Oxidation Protective Hybrid Coating for Thermoelectric Materials

Low-Cost Magnesium-Based Thermoelectric Materials: Progress, Challenges, and Enhancements

Magnesium-based thermoelectric (TE) materials have attracted considerable interest due to their high ZT values, coupled with their low cost, widespread availability, nontoxicity, and low density. In this review, we provide a succinct overview of the advances and strategies pertaining to the development of Mg-based materials aimed at enhancing their performance. Following this, we delve into the major challenges posed by the severe working conditions, such as high temperature and thermal cycling, which adversely impact the behavior and long-term stability of the TE modules. Challenges include issues like the lack of mechanical strength, chemical instability, and unreliable contact. Subsequently, we focus on the key methodologies aimed at addressing these challenges to facilitate the broader application of the TE modules. These include boosting the mechanical strength, especially the toughness, through grain refining and additions of second phases. Furthermore, strategies targeted at enhancing the chemical stability through coatings and modifying the microstructure, as well as improving the contact design and materials, are discussed. In the end, we highlight the perspectives for boosting the practical applications of Mg-based TE materials in the future.

Probing High-Temperature Oxidation of Thermoelectric Phases Yb14MSb11 (M = Mg, Mn, Zn)

Yb₁₄MSb₁₁ (M = Mg, Mn, Zn) are p-type Zintl phases with high thermoelectric efficiencies at 1000 °C and melting points above 1200 °C under vacuum and/or inert atmosphere. In a thermoelectric generator, even within a vacuum jacket, small amounts of oxygen may be present, and therefore, elucidating chemical reactions in the presence of air or oxygen provides a framework for engineering design. The oxidation of Yb₁₄MSb₁₁ was investigated from room temperature to 1000 °C in dry air with thermogravimetric/differential scanning calorimetry (TG/DSC) on small pellets and visually after heat treatment to 1000 °C under ambient conditions on large pellets. Scanning electron microscopy/energy-dispersive spectroscopy (SEM/EDS) and powder X-ray diffraction provide identification of the oxidation products. In the presence of dry air, Yb₁₄MSb₁₁ initially oxidizes initially slowly at room temperature with a sweeping exotherm and weight gain with rapid oxidation at 400 °C, after which the exotherm signal plateaus at about 600 °C, with M = Zn showing the smallest overall exothermic curve. All samples showed a paired endo-/exotherm at 785-803 °C, consistent with the melting/solidification of YbSb₂, which in the case of M = Mg, Mn extrudes from the sample. The various sections of the pellets─outer layer, inner layer, and core are analyzed, and oxidation reactions are proposed. After cycling to 1000 °C, the outer layer is composed of Yb₂O₃ with small amounts of the corresponding metal oxides. The inner layer shows delamination by inward diffusion of oxygen and outward diffusion of Sb or Sb oxide-containing phases, and the core shows Yb₁₄MSb₁₁. Yb₁₄ZnSb₁₁ shows the best resistance to oxidation and may provide a promising material for further passivation optimization.

Overcoming Asymmetric Contact Resistances in Al-Contacted Mg2(Si,Sn) Thermoelectric Legs

Thermoelectric generators are a reliable and environmentally friendly source of electrical energy. A crucial step for their development is the maximization of their efficiency. The efficiency of a TEG is inversely related to its electrical contact resistance, which it is therefore essential to minimize. In this paper, we investigate the contacting of an Al electrode on Mg2(Si,Sn) thermoelectric material and find that samples can show highly asymmetric electrical contact resistivities on both sides of a leg (e.g., 10 µΩ·cm2 and 200 µΩ·cm2). Differential contacting experiments allow one to identify the oxide layer on the Al foil as well as the dicing of the pellets into legs are identified as the main origins of this behavior. In order to avoid any oxidation of the foil, a thin layer of Zn is sputtered after etching the Al surface; this method proves itself effective in keeping the contact resistivities of both interfaces equally low (<10 µΩ·cm2) after dicing. A slight gradient is observed in the n-type leg's Seebeck coefficient after the contacting with the Zn-coated electrode and the role of Zn in this change is confirmed by comparing the experimental results to hybrid-density functional calculations of Zn point defects.

Advanced Thermoelectric Design: From Materials and Structures to Devices

The long-standing popularity of thermoelectric materials has contributed to the creation of various thermoelectric devices and stimulated the development of strategies to improve their thermoelectric performance. In this review, we aim to comprehensively summarize the state-of-the-art strategies for the realization of high-performance thermoelectric materials and devices by establishing the links between synthesis, structural characteristics, properties, underlying chemistry and physics, including structural design (point defects, dislocations, interfaces, inclusions, and pores), multidimensional design (quantum dots/wires, nanoparticles, nanowires, nano- or microbelts, few-layered nanosheets, nano- or microplates, thin films, single crystals, and polycrystalline bulks), and advanced device design (thermoelectric modules, miniature generators and coolers, and flexible thermoelectric generators). The outline of each strategy starts with a concise presentation of their fundamentals and carefully selected examples. In the end, we point out the controversies, challenges, and outlooks toward the future development of thermoelectric materials and devices. Overall, this review will serve to help materials scientists, chemists, and physicists, particularly students and young researchers, in selecting suitable strategies for the improvement of thermoelectrics and potentially other relevant energy conversion technologies.