Thermoelectric Properties of the Bi2Te3 Compound Prepared by an Aqueous Chemical Method Followed by Hot Pressing

Bi₂Te₃ powders were synthesized at 453 K from Bi(NO₃)₃·5H₂O and Te by an aqueous chemical method, then bulk material was fabricated by hot pressing at 673 K. To investigate the effect of microstructure on the transport properties of electrons and phonons, the thermoelectric performance of the hot-pressed sample and a zone-melted crystal with similar chemical composition was measured and compared. Strong grain-boundary scattering caused a significant decrease of the thermal conductivity; however, the maximum ZT value of the hot-pressed sample was 0.28, which was lower than that of the zone-melted crystal. Further improvement could be obtained through optimization of both chemical composition and microstructure.     

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.     

Eutectic Microstructure and Thermoelectric Properties of Mg2Sn

Ingots of undoped and Ag-doped Mg₂Sn were prepared from the melt using a rocking Bridgman furnace at different cooling rates: slow cooling (0.1 K/min), moderate cooling (1 K/min), and rapid quenching. The ingots show very different microstructure and thermoelectric properties. Slow-cooled ingots consist of large Mg₂Sn crystals with minor inclusions. Moderate-cooled ingots show significant variation in composition and microstructure, with Mg-rich material at the topmost section of the ingot and Sn-rich material at the bottom surface of the ingot. Rapid quenching results in ingots with finely dispersed Mg + Mg₂Sn eutectic microstructure in the form of lamellae 200 nm to 500 nm in thickness. Measurements of the Seebeck coefficient and electrical conductivity in the temperature range of T = 80 K to 700 K were carried out to establish correlations between the microstructure and the thermoelectric properties.     

Effects of Synthesis and Spark Plasma Sintering Conditions on the Thermoelectric Properties of Ca3Co4O9+δ

Ca₃Co₄O_9+δ samples were synthesized by solid-state (SS) and sol–gel (SG) reactions, followed by spark plasma sintering under different processing conditions. The synthesis process was optimized and the resulting materials characterized with respect to their microstructure, bulk density, and thermoelectric transport properties. High power factors of about 400 μW/m·K² and 465 μW/m·K² (at 800°C) were measured for SS and SG samples, respectively. The improved thermoelectric performance of the SG sample is believed to originate from the smaller particle sizes and better grain alignment. The SG method is suggested to be a beneficial means of obtaining high-performance thermoelectric materials of Ca₃Co₄O_9+δ type.     

The Influence of Grain Boundary Scattering on Thermoelectric Properties of Mg2Si and Mg2Si0.8Sn0.2

The temperature dependences of the Seebeck coefficient, and electrical and thermal conductivities of bulk hot-pressed Sb-doped n-type Mg₂Si and Mg₂Si_0.8Sn_0.2 samples were measured in the temperature range from 300 K to 850 K together with the Hall coefficients at room temperature. The features of the complex band structure and scattering mechanisms were analyzed based on experimental data within the relaxation-time approximation. Based on the obtained model parameters, the possibility of improvement of the thermoelectric figure of merit due to nanostructuring and grain boundary scattering was theoretically analyzed for both Mg₂Si and the solid solution.     

Thermoelectric Properties of Highly Deformed and Subsequently Annealed p-Type (Bi0.25Sb0.75)2Te3 Alloys

The effects of mechanical deformation and subsequent annealing on the thermoelectric properties and microstructure have been investigated for p-type (Bi_0.25Sb_0.75)₂Te₃ alloys prepared by melting followed by quenching. The mechanically deformed pellets were prepared by repetition of cold-pressing of quenched samples at room temperature. Cold-pressed pellets were then annealed at 300°C in vacuum, and the thermoelectric properties and microstructure were traced through the course of the heat treatment. For the heavily deformed samples, the Seebeck coefficient rapidly increased at the very early stage of annealing and did not change as the annealing time increased, due to recrystallization of a new δ-phase which equilibrated at the annealing temperature of 300°C (δ₃₀₀-phase). At the initial stage of annealing (recovery stage), the electrical resistivity sharply increased, probably due to the interaction of antistructural defects with vacancies produced during the cold-pressing treatment. However, for the lightly deformed samples, recrystallization occurred only at some portion of the grain boundaries, and the newly generated δ₃₀₀-phase slowly replaced the original, as-solidified δ_ingot-phase.     

Influence of Sedimentation of Atoms on Structural and Thermoelectric Properties of Bi-Sb Alloys

Functionally graded thermoelectric materials (FGTMs) have been prepared by sedimentation of atoms under a strong gravitational field. Starting samples of Bi _x Sb_1?x alloys with different composition x were synthesized by melting of metals and subsequent annealing of quenched samples. The thermoelectric properties (Seebeck coefficient, electrical conductivity) of the starting materials were characterized over the temperature range from 300 K to 525 K. Strong gravity experiments were performed in a unique ultracentrifuge apparatus under acceleration of over 0.5 × 10⁶ G at temperatures of 538 K and 623 K. Changes of the microstructure and chemical composition were analyzed using scanning electron microscopy with energy-dispersive x-ray spectroscopy analysis. The distribution of the Seebeck coefficient of the Bi-Sb alloys was characterized by scanning thermoelectric microprobe. As a result of sedimentation, large changes in chemical composition (x = 0.45 to 1) were obtained. It was found that the changes in chemical composition were correlated with alterations of the Seebeck coefficient. The obtained experimental data allowed the development of a semiempirical model for the selection of optimal processing parameters for preparation of Bi-Sb alloys with required thermoelectric properties.     

The Effect of Grain Size and Density on the Thermoelectric Properties of Bi2Te3-PbTe Compounds

The effects of microstructure on thermoelectric properties were investigated in Bi₂Te₃-PbTe compounds of different grain size and density. Powders of two different sizes [0.1 μm to 1 μm (micropowder) and <50 nm (nanopowder)] were prepared from Bi₂Te₃-PbTe ingots by ball milling and high-energy ball milling. Three different samples were spark plasma sintered from each powder and the mixture of the two powders. The grain size and relative density of the sintered samples varied from 100 nm to a few micrometers and 89.7% to 97.3%, respectively. The dimensionless figure of merit zT of the sample sintered from nanopowder was about 0.50 at 500 K, being about 3.3 times larger than that of the sample sintered from micropowder (～0.15 at 500 K), when the relative density of the former and the latter were 89.7% and 97.3%, respectively. The improved thermoelectric performance of the samples may originate from the decrease of the thermal conductivity, which was caused by the decrease of the grain size and the increase of the amount of pores.     

Thermal Conductivity of Exfoliated p-Type Bismuth Antimony Telluride

A bulk p-type thermoelectric compound with nominal composition Bi_0.5Sb_1.5 Te₃ has been exfoliated using dimethyl sulfoxide as a solvent. Samples have been prepared from the exfoliated platelets by pressing followed by sintering or hot pressing. The exfoliated nanoplatelets have been characterized for size distribution and composition using scanning electron microscopy and energy-dispersive spectrometry. The smallest size platelet was 40 nm, and the maximum in the size distribution was near 80 nm. The exfoliated platelets and sintered sample showed significant deficiency in Sb and Te. The nonstoichiometry in the composition of the exfoliated platelets indicates that the mechanism of exfoliation may not be between quintuplets only, with other layers also being active. The composition of the hot-pressed sample remained closer to that of the bulk. Results of x-ray diffraction indicated the presence of Bi₂Te₃ and Bi_0.5Sb_1.5Te₃ phases and pure Te and Sb. Residual porosity was observed in the hot-pressed and sintered samples. The thermal conductivity of the samples was measured by transient thermoreflectance. The results showed that the thermal conductivity of the hot-pressed sample was reduced by a factor of two compared with that of the bulk as a result of the presence of a high density of interfaces and residual porosity. The thermal conductivity of the sintered sample showed an increase above that of the bulk sample, which is explained by the change in composition due to loss of Sb and Te.     

Ab Initio Calculations and Measurements of Thermoelectric Properties of V2O5 Films

Density functional theory and the Boltzmann transport equation were used to calculate the thermoelectric transport coefficients for bulk V₂O₅ and MV₂O₅ (M = Cr, Ti, Na, Li). The structural relaxation for the given compounds based on the ABINIT code was observed. The temperature dependences of the Seebeck coefficients as well as electrical and thermal electrical conductivities of all relaxed structures displayed anisotropic behavior. Electrooptical measurements of thermoelectric properties were carried out on V₂O₅ thin films obtained by thermal evaporation with different post-annealing treatments. A Seebeck coefficient of ?148 μV/K at T = 300 K was obtained in the in-plane direction for V₂O₅ thin films with thickness less than 100 nm.     

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.     

Preparation by Poly(Acrylic Acid) Sol–Gel Method and Thermoelectric Properties of γ-Na<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>CoO<Subscript>2</Subscript> Bulk Materials

γ-Na _x CoO₂ single-phase powders have been synthesized by a poly(acrylic acid) (PAA) sol–gel (SG) method, and γ-Na _x CoO₂ bulk ceramic fabricated using spark plasma sintering. The effects of the PAA concentration on the sample phase composition and morphology were investigated. The thermoelectric properties of the γ-Na _x CoO₂ bulk ceramic were also studied. The results show that the PAA concentration did not significantly affect the crystalline phase of the product. However, agglomeration of γ-Na _x CoO₂ crystals was suppressed by the steric effect of PAA. The Na _x CoO₂ bulk ceramic obtained using the PAA SG method had higher crystallographic anisotropy, better chemical homogeneity, and higher density than the sample obtained by solid-state reaction (SSR), leading to improved thermoelectric performance. The PAA SG sample had power factor (in-plane PF = σS ²) of 0.61 mW m^?1 K^?2 and dimensionless figure of merit (ZT) along the in-plane direction of 0.19 at 900 K, higher than for the SSR sample (in-plane PF = 0.51 mW m^?1 K^?2, in-plane ZT = 0.17). These results demonstrate that a simple and feasible PAA SG method can be used for synthesis of Na _x CoO₂ ceramics with improved thermoelectric properties.     

Low Thermal Conductivity and High Thermoelectric Performance in In<Subscript>4</Subscript>Se<Subscript>3−<Emphasis Type="Italic">x</Emphasis></Subscript> with Phase-Separated Indium Inclusions

We report the thermoelectric properties of undoped hot-pressed In₄Se_3?x (x = 0.05). Stoichiometric imbalance due to selenium deficiency in In₄Se₃ was found to create a secondary phase of elemental indium in the host material. Heat treatment drove grain growth and increased the indium solubility in In₄Se₃. Indium-rich domains at grain surfaces/boundaries in untreated samples were found to redistribute inside the grains and their junctions after heat treatment. Due to enhanced phonon scattering by secondary phase of indium, very low values of thermal conductivity were observed for all samples, leading to a maximum thermoelectric figure of merit (zT) of 1.13 at 723 K along the hot-pressing direction for the heat-treated sample.     

Thermoelectric Properties of Hot-Pressed and PECS-Sintered Magnesium-Doped Copper Aluminum Oxide

Copper aluminum oxide (CuAlO₂) is considered as a potential candidate for thermoelectric applications. Partially magnesium-doped CuAlO₂ bulk pellets were fabricated using solid-state reactions, hot-pressing, and pulsed electric current sintering (PECS) techniques. X-ray diffraction and scanning electron microscopy were adopted for structural analysis. High-temperature transport property measurements were performed on hot-pressed samples. Electrical conductivity increased with Mg doping before secondary phases became significant, while the Seebeck coefficient displayed the opposite trend. Thermal conductivity was consistently reduced as the Mg concentration increased. Effects of Mg doping, preparation conditions, and future modification on this material’s properties are discussed.     

Investigation on Indium-Filled Skutterudite Materials Prepared by Combining Hydrothermal Synthesis and Hot Pressing

Indium-filled CoSb₃ materials have been investigated by combining hydrothermal synthesis and hot pressing. The materials were prepared as follows. Corresponding nanopowders were synthesized by a hydrothermal synthesis method, in some cases followed by melting at 1373 K, and then hot pressed at 923 K. The phase composition and microstructure of the bulk materials were analyzed by conventional methods, such as x-ray diffraction (XRD), scanning electron microscopy (SEM), and field-emission SEM equipped with energy-dispersive x-ray spectroscopy (EDS). The thermoelectric properties of the bulk materials were measured from room temperature to 773 K. The results reveal that indium can be successfully filled into the voids of the CoSb₃ structure when the sample preparation procedure contains melting. The influence of the processing on the thermoelectric properties of the materials is also discussed.     

Manufacturing Processes for TiO x -Based Thermoelectric Modules: from Suboxide Synthesis to Module Testing

An overview of different TiO _x synthesis methods with regard to enhancement of thermoelectric properties and transfer of the synthesis process to cost-efficient methods as well as joining techniques for module manufacture is presented. Different synthesis routes were applied and investigated, namely synthesis of TiO _x via reduction with less gas formation by mixing TiO₂ and TiC [powder-derived (PD)-TiO _x ], a bottom-up approach via a precursor route for synthesizing TiO _x directly [precursor-derived (PDC)-TiO _x ], and the combination by mixing TiO₂ with precursor (PDC-TiO _x /TiO₂). All the approaches resulted in adjustable phase composition with different oxygen contents and, therefore, adjustable electrical properties as well as different microstructures to enhance the physical and thermoelectric properties. The electrical resistivity could be adjusted from 1 mΩ cm to 1000 mΩ cm through the oxygen content of TiO _x . The research included investigations of cost-efficient production processes for thermoelectric material such as spray-drying, spark plasma sintering, hot pressing or pressureless sintering in terms of shaping, sintering, and machining, as well as joining techniques to build a complete thermoelectric module. To realize thermal and electrical connections, technologies for joining and packaging were developed. For a first demonstration of the feasibility of TiO _x -based thermoelectric modules for use at high temperatures, a unileg n-type module with footprint of 30 mm × 30 mm was designed. Low-volume fabrication yielded more than 20 single modules. Finally, the modules were successfully tested under conditions close to those of the desired applications with hot-side temperature up to 600°C.     

Thermoelectric Properties for a Suspended Microribbon of Quasi-One-Dimensional TiS<Subscript>3</Subscript>

Transition-metal trichalcogenides MX₃ (M = Ti, Zr, Nb, Ta; X = S, Se) are well-known inorganic quasi-one-dimensional conductors. Among them, we have investigated the thermoelectric properties of titanium trisulfide TiS₃ microribbon. The electrical resistivity ρ, thermal conductivity κ, and thermoelectric power S were measured using 3ω method. The weight mean values were found to be ρ = 5 mω m and κ = 10 W K^?1 m^?1 along the one-dimensional direction (b-axis) of the TiS₃ microribbon. Combined with the thermoelectric power S = ?530 μV K^?1, the figure of merit was calculated as ZT = 0.0023. This efficiency is the same as that of randomly oriented bulk TiS₃. We also estimated the anisotropy of σ and κ using the present results and those for randomly oriented bulk material. The obtained weak anisotropy for TiS₃ is attributable to strong coupling between triangular columns consisting of TiS₃ units. These experimental results are consistent with theoretical results obtained using density functional theory (DFT) calculations.     

Effects of Synthesis Temperature on the Microstructure and Thermoelectric Properties of Te-Se Codoped Skutterudites

Skutterudite compounds Co₄Sb_11.3Te_0.5Se_0.2 were synthesized by solid-state reaction at different temperatures (853 K, 903 K, 953 K, and 1003 K) with subsequent spark plasma sintering. x-Ray diffraction, field-emission scanning electron microscopy, and electron probe microanalysis were utilized to analyze the phase structure, microstructure, and actual compositions of the samples. The results showed that the actual composition and the grain size vary with the synthesis temperature. The thermoelectric properties of all samples were measured in the temperature range of 300 K to 800 K. As the synthesis temperature increases, the electrical conductivity increases rapidly, the absolute Seebeck coefficient falls, and the thermal conductivity first decreases and then increases. The highest dimensionless figure of merit ZT was achieved for the sample synthesized at 953 K, exceeding 1.0 at high temperature.     

Enhanced Thermoelectric Properties of Antimony Telluride Thin Films with Preferred Orientation Prepared by Sputtering a Fan-Shaped Binary Composite Target

p-Type antimony telluride (Sb₂Te₃) thermoelectric thin films were deposited on BK7 glass substrates by ion beam sputter deposition using a fan-shaped binary composite target. The deposition temperature was varied from 100°C to 300°C in increments of 50°C. The influence of the deposition temperature on the microstructure, surface morphology, and thermoelectric properties of the thin films was systematically investigated. x-Ray diffraction results show that various alloy composition phases of the Sb₂Te₃ materials are grown when the deposition temperature is lower than 200°C. Preferred c-axis orientation of the Sb₂Te₃ thin film became obvious when the deposition temperature was above 200°C, and thin film with single-phase Sb₂Te₃ was obtained when the deposition temperature was 250°C. Scanning electron microscopy reveals that the average grain size of the films increases with increasing deposition temperature and that the thin film deposited at 250°C shows rhombohedral shape corresponding to the original Sb₂Te₃ structure. The room-temperature Seebeck coefficient and electrical conductivity range from 101 μV K^?1 to 161 μV K^?1 and 0.81 × 10³ S cm^?1 to 3.91 × 10³ S cm^?1, respectively, as the deposition temperature is increased from 100°C to 300°C. An optimal power factor of 6.12 × 10^?3 W m^?1 K^?2 is obtained for deposition temperature of 250°C. The thermoelectric properties of Sb₂Te₃ thin films have been found to be strongly enhanced when prepared using the fan-shaped binary composite target method with an appropriate substrate temperature.