Thermoelectric Properties of Cu-doped Bi<Subscript>0.4</Subscript>Sb<Subscript>1.6</Subscript>Te<Subscript>3</Subscript> Prepared by Hot Extrusion

Cu_0.003Bi_0.4Sb_1.6Te₃ alloys were prepared by using encapsulated melting and hot extrusion (HE). The hot-extruded specimens had the relative average density of 98%. The (00l) planes were preferentially oriented parallel to the extrusion direction, but the specimens showed low crystallographic anisotropy with low orientation factors. The specimens were hot-extruded at 698 K, and they showed excellent mechanical properties with a Vickers hardness of 76 Hv and a bending strength of 59 MPa. However, as the HE temperature increased, the mechanical properties degraded due to grain growth. The hot-extruded specimens showed positive Seebeck coefficients, indicating that the specimens have p-type conduction. These specimens exhibited negative temperature dependences of electrical conductivity, and thus behaved as degenerate semiconductors. The Seebeck coefficient reached the maximum value at 373 K and then decreased with increasing temperature due to intrinsic conduction. Cu-doped specimens exhibited high power factors due to relatively higher electrical conductivities and Seebeck coefficients than those of undoped specimens. A thermal conductivity of 1.00 Wm^?1 K^?1 was obtained at 373 K for Cu_0.003Bi_0.4Sb_1.6Te₃ hot-extruded at 723 K. A maximum dimensionless figure of merit, ZT _max = 1.05, and an average dimensionless figure of merit, ZT _ave = 0.98, were achieved at 373 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.     

Electronic Structure and Thermoelectric Properties of the Delafossite-Type Oxides CuFe<Subscript>1–<Emphasis Type="Italic">x</Emphasis></Subscript>Ni<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>O<Subscript>2</Subscript>

In this study, we investigated the chemical composition and electronic structure of the delafossite-type oxides CuFeO₂ (CFO) and CuFe_0.98Ni_0.02O₂ (CFNO). The hole carrier density in the Cu and FeO₂ layers of CFNO was found to be different from that of CFO, leading to the enhancement of electrical conductivity by Ni substitution. In addition, thermoelectric properties were found to be affected by the surface treatment, possibly due to some surface contamination. An etched CFNO (E-CFNO) exhibited a higher electrical conductivity and a higher Seebeck coefficient relative to the polished CFNO (P-CFNO). The thermal conductivity did not change much between E-CFNO and P-CFNO. As a result, the thermoelectric performance of E-CFNO was higher than that of P-CFNO. This result indicates that etching is needed when we use CFNO as a p-leg in thermoelectric generators.     

Thermoelectric Properties of the One-Dimensional Cobalt Oxide CaCo<Subscript>2</Subscript>O<Subscript>4</Subscript>

In this work we studied the crystal structure and physical properties of the new one-dimensional cobalt oxide CaCo₂O_4+δ . The CaCo₂O_4+δ phase crystallizes as a calcium-ferrite-type structure, which consists of a corner- and edge-shared CoO₆ octahedron network including one-dimensional double chains. The specific-heat Sommerfeld constant γ was found to be 4.48(7) mJ/mol K². This result suggests that the CaCo₂O_4+δ phase has a finite density of states at the Fermi level. Metallic temperature dependence of the Seebeck coefficient S with a large thermoelectric power (S = 151 μV/K at 387 K) was observed. The origin of the large thermoelectric power may be attributed to the quasi one-dimensional character of the energy band near the valence band maximum in CaCo₂O_4+δ .     

Electrodeposition and Thermoelectric Characteristics of Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript> and Sb<Subscript>2</Subscript>Te<Subscript>3</Subscript> Films for Thermopile Sensor Applications

A thermopile sensor was processed on a glass substrate by electrodeposition of n-type bismuth telluride (Bi-Te) and p-type antimony telluride (Sb-Te) films. The n-type Bi-Te film electrodeposited at −50 mV in a 50 mM electrolyte with a Bi/(Bi + Te) mole ratio of 0.5 exhibited a Seebeck coefficient of −51.6 μV/K and a power factor of 7.1 × 10⁻⁴ W/K² · m. The p-type Sb-Te film electroplated at 20 mV in a 70 mM solution with an Sb/(Sb + Te) mole ratio of 0.9 exhibited a Seebeck coefficient of 52.1 μV/K and a power factor of 1.7 × 10⁻⁴ W/K² · m. A thermopile sensor composed of 196 pairs of the p-type Sb-Te and the n-type Bi-Te thin-film legs exhibited sensitivity of 7.3 mV/K.     

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 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.     

Effect of Cyclic Thermal Loading on the Microstructure and Thermoelectric Properties of CoSb<Subscript>3</Subscript>

The effect of cyclic thermal loading on the microstructure and thermoelectric properties of CoSb₃ was investigated. The microstructures of the samples were characterized by x-ray diffractometry, scanning electron microscopy, energy dispersive x-ray spectrometry and density measurements. The electrical conductivity, the Seebeck coefficient and the thermal conductivity were measured from room temperature to 800 K. Under cyclic thermal loading, antimony partially volatilized from the surface of the sample, and the density obviously decreased. After 2000 cycles, the phase composition of the sample remained stable, and the average grain size did not change significantly. Moreover, the electrical conductivity varied only slightly, except in the low temperature region. The Seebeck coefficient decreased slightly. However, the thermal conductivity changed remarkably with increasing numbers of thermal cycles.     

Characterization of Ge<Subscript>22</Subscript>Sb<Subscript>22</Subscript>Te<Subscript>56</Subscript> and Sb-Excess Ge<Subscript>15</Subscript>Sb<Subscript>47</Subscript>Te<Subscript>38</Subscript> Chalcogenide Thin Films for Phase-Change Memory Applications

A phase-change memory device that utilizes an antimony (Sb)-excess Ge₁₅Sb₄₇Te₃₈ chalcogenide thin film was fabricated and its electrical properties were measured and compared with a similar device that uses Ge₂₂Sb₂₂Te₅₆. The resulting electrical characteristics exhibited I _reset values of 14 mA for Ge₂₂Sb₂₂Te₅₆ and 10.6 mA for Ge₁₅Sb₄₇Te₃₈. Also, the set operation time (t _set) for the device using Ge₁₅Sb₄₇Te₃₈ films was 140 ns, which was more than twice as fast as the Ge₂₂Sb₂₂Te₅₆ device. The relationship between the microstructure and the improved electrical performance of the device was examined by means of transmission electron microscopy (TEM).     

First-Principles Study of Semiconducting Clathrate Ba<Subscript>8</Subscript>Al<Subscript>16</Subscript>Ge<Subscript>30</Subscript>

We calculated electronic structures of Ba₈Al₁₆Ge₃₀ in some Al-Ge framework configurations without nearest-neighbor Al-Al bonds by using a first-principles method. The calculated band structures are similar in outline but different in detail. We also calculated thermoelectric properties by using the electronic structures to analyze the experimental results on a sintered Ba₈Al₁₆Ge₃₀ sample. The calculated properties nearly agree with the experimental results; however, the calculated temperature dependences of electrical conductivity are slightly different from one another, because of differences in electronic structure. In this paper, we discuss the temperature dependence from the viewpoints of nonparabolic band effects.     

Effect of Deformation Temperature on Texture and Thermoelectric Properties of Bi<Subscript>0.5</Subscript>Sb<Subscript>1.5</Subscript>Te<Subscript>3</Subscript> Prepared by Hot-Press Deformation

The effects of deformation temperature on texture and thermoelectric properties of p-type Bi_0.5Sb_1.5Te₃ sintered materials were investigated. The sintered materials were prepared by mechanical alloying and hot-press sintering. The hot-press deformation was performed at 723 K and 823 K by applying mechanical pressure in a graphite die. Then, the materials were extruded in the direction opposite to the direction of applied pressure. X-ray diffraction and electron backscattered diffraction patterns showed that the hexagonal c-plane tended to align along the extruded direction when the samples were deformed at high temperatures. The thermoelectric power factor was increased by high-temperature hot-press deformation because of the low electrical resistivity that originated from the c-plane orientation.     

Thermoelectric properties of Bi<Subscript>0.5</Subscript>Sb<Subscript>1.5</Subscript>Te<Subscript>3</Subscript> ribbons prepared by melt spinning

The results of studying the thermoelectric properties of p-type Bi_0.5Sb_1.5Te₃ alloy samples prepared by melt spinning quenching are presented. The material after melt spinning is shaped as thin ribbons and has a quasi-amorphous structure. The thermoelectric properties (thermoelectric power and electrical resistance) and crystallization processes of as-prepared melt-spun ribbons are studied at 300–800 K for the first time. The stability range of the initial state, the crystallization-onset temperature, and the effect of thermal annealing on the thermoelectric-power factor of the alloy are determined.     

Nernst-ettingshausen tensor in Sb<Subscript>2</Subscript>Te<Subscript>3</Subscript> single crystal

On one Sb₂Te₃ single crystal, the temperature dependences of all three independent components of the Nernst-Ettingshausen tensor (Q _ikl ) are measured in the temperature range of 85–450 K, all three components being negative. Alongside with the Nernst-Ettingshausen effect, the anisotropy of the Hall (R _ikl ) and Seebeck (S _ij ) coefficients and the conductivity (σ _ii ) is also investigated. The carried-out analysis of the experimental data on the Nernst-Ettingshausen and Seebeck effects indicates that there is the mixed scattering mechanism with the participation of acoustic phonons and impurity ions, the relative contributions of these mechanisms varying with temperature. In the relaxation-time-tensor approximation, the values of the effective scattering parameter (r) are determined. The obtained values point to the dominant scattering at acoustic phonons in the cleavage plane and to the substantial contribution of charged ions to the scattering along the trigonal axis c ₃. It is shown that it is possible to explain the major features of experimental data on the Nernst-Ettingshausen effect within the two-valence-band model with the participation of several groups of holes in the transport phenomena.     

Preparation and Thermoelectric Properties of Nanoporous Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript>-Based Alloys

n-Type nanoporous Bi₂Te₃-based thermoelectric materials with different porosity ratios have been prepared by spark plasma sintering (SPS). The microstructure and phase morphology have been analyzed by x-ray diffraction (XRD) and field-emission scanning electron microscopy (FESEM), and the thermoelectric properties of the SPS samples have been measured. Experimental results show that the nanoporous structures lying in the sheet layers and among the plate grains of the Bi₂Te₃ bulk material can lead to an increase in the Seebeck coefficient and a decrease in the thermal conductivity, thus leading to an enhanced figure of merit.     

Thermoelectric Properties of B-Site Substituted LaRhO<Subscript>3</Subscript>

We have prepared Ni- and Mg-substituted LaRhO₃ and have measured the thermoelectric properties. Ni substitution decreases resistivity at room temperature, while Mg substitution decreases resistivity up to 5%, where the Mg substitution increases the carrier concentration, as confirmed by Hall coefficient measurements.     

Thermal Stability of Thermoelectric Zn<Subscript>4</Subscript>Sb<Subscript>3</Subscript>

The thermal stability of the thermoelectric Zn₄Sb₃ has been investigated by synchrotron power diffraction measurements in the temperature range of 300 K to 625 K in a capillary sealed under Ar. Data were also collected in air on a 1% Cd-doped sample. Rietveld refinements of the data indicate that a variety of impurity phases are formed. After heat treatment, more than 85% of the Zn₄Sb₃ phase remains in the 1% Cd-doped sample heated in air, and 97% remains in the undoped Zn₄Sb₃ heated in Ar. These stabilities are better than those previously observed in pure samples heated in air. This suggests that doping, as well as oxygen or oxidation impurities, play important roles in the thermal stability of this compound.     

Effect of Composition on Thermoelectric Properties in PbTe-Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript> Composites

The solidification of alloys in the Bi₂Te₃-PbTe pseudobinary system at off- and near-eutectic compositions was investigated for their microstructure and thermoelectric properties. Dendritic and lamellar structures were clearly observed due to the phase separation and the existence of a metastable ternary phase. In this system, three phases with different compositions were observed: binary Bi₂Te₃, PbTe, and metastable PbBi₂Te₄. The Seebeck coefficient, electrical resistivity, and thermal conductivity of ternary alloys as well as binary compounds were measured. The phonon thermal conductivities of Pb-Bi-Te alloys were lower than those in binary PbTe and Bi₂Te₃, which could have resulted from the increased interfacial area between phases due to the existence of the metastable ternary phase and the resultant phase separation.     

Thermoelectric Properties of Spark Plasma-Sintered In<Subscript>4</Subscript>Se<Subscript>3</Subscript>-In<Subscript>4</Subscript>Te<Subscript>3</Subscript>

We report the thermoelectric properties of spark plasma-sintered In₄Se₃-In₄Te₃ materials. For comparison, pure In₄Se₃ and In₄Se₃ (80 wt.%)/In₄Te₃ (20 wt.%) mixture samples were prepared. In₄Se₃ and In₄Te₃ powders were synthesized by a conventional melting process in evacuated quartz ampoules, and a spark plasma method was used for the sintering of the pure In₄Se₃ and mixture samples. Thermoelectric and structural characterizations were carried out, and the mixing effect of In₄Se₃ and In₄Te₃ on the thermoelectric properties was investigated.     

Improvement of Thermoelectric Power Factor of Hydrothermally Prepared Bi<Subscript>0.5</Subscript>Sb<Subscript>1.5</Subscript>Te<Subscript>3</Subscript> Compared with its Solvothermally Prepared Counterpart

We report on the successful hydrothermal synthesis of Bi_0.5Sb_1.5Te₃, using water as the solvent. The products of the hydrothermally prepared Bi_0.5 Sb_1.5Te₃ were hexagonal platelets with edges of 200–1500 nm and thicknesses of 30–50 nm. Both the Seebeck coefficient and electrical conductivity of the hydrothermally prepared Bi_0.5Sb_1.5Te₃ were larger than those of the solvothermally prepared counterpart. Hall measurements of Bi_0.5Sb_1.5Te₃ at room temperature indicated that the charge carrier was p-type, with a carrier concentration of 9.47 × 10¹⁸ cm⁻³ and 1.42 × 10¹⁹ cm⁻³ for the hydrothermally prepared Bi_0.5Sb_1.5Te₃ and solvothermally prepared sample, respectively. The thermoelectric power factor at 290 K was 10.4 μW/cm K² and 2.9 μW/cm K² for the hydrothermally prepared Bi_0.5Sb_1.5Te₃ and solvothermally prepared sample, respectively.     

Thermoelectric Characterization of Zone-Melted and Quenched Zn<Subscript>4</Subscript>Sb<Subscript>3</Subscript>

Because of its complex structure, Zn₄Sb₃ exhibits relatively low thermal conductivity. This, in combination with large values of the Seebeck coefficient and moderate to high electrical conductivity, makes the material especially interesting for thermoelectric application in temperatures up to 400°C. The phase purity and thermal stability of Zn₄Sb₃ are major issues for its thermoelectric performance and are strongly dependent on the synthesis method, atmosphere, density, and grain size. Therefore, Zn₄Sb₃ was prepared by both zone melting and quenching in this study, and pressed samples from crushed powders of three different grain sizes were compared. The effect of thermal cycling was studied, along with repeated structural analysis and Seebeck mapping. It was found that zone melting leads to improved thermal stability regarding decomposition via Zn loss, which finally may result in the formation of ZnSb. Larger grain size seems to reduce the degradation, because of lower concentration of grain boundaries, thus hindering diffusion inside the material.