Thermoelectric Properties of (Bi1−x Sb x )2S3 with Orthorhombic Structure

Thermoelectric properties of the substitution system (Bi_1?x Sb _x )₂S₃ have been investigated, where binary Bi₂S₃ and Sb₂S₃ are narrow-gap semiconductors. It is confirmed that metallic conduction, originating from mobile electrons due to production of sulfur vacancies, is observed in Bi₂S₃ over a wide temperature range below room temperature. In Sb₂S₃, mobile carriers are not created and insulating behavior is observed because of the considerably wide bandgap. Change of the carrier number by substitution of antimony contributes strongly to the thermoelectric properties (resistivity and Seebeck coefficient). As a result, the nondimensional figure of merit, ZT, decreases monotonically with increasing antimony content. The maximum value of ZT is obtained in Bi₂S₃ as ZT ≈ 0.1 at room temperature. It is pointed out that control of the carrier number, which is achieved by production of sulfur vacancies, is important to achieve high thermoelectric performance in the (Bi_1?x Sb _x )₂S₃ system. It is possible that the thermoelectric efficiency could be improved by control of the carrier concentration in the bismuth-rich region, including pure binary Bi₂S₃.     

Solid-State Synthesis and Thermoelectric Properties of Mg2Si1−x Sn x

Mg₂Si_1?x Sn _x (0 ≤ x ≤ 1) solid solutions have been successfully prepared by mechanical alloying and hot pressing as a solid-state synthesis route. All specimens were identified as phases with antifluorite structure. The electrical conduction changed from n-type to p-type at room temperature for x ≥ 0.5 due to the intrinsic properties of Mg₂Sn. The absolute value of the Seebeck coefficient decreased with increasing temperature, and the electrical conductivity increased with increasing temperature; this is indicative of nondegenerate semiconducting behavior. The thermal conductivity was reduced by Mg₂Si-Mg₂Sn solid solution due to phonon scattering by the alloying effect.     

Thermoelectric Properties of Light-Element-Containing Zintl Compounds CaZn2−x Cu x P2 and CaMnZn1−x Cu x P2 (x = 0.0–0.2)

Light-element-containing CaAl₂Si₂-type Zintl phases CaZn_2?x Cu _x P₂ and CaMnZn_1?x Cu _x P₂ (x = 0.0–0.2) have been synthesized by solid-state reaction. Electrical resistivity (ρ), Seebeck coefficient (α), and thermal conductivity (κ) were measured over a wide temperature (T) range (80–1000 K) to evaluate the thermoelectric potential of these materials. Below 300 K, the power factor (PF; α ²/ρ) is very small. Above 600 K, however, PF increases rapidly for all compositions because of a rapid increase of α and a simultaneous decrease of ρ. The measured large α is consistent with the wider band gap expected for these compositions. Compared with the pure compounds, larger PF values are observed for the Cu-substituted compounds; the largest observed PF is ～0.5 mW/m K². The thermal conductivity is found to be rather low, despite the presence of light elements, and is in the range 1.0–1.5 W/m K at 1000 K. Because of the combination of low κ and moderate PF values, the dimensionless figure of merit ZT = α ² T/ρκ reaches a maximum of 0.4 for CaZn_1.9Cu_0.1P₂.     

Effect of Fe Substitution on Thermoelectric Properties of Fe x In4−x Se3 Compounds

Starting from elemental powder mixtures of Fe _x In_4?x Se₃ (x = 0, 0.05, 0.1, 0.15), polycrystalline In₄Se₃-based compounds with homogeneous microstructures were prepared by mechanical alloying (MA) and hot pressing (HP). With the increase of x from 0 to 0.15, the electrical resistivity and the absolute value of the Seebeck coefficient increased, while the thermal conductivity first decreased and then increased. The maximal dimensionless figure of merit ZT of 0.44 was obtained for the Fe _x In_4?x Se₃ (x = 0.05) sample at 723 K.     

Structural and Thermoelectric Properties of Bi1−x Sb x Nanoalloys Prepared by Mechanical Alloying

Bi_1?x Sb _x nanoparticles were prepared by mechanical alloying and compacted using different techniques. The influence of the composition as well as the pressing conditions on the thermoelectric performance was investigated. A strong dependence of the thermoelectric properties on the composition was found, which deviates from the behavior of single crystals. The results indicate a significant change in the band structure of the material induced by the reduced size. The influence of the pressing conditions on the thermoelectric properties also showed composition dependence. The results show that the compacting method has to be chosen carefully.     

Thermoelectric Properties of CdTe1−x Cl x Material Prepared by Spark Plasma Sintering Method

CdTe compound is a prospective thermoelectric material due to its high Seebeck coefficient and low thermal conductivity. In the present study, we optimized its carrier concentration by substituting Cl on the Te site in order to improve the electrical conductivity and decrease the lattice thermal conductivity. The polycrystalline CdTe_1?x Cl _x (x = 0.005, 0.01, 0.03, 0.05) samples were fabricated by solid state reaction followed with spark plasma sintering, and the relative densities of the sintered samples were higher than 98%. Thermoelectric properties, including Seebeck coefficient (α), electrical conductivity (σ). and thermal conductivity (κ), were measured in the temperature range of 300–700 K. The increase of Cl content (x) caused an increase of σ, and the maximum ZT value of 0.2 was obtained at about 630 K for the CdTe_0.97Cl_0.03 sample.     

Thermoelectric Properties of Y1−x Ag x BaCo4O7+δ Ceramics

The thermoelectric properties of the Ag-doped ceramics Y_1?x Ag _x BaCo₄O_7+δ (x = 0.0, 0.05, 0.1, 0.15, and 0.2) were investigated from 373 K to 973 K. The results show that the doping of Ag can reduce the electrical resistivity. The Seebeck coefficients of the samples decrease when the Ag doping amount is small, but increase when the Ag doping amount is large. The activation energy of the electrical conductivity was calculated using Arrhenius plots, and it was found that the activation energy descends with increase of the Ag doping amount. According to the power factors, the optimum Ag doping amount is x = 0.15, which results in a higher power factor of 81 μW m^?1 K^?2 at 973 K, 72.7% higher than for the sample without Ag doping.     

Thermoelectric Properties of Hot-Pressed β-K2Bi8Se13−x S x Materials

In this work, hot-pressed pellets of the K₂Bi₈Se₁₃ family of compounds were prepared for the first time. The pellet fabrication of selected members of the K₂Bi₈Se_13?x S _x series was studied. Sintering parameters, such as temperature, pressure, and duration, were investigated based on a statistical design- of-experiments approach to identify the optimum conditions for fabrication of high-quality pellets. These optimum conditions were then applied for the K₂Bi₈Se_13?x S _x series, and the thermoelectric properties of the stoichiometric members for x = 0, 4, 6, and 8 were studied. Doping experiments were also investigated using sulfur excess in the x = 6 member in an attempt to modify its properties.     

Thermoelectric Properties of Nanostructured Bismuth-Doped Lead Telluride Bi x (PbTe)1−x Prepared by Co-Ball-Milling

In this work we present a simple method to synthesize nanostructured, bismuth-doped lead telluride by co-ball-milling. The obtained nanopowders were compacted via either a cold pressing/annealing approach or by hot pressing. The two compacting methods were compared regarding sample density. Series with bismuth content up to 6 at.% were characterized by measuring the thermoelectric transport properties over a wide temperature range between 123 K and 773 K using two different techniques for the Seebeck coefficient and electrical conductivity. A decreasing thermal conductivity and increasing electrical conductivity were found with increasing doping level. The best results were obtained for samples with 5 at.% and 6 at.% bismuth, showing a maximum ZT value of 1.1 at 773 K. Transmission electron microscopy study was performed to analyze the microstructure of the nanopowders, suggesting that, in addition to n-type doping of the lead telluride matrix, segregation effects occur and the samples consist of multiple phases.     

Enhanced Thermoelectric Performance of Nonstoichiometric Compounds Cu3−x SbSe4 by Cu Deficiencies

This work revealed that the Cu-deficient ternary compounds Cu_3?x SbSe₄ free of Te and Pb exhibit enhanced thermoelectric performance. Cu_3?x SbSe₄ (x = 0, 0.025, 0.050, 0.075) polycrystalline materials with high phase purity were fabricated by a facile method combining mechanical alloying and spark plasma sintering. Effects of Cu deficiencies on crystal structures, microstructures, element chemical states, and thermoelectric properties were systematically studied. High carrier concentration was obtained for the compositions Cu_2.95SbSe₄ and Cu_2.925SbSe₄ due to additional Cu vacancies, contributing to a remarkable increase in electrical conductivity. Together with a satisfactorily large Seebeck coefficient above 300 μV/K, a high power factor of about 890 μW/m-K² at 523 K was achieved for Cu_2.95SbSe₄ and Cu_2.925SbSe₄, almost 60% larger than that of the stoichiometric sample with x = 0. The maximum ZT value was increased to 0.50 at 673 K in the Cu_2.925SbSe₄ sample sintered at a high temperature (703 K); this is the highest value reported so far for the undoped Cu₃SbSe₄ system.     

Structural and Magnetic Study of Cu x FeCr1−x O2 Oxides Under High External Magnetic Fields

The structural, electronic, and magnetic behaviors of Cu _x FeCr_1?x O₂ polycrystals are investigated. Investigations are conducted for increasing chromium substitution, according to varying x values in the formula versus copper, for x = 0, 0.2, 0.4, 0.6, 0.8, and 1. The magnetic response of polycrystalline samples under increasing external magnetic field from 0.4 T to 5 T is also studied. The partial crystal structure deformation/transition from delafossite CuFeO₂ structure to corundum-type FeCrO₃ structure containing CrO₂ and Cr₂O₃ blocks is determined. The change in the crystal structure geometry with increasing Cr substitution is observed. Besides, prominent changes in magnetic ordering are observed from antiferromagnetic (x = 1, 0.8, and 0.6) to ferromagnetic ordering (x = 0.4 and 0.2) for high applied external magnetic fields above 2 T.     

Thermoelectric Properties of Ga-Doped Ba8Al x Si46−x Clathrate

Single-crystalline Ba₈Al _x Ga _y Si_46?x?y clathrates were synthesized by the arc melting method and Czochralski method without subsequent treatment, and their thermoelectric properties were compared with those of Ba₈Al _x Ga _y Si_46?x?y and Ba₈Al _x Si_46?x clathrates with almost the same carrier concentration as estimated from the similar Seebeck coefficient and the Zintl concept. The resistivity of Ba_7.8Al_5.3Ga_7.4Si_33.3 was lower than that of Ba_7.9Al_12.6Si_33.4. The specific electrical resistance of Ba_7.9Al_12.6Si_33.4 and Ba_7.8Al_5.3Ga_7.4Si_33.3 was 0.573 mΩ cm and 0.282 mΩ cm at 750 K, respectively. The band structure of Ba₈Al₈Ga₈Si₃₀ and Ba₈Al₁₆Si₃₀ was estimated by first-principle calculations using density functional theory with the local density approximation. Based on these calculations, it was found that the shape of the bottom of the conduction band for Ba₈Al _x Si_46?x clathrate changed slightly on Ga doping and the radius of curvature of the bottom of the conduction band for Ba₈Al₈Ga₈Si₃₀ clathrate was lower than that for Ba₈Al₁₆Si₃₀ clathrate. These results indicate that the mobility was enhanced by Ga doping of Ba₈Al _x Si_46?x clathrate. We also synthesized single-crystalline Ga-doped Ba₈Al _x Si_46?x clathrate. The electrical resistivity decreased dramatically due to the single-crystallization because of reduced electron scattering on grain boundaries. These results suggest that Ga doping and single-crystallization are effective for improvement of the thermoelectric properties of Ba₈Al _x Si_46?x clathrate.     

Improved Thermoelectric Properties of Se-Doped n-Type PbTe1−x Se x (0 ≤ x ≤ 1)

Enhancement of the thermoelectric figure of merit is of prime importance for any thermoelectric material. Lead telluride has received attention as a potential thermoelectric material. In this work, the effect of Se substitution has been systematically investigated in PbTe_1?x Se _x . The thermoelectric properties of synthesized alloys were measured in the temperature range of 300 K to 873 K. For the particular composition of x = 0.5, α was highest at ~292 μV/K, while k was lowest at ~0.75 W/m-K, resulting in the highest dimensionless figure of merit of ZT ≈ 0.95 at 600 K. The increase in thermopower for x = 0.5 can be attributed to the high distortion in the crystal lattice which leads to the formation of defect states. These defect states scatter the majority charge carriers, leading to high thermopower and high electrical resistivity. The dramatic reduction of the thermal conductivity for x = 0.5 can be attributed to phonon scattering by defect states.     

Enhanced Thermoelectric Properties of Hole-Doped Bi2−x Ba x Sr2Co2O y Ceramics

The influence of Ba doping on the thermoelectric properties of Bi_2?x Ba _x Sr₂ Co₂O _y (x = 0.00, 0.025, 0.05, 0.075, 0.10, 0.125, and 0.15) samples prepared by the solid-state reaction method was investigated from 333 K to 973 K. For the samples with x ≤ 0.075, the electrical resistivity decreased with increase of the Ba doping amount due to p-type doping and they exhibited metallic electrical conductivity behavior, whereas the samples with x ≥ 0.10 exhibited semiconductor-like electrical conductivity behavior. The Seebeck coefficients of all the samples decreased with increase of the Ba doping amount. The thermal conductivity first decreased for x ≤ 0.075, then increased with higher Ba doping amounts. As an overall result, the dimensionless figure of merit (ZT) of Bi_1.925Ba_0.075Sr₂Co₂O _y reached the maximum value of 0.245 at 973 K, being 41% higher than that of the undoped sample.     

Thermoelectric Properties of Ca3−x Dy x Co3.95Ga0.05O9+δ

Ca₃Co₄O_9+δ and Ca_3?x Dy _x Co_3.95Ga_0.05O_9+δ (x = 0.05, 0.10) samples were prepared by conventional solid-state synthesis, and their thermoelectric properties measured at 25 K to 300 K. The x-ray diffraction patterns revealed that all the samples are single phase. The thermopower of all the samples was positive, indicating that the predominant carriers are holes over the entire temperature range. The highest power factor among all the samples (2.45 μW cm^?1 K^?2 at 132 K) was obtained for Ca_2.9Dy_0.1Co_3.95Ga_0.05O_9+δ , being about 80% higher than that of the undoped sample. Ca_2.9Dy_0.1Co_3.95Ga_0.05O_9+δ had the highest dimensionless figure of merit of 0.033 at 300 K, representing an improvement of about 74% compared with undoped Ca₃Co₄O_9+δ .     

Thermoelectric Properties of Ca3−x Ag x Co3.95Fe0.05O9+δ (0 ≤ x ≤ 0.3)

Polycrystalline samples of Ca_3?x Ag _x Co_3.95Fe_0.05O_9+δ (x = 0.0, 0.1, 0.2, and 0.3) have been prepared by conventional solid-state synthesis. The x-ray diffraction patterns reveal that all the samples are single phase. The thermopower of all the samples is positive, indicating that the predominant carriers are holes over the entire temperature range. The electrical resistivity and thermopower simultaneously decrease with increasing Ag content. The highest power factor (2.66 μW cm^?1 K^?2 at 150 K) is reached for the Ca_2.8Ag_0.2Co_3.95Fe_0.05O_9+δ , representing an improvement of about 16 % compared to the undoped sample. These results suggest that the Ag is an effective doping element for enhancing the thermoelectric properties of Ca₃Co_3.95Fe_0.05O_9+δ.