Rapid One-Step Synthesis and Compaction of High-Performance n-Type Mg3 Sb2 Thermoelectrics

Janus-like Structure and Resonance Level Actualized Ultralow Lattice Thermal Conductivity and Enhanced ZTave in Mg3(Sb, Bi)2-Based Zintls

Grain boundary (GB) engineering includes grain size and GB segregation. Grain size has been proven to affect the electrical properties of Mg3(Sb, Bi)2 at low temperatures. However, the formation mechanism of GB segregation and what kind of GB segregation is beneficial to the performance are still unclear. Here, the Ga/Bi cosegregation at GBs and Mg segregation within grains optimize the transport of electrons and phonons simultaneously. Ga/Bi cosegregation promotes the formation of Janus-like structures due to the diverse ordering tendencies of liquid Mg3Sb2 and Mg3Bi2 and the absence of a solid solution of Ga/Bi. The Janus-like structure significantly reduces the room-temperature lattice thermal conductivity by introducing diverse microdefects. Meanwhile, a coherent interface between the nano Mg segregation region and the matrix is formed, which reduces the thermal conductivity without affecting the carrier transport. Furthermore, the band structure calculations show that Ga doping introduces the resonance level, increasing the Seebeck coefficient. Finally, the lattice thermal conductivity reaches ∼0.4 W m-1 K-1, and a high average ZT of 1.21 between 323 and ∼773 K is achieved for Mg3.2Y0.02Ga0.03Sb1.5Bi0.5. This work provides guidance for improving the thermoelectric performance via designing cosegregation.

High-performance Mg3Sb2-based thermoelectrics with reduced structural disorder and microstructure evolution

Mg3Sb2-based thermoelectrics show great promise for next-generation thermoelectric power generators and coolers owing to their excellent figure of merit (zT) and earth-abundant composition elements. However, the complexity of the defect microstructure hinders the advancement of high performance. Here, the defect microstructure is modified via In doping and prolonged sintering time to realize the reduced structural disorder and microstructural evolution, synergistically optimizing electron and phonon transport via a delocalization effect. As a result, an excellent carrier mobility of ~174 cm2 V-1 s-1 and an ultralowκ l a t of ~0.42 W m-1 K-1 are realized in this system, leading to an ultrahigh zT of ~2.0 at 723 K. The corresponding single-leg module demonstrates a high conversion efficiency of ~12.6% with a 425 K temperature difference, and the two-pair module of Mg3Sb2/MgAgSb displays ~7.1% conversion efficiency with a 276 K temperature difference. This work paves a pathway to improve the thermoelectric performance of Mg3Sb2-based materials, and represents a significant step forward for the practical application of Mg3Sb2-based devices.

Simultaneously Boosting Thermoelectric and Mechanical Properties of n-Type Mg3Sb1.5Bi0.5-Based Zintls through Energy-Band and Defect Engineering

Incorporating donor doping into Mg3Sb1.5Bi0.5 to achieve n-type conductivity is one of the crucial strategies for performance enhancement. In pursuit of higher thermoelectric performance, we herein report co-doping with Te and Y to optimize the thermoelectric properties of Mg3Sb1.5Bi0.5, achieving a peak ZT exceeding 1.7 at 703 K in Y0.01Mg3.19Sb1.5Bi0.47Te0.03. Guided by first-principles calculations for compositional design, we find that Te-doping shifts the Fermi level into the conduction band, resulting in n-type semiconductor behavior, while Y-doping further shifts the Fermi level into the conduction band and reduces the bandgap, leading to enhanced thermoelectric performance with a power factor as high as >20 μW cm-1 K-2. Additionally, through detailed micro/nanostructure characterizations, we discover that Te and Y co-doping induces dense crystal and lattice defects, including local lattice distortions and strains caused by point defects, and densely distributed grain boundaries between nanocrystalline domains. These defects efficiently scatter phonons of various wavelengths, resulting in a low thermal conductivity of 0.83 W m-1 K-1 and ultimately achieving a high ZT. Furthermore, the dense lattice defects induced by co-doping can further strengthen the mechanical performance, which is crucial for its service in devices. This work provides guidance for the composition and structure design of thermoelectric materials.

Solid-State Janus Nanoprecipitation Enables Amorphous-Like Heat Conduction in Crystalline Mg3 Sb2 -Based Thermoelectric Materials

Solid-state precipitation can be used to tailor material properties, ranging from ferromagnets and catalysts to mechanical strengthening and energy storage. Thermoelectric properties can be modified by precipitation to enhance phonon scattering while retaining charge-carrier transmission. Here, unconventional Janus-type nanoprecipitates are uncovered in Mg₃ Sb_1.5 Bi_0.5 formed by side-by-side Bi- and Ge-rich appendages, in contrast to separate nanoprecipitate formation. These Janus nanoprecipitates result from local comelting of Bi and Ge during sintering, enabling an amorphous-like lattice thermal conductivity. A precipitate size effect on phonon scattering is observed due to the balance between alloy-disorder and nanoprecipitate scattering. The thermoelectric figure-of-merit ZT reaches 0.6 near room temperature and 1.6 at 773 K. The Janus nanoprecipitation can be introduced into other materials and may act as a general property-tailoring mechanism.

Insight into the Strategies for Improving the Thermal Stability of Efficient N-Type Mg3Sb2-Based Thermoelectric Materials

N-type Mg₃(Sb,Bi)₂ compounds have recently been demonstrated as promising low-cost efficient thermoelectric materials in low and intermediate temperature ranges; however, the thermal stability of this type of material still poses a great challenge for practical applications. In this work, we conduct a systematic investigation of the thermal stability of several high-performing n-type Mg₃(Sb,Bi)₂-based thermoelectric materials in both bulk and powdered forms using X-ray and neutron diffraction. It is found that the bulk sample exhibits a much slower degradation rate based on the evolution of the secondary Bi/Sb phase in comparison with the powdered sample, revealing a clear kinetic effect. Moreover, the surface of the bulk sample will gradually become Mg-poor or Bi-rich even at room temperature when exposed to air for a long time, highlighting the importance of surface encapsulation for applications. An underlying mechanism based on the Mg loss/migration is proposed to account for the property degradation. Importantly, to address the property degradation, we discuss possible solutions and propose Mg-vapor annealing as an effective approach to enhance thermal stability by suppressing the Mg loss/migration through saturating grains and grain boundaries with elemental Mg. We expect a combination of the Mg-vapor annealing and surface coating to further improve the long-term thermal stability. This work sheds light on the strategies for enhancing the long-term stability of n-type Mg₃Sb₂-based thermoelectrics for practical applications.

Improved Thermoelectric Properties of N-Type Mg3Sb2 through Cation-Site Doping with Gd or Ho

The success of n-type doping has attracted strong research interest for exploring effective n-type dopants for Mg₃Sb₂ thermoelectrics. Herein, we experimentally study Gd and Ho as n-type dopants for Mg₃Sb₂ thermoelectrics. The synthesis, structural characterization, and thermoelectric properties of Gd-doped, Ho-doped, (Gd, Te)-codoped, and (Ho, Te)-codoped Mg₃Sb₂ samples are reported. It is found that Gd and Ho are effective n-type cation-site dopants showing a higher doping efficiency as well as a superior carrier concentration in comparison with anion-site doping with Te, consistent with the previous theoretical prediction. For n-type Mg₃Sb₂ samples doped with Gd or Ho, optimal thermoelectric figure of merit zT values of ∼1.26 and ∼0.94 at 725 K are obtained, respectively, in Mg_3.5Gd_0.04Sb₂ and Mg_3.5Ho_0.04Sb₂, which are superior to many reported Te-doped Mg₃Sb₂ without alloying with Mg₃Bi₂. By codoping with Gd (or Ho) and Te, reduced thermal conductivity and enhanced power factor values are achieved at high temperatures, which results in enhanced peak zT values well above unity at 725 K. This work reveals Gd and Ho as effective n-type dopants for Mg₃Sb₂ thermoelectric materials.

Probing Efficient N-Type Lanthanide Dopants for Mg3Sb2 Thermoelectrics

The recent discovery of n-type Mg3Sb2 thermoelectrics has ignited intensive research activities on searching for potential n-type dopants for this material. Using first-principles defect calculations, here, a systematic computational screening of potential efficient n-type lanthanide dopants is conducted for Mg3Sb2. In addition to La, Ce, Pr, and Tm, it is found that high electron concentration (≳1020 cm-3 at the growth temperature of 900 K) can be achieved by doping on the Mg sites with Nd, Gd, Ho, and Lu, which are generally more efficient than other lanthanide dopants and the anion-site dopant Te. Experimentally, Nd and Tm are confirmed as effective n-type dopants for Mg3Sb2 since doping with Nd and Tm shows higher electron concentration and thermoelectric figure of merit zT than doping with Te. Through codoping with Nd (Tm) and Te, simultaneous power factor improvement and thermal conductivity reduction are achieved. As a result, high zT values of ≈1.65 and ≈1.75 at 775 K are obtained in n-type Mg3.5Nd0.04Sb1.97Te0.03 and Mg3.5Tm0.03Sb1.97Te0.03, respectively, which are among the highest values for n-type Mg3Sb2 without alloying with Mg3Bi2. This work sheds light on exploring promising n-type dopants for the design of Mg3Sb2 thermoelectrics.

N-Type Mg3Sb2-x Bi x Alloys as Promising Thermoelectric Materials

N-type Mg₃Sb_2-x Bi _x alloys have been extensively studied in recent years due to their significantly enhanced thermoelectric figure of merit (zT), thus promoting them as potential candidates for waste heat recovery and cooling applications. In this review, the effects resulting from alloying Mg₃Bi₂ with Mg₃Sb₂, including narrowed bandgap, decreased effective mass, and increased carrier mobility, are summarized. Subsequently, defect-controlled electrical properties in n-type Mg₃Sb_2-x Bi _x are revealed. On one hand, manipulation of intrinsic and extrinsic defects can achieve optimal carrier concentration. On the other hand, Mg vacancies dominate carrier-scattering mechanisms (ionized impurity scattering and grain boundary scattering). Both aspects are discussed for Mg₃Sb_2-x Bi _x thermoelectric materials. Finally, we review the present status of, and future outlook for, these materials in power generation and cooling applications.

High-Performance Mg3Sb2-x Bi x Thermoelectrics: Progress and Perspective

Since the first successful implementation of n-type doping, low-cost Mg₃Sb_2-x Bi _x alloys have been rapidly developed as excellent thermoelectric materials in recent years. An average figure of merit zT above unity over the temperature range 300-700 K makes this new system become a promising alternative to the commercially used n-type Bi₂Te_3-x Se _x alloys for either refrigeration or low-grade heat power generation near room temperature. In this review, with the structure-property-application relationship as the mainline, we first discuss how the crystallographic, electronic, and phononic structures lay the foundation of the high thermoelectric performance. Then, optimization strategies, including the physical aspects of band engineering with Sb/Bi alloying and carrier scattering mechanism with grain boundary modification and the chemical aspects of Mg defects and aliovalent doping, are extensively reviewed. Mainstream directions targeting the improvement of zT near room temperature are outlined. Finally, device applications and related engineering issues are discussed. We hope this review could help to promote the understanding and future developments of low-cost Mg₃Sb_2-x Bi _x alloys for practical thermoelectric applications.