Thermoelectric Properties of Zr<Subscript>3</Subscript>Mn<Subscript>4</Subscript>Si<Subscript>6</Subscript> and TiMnSi<Subscript>2</Subscript>

The Seebeck coefficient, electrical resistivity, and thermal conductivity of Zr₃Mn₄Si₆ and TiMnSi₂ were studied. The crystal lattices of these compounds contain relatively large open spaces, and, therefore, they have fairly low thermal conductivities (8.26 Wm⁻¹ K⁻¹ and 6.63 Wm⁻¹ K⁻¹, respectively) at room temperature. Their dimensionless figures of merit ZT were found to be 1.92 × 10⁻³ (at 1200 K) and 2.76 × 10⁻³ (at 900 K), respectively. The good electrical conductivities and low Seebeck coefficients might possibly be due to the fact that the distance between silicon atoms in these compounds is shorter than that in pure semiconductive silicon.     

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.     

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.     

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.     

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.     

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).     

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.     

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.     

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 In<Subscript>0.3</Subscript>Ga<Subscript>0.7</Subscript>N Alloys

We report on the experimental investigation of the potential of InGaN alloys as thermoelectric (TE) materials. We have grown undoped and Si-doped In_0.3Ga_0.7N alloys by metalorganic chemical vapor deposition and measured the Seebeck coefficient and electrical conductivity of the grown films with the aim of maximizing the power factor (P). It was found that P decreases as electron concentration (n) increases. The maximum value for P was found to be 7.3 × 10⁻⁴ W/m K² at 750 K in an undoped sample with corresponding values of Seebeck coefficient and electrical conductivity of 280 μV/K and 93␣(Ω cm)⁻¹, respectively. Further enhancement in P is expected by improving the InGaN material quality and conductivity control by reducing background electron concentration.     

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 Heavily Doped <Emphasis Type="Italic">n</Emphasis>-Type SrTiO<Subscript>3</Subscript> Bulk Materials

Three Ta-doped strontium titanates were prepared as potential candidates for n-type thermoelectric oxides. The purity of the polycrystalline samples of SrTi_1−x Ta _x O₃ (x = 0.05 to 0.14) were characterized by means of powder x-ray diffraction and electron probe micro analysis (EPMA). We present the results of Seebeck coefficient, electrical conductivity, and thermal conductivity measurements performed at high temperatures.     

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.     

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 Ca-Filled CoSb<Subscript>3</Subscript>-Based Skutterudites Synthesized by Mechanical Alloying

Ca _z Co_4−x (Fe/Mn) _x Sb₁₂ skutterudites were prepared by mechanical alloying and hot pressing. The phases of mechanically alloyed powders were identified as γ-CoSb₂ and Sb, but they were transformed to δ-CoSb₃ by annealing at 873 K for 100 h. All specimens had a positive Hall coefficient and Seebeck coefficient, indicating p-type conduction by holes as majority carriers. For the binary CoSb₃, the electrical conductivity behaved like a nondegenerate semiconductor, but Ca-filled and Fe/Mn-doped CoSb₃ showed a temperature dependence of a degenerate semiconductor. While the Seebeck coefficient of intrinsic CoSb₃ increased with temperature and reached a maximum at 623 K, the Seebeck coefficient increased with increasing temperature for the Ca-filled and Fe/Mn-doped specimens. Relatively low thermal conductivity was obtained because fine particles prepared by mechanical alloying lead to phonon scattering. The thermal conductivity was reduced by Ca filling and Fe/Mn doping. The electronic thermal conductivity was increased by Fe/Mn doping, but the lattice thermal conductivity was decreased by Ca filling. Reasonable thermoelectric figure-of-merit values were obtained for Ca-filled Co-rich p-type skutterudites.     

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 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.     

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.     

Thermal Conductivity and Other Transport Properties of Mg<Subscript>2</Subscript>Sn:Ag Crystals

The temperature dependence of the thermal conductivity κ(T), electrical resistivity ρ(T), and Seebeck coefficient S(T) of Mg₂Sn:Ag crystals with 0 at.% to 1 at.% Ag content were measured at T = 2 K to 400 K. The crystals were cut from ingots that were prepared by the vertical Bridgman method. Undoped samples show a dramatic κ ∝ T ³ rise at low temperatures to a peak value κ _15K = 477 W m⁻¹ K⁻¹. This leads to exceptionally large phonon drag effects causing giant thermopower with S rising sharply to a peak value S _20K = 3000 μV K⁻¹. At higher temperatures S decreases and changes sign to intrinsic values S ≈ −60 μV K⁻¹. The addition of Ag changes the transport properties as follows: (a) κ decreases systematically, the peak shifts to 30 K and falls to 7 W m⁻¹ K⁻¹; (b) ρ changes from high to low values; (c) S(T) changes to a linear dependence with S _300K ≈ 150 μV K⁻¹ to 200 μV K⁻¹.     

Thermoelectric Properties of the Narrow-Gap Intermetallic Compound Ga<Subscript>2</Subscript>Ru: Effect of Re Substitution for Ru Atoms

In this paper, the effect of hole doping on the thermoelectric properties of the binary narrow-gap semiconducting intermetallic compound Ga₂Ru in the temperature range from 373 K to 973 K was investigated. We synthesized sintered pellets by spark plasma sintering (SPS) after arc-melting and succeeded in preparing crack-free samples. The maximum dimensionless figure of merit ZT _max was 0.50 at 773 K for the sintered Ga₂Ru alloy. The temperature dependence of the electrical resistivity and its magnitude at 373 K dramatically changed from negative (~11,000 μΩcm) to positive (~200 μΩcm) upon hole doping by the substitution of Re for Ru atoms. Also, the Seebeck coefficient at 373 K changed from 300 μV/K to 75 μV/K. These changes were identified by the increase in carrier concentrations observed by Hall- effect measurements. In particular, large power factors (2.0 mW/m K² to 3.0 mW/m K²) were obtained over a wide temperature range from 373 K to 973 K upon Re substitution. The lattice thermal conductivity beneficially decreased with increasing Re concentration as a result of an alloying effect.