Thermoelectric Properties of Nb-Doped Ordered Mesoporous TiO<Subscript>2</Subscript>

Mesoporous materials have pores with diameters between 2 nm and 50 nm, the presence of which generally decreases the thermal conductivity of the material. By incorporating mesoporous structures into thermoelectric materials, the thermoelectric properties of these materials can be improved. Although TiO₂ is an ordinary insulator, reduced TiO₂ shows better electrical conductivity and is therefore a potential thermoelectric material. Furthermore, the addition of a dopant to TiO₂ can improve its electrical conductivity. We hypothesized that, by doping ordered mesoporous TiO₂ films with niobium, we would be able to minimize the thermal conductivity and maximize the electrical conductivity. To investigate the effects of Nb doping and a mesoporous structure on the thermoelectric characteristics of TiO₂ films, Nb-doped mesoporous films were investigated using x-ray diffraction, ellipsometry, four-point probe measurements, and thermal conductivity analysis. We found that Nb doping of ordered mesoporous TiO₂ films improved their thermoelectric properties.     

Synthesis and Thermoelectric Properties of “Nano-Engineered” CoSb<Subscript>3</Subscript> Skutterudite Materials

Because of their good electrical transport properties, skutterudites have been widely studied as potential next-generation thermoelectric (TE) materials. One of the main obstacles to further improving their thermoelectric performance has been reducing their relatively high thermal conductivity. To some extent, this hindrance has been partially resolved by filling the voids found in the skutterudite structure with so-called “rattling” atoms. It has been predicted that reducing the dimensionality in a TE material would have a positive effect in enhancing its thermoelectric properties, for example increasing the thermopower and reducing the thermal conductivity. Introducing nanoparticles into the skutterudite materials could therefore have favorable effects on their electrical properties and should also reduce lattice thermal conductivity by introducing extra scattering centers throughout the sample. Nanoparticles may also be used in conjunction with void filling for further reduction of the thermal conductivity of skutterudites. Cobalt triantimonide (CoSb₃) samples with different amounts of embedded nanoparticles have been grown, and the electrical and thermal transport properties for these composites have been measured from 10 K to 650 K. The synthetic techniques and electrical and thermal transport data are discussed in this paper.     

Re-Doped p-Type Thermoelectric SnSe Polycrystals with Enhanced Power Factor and High ZT > 2

Thermoelectric technology enables the direct interconversion between heat and electricity. SnSe has received increasing interest as a new promising thermoelectric compound due to its exceptionally high performance reported in crystals. SnSe possesses intrinsic low thermal conductivity as a congenital advantage for thermoelectric, but high thermoelectric performance can be hardly achieved due to the difficulty to realize efficient doping to raise its low carrier concentration to an optimal level. In this work, it is found that a series of rare earth elements are effective dopants for SnSe, which can greatly improve the electrical transport properties of p-type polycrystalline SnSe. In particular, the remarkable enhancement in electrical conductivity and power factor is achieved by Na/Er co-doping at 873 K. The lattice thermal conductivity is reduced due to the presence of abundant defects (dislocations, stacking faults, and twin boundaries). Consequently, a peak thermoelectric figure of merit ZT (2.1) as well as a high average ZT (0.77) are achieved in polycrystalline SnSe.     

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 Properties of Sn-S Bulk Materials Prepared by Mechanical Alloying and Spark Plasma Sintering

To study the possibility of SnS as an earth-abundant and environmentally friendly thermoelectric material, the electrical and thermal transport properties of bulk materials prepared by combining mechanical alloying and spark plasma sintering were investigated. It was revealed that SnS has potential as a good thermoelectric material, benefiting from its intrinsically low thermal conductivity below 1.0 W/m/K above 400 K and its high Seebeck coefficient over 500 μV/K. Although the highest ZT value was 0.16 at 823 K in the pristine sample, further enhancement can be expected through chemical doping to increase the electrical conductivity. It was also revealed that changing the stoichiometric ratio and sintering temperature had less apparent influence on the microstructure and thermoelectric properties of SnS because redundant S in the powders decomposed during the sintering process.     

Synthesis and Transport Properties of In<Subscript>4</Subscript>(Se<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Te<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>)<Subscript>3</Subscript>

Polycrystalline samples of In₄(Se_1−x Te _x )₃ were synthesized by using a melting–quenching–annealing process. The thermoelectric performance of the samples was evaluated by measuring the transport properties from 290 K to 650 K after sintering using the spark plasma sintering (SPS) technique. The results indicate that Te substitution can effectively reduce the thermal conductivity while maintaining good electrical transport properties. In₄Te₃ shows the lowest thermal conductivity of all compositions tested.     

Zintl Phases as Thermoelectric Materials: Tuned Transport Properties of the Compounds CaxYb1–xZn2Sb2

Zintl phases are ideal candidates for efficient thermoelectric materials, because they are typically small‐bandgap semiconductors with complex structures. Furthermore, such phases allow fine adjustment of dopant concentration without disrupting electronic mobility, which is essential for optimizing thermoelectric material efficiency. The tunability of Zintl phases is demonstrated with the series Ca_xYb_1–xZn₂Sb₂ (0 ≤ x ≤ 1). Measurements of the electrical conductivity, Hall mobility, Seebeck coefficient, and thermal conductivity (in the 300–800 K temperature range) show the compounds to behave as heavily doped semiconductors, with transport properties that can be systematically regulated by varying x. Within this series, x = 0 is the most metallic (lowest electrical resistivity, lowest Seebeck coefficient, and highest carrier concentration), and x = 1 is the most semiconducting (highest electrical resistivity, highest Seebeck coefficient, and lowest carrier concentration), while the mobility is largely independent of x. In addition, the structural disorder generated by the incorporation of multiple cations lowers the overall thermal conductivity significantly at intermediate compositions, increasing the thermoelectric figure of merit, zT. Thus, both zT and the thermoelectric compatibility factor (like zT, a composite function of the transport properties) can be finely tuned to allow optimization of efficiency in a thermoelectric device.     

Transport Properties of Ni and PbTe Under Pressure

The high-pressure transport properties have been determined for nickel and PbTe. Nickel shows a reduction in electrical resistivity, an increase in thermal conductivity, and a variable effect on the Seebeck coefficient with pressure. In PbTe, a dramatic decrease in resistivity and a slow increase in thermal conductivity have been observed with increasing pressure. The three transport properties in PbTe are affected by a pressure-induced structural phase transition. The measurements show that the high-pressure phase is likely a more effective thermoelectric material than the ambient-pressure phase.     

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.     

Synthesis and Characterization of <Emphasis Type="Italic">p</Emphasis>-Type Pb<Subscript>0.5</Subscript>Sn<Subscript>0.5</Subscript>Te Thermoelectric Power Generation Elements by Mechanical Alloying

A mechanical alloying (MA) process to transform elemental powders into solid Pb_0.5Sn_0.5Te with thermoelectric functionality comparable to melt-alloyed material is described. The room-temperature doping level and mobility as well as temperature-dependent electrical conductivity, Seebeck coefficient, and thermal conductivity are reported. Estimated values of lattice thermal conductivity (0.7 W m⁻¹ K⁻¹) are lower than some reports of functional melt-alloyed PbSnTe-based material, providing evidence that MA can engender the combination of properties resulting in highly functional thermoelectric material. Though doping level and Sn composition have not been optimized, this material exhibits a ZT value >0.5 at 550 K.     

Solid-State Synthesis of Te-Doped Mg<Subscript>2</Subscript>Si

Te-doped Mg₂Si (Mg₂Si:Te _m , m = 0, 0.01, 0.02, 0.03, 0.05) alloys were synthesized by a solid-state reaction and mechanical alloying. The electronic transport properties (Hall coefficient, carrier concentration, and mobility) and thermoelectric properties (Seebeck coefficient, electrical conductivity, thermal conductivity, and figure of merit) were examined. Mg₂Si was synthesized successfully by a solid-state reaction at 673 K for 6 h, and Te-doped Mg₂Si powders were obtained by mechanical alloying for 24 h. The alloys were fully consolidated by hot-pressing at 1073 K for 1 h. All the Mg₂Si:Te _m samples showed n-type conduction, indicating that the electrical conduction is due mainly to electrons. The electrical conductivity increased and the absolute value of the Seebeck coefficient decreased with increasing Te content, because Te doping increased the electron concentration considerably from 10¹⁶ cm⁻³ to 10¹⁸ cm⁻³. The thermal conductivity did not change significantly on Te doping, due to the much larger contribution of lattice thermal conductivity over the electronic thermal conductivity. Thermal conduction in Te-doped Mg₂Si was due primarily to lattice vibrations (phonons). The thermoelectric figure of merit of intrinsic Mg₂Si was improved by Te doping.     

Electronic,optical and thermoelectric properties of XNMg3 (X=P,As, Sb,Bi) compounds

Different exchange correlation potential approximations are used to examine electronic, optical, and thermoelectric properties of XNMg₃(X=P, As, Sb, and Bi) antiperovskite compounds. Band structures of the compounds are direct in nature. Within a high-energy range (2–6 eV), these materials exhibit maximum levels of optical conductivity, and these materials may therefore be used in radiation detectors and solar cells. Optical properties such as dielectric function, optical conductivity, reflectivity, refractive indices and absorption coefficients vary in transitions from P to Bi. Furthermore, calculated peaks of conductivity and absorption coefficient values decrease with increasing photon energy. With respect to thermoelectric properties, electrical conductivity, Seebeck coefficient and thermal conductivity levels vary with increase in temperature, thus resulting in the formation of thermoelectric materials.     

The Effects of Polysilastyrene and Au Additions on the Thermoelectric Properties of <Emphasis Type="Italic">β</Emphasis>-SiC/Si Composites

Recently, Yamaguchi et al. proposed a self-cooling device that does not require additional power circuits for cooling because it is Peltier-cooled using its own current in conjunction with a thermoelectric material. Silicon carbide is a promising thermoelectric material for this technology since its electrical conductivity, thermal conductivity, and Seebeck coefficient are higher than those of conventional thermoelectric materials. This study investigates the effects of polysilastyrene and Au additions on the thermoelectric properties of p-type β-SiC/Si polycrystalline semiconductor composites in order to assess whether their addition improves the performance of self-cooling devices.     

Intrinsically Low Lattice Thermal Conductivity and Anisotropic Thermoelectric Performance in In-doped GeSb2Te4 Single Crystals

Layer-structured GeSb₂Te₄ is a promising thermoelectric candidate, while its anisotropy of thermal and electrical transport properties is still not clear. In this study, Ge_1–xIn_xSb₂Te₄ single crystals are grown by Bridgman method, and their anisotropic thermoelectric properties are systematically investigated. Lower electrical conductivity and higher Seebeck coefficient are observed in the c-axis due to the higher effective mass in this direction. Intrinsically low lattice thermal conductivity is also observed in the c-axis due to the weak chemical bonding and the strong lattice anharmonicity proved by density functional theory calculation. Indium doping introduces an impurity band in the bandgap of GeSb₂Te₄ and leads to the locally distorted density of states near the Fermi level, which contributes to enhanced Seebeck coefficient and improved power factor. Ultimately, a peak zT value of 1 at 673 K and an average zT value of 0.68 within 323–773 K are obtained in Ge_0.93In_0.07Sb₂Te₄ along the c-axis direction, which are 54% and 79% higher than that of the pristine GeSb₂Te₄ single crystal, respectively. This study clarified the origin of intrinsic low lattice thermal conductivity and anisotropy transport properties in GeSb₂Te₄, and shed light on the performance optimization of other layered thermoelectric materials.     

Low-Temperature Thermoelectric Properties of Fe2VAl with Partial Cobalt Doping

Ternary metallic alloy Fe₂VAl with a pseudogap in its energy band structure has received intensive scrutiny for potential thermoelectric applications. Due to the sharp change in the density of states profile near the Fermi level, interesting transport properties can be triggered to render possible enhancement in the overall thermoelectric performance. Previously, this full-Heusler-type alloy was partially doped with cobalt at the iron sites to produce a series of compounds with n-type conductivity. Their thermoelectric properties in the temperature range of 300?K to 850?K were reported. In this research, efforts were made to extend the investigation on (Fe_1?x Co _x )₂VAl to the low-temperature range. Alloy samples were prepared by arc-melting and annealing. Seebeck coefficient, electrical resistivity, and thermal conductivity measurements were performed from 80?K to room temperature. The effects of cobalt doping on the material??s electronic and thermal properties are discussed.     

Embedding Aligned Graphene Oxides in Polyelectrolytes to Facilitate Thermo-Diffusion of Protons for High Ionic Thermoelectric Figure-of-Merit

Recently emerged ionic thermoelectric conversion with the Soret effect is advantageous in providing large thermopower on the order of ≈ 1–10 mV K⁻¹, but the origin of the large thermopower and the methodology of attaining superior thermoelectric performances are yet to be disclosed. Here, key parameters and their optimization for outstanding thermoelectric responses with polystyrene sulfonic acid (PSS-H) are unveiled. It is found that the thermo-diffusion of water boosts proton transport, playing a key role in obtaining large ionic thermopower by promoting unidirectional migration of protons from a hotter to colder side. When graphene oxide (GO) embedded in PSS-H is aligned along the transport direction of protons and water, their diffusion along the in-plane direction of GO is promoted, enlarging both ionic thermopower and ionic electrical conductivity. PSS-H containing 3 wt% GO possesses extremely large ionic power factors up to 1.8 mW m⁻¹ K⁻² and ionic figure-of-merits up to 0.85 at 23 ° C. This study provides not only preeminent thermoelectric performances based on ion transport but also identifies the influence of the key parameters on thermoelectric properties, suggesting controllability of thermo-diffusion of ions depending on inclusions in base materials, which will draw numerous subsequent research with their alterations.     

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.     

Power Generator Modules of Segmented Bi2Te3 and ErAs:(InGaAs)1−x (InAlAs) x

The thermoelectric properties of ErAs:InGaAlAs were characterized by variable-temperature measurements of thermal conductivity, electrical conductivity, and Seebeck coefficient from 300 K to 600 K, which shows that the ZT(, where and T are the Seebeck coefficient, electrical conductivity, thermal conductivity, and absolute temperature, respectively) of the material is greater than 1 at 600 K. Power generator modules of segmented elements of 300 μm Bi₂Te₃ and 50 μm thickness ErAs:(InGaAs)_1−x (InAlAs) _x were fabricated and characterized. The segmented element is 1 mm × 1 mm in area, and each segment can work at different temperature ranges. An output power up to 5.5 W and an open-circuit voltage over 10 V were measured. Theoretical calculations were carried out and the results indicate that the performance of the thermoelectric generator modules can be improved further by improving the thermoelectric properties of the element material, and reducing the electrical and thermal parasitic losses.     

A Comparison Between the Effects of Sb and Bi Doping on the Thermoelectric Properties of the Ti0.3Zr0.35Hf0.35NiSn Half-Heusler Alloy

We deal here with Sb and Bi doping effects of the n-type half-Heusler (HH) Ti_0.3Zr_0.35Hf_0.35NiSn alloy on the measured thermoelectric properties. To date, the thermoelectric effects upon Bi doping on the Sn site of HH alloys have rarely been reported, while Sb has been widely used as a donor dopant. A comparison between the measured transport properties following arc melting and spark plasma sintering of both Bi- and Sb-doped samples indicates a much stronger doping effect upon Sb doping, an effect which was explained thermodynamically. Due to similar lattice thermal conductivity values obtained for the various doped samples, synthesized in a similar experimental route, no practical variations in the thermoelectric figure of merit values were observed between the various investigated samples, an effect which was attributed to compensation between the power factor and electrical thermal conductivity values regardless of the various investigated dopants and doping levels.     

Effect of Nanoparticles on Electron and Thermoelectric Transport

Recent experimental results have shown that adding nanoparticles inside a bulk material can enhance the thermoelectric performance by reducing the thermal conductivity and increasing the Seebeck coefficient. In this paper we investigate electron scattering from nanoparticles using different models. We compare the results of the Born approximation to that of the partial-wave method for a single nanoparticle scattering. The partial-wave method is more accurate for particle sizes in the 1 nm to 5 nm range where the point scattering approximation is not valid. The two methods can have different predictions for the thermoelectric properties such as the electrical conductivity and the Seebeck coefficient. To include a random distribution of nanoparticles, we consider an effective medium for the electron scattering using the coherent potential approximation. We compare various theoretical results with the experimental data obtained with ErAs nanoparticles in an InGaAlAs matrix. Reasonably good agreement is found between the measured and theoretical electrical conductivity and Seebeck data in the 300 K to 850 K temperature range.