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.     

Preparation and Thermoelectric Properties of LaGd<Subscript>1+<Emphasis Type="Italic">x</Emphasis></Subscript>S<Subscript>3</Subscript> and SmGd<Subscript>1+<Emphasis Type="Italic">x</Emphasis></Subscript>S<Subscript>3</Subscript>

Thermoelectric materials are attractive since they can recover waste heat directly in the form of electricity. In this study, the thermoelectric properties of ternary rare-earth sulfides LaGd_1+x S₃ (x = 0.00 to 0.03) and SmGd_1+x S₃ (x = 0.00 to 0.06) were investigated over the temperature range of 300 K to 953 K. These sulfides were prepared by CS₂ sulfurization, and samples were consolidated by pressure-assisted sintering to obtain dense compacts. The sintered compacts of LaGd_1+x S₃ were n-type metal-like conductors with a thermal conductivity of less than 1.7 W K⁻¹ m⁻¹. Their thermoelectric figure of merit ZT was improved by tuning the chemical composition (self-doping). The optimized ZT value of 0.4 was obtained in LaGd_1.02S₃ at 953 K. The sintered compacts of SmGd_1+x S₃ were n-type hopping conductors with a thermal conductivity of less than 0.8 W K⁻¹ m⁻¹. Their ZT value increased significantly with temperature. In SmGd_1+x S₃, the ZT value of 0.3 was attained at 953 K.     

Influence of Doping on Structural and Thermoelectric Properties of AgSbSe<Subscript>2</Subscript>

A series of samples with nominal compositions of AgSb_1−x Sn _x Se₂ (with x = 0.0, 0.1, 0.2, and 0.3) and AgSbSe_2−y Te _y (with y = 0.0, 0.25, 0.5, 0.75, and 1.0) were prepared. The crystal structure of both single crystals and polycrystalline samples was analyzed using x-ray and neutron diffractometry. The electrical conductivity, thermal conductivity, and Seebeck coefficient were measured within the temperature range from 300 K to 700 K. In contrast to intrinsic AgSbSe₂, samples doped with Sn and Te exhibit apparent semiconducting properties (E _g = 0.3 eV to 0.5 eV), lower electrical conductivity, and higher values of the Seebeck coefficient for a small amount of Sn (x = 0.1). Further doping leads to decrease of the thermoelectric power and increase of the electrical conductivity. In order to explain electron transport behavior observed in pure and doped AgSbSe₂, electronic structure calculations were performed by the Korringa–Kohn–Rostoker method with coherent potential approximation (KKR–CPA).     

Thermoelectric Properties of Co<Subscript>4</Subscript>Sb<Subscript>12</Subscript> Skutterudite Materials with Partial In Filling and Excess In Additions

The properties of Co₄Sb₁₂ with various In additions were studied. X-ray diffraction revealed the presence of the pure δ-phase of In_0.16Co₄Sb₁₂, whereas impurity phases (γ-CoSb₂ and InSb) appeared for x = 0.25, 0.40, 0.80, and 1.20. The homogeneity and morphology of the samples were observed by Seebeck microprobe and scanning electron microscopy, respectively. All the quenched ingots from which the studied samples were cut were inhomogeneous in the axial direction. The temperature dependence of the Seebeck coefficient (S), electrical conductivity (σ), and thermal conductivity (κ) was measured from room temperature up to 673 K. The Seebeck coefficient of all In-added Co₄Sb₁₂ materials was negative. When the filler concentration increases, the Seebeck coefficient decreases. The samples with In additions above the filling limit (x = 0.22) show an even lower Seebeck coefficient due to the formation of secondary phases: InSb and CoSb₂. The temperature variation of the electrical conductivity is semiconductor-like. The thermal conductivity of all the samples decreases with temperature. The central region of the In_0.4Co₄Sb₁₂ ingot shows the lowest thermal conductivity, probably due to the combined effect of (a) rattling due to maximum filling and (b) the presence of a small amount of fine-dispersed secondary phases at the grain boundaries. Thus, regardless of the non-single-phase morphology, a promising ZT (S ² σT/κ) value of 0.96 at 673 K has been obtained with an In addition above the filling limit.     

Thermoelectric Properties of Sb-Doped Mg<Subscript>2</Subscript>Si<Subscript>0.3</Subscript>Sn<Subscript>0.7</Subscript>

Mg₂(Si_0.3Sn_0.7)_1−y Sb _y (0 ≤ y ≤ 0.04) solid solutions were prepared by a two-step solid-state reaction method combined with the spark plasma sintering technique. Investigations indicate that the Sb doping amount has a significant impact on the thermoelectric properties of Mg₂(Si_0.3Sn_0.7)_1−y Sb _y compounds. As the Sb fraction y increases, the electron concentration and electrical conductivity of Mg₂(Si_0.3Sn_0.7)_1−y Sb _y first increase and then decrease, and both reach their highest value at y = 0.025. The sample with y = 0.025, possessing the highest electrical conductivity and one of the higher Seebeck coefficient values among all the samples, has the highest power factor, being 3.45 mW m⁻¹ K⁻² to 3.69 mW m⁻¹ K⁻² in the temperature range of 300 K to 660 K. Meanwhile, Sb doping can significantly reduce the lattice thermal conductivity (κ _ph) of Mg₂(Si_0.3Sn_0.7)_1−y Sb _y due to increased point defect scattering, and κ _ph for Sb-doped samples is 10% to 20% lower than that of the nondoped sample for 300 K < T < 400 K. Mg₂(Si_0.3Sn_0.7)_0.975Sb_0.025 possesses the highest power factor and one of the lower κ _ph values among all the samples, and reaches the highest ZT value: 1.0 at 640 K.     

Thermoelectric Properties of Co<Subscript>4</Subscript>Sn<Subscript>6</Subscript>Se<Subscript>6</Subscript> Ternary Skutterudites

A ternary ordered variant of the skutterudite structure, the Co₄Sn₆Se₆ compound, was prepared. Polycrystalline samples were prepared by a modified ceramic method. The electrical conductivity, the Seebeck coefficient and the thermal conductivity were measured over a temperature range of 300–800 K. The undoped Co₄Sn₆Se₆ compound was of p-type electrical conductivity and had a band gap E _g of approximately 0.6 eV. The influence of transition metal (Ni and Ru) doping on the thermoelectric properties was studied. While the thermal conductivity was significantly lowered both for the undoped Co₄Sn₆Se₆ compound and for the doped compounds, as compared with the Co₄Sb₁₂ binary skutterudite, the calculated ZT values were improved only slightly.     

Beneficial Effect of S-Filling on Thermoelectric Properties of S<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Co<Subscript>4</Subscript>Sb<Subscript>11.2</Subscript>Te<Subscript>0.8</Subscript> Skutterudite

In this work, Te-doped and S-filled S _x Co₄Sb_11.2Te_0.8 (x = 0.1, 0.15, 0.2, 0.25, 0.3, 0.4) skutterudite compounds have been prepared using solid state reaction and spark plasma sintering. Thermoelectric measurements of the consolidated samples were examined in a temperature range of 300–850 K, and the influences of S-addition on the thermoelectric properties of S _x Co₄Sb_11.2Te_0.8 skutterudites are systematically investigated. The results indicate that the addition of sulfur and tellurium is effective in reducing lattice thermal conductivity due to the point-defect scattering caused by tellurium substitutions and the cluster vibration brought by S-filling. The solubility of tellurium in skutterudites is enhanced with sulfur addition via charge compensation. The thermal conductivity decreases with increasing sulfur content. The highest figure of merit, ZT = 1.5, was obtained at 850 K for S_0.3Co₄Sb_11.2Te_0.8 sample, because of the low lattice thermal conductivity.     

Thermal Stability of Barium and Indium Double-Filled Skutterudite Ba<Subscript>0.3</Subscript>In<Subscript>0.2</Subscript>Co<Subscript>3.95</Subscript>Ni<Subscript>0.05</Subscript>Sb<Subscript>12</Subscript> Coated by SiO<Subscript>2</Subscript> Nanoparticles

Filled skutterudite thermoelectric (TE) materials have been extensively studied to search for better TE materials in the past decade. However, there is no detailed investigation about the thermal stability of filled skutterudite TE materials. The evolution of microstructure and TE properties of nanostructured skutterudite materials fabricated with Ba_0.3In_0.2Co_3.95Ni_0.05Sb₁₂/SiO₂ core–shell composite particles with 3 nm thickness shell was investigated during periodic thermal cycling from room temperature to 723 K in this work. Scanning electronic microscopy and electron probe microscopy analysis were used to investigate the microstructure and chemical composition of the nanostructured skutterudite materials. TE properties of the nanostructured skutterudite materials were measured after every 200 cycles of quenching in the temperature range from 300 K to 800 K. The results show that the microstructure and composition of Ba_0.3In_0.2Co_3.95Ni_0.05Sb₁₂/SiO₂ nanostructured skutterudite materials were more stable than those of single-phase Ba_0.3In_0.2Co_3.95Ni_0.05Sb₁₂ bulk materials. The evolution of TE properties indicates that the electrical and thermal conductivity decrease along with an increase in the Seebeck coefficient with increasing quenching up to 2000 cycles. As a result, the dimensionless TE figure of merit (ZT) of the nanostructured skutterudite materials remains almost constant. It can be concluded that these nanostructured skutterudite materials have good thermal stability and are suitable for use in solar power generation systems.     

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.     

Thermoelectric Properties of Zintl Compound YbZn<Subscript>2</Subscript>Sb<Subscript>2</Subscript> with Mn Substitution in Anionic Framework

The thermoelectric properties of the Zintl compound YbZn₂Sb₂ with isoelectronic substitution of Zn by Mn in the anionic (Zn₂Sb₂)²⁻ framework have been studied. The p-type YbZn_2−x Mn _x Sb₂ (0.0 ≤ x ≤ 0.4) samples were prepared via melting followed by annealing and hot-pressing. Thermoelectric property measurement showed that the Mn substitution effectively lowered the thermal conductivity for all the samples, while it significantly increased the Seebeck coefficient for x < 0.2. As a result, a dimensionless figure of merit ZT of approximately 0.61 to 0.65 was attained at 726 K for x = 0.05 to 0.15, compared with the ZT of ~0.48 in the unsubstituted YbZn₂Sb₂.     

Thermoelectric Properties of SnO<Subscript>2</Subscript> Ceramics Doped with Sb and Zn

Polycrystalline SnO₂-based samples (Sn_0.97−x Sb_0.03Zn _x O₂, x = 0, 0.01, 0.03) were prepared by solid-state reactions. The thermoelectric properties of SnO₂ doped with Sb and Zn were investigated from 300 K to 1100 K. X-ray diffraction (XRD) analysis revealed all XRD peaks of all the samples as identical to the rutile structure, except for the x = 0.03 sample, which had a small amount of Zn₂SbO₄ as a secondary phase. We found that the power factor of the x = 0.03 sample was significantly improved due to the simultaneous increase in the electrical conductivity and the Seebeck coefficient. A power factor value of ∼2 × 10⁻⁴ W m⁻¹ K⁻² was obtained for the x = 0.03 sample at 1060 K, 126% higher than that for the undoped sample.     

Thermoelectric Properties of Mn-Doped Ca<Subscript>5</Subscript>Al<Subscript>2</Subscript>Sb<Subscript>6</Subscript>

Ca₅Al₂Sb₆ is a relatively inexpensive Zintl compound exhibiting promising thermoelectric efficiency at temperatures suitable for waste heat recovery. Motivated by our previous studies of Ca₅Al₂Sb₆ doped with Na and Zn, this study focuses on doping with Mn²⁺ at the Al³⁺ site. While Mn is a successful p-type dopant in Ca₅Al₂Sb₆, we find that incomplete dopant activation yields lower hole concentrations than obtained with either previously investigated dopant. High-temperature Hall effect and Seebeck coefficient measurements show a transition from nondegenerate to degenerate semiconducting behavior in Ca₅Al_2−x Mn _x Sb₆ samples (x = 0.05, 0.1, 0.2, 0.3, 0.4) with increasing Mn content. Ultimately, no improvement in zT is achieved via Mn doping, due in part to the limited carrier concentration range achieved.     

Cage-Shaped Mo<Subscript>9</Subscript> Chalcogenides: Promising Thermoelectric Materials with Significantly Low Thermal Conductivity

Thermoelectric properties of molybdenum selenides containing Mo₉ clusters have been investigated between 300 K and 800 K. Ag _x Mo₉Se₁₁ (x = 3.4 and 3.8) have been synthesized by solid-state reaction and spark plasma sintering. X-ray diffraction and scanning electron microscopy reveal high purity and good homogeneity of the samples. The thermoelectric power of the samples is positive over the whole investigated temperature range, indicating that the majority of charge carriers are holes. The Seebeck coefficient increases with temperature, and the temperature coefficient of the resistivity is positive. Significantly low thermal conductivity, comparable to values reported for state-of-the-art thermoelectric materials, is observed in this new system, and this is assumed to be associated with the rattling effect from the Ag filler atoms. It has been demonstrated that the electrical and thermal properties correlate to the Ag concentration. For x = 3.8, a promising dimensionless thermoelectric figure of merit of ∼0.7 is obtained at 800 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.     

First-Principles and Experimental Studies of Y-Doped Mg<Subscript>2</Subscript>Si Prepared Using Field-Activated Pressure-Assisted Synthesis

The thermoelectric properties of Y-doped (1000 ppm, 2000 ppm, 3000 ppm) Mg₂Si fabricated using field-activated pressure-assisted synthesis (FAPAS) have been characterized using measurements of electrical resistivity (ρ), Seebeck coefficient (S), and thermal conductivity (κ) at temperatures ranging from 285 K to 810 K. The Y-doped Mg₂Si samples were n-type in the measured temperature range. A first-principles calculation revealed that the Y atoms were expected to be primarily located at Mg sites. In sample doped with 2000 ppm Y, which exhibited the best electrical and thermal conductivity, the absolute value of the Seebeck coefficient increased in the temperature range of 320 K to 680 K, being higher than that of undoped Mg₂Si. Moreover, this sample exhibited a higher level of electrical conductivity and a higher power factor. In addition, introduction of Y decreased the thermal conductivity appreciably, indicating that Y dopants are favorable for improving the properties of Mg₂Si.     

Thermoelectric Properties of Layer-Antiferromagnet CuCrS<Subscript>2</Subscript>

We have performed a detailed study of the electrical and thermal conductivities and thermoelectric power behavior of an antiferromagnetic-layer compound of chromium, CuCrS₂, from 15 K to 300 K. Unlike previous studies, we find noninsulating properties and sensitive dependence on the preparation method, the microstructure, and the flaky texture formed in polycrystalline samples after extended sintering at high temperatures. Flakes are found to be metallic, with strong localization effects in the conductivity on cooling to low temperatures. The antiferromagnetic transition temperature T _N (=40 K) remains essentially unaffected. The Seebeck coefficient is found to be in the range of 150 μV/K to 450 μV/K, which is exceptionally large, and becomes temperature independent at high temperatures, even for specimens with low resistivity values of 5 mΩ cm to 200 mΩ cm. We find the thermal conductivity κ to be low, viz. 5 mW/K cm to 30 mW/K cm. This can be attributed mostly to the dominance of lattice conduction over electronic conduction. The value of κ is further reduced by disorder in Cu occupancy in the quenched phase. We also observe an unusually strong dip in κ at T _N, which is probably due to strong magnetocrystalline coupling in these compounds. Finally we discuss the properties of CuCrS₂ as a heavily doped Kondo-like insulator in its paramagnetic phase. The combination of the electronic properties observed in CuCrS₂ makes it a potential candidate for various thermoelectric applications.     

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.     

Thermoelectric Properties of Si<Subscript>2</Subscript>Ti-Type Al-(Mn,Cr,Fe)-Si Alloys

To optimize the thermoelectric properties of Si₂Ti-type Al₃₂Mn₃₄Si₃₄ (C54-phase), which possesses a large absolute Seebeck coefficient |S| exceeding 300 μV/K with negative sign, we partially substituted Cr and Fe for Mn, and succeeded in decreasing the number of valence electrons (in the case of Cr) without observing precipitation of secondary phases. A large, positive Seebeck coefficient exceeding 200 μV/K was observed for Al₃₂Cr _x Mn_34−x Si₃₄ (1 ≤ x ≤ 2.5), which consists almost solely of the C54-phase. The increase of hole concentration caused by Cr substitution for Mn was confirmed by both the reduction in electrical resistivity and the sign reversal of the Seebeck coefficient. The largest ZT-value for positive Seebeck coefficient (p-type behavior) was obtained for Al₃₂Cr_2.5Mn_31.5Si₃₄, with the resulting ZT-value reaching a magnitude twice as large as the largest ZT-value of the ternary compound Al₃₃Mn₃₄Si₃₃ possessing p-type behavior.     

Magnetic properties of the FeIn<Subscript>2</Subscript>S<Subscript>4</Subscript> ternary-compound crystals

Crystals of the ternary compound FeIn₂S₄ are grown by directional crystallization of a melt (the horizontal Bridgman method). Composition of the crystals and their crystal structure are determined. Magnetic properties of the FeIn₂S₄ crystals are studied in the temperature range 4–310 K in magnetic fields of 0–140 kOe. It is shown that the crystals under study are paramagnets up to ∼12 K and their specific magnetic moment monotonically increases with decreasing temperature. The antiferromagnetic character of indirect interactions between Fe²⁺ cations is established. The most probable causes and the mechanism of the formation of the magnetic state in the FeIn₂S₄ crystals are discussed.     

Thermoelectric Properties of Ag-doped Mg<Subscript>2</Subscript>Ge Thin Films Prepared by Magnetron Sputtering

Silver doped p-type Mg₂Ge thin films were grown in situ at 773 K using magnetron co-sputtering from individual high-purity Mg and Ge targets. A sacrificial base layer of silver of various thicknesses from 4 nm to 20 nm was initially deposited onto the substrate to supply Ag atoms, which entered the growing Mg₂Ge films by thermal diffusion. The addition of silver during film growth led to increased grain size and surface microroughness. The carrier concentration increased from 1.9 × 10¹⁸ cm⁻³ for undoped films to 8.8 × 10¹⁸ cm⁻³ for the most heavily doped films, but it did not reach saturation. Measurements in the temperature range of T = 200–650 K showed a positive Seebeck coefficient for all the films, with maximum values at temperatures between 400 K and 500 K. The highest Seebeck coefficient of the undoped film was 400 μV K⁻¹, while it was 280 μV K⁻¹ for the most heavily doped film at ∼400 K. The electrical conductivity increased with silver doping by a factor of approximately 10. The temperature effects on power factors for the undoped and lightly doped films were very limited, while the effects for the heavily doped films were substantial. The power factor of the heavily doped films reached a non-optimum value of ∼10⁻⁵ W cm⁻¹ K⁻² at 700 K.