N-type Mg3Sb2 Research Articles

Enhanced thermoelectric performance of n-type Mg3.2Sb1.5Bi0.5 by rare-earth elements (Er, Tb, Tm) doping into Mg site

The n-type Mg3.2Sb1.5Bi0.5 has emerged as a promising candidate for thermoelectric materials and has attracted significant research interest. However, effective n-type dopants have been predominantly limited to chalcogens. In our work, we found that rare-earth elements (Er, Tb, Tm) are highly effective dopants for n-type Mg3.2Sb1.5Bi0.5. Through a combination of theoretical predictions and experimental validations, we demonstrate that these dopants significantly enhance the carrier concentration, leading to improved thermoelectric performance. Specifically, we employed direct ball milling and spark plasma sintering to introduce rare-earth elements at the Mg sites. Experimental results show that doping rare-earth elements in n-type Mg3.2Sb1.5Bi0.5 has a higher carrier concentration ( ≈ 8.4 × 1019 cm−3), which is higher than that of chalcogen-doped samples ( ≈ 2 × 1019 cm−3). High power factor (S2σ) ≈ 16 μW cm−1K−2 was obtained in rare-earth elements (Er, Tb, Tm) doped samples. Combined with the lowered lattice thermal conductivity due to the introduction of effective phonon scattering centers in Mg3.2−xAxSb1.5Bi0.5 (A= Er, Tb, Tm) sample. Finally, the ZT values of all doped samples reached ≈ 1.44 at 750 K. The highest peak ZT is ≈ 1.65 at 750 K for Mg3.195Er0.005Sb1.5Bi0.5. The rare-earth elements (Er, Tb, Tm) doped n-type Mg3Sb1.5Bi0.5 has comparable thermoelectric performance with the chalcogen-doped n-type Mg3Sb1.5Bi0.5, exhibiting promising potential for practical applications. This present work offers a comprehensive understanding of the effect of cation site doping improves the thermoelectric properties of n-type Mg3Sb2.

Efficient first-principles electronic transport approach to complex band structure materials: the case of n-type Mg3Sb2

We present an efficient method for accurately computing electronic scattering rates and transport properties in materials with complex band structures. Using ab initio simulations, we calculate a limited number of electron–phonon matrix elements, and extract scattering rates for acoustic and optical processes based on deformation potential theory. Polar optical phonon scattering rates are determined using the Fröhlich model, and ionized impurity scattering rates are derived from the Brooks-Herring theory. Subsequently, electronic transport coefficients are computed within the Boltzmann transport theory. We exemplify our approach with n-type Mg3Sb2, a promising thermoelectric material with a challenging large unit cell and low symmetry. Notably, our method attains competitive accuracy, requiring less than 10% of the computational cost compared to state-of-the-art ab initio methods, dropping to 1% for simpler materials. Additionally, our approach provides explicit information on individual scattering processes, offering an alternative that combines efficiency, robustness, and flexibility beyond the commonly employed constant relaxation time approximation with the accuracy of fully first-principles calculations.

Inhibiting Mg Diffusion and Evaporation by Forming Mg-Rich Reservoir at Grain Boundaries Improves the Thermal Stability of N-Type Mg3 Sb2 Thermoelectrics.

N-type Mg3 Sb2 -based thermoelectric materials show great promise in power generation due to their mechanical robustness, low cost of Mg, and high figure of merit (ZT) over a wide range of temperatures. However, their poor thermal stability hinders their practical applications. Here, MgB2 is introduced to improve the thermal stability of n-type Mg3 Sb2 . Enabled by MgB2 decomposition, extra Mg can be released into the matrix for Mg compensation thermodynamically, and secondary phases of Mg─B compounds can kinetically prevent Mg diffusion along grain boundaries. These synergetic effects inhibit the formation of Mg vacancies at elevated temperatures, thereby enhancing the thermal stability of n-type Mg3 Sb2 . Consequently, the Mg3.05 (Sb0.75 Bi0.25 )1.99 Te0.01 (MgB2 )0.03 sample exhibits negligible variation in thermoelectric performance during the 120-hour continuous measurement at 673 K. Moreover, the ZT of n-type Mg3 Sb2 can be maintained by adding MgB2 , reaching a high average ZT of ≈1.1 within 300-723K. An eight-pair Mg3 Sb2 -GeTe-based thermoelectric device is also fabricated, achieving an energy conversion efficiency of ≈5.7% at a temperature difference of 438 K with good thermal stability. This work paves a new way to enhance the long-term thermal stability of n-type Mg3 Sb2 -based alloys and other thermoelectrics for practical applications.

Mechanical Compatibility between Mg3(Sb,Bi)2 and MgAgSb in Thermoelectric Modules.

Thermoelectric (TE) modules are exposed to temperature gradients and repeated thermal cycles during their operation; therefore, mechanically robust n- and p-type legs are required to ensure their structural integrity. The difference in the coefficients of thermal expansion (CTEs) of the two legs of a TE module can cause stress buildup and the deterioration of performance with frequent thermal cycles. Recently, n-type Mg3Sb2 and p-type MgAgSb have become two promising components of low-temperature TE modules because of to their high TE performance, nontoxicity, and abundance. However, the CTEs of n-Mg3Sb2 and p-MgAgSb differ by approximately 10%. Furthermore, the oxidation resistances of these materials at increased temperatures are unclear. This work manipulates the thermal expansion of Mg3Sb2 by alloying it with Mg3Bi2. The addition of Bi to Mg3Sb2 reduces the coefficient of linear thermal expansion from 22.6 × 10-6 to 21.2 × 10-6 K-1 for Mg3Sb1.5Bi0.5, which is in excellent agreement with that of MgAgSb (21 × 10-6 K-1). Furthermore, thermogravimetric data indicate that both Mg3Sb1.5Bi0.5 and MgAgSb are stable in air and Ar at temperatures below ∼570 K. The results suggest the compatibility and robustness of Mg3Sb1.5Bi0.5 and MgAgSb as a pair of thermoelectric legs for low-temperature TE modules.

Realizing Excellent Structural and Thermoelectric Performance in Mg3Sb2-Based Alloys by Manipulating Mg Intrinsic Migration Kinetics with Interstitial Ni Doping

N-type Mg3Sb2 is attracting increasing focus for its outstanding room-temperature (RT) thermoelectric (TE) performance; however, achieving reliable n-type conduction remains challenging due to negatively charged Mg vacancies. Doping with compensation charges is generally used but does not fundamentally resolve the high intrinsic activity and easy formation of Mg vacancies. Herein, a robust structural and thermoelectric performance is obtained by manipulating Mg intrinsic migration activity by precisely incorporating Ni at the interstitial site. Density functional theory (DFT) indicates that a strong performance originates from a significant thermodynamic preference for Ni occupying the interstitial site across the complete Mg-poor to -rich window, which dramatically promotes the Mg migration barrier and kinetically immobilizes Mg. As a result, the detrimental vacancy-associated ionized scattering is eliminated with a leading room-temperature ZT up to 0.85. This work reveals that interstitial occupation in Mg3Sb2-based materials is a novel approach promoting both structural and thermoelectric performance.

Enhanced Thermoelectric Efficiency in P-Type Mg3Sb2: Role of Monovalent Atoms Codoping at Mg sites

Due to natural abundance, low cost, and compatibility with sustainable green technology, Mg3Sb2-based Zintl compounds are comprehensively explored as potential thermoelectric materials for near-room temperature applications. The effective use of these materials in thermoelectric devices requires both p and n-type Mg3Sb2 having comparable thermoelectric efficiency. However, p-type Mg3Sb2 has inferior thermoelectric efficiency efficiency compared to its n-type counterpart due to low electrical conductivity . Here, we show that codoping of monovalent atoms (Li-Ag, and Na-Ag) at the Mg site of Mg3Sb2 produces a synergistic effect and boosts the electrical conductivity, which enhances the thermoelectric properties of p-type Mg3Sb2. While, Ag prefers to occupy the Mg2 site, Li and Na are favorable at the Mg1 site of Mg3Sb2 lattice. Compared to Li-Ag codoping, Na-Ag codoping in Mg3Sb2 is found to be more effective for increasing the charge carrier concentration and significantly augmenting the electrical conductivity. The dominance of the three-phonon scattering mechanism in Li and Li-Ag doped Mg3Sb2 and the four-phonon scattering process for the Na and Na-Ag doped Mg3Sb2 are confirmed. Due to the simultaneous increase in electrical conductivity and decrease in thermal conductivity, the zT value ∼0.8 at 675 K achieved for Mg2.975Na0.02Ag0.005Sb2 is the highest value among p-type Mg3Sb2. Our work shows a constructive approach to enhance the zT of p-type Mg3Sb2 via monovalent atoms codoping at the Mg sites.

First-principles study of O2 and H2O adsorption on the Mg3Sb2(10-11) surface.

The degeneration of its thermoelectric properties in air is one of the factors that limit the practical application of high-performance n-type Mg3Sb2. In this work, first-principles calculations are conducted to study the adsorption of O2 and H2O on the Mg3Sb2(10-11) surface, as well as the effect of two strategies based on a terminating atom and Al doping on the adsorption performance. The calculated results show that the adsorbates prefer to adsorb on the bridge site of the surface because of the interaction with the outermost Mg atom. Sb termination can weaken the adsorption actions, due to the decreased interaction between the adsorbates and Mg atoms resulting from the decreased O-Mg bond strength. In addition, Al doping makes O2 prefer to interact with an Al atom rather than a Mg atom, which is beneficial for reducing Mg loss and thus improving the performance stability. This work aims to provide insight into improving the thermoelectric performance stability of Mg3Sb2-based materials.

The Importance of Mg–Sb Interactions in Achieving High Conduction Band Degeneracy in Mg3Sb2 for High n-Type Thermoelectric Performance

In recent years, Mg3Sb2 has attracted much attention due to its excellent performance as an n-type thermoelectric material. The high performance of n-type Mg3Sb2 is largely due to the high valley degeneracy (NV) of the electron transport edge, which is the result of the conduction band minimum (CBM) being located at a low-symmetry, six-fold degenerate point (inside the ΓALM plane) in the first Brillouin zone. Furthermore, Bi alloying on the Sb site is well-known to improve electron mobility, and hence, performance. In previous works, this low-symmetry CBM has been attributed to interactions between the tetrahderally and octahedrally coordinated Mg-sites. However, these Mg–Mg interactions cannot explain changes in the shape of the CBM upon Bi alloying, while Mg–Sb (and Mg–Bi) interactions could explain them. Prior studies have neglected to fully consider the importance of the Sb orbitals in the existence of the low-symmetry CBM because at the CBM, there is very little contribution from the Sb orbitals. In this work, we use first-principles density functional theory and tight-binding calculations to show that the low-symmetry CBM is due to Mg-s–Sb-p interactions that are weakest at or near the CBM. Controlling the cation-s–anion-p interactions through anion- or cation-site alloying can be used as an engineering strategy to optimize thermoelectric performance.

Suppression of Interfacial Diffusion in Mg3Sb2 Thermoelectric Materials through an Mg4.3Sb3Ni/Mg3.2Sb2Y0.05/Mg4.3Sb3Ni-Graded Structure.

The Zintl compound, n-type Mg3Sb2, has been extensively investigated as a promising thermoelectric material. However, performance degradation caused by the loss of Mg element during device preparation and service is a main disadvantage in its utilization in thermoelectric devices. To suppress volatilization, diffusion, or reaction of Mg, we designed a graded concentration junction to control the interfacial elemental diffusion and improve the stability of the thermoelectric joint. We utilized the reaction product at the Ni/Mg3.2Sb2Y0.05 interface, the phase Mg4.3Sb3Ni, as a barrier layer material, and prepared Mg4.3Sb3Ni/Mg3.2Sb2Y0.05/Mg4.3Sb3Ni junctions. The results show that the interface behavior of the thermoelectric junction is optimized by the gradation of elemental concentration, thermal expansion coefficient, and work function. The Mg4.3Sb3Ni/Mg3.2Sb2Y0.05/Mg4.3Sb3Ni single-leg device showed high thermal stability at 673 K for 20 days, the contact resistance was stable at around 10 μΩ cm2, and the shear strength was maintained at about 20 MPa. The conversion efficiency of its single-leg device maintains nearly 90% of the best performance after aging at 673 K for 20 days.

Significantly enhanced thermoelectric figure of merit of n-type Mg3Sb2-based Zintl phase compounds via co-doped of Mg and Sb site

N-type Mg3Sb2 thermoelectric material has a complex crystal structure and electronic structure, which is a new thermoelectric material with excellent application prospects. The state density and band structure of Mg24Sb16 and Bi/Se/Pr–Se/Nd–Se doped Mg24Sb16 are first-principles calculated. Se/Pr–Se/Nd–Se doped increases the density of states around the Fermi level. Bi-doped does not impact the band structure because the electronic structure between Sb and Bi are similar. The band gap of Pr–Se/Nd–Se doped becomes narrower, which is more conducive to carrier transport and increases carrier concentration than that of single-doped Se. The experimental results show that Pr and Nd replace Mg to provide more electrons for the system. The carrier concentration of the co-doped sample increases significantly, thus improving the power factor and optimizing the samples' electrical properties. The highest ZT value of the Mg3.2Sb1.5Bi0.49Se0.01 sample is 1.21. Pr–Se/Nd–Se co-doped ZT values are 1.67 and 1.74, respectively, 38% and 43% higher than the Mg3.2Sb1.5Bi0.49Se0.01 sample. Multi-element doped is an effective strategy to improve the thermoelectric properties of materials.

Large improvement in thermoelectric performance of pressure-tuned Mg3Sb2.

The Mg3Sb2-based Zintl compound is a promising candidate for a high-performance thermoelectric material with the advantage of the component elements being low cost, non-toxic and earth-abundant. Here, we investigate the influence of pressure on the electronic structure and p-type and n-type thermoelectric transport properties of Mg3Sb2 by using density functional theory and Boltzmann transport theory. The energy gaps first increase and then decrease with the increasing of pressure, and a peak value of the valley degeneracy of conduction band occurs at 4 GPa. Based on the calculated band structures, the zT (figure of merit) values of p-type Mg3Sb2 under pressure are significantly enhanced, which predominantly originates from the boosted PF (power factor) contributed by the increased carrier's relaxation time. When the carrier concentration reaches 1 × 1020 cm−3, the PF of p-type Mg3Sb2 at 4 GPa is increased by 35% relative to that of the compound at 0 GPa, thus leading to a considerably improved zT of ∼0.62 at 725 K. Under the same conditions, due to the increased density of states effective mass, the n-type Mg3Sb2 exhibits a highest PF of ∼19 μW cm−1 K−2 and a peak zT of 1.7. Therefore, pressure tuning is an effective method to improve the p-type and n-type thermoelectric transport performance of Mg3Sb2-based Zintl compounds. This work on Mg3Sb2 under pressure may provide a new mechanism for the experimenters towards the enhancement of the thermoelectric performance of materials.

The challenge of tuning the ratio of lattice/total thermal conductivity toward conversion efficiency vs power density

Minimizing the lattice thermal conductivity of thermoelectric materials is essential for preserving the temperature difference during the operation of thermoelectric devices incorporating these materials. During the past two decades, there has been substantial improvement in the thermoelectric figure of merit (zT) due to reduced lattice thermal conductivity. Employing alloying effects in solid-solution compounds is the most common and practical approach for inhibiting lattice thermal conductivity. This Perspective takes the n-type Mg3Sb2−xBix thermoelectric alloys as examples, addressing their lattice thermal conductivity and corresponding zT as functions of their Bi concentration. Additionally, we seek to understand the effect of the lattice contribution to total thermal conductivity for most thermoelectric materials currently being researched. The lattice/total thermal conductivity ratio at the temperature corresponding to the peak zT shows weak material dependence, widely ranging from 0.5 to 0.75, which implies that the lattice thermal conductivity of most thermoelectric materials can be decreased further to improve thermoelectric performance. On the other hand, thermoelectric materials with relatively low ratios exhibit high power factors in their operating temperature ranges, which is ascribed to their excellent electrical performance. These observations provide guidelines to tune transport properties for future applications in thermoelectric power generation.

Realizing Cd and Ag codoping in p-type Mg3Sb2 toward high thermoelectric performance

Mg3Sb2 has attracted intensive attention as a typical Zintl-type thermoelectric material. Despite the exceptional thermoelectric performance in n-type Mg3Sb2, the dimensionless figure of merit (zT) of p-type Mg3Sb2 remains lower than 1, which is mainly attributed to its inferior electrical properties. Herein, we synergistically optimize the thermoelectric properties of p-type Mg3Sb2 materials via codoping of Cd and Ag, which were synthesized by high-energy ball milling combined with hot pressing. It is found that Cd doping not only increases the carrier mobility of p-type Mg3Sb2, but also diminishes its thermal conductivity (κtot), with Mg2.85Cd0.5Sb2 achieving a low κtot value of ∼0.67 W m−1 K−1 at room temperature. Further Ag doping elevates the carrier concentration, so that the power factor is optimized over the entire temperature range. Eventually, a peak zT of ∼0.75 at 773 K and an excellent average zT of ∼0.41 over 300 − 773 K are obtained in Mg2.82Ag0.03Cd0.5Sb2, which are ∼240% and ∼490% higher than those of pristine Mg3.4Sb2, respectively. This study provides an effective pathway to synergistically improve the thermoelectric performance of p-type Mg3Sb2 by codoping Cd and Ag, which is beneficial to the future applications of Mg3Sb2-based thermoelectric materials.

Nearly isotropic transport properties in anisotropically structured n-type single-crystalline Mg3Sb2

As a top candidate for competing with the conventional n-Bi2Te3 thermoelectrics, Mg3Sb2-based materials have attracted increasing attentions for low-grade (<300 °C) waste heat recovery applications, due to the high thermoelectric performance, low cost, abundance and nontoxicity. Because of its anisotropic crystal structure as that of Bi2Te3, possible property anisotropy and the resultant similar requirement of texturing for preferential performance remain not entirely clear Mg3Sb2-thermoelectrics at working temperatures. This motivates the current work to focus on the transport-property anisotropy of n-type Mg3Sb2, centimeter-sized single crystals of which are successfully grown by a flux-assisted vertical Bridgman technique with a post-annealing under a Mg vapor pressure. This enables a revelation of nearly isotropic transport properties in this anisotropically structured material, guaranteeing reasonably high performances in polycrystalline materials achievable by cheap and scalable processing approaches such as powder-metallurgy. In addition, this work uncovers the strong lattice anharmonicity and large band degeneracy are respectively the origins for the low lattice thermal conductivity and superior electronic performance.

Theoretical Study on Thermoelectric Properties and Doping Regulation of Mg3X2 (X = As, Sb, Bi)

For searching both high-performances and better fits for near-room temperature thermoelectric materials, we here carried out a theoretical study on thermoelectric properties and doping regulation of Mg3X2 (X = As, Sb, Bi) by the combined method of first principle calculations and semi-classical Boltzmann theory. The thermoelectric properties of n-type Mg3As2, Mg3Sb2, and Mg3Bi2 were studied, and it was found that the dimensionless figures of merit, zT, are 2.58, 1.38, 0.34, and the p-type ones are 1.39, 0.64, 0.32, respectively. Furthermore, we calculated the lattice thermal conductivity of doped structures and screened out the structures with a relatively low formation energy to study the phonon dispersion and thermal conductivity in Mg3X2 (X = As, Sb, Bi). Finally, high thermoelectric zT and ultralow thermal conductivity of these doped structures was discussed.

Improved Thermoelectric Properties of N-Type Mg3Sb2 through Cation-Site Doping with Gd or Ho.

The success of n-type doping has attracted strong research interest for exploring effective n-type dopants for Mg3Sb2 thermoelectrics. Herein, we experimentally study Gd and Ho as n-type dopants for Mg3Sb2 thermoelectrics. The synthesis, structural characterization, and thermoelectric properties of Gd-doped, Ho-doped, (Gd, Te)-codoped, and (Ho, Te)-codoped Mg3Sb2 samples are reported. It is found that Gd and Ho are effective n-type cation-site dopants showing a higher doping efficiency as well as a superior carrier concentration in comparison with anion-site doping with Te, consistent with the previous theoretical prediction. For n-type Mg3Sb2 samples doped with Gd or Ho, optimal thermoelectric figure of merit zT values of ∼1.26 and ∼0.94 at 725 K are obtained, respectively, in Mg3.5Gd0.04Sb2 and Mg3.5Ho0.04Sb2, which are superior to many reported Te-doped Mg3Sb2 without alloying with Mg3Bi2. By codoping with Gd (or Ho) and Te, reduced thermal conductivity and enhanced power factor values are achieved at high temperatures, which results in enhanced peak zT values well above unity at 725 K. This work reveals Gd and Ho as effective n-type dopants for Mg3Sb2 thermoelectric materials.

Monolayer square-Ag2X (X = S, Se): Excellent n-type thermoelectric materials with high power factors

Thermoelectric devices based on thermoelectric materials, which can be utilized to transform heat to electric energy, are regarded as the clean and reliable energy-harvestings that reduce greenhouse gas and noxious gas missions. In this work, thermoelectric properties of square-Ag2X (X = S, Se) monolayers have been systematically studied through uniting Boltzmann transport theory and first-principles. The results show that square-Ag2X (X = S, Se) monolayers possess ultrahigh electrical conductivities due to their ultralight electron effective masses, together with high Seebeck coefficients, resulting in ultrahigh power factors (0.014 and 0.018 W/m K2). Moreover, phonon transport calculations unveil that square-Ag2X (X = S, Se) monolayers exhibit low lattice thermal conductivities (3.46 and 2.33 W/m K) at 300 K, which can be traced back to their small Debye temperature, low acoustic group velocities, short phonon relaxation times and large Grüneisen parameters. Accordingly, the obtained figure of merit of monolayer square-Ag2Se for optimal n-type doping at 700 K can approach 2.81, which is larger than the well-known n-type thermoelectric monolayer SnSe (2.32) and Mg3Sb2 (2.21). Therefore, the excellent thermoelectric properties of square-Ag2Se monolayer along with its all-round stability verified by the phonon dispersion, ab initio molecular dynamic simulation and accordance of Born–Huang criteria highlight its prominent potential for thermoelectric applications.

Thermoelectric properties of yttrium-doped Mg3(Sb,Bi)2 synthesized by melting method

A thermoelectric compound of n-type Mg3Sb2 is a promising candidate as an element of practical thermoelectric modules. We established a low-cost synthesis-procedure for Y-doped Mg3(Sb,Bi)2 that can be easily scaled up for mass production. Samples of Mg3+δYxSb1.5Bi0.5 were synthesized via a melting method using a stainless-steel container for sealing the melt, and their thermoelectric properties depending on the Mg and Y concentrations were characterized. The highest dimensionless figure-of-merit (ZT) was obtained with the Mg3.3Y0.02Sb1.5Bi0.5 sample, which exhibited a value of ZT = 1.16 at T = 759 K.

Improved thermoelectric performance of n-type Mg3Sb2–Mg3Bi2 alloy with Co element doping

n-type Mg3Sb2–Mg3Bi2 alloy shows as a potential new thermoelectric material (TE) and has been widely researched recently. The pure phase n-type Mg3·20(Sb0·3Bi0.7)1.99Te0.01 were prepared by adjusting Mg content with the Bi impurity phase being effectively suppressed. Then, Co element was doped into the pure phase and the electrical conductivity of samples were improved. With a high power factor of 20.3 μW cm−1K−2 for Mg3·185Co0·015(Sb0·3Bi0.7)1.99Te0.01 at 525 K. Additionally, it was found that the phonon scattering is enhanced due to the larger atomic mass of Co comparing to Mg and the lattice thermal conductivity is reduced. As a result, a high ZT value of ~ 1.03 at 525 K is achieved for the Mg3·185Co0·015(Sb0·3Bi0.7)1.99Te0.01.

Enhanced Average Thermoelectric Figure of Merit of p-Type Zintl Phase Mg2ZnSb2 via Zn Vacancy Tuning and Hole Doping.

Isoelectronic Zn substitution at the Mg site has been proved to be effective in regulating the carrier concentration of p-type Mg3Sb2 Zintl phase. However, the reported thermoelectric performance is still unsatisfactory compared with that of n-type Mg3Sb2 due to the poor electrical transport properties. Here, we report an enhanced average ZT through improving low-temperature ZTs by introducing Zn vacancy followed suppressing the bipolar effect by doping. First, the Zn vacancy simultaneously increases the power factor and decreases the thermal conductivity, leading to a peak ZT value of ∼0.52 at 773 K in Mg2Zn0.98Sb2. Additionally, doping Li or Ag at the Mg site is identified as a high-efficiency strategy for further increasing the carrier concentration and hence suppressing the bipolar effect. Finally, a peak ZT of ∼0.73 at 773 K and an average ZT of ∼0.46 between 300 and 773 K were obtained in Mg1.98Li0.02Zn0.98Sb2.