![(a) zT comparison of n-type Mg3.2Nd0.03Sb1.5Bi0.5 and other n-type Mg3Sb2-based thermoelectric materials [14,18,25,26,28,29,31]. (b) Mg/Nd/Pr/Y/Chalcogen doping content dependence of power factor of Mg3Sb2-based compounds at 725 K [17,18,25,26,29,31]. The blue squares are Nd doped samples from this work.](https://www.researchgate.net/publication/343825746/figure/fig1/AS:927894872272898@1598238785654/a-zT-comparison-of-n-type-Mg32Nd003Sb15Bi05-and-other-n-type-Mg3Sb2-based_Q320.jpg)
![(a) Temperature dependence of Hall carrier concentration of n-type Mg3.2NdxSb1.5Bi0.5 (x = 0.01, 0.02, 0.03 and 0.04) samples. The pink stars are the predicted maximal achievable carrier concentration of n-type Nd-doped Mg3Sb2 under Mg-rich condition using first-principles defect calculations with density functional theory (DFT). (b) Defect formation energies of charged point defects as a function of Fermi level in Mg3Sb2 for n-type doping with Nd under Mg-rich condition. The shade region represents the conduction band. The green vertical line represents the Fermi level at 700 K. (c) Mg/Nd/Pr/La/Chalcogen doping content dependence of carrier concentration of Mg3Sb2-based compounds at 300 K [18,25,26,28,29,31]. The blue squares are Nd doped samples from this work.](https://www.researchgate.net/publication/343825746/figure/fig2/AS:927895144902658@1598238850246/a-Temperature-dependence-of-Hall-carrier-concentration-of-n-type-Mg32NdxSb15Bi05-x_Q320.jpg)
![Temperature dependence of (a) total thermal conductivity, (b) Lorenz number, (c) electronic thermal conductivity and (d) lattice thermal conductivity of n-type Mg3.2NdxSb1.5Bi0.5 samples. In (d), the black short dash line represents the previously reported Cahill’s minimum lattice thermal conductivity of Mg3Sb1.5Bi0.5 [29].](https://www.researchgate.net/publication/343825746/figure/fig5/AS:927895325265921@1598238893960/Temperature-dependence-of-a-total-thermal-conductivity-b-Lorenz-number-c_Q320.jpg)
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Point defect engineering and machinability in n-type Mg3Sb2-based materials
Abstract and Figures
Approaching practical thermoelectric devices require high-performance and machinable thermoelectric materials. However, the currently available materials are usually brittle. In this work, Nd-doped Mg3Sb2-based compounds exhibit not only excellent thermoelectric performance but also superior machinability. Mg3.2Nd0.03Sb1.5Bi0.5 exhibits a high power factor of 20.6 μW cm-1 K-2 at 725 K and a peak zT of 1.8, which mainly originates from the increased n of ~8 × 1019 cm-3 by Nd/Mg substitutional defects. Defect calculations predict that other rare earth elements (Sm, Gd, Tb, Dy and Ho) have the same effect as Nd on Mg3Sb2 and the predicted highest achievable electron concentrations at 700 K are ~1020 cm-3 . The measured hardness, Young's modulus and fracture toughness of Mg3.2Nd0.03Sb1.5Bi0.5 are 1.1 GPa, 49.8 GPa and 1.4 MPa m1/2 , respectively. In addition, the sample can be easily machined into the dog-bone shape with external thread at both ends, indicating the excellent machinability of Mg3Sb2-based materials. This work suggests a bright future of Mg3Sb2-based thermoelectric materials for practical applications and device fabrication.
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... 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 Mg 3 Sb 2 . Various dopants reported on increasing the Hall carrier concentration of Mg 3 Sb 2−x Bi 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 Mg 3 Sb 2−x Bi x , enabling an increased Hall carrier concentration ∼1 × 10 20 cm −3 (Fig. 4); thus, an improved zT ∼ 1.8 has been realized. ...
... Recently, the plasticity and flexibility of Mg 3 Sb 2−x Bi x based thermoelectrics were studied. 82,92 It is found that wires of Mg 3 Sb 2−x Bi x compounds obtained during the lathing are thin and twisty. 82 Besides, compressive stress-strain measurements of Mg 3 Sb 2−x Bi x show that its strain could reach ∼30% at 573 K, demonstrating promising plasticity. ...
... 82,92 It is found that wires of Mg 3 Sb 2−x Bi x compounds obtained during the lathing are thin and twisty. 82 Besides, compressive stress-strain measurements of Mg 3 Sb 2−x Bi x show that its strain could reach ∼30% at 573 K, demonstrating promising plasticity. 92 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. ...
Effective strategies such as manipulation of carrier scattering mechanism, introduction of phonon scattering sources, and optimization of interface layer for improving Mg 3 Sb 2 thermoelectric materials and devices are summarized.
... Herein, we demonstrate ADP characterization for thermoelectric materials based on ultrafast spectroscopy of coherent acoustic phonon (CAP) dynamics. Mg 3 Sb 2 , an attractive thermoelectric material due to ideal performance [12][13][14][15] and intriguing transport properties [16,17], is selected as the modeling system because carrier-acoustic phonon scattering is a key concern in its development [18][19][20] and its nearly isotropic structure [21] facilitates the phonon dynamics analysis. Through the relation between the amplitude of the CAP-induced oscillatory transient signal and the excitation photon energy, we show that the ADP mechanism and the thermoelastic (TE) effect [22,23] contribute comparably to CAP generation. ...
... The amplitude of the oscillatory signal A osc then follows Eq. (14). ...
... This analytic expression with diffusion neglected facilitates the discussion on the role of E pump and will be modified later. According to Eq. (14), when the probe parameters are fixed, A osc is directly related to η 0 . The contributions have different expressions for the ADP mechanism η 0,ADP and the TE effect η 0,TE because the former is proportional to the excited carrier number density while the latter to the kinetic energy density within the carriers. ...
Acoustic deformation potential (ADP) quantifies carrier-acoustic phonon coupling and is essential for dissecting transport physics in thermoelectrics. Herein, we report the use of ultrafast spectroscopy of coherent acoustic phonons (CAPs) to characterize the ADP of thermoelectric materials, using Mg3Sb2 as an example. The photon energy-dependent amplitudes of the CAP-induced oscillatory reflectance were used to determine the ADP coupling constant, agreeing well with that from first-principles calculations. This method relies on the transient Coulombic interaction between carriers and acoustic phonons, free of influence from other scattering channels. It is shown that the method is particularly feasible for the study of thermoelectric materials, because their common features of strong phonon anharmonicity and small band gaps make the measurement insensitive to the uncertainty of carrier diffusion coefficients, ensuring its accuracy.
... The synergic effects of reduced bandgap, increased band curvature, strengthened phonon scattering, and enhanced grain size by Bi alloying make Mg 3 Sb 2-x Bi x (x = 1.4-1.75) exhibit zT values above 0.6 at room temperature 34,35 , which approach to that of the state-of-the-art n-type Bi 2 (Te, Se) 3 and are much superior to the plastic Ag 2 (Te, Se, S) and organic TE materials 31,[36][37][38][39] . ...
... However, these advancements have predominantly focused on rigid TE modules, neglecting the potential application of Mg 3 Sb 2-x Bi x in flexible electronics. Given its notable TE performance and potential plasticity at room temperature 36 , there exists a compelling opportunity to explore the suitability of Mg 3 Sb 2-x Bi x for power generation in flexible electronics. Achieving simultaneous high TE performance and plasticity in Mg 3 Sb 2-x Bi x remains a challenge. ...
Low-cost thermoelectric materials with simultaneous high performance and superior plasticity at room temperature are urgently demanded due to the lack of ever-lasting power supply for flexible electronics. However, the inherent brittleness in conventional thermoelectric semiconductors and the inferior thermoelectric performance in plastic organics/inorganics severely limit such applications. Here, we report low-cost inorganic polycrystalline Mg3Sb0.5Bi1.498Te0.002, which demonstrates a remarkable combination of large strain (~ 43%) and high figure of merit zT (~ 0.72) at room temperature, surpassing both brittle Bi2(Te,Se)3 (strain ≤ 5%) and plastic Ag2(Te,Se,S) and organics (zT ≤ 0.4). By revealing the inherent high plasticity in Mg3Sb2 and Mg3Bi2, capable of sustaining over 30% compressive strain in polycrystalline form, and the remarkable deformability of single-crystalline Mg3Bi2 under bending, cutting, and twisting, we optimize the Bi contents in Mg3Sb2-xBix (x = 0 to 1) to simultaneously boost its room-temperature thermoelectric performance and plasticity. The exceptional plasticity of Mg3Sb2-xBix is further revealed to be brought by the presence of a dense dislocation network and the persistent Mg-Sb/Bi bonds during slipping. Leveraging its high plasticity and strength, polycrystalline Mg3Sb2-xBix can be easily processed into micro-scale dimensions. As a result, we successfully fabricate both in-plane and out-of-plane flexible Mg3Sb2-xBix thermoelectric modules, demonstrating promising power density. The inherent remarkable plasticity and high thermoelectric performance of Mg3Sb2-xBix hold the potential for significant advancements in flexible electronics and also inspire further exploration of plastic inorganic semiconductors.
... Additionally, these compounds are low-cost, environmentally friendly, and mechanically robust, making them distinctly potential substitutes for commercial Bi 2 Te 3 -based alloys in large-scale applications [9]. To further enhance the thermoelectric performance of the n-type Mg 3 Sb 2 -based materials, different dopants (Te [10][11][12][13], Se [14], La [15,16], Y [17,18], Nd [19], etc.) have been theoretically or experimentally explored to optimize their carrier concentration for achieving a higher zT. Furthermore, it has been shown that tuning the carrier scattering mechanism of the Mg 3 Sb 2 -based thermoelectric materials by introducing transition metals [20,21] or manipulating the grain size [22,23] can significantly enhance their carrier mobility near room temperature and thus raise their average zT to ~1.2 over the temperature range between 300 K and 773 K [24]. ...
... Moreover, as shown in Figure S2, Mg 3.1 Mn 0.1 Sb 1.5 Bi 0.49 Te 0.01 exhibits higher mid-temperature-range zT values than most other state-of-the-art n-type thermoelectric materials. Considering its low cost [45] and outstanding mechanical robustness [19,42], Mg 3.1 Mn 0.1 Sb 1.5 Bi 0.49 Te 0.01 is a highly competitive candidate for medium-temperature thermoelectric power generation applications. ...
... Recently, the n-type Mg 3 Sb 2 compound has garnered significant attention because of its unique electronic band structure, characterized by sixfold degeneracies in the M and L bands and twofold degeneracies in the K band [16]. This compound is particularly attractive for practical applications owing to its low-cost components and favorable mechanical properties [17,18]. However, a critical challenge lies in the high vapor pressure of magnesium, which results in uncontrollable electron "killer" Mg vacancies (V Mg ″ ) in Mg 3 Sb 2 [19]. ...
Improving the power factor (PF) of thermoelectric materials is crucial for increasing the output power density and broadening practical applications. The near-room-temperature electrical performance of Mg3(Sb,Bi)2-based alloys is hindered due to the presence of Mg vacancies and grain boundary scattering, resulting in a lower power factor. In this study, we introduced excess Mg into Mg3(Sb,Bi)2 alloy during the hot-pressing process, triggering a liquid phase sintering process, which can effectively fill the Mg vacancies and increase the average grain size (Dave) to significantly reduce grain boundary scattering. This leads to enhanced room-temperature electrical conductivity (σ) without detrimental effects on the Seebeck coefficient (S), thus yielding a high average PF of ~25.3 μW·cm⁻¹·K⁻² and an average figure of merit (ZT) of ~1.03 within the temperature range of 323‒623 K. Moreover, different amounts of W were further added, and density-functional theory (DFT) calculations reveal that W segregation at grain boundaries (GBs) reduces interfacial potential barriers, leading to improved S and σ. Consequently, an ultrahigh average PF of ~26.2 μW·cm⁻¹·K⁻² was attained in the W0.06Mg3.2Sb1.5Bi0.49Te0.01–4% Mg alloys. Additionally, the mechanical properties (Vickers hardness and fracture toughness) were also enhanced compared with those of the pristine Mg3(Sb,Bi)2 alloy. This dual-modified approach can significantly boost the TE performance and mechanical stability, advancing Mg3(Sb,Bi)2-based materials for practical applications.
... The plasticity was initially manifested in Mg 3.2 Nd 0.03 Sb 1.5 Bi 0.5 , which could be easily machined to the dog-bone shape with external thread at both ends, and thin and winding wires were obtained during the lathing. 57 Later, the compressive stress-strain curve of TM 0.01 Mg 3 Sb 1.5 Bi 0.5 (TM ¼ CrMnFeCoCu) was obtained by mechanical test, quantifying the plasticity. 58 Benefiting from the decent plasticity, cold-deformation was applied to improve TE performance of Mg 3 (Sb,Bi) 2 -based compounds by enhancing texture, but the room temperature zT was still inferior. ...
The rapid growth of wearable electronics, health monitoring, and the Internet of Things has created a tremendous demand for flexible semiconductors and gadgets. Thermoelectric (TE) semiconductors that enable direct conversion between heat and electricity have been utilized as power generators, but their intrinsic brittleness inhibits the application for powering flexible/wearable electronics. The plastic inorganic semiconductors discovered in recent years offer a different option for flexible TE technologies when these materials combine good plasticity and high TE performance at room temperature. In this Perspective, we discuss how room-temperature plasticity affects the manufacturing process and real-world uses in flexible electronics, the trade-off between plasticity and thermoelectric performance, and the underlying deformation mechanisms. Further outlook on the discovery of viable plastic inorganic semiconductors is proposed.
... It had long been considered a p-type semiconductor with undistinguished performance, but Te-doped n-type Mg 3 (Sb, Bi) 2 -based alloys were recently reported to show high thermoelectric performance, demonstrating good potential for both power generation and cooling applications (Tamaki et al. 2016;Zhang et al. 2017). Subsequently, various strategies, including tuning the carrier scattering mechanism Shuai et al. 2017b), defect engineering Li et al. 2019Li et al. , 2020a and doping (Shi et al. , 2019cShu et al. 2019), have been successfully applied in Mg 3 Sb 2 -based alloys, achieving a state-of-the-art average zT value up to 1.1 in the range of 300~500 K (Imasato et al. 2019a;Shi et al. 2019a;Shang et al. 2020a;Wood et al. 2019). ...
... Such a characteristics has been well studied in many typical thermoelectric materials, PbTe, [5] SnTe, [6] Skutterudites, [7] Mg 3 Sb 2 . [8] Previous studies also reveal an interesting correlation between decreasing k lat and increasing CN in a crystal structure. [4] Generally, as the CN increases, the bond length gradually lengthens and the bond strength weakens, leading to a slower speed of sound (v m ), by which the phonons (waves) propagate through this lattice. ...
The discovery of compounds with low thermal conductivity and the understanding of their microscopic mechanisms are of great challenges and scientific significance. Herein, we report a unique ternary sulfide compound, Cu3BiS3, in which all Cu atoms are coordinated within a two‐dimensional [CuS3] triangle plane. This local coordination leads to efficient out‐of‐plane phonon scattering and an ultralow thermal conductivity. Through DFT phonon spectrum calculations and analyses, we reveal that the lowest vibration frequency decreases from 2 THz for high‐dimensional [CuS4] tetrahedral coordinated Cu atoms in CuBiS2 (CN=4, with an average Cu−S bond length of 2.328 Å) to 1.5 THz for low‐dimensional [CuS3] triangular coordinated Cu atoms in Cu3BiS3 (CN=3, with a shorter Cu−S bond length of 2.285 Å). This is due to the out‐of‐plane thermal vibration of the Cu atoms in the latter. Consequently,Cu3BiS3 exhibits one of the lowest values of κlat (0.32 W/m K) among its peer, with a 36 % reduction compared to CuBiS2 (0.50 W/m K). This groundbreaking discovery highlights the significant role of 2D local coordination in reducing thermal conductivity through characteristic out‐of‐plane phonon scattering, while also contributing to a large Grüneisen parameter (2.06) in Cu3BiS3.
... first reported the high-performance n-type transport with the favorable six-fold energy pockets regarding the conduction band and the ground-breaking ZT values for such materials in 2016, simultaneously. Afterwards, many strategies, such as energy band engineering [6], defect designing [16], doping [17], boundary modification [18], etc., have been implemented. Kuo et al. [19]. ...
High-performance n-type Mg 3 Sb 2-based thermoelectric materials have attracted much attention due to the six-fold degenerate carrier pockets in the conduction band. This study uncovers the electrical and thermal transport behaviors in different directions with respect to the spark plasma sintering (SPS) pressure that is rarely investigated. Results indicate that the grain size in the ⊥P direction (perpendicular to the SPS pressure direction) is slightly larger than that in the ‖P direction. In addition, a moderate texture is observed, reflecting a preferential orientation of the (001)-plane in the ⊥P direction. The synergy of both grain size and the texture leads to an improved electrical conductivity for Mg 3.24 Sb 1.5 Bi 0.49 Te 0.01 compounds with lamellar crystal structures. Consequently, the highest thermoelectric figure of merit ZT peak ~1.71 and ZT avg ~1.12 (almost the best ZT values ever reported for Te-doped Mg 3 Sb 2-based materials) are achieved in the ⊥P direction compared to the ‖P direction. As one of the most promising solutions to partially alleviate the energy crisis, the thermoelectric technique can directly convert waste heat and electricity in a quiet and free-emission way. The dimensionless thermoelectric figure of merit ZT, which governs the conversion effi
... The hardness of as-fabricated TM 0.01 Mg 3 Sb 1.5 Bi 0.5 sample is 1.29 GPa, which is better than that of promising thermoelectric materials, such as Bi 2 Te 3 (1.14 GPa) [80], Cu 1.8 S-Ru 0.01 (1.01 GPa) [81], and Nd-doped Mg 3 Sb 1.5 Bi 0.5 (1.11 GPa) [82]. The multiple dopants at the interstitial site may strengthen the chemical bonding between adjacent layers ( Fig. 6 (b)), a similar strengthening effect was also observed in MgB 2 doped in GeTe matrix [83]. ...
Mg3Sb2-based thermoelectric materials have attracted much interest since the discovery of their excellent n-type thermoelectric performance with excess Mg content and proper dopant. Herein, we proposed a doping diagram for Mg3Sb2-based material in a binary parameters space of electronegativity difference and atomic radius difference of extrinsic elements with Mg atom. Based on this doping diagram, we designed a multiple interstitial doped Mg3Sb1.5Bi0.5 material, and achieved a high dimensionless figure of merit value of 0.75 at room temperature and 1.83 at 500 °C, respectively. The multiple interstitial dopants significantly suppress the formation of Mg vacancy, which is of great importance in protecting the electrical transport channel and improving the thermal stability. Besides, the mechanical properties are also enhanced due to the random distribution of elements in the matrix, impeding the migration of dislocations. Our work clearly shows the benefits of maximized sub-lattice disordering through the multi-elements doping strategy in the Mg3Sb1.5Bi0.5 material.
... ZT ZT = S 2 σT/κ Thermoelectric materials that can directly realize the conversion between heat and electricity via the Seebeck effect and Peltier effect have been explored to use in the Internet of Things (IoTs). [1][2][3] Conventional strategies, such as band engineering, [4][5][6] nanostructure engineering, [7,8] and defect engineering [9,10] are commonly used to boost their thermoelectric performance, which was determined by the dimensionless figure of merit, , termed as , where S, σ, T, and κ are Seebeck coefficient, electrical conductivity, absolute temperature, and thermal conductivity, respectively. Recently, introducing another dimensional discipline -magnetism into thermoelectrics attracted intensive attention. ...
Introducing magnetic elements or nanoparticles into the thermoelectric matrix is of great importance to regulate the thermoelectric performance and evaluate the magnetic-thermoelectric effect. While, the limitation of solid solution ability of magnetic elements in thermoelectric materials impedes the development of magnetic thermoelectric matrix. Herein, we have applied high entropy strategy to alloy a large amount of Cr elements into the Bi2Se3 sub-lattice, and successfully obtained a single-phase magnetic thermoelectric material in the nominal composition of Bi0.6Sb0.6In0.4Cr0.4Se3. The Magnetization loop curves of Bi0.6Sb0.6In0.4Cr0.4Se3 sample shows obvious ferromagnetic behavior with a coercivity of 2000 Oe and residual magnetization of 0.22 emu g-1 at 2 K. The temperature dependence of zero-field-cooled magnetic susceptibility and field-cooled magnetic susceptibility reveals a transition from ferromagnetism to paramagnetism at 61 K. These findings indicate that a magnetic Bi2Se3 based thermoelectric material is successfully obtained. The corresponding structure, magnetic and thermoelectric properties are also carefully discussed. This work offers a new avenue to achieve a magnetic thermoelectric material through high entropy strategy.
Improving the power factor is a crucial parameter in enhancing thermoelectric performance, making it essential to find an effective strategy for its enhancement. This study examines n-type Mg3Sb1.5Bi0.5-based thermoelectric materials doped with Zn and Se. Se is added to adjust the carrier concentration, while Zn is introduced into Mg3.2Sb1.5Bi0.49Se0.01 to manipulate the carrier scattering mechanism. Experimental results indicate a significant increase in carrier mobility from 42.21 cm² V− 1 s− 1 to 73.92 cm² V− 1 s− 1, leading to a substantial enhancement in electrical conductivity and power factor across the entire temperature range under investigation. Additionally, due to reduced lattice thermal conductivity resulting from the introduction of efficient phonon scattering centers in the Zn and Se co-doped sample, Mg3.18Zn0.02Sb1.5Bi0.49Se0.01 attains a maximum ZT value of 1.77 at 623 K, resulting in a notable average ZT ≈ 1.24 over the temperature range of 300 to 673 K. Given its cost-effectiveness and low toxicity, this material is anticipated to replace the commercially available n-type Bi2Te3-based thermoelectric materials commonly used at moderate and low temperatures.
B and Te co-doped samples of Mg 3.2 B y SbBi 1− x Te x achieved the lowest κ − κ E of ∼0.49 W m ⁻¹ K ⁻¹ and a maximum zT of ∼1.54.
Defect engineering is an effective method for tuning the performance of thermoelectric materials and shows significant promise in advancing thermoelectric performance. Given the rapid progress in this research field, this Review summarizes recent advances in the application of defect engineering in thermoelectric materials, offering insights into how defect engineering can enhance thermoelectric performance. By manipulating the micro/nanostructure and chemical composition to introduce defects at various scales, the physical impacts of diverse types of defects on band structure, carrier and phonon transport behaviors, and the improvement of mechanical stability are comprehensively discussed. These findings provide more reliable and efficient solutions for practical applications of thermoelectric materials. Additionally, the development of relevant defect characterization techniques and theoretical models are explored to help identify the optimal types and densities of defects for a given thermoelectric material. Finally, the challenges faced in the conversion efficiency and stability of thermoelectric materials are highlighted and a look ahead to the prospects of defect engineering strategies in this field is presented.
Magnesium-based thermoelectric (TE) materials have attracted considerable interest due to their high ZT values, coupled with their low cost, widespread availability, nontoxicity, and low density. In this review, we provide a succinct overview of the advances and strategies pertaining to the development of Mg-based materials aimed at enhancing their performance. Following this, we delve into the major challenges posed by the severe working conditions, such as high temperature and thermal cycling, which adversely impact the behavior and long-term stability of the TE modules. Challenges include issues like the lack of mechanical strength, chemical instability, and unreliable contact. Subsequently, we focus on the key methodologies aimed at addressing these challenges to facilitate the broader application of the TE modules. These include boosting the mechanical strength, especially the toughness, through grain refining and additions of second phases. Furthermore, strategies targeted at enhancing the chemical stability through coatings and modifying the microstructure, as well as improving the contact design and materials, are discussed. In the end, we highlight the perspectives for boosting the practical applications of Mg-based TE materials in the future.
Magnesium-based thermoelectric (TE) materials have attracted considerable interest due to their high ZT values, coupled with their low cost, widespread availability, nontoxicity, and low density. In this review, we provide a succinct overview of the advances and strategies pertaining to the development of Mg-based materials aimed at enhancing their performance. Following this, we delve into the major challenges posed by the severe working conditions, such as high temperature and thermal cycling, which adversely impact the behavior and long-term stability of the TE modules. Challenges include issues like the lack of mechanical strength, chemical instability, and unreliable contact. Subsequently, we focus on the key methodologies aimed at addressing these challenges to facilitate the broader application of the TE modules. These include boosting the mechanical strength, especially the toughness, through grain refining and additions of second phases. Furthermore, strategies targeted at enhancing the chemical stability through coatings and modifying the microstructure, as well as improving the contact design and materials, are discussed. In the end, we highlight the perspectives for boosting the practical applications of Mg-based TE materials in the future.
Here, a high peak ZT of ≈2.0 is reported in solution‐processed polycrystalline Ge and Cd codoped SnSe. Microstructural characterization reveals that CdSe quantum dots are successfully introduced by solution process method. Ultraviolet photoelectron spectroscopy evinces that CdSe quantum dots enhance the density of states in the electronic structure of SnSe, which leads to a large Seebeck coefficient. It is found that Ge and Cd codoping simultaneously optimizes carrier concentration and improves electrical conductivity. The enhanced Seebeck coefficient and optimization of carrier concentration lead to marked increase in power factor. CdSe quantum dots combined with strong lattice strain give rise to strong phonon scattering, leading to an ultralow lattice thermal conductivity. Consequently, high thermoelectric performance is realized in solution‐processed polycrystalline SnSe by designing quantum dot structures and introducing lattice strain. This work provides a new route for designing prospective thermoelectric materials by microstructural manipulation in solution chemistry.
Incorporating donor doping into Mg3Sb1.5Bi0.5 to achieve n-type conductivity is one of the crucial strategies for performance enhancement. In pursuit of higher thermoelectric performance, we herein report co-doping with Te and Y to optimize the thermoelectric properties of Mg3Sb1.5Bi0.5, achieving a peak ZT exceeding 1.7 at 703 K in Y0.01Mg3.19Sb1.5Bi0.47Te0.03. Guided by first-principles calculations for compositional design, we find that Te-doping shifts the Fermi level into the conduction band, resulting in n-type semiconductor behavior, while Y-doping further shifts the Fermi level into the conduction band and reduces the bandgap, leading to enhanced thermoelectric performance with a power factor as high as >20 μW cm–1 K–2. Additionally, through detailed micro/nanostructure characterizations, we discover that Te and Y co-doping induces dense crystal and lattice defects, including local lattice distortions and strains caused by point defects, and densely distributed grain boundaries between nanocrystalline domains. These defects efficiently scatter phonons of various wavelengths, resulting in a low thermal conductivity of 0.83 W m–1 K–1 and ultimately achieving a high ZT. Furthermore, the dense lattice defects induced by co-doping can further strengthen the mechanical performance, which is crucial for its service in devices. This work provides guidance for the composition and structure design of thermoelectric materials.
Thermoelectric (TE) generators (TEGs) have recently attracted significant attention for harvesting ubiquitous heat energy to power industrial sensor nodes or wearable and implantable electronics. Such applications require that a small, fitted heat sink be incorporated into a tiny harvester to ensure conformal deployment and ease of use. Since the small heat sink will exhibit very poor heat rejection performance, the optimal structural design of the TEG device and the proper selection of its constituent TE materials are extremely important to ensure that sufficient electric power is provided to the connected post-stage circuits. Regrettably, the traditional forward design strategies, i.e., material-to-device and device-to-harvester, are often disconnected from one another and fail to form a complete chain, thereby presenting a fundamental challenge in achieving ultrahigh-power-density harvesters. Here we present a zero-dimensional model for a tiny harvester to establish a set of new metrics (not limited to the figure of merit zT) that can be used to determine the optimal TE material and to directly deduce the optimal configuration of the TEG for guaranteeing the maximum output power at various load conditions, making backward design strategy possible. Two tiny harvesters are accordingly fabricated and exhibit areal power densities 52 and 94 times that of a harvester incorporating a commercial TEG, with identical amounts of TE material used for each.
The discovery of compounds with low thermal conductivity and the understanding of their microscopic mechanisms are of great challenges and scientific significance. Herein, we report a unique ternary sulfide compound, Cu 3 BiS 3 , in which all Cu atoms are coordinated within a two‐dimensional [CuS 3 ] triangle plane. This local coordination leads to efficient out‐of‐plane phonon scattering and an ultralow thermal conductivity. Through DFT phonon spectrum calculations and analyses, we reveal that the lowest vibration frequency decreases from 2 THz for high‐dimensional [CuS 4 ] tetrahedral coordinated Cu atoms in CuBiS 2 (CN=4, with an average Cu−S bond length of 2.328 Å) to 1.5 THz for low‐dimensional [CuS 3 ] triangular coordinated Cu atoms in Cu 3 BiS 3 (CN=3, with a shorter Cu−S bond length of 2.285 Å). This is due to the out‐of‐plane thermal vibration of the Cu atoms in the latter. Consequently,Cu 3 BiS 3 exhibits one of the lowest values of κ lat (0.32 W/m K) among its peer, with a 36 % reduction compared to CuBiS 2 (0.50 W/m K). This groundbreaking discovery highlights the significant role of 2D local coordination in reducing thermal conductivity through characteristic out‐of‐plane phonon scattering, while also contributing to a large Grüneisen parameter (2.06) in Cu 3 BiS 3 .
N‐type Mg3Sb2‐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 Mg3Sb2. 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 Mg3Sb2. Consequently, the Mg3.05(Sb0.75Bi0.25)1.99Te0.01(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 Mg3Sb2 can be maintained by adding MgB2, reaching a high average ZT of ≈1.1 within 300–723 K. An eight‐pair Mg3Sb2‐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 Mg3Sb2‐based alloys and other thermoelectrics for practical applications.
Mg3(Sb1-xBix)2 alloy has been extensively studied in the last 5 years due to its exceptional thermoelectric (TE) performance. The absence of accurate force field for inorganic alloy compounds presents great challenges for computational studies. Here, we explore the atomic microstructure, thermal, and elastic properties of the Mg3(Sb1-xBix)2 alloy at different solution concentrations through atomic simulations with a highly accurate machine learning interatomic potential (ML-IAP). We find atomic local ordering in the optimized structure with the Bi-Bi pair inclined to join adjacent layers and Sb-Sb pair preferring to stay within the same layer. The thermal conductivity changes with the solution concentrations can be correctly predicted through ML-IAP-based molecular dynamics simulations. Spectral thermal conductance analysis shows that the continuous movement of low-frequency peak to high frequency is responsible for the reduction of the thermal conductivity upon alloying. Elastic calculations reveal that similar to the thermal conductivity, solid solution alloying can reduce the overall elastic properties at both Mg3Sb2 and Mg3Bi2 ends, while anisotropic behavior is clearly observed with linear interpolation relationship upon alloying along the interlayer direction and nonlinearity along the intralayer direction. Although the atomic local ordering shows little effects on the properties of the Mg3(Sb1-xBix)2 alloy with only two alloying elements, it possesses potential important impacts on multiprincipal element inorganic TE alloys. This work provides a recipe for computational studies on the TE alloy systems and thus can accelerate the discovery and optimization of TE materials with high TE performance.
In recent decades, improvements in thermoelectric material performance have made it more practical to generate electricity from waste heat and to use solid‐state devices for refrigeration. However, despite the development of successful strategies to enhance the figure‐of‐merit zT, optimizing devices for large‐scale applications remains challenging. High zT values do not guarantee excellent device performance, and maintaining high zT over a wide temperature range is difficult. Thus, device‐level structural optimization is crucial for maximizing overall energy conversion efficiency. Proper interfacial and structure design strategies, including contact layer selection, multi‐stage optimization, and size matching for the n‐ and p‐type thermoelectric legs, are necessary for advancing device performance. Additionally, thermal stability issues, device assembly techniques, mechanical properties, and manufacturing costs are crucial considerations for large‐scale applications. To achieve actual applications, the thermoelectric community must look beyond simply aiming for high zT values. This article focuses on modules based on n‐type Mg3(Sb, Bi)2, one of the most promising commercially available thermoelectric materials, and discusses the influence of various parameters on the modules and on the corresponding device‐level optimization strategies.
N‐type Mg3(Bi, Sb)2‐based thermoelectric (TE) alloys show great promise for solid‐state power generation and refrigeration, owing to their excellent figure‐of‐merit (ZT) and using cheap Mg. However, their rigorous preparation conditions and poor thermal stability limit their large‐scale applications. Here, this work develops an Mg compensating strategy to realize n‐type Mg3(Bi, Sb)2 by a facile melting‐sintering approach. “2D roadmaps” of TE parameters versus sintering temperature and time are plotted to understand the Mg‐vacancy‐formation and Mg‐diffusion mechanisms. Under this guidance, high weight mobility of 347 cm² V⁻¹ s⁻¹ and power factor of 34 µW cm⁻¹ K⁻² can be obtained for Mg3.05Bi1.99Te0.01, and a peak ZT≈1.55 at 723 K and average ZT≈1.25 within 323–723 K can be obtained for Mg3.05(Sb0.75Bi0.25)1.99Te0.01. Moreover, this Mg compensating strategy can also improve the interfacial connecting and thermal stability of corresponding Mg3(Bi, Sb)2/Fe TE legs. As a consequence, this work fabricates an 8‐pair Mg3Sb2‐GeTe‐based power‐generation device reaching an energy conversion efficiency of ≈5.0% at a temperature difference of 439 K, and a one‐pair Mg3Sb2‐Bi2Te3‐based cooling device reaching −10.7 °C at the cold side. This work paves a facile way to obtain Mg3Sb2‐based TE devices at low cost and also provides a guide to optimize the off‐stoichiometric defects in other TE materials.
Although a high figure of merit (zT) over a wide range of temperatures has been shown in n‐type Zintl Mg3(Sb, Bi)2, further improvement of its near‐room‐temperature performance is still required to promote its application in next‐generation thermoelectric coolers and power generators. Here, a novel strategy of phonon spectra mismatch for enhancing the thermoelectric performance of Mg3(Sb, Bi)2 is reported by incorporating multi‐walled carbon nanotubes (MWCNT). The introduction of very small amounts of MWCNT generates a large interfacial thermal resistance and thus significantly reduces the lattice thermal conductivity of the resulting composites, while maintaining a high power factor. As a result, a zT of ≈1.5 at 573 K and an average zT of 1.14 between 323 and 573 K can be achieved in the 0.5 wt% MWCNT composite. Moreover, a high conversion efficiency of ≈8.1% under a temperature difference of 283 K is realized in a resulting single‐leg device, making it a promising candidate material for low‐grade heat recovery. This study provides not only a material with a high near‐room‐temperature zT but also a unique insight into the design of high‐performance thermoelectric materials via compositing to exploit the large difference in phonon spectra between the components of the composite.
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.
Mg3Sb1.5Bi0.5-based Zintl compounds have attracted extensive attention as potential thermoelectric materials due to their earth-abundant elements. However, pure and intrinsic Mg3Sb1.5Bi0.5 manifests a poor thermoelectric performance because of its low electrical conductivity of about 3 × 102 S/m at room temperature. In this work, In and Se co-doping was carried out to optimize the thermoelectric performance of n-type Mg3Sb1.5Bi0.5-based material. The experimental results revealed that the carrier concentration and mobility of Mg3Sb1.5Bi0.5 significantly increased after In and Se co-doping, leading to an improvement of power factor. Simultaneously, lattice thermal conductivity was significantly reduced due to the large mass difference between In and Mg. A maximum zT of 1.64 at 723 K was obtained for the Mg3.17In0.03Sb1.5Bi0.49Se0.01 sample. And an average zT value of about 1.1 between 300 and 723 K was achieved, which insures its possible application at medium temperature range as a non-toxic and low-cost TE material.
In this paper, we are studied the electronic, structural, optical, dielectric and thermodynamic properties of Bi2CaX2 (X = Mg and Mn) with full-potential linearized augmented plane-wave (FP-LAPW) method, by using the “WIEN2k” code. The lattice constants of Bi2CaX2 compounds are calculated by using the GGA-PBE approximation. We are found the first one is semi-conductor character with direct gap Eg = 0.580 eV and the second is metallic character. The reflectivity, dielectric function, absorption coefficient, energy loss function and optical conductivity are obtained. The values obtained at room temperature and at P = 0 GPa are 215.4997 and 180.8549 J/(mol.K) for Bi2CaMg2 and Bi2CaMn2, respectively. This large entropy indicates that these compounds are not ordered. The thermal expansion coefficient, the thermal entropy and the Debye temperatures have also been determined. The elastic parameters are also studied and discussed.
Experimental results show an intriguing phenomenon that although Bi and Sb have the same number of valence electrons, Bi/Sb substitution increases the electron concentration of n-type Mg3Sb2-based materials. Using a combination of theoretical calculations and experimental synthesis, this work reveals the physical mechanism of the effect of Bi doping on carrier concentration. The increase in electron concentration mainly originates from the enhanced degree of ionization of donor impurity because of the decrease of conductivity effective mass and increase of dielectric constant caused by the narrowing of bandgap with Bi doping. Based on the collaborative optimization of the electrical and thermal transports, n-type Mg3.175Mn0.025Sb1.48Bi0.48Te0.04 exhibits the best thermoelectric performance with a peak zT of 1.85 at 725 K and an average zT of 1.21. This work demonstrates an effective strategy of bandgap engineering for the optimization of carrier concentration and provides insightful guidance for designing other thermoelectric materials.
AgSbTe2-based ternary chalcogenides show excellent thermoelectric performance at low- and middle-temperature ranges, yet their practical applications are greatly limited by their intrinsic poor thermodynamic stability. In this work, we demonstrate that AgSbTe2-based ternary chalcogenides can be stabilized for service below their decomposition threshold. A series of AgxSb2-xTe3-x (x = 1.0, 0.9, 0.8, and 0.7) samples have been prepared by the melt-quenching method. Among them, phase pure Ag0.9Sb1.1Te2.1 is verified by comprehensive structural characterizations from macroscale by X-ray diffraction to microscale by energy-dispersive spectroscopy and then to sub-nanometer scale by atom probe tomography. This composition is further chosen for the stability investigation. The decomposition threshold of Ag0.9Sb1.1Te2.1 appears around 473 K. Below this temperature, the chemical compositions and thermoelectric properties are barely changed even after 720 h annealing at 473 K. The figure-of-merit (zT) value of Ag0.9Sb1.1Te2.1 below the decomposition threshold is very competitive for real applications even compared with Bi2Te3-based alloys. The average zT of Ag0.9Sb1.1Te2.1 at 300–473 K reaches 0.84, which is higher than most other thermoelectric materials in a similar temperature range, promising applications in miniaturized refrigeration and power generation near room temperature.
n-type Mg3Sb2-Mg3Bi2 alloys have been investigated as one of the most promising thermoelectric materials. To achieve high performance, a detailed understanding of the microstructure is required. Although Mg3Sb2-Mg3Bi2 is usually considered to be a complete solid solution, nanosized compositional fluctuations were observed within a matrix and in the vicinity of the grain boundary. As an inhomogeneous microstructure can be beneficial or detrimental to thermoelectric performance, it is important to investigate the evolution of compositional variations for the engineering and long-term use of these materials. Using scanning transmission electron microscopy and atom probe tomography, a Bi-rich phase and compositional fluctuations are observed in sintered and annealed samples. After annealing, the broad intergranular phase was sharpened, resulting in a greater compositional change in the intergranular region. Annealing considerably reduces the fluctuations of Bi and Mg content within the grain as observed in atom probe tomography. Weighted mobility and lattice thermal conductivity were both increased as a result of the homogenized matrix phase. The combined microstructure features of intragrain and grain boundary effects resulted in an increased thermoelectric figure-of-merit zT of Mg3Sb0.6Bi1.4. These findings imply that the optimization of thermal and electrical properties can be realized through microstructure tuning.
SnSe is considered as one of the most promising candidates for thermoelectric application because of its attractive performance recently. In this work, a high magnetic field (5 T) is applied during the solution synthesis process. With the assist of the high magnetic field, nanopores and quantum dots (Sn, Se) are induced in the Ga doped p-type polycrystalline SnSe. It is found that an ultralow lattice thermal conductivity of 0.19 W m⁻¹ K⁻¹ is caused by enhanced phonon scattering due to the presence of nanopores, quantum dots, lattice strain in dislocation networks. The enhanced density of states induced by Sn and Se quantum dots contributes to enhanced Seebeck coefficient. The enhanced electrical conductivity and Seebeck coefficient give rise to a significant enhancement in power factor over the whole temperature range. The maximum power factor reaches to 6.12 μW cm⁻¹ K⁻² at 873 K for the sample prepared under high magnetic field. Consequently, a peak ZT of 2.0 as well as a high average ZT of 0.74 (300-873 K) is achieved in 5T-NP/QD Sn0.975Ga0.025Se sample. This study provides an important direction for developing high performance thermoelectric materials by structural manipulation with the aid of high magnetic field in solution chemistry.
Mg3Bi2-based materials exhibit high thermoelectric performance at ambient temperature benefitting from their low lattice thermal conductivity. However, the underlying physics of this low lattice thermal conductivity in a simple hexagonal crystal structure of Mg3Bi2 remain puzzling. Understanding the microscopic thermal transport behavior and its correlation with bonding, lattice dynamics are critical to improve the thermoelectric performance and design new promising functional materials. In this work, the giant anharmonic phonon modes are experimentally observed via measuring temperature-dependent inelastic x-ray scattering (IXS) for Mg3Bi2 single-crystal, which is also verified by the anomalously large Grüneisen parameters and frozen phonon potential calculations. Furthermore, we propose that the giant anharmonicty is associated with the asymmetric Bismuth 6s lone-pair electrons. The present work builds a microscopic connection between electronic structure and giant anharmonic phonon scattering, providing new insights on the low lattice thermal conductivity of Mg3Bi2, and paves the way to design novel high-efficient thermoelectrics for application in energy recycling and refrigeration.
Numerous efforts have been paid on n-type Mg3Sb2-based Zintl compounds with exceptional thermoelectric performance, but seldom on p-type sample with poor electrical transports. In this work, we investigate the electronic structure and transport properties of p-type Mg3Sb2 by using first-principles method and Boltzmann transport theory. Firstly, the slightly higher low-temperature electrical conductivity for theoretical calculations than experimental results suggest that different from n-type sample, the contribution of eliminating grain boundary scattering to electrical transports is weak in p-type Mg3Sb2. Secondly, the calculated higher Seebeck coefficient along x-axis and higher electrical conductivity along z-axis reveal the anisotropy of electrical transports, and this phenomenon may be ascribed to the anisotropic carrier's effective masses. Because the gradual leading role of Seebeck coefficient as temperature increasing, the peak power factor along x-axis exceeds that along z-axis at temperature above ∼500 K, which indicates that further improvement of the electrical performance can be expected through anisotropic transports. Thirdly, the effect of the adsorption of oxygen atom on Mg3Sb2 (001) surface on the electronic structure are investigated. This work aims to provide new insight into the optimization of p-type electrical transport property, thereby closing the gap with n-type property for developing Mg3Sb2-based thermoelectric devices.
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.
Over the past two decades, we have witnessed a strong interest in developing Mg3Sb2 and related CaAl2Si2-type materials for low- and intermediate-temperature thermoelectric applications. In this review, we discuss how computations coupled with experiments provide insights for understanding chemical bonding, electronic transport, point defects, thermal transport, and transport anisotropy in these materials. Based on the underlying insights, we examine design strategies to guide the further optimization and development of thermoelectric Mg3Sb2-based materials and their analogs. We begin with a general introduction of the Zintl concept for understanding bonding and properties and then reveal the breakdown of this concept in AMg2X2 with a nearly isotropic three-dimensional chemical bonding network. For electronic transport, we start from a simple yet powerful atomic orbital scheme of tuning orbital degeneracy for optimizing p-type electrical properties, then discuss the complex Fermi surface aided by high valley degeneracy, carrier pocket anisotropy, and light conductivity effective mass responsible for the exceptional n-type transport properties, and finally address the defect-controlled carrier density in relation to the electronegativity and bonding character. Regarding thermal transport, we discuss the insight into the origin of the intrinsically low lattice thermal conductivity in Mg3Sb2. Furthermore, the anisotropies in electronic and thermal transport properties are discussed in relation to crystal orbitals and chemical bonding. Finally, some specific challenges and perspectives on how to make further developments are presented.
The urgent need for eco-friendly, stable, long lifetime power sources is driving the booming market for miniaturized and integrated electronics including wearable and medical implantable devices. Flexible thermoelectric materials and devices are receiving increasing attention, due to their capability to convert heat into electricity directly by conformably attaching onto heat sources. Polymer-based flexible thermoelectric materials are particular fascinating because of their intrinsic flexibility, affordability, and low toxicity. There are other promising alternatives including inorganic-based flexible thermoelectrics that have high energy conversion efficiency, large power output, and relatively high-temperature stability. In this review, we summarize the state-of-the-art in the development of flexible thermoelectric materials and devices, including exploring the fundamentals behind the performance of flexible thermoelectric materials and devices by relating materials chemistry and physics to properties. By taking insights from carrier and phonon transport, we highlight the limitations of high-performance flexible thermoelectric materials and the underlying mechanisms associated with each optimization strategy. Finally, we conclude by discussing the remaining challenges in flexible thermoelectric materials and provide suggestions and a framework to guide future development, which may pave the way for a bright future for flexible thermoelectric devices in the energy market.
Bi2Te3 alloys have been the most widely used n-type material for low temperature thermoelectric power generation for over 50~years, thanks to the highest efficiency in the 300-500 K temperature range relevant for low-grade waste-heat recovery. Here we show that n-type Mg3Sb0.6Bi1.4, with a thermoelectric figure-of-merit zT of 1.0-1.2 at 400-500 K, finally surpasses n-type Bi2Te3. This exceptional performance is achieved by tuning the alloy composition of Mg3(Sb1-xBix)2. The two primary mechanisms of the improvement are the band effective-mass reduction and enhanced grain size as the Mg3Bi2 content increases. The benefit of the effective-mass reduction is only effective up to the optimum composition Mg3Sb0.6Bi1.4, after which a different band dominates charge transport. The larger grains are important for minimizing grain-boundary electrical resistance. Considering the limited choice for low temperature n-type thermoelectric materials, the development of Mg3Sb0.6Bi1.4 is a significant advancement towards sustainable heat recovery technology.
Ductility is common in metals and metal-based alloys, but is rarely observed in inorganic semiconductors and ceramic insulators. In particular, room-temperature ductile inorganic semiconductors were not known until now. Here, we report an inorganic α-Ag2S semiconductor that exhibits extraordinary metal-like ductility with high plastic deformation strains at room temperature. Analysis of the chemical bonding reveals systems of planes with relatively weak atomic interactions in the crystal structure. In combination with irregularly distributed silver-silver and sulfur-silver bonds due to the silver diffusion, they suppress the cleavage of the material, and thus result in unprecedented ductility. This work opens up the possibility of searching for ductile inorganic semiconductors/ceramics for flexible electronic devices.
While there has been good progress in finding high thermoelectric efficiency (zT) p-type Zintl compounds, high zT n-type Zintl compounds have eluded discovery for 10 years despite the theoretical predictions that these would make even better thermoelectric materials. Here we show that even in a “line compound” multiple thermodynamic states exist that profoundly affect the electronic properties by suppressing the formation of unwanted defects differently. To form the desired n-type Mg_3Sb_2-based compound it is most critical to make the Mg-excess thermodynamic state. This understanding suggests a synthesis strategy we call “phase boundary mapping” that could be counter-intuitive from the normal perspective that favors thermodynamic phase purity: to add excess constituents until the impurity phase is identified. This strategy can help discover many different versions of any compound, even ones considered to be a “line compound” with no measurable compositional variation.
Mg3Sb2-Mg3Bi2 alloys show excellent thermoelectric properties. The benefit of alloying has been attributed to the reduction in lattice thermal conductivity. However, Mg3Bi2-alloying may also be expected to significantly change the electronic structure. By comparatively modeling the transport properties of n- and p-type Mg3Sb2-Mg3Bi2 and also Mg3Bi2-alloyed and non-alloyed samples, we elucidate the origin of the highest zT composition where electronic properties account for about 50 % of the improvement. We find that Mg3Bi2 alloying increases the weighted mobility while reducing the band gap. The reduced band gap is found not to compromise the thermoelectric performance for a small amount of Mg3Bi2 because the peak zT in unalloyed Mg3Sb2 is at a temperature higher than the stable range for the material. By quantifying the electronic influence of Mg3Bi2 alloying, we model the optimum Mg3Bi2 content for thermoelectrics to be in the range of 20-30%, consistent with the most commonly reported composition Mg3Sb1.5Bi0.5.
In order to locate the optimal carrier concentrations for peaking the thermoelectric performance in p-type group IV monotellurides, existing efforts focus on aliovalent doping, either to increase (in PbTe) or to decrease (in SnTe and GeTe) the hole concentration. The limited solubility of aliovalent dopants usually introduces insufficient phonon scattering for thermoelectric performance maximization. With a decrease in the size of cation, the concentration of holes, induced by cation vacancies in intrinsic compounds, increases rapidly from ≈1018 cm−3 in PbTe to ≈1020 cm−3 in SnTe and then to ≈1021 cm−3 in GeTe. This motivates a strategy here for reducing the carrier concentration in GeTe, by increasing the mean size of cations and vice-versa decreasing the average size of anions through isovalent substitutions for increased formation energy of cation vacancy. A combination of the simultaneously resulting strong phonon scattering due to the high solubility of isovalent impurities, an ultrahigh thermoelectric figure of merit, zT of 2.2 is achieved in GeTe–PbSe alloys. This corresponds to a 300% enhancement in average zT as compared to pristine GeTe. This work not only demonstrates GeTe as a promising thermoelectric material but also paves the way for enhancing the thermoelectric performance in similar materials.
Widespread application of thermoelectric devices for waste heat recovery requires low-cost high-performance materials. The currently available n-type thermoelectric materials are limited either by their low efficiencies or by being based on expensive, scarce or toxic elements. Here we report a low-cost n-type material, Te-doped Mg3Sb1.5Bi0.5, that exhibits a very high figure of merit zT ranging from 0.56 to 1.65 at 300−725 K. Using combined theoretical prediction and experimental validation, we show that the high thermoelectric performance originates from the significantly enhanced power factor because of the multi-valley band behaviour dominated by a unique near-edge conduction band with a sixfold valley degeneracy. This makes Te-doped Mg3Sb1.5Bi0.5 a promising candidate for the low- and intermediate-temperature thermoelectric applications.
Thermoelectric technology, harvesting electric power directly from heat, is a promising environmentally-friendly means of
energy savings and power generation. The thermoelectric efficiency is determined by the device dimensionless figure of merit
ZTdev, and optimizing this efficiency requires maximizing ZT values over a broad temperature range. Herein, we report a record high ZTdev ∼1.34, with ZT ranging from 0.7 to 2.0 at 300-773K, realized in hole doped SnSe crystals. The exceptional performance arises from the ultra-high
power factor, which comes from a high electrical conductivity and a strongly enhanced Seebeck coefficient enabled by the contribution
of multiple electronic valence bands present in SnSe. SnSe is a robust thermoelectric candidate for energy conversion applications
in the low and moderate temperature range.
Deformable semiconductors
Semiconductors are usually brittle and do not deform easily. Wei et al. found that bulk single crystals of indium selenide instead have excellent flexibility (see the Perspective by Han). The deformability comes from the compliant intralayer bonding between indium and selenium. The authors used these observations along with a previously discovered silver sulfide to determine a deformability factor for materials that may help find other deformable semiconductors.
Science , this issue p. 542 ; see also p. 509
Thermoelectric technology has been applied in the fields of industrial waste heat power generation, special spatial power supply, semiconductor chip cooling, advanced refrigeration, etc. Comparing to other thermoelectric material systems, bismuth telluride (Bi2Te3) based thermoelectric materials are widely used in industrial and commercial fields due to their stable zT values and relatively high conversion efficiencies at room temperature range. However, the low hardness and easy dissociation of Bi2Te3 materials lead to the waste of raw materials in industrial processing. Therefore, substantial efforts have been made to find a balance between the zT value and the hardness of bismuth telluride. In this study, the mechanical properties of commercial Bi2Te3 were significantly enhanced by dispersing the second phase of the SiC particles in the matrix via a combined process of mechanical alloying (MA) and spark plasma sintering (SPS). Although the zT value of the composite was slightly decreased, the average hardness was increased from ∼1.2 to 1.7 GPa, and the modulus of the composite was also improved. The thermal expansion rate and thermal expansion coefficients were also measured. The mechanism of the enhanced mechanical properties of the Bi2Te3 was discussed in detail. This work offered a simple strategy for improving the machinability of commercial Bi2Te3, which is of high importance for the large scale industrial application of thermoelectric materials.
Using first principles calculations, we study the conduction band alignment, effective mass, and Fermi surface complexity factor of n-type Mg3Sb2 – xBix (x = 0, 1, and 2) from the full ab initio band structure. We find that with an increase in the Bi content, the K and M band minima move away from the conduction band minimum CB1 while the singly-degenerate Г band minimum shifts rapidly downward and approaches the conduction band minimum. However, the favorable sixfold degenerate CB1 band minimum keeps dominating the conduction band minimum and there is no band crossing between the Г and CB1 band minima. In addition, we show that the connection of the CB1 carrier pockets with the energy level close to the band minimum M can strongly enhance the carrier pocket anisotropy and Fermi surface complexity factor, which is likely the electronic origin for the local maximum in the theoretical power factor. Our calculations also show that the density of states effective mass, Seebeck coefficient, and Fermi surface complexity factor decrease with an increase in the Bi content, which is unfavorable to the electrical transport. In contrast, reducing the conductivity effective mass with an increase in the Bi content is beneficial to the electrical transport by improving carrier mobility and weighted mobility as long as the detrimental bipolar effect is insignificant. As a result, in comparison with n-type Mg3Sb2, n-type Mg3SbBi shows higher power factors and a much lower optimal carrier concentration for the theoretical power factor at 300 K, which can be easily achieved by the experiment.
N-type Mg3Sb2-based Zintl compounds have recently been discovered to be a promising class of thermoelectric materials. Effective n-type dopants are crucial for realizing high thermoelectric performance. Here, using first-principles defect calculations, we investigate that Tm and Ce are effective n-type dopants in Mg3Sb2 and explain why n-type conduction can be successfully achieved by a simple doping without extra Mg. Under Mg-rich condition, the maximal achievable free carrier concentrations for Tm and Ce substitution on Mg sites at 750 K exceed 1020 cm-3, approaching to the optimal carriers of ~1020 cm-3. Under Mg-poor condition, the miraculous n-type conduction achieved by a simple extrinsic n-type doping can be explained by the sufficiently low defect formation energy for the substitutional donor defect.
The introduction of point defect by extrinsic doping is an effective way to the optimization of carrier concentration. Here, we theoretically and experimentally find that Pr is a more effective dopant in Mg3Sb2 compared to Te. Using first-principles defect calculations, the predicted highest carrier concentration with Pr doping at 725 K can be up to ~9.3 × 1019 cm-3, consistent with our experimental measurement. In addition, the point defect introduced by Pr substitution on Mg sites leads to the lattice thermal conductivity to be reduced to as low as 0.429 W m-1 K-1. By optimizing the Pr doping concentration, Mg3.2Pr0.02Sb1.5Bi0.5 exhibits a peak zT value of 1.70 at 725 K.
Thermoelectric materials have a large Peltier effect, making them attractive for solid-state cooling applications. Bismuth telluride (Bi2Te3)-based alloys have remained the state-of-the-art room-temperature materials for many decades. However, cost partially limited wider use of thermoelectric cooling devices because of the large amounts of expensive tellurium required. We report n-type magnesium bismuthide (Mg3Bi2)-based materials with a peak figure of merit (ZT) of ~0.9 at 350 kelvin, which is comparable to the commercial bismuth telluride selenide (Bi2Te3-
x
Se
x
) but much cheaper. A cooling device made of our material and p-type bismuth antimony telluride (Bi0.5Sb1.5Te3) has produced a large temperature difference of ~91 kelvin at the hot-side temperature of 350 kelvin. n-type Mg3Bi2-based materials are promising for thermoelectric cooling applications.
Mg3Sb2-based thermoelectric materials have drawn much attention in the last couple of years due to their high peak figure of merit, ZT. However, there have yet been no reports focused on fabricating Mg3Sb2-based thermoelectric devices and measuring their conversion efficiencies. Here we report the successful production of contact layers on the recently reported Mg3Sb2-based thermoelectric material using a one-step hot-press technique to achieve good bonding strength and low electrical contact resistance between the thermoelectric material and the contact layers, which are both very important for real applications. The conversion efficiency is measured to be up to 10.6% at a temperature difference of 400 °C from 100 to 500 °C, suggesting that Mg3Sb2-based thermoelectric materials have a good potential for mid-temperature heat conversion, which is supported by the conversion efficiency extracted from the finite difference method. Although a high average ZT, (ZT)avg of 0.96 is measured, we further experimentally demonstrate engineering ZT, (ZT)eng, is more accurate than (ZT)avg, in predicting conversion efficiency. Another recently reported half-Heusler material ZrCoBi0.65Sb0.15Sn0.2, with high peak ZT is also tested to verify this assertion.
The thermoelectric figure of merit reported for n-type Mg 3 (Sb, Bi) 2 compounds has made these materials of great engineering significance, increasing the need for accurate evaluations of their thermal conductivity. Thermal conductivity is typically derived from measurements of thermal diffusivity and determination of the specific heat capacity. The uncertainty in this method (often 10% or more) is frequently attributed to measurement of heat capacity such that estimated values are often more accurate. Inconsistencies between reported thermal conductivity of Mg 3 (Sb, Bi) 2 compounds may be attributed to the different values of heat capacity measured or used to calculate thermal conductivity. The high anharmonicity of these materials can lead to significant deviations at high temperatures from the Dulong-Petit heat capacity, which is often a reasonable substitute for measurements at high temperatures...
Band convergence has been proven as an effective approach for enhancing thermoelectric performance, particularly in p-type IV-VI semiconductors, where the superior electronic performance originates from the contributions of both L and Σ band valleys when they converge to have a small energy offset. When alloying with cubic IV-VI semiconductors, CdTe has been found as an effective agent for achieving such a band convergence. This work focuses on the effect of CdTe-alloying on the thermoelectric transport properties of GeTe, where the carrier concentration can be tuned in a broad range through a Bi-doping on Ge site. It is found that CdTe-alloying indeed helps to converge the valence bands of GeTe in both low-T rhombohedral and high-T cubic phases for an increase in Seebeck coefficient with a decrease in mobility. In addition, the strong phonon scattering due to the existence of high-concentration Cd/Ge and Bi/Ge substitutions leads the lattice thermal conductivity to be reduced to as low as 0.6 W/m-K. These lead to an effectively increased average thermoelectric figure of merit (zTave~1.2) in 300-800 K, which is higher than that of many IV-VI materials with CdTe-alloying or alternatively with MnTe-, MgTe-, SrTe-, EuTe- or YbTe-alloying for a similar band convergence effect.
N-type Mg3Sb2-based Zintl compounds with multi-valley conduction bands and low thermal conductivity exhibit high thermoelectric performance. Here, two strategies based on solid solution map and biaxial strain engineering are investigated for the optimization of thermoelectric performance by using the first-principles method and Boltzmann transport theory. In Mg3(Sb,Bi)2 solid solution, increasing Bi content leads to a decreased Seebeck coefficient and increased carrier mobility due to the changes in the electronic structure, which indicates that the optimal Bi content is urgently needed for the best electronic transport. The decreasing lattice thermal conductivity is mainly ascribed to the changes in Grüneisen parameter and Debye temperature. The maximum ZT of Mg3SbBi with the optimal carrier concentration of 3.38 × 1020 cm-3 at 725 K can be up to 2.75 or even higher when the thermal conductivity decreases from the experimental value through its low bound value. Additionally, biaxial strain engineering is also effective in the enhancement of the thermoelectric performance.
n-type conduction in Mg 3 Sb 1.5 Bi 0.5 system is achieved with La-doping on cation site with peak zT>1. La-doped samples exhibit much higher doping efficiency and dopability compared to other chalcogen-doped samples. This...
Zintl phases are ideal candidates for thermoelectric applications due to their rich chemistry and structural complexity. However, the persistent p-type conduction due to intrinsic defects strongly restricts their practical applications. Recently, several typical n-type Zintl materials have been designed, where Te-doped Mg3Sb1.5Bi0.5 as the most promising. To enhance its overall thermoelectric performance, we introduce Mn to synergistically optimize the electrical and thermal transport properties. Both experimental and computational results demonstrate that multiple conduction bands with high band degeneracy are responsible for the enhanced Seebeck coefficient. Mn doping on Mg sites changes the low-temperature carrier scattering mechanism from ionized impurity scattering to mixed scattering with acoustic phonons and ionized impurities, resulting in a significant enhancement of carrier mobility and therefore power factor. Simultaneously, the total thermal conductivity is observably reduced after Mn doping. We employed aberration-corrected scanning transmission electron microscopy (Cs-corrected STEM) to thoroughly investigate its hierarchical microstructure, including sub-micron grains, nanoscale Bi precipitates segregated at grain boundaries, nanoscale endotaxial Bi-rich precipitates within the Mg3Sb2 based matrix, as well as the resulting strain fields around these defects. The synergistic optimization of electrical and thermal transport contributes to extraordinary performance, namely a peak ZT ~ 1.85 at 723 K and an average ZT ~ 1.25 (from 300 K to 723 K), which are the highest ever reported in any n-type thermoelectric material.
Recent, and somewhat surprising, successful n-type doping of Mg3Sb2 was the key to realizing high thermoelectric performance in this material. Herein, we use first-principles defect calculations to investigate different extrinsic n-type doping strategies for Mg3Sb2 and to reveal general chemical trends in terms of dopant solubilities and maximal achievable electron concentrations. In agreement with experiments, we find that Sb substitution is an effective doping strategy, with Se and Te doping predicted to yield up to ~8×10¹⁹ cm⁻³ electrons. However, we also find that Mg substitution with trivalent (or higher) cations can be even more effective; in particular, the predicted highest achievable electron concentration ~4×10²⁰ cm⁻³ with La as an extrinsic dopant exceeds that of Se and Te doping. Interstitial doping (Li, Zn, Cu, Be) is found to be largely ineffective either due to self-compensation (Li) or high formation energy (Zn, Cu, Be). Our results offer La as an alternative dopant to Te and Se and reinforce the need for careful phase boundary mapping in achieving high electron concentrations in Mg3Sb2.
Thermoelectric materials offer an alternative opportunity to tackle the energy crisis and environmental problems by enabling the direct solid-state energy conversion. As a promising candidate with full potentials for the next generation thermoelectrics, tin selenide (SnSe) and its associated thermoelectric materials have been attracted extensive attentions due to their ultralow thermal conductivity and high electrical transport performance (power factor). To provide a thorough overview of recent advances in SnSe-based thermoelectric materials that have been revealed as promising thermoelectric materials since 2014, here, we first focus on the inherent relationship between the structural characteristics and the supreme thermoelectric performance of SnSe, including the thermodynamics, crystal structures, and electronic structures. The effects of phonon scattering, pressure or strain, and oxidation behavior on the thermoelectric performance of SnSe are discussed in detail. Besides, we summarize the current theoretical calculations to predict and understand the thermoelectric performance of SnSe, and provide a comprehensive summary on the current synthesis, characterization, and thermoelectric performance of both SnSe crystals and polycrystals, and their associated materials. In the end, we point out the controversies, challenges and strategies toward future enhancements of the SnSe thermoelectric materials.
Self-assembled 3D flower-like hierarchical Ti-doped Cu3SbSe4 microspheres have been synthesised by a microwave-assisted solvothermal method. Through detailed structural characterization and analysis, we elucidate that the growth temperature is the key factor to tailor the hierarchical Cu3SbSe4 microspheres assembled from nanoparticles and nanosheets. Such unique structures can strengthen phonon scatterings, leading to an ultralow thermal conductivity of 0.38 W m⁻¹ K⁻¹ at 623 K in the Ti-doped Cu3Sb0.93Ti0.07Se4 sample. Ti doping can also increase the hole concentration to the optimal level and consequently enhance the power factor in the Cu3Sb0.96Ti0.04Se4 sample, resulting in a promising peak zT of ~ 0.59 at 623 K, which approximately doubles that of the pristine Cu3SbSe4. This study demonstrates a facile wet chemical method to synthesise large-scale Cu3SbSe4-based thermoelectric materials, which provides an alternative methodology to fabricate non-toxic thermoelectric materials.
n-type Mg3Sb1.5Bi0.5 has recently been discovered to be a promising thermoelectric material, yet the effective n-type dopants are mainly limited to the chalcogens. This may be attributed to the limited chemical insight into the effects from different n-type dopants. By comparing the effects of different chalcogen dopants Q (Q = S, Se, and Te) on thermoelectric properties, it is found that the chalcogen dopants Q become more efficient with decreasing electronegativity difference between Q and Mg, which is mainly due to the increasing carrier concentration and mobility. Using density functional theory calculations, it is shown that the improving carrier concentration originates from the increasing doping limit induced by the stabilizing extrinsic defect. Moreover, the increasing electron mobility with decreasing electronegativity difference between Q and Mg is attributed to the smaller effective mass resulting from the enhancing chemical bond covalency, which is supported by the decreasing theoretical density of states. According to the above trends, a simple guiding principle based on electronegativity is proposed to shed new light on n-type doping in Zintl antimonides.
N-type Mg3Sb2-based Zintl compounds are proved to be high-performance thermoelectric materials with multiple degenerate valleys and low lattice thermal conductivity. Here, we investigate the electronic band structure and the thermoelectric properties of n-type Mg3Sb2 using the first-principles density functional theory. A high ZT of 3.1 at 725K is obtained when the minimum lattice thermal conductivity and the optimal carrier concentration are reached. The calculated thermoelectric performance demonstrate that Mg3Sb2 possesses isotropic character in thermoelectric transport. Furthermore, the calculated lattice thermal conductivity кL reveals that the unusually low кL in Mg3Sb2 predominantly originates from the large Grüneisen parameter .
Complex structures with versatile chemistry provide considerable chemical tunability of the transport properties. Good thermoelectric materials are generally extrinsically doped semiconductors with optimal carrier concentrations, while charged intrinsic defects (e.g., vacancies, interstitials) can also adjust the carriers, even in the compounds with no apparent deviation from a stoichiometric nominal composition. Here we report that in Zintl compounds Mg3+xSb1.5Bi0.5, the carrier concentration can be tuned from p-type to n-type by simply altering the initial Mg concentration. The spherical aberration-corrected (CS-corrected) high angle annular dark field scanning transmission electron microscope (HAADF-STEM) and energy-dispersive X-ray spectroscopy (EDX) mapping analysis show that the excess Mg would form a separate Mg-rich phase after Mg vacancies have been essentially compensated. Additionally, a slight Te doping at Bi site on Mg3.025Sb1.5Bi0.5 has enabled good n-type thermoelectric properties, which is comparable to the Te-doped Mg-rich sample. The actual final composition of Mg3.025Sb1.5Bi0.5 analyzed by EPMA is also close to the stoichiometry Mg3Sb1.5Bi0.5, answering the open question whether excess Mg is prerequisite to realize exceptionally high n-type thermoelectric performance by different sample preparation methods. The motivation for this work is first to understand the important role of vacancy and then to guide for discovering more promising n-type Zintl thermoelectric materials.
Thermoelectric figure of merit ZT has been greatly improved in the past decade via band engineering to enhance power factor or nanostructuring to reduce thermal conductivity, but less attention has been paid to other significant factors, e.g., carrier scattering mechanism, bipolar effect, etc. Here we show that Te doping on the Sb site, as an n-type strong donor, could significantly suppress the high-temperature bipolar effect in the nanostructured Zintl Zr3Ni3Sb4, which can be ascribed to the combination of high majority-carrier concentration and enlarged band gap. A relatively good ZT of ∼0.6 at 773 K for Te doping can be achieved and that is almost double of the previous reported ZT by Cu doping. In addition, the role of carrier scattering mechanism on the low-temperature electrical transport properties is also pointed out, where both carrier mobility and power factor of Te doping, due to the detrimental effect of ionized impurity scattering, are lower than that of Cu doping in which the mixed acoustic phonon and ionized impurity scattering dominates.
Point defects, which scatter the electronic carriers as well as phonons, play a vital role in the transport properties of thermoelectric materials. Therefore, the defect engineering can be utilized for tuning the thermoelectric properties. Mg vacancies, as the dominant defects in the n-type Mg3Sb2-based materials, can greatly impact the transport properties of this compound. Here we demonstrate that the Mg vacancies in the n-type Mg3Sb2-based materials can be successfully manipulated by simply tuning the preparation conditions. A substantial enhancement in the Hall mobility is obtained, from ~39 cm2 V-1 s-2 to ~128 cm2 V-1 s-2, an increase of ~228%. The significantly improved Hall mobility noticeably boosts the power factor from ~6 µW cm-1 K-2 to ~20 µW cm-1 K-2 and effectively enhances the thermoelectric figure of merit. Our results demonstrate that defect engineering could be very effective in improving the thermoelectric performance of n-type Mg3Sb2-based materials.
We develop a model based on first principles calculations and experimental data for the thermoelectric properties of p-type CaMg2Bi2. The thermoelectric performance was assessed and optimized based on the model. The model predicts bipolar reduction of ZT at high temperatures (e.g. > 800 K). This indicates the thermoelectric performance of the material can be further optimized by tuning the carrier concentration beyond the bipolar regime.
Thermoelectric materials have attracted extensive interest in the last two decades due to their potential applications in waste-heat recovery from industrial processes, automobiles, and renewable energy sources. Among the various candidate materials, Zintls have recently gained significant interest because of their high thermoelectric figure of merit (ZT) for potential use in thermoelectric power generation. In this review, we first briefly summarize some of the most intensely studied Zintl families with unique and diverse anionic frameworks, ranging from isolated moieties to one-dimensional (1D) chains of tetrahedra or frames of ribbons to 2D layered structures, along with discussion of their challenges and possibilities for further improvements. Second, the 2D layered CaAl2Si2-type Zintl phases are discussed in more detail, from fundamental crystal structure and electronic band structure to the approaches that have been successfully used to enhance the thermoelectric performance. Finally, we provide an overview of the recent progress in thermoelectric Zintl materials, particularly the most recent exciting development in achieving high ZTs in n-type Zintls, and what can be realistically expected for advancing this class of materials into practical applications.
Thermoelectric materials, capable of converting heat directly into electricity without moving parts, provide a promising renewable solid-state solution for waste heat harvesting. However, currently available commercial thermoelectric materials PbTe and Bi2Te3 are based on tellurium, an extremely scarce and high-cost element on earth, which prohibits large scale applications. Herein, we present a systematic study on a new low-cost Te-free material, n-type Se-doped Mg3.07Sb1.5Bi0.5, by combining the structure and property characterization with electronic structure and electrical transport modelling. Compared with pure Mg3Sb2, Se-doped Mg3.07Sb1.5Bi0.5 shows considerably enhanced power factor as well as much lower thermal conductivity. The excellent electrical transport originates from a nontrivial near-edge conduction band with six conducting carrier pockets and a light conductivity effective mass as well as the weak contribution from a secondary conduction band with a valley degeneracy of 2. The accurate location of the conduction band minimum is revealed from the Fermi surface, which appears to be crucial for the understanding of the electronic transport properties. In addition, the low thermal conductivity is induced by the point defect scattering. As a result, an optimal zT of 1.23 at 725 K is obtained in Mg3.07Sb1.5Bi0.48Se0.02. The high zT, as well as the earth-abundant constituent elements, makes the low-cost Se-doped Mg3.07Sb1.5Bi0.5 a promising candidate for the intermediate-temperature thermoelectric application. Moreover, the systematic electronic structure and transport modelling provide an insightful guidance for the further optimization of this material and other related Zintl compounds.
High thermoelectric power factor not only enables potentially high figure of merit ZT but also leads to large output power density, hence it is pivotal to find an effective route to improve power factor. Previous reports on manipulation of carrier scattering mechanisms (e.g., ionization scattering) were mainly focused on enhancing the Seebeck coefficient. On the contrary, here we demonstrate that by tuning the carrier scattering mechanism in n-type Mg3Sb2-based materials, it is possible to noticeably improve the Hall mobility, from ~19 cm2 V-1 s-1 to ~77 cm2 V-1 s-1, and hence substantially increasing the power factor by a factor of 3, from ~5 to ~15 µW cm-1 K-2. The enhancement in mobility is mainly due to the reason that ionization scattering has been shifted into mixed scattering between ionization scattering and acoustic phonon scattering, which less effectively scatters the carriers. The strategy of tuning carrier scattering mechanism to improve the mobility should be widely applicable to various materials systems for achieving better thermoelectric performance.
Thermoelectric performance in the layered Zintl phase n-type Mg3+ δ (Sb,Bi)2 is reported. Insertion of the excess Mg into the compounds is crucial for realizing n-type carrier transport with multivalley and isotropic character. An excellent ZT of 1.51 ± 0.06 at 716 K is achieved in the sintered polycrystals at the composition of Mg3.2 Sb1.5 Bi0.49 Te0.01 .
Mechanical robustness plays an important role in the manufacturing and assembling processes as well as reliable operation for thermoelectric devices. Herein, we first give a detailed study on the mechanical properties of nanostructured α-MgAgSb. The corresponding Young's modulus, nanoindentation hardness, compressive strength, and fracture toughness are 55.0 GPa, 3.3 GPa, 389.6 MPa, and 1.1 MPa m1/2, respectively, which have a close relationship with the intricate microstructure. Compared with other p-type thermoelectric materials, the good mechanical properties of nanostructured α-MgAgSb further highlight the realistic prospect for power generation.
SiC nano-powders and nano-wires with excellent toughness as well as high strength were incorporated in Mg2.16(Si0.3Sn0.7)0.98Sb0.02. The effect of the morphology and phase fraction of nano-SiC additives on the thermoelectric (TE) as well as mechanical properties of the composite was characterized in detail. It is found that, due to the pinning effect, fiber bridging and fiber pull-out mechanisms, the fracture toughness and the compressive strength of the composite with 0.8at.% SiC nano-powders or nano-wires are improved by about 50% and 30%, respectively. And the TE properties changed little, with a maximum ZT value of ~1.20 at 750K.
Driven by the prospective applications of thermoelectric materials, massive efforts have been dedicated to enhancing the conversion efficiency. The latter is governed by the figure of merit (ZT), which is proportional to the power factor (S(2)σ) and inversely proportional to the thermal conductivity (κ). Here, we demonstrate the synthesis of high-quality ternary Bi2Te3-xSex nanoplates using a microwave-assisted surfactant-free solvothermal method. The obtained n-type Bi2Te2.7Se0.3 nanostructures exhibit a high ZT of 1.23 at 480 K measured from the corresponding sintered pellets, in which a remarkably low κ and a shift of peak S(2)σ to high temperature are observed. By detailed electron microscopy investigations, coupled with theoretical analysis on phonon transports, we propose that the achieved κ reduction is attributed to the strong wide-frequency phonon scattering. The shifting of peak S(2)σ to high temperature is due to the weakened temperature dependent transport properties governed by the synergistic carrier scattering and the suppressed bipolar effects by enlarging the band gap.
SnTe, a lead-free rock-salt analogue of PbTe, having valence band structure similar to PbTe, recently has attracted attention for thermoelectric heat to electricity generation. However, pristine SnTe is a poor thermoelectric material because of very high hole concentration resulting from intrinsic Sn vacancies, which give rise to low Seebeck coefficient and high electrical thermal conductivity. In this report, we show that SnTe can be optimized to be a high performance thermoelectric material for power generation by controlling the hole concentration and significantly improving the Seebeck coefficient. Mg (2−10 mol %) alloying in SnTe modulates its electronic band structure by increasing the band gap of SnTe and results in decrease in the energy separation between its light and heavy hole valence bands. Thus, solid solution alloying with Mg enhances the contribution of the heavy hole valence band, leading to significant improvement in the Seebeck coefficient in Mg alloyed SnTe, which in turn results in remarkable enhancement in power factor. Maximum thermoelectric figure of merit, ZT, of ∼1.2 is achieved at 860 K in the high quality crystalline ingot of p-type Sn 0.94 Mg 0.09 Te.
Bismuth telluride based thermoelectric materials have been commercialized for a wide range of applications in power generation and refrigeration. However, the poor machinability and susceptibility to brittle fracturing of commercial ingots often impose significant limitations on the manufacturing process and durability of thermoelectric devices. In this study, melt spinning combined with a plasma-activated sintering (MS-PAS) method is employed for commercial p-type zone-melted (ZM) ingots of Bi0.5Sb1.5Te3. This fast synthesis approach achieves hierarchical structures and in-situ nanoscale precipitates, resulting in the simultaneous improvement of the thermoelectric performance and the mechanical properties. Benefitting from a strong suppression of the lattice thermal conductivity, a peak ZT of 1.22 is achieved at 340 K in MS-PAS synthesized structures, representing about a 40% enhancement over that of ZM ingots. Moreover, MS-PAS specimens with hierarchical structures exhibit superior machinability and mechanical properties with an almost 30% enhancement in their fracture toughness, combined with an eightfold and a factor of six increase in the compressive and flexural strength, respectively. Accompanied by an excellent thermal stability up to 200 °C for the MS-PAS synthesized samples, the MS-PAS technique demonstrates great potential for mass production and large-scale applications of Bi2Te3 related thermoelectrics.
Direct synthesis using elemental powders has been used to produce single-phase polycrystalline ε-Zn4Sb3 specimens as well as composite specimens having ε-Zn4Sb3 (majority phase) and Zn (minority phase). The effect of the Zn phase on the elastic, thermoelectric and mechanical properties was investigated in this alloy system. Thermoelectric properties of single-phase Zn4Sb3 at an ambient temperature are comparable to the published data for the sample prepared by a hot-pressing of ingot-melted alloy powders. Transport properties at room temperature were also evaluated. In addition, Young’s modulus and the bulk modulus of polycrystalline Zn4Sb3 were measured using a resonant-ultrasonic technique. The fracture toughness in this alloy system was determined by measuring the length of cracks that formed at the corners of pyramidal indentations used for hardness tests. It is shown that the addition of Zn increases the fracture toughness, but this is achieved at the cost of reducing the thermoelectric figure of merit.
Highly dense n-type Bi2Te3 dispersed with xvol% nano-SiC particles (x=0, 0.1, 0.5, 1.0) thermoelectric materials were fabricated by mechanical alloying (MA) and spark plasma sintering (SPS) method. The effects of nano-SiC addition on the thermoelectric and mechanical properties were studied. Compared with Bi2Te3 matrix, Bi2Te3 materials dispersed with nano-SiC show increased Seebeck coefficient, decreased electrical conductivity, and reduced thermal conductivity as well in the measured temperature range of 323–523K. The maximum dimensionless figure of merit (ZTmax) was improved from 0.99 for Bi2Te3 sample to 1.04 for 0.1vol% SiC dispersed Bi2Te3 sample at 423K. The Vickers hardness increased linearly from 0.62 to 0.79GPa as 1.0vol% SiC was added. The highest fracture toughness of 1.34MPam1/2 was obtained for 0.1vol% SiC addition, and the highest Young's modulus of 42.7GPa was obtained for 0.5vol% SiC added sample.
We present a detailed description and comparison of algorithms for performing ab-initio quantum-mechanical calculations using pseudopotentials and a plane-wave basis set. We will discuss: (a) partial occupancies within the framework of the linear tetrahedron method and the finite temperature density-functional theory, (b) iterative methods for the diagonalization of the Kohn-Sham Hamiltonian and a discussion of an efficient iterative method based on the ideas of Pulay's residual minimization, which is close to an order N-atoms(2) scaling even for relatively large systems, (c) efficient Broyden-like and Pulay-like mixing methods for the charge density including a new special 'preconditioning' optimized for a plane-wave basis set, (d) conjugate gradient methods for minimizing the electronic free energy with respect to all degrees of freedom simultaneously. We have implemented these algorithms within a powerful package called VAMP (Vienna ab-initio molecular-dynamics package), The program and the techniques have been used successfully for a large number of different systems (liquid and amorphous semiconductors, liquid simple and transition metals, metallic and semi-conducting surfaces, phonons in simple metals, transition metals and semiconductors) and turned out to be very reliable.
A simple formulation of a generalized gradient approximation for the exchange and correlation energy of electrons has been proposed by Perdew, Burke, and Ernzerhof (PBE) [Phys. Rev. Lett. 77, 3865 (1996)]. Subsequently Zhang and Yang [Phys. Rev. Lett. 80, 890 (1998)] have shown that a slight revision of the PBE functional systematically improves the atomization energies for a large database of small molecules. In the present work, we show that the Zhang and Yang functional (revPBE) also improves the chemisorption energetics of atoms and molecules on transition-metal surfaces. Our test systems comprise atomic and molecular adsorption of oxygen, CO, and NO on Ni(100), Ni(111), Rh(100), Pd(100), and Pd(111) surfaces. As the revPBE functional may locally violate the Lieb-Oxford criterion, we further develop an alternative revision of the PBE functional, RPBE, which gives the same improvement of the chemisorption energies as the revPBE functional at the same time as it fulfills the Lieb-Oxford criterion loc
Lead sulfide, a compound consisting of elements with high natural abundance, can be converted into an excellent thermoelectric material. We report extensive doping studies, which show that the power factor maximum for pure n-type PbS can be raised substantially to ~12 μW cm(-1) K(-2) at >723 K using 1.0 mol % PbCl(2) as the electron donor dopant. We also report that the lattice thermal conductivity of PbS can be greatly reduced by adding selected metal sulfide phases. The thermal conductivity at 723 K can be reduced by ~50%, 52%, 30%, and 42% through introduction of up to 5.0 mol % Bi(2)S(3), Sb(2)S(3), SrS, and CaS, respectively. These phases form as nanoscale precipitates in the PbS matrix, as confirmed by transmission electron microscopy (TEM), and the experimental results show that they cause huge phonon scattering. As a consequence of this nanostructuring, ZT values as high as 0.8 and 0.78 at 723 K can be obtained for nominal bulk PbS material. When processed with spark plasma sintering, PbS samples with 1.0 mol % Bi(2)S(3) dispersion phase and doped with 1.0 mol % PbCl(2) show even lower levels of lattice thermal conductivity and further enhanced ZT values of 1.1 at 923 K. The promising thermoelectric properties promote PbS as a robust alternative to PbTe and other thermoelectric materials.
Generalized gradient approximations (GGA{close_quote}s) for the exchange-correlation energy improve upon the local spin density (LSD) description of atoms, molecules, and solids. We present a simple derivation of a simple GGA, in which all parameters (other than those in LSD) are fundamental constants. Only general features of the detailed construction underlying the Perdew-Wang 1991 (PW91) GGA are invoked. Improvements over PW91 include an accurate description of the linear response of the uniform electron gas, correct behavior under uniform scaling, and a smoother potential. {copyright} {ital 1996 The American Physical Society.}
Hybrid Fock exchange/density functional theory functionals have shown to be very successful in describing a wide range of molecular properties. For periodic systems, however, the long-range nature of the Fock exchange interaction and the resultant large computational requirements present a major drawback. This is especially true for metallic systems, which require a dense Brillouin zone sampling. Recently, a new hybrid functional [HSE03, J. Heyd, G. E. Scuseria, and M. Ernzerhof, J. Chem. Phys. 118, 8207 (2003)] that addresses this problem within the context of methods that evaluate the Fock exchange in real space was introduced. We discuss the advantages the HSE03 functional brings to methods that rely on a reciprocal space description of the Fock exchange interaction, e.g., all methods that use plane wave basis sets. Furthermore, we present a detailed comparison of the performance of the HSE03 and PBE0 functionals for a set of archetypical solid state systems by calculating lattice parameters, bulk moduli, heats of formation, and band gaps. The results indicate that the hybrid functionals indeed often improve the description of these properties, but in several cases the results are not yet on par with standard gradient corrected functionals. This concerns in particular metallic systems for which the bandwidth and exchange splitting are seriously overestimated.