Phase transformations in the Zn–Cd–Sb system

Phase equilibria of the Cd–Sb–Zn system have been investigated by metallographic examinations, DSC, XRD and WDS measurements. At 250 °C, the ternary diagram shows two three-phase fields, (Zn)+(Cd)+Zn₄Sb₃ and (Cd)+ Zn₄Sb₃+(Zn,Cd)Sb. Continuous solid solution has been found between ZnSb and CdSb. Solubility of Cd in Sb₃Zn₄ was determined to be about 43 at.%. A variant of the reaction scheme is proposed for the Cd–Sb–Zn system to understand phase relations observed at 250 °C.     

Mechanical and thermoelectric properties of Zn4Sb3 and Zn4Sb3+Zn directly synthesized using elemental powders

Direct synthesis using elemental powders has been used to produce single-phase polycrystalline ε-Zn₄Sb₃ specimens as well as composite specimens having ε-Zn₄Sb₃ (majority phase) and Zn (minority phase). The effect of the Zn phase on the elastic, thermoelectric and mechanical properties was investigated in this alloy system. Thermoelectric properties of single-phase Zn₄Sb₃ at an ambient temperature are comparable to the published data for the sample prepared by a hot-pressing of ingot-melted alloy powders. Transport properties at room temperature were also evaluated. In addition, Young’s modulus and the bulk modulus of polycrystalline Zn₄Sb₃ were measured using a resonant-ultrasonic technique. The fracture toughness in this alloy system was determined by measuring the length of cracks that formed at the corners of pyramidal indentations used for hardness tests. It is shown that the addition of Zn increases the fracture toughness, but this is achieved at the cost of reducing the thermoelectric figure of merit.     

Effects of Ag doping on thermoelectric properties of Zn4Sb3 at low temperatures

The thermoelectric properties of Ag-doped compounds (Zn_1?xAg_x)₄Sb₃ (x = 0, 0.0025, 0.005, 0.01) have been studied at the temperatures from 15 to 300 K. The results indicate that low-temperature (T < 300 K) thermal conductivity of the moderately doped (Zn_1?xAg_x)₄Sb₃ (x = 0.0025 and 0.005) reduced remarkably as compared with that of Zn₄Sb₃ due to enhanced impurity (dopant) scattering of phonons. Electrical resistivity and Seebeck coefficient were found to increase first and then decrease obviously with the increase in the Ag content, which could be ascribed to the change of carrier concentration presumably due to different Zn positions occupied by Ag upon increasing doping content. Moreover, the lightly doped compound (Zn_0.995Ag_0.005)₄Sb₃ exhibited the best thermoelectric performance due to the improvement in both its electrical resistivity and thermal conductivity, whose figure of merit (at 300 K), ZT, is about 1.3 times larger than that of β-Zn₄Sb₃ obtained in the present study. Present results suggest that proper Ag doping in Zn₄Sb₃ is a promising way of improving its thermoelectric properties.     

Electrical Transport Properties of Single-Crystalline β-Zn<Subscript>4</Subscript>Sb<Subscript>3</Subscript> Prepared Through the Zn-Sn Mixed-Flux Method

β-Zn₄Sb₃ is a promising p-type thermoelectric material for utilization in moderate temperatures. This study prepares a group of single-crystalline β-Zn₄Sb₃ samples using the Zn-Sn mixed-flux method based on the stoichiometric ratios of Zn_4+x Sb₃Sn _y . The effect of Zn-to-Sn proportion in the flux on the structure and electrical transport properties is investigated. All samples are strip-shaped single crystals of different sizes. The actual Zn content of the present samples is improved (>3.9) compared with that of the samples prepared through the Sn flux method. Larger lattice parameters are also obtained. The carrier concentration of all the samples is in the order of over 10¹⁹ cm^?3. With increasing Sn rate in the flux, this carrier concentration decreases, whereas mobility is significantly enhanced. The electrical conductivity and Seebeck coefficients of all the samples exhibit a behavior that of a degenerate semiconductor transport. Electrical conductivity initially increases and then decreases as the Sn ratio in the flux increases. The electrical conductivity of the x:y = 5:1 sample reaches 6.45 × 10⁴ S m^?1 at 300 K. Benefitting from the electrical conductivity and Seebeck coefficient, the flux proportion of the x:y = 7:1 sample finally achieves the highest power factor value of 1.4 × 10^?3 W m^?1 K^?2 at 598 K.     

Effects of Te doping on the transport and thermoelectric properties of Zn4Sb3

The transport and thermoelectric properties of Te-doped Zn₄Sb₃ compounds, Zn₄(Sb_1?xTe_x)₃ (x = 0, 0.005 and 0.02), were investigated. The results showed that thermal conductivity of the Te-doped compounds were reduced remarkably as compared to that of Zn₄Sb₃ presumably due to enhanced impurity (dopant) scattering of phonons. Thermopower S was found to decrease with increase in Te content, which could be ascribed to the excess Zn in the doped compounds acting as p-type dopant that leads to an increase in the carrier concentration. Moreover, it was found that the β to α phase transition of Zn₄Sb₃ could be completely prohibited by Te doping. The figure of merit, ZT, for doped compounds was greater than the un-doped Zn₄Sb₃ for the temperature range investigated. In particular, the ZT of Zn₄(Sb_0.995Te_0.005)₃ reached a value of 1.08 at 680 K, which is 69% greater than that of the un-doped Zn₄Sb₃ obtained in this study.     

Optimizing thermoelectric performance of Cd-doped β-Zn4Sb3 through self-adjusting carrier concentration

Crack-free Zn_3.96+xCd_0.04Sb₃ (x = ?0.05, 0.0, 0.05 and 0.1) ingots were successfully synthesized by a melting followed by a precisely controlled slow cooling process. The facile control of Zn content realizes the effective self-adjustment of carrier concentration, as well as the optimization of the thermoelectric figure of merit. The Zn-deficiency and stoichiometric samples are single phase, whereas a slight metal Zn phase can be detected in other two Zn-rich samples existing as forms of numerous evenly distributed nano-clusters with size of 20–50 nm and a spot of micro-scale precipitations embedded in the matrix. In particular, these multi-scale microstructures combined with the subtle variation of interstitial Zn apparently intensify phonon scattering and give rise to a “phonon-glass” feature of Zn-rich samples. However, Zn-deficiency sample benefiting from high Seebeck coefficient, shows a high power factor (>1.0 mW m^?1 K^?1) in the entire temperature range and a maximum value of 1.26 mW m^?1 K^?1 at 660 K. As a result, the enhanced effective hole mass by a slight Cd-doping coupled with the extremely low lattice thermal conductivity originated from crystalline complexities lead to a high figure of merit of 1.23 at 660 K for Zn_3.91Cd_0.04Sb₃ sample, which is comparable with the highest value reported by T. Caillat et al. [T. Caillat et al. J Phys Chem Solids 1997; 58: 1119?25]. Furthermore, this study demonstrates a simple and easily-industrialized melting combined with slow cooling technique making the high performance β-Zn₄Sb₃ a promising candidate for low-grade waste heat recovery.     

Thermoelectric properties of hot-pressed Zn4Sb3−xTex

A series of samples have been fabricated through vacuum melting method followed by hot-pressing for Zn₄Sb_3−xTe_x (x = 0.02–0.08), XRD patterns indicated that all the samples were single-phased β-Zn₄Sb₃. Electrical conductivity and Seebeck coefficient were evaluated in the temperature range of 300–700 K, showing p-type conduction. The thermoelectric figure of merit (ZT) was increased with the increase of Te content. ZT values of 0.8 and 1.0 were obtained at 673 K for Zn_4.08Sb₃ and Zn₄Sb_2.92Te_0.08, respectively.     

Microstructure and thermoelectric properties of Sb doped Hf0.25Zr0.75NiSn Half-Heusler compounds with improved carrier mobility

Half-Heusler (HH) semiconductor alloys are being widely investigated due to their promising potential for thermoelectric (TE) power generation applications. Sb is an effective doping element for n-type ZrNiSn half-Heuslers alloys. HH thermoelectric materials Hf_0.25Zr_0.75NiSn_1−xSb_x (0 ≤ x ≤ 0.03) were synthesized by induction melting combined with plasma activated sintering (PAS) technique. X-ray diffraction concluded that single-phase HH compounds without compositional segregations were obtained. Presence of bended lamellar structures was revealed by the FESEM. Sb doping significantly enhanced the electrical conductivity, power factor and carrier concentration of the alloys. An increase in the carrier mobility was also observed. Consequently, optimum values of 4.36 × 10⁻³ W/mK² and 4.7 × 10²⁰ cm⁻³ were achieved for power factor and carrier concentration, respectively. As a result, a ZT value of 0.83 at 923 K was obtained which is about 67% improvement compared to the un-doped sample.     

Thermodynamic assessment of the Gd–Sb system

A thermodynamic assessment of the binary Gd–Sb system was performed through the CALPHAD approach (CALculation of PHAse Diagram) based on the evaluation of all phase diagram data and available thermodynamic data in the literature. The liquid, hcp-A3 (αGd) and bcc-A2 (βGd) phases were described by a substitutional solution model. All the intermediate phases, Gd₅Sb₃, Gd₄Sb₃, βGdSb, αGdSb and Gd₁₆Sb₃₉, were treated as stoichiometric compounds. A set of self-consistent thermodynamic parameters of the Gd–Sb system has been obtained. A good agreement is obtained between the experimental and the calculated phase diagrams.     

Preparation and thermal conductivities of filled tetrahedral LixIn1−xZnxSb and the related compounds

Li-filling in tetrahedral InSb and related compounds was attempted to investigate its effect on their thermal conductivities. Li-filled Li_0.2In_0.8Zn_0.2Sb, Li_0.4In_0.6Zn_0.4Sb, LiZnSb, and Li_0.16Ga_0.84Zn_0.16Sb sintered samples were prepared by powder metallurgy. The filled samples had much lower room temperature lattice thermal conductivities than those of the corresponding unfilled materials; the values of the Li_0.4In_0.6Zn_0.4Sb, LiZnSb, and Li_0.16Ga_0.84Zn_0.16Sb were 23, 45, and 72 mW cm⁻¹K⁻¹, respectively. Filled tetrahedral compounds such as Li_xIn_1−xZn_xSb might be good thermoelectric materials.     

Constitution of the systems {V,Nb,Ta}-Sb and physical properties of di-antimonides {V,Nb,Ta}Sb2

The binary phase diagrams {V,Nb,Ta}-Sb below 1450 °C were studied by means of XRPD, EPMA, and DTA measurements. In the V-Sb system, five stable binary phases were observed in this investigation: V_3+xSb_1−x, ℓT-V₃Sb₂, hT-V_2−xSb, V_7.46Sb₉, V_1−xSb₂. The V-Sb phase diagram is characterized by two degenerate eutectic reactions: L↔V_3+xSb_1−x+(V) (T > 1450  °C at 18.1 at.% Sb) and L↔V_1−xSb₂+(Sb) (T=(621 ± 5)°C at ∼99 at.% Sb), three peritectic reactions: L + V_3+xSb_1−x↔hT-V_2−xSb (T=(1230 ± 10)°C at ∼42 at.% Sb), L + hT-V_2−xSb↔V_7.46Sb₉ (T=(920 ± 10)°C at ∼87 at.% Sb), and L + V_7.46Sb₉↔V_1−xSb₂ (T=(869 ± 5)°C at ∼88 at.% Sb), a peritectoid reaction: V_3+xSb_1−x + hT-V_2−xSb↔ℓT-V₃Sb₂ at (875 ± 25)°C, a eutectoid reaction: hT-V_2−xSb↔ℓT-V₃Sb₂+V_7.46Sb₉ at (815 ± 15)°C and congruent melting of V_3+xSb_1−x (T > 1450 °C). An X-ray single crystal study of V₅Sb₄C_1−x proved the existence of interstitial elements in the octahedral voids of a partially filled Ti₅Te₄-type structure (x∼0.5; R_F2 = 0.0101), therefore this phase (earlier labeled “V₅Sb₄”) was excluded from the binary equilibrium phase diagram. V₅Sb₄C_1−x is the first representative of a filled Ti₅Te₄-type structure.A re-investigation of the Nb-Sb system removed the contradiction between the hitherto reported phase diagrams and confirmed the version derived by Melnyk et al. (see ref. [1]).Three binary phases exist in the Ta-Sb system: Ta_3+xSb_1−x, Ta₅Sb₄, TaSb₂. Due to instrumental limits (≤1450 °C), only the peritectic reaction of TaSb₂: L + Ta₅Sb₄ ↔ TaSb₂ ((1080 ± 10)°C at ∼92 at.% Sb) and a degenerate Sb-rich eutectic reaction (L↔TaSb₂+(Sb); (622 ± 5)°C; ∼99 at.% Sb) have been determined.Physical properties (mechanical and transport properties) of binary di-antimonides were investigated with respect to a potential use of these metals either as diffusion barriers or electrodes for thermoelectric devices based on skutterudites. All group-V metal di-antimonides have low metallic-type resistivity and relatively high thermal conductivity. Magnetic field has little influence on the resistivity of V_1−xSb₂ at low temperature, while on {Nb,Ta}Sb₂ it increases the resistivity, especially on NbSb₂. The coefficient of thermal expansion (CTE) decreases from V_1−xSb₂ to TaSb₂, particularly the CTE value of NbSb₂ is in the range of average n-type filled skutterudites. In contrast to the CTE value, elastic moduli increase from V_1−xSb₂ to TaSb₂. The value for V_1−xSb₂ is in the range of Sb-based skutterudites, whereas the values for {Nb,Ta}Sb₂ are significantly higher.     

Crystal structures and phase transformation of deuterated lithium imide, Li2ND

Half-Heusler compounds of Sn-doped TiCoSb “TiCoSn_xSb_1−x (x = 0.0, 0.01, and 0.05)” were prepared and their thermoelectric properties were measured above room temperature. From the EDX analysis, all the samples have three phases: the TiCoSn_xSb_1−x, Co-rich, and Ti-rich phases. The values of the thermoelectric power increase with Sn doping, and a positive thermoelectric power is obtained in the sample of TiCoSn_0.05Sb_0.95. The thermal conductivity decreases both with increasing temperature and with Sn content. The maximum value of ZT for p-type material is 0.030 at 988 K in the sample of TiCoSn_0.05Sb_0.95.     

Experimental Investigation of the Zn-Al-Sb System at 450 °C

An isothermal section of the Zn-Al-Sb ternary system at 450 °C has been established by equilibrating 11 samples of different compositions and phase identification by optical and scanning electron microscopy with energy dispersive spectroscopy, x-ray diffraction after quenching to room temperature. Five three-phase regions exist in the system at 450 °C. No ternary compound has been found. And, the compound AlSb with 2.1 at.% Zn can equilibrate with all phases in the system. Experimental results indicate that the solubility of Sb in α-Al phase is too limited to be detected. The SbZn, Sb₂Zn₃, and Sb₃Zn₄ phases, which have little Al solubility (at most 3.3 at.%), were observed in the system.     

Semimetal-semiconductor transitions in bismuth-antimony films and nanowires induced by size quantization

In this paper we present the experimental results of an investigation of the electrical transport, thermoelectrical properties, the Shubnikov de Haas oscillations of Bi_{1 ? x} Sb _x films (0 < x < 0.04) grown by the vacuum thermal evaporation and nanowires prepared by a modified Ulitovsky-Teilor technique. The results of the X-ray diffraction indicate that the trigonal axes were perpendicular to the film plane and the single Bi-2 at % Sb nanowires with the diameter 100–1000 nm were represented by single crystals in a glass capillary with (1011) orientation along the wire axis. The investigations of the Shubnikov de Haas oscillations on Bi-2 at % Sb wires with d > 600 nm show that overlapping of L and T bands was twice smaller than that in pure Bi. The temperature dependences of thin semimetalic Bi-3 at % Sb films and Bi-2 at % Sb wires show a semiconducting behavior. The semimetal-semiconductor transition induced by the quantum confinement effect is observed in semimetal Bi_{1 ? x} Sb _x films and nanowires at the diameters up to five times greater than those in the pure Bi. That experimental fact, on the one hand, will allow observing the display quantum confinement effect at higher temperatures on nanowires of the same diameters, and, on the other hand, will allows separating effects connected with the surface state and the quantum size effects. In addition, the thermoelectric properties and thermoelectric efficiency of bismuth-antimony wires are considered and a possibility to use them in thermoelectric converters of energy is discussed.     

Enhanced thermoelectric performance via randomly arranged nanopores: Excellent transport properties of YbZn2Sb2 nanoporous materials

Fabricating nanoporous bulk thermoelectric (TE) materials with periodically arranged nanopores is highly challenging and expensive, although TE materials exhibit high power factors (α²σ) and low thermal conductivities (κ). Enhanced TE performance via randomly arranged nanopores is demonstrated with a YbZn₂Sb₂ nanoporous material (nPM) fabricated by a combination of melt quenching and two stage spark plasma sintering in less than 10 h. Measurement of the electrical conductivity, Hall mobility, Seebeck coefficient, and thermal conductivity show that simultaneously enhancing α²σ and reducing κ can realize in the YbZn₂Sb₂ nPM with randomly arranged nanopores about 50-200 nm in diameter. Compared with YbZn₂Sb₂ dense bulk materials (dBM) fabricated by a conventional method taking more than 180 h, α²σ at 300 K increases by 122%, κ at 300 K decreases by 29%, and the maximum ZT value at 775 K reaches 0.67, increasing by 46% for the nPM725 sample. This work shows that a periodic arrangement of nanopores is not essential for the fabrication of attractive TE materials, which offers a wider approach to nanostructure engineering to improve TE performance.     

Synthesis and Structural Characterization of Sb-Doped TiFe<Subscript>2</Subscript>Sn Heusler Compounds

Ternary Heusler compounds form a numerous class of intermetallics, which include two families with general compositions ABC and AB₂C, usually referred to as half- and full-Heusler compounds, respectively. Given their tunable electronic properties, made possible by adjusting the chemical composition, these materials are currently considered for the possible use in sustainable technologies such as solar energy and thermoelectric conversion. According to theoretical predictions, Sb substitution in the TiFe₂Sn full-Heusler compound is thought to yield band structure modifications that should enhance the thermoelectric power factor. In this work, we tested the phase stability and the structural and microstructural properties of such heavily doped compounds. We synthesized polycrystalline TiFe₂Sn_1?xSb_x samples, with x?=?0, 0.1, 0.2 and 1.0 by arc melting, followed by an annealing treatment. The structural characterization, performed by x-ray powder diffraction and microscopy analyses, confirmed the formation of the pseudo-ternary Heusler structure (cF16, Fm-3m, prototype: MnCu₂Al) in all samples, with only few percent amounts of secondary phases and only slight deviations from nominal stoichiometry. With increasing Sb substitution, we found a steady decrease in the lattice parameter, confirming that the replacement takes place at the Sn site. Quite unusually, the as-cast samples exhibited a higher lattice contraction than the annealed ones. The fully substituted x?=?1.0 compound, again adopting the MnCu₂Al structure, does not form as stoichiometric phase and turned out to be strongly Fe deficient. The physical behavior at room temperature indicated that annealing with increasing temperature is beneficial for electrical and thermoelectrical transport. Moreover, we measured a slight improvement in electrical and thermoelectrical properties in the x?=?0.1 sample and a suppression in the x?=?0.2 sample, as compared to the undoped x?=?0 sample.     

Thermoelectric properties of Zn4Sb3 intermetallic compound doped with Aluminum and Silver

The β-phase Zn₄Sb₃ has attracted much attention because of its high thermoelectric performance in the intermediate temperature range thanks to disorder in the Zn lattice site. In this work are presented structural, thermal, electric and thermoelectric characterization of Zn₄Sb₃ pure and Ag, Al doped, prepared by a simple synthesis. Structural and microstructural analyses reveal homogeneous one-phases having compositions in agreement with the nominal ones. After thermoelectric characterization, Ag doping results mostly effective in lowering the resistivity and Seebeck coefficient value, by introducing holes in the system. On the other hand, the Al substitution yields a very small decrease of the Seebeck coefficient but, at the same time, a significant decrease of the thermal conductivity mainly due to the depressed phonon contribution. The thermal conductivity behavior is the main responsible for the good thermoelectric performances of (Zn_0.99Al_0.01)₄Sb₃, whose thermoelectric figure of merit reaches the encouraging value of 0.23 at 260 K.     

High-performance half-Heusler thermoelectric materials Hf1−x ZrxNiSn1−ySby prepared by levitation melting and spark plasma sintering

Half-Heusler thermoelectric materials Hf_1?xZr_xNiSn_1?ySb_y (x = 0, 0.25, 0.4, 0.5; y = 0.02, 0.04, 0.06) have been prepared by levitation melting followed by spark plasma sintering or hot pressing. X-ray diffraction analysis and scanning electron microscopy observation show that single-phased half-Heusler compounds without compositional segregations have been obtained by levitation melting in a time-efficient manner. A small amount of Sb doping can improve the electrical power factor but undesirably increases the thermal conductivity due to the increased carrier thermal conductivity. The isoelectronic substitution of Zr for Hf substantially decreased the lattice thermal conductivity. A state-of-the-art ZT value of 1.0 has been attained at 1000 K for the levitation-melted and spark-plasma-sintered Hf_0.6Zr_0.4NiSn_0.98Sb_0.02, which is one of the highest achieved ZT values for half-Heusler thermoelectric alloys.     

定向凝固Ag掺杂Mg3Sb2合金热电性能

通过定向凝固方法可以高效制备Mg₃Sb₂晶体,根据凝固理论计算了平界面生长临界速率,在此速率下可以有效抑制第二相Sb的析出。对不同的凝固速率下的Mg₃Sb₂晶体微观组织进行了分析,表明凝固速率为5μm·s^-1时可以有效减少Mg空位的出现,并在晶体中获得过量Mg原子,有利于更好地提升热电性能。通过消除晶界和Ag元素掺杂有效提升了Mg₃Sb₂晶体的载流子迁移率和浓度,在测试温度区间(300～800K)内,最大电导率值可达309S·cm^-1,同时保持了较高的Seebeck系数值,从而获得了更好的电子传输性能(PF_max=1.2mW·m^-1·K^-2),通过Hall测试和第一性原理计算对此结果进行了验证。Ag掺杂浓度为2.5at%下相应的热电优值最高可以达到0.67,此方法为Mg₃Sb₂基热电材料性能优化提供了新的...     

The Zn-Rich Corner of the 450 °C Isothermal Section of the Zn-Fe-Ni-Sb Quaternary System

The 450 °C isothermal section of the Zn-Fe-Ni-Sb quaternary system with Zn fixed at 93 at.% has been studied experimentally using scanning electron microscopy coupled with energy dispersive x-ray spectroscopy and x-ray diffraction. The (L + T-FeNiZn) field is found to be coexistent with all other phase fields in the section, except the (ζ-FeZn + Sb₂Zn₃) field. The Sb solubility in the ζ-FeZn phase is very limited, and it almost insoluble in the δ-NiZn phase. The solubilities of Fe and Ni in the Sb₂Zn₃ phase are 0.6 and 0.9 at.%, respectively. The maximum solubility of Sb in the γ-NiZn phase is 1.9 at.%. Three four-phase regions and two three-phase regions have been confirmed experimentally in this isothermal section. In addition, no new phase was found in the study.