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Muhammad Fasih Aamir, [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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[Process optimization of contact interface layer for maximizing the performance of Mg3(Sb,Bi)2 based thermoelectric compounds](https://mdr.nims.go.jp/datasets/527a8da8-80a8-47a0-96f7-384ae6af5420)
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Process optimization of contact interface layer for maximizing the performance of Mg3(Sb,Bi)2 based thermoelectric compoundsAs featured in: Showing research from Thermoelectric Materials Group, Research Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), Japan. Process optimization of contact interface layer for maximizing the performance of Mg 3 (Sb,Bi) 2 based thermoelectric compounds This work demonstrates the reduction of specific contact resistivity ( c) through the optimization of sintering temperatures, leading to a maximum conversion efficiency ( max) of 9.3% at T = 380 K for SSf/Mg3Sb1.5Bi0.5/SSf thermoelectric (TE) single leg, emphasizing the potential of stainless-steel foil (SSf) interface layer for thermoelectric applications.Image reproduced by permission of Takao Mori from J . Mater . Chem . C , 2025, 13 , 10567. See Raju Chetty, Takao Mori et al ., J . Mater . Chem . C , 2025, 13 , 10567.Materials for optical, magnetic and electronic devicesJournal of Materials Chemistry Crsc.li/materials-c PAPER Junghun Kim, Dongryul Lee et al . Annealing-free Ohmic contact of -Ga 2 O 3 via nitrogen plasma treatment ISSN 2050-7526Volume 13Number 217 June 2025Pages 10439–10956rsc.li/materials-cRegistered charity number: 207890This journal is © The Royal Society of Chemistry 2025 J. Mater. Chem. C, 2025, 13, 10567–10575 | 10567Cite this: J. Mater. Chem. C, 2025,13, 10567Process optimization of contact interface layer formaximizing the performance of Mg3(Sb,Bi)2 basedthermoelectric compounds†Muhammad Fasih Aamir,ab Raju Chetty, *a Jayachandran Babu a andTakao Mori *abMg3(Sb,Bi)2 based compounds exhibit promising thermoelectric (TE) performance within the 300–700 K range, making them suitable for mid-temperature applications; yet achieving optimal electricalcontact between the TE material and the contact material is crucial. One-step sintering has emergedas a widely used technique for establishing these contacts in Mg3(Sb,Bi)2 compounds, thoughvariations in process parameters can impact contact quality and, consequently TE conversionefficiency. Therefore, this study explores the optimization of Mg3(Sb,Bi)2 compounds using sparkplasma sintering with stainless steel (SS) 304 contacts at three different temperatures of 973 K, 1023 K,and 1073 K. By increasing the sintering temperature from 973 K to 1073 K, a significant reduction inthe specific contact resistivity (rc) by B60% is realized, without compromising TE properties.Furthermore, it was found that replacing SS powder (SSp) with SS foil (SSf) could lead to more uniformand dense layers, achieving a lower specific rc value of 8.2 mO cm2 at the interface. A maximumconversion efficiency (Zmax) of B9.3% was obtained at a temperature difference (DT) of B380 K forSSf/Mg3(Sb,Bi)2/SSf sintered at 1073 K. Moreover, thermal aging for 30 days at 673 K confirms therobustness of SSf/Mg3(Sb,Bi)2/SSf contacts with negligible degradation of TE properties and conversionefficiency of the TE single leg.1. IntroductionApproximately 66% of the energy used in factories, powerplants, and other appliances is lost as heat.1 Thermoelectric(TE) technology is a promising solution to convert this wasteheat into electrical power while reducing the environmentalimpact of CO2 emission.2 TE devices offer benefits like zeropollution, silent operation, no moving parts, precise tempera-ture control, and long service life, making them suitable forvarious applications, including energy harvesting, refrigeration,and space exploration.3–7Conventional TE devices mostly comprise n-type and p-typecompounds, connected electrically in series and thermally inparallel via metal electrodes.8 When a temperature difference isapplied across the TE device, a thermovoltage (V) is generateddue to the Seebeck effect.9 A connected electrical load causescurrent flow (I) through the TE device, which generates theuseful electric power (P = V � I).10 The power conversionefficiency (Z) of TE devices is defined as Z ¼ PQin, where Qin isthe heat flow on the hot side.11 The relationship betweenmaximum conversion efficiency and TE material propertiesfollows the equation12,13Zmax ¼DTTHffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1þ ZTp� 1ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1þ ZTpþ TCTH(1)where ZT is the dimensionless figure of merit of TE materials,defined as ZT ¼ S2sTk, where S = Seebeck coefficient, s =electrical conductivity, k = total thermal conductivity, and T =absolute temperature.14 Several TE materials such as Bi2Te3,15–19PbTe,20–27 SiGe,28–31 skutterudites32–38 and half-Heuslers39–43 haveshown promising ZT values.44,45 To realize high conversionefficiencies in TE devices, the electrical contacts atvarious interfaces play a crucial role despite the high materialZT values.46 Xiong et al. suggested a relationship between thea Research Center for Materials Nanoarchitectonics (MANA), National Institute forMaterials Science (NIMS), 1-1 Namiki, Tsukuba 305-0044, Japan.E-mail: chetty.raju@nims.go.jp, mori.takao@nims.go.jpb Graduate School of Science and Technology, University of Tsukuba, 1-1-1Tennodai, Tsukuba, Ibaraki 305-8577, Japan† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5tc00851dReceived 26th February 2025,Accepted 26th April 2025DOI: 10.1039/d5tc00851drsc.li/materials-cJournal ofMaterials Chemistry CPAPEROpen Access Article. Published on 01 May 2025. Downloaded on 11/8/2025 3:44:56 PM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article OnlineView Journal | View Issuehttps://orcid.org/0000-0003-1072-8241https://orcid.org/0000-0002-1182-6655https://orcid.org/0000-0003-2682-1846http://crossmark.crossref.org/dialog/?doi=10.1039/d5tc00851d&domain=pdf&date_stamp=2025-05-20https://doi.org/10.1039/d5tc00851dhttps://doi.org/10.1039/d5tc00851dhttps://rsc.li/materials-chttp://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d5tc00851dhttps://pubs.rsc.org/en/journals/journal/TChttps://pubs.rsc.org/en/journals/journal/TC?issueid=TC01302110568 | J. Mater. Chem. C, 2025, 13, 10567–10575 This journal is © The Royal Society of Chemistry 2025TE material ZT and the device ZT (ZTD) according to theequationZTD ¼LLþ 2rcsZT (2)where L, s and rc are the length, electrical conductivity of theTE compound, and the specific contact resistivity across the TEcompound/electrode interface.47 Typically, the TE compound/electrode contact should exhibit ohmic behavior with a contactresistivity of rc r 10�10 O m2 to achieve high ZTD values andconversion efficiency.48Mg3(Sb,Bi)2 compounds have recently emerged as promisingTE materials with good ZT at mid-range temperatures.49–56 Thehigh ZT values in these compounds have led to the realizationof high TE conversion efficiencies.57–61 However, the conver-sion efficiencies are still lower than expected due to the highercontact resistivities at the interfaces.57 Therefore, several stu-dies have been conducted to reduce the rc by interfacialengineering of contact layers.48,51,53,55 Pure metals like Ti,61Fe,59 Ni,62 and Nb63 are explored as contact interface layers forthe Mg3(Sb,Bi)2 compounds. Amongst them, Fe is the mostlyused contact layer for Mg3(Sb,Bi)2. However, various reportsshow an inconsistency in the rc that varies between 2.5 � 10�10and 43.6 � 10�10 O m2.48,64–70 Not to mention that the incom-patibility of Fe with the solder and the high specific contactresistivity (60 mO cm2) at various interfaces after moduleoperation is problematic.65 While the rc of 9.7 mO cm2 wasobserved in the case of Nb/Mg3(Sb,Bi)2, which increased up to26 mO cm2 after module operation at 773 K for 360 h.63 In caseof the Ni contact interface layer, the rc (18.56 mO cm2)was also increased by 700% after aging at 673 K.71 Apart fromthe above, metal alloys such as CuNi,60 Mg2Ni,72 NiFe,73FeMgCrTiMn,74 Fe7Mg2Cr & Fe7Mg2Ti,48 stainless steel(SS304),68 Mg3.4Sb3Ni71 and Mg2Cu75 were also employed ascontact layers to lower the rc. In addition to the lowering ofrc, developing a thermally stable contact interface layer iscrucial to realizing long-term TE device operation withoutdeteriorating the conversion efficiency.In this study, we focused on optimizing the fabricationprocess by varying the sintering temperature to lower the rcand realize high conversion efficiency. Here we selected stain-less steel 304 (SS) as a contact interface layer for the n-typeMg3(Sb,Bi)2 compound and fabricated via one-step sintering atdifferent temperatures from 973 K to 1073 K. A significantreduction in rc occurred from 19.7 mO cm2 to 7.9 mO cm2 byincreasing the sintering temperature from 973 K to 1073 K. Thisis mainly attributed to strong adhesion between the SS andMg3(Sb,Bi)2 at a higher sintering temperature. Furthermore,replacing SS powder (SSp) with SS foil (SSf) led to more uniformand dense interface layers. Consequently, a maximum conver-sion efficiency (Zmax) of 9.3% is achieved at a temperaturedifference of 380 K. The long-term thermal stability of theSSf/Mg3(Sb,Bi)2/SSf TE single leg was tested under isothermal(673 K) aging for 30 days, revealing a negligible diffusionbetween the SSf and Mg3(Sb,Bi)2. As a result, no significantvariation in the Zmax value was found, confirming the goodstability of SSf as the contact interface layer for the Mg3(Sb,Bi)2compounds.2. Experimental techniques2.1. Preparation of Mg3(Sb,Bi)2 TE materials and contactingBall milling (Sample Prep 8000, SPEX) was used to prepare theMg3.2Sb1.5Bi0.49Te0.01Cu0.01 bulk material (abbreviated as‘Mg3Sb1.5Bi0.5’). Ball-milled powders were loaded inside a gra-phite die of 10 mm in inner diameter and compacted using aspark plasma sintering (SPS-1080 System, SPS SYNTEX INC).The single TE legs were fabricated by sandwiching the ball-milled Mg3Sb1.5Bi0.5 powder between the layers of stainlesssteel 304 (SS) foils/powders. One-step sintering was carriedout with SPS at different sintering temperatures (973 K,1023 K, and 1073 K) under 60 MPa uniaxial pressure for5 minutes. The prepared samples were diced into cuboid shapesusing a wire saw. Mechanical polishing was done for the samplesprepared for microstructural evaluation. For thermal stabilitystudies, the samples were placed inside a sealed quartz ampouleand isothermally aged for 7, 15, and 30 days at 673 K.CharacterizationThe phase purity of sintered pellets was characterized via X-raydiffraction (MiniFlex600, Rigaku Corporation). Microstructureanalysis was carried out using a scanning electron microscopeequipped with an EDX detector (SU8000, Hitachi High-Technologies/Bruker).An approximately 3 mm� 3 mm� 9 mm cuboid sample wasprepared for the combined Seebeck coefficient (S) and electricalconductivity (s) measurements (ZEM-3, Advance Riko) and a10 mm diameter cylindrical sample with 2 mm thickness forthermal diffusivity (l) measurements (LFA 467, Netzsch). Ther-mal conductivity (k) was estimated from the relation k = dCplwhere ‘d’ is the mass density of the sample measured using theArchimedes technique and ‘Cp’ is the specific heat at constantpressure calculated from the Dulong–Petit limit.Cuboid-shaped samples of approximately 3 � 3 � 5 mm3 involume were subjected to a resistance profiler to check thespecific contact resistivity (rc) which was obtained by measur-ing the contact resistance jump (DR) at the interfaces using theequation, rc = DR�A, where ‘A’ is the cross-sectional area. Thepower generation characteristics of the SS/Mg3Sb1.5Bi0.5/SSsample were measured using Mini PEM (Advance Riko).3. Results and discussion3.1. XRD and TE properties of Mg3Sb1.5Bi0.5The XRD patterns of Mg3Sb1.5Bi0.5 pellets sintered at tempera-tures between 973 K and 1073 K are displayed in Fig. 1a. Thediffraction peaks align with those of the Mg3Sb1.5Bi0.5 phase,with no secondary phases for all the samples. XRD analysisreveals that all the sintered pellets exhibit a single phase.Fig. 1b shows the temperature-dependent electrical conduc-tivity (s) of the Mg3Sb1.5Bi0.5 pellets sintered between 973 K andPaper Journal of Materials Chemistry COpen Access Article. Published on 01 May 2025. Downloaded on 11/8/2025 3:44:56 PM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article Onlinehttp://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d5tc00851dThis journal is © The Royal Society of Chemistry 2025 J. Mater. Chem. C, 2025, 13, 10567–10575 | 105691073 K. The s values of all the samples decrease with increasingtemperature, indicating a degenerate semiconducting beha-vior, which is consistent with a previous report.57 The s valuesvary from B5.2 � 104 S m�1 at 373 K to B2.6 � 104 S m�1 at673 K in the whole temperature range and lie within the rangeof error bar for all the samples, confirming the minimalsintering temperature effect on s. Similarly, very little sinteringinfluence is observed for the Seebeck coefficient (S), rangingfrom �223 mV K�1 to �279 mV K�1 between 373 K and 673 K forall the pellets (Fig. 1c). However, at room temperature, there is aslight variation in the s and S of Mg3Sb1.5Bi0.5, which is attributedto the changes in the grain size with the sintering temperatures.These values are in good agreement with the previous report.57A maximum power factor (S2s) of B2.2 mW m�1 K�2 at 423 K isobtained for all the samples (Fig. 1d). The total thermal con-ductivity (ktotal) of Mg3Sb1.5Bi0.5 samples sintered at 973 K to1073 K reduced from 1.13 W m�1 K�1 at room temperature to0.81 W m�1 K�1 at 673 K (Fig. 1e). The maximum figure of merit(ZT) calculated was B1.4 at 673 K, which is consistent with theprevious report57 (ZT B1.4) for the same compound sintered at973 K (Fig. 1f).3.2. SSp/f/Mg3Sb1.5Bi0.5/SSp/f contacts3.2.1. Specific contact resistivity. Our initial studies werecarried out using stainless steel powder (SSp) and were laterchanged to 0.5 mm thick foil (SSf) due to its high density,Fig. 1 (a) XRD of Mg3Sb1.5Bi0.5 sintered at 973 K, 1023 K and 1073 K compared with the literature.57 Thermoelectric properties of Mg3Sb1.5Bi0.5 sintered at973 K, 1023 K and 1073 K which includes (b) electrical conductivity (s), (c) Seebeck coefficient (S), (d) power factor (S2s), (e) total thermal conductivity(ktotal) and (f) figure of merit (ZT).Journal of Materials Chemistry C PaperOpen Access Article. Published on 01 May 2025. Downloaded on 11/8/2025 3:44:56 PM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article Onlinehttp://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d5tc00851d10570 | J. Mater. Chem. C, 2025, 13, 10567–10575 This journal is © The Royal Society of Chemistry 2025process simplicity, and diffusion passivation characteristics(Fig. S1, ESI†). Mg3Sb1.5Bi0.5 powders were sandwiched betweenSSp/f layers and sintered at temperatures between 973 K and1073 K (Fig. 2a). All the TE disks (SSp/f/Mg3Sb1.5Bi0.5/SSp/f)prepared at various sintering temperatures show good bondingand are crack-free without delamination after dicing. Thespecific contact resistivity (rc) of the samples at three differentsintering temperatures (973–1073 K) is measured using aresistance profiler with a circuit diagram shown in Fig. 2b. Asudden change in the slope at the SSf/Mg3Sb1.5Bi0.5 interfacewas characterized from the line scan as shown in Fig. 2c. Thefive-line scans are carried out at different sections of contactsand the arithmetic average is reported in Fig. 2d. It is observedthat increasing the sintering temperature from 973 K to 1073 Kresults in lowering of the rc from 19.7 mO cm2 to 7.9 mO cm2 forthe SSp/Mg3Sb1.5Bi0.5 samples. The combined effect of improvedsinter-bonding of SSp particles and enhanced controlled diffu-sion helps to lower the rc over B60%. However, a significantdiffusion between the TE materials and contact layers during theaging and long-term operation is a serious concern.59,68 Thus, SSpis replaced with SSf, which results in the rc of 10.7 mO cm2(B46% reduction compared to the SSp sample) for the samplesintered at 973 K. High density of the foils (usually prepared byforging, rolling, and annealing) compared to sintered powdersresults in lower rc for samples sintered even at 1023 K. However,with an increase in the sintering temperature to 1073 K,the rc lowers to B23% for SSf/Mg3Sb1.5Bi0.5 samples reachingB8.2 mO cm2. It is noteworthy that the s calculated from theFig. 2 (a) Schematic diagram of die preparation by using stainless steel 304 powder and foil. (b) Circuit diagram of resistance profiler. (c) Method tocalculate the specific contact resistivity (rc). (d) Specific contact resistivity (rc) of powder and foil fabricated SS/Mg3Sb1.5Bi0.5/SS sintered at 973 K, 1023 Kand 1073 K. (e) Electrical conductivity (s) comparison of powder/foil fabricated SS/Mg3Sb1.5Bi0.5/SS by using resistance scan and Mg3Sb1.5Bi0.5 by using afour probe method (ZEM-3 data).Paper Journal of Materials Chemistry COpen Access Article. Published on 01 May 2025. Downloaded on 11/8/2025 3:44:56 PM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article Onlinehttp://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d5tc00851dThis journal is © The Royal Society of Chemistry 2025 J. Mater. Chem. C, 2025, 13, 10567–10575 | 10571slope of the resistance line scan on the Mg3Sb1.5Bi0.5 for allsamples (Fig. 2e and Tables S1, ESI†) is almost consistent withthe s measured by using the four-probe method (ZEM-3). Thisresult further confirms the negligible influence of sinteringtemperature on the s of TE material.3.2.2. Microstructure. Fig. S2 (ESI†) shows the SEM+EDXmapping of stainless steel 304 powder (SSp) contacts sinteredbetween 973 K and 1073 K. Microstructure analysis reveals thediffusion of the elemental Sb and Bi into the SSp contact layerB100 mm for the SSp/Mg3(Bi,Sb)2/SSp sample. Also, the SSpregion is porous, which results from the low densification ofthe powder form of stainless steel during the sintering. Qu et al.also reported the quick diffusion of Mg and Bi from the TEmaterial to the non-dense Fe contact layers (4100 mm mixedlayer) for Fe/Bi-rich Mg3(Bi,Sb)2.59 The microstructure and EDXmapping of the foil fabricated SSf/Mg3Sb1.5Bi0.5/SSf contactssintered at 973 K, 1023 K, and 1073 K are shown in Fig. 3.Similar microstructural features were observed at all sinteringtemperatures (973 K–1073 K), and the interfaces were crack-freeand uniform. The samples sintered at 973 K and 1023 K showinhomogeneous distribution of Bi at the interfaces, whichmight result in off-stoichiometry in the chemical composition.However, the changes in the TE properties are minimal withinthe error limit (Fig. 1), while the 1073 K sintered sample showshomogeneous distribution of all the elements. Thus, we inves-tigated the power generation characteristics and thermal stabilityof the 1073 K sintered sample. The sharp boundaries between SSfand Mg3Sb1.5Bi0.5 indicate the absence of notable atomicdiffusion or reaction layer formation, indicating the effective-ness of SSf as a suitable contact material for Mg3Sb1.5Bi0.5.These results confirm that SSf contact interface layers preventdiffusion, leading to uniform contacts without cracks or pores(Fig. S3–S5, ESI†).The microstructural analysis and electrical contact resistivityreveal that the SSf is promising as a contact interface layersintered at 1073 K. Thus, we investigated the influence of thecontact interface layer on the TE properties of the Mg3Sb1.5Bi0.5disk sintered at 1073 K by removing the SSf contact layers(Fig. S6a, ESI†). Fig. S6(b)–(f) (ESI†) shows the comparisonof temperature-dependent s, S, S2s, ktotal, and ZT of theMg3Sb1.5Bi0.5 sample and SSf/Mg3Sb1.5Bi0.5/SSf. Note that theTE properties were measured after removing the SSf in the latersample. The TE properties of foil-removed Mg3Sb1.5Bi0.5 areconcurrent with the Mg3Sb1.5Bi0.5 sample without foil as well aswith the previous report57 on the same compound.3.2.3. Power generation characteristics of TE single leg SSf/Mg3Sb1.5Bi0.5/SSf. Fig. 4 shows the power generation character-istics, including terminal voltage (V), electrical power output(P), output heat flow (Qout) from the cold side, and conversionefficiency (Z) of TE single leg SSf/Mg3Sb1.5Bi0.5/SSf as a functionof electrical current (I). The hot-side temperature (Th) of the TEleg varied between 323 K and 673 K, while the cold-sidetemperature was maintained at B293–296 K. The open circuitvoltage (Voc), obtained from the intercept of V–I plot, increasesfrom 4.8 mV at Th = 323 K to 89 mV at Th = 673 K (Fig. 4a). Theinternal resistance (Rin) is obtained by the slope of the V–I plot,which increases from 7.5 mO to 13.4 mO as Th increases from323 K to 673 K. This is attributed to the decrease of s as thetemperature increases (Fig. 1b). Maximum electrical outputpower (Pmax) is obtained when the electronic load matchesthe internal resistance of the TE leg. The Pmax increases from0.7 mW at Th = 323 K to 146 mW at Th = 673 K (Fig. 4b). Theopen circuit heat flow (Qoc) obtained from the intercept of theQout–I plot, which increases from 195.9 mW at 323 K to1960 mW at 673 K. Fig. 4c shows that the Qout increases withI at every rise in DT due to Peltier heat and Joule heat, which areproportional to I and I2, respectively.76 At Th = 323 K, themaximum conversion efficiency (Zmax) of B0.46% is obtained,which reaches B9.3% as the Th rises to 673 K (Fig. 4d). Thisresult is comparable with the previously reported Zmax of thesingle-leg TE Mg3(Sb,Bi)2 with SS304 powder used as contactinterface layers.68 The Zmax of B9.3% obtained in this study isalso comparable to the efficiencies of single-leg TE Mg3(Sb,Bi)2with other contact interface layers51,52,55,60,62,66,74,77–79 (Fig. 4e).However, long-term thermal stability analyses of these highefficiencies are still lacking in previous studies.Fig. 3 SEM and EDX of SSf/Mg3Sb1.5Bi0.5/SSf sintered at (a) 973 K (b) 1023 K and (c) 1073 K.Journal of Materials Chemistry C PaperOpen Access Article. Published on 01 May 2025. Downloaded on 11/8/2025 3:44:56 PM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article Onlinehttp://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d5tc00851d10572 | J. Mater. Chem. C, 2025, 13, 10567–10575 This journal is © The Royal Society of Chemistry 20253.3. Thermal stability evaluation of SSf/Mg3Sb1.5Bi0.5/SSfThe thermal stability of SSf/Mg3Sb1.5Bi0.5/SSf was systematicallyevaluated by performing an aging test at 673 K over 7, 15, and30 days and the corresponding microstructural investigation.The microstructural analysis showed a stable, crack-free con-tact interface, with no evidence of element diffusion after agingfor 30 days (Fig. S7–S9, ESI†). Moreover, the stability of theinterface was evaluated by measuring the specific contactresistivity (rc) of the SSf/Mg3Sb1.5Bi0.5/SSf aged at 673 K for 0,7, 15, and 30 days (Fig. S10, ESI†). The specific contactresistivity (rc) was B8.2 mO cm2 and showed variations duringthe thermal aging at 673 K. Initially, the specific contactresistivity (rc) decreased from B8.2 mO cm2 to B6 mO cm2after 7 days’ aging. This trend is similar to the previous reportof Fe foil and Bi-rich Mg3(Sb,Bi)2, due to the formation of a thinintermediate layer after 3 days of aging at 573 K.59 However, wedid not observe any intermediate layer of SS foil and Sb-richMg3(Sb,Bi)2 within the detection limit of SEM. The initialdecrease in specific contact resistivity (rc) might be due tothe chemical reaction with nano intermediate layer formation.The volatile Mg loss at the interfaces may cause a change in thechemical composition,61 which could result in slight changesin the carrier concentration and further rise in specific contactresistivity (rc) up to B14.8 mO cm2 after 30 days’ aging (Fig. 5a).It is noteworthy that the aging test performed in this study is at ahigher temperature (673 K) for a long time (30 days) as comparedto the previous reports48,59,71,74,75,77 on the Mg3(Sb,Bi)2-basedTE legs. For example, the rc increased from B5.6 mO cm2 toB11 mO cm2 for the Mg3.2Sb1.5Bi0.49Te0.01/SS powder afterperforming an aging test at a lower temperature (523 K) andfor a smaller period (200 hours, i.e., 9 days).68 Furthermore, thepower generation characteristics of the SSf/Mg3Sb1.5Bi0.5/SSf werealso evaluated after aging at 673 K for 7, 15, and 30 days (Fig. S11,ESI†). No significant variation in the internal resistance (Rin) ofTE single leg SSf/Mg3Sb1.5Bi0.5/SSf is observed after aging of 30days. The Rin slightly increased from 13.4 mO to 14.2 mO after 30days of aging (Fig. 5b). Moreover, no influence of aging on the Vocindicates the good chemical and thermal stability of the SSf/Mg3Sb1.5Bi0.5/SSf. The maximum power density (Pd(max) B1.7 Wcm�2) shows minimal variation after 30 days of annealing at673 K (Fig. 5c). The maximum conversion efficiency (Zmax) of thesingle leg drops slightly from 9.3% to 8.9% after 30 days ofannealing at 673 K, indicating the good thermal stability (Fig. 5d).Additionally, the TE properties of SSf/Mg3Sb1.5Bi0.5/SSf are con-sistent before and after aging of 30 days (Fig. S12, ESI†). Overall,the negligible influence on the TE properties, microstructure,specific contact resistivity (rc), power generation characteristicsof the SSf/Mg3Sb1.5Bi0.5/SSf are observed, confirming the excellentstability of material and contact interface layer at high tempera-tures (Fig. S13, ESI†), making it a promising candidate for longterm thermoelectric device applications.4. ConclusionsIn this work, we demonstrated the influence of sintering condi-tions on the optimization of contact layers for the Mg3Sb1.5Bi0.5.At a higher sintering temperature (1073 K), the contact betweenthe SS and Mg3Sb1.5Bi0.5 is improved due to the increasedFig. 4 Power generation characteristics of SSf/Mg3Sb1.5Bi0.5/SSf sintered at 1073 K, which includes (a) terminal voltage (V), (b) power output (P), (c)output heat flow (Qout) at cold side, (d) conversion efficiency (Z) at different Th, (e) maximum conversion efficiency (Zmax) compared with the reportedliterature.51,52,55,60,62,64,66,74,77–79Paper Journal of Materials Chemistry COpen Access Article. Published on 01 May 2025. Downloaded on 11/8/2025 3:44:56 PM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article Onlinehttp://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d5tc00851dThis journal is © The Royal Society of Chemistry 2025 J. Mater. Chem. C, 2025, 13, 10567–10575 | 10573adhesive strength and results in a significant reduction (B60%)in the specific contact resistivity (rc) at their interface. A uniform,crack-free interface with low rc led to a maximum conversionefficiency (Zmax) of 9.3% at a temperature difference (DT) of 380 Kfor SSf/Mg3Sb1.5Bi0.5/SSf TE single leg sintered at 1073 K. More-over, our work reveals that the Mg3Sb1.5Bi0.5-based TE single legshows the good thermal stability without much degradation inthe TE properties and power generation characteristics afteraging at 673 K for 30 days. This work facilitates the advancementin contact layer optimization through process engineering for theMg3(Sb,Bi)2-based compounds.Data availabilityData will be made available upon reasonable request to thecorresponding author.Conflicts of interestThe authors declare that they have no competing financialinterests or personal relationships that could have appearedto influence the work reported in this paper.AcknowledgementsThis work was supported by the JST Mirai Program grantnumber JPMJMI19A1. We also acknowledge the support ofthe MEXT fellowship to M. F. A. Institutional support fromthe JSPS WPI Academy Program is also acknowledged.References1 T. Mori and S. Priya, MRS Bull., 2018, 43, 176–180.2 L. E. Bell, Science, 2008, 321, 1457–1461.3 D. Champier, Energy Convers. Manage., 2017, 140, 167–181.4 F. J. DiSalvo, Science, 1999, 285, 703–706.5 I. Petsagkourakis, K. Tybrandt, X. Crispin, I. Ohkubo,N. Satoh and T. Mori, Sci. Technol. Adv. Mater., 2018, 19,836–862.6 R. O’Brien, R. 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