parameters in n-type (Hf_0.6Zr_0.4)NiSn_0.99Sb_0.01 half-Heusler (hH) alloys through multi-scale nanocomposite architecture comprising of tungsten nanoinclusions. The tungsten nanoparticles not only assist electron injection, thereby improving electrical conductivity, but also enhance the Seebeck coefficient through energy filtering effect. The microstructure comprises of disordered phases with varying size of microstructural features, which assists in effective scattering of heat-carrying phonons over diverse mean-free-path ranges. Cumulatively, these effects are shown to result in outstanding thermoelectric performance of zT_max ∼ 1.4 at 773 K and zT_avg ∼ 0.93 between 300 and 973 K. Using this material, a TE generator is demonstrated, which exhibits high power density of 13.93 W cm⁻² and conversion efficiency of 10.7% under ΔT = 674 K. The fundamental material design principle for TE nanocomposites demonstrated here can be generalized and extended to other TE systems.     

Lattice Distortions and Multiple Valence Band Convergence Contributing to High Thermoelectric Performance in MnTe

Here, a new route is proposed for the minimization of lattice thermal conductivity in MnTe through considerable increasing phonon scattering by introducing dense lattice distortions. Dense lattice distortions can be induced by Cu and Ag dopants possessing large differences in atom radius with host elements, which causes strong phonon scattering and results in extremely low lattice thermal conductivity. Density functional theory (DFT) calculations reveal that Cu and Ag codoping enables multiple valence band convergence and produces a high density of state values in the electronic structure of MnTe, contributing to the large Seebeck coefficient. Cu and Ag codoping not only optimizes the Seebeck coefficient but also substantially increases the carrier concentration and electrical conductivity, resulting in the significant enhancement of power factor. The maximum power factor reaches 11.36 µW cm⁻¹K⁻² in Mn_0.98Cu_0.04Ag_0.04Te. Consequently, an outstanding ZT of 1.3 is achieved for Mn_0.98Cu_0.04Ag_0.04Te by these synergistic effects. This study provides guidelines for developing high-performance thermoelectric materials through the rational design of effective dopants.     

Effect of melt-spun powder additions on the thermoelectric properties of bismuth and antimony chalcogenides

We have studied the thermoelectric properties of p-type Bi_0.5Sb_1.5Te₃ and n-type Bi₂Te_2.4Se_0.6 solid solutions prepared by vacuum hot pressing of mixtures of powders differing in particle size composition. The powders were prepared by mechanically grinding ingots and by ultrarapid melt cooling (melt spinning). Fracture surfaces of samples were examined by scanning electron microscopy. The samples consisted of large, layered particles of the major component (up to hundreds of microns in size) and small flakes (a few to tens of microns in size) of the component prepared by melt spinning. The microstructure of the materials was examined under an optical microscope. On grain boundaries in the p-type materials, we observed telluriumbased eutectic precipitates in the form of a white phase. In some of the n-type samples, the Bi₂Te_2.4Se_0.6 solid solution was found to undergo ordering, resulting in the formation of the ternary compound Bi₂Te₂Se. The Seebeck coefficient, electrical conductivity, and thermal conductivity of the materials were measured in the temperature range 100–700 K. The addition of 40 wt % powder prepared by melt spinning to hot-pressed p-type samples was shown to have no effect on their thermoelectric figure of merit (ZT)_max, which was 1.0 at 350 K. On the addition of 20 wt % powder prepared by melt spinning, we obtained (ZT)_max = 1.1 at 550 K in a hot-pressed n-type sample.     

Engineering Multiple Microstructural Defects for Record-Breaking Thermoelectric Properties of Chalcopyrite Cu1-xAgxGaTe2

Defect engineering for vacancies, holes, nano precipitates, dislocations, and strain are efficient means of suppressing lattice thermal conductivity. Multiple microstructural defects are successfully designed in Cu_1-xAg_xGaTe₂ (0 ≤ x ≤ 0.5) solid solutions through high-ratio alloying and vibratory ball milling, to achieve ultra-low thermal conductivity and record-breaking thermoelectric performance. Extremely low total thermal conductivities of 1.28 W m⁻¹ K⁻¹ at 300 K and 0.40 W m⁻¹ K⁻¹ at 873 K for the Cu_0.5Ag_0.5GaTe₂ are observed, which are ≈79% and ≈58% lower than that of the CuGaTe₂ matrix. Multiple phonon scattering mechanisms are collectively responsible for the reduction of thermal conductivity in this work. On one hand, large amounts of nano precipitates and dislocations are formed via vibrating ball milling followed by the low-temperature hot press, which can enhance phonon scattering. On the other hand, the difference in atomic sizes, distorted chemical bonds, elements fluctuation, and strained domains are caused by the high substitution ratio of Ag and also function as a center for the strong phonon scattering. As a result, the Cu_0.7Ag_0.3GaTe₂ exhibits a record high ZT_max of ≈1.73 at 873 K and ZT_ave of ≈0.69 between 300–873 K, which are the highest values of CuGaTe₂-based thermoelectric materials.     

Nanostructure design for drastic reduction of thermal conductivity while preserving high electrical conductivity

The design and fabrication of nanostructured materials to control both thermal and electrical properties are demonstrated for high-performance thermoelectric conversion. We have focused on silicon (Si) because it is an environmentally friendly and ubiquitous element. High bulk thermal conductivity of Si limits its potential as a thermoelectric material. The thermal conductivity of Si has been reduced by introducing grains, or wires, yet a further reduction is required while retaining a high electrical conductivity. We have designed two different nanostructures for this purpose. One structure is connected Si nanodots (NDs) with the same crystal orientation. The phonons scattering at the interfaces of these NDs occurred and it depended on the ND size. As a result of phonon scattering, the thermal conductivity of this nanostructured material was below/close to the amorphous limit. The other structure is Si films containing epitaxially grown Ge NDs. The Si layer imparted high electrical conductivity, while the Ge NDs served as phonon scattering bodies reducing thermal conductivity drastically. This work gives a methodology for the independent control of electron and phonon transport using nanostructured materials. This can bring the realization of thermoelectric Si-based materials that are compatible with large scale integrated circuit processing technologies.     

Lattice Dislocations Enhancing Thermoelectric PbTe in Addition to Band Convergence

Phonon scattering by nanostructures and point defects has become the primary strategy for minimizing the lattice thermal conductivity (κ_L) in thermoelectric materials. However, these scatterers are only effective at the extremes of the phonon spectrum. Recently, it has been demonstrated that dislocations are effective at scattering the remaining mid‐frequency phonons as well. In this work, by varying the concentration of Na in Pb_0.97Eu_0.03Te, it has been determined that the dominant microstructural features are point defects, lattice dislocations, and nanostructure interfaces. This study reveals that dense lattice dislocations (≈4 × 10¹² cm^?2) are particularly effective at reducing κ_L. When the dislocation concentration is maximized, one of the lowest κ_L values reported for PbTe is achieved. Furthermore, due to the band convergence of the alloyed 3% mol. EuTe the electronic performance is enhanced, and a high thermoelectric figure of merit, zT , of ≈2.2 is achieved. This work not only demonstrates the effectiveness of dense lattice dislocations as a means of lowering κ_L, but also the importance of engineering both thermal and electronic transport simultaneously when designing high‐performance thermoelectrics.     

Phase diagram of the TlSe-SmSe system and transport properties of TlSmX<Subscript>2</Subscript> (X = S,Se, Te) crystals

We have studied phase equilibria in the TlSe-SmSe system and showed that it contains a congruently melting compound of composition TlSmSe₂ (1: 1 ratio of the constituent selenides) and a TlSe-based solid solution series. The electrical and thermophysical properties of TlSmX₂ (X = S, Se, Te) crystals have been investigated, and their conductivity type, band gap, thermal conductivity, and thermoelectric power have been determined. The results are used to clarify the mechanisms of carrier and phonon scattering in the crystals.     

Lattice dynamic study on 3d transition metal intercalates M x TiS2 (M=Mn,Fe, Co,and Ni) using point-contact spectroscopy

The derivatives of the current-voltage characteristics of a homojunction point contact of 1T-CdI₂-type layered crystal TiS₂ and its intercalation compounds M_1/4TiS₂ (M=Mn, Fe, Co, and Ni) have been measured at 1.4 K. With reference to the available data on lattice dynamics, we have identified various inter- and intralayer acoustic and optical phonon modes. The acoustic phonon modes are strongly anisotropic compared with the optical ones in these materials. The variations of the acoustic phonon energies upon intercalation of 3d metals are strongly correlated with those of the interlayer spacingc, for which qualitative discussions are given.