n‐Type Mg3Sb2‐Mg3Bi2 alloys are some of the most promising thermoelectric materials in the low–mid temperature range. While discovered relatively recently, these materials have garnered intense attention, and numerous papers from the international thermoelectric community have been published in a relatively short period of time. As with all materials, detailed insights into the underlying mechanisms that contribute to these alloys’ distinguished thermoelectric properties are important for future researchers to push the performance of this material to new heights. Herein, experimental studies on the role defects, synthesis conditions, electronic band structure, and microstructure along with future prospects arecompiled to establish a guide for fully exploiting the potential of this material system. Considering the limited number of n‐type thermoelectric materials with this performance for low‐grade heat recovery and cooling technologies, further development of the Mg3Sb2‐Mg3Bi2 alloys is an important step toward commercial applications of thermoelectric materials, including cooling technologies and waste heat recovery applications.
Thermoelectric materials enable the mutual energy conversion of waste heat and electricity, critical to relieve global energy crisis. Hightemperature thermoelectric materials are special species due to their high-temperature stability and noticeable energy conversion efficiency.Here, we report a systematic investigation on high-temperature thermoelectric gadolinium selenides, cubic Gd 3-x Se 4 (x = 0.16, 0.21 and 0.25) and orthorhombic Gd 2 Se 3-y (y = 0.02, 0.06 and 0.08). High energy synchrotron x-ray diffraction and total scattering are used to investigate the crystallographic and local structures. The atomic-scale cluster of Gd vacancy in cubic Gd 2.84 Se 4 is observed by employing the reverse Monte Carlo simulation. For cubic Gd 3-x Se 4 , its carrier concentration is tuned and multiple conduction bands are incorporated by adjusting Gd vacancy. Experimentally, the gradual increase in effective masses is evidently observed in cubic Gd 3-x Se 4 , which is consistent with the density functional theory (DFT) calculation. A reasonable peak zT value of 0.27 is achieved at 850 K for Gd 2.84 Se 4 . On the other hand, adjusting Se vacancy enables the optimization of electron concentration for orthorhombic Gd 2 Se 3-y phase. Its low deformation potential (Ξ = 12eV) with single conduction band gives rise to enhanced electron mobility and higher peak zT value of 0.54 at 850 K for Gd 2 Se 2.98 . In addition, a higher zT of 1.2 at1200 K is reasonably predicted for Gd 2 Se 2.98 by using quality factor analysis. This work not just presents a systematic crystallographic investigation of gadolinium selenides, but also provides a deep insight into the charge transport and phonon scattering mechanisms. This study facilitates the exploration of more hightemperature thermoelectric materials.
Layered Zintl phases with A2MPn2 stoichiometry are an underexplored class of potential thermoelectric materials with complex and diverse chemistry. The solid solution Yb2–x Eu x CdSb2 is an example of the promise these compounds hold, as one composition, Yb1.64Eu0.36CdSb2, has reported one of the highest zTs of any Zintl phase material at 525 K. The present study examines changes in structure and bonding of this solid solution that impacts thermoelectric performance. Pair distribution function analysis is combined with electronic structure modeling to take a chemical bonding-based approach to deconvolute the impact of defects on thermal and electronic properties in Yb2–x Eu x CdSb2. Yb2–x Eu x CdSb2 (x = 0, 0.1, 0.2, 0.3, 0.4, 0.5) samples were synthesized, and thermoelectric properties and defect chemistry were investigated. Samples from the middle of the series x = 0.2 and 0.3 were found to be the most highly defected, exhibiting Yb and Sb vacancies, as well as distortions in the Yb–Sb coordination spheres that correlate with thermoelectric properties. The highest efficiency is reported for x = 0.4 (zT ≈ 0.9 at 525 K), and the thermoelectric quality factor predicts that x = 0.3 could achieve zT > 2 by synthetically tuning the defect structure and thereby carrier concentration. The strategy of investigating local structure outlined in this study can be applied to a variety of other thermoelectric materials to provide insight into the hidden role of defect chemistry in understanding the structure–property relationships in extended solids.
<pre>The physics of heat conduction puts practical </pre><pre>limits on many technological fields such as </pre><pre>energy production, storage, and conversion, as</pre><pre>well as high-power and high-frequency </pre><pre>electronics. Heat conduction in simple, </pre><pre>defect-free crystals is generally well </pre><pre>understood and seems to be well described by </pre><pre>the phonon-gas model (PGM), where phonon </pre><pre>wave-packets are viewed as heat carrying</pre><pre>particles which propagate their mean free </pre><pre>path before being scattered. It is widely</pre><pre>appreciated that the PGM does not describe </pre><pre>the full vibrational spectrum in amorphous </pre><pre>materials, since this picture likely breaks </pre><pre>down at higher frequencies. Furthermore, it </pre><pre>has been shown that the PGM also breaks down </pre><pre>in certain defective and anharmonic crystals,</pre><pre>not only in the amorphous limit. In this work, </pre><pre>in an attempt to bridge our understanding </pre><pre>between crystal-like (described by the PGM)</pre><pre>and amorphous-like heat conduction, we study </pre><pre>structurally-complex crystalline YB<sub>14</sub>(Mn,Mg)SB<sub>11</sub> </pre><pre>experimentally using inelastic neutron </pre><pre>scattering and computationally using a </pre><pre>two-channel lattice dynamical approach. </pre><pre>One channel is the commonly considered PGM, </pre><pre>and the second we call the diffuson-channel </pre><pre>since it is mathematically the same mechanism </pre><pre>through which diffusons were defined. Our </pre><pre>results show that the diffuson-channel </pre><pre>dominates in YB<sub>14</sub>MnSb<sub>11</sub> above 300 K, which is </pre><pre>a champion thermoelectric material above 800 K. </pre><pre>We demonstrate a method for the rational </pre><pre>design of amorphous-like heat conduction by </pre><pre>considering the energetic proximity phonon modes </pre><pre>and modifying them through chemical means.</pre>
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