Advances in Mg₃Sb₂ thermoelectric materials and devices

Abstract

Thermoelectric technology offers a green-viable and carbon-neutral solution for energy problems by directly converting waste heat to electricity. For years, Bi₂Te₃-based compounds have been the main choice materials for commercial thermoelectric devices. However, Bi₂Te₃ comprises scarce and toxic tellurium (Te) elements, which might limit its large-scale application. Recently, Mg₃Sb₂ compounds have drawn increasing attention as an alternative to Bi₂Te₃ thermoelectrics due to their excellent thermoelectric performance. Enabled by effective strategies such as optimizing carrier concentration, introducing point defects, and manipulating carrier scattering mechanisms, Mg₃Sb₂ compounds have realized an improved thermoelectric performance. In this review, optimizing strategies for both Mg₃Sb₂-based thermoelectric materials and devices are discussed. Moreover, the flexibility and plasticity of Bi-alloyed Mg₃Sb₂ mainly stemming from the dense dislocations are outlined. The above strategies summarized here for enhancing Mg₃Sb₂ thermoelectrics are believed to be applicable to many other thermoelectrics.

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Introduction

Thermoelectric technology with the advantages of portable, emission-free, and quiet device design enables conversion from waste heat to electricity, which has been regarded as one of the promising solutions to energy problems. Until now, major high-performance materials used for thermoelectric devices usually contained toxic or high-cost elements (such as Pb, Ge, and Te).^1–4 Besides, the realizable thermoelectric conversion efficiencies are still inferior to those of other renewable technologies such as photovoltaic⁵ and lithium batteries,⁶ which limits the large-scale application of thermoelectric technology. The key challenges for improving thermoelectric conversion efficiency are the complexity of thermoelectric device fabrication and the enhancement of the thermoelectric performance of the materials, which could be determined by the thermoelectric figure of merit, zT = S²σT/(κ_e + κ_L), where σ, S, T, κ_e, and κ_L are the electrical conductivity, Seebeck coefficient, absolute temperature, and the electronic and lattice components of thermal conductivity, respectively.⁷

Many strategies including band structure manipulation,^8–10 carrier concentration optimization^11,12 and lattice thermal conductivity reduction by the introduction of point defects,^13,14 dislocation,^15,16 strong anharmonicity,^17,18 complex crystal structures,^19–21 and weak atomic bonding^22,23 have been proven to be effective at improving the thermoelectric performance of materials such as PbTe,⁴ GeTe,^1,24 half-Heusler,^25,26 and CoSb₃ materials.²⁷ All the above thermoelectric materials mentioned successfully realize zT-improvements at mid- to high-temperature (T > ∼600 K), while there exist limitations on the low-grade waste heat application (T < ∼600 K) for these materials. For a century, the only choice for commercial thermoelectric devices for low-grade waste heat thermoelectric power generation and solid-state cooling is Bi₂Te₃-based thermoelectrics because of the facile fabrication process of devices and improved thermoelectric performance near room temperature. However, Bi₂Te₃-based thermoelectrics contain the toxic and scarce element tellurium (Te), which might limit its large-scale application. Additionally, the maximal thermoelectric performance of Bi₂Te₃ appears at ∼400–500 K, and then the thermoelectric performance degrades when the temperature is above 500 K, which could be understood from the bipolar effect. Therefore, there is an urgent need to explore alternative non-toxic, low-cost, and abundant materials for Bi₂Te₃.

Recently, Mg-based thermoelectrics including MgAgSb,^28,29 Mg₂Si,²⁹ and Mg₃Sb₂^30–32 compounds have drawn increasing attention due to their outstanding thermoelectric performance. In MgAgSb-based compounds, a higher phase purity and small grain size enable a zT close to 1 near room temperature and a zT of ∼1.4 at 475 K, demonstrating its potential to replace Bi₂Te₃ thermoelectrics.²⁸ In addition to MgAgSb thermoelectrics, Mg₂(Si,Sn) with a zT ∼ 1.3 at 700 K is a promising alternative choice for Bi₂Te₃ as well.³³ In order to prove the high-performance of these Mg-based materials, various thermoelectric devices have been fabricated. A thermoelectric conversion efficiency of ∼7.5%²⁹ is demonstrated in a Mg₂(Si,Sn) device at 600 K, indicating its possible application in low-grade waste heat. Among the above Mg-based thermoelectrics, Mg₃Sb₂-based compounds showing similar laminar structural features with local weak bonding to those of Bi₂Te₃ realize intrinsically low lattice thermal conductivity.³⁴ They once became star thermoelectric materials due to their ultra-low prices³⁵ (Fig. 1a) and extraordinary thermoelectric performances,^30,31 which motivated us to focus on Mg₃Sb₂ thermoelectrics in this review.

Fig. 1 (a) Cost ($ per kg) of raw materials for thermoelectrics.⁴⁰ Copyright 2020, Wiley. (b) Peak figure of merit (zT_peak) for Mg₃Sb_2−xBi_x thermoelectrics.^{36,37,39,41,45,47–74}

The thermoelectric performance of Mg₃Sb₂ and Mg₃Bi₂ compounds was first reported in 2003.³⁶ In the following years, Mg₃Sb₂-based compounds were studied as p-type thermoelectric materials. Until 2016, n-type Mg₃Sb_2−xBi_x with a significant zT ∼ 1.5 at 700 K has been reported,³⁷ which led to Mg₃Sb_2−xBi_x once becoming the most efficient and popular among Zintl compounds. The higher zT realized in n-type Mg₃Sb₂ compared to that of p-type Mg₃Sb₂ mainly stems from the superior power factor induced by the larger conduction band degeneracy (N_c = 6) in n-type Mg₃Sb₂ as compared to valence band degeneracy (N_v = 1).³⁸ With further optimization by effective strategies such as optimization of carrier concentration,^39–41 manipulation of the carrier scattering mechanism,^42,43 strengthened phonon scattering,⁴⁴ an improved peak zT ∼ 1.8 at high temperature⁴⁵ (Fig. 1b) and a peak zT of ∼1.02 near room temperature⁴⁶ in n-type Mg₃Sb_2−xBi_x compounds were realized. Such a high zT near room temperature mainly stems from the manipulation of the carrier scattering mechanism for enhancing the mobility of the Mg₃Sb_2−xBi_x compounds, the reduction of the lattice thermal conductivity through tuning the Bi/Sb ratio, and the optimization of the carrier concentration by doping heterovalent atoms, making Mg₃Sb_2−xBi_x a promising alternative to n-type Bi₂Te₃.

In this review, representative strategies for improving Mg₃Sb₂ thermoelectric materials and devices are summarized. At a material level, electronic strategies such as carrier concentration optimization, carrier scattering mechanism manipulation, and thermal strategies by the introduction of a phonon scattering source are outlined. At a device level, the methods adopted for selecting contact layers for devices and prospects for cooling and electricity generation are discussed. In addition, Mg₃Sb_2−xBi_x-based plastic semiconductors show a possibility for flexible device application. This review is expected to inspire new insights into advancing Mg₃Sb₂ thermoelectrics, which may be useful for improving other thermoelectric materials.

Materials properties

According to the Zintl compound perspective, Mg₃Sb₂ is categorized as a CaAl₂Si₂-type compound, and its crystal structure is shown in Fig. 2a. It is recognized that Mg atoms occupy two different sites in the lattice. One (Mg1 atom) is located at the octahedral site surrounded by six equivalent Sb atoms and the other (Mg2 atom) is at the tetrahedral site surrounded by four asymmetric Sb atoms. The [Mg₂Sb₂]²⁻ polyanion framework is covalently bonded in the ab-plane, while Mg2 sits at the interlayers of polyanions and is ionically bonded to the polyanions. Such different bond strength between inter- and intra-layers allows an isotropic atomic structure only along the crystallographic ab plane but leaves a strong difference along the c direction. The bond strength between the inter- and intra-layers helps in understanding the anisotropic of Hall mobility, lattice thermal conductivity, and overall thermoelectric performance along ab-plane and c-axis direction (Fig. 2b).^75,76

Fig. 2 (a) Crystal structure of Mg₃Sb₂. (b) Anisotropy factors of Hall mobility, lattice thermal conductivity, and zT for single-crystalline Mg₃Sb₂.⁷⁶

Vacancies at Mg and Sb sites are pronounced in Mg₃Sb₂ compounds, which could be tuned by its stoichiometry. Low-energy electron acceptor defect V_Mg²⁻ (Mg_Mg → V_Mg²⁻ + 2h⁺) in Mg-deficiency case produces holes h⁺, resulting in a reduction of the energy at the Fermi level (E_F) close to equilibrium E_F and vacancy concentration³⁰ (Fig. 3a). In a Mg-deficiency case, even if introducing n-type dopants (such as Te), the intrinsic acceptor defects pin the E_F, thus making material to be always of p-type.³⁰ Fortunately, excess Mg still allows a single phase thermodynamically according to the binary phase diagram (Fig. 3b). n-type Mg₃Sb₂ is realizable in the Mg-excess case, which could be explained by the enhanced Mg vacancy formation (Fig. 3a).

Fig. 3 (a) Mg-vacancy formation energy as a function of Fermi level (E_F).³⁰ (b) Calculated Mg–Sb phase diagram³⁰ for Mg_3+xSb₂. (c) Calculated band structure for Mg₃Sb₂.⁵⁵ (a) and (b) Adapted with permission, Copyright 2017, Cell Press.³⁰ (c) Reproduced under the terms of the CC-BY Creative Commons Attribution 4.0 International License.⁵⁵

Mg defects play pivotal roles in the thermoelectric properties of Mg₃Sb₂.⁷⁷ Until now, different methods have been adopted to manipulate Mg vacancies for improving n-type Mg₃Sb₂ thermoelectrics. Mn-doping is found to increase the formation energies of vacancy at the Mg1 site, helping to stabilize the Mg atom via suppressing its evaporation during the sintering process, thus reducing the concentration of Mg vacancies.⁷⁸ Moreover, Bi-deficiency leads to the coalescence of the Mg vacancies, which reduces the scattering of charge carriers and strengthens the phonon scattering.⁷⁹

The calculated band structure reveals that Mg₃Sb₂ is an indirect semiconductor, with a band gap of ∼0.8 eV (Fig. 3c). The band gap of Mg₃Sb₂ reported by other calculation methods ranges from 0.4 to 0.7 eV.^32,53 It is observed that the valence band maximum (VBM) is located at Γ point, the center of the first Brillouin zone, which is attained from the p-orbitals of Sb. The valence band at the Γ point only offers 1 band degeneracy (N_v) in the Mg₃Sb₂ compound. In contrast, the conduction bands (CBM) involve a higher degeneracy band (N_c = 6) along the L–M direction in the Brillouin zone. In a higher carrier concentration case, a possible conduction band (N_c = 2) located at K point can participate in electronic transport, resulting in a total N_v = 8. Higher band degeneracy enables more carriers to contribute to electronic transport, thus improving the electrical conductivity without sacrificing the Seebeck coefficient. Higher N_c of conduction bands than that of valence bands helps to explain the superior thermoelectric performance of n-type Mg₃Sb₂ compounds than that of the p-type Mg₃Sb₂ (Fig. 1b). In order to tune the band edge of CBM for improving n-type Mg₃Sb₂ thermoelectrics, some ionic cations such as Yb and Ca have been considered to dissolve at the octahedral Mg sites for reducing the bonding covalency of CBM in a theoretical study.⁸⁰

Materials optimization strategy

Optimizing the Hall carrier concentration is utilized on both p-type and n-type Mg₃Sb₂ thermoelectrics. Pristine Mg₃Sb₂ shows a p-type characteristic conduction due to the unavoidable vacancies at Mg sites. However, the low Hall carrier concentration results in poor electrical performance. In order to enhance the Hall carrier concentration, different element species are doped such as Pb⁴⁸ and Bi⁴⁷ at anion sites and Ag,⁵² Li,^63–65 and Na⁵⁰ at cation sites. Co-doping Na/Zn is found to synergistically optimize the carrier concentration along with manipulation of the band structure.⁸¹ According to the calculated band structure of Mg₃Sb₂ (Fig. 3c), the electrical performance of p-type Mg₃Sb₂ is strongly dependent on Γ bands, which are dominated by the p orbitals (including p_x,y, and p_z orbitals) of Sb anions. Zn-doping leads to substitution at the Mg2 site for aligning p_x,y, and p_z orbitals, resulting in a high band degeneracy.⁸¹ Moreover, Li/Cd⁶⁵ and Na/Cd⁷² co-doping is used to synergistically optimize p-type thermoelectrics as well.

During the synthesis process of Mg₃Sb₂ samples, loss of Mg at high temperatures due to its high vapor pressure and reactivity leads to Mg₃Sb₂ generally showing p-type conduction characteristics. The addition of an optimized amount of excess Mg turns material into n-type conduction, while it is insufficient to optimize the Hall carrier concentration of Mg₃Sb₂. Various dopants reported on increasing the Hall carrier concentration of Mg₃Sb_2−xBi_x thermoelectrics with Mg excess, such as chalcogenide (S, Se, and Te)^53,59 and rare-earth elements^{39,41,66,67,82,83} (Gd, La, Tm, Ho, Nd, Ce, and Pr). Additionally, Y shows a high doping efficiency on Mg₃Sb_2−xBi_x, enabling an increased Hall carrier concentration ∼1 × 10²⁰ cm⁻³ (Fig. 4); thus, an improved zT ∼ 1.8 has been realized.⁵⁷

Fig. 4 Maximal Hall carrier concentration for n-type^{39,41,57–59,66,67,73} and p-type^{47,48,50,52,63,65,81} Mg₃Sb_2−xBi_x thermoelectrics doped with various elements at room temperature.

Bi-alloying is widely used in advancing n-type Mg₃Sb₂ thermoelectrics, of which the main purpose is to suppress the lattice thermal conductivity and inertial effective mass (). Existing publications have shown that Mg₃Sb_1.5Bi_0.5 based materials have excellent thermoelectric performance at room temperature.^55,78 Zhang et al. found that Mg₃Sb_1.5Bi_0.5 with Te-doping is promising for low- and intermediate-temperature thermoelectric applications.⁵⁵ Shu et al. reported that Mg₃Sb_1.5Bi_0.5 shows two times higher mechanical toughness than n-type Bi₂Te₃.⁷⁸ Later, Mg₃SbBi with a high average zT below 600 K was recognized as a strong candidate to replace the conventional n-type Bi₂Te₃-based thermoelectrics.⁶²

The schematic band structure, calculated band gap, weighted mobility (μ_w), lattice thermal conductivity (more precisely κ − LσT) and zT of Mg₃Sb_2−xBi_x (0 ≤ x ≤ 2) are shown in Fig. 5a–d. It is found that μ_w increases and (κ − LσT) reduces with x increasing from 0 to 1 for the Mg₃Sb_2−xBi_x compounds. The increase of μ_w could be understood by the reduced inertial effective mass ().⁴⁴ In addition, large mass/strain fluctuations of Bi/Sb alloying defects and the strong reduction in sound velocity⁴⁴ lead to a lattice thermal conductivity reduction. These result in a zT-improvement with x increasing from 0 to 1 for the Mg₃Sb_2−xBi_x compounds. When x is above 1, a reduction of zT appears mainly due to the bipolar effect induced by Bi alloying. Mg₃Bi₂ is approximately a degenerate semiconductor with slightly overlapped conduction and valence bands (Fig. 5a). This could explain why Bi-alloying leads to a reduction of the band gap (Fig. 5b), resulting in a reduction of μ_w and zT and an increase of lattice thermal conductivity with the increase of x from 1 to 2 due to the bipolar effect (Fig. 5c–e). A maximal μ_w and minimal (κ − LσT) enable a maximal zT ∼ 0.8 that appears at x = 1, which is competitive with those of Bi₂Te₃ thermoelectrics.⁴⁴

Fig. 5 (a) The schematic band structure and (b) calculated band gap⁴⁴ for Mg₃Sb_2−xBi_x alloys. (c) Weighted mobility (μ_w),^32,44 (d) lattice thermal conductivity (more precisely κ − LσT)^32,44 and (e) zT ^32,44 of Mg₃Sb_2−xBi_x (0 ≤ x ≤ 2) alloys at 300 K.

In addition to Bi-alloying and doping various elements for tuning the carrier concentration, manipulation of the carrier scattering mechanism is crucial for enhancing the thermoelectric performance of Mg₃Sb_2−xBi_x alloys, especially near room temperature. The mobility of n-type Mg₃Sb₂-based materials is firstly attributed to ionization scattering,⁸⁴ which sacrifices its mobility and limits its enhancement of thermoelectric performance as compared to the cases of pure acoustic phonon scattering. Nevertheless, Te-doped Mg₃Sb₂ single crystals without ionized impurity scattering prove that the thermally activated mobility previously observed in polycrystalline materials was caused by grain boundary resistance.⁸⁵

As shown in Fig. 6a, the Hall mobility increases with the grain size enlarging. In order to enhance the mobility, many efforts are focused on enlarging the grain size. Changing the sintering process is one way to obtain coarse-grain Mg₃Sb_2−xBi_x alloys. Shi et al. used melting and hot-deforming techniques to increase the grain size.⁵⁷ Jia et al. reported that increasing the sintering temperature enables an increase in grain size.⁴³ Optimizing the sample synthesis method is another way to enlarge grain size. Chen et al. grew coarse-grained Mg₃Bi_2−xSb_x crystals with an average grain size of ∼800 μm, leading to a high Hall carrier mobility of ∼210 cm² V⁻¹ s⁻¹.⁴² Note that changing both the temperature and time of synthesizing Mg₃Sb₂-based compounds has the effect of increasing grain sizes. Enhancing the grain size with other heat treatment techniques at various times and temperatures is also expected. Besides, other dopants such as Nb⁴⁰ and Co⁴³ have the effect of increasing the Hall mobility by grain boundary engineering strategies. Nb-doping is recognized to wet the grain boundary and then speed up the grain growth, resulting in a reduced grain boundary resistivity⁴⁰ (Fig. 6b). As shown in Fig. 6c, Co-doping enables an increased Hall mobility without enlarging the grain size, which explains that Co-doping affects the electron scattering mechanism by intentionally introducing the point defects.⁴³

Fig. 6 (a) Grain size dependent Hall mobility at 300 K for Mg₃Sb_2−xBi_x thermoelectircs.^{32,42,43,45,86–88} (b) Scanning electron microscope (SEM) image of a needle-shaped atom probe tomography (APT) specimen for Mg₃Sb₂-Nb samples.⁴⁰ (c) Temperature-dependent Hall mobility for Mg₃Sb_2−xBi_x-Te compounds with and without Co-doping.⁴³ (d) zT for Mg₃Sb_2−xBi_x thermoelectrics at 300 K.^{45,46,57,62,86,88–91} (b) Reproduced under the terms of the CC-BY Creative Commons Attribution 4.0 International License.⁴⁰ (c) Reproduced under the terms of the CC-BY Creative Commons Attribution 4.0 International License.⁴³

Combining the strategies of n_H optimization, Bi-alloying, and manipulation of the scattering mechanism, the zT at both room temperature (Fig. 6d) and high temperatures (Fig. 1b) of Mg₃Sb_2−xBi_x thermoelectrics has been significantly improved. Since 2019, the thermoelectric performance of Mg₃Sb_2−xBi_x thermoelectrics near room temperature has been a focus of research. Up to now, Mg₃Sb₂-based thermoelectrics have been recognized as one of the most promising alternative choices to Bi₂Te₃-based thermoelectrics due to its non-toxic elements and exceptional near-room temperature thermoelectric performance. Shi et al. realized a zT ∼ 0.8 at 300 K and ∼1.3 at 500 K by Sc-doping.⁸⁹ Y-doped single-crystal Mg₃Sb_2−xBi_x achieved a zT ∼ 0.8 near room temperature mainly due to high weighted mobility.⁹⁰ Besides, co-doping of elements such as Mo/Te,⁴⁵ Nb/Te,⁴⁶ Cu/Te,⁹¹ and Ti/Te⁸⁶ allowed synergetic effects on enhancing thermoelectric performance.

In addition to the high thermoelectric performance of materials for enhanced conversion efficiency and refrigeration capacity, the good thermal stability of Mg₃Sb_2−xBi_x materials is also important to ensure the long-term performance of devices. Mg₃Sb₂-based thermoelectrics are sensitive to water vapor and oxygen, and they tend to decompose spontaneously to Bi/Sb and Mg(OH)₂ in humid ambient air.⁹⁶ Such decomposition leads to Mg deficiency in the matrix, thus a significant deterioration of thermoelectric performance for both materials and devices. In order to extend the service life of Mg₃Sb₂-based thermoelectrics, different effective strategies have been adopted. By eliminating moisture in the air, the thermoelectric properties of Mg₃Sb_2−xBi_x compounds remained nearly unchanged for two months.⁹⁶ For Mg₃Sb_2−xBi_x thermoelectrics, boron nitride⁹⁵ or polydimethylsiloxane⁹⁷ was coated onto the surfaces to avoid Mg loss in humid ambient air and enhance the reliability of the devices. In addition to isolating Mg₃Sb_2−xBi_x and humid ambient air, inhibiting the formation of Mg vacancies by doping elements such as Er,⁷¹ Co,⁷¹ Mg₂B,⁹³ Mn,⁹⁴ and CrMnFeCoCu⁹² has been found to effectively improve thermal stability (Fig. 7), paving a path for further long-term device application.

Fig. 7 Long-term electronic property measurements including PF/PF₀ (a), S/S₀ (b), and σ/σ₀ for Mg₃Sb_2−xBi_x-1%Te thermoelectrics alloyed with MnFeCoCu,⁹² Co,⁷¹ Er,⁷¹ MgB₂,⁹³ Mn,⁹⁴ or coated with BN⁹⁵ at 673 K.

Mg₃Sb_2−xBi_x based thermoelectric devices

Various thermoelectric devices are fabricated to demonstrate the high-zT of Mg₃Sb_2−xBi_x compounds. Selecting a candidate contact layer for Mg₃Sb_2−xBi_x thermoelectric devices is the first and crucial step because of its possibility of impacting the thermal stability, interface resistivity, and thermal resistance of the devices. Up to now, strategies such as the high-throughput method⁹⁸ (Fig. 8a) and the calculation of the phase diagram method⁹⁹ (Fig. 8b) were used to select contact layers for Mg₃Sb_2−xBi_x thermoelectric devices, in which the coefficient of thermal expansion, shear strengths, and contact resistivity are taken into consideration. Both metal elements and alloys as contact layers for Mg₃Sb_2−xBi_x thermoelectric devices are summarized in Fig. 8c.

Fig. 8 (a) High-throughput method⁹⁸ and (b) calculation of the phase diagram method⁹⁹ for screening the Mg₃Sb_2−xBi_x contact layer. (c) Contact resistivity of several contact materials for Mg₃Sb_2−xBi_x thermoelectrics.^99–102 (a) Adapted with permission, Copyright 2022, Elsevier.⁹⁸ (b) Reproduced under the terms of the CC-BY Creative Commons Attribution 4.0 International License.⁹⁹

Several types of metal powders were mixed and then co-sintered, which could be defined as a high-throughput method for quickly investigating interfacial reaction and diffusion between pure metals and Mg₃Sb_2−xBi_x alloys (Fig. 8a). These metals start from a chemical reaction with Mg₃Sb_2−xBi_x and then diffuse into the matrix material. Rapid diffusion causes deterioration of the electrical and thermal properties of devices; thus they are not good choices for contact layers. The interfacial reaction energy and activation energy barrier calculated from first principles can help us to better understand the atomic migration, and select candidate contact layers.¹⁰³ Experimentally, Ni and Fe demonstrate relatively lower contact interface resistivity, which is the reason for being the primary choices as contact layers in many previous publications.^44,100 The general design strategy by considering the coefficient of thermal expansion, shear strengths, and contact resistivity was then adopted to select contact layers such as the Fe₇Mg₂Cr,⁹⁸ Fe₇Mg₂Ti,⁹⁸ and FeCrTiMnMg compounds.¹⁰¹ Fe₇Mg₂Cr and Fe₇Mg₂Ti as contact layers displayed a low contact resistivity (<10 μΩ cm²) after aging treatment at 723 K for 15 days.⁹⁸ An FeCrTiMnMg interface exhibits low contact resistivity of ∼4.0 μΩ cm² and high shear strength of 36.7 MPa.¹⁰¹

Another typical method of exploring candidate contact layers is from phase diagrams. Mg₂Cu is thermodynamically stable in the phase region of Mg-Mg₂Cu-Mg₃Bi_1.5Sb_0.5, making it a more reliable choice of contact layer than Fe.¹⁰⁴ Mg₂Cu with a low melting point (853 K) could be facilely hot-pressed on Mg₃Sb_2−xBi_x without sacrificing thermoelectric properties.¹⁰⁴ Later, Yin et al. explored the calculation of the phase diagram method to describe the thermodynamic properties of materials quantitatively and then screened Mg₂Ni as a thermodynamically stable contact layer for Mg₃Sb_2−xBi_x-based thermoelectric devices.⁹⁹

Mg₃Sb_2−xBi_x compounds achieve excellent thermoelectric performance at both room and high temperatures, which enables the possibility of application on both power generation and cooling. As illustrated in Fig. 9a, single leg with high thermoelectric conversion efficiency is realizable, demonstrating that the high zT attained from the materials is reliable. In addition, Mg₃Sb_2−xBi_x based thermoelectric modules are successfully fabricated and a maximal thermoelectric conversion efficiency of ∼14.5% is demonstrated in Mg₃Sb_2−xBi_x/TAGS modules.¹¹¹ It is noteworthy that the conversion efficiency of Mg₃Sb_2−xBi_x based thermoelectrics is competitive with that of commercial Bi₂Te₃ near room temperature. At higher temperatures, Mg₃Sb_2−xBi_x based thermoelectrics overpower Bi₂Te₃ thermoelectrics in terms of conversion efficiency. Additionally, the long-term stability of Mg₃Sb_2−xBi_x thermoelectric devices may be achieved by selecting reliable contact layers and improving the thermal stability of materials. Doping with MgB₂ ⁹³ leads to nearly unchanged output power in Mg₃Sb_2−xBi_x based thermoelectric single leg over time. By adopting a suitable diffusion barrier layer such as FeCrTiMnMg,¹⁰¹ the Mg₃Sb_2−xBi_x thermoelectric device also showed high reliability.

Fig. 9 (a) Temperature-dependent thermoelectric conversion efficiency^{45,86,88,91,95,99–101,105–111} and (b) cooling temperature for Mg₃Sb_2−xBi_x thermoelectrics.^{42,104,112–115}

Fig. 9b shows the cooling temperature difference achieved in Mg₃Sb_2−xBi_x based thermoelectric devices. Mao et al. investigated its cooling performance for the first time by constructing a uni-couple consisting of n-type Mg_3.2Sb_0.5Bi_1.498Te_0.002 and p-type Bi_0.5Sb_1.5Te₃, of which a maximal cooling temperature of ∼90 K was realized.¹¹³ Later, MgAgSb thermoelectrics were chosen as p-type legs subsequently for assembling Te-free modules.¹¹⁴ Noting that much effort has been devoted to advancing conversion efficiency, there is still room for improving the cooling capability of Mg₃Sb_2−xBi_x based thermoelectrics.

Plasticity and flexibility

Many plastic and flexible semiconductors such as InSe,¹¹⁷ Ag₂S,¹¹⁸ GaSe,¹¹⁹ and SnSe₂ ¹²⁰ have been uncovered and have drawn much attention due to their possible applications in wearable health monitor equipment. Among these semiconductors, only Ag₂(S, Se, and Te) and SnSe₂ exhibit the possibility of thermoelectric application near room temperature. Recently, the plasticity and flexibility of Mg₃Sb_2−xBi_x based thermoelectrics were studied.^82,92 It is found that wires of Mg₃Sb_2−xBi_x compounds obtained during the lathing are thin and twisty.⁸² Besides, compressive stress–strain measurements of Mg₃Sb_2−xBi_x show that its strain could reach ∼30% at 573 K, demonstrating promising plasticity.⁹² Zhao et al. revealed that dense gliding of dislocations is the main mechanism for the plasticity of the Mg₃Bi₂ thermoelectrics.¹²¹ Edge dislocations with Burgers vector b = c [0001] corresponding to the slip plane (1210) and edge dislocations with Burgers vector b = c/2 [0001] corresponding to the slip plane (0110) were observed, suggesting a slip of the atomic plane along the [0001] direction during plastic deformation. This led to a large tensile strain of ∼100% along the (0001) plane.¹²¹ Both in-plane and out-of-plane flexible thermoelectric devices with Cu as electrodes were fabricated and the output power is shown in Fig. 10.¹¹⁶ The interface between Cu and materials limits the high output power density of the devices; therefore, it is important to explore suitable electrodes for improving the flexible Mg₃Sb_2−xBi_x thermoelectric devices.

Fig. 10 Current-dependent voltage (V) and output power density (P/A) for in-plane and out-of-plane Mg₃Sb_0.5Bi_1.498Te_0.002 thermoelectric devices.¹¹⁶ Reproduced under the terms of the CC-BY Creative Commons Attribution 4.0 International License.¹¹⁶

Summary and outlook

Improvements in Mg₃Sb_2−xBi_x materials using strategies such as manipulation of carrier concentration, carrier scattering mechanism, and suppression of lattice thermal conductivity synergistically enable a high thermoelectric conversion efficiency and large cooling capacity. Here, we summarize the various strategies for enhancing the thermoelectric performance of Mg₃Sb_2−xBi_x materials and outline thermoelectric conversion efficiency and thermoelectric cooling capability in Mg₃Sb_2−xBi_x based devices. With competitive thermoelectric conversion efficiency and cooling temperature to those of commercial Bi₂Te₃ devices, Mg₃Sb_2−xBi_x based devices are becoming a popular option for refrigeration and power generation near room temperature. The stability of both materials and devices is crucial for the long-term service of Mg₃Sb_2−xBi_x thermoelectrics. The thermal stability of n-type Mg₃Sb_2−xBi_x materials is closely related to the Mg vacancies; an increase in Mg-vacancy formation energy enhances thermal stability. Such Mg-vacancy formation energy can be calculated using DFT, which is believed to be useful for quickly selecting doping elements for enhancing the thermal stability of Mg₃Sb_2−xBi_x materials. Additionally, the plasticity and flexibility of Mg₃Sb_2−xBi_x thermoelectrics are discussed, demonstrating its promising application in wearable devices. It is believed that this review provides insights into advancing Mg₃Sb_2−xBi_x materials and devices, which can also be useful for other thermoelectrics.

Data availability

Data are available on reasonable request from the authors.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work is supported by the Research Grants Council of Hong Kong (C7002-22Y and 17318122).

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Footnote

† These two authors contributed equally to this work and are listed alphabetically.

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