Realizing Ultrahigh Near-Room-Temperature Thermoelectric Figure of Merit for N-Type Mg<sub>3</sub>(Sb,Bi)<sub>2</sub> through Grain Boundary Complexion Engineering with Niobium
Melis Özen, Arda Baran Burcak, Duncan Zavanelli, Minsu Heo, Mujde Yahyaoglu, Yahya Öz, Ulrich Burkhardt, Hyun‐Sik Kim, G. Jeffrey Snyder, Umut Aydemir
Abstract
High Resolution Image Download MS PowerPoint Slide Despite decades of extensive research on thermoelectric materials, Bi 2 Te 3 alloys have dominated room-temperature applications. However, recent advancements have highlighted the potential of alternative candidates, notably Mg 3 Sb 2 –Mg 3 Bi 2 alloys, for low- to mid-temperature ranges. This study optimizes the low-temperature composition of this alloy system through Nb addition (Mg 3.2– x Nb x (Sb 0.3 Bi 0.7 ) 1.996 Te 0.004 ), characterizing composition, microstructure, and transport properties. A high Mg 3 Bi 2 content improves the band structure by increasing weighted mobility while enhancing the microstructure. Crucially, it suppresses detrimental grain boundary scattering effects for room-temperature applications. While grain boundary scattering suppression is typically achieved through grain growth, our study reveals that Nb addition significantly reduces grain boundary resistance without increasing grain size. This phenomenon is attributed to a grain boundary complexion transition, where Nb addition transforms the highly resistive Mg 3 Bi 2 -rich boundary complexion into a less resistive, metal-like interfacial phase. This marks the rare demonstration of chemistry noticeably affecting grain boundary interfacial electrical resistance in Mg 3 Sb 2 –Mg 3 Bi 2 . The results culminate in a remarkable advancement in zT, reaching 1.14 at 330 K. The device ZT is found to be 1.03 at 350 K, which further increases to 1.24 at 523 K and reaches a theoretical maximum device efficiency (η max ) of 10.5% at 623 K, underscoring its competitive performance. These findings showcase the outstanding low-temperature performance of n -type Mg 3 Bi 2 –Mg 3 Sb 2 alloys, rivaling Bi 2 Te 3, and emphasize the critical need for continued exploration of complexion phase engineering to advance thermoelectric materials further.