Enhancing Interfacial Stability and Mechanical Strength of a CoSb3-Based Thermoelectric Junction Using Ti-Based Alloy Barrier Layers

CoSb3-based filled skutterudites (SKDs) are among the most promising materials for power generation. However, the poor interfacial stability and mechanical strength severely limit their practical application when joined with Cu electrodes. In this study, we propose multiphase Ti-based alloy barrier layers for CoSb3-based thermoelectric junctions to prevent the continuous brittle TiCoSb phase formation. Following the principles of coefficient of thermal expansion matching, we designed three types of Ti80-xNbxCo20 (x = 0, 5, and 10, at.%) barrier layers with the thin intermetallic compound (IMC) layers (<20 μm). Transmission electron microscopy analysis revealed that the interfacial microstructure of the Ti75Nb5Co20/Ce-SKD junction comprises Ti5Sb3, Ti5CoSb3, TiCoSb, and TiSb2 phases, as well as unreacted TiCo, Ti2Co, and Ti(Nb)ss phases, demonstrating a uniform staggered distribution state. After aging tests, the IMC thickness increased gradually from 7 to 12 μm, and the interfacial contact resistivity increased from 7.59 to 15.46 μΩ·cm2. A Cu layer was chosen as a buffer during the brazing process to prevent the formation of cracks and holes. After aging for 360 h at 823 K, the shear strength of the brazed joints remained at ∼21 MPa. Our results demonstrate that the Cu/CuSnP/Cu/Ti75Nb5Co20/Ce-SKD brazed joint exhibits excellent interfacial stability and satisfactory mechanical strength.

Investigation of the Effect of Double-Filler Atoms on the Thermoelectric Properties of Ce-YbCo4Sb12

Skutterudite compounds have been studied as potential thermoelectric materials due to their high thermoelectric efficiency, which makes them attractive candidates for applications in thermoelectric power generation. In this study, the effects of double-filling on the thermoelectric properties of the Ce_xYb_0.2-xCo₄Sb₁₂ skutterudite material system were investigated through the process of melt spinning and spark plasma sintering (SPS). By replacing Yb with Ce, the carrier concentration was compensated for by the extra electron from Ce donors, leading to optimized electrical conductivity, Seebeck coefficient, and power factor of the Ce_xYb_0.2-xCo₄Sb₁₂ system. However, at high temperatures, the power factor showed a downturn due to bipolar conduction in the intrinsic conduction regime. The lattice thermal conductivity of the Ce_xYb_0.2-xCo₄Sb₁₂ skutterudite system was clearly suppressed in the range between 0.025 and 0.1 for Ce content, due to the introduction of the dual phonon scattering center from Ce and Yb fillers. The highest ZT value of 1.15 at 750 K was achieved for the Ce_0.05Yb_0.15Co₄Sb₁₂ sample. The thermoelectric properties could be further improved by controlling the secondary phase formation of CoSb₂ in this double-filled skutterudite system.

High-Performance Skutterudite/Half-Heusler Cascaded Thermoelectric Module Using the Transient Liquid Phase Sintering Joining Technique

Thermoelectric (TE) materials have made rapid advancement in the past decade, paving the pathway toward the design of solid-state waste heat recovery systems. The next requirement in the design process is realization of full-scale multistage TE devices in the medium to high temperature range for enhanced power generation. Here, we report the design and manufacturing of full-scale skutterudite (SKD)/half-Heusler (hH) cascaded TE devices with 49-couple TE legs for each stage. The automated pick-and-place tool is employed for module fabrication providing overall high manufacturing process efficiency and repeatability. Optimized Ti/Ni/Au coating layers are developed for metallization as the diffusion barrier and electrode contact layers. The Cu-Sn transient liquid phase sintering technique is utilized for SKD and hH stages, which provides a high strength bonding and very low contact resistance. A remarkably high output power of 38.3 W with a device power density of 2.8 W·cm^-2 at a temperature gradient of 513 °C is achieved. These results provide an avenue for widespread utilization of TE technology in waste heat recovery applications.

Enhanced Thermoelectric Performance of Yb-Filled Skutterudite with Bottom-Up Formed CoSi2 Nanoparticles

It is known that Yb-filled skutterudite with excellent thermoelectric performance is promising for a power generation device in the intermediate temperature region. Here we created a new approach to obtain nanostructured materials by adding Si to Co-overstoichiometric Yb-filled skutterudite through high-energy ball milling, which embedded bottom-up formed CoSi₂ nanoparticles into grain-refining Yb_0.25Co₄Sb₁₂, synergistically resulting in the enhanced thermoelectric properties and room-temperature hardness. On one hand, the abundant grain boundaries and phase interfaces effectively blocked the propagation of medium-low frequency phonons, resulting in a lower lattice thermal conductivity. On the other hand, phase interfaces barrier nicely screened a portion of low-energy electrons, leading to an improved power factor. As a result, an enhanced peak ZT value of ∼1.43 at 823 K and a promising average ZT of ∼1.00 between 300 and 823 K were achieved in the Yb_0.25Co₄Sb₁₂/0.05CoSi₂ sample. Meanwhile, such nanostructures also enhanced the hardness through the collective contributions of second phase and fine grain strengthening, which made skutterudite more competitive in practical application.

Ce Filling Limit and Its Influence on Thermoelectric Performance of Fe3CoSb12-Based Skutterudite Grown by a Temperature Gradient Zone Melting Method

CoSb3-based skutterudite is a promising mid-temperature thermoelectric material. However, the high lattice thermal conductivity limits its further application. Filling is one of the most effective methods to reduce the lattice thermal conductivity. In this study, we investigate the Ce filling limit and its influence on thermoelectric properties of p-type Fe3CoSb12-based skutterudites grown by a temperature gradient zone melting (TGZM) method. Crystal structure and composition characterization suggests that a maximum filling fraction of Ce reaches 0.73 in a composition of Ce0.73Fe2.73Co1.18Sb12 prepared by the TGZM method. The Ce filling reduces the carrier concentration to 1.03 × 1020 cm-3 in the Ce1.25Fe3CoSb12, leading to an increased Seebeck coefficient. Density functional theory (DFT) calculation indicates that the Ce-filling introduces an impurity level near the Fermi level. Moreover, the rattling effect of the Ce fillers strengthens the short-wavelength phonon scattering and reduces the lattice thermal conductivity to 0.91 W m-1 K-1. These effects induce a maximum Seebeck coefficient of 168 μV K-1 and a lowest κ of 1.52 W m-1 K-1 at 693 K in the Ce1.25Fe3CoSb12, leading to a peak zT value of 0.65, which is 9 times higher than that of the unfilled Fe3CoSb12.

Effect of ZnO and SnO2 Nanolayers at Grain Boundaries on Thermoelectric Properties of Polycrystalline Skutterudites

Nanostructuring is considered one of the key approaches to achieve highly efficient thermoelectric alloys by reducing thermal conductivity. In this study, we investigated the effect of oxide (ZnO and SnO₂) nanolayers at the grain boundaries of polycrystalline In_0.2Yb_0.1Co₄Sb₁₂ skutterudites on their electrical and thermal transport properties. Skutterudite powders with oxide nanolayers were prepared by atomic layer deposition method, and the number of deposition cycles was varied to control the coating thickness. The coated powders were consolidated by spark plasma sintering. With increasing number of deposition cycle, the electrical conductivity gradually decreased, while the Seebeck coefficient changed insignificantly; this indicates that the carrier mobility decreased due to the oxide nanolayers. In contrast, the lattice thermal conductivity increased with an increase in the number of deposition cycles, demonstrating the reduction in phonon scattering by grain boundaries owing to the oxide nanolayers. Thus, we could easily control the thermoelectric properties of skutterudite materials through adjusting the oxide nanolayer by atomic layer deposition method.

Enhanced Interfacial Reliability and Mechanical Strength of CoSb3-Based Thermoelectric Joints with Rationally Designed Diffusion Barrier Materials of Ti-Based Alloys

To achieve high-performance thermoelectric (TE) devices, constructing a good interfacial connection between TE materials and electrodes is as important as having high figure-of-merit TE materials. Although CoSb₃-based TE devices have received great attention for power generation recently, the limited long-term service stability is the main obstruct for their applications. In this work, we have prepared two kinds of Ti-based alloys (Ti_83.7Al_10.7Si_5.6 and Ti₇₄Ni₂₆) as the diffusion barrier layer of CoSb₃-based TE joints by the spark plasma sintering method and have systematically investigated their interfacial behaviors during the aging process. The performances of contact resistivity and mechanical strength for Ti₇₄Ni₂₆/Yb_0.4Co_3.8Fe_0.2Sb₁₂ TE joints are good before aging treatment but gradually deteriorate during the aging process, which should be ascribed to the phase-transition-induced negative thermal expansion in Ti-Ni alloys. On the other hand, Ti_83.7Al_10.7Si_5.6/Yb_0.4Co_3.8Fe_0.2Sb₁₂ TE joints show both low contact resistivity (<10 μΩ·cm²) and high mechanical strength (>20 MPa) before and after 16-day aging at 500 °C, which is originated from the matching of the coefficient of thermal expansion (CTE) and the formation of network structures in Ti-Al-Si alloys. We have also prepared an eight-couple TE module of p-Ge_0.9Sb_0.1TeB_0.01/n-Yb_0.4Co_3.8Fe_0.2Sb₁₂ and have measured its corresponding device performance. Our work has demonstrated that the matched CTE and network structures in the Ti-Al-Si alloy are key to obtain high-performance CoSb₃-based TE joints for long-term service.

Phase Formation Behavior and Thermoelectric Transport Properties of P-Type YbxFe3CoSb12 Prepared by Melt Spinning and Spark Plasma Sintering

Formation of multiple phases is considered an effective approach for enhancing the performance of thermoelectric materials since it can reduce the thermal conductivity and improve the power factor. Herein, we report the in-situ generation of a submicron-scale (~500 nm) heterograin structure in p-type Yb-filled (Fe,Co)₄Sb₁₂ skutterudites during the melt spinning process. Mixed grains of Yb_xFe_3−yCo_1+ySb₁₂ and Yb_zFe_3+yCo_1−ySb₁₂ were formed in melt spun ribbons due to uneven distribution of cations. By the formation of interfaces between two different grains, the power factor was enhanced due to the formation of an energy barrier for carrier transport, and simultaneously the lattice thermal conductivity was reduced due to the intensified boundary phonon scattering. A high thermoelectric figure of merit zT of 0.66 was obtained at 700 K.

Solid-Liquid Interdiffusion (SLID) Bonding of p-Type Skutterudite Thermoelectric Material Using Al-Ni Interlayers

Over the past few years, significant progress towards implementation of environmentally sustainable and cost-effective thermoelectric power generation has been made. However, the reliability and high-temperature stability challenges of incorporating thermoelectric materials into modules still represent a key bottleneck. Here, we demonstrate an implementation of the Solid-Liquid Interdiffusion technique used for bonding Mm_y(Fe,Co)₄Sb₁₂ p-type thermoelectric material to metallic interconnect using a novel aluminium–nickel multi-layered system. It was found that the diffusion reaction-controlled process leads to the formation of two distinct intermetallic compounds (IMCs), Al₃Ni and Al₃Ni₂, with a theoretical melting point higher than the initial bonding temperature. Different manufacturing parameters have also been investigated and their influence on electrical, mechanical and microstructural features of bonded components are reported here. The resulting electrical contact resistances and apparent shear strengths for components with residual aluminium were measured to be (2.8 ± 0.4) × 10⁻⁵ Ω∙cm² and 5.1 ± 0.5 MPa and with aluminium completely transformed into Al₃Ni and Al₃Ni₂ IMCs were (4.8 ± 0.3) × 10⁻⁵ Ω∙cm² and 4.5 ± 0.5 MPa respectively. The behaviour and microstructural changes in the joining material have been evaluated through isothermal annealing at hot-leg working temperature to investigate the stability and evolution of the contact.

Synthesis and Thermoelectric Properties of Charge-Compensated SyPdxCo4-xSb12 Skutterudites

Recently, the electronegative elements (e.g., S, Se, Cl, and Br) filled skutterudites have attracted great attention in thermoelectric community. Via doping of some electron donors at the Sb sites, these electronegative elements can be filled into the voids of CoSb₃ forming thermodynamically stable compounds, which greatly extends the scope of filled skutterudites. In this study, we show that doping appropriate elements at the Co sites can also stabilize the electronegative elements in the voids of CoSb₃. A series of S_yPd_xCo_4-xSb₁₂ compounds were successfully fabricated by a traditional solid state reaction method combined with a spark plasma sintering technique. The phase composition and electrical and thermal transport properties were systematically characterized, and the related mechanisms were deeply discussed. It is found that the charge compensation between Pd doping and S filling is the main reason for the formation of thermodynamically stable S_yPd_xCo_4-xSb₁₂ compounds. Filling S element in the voids of CoSb₃ provides additional holes to reduce the carrier concentration while scarcely affecting the carrier mobility. However, doping Pd at the Co sites not only changes the carrier scattering mechanism but also deteriorates the carrier mobility. Low lattice thermal conductivities are observed in these S_yPd_xCo_4-xSb₁₂ compounds, which are attributed to the low resonant frequency of the S element. Finally, a maximal figure of merit of 0.85 is obtained for S_0.05Pd_0.25Co_3.75Sb₁₂ at 700 K.

Compound Defects and Thermoelectric Properties of Self-Charge Compensated Skutterudites SeyCo4Sb12-xSex

In the past two decades, many studies have focused on the effects of electropositive guest fillers on the electrical and thermal transport properties in CoSb₃-based skutterudites. Recently, some electronegative elements such as S, Se, Cl, and Br have been filled into the voids in CoSb₃ with a small amount of n-type dopant Te on the Sb sites. In this report, self-charge compensated skutterudites Se_yCo₄Sb_12-xSe_x (0 < x + y < 0.9) with Se occupying two different atomic sites have been fabricated by a traditional melting-annealing process combined with a spark plasma sintering method. Phase purity was determined by X-ray diffraction, and the microstructures were examined by scanning electron microscopy. The temperature dependencies of the electrical and thermal transport properties were characterized. Se could enter both the void and Sb sites in CoSb₃ with a solubility limit around 0.6. The Se content has little effect on bandgap. Similar to Ga dual-site occupied Ga_yCo₄Sb_12-xGa_x (y = 2x), a typical semiconducting electrical property with a low carrier concentration as well as a large Seebeck coefficient is observed. A correlation between the large Seebeck coefficient and the carrier scattering mechanism has been proposed. In addition, a largely reduced room temperature lattice thermal conductivity is obtained with a minimum value of 2.1 Wm^-1 K^-1 for Se_0.2Co₄Sb_11.6Se_0.4. The effects of Se on lattice thermal conductivity and filler resonant frequency are discussed.

Synthesis and high temperature thermoelectric properties of Yb0.25Co4Sb12-(Ag2Te) x (Sb2Te3)1-x nanocomposites

Nanocomposites are becoming a new paradigm in thermoelectric study: by incorporating nanophase(s) into a bulk matrix, a nanocomposite often exhibits unusual thermoelectric properties beyond its constituent phases. To date most nanophases are binary, while reports on ternary nanoinclusions are scarce. In this work, we conducted an exploratory study of introducing ternary (Ag2Te)x(Sb2Te3)1-x inclusions in the host matrix of Yb0.25Co4Sb12. Yb0.25Co4Sb12-4wt% (Ag2Te)x(Sb2Te3)1-x nanocomposites were prepared by a melting-milling-hot-pressing process. Microstructural analysis showed that poly-dispersed nanosized Ag-Sb-Te inclusions are distributed on the grain boundaries of Yb0.25Co4Sb12 coarse grains. Compared to the pristine nanoinclusion-free sample, the electrical conductivity, Seebeck coefficient, and thermal conductivity were optimized simultaneously upon nanocompositing, while the carrier mobility was largely remained. A maximum ZT of 1.3 was obtained in Yb0.25Co4Sb12-4wt% (Ag2Te)0.42(Sb2Te3)0.58 at 773 K, a ~ 40% increase compared to the pristine sample. The electron and phonon mean-free-path were estimated to help quantify the observed changes in the carrier mobility and lattice thermal conductivity.

Enhancement of thermoelectric efficiency of CoSb3-based skutterudites by double filling with K and Tl

The high-temperature thermoelectric properties of thallium (Tl) and potassium (K) double-filled cobalt antimonide (CoSb3)-based skutterudites with nominal compositions TlxK0.3Co4Sb12 (x = 0.1 - 0.3) were investigated. The filling fraction of Tl in CoSb3 was enhanced by co-filling with K, which resulted in all of the samples showing the filled-skutterudite single phase. Owing to the high filling ratio, the carrier concentration in the sample with x = 0.3 was as high as 4.3 × 10(20) cm(-3) at room temperature. Furthermore, quite low lattice thermal conductivity (as low as 0.9 Wm(-1)K(-1)) was obtained for the sample with x = 0.3, probably because of strong phonon scattering by the Tl and K co-rattling effect, which resulted in a maximum zT of around one at 773 K.