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[Raju Chetty](https://orcid.org/0000-0003-1072-8241), [Jayachandran Babu](https://orcid.org/0000-0002-1182-6655), [Takao Mori](https://orcid.org/0000-0003-2682-1846)

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This document is the unedited Author’s version of a Submitted Work that was subsequently accepted for publication in ACS Applied Energy Materials, copyright © 2024 American Chemical Society after peer review. To access the final edited and published work see https://doi.org/10.1021/acsaem.4c02794.[In Copyright](http://rightsstatements.org/vocab/InC/1.0/)

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[Improving Thermoelectric Conversion Efficiency of Mg<sub>3</sub>(Sb, Bi)<sub>2</sub>-Based TE Materials via Interface Contact Layer Optimization](https://mdr.nims.go.jp/datasets/eda3820a-5a77-4d20-b6c0-1f6e3bbc469e)

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Template for Electronic Submission to ACS JournalsImproving thermoelectric conversion efficiency of Mg3(Sb, Bi)2-based TE materials via interface contact layer optimization Raju Chettya, Jayachandran Babua, Takao Moria,b,*a Research Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), Tsukuba, 305-0044 (Japan)b Graduate School of Pure and Applied Sciences, University of Tsukuba, Tsukuba, 305-8577 (Japan)Corresponding Author* Corresponding author: MORI.Takao@nims.go.jp1. ABSTRACTMg3(Sb, Bi)2-based compounds have recently attracted intense attention as thermoelectric (TE) materials for power generation and cooling applications because of their high TE performance. The contact interface layers play a crucial role in achieving high conversion efficiency of TE devices. Iron contacts have often been used for the Mg3(Sb, Bi)2 compound, however, a large drawback for device fabrication is their incompatibility with solder. In this study, we developed Cupronickel as a potential interface contact layer for Mg3(Sb, Bi)2. A crack-free interface with low specific contact resistance of ~5 µΩcm2, enables a maximum conversion efficiency (max) of ~8% for the single-leg Cupronickel /Mg3(Sb, Bi)2/Cupronickel. Additionally, a max of ~7.8% is realized for a 2-pair module of Mg3(Sb, Bi)2 and MgAgSb at a temperature difference (ΔT) of 277 K. The optimization of the Cupronickel contact layer in this study has the potential to enhance the conversion efficiency of Mg3(Sb, Bi)2-based compounds.Keywords: Cupronickel, contact layer, Mg3(Sb, Bi)2, power generation, conversion efficiency, Mg3(Sb, Bi)22. INTRODUCTIONThermoelectric (TE) technology offers a promising solution for thermal management, contributing to zero-carbon emissions, and powering a myriad Internet of Things (IoT) sensors1. TE devices can significantly enhance energy efficiency and reduce greenhouse gas emissions by converting waste heat directly into electricity through the Seebeck effect2,3. As research advances, the development of high-performance, durable thermoelectric materials will be crucial in realizing the full potential of this technology for a sustainable, zero-carbon future4–6.Recently, Mg3(Sb, Bi)2-based compounds have garnered significant attention for TE power generation and cooling applications due to their high TE materials performance, cost-effectiveness, and abundance of elements7–12. Tamaki et al. reported13 a high thermoelectric figure of merit (ZT) of ~1.5 at 716 K for the n-type n-type Mg3(Sb, Bi)2-based compounds. Following this report, numerous efforts have been made to enhance the ZT of these compounds by adopting various approaches14–30. For example, significant improvement in the ZT have been achieved through interstitial doping22, microstructure control24, grain boundary engineering20,26,28, reducing structural disorder and microstructure evolution29, band engineering30, etc. However, progress towards the module development has not kept pace with materials development due to the challenges involved in designing the interface contact layer optimization11,31–33. Critical parameters related to the interfaces include high electrical and thermal contact resistances, interatomic diffusion, and crack formation when the TE modules operate at high temperatures. Establishing an effective contact between the TE materials and the contact layers with low specific contact resistances is crucial for suppressing the Joule heat loss in the TE module. A relationship between the material’s average ZT (ZTavg) and the device ZT (ZTD) is expressed as follows34:  (1)where L, σ, and c are the length, electrical conductivity, and specific contact resistance of the TE leg, respectively. Thus, significant focus has been directed toward developing contact interface materials for the recently emerging TE materials12,35–37. Metallic elements Fe38–40, Ni41,42, and Nb43 are used as contact layers for n-type Mg3(Sb, Bi)2-based compounds, with Fe being the most commonly used. For the Fe contact layer, a wide range of c values (~2.5 – 26.6  cm2) has been reported38–40,44–47. However, a drastic increase in c (~60  cm2) is observed after TE module operation, likely due to interatomic diffusion and chemical reactions at the  Fe/ Mg3(Sb, Bi)2 interface39. Furthermore, a significant drawback for device fabrication is Fe contact’s incompatibility with solder. Nb is used as a contact layer for Mg3(Sb, Bi)2, with c of 9.7  cm2, which increases to 26  cm2 after the TE module operation at 773 K for 360 h43. Ni is also investigated as a contact layer for Mg3(Sb, Bi)2. It is reported that the c at the Ni/Mg3.2Sb2Y0.05 interface is 18.56  cm2, which increases by 700% after aging at 673 K48. Despite showing the low c values for the metal contact layers, significant increase in c is found after thermal aging due to the chemical reaction and interdiffusion between the TE material and contact layer. Some metallic alloys such as 304 Stainless Steel (SS)45, Mg2Cu49, Mg4.3Sb3Ni48, and NiFe50 are reported with low c values. Moreover, the chemical reaction between the Ni and Mg3.2Sb2Y0.05 forms a buffer layer of Mg4.3Sb3Ni, which shows a c of ~ 10  cm2 after aging at 673 K for 20 days48. In another case, the NiFe contact layer exhibits a c of ~ 13  cm2 with good thermal stability after aging at 573 K for 2100 h50. Inspired by these studies, Wu et al. proposed a general alloying strategy to identify appropriate contact layers for Mg3(Sb, Bi)2 compounds by sequentially optimizing (1) bonding strength (σS), (2) bonding strength and specific contact resistance (c), and (3) σS, c, and thermal stability. Using this approach, two types of ternary alloys, Fe7Mg2Cr and Fe7Mg2Ti, with c < 5  cm2 were obtained47. These alloys shows good thermal stability with c < 10  cm2 after aging at 673 K for 15 days. Furthermore, a multi-component Fe0.2Mg0.2Cr0.2Ti0.2Mn0.2 alloy developed using the same approach, demonstrates a c of 4  cm2 with enhanced thermal stability51.Motivated by the alloying approach, we developed a Cu-Ni alloy (Cupronickel)-based contact layer for the n-type Mg3.2Sb1.5Bi0.49Te0.01Cu0.01 compound. In this study, we employ a two-step sintering approach to fabricate a Cupronickel/Mg3.2Sb1.5Bi0.49Te0.01Cu0.01/Cupronickel single TE leg. Microstructural investigations confirmed the formation of a crack-free and well-bonded interface between the Cupronickel and Mg3.2Sb1.5Bi0.49Te0.01Cu0.01. As a result, a low c of ~5  cm2 is obtained at the interface between the Cupronickel and Mg3.2Sb1.5Bi0.49Te0.01Cu0.01. Consequently, the single TE leg demonstrated a max of ~8% at a ΔT of 277 K. Moreover, the n-type compound with an optimized contact layer is coupled with the p-type MgAgSb-based compound with Ag contact to fabricate a two-pair TE module. This two-pair module demonstrates a max of ~7.8% at a ΔT of 277 K, which is promising for low-grade thermal management applications.3. EXPERIMENTAL SECTIONHigh purity elements Mg (99.999%, 5N plus), Sb (99.999%, 5N plus), Bi (99.999%, 5N plus), Te (99.999%, 5N plus), Cu (99.99%, Sigma-Aldrich), Ag (99.99%, Furuuchi Chemical) were weighed with the chemical composition of n-type Mg3.2Sb1.5Bi0.49Te0.01Cu0.01 and p-type Mg0.99Cu0.01Ag0.97Sb0.99. The sample mixtures were loaded into a stainless-steel ball milling jar inside an Ar-filled glove box. The n-type sample was prepared by one-step high-energy ball milling, whereas the p-type was prepared by two-step ball milling. A Cupronickel (Cu70Ni30, Nilaco Corporation) was used as a contact layer for the n-type Mg3.2Sb1.5Bi0.49Te0.01Cu0.01 and Ag (Kojundo) was used as a contact layer for the p-type Mg0.99Cu0.01Ag0.97Sb0.99. The ball-milled Mg3.2Sb1.5Bi0.49Te0.01Cu0.01 powder was sandwiched between the Cupronickel in the graphite die with an inner diameter of 10 mm and immediately sintered by one-step spark plasma sintering SPS (SPS, SPS-1080 System, SPS SYNTEX INC) at 1023 K for 5 minutes under a pressure of ~60 MPa. For the two-step sintering method, the Mg3.2Sb1.5Bi0.49Te0.01Cu0.01 was initially sintered at 1023 K for 5 minutes under a pressure of ~60 MPa. The obtained n-type pellet was polished and placed between the Cupronickel and sintered at 773 K for 10 minutes under the pressure of ~80 MPa. The ball-milled fine powder of p-type Mg0.99Cu0.01Ag0.97Sb0.99 is sandwiched between the Ag and sintered at 573 K for 5 minutes under the pressure of ~80 MPa. In addition, the commercial Cupronickel was sintered at 773 K for 10 minutes under the pressure of 80 MPa to measure the thermoelectric properties to include in the simulations. The microstructures and chemical compositions of the sintered compacts were examined using scanning electron microscopy (SEM; 15 kV, Miniscope TM3030Plus, Hitachi High- Technologies) coupled with energy-dispersive X-ray spectroscopy (EDX; Quantax70, Bruker).The Seebeck coefficient (S) and electrical conductivity (σ) of the sintered pellets (after removing the contact layers) were simultaneously measured in the temperature range of 300 K to 573 K. The S and σ measurements were conducted using the differential and four-probe methods, respectively (ZEM-3, Advance Riko). The relative uncertainty of the measurements was estimated to be 5%.The total thermal conductivity κtotal of the sintered pellets was determined by combining the measured density dmeas (using Archimedes method), measured thermal diffusivity D, and heat capacity Cp using the expression κtotal = dmeasDCp. The D was directly measured, while the Cp was obtained from the reference sample (Pyroceram 9606, Netzsch) using the laser flash method (Netzsch LFA 467, Germany) under an Ar gas flow of 50 ml min−1 over the temperature range of 300 K–573 K for p-type and for n-type samples. The relative uncertainty of the measurements was estimated to be 6%.Thermoelectric legs of n-type with Cupronickel contact layers and p-type with Ag contact layers were prepared by polishing and dicing the sintered pellets. The cross-section of each leg is ~3 mm × ~3 mm. Each leg's total length, including the contact layers, is ~5 mm. The two-pair thermoelectric module was fabricated using the p-type Mg0.99Cu0.01Ag0.97Sb0.99 and n-type Mg3.2Sb1.5Bi0.49Te0.01Cu0.01 TE legs. The insulated metal substrate (10 mm × 10 mm × 0.5 mm) was used on the cold side, consisting of a Cu baseplate, 80 μm insulated polymer film, and 210 μm thick Cu patterns. The p- and n-type legs were alternately arranged onto the Cu patterns. On the hot side, the p- and n-type legs were connected using Cu electrodes of 10 mm × ~4 mm × ~1 mm. A liquid In-Ga eutectic alloy was applied to connect the TE legs and Cu interconnecting electrodes at the hot and cold sides. Four Cu wires were soldered to the Cu pattern: two for supplying the electrical current and another two for measuring the terminal voltage. The two-pair thermoelectric module was electrically insulated by placing the aluminum nitride (AlN) plate (10 mm length × 10 mm width × 1 mm thick) on the top of the module. A resistance scanning probe was employed to measure the electrical resistance R of the p- and n-type TE legs. A movable probe measures the voltage along the sample length when the alternating current is applied across the element.The power generation characteristics terminal voltage (V), electrical output power (P), heat dissipated from the cold side (Qout), and conversion efficiency (η) as a function of electrical current I for a TE single-leg and two-pair module were measured under vacuum using the commercial apparatus Mini-PEM (Advance Riko). The cold-side temperature was maintained between 293 K–296 K. For the single leg, the hot-side temperatures of 373 K, 473 K, and 573 K were maintained. The two-pair module maintained the hot-side temperatures at 323 K, 373 K, 423 K, 473 K, 523 K, and 573 K. A graphite sheet was used at the hot and cold side of the TE module to reduce thermal contact resistance. A uniaxial pressure of ~1 MPa was applied to minimize electrical and thermal contact resistances. The conversion efficiency was evaluated by measuring the electrical output power (P) and output heat flow (Qout) using the following equation:The power generation characteristics V, P, Q, and η of the two pair TE module were simulated using three-dimensional finite-element method (FEM) in COMSOL Multiphysics with Heat Transfer Module. A geometrical model with the same dimensions as the TE module was built in the software. The measured temperature-dependent electrical (S and σ) and thermal properties (κtotal) of the p-type Mg0.99Cu0.01Ag0.97Sb0.99, n-type Mg3.2Sb1.5Bi0.49Te0.01Cu0.01 and Cu70Ni30 were used for the simulation. The simulations did not include the electrical and thermal contact resistances at various interfaces of the TE module. 4. RESULTS AND DISCUSSIONIn this study, the Cupronickel (Cu70Ni30) was selected as the interfacial contact layer for the Mg3.2Sb1.5Bi0.49Te0.01Cu0.01 compound. Cupronickel possesses the characteristics of a good contact layer, such as a coefficient of thermal expansion (CTE) of ~16.2  10−6 K-1 (293 K – 573 K)52 that is comparable to the Mg3(Sb, Bi)2 compound (~23  10−6 K−1)47, high thermal conductivity of ~29 W m−1 K−1 (293 K)52, high electrical conductivity of ~2.6  106 S m−1 (293 K)52. A single leg of Cupronickel/Mg3.2Sb1.5Bi0.49Te0.01Cu0.01/Cupronickel was fabricated by one-step sintering at 1023 K for 5 minutes. During sintering, Cupronickel underwent vigorous reactions with the TE material, Figure 1. (a) Scanning electron microscopy backscattered electron (SEM-BSE) image of Cu70Ni30/Mg3.2Sb1.5Bi0.49Te0.01Cu0.01, (b) Electrical resistance R of the thermoelectric leg Cu70Ni30/Mg3.2Sb1.5Bi0.49Te0.01Cu0.01/Cu70Ni30, and (c) Comparison of specific contact resistance c with metal 38–41,43–48 and metallic alloys45,47–51.resulting in the melting of reactive phases at ~848 K (Figure S1a). To limit the chemical reaction between TE material and Cupronickel, A two-step sintering method was employed to fabricate the single leg (experimental methods). The Cupronickel bonded effectively with the Mg3.2Sb1.5Bi0.49Te0.01Cu0.01 after the two-step sintering (Figure S1b). Figure 1a shows an SEM image with elemental mapping of the crack-free interface between the Cupronickel and TE material. Establishing the crack-free interface at the junction is due to the relatively good CTE match between the Cupronickel and Mg3.2Sb1.5Bi0.49Te0.01Cu0.01. At the interface between Cupronickel and Mg3.2Sb1.5Bi0.49Te0.01Cu0.01, an intermediate layer corresponding to the Mg-Ni-Cu alloy with a thickness of ~1 m is formed (Figure S2). This is mainly due to a chemical reaction between the Cupronickel and TE material with limited diffusion. To verify the influence of this intermediate layer on the TE properties of Mg3.2Sb1.5Bi0.49Te0.01Cu0.01, we measured the TE properties of a single leg by removing the contact layer from both sides of the TE material (Figure S3). The electrical and thermal properties of the Mg3.2Sb1.5Bi0.49Te0.01Cu0.01 sample in this study are in good agreement with our previous report22. This result indicates that the intermediate layer has minimal impact on the TE properties of Mg3.2Sb1.5Bi0.49Te0.01Cu0.01; however, it facilitates strong adhesion between the TE material and contact layer.Microstructural investigations revealed that the Cupronickel acts as a good contact layer for the Mg3.2Sb1.5Bi0.49Te0.01Cu0.01 compound. Further, we evaluated the contact resistance at the interface between Cupronickel and Mg3.2Sb1.5Bi0.49Te0.01Cu0.01 to verify the reliability of electrical contact. Figure 1b shows the electrical resistance (R) versus probe distance (x) for the TE leg composed of Cupronickel/Mg3.2Sb1.5Bi0.49Te0.01Cu0.01/Cupronickel at room temperature. No sudden increase in R at the interfaces indicates good electrical contact between the Cupronickel and Mg3.2Sb1.5Bi0.49Te0.01Cu0.01. As a result, a low specific contact resistance (c = Rc  A, where A and Rc is the cross-sectional area and the contact resistance, respectively) of ~5  cm2 is achieved at their interfaces. The c value obtained in this study is comparable to the metallic alloy contact layers 45,47–51 and relatively lower than the metal contact layer 38–41,43–48,53 for the Mg3(Sb, Bi)2 compounds (Figure 1c). Further, we performed a thermal stability test of the single-leg Cupronickel/Mg3.2Sb1.5Bi0.49Te0.01Cu0.01/Cupronickel by annealing at 573 K for 360 hours. The microstructure analysis shows that the intermediate layer thickness is almost unchanged (~1 m before and ~3 m after annealing). Additionally, no other secondary phases are present after the 360-hour annealing (Figure S4), indicating good thermal stability of the contact layers. In addition, Figure 2. (a-d) Measured terminal voltage (V), electrical output power (P), heat dissipated from the cold side (Qout), and conversion efficiency (η) as a function of electrical current I for a TE single-leg Cupronickel/Mg3.2Sb1.5Bi0.49Te0.01Cu0.01/Cupronickel. (e) Comparison of maximum conversion efficiency (ηmax) of single TE leg in this work with the reported Mg-Sb-based single TE legs26,29,40,41,50,51,54,55. The inset shows the image of a single-leg measurement system (Mini-PEM).the electrical contact resistance measurement revealed that the specific contact resistance is almost unchanged before (~5.1  cm2) and after aging (~4.6  cm2) (Figure S5).Figure 2 displays the terminal voltage (V), electrical output power (P), heat dissipated from the cold side (Qout), and conversion efficiency (η) as a function of electrical current I for a TE single-leg Cupronickel/Mg3.2Sb1.5Bi0.49Te0.01Cu0.01/Cupronickel. The internal resistance (Rin) of the single leg is determined from the slope of the V–I plot in Figure 2a. The Rin value increases from ~9.9 m at ΔT of 78 K to 11.5 m at ΔT of 272 K, resulting from the increase of electrical resistivity () with temperature for the Mg3.2Sb1.5Bi0.49Te0.01Cu0.01. An open circuit voltage (Voc, i.e., effective Seebeck coefficient of the single leg) is obtained from the intercept of the V–I plot in Figure 2a. The Voc increases from 14.5 mV at 78 K to 61.5 mV at ΔT of 272 K, because the Seebeck coefficient increases with temperature for the Mg3.2Sb1.5Bi0.49Te0.01Cu0.01. Figure 2b displays the P as a function of I. The maximum output power (Pmax) occurs when the external load matches with the TE single leg’s Rin. The Pmax of the Mg3.2Sb1.5Bi0.49Te0.01Cu0.01 increases from 5.3 mW at 78 K to 82.8 mW at ΔT of 272 K. Figure 2c displays the Qout increases with I at each ΔT. The rise in Qout results from Peltier heat and Joule heating, which are proportional to the I and I2, respectively56. The open circuit heat flow (Qoc) is determined from the Qout–I plot, which increases from 0.25 W at 78 K to 0.77 W at ΔT of 272 K. A maximum conversion efficiency (max) of ~8% is achieved for the single-leg at ΔT of 272 K (Figure 2d), which is comparable with the reported Mg3(Sb, Bi)2-based single legs fabricated with different contact layers26,29,40,41,45,51,54,55 (Figure 2e). The realization of high conversion efficiency in the single-leg, due to the low specific contact resistance achieved through optimization of the contact layer, allowed for the fabrication of the TE module. A counterpart p-type with the chemical composition of Mg0.99Cu0.01Ag0.97Sb0.99 was selected because of its comparable TE properties (Figure S6). This p-type compound, which has a room temperature α phase, is chosen in this study for its high thermoelectric performance near room temperature.  The TE properties show good reproducibility and are comparable with our previously reported data22 (Figure S6). We fabricated a two-pair module composed of n-type Cupronickel/Mg3.2Sb1.5Bi0.49Te0.01Cu0.01/Cupronickel and p-type Ag/Mg0.99Cu0.01Ag0.97Sb0.99/Ag (see experimental methods) to verify the performance of the TE module. The quality of the contact layer formation for the n- and p-type is verified by the SEM with elemental mapping (Figure S7) and resistance scan (Figure S8). Figure 3 (a)-(d) shows the V, P, Qout, and  as a function of I for the two-pair module at different temperature differences. As a result of low Rin (~61 m) and higher Voc (~243 mV), a Pmax of ~242 mW is obtained at ΔT of 277 K for the two-pair module. AFigure 3. (a-d) Measured V, P, Qout, and  as a function of I for the two-pair module at different temperature differences. (e) Comparison of max of the TE module in this work with the Bi2Te3-based 36,57–63 and Mg-Sb-based TE modules22,29. The inset is the image of the two-pair module. max of ~7.8% is realized at ΔT of 277 K for the two-pair n-type Cupronickel/Mg3.2Sb1.5Bi0.49Te0.01Cu0.01/Cupronickel and p-type Ag/Mg0.99Cu0.01Ag0.97Sb0.99/Ag module. It is noteworthy to mention that the development of a Cupronickel-based contact layer allows for significant a improvement in max of ~7.8% at ΔT of 277 K, which is superior to the reported max of ~7.3% at ΔT of 315 K for the same Mg0.99Cu0.01Ag0.97Sb0.99/Mg3.2Sb1.5Bi0.49Te0.01Cu0.01-based TE module22. Moreover, this max value is comparable with the long-lasting champion Bi2Te3-based thermoelectric power generator modules22,36,57–62(Figure 3e). The simulated power generation characteristics and temperature distribution model of the two-pair Cupronickel/Mg3.2Sb1.5Bi0.49Te0.01Cu0.01/Cupronickel and p-type Ag/Mg0.99Cu0.01Ag0.97Sb0.99/Ag module are shown in Figure S9. The contact resistances at various material interfaces are not included in the simulation. The simulated Voc, Pmax, Rin, Qoc, and max values as a function of ΔT are compared with the measured data (Figure S10). At each ΔT, the measured Voc values are close to the simulated values (Figure S10a), indicating good electrical and thermal contacts at the interfaces under the open circuit conditions. The measured Pmax values are still lower than the simulated values (Figure S10b). At ΔT of 277 K, the measured Pmax is ~0.25, which is 30% lower than the simulated Pmax of ~0.36 W. This is mainly attributed to the higher measured Rin values compared to the simulated values (Figure S10c). At ΔT of 277 K, the measured Rin is ~ 0.061 , which is higher than the simulated Rin of ~0.04 . The higher measured Rin values result from larger contact resistances at various material interfaces in the TE module. At each ΔT, the measured Qoc values are close to the simulated values (Figure S10d), indicating good thermal contacts at the interfaces under the open circuit conditions. The measured max of ~7.8% is still lower than the simulated max of ~10% at ΔT of 277 K for the two-pair TE module (Figure S10e). This result indicates that there is further scope for improving Pmax and max by contact layer interfacial engineering and joining technology to reduce the Rin of the TE module. It is important to note that this study can facilitate the development of new metallic alloys as contact layers for the Mg3(Sb, Bi)2-based compounds and the fabrication of high-performance TE modules.5. ConclusionsIn summary, we have developed a new contact interface layer based on the Cupronickel (Cu70Ni30) for the n-type Mg3(Sb, Bi)2 compound, which has previously often used Fe contacts that are incompatible with device fabrication using solder. A two-step SPS sintering allowed the successful fabrication of an n-type single TE leg of Cupronickel / Mg3.2Sb1.5Bi0.49Te0.01Cu0.01 / Cupronickel. A homogeneous and crack-free interface is formed between the Cupronickel and Mg3.2Sb1.5Bi0.49Te0.01Cu0.01 confirmed by the microstructural investigations. As a result, a low specific contact resistance (c) of ~5  cm2 is obtained at their interface. This crack-free interface and low c led to a maximum conversion efficiency (max) of ~8% achieved at a temperature difference (ΔT) of 277 K for the Mg3.2Sb1.5Bi0.49Te0.01Cu0.01 single leg with Cupronickel contact layer. We fabricated a two-pair thermoelectric module by coupling a high-performance n-type TE leg with a p-type MgAgSb-based TE leg. This two-pair module demonstrates a max~7.8% at ΔT of 277 K, which shows great potential for low-grade waste heat recovery.ASSOCIATED CONTENTAUTHOR INFORMATION:Corresponding AuthorTakao Mori - Research Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), Tsukuba, 305-0044 (Japan); Graduate School of Pure and Applied Sciences, University of Tsukuba, Tsukuba, 305-8577 (Japan);https://orcid.org/0000-0003-2682-1846; Email: MORI.Takao@nims.go.jp AuthorsRaju Chetty - Research Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), Tsukuba, 305-0044 (Japan);https://orcid.org/0000-0003-1072-8241;Jayachandran Babu - Research Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), Tsukuba, 305-0044 (Japan);https://orcid.org/0000-0002-1182-6655;SUPPORTING INFORMATION:The sintering curve for the n-type TE materials, scanning electron microscopy backscattered electron images of both p- and n-type TE single legs, TE properties of p- and n-type TE compounds, Resistance scan of both p- and n-type TE legs, simulated and experimental power generation characteristics of the two-pair module.  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