No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
Scalable synthesis of n-type Mg3Sb2-xBix for thermoelectric applications
Abstract
The highly efficient n-type Mg3Sb2-xBix thermoelectric materials hold great promise for application in power generation as well as refrigeration. Currently, n-type Mg3Sb2-xBix compounds with high zTs can be easily reproduced on a laboratory scale with ∼10 g per batch. However, scaling up the synthesis of Mg3Sb2-xBix with uniform high thermoelectric performance, which is critical for promoting this compound for practical applications, has yet to be achieved. Here we report a scalable preparation method based on Simoloyer ball-milling, which allows us to obtain over 1 kg Mg3.1Sb1.5Bi0.49Te0.01 powder in a single batch. Subsequently, samples with different diameters (ranging from a half inch to two inches) were successfully prepared and their thermoelectric performance was found to be comparable. In addition, a two-inch sample was sectioned into several parts, and the thermoelectric properties of the separate parts are also similar, indicating the high uniformity of the prepared large-scale sample. Importantly, the single-leg Mg3.1Sb1.5Bi0.49Te0.01 attains a high energy conversion efficiency of ∼12.9% under a temperature difference of ∼480 K at the hot-side temperature of 773 K. This study represents a step toward the practical application of Mg3Sb2-xBix for thermoelectric power generation.
... Compared with quick and significant progress in the Mg 3 Sb 2 -based TE materials, the Mg 3 Sb 2based devices progress hardly due to the challenges in electrode interface design and device assembling [24,25] . As a result, there is still a yawning gap between theoretical and experimental conversion efficiency (η), and output power density (ω) for the reported Mg 3 Sb 2 -based TE devices [18][19][20][26][27][28][29][30][31][32][33][34][35][36][37][38] . ...
... Interface contact problems have been extensively studied in classically commercial Bi 2 Te 3 devices [68,69] , but have not re-ceived sufficient attention in Mg 3 Sb 2 -based TE devices. Recently, researchers have attempted to use Fe [9,18,19,27,28,32] , Ni [9,33] , and Nb [34] as the TEiM layer for Mg 3 Sb 2 -based TE power generation devices. For instance, Ren et al. [31] , Pei et al. [30] , and Schierning et al. [18] respectively reported that the ρ c of Fe/Mg 3 (Sb, Bi) 2 interfaces are approximately 2.5, 14.6, and 26.6 µΩ cm 2 (Fig. 2). ...
... al. applied 304SS as TEiM instead of single-element Fe and obtained an excellent single-leg device with a η of 9% at a T h of 400 °C in 2020 [29] . Further development begins in 2021; TE module devices with p-type Bi 2 Te 3 [9,10,31,35,48,62] , MgAgSb [18,20,32,38,72] , CdSb [19] , GeTe [26,30] , and SKD [34] were reported sequentially and yielded great success. During this flourishing period, the key factors considering practice application are gradually concerned, including scalable fabrication [33] , electrical stability [30,62] , chemical stabil-ity [63] , and thermal stability [28,[34][35][36]38,62] . ...
Thermoelectric power generators enable the direct conversion between waste heat and electricity near room temperatures, providing an environmentally friendly solution toward mitigating the ever-increasing global energy issues. Over the past years, we have witnessed significant advances in Mg3Sb2-based thermoelectric conversion materials. However, the device-relative efforts lag behind the materials-level works. In this mini-review, we summarize the advances in Mg3Sb2-based thermoelectrics from materials to devices. Further, we shine some light on the device-level challenge, including the design of thermoelectric interface materials, the stability issue, and the system-level full-parameter optimization. Finally, we discuss the new application scenarios exploration to inspire confidence in device-level efforts towards practical applications.
... However, n-type Mg 3+δ Sb 2-x Bi x materials have unparalleled advantages over the n-type Bi 2 Te 3-y Se y compounds in terms of material costs and mechanical strength, leading to an alternative solution of pairing n-type Mg 3+δ Sb 2-x Bi x materials with p-type Bi 2 Te 3 -based materials for TE device assembly. Since the stability and mass production of n-type Mg 3+δ Sb 2-x Bi x have been explored recently [5][6][7] , seeking the proper contacts for n-type Mg 3+δ Sb 2-x Bi x is essential to guarantee long service life and high conversion efficiency for the assembled TE devices. ...
... High contact resistance could thus severely compromise the performance of a thermoelectric device due to the accompanying Joule heating. To verify the performance improvement of a TEG that could result from low contact resistance, a unicouple was assembled with NiFe/Mg 3+δ Bi 1.5 Sb 0.5 as the n-type leg and a p-type counterpart made via one-step pressing using Bi 0.4 Sb 1.6 Te 3 as the TE material and Fe as the contact material, and its energy conversion efficiency was measured using a homemade setup as shown in Figure 6A [5,21] . The thermoelectric transport properties of the TE materials were properly optimized toward room-temperature operation as shown in Supplementary Figure 7. ...
... The highest efficiency obtained was 6% at a temperature difference of 150 K and a hot-side temperature of 448 K. With such a decent efficiency, this unicouple already outperforms an intermediate TEG with a temperature difference of over 400 K [5,42,43] . ...
Reliable metal alloy contact for Mg 3+ Bi 1.5 Sb 0.5 thermoelectric devices. Soft Sci 2022;2:13. https://dx. Abstract Proper contacts between thermoelectric (TE) materials and electrodes are critical for TE power generation or refrigeration. The Bi-rich n-type Zintl material Mg 3+δ Bi 2-x Sb x exhibits very good TE performance near room temperature, which makes Mg 3+δ Bi 2-x Sb x-based compounds highly promising candidates to replace the Bi 2 Te 3-y Se y alloys, but ideal contacts that can match their TE performance have not yet been well studied. Here we investigate different metal (Ni and Fe) and metal alloy (NiFe, NiCr, NiCrFe, and stainless steel) contacts on n-type Mg 3+δ Bi 1.5 Sb 0.5. It is first shown that the low Schottky barrier and narrow depletion region resulting from the band degeneracy and high carrier concentration of a heavily doped TE material are beneficial for the formation of a low-resistivity ohmic contact with a metal or a metal alloy. Most fully optimized TE materials can take advantage of this. Second, it is found that the NiFe/Mg 3+δ Bi 1.5 Sb 0.5 contact exhibits excellent thermal stability and the lowest ohmic contact resistivity among those studied after aging for over 2100 h, which is attributed to the formation of metallic NiMgBi between the NiFe and Mg 3+δ Bi 1.5 Sb 0.5 layers. As a buffer phase, NiMgBi can effectively prevent elemental diffusion without negatively affecting the electron transport. Benefiting from such low contact resistance, a Mg 3+δ Bi 1.5 Sb 0.5 /Bi 0.4 Sb 1.6 Te 3 unicouple exhibits competitive conversion efficiency, 6% with a 150 K temperature difference and a hot-side temperature of 448 K.
... More importantly, Mg 3 Sb 1.5 Bi 0.5 exhibited excellent n-type TE properties at middle-high temperatures [ 20 , 21 ]. Recently, Fe [22][23][24][25] and Ni [24] have been applied in the fabrication of Mg 3 (Sb, Bi) 2 devices with remarkable results, although a detailed analysis of the contact interface was not conducted. Liang [26] and Ying [27] reported ρ c values for the Fe/Mg 3 (Sb, Bi) 2 interfaces of approximately 2.5 and 26.6 μ cm 2 , respectively. ...
... More importantly, Mg 3 Sb 1.5 Bi 0.5 exhibited excellent n-type TE properties at middle-high temperatures [ 20 , 21 ]. Recently, Fe [22][23][24][25] and Ni [24] have been applied in the fabrication of Mg 3 (Sb, Bi) 2 devices with remarkable results, although a detailed analysis of the contact interface was not conducted. Liang [26] and Ying [27] reported ρ c values for the Fe/Mg 3 (Sb, Bi) 2 interfaces of approximately 2.5 and 26.6 μ cm 2 , respectively. ...
The reliability of thermoelectric power generators (TEGs) depends heavily on the contact interface between thermoelectric (TE) materials and electrodes. We propose a general alloying approach for generating TE interface materials (TEiMs) for n-type Mg3Sb1.5Bi0.5 systems. The TEiM serves as a metallisation layer or barrier that precedes the soldering assembly. We first selected Fe from 15 elements as the matrix element considering the criteria of high bonding strength and low interfacial resistivity. Following the principles of high bonding propensity, coefficient of thermal expansion (CTE) matching, diffusion passivation, and dopant inactivation, two types of ternary alloys (Fe7Mg2Cr and Fe7Mg2Ti) with shear strengths (σs) of > 40 MPa and the specific contact resistivities (ρc) of < 5 μΩ cm² were obtained. Furthermore, the thermal stability of the TEiM/Mg3Sb1.5Bi0.5 contact interface was investigated employing aging treatment. The contact interface exhibited high shear strength (σs > 30 MPa), low specific contact resistivity (ρc < 10 μΩ cm²), and excellent thermal stability after aging treatment at 400°C for 15 days. The general TEiM design strategy presented herein will contribute to further optimization of contact interfaces in TEG devices.
... Among them, κ ele is directly proportional to σ, T, and the Lorenz number L according to the Wiedemann-Franz law, κ ele = LσT. 1 Considering that κ lat is an independent parameter with electrical properties, numerous strategies for reducing κ lat have attracted extensive attention from researchers for realizing higher ZT in various thermoelectric systems. [25][26][27][28][29][30][31][32][33][34][35] In the theory of condensed matter physics, κ lat mainly originates from two parts: the anharmonicity of atom bonding and extrinsic crystal defects with multiple dimensions and scales. 25,[36][37][38][39][40] In solids, atoms vibrate around their equilibrium positions, and harmonic vibration in crystal lattice is characterized as "phonon" in quantum theory, which can also be regarded as a heatcarrying quasiparticle. ...
... 25,[63][64][65][66] Simultaneously, these extrinsic defects play a key role in strengthening phonon scattering and reducing κ lat . 37,67,68 Zero-dimensional (0D) point defects, such as atomic substitutions, vacancies, and interstitial and filling atoms in specific structures, can strongly scatter high-frequency phonons by inducing atomic-scale lattice distortions and high-density strains around them, 22,32,[69][70][71][72] thereby decreasing κ lat . One-dimensional (1D) linear defects, mainly existing as edge dislocations in thermoelectric materials, scatter mid-frequency phonons. ...
Heat transport has various applications in solid materials. In particular, the thermoelectric technology provides an alternative approach to traditional methods for waste heat recovery and solid‐state refrigeration by enabling direct and reversible conversion between heat and electricity. For enhancing the thermoelectric performance of the materials, attempts must be made to slow down the heat transport by minimizing their thermal conductivity (κ). In this study, a continuously developing heat transport model is reviewed first. Theoretical models for predicting the lattice thermal conductivity (κlat) of materials are summarized, which are significant for the rapid screening of thermoelectric materials with low κlat. Moreover, typical strategies, including the introduction of extrinsic phonon scattering centers with multidimensions and internal physical mechanisms of materials with intrinsically low κlat, for slowing down the heat transport are outlined. Extrinsic defect centers with multidimensions substantially scatter various‐frequency phonons; the intrinsically low κlat in materials with various crystal structures can be attributed to the strong anharmonicity resulting from weak chemical bonding, resonant bonding, low‐lying optical modes, liquid‐like sublattices, off‐center atoms, and complex crystal structures. This review provides an overall understanding of heat transport in thermoelectric materials and proposes effective approaches for slowing down the heat transport to depress κlat for the enhancement of thermoelectric performance.
image
... In a later study, Ni was found to play a role similar to that of Fe as a contact layer (Mao et al. 2019a). Ni can also ensure a low electrical contact resistance for soldering once it is electroplated on both sides of the Mg 3 (Sb, Bi) 2 thermoelectric leg (Shang et al. 2020b;Xu et al. 2021). On the other hand, the Fe contact layer could degrade upon long-term aging, which may be attributed to the mismatched CTE values and the undesirable interface reaction between the thermoelectric material and Fe. ...
... This weak dependence can be traced back to the stiffening of low-lying phonons and diminished anharmonicity at high temperatures, as indicated by the authors [36]. Mg 3 Sb 2 and Mg 3 Bi 2 can be combined in different stoichiometric ratios [37][38][39][40]. The combination of these compounds has been found to lower lattice thermal conductivity [41]. ...
Mg 3 (Bi x Sb 1−x) 2 (0 ≤ x ≤ 1) nanocomposites are a highly appealing class of thermoelectric materials that hold great potential for solid-state cooling applications. Tuning of the lattice thermal conductivity is crucial for improving the thermoelectric properties of these materials. Hereby, we investigated the lattice thermal conductivity of Mg 3 (Bi x Sb 1−x) 2 nanocomposites with varying Bi content (x = 0.0, 0.25, 0.5, 0.75, and 1.0) using first-principles calculations. This study reveals that the lattice thermal conductivity follows a classical inverse temperature-dependent relationship. There is a significant decrease in the lattice thermal conductivity when the Bi content increases from 0 to 0.25 or decreases from 1.0 to 0.75 at 300 K. In contrast, when the Bi content increases from 0.25 to 0.75, the lattice thermal conductivity experiences a gradual decrease and reaches a plateau. For the nanohybrids (x = 0.25, 0.5, and 0.75), the distribution patterns of the phonon group velocity and phonon lifetime are similar, with consistent distribution intervals. Consequently, the change in lattice thermal conductivity is not pronounced. However, the phonon group speed and phonon lifetime are generally lower compared to those of the pristine components with x = 0 and x = 1.0. Our results suggest that the lattice thermal conductivity is sensitive to impurities but not to concentrations. This research provides valuable theoretical insights for adjusting the lattice thermal conductivity of Mg 3 (Bi x Sb 1−x) 2 nanocomposites.
... At a single-leg level, efforts have been made in terms of scalable synthesis of n-type Mg 3 (Sb,Bi) 2 , design of reliable junction interfaces, and screening of barrier layers. [13][14][15] A noteworthy result is that a single-leg efficiency of ~10% could be achieved at a temperature difference of 400 K with a heat source temperature of 700 K, [14] indicating good potential for medium-temperature power generation applications. At a unicouple or module level, different p-type TE compounds, such as Bi 2 Te 3 , MgAgSb, GeTe, CdSb, and CoSb 3 , have been used for pairing with n-Mg 3 Sb 2 . ...
Thermoelectric modules can convert waste heat directly into useful electricity, providing a clean and sustainable way to use fossil energy more efficiently. Mg3Sb2-based alloys have recently attracted considerable interest from the thermoelectric community due to their nontoxic nature, abundance of constituent elements, and excellent mechanical and thermoelectric properties. However, robust modules based on Mg3Sb2 have progressed less rapidly. Here, we develop multiple-pair thermoelectric modules consisting of both n-type and p-type Mg3Sb2-based alloys. Thermoelectric legs based on the same parent fit in each other in terms of thermomechanical properties, facilitating module fabrication and ensuring low thermal stress. By adopting a suitable diffusion barrier layer and developing a new joining technique, an integrated all Mg3Sb2-based module demonstrates a high efficiency of 7.5% at a temperature difference of 380 K, exceeding the state-of-the-art same-parent thermoelectric modules. Moreover, the efficiency remains stable during 150 thermal cycling shocks (∼225 h), demonstrating excellent module reliability.
... Furthermore, various binary and ternary of Mg3X2-based Zintl compounds were investigated, such as Mg3Bi2 [36][37], Mg3Sb2 [38][39] and Mg3Sb2-xBix [40][41][42][43]. In addition, J. Xin et al. [44] have studied p-type Mg3X2 (X= Sb, Bi) single crystals by using a self-flux method and the Debye-Callaway model. ...
Using the density functional theory (DFT) in combination with Boltzmann transport theory, the influence of Mg concentrations (x) doping on the thermoelectric properties of Hg1-xMgxSe ternary alloys was systematically investigated. The generalized gradient approximations of Perdew-Burke-Ernzerhof (GGA-PBE) have been used to illustrate the exchange correlation potential. Various thermoelectric transport parameters, such as the Seebeck coefficient (S), the thermal conductivity over relaxation time, the electrical conductivity over relaxation time, the power factor (PF) and the figure of merit (ZT) have been deduced and discussed. The obtained results of thermoelectric properties show that the studied materials can be useful for room temperature thermoelectric devices. It is also found that Mg compositions can increase the thermal efficiency of the HgSe alloy.
... 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]. ...
... These compounds' relatively inexpensive constituent elements, robust mechanical properties, and simple scalable synthesis make n-type Mg 3+x (Sb, Bi) 2 a highly promising class of candidates for commercial applications. As a result, the development of thermoelectric devices based on these materials is rapidly evolving [10,[20][21][22][23][24][25][26][27][28][29][30]. Despite past advances, many scientific and engineering obstacles are encountered during thermoelectric device development using n-type Mg 3+x (Sb, Bi) 2 -based materials, particularly regarding their thermal instability, which is a critical but often ignored problem. ...
N-type Mg3+x(Sb, Bi)2-based thermoelectrics have quickly attracted considerable interest because of their excellent thermoelectric performance over a wide temperature range. Most studies on these compounds have thus far focused on improving their thermoelectric performance, with little consideration given to the equally essential issue of thermal stability. Mg3+x(Sb, Bi)2 is highly disordered due to having many kinds of defects, resulting in features like low thermal conductivity. However, the lattice distortion introduced by defects and the evolution of non-equilibrium defects both may impair the thermal stability of these thermoelectric materials. Additionally, incorporating Mg as the most prominent element in Mg3+x(Sb, Bi)2 has a significant impact on the compound’s initial defect concentration and its stability performance at high temperatures due to Mg loss resulting from the element’s high vapor pressure. Here we used in situ stability testing to reveal the evolution of intrinsic defects in n-type Mg3+xSb1.5Bi0.49Te0.01. A low-temperature annealing treatment was employed to improve stability by regulating non-equilibrium defects. Results from both experiments and theoretical calculations show that filling vacancy defects with transition metals rather than with additional excess Mg is effective in improving thermal stability due to the resulting enhanced chemical bonding and increased defect formation energy. This study has important implications for understanding and overcoming instability in other similar thermoelectric materials.
... N-type Mg 3 Sb 2 -based materials exhibit outstanding thermoelectric performance [25][26][27][28][29][30][31][32][33][34]. Remarkable device performance for power generation and cooling has been demonstrated [33,[35][36][37][38][39][40]. ...
The thermoelectric parameters are essentially governed by electron and phonon transport. Since the carrier scattering mechanism plays a decisive role in electron transport, it is of great significance for the electrical properties of thermoelectric materials. As a typical example, the defect-dominated carrier scattering mechanism can significantly impact the room-temperature electron mobility of n-type Mg 3 Sb 2 -based materials. However, the origin of such a defect scattering mechanism is still controversial. Herein, the existence of the Mg vacancies and Mg interstitials has been identified by synchrotron powder X-ray diffraction. The relationship among the point defects, chemical compositions, and synthesis conditions in Mg 3 Sb 2 -based materials has been revealed. By further introducing the point defects without affecting the grain size via neutron irradiation, the thermally activated electrical conductivity can be reproduced. Our results demonstrate that the point defects scattering of electrons is important in the n-type Mg 3 Sb 2 -based materials.
... Due to the inevitable heat loss under high temperatures, a precise measurement of the hot side heat flux is challenging. Alternatively, considering the relationship among heat fluxes and power output, i.e., Q h = P + Q c , the cold side heat flux could be measured instead, and the conversion efficiency can be rewritten as η = P/ (P + Q c ). [41,52,53] Compared to the measurement of the hot side heat flux, the cold side heat flux measurement has the advantage of fewer heat losses caused 048502-5 by conduction, convection, and radiation due to the comparable temperature to the surroundings. Figures 2(a) and 2(c) depict the output power and efficiency measurement systems, in which a thermoelectric device is placed between a heat sink unit and a heater part made of nickel ( Fig. 2(a), a commercial system by ULVAC [54] ) or copper (Fig. 2(b) by AIST [55] and Fig. 2(c) by lab-made system [17,56] ). ...
Thermoelectric power generation provides us the unique capability to explore the deep space and holds promise for harvesting the waste heat and providing a battery-free power supply for IoTs. The past years have witnessed massive progress in thermoelectric materials, while the module-level development is still lagged behind. We would like to shine some light on the module-level design and characterization of thermoelectric power generators (TEGs). In the module-level design, we review material selection, thermal management, and the determination of structural parameters. We also look into the module-level characterization, with particular attention on the heat flux measurement. Finally, the challenge in the optimal design and reliable characterization of thermoelectric power generators is discussed, together with a calling to establish a standard test procedure.
... As a result, the 20-mm pellets with stable, uniform distribution of high thermoelectric properties would lay the basis for the mass production of Mg 3.2 Bi 1.4975 Sb 0.5 Te 0.0025 ll and fabrication of high-performance cooling modules. 52 The post hot deformation process developed in this work, simple but effective, truly triggers the transformation of Mg 3 (Sb,Bi) 2 from material study to real device applications. ...
Full-scale thermoelectric cooling modules (7, 31, and 71 pairs) based on n-type Mg 3.2 Bi 1.4975 Sb 0.5 Te 0.0025 and p-type (Sb 0.75 Bi 0.25) 2 (Te 0.97 Se 0.03) 3 have been successfully fabricated in our work. Enhanced stability of Mg 3 (Bi,Sb) 2 material was realized by introducing a hot deformation process, and meanwhile highly reliable Mg 2 Cu contact layer to Mg 3 (Bi,Sb) 2 material was developed. The superior thermoelectric cooling performance and service durability were achieved in our module, a striking benefit of 23% higher performance-cost ratio over commercial Bi 2 Te 3 enables it the promising candidate for next-generation thermoelectric cooling.
... As a result, the 20-mm pellets with stable, uniform distribution of high thermoelectric properties would lay the basis for the mass production of Mg 3.2 Bi 1.4975 Sb 0.5 Te 0.0025 ll and fabrication of high-performance cooling modules. 52 The post hot deformation process developed in this work, simple but effective, truly triggers the transformation of Mg 3 (Sb,Bi) 2 from material study to real device applications. ...
Full-scale thermoelectric cooling modules (7, 31, and 71 pairs) based on n-type Mg 3.2 Bi 1.4975 Sb 0.5 Te 0.0025 and p-type (Sb 0.75 Bi 0.25) 2 (Te 0.97 Se 0.03) 3 have been successfully fabricated in our work. Enhanced stability of Mg 3 (Bi,Sb) 2 material was realized by introducing a hot deformation process, and meanwhile highly reliable Mg 2 Cu contact layer to Mg 3 (Bi,Sb) 2 material was developed. The superior thermoelectric cooling performance and service durability were achieved in our module, a striking benefit of 23% higher performance-cost ratio over commercial Bi 2 Te 3 enables it the promising candidate for next-generation thermoelectric cooling.
... As one of the most promising types of TE material, inorganic materials, such as Bi 2 Te 3 , 35,36 PbTe, 37 half-Heuslers, 38 and Zintls, [39][40][41] have dominated the development of the TE field because of their excellent performance. The Bi 2 Te 3 -based compounds have demonstrated room-temperature zT values of >1 and high energy conversion efficiency values up to $6.6% at a DT value of 225 K. 42 Recent reports 43-45 on Ag 2 S-based materials with metal-like ductility and decent thermoelectric performance have attracted much attention, indicating good potential for flexible thermoelectric modules to be made. ...
Recently, flexible thermoelectric (TE) materials and devices have attracted extensive attention due to their capability to convert heat into electricity directly and their conformal contact with arbitrarily shaped heat sources, demonstrating great promise for application in selfpowered portable/wearable low power consuming electronics. Here, we review the state of the art in the development of flexible TEs, including TE modules and materials themselves. The remaining challenges that limit the practical application of flexible TE devices are discussed, and possible solutions and suggestions to guide future development are also provided in this perspective.
n‐Type Mg3(Sb, Bi)2 shows great prospects in mid‐temperature power generation due to its extraordinary thermoelectric (TE) performance, low‐cost, and environment‐friendly attributes. However, its practical application is hindered by the slow progress of its p‐type counterpart. Herein, a high ZT of ≈0.9 at 773 K is achieved for p‐type Na0.01Mg1.7Zn1Yb0.25Sb2 through manipulation of the electronic transport properties. Then thermal expansion matched barrier material Fe75Sb25 is subtly obtained for Na0.01Mg1.7Zn1Yb0.25Sb2. The contact resistivity no longer increases significantly after aging for 72 hours at 673 K and tends to stabilize, remaining below ≈17 µΩ cm² after 168 hours. Paired with the high‐performance n‐type Mg3(Sb, Bi)2, the transient liquid phase bonding technique is adopted to connect the hot side of the all‐Mg3Sb2‐based legs to electrode at low temperature, which enables service at high temperature. This all‐Mg3Sb2‐based module displays a high efficiency of ≈8.3% at a temperature difference of ≈430 K and shows good thermal stability when the hot side temperature is maintained at 673 K. This work merits the great potential of the all‐Mg3Sb2‐based device for heat recovery.
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.
The rapid advancement of Internet of Things (IoT) technology and AI microchips has increased the demand for efficient and cost‐effective cooling solutions. However, traditional Bi2Te3‐based thermoelectric modules face the challenge of low abundance tellurium (Te). Although Te‐free modules using MgAgSb have been explored, they still suffer from a low price‐to‐performance ratio. To address these issues, this study investigates the development of alternative thermoelectric materials that are both cost‐effective and Te free. In this work, the potential of cost‐effective Te‐free alternatives is explored for thermoelectric applications by developing a high‐performance module composed of amorphous carbon‐modulated Mg3(Bi,Sb)2 and electron‐poor CdSb. The modules of CdSb/Bi2Te3 and CdSb/Mg3(Bi,Sb)2 demonstrate superior refrigeration performance, achieving a maximum temperature difference (ΔTmax) of 49.2 and 46 K, respectively. Notably, the material cost of CdSb/Mg3(Bi,Sb)2 module is only 5.5% of Te‐free modules built on MgAgSb, highlighting a significant economic advantage. This work provides a viable, ultralow‐cost approach to meet general refrigeration needs, thereby enhancing the practical value and application potential of thermoelectric materials.
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.
The successful deployment of thermoelectric materials necessitates the concurrent development of high-performance p-type and n-type pairs situated within an identical matrix. Nevertheless, limiting by the low dopant solubility, the conventional doping often cannot transfer the Fermi level to the opposite carrier type. Here, the solubility limit of donor dopants was enhanced to achieve n-type GeSe by inducing additional cationic vacancies through raising crystal symmetry. Converting the intrinsic p-type nature of GeSe to n-type poses significant challenges, primarily due to the exceedingly low dopant solubility within its native orthorhombic structure. To overcome this, the In2Te3 alloying was initially employed to transition GeSe from orthorhombic to rhombohedral structure, simultaneously generating a large number of Ge vacancies. Following this, the introduction of Pb acts to mitigate the excessive Ge vacancies, steering the material toward a weak p-type character. Crucially, the elevated Ge vacancy concentration serves to extend the solubility limit of Bi donor dopant, which not only promotes the formation of cubic phase, but also enables the p–n type transition. As a result, a peak zT of 0.18 at 773 K was attained for the n-type cubic Ge0.55Bi0.2Pb0.25Se(In2Te3)0.1, marking an 18-fold enhancement in comparison with its n-type orthorhombic counterpart. This work attests to the efficacy of introducing vacancies through enhancing crystal symmetry as an effective means to expand dopant solubility, thereby offering valuable insights into the achievement of compatible p- and n-type chalcogenides within the same matrix.
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.
This work demonstrates a robust magnetism-induced diffuse scattering effect in improving the room-temperature thermoelectric performance.
Mg3(Sb,Bi)2-based Zintl compounds have attracted lots of attention due to its high thermoelectric performance and good mechanical property. However, thermally activated mobility, which is ascribed to the grain boundary scattering,...
Mg3Sb2-based n-type materials are consisted of earth-abundant elements and possess comparable thermoelectric properties with n-type Bi2Te3 at low temperatures, which make them promising candidates for cooling and power generation applications in terms of cost and performance. Substitution of Sb atom with chalcogen elements (Te, Se S) is a conventional method for n-type doping, but doping cations such as rare-earth elements and transition metals is also widely studied for its unique advantages. In this study, La and Mn were selected for co-doping of Mg3SbBi, and the thermoelectric performances of the doped materials were investigated. Mg3La0.005MnxSbBi (0 x 0.015) polycrystalline samples were made by sintering the fine powders of the mother alloy after arc melting, in which elemental Mn and LaSb compound were included for n-type dual doping. Considering the loss of Mg at elevated temperatures by vaporization, the molar ratio of Mg, Sb, and Bi in the mixture for arc melting was set to 4 : 1 : 1 with excess Mg. Analysis shows that all the samples are n-type, and the electrical conductivity of Mg3La0.005Mn0.015SbBi increased by 62% from the Mn-free Mg3La0.005SbBi at 298 K. In addition, the lattice thermal conductivity ( lat ) decreased with increasing Mn content in the measured temperature range of 298-623 K. The minimum value of lat was about 0.60 W m-1K-1 in Mg3La0.005Mn0.015SbBi at 523 K, which is about 19% smaller than that of the Mn-free sample. As a result of these enhancements in thermoelectric performance, the maximum figure of merit ( zTmax ) of 1.12 was obtained in Mg3La0.005Mn0.01SbBi and Mg3La0.005Mn0.015SbBi at 573 K, and the zT at 298 K increased by 73% to 0.35 in Mg3La0.005Mn0.015SbBi compared to Mn-free Mg3La0.005SbBi, which is beneficial to room-temperature applications.
Mg3(Sb, Bi)2‐based materials possess excellent room‐temperature thermoelectric performance, while poor interfacial behaviors occur when connected with metal electrodes due to the strong chemical activity and volatility of Mg element. In this study, a high efficiency of 7.1% under a temperature difference of 230 K is achieved in n‐Mg3(Sb, Bi)2/p‐Bi2Te3 thermoelectric module. When changing the interfacial layer from Fe powder to Fe foil, it effectively prevents a significant diffusion of both Mg and Bi elements from the material matrix to the interfacial layer, resulting in an extremely low contact resistivity ≈3.4 µΩ cm² that is almost one order lower than of that of Fe powder/Mg3(Sb, Bi)2 junction ≈30 µΩ cm². Particularly, a thin diffusion layer with a width of ≈2 µm is initially observed in the unannealed Fe foil/Mg3(Sb, Bi)2 junction. Even after thermal aging at 573 K for 28 days, the diffusion‐layer width is basically unchanged and its corresponding contact resistivity maintained as low as ≈5.8 µΩ cm². Overall, this work provides deep insights into interfacial design and paves the way for high‐performance and sustainable low‐grade waste heat recovery.
Mg3Sb2-based materials are promising candidates to replace n-type Bi2Te3 for cooling and power generation at low temperatures. Generally, the thermoelectric performance of a material is sensitively affected by synthesis process parameters, and among them, sintering temperature ( Ts ) is a critical one. In this study, n-type Mg3SbBi0.99Te0.01 polycrystalline samples were fabricated by mechanical alloying and spark plasma sintering (SPS), and the effects of varying Ts (923 – 1073 K) on the thermoelectric properties were investigated. Sintering Mg3SbBi0.99Te0.01 at an elevated temperature of 1073 K resulted in a notable increase in electrical conductivity at low temperatures below about 423 K. This is ascribed to a sharp reduction in carrier scattering by ionized impurities. For the same reason, the carrier mobility increased sharply at a Ts of 1073 K, which is a critical temperature for sintering in this study. Moreover, the Seebeck coefficient increased and thermal conductivity decreased simultaneously by raising Ts , resulting in the maximum power factor ( PFmax ) of 2.2 × 10-3 W m-1K-2 and the maximum dimensionless figure of merit ( zTmax ) of 1.20 in the sample sintered at 1073 K. Therefore, when Ts was raised from 923 K to 1073 K, the PFmax and zTmax increased by 29 % and 64 %, respectively. This improvement in performance is attributed to the annihilation of defects generated during the mechanical alloying process, which was confirmed by microstructure analysis by transmission electron microscopy (TEM).
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.
By using the density functional theory (DFT) in combination with Boltzmann transport theory, the influence of Mg concentrations (x) doping on the thermoelectric properties of Hg1−xMgxSe ternary alloys was systematically investigated. The generalized gradient approximations of Perdew–Burke–Ernzerhof (GGA-PBE) have been used to illustrate the exchange correlation potential. The thermodynamic stability of the studied compounds was analyzed in terms of formation energy. Various thermoelectric transport parameters, such as the Seebeck coefficient (S), the thermal conductivity over relaxation time (k/τ), the electrical conductivity over relaxation time (σ/τ), the power factor (PF) and the figure of merit (ZT) have been deduced and discussed. The obtained results of thermoelectric properties show that the studied materials can be useful for thermoelectric devices at room temperature. It can also be seen that Mg concentrations can increase the thermal efficiency of the HgSe alloy.
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.
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.
Bi2Te3‐based devices have long dominated the commercial market for thermoelectric cooling applications, but their narrow operating temperature range and high cost have limited their possible applications for conversion of low‐grade heat into electric power. The recently developed n‐type Mg3Sb2‐based compounds exhibit excellent transport properties across a wide temperature range, have low material costs, and are nontoxic, so it would be possible to substitute the conventional Bi2Te3 module with a reliable and low‐cost all‐Mg3Sb2‐based thermoelectric device if a good p‐type Mg3Sb2 material can be obtained to match its n‐type counterpart. In this study, by comprehensively regulating the carrier concentration, carrier mobility, and lattice thermal conductivity, the thermoelectric performance of p‐type Mg3Sb2 is significantly improved through Na and Yb doping in Mg1.8Zn1.2Sb2. Moreover, p‐ and n‐type Mg3Sb2 are similar in terms of their coefficients of thermal expansion and their good performance stability, thus allowing the construction of a reliable all‐Mg3Sb2‐based unicouple. The decent conversion efficiency (≈5.5% at the hot‐side temperature of 573 K), good performance stability, and low cost of this unicouple effectively promote the practical application of Mg3Sb2‐based thermoelectric generators for low‐grade heat recovery.
Realizing high‐temperature thermal stability in thermoelectric (TE) generators is a critical challenge. In this study, a synergistic interface and surface optimization strategy is implemented to enhance Mg3Sb1.5Bi0.5 TE generator performance by employing FeCrTiMnMg thermoelectric interface materials and the MgMn‐based alloy protective coating. The competitive output power density (ω) of 1.7 W cm⁻² and a conversion efficiency (η) of 13% for the single‐leg device are achieved at hot‐side temperature (Th) and cold‐side temperature (Tc) of 500 and 5 °C, respectively. An ω of 0.8 W cm⁻² and η of 6% for the two‐couple TE devices with p‐type commercial Bi2Te3 are also realized, values that are competitive with the commercial Bi2Te3 device. Additionally, the single‐leg device shows a high stable η for over 100 h when the Th and Tc are 400 and 5 °C, respectively, with an change rate (Δηmax/ηmax,o) of <3%. In situ transmission electron microscopy analysis further reveals that the high stability results from the effectively sluggish interdiffusion and reduced Mg evaporation that decrease the chemical potential gradient, reduce the saturated vapor pressure, and increase the diffusion activation energy barrier. This study provides a general technique route for boosting the high‐temperature thermal stability of TE generator.
Driven by the intensive efforts in the development of high‐performance GeTe thermoelectrics for mass‐market application in power generation and refrigeration, GeTe‐based materials display a high figure of merit of > 2.0 and an energy conversion efficiency beyond 10%. However, there still needs a comprehensive review on GeTe, from fundamentals to devices. In this regard, we timely review the latest progress on the state‐of‐the‐art GeTe. We fundamentally analyze the phase transition, intrinsic high carrier concentration, and multiple band edges of GeTe from the perspectives of the native atomic orbital, chemical bonding, and lattice defects. Then, the fabrication methods are summarized with a focus on large‐scale production. Afterward, we comprehensively review the strategies for enhancing electronic transports of GeTe by energy filtering effect, resonance doping, band convergence, and Rashba band splitting, and the methods for strengthening phonon scatterings via nanoprecipitates, planar vacancies, and superlattices. Besides, the device assembly and performance are highlighted. In the end, we conclude and propose future research directions, which will enlighten the development of broader thermoelectric materials. This article is protected by copyright. All rights reserved
In this work, we focused on the matrix material with a composition of Mg3Sb1.8Bi0.2. The effect of Pb doping on the thermoelectric properties of Mg3Sb1.8‐yBi0.2Pby (y=0, 0.005, 0.01, 0.02) compounds was investigated. Mechanical alloying followed by spark plasma sintering was employed to synthesize the corresponding bulk materials. The x‐ray diffraction results indicate that the doping limit of Pb in Mg3Sb1.8Bi0.2 is 0.01～0.02. Pb doping significantly increases the carrier concentration up to >1019 cm‐3, while decreasing the Seebeck coefficient. Moreover, the thermal conductivity of the matrix material is reduced upon Pb doping at 400‐700 K. As a result, the thermoelectric figure of merit is improved in the low‐temperature range, and a ZT value of 0.039 is obtained for the y=0.005 sample at 500 K, which is 77% higher than that of the undoped sample.
Thermoelectrics is defined as direct energy conversion between the heat and electricity associated with thermoelectric power generation and refrigeration. Significant progress has been made in the theoretical understanding, materials and device preparation technology, and performance improvement of thermoelectrics since the Seebeck effect was found 200 years ago. Herein, the article starts with the basic physics concepts underlying the thermoelectric phenomenon, and the key descriptor for evaluating thermoelectric performance, the thermoelectric figure of merit (ZT), is introduced. The corresponding issues and strategies for pursuing higher ZT are discussed in detail, and a brief introduction of organic thermoelectric materials is given. The leading engineering challenges from the material to the device are further elaborated based on the state-of-the-art device design. This article aims to provide the reader with a comprehensive overview of the scope of activities related to this area of scientific endeavor.
The performance of thermoelectric materials has been improved considerably in recent decades, making the concept of generating energy from waste heat via solid‐state thermoelectric devices more realistic. The construction of multi‐stage modular structures based on complex parameter optimization to maximize the efficiency of each material over its optimal operating temperature range has become an effective strategy for improving device performance. Here, multi‐segmented n‐type Mg3(Sb, Bi)2 with low‐contact‐resistance buffer layers is first fabricated, and phase‐transition‐suppressed cubic p‐type GeTe with enhanced thermoelectric performance is subsequently designed to match the segmented n‐type legs. A 3D finite‐element analysis model is then used to optimize the module size, providing higher energy conversion efficiency with an optimal average figure of merit over the entire operating temperature range. As a result, the prepared segmented‐Mg3(Sb, Bi)2/cubic‐GeTe module exhibits a high conversion efficiency of (12.8 ± 0.8)% at a hot‐side temperature of 773 K with a temperature difference of ≈480 K, which is also comparable to that of previously reported thermoelectric modules. This study increases the number of matching combinations among n‐/p‐type thermoelectric materials and further broadens the potential candidate material library for segmented thermoelectric devices.
Thermoelectric materials, which can directly convert the thermal energy into electricity, play an important role in the waste heat recovery. Sb2Si2Te6 is a promising medium-temperature thermoelectric material for the power...
In the current thermoelectric research framework, the material used to assemble a module is selected simply based on its transport properties while assuming that thermoelectric junction contact resistivity values are nearly constant for a specific material matrix, which raises concerns regarding incorrect material selection and thus deficient device performance. Here, by analyzing the thermoelectric performance of a series of Mg3.2SbxBi1.99-xTe0.01-based compounds, as well as their electrical contact resistivity across the Fe/Mg3.2SbxBi1.99-xTe0.01/Fe interface, we show that the contact resistivity in the n-type Mg3.2SbxBi2-x thermoelectric single leg is composition-dependent and then demonstrate that the high contact resistivity of a junction based on a material with even the highest figure of merit (zT) may considerably restrict performance at the device level. To overcome this dilemma, a multi-layered single leg design is proposed and investigated with the goal to reduce contact resistivity and thus maximize the conversion efficiency of thermoelectric devices.
Thermoelectric devices enable the direct conversion of heat flux into electrical energy, which have attracted considerable research interests for energy harvesting to address the challenges of energy sustainability. Owing to the emerging concepts or strategies, the dimensionless thermoelectric figure of merit (ZT), dominating the device’s conversion efficiency, has been significantly boosted during the last two decades. However, thermoelectric materials remain stagnant for practical applications. In this review, future challenges from a material perspective are discussed and emphasized. It includes fundamental theories, design criteria, material synthesis, and properties measurement. Our review tries to point out these important research directions in the near future, thereby enabling rationally developing thermoelectric science and pushing thermoelectric devices for large-scale applications.
P-type Mg3Sb2-based Zintl compounds remain largely unexplored due to their inferior ZT values compared to n-type counterparts. The intrinsic p-type Mg3Sb1.5Bi0.5 manifests relatively poor thermoelectric performance because of its low electrical conductivity of about 3 × 10² S m⁻¹ at room temperature. In this work, Ge is doped on the Bi site of Mg3Sb1.5Bi0.5, where the carrier scattering mechanism change from ionized impurity scattering to mixed ionized impurity and acoustic phonon scattering. A significant improvement in Hall mobility from ∼1.8 to ∼24 cm² V⁻¹ s⁻¹ is obtained, thus leading to a notably enhanced electrical conductivity of 2.2 × 10⁴ S m⁻¹ from 2.9 × 10² S m⁻¹. A simultaneous reduction in lattice thermal conductivity is also achieved. Collectively, a maximum ZT value of 0.5 is obtained at 723 K in Mg3Sb1.5Bi0.47Ge0.03 with a carrier concentration of 5.02 × 10¹⁹ cm⁻³.
Wearable thermoelectric generators have great potential to be utilized as the power supply for wearable electronics. However, the limited temperature difference across the thermoelectric generators significantly degrades the output performance, which is anticipated to be improved by enhancing the thermal radiation at the cold side without extra energy consumption. In this paper, the impact of thermal radiation on the performance of thermoelectric generators in different environments is simulated and the enhanced performance in a wearable thermoelectric generator combined with a radiative cooling coating is experimentally verified. Compared with the pristine device, the wearable thermoelectric generator with radiative cooling coating can not only achieve an ≈128% improvement of output power in exposed environments, but also exhibit an ≈96% improvement of output power in non‐exposed environments. The indoor output performance of the wearable thermoelectric generator with a radiative cooling coating due to its stable voltage output is extensively investigated, which shows an output power density of ≈5.5 μW cm–2 at the indoor temperature of 295 K, doubled that without a radiative cooling coating. This work paves a new way for further enhancing the performance of thermoelectric generators via passive radiative cooling.
Room-temperature thermoelectric materials provide promising solutions for energy harvesting from the environment, and deliver a maintenance-free power supply for the internet-of-things (IoTs). The currently available Bi2Te3 family discovered in the 1950s, still dominates industrial applications, however, it has serious disadvantages of brittleness and the resource shortage of tellurium (1 × 10⁻³ ppm in the earth's crust). The novel Mg3Sb2 family has received increasing attention as a promising alternative for room-temperature thermoelectric materials. In this review, the development timeline and fabrication strategies of the Mg3Sb2 family are depicted. Moreover, an insightful comparison between the crystallinity and band structures of Mg3Sb2 and Bi2Te3 is drawn. An outlook is presented to discuss challenges and new paradigms in designing room-temperature thermoelectric materials.
n-type Mg3Sb2-based materials have become a top candidate for efficient thermoelectric applications within 300-700K, due to its high band degeneracy, inherently high carrier mobility and low lattice thermal conductivity, as well as its advantages of less toxicity and abundance. Existing works showed that Mg3Bi2-alloying largely help ensure the exceptional performance, leaving a key issue to be uncovered on the primary mechanisms favoring or limiting the thermoelectric performance of Mg3Sb2-xBix alloys. Here we focus on the alloy composition dependent transport properties at various temperatures, with a large volume of experimental data. It is revealed that, with increasing x, the reduction in both inertial mass and lattice thermal conductivity is significantly beneficial, but the closure in band gap leads to a strong compensation due to the bipolar effect. Such a compromise between band structure and phonon scattering results in optimal Mg3Bi2-alloying concentrations to be about 50% ∼ 75% at 300 K, 50% ∼ 60% at 450K and 50% at 600 K, which successfully guiding this work to realize extraordinary thermoelectric figure of merit at these temperatures.
N-type Mg3Sb2-x Bi
x
alloys have been extensively studied in recent years due to their significantly enhanced thermoelectric figure of merit (zT), thus promoting them as potential candidates for waste heat recovery and cooling applications. In this review, the effects resulting from alloying Mg3Bi2 with Mg3Sb2, including narrowed bandgap, decreased effective mass, and increased carrier mobility, are summarized. Subsequently, defect-controlled electrical properties in n-type Mg3Sb2-x Bi
x
are revealed. On one hand, manipulation of intrinsic and extrinsic defects can achieve optimal carrier concentration. On the other hand, Mg vacancies dominate carrier-scattering mechanisms (ionized impurity scattering and grain boundary scattering). Both aspects are discussed for Mg3Sb2-x Bi
x
thermoelectric materials. Finally, we review the present status of, and future outlook for, these materials in power generation and cooling applications.
Since the first successful implementation of n-type doping, low-cost Mg3Sb2-x Bi
x
alloys have been rapidly developed as excellent thermoelectric materials in recent years. An average figure of merit zT above unity over the temperature range 300-700 K makes this new system become a promising alternative to the commercially used n-type Bi2Te3-x Se
x
alloys for either refrigeration or low-grade heat power generation near room temperature. In this review, with the structure-property-application relationship as the mainline, we first discuss how the crystallographic, electronic, and phononic structures lay the foundation of the high thermoelectric performance. Then, optimization strategies, including the physical aspects of band engineering with Sb/Bi alloying and carrier scattering mechanism with grain boundary modification and the chemical aspects of Mg defects and aliovalent doping, are extensively reviewed. Mainstream directions targeting the improvement of zT near room temperature are outlined. Finally, device applications and related engineering issues are discussed. We hope this review could help to promote the understanding and future developments of low-cost Mg3Sb2-x Bi
x
alloys for practical thermoelectric applications.
Solid-state thermoelectric devices can directly convert electricity into cooling or enable heat pumping through the Peltier effect. The commercialization of thermoelectric cooling technology has been built on the Bi2Te3 alloys, which have had no rival for the past six decades around room temperature. With the discovery and development of more promising materials, it is possible to reshape thermoelectric cooling technology. Here we review the current status of, and future outlook for, thermoelectric cooling materials.
The recently discovered n-type Mg3Sb2-xBix alloys with high thermoelectric performance hold great potential for applications in waste heat recovery due to their high average zT in the temperature range between 300 and 773 K. However, systematic studies of their thermal stability remain lacking and are significant for these thermoelectric materials since the possible degradation of their thermoelectric performance could greatly limit their practical applications. Here we studied the thermal stability of the Mg3Sb2-xBix alloys via in situ measurements of their thermoelectric properties at different temperatures, along with microstructural and composition characterizations. Our results show that Mg3Sb2-xBix alloys are unstable when the temperature is above 673 K due to significant Mg loss and changed microstructures. By coating Mg3Sb2-xBix alloys with boron nitride, the Mg loss can be effectively suppressed, thus greatly improving their thermal stability. Additionally, energy conversion efficiency measurements validated the high thermoelectric performance of the Mg3Sb2-xBix alloys and further confirmed the improved thermal stability of the boron-nitride-coated sample. Therefore, our study provides an effective strategy for improving the thermal stability of Mg3Sb2-xBix, thus promoting it as a promising candidate for thermoelectric power generation.
The rapid growth of the thermoelectric cooler market makes the development of novel room temperature thermoelectric materials of great importance. Ternary n-type Mg3(Bi,Sb)2 alloys are promising alternatives to the state-of-the-art Bi2(Te,Se)3 alloys but grain boundary resistance is the most important limitation. n-type Mg3(Bi,Sb)2 single crystals with negligible grain boundaries are expected to have particularly high zT but have rarely been realized due to the demanding Mg-rich growth conditions required. Here, we report, for the first time, the thermoelectric properties of n-type Mg3(Bi,Sb)2 alloyed single crystals grown by a one-step Mg-flux method using sealed tantalum tubes. High weighted mobility ~140 cm2V-1s-1 and a high zT of 0.82 at 315 K are achieved in Y-doped Mg3Bi1.25Sb0.75 single crystals. Through both experimental angle-resolved photoemission spectroscopy and theoretical calculations, we denote the origin of the high thermoelectric performance from a point of view of band widening effect and electronegativity, as well as the necessity to form high Bi/Sb ratio ternary Mg3(Bi,Sb)2 alloys. The present work paves the way for further development of Mg3(Bi,Sb)2 for near room temperature thermoelectric applications.
Considering the need for large quantities of high efficiency thermoelectric materials for industrial applications, a scalable synthesis method for high performance magnesium silicide based materials is proposed. The synthesis procedure consists of a melting step followed by high energy ball milling. All the materials synthesized via this method demonstrated not only high functional homogeneity but also high electrical conductivity and Seebeck coefficients of around 1000 Ω−1 cm−1 and −200 μV K−1 at 773 K, respectively. The measured values were similar for all the samples extracted from the ∅50 mm and ∅70 mm compacted pellets and were stable upon thermal cycling. Thermal stability experiments from 168 hours to 720 hours at 723 K revealed no significant change in the material properties. The low thermal conductivity of ∼2.5 W m−1 K−1 at 773 K led to a maximum figure of merit, zTmax, of 1.3 at the same temperature and an average value, zTavg, of 0.9 between 300 K and 773 K, which enables high efficiency in future silicide-based thermoelectric generators.
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.
Over the past years, thermoelectric Mg3Sb2 alloys particularly in n‐type conduction, have attracted increasing attentions for thermoelectric applications, due to the multivalley conduction band, abundance of constituents, and less toxicity. However, the high vapor pressure, causticity of Mg, and the high melting point of Mg3Sb2 tend to cause the inclusion in the materials of boundary phases and defects that affect the transport properties. In this work, a utilization of tantalum‐sealing for melting enables n‐type Mg3Sb2 alloys to show a substantially higher mobility than ever reported, which can be attributed to the purification of phases and to the coarse grains. Importantly, the inherently high mobility successfully enables the thermoelectric figure of merit in optimal compositions to be highly competitive to that of commercially available n‐type Bi2Te3 alloys and to be higher than that of other known n‐type thermoelectrics at 300–500 K. This work reveals Mg3Sb2 alloys as a top candidate for near‐room‐temperature thermoelectric applications. A tantalum‐sealing and hot‐deforming technique is developed for making n‐type Mg3Sb2–Mg3Bi2 alloys with coarse grains and minimal oxidation, which leads to a revelation of an inherently high mobility for extraordinary thermoelectric performance. Interestingly, these alloys with a nontoxic and cheap composition, show a great potential for thermoelectric applications, as an alternative to conventional Bi2Te3 thermoelectrics.
Materials with high zT over a wide temperature range are essential for thermoelectric applications. n‐Type Mg3Sb2‐based compounds have been shown to achieve high zT at 700 K, but their performance at low temperatures (<500 K) is compromised due to their highly resistive grain boundaries. Syntheses and optimization processes to mitigate this grain‐boundary effect has been limited due to loss of Mg, which hinders a sample's n‐type dopability. A Mg‐vapor anneal processing step that grows a sample's grain size and preserves its n‐type carrier concentration during annealing is demonstrated. The electrical conductivity and mobility of the samples with large grain size follows a phonon‐scattering‐dominated T−3/2 trend over a large temperature range, further supporting the conclusion that the temperature‐activated mobility in Mg3Sb2‐based materials is caused by resistive grain boundaries. The measured Hall mobility of electrons reaches 170 cm2 V−1 s−1 in annealed 800 °C sintered Mg3 + δSb1.49Bi0.5Te0.01, the highest ever reported for Mg3Sb2‐based thermoelectric materials. In particular, a sample with grain size >30 mm has a zT 0.8 at 300 K, which is comparable to commercial thermoelectric materials used at room temperature (n‐type Bi2Te3) while reaching zT 1.4 at 700 K, allowing applications over a wider temperature scale. By developing a novel annealing technique in Mg vapor, the high resistance grain boundaries of Mg3Sb1.49Bi0.5Te0.01 can be effectively eliminated. This increases the room temperature thermoelectric figure of merit, zT, from 0.3 to 0.8, making it the first real competitor to state‐of‐the‐art Bi2Te3‐based n‐type thermoelectric materials.
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.
The Bi2Te3−xSex family has constituted n‐type state‐of‐the‐art thermoelectric materials near room temperature (RT) for more than half a century, which dominates the active cooling and novel heat harvesting application near RT. However, the drawbacks of a brittle nature and Te‐content restricts the possibility for exploring potential applications. Here, it is shown that the Mg3+δSbxBi2−x family ((ZT)avg = 1.05) could be a promising substitute for the Bi2Te3−xSex family ((ZT)avg = 0.9–1.0) in the temperature range of 50–250 °C based on the comparable thermoelectric performance through a synergistic effect from the tunable bandgap using the alloy effect and the suppressible Mg‐vacancy formation using an interstitial Mn dopant. The former is to shift the optimal thermoelectric performance to near RT, and the latter is helpful to partially decouple the electrical transport and thermal transport in order to get an optimal RT power factor. The positive temperature dependence of the bandgap suggests this family is also a superior medium‐temperature thermoelectric material for the significantly suppressed bipolar effect. Furthermore, a two times higher mechanical toughness, compared with the Bi2Te3−xSex family, allows for a promising substitute for state‐of‐the‐art n‐type thermoelectric materials near RT.
Thermoelectric materials are capable of converting waste heat into electricity. The dimensionless figure-of-merit (ZT), as the critical measure for the material's thermoelectric performance, plays a decisive role in the energy conversion efficiency. Half-Heusler materials, as one of the most promising candidates for thermoelectric power generation, have relatively low ZTs compared to other material systems. Here we report the discovery of p-type ZrCoBi-based half-Heuslers with a record-high ZT of ∼1.42 at 973 K and a high thermoelectric conversion efficiency of ∼9% at the temperature difference of ∼500 K. Such an outstanding thermoelectric performance originates from its unique band structure offering a high band degeneracy (Nv) of 10 in conjunction with a low thermal conductivity benefiting from the low mean sound velocity (vm ∼2800 m s-1). Our work demonstrates that ZrCoBi-based half-Heuslers are promising candidates for high-temperature thermoelectric power generation.
Thermally activated mobility near room temperature is a signature of detrimental scattering that limits the efficiency and figure-of-merit zT in thermoelectric semiconductors. This effect has been observed dramatically in Mg_3Sb_2-based compounds, but also to a lesser extent in other thermoelectric compounds. Processing samples differently or adding impurities such that this effect is less noticeable produces materials with a higher zT. Experiments suggest that the behavior is related to grain boundaries, but impurity scattering has also been proposed. However, conventional models using Matthissen's rule are not able to explain the dramatic change in the temperature dependency of conductivity or drift mobility which is observed in Mg3Sb2-based compounds. We find that it is essential to consider the grain boundary region as an effectively separate phase rather than a scattering center, taking into account the weaker screening in semiconductors compared with classical metals. By modeling a grain boundary phase with a band offset, we successfully reproduce the experimentally observed conductivity versus temperature and thermopower versus conductivity relations, which indicate an improved description of transport. The model shows good agreement with measured grain size dependencies of conductivity, opening up avenues for quantitatively engineering materials with similar behavior. Model estimates predict room for >60% improvement in the room temperature zT of Mg_(3.2)Sb_(1.5)Bi_(0.49)Te_(0.01) if the grain boundary resistance could be eliminated.
The thermoelectric performance of Mg3+xSb1.5Bi0.49Te0.01 was improved by reducing the amount of excess Mg (x = 0.01-0.2). A 20% reduction in effective lattice thermal conductivity at 600 K was observed by decreasing the nominal x from 0.2 to 0.01 in Mg3+xSb1.5Bi0.49Te0.01, leading to a 20% improvement in the figure-of-merit zT. Since materials with different amounts of Mg have similar electronic properties, the enhancement is attributed primarily to the reduction in thermal conductivity. It is known that excess Mg is required to make n-type Mg3+xSb1.5Bi0.49Te0.01; however, too much excess Mg in the material increases the thermal conductivity and is therefore detrimental for the overall thermoelectric performance of the material.
Zintl compound n-type Mg3(Sb,Bi)2 was recently found to exhibit excellent thermoelectric figure of merit zT (∼1.5 at around 700 K). To improve the thermoelectric performance in the whole temperature range of operation from room temperature to 720 K, we investigated how the grain size of sintered samples influences electronic and thermal transport. By increasing the average grain size from 1.0 μm to 7.8 μm, the Hall mobility below 500 K was significantly improved, possibly due to suppression of grain boundary scattering. We also confirmed that the thermal conductivity did not change by increasing the grain size. Consequently, the sample with larger grains exhibited enhanced average zT. The calculated efficiency of thermoelectric power generation reaches 14.5% (ΔT = 420 K), which is quite high for a polycrystalline pristine material.
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.
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.
Significance
Thermoelectric materials generate electricity from temperature gradients. The dimensionless figure of merit, ZT = S ² ρ ⁻¹ κ ⁻¹ T , is calculated from the Seebeck coefficient ( S ), electrical resistivity ( ρ ), and thermal conductivity ( κ ). The calculated efficiency based on ZT using the conventional formula is not reliable in some cases due to the assumption of temperature-independent S , ρ , and κ . We established a new efficiency formula by introducing an engineering figure of merit ( ZT ) eng and an engineering power factor ( PF ) eng to predict reliably and accurately the efficiency of materials at a large temperature difference between the hot and cold sides, unlike the conventional ZT and PF providing performance only at specific temperatures. These new formulas will profoundly impact the search for new thermoelectric materials.
Mechanical alloying (MA) is a solid-state powder processng technique involving repeated welding, fracturing, and rewelding of powder particles in a high-energy ball mill. Originally developed to produce oxide-dispersion strengthened (ODS) nickel- and iron-base superalloys for applications in the aerospace industry, MA has now been shown to be capable of synthesizing a variety of equilibrium and non-equilibrium alloy phases starting from blended elemental or prealloyed powders. The non-equilibrium phases synthesized include supersaturated solid solutions, metastable crystalline and quasicrystalline phases, nanostructures, and amorphous alloys. Recent advances in these areas and also on disordering of ordered intermetallics and mechanochemical synthesis of materials have been critically reviewed after discussing the process and process variables involved in MA. The often vexing problem of powder contamination has been analyzed and methods have been suggested to avoid/minimize it. The present understanding of the modeling of the MA process has also been discussed. The present and potential applications of MA are described. Wherever possible, comparisons have been made on the product phases obtained by MA with those of rapid solidification processing, another non-equilibrium processing technique.
The theoretical work done by Lyndon Hicks and Mildred Dresselhaus 20
years ago on the effect of reduced dimensionality on thermoelectric
efficiency has had deep implications beyond the initial expectations.
Recently a significant figure-of-merit (ZT) improvement in the most-studied existing thermoelectric materials has been achieved by creating nanograins and nanostructures in the grains using the combination of high-energy ball milling and a direct-current-induced hot-press process. Thermoelectric transport measurements, coupled with microstructure studies and theoretical modeling, show that the ZT improvement is the result of low lattice thermal conductivity due to the increased phonon scattering by grain boundaries and structural defects. In this article, the synthesis process and the relationship between the microstructures and the thermoelectric properties of the nanostructured thermoelectric bulk materials with an enhanced ZT value are reviewed. It is expected that the nanostructured materials described here will be useful for a variety of applications such as waste heat recovery, solar energy conversion, and environmentally friendly refrigeration.
Thallium doping into lead telluride has been demonstrated to increase the dimensionless thermoelectric figure-of-merit (ZT) by enhancing Seebeck coefficient due to the creation of resonant states close to Fermi level without affecting the thermal conductivity. However, the process is tedious, energy consuming, and small in quantities since it involves melting, slow cooling for crystal growth, long time annealing, post-crushing and hot pressing. Here we show that a similar ZT value about 1.3 at 400 °C is achieved on bulk samples with grain sizes of 3–7 μm by ball milling a mixture of elemental thallium, lead, and tellurium and then hot pressing the ball milled nanopowders.
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.
A thermoelectric material’s efficiency in converting heat to electricity is its most important factor for considering the material for practical applications. Therefore, accurately measuring the conversion efficiency of any thermoelectric material is essential. However, the electrical contact layers without resistance between a thermoelectric material and electrodes, which are very difficult if not impossible to make, are required to reliably demonstrate the actual conversion efficiency. Here we report a double four-point probe method that can reliably measure the conversion efficiency of any thermoelectric material without the need to make the contact layers. In this method, copper foils acting as electrodes are mechanically pressed on the ends of the thermoelectric leg to conduct the measurement. Conversion efficiency for Mg3Sb2-based thermoelectric materials was measured to be ∼12.4% at a temperature difference of ∼420 °C, consistent with the predicted value from COMSOL Multiphysics, a finite element-analysis tool that reliably predicts the measured efficiencies. This method can be applied to any thermoelectric material without undertaking the tedious contact-fabrication process, which is important for rapidly screening thermoelectric materials, although practical applications still require reliable contacts.
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.
Identifying strategies for beneficial band engineering is crucial for the optimization of thermoelectric (TE) materials. In this study, we demonstrate the beneficial effects of ionic dopants on n‐type Mg3Sb2. Using the band‐resolved projected crystal orbital Hamilton population, the covalent characters of the bonding between Mg atoms at different sites are observed. By partially substituting the Mg at the octahedral sites with more ionic dopants, such as Ca and Yb, the conduction band minimum (CBM) of Mg3Sb2 is altered to be more anisotropic with an enhanced band degeneracy of 7. The CBM density of states of doped Mg3Sb2 with these dopants is significantly enlarged by band engineering. The improved Seebeck coefficients and power factors, together with the reduced lattice thermal conductivities, imply that the partial introduction of more ionic dopants in Mg3Sb2 is a general solution for its n‐type TE performance. © 2019 Wiley Periodicals, Inc.
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.
N-type Mg 3 Sb 2-based Zintl compounds have attracted considerable interest in recent years for their high thermoelectric performance. Mg 3 Sb 2-based compounds inherently have p-type transport properties because of the presence of intrinsic Mg vacancies. Therefore, eliminating Mg vacancies and increasing the electron concentration are crucial for achieving high-performance n-type Mg 3 Sb 2-based materials. The addition of excess Mg in the initial composition and the doping of chalcogens (Te, Se, and S) at the Sb site have been the primary methods used to date. Here, we demonstrate that n-type conduction was successfully achieved by a simple Y doping at the Mg site without adding extra Mg in the initial composition. Neutron diffraction analysis shows that Y preferentially goes to the Mg (Ⅱ) site and that almost all of the Mg vacancies are eliminated, giving rise to a strong donor effect to create n-type conduction. By eliminating the vacancies, the carrier concentration is easily optimized through the combination of Y and Mg, leading to a peak ZT of ~1.8 at 773 K in Mg 3.02 Y 0.02 Sb 1.5 Bi 0.5 .
Thermoelectric generators, capable of directly converting heat into electricity, hold great promise for tackling the ever-increasing energy sustainability issue. The thermoelectric energy conversion efficiency is heavily dependent upon the materials’ performance that is quantified by the dimensionless figure-of-merit (ZT). Therefore, the central issue in the research of thermoelectric materials lies in continuously boosting the ZT value. Although thermoelectric effects were discovered in 19th century, it was only until 1950s when classic materials like Bi2Te3 and PbTe were developed and basic science of thermoelectrics was established. However, the research of thermoelectrics did not take a smooth path but a rather tortuous one with ups and downs. After hiatus in 1970s and 1980s, relentless efforts starting from 1990s were devoted to understanding the transport and coupling of electrons and phonons, identifying strategies for improving the thermoelectric performance of existing materials, and discovering new promising compounds. Rewardingly, substantial improvements in materials’ performance have been achieved that broke the ZT limit of unity. Meanwhile, advancements in fundamental understanding related to thermoelectrics have also been made. In this Review, recent advances in the research of thermoelectric materials are overviewed. Herein, strategies for improving and decoupling the individual thermoelectric parameters are first reviewed, together with a discussion on open questions and distinctly different opinions. Recent advancements on a number of good thermoelectric materials are highlighted and several newly discovered promising compounds are discussed. Existing challenges in the research of thermoelectric materials are outlined and an outlook for the future thermoelectrics research is presented. The paper concludes with a discussion of topics in other fields but related to thermoelectricity.
Cheap and non-toxic n-type Mg3Sb2 based materials exhibit outstanding thermoelectric properties, but for actual applications it is essential to scrutinize their behavior under high temperature conditions. Here, powder samples of nominal composition Mg3Sb1.475Bi0.475Te0.05 have been subjected to repeated thermal cycling in the temperature range of 300 – 725 K, while measuring separate synchrotron powder X-ray diffraction and X-ray total scattering data. Approximately 11 wt% elemental bismuth crystallizes as a secondary phase after the first thermal cycle, but the evolution stagnates at approximately 15 wt% after the 10th thermal cycle. A significant decrease is found in the unit cell parameter of the Mg3Sb1.475Bi0.475Te0.05 phase after the first thermal cycle, indicating that bismuth release from the crystal structure. This is corroborated by the total scattering data, which do not detect an initial amorphous bismuth phase. In addition, STEM-EDS reveal a homogeneous distribution of antimony and bismuth in the as-synthesized particles, while a clear growth of pure bismuth is observed in a sample having been annealed at 725 K for one hour.
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.
Over the past couple of decades, thermoelectric Mg3Sb2 and its derivatives have attracted increasing attention for thermoelectric applications. This is enabled by the richness in composition for manipulating both electronic and thermal properties and by the intrinsic low lattice thermal conductivity. With existing efforts on these materials, the thermoelectric figure of merit has been significantly improved to compete with conventional thermoelectrics, while many of these materials keep the compositions cheap and less‐toxic elements only. Here, not only the control of defects, band structure, electronic transport properties, and lattice thermal conductivity for these materials, but also the proven strategies on transport property manipulation are summarized. These strategies are well demonstrated for advancing thermoelectric Mg3Sb2 and its derivatives, and the principles used are believed to be equally applicable for many other thermoelectric materials. In addition, perspectives for possible further advancements in this class of thermoelectric materials are shown.
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.
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.
Strategies for efficient thermoelectrics
Thermoelectric materials convert heat into electricity and can provide solid-state cooling for spot-sized refrigeration. One important barrier for adopting these materials beyond niche applications is their low efficiency. He and Tritt review the mechanisms and strategies for improving thermoelectric efficiency. They discuss how to report material performance and highlight the most promising materials. With new materials and strategies for performance enhancement, thermoelectrics are poised to alter the renewable energy landscape.
Science , this issue p. eaak9997
Significance
Higher carrier mobility can contribute to a larger power factor, so it is important to identify effective means for achieving higher carrier mobility. Since carrier mobility is governed by the band structure and the carrier scattering mechanism, its possible enhancement could be obtained by manipulating either or both of these. Here, we report a substantial enhancement in carrier mobility by tuning the carrier scattering mechanism in n-type Mg 3 Sb 2 -based materials. The ionized impurity scattering in these materials has been shifted into mixed scattering by acoustic phonons and ionized impurities. Our results clearly demonstrate that the strategy of tuning the carrier scattering mechanism is quite effective for improving the mobility and should also be applicable to other material systems.
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.
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 energy conversion system has great appeal in term of its silence, simplicity and reliability as compared with traditional power generator and refrigerator. The past two decades witnessed a significantly increased academic activities and industrial interests in thermoelectric materials. One of the most important impetuses for this boost is the concept of “nano”, which could trace back to the pioneer works of Mildred S. Dresselhaus at 1990s. Although the pioneer passed away, the story about the nano thermoelectric materials is still continuous. In this perspective, we will review the main mile stones along the concept of thermoelectric nanocomposites, and then discuss some new trends, strategies and opportunities.
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.
While considerable efforts have been made to develop and improve thermoelectric materials, research on thermoelectric modules is at a relatively early stage because of the gap between material and device technologies. In this review, we discuss the cumulative temperature dependence model to reliably predict the thermoelectric performance of module devices and individual materials for an accurate evaluation of the p-n configuration compared to the conventional model used since the 1950s. In this model, the engineering figure of merit and engineering power factor are direct indicators, and they exhibit linear correlations to efficiency and output power density, respectively. To reconcile the strategy for high material performance and the thermomechanical reliability issue in devices, a new methodology is introduced by defining the engineering thermal conductivity. Beyond thermoelectric materials, the device point of view needs to be actively addressed before thermoelectric generators can be envisioned as power sources.
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 .
High thermoelectric performance of mechanically robust p-type Bi2Te3-based materials prepared by melt spinning (MS) combined with plasma-activated sintering (PAS) method can be obtained with small, laboratory grown samples. However, large-size samples are required for commercial applications. Here, large-size p-type Bi2Te3-based ingots with 30, 40, and 60 mm in diameter are produced by MS-PAS, and the influence of temperature distribution during the sintering process on the composition and thermoelectric properties is systematically studied for the first time. Room-temperature scanning Seebeck Microprobe results show that the large-size ingot is inhomogeneous, induced by ellipsoidal-shape-distributed temperature field during the sintering process, which is verified by finite-element analysis. Although some temperature differences are unavoidable in the sintering process, homogeneity and mechanical properties of ingots can be improved by appropriately extending the sintering time and design of graphite die. Samples cut from ingots attain the peak ZT value of 1.15 at 373 K, about 17% enhancement over commercial zone-melted samples. Moreover, the compressive and bending strengths are improved by several times as well. It is important to ascertain that large-size p-type Bi2Te3-based thermoelectric materials with high thermoelectric performance can be fabricated by MS-PAS.
Traditional processes of making contacts (metallization layer) onto bulk crystalline Bi2Te3-based materials do not work for nanostructured thermoelectric materials either because of weak bonding strength or an unstable contact interface at temperatures higher than 200 °C. Hot pressing of nickel contact onto nanostructured thermoelectric legs in a one-step process leads to strong bonding. However, such a process results in large contact resistance in n-type Ni/Bi[subscript 2]Te[subscript 2.7]Se[subscript 0.3]/Ni legs, although not in p-type Ni/Bi[subscript 0.4]Sb[subscript 1.6]Te[subscript 3]/Ni legs. A systematic study was carried out to investigate the detailed reaction and diffusion at the interface of the nickel layer and n-type Bi[subscript 2]Te[subscript 3]-based thermoelectric material layer. We found that a p-type region formed within the n-type Bi[subscript 2]Te[subscript 2.7]Se[subscript 0.3] during hot pressing due to Te deficiency and Ni doping, leading to a large contact resistance.
Traditional processes of making contacts (metallization layer) onto bulk crystalline Bi2Te3-based materials do not work for nanostructured thermoelectric materials either because of weak bonding strength or an unstable contact interface at temperatures higher than 200 °C. Hot pressing of nickel contact onto nanostructured thermoelectric legs in a one-step process leads to strong bonding. However, such a process results in large contact resistance in n-type Ni/Bi2Te2.7Se0.3/Ni legs, although not in p-type Ni/Bi0.4Sb1.6Te3/Ni legs. A systematic study was carried out to investigate the detailed reaction and diffusion at the interface of the nickel layer and n-type Bi2Te3-based thermoelectric material layer. We found that a p-type region formed within the n-type Bi2Te2.7Se0.3 during hot pressing due to Te deficiency and Ni doping, leading to a large contact resistance.
Cryomilling, the mechanical attrition of powders within a cryogenic medium, is a method of strengthening materials through grain size refinement and the dispersion of fine, nanometer-scale particles. The technique was developed as a means to decrease both the size of these particles and their spacing within a metallic matrix to increase threshold creep stress and intermediate temperature performance. More recent work has been concerned with increasing the strength of lightweight structural materials. In this overview paper, the available literature is reviewed that covers the microstructural evolution during cryomilling, consolidation and processing, the thermal stability of the microstructure, and mechanical properties of consolidated materials. The properties of cryomilled materials are compared to those results for powders and consolidated materials generated by mechanical alloying, milling at ambient temperatures and other means to produce fine grained materials.
Thermoelectric materials are solid-state energy converters whose combination of thermal, electrical, and semiconducting properties
allows them to be used to convert waste heat into electricity or electrical power directly into cooling and heating. These
materials can be competitive with fluid-based systems, such as two-phase air-conditioning compressors or heat pumps, or used
in smaller-scale applications such as in automobile seats, night-vision systems, and electrical-enclosure cooling. More widespread
use of thermoelectrics requires not only improving the intrinsic energy-conversion efficiency of the materials but also implementing
recent advancements in system architecture. These principles are illustrated with several proven and potential applications
of thermoelectrics.