Rapid One‐Step Synthesis and Compaction of High‐Performance n‐Type Mg₃Sb₂ Thermoelectrics

Abstract

n‐type Mg3Sb2‐based compounds have emerged as a promising class of low‐cost thermoelectric materials due to their extraordinary performance at low and intermediate temperatures. However, so far, high thermoelectric performance has merely been reported in n‐type Mg3Sb2‐Mg3Bi2 alloys with a large amount of Bi. Moreover, current synthesis methods of n‐type Mg3Sb2 bulk thermoelectrics involve multi‐step processes that are time‐ and energy‐consuming. Herein, we report a fast and straightforward approach to fabricate n‐type Mg3Sb2 thermoelectrics using spark plasma sintering, which combines the synthesis and compaction in one step. Using this method, we achieve a high thermoelectric figure of merit zT of about 0.4–1.5 at 300–725 K in n‐type (Sc, Te)‐co‐doped Mg3Sb2 without alloying with Mg3Bi2. In comparison with the currently reported synthesis methods, the complexity, process time, and cost of our method are significantly reduced. This work demonstrates a simple, low‐cost route for the potential large‐scale production of n‐type Mg3Sb2 thermoelectrics.

Probing Efficient N‐Type Lanthanide Dopants for Mg3Sb2 Thermoelectrics

“…With the experimental data of the Nd-, Tm-, and Y-doped samples, we can compare the results with those of the reported Te-and Sc-doped samples prepared using a similar synthesis method. [30] It is found that n-type Nddoped and Tm-doped Mg 3 Sb 2 samples show slightly lower electron concentrations than those of the Y-doped samples but higher electron concentrations than those of Sc-doped and Te-doped samples (see Figure 2c), which is consistent with the theoretical calculation. The room temperature electron concentrations of ntype Mg 3.5 Nd y Sb 2 samples vary from 1.46 × 10 19 (y = 0.01) to 3.17 × 10 19 cm −3 (y = 0.03), while the electron concentrations vary from 7.32 × 10 18 to 2.36 × 10 19 cm −3 in Mg 3.5 Tm y Sb 2 samples.…”

“…Although the Nd-doped samples have been sintered at a high temperature of 800°C followed by annealing at 615°C under the Mg-rich environment [11] (see the Experimental Section) to minimize the grain boundary scattering, the electron mobility of Nd-doped Mg 3 Sb 2 samples still shows a temperature dependence of T 1.5 at low temperatures. This is very different from the Y-doped, Sc-doped, [30] or Te-doped [30] samples synthesized using a similar method, which show dominant acoustic phonon scattering at low temperatures ( Figure S14, Supporting Information). The increasing temperature dependence of the mobility at low temperatures results in poor room-temperature electron mobility (≈20.8-32.3 cm 2 V −1 s −1 ) of Nd-doped samples in comparison with those of Te-doped samples (≈51.0-73.8 cm 2 V −1 s −1 ).…”

“…Recent defect calculations [22][23][24][25] predicted that the group-3 elements (Sc and Y) as well as several lanthanides including La, Pr, Ce, and Tm are efficient n-type cation-site dopants for Mg 3 Sb 2 and doping with these elements on the Mg sites is able to achieve higher electron concentration than doping with chalcogens on the anion sites. The subsequent experiments [24,[26][27][28][29][30][31][32] confirmed Sc, Y, La, Pr, and Ce as efficient n-type dopants on the Mg sites for Mg 3+ Sb 2−x Bi x . Despite these significant theoretical and experimental efforts on exploring efficient n-type dopants, the n-type doping behavior of many other lanthanide dopants in Mg 3 Sb 2 remains largely unexplored so far.…”

“…To verify the theoretical prediction, here, we experimentally examine the potential of Nd and Tm as n-type dopants for Mg 3 Sb 2 . The Nd-doped, Tm-doped, (Nd, Te)-codoped, and (Tm, Te)-codoped Mg 3 Sb 2 bulk samples were prepared by the previously proposed spark plasma sintering (SPS) method [30,38] followed by annealing in the Mg-rich condition. [11] According to our previous work, [30] the specific amount of excess Mg (Mg 3.5 ) is used to compensate for the evaporation loss of Mg so that the as-SPS-pressed pellets show no Mg deficiency.…”

“…The energy dispersive X-ray spectroscopy (EDS) elemental mapping of the Mg 3.5 Nd 0.03 Sb 1.97 Te 0.03 sample shows that the sample is virtually homogeneous with nearly uniform distributions of the constituent elements ( Figure S9, Supporting Information). The chemical composition measured by scanning [30] and Te [30] ) in Mg 3+ Sb 2 without the Mg 3 Bi 2 alloying. The data of Nd-, Tm-, and Y-doped samples are from this work.…”