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

Abstract

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.

Keywords: Janus nanoprecipitation; Mg3Sb2; atom probe tomography; low thermal conductivity; thermalelectrics.

Figure 1

Microstructure of Mg_3.2Sb_1.47Bi_0.5Te_0.02Ge_0.01 specimen (Ge‐0.01). a,b) STEM image, revealing a high density of nanoscale precipitates. The insets show the SAED pattern of the corresponding microstructure along the <100> zone axis. c) APT reconstructions showing the elemental distribution (Mg, pink; Ge, red; Bi, teal, Sb and Te atoms are omitted, for clarity). d) Close‐up of a subregion from c, highlighting the 3D structure of Bi/Ge‐rich Janus nanoprecipitates. The dimension of the cuboid region of interest is 30 × 30 × 10 nm³. e) 1D composition profile along the arrow in (d).

Figure 2

Microstructure of the Mg_3.2Sb_1.39Bi_0.5Te_0.06Ge_0.05 specimen (Ge‐0.05). a) STEM image, revealing high‐density nanoprecipitates. The insets show the corresponding SAED pattern. b) Magnified view of a Bi‐rich precipitate. c) APT reconstructions showing the elemental distribution (Mg, pink; Ge, red; Bi, teal); the Ge‐rich and Bi‐rich precipitates are depicted by iso‐composition surfaces of 10 at% Ge and 30 at% Bi, respectively. Both precipitates connect side‐by‐side forming Janus precipitates. d) 1D composition profile calculated along the horizontal arrow across two Ge‐rich precipitates. e) 1D composition profile calculated along the vertical arrow across the Janus precipitate from Ge‐rich to Bi‐rich part.

Figure 3

Mechanism of Janus precipitation. a) Bi‐Ge binary phase diagram.^{[ 34 ]} Red‐colored areas denote the Ge‐doping limitation for Mg_3.2Sb_1.5Bi_0.5 compounds. b–d) Schematic plots of the co‐precipitation process during sintering and cooling, which can be divided into 3 stages. e,f) Experimental observation from APT reconstructions for correlative stage II, and III.

Figure 4

Thermoelectric performance of n‐type Ge‐doped Mg_3.2Sb_1.5Bi_0.5. a) A comparison of total thermal conductivity κ _tot among several polycrystalline n‐type Mg₃(Sb,Bi)₂ materials: Mg_3.2Sb₂,^{[ 35 ]} Mg_3.2Sb_1.5Bi_0.5,^{[ 11 ]} transition‐metal (Nb,^{[ 13 ]} Co,^{[ 14 ]} Mn,^{[ 9} , ^{31 ]} Sc,^{[ 36} , ^{37 ]} Y^{[ 30 ]}) doped Mg₃(Sb,Bi)₂, and Janus precipitation Mg_3.2Sb_0.5Bi_0.5. b) Temperature dependence of lattice thermal conductivity κ _lat of dopant‐free Mg_3.2Sb_1.5Bi_0.5 (no precipitates), Mg_3.15Mn_0.05Sb_1.5Bi_0.5 [single‐type Bi‐rich precipitates, ref.[31]] and Mg_3.2Sb_1.47Bi_0.5Te_0.02Ge_0.01 (Bi/Ge‐rich Janus precipitates). The solid purple and red lines in (b) show the temperature‐dependent lattice thermal conductivity κ _lat calculated with the Callaway model and input parameters obtained from STEM and APT investigation of Ge‐free and Ge‐0.01 doping samples. c) Figure of merit, ZT of above three Mg_3.2Sb_1.5Bi_0.5‐based samples in comparison with state‐of‐the‐art n‐type thermoelectric material Bi₂Te_2.3Se_0.7.^{[ 38 ]}