Revelation of Inherently High Mobility Enables Mg₃Sb₂ as a Sustainable Alternative to n‐Bi₂Te₃ Thermoelectrics

Abstract

Over the past years, thermoelectric Mg 3 Sb 2 alloys particularly in n‐type conduction, have attracted increasing attentions for thermoelectric applications, due to the multivalley conduction band, abundance of constituents, and less toxicity. However, the high vapor pressure, causticity of Mg, and the high melting point of Mg 3 Sb 2 tend to cause the inclusion in the materials of boundary phases and defects that affect the transport properties. In this work, a utilization of tantalum‐sealing for melting enables n‐type Mg 3 Sb 2 alloys to show a substantially higher mobility than ever reported, which can be attributed to the purification of phases and to the coarse grains. Importantly, the inherently high mobility successfully enables the thermoelectric figure of merit in optimal compositions to be highly competitive to that of commercially available n‐type Bi 2 Te 3 alloys and to be higher than that of other known n‐type thermoelectrics at 300–500 K. This work reveals Mg 3 Sb 2 alloys as a top candidate for near‐room‐temperature thermoelectric applications.

Metallic n‐Type Mg3Sb2 Single Crystals Demonstrate the Absence of Ionized Impurity Scattering and Enhanced Thermoelectric Performance

“…n‐Type Mg 3 Sb 2 –Mg 3 Bi 2 alloys are one of the most potent thermoelectric materials for low‐ (near room temperature) to mid‐temperature range [ 1–9 ] because of their highly degenerate conduction band structure [ 1,4 ] and extremely low phonon thermal conductivity. [ 10–11 ] Since the discovery of n‐type Mg 3 Sb 1.5 Bi 0.5 with zT of 1.6 at 700 K in 2016, [ 1 ] extensive research have been conducted to improve their thermoelectric performance via the engineering of the electronic band structure, [ 5,12 ] chemical doping, [ 13–16 ] bonding [ 17–19 ] and modulation of microstructure.…”

“…[ 20 ] Presumably because all previous (polycrystalline) samples still show some increasing trend of electrical conductivity with temperature below room temperature, many recent reports still claim the predominance of ionized impurity scattering at low temperature. [ 3,6,23–25,31–35 ]…”

“…It is worth noting that the elimination of the grain boundary effect near room temperature is extremely important since this system is now competitive with commercial Bi 2 Te 3 ‐based thermoelectric materials for low‐grade heat recovery and cooling applications. [ 5–8,21 ] Although computational studies predict nearly isotropic thermoelectric transport properties, [ 1,18 ] this should be validated. Here, we only measure c ‐axis thermal conductivity in single crystals, but as this measured thermal conductivity is similar to polycrystalline samples, we have no reason to believe the thermal conductivity in the ab‐plane is significantly different (within the ≈30% experimental uncertainty reported here).…”

High-Performance Mg 3 Sb 2- x Bi x Thermoelectrics: Progress and Perspective

“…Aiming at improving the μ and σ near RT by suppressing the grain boundary resistance, one direct way is to simply reduce the number of grain boundaries to get coarse-grained polycrystalline samples, which can be achieved by either increasing the sintering temperature [ 78 , 82 ] or post-annealing under Mg atmosphere if initial grain sizes are small [ 82 ]. Furthermore, melting followed by a sintering process has recently been reported as an effective way to prepare samples with large grain sizes [ 73 , 92 , 177 ]. Some experimental results also indicate that Mg 3 Sb 2- x Bi x samples with high Bi content can have large grain sizes under the same preparing condition [ 47 ], which is probably due to their low melting points.…”

N-Type Mg 3 Sb 2- x Bi x Alloys as Promising Thermoelectric Materials

“…For example, the room-temperature electrical conductivity is ~4 × 10 4 S m −1 for the sample with an average grain size of ~7.8 μ m, and it is ~1 × 10 4 S m −1 for the sample with an average grain size of ~1.0 μ m [ 53 ]. Similarly, the grain size of Mg 3 Sb 2- x Bi x alloys was increased by annealing [ 54 ] or hot deforming [ 27 , 34 , 55 ], and improvement in mobility was also observed. It should be noted that the defects would be also reduced, in addition to the increasing grain size, by increasing the sintering temperature or by annealing.…”