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Although a high figure of merit (zT) over a wide range of temperatures has been shown in n‐type Zintl Mg3(Sb, Bi)2, further improvement of its near‐room‐temperature performance is still required to promote its application in next‐generation thermoelectric coolers and power generators. Here, a novel strategy of phonon spectra mismatch for enhancing the thermoelectric performance of Mg3(Sb, Bi)2 is reported by incorporating multi‐walled carbon nanotubes (MWCNT). The introduction of very small amounts of MWCNT generates a large interfacial thermal resistance and thus significantly reduces the lattice thermal conductivity of the resulting composites, while maintaining a high power factor. As a result, a zT of ≈1.5 at 573 K and an average zT of 1.14 between 323 and 573 K can be achieved in the 0.5 wt% MWCNT composite. Moreover, a high conversion efficiency of ≈8.1% under a temperature difference of 283 K is realized in a resulting single‐leg device, making it a promising candidate material for low‐grade heat recovery. This study provides not only a material with a high near‐room‐temperature zT but also a unique insight into the design of high‐performance thermoelectric materials via compositing to exploit the large difference in phonon spectra between the components of the composite. [ABSTRACT FROM AUTHOR]

The performance of thermoelectric materials has been improved considerably in recent decades, making the concept of generating energy from waste heat via solid‐state thermoelectric devices more realistic. The construction of multi‐stage modular structures based on complex parameter optimization to maximize the efficiency of each material over its optimal operating temperature range has become an effective strategy for improving device performance. Here, multi‐segmented n‐type Mg3(Sb, Bi)2 with low‐contact‐resistance buffer layers is first fabricated, and phase‐transition‐suppressed cubic p‐type GeTe with enhanced thermoelectric performance is subsequently designed to match the segmented n‐type legs. A 3D finite‐element analysis model is then used to optimize the module size, providing higher energy conversion efficiency with an optimal average figure of merit over the entire operating temperature range. As a result, the prepared segmented‐Mg3(Sb, Bi)2/cubic‐GeTe module exhibits a high conversion efficiency of (12.8 ± 0.8)% at a hot‐side temperature of 773 K with a temperature difference of ≈480 K, which is also comparable to that of previously reported thermoelectric modules. This study increases the number of matching combinations among n‐/p‐type thermoelectric materials and further broadens the potential candidate material library for segmented thermoelectric devices. [ABSTRACT FROM AUTHOR]

Minimizing the lattice thermal conductivity of thermoelectric materials is essential for preserving the temperature difference during the operation of thermoelectric devices incorporating these materials. During the past two decades, there has been substantial improvement in the thermoelectric figure of merit (zT) due to reduced lattice thermal conductivity. Employing alloying effects in solid-solution compounds is the most common and practical approach for inhibiting lattice thermal conductivity. This Perspective takes the n-type Mg3Sb2−xBix thermoelectric alloys as examples, addressing their lattice thermal conductivity and corresponding zT as functions of their Bi concentration. Additionally, we seek to understand the effect of the lattice contribution to total thermal conductivity for most thermoelectric materials currently being researched. The lattice/total thermal conductivity ratio at the temperature corresponding to the peak zT shows weak material dependence, widely ranging from 0.5 to 0.75, which implies that the lattice thermal conductivity of most thermoelectric materials can be decreased further to improve thermoelectric performance. On the other hand, thermoelectric materials with relatively low ratios exhibit high power factors in their operating temperature ranges, which is ascribed to their excellent electrical performance. These observations provide guidelines to tune transport properties for future applications in thermoelectric power generation. [ABSTRACT FROM AUTHOR]

The recently discovered n-type Mg 3 Sb 2- x Bi x alloys with high thermoelectric performance hold great potential for applications in waste heat recovery due to their high average zT in the temperature range between 300 and 773 K. However, systematic studies of their thermal stability remain lacking and are significant for these thermoelectric materials since the possible degradation of their thermoelectric performance could greatly limit their practical applications. Here we studied the thermal stability of the Mg 3 Sb 2- x Bi x alloys via in situ measurements of their thermoelectric properties at different temperatures, along with microstructural and composition characterizations. Our results show that Mg 3 Sb 2- x Bi x alloys are unstable when the temperature is above 673 K due to significant Mg loss and changed microstructures. By coating Mg 3 Sb 2- x Bi x alloys with boron nitride, the Mg loss can be effectively suppressed, thus greatly improving their thermal stability. Additionally, energy conversion efficiency measurements validated the high thermoelectric performance of the Mg 3 Sb 2- x Bi x alloys and further confirmed the improved thermal stability of the boron-nitride-coated sample. Therefore, our study provides an effective strategy for improving the thermal stability of Mg 3 Sb 2- x Bi x , thus promoting it as a promising candidate for thermoelectric power generation. Image, graphical abstract [ABSTRACT FROM AUTHOR]

Mg 3 Sb 2 -based Zintl compounds show great promise for thermoelectric power generation due to their favorable properties, such as high thermoelectric performance over a wide range of temperatures, low cost, and mechanical robustness. However, their practical application is severely impeded by the low thermal stability induced by significant Mg loss at elevated temperatures. Here, with the goal to intrinsically enhance the thermal stability of Mg 3 Sb 2 -based materials, a strategy of Mn doping at the Mg site is investigated and is found to be effective. In addition to having a high average zT , Mn-doped Mg 3 Sb 1.5 Bi 0.5 exhibits negligible variation in electrical performance throughout a 40-hour continuous electrical properties measurement at 673 K. Microstructure and composition analyses verify the high structural stability of the Mn-doped sample following the long-term in situ measurement. Density functional theory (DFT) calculations show that the enhanced thermal stability of the Mn-doped compound results from the stronger bonding between Mn and Mg atoms in comparison to the Mg-Mg bonds, which can significantly suppress the formation of Mg vacancies. This study demonstrates a novel approach to the development of reliable thermoelectric materials with both a high zT and intrinsically improved thermal stability for applications. [Display omitted] [ABSTRACT FROM AUTHOR]