Fast Fabrication of High-Performance CoSb3-Based Thermoelectric Skutterudites via One-Step Yb-Promoted Peritectic Solidification

As a most promising mid-temperature thermoelectric material, CoSb₃-based bulk material exhibits an applicable figure-of-merit (ZT) of more than one. However, their fabrication is historically time-consuming due to the long-time solid-state phase transitions from CoSb₂ to CoSb₃. To overcome this challenge, here, a fast one-step process is developed to fabricate n-type Yb-doped CoSb₃ with stable ZT of 1.12 at 765 K in <5 h. Experiments confirm Yb promotes peritectic reactions of CoSb + Liquid → CoSb₂ and CoSb₂ + Liquid → CoSb₃, optimizes power factor, and suppresses thermal conductivity. Moreover, the dense grains, induced by the one-step crystallization, result in outstanding mechanical properties with a Young's modulus of 171.4 GPa and a hardness of 8.8 GPa in the Yb-doped CoSb₃. This study indicates that the fast one-step fabrication route can effectively promote the practical applications of CoSb₃-based thermoelectrics and provide guidance for thermoelectric fabrication via rational phase design.     

Thermal Decomposition of Thermoelectric Material CoSb3: A Thermogravimetry Kinetic Analysis

The thermal decomposition of the thermoelectric CoSb₃ alloy was investigated using thermogravimetry (TG). TG curves obtained in inert gas flow with different heating rates were used to perform kinetic analysis based on the Arrhenius equation. Kinetic parameters, such as the effective activation energy, the pre-exponential factor, and the kinetic model function $ f(\alpha ) $ , were obtained using the Freeman–Carroll method, the multiheating rates method, and the Coats–Redfern equation. The activation energy was found to be around 200 kJ/mol, and the reaction mechanism for the decomposition of CoSb₃ alloy mostly obeys the second-order chemical decomposition process $ f(\alpha ) = (1 - \alpha )^{2} $ .     

Practical Contact Resistance Measurement Method for Bulk Bi2Te3-Based Thermoelectric Devices

The impact of contact resistance on thermoelectric (TE) device performance grows more significant as devices are scaled down. To improve and understand the effects of contact resistance on bulk TE device performance, a reliable experimental measurement method is needed. There are many popular methods to extract contact resistance, but they are only well suited for measuring metal contacts on thin films and do not necessarily translate to measuring contact resistance on bulk TE materials. The authors present a measurement technique that precisely measures contact resistance on bulk TE materials by making and testing stacks of bulk, metal-coated TE wafers using TE industry-standard processes. An equation that uses the Z of the stacked device to extract the contact resistance is used to reduce the sensitivity to resistivity variations of the TE material. Another advantage of this technique is that it exploits realistic TE device manufacturing techniques and results in an almost device-like structure. The lowest contact resistivity measured was 1.1 × 10^?6 Ω cm² and 1.3 × 10^?6 Ω cm² for n- and p-type materials, respectively using a newly developed process at 300 K. The uncertainty in the contact resistivity values for each sample was 10% to 20%, which is quite good for measurements in the 10^?6 Ω cm² range.     

Influence of ZnO Inclusions on the Low-Temperature Thermoelectric Properties of CoSb3

CoSb₃ composites with different amounts of ZnO nanoparticles (2?wt.% to 12?wt.%) were prepared from nanosized ZnO (commercial) and micron-sized CoSb₃ (obtained via solid-state reaction) particles mixed in solution and freeze dried. The resulting powders were densified by spark plasma sintering. The samples were characterized by x-ray diffraction and scanning electron microscopy. It was found that ZnO forms micron-sized clusters at the grain boundaries of the matrix material. The thermoelectric properties (electrical resistivity, thermopower, and thermal conductivity) were measured in the 2?K to 300?K temperature range. Both the electrical and thermal conductivities were observed to decrease with increasing ZnO content. The dimensionless figure of merit ZT was improved by up to 30% at 300?K for the sample containing 2?wt.% ZnO.     

The Impact of Nanostructuring on the Thermal Conductivity of Thermoelectric CoSb3

The high concentration of grain boundaries provided by nanostructuring is expected to lower the thermal conductivity of thermoelectric materials, which favors an increase in their thermoelectric figure‐of‐merit, ZT. A novel chemical alloying method has been used for the synthesis of nanoengineered‐skutterudite CoSb₃. The CoSb₃ powders were annealed for different durations to obtain a set of samples with different particle sizes. The samples were then compacted into pellets by uniaxial pressing under various conditions and used for the thermoelectric characterization. The transport properties were investigated by measuring the Seebeck coefficient and the electrical and thermal conductivities in the temperature range 300 K to 650 K. A substantial reduction in the thermal conductivity of CoSb₃ was observed with decreasing grain size in the nanometer region. For an average grain size of 140 nm, the thermal conductivity was reduced by almost an order of magnitude compared to that of a single crystalline or highly annealed polycrystalline material. The highest ZT value obtained was 0.17 at 611 K for a sample with an average grain size of 220 nm. The observed decrease in the thermal conductivity with decreasing grain size is quantified using a model that combines the macroscopic effective medium approaches with the concept of the Kapitza resistance. The compacted samples exhibit Kapitza resistances typical of semiconductors and comparable to those of Si–Ge alloys.     

Thermoelectric Performance Optimization in p-Type Ce y Fe3CoSb12 Skutterudites

In this study, we investigated the impact of the Ce filling fraction on the thermoelectric properties of p-type filled skutterudites Ce _y Fe₃CoSb₁₂ (y = 0.6 to 1.0). The electrical conductivity decreases gradually with increasing y, while the Seebeck coefficient displays an opposite variation tendency, consistent with the expected electron donor role of the Ce filler in this compound. The overall power factors are invariable among all the samples. Alteration of the Ce filling fraction exerts little influence on the phonon transport, but the total thermal conductivity markedly declined with increasing y due to the reduced contribution to heat transfer from carriers. As a consequence, the maximum thermoelectric figure of merit ZT reaches ～0.8 for the sample with y = 0.9, comparable to that of pure Fe-based skutterudite CeFe₄Sb₁₂; more importantly, the former possesses a much larger average ZT between 300 K and 800 K than the latter, showing superior potential for use in intermediate-temperature thermoelectric power generation applications. Further enhancement of ZT in p-type Fe₃CoSb₁₂-based skutterudites could be realized via nanostructuring or a multiple-filling approach.     

Effects of Annealing on Microstructure and Thermoelectric Properties of Nanostructured CoSb3

Nanocrystallization of thermoelectric materials is an effective way to reduce their thermal conductivity, but so far the thermal stability of nanostructured thermoelectric materials has been little studied. Effects of annealing treatment on the microstructure and the thermoelectric properties of nanostructured CoSb₃ were investigated in this work. Samples with average grain size of 300 nm were prepared by spark plasma sintering of high-energy ball-milled nanosized CoSb₃ powders. The study shows that annealing has a very significant impact on the grain size of the samples. The grain size of the sample with 100 h annealing is three times greater than before annealing. The major phase in the 150-h-annealed sample is still skutterudite, except for a trace amount of Sb phase. With increasing annealing time, the density reduces slightly. In addition, the power factor of the sample decreases, thus resulting in a decrease of the thermoelectric figure of merit.     

Thermoelectric Properties of Sb2Te3-Based Nanocomposites with Graphite

Semiconductors - Antimony-telluride-based nanocomposite samples containing different weight fractions of graphite (Sb2Te3 + x% graphite, where x = 0.0, 0.5, 1.0, and 2.5%) are synthesized and...     

Enhanced Thermoelectric Performance of p-Type Mg3Sb2 for Reliable and Low-Cost all-Mg3Sb2-Based Thermoelectric Low-Grade Heat Recovery

Bi₂Te₃-based devices have long dominated the commercial market for thermoelectric cooling applications, but their narrow operating temperature range and high cost have limited their possible applications for conversion of low-grade heat into electric power. The recently developed n-type Mg₃Sb₂-based compounds exhibit excellent transport properties across a wide temperature range, have low material costs, and are nontoxic, so it would be possible to substitute the conventional Bi₂Te₃ module with a reliable and low-cost all-Mg₃Sb₂-based thermoelectric device if a good p-type Mg₃Sb₂ material can be obtained to match its n-type counterpart. In this study, by comprehensively regulating the carrier concentration, carrier mobility, and lattice thermal conductivity, the thermoelectric performance of p-type Mg₃Sb₂ is significantly improved through Na and Yb doping in Mg_1.8Zn_1.2Sb₂. Moreover, p- and n-type Mg₃Sb₂ are similar in terms of their coefficients of thermal expansion and their good performance stability, thus allowing the construction of a reliable all-Mg₃Sb₂-based unicouple. The decent conversion efficiency (≈5.5% at the hot-side temperature of 573 K), good performance stability, and low cost of this unicouple effectively promote the practical application of Mg₃Sb₂-based thermoelectric generators for low-grade heat recovery.     

Fabrication and Thermoelectric Power Factor of CoSb3 Prepared using Modified Polyol Process and Evacuated-and-Encapsulated Sintering

Various reaction temperatures have been investigated to stabilize the CoSb₃ phase obtained by a modified polyol process using a refluxing condenser. The resulting powders were cold-pressed and sintered in an evacuated-and-encapsulated ampoule for transport measurements. Single-phase CoSb₃ could be produced for the reaction carried out at 448 K for a short duration of 15 min followed by evacuated-and-encapsulated sintering at 848 K for 5 h. In comparison with the sample sintered at 798 K, the sample sintered at 848 K exhibits lower resistivity and thermopower. Due to the sign crossover of the thermopower, the latter sample shows lower power factor for temperatures above 475 K.     

Nb-Mediated Grain Growth and Grain-Boundary Engineering in Mg3Sb2-Based Thermoelectric Materials

The poor carrier mobility of polycrystalline Mg₃Sb₂ at low temperatures strongly degrades the thermoelectric performance. Ionized impurities are initially thought to dominate charge carrier scattering at low temperatures. Accordingly, the increased electrical conductivity by replacing Mg with metals such as Nb is also attributed to reduced ionized impurity scattering. Recent experimental and theoretical studies challenge this view and favor the grain boundary (GB) scattering mechanism. A reduction of GB scattering improves the low-temperature performance of Mg₃(Sb, Bi)₂ alloys. However, it is still elusive how these metal additions reduce the GB resistivity. In this study, Nb-free and Nb-added Mg₃Sb₂ are studied through diffraction, X-ray absorption spectroscopy, solid-state nuclear magnetic resonance spectroscopy, and atom probe tomography. It is shown that Nb does not enter the Mg₃Sb₂ matrix and remains in the metallic state. Besides, Nb diffuses along the GB forming a wetting layer, which modifies the interfacial energy and accelerates grain growth. The GB resistivity appears to be reduced by Nb-enrichment, as evidenced by modeling the electrical transport properties. This study not only confirms the GB scattering in Mg₃Sb₂ but also reveals the hitherto hidden role of metallic additives on enhancing grain growth and reducing the GB resistivity.     

Nanostructure, Excitations, and Thermoelectric Properties of Bi2Te3-Based Nanomaterials

The effect of dimensionality and nanostructure on thermoelectric properties in Bi₂Te₃-based nanomaterials is summarized. Stoichiometric, single-crystalline Bi₂Te₃ nanowires were prepared by potential-pulsed electrochemical deposition in a nanostructured Al₂O₃ matrix, yielding transport in the basal plane. Polycrystalline, textured Sb₂Te₃ and Bi₂Te₃ thin films were grown at room temperature using molecular beam epitaxy and subsequently annealed at 250°C. Sb₂Te₃ films revealed low charge carrier density of 2.6?×?10¹⁹?cm^?3, large thermopower of 130???V?K^?1, and large charge carrier mobility of 402?cm²?V^?1?s^?1. Bi₂(Te_0.91Se_0.09)₃ and (Bi_0.26Sb_0.74)₂Te₃ nanostructured bulk samples were prepared from as-cast materials by ball milling and subsequent spark plasma sintering, yielding grain sizes of 50?nm and thermal diffusivities reduced by 60%. Structure, chemical composition, as well as electronic and phononic excitations were investigated by x-ray and electron diffraction, nuclear resonance scattering, and analytical energy-filtered transmission electron microscopy. Ab?initio calculations yielded point defect energies, excitation spectra, and band structure. Mechanisms limiting the thermoelectric figure of merit ZT for Bi₂Te₃ nanomaterials are discussed.