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[Airan Li](https://orcid.org/0009-0004-7318-4821), [Longquan Wang](https://orcid.org/0009-0009-9910-9770), Jiankang Li, Xinzhi Wu, [Takao Mori](https://orcid.org/0000-0003-2682-1846)

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[Self-optimized contact in air-robust thermoelectric junction towards long-lasting heat harvesting](https://mdr.nims.go.jp/datasets/2cd787c5-840b-40cf-a37a-eb5279e27fbf)

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Self-optimized contact in air-robust thermoelectric junction towards long-lasting heat harvestingArticle https://doi.org/10.1038/s41467-025-56861-3Self-optimized contact in air-robustthermoelectric junction towards long-lastingheat harvestingAiran Li 1,3, Longquan Wang1,2,3, Jiankang Li1,2, Xinzhi Wu1 & Takao Mori 1,2Ensuring long-term reliable contacts in thermoelectric devices is particularlychallenging due to their operation under high temperatures and has been oneof the large obstacles in the field for application. Typically, thermodynamicallydriven atomic diffusion and reactions often degrade the contacts, leading toincreased contact resistivity and ultimately limiting the device’s lifespan. Here,we report an unconventional self-optimized contact resistivity mechanism inthe Sb/MgAgSb junction. Mg diffusion from MgAgSb to Sb does not degradebut instead optimizes its contact resistivity even after aging in air for 30 days.This unexpected automatic optimization arises from an increased carrierconcentration in MgAgSb, which enhances electron tunneling across theinterface, effectively reducing the contact resistivity. Leveraging the self-optimized contact in Sb/MgAgSb and stable thermoelectric performance ofMgAgSb, a two-pair thermoelectric device employing 100-day air-aged Sb/MgAgSb achieves an impressive conversion efficiency of 8.1% and a rare powerdensity of 0.41W cm-2 under 294 K temperature gradient. These resultsunderscore its significant potential for robust, long-term heat harvesting. Theself-optimization mechanism identified in this work also offers valuableinsights for designing future junctions for high-temperature applications.Metal-semiconductor contacts are essential in electronic systems dueto their critical role in controlling the flow of electrical signals at theinterface1,2. The energy barriers formed between the metal and semi-conductor typically determine whether the contact behaves as a rec-tifying or ohmic junction, directly influencing current flow, powerconsumption, and device efficiency, particularly in transistors, diodes,and thermoelectrics3–6. Thermoelectric (TE) devices, which enable thedirect conversion of heat into electricity based on the Seebeck effect,offer great potential for sustainably powering numerous sensors in theInternet of Things (IoT) and contributing to carbon-neutral goals viawaste heating energy harvesting7–9. As global energy demand con-tinues to rise, leveraging waste heat presents a significant opportunityto improve energy efficiency, underscoring the importance of advan-cing TE technologies to enhance energy utilization. However, unlikemost electronic devices that typically operate around roomtemperature, TEdevices usually function under high temperatures andtheir gradients, which demands that TE materials and numerous con-tacts within the device endure harsh conditions for extended periodswithout obvious performance degradation10,11.The performance of TE materials is typically evaluated by thedimensionless figure of merit, zT = S2σT/κ, where S is the Seebeckcoefficient, σ is electrical conductivity, T is the absolute temperatureand κ is thermal conductivity. Significant progress has beenmade withthe development of high-performance TE materials, advancing TEtechnology in both power generation and solid-state cooling12–19.However, for practical applications, the stability of TE materials underhigh temperatures, electric fields and their gradient is just as critical asthe materials’ performance. At elevated temperatures or under strongelectric fields, issues such as element evaporation, environmentalreactions, and ion migration can degrade the performance of TEReceived: 2 December 2024Accepted: 5 February 2025Check for updates1Research Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), Tsukuba, Japan. 2Graduate School of Pure andApplied Sciences, University of Tsukuba, Tsukuba, Japan. 3These authors contributed equally: Airan Li, Longquan Wang. e-mail: MORI.Takao@nims.go.jpNature Communications |         (2025) 16:1502 11234567890():,;1234567890():,;http://orcid.org/0009-0004-7318-4821http://orcid.org/0009-0004-7318-4821http://orcid.org/0009-0004-7318-4821http://orcid.org/0009-0004-7318-4821http://orcid.org/0009-0004-7318-4821http://orcid.org/0000-0003-2682-1846http://orcid.org/0000-0003-2682-1846http://orcid.org/0000-0003-2682-1846http://orcid.org/0000-0003-2682-1846http://orcid.org/0000-0003-2682-1846http://crossmark.crossref.org/dialog/?doi=10.1038/s41467-025-56861-3&domain=pdfhttp://crossmark.crossref.org/dialog/?doi=10.1038/s41467-025-56861-3&domain=pdfhttp://crossmark.crossref.org/dialog/?doi=10.1038/s41467-025-56861-3&domain=pdfhttp://crossmark.crossref.org/dialog/?doi=10.1038/s41467-025-56861-3&domain=pdfmailto:MORI.Takao@nims.go.jpwww.nature.com/naturecommunicationsmaterials, hindering their practical use20–23. In addition to the TEmaterials themselves, ensuring the stability of contacts between TEmaterials and electrodes poses an even greater difficulty.Contacts always involve materials with differing chemical com-positions, which often leads to thermodynamic atomic diffusion andreaction, especially under high temperatures and their gradients. Thisatomic diffusion and reaction alter the contact properties and oftendegrades the device’s performance24–31. To mitigate atomic diffusionand reaction, TE interface materials (TEiM) are commonly used infabricating TE legs, forming a “TEiM/TE material/TEiM” sandwichedstructure with two junctions on either side32–34. Generally, TEiMs areselected from metals or alloys due to their high σ and κ, which helpminimize energy loss during heat and electricity transport. Since TEmaterials are typically semiconductors,metal-semiconductor contactsare formed between TEiMs and TEmaterials. In addition to the σ and κof TEiMs, establishing ohmic contacts with low contact resistivity ρcbetween TEiMs and TE materials is crucial.When considering ρc in TEiM/TE material, the effective TE perfor-mancewill shift from thematerial’s zT to zTeff = zT × L/(L+2ρcσ), where Lis the length of TE leg35–37. According to this formula, high ρc will degradethe performance of the TE leg and device. Therefore, it is essential toachieve low ρc, especially the long-term low ρc between TEiM and TEmaterial to ensure stable and efficient device performance. However,the behavior of ρc in the TE leg is very complex. During fabrication,uncontrollable reactions between TEiM and TE materials usually occur,challenging the theoretical prediction of ρc on the one hand. Foranother, ρc is not a constant after fabrication and usually degrades dueto atomic diffusion and reaction under high temperatures over time25,31.Although selecting or designing suitable TEiMs can mitigate ρcdegradation6,36,38–42, the thermodynamically driven atomic diffusion andreaction, especially at high temperatures, always challenges theachievements of long-term reliable contacts in TE legs.MgAgSb has attracted considerable attention due to its high TEperformance near room temperature43–49. Initially, Ag has been used asthe TEiM for fabrication of its TE leg, achieving low ρc and high con-version efficiency η of 8.5% in both single TE leg and two-pair TEdevice50–54. However, a recent study has shown that ρc in Ag/MgAgSbincreases substantially from ~6.1 to 1006.0μΩ cm2 after 12 h of aging,challenging its long-term usage6. It is pointed out that Sb will diffusefromMgAgSb toAgTEiM, causingAg3Sb impurity phases and cracks atthe interface, which significantly contributes to the increased ρc in Ag/MgAgSb6,55. To address the issues with Ag as TEiM, a screened semi-metal, MgCuSb, and a developed alloy, MgAgMn0.1, have beenrespectively incorporated intoMgAgSb. Both the TE junctions achievea long-term low ρc and facilitate the improvement of η6,56.Here, to prevent Sb diffusion fromMgAgSb to TEiM, we have theidea to directly use Sb as the TEiM to develop Sb/MgAgSb TE junc-tions. In contrast to other TE junctions that typically show degradedρc after aging, we demonstrate that this junction exhibits self-optimized ρc when exposed to 573 K, even in air. We analyze thestructural evolution of the junction and find that Mg diffuses fromMgAgSb into the Sb TEiM, which leads to Mg deficiency in MgAgSb,increasing carrier concentration and reducing ρc. Importantly, thisMg diffusion occurs only near the interface, which does not sig-nificantly affect the performance ofMgAgSb away from the interface.For Sb TEiM, we reveal that it manifests decent σ, κ, low-cost, andexcellent weldability. Using the 100-day air-aged Sb/MgAgSb junc-tions, we fabricate a two-pair Mg-based TE device, which achieves amaximum η of 8.1% and power density of 0.41W cm-2 under a 294 Ktemperature gradient. This high output performance from long-termaged TE legs is rarely reported, highlighting their significant potentialfor lasting air-robust heat harvesting. The self-optimization phe-nomenon we discover can also inspire future designs of other junc-tions for long-term usage and pave the way for the long-awaitedwide-scale application of TE power generation.ResultsSelf-optimized ρc in Sb/MgAgSb/Sb with high air-resistanceDue to Sb diffusion and increased cracks at the Ag/MgAgSb junctions,a substantial increase in ρc has been observed6. Literature indicatesthat Sb deficiency in MgAgSb is detrimental to its performance46. Tosuppress Sb diffusion from MgAgSb to the TEiM, we directly employSb as the TEiM in MgAgSb, forming the Sb/MgAgSb TE junction.Generally, long-term low ρc is significantlymore desirable thanmerelyachieving an initial low ρc in the junction. Therefore, the Sb/MgAgSbjunction was aged at 573 K in the air for periods ranging from 7 to30 days to investigate changes in ρc.As shown in Fig. 1a, two distinct regions can be identified in theprobe distance dependence of resistance, corresponding to the SbTEiM and MgAgSb. Notably, when probing from Sb to MgAgSb, anincrease in resistance can be observed. For the as-fabricated Sb/MgAgSb (0 days), a rapid initial increase in resistance is noticed at theinterface, followed by a milder increase in the MgAgSb away from theinterface. This rapid initial increase is attributed to the marked ρcbetween the Sb TEiM and MgAgSb. As aging progresses, the rapidinitial increase in resistance near the interface diminishes, indicating areduction in ρc between the Sb TEiM and MgAgSb. The inset of Fig. 1apresents a zoomed-in view of the probe distance dependence ofresistance, detailing the gradual optimization of ρc with aging. It isnoteworthy that the resistivity of MgAgSb remains unchanged, whichis evidenced by the consistent slope in the MgAgSb region. Theunchanged resistivity in MgAgSb also indicates its excellent stabilityin air.Based on multiple measurements and averaging the ρc at bothsides of the Sb/MgAgSb/Sb TE leg (Supplementary Figs. 1–4), ρc isfound to decrease from 20.8μΩ cm2 to 7.9μΩ cm2 after 30 days ofaging (Fig. 1b). It is worth noting that in contrast to the significantincrease in ρc in Ag/MgAgSb with aging, the ρc of the Sb/MgAgSbjunction gradually decreases over time. Although the ρc of Sb/MgAgSbis relatively higher compared to MgCuSb and MgAgMn0.1 (Supple-mentary Fig. 5), it is important to note that the Sb/MgAgSb junctionwas aged at 573 K, a higher temperature than the aging conditions usedfor Ag, MgAgMn0.1, and MgCuSb. Moreover, unlike the vacuumenvironments used for Ag, MgAgMn0.1, andMgCuSb6,50,51,53,56,57, the Sb/MgAgSb junction was aged directly in air. The ability to withstand highair resistance at elevated temperatures allows Sb/MgAgSb tomaximizethe performance of MgAgSb under such challenging conditions,highlighting its strong potential for practical applications. Moreinterestingly, this self-optimized ρcwith aging hasnotbeen reported inprevious studies, as shown in Fig. 1c, where nearly all TE junctionsexhibit degraded ρc over time6,36,37,39,41,42,56–65. The optimized ρc in thiswork suggests that the aging will not always degrade contact proper-ties. On the contrary, if appropriatemetals, alloys, or intermetallics canbe identified or designed, long-term reliable contacts with low ρc areachievable.Mg diffusion from MgAgSb to Sb at the junctionThe self-optimized ρc at the interface, along with the unchangedresistivity away from it, suggests that the changes in properties in Sb/MgAgSb occur only near the interface and do not affect the propertiesfarther away. Therefore, it is necessary to investigate how structureevolves near the interface in Sb/MgAgSb junctions over time withaging. Scanning electron microscopy (SEM) equipped with energy-dispersive spectroscopy (EDS) was employed, and Fig. 2a shows theSEM images of Sb/MgAgSb junctions after 0 days, 7 days and 30 daysof aging. Line scans of Mg, Ag and Sb across the interface, shown inFig. 2b, indicate obvious Mg diffusion from MgAgSb into Sb TEiM.Figure 2c–e display EDS mappings for Mg, Ag, and Sb, providing aclearer view of the element’s evolution with aging time. Initially, a clearinterface can be observed, with Sb serving as TEiM on the left andMgAgSb as TEmaterial on the right. As aging progresses, Mg graduallyArticle https://doi.org/10.1038/s41467-025-56861-3Nature Communications |         (2025) 16:1502 2www.nature.com/naturecommunicationsdiffuses from the MgAgSb into the Sb TEiM, creating a Mg-rich regionabout 10μm from the original interface. While Ag shows minimal dif-fusion, an Sb-deficient region forms in the same Mg-rich region in theSb TEiM. Additionally, some scattered Sb-rich phases are detectedwithin the MgAgSb.To further investigate the microstructure of the Sb/MgAgSbjunction, an enlarged view is provided in Fig. 2f, whereMg-richwith Sb-deficient areas in the Sb TEiM, aswell as scattered Sb-rich phases in theMgAgSb region are more obvious. Additionally, intensified Ag signalsin the Ag mapping can be noticed within MgAgSb, suggesting theformation of Ag-rich phases. This is likely due to Mg diffusion, whichleads to Mg deficiency and consequently causes Ag-rich and Sb-richareas inMgAgSb. The overlappedmappings ofMg, Ag, and Sb in Fig. 2foffer a more comprehensive view of the phases present in the Sb/MgAgSb junction. Further semi-quantitative spot analysis was con-ducted to determine the specific composition at the Sb/MgAgSbjunction. As shown in Supplementary Fig. 6, the composition of TEiMand TE material away from the interface is predominately Sb andMgAgSb, respectively. Near the interface, the composition of TEiM andTE material is found to be mainly Sb and MgAgSb with large amountsof Ag3Sb, respectively (Supplementary Fig. 7). The formation of theseimpurities is attributed toMgdiffusion fromMgAgSb to Sb,which alsoleads to the formation of Mg3Sb2 in Sb TEiM, as shown in Supple-mentary Fig. 8.Historically, Ag3Sb has been considered detrimental to the per-formance of MgAgSb66, and will increase ρc6,55. The content of Ag3Sbnear the interface is revealed to progressively increase with aging dueto continued Mg diffusion (Supplementary Fig. 9). However, in thisstudy, the presence of Ag3Sb near the interface does not increase ρc.On the contrary, a reduction in ρc is observed. Notably, cracks havefrequently been found at the interface besides Ag3Sbwhen using Ag asTEiM6. However, no cracks were found at the interface in this work.Importantly, the Mg diffusion observed in the Sb/MgAgSb junctionmay contribute to stronger bonding between Sb and MgAgSb.Despite the evident Mg diffusion observed in Fig. 2,a questionarises as towhy thediffusionof Sb towardMgAgSb andAg towardSb isnot as pronounced, given their chemical potential differences on bothsides. We propose that Mg, being chemically active and having a smallatomic radius, can readily diffuse from MgAgSb to Sb, leading to theformation of Mg3Sb2. Simultaneously, the resulting Mg deficiency inMgAgSb facilitates the formation of Ag3Sb and SbwithinMgAgSb. TheSb impurities formed at the MgAgSb interface may act as a barrier,inhibiting the diffusion of Sb from the Sb TEiM intoMgAgSb, while theformation of Ag3Sb at the interface may similarly restrict Ag diffusiontoward Sb.The mechanism behind reduced ρcAlthough significant Mg diffusion fromMgAgSb into Sb is observed inthe Sb/MgAgSb junction, the underlyingmechanism for the reducedρcremains unclear. Based on the observation in Fig. 2a–f, it is evident thatMg diffusion fromMgAgSb to Sb becomes significant with aging. Thediffused Mg reacts with Sb to form Mg3Sb2 impurities, while thedepletion of Mg in MgAgSb leads to the decomposition of MgAgSbinto Ag3Sb and Sb. Figure 3a provides a schematic diagram illustratingthe structural changes with aging. To investigate whetherMgdiffusionplays a key role in reducing ρc, we deliberately reduced theMg content0.0 0.2 0.4 0.6 0.8 1.00.00.61.21.82.40 5 10 15 20 25 30030060090012000 10 20 300510152025Fe-Mo/NbFeSbCoSi2/ZrCoSbTi/CoSb3Fe Alloy/CoSb3Ti/Mo/CoSb3TiAl/CoSb3This work0.2 0.3 0.4 0.50.00.40.8Probe distance (mm)0 days7 days15 days30 daysProbe distance (mm)Sb (573 K in air)MgAgMn0.1 (523 K)MgCuSb (553 K)Ag (553 K, Xie et al.)Ag (553 K, Zhang et al.)c2 )Aging time (day)573 K in airSb/MgAgSb/Sbc2 )Aging time (day)0 10 20 30 40101600.3This workNi/Bi2Te3Sb/Bi2Te3Ag/MASMgCuSb/MASMgAgMn0.1/MASFe foil/Mg3Sb2Fe/Mg3Sb2Fe7Mg2Ti/Mg3Sb2304/Mg3Sb2Fe-Sb/ZintlMg2Ni/ZintlAging time (day)c/c0a cbOptimizedDegradedFig. 1 | Self-optimized ρc in Sb/MgAgSb/Sb aged at 573K in air. a Probe distancedependence of resistance in Sb/MgAgSb TE junctions after 0–30 days aging;b aging time dependenceofρc in Sb/MgAgSb junction compared to otherMgAgSb-based TE junctions from literature6,56,57; c aging time dependence of ρc/ρc0 ratio inSb/MgAgSband its comparison to TE junctions from literature6,36,37,39,41,42,56–65, whereρc0 represents the initial value of ρc.Article https://doi.org/10.1038/s41467-025-56861-3Nature Communications |         (2025) 16:1502 3www.nature.com/naturecommunicationsin MgAgSb to simulate this Mg diffusion and synthesized Mg-deficientMg1-xAgSb (x =0.05 and 0.1). The X-ray diffraction (XRD) patterns oftheseMg-deficient samples, shown in Fig. 3b, reveal impurity phases ofAg3Sb and Sb, consistent with the EDS analysis. Using these Mg-deficient samples, we fabricated Sb/Mg1-xAgSb junctions to examinechanges in ρc. As shown in Fig. 3c, the ρc is indeed reduced withdecreasingMg content inMg1-xAgSb, confirmingMgdeficiency plays acrucial role in lowering ρc.Considering that Mg3Sb2 forms in Sb TEiM at a distance from theinterface, the contact between Sb and MgAgSb gradually evolves intothe contact between Sb and MgAgSb with Ag3Sb. Based on their den-sity of states (DOS) near Fermi level EF (Fig. 3d), it can be found thatMgAgSb is a semiconductor, Ag3Sb is a typical metal, and Sb is asemimetal. Typically, there are no energy barriers at the junctionbetween ametal and a semimetal,whichmeans the increasedpresenceof Ag3Sb metallic phases at the interface may help reduce ρc. More-over, according to metal-semiconductor contact theory, ρc dependson both the barrier height (ϕB) and carrier concentration n, with therelationship simplified as ρc ~ exp(qϕB/n1/2), where q is the elementalcharge29,33,67. Therefore, increasing n can help reduce ρc68,69. Figure 3e, fdemonstrates that, as the Mg content decreases, S of Mg1-xAgSbdecreases, while σ increases, indicating that n increases with reducedMg content. This suggests that Mg deficiency not only results in theformation of Ag3Sb and Sb impurities but also significantly enhances nin Mg1-xAgSb. Similarly, in the Sb/MgAgSb junction, Mg diffusion fromMgAgSb to Sbpromotes the formation of themetallic Ag3Sbphase andenhances n at the interface, thereby contributing to the reductionin ρc.Intrinsically, n influences the electron transport mechanismacross the interface. In a metal/p-type semiconductor contact, aSchottky barrier ϕB forms when the work function of the metal (ϕm) islower than that of the semiconductor (ϕs), as shown in Fig. 3g. For Sb/MgAgSb, such a barrier is formed due to the work functions of Sb(4.40 eV) andMgAgSb (4.56 eV)4,6. Typically, electron transport acrossthe interface follows thermionic emission when the n of thematerial islow or moderate. If n increases, EFS will approach the valence bandedge (EVS) of the semiconductor, which will make the width of thedepletion layer W narrower (Fig. 3h). At this point, electron tunnelingeffects become more significant, allowing for a much higher currentand lowering ρc. Notably, in this study,Mg thermodynamically diffusesfromMgAgSb into the Sb TEiM, naturally forming a layer with high n atthe interface. This high n layer directly bonds to the Sb TEiM, whichSEM0 23501k2k25Ag25Mg Mg MgAg Ag Ag0 235SEM SEM0 235SbC.P.S.0 days 7 days 30 daysSb2525MgSb SbAg3SbMg3Sb2SbMg-rich phasesSb-rich phasesMg diffusionAg-rich phasesabcdefFig. 2 | Structural analysis of Sb/MgAgSb junction. a SEM images, b line scan of Mg, Ag, and Sb, and elemental mappings of cMg,d Ag, and e Sb in Sb/MgAgSb junctionafter 0 days, 7 days, and 30 days of aging; f enlarged view of elemental mappings ofMg, Ag, Sb, and overlappedMg, Ag, Sb in Sb/MgAgSb junction after 30 days of aging.Article https://doi.org/10.1038/s41467-025-56861-3Nature Communications |         (2025) 16:1502 4www.nature.com/naturecommunicationsreduces theW of the depletion layer and promotes electron tunneling.The schematic diagram in Sb/MgAgSb after aging is illustratedin Fig. 3i.However, despite the ρc being reduced, there are concernsabout the alerted performance of MgAgSb near the interface. In Sb/MgAgSb junction, it is noteworthy that the overall composition ofMgAgSb away from the interface does not significantly change after30 days of aging, as shown in Supplementary Fig. 10. Consequently,the performance of MgAgSb away from the interface remains largelyunchanged, as evidenced by the consistent Seebeck coefficient inboth the Sb/MgAgSb/Sb junction and the MgAgSb material at dif-ferent aging times (Supplementary Fig. 11). Thus, the Mg diffusion inSb/MgAgSb only forms a thin high-n layer, just micrometers thicknear the interface, which helps reduce ρc when in contact with Sb.Heavily doping the surface layer of a semiconductor is a commonstrategy in semiconductor devices to reduce the barrier width andfacilitate ohmic contacts between themetal and semiconductor. Thisapproach may be effective for achieving low ρc in TE junctions in thefuture.Efficient η of Sb/MgAgSb-based TE deviceTypically, high σ and κ are desired for the TEiM tominimize energy lossduring heat and electricity transport. Here, we compare Sb TEiM withother TEiMs used in MgAgSb. As shown in Fig. 4a, b, Sb exhibitsmoderate values of both σ and κ compared to Ag, MgAgMn0.1, andMgCuSb6,56,70. Based on the σ, κ, S (as shown in Supplementary Fig. 12)of these TEiMs, we simulated themaximum conversion efficiency ηmaxand maximum power Pmax of a single MgAgSb TE leg. The TEiM wasassumed to have a height of 0.5mm at both ends of the leg withoutconsidering ρc. As shown in Supplementary Fig. 13, despite large dif-ferences in σ and κ, the variation inηmax and Pmax of these single TE legsare relatively small. For MgCuSb sintered at 573 K, which has lower σand κ, it demonstrates correspondingly lower ηmax and Pmax. Sincepower output is accumulated acrossmultiple legs in a TE device, whichusually consists of 30 or more TE legs, high σ and κ are always desir-able. But if the TEiM layer is made thinner, its impact on ηmax and Pmaxwill diminish.Cost is another factor that could be considered, especially withpotential large-scale applications. Figure 4c shows that Sb andMgCuSb are much more cost-effective than Ag and MgAgMn0.171. Inaddition, weldability is an important aspect of the TEiM that should beconsidered. A key functionof TEiMs is to facilitate the joiningof TE legswith electrodes.Without properweldability, additional layers are oftenrequired for the joining, which complicates the fabrication process.Compared to MgAgMn0.1 and MgCuSb, Sb demonstrates excellentweldability. The inset in Fig. 4d shows that conventional Sn-basedsolder can easily wet the surface of Sb. Moreover, as shown in Fig. 4d,the joint between Sb/MgAgSb and Cu electrode made with this solderexhibits very low ρc, further highlighting the advantages of using Sb asa promising TEiM for future practical applications.300 400 500 600100150200250300 400 500 6000.50.70.91.11.31.5100.00.71.42.12.8-2 -1 0 1 2x = 0x = 0.05x = 0.10S-1)T (K)Mg1-xAgSb(104Sm-1)T (K)x = 0x = 0.05x = 0.10Sb/MgAgSbSb/Mg0.95AgSbSb/Mg0.90AgSbProbe distance (mm)Intensity(a.u.)2 (°)Mg1-xAgSbSimulatedx = 0x = 0.05x = 0.10Sb MgAgSb MgAgSbSb Mg3Sb2+SbMgAg3Sb+Sb+MgAgSb Sb Ag3SbIncreased nNarrowed WEnahnced e- tunnelingIncreased n at surface layer Enahnced e- tunnelingNormal state Schottky barrierEFMgAgSbSemiconductorMetalDensityofstates(a.u.)EFAg3SbE (eV)a bc de fghiEFSbSemimetalFig. 3 | Reduced ρc from increased carrier concentration for enhanced electrontunneling. a Schematic diagram of structural changes in Sb/MgAgSb with aging;b XRDpatterns of Mg1-xAgSb and c probe distance dependence of resistance in Sb/Mg1-xAgSbTE junctions;dDOSofMgAgSb,Ag3SbandSb; temperaturedependenceof e S and f σ ofMg1-xAgSb; energy band diagram of the contact betweenmetal andg normal p-type semiconductor, h p-type semiconductor with increased n andi p-type semiconductor with a high n layer, specifically for the diagram in Sb/MgAgSb after aging.Article https://doi.org/10.1038/s41467-025-56861-3Nature Communications |         (2025) 16:1502 5www.nature.com/naturecommunicationsTo further validate the reliability of the Sb/MgAgSb TE junctionsdeveloped in this study, two-pair Mg-based TE devices are fabricated,with the schematic diagram of the module’s dimensions shown inSupplementary Fig. 14. The n-type legs were made of Mg3Sb0.6Bi1.4,with their TE performance detailed in Supplementary Fig. 15. The as-fabricated Sb/MgAgSb/Sb TE legs without aging were used for initialtesting. Optical images of themeasurement set-up and the two-pair TEdevice are shown in Fig. 4e. Supplementary Fig. 16 shows the voltage(V), power output (P), heat flow (Q), and conversion efficiency (η) ofthe device under varying ΔT and applied current (I), where ηmaxreaches approximately 7.7% at a ΔT of 276K. After aging Sb/MgAgSb/Sb legs at 573K in air for 30 and 60 days, they are coupled with n-typeMg3Sb0.6Bi1.4 tomake another two two-pair TE devices. Measurementsin Supplementary Figs. 17–18 show that ηmax of them reach 7.9% atΔT ~ 275 K. Furthermore, Sb/MgAgSb/Sb legs were aged at 573 K in theair for an additional 40 days (totaling 100days). I-dependentV, P andQunder different ΔTs measured in its corresponding two-pair TE deviceis shown in Fig. 4f, g. It reveals that ηmax can reach 8.1% under ΔT ~294 K. While this value is slightly lower than the state-of-the-art Ag,MgCuSb, and MgAgMn0.1 used as TEiMs in MgAgSb6,54,56, several fac-tors should be considered. Excluding variations in measurement con-ditions andpotential errors across differentmeasurement systems, theshorter height of the TE legs in this study compared to those in theliterature6,54,56 likely reduces efficiency but enhances power0 2 4 6 80.000.050.100.150.200.250 2 4 6 80.00.20.40.60 2 4 6 802468100 200 3003691202004006008000 1 2 30123410 100 10000.0010.010.113300 400 500 600100101102103104300 400 500 6001001011021030.00.30.604812V(V)I (A) I (A)P(W)79 K 128 K178 K 227 K275 K 294 KbMgAgSbQ(W)eMg3Bi1.4Sb0.6HeaterCoolerSb 304 SSCu79 K 128 K178 K 227 K275 K 294 KPositive viewmax(%)T (K)100 days60 days30 days0 daysPs+QsPm+Qs10.6%8.1%9.5%h i j593 K293 KPrice(USD/kg)4095215.8 5.5MgAgMn0.1 Ag Sb MgCuSb0 30 60 100Probe distance (mm)Sn-basedsolderCu Sb SolderYing et al.Xie et al.100 daysMSB/ZintlMSB/MSBHHPbTeBi2Te3max(Wcm-2)T (K)af100-300 K(104Sm-1)T (K)MgCuSb (S573)MgCuSb (S773)MgCuSb (Xie et al.)SbMgAgMn0.1Agg(Wm-1K-1 )T (K)dcMgCuSb (S573)MgCuSb (S773)MgAgMn0.1AgSbPmax(W)Aging time (days)h i jTh = 573 Kmax(%)h iTh = 573 KFig. 4 | Conversion efficiency two-pair TE device based on Sb/MgAgSb TEjunction. T dependence of a σ, b κ and c price of Sb and other TEiMs in MgAgSb,where σ of Ag, MgAgMn0.1 are sourced from literature6,56,70, while κ of Ag andMgAgMn0.1 are calculated based Wiedemann-Franz law; d probe distance depen-dence of resistance for the joint between Sb/MgAgSb and Cu electrode made withSn-based solder, with an optical image of the Sb/MgAgSb/Sb leg wetted by Sn-based solder included in the inset; e optical image of two-pair TE device and themeasurement set-up; I dependence of f V and P, and gQ under different ΔTs of thetwo-pair TE device based on Sb/MgAgSb/Sb legs after 100 days of aging in air;h aging time dependence of Pmax and ηmax when Th is 573K; i ΔT dependence ofηmax in MgAgSb/Mg3Sb0.6Bi1.4 two-pair TE devices, where Pm, Ps and Qs representmeasured power, simulated power and simulated heat flow, respectively; j ΔTdependence of ωmax in MgAgSb/Mg3Sb0.6Bi1.4 two-pair TE device and its compar-ison with other two-pair TE devices in the literature6,42,51,56,73–75, where the abbre-viations MSB, HH and BST refer to Mg3(Sb,Bi)2, half-Heusler and (Bi,Sb)2Te3,respectively.Article https://doi.org/10.1038/s41467-025-56861-3Nature Communications |         (2025) 16:1502 6www.nature.com/naturecommunicationsgeneration, contributing to the impressive high Pmax of 0.41Wachieved in this work. More importantly, themaintained Pmax and ηmaxin these two-pair TE devices based on Sb/MgAgSb after 0, 30 days,60 days, and 100 days of air-aging sufficiently confirms the long-termreliable contacts in Sb/MgAgSb, as well as emphasizing the excellentstability of MgAgSb despite some Mg diffusion at the inter-face (Fig. 4h).To explore further potential improvements, a simulation is con-ducted to evaluate the performance of the two-pair TE device, takinginto account the TE performance of MgAgSb and Mg3Sb0.6Bi1.4, ρc ofboth n-type and p-type legs, as well as the properties of the TEiMs. Theinset in Fig. 4i presents the simulated two-pair TE device under a hotside temperature of 593 K and a cold side temperature of 293 K. Thesimulation results indicate that the ηmax could reach 10.6%. Detailedsimulated I-dependent of V, P, Q, and η is shown in SupplementaryFig. 19. Generally, the measured P tends to be lower, while the mea-sured Q tends to be higher than the simulated results. In practicalmeasurements, P can usually be accurately assessed, but preciselymeasuringQ is often challenging72. To better illustrate the potential ofthe developed two-pair TE device, the simulated heat flow Qs andmeasured power output Pm were used to calculate ηmax. When ΔT is300K, the two-pair TE device can demonstrate ηmax of 9.5%.Supplementary Fig. 20 displays the discrepancies in open-circuitvoltageV0, internal resistanceR, heatflowwithout currentQ0, andPmaxbetween the measurements and simulations, indicating potentialroom for further improvement of the developed two-pair TE device. Infact, the achieved maximum power density ωmax of 0.41W cm-2 andηmax of 8.1% in our present MgAgSb/Mg3Sb0.6Bi1.4-based TE device isalso highly impressive when compared to other two-pair TE devicesunder similar temperature gradients6,42,51,56,73–75, especially in the100–300K range (Fig. 4j and Supplementary Fig. 21). Furthermore, adistinguishing feature of Sb/MgAgSb/Sb is its exceptional durability.The junction remains stable even after 100 days of aging in air, acharacteristic rarely reported in the literature for other TE materials.This highlights its strong potential for long-term applications.DiscussionIn summary, we report for the first time a self-optimized contactresistivity ρc in Sb/MgAgSb TE junctions. Dramatically different fromconventional TE junctions that typically experience increased resis-tivity after high-temperature aging, ρc of Sb/MgAgSb junction gradu-ally optimized when exposed to 573 K, even in air. Mg diffusion fromMgAgSb into the Sb TEiM is revealed, which leads to Mg deficiency inMgAgSb, resulting in increased carrier concentration n. This increasedn enhances electron tunneling between MgAgSb and the Sb TEiM,effectively reducing ρc. The Mg diffusion is revealed to occur near theinterface, which does not affect the performance of the main bulkMgAgSb material. For Sb TEiM, it is revealed to possess decent σ, κ,low-cost, and excellent weldability, which allows for its direct bondingto electrodes without the need for additional layers. The practicalapplication of our developed TE junction is highlighted by the fabri-cation of a two-pair Mg-based TE device, which achieves ωmax of0.41Wcm-2 and ηmax of 8.1% under a 294K temperature difference,even after Sb/MgAgSb/SbTE legswere aged at 573 K in air for 100days.This finding underscores the significant potential of the developed Sb/MgAgSb junctions for long-term efficient low-grade heat harvesting.Additionally, the self-optimizationphenomenonobserved in this studyoffers valuable insights that could inspire the design of future TEjunctions aimed at long-term applications, and thereby paving the wayfor the long-awaited wide-scale application of TE power generation.MethodsMaterials synthesisMg1-xAg0.97Sb0.99 with 0.625wt% C18H36O2 (x = 0, 0.05, 1) (denoted asMg1-xAgSb in the main text and below), MgCuSb andMg3.2In0.02Sb0.595Bi1.4Te0.005 (denoted asMg3Sb0.6Bi1.4 in themain textand below) were synthesized by using Mg turnings (99.95%), Agpowers (99.99%) Sb shots (99.999%), Cu powder (99.999%), Bi shots(99.999%), Te shots (99.999%) and In powders (99.99%). The rawmaterials were weighted stoichiometrically and then put into the ball-milling jaw with the inside of argon. Then, the jars were mechanicallyalloyed for 5 h, 20 h, and 5 h for Mg1-xAgSb MgCuSb andMg3.2In0.02Sb0.595Bi1.4Te0.005, respectively (SPEX-8000D). Theobtained powders from the jars were then consolidated into the bulkby vacuumspark plasmasintering.Mg1-xAgSbwas sintered under 573 Kand 60MPa for 5min (SPS-322Lx, Dr. Sintering). MgCuSb was sinteredunder 573K or 773 K and 60MPa for 5min (SPS-322Lx, Dr. Sintering).Mg3.2In0.02Sb0.595Bi1.4Te0.005 was sintered under 973 K and 60MPa for10min (SPS-1080 System, SPS SYNTEX INC).Characterization and measurementsX-ray diffractometer (SmartLab3, Rigaku) with Cu Kα radiation (40 kVand 15mA) was used to characterize the phases of the obtained Mg1-xAgSb samples. ZEM-3 (Advance Riko) was employed to measureelectrical transport properties (S and σ) of the samples, which have anuncertainty of ±5%. LFA467 (Netzsch) was used to determine thethermal diffusivity D of the samples, with an uncertainty of ±3%. The κwas calculated using the formula κ =DρCp, where ρ and Cp representthe density and heat capacity of the samples, obtained by the Archi-medes method and the Dulong-Petit law, respectively. The figure ofmerit zT was calculated based on above S, σ and κ using the equationzT = S2σT/κ.TE junction fabrication and characterizationSb/MgAgSb/Sb leg was fabricated by sandwiching two layers of Sb asthe interface material and then sintered by SPS under 573 K and60MPa for 5min. The obtained Sb/MgAgSb/Sbwere cut into dice withdimensions of ~3.8 × 3.8 × 6mm3. Sb/MgAgSb TE junctions were agedat 573 K in the air for 0, 7, 15, and 30 days for characterization andcontact resistivity measurements. The contact resistance of the Sb/MgAgSb TE junctions was measured by a 2-axis resistance distributionmeasurement instrument (S1331, Mottainai energy). The compositiondistribution of these TE junctions was investigated by using scanningelectron microscopy (FESEM, Hitachi SU8000) equipped with anenergy dispersive spectrometer (EDS, XFlash FlatQUAD 5060 F).TE device fabrication, measurement, and simulationTwo-pair TE device was assembled using p-type MgAgSb and n-typeMg3Sb0.6Bi1.4 TE legs. Sb and 304 stainless steel were used as TEiMs forMgAgSb andMg3Sb0.6Bi1.4, respectively. Mini-PEM (Advance Riko) wasused to measure the output power and conversion efficiency the two-pair TE device. The measurement was conducted under vacuum con-ditions. The temperature gradient was established by maintaining thecold-side temperature at 293 K, while the hot-side temperatures variedfrom 373 K to 593 K. The finite-element simulations were performedwith COMSOL Multiphysics® software to simulate the conversionefficiency and power of the TE leg with different TEiM and the two-pairTE device, the thermal contact resistance between the multiple inter-faces of the TE device was not considered in this work.First-principles calculationsFirst-principles calculations were performed to calculate the densityof states (DOS) of MgAgSb, Ag3Sb and Sb. These calculations wereconducted using software Vienna ab initio Simulation Package(VASP) with the projector augmented-wave method76,77. Here, thegeneralized gradient approximation with the Perdew-Burke-Ernzerhof functional GGA-PBE and modified Becke-Johnson (mBJ)were used as exchange-correlation functionals to achieve a moreaccurate estimation of the bandgap78,79. 500 eV was used as plane-wave energy cutoff. The convergence criteria forHellmann–FeynmanArticle https://doi.org/10.1038/s41467-025-56861-3Nature Communications |         (2025) 16:1502 7www.nature.com/naturecommunicationsforce on each atom energy and total energy were set to 0.001 eV Å-1and 10-8eV, respectively. Geometry relaxation was first performedusing Gamma-centered k-point sampling with k = 30/L, where L is thelattice parameter of crystal. The relaxed structure was then used forself-consistent static calculations with k = 60/L. VASPKIT80 has beenused to post-process the calculated data for obtaining the DOS ofMgAgSb, Ag3Sb and Sb.Data availabilityAll data generated or analyzed during this study are included in thispublished article (and its supplementary information file).References1. Cohen, S. S. & Gildenblat, G. S. Metal-semiconductor contacts anddevices (Academic Press, 1986).2. Liu, Y. et al. Approaching the Schottky–Mott limit in van der Waalsmetal–semiconductor junctions. Nature 557, 696–700 (2018).3. 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T.M.supervised the whole project. A.L., L.W., J.L., X.W., and T. M. discussed,reviewed and edited the manuscript.Competing interestsT.M. andA.L. have filed one Japanese patent application (2024−204680)on the work described here. The remaining authors declare no com-peting interests.Article https://doi.org/10.1038/s41467-025-56861-3Nature Communications |         (2025) 16:1502 9https://en.wikipedia.org/wiki/Prices_of_chemical_elements/https://en.wikipedia.org/wiki/Prices_of_chemical_elements/www.nature.com/naturecommunicationsAdditional informationSupplementary information The online version containssupplementary material available athttps://doi.org/10.1038/s41467-025-56861-3.Correspondence and requests for materials should be addressed toTakao Mori.Peer review information Nature Communications thanks Jamal-DeenMusah, and the other, anonymous, reviewer(s) for their contribution tothe peer review of this work. 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To view a copy of this licence, visit http://creativecommons.org/licenses/by-nc-nd/4.0/.© The Author(s) 2025Article https://doi.org/10.1038/s41467-025-56861-3Nature Communications |         (2025) 16:1502 10https://doi.org/10.1038/s41467-025-56861-3http://www.nature.com/reprintshttp://creativecommons.org/licenses/by-nc-nd/4.0/http://creativecommons.org/licenses/by-nc-nd/4.0/www.nature.com/naturecommunications Self-optimized contact in air-robust thermoelectric junction towards long-lasting heat harvesting Results Self-optimized ρc in Sb/MgAgSb/Sb with high air-resistance Mg diffusion from MgAgSb to Sb at the junction The mechanism behind reduced ρc Efficient η of Sb/MgAgSb-based TE device Discussion Methods Materials synthesis Characterization and measurements TE junction fabrication and characterization TE device fabrication, measurement, and simulation First-principles calculations Data availability References Acknowledgements Author contributions Competing interests Additional information