Thermoelectric Properties of <Emphasis Type="Italic">n</Emphasis>-Type Multiple-Filled Skutterudites

Filled skutterudites are prospective intermediate temperature materials for␣thermoelectric power generation. CoSb₃-based n-type filled skutterudites have good electrical transport properties with power factor values over 40 μW/cm K² at elevated temperatures. Filling multiple fillers into the crystallographic voids of skutterudites would help scatter a broad range of lattice phonons, thus resulting in lower lattice thermal conductivity values. We report the thermoelectric properties of n-type multiple-filled skutterudites between 5 K and 800 K. The combination of different fillers inside the voids of the skutterudite structure shows enhanced phonon scattering, and consequently a strong suppression of the lattice thermal conductivity. Very good power factor values are achieved in multiple-filled skutterudite compared with single-element-filled materials. The dimensionless thermoelectric figure of merit for n-type filled skutterudites is improved through multiple-filling in a wide temperature range.     

Thermoelectric Performance Optimization in p-Type Ce y Fe3CoSb12 Skutterudites

In this study, we investigated the impact of the Ce filling fraction on the thermoelectric properties of p-type filled skutterudites Ce _y Fe₃CoSb₁₂ (y = 0.6 to 1.0). The electrical conductivity decreases gradually with increasing y, while the Seebeck coefficient displays an opposite variation tendency, consistent with the expected electron donor role of the Ce filler in this compound. The overall power factors are invariable among all the samples. Alteration of the Ce filling fraction exerts little influence on the phonon transport, but the total thermal conductivity markedly declined with increasing y due to the reduced contribution to heat transfer from carriers. As a consequence, the maximum thermoelectric figure of merit ZT reaches ～0.8 for the sample with y = 0.9, comparable to that of pure Fe-based skutterudite CeFe₄Sb₁₂; more importantly, the former possesses a much larger average ZT between 300 K and 800 K than the latter, showing superior potential for use in intermediate-temperature thermoelectric power generation applications. Further enhancement of ZT in p-type Fe₃CoSb₁₂-based skutterudites could be realized via nanostructuring or a multiple-filling approach.     

Synthesis and Thermoelectric Properties of the Double-Filled Skutterudite Yb<Subscript>0.2</Subscript>In<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript>Co<Subscript>4</Subscript>Sb<Subscript>12</Subscript>

Filled skutterudites have long been singled out as one of the prime examples of phonon glass electron crystal materials. Recently the double-filling approach in these materials has been attracting increased attention. In this study, Yb_0.2In _y Co₄Sb₁₂ (y = 0.0 to 0.2) samples have been prepared by a simple melting method and their thermoelectric properties have been investigated. The power factor is increased dramatically when increasing the In content, while the lattice thermal conductivity is lowered considerably, leading to a large increase of the ZT value. A state-of-the-art ZT value of 1.0 is attained in Yb_0.2In_0.2Co₄Sb₁₂ at 750 K.     

Low Thermal Conductivity and Enhanced Thermoelectric Performance in In and Lu Double-Filled CoSb<Subscript>3</Subscript> Skutterudites

The thermoelectric properties of indium (In) and lutetium (Lu) double-filled skutterudites In _x Lu _y Co₄Sb₁₂ prepared by high-pressure synthesis were investigated in detail from 4 K to 365 K. Our results indicate that In and Lu double filling can remarkably reduce the thermal conductivity, and substantially improve the thermoelectric performance. A thermoelectric figure of merit of ZT = 0.27 for In_0.13Lu_0.05Co_4.02Sb₁₂ was achieved at 365 K, being larger by one order of magnitude than that for CoSb₃. It is thought that the large difference in resonance frequencies of the In and Lu elements broadens the range of normal phonon scattering in the multifilled skutterudites, helping to achieve an even lower lattice thermal conductivity. This investigation suggests that an effective way to improve the thermoelectric performance of skutterudite materials is to use In and Lu double filling.     

Microstructure and Thermoelectric Properties of Yb-Filled Skutterudites Prepared by Rapid Solidification

Single-phase nanostructured bulk Yb_0.2Co₄Sb₁₂ skutterudites have been prepared by combining a melt spinning technique with spark plasma sintering. The effects of a pre-annealing process on the microstructure and phase composition of ribbon samples and bulk materials are investigated. After the pre-annealing process, average grain size increases from 200 nm to 300 nm for ribbon samples and from 250 nm to 350 nm for bulk materials, and nearly single-phase skutterudites have formed. Because of the nanostructure, the thermal conductivity of bulk skutterudites notably decreases 25% at 800 K. As␣a result, the ZT values are improved compared with starting material prepared by the traditional method.     

Thermoelectric Properties of Skutterudites Co4−x Ni x Sb11.9−y Te y Se0.1

CoSb₃-based skutterudites with substitution of Ni atoms for Co, and substitution of Te and Se atoms for Sb were successfully prepared by solid-state reaction and spark plasma sintering. According to x-ray diffraction analysis the major phase of all the samples had a CoSb₃-type structure, although back-scattered electron images showed that small amounts of impurity phases were present in all the samples. The temperature-dependent transport properties were characterized over the temperature range 300–800 K for all the samples. It was found that appropriate substitution with Ni, Te, and Se effectively improved the power factor and reduced the thermal conductivity. As a result, Ni, Te, and Se-tri-doped CoSb₃ materials with enhanced thermoelectric figures of merit, ZT, were obtained. The highest ZT was greater than 1.1 at high temperature.     

Bulk Nanostructured Thermoelectric Materials: Preparation,Structure and Properties

Bulk nanostructured materials have recently emerged as a new paradigm for improving the performance of existing thermoelectric materials. Here, we fabricated two kinds of bulk nanostructured thermoelectric materials by a bottom-up strategy and an in situ precipitation method, respectively. Binary PbTe was fabricated by a combination of chemical synthesis and hot pressing. The grain sizes of the hot pressed bulk samples varied from 200 nm to 400 nm, which significantly contributed to the reduction of thermal conductivity due to the enhanced boundary phonon scattering. The highest figure of merit ZT of the binary PbTe sample reached 0.8 at 580 K. Mg₂(Si,Sn) solid solutions have shown great promise for thermoelectric application, due to good thermoelectric properties, non-toxicity, and abundantly available constituent elements. The nanoscale microstructure observation of the compounds showed the existence of nanophases formed in situ, which is believed to be related to the relatively low lattice thermal conductivity in this material system. The highest ZT of Sb-doped Mg₂(Si,Sn) samples reached 1.1 at 770 K.     

Effects of YSZ Additions on Thermoelectric Properties of Nb-Doped Strontium Titanate

The effect of yttria-stabilized zirconia (YSZ) with a low thermal conductivity on the thermoelectric properties of Nb-doped SrTiO₃ bulk materials, which were fabricated by the conventional normal pressure sintering method in an Ar atmosphere, was examined. YSZ additions reduced the thermal conductivity but significantly enhanced the electrical conductivity. However, the Seebeck coefficient was nearly independent of YSZ content. Thus, the ZT value was enhanced, and a sample with 3 wt.% YSZ displayed the maximum ZT value, 0.21, at 900 K. Additionally, the reason for the reduced thermal conductivity and enhanced electrical conductivity by YSZ additions was investigated in detail.     

Engineering Thermal Conductivity for Balancing Between Reliability and Performance of Bulk Thermoelectric Generators

The nanostructuring approach has significantly contributed to the improving of thermoelectric figure‐of‐merit (ZT) by reducing lattice thermal conductivity. Even though it is an effective method to enhance ZT, the drastically lowered thermal conductivity in some cases can cause thermomechanical issues leading to decreased reliability of thermoelectric generators. Here, an engineering thermal conductivity (κ_eng) is defined as a minimum allowable thermal conductivity of a thermoelectric material in a module, and is evaluated to avoid thermomechanical failure and thermoelectric degradation of a device. Additionally, there is dilemma of determining thermoelectric leg length: a shorter leg is desired for higher W kg^?1, W cm^?3, and W The nanostructuring approach has significantly contributed to the improving of thermoelectric figure‐of‐merit (ZT) by reducing lattice thermal conductivity. Even though it is an effective method to enhance ZT, the drastically lowered thermal conductivity in some cases can cause thermomechanical issues leading to decreased reliability of thermoelectric generators. Here, an engineering thermal conductivity (κ_eng) is defined as a minimum allowable thermal conductivity of a thermoelectric material in a module, and is evaluated to avoid thermomechanical failure and thermoelectric degradation of a device. Additionally, there is dilemma of determining thermoelectric leg length: a shorter leg is desired for higher W kg^?1, W cm^?3, and W $^?1, but it raises the thermomechanical vulnerability issue. By considering a balance between the thermoelectric performance and thermomechanical reliability issues, it is discussed how to improve device reliability of thermoelectric generators and the engineering thermal conductivity of thermoelectric materials.     

Development of Skutterudite Thermoelectric Materials and Modules

Multifilling with La, Ba, Ga, and Ti in p-type skutterudite and Yb, Ca, Al, Ga, and In in n-type skutterudite remarkably reduces their thermal conductivity, resulting in enhancement of their dimensionless figure of merit ZT to ZT?=?0.75 for p-type (La,Ba,Ga,Ti)₁(Fe,Co)₄Sb₁₂ and ZT?=?1.0 for n-type (Yb,Ca,Al,Ga,In)_0.7(Co,Fe)₄Sb₁₂. A thermoelectric module technology suitable for these skutterudites including diffusion barrier and electrode materials has been established. The diffusion barrier materials allow the electrode to coexist stably with the p/n skutterudites in the module??s working temperature range of room temperature to 600°C. Under conditions of hot/cold-side temperatures of 600°C/50°C, a skutterudite module with size of 50?mm?×?50?mm?×?7.6?mm exhibited generation performance of 32?W power output and 8% thermoelectric conversion efficiency.     

Thermoelectric Properties of Triple-Filled BaxYbyInzCo4Sb12 Skutterudites

Indium-filled skutterudites are promising power generation thermoelectric materials due to the presence of an InSb nanostructure that lowers the thermal conductivity. In this work, we have investigated thermoelectric properties of triple-filled Ba _x Yb _y In _z Co₄Sb₁₂ (0 ≤ x, y, z ≤ 0.14 actual) compounds by measuring their Seebeck coefficient, electrical conductivity, thermal conductivity, and Hall coefficient. All samples were prepared by a melting–annealing–spark plasma sintering method, and their structure was characterized by x-ray diffraction and transmission electron microscopy (TEM). TEM results show the development of an InSb nanostructure with a grain size of 30 nm to 500 nm. The nanostructure is present in all samples containing In and is also detected by specific heat measurements. The Seebeck and Hall coefficients indicate that the compounds are n-type semiconductors. Electrical conductivity increases with increasing Ba content. Thermal conductivity is strongly suppressed upon the presence of In in the skutterudite structure, likely due to enhanced boundary scattering of phonons on the nanometer-scale InSb inclusions. The highest thermoelectric figure of merit is achieved with Ba_0.09Yb_0.07In_0.06Co₄Sb_11.97, reaching ZT = 1.25 at 800 K.     

On Intensifying Carrier Impurity Scattering to Enhance Thermoelectric Performance in Cr‐Doped CeyCo4Sb12

The beneficial effect of impurity scattering on thermoelectric properties has long been disregarded even though possible improvements in power factor have been suggested by Ioffe more than a half century ago. Here it is theoretically and experimentally demonstrated that proper intensification of ionized impurity scattering to charge carriers can benefit the thermoelectric figure of merit (ZT) by increasing the Seebeck coefficient and decreasing the electronic thermal conductivity. The optimal strength of ionized impurity scattering for maximum ZT depends on the Fermi level and the density of states effective mass. Cr‐doping in Ce_yCo₄Sb₁₂ progressively increases the strength of ionized impurity scattering, and significantly improves the Seebeck coefficient, resulting in high power factors of 45 μW cm^?1 K^?2 with relatively low electrical conductivity. This effect, combined with the increased Ce‐filling fraction and thus decreased lattice thermal conductivity by charge compensation of Cr‐dopant, gives rise to a maximum ZT of 1.3 at 800 K and a large average ZT of 1.1 between 500 and 850 K, ≈30% and ≈20% enhancements as compared with those of Cr‐free sample, respectively. Furthermore, this study also reveals that carrier scattering parameter can be another fundamental degree of freedom to optimize electrical properties and improve thermal‐to‐electricity conversion efficiencies of thermoelectric materials.     

High Thermoelectric Performance of Dually Doped ZnO Ceramics

A marked improvement in the thermoelectric performance of dense ZnO ceramics is achieved by employing a third element as a co-dopant with Al. Dual doping of ZnO with Al and Ga results in a drastic decrease in the thermal conductivity of the oxide, while the decrease in the electrical conductivity is relatively small. With the aid of a significant enhancement in the thermopower, the dually doped oxide shows thermoelectric figure of merit values, ZT, values of 0.47 at 1000 K and 0.65 at 1247 K at the composition Zn_0.96Al_0.02Ga_0.02O. These results appear to be the highest ZT values so far reported for bulk n-type oxides. Microscopic observation of the samples reveals a granular texture in the densely sintered oxide matrix, suggesting that considerable reduction of the thermal conductivity while maintaining high electrical conductivity could be achieved by such a bulk nanocomposite structure in the samples.     

Nanostructure and High Thermoelectric Performance in Nonstoichiometric AgPbSbTe Compounds: the Role of Ag

High-performance nanostructured Ag_1−x Pb_22.5SbTe₂₀ thermoelectric materials have been fabricated using mechanical alloying and spark plasma sintering. A decrease in Ag content causes a great reduction in thermal conductivity and a prominent increase in ZT value. A minimum thermal conductivity of 0.86 W/m K and a high ZT value of 1.5 (700 K) have been obtained for the Ag_0.4Pb_22.5SbTe₂₀ sample. The smaller and denser nanoscopic regions with reduced Ag content are thought to enhance phonon scattering, resulting in decreased thermal conductivity and enhanced thermoelectric performance.     

Hydride Anion Substitution Boosts Thermoelectric Performance of Polycrystalline SrTiO3 via Simultaneous Realization of Reduced Thermal Conductivity and High Electronic Conductivity

The development of environmentally benign thermoelectric materials with high energy conversion efficiency (ZT) continues to be a long-standing challenge. So far, high ZT has been achieved using heavy elements to reduce lattice thermal conductivity (κ_lat). However, it is not preferred to use such elements because of their environmental load and high material cost. Here a new approach utilizing hydride anion (H⁻) substitution to oxide ion is proposed for ZT enhancement in thermoelectric oxide SrTiO₃ bulk polycrystals. Light element H⁻ substitution largely reduces κ_lat from 8.2 W/(mK) of SrTiO₃ to 3.5 W/(mK) for SrTiO_3−xH_x with x = 0.216. The mass difference effect on phonon scattering is small in the SrTiO_3−xH_x, while local structure distortion arising from the distributed Ti−(O,H) bond lengths strongly enhances phonon scattering. The polycrystalline SrTiO_3−xH_x shows high electronic conductivity comparable to La-doped SrTiO₃ single crystal because the H⁻ substitution does not form a grain boundary potential barrier and thus suppresses electron scattering. As a consequence, SrTiO_3−xH_x bulk exhibits maximum ZT = 0.11 at room temperature and the ZT value increases continuously up to 0.22 at T = 657 K. The H⁻ substitution idea offers a new approach for ZT enhancement in thermoelectric materials without utilizing heavy elements.     

Microstructure and Thermoelectric Transport Properties of Type I Clathrates Ba<Subscript>8</Subscript>Sb<Subscript>2</Subscript>Ga<Subscript>14</Subscript>Ge<Subscript>30</Subscript> Prepared by Ultrarapid Solidification Process

Type I clathrate bulk materials Ba₈Sb₂Ga₁₄Ge₃₀ were prepared by the melt spinning (MS) technique combined with the spark plasma sintering (SPS) method. The microstructure and thermoelectric transport properties of the compounds were investigated. The results show that the grain size decreases greatly compared with materials obtained by the traditional melting and SPS method. The electrical conductivity increases greatly and the lattice thermal conductivity decreases significantly with increasing roller linear speed. The maximum thermoelectric dimensionless figure of merit ZT of 1.05 is obtained at 950 K for the sample prepared by melt spinning with a roller linear speed of 40 m/s.     

Thermoelectric properties of Ce x Nd y Co4Sb12 skutterudites

Experimental results of studying the thermoelectric properties of Co₄Sb₁₂, Ce_0.1Nd_0.5Co₄Sb₁₂, and Ce_0.5Nd_0.1Co₄Sb₁₂ prepared by induction melting are presented. The thermoelectric figure of merit ZT of the studied Co₄Sb₁₂ is approximately two times higher than ZT of unfilled skutterudites prepared by the conventional solid-phase synthesis method. The figure of merit of Ce_0.1Nd_0.5Co₄Sb₁₂ and Ce_0.5Nd_0.1Co₄Sb₁₂ appears lower than ZT of Co₄Sb₁₂ due to the presence of an impurity phase of metal antimony in the first two samples. It is assumed that the thermoelectric properties of filled skutterudites can be significantly improved by optimizing the induction melting method.

     

Charge-Compensated <Emphasis Type="Italic">n</Emphasis>-Type Skutterudites

Ba+Yb double-filled n-type skutterudites with a modest degree of charge compensation by Fe on the Co lattice have been synthesized and compacted by spark-plasma sintering, and their thermoelectric properties evaluated at temperatures up to 800 K. Although this approach to making n-type skutterudites seems counterintuitive, the presence of Fe leads to a reduction in the thermal conductivity while it preserves a robust Seebeck coefficient. Consequently, a high ZT in excess of 1.3 was achieved at 800 K in these Fe-containing n-type skutterudite compounds.     

Oxide Thermoelectric Materials: A Structure–Property Relationship

Recent demand for thermoelectric materials for power harvesting from automobile and industrial waste heat requires oxide materials because of their potential advantages over intermetallic alloys in terms of chemical and thermal stability at high temperatures. Achievement of thermoelectric figure of merit equivalent to unity (ZT ≈ 1) for transition-metal oxides necessitates a second look at the fundamental theory on the basis of the structure–property relationship giving rise to electron correlation accompanied by spin fluctuation. Promising transition-metal oxides based on wide-bandgap semiconductors, perovskite and layered oxides have been studied as potential candidate n- and p-type materials. This paper reviews the correlation between the crystal structure and thermoelectric properties of transition-metal oxides. The crystal-site-dependent electronic configuration and spin degeneracy to control the thermopower and electron–phonon interaction leading to polaron hopping to control electrical conductivity is discussed. Crystal structure tailoring leading to phonon scattering at interfaces and nanograin domains to achieve low thermal conductivity is also highlighted.     

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

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.