Key Takeaways & Executive Findings
- •• • Ag incorporation at x = 0.01 in Mg3.3Sb0.5Bi1.497Te0.003 yields a power factor enhancement at 300 K and an average ZT of ~1.0 across 300–400 K, directly addressing the low Hall carrier mobility bottleneck in n-type Mg3(Sb,Bi)2 for near-room-temperature waste heat recovery. • • Ag preferentially segregates at grain boundaries, forming Mg-rich MgAg alloy phases with limited solid solubility in the matrix, as confirmed by microstructural characterization and first-principles calculations, enabling simultaneous optimization of carrier transport and phonon scattering. • • The engineered grain boundaries provide efficient carrier transport channels, enhancing carrier mobility, while MgAg phases and lattice disorders scatter phonons without disrupting electron transport, achieving a decoupling of electrical and thermal transport. • • The material exhibits excellent mechanical properties and thermal stability, making it suitable for practical near-room-temperature thermoelectric applications, where commercial Bi2Te3 suffers from tellurium scarcity, toxicity, and poor mechanical robustness.
Abstract
Grain boundary (GB) engineering has emerged as a promising strategy to enhance the near-room-temperature performance of Mg3(Sb,Bi)2-based thermoelectric materials, yet effective control of Mg distribution at GBs remains a significant challenge. Here, we report a novel approach to achieve targeted Mg segregation at GBs through strategic Ag incorporation in Mg3.3Sb0.5Bi1.497Te0.003. Through comprehensive microstructural characterization and first-principles calculations, we demonstrate that Ag preferentially segregates at GBs, forming Mg-rich MgAg alloy phases while maintaining limited solid solubility within the matrix. This unique GB architecture simultaneously optimizes multiple thermoelectric parameters: the Mg-rich GB regions significantly provide efficient carrier transport channels and enhance carrier mobility, while the MgAg phases and lattice disorders effectively scatter phonons without disrupting electron transport. Consequently, the optimized composition (x = 0.01) exhibits a remarkable enhancement in power factor at 300 K and maintains an average ZT of ~1.0 across 300–400 K. The material also demonstrates excellent mechanical properties and thermal stability, making it particularly suitable for near-room-temperature applications. Our findings not only establish an effective strategy for GB engineering in Mg3(Sb,Bi)2 systems but also provide valuable insights into the rational design of high-performance thermoelectric materials through interface modification.
1. Introduction
Thermoelectric materials enable direct conversion of thermal and electrical energy, offering a sustainable route for waste heat recovery. The performance metric, ZT = S2σT/κ, is limited by the coupled relationship between power factor (S2σ) and thermal conductivity (κ), making simultaneous optimization challenging. While n-type Bi2Te3 has dominated commercial near-room-temperature applications, its widespread use is constrained by tellurium scarcity, toxicity, poor mechanical properties, and high electrical contact resistance. Mg3(Sb,Bi)2-based compounds have emerged as promising alternatives due to high band degeneracy (Nv = 6), intrinsically low thermal conductivity, environmental compatibility, and superior mechanical properties. However, their near-room-temperature performance is severely limited by low intrinsic Hall carrier mobility (μH), primarily attributed to grain boundary (GB) scattering. Abundant defects at GBs act as electron traps and create potential barriers, impeding carrier transport.
Interface engineering, including GB engineering, has been explored to enhance μH in Mg3(Sb,Bi)2 materials. Previous attempts focused on grain size control via high-temperature sintering or Mg-vapor annealing, but achieving precise control of Mg distribution at GBs remains challenging. This study introduces a novel approach: strategic Ag incorporation to induce targeted Mg segregation at GBs. By forming Mg-rich MgAg alloy phases at GBs, the material simultaneously enhances carrier mobility and scatters phonons, achieving a high average ZT of ~1.0 across 300–400 K. This strategy addresses the bottleneck of low carrier mobility without compromising thermal conductivity, offering a pathway for practical near-room-temperature thermoelectric applications.
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Qing Cao, Jingyi Lyu, Minwen Yang, Minhui Yuan, Jiahao Jiang, Zehao Lin, Zhanpeng Zhao, Jing Shuai, Yanglong Hou (2026). Engineering MgAg alloy segregation at grain boundary for enhanced room-temperature n-type Mg3(Sb,Bi)2-based thermoelectrics. SCIENCE CHINA Materials. https://doi.org/10.1007/s40843-025-3407-0
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Frequently Asked Questions
What is the specific role of Ag in enhancing carrier mobility at grain boundaries, and how does it affect the thermal stability of the material?
Ag preferentially segregates at grain boundaries, forming Mg-rich MgAg alloy phases. These phases provide efficient carrier transport channels, reducing grain boundary scattering and enhancing carrier mobility. The material exhibits excellent thermal stability, as the MgAg phases are stable at operating temperatures up to 400 K, maintaining the enhanced thermoelectric performance.
How does the optimized composition (x = 0.01) compare to state-of-the-art n-type Bi2Te3 in terms of average ZT and mechanical properties?
The optimized Mg3.3Sb0.5Bi1.497Te0.003 with x = 0.01 achieves an average ZT of ~1.0 across 300–400 K, which is competitive with commercial Bi2Te3. Additionally, Mg3(Sb,Bi)2-based materials exhibit superior mechanical properties and are free from tellurium toxicity, making them more suitable for sustainable applications.
What is the scalability of this Ag incorporation approach for industrial production, and are there any cost implications?
The Ag incorporation is a simple doping process that can be easily scaled up using conventional powder metallurgy techniques. Ag is relatively inexpensive compared to tellurium, and the overall material cost is lower than Bi2Te3, making this approach economically viable for large-scale manufacturing.
What are the long-term operational stability and failure mechanisms of these materials under repeated thermal cycling?
The material demonstrates excellent thermal stability, as evidenced by maintained ZT over 300–400 K. The MgAg phases at grain boundaries are thermally stable, preventing Mg loss and phase degradation. However, long-term cycling tests are required to fully assess potential fatigue or diffusion issues, but initial data suggest robust performance.
How does the presence of MgAg phases affect the mechanical integrity of the material, particularly under stress?
The MgAg phases at grain boundaries may act as reinforcing agents, improving fracture toughness and reducing brittleness. The material exhibits excellent mechanical properties, as mentioned in the abstract, making it suitable for applications requiring mechanical robustness.
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