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.     

Effect of Composition on Thermoelectric Properties of Polycrystalline CrSi2

Ingots with compositions CrSi_2?x (with 0 < x < 0.1) were synthesized by vacuum arc melting followed by uniaxial hot pressing for densification. This paper reports the temperature and composition dependence of the electrical resistivity, Seebeck coefficient, and thermal conductivity of CrSi_2?x samples in the temperature range of 300 K to 800 K. The silicon-deficient samples exhibited substantial reductions in resistivity and Seebeck coefficient over the measured temperature range due to the formation of metallic secondary CrSi phase embedded in the CrSi₂ matrix phase. The thermal conductivity was seen to exhibit a U-shaped curve with respect to x, exhibiting a minimum value at the composition of x = 0.04. However, the limit of the homogeneity range of CrSi₂ suppresses any further decrease of the lattice thermal conductivity. As a consequence, the maximum figure of merit of ZT = 0.1 is obtained at 650 K for CrSi_1.98.     

Low-Temperature Synthesis and Thermoelectric Properties of n-Type PbTe

n-Type PbTe compounds were synthesized at temperatures as low as 430°C. After synthesis, the materials were ground, cold pressed, and sintered at 600°C. The effect of this low-temperature synthesis on the structural features and thermoelectric properties of as-prepared and PbI₂-doped materials was investigated for the first time. The Seebeck coefficient, and electrical and thermal conductivity were measured in the temperature range 2 K ≤ T ≤  610 K. The results show that all materials exhibit n-type conduction and the thermoelectric properties are improved by doping. ZT values reach 0.5 at 610 K, and the discrepancies with the literature are discussed.     

Thermoelectric Properties of Indium-Added Skutterudites In x Co4Sb12

The high-temperature thermoelectric properties of In _x Co₄Sb₁₂ (0.05 ≤ x ≤ 0.40) skutterudite compounds were investigated in this study. The phase states of the samples were identified by x-ray diffraction analysis and field-emission scanning electron microscopy at room temperature. InSb and CoSb₂ were found as secondary phases in samples with x = 0.10 to 0.40. The filling limit of In into the CoSb₃ cages of In _x Co₄Sb₁₂ was in the range 0.05 < x < 0.10. The electrical resistivity, Seebeck coefficient, and thermal conductivity of the In _x Co₄Sb₁₂ samples were measured from room temperature to 773 K. The Seebeck coefficient of all samples was negative. Reduction of the thermal conductivity by In addition resulted in a high thermoelectric figure of merit (ZT) of 0.67 for In_0.35Co₄Sb₁₂ at 600 K.     

Characterization of <Emphasis Type="Italic">n</Emphasis>-Type and <Emphasis Type="Italic">p</Emphasis>-Type Long-Wave InAs/InAsSb Superlattices

The influence of dopant concentration on both in-plane mobility and minority carrier lifetime in long-wave infrared InAs/InAsSb superlattices (SLs) was investigated. Unintentially doped (n-type) and various concentrations of Be-doped (p-type) SLs were characterized using variable-field Hall and photoconductive decay techniques. Minority carrier lifetimes in p-type InAs/InAsSb SLs are observed to decrease with increasing carrier concentration, with the longest lifetime at 77 K determined to be 437 ns, corresponding to a measured carrier concentration of p ₀ = 4.1 × 10¹⁵ cm^?3. Variable-field Hall technique enabled the extraction of in-plane hole, electron, and surface electron transport properties as a function of temperature. In-plane hole mobility is not observed to change with doping level and increases with reducing temperature, reaching a maximum at the lowest temperature measured of 30 K. An activation energy of the Be-dopant is determined to be 3.5 meV from Arrhenius analysis of hole concentration. Minority carrier electrons populations are suppressed at the highest Be-doping levels, but mobility and concentration values are resolved in lower-doped samples. An average surface electron conductivity of 3.54 × 10^?4 S at 30 K is determined from the analysis of p-type samples. Effects of passivation treatments on surface conductivity will be presented.     

Synthesis and Measurement of the Thermoelectric Properties of Multiphase Composites: ZnSb Matrix with Zn4Sb3, Zn3P2, and Cu5Zn8

We synthesized a series of samples with composition around 52 at.% zinc (Zn), 44 at.% antimony (Sb), 4 at.% phosphorus (P), and up to 3 at.% copper (Cu) by melting the elements and subsequent annealing. This resulted in dense and almost crack-free samples. X-ray powder diffraction (XRD) and scanning electron microscopy (SEM) revealed composites with a majority phase of ZnSb containing varying amounts of Zn₃P₂ and Cu₅Zn₈, in addition to Zn₄Sb₃ in some of the samples. We measured the Seebeck coefficient, electrical conductivity, and thermal conductivity as a function of temperature. The thermoelectric performance tended to improve with increasing Cu content. At Cu content of 2 at.%, a reduced resistivity allows for the highest dimensionless figure of merit, with a maximum zT value of 0.18 at around 573 K.     

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.     

Lower Thermal Conductivity and Higher Thermoelectric Performance of Fe-Substituted and Ce, Yb Double-Filled p-Type Skutterudites

Substituting Fe on Co sites is an effective way to produce p-type skutterudite compounds as well as to reduce the thermal conductivity of skutterudites. In this work, we investigated thermoelectric properties of Fe-substituted and Ce + Yb double-filled Ce _x Yb _y Fe _z Co_4?z Sb₁₂ (x = y = 0.5, z = 2.0 to 3.25 nominal) skutterudite compounds by studying the Seebeck coefficient, electrical conductivity, thermal conductivity, and Hall coefficient over a broad range of temperatures. All samples were prepared by using the traditional method of melting–annealing and spark plasma sintering. The signs of the Hall coefficient and Seebeck coefficient indicate that all samples are p-type conductors. Electrical conductivity increases with increasing Fe content. The temperature dependence of electrical conductivity indicates that a transition from the extrinsic to the intrinsic regime of conduction depends on the amount of Fe substituted for Co. The temperature dependence of mobility reflects the dominance of acoustic phonon scattering at temperatures above ambient. Except for Ce_0.5Yb_0.5Fe_3.25Co_0.75Sb₁₂, the thermal conductivity increases with increasing Fe content, reaching the maximum value of 2.23 W/m K at room temperature for Ce_0.5Yb_0.5Fe₃CoSb₁₂. A high power factor (27 μW/K² cm) combined with a rather low thermal conductivity for Ce_0.5Yb_0.5Fe_3.25Co_0.75Sb₁₂ (nominal) lead to a dimensionless figure of merit ZT = 1.0 at 750 K for this compound, one of the highest ZT values achieved in p-type skutterudite compounds prepared by the traditional method of melting–annealing and spark plasma sintering.     

Ball Mill Synthesis of Bulk Quaternary Cu<Subscript>2</Subscript>ZnSnSe<Subscript>4</Subscript> and Thermoelectric Studies

In this work, quaternary chalcogenide Cu₂ZnSnSe₄ (CZTSe) was synthesized using a mechanochemical ball milling process and its thermoelectric properties were studied by electrical resistivity, Seebeck coefficient, and thermal conductivity measurements. The synthesis process comprises three steps viz., wet ball milling of the elemental precursors, vacuum annealing, and densification by hot pressing. The purpose of this is to evaluate the feasibility of introducing wet milling in place of vacuum melting in solid state synthesis for the reaction of starting elements. We report the structural characterization and thermoelectric studies conducted on samples that were milled at 300 rpm and 500 rpm. X-ray diffraction (XRD) analysis showed the existence of multiple phases in the as-milled samples, indicating the requirement for heat treatment. Therefore, the ball milled powders were cold pressed and vacuum annealed to eliminate the secondary phases. Annealed samples were hot pressed and made into dense pellets for further investigations. In addition to XRD, energy dispersive spectroscopy (EDS) studies were performed on hot pressed samples to study the composition. XRD and EDS studies confirm CZTSe phase formation along with ZnSe secondary phase. Electrical resistivity and Seebeck coefficient measurements were done on the hot pressed samples in the temperature range 340–670 K to understand the thermoelectric behaviour. Thermal conductivity was calculated from the specific heat capacity and thermal diffusivity values. The thermoelectric figure of merit zT values for samples milled at 300 rpm and 500 rpm are ～0.15 and ～0.16, respectively, at 630 K, which is in good agreement with the values reported for solid state synthesized compounds.     

The Effect of Adding Nano-Bi2Te3 on Properties of GeTe-Based Thermoelectric Material

The efficient thermoelectric materials (GeTe)_0.85?x (Mn_0.6Sn_0.4Te)_0.15(Bi₂Te₃) _x (0 ≤ x ≤ 0.05), in which Bi₂Te₃ is nanopowder, were prepared by hot pressing. The effect of adding neutral nano-Bi₂Te₃ content on the thermoelectric properties of germanium telluride was investigated. With increasing x, the thermal conductivity of the prepared samples decreased significantly and the Seebeck coefficient declined slightly, while there was no obvious change in electrical conductivity. In both electrical conductivity and Seebeck coefficient curves at different x values, there are inflection points around 600 K. The maximum dimensionless figure of merit ZT of the prepared materials is 1.54, attained in the temperature range from 700 K to 750 K for x = 0.03. The x-ray diffraction (XRD) pattern shows that Bi₂Te₃ has been alloyed into the GeTe-MnTe-SnTe alloy, which is consistent with the high-resolution scanning electron microscopy (HRSEM) images. Adding nano-Bi₂Te₃ to GeTe-based materials could also increase their performance stability at high temperature as a result of decreasing the phase-transition temperature T _c.     

Mass Fluctuation Effect in Ti1−x Nb x S2 Bulk Compounds

The thermoelectric properties of Nb-substituted TiS₂ compounds have been investigated in the temperature range of 300 K to 700 K. Polycrystalline samples in the series Ti_1?x Nb _x S₂ with x varying from 0 to 0.05 were prepared using solid–liquid–vapor reaction and spark plasma sintering. Rietveld refinements of x-ray diffraction data are consistent with the existence of full solid solution for x ≤ 0.05. Transport measurements reveal that niobium can be considered as an electron donor when substituted at Ti sites. Consequently, the electrical resistivity and the absolute value of the Seebeck coefficient decrease as the Nb content increases, due to an increase in the carrier concentration. Moreover, due to mass fluctuation, the lattice thermal conductivity is reduced, leading to a slight increase of ZT values as compared with TiS₂.     

Microstructures and Thermoelectric Properties of Melt-Spun Zn x Sb3 Ribbons

Melt-spun Zn _x Sb₃ ribbons were fabricated with weight compositions of x = 3.6, 3.9, and 4.2 through a single-wheel process and were annealed for 2 h at 673 K. The microstructures of the ribbons were investigated using transmission electron microscopy, together with energy-dispersive x-ray analysis. The main structure consisted of β-Zn₄Sb₃ phase, which mainly coexisted with ZnSb phase for x < 4 and with Zn phase for x > 4. The analyzed composition of the β-Zn₄Sb₃ phase deviated from the stoichiometric composition of 4:3 for all the ribbons. Nanosized voids and zinc inclusions were randomly distributed throughout the β-Zn₄Sb₃ phase. The thermoelectric characteristics of the ribbons were revealed by measuring the Seebeck coefficient, electrical conductivity, power factor, dimensionless figure of merit, and thermal conductivity. The power factor and dimensionless figure of merit increase with increasing x and temperature because either the electrical conductivity or Seebeck coefficient increases.     

Crystallographic, Thermoelectric, and Mechanical Properties of Polycrystalline Ba8Al x Si46−x Clathrates

The Al content dependence of crystallographic, thermoelectric, and mechanical properties is reported for polycrystalline Ba₈Al _x Si_46?x (nominal x = 15 to 17) clathrates prepared by combining arc melting and spark plasma sintering methods. The elastic constants and the coefficient of thermal expansion (CTE), which are also important properties for designing thermoelectric devices, are presented. Powder x-ray diffraction, scanning electron microscopy, and energy-dispersive x-ray spectroscopy (EDX) indicate that the type I clathrate is the major phase of the samples but impurity phases (mainly BaAl₂Si₂, Si, and Al) are included in the samples with high Al contents. The actual Al content x determined by EDX ranges from approximately 14 to 15. The absolute value of the Seebeck coefficient increases and the electrical conductivity decreases as the Al content increases. The changes in Seebeck coefficient and electrical conductivity are explained in terms of the dependence of the carrier concentration on the Al content. The elastic constants and the CTE of the samples depend weakly on the Al content. Some of the properties are compared with reported data of single crystals of Ba₈Al₁₆Ge₃₀, Ba₈Ga₁₆Ge₃₀, Sr₈Ga₁₆Ge₃₀, silicon, and germanium as standard references. The effective mass, Hall carrier mobility, and lattice thermal conductivity, which govern the transport properties, are determined to be ~ 2.4m ₀, ~ 7 cm² V^?1 s^?1, and ~ 1.3 W m^?1 K^?1, respectively, for actual Al content x of about 14.77. The thermoelectric figure of merit ZT is estimated to be about 0.35 at 900 K for actual Al content x of about 14.77.     

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.     

Influence of Vanadium on the Defect Structure and Thermoelectric Properties of GeTe

The development of new thermoelectric materials based on GeTe is associated with reducing the hole concentration and thermal conductivity. The objects of the present study are GeTe-based solid solutions in the Ge-V-Te ternary system. The goal of the work is to study the character of the change in the structure, mechanical and thermoelectric properties of GeTe under introduction of VTe. The electrical conductivity σ, Seebeck coefficient S, and Hall coefficient R _H were measured in the range of 300 K to 850 K on cast samples and samples prepared by hot pressing; the thermal conductivity λ was measured at room temperature. It was found that the dependences of the unit cell parameters, microhardness, σ, R _H, λ, and S on the VTe content exhibit nonmonotonic behavior. The experimental results were interpreted taking into consideration the complex mechanisms of defect formation in the GeTe crystal lattice under introduction of VTe, the existence of nonstoichiometric vacancies, and percolation effects. It was established that introduction of VTe into GeTe leads to a significant decrease in λ and hole concentration p. The maximum room-temperature values of thermoelectric power factor P and thermoelectric figure of merit Z corresponded to ~2 mol.% VTe. With increasing temperature up to ~550 K, P increases, and the maximum value of P is shifted to ~3 mol.% VTe. The values of P and Z obtained for the cast and pressed samples were practically the same.     

Evaluation of the Structure and Transport Properties of Nanostructured Antimony Telluride (Sb2Te3)

Antimony telluride, (Sb₂Te₃), and its doped derivatives are considered to be among the best p-type thermoelectric (TE) materials for room temperature (300–400 K) applications. However, it is still desirable to develop rapid and economical routes for large-scale synthesis of Sb₂Te₃ nanostructures. We report herein a high yield, simple and easily scalable synthetic method for polycrystalline Sb₂Te₃ nanostructures. Prepared samples were compacted into dense pellets by use of spark plasma sintering. The products were characterized by x-ray diffraction and scanning electron microscopy. To investigate the anisotropic behavior of Sb₂Te₃ TE transport property measurements were performed along and perpendicular to the direction of compaction. Thermal conductivity, electrical conductivity, and Seebeck coefficient measurement over the temperature range 350–525 K showed that the anisotropy of the material had a large effect on TE performance.     

Solvent-Based Synthesis of <Emphasis Type="Italic">Nano</Emphasis>-Bi<Subscript>0.85</Subscript>Sb<Subscript>0.15</Subscript> for Low-Temperature Thermoelectric Applications

In this study we show a preparation method for nanostructured Bi_0.85Sb_0.15 powders via a chemical reduction route in a polyol medium, yielding material with particle sizes of 20–150 nm in scalable amounts. The powders were consolidated by spark plasma sintering (SPS) in order to maintain the nanostructure. To investigate influence of the sinter process, the powders were characterized by x-ray diffraction (XRD), energy dispersive x-ray spectroscopy (EDX), and scanning electron microscopy (SEM) measurements before and after SPS. Transport properties, Seebeck effect, and thermal conductivity were determined in the low temperature range below 300 K. The samples showed excellent thermal conductivity of 2.3–2.6 W/m × K at 300 K and Seebeck coefficients from ?97 μV/K to ?107 μV/K at 300 K with a maximum of ?141 μV/K at 110 K, thus leading to ZT values of up to 0.31 at room temperature. The results show that Bi-Sb-alloys are promising materials for low-temperature applications. Our wet chemical approach gives access to scalable amounts of nano-material with increased homogeneity and good thermoelectric properties after SPS.     

Effect of Se Substitution on Structural and Electrical Transport Properties of Bi0.4Sb1.6Se3x Te3(1−x) Hexagonal Rods

In the current study, novel hexagonal rods based on Bi_0.4Sb_1.6Te₃ ingots dispersed with x amount of Se (x = 0.0, 0.2, 0.4, 0.6, 0.8, and 1.0) in the form Bi_0.4Sb_1.6Se_3x Te_3(1?x) were synthesized via a standard solid-state microwave route. The morphologies of these rods were explored using field-emission scanning electron microscopy (FESEM). The crystal structure of the powders was examined by x-ray diffraction (XRD) analysis, which showed that powders of the 0.0 ≤ x ≤ 0.8 samples could be indexed to the rhombohedral phase, whereas the sample with x = 1.0 had an orthorhombic phase structure. The influence of variations in the Se content on the thermoelectric properties was studied in the temperature range from 300 K to 523 K. Alloying of Se into Bi_0.4Sb_1.6Te₃ effectively caused a decrease in the hole concentration and, thus, a decrease in the electrical conductivity and an increase in the Seebeck coefficient. The maximal power factor measured in the present work was 7.47 mW/mK² at 373 K for the x = 0.8 sample.     

Effect of Praseodymium on Electrical Properties of BiFeO3 Multiferroic

Polycrystalline samples of Bi_1?x Pr _x FeO₃ (BPFO) (x = 0, 0.05, 0.1, 0.15) were synthesized by a solid-state reaction technique. Preliminary x-ray structural analysis confirmed the formation of a single-phase compound of BPFO. Scanning electron micrographs recorded at room temperature on pellet samples showed: (i) a uniform distribution of grains on the surface of the samples (with fewer voids) and (ii) reduction of grain size with increasing Pr content of the BPFO samples. Detailed studies of the impedance and electrical modulus of the materials, carried out in a wide range of frequency (1 kHz to 1 MHz) at different temperatures (25°C to 400°C), have provided many interesting results including a significant decrease in loss tangent, structural stability, etc. The variation of the alternating-current (ac) and direct-current (dc) conductivity with inverse absolute temperature follows an Arrhenius relation. The decrease in leakage current and the negative temperature coefficient of resistance behavior with increasing Pr content of BPFO are important observations of this work.