Grain boundary plays a vital role in thermoelectric transports, leading to distinct properties between single crystals and polycrystals. Manipulating the grain boundary to realize good thermoelectric properties in polycrystals similar as those of single crystals is a long-standing task, but it is quite challenging. Herein, we develop a liquid-phase sintering strategy to successfully introduce Mg 2 Cu nano-sintering-aid into the grain boundaries of Mg 3 (Bi, Sb) 2 -based materials. The nano-aid helps to enlarge the average grain size to 23.7 μm and effectively scatter phonons, leading to excellent electrical transports similar as those of single crystals and ultralow lattice thermal conductivity as well as exceptional thermoelectric figure of merit (1.5 at 500 K) and conversion efficiency (7.4% under temperature difference of 207 K). This work provides a simple but effective strategy for the fabrication of high-performance polycrystals for large-scale applications.
While transition metal doping is widely adopted to optimize Mg3(Bi, Sb)2-based thermoelectrics, conventional high-concentration doping inevitably induces detrimental grain boundary segregation, creating a convoluted mixed effect that obscures the intrinsic regulatory roles of the dopants. To systematically unveil these fundamental impacts, we establish a low-concentration doping paradigm. By synergizing a precisely controlled low-concentration strategy with an optimized synthesis process, 3d transition metals (Mn, Fe, Co, Cu) are homogeneously incorporated into the bulk lattice without boundary enrichment. Based on this homogeneous microstructural framework, distinct doping effects are systematically revealed. Fe and Co act as highly efficient electron donors, elevating the carrier concentration to >1.5 × 1020 cm–3. In contrast, Mn fundamentally optimizes the carrier scattering mechanism via effective mass reduction, leading to a remarkable 2.6-fold enhancement in carrier mobility. Consequently, the lightly Mn-doped sample achieves an optimal balance, yielding a peak power factor of ∼30.7 μW/(K2cm) and a maximum zT of 1.03. Furthermore, this homogeneously doped sample demonstrates optimized macroscopic durability, retaining consistent electrical properties after 3-month ambient air exposure and withstanding 50 rigorous thermal shock cycles. Ultimately, this work clarifies how homogeneously incorporated dopants regulate transport properties and stabilize the crystal matrix, establishing precise low-concentration as a robust framework for high-performance and durable thermoelectrics.
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