Thermoelectric Properties of NdGd<Subscript>1+<Emphasis Type="Italic">x</Emphasis></Subscript>S<Subscript>3</Subscript> Prepared by CS<Subscript>2</Subscript> Sulfurization

Ternary rare-earth sulfides NdGd_1+x S₃, where 0 ≤ x ≤ 0.08, were prepared by sulfurizing Ln₂O₃ (Ln = Nd, Gd) with CS₂ gas, followed by reaction sintering. The sintered samples have full density and homogeneous compositions. The Seebeck coefficient, electrical resistivity, and thermal conductivity were measured over the temperature range of 300 K to 950 K. All the sintered samples exhibit a negative Seebeck coefficient. The magnitude of the Seebeck coefficient and the electrical resistivity decrease systematically with increasing Gd content. The thermal conductivity of all the sintered samples is less than 1.9 W K⁻¹ m⁻¹. The highest figure of merit ZT of 0.51 was found in NdGd_1.02S₃ at 950 K.     

Crystal Structure and High-Temperature Thermoelectric Properties of the Mo<Subscript>3−<Emphasis Type="Italic">x</Emphasis></Subscript>Ru<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Sb<Subscript>7</Subscript> Compounds

Zintl phases are currently receiving great attention for their thermoelectric potential typified by the discovery of a high ZT value in Yb₁₄MnSb₁₁-based compounds. Herein, we report on the crystallographic characterization via neutron and x-ray diffraction experiments, and on the thermoelectric properties measured in the 300 K to 1000 K temperature range, of Mo₃Sb₇ and its isostructural compounds Mo_3−x Ru _x Sb₇. Even though Mo₃Sb₇ displays rather high ZT values given its metallic character, the partial substitution of Mo by Ru substantially improves its thermoelectric properties, resulting in a ZT value of ∼0.45 at 1000 K for x = 0.8.     

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.     

High-Performance (Ag<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>SbTe<Subscript><Emphasis Type="Italic">x</Emphasis>/2+1.5</Subscript>)<Subscript>15</Subscript>(GeTe)<Subscript>85</Subscript> Thermoelectric Materials Prepared by Melt Spinning

A new preparation process combining melt spinning and hot pressing has been developed for the (Ag _x SbTe _x/2+1.5)₁₅(GeTe)₈₅ (TAGS-85) system. Compared with samples prepared by the traditional air-quenching and hot-pressing method, electrical conductivity and thermal conductivity are lowered. The thermoelectric performance of the TAGS-85 samples varied with changing Ag content and reached the highest ZT of 1.48 when x was 0.8 for the melt-spun sample, compared with the maximum ZT of 1.36 for the air-quenched sample. The Seebeck coefficient of the melt-spun TAGS-85 alloys was improved, while both the electrical conductivity and thermal conductivity were decreased. The net result of this process is to effectively enlarge the temperature span of ZT > 1, which will benefit industrial application.     

Electrodeposition of <Emphasis Type="Italic">p</Emphasis>-Type Sb<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Te<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript> Thermoelectric Films

Thermoelectric Sb _x Te _y films were potentiostatically electrodeposited in aqueous nitric acid electrolyte solutions containing different concentrations of TeO₂. Stoichiometric Sb _x Te _y films were obtained by applying a voltage of −0.15 V versus saturated calomel electrode (SCE) using a solution consisting of 2.4 mM TeO₂, 0.8 mM Sb₂O₃, 33 mM tartaric acid, and 1 M HNO₃. The nearly stoichiometric Sb₂Te₃ films had a rhombohedral structure, R[`3]m R\bar{3}m , with a preferred orientation along the (015) direction. The films had hole concentration of 5.8 × 10¹⁸/cm³ and exhibited mobility of 54.8 cm²/Vs. A more negative potential resulted in higher Sb content in the deposited Sb _x Te _y films. Furthermore, it was observed that the hole concentration and mobility decreased with increasingly negative deposition potential, and eventually showed insulating properties, possibly due to increased defect formation. The absolute value of the Seebeck coefficient of the as-deposited Sb₂Te₃ thin film at room temperature was 118 μV/K.     

Microstructure and Thermoelectric Properties of <Emphasis Type="Italic">n</Emphasis>- and <Emphasis Type="Italic">p</Emphasis>-Type Doped Mg<Subscript>2</Subscript>Sn Compounds Prepared by the Modified Bridgman Method

Mg₂Sn compounds were prepared by the modified vertical Bridgman method, and were doped with Bi and Ag to obtain n- and p-type materials, respectively. Excess Mg was also added to some of the ingots to compensate for the loss of Mg during the preparation process. The Mg₂Sn samples were characterized by x-ray diffraction (XRD) and scanning electron microscopy (SEM), and their power factors were calculated from the Seebeck coefficient and electrical conductivity, measured from 80 K to 700 K. The sample prepared with 4% excess Mg, which contains a small amount of Mg₂Sn + Mg eutectic phase, had the highest power factor of 12 × 10⁻³ W m⁻¹ K⁻² at 115 K, while the sample doped with 2% Ag, in which a small amount of eutectics also exists, has a power factor of 4 × 10⁻³ W m⁻¹ K⁻² at 420 K.     

Preparation and Thermoelectric Properties of La-Doped SrTiO<Subscript>3</Subscript> Ceramics

Single-phase polycrystalline La _x Sr_1−x TiO₃ (x = 0, 0.04, 0.06, 0.08, and 0.12) ceramics were prepared by the conventional solid-state reaction method using high-activity hydroxides as the raw materials. The electrical conductivity of all the samples increased with increasing x value and decreased with measurement temperature, while the thermal conductivity decreased with increasing x value and measurement temperature. The La_0.12Sr_0.88TiO₃ sample showed the lowest thermal conductivity of 2.45 W m⁻¹ K⁻¹ at 873 K and the largest ZT of 0.28 at 773 K. The present work revealed that hydroxides with high activity as raw materials are beneficial to improve the thermoelectric properties, especially to decrease the thermal conductivity.     

Thermoelectric Performance of Zn and Nd Co-doped In<Subscript>2</Subscript>O<Subscript>3</Subscript> Ceramics

Polycrystalline In₂O₃ ceramics co-doped with Zn and Nd were prepared by the spark plasma sintering (SPS) process, and microstructure and thermoelectric (TE) transport properties of the ceramics were investigated. Our results indicate that co-doping with Zn²⁺ and Nd³⁺ shows a remarkable effect on the transport properties of In₂O₃-based ceramics. Large electrical conductivity (~130 S cm⁻¹) and thermopower (~220 μV K⁻¹) can be observed in these In₂O₃-based ceramic samples. The maximum power factor (PF) reaches 5.3 × 10⁻⁴ W m⁻¹ K⁻² at 973 K in the In_1.92Nd_0.04Zn_0.04O₃ sample, with a highest ZT of ~0.25.     

Thermoelectric Properties of <Emphasis Type="Italic">p</Emphasis>-Type Mg<Subscript>2.00</Subscript>Si<Subscript>0.25</Subscript>Sn<Subscript>0.75</Subscript> with Li and Ag Double Doping

Mg₂Si_1−x Sn _x -system solid solutions are ecofriendly semiconductors that are promising materials for thermoelectric generators in the middle temperature range. To produce a thermoelectric device, high-performance p- and n-type materials must be balanced. In this paper, p-type Mg_2.00Si_0.25Sn_0.75 with Li and Ag double doping was prepared by the liquid–solid reaction method and hot-pressing. Effects of Li and Ag double doping on thermoelectric properties were investigated in the temperature range from room temperature to 850 K. All sintered compacts were identified as single-phase solid solutions with anti-fluorite structure. The carrier concentration increased with the double doping. The temperature dependence of resistivity of the double-doped samples was similar to that of a metal. The seebeck coefficient increased with temperature to a maximum value and then decreased in the intrinsic region. Thermal conductivity decreased linearly with increasing temperature, reaching a minimum near the intrinsic region, and then increased rapidly because of the contribution of the bipolar component. The dimensionless figure of merit reached 0.32 at 610 K for Mg_2.00Si_0.25Sn_0.75 double-doped with Li-5000 ppm and Ag-20000 ppm.     

Preparation of Well-Tiled <Emphasis Type="Italic">γ</Emphasis>-Na<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Co<Subscript>2</Subscript>O<Subscript>4</Subscript> by a Novel and Simple Sodium Alginate Gel Method and Its Electrical Properties

In this paper, a novel and simple sodium alginate (SA) gel method was developed to prepare γ-Na _x Co₂O₄. This method involved the chemical gelling of SA in the presence of Co²⁺ ions by cross-linking. After calcining at 700°C to 800°C, single-phase γ-Na _x Co₂O₄ crystals were obtained. The arrangement of about 1 μm to 4 μm flaky particles exhibited a well-tiled structure along the plane direction of the flaky particles. SA not only acted as the control agent for crystal growth, but also provided a Na source for the γ-Na _x Co₂O₄ crystals. The electrical properties of γ-Na _x Co₂O₄ ceramics prepared via ordinary sintering after cold isostatic pressing were investigated. The Seebeck coefficient and power factor of the bulk material were 177 μV K⁻¹ and 4.3 × 10⁻⁴ W m⁻¹ K⁻² at 850 K, respectively.     

Influence of Nanoinclusions on Thermoelectric Properties of <Emphasis Type="Italic">n</Emphasis>-Type Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript> Nanocomposites

n-Type Bi₂Te₃ nanocomposites with enhanced figure of merit, ZT, were fabricated by a simple, high-throughput method of mixing nanostructured Bi₂Te₃ particles obtained through melt spinning with micron-sized particles. Moderately high power factors were retained, while the thermal conductivity of the nanocomposites was found to decrease with increasing weight percent of nanoinclusions. The peak ZT values for all the nanocomposites were above 1.1, and the maximum shifted to higher temperature with increasing amount of nanoinclusions. A maximum ZT of 1.18 at 42°C was obtained for the 10 wt.% nanocomposite, which is a 43% increase over the bulk sample at the same temperature. This is the highest ZT reported for n-type Bi₂Te₃ binary material, and higher ZT values are expected if state-of-the-art Bi₂Te_3−x Se _x materials are used.     

Thermoelectric Properties of Half-Heusler Bismuthides ZrCo<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Ni<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Bi (<Emphasis Type="Italic">x</Emphasis> = 0.0 to 0.1)

The usefulness of half-Heusler (HH) alloys as thermoelectrics has been mainly limited by their relatively large thermal conductivity, which is a key issue despite their high thermoelectric power factors. In this regard, Bi-containing half-Heusler alloys are particularly appealing, because they are, potentially, of low thermal conductivity. One such a material is ZrCoBi. We prepared pure and Ni-doped ZrCoBi by a solid-state reaction. To evaluate thermoelectric potential we measured electrical resistivity (ρ = 1/σ) and thermopower (σ) up to 1000 K and thermal conductivity (κ) up to 300 K. Our measurements indicate that for these alloys resistivity of approximately a few mΩ cm and thermopower larger than a hundred μV K⁻¹ are possible. Low κ values are also possible. On the basis of these data we conclude that this system has a potential to be optimized further, despite the low power factors (α ² σT) we have currently measured.     

Thermoelectric Properties of Co-Doped TiS<Subscript>2</Subscript>

The thermoelectric properties of cobalt-doped compounds Co _x Ti_1−x S₂ (0 ≤ x ≤ 0.3) prepared by solid-state reaction were investigated from 5 K to 310 K. It was found that the electric resistivity ρ and absolute thermopower |S| for all the doped compounds decreased significantly with increasing Co content over the whole temperature range investigated. The increased lattice thermal conductivity of the doped compounds would imply enhancement of the acoustic velocity. Moreover, the ZT value of the doped compounds was improved over the whole temperature range investigated, and specifically reached 0.03 at 310 K for Co_0.3Ti_0.7S₂, being about 66% larger than that of TiS₂.     

Effect of Hydrogen Free Radicals on Hg<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Cd<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Te

The effects of atomic hydrogen (H) and Br/methanol etching on Hg_1−x Cd _x Te films were investigated using x-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). Exposure of an as-received Hg_1−x Cd _x Te sample to H + H₂ resulted in H-induced TeO₂ reduction. The oxide reduction was first order with respect to H + H₂ exposure. Exposure to H + H₂ after etching the Hg_1−x Cd _x Te film in a Br/methanol solution induced Hg and C depletion. Hg and C removal was also observed after completely reducing the TeO₂ on the as-received sample. The removal process was hindered by the formation of a Cd-rich overlayer on both etched and unetched surfaces.     

Thermoelectric Power and <Emphasis Type="Italic">ZT</Emphasis> in Conducting Organic Semiconductor

A recent report on poly(3,4-ethylenedioxythiophene-tosylate) (PEDOT.Tos) suggested that the thermoelectric figure of merit (ZT) could be enhanced when the percentage oxidation was chemically altered. This invokes the question of whether the carrier density or the mobility was modified. In this work, we analyzed data reported by Bibnova et al. (Nat. Mater. 10, 429, 2011) and extracted the transport parameters using three-dimensional (3D) and two-dimensional (2D) models. Our results indicate that the increase in the power factor (S ² σ) was due primarily to upward extension in the range of thermoelectric power. A changeover from lattice scattering to ionized impurity scattering in PEDOT.Tos allowed the equation governing the thermoelectric power to be valid at higher carrier densities, resulting in an increase in the power factor. ZT was also enhanced in PEDOT.Tos due to the low intrinsic thermal conductivity (~0.37 W/m K). The peak value of ZT (~0.3) was found close to the regime where the semiconductor turned “metallic,” beyond which ZT would decrease. We are of the opinion that charge-to-charge scattering (which normally would lower the power factor in highly doped semiconductors) remain subdued in PEDOT.Tos due potentially to electronic screening and a lack of long-range order. We used the reported data to compute the carrier density and mobility assuming ionized impurity scattering and found the peak power factor to occur for carrier density of ~1 × 10²⁶ m⁻³ and mobility of ~5 × 10⁻⁴ m²/V s.     

Features of the Behavior of the Figure of Merit for <Emphasis Type="Italic">p</Emphasis>-Type Solid Solutions Based on Bismuth and Antimony Chalcogenides

Thermoelectric and galvanomagnetic properties of p-type solid solutions based on bismuth and antimony chalcogenides (Bi,Sb)₂(Te,Se)₃ have been studied to analyze the features of the figure of merit Z. The increase of Z and ZT for the p-Bi_2−x Sb _x Te₃ composition at x = 1.6 in the temperature interval of 370 K to 550 K was shown to be defined by the increase of the density-of-states effective mass, the slope of the temperature dependence of the carrier mobility, and the reduction of the lattice thermal conductivity for optimal charge carrier concentration. High carrier mobility and low lattice thermal conductivity provide the increase of Z and ZT in the p-Bi_2−x Sb _x Te_3−y Se _y (x = 1.3, y = 0.06) solid solution in the interval from 300 K to 370 K. The growth of Z in these compositions is determined by the increase of the compression of the constant-energy ellipsoids along binary and bisector directions, and by the change of the tilt angle Θ between the principal axes of the ellipsoids and the crystallographic axes.     

Thermoelectric Properties of Chevrel-Phase Sulfides M<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Mo<Subscript>6</Subscript>S<Subscript>8</Subscript> (M: Cr,Mn, Fe,Ni)

Chevrel-phase sulfides M _x Mo₆S₈ (M, Cr, Mn, Fe, Ni; x: 1.3, 2.0) were prepared by reacting appropriate amounts of M, Mo, and MoS₂ powders. The samples were then consolidated by pressure-assisted sintering to fabricate dense compacts. While Cr_1.3Mo₆S₈ crystallized in a triclinic structure, Mn_1.3Mo₆S₈, Fe_1.3Mo₆S₈, and Ni_2.0Mo₆S₈ crystallized in a hexagonal structure. The Seebeck coefficient, electrical resistivity, and thermal conductivity of the sintered samples were measured over the temperature range of 300 K to 973 K. All the samples exhibited a positive Seebeck coefficient. The Seebeck coefficient, electrical resistivity, and thermal conductivity of M_1.3Mo₆S₈ (M: Cr, Mn, Fe) were almost identical and increased with temperature. However, the corresponding values and temperature dependent behavior of Ni_2.0Mo₆S₈ were different from those of M_1.3Mo₆S₈ (M: Cr, Mn, Fe). For Ni_2.0Mo₆S₈, as temperature increased, the Seebeck coefficient and thermal conductivity increased while the electrical resistivity decreased. The highest value of the thermoelectric figure of merit (0.17) was observed in Cr_1.3Mo₆S₈ at 973 K.     

Enhancing Thermoelectric Properties of Polycrystalline Bi<Subscript>2</Subscript>S<Subscript>3</Subscript> by Optimizing a Ball-Milling Process

Bismuth sulfide (Bi₂S₃) polycrystalline samples were fabricated by mechanical alloying (MA) combined with spark plasma sintering (SPS). The microstructure and electrical transport properties were investigated with special emphasis on the influence of the ball-milling process. Bi₂S₃ compound powders could be readily synthesized directly from elemental powders under all the investigated conditions, and highly dense n-type bulk Bi₂S₃ samples with high density (>95%) were fabricated by the subsequent SPS process. Changing the MA conditions had no apparent influence on the microstructure or phase structure of the MA-derived Bi₂S₃ powders, but the electrical properties and thermopower of the SPS-sintered Bi₂S₃ bulk samples were greatly dependent on the MA speed and time. The power factor of Bi₂S₃ was increased to 233 μW K⁻² m⁻¹ at 573 K by optimizing the ball-milling process. This power factor is higher than values reported to date for Bi-S binary samples without texture.     

Phonon-Assisted Tunneling from <Emphasis Type="Italic">Z</Emphasis><Subscript>1</Subscript>/<Emphasis Type="Italic">Z</Emphasis><Subscript>2</Subscript> in 4H-SiC

n-Type 4H-SiC bulk samples with a net doping concentration of 2.5 × 10¹⁷ cm⁻³ were irradiated at room temperature with 1-MeV electrons. The high doping concentration plus a reverse bias of up to −13 V ensures high electric field in the depletion region. The dependence of the emission rate on the electric field in the depletion region was measured using deep-level transient spectroscopy (DLTS) and double-correlation deep-level transient spectroscopy (DDLTS). The experimental data are adequately described by the phonon-assisted tunneling model proposed by Karpus and Pere.     

Epitaxial growth and properties of Mg<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Zn<Subscript>1-<Emphasis Type="Italic">x</Emphasis></Subscript>O films produced by pulsed laser deposition

The Mg _x Zn_1-x O thin films with a Mg content corresponding to x = 0–0.45 are grown by pulsed laser deposition on ablation of ceramic targets. The conditions for epitaxial growth of the films on the single-crystal Al₂O₃ (00.1) substrates are established. The record limit of solubility of Mg in hexagonal ZnO, x = 35 is attained. In this case, the lattice mismatch for the parameter a of the ZnO and Mg_0.35Zn_0.65O films does not exceed 1%, whereas the band gaps of the films differ by 0.78 eV. The surface roughness of the films corresponds to 0.8–1.5 nm in the range of x = 0–0.27.