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As a field-assisted technique, spark plasma sintering (SPS) enables densification of specimens in a very short period of time compared to other sintering techniques. For high performance thermoelectric material synthesis, SPS is widely used to fabricate nanograin-structured thermoelectric materials by rapidly densifying the nanopowders suppressing grain growth. However, the microstructural evolution behavior of thermoelectric materials by SPS, another important process during sintering, has been rarely studied. Here, we explore SPS as a tool to control the microstructure by long-time SPS. Using p-type (Bi 0.25 Sb 0.75 ) 2 Te 3 thermoelectric materials as a model system, we systematically vary SPS temperature and time to understand the correlations between SPS conditions, microstructural evolution, and the thermoelectric properties. Our results show that the relatively low eutectic temperature (∼420 °C) and the existence of volatile tellurium (Te) are critical factors to determine both microstructure and thermoelectric property. In the liquid-phase sintering regime, rapid evaporation of Te leads to a strong dependence of thermoelectric property on SPS time. On the other hand, in the solid-phase sintering regime, there is a weak dependence on SPS time. The optimum thermoelectric figure-of-merit (Z) of 2.93 × 10 −3 /K is achieved by SPS at 500 °C for 30 min. Our results will provide an insight on the optimization of SPS conditions for materials containing volatile elements with low eutectic temperature.

Thermoelectric devices have attracted a great attention for renewable energy harvesters and solid-state coolers. For practical applications, the mechanical properties of thermoelectric materials become critical for the device reliability, a persistent performance with a long time and high operation cycles. Bi–Te based single-crystals, mostly used in commercial thermoelectric devices, are intrinsically brittle with weak van der Waals bonding, often leading to device failures such as crack and debonding during fabrication and operation. Thus, it is highly desirable to enhance the mechanical property of Bi–Te based alloys as well as the thermoelectric property. Here, we investigate the effect of B 4 C nanoparticles (less than 0.5 wt%) dispersed in p -type Bi 0.4 Sb 1.6 Te 3 matrix on the mechanical properties. X-ray diffraction (XRD) result confirms that B 4 C-dispersed Bi 0.4 Sb 1.6 Te 3 has a single phase. We observe that the grain size of Bi 0.4 Sb 1.6 Te 3 becomes decreased with the B 4 C nanoparticle concentration by electron backscatter diffraction (EBSD) technique. Hardness, Young’s modulus, and flexural strength of B 4 C-dispersed Bi 0.4 Sb 1.6 Te 3 are enhanced, compared to the B 4 C-free Bi 0.4 Sb 1.6 Te 3 polycrystals. On the other hand, the thermoelectric figure-of-merit of B 4 C-dispersed Bi 0.4 Sb 1.6 Te 3 is almost identical to that of the pure Bi 0.4 Sb 1.6 Te 3 . Such enhancements of the mechanical properties of the B 4 C-dispersed Bi 0.4 Sb 1.6 Te 3 are attributed to the grain boundary hardening and second-phase hardening. Beyond thermoelectric materials, our result implies that the grain refinement by nanoparticle dispersion is a simple and promising way to strengthen the mechanical properties of other brittle materials with layered structure.

Bi Te -Bi Se alloy which is typical n-type thermoelectric material were grown by traveling heater method (THM) technique. We investigate the effect of the composition of 100-x(Bi Te ) - x(Bi Se ) and doping of n-type dopants such as SbI and CdCl . Maximum figure of merit of Bi Te -Bi Se alloy was observed with CdCl 0.1 wt% (Z: 2.73 × 10 / K) and SbI 0.05 wt% (Z: 2.29 × 10 /K). Deviation along the length of Bi Te -Bi Se ingot grown by THM method is low, which indicates that the ingot is very homogenized. Also we observed the close relationship of between anisotropy ratio and dopant in the 90(Bi Te ) - 10(Bi Se ) alloys. And we confirmed the fact that anisotropy ratio exerts thermoelectric performance in Bi Te based n-type thermoelectric material.

An accurate and rapid characterization of a thermoelectric module (TEM) is critical to understand the problems in module design and fabrication. We describe an apparatus and a method that directly measure the cooling performance of a TEM such as current for maximum cooling (Imax), maximum cooling power (Qc,max), and maximum temperature difference (ΔTmax). The apparatus is designed based on a finite element model to ensure a simple heat flow measurement. We evaluate the module performance metrics based on differential measurement between the cooling powers with temperature difference across the module under transient conditions. The use of transient data reduces measurement time, and the use of a differential technique enables compensation of the thermal losses. The measured data fit well with conventional theoretical relations for the TEM performance metrics. We test a commercial TEM and validate the results using the Harman method.

In the present work, 0.01–0.05wt.% Sn-doped Bi2(Te0.9Se0.1)3 alloys were prepared by mechanical deformation followed by hot pressing, and their thermoelectric properties were studied. We observed that the Sn element is a very effective dopant as an acceptor to control the carrier concentration in the n-type Bi2(Te0.9Se0.1)3 alloys to optimize their thermoelectric property. The n-type carrier concentration can be controlled from 4.2 × 1019/cm3 to 2.4 × 1019/cm3 by 0.05wt.% Sn-doping. While the Seebeck coefficient and the electrical resistivity are both increased with doping, the power factor remains the same. Therefore, we found that the thermoelectric figure-of-merit becomes maximized at 0.75 when the thermal conductivity has a minimum value for the 0.03wt.% Sn-doped sample.

In this work, p-type 20%Bi2Te3–80%Sb2Te3 bulk thermoelectric (TE) materials were prepared by mechanical deformation (MD) of pre-melted ingot and by mechanical alloying (MA) of elemental Bi, Sb, and Te granules followed by cold-pressing. The dependence on annealing time of changes of microstructure and TE properties of the prepared samples, including Seebeck coefficient, electrical resistivity, thermal conductivity, and figure-of-merit, was investigated. For both samples, saturation of the Seebeck coefficient and electrical resistivity were observed after annealing for 1 h at 380°C. It is suggested that energy stored in samples prepared by both MA and MD facilitated their recrystallization within short annealing times. The 20%Bi2Te3–80%Sb2Te3 sample prepared by MA followed by heat treatment had higher a Seebeck coefficient and electrical resistivity than specimens fabricated by MD. Maximum figures-of-merit of 3.00 × 10−3/K and 2.85 × 10−3/K were achieved for samples prepared by MA and MD, respectively.

The thermoelectric properties of I-doped Bi2Te3 films grown by metal-organic chemical vapor deposition have been studied. I-doped epitaxial (00l) Bi2Te3 films were successfully grown on 4° tilted GaAs (001) substrates at 360 °C. I concentration in the Bi2Te3 films was easily controlled by the variation in a flow rate of H2 carrier gas for the delivery of an isopropyliodide precursor. As I ions in the as-grown Bi2Te3 films were not fully activated, they did not influence the carrier concentration and thermoelectric properties. However, a post-annealing process at 400 °C activated I ions as a donor, accompanied with an increase in the carrier concentration. Interestingly, the I-doped Bi2Te3 films after the post-annealing process also exhibited enhancement of the Seebeck coefficient at the same electron concentration compared to un-doped Bi2Te3 films. Through doping I ions into Bi2Te3, the thermopower was also enhanced in Bi2Te3, and a high power factor of 5 × 10−3 W K−2 m−1 was achieved.

The effects of mechanical deformation and subsequent annealing on the thermoelectric properties and microstructure have been investigated for p-type (Bi0.25Sb0.75)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 δingot-phase.

We present the effects of In4Se3 addition on thermoelectric properties of n-type Bi2Te2.7Se0.3. In this study, polycrystalline (In4Se3)x-(Bi2Te2.7Se0.3)1−x pellets were prepared by mechanical alloying followed by spark plasma sintering (SPS). The thermoelectric properties such as Seebeck coefficient and electrical and thermal conductivities were measured in the temperature range of 300 K to 500 K. Addition of In4Se3 into Bi2Te2.7Se0.3 resulted in segregation of In4Se3 phase within Bi2Te2.7Se0.3 matrix. The Seebeck coefficient of the (In4Se3)x-(Bi2Te2.7Se0.3)1−x samples exhibited lower values compared with that of pure Bi2Te2.7Se0.3 phase. This reduction of Seebeck coefficient in n-type (In4Se3)x-(Bi2Te2.7Se0.3)1−x is attributed to the formation of unwanted p-type phases by interdiffusion through the interface between (In4Se3)x and (Bi2Te2.7Se0.3)1−x as well as consequently formed Te-deficient matrix. However, the decrease in electrical resistivity and thermal conductivity with addition of In4Se3 leads to an enhanced thermoelectric figure of merit (ZT) at a temperature range over 450 K: a maximum ZT of 1.0 is achieved for the n-type (In4Se3)0.03-(Bi2Te2.7Se0.3)0.97 sample at 500 K.

The effect of mechanical deformation and annealing on thermoelectric properties of p-type (Bi0.225Sb0.775)Te3was performed. The ingots were prepared by melting, followed by quenching method using source materials with compositions of (Bi0.225Sb0.775)2Te3. Rectangular shaped specimens (5×5×12 mm3) were cut from ingots and then cold-pressed at 700 MPa for 2 to 20 times by changing the press direction perpendicular to previous one. The cold-pressed samples have been annealed in a quartz ampoule at 573 K. The grain size of the samples was controlled by the number of cold-pressing process and annealing time. Fine grain structure with a grain size of not more than 10 μm is obtained in highly deformed samples. The Seebeck coefficient of the deformed samples were gradually increased with annealing and converged to the similar value of about 225 μV/K after 30 hrs. The small grain size in highly deformed sample enables a rapid increase of Seebeck coefficient with annealing time (~2 hrs.), indicating that the thermal energy needed to recrystallize in highly deformed specimens is lower than that in low deformed specimens.Zvalues are rapidly increased with annealing time especially in highly deformed alloys, and converge to about3.0×10−3/K at room temperature. A higher thermoelectric performance could be expected by the optimization of composition and microstructural adjustment. The present study experimentally demonstrates a simple and cost-effective method for fabricating Bi-Te-based alloys with higher thermoelectric performance.

Accuracy of the Harman measurement largely depends on the heat transfer between the sample and its surroundings, so-called parasitic thermal effects (PTEs). Similar to the material evaluations, measuring thermoelectric modules (TEMs) is also affected by the PTEs especially when measuring under atmospheric condition. Here, we study the correction methods for the Harman measurements with systematically varied samples (both bulk materials and TEMs) at various conditions. Among several PTEs, the heat transfer via electric wires is critical. Thus, we estimate the thermal conductance of the electric wires, and correct the measured properties for a certain sample shape and measuring temperature. The PTEs are responsible for the underestimation of the TEM properties especially under atmospheric conditions (10–35%). This study will be useful to accurately characterize the thermoelectric properties of materials and modules.

Interfaces, such as grain boundaries in a solid material, are excellent regions to explore novel properties that emerge as the result of local symmetry-breaking. For instance, at the interface of a layered-chalcogenide material, the potential reconfiguration of the atoms at the boundaries can lead to a significant modification of the electronic properties because of their complex atomic bonding structure. Here, we report the experimental observation of an electron source at 60° twin boundaries in Bi2Te3, a representative layered-chalcogenide material. First-principles calculations reveal that the modification of the interatomic distance at the 60° twin boundary to accommodate structural misfits can alter the electronic structure of Bi2Te3. The change in the electronic structure generates occupied states within the original bandgap in a favourable condition to create carriers and enlarges the density-of-states near the conduction band minimum. The present work provides insight into the various transport behaviours of thermoelectrics and topological insulators., Grain boundaries in polycrystalline materials may offer the opportunity to explore physical phenomena that do not normally occur within the crystal grains. Here, the authors show that twin boundaries in Bi2Te3 works as an electron supply for the whole bulk material.

Although the Harman method evaluates the thermoelectric figure-of-merit in a rapid and simple fashion, the accuracy of this method is affected by several electrical and thermal extrinsic factors that have not been thoroughly investigated. Here, we study the relevant extrinsic effects and a correction scheme for them. A finite element model simulates the electrical potential and temperature fields of a sample, and enables the detailed analysis of electrical and thermal transport. The model predicts that the measurement strongly depends on the materials, sample geometries, and contact resistance of the electrodes. To verify the model, we measure the thermoelectric properties of Bi2-Te3 based alloys with systematically varied sample geometries and either with a point or a surface current source. By comparing the model and experimental data, we understand how the measurement conditions determine the extrinsic effects, and, furthermore, able to extract the intrinsic thermoelectric properties. A correction scheme is proposed to eliminate the associated extrinsic effects for an accurate evaluation. This work will help the Harman method be more consistent and accurate and contribute to the development of thermoelectric materials.

Alloys of Bi2Te3 rich side of Bi2Te3-Bi2Se3 were prepared by the zone melting method and the hot pressing method in order to compare their thermoelectric properties. When specimens were hot pressed thermoelectric properties changed as a function of particle size, pressing time and hot pressing temperature. The reasons for the variation of the thermoelectric properties were investigated by examining influences of following parameters; oxidation, mechanical deformation during pulverization and the hot pressing temperature. Thermoelectric properties of zone melted ingot were largely affected by dopants when composition is fixed, whereas thermoelectric properties of hot pressed material were mainly related with the variation of the carrier concentrations caused by the generation of electrically active defects from many sources. Defects induced by the mechanical deformation and oxygen causes generation of donors. Defect concentration is also altered with different hot pressing temperatures depending on the amount of previously received mechanical deformation.

The influences of zone melting growth speed on the thermoelectric properties of p-type bismuth telluride alloys were investigated. When the growth speed was fast, thermoelectric properties were uniform along ingots, as the composition of the molten zone did not change significantly. On the other hand, when growth speed was slow, thermoelectric properties varied along ingots due to the change of the solidified composition determined by the composition of a molten zone. The maximum figure of merit appeared in the middle of an ingot grown at slow growth speed. In the present study, the molten zone leveling method was developed as an attempt to extend theregion with high figure of merit. The basic idea of the method was to preserve a uniform composition of liquid at the solid-liquid interface during growth of an ingot so as to obtain a uniform composition of precipitated solids. Ingots composed of two sections with different compositions were used to control the accumulation of Te within the molten zone. As a result, high figure of merit could be obtained over more than 90% of ingot when zone melting growth speed was slow.

Thermoelectric properties of the hot-pressed p-type (Bi0.25Sb0.75)2Te3 alloy were characterized with variation of the hot-pressing temperature and the starting powder size. The roles of the factors which affect the Seebeck coefficient of the hot-pressed (Bi0.25Sb0.75)2Te3 alloy has been elucidated in this study. The donor-like behavior of oxygen could be one of the possible explanations for the higher Seebeck coefficient of the hot-pressed (Bi0.25Sb0.75)2Te3 alloy. Te vacancies formed by mechanical deformation during the powdering process significantly promote the diffusion of second phase Te atoms into their lattice sites so that the matrix Te solubility approaches its equilibrium value at a given temperature in a relatively short length of time. Using the Seebeck coefficient at various hot-pressing temperatures, the micro-phase diagram near the stoichiometric composition of (Bi0.25Sb0.75)2Te3 was evaluated.

Effects of excess Te on the thermoelectric properties of p-type 25% Bi2Te3-75% Sb2Te3 single crystal and hot-pressed sinter were characterized and understood with the micro-phase diagram near the stoichiometric composition obtained by measuring the equilibrium Seebeck coefficient. Thermoelectric properties of the 25% Bi2Te3-75% Sb2Te3 single crystal were varied with the amount of excess Te, as δ-phase of the single crystal becomes less Te-deficient with adding more excess Te. However, thermoelectric properties of the hot-pressed sinter were not varied with the amount of excess Te, because the composition of δ-phase is not changed with the amount of excess Te. While a maximum figure-of-merit of 2.39 × 10−3/K at 300 K was obtained for the 25% Bi2Te3-75% Sb2Te3 single crystal by adding 6 wt % excess Te, the hot-pressed 25% Bi2Te3-75% Sb2Te3 sinter exhibited the figure-of-merit of 2.97 × 10−3/K regardless of the excess Te amount.

Thermoelectric properties of the hot-pressed Bi2(Te0.85Se0.15)3 alloy were investigated with the addition of BN powders as phonon scattering centers. The Seebeck coefficient and electrical resistivity of the alloy increased as the volume fraction of BN increased. Although the thermal conductivity of the alloy decreased as the volume fraction of BN increased due to the reduction of Kel, the lattice thermal conductivity varied little. The figure-of-merit of the alloys, 1.6×l0-3/K without the addition of BN, decreased as the volume fraction of BN increased because the increment of the electrical resistivity was much larger than the decrement of the thermal conductivity due to grain refinement.

The p-type (100− x )% Bi 2 Te 3 – x % Sb 2 Te 3 (75≤ x ≤85) and 20% Bi 2 Te 3 –(80− y )% Sb 2 Te 3 – y % Sb 2 Se 3 (0≤ y ≤7) alloys were fabricated by mechanical alloying and hot pressing, and their thermoelectric properties were characterized. Among the hot-pressed (100− x )% Bi 2 Te 3 – x % Sb 2 Te 3 alloys, the 20% Bi 2 Te 3 –80% Sb 2 Te 3 alloy exhibited a maximum figure-of-merit of 3.05×10 −3 /K. The Seebeck coefficient and the electrical resistivity of the hot-pressed 20% Bi 2 Te 3 –(80− y )% Sb 2 Te 3 – y % Sb 2 Se 3 alloys increased with increasing the Sb 2 Se 3 content due to the reduction of the hole concentration. The figure-of-merit of the hot-pressed 20% Bi 2 Te 3 –(80− y )% Sb 2 Te 3 – y % Sb 2 Se 3 alloys decreased from 3.05×10 −3 /K to 2.5×10 −3 /K with increasing the Sb 2 Se 3 content up to 7 mol%, because the increment of the electrical resistivity was much larger than the decrement of the thermal conductivity and the increment of the Seebeck coefficient.

Thermoelectric properties of the (Bi,Sb){sub 2}Te{sub 3} and Bi{sub 2}(Te,Se){sub 3} single crystals were well characterized for the whole composition range of the Bi{sub 2}Te{sub 3}-Sb{sub 2}Te{sub 3} and Bi{sub 2}Te{sub 3}-Bi{sub 2}Se{sub 3} systems. However, the same work has been seldom conducted for the hot-pressed alloys. Thermoelectric properties of the hot-pressed (Bi,Sb){sub 2}Te{sub 3} and Bi{sub 2}(Te,Se){sub 3} alloys have been investigated mainly at the compositions of (Bi{sub 0.25}Sb{sub 0.75}){sub 2}Te{sub 3} and Bi{sub 2}(Te{sub 0.95}Se{sub 0.05}){sub 3} where the maximum figure-of-merits of p-type and n-type, respectively, were obtained for the single crystals with doping. In this study, thermoelectric properties of the hot-pressed (Bi,Sb){sub 2}(Te,Se){sub 3} alloys, prepared by powder metallurgy such as melting/grinding and mechanical alloying, were measured at room temperature and compared with the characteristics of the single crystals grown by the Bridgman technique for a wide range of compositions of the alloys.

Thermoelectric properties of the 0.05 wt.% SbI3-doped n-type Bi2(Teo.95Seo.o5)3 alloy, fabricated by hot pressing at temperatures ranging from 350°C to 550°C, were characterized. The electron concentration of the alloy decreased as the hot pressing temperature increased due to the annealing-out of the excess Te vacancies. When hot pressed at 350°C, a figure-of-merit of 0.75x10-3/K was obtained due to the low Seebeck coefficient of -145 µV/K and relatively high electrical resistivity of 2.05 mΩ-cm. Upon increasing the hot pressing temperature, however, the figure-of-merit was improved mainly due to the increase of the Seebeck coefficient. A maximum figure-of-merit of 2.1x10-3/K was obtained by hot pressing at 550°C.

Thermoelectric properties of the (Bi0.2Sb0.8)2Te3 alloy, fabricated by mechanical alloying and hot-pressing, were investigated with addition of BN and WO3 powders as phonon scattering centers. The Seebeck coefficient and the electrical resistivity of the hot-pressed (Bi0.2Sb0.8)2Te3 alloy increased with increasing the volume fraction of BN and WO3. Although the thermal conductivity of the hot-pressed (Bi0.2Sb0.8)2Te3 alloy decreased with increasing the volume fraction of BN and WO3 due to the reduction of κel, the lattice thermal conductivity was not lowered. The figure-of-merit of the hot-pressed (Bi0.2Sb0.8)2Te3 alloy, 3.05 × 10−3/K without addition of BN and WO3, decreased with increasing the volume fraction of BN and WO3, because the increment of the electrical resistivity was much larger than the decrement of the thermal conductivity due to the grain refinement.

Bi{sub 2}Te{sub 3}-Sb{sub 2}Te{sub 3}-Sb{sub 2}Se{sub 3} alloys with Sb{sub 2}Te{sub 3} and Sb{sub 2}Se{sub 3} contents up to 10 mol%, e.g., the 90% Bi{sub 2}Te{sub 3}-5% Sb{sub 2}Te{sub 3}-5% Sb{sub 2}Se{sub 3} alloy, are among the best n-type thermoelectric materials for Peltier coolers used near room temperature. In this work, the electrical and thermoelectric properties of Sbl{sub 3}doped 90% Bi{sub 2}Te{sub 3}-5% Sb{sub 2}Te{sub 3}-5% Sb{sub 2}Se{sub 3} alloys were characterized at temperatures ranging from 80K to 600K. The temperature dependencies of the Hall coefficient, carrier mobility, Seebeck coefficient and thermal conductivity were measured, and the scattering parameter and bandgap energy were determined.

The temperature dependences of the Seebeck coefficient, resistivity, Hall coefficient, and carrier mobility of SbI3- and CuBr-doped 85% Bi2Te3-15% Bi2Se3 single crystals have been characterized at temperatures ranging from 77 to 600 K, and the degenerate temperature, scattering parameter, bandgap energy, and the effective masses of the electron and hole have been determined. The degenerate temperature of the 85% Bi2Te3-15% Bi2Se3 alloy is 103 K, and the scattering parameter is determined to be 0.1. The ratio of the electron to hole mobility b (= μ e μ h ) is 1.45, and the bandgap energy EG at 0 K of the 85% Bi2Te3-15% Bi2Se3 alloy is 0.245 eV which is higher than EG of the pure Bi2Te3. The effective mass of the electron and hole in the 85% Bi2Te3-15% Bi2Se3 alloy are me = 0.056 mo and mh = 0.065 mo, respectively.

The temperature dependence of the Hall mobility, Seebeck coefficient, electrical resistivity, thermal conductivity, and figure-of-merit of the SbI3 and CuBr-doped 85% Bi2Te3-15% Bi2Se3 single crystals have been characterized at temperatures ranging from 77 K to 600 K. The scattering parameter in 85% Bi2Te3-15% Bi2Se3 single crystal was determined as 0.1 from the temperature dependence of the carrier mobility. With increasing the amount of Sbl3 or CuBr doping, the Seebeck coefficient of 85% Bi2Te3-15% Bi2Se3 decreased and the temperature at which the Seebeck coefficient shows a maximum shifted to higher temperature. Compared to the Sbl3-doped specimens, the CuBr-doped single crystals exhibited higher (m* / m0)3/2 μc, implying that CuBr is a more effective dopant to improve the material factor and thus the figure-of-merit of 85% Bi2Te3-15% Bi2Se3. The maximum figure-of-merit of 2.0 × 10−3/K and 2.2 × 10−3/K was obtained for 0.1 wt % Sbl3-doped specimen and 0.03 wt % CuBr-doped specimen, respectively.

P-type (Bi0.25Sb0.75)2Te3 powders were fabricated by melting/grinding and mechanical alloying processes. Thermoelectric properties of the hot-pressed (Bi0.25Sb0.75)2Te3 were characterized with the powder processing method, powder size, hot pressing temperature, and the amount of excess-Te dopant. Specimens fabricated by melting/grinding exhibited lower Seebeck coefficient, lower electrical resistivity and higher thermal conductivity, compared to the specimens prepared by mechanical alloying. 3 wt.% excess Te-doped (Bi0.25Sb0.75)2 Te3, fabricated by melting/grinding and hot pressing at 550°C, exhibited a figure-merit of 3.2 x 10-3/K. For 1 wt.% excess Te-doped specimen prepared by mechanical alloying and hot pressing at 550°C, a figure-merit of 3.05 x 10-3/K was obtained.

The Seebeck coefficient and electrical conductivity of the 25%Bi 2 Te 3 -75%Sb 2 Te 3 solid solution prepared by hot-pressing method were measured and the effects of particle size, oxidation, hot-pressing temperature and time on the Seebeck coefficient of the polycrystalline solid solution were examined in detail. It has been found that the mechanical deformation during pulverization or pressing processes is very important in controlling the Seebeck coefficient. The role of the anion vacancies formed by the mechanical deformation as well as the change of the antistructure defect concentration were discussed. By optimizing particle size and hot-pressing temperature, the figure of merit about 2.9 x 10 −3 K −1 could be obtained for the P -type polycrystalline 25%Bi 2 Te 3 -75%Sb 2 Te 3 solid solutions without excess Te addition.

The PbTe intermetallic compound could be fabricated by mechanical alloying of elemental Pb and Te powders for 2 minutes at ball-to-powder weight ratio of 2 : 1. The lattice parameter of PbTe processed by mechanical alloying, 0.6462 nm, was in excellent agreement with the value of 0.6458 nm which was reported for PbTe powder fabricated by melting and grinding. In situ observation of the abrupt temperature rise during the ball milling process indicated that the PbTe intermetallic compound was formed by a self-sustained reaction rather than by diffusional reactions. There was no tendency for PbTe crystalline powders to be amorphized by mechanical alloying.

The thermoelectric properties and Hall effects of a number of undoped Bi 2 Te 3 -Sb 2 Te 3 pseudo-binary and Bi 2 Te 3 -Sb 2 Te 3 -5%Sb 2 Se 3 pseudo-ternary single crystal alloys grown by the Bridgman method were measured over the temperature range from 77 to 580 K. All of the alloys showed p -type extrinsic conduction and the analysis of the Seebeck coefficient and electrical conductivity in the extrinsic range indicated partial degeneracy at the lower temperatures. The Seebeck coefficient and energy gap of the pseudo-ternary alloy increased and the electrical conductivity decreased with the addition of Sb 2 Se 3 . It was proved by the Hall measurement that the carrier concentration increased with increasing amount of Sb 2 Te 3 , and decreased with the addition of Sb 2 Se 3 , while the Hall mobility was constant except for the Sb 2 Te 3 -rich region at room temperature. The scattering of carriers in these alloys can be expressed as acoustic-mode lattice scattering and two-phonon processes. The figure of merit shows a maximum at a carrier concentration of about 4.0 × 10 19 cm −3 at room temperature.

Bi/sub 2/Te/sub 3/-Sb/sub 2/Te/sub 3/ alloys were prepared by the mechanical alloying method and sintered at the temperatures ranging from 50 to 550/spl deg/C to ensure the phase equilibrium at low temperatures. A micro-phase diagram near the stoichiometric composition was evaluated with the carrier concentration which was estimated from the values of the Seebeck coefficient. Singular properties of the Seebeck coefficient and the electrical resistivity observed at the composition of 20% Bi/sub 2/Te/sub 3/-80% Sb/sub 2/Te/sub 3/ were also discussed.

Thermoelectric properties of the polycrystalline (Bi/sub 1-x/Sb/sub x/)/sub 2/Te/sub 3/ (0/spl les/x/spl les/1) alloys, prepared by various powder metallurgy such as mechanical alloying and melting/grinding, were measured and compared with the characteristics of single crystals. Contrary to the p-type behavior of the single crystals in all compositions of 0/spl les/x/spl les/1, a p-n transition occurred at about 0.67 mol% Sb/sub 2/Te/sub 3/ for the alloys prepared by powder metallurgy. Among the p-type (Bi/sub 1-x/Sb/sub x/)/sub 2/Te/sub 3/ (x/spl ges/0.7) processed by powder metallurgy, (Bi/sub 0.2/Sb/sub 0.8/)/sub 2/Te/sub 3/ exhibited the highest figure-of-merit of 3.05/spl times/10/sup -3//K. Thermoelectric properties of the hot-pressed (Bi/sub 0.2/Sb/sub 0.8/)/sub 2/Te/sub 3/ with addition of BN powders as scattering centers were also investigated.

Thermoelectric properties of the hot-pressed Bi/sub 2/(Te/sub 1-x/Se/sub x/)/sub 3/ alloys, prepared by melting/grinding and mechanical alloying, were investigated with the Bi/sub 2/Se/sub 3/ content. Whereas a p-n transition occurred at about 30 mol% Bi/sub 2/Se/sub 3/ in Bi/sub 2/(Te/sub 1-x/Se/sub x/)/sub 3/ single crystals, the hot-pressed specimens exhibited n-type conduction for all compositions of 0/spl les/x/spl les/1. Among the hot-pressed specimens, a maximum figure-of-merit of 1.92/spl times/10/sup -3//K was obtained with the Bi/sub 2/(Te/sub 0.85/Se/sub 0.15/)/sub 3/ composition. Thermoelectric properties of the mechanically-alloyed and hot-pressed Bi/sub 2/(Te/sub 0.85/Se/sub 0.15/)/sub 3/ were also investigated with addition of BN powders as scattering centers.

Application of the two-stage modules has been widely considered to obtain a large temperature difference, required for devices such as a freezer, by thermoelectric cooling. Due to the complexity of the formulas for maximum parameters, however, design of the application devices in practical use has been limited. Generalized formulas have been derived to express the maximum temperature difference and maximum current at /spl Delta/T/sub max/-mode for two-stage thermoelectric modules connected electrically in series or in parallel: /spl Delta/T/sub max/=T/sub k/(a/c)(M'-1/M'+1), I/sub max/=2T/sub h/(a/b)(e/sub 1//R/sub 1/)(1/M'+1), and M'=(1+2T/sub k/(ac/b)((Z/sub 1/)/(K/sub 2//K/sub 1/)))/sup 1/2/. Corresponding to these expressions, /spl Delta/T/sub max/=T/sub h/-(M'-1)/(Z/sub 1/)=T/sub h/(M'-1)/(M'+1), I/sub max/=(K/sub 1/)/(e/sub 1/)(M'-1)=2T/sub h/(e/sub 1/)/(R/sub 1/)(1)/(M'+1), and M'=(1+2/spl middot/Z/sub 1//spl middot/T/sub k/)/sup 1/2/ for one-stage modules.

Bi-doped n-type PbTe powders were fabricated by mechanical alloying and melting/grinding to compare the thermoelectric properties of the alloys with powder processing methods. Hot pressing of the PbTe powders was conducted at 650/spl deg/C and 750/spl deg/C for 1 hour in vacuum. When measured at temperatures ranging from 25/spl deg/C to 450/spl deg/C, PbTe samples fabricated by mechanical alloying exhibited more negative Seebeck coefficient, higher electrical resistivity and lower thermal conductivity, compared to ones prepared by melting/grinding. The temperature for the maximum figure-of-merit shifted to lower temperature for the specimens fabricated by mechanical alloying. When hot pressed at 650/spl deg/C, 0.3 wt% Bi-doped PbTe fabricated by mechanical alloying and melting/grinding exhibited a maximum figure-of-merit of 1.33/spl times/10/sup -3//K at 200/spl deg/C and 1.07/spl times/10/sup -3//K at 350/spl deg/C, respectively.

(Bi/sub 1-x/Sb/sub x/)/sub 2/Te/sub 3/ (0.75/spl les/x/spl les/0.85) thermoelectric materials were fabricated by mechanical alloying and hot pressing at 550/spl deg/C for 30 minutes with and without reduction treatment of the mechanically alloyed powders. When the mechanically alloyed (Bi/sub 1-x/Sb/sub x/)/sub 2/Te/sub 3/ powders were hot-pressed without a reduction treatment, a maximum figure-of-merit of 2.92/spl times/10/sup -3//K at 300 K was obtained for x=0.8 composition. Although the electrical resistivity of the mechanically alloyed (Bi/sub 1-x/Sb/sub x/)/sub 2/Te/sub 3/ decreased with a reduction treatment of the powders, the figure-of-merit was lowered due to the substantial decrease of the Seebeck coefficient. With addition of 1 wt% excess Te, the figure-of-merit of the reduction-treated (Bi/sub 0.2/Sb/sub 0.8/)/sub 2/Te/sub 3/ could be improved to be 3.33/spl times/10/sup -3//K.

Despite high thermoelectric performance, the commercial applications of Bi/sub 2/Te/sub 3/-Sb/sub 2/Te/sub 3/ based thermoelectrics are limited mainly because of production cost due to complicated processing steps. In this study, a new approach to the powder-extrusion-sintering method, which is intended to fabricate a sound sintered material by the simultaneous application of pressure and heat, was investigated. This method is expected to be suitable for low cost production of Bi/sub 2/Te/sub 3/-Sb/sub 2/Te/sub 3/ materials because of the possibility of semi-continuous fabrication processing. It was possible to achieve high density (/spl ges/95% of theoretical density) products by adjusting variables, such as; die angle, die hole length, powder size, extrusion pressure and temperature. Thermoelectric properties of sintered specimens were measured, and discussed to achieve optimum fabrication conditions.

To investigate the effects of solid solution alloying on thermoelectric properties of PbTe-based thermoelectric materials, 0.3 wt% Bi-doped (Pb/sub 1-x/Ge/sub x/)Te alloy powders (0/spl les/x/spl les/0.15) were fabricated by "melting/grinding" process, and hot-pressed at 650/spl deg/C and 750/spl deg/C for 1 hour in vacuum. At temperatures ranging from 25/spl deg/C to 450/spl deg/C, the (Pb/sub 1-x/Ge/sub x/)Te alloys exhibited more negative Seebeck coefficient, higher electrical resistivity and lower thermal conductivity with increasing GeTe content. The temperature, where the figure-of-merit of (Pb/sub 1-x/Ge/sub x/)Te reached a maximum, was lowered with increasing GeTe content x. When hot-pressed at 650/spl deg/C, 0.3 wt% Bi-doped PbTe and (Pb/sub 0.9/Ge/sub 0.1/)Te exhibited a maximum figure-of-merit of 1.07/spl times/10/sup -3//K at 400/spl deg/C and 1.25/spl times/10/sup -3//K at 300/spl deg/C, respectively.

Hot pressed Bi/sub 2/Te/sub 3/-Bi/sub 2/Se/sub 3/ alloys show different thermoelectric properties compared to alloys grown by the ordinary zone melting growth method. When specimens were hot pressed, thermoelectric properties changed according to different particle size, pressing time and hot pressing temperature. The effects of these individual parameters on the thermoelectric properties of hot pressed materials were examined. Special emphasis was put on carrier generation mechanisms related to the following parameters; oxidation, mechanical deformation during pulverization and the hot pressing temperature. It was found that all processing parameters influence the generation of electrically active defects. Defects induced by the mechanical deformation and oxygen cause generation of donors. Defect concentration is also altered with different hot pressing temperatures depending on the amount of previously received mechanical deformation. A figure of merit of 2.4/spl times/10/sup -3/ K/sup -1/ could be obtained at an optimal process condition.

Utilizing internal energy artificially implemented by cold-pressing in the specimens, we demonstrate a way to synthesize high-quality bulk thermoelectric materials at otherwise too low a temperature to approach to an equilibrium state. This low-temperature synthesis technique will provide a new opportunity to integrate high-performance thermoelectric materials into various electronic devices for a built-in energy source, as well as to develop low-cost fabrication methods.

Thermoelectric properties of polycrystalline Bi2(Te1−xSex)3 (0.05 ≤ x ≤ 0.25), fabricated by mechanical alloying and hot pressing, have been investigated. Formation of n-type Bi2(Te0.9 Se0.1)3 alloy powders was completed by mechanical alloying for 3 hours at ball-to-material ratio of 5: 1, and processing time for Bi2(Te1−xSex)3 formation increased with Bi2Se3 content x. Figure-of-merit of Bi2(Te0.9Se0.1) was markedly increased by hot pressing at temperatures above 450°C, and maximum value of 1.9 × 10−3/K was obtained by hot pressing at 550°C. With addition of 0.015 wt% Bi as acceptor dopant, figure-of-merit of Bi2 (Te0.9Se0.1)3, hot pressed at 550°C, could be improved to 2.1 × 10−3/K. When Bi2(Te1−xSex)3 was hot pressed at 550°C, figure-of-merit increased from 1.14 × 103/K to 1.92 × 10−3/K with increasing Bi2Se3 content x from 0.05 to 0.15, and then decreased to 1.30 × 103/K for x = 0.25 composition.

Thermoelectric properties of polycrystalline (Bi1−xSbx)2Te3 (0.75 ≤ x ≤ 0.85), fabricated by mechanical alloying and hot pressing methods, have been investigated. Formation of (Bi0.25Sb0.75)2Te3 alloy powder was completed by mechanical alloying for 5 hours at ball- to-material ratio of 5: 1, and processing time for (Bi1−xSbx)2Te3 formation increased with Sb2Te3 content x. When (Bi0.25Sb0.75)2Te3 was hot pressed at temperatures ranging from 300°C to 550°C for 30 minutes, figure-of-merit increased with hot pressing temperature and maximum value of 2.8 × 10−3/K could be obtained by hot pressing at 550°C. When hot pressed at 550°C, (Bi0.2Sb0.8)2Te3 exhibited figure-of-merit of 2.92 × 10−3/K, which could be improved to 2.97 × 10−3/K with addition of 1 wt% Sb as acceptor dopant.

The Bi2Te3‐Bi2Se3 solid solutions were grown by the zone melting method and material factor, μ0(m*/m0)3/2/kph, of these crystals were calculated from the measured values of the thermal conductivity, Seebeck coefficient, and electrical resistivity. It was found that Bi2Se3 content of this solid solution must be between 6 and 10 mol % to obtain high figure of merit near room temperature. To optimize carrier concentration Cdl2 or CdCl2 was doped to 90%Bi2Te3‐10%Bi2Se3 solid solution and their doping characteristics were compared. The effects of process parameters on the thermoelectric properties of these n‐type solid solutions were also reported. The figure of merit of 2.85×10−3 K−1 could be obtained by optimizing process condition.

0.5 at% Te-doped ()Te and () powders were fabricated by mechanical alloying process. 0.5 at% NaTe-doped ()Te powders were charged at one end of mold and () powders were charged at the other end of a mold. Then these powders were hot-pressed to form p-type ()Te/() functional gradient materials with the segment ratios (the ratio of ()Te to () ) of 1:2, 1:1, and 2:1. Power generation characteristics of the ()Te/() were measured. When the temperature difference ΔT at both ends of the specimen was larger than , the ()Te/() with the segment ratios of 1:2 and 1:1 exhibited larger output power than those of the () and 0.5 at% Te-doped ()Te alloys. The maximum output power of the ()Te/() predicted with the measured Seebeck coefficient and the estimated electrical resistivity was in good agreement with the measured maximum output power.