Ordered-Defect Sulfides as Thermoelectric Materials

The thermoelectric behavior of the transition-metal disulfides n-type NiCr₂S₄ and p-type CuCrS₂ has been investigated. Materials prepared by high-temperature reaction were consolidated using cold-pressing and sintering, hot-pressing in graphite dies or spark-plasma sintering in tungsten carbide dies. The consolidation conditions have a marked influence on the electrical transport properties. In addition to the effect on sample density, altering the consolidation conditions results in changes to the sample composition, including the formation of impurity phases. Maximum room-temperature power factors were 0.18 mW m^?1 K^?2 and 0.09 mW m^?1 K^?2 for NiCr₂S₄ and CuCrS₂, respectively. Thermal conductivities of ca. 1.4 W m^?1 K^?1 and 1.2 W m^?1 K^?1 lead to figures of merit of 0.024 and 0.023 for NiCr₂S₄ and CuCrS₂, respectively.     

Effect of Ce Substitution for Sb on the Thermoelectric Properties of AgSbTe2 Compound

We have prepared Ce-doped polycrystalline AgSbTe_2.01 compounds from high-purity elements by a melt-quench technique followed by spark plasma sintering, and their thermoelectric transport properties have been investigated in the temperature range of 300 K to 625 K. The actual concentration of Ce was much less than the initial composition, but roughly proportional to it. Small additions of Ce shifted the composition of the homogeneity range from the nearly ideal atomic ratio Ag:Sb:Te = 0.98:1.02:2.01 toward Sb rich (Ag poor), and led to the reemergence of Ag₂Te impurity in AgSbTe₂ compound. The Ce-doped samples possessed lower electrical conductivity compared with the undoped AgSbTe_2.01 compound at room temperature, but the carrier mobility and effective mass were essentially constant, indicating intact band structure near the covalent band maximum upon Ce substitution for Sb. Due to the decrease of lattice vibration anharmonicity resulting from Ce substitution for Sb, the lattice conductivity of the Ce-doped samples was about 0.1 W m^?1 K^?1 higher than that of the AgSbTe_2.01 sample, and the magnitude spanned the range from 0.30 W m^?1 K^?1 to 0.55 W m^?1 K^?1. A ZT of 1.20 was achieved at about 615 K for the AgSb_0.99Ce_0.01Te_2.01 sample.     

Effects of Al/Sb Double Doping on the Thermoelectric Properties of Mg2Si0.75Sn0.25

Al/Sb double-doped Mg₂Si_0.75Sn_0.25 materials were prepared by liquid–solid reaction synthesis and the hot-pressing technique. The effects of Al/Sb double doping on the thermoelectric properties were investigated at temperatures between room temperature and 900 K, and the resistivity and Hall coefficient were investigated at 80 K to 900 K. Al/Sb double-doped samples were found to be n-type semiconductors in the investigated temperature range. The absolute Seebeck coefficient (α), resistivity (ρ), and thermal conductivity (κ) for Al/Sb double-doped samples at room temperature were in the ranges of 152.5 μV K^?1 to 109.2 μV K^?1, 2.92 × 10^?5 Ω m to 1.29 × 10^?5 Ω m, and 2.50 W K^?1 m^?1 to 2.86 W K^?1 m^?1, respectively. The absolute values of α increased with increasing temperature up to a maximum, and decreased thereafter. This could be attributed to mixed carrier conduction in the intrinsic region. κ decreased linearly with increasing temperature to a minimum near the intrinsic region, then increased rapidly because of bipolar components. The highest ZT value measured was 0.94 at 850 K for Mg_1.9975Al_0.0025Si_0.75Sn_0.2425Sb_0.0075. Sb doping was effective for enhancement of ZT, because of a remarkable increase in the carrier concentration. However, Al doping was almost ineffective for enhancing ZT.     

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.     

Thermoelectric Properties of Polycrystalline SrZn2Sb2 Prepared by Spark Plasma Sintering

Compact polycrystalline samples of SrZn₂Sb₂ [space group $ P\overline{3} m1 $ , a = 4.503(1) Å, c = 7.721(1) Å] were prepared by spark plasma sintering. Thermoelectric performance, Hall effect, and magnetic properties were investigated in the temperature range from 2 K to 650 K. The thermoelectric figure of merit ZT was found to increase with temperature up to ZT = 0.15 at 650 K. At this temperature the material showed a high Seebeck coefficient of +230 μV K^?1, low thermal conductivity of 1.3 W m^?1 K^?1, but rather low electrical conductivity of 54 S cm^?1, together with a complex temperature behavior. SrZn₂Sb₂ is a diamagnetic p-type conductor with a carrier concentration of 5 × 10¹⁸ cm^?3 at 300 K. The electronic structure was calculated within the density-functional theory (DFT), revealing a low density of states (DOS) of 0.43 states eV^?1 cell^?1 at the Fermi level.     

Reducing Lattice Thermal Conductivity of the Thermoelectric Compound AgSbTe2 (P4/mmm) by Lanthanum Substitution: Computational and Experimental Approaches

In this study we performed lattice dynamics first-principles calculations for the promising thermoelectric (TE) compound AgSbTe₂, and estimated the stability of its three polymorphs over a wide temperature range from 0 to 600 K. We calculated the vibrational density of states of the AgSbTe₂ (P4/mmm) phase. The results suggested that formation of substitutional defects at Ag-sublattice sites impedes lattice vibrations, thereby reducing lattice thermal conductivity. We focused on calculations based on the Debye approximation for the compound La_0.125Ag_0.875SbTe₂, and predicted reduction of the average sound velocity from 1684 to 1563 m s^?1 as a result of La doping. This is manifested as a ca. 14% reduction in thermal conductivity. To confirm the results from computation we produced two Ag–Sb–Te-based alloys, a ternary alloy without La addition and a quaternary alloy containing La. We measured the thermal conductivity of both alloys by use of the laser flash analysis method, and, as a result of La alloying, observed a reduction in thermal conductivity from 0.92 to 0.71 W m^?1 K^?1 at 573 K, as calculated from first principles.     

Ultrafine-Grain and Isotropic Cu/SAC305/Cu Solder Interconnects Fabricated by High-Intensity Ultrasound-Assisted Solidification

With the miniaturization of portable electronic devices, the size of solder joint interconnects is decreasing to micrometer levels. These joints possess only several or even one or two grains, resulting in anisotropy and failure issues. Direct ultrasound-assisted solidification of Cu/SAC305/Cu interconnects for grain refinement and fabrication of isotropic solder joints is presented herein. These joints consist of many β-Sn grains. The average cross-sectional area of the Sn-rich phase is significantly reduced by up to 99% when compared with conventional as-reflowed samples. The ultrasonic power density exhibits a threshold value for affecting the microstructures. Below 200 W cm^?2, the β-Sn grains were refined and had circular shape. The Ag₃Sn phase grew in a manner similar to branched coral to sizes reaching 30 μm, or as rods aggregated together with Cu₆Sn₅ tube fragments. Above 200 W cm^?2, the microstructures were coarsened and Ag₃Sn had plate-like shape. The thickness of Cu₆Sn₅ intermetallic layers at the Cu/solder interfaces was reduced by more than 26%. The relationships among the ultrasonic power, nucleation rate, local temperature drop, and pressure were identified. At the highest power density of 267 W cm^?2, the nucleation rate was about 4.05 × 10¹⁴ m^?3 s^?1, the local temperature drop was 248 K, and the local pressure was on the order of several GPa.     

Fabrication of a Flexible Bismuth Telluride Power Generation Module Using Microporous Polyimide Films as Substrates

In this study, we investigated the effect of the structure of microporous p-type (Bi_0.4Te₃Sb_1.6) and n-type (Bi_2.0Te_2.7Se_0.3) BiTe-based thin films on their thermoelectric performance. High-aspect-ratio porous thin films with pore depth greater than 1 μm and pore diameter ranging from 300 nm to 500 nm were prepared by oxygen plasma etching of polyimide (PI) layers capped with a heat-resistant block copolymer, which acted as the template. The cross-plane thermal conductivities of the porous p- and n-type thin films were 0.4 W m^?1 K^?1 and 0.42 W m^?1 K^?1, respectively, and the dimensionless figures of merit, ZT, of the p- and n-type BiTe films were estimated as 1.0 and 1.0, respectively, at room temperature. A prototype thermoelectric module consisting of 20 pairs of p- and n-type strips over an area of 3 cm × 5 cm was fabricated on the porous PI substrate. This module produced an output power of 0.1 mW and an output voltage of 0.6 V for a temperature difference of 130°C. The output power of the submicrostructured module was 1.5 times greater than that of a module based on smooth BiTe-based thin films. Thus, the thermoelectric performance of the thin films was improved owing to their submicroscale structure.     

Effect of Cooling Conditions on the Microstructure and Thermoelectric Properties of Zn/Si-Codoped InSb

InSb is a good candidate thermoelectric (TE) material owing to its high carrier mobility and narrow band gap around 0.18 eV. However, a high figure of merit (ZT) value has not been achieved with InSb because of its high lattice thermal conductivity (κ _lat). To reduce the κ _lat of InSb, we prepared a ZnIn₁₈SiSb₂₀ alloy by Zn/Si codoping into the In lattice sites of InSb. Polycrystalline samples of ZnIn₁₈SiSb₂₀ were prepared by a solid-state reaction method combined with hot pressing. To investigate the microstructures and TE properties resulting from different cooling conditions, samples were prepared by water quenching or slow cooling after an annealing process. The different cooling conditions led to different ZnIn₁₈SiSb₂₀ microstructures and TE properties. The electrical transport properties showed that both samples exhibited metal-like behavior and p-type conduction. The thermal conductivity values of the quenched and slow-cooled samples at room temperature were 8.7 W m^?1 K^?1 and 11.7 W m^?1 K^?1, respectively. A maximum ZT value of 0.23 was obtained at 723 K for the quenched ZnIn₁₈SiSb₂₀ sample.     

Effects of Second Phase Yb5Sb3 on the Thermoelectric Properties of YbAl3

The compound YbAl₃ exhibits a very high power factor but also rather a large thermal conductivity, leading to a low figure of merit. The second phase Yb₅Sb₃ was introduced in the YbAl₃ matrix to reduce its thermal conductivity. The composites (YbAl₃)_1?x (Yb₅Sb₃) _x with x = 0, 0.01, 0.05, 0.10, and 0.20 were synthesized by high frequency induction melting, annealing treatment, and spark plasma sintering. The thermoelectric properties of the composites were evaluated. The composites are of n-type conduction. The pure YbAl₃ obtained in this work shows a high power factor of 11,500 μW m^?1 K^?2 but also a high thermal conductivity of 19.6 W m^?1 K^?1. However, the existence of Yb₅Sb₃ compound in the YbAl₃ matrix enhances the electrical resistivity and the absolute Seebeck coefficient of the composite, but significantly reduces its thermal conductivity in the temperature range considered, thereby enhancing the figure of merit. The highest ZT value of 0.23 may be obtained in the sample (YbAl₃)_0.95(Yb₅Sb₃)_0.05 at room temperature, which is apparently higher than that of pure YbAl₃.     

Thermal Characterization of Polycrystalline SiC

A study is made using fabricated thermal resistors in combination with two-dimensional (2D) electrothermal simulations to determine the thermal conductivity of polycrystalline SiC, single-crystalline SiC, and Si. The results show that the poly-SiC substrate has thermal conductivity of κ _poly-SiC = 2.7 W K^?1 cm^?1, which is significantly lower than that of single-crystalline SiC.     

Effects of Ni, Pd, and Pt Substitutions on Thermoelectric Properties of CoSi Alloys

Ni-, Pd-, and Pt-substituted CoSi samples have been prepared by an arc melting and annealing procedure. The x-ray diffraction and scanning electron microscopy results show that Ni and Pd are effective n-type dopants for CoSi, while Pt is immiscible with CoSi and forms an impurity phase with a possible chemical formula of PtCoSi₂. The thermoelectric properties were measured from 80 K to 300 K. For Ni- and Pd-doped samples, the electrical resistivity and Seebeck coefficient decrease simultaneously due to the increasing carrier concentration. For Pt-substituted samples, the electrical resistivity also decreases. However, this reduction is thought to be not due to an increase in carriers but rather to originate from the effect of the Pt-rich impurity phase at the grain boundaries. The Seebeck coefficient is not affected by 1% Pt substitution; however, further increase of the Pt level also causes a decrease in the Seebeck coefficient. The room-temperature power factor is 63 μW K^?2 cm^?1 for pure CoSi and 73 μW K^?2 cm^?1 for the Co_0.99Pt_0.01Si sample. Although the thermal conductivity is reduced for both n-type-doped and Pt-substituted samples around 80 K, the room-temperature values are still close to that of pure CoSi. As a result, ZT of 0.13 is obtained at room temperature for Co_0.99Pt_0.01Si, an 18% increase compared with CoSi.     

Thermoelectric Properties of Polyaniline Films with Different Doping Concentrations of (±)-10-Camphorsulfonic Acid

The temperature dependence of the thermoelectric properties was investigated for polyaniline (PANI) films doped with different concentrations of (±)-10-camphorsulfonic acid (CSA) with molar ratio x of CSA to two phenyl-nitrogen units of x = 1 to 0.2. All PANI-CSA films exhibit p-type conduction. The temperature dependence of the electrical conductivity of the films with low CSA concentrations is consistent with a transport mechanism of variable-range hopping. On the other hand, the Seebeck coefficient above room temperature shows a linear increase with temperature, attributed to the metallic nature of PANI-CSA. As the CSA concentration decreases, the absolute value of the Seebeck coefficient increases while the electrical conductivity extremely decreases, probably due to the changes not only in the carrier concentration but also in the degree of structural disorder. The power factor increases monotonically with increasing CSA concentration toward x = 1 (the maximum limit). The thermal conductivity value of CSA-PANI film with x = 1 is as low as about 0.20 W m^?1 K^?1 in the through-plane direction and about 0.67 W m^?1 K^?1 in the in-plane direction. The thermoelectric figure of merit ZT in the in-plane direction is estimated to be approximately 1 × 10^?3 for x = 1.     

Effects of Reaction Temperature on Thermoelectric Properties of p-Type Nanostructured Bi2−x Sb x Te3 Prepared Using Hydrothermal Method and Evacuated-and-Encapsulated Sintering

We report fabrication of nanostructured Bi_2?x Sb _x Te₃ using hydrothermal method followed by cold-pressing and evacuated-and-encapsulated sintering techniques. To obtain lower resistivity, the reaction temperature in the hydrothermal synthesis is investigated, and the effects on the ZT values of Bi_2?x Sb _x Te₃ are reported. Both the x = 1.52 and 1.55 samples hydrothermally synthesized at 160°C show lower resistivity than the x = 1.55 sample hydrothermally synthesized at 140°C. However, the power factor is lower for the samples synthesized at 160°C due to the accompanying smaller thermopower. All three samples exhibit remarkably low thermal conductivity of around 0.41 W m^?1 K^?1 at room temperature. The peak ZT value occurs at 270 K for all three samples, being ZT = 1.75, 1.29, and 1.17 for x = 1.55 (synthesized at 140°C), 1.55 (synthesized at 160°C), and 1.52 (synthesized at 160°C), respectively.     

Enhancing the Thermoelectric Properties of Ca3Co4O9 Thin Films by the Addition of a Nanoscale NbN x Second Phase

High-quality Ca₃Co₄O₉ (CCO) thin films have been epitaxially grown on c-cut Al₂O₃ single crystal substrates using pulsed laser deposition (PLD). Different doses of Nb ions were injected into the films using an ion beam injection technique, and a nanoscale NbN _x second phase was generated in the films after annealing in pure N₂. The resistivity and Seebeck coefficient of the films were measured in the temperature range 175–375 K. The results demonstrated that the power factor of the films increases when injected with appropriate quantities of Nb. When the injected Nb concentration was 1.46 × 10²⁰/cm³, the power factor of the film reached 0.17 mW/m K² at room temperature, which is nearly twice as large as that for pure CCO film. A maximum value of 0.22 mW/m K² was obtained at 375 K.     

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.     

Thermoelectric Properties of p-Type Mg2Si0.25Sn0.75 Doped with Sodium Acetate and Metallic Sodium

We have investigated the thermoelectric properties of p-type Na-doped Mg₂ Si_0.25Sn_0.75 solid solutions prepared by liquid–solid reaction and hot-pressing methods. Na was introduced into Mg₂Si_0.25Sn_0.75 by using either sodium acetate (CH₃COONa) or metallic sodium (2 N). The samples doped with sodium acetate consisted of phases with antifluorite structure and a small amount of MgO as revealed by x-ray diffraction, whereas the sample doped with metallic sodium contained the Sn, MgO, and Mg₂SiSn phases. The hole concentrations of Mg_1.975Na_0.025Si_0.25Sn_0.75 doped by sodium acetate and metallic sodium were 1.84 × 10²⁵ m^?3 and 1.22 × 10²⁵ m^?3, respectively, resulting in resistivities of 4.96 × 10^?5 Ω m (sodium acetate) and 1.09 × 10^?5 Ω m (metallic sodium). The Seebeck coefficients were 198 μV K^?1 (sodium acetate) and 241 μV K^?1 (metallic sodium). The figures of merit for Mg_1.975Na_0.025Si_0.25Sn_0.75 were 0.40 × 10^?3 K^?1 (sodium acetate) and 0.25 × 10^?3 K^?1 (metallic sodium) at 400 K. Thus, sodium acetate is a suitable Na dopant for Mg₂Si_1?x Sn _x .     

Thermopower Enhancement of Bi2Te3 Films by Doping I Ions

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.     

Low-Temperature,Solution-Based,Scalable Synthesis of Sb2Te3 Nanoparticles with an Enhanced Power Factor

Nanostructured thermoelectric (TE) materials, for example Sb₂Te₃, PbTe, and SiGe-based semiconductors, have excellent thermoelectric transport properties and are promising candidates for next-generation TE commercial application. However, it is a challenge to synthesize the corresponding pure nanocrystals with controlled size by low-temperature wet-chemical reaction. Herein, we report an alternative versatile solution-based method for synthesis of plate-like Sb₂Te₃ nanoparticles in a flask using SbCl₃ and Te powders as raw materials, EDTA-Na₂ as complexing agent, and NaBH₄ as reducing agent in the solvent (distilled water). To investigate their thermoelectric transport properties, the obtained powders were cold compacted into cuboid prisms then annealed under a protective N₂ atmosphere. The results showed that both the electrical conductivity (σ) and the power factor (S ² σ) can be enhanced by improving the purity of the products and by increasing the annealing temperature. The highest power factor was 2.04 μW cm^?1 K^?2 at 140°C and electrical conductivity remained in the range 5–10 × 10³ S m^?1. This work provides a simple and economic approach to preparation of large quantities of nanostructured Sb₂Te₃ with excellent TE performance, making it a fascinating candidate for commercialization of cooling devices.     

Crystal Structure and Physical Properties of Skutterudites SrPd4Sn x Sb12−x

A Pd-based skutterudite phase SrPd₄Sn _x Sb_12?x in which the 8c sites of the structure are occupied solely by Pd atoms and with a homogeneity range of 4.3(2) ≤ x ≤ 5.8(2) (wavelength dispersive x-ray spectroscopy) has been synthesized by solid-state reaction then spark-plasma sintering (SPS). It crystallizes as the filled-skutterudite structure type, electronically compensated by substitution of Sn for Sb at framework (24g) positions. The unit cell decreases substantially with increasing nominal and detected Sn content. Magnetization, specific heat, Hall effect, electrical resistivity, thermopower, and thermal conductivity were measured for the SPS-treated samples. SrPd₄Sn _x Sb_12?x is a diamagnetic material. Depending on composition it occurs in the metallic or semiconducting states. Hall effect data show that the type and concentration of most of the carriers depend on the Sn/Sb atomic ratio. The thermal conductivity of SrPd₄Sn _x Sb_12?x is approximately 3.0–5.0 W K^?1 m^?1 at 300 K. The Seebeck coefficient is negative throughout the temperature range covered, reaching approximately ?20 μV K^?1 at 300 K.