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

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Global softening to manipulate sound velocity for reliable high-performance MgAgSb thermoelectrics8810 |  Energy Environ. Sci., 2024, 17, 8810–8819 This journal is © The Royal Society of Chemistry 2024Cite this: Energy Environ. Sci.,2024, 17, 8810Global softening to manipulate sound velocityfor reliable high-performance MgAgSbthermoelectrics†Airan Li, ‡a Longquan Wang,‡ab Jiankang Liab and Takao Mori *abHigh-performance thermoelectric materials at room temperature are eagerly pursued due to theirpromising applications in the Internet of Things for sustainable power supply. Reducing sound velocityby softening chemical bonds is considered an effective approach to lowering thermal conductivity andenhancing thermoelectric performance. Here, different from softening chemical bonds at the atomicscale, we introduce a global softening strategy, which macroscopically softens the overall material tomanipulate its sound velocity. This is demonstrated in MgAgSb, one of the most promising p-typethermoelectric materials at room temperature to replace (Bi,Sb)2Te3, that the addition of inherently softorganic compounds can easily lower its sound velocity, leading to an obvious reduction in latticethermal conductivity. Despite a simultaneous reduction of the power factor, the overall thermoelectricquality factor B is enhanced, enabling softened MgAgSb by C18H36O2 addition to achieve a figure ofmerit zT value of B0.88 at 300 K and a peak zT value of B1.30. Consequently, an impressive averagezT of B1.17 over a wide temperature range has been realized. Moreover, this high-performance MgAgSbis verified to be highly repeatable and stable. With this MgAgSb, a decent conversion efficiency of 8.6%for a single thermoelectric leg and B7% for a two-pair module have been achieved under a temperaturedifference of B276 K, indicating its great potential for low-grade heat harvesting. This work will notonly advance MgAgSb for low-grade power generation, but also inspire the development of high-performance thermoelectrics with global softening in the future.Broader contextThermoelectrics can harvest waste heat from the environment to power numerous sensors in the Internet of Things, aiding the realization of a carbon-neutralsociety. Efficient thermal-electricity transfer requires thermoelectric materials to exhibit not only an excellent power factor but also an extremely low thermalconductivity. Traditionally, reducing sound velocity by softening chemical bonds at atomic scale is effective to decrease thermal conductivity and enhancethermoelectric performance, but this approach often involves trial and error to find suitable doping or alloying elements. In this work, we adopt a moremacroscopic method by incorporating soft organic compounds, which could globally soften the materials and intentionally reduce sound velocity of material.We demonstrate that in MgAgSb, a promising p-type thermoelectric material with intrinsically low thermal conductivity at room temperature, adding softorganic compounds like fatty acid C18H36O2 can effectively reduce its sound velocity, significantly lowering its lattice thermal conductivity further andimproving its thermoelectric performance. Importantly, the addition of C18H36O2 also enhances the reproducibility of high-performance MgAgSb. Based on thisreliable MgAgSb, high conversion efficiency can be achieved, showcasing its potential for practical application in the Internet of Things in the future.IntroductionThermoelectric (TE) technology, which enables the mutualconversion of heat and electricity, holds great promise forpower generation by harvesting waste heat.1,2 With the rapiddevelopment of the Internet of Things (IoTs) recently, therearises a growing demand for high-performance TE materialsnear room temperature to sustainably power the numeroussensors.3 However, the limited kinds of high-performanceTE materials at room temperature hinder their widespreada Research Center for Materials Nanoarchitectonics (MANA), National Institute forMaterials Science (NIMS), Namiki 1-1, Tsukuba, 305-0044, Japan.E-mail: MORI.Takao@nims.go.jpb Graduate School of Pure and Applied Sciences, University of Tsukuba,Tennodai 1-1-1, Tsukuba, 305-8671, Japan† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4ee03521f‡ These authors contribute equally to this work.Received 7th August 2024,Accepted 6th October 2024DOI: 10.1039/d4ee03521frsc.li/eesEnergy &EnvironmentalSciencePAPERhttps://orcid.org/0009-0004-7318-4821https://orcid.org/0000-0003-2682-1846http://crossmark.crossref.org/dialog/?doi=10.1039/d4ee03521f&domain=pdf&date_stamp=2024-10-11https://doi.org/10.1039/d4ee03521fhttps://doi.org/10.1039/d4ee03521fhttps://rsc.li/eesThis journal is © The Royal Society of Chemistry 2024 Energy Environ. Sci., 2024, 17, 8810–8819 |  8811application. Generally, the TE performance of a material isjudged by the figure of merit, zT = S2sT/k, where S is theSeebeck coefficient, s is electrical conductivity, T is the absolutetemperature and k is thermal conductivity. According to thisformula, outstanding TE materials necessitate not only a super-ior power factor PF = S2s but also an ultralow k.In the past, various strategies have been employed toenhance zT by improving the PF.4 These strategies include,but are not limited to, band engineering,5,6 carrier scatteringmanipulation7–9 and carrier concentration optimization,10,11which have led to the advancement of numerous high-performance TE materials since the last century.12 Besidesenhancing PF, decreasing k is also a viable approach to boost-ing TE performance.13–15 For most bulk materials, k iscomposed of electronic thermal conductivity ke and latticethermal conductivity kL, where ke depends on the transportof carriers (electrons and holes), and kL depends on thetransport of phonons. According to the Wiedmann–Franz law,ke = LsT (L is the Lorenz number), ke has a strong positiverelationship with s. Therefore, kL is considered an independentTE parameter beyond S, s and ke. Due to the complex relation-ship between S, s and ke, the strategy targeting a decrease in kLis very attractive.In bulk materials, kL is governed by the phonon transportand can be described by the ‘‘phonon gas’’ model, which givesrise to the formula = 1/3 � cvvg2t, where cv, vg, and t representspecific heat, phonon group velocity, and phonon relaxationtime, respectively. This suggests that decreasing kL or findingmaterials with inherently low kL requires materials to possesslow cv, vg and t. Typically, materials with liquid-like ions canexhibit a reduction of cv,16 and it is challenging to manipulatethe cv, because the energy carried by atoms approaches theDulong–Petit limit of 3kBT per atom in bulk materials at hightemperatures. Therefore, decreasing vg and/or t become themain focus to obtain low kL material. Phonon relaxation time treflects the scattering process of phonons, including bothintrinsic and extrinsic contributions. Intrinsic phonon scatter-ing (Umklapp scattering, U) requires materials to possess largeanharmonicity, which is an important characteristic of manyhigh-performance TE materials, such as PbTe,17 SnSe18 andMg3Sb2.19,20 Besides intrinsic U scattering, extrinsic phononscattering by point defects, dislocations, grain boundaries,nanoporous and nanoprecipitate has been frequently intro-duced into the material to decrease kL, leading to significantadvancements in TE materials to achieve high zT values.21–24It is worth noting that strengthening phonon scattering canalso influence vg.25 Generally, phonons in materials can becategorized as acoustic phonons and optical phonons. Sincethe velocity of optical phonons approaches 0, the main con-tribution to vg comes from acoustic phonons. Usually, thevelocity of acoustic phonons can be approximated by the soundvelocity vs due to their equivalence in the long wavelength limit.At the microscopic atomic scale level, vs is influenced by thechemical bonds.26,27 Low force constants between atoms andlarge atomic mass are beneficial for achieving low vs.28 Dopingto soften chemical bonds has been demonstrated to effectivelydecrease kL and enhance the performance of TE materials.15 Onthe other hand, from a more macroscopic perspective, vs isdetermined by the strength of the overall sound transportmedium, meaning the softer the medium, the lower the vs.This suggests that if the material can be globally softened, adecrease in kL can also be expected, which however, remains tobe verified.Traditionally, methods such as alloying, work hardening,grain refinement, and second-phase strengthening are com-monly used to increase materials’ strength. The rule of mix-tures serves as a guideline when adding a second phase into thematrix to form composites, predicting the composite’s strengthby averaging the strengths of the matrix and second phase,weighted by their respective proportions.29 Incorporatingstrong, hard second phases can significantly enhance materi-als’ strength, as seen when adding strong SiC to Bi2Te3-basedmaterials can improve their mechanical properties.30,31Conversely, adding a soft second phase can reduce overall mate-rial strength, potentially lowering vs and leading to low kL and highzT. MgAgSb, regarded as one of the most promising p-type TEmaterials for replacing (Bi,Sb)2Te3 at room temperature, hasgarnered significant attention in the past.32–34 Due to its highband degeneracy and inherently low kL, MgAgSb can achievea zT value of B0.7 at room temperature and a peak zT value ofB1.2.35–39 Factors such as hierarchical weak chemical bonds,atomic disorder, local structural distortion and crystal-liquidduality have been identified as important contributors to itsintrinsically low kL.28,40–43 Recently, efforts to further lower kLhave been realized by nanostructuring and introducing nano-pores into MgAgSb, which strengthen phonon scattering andresult in an enhanced zT value in the temperature range from300 K to 473 K.44 However, this ultrafine-grained and nanopor-ous MgAgSb faces the risk of grain growth at high tempera-tures, making it suitable for cooling applications at lowtemperatures rather than power generation at relatively hightemperatures. Nevertheless, decreasing kL could be an effectiveway to improve the TE performance of MgAgSb.In this work, we introduce and adopt a global softeningstrategy to decrease vs of MgAgSb, aiming to reduce its kL andenhance its TE performance. Firstly, by adding inherently softorganic compounds, we demonstrate that the vs of MgAgSb canbe efficiently decreased. Then, focusing on the addition ofC18H36O2, we find that the vs of MgAgSb can be graduallymanipulated when adjusting the C18H36O2 content. Despite adecreased PF, the overall TE quality factor B gets enhanced dueto the much-decreased kL, resulting in a high zT of B0.88 at300 K and a peak zT of B1.30. Further, we reveal that this high-performance MgAgSb is highly repeatable and stable as a resultof C18H36O2 addition. Finally, using this high-performanceMgAgSb, we achieve a high conversion efficiency Z of B8.6%and B7% for a single TE leg and a two-pair module, respec-tively, showing great potential for harvesting low-grade heat.This reliable high-performance MgAgSb paves the way for itspractical usage for power generations in the future, and theproposed global softening strategy can also stimulate perfor-mance enhancement in other TE systems.Paper Energy & Environmental Science8812 |  Energy Environ. Sci., 2024, 17, 8810–8819 This journal is © The Royal Society of Chemistry 2024Results and discussionsReduced sound velocity for high TE performanceDue to the close relationship between phonon group velocityand vs, kL is closely related to vs. As shown in Fig. 1a, the curveof vs versus kL for most semiconductors reveals a positiveproportion of kL to vs,28,45,46 indicating that materials withlow vs tend to possess low kL and have the potential to behigh-performance TE materials. MgAgSb is one such materialwith an intrinsically low vs of B1921 m s�1 (or 1844 m s�1),depending on the synthesis condition and composition.28,36This low vs gives rise to the low kL of MgAgSb (B0.6 W m�1 K�1).In this work, one-step ball milling has been used to synthesizeMgAgSb, resulting in its vs of 1826 m s�1 and kL of 0.58 W m�1 K�1,similar to the literature.As mentioned above, vs is strongly dependent on soundtransport medium. Therefore, globally softening the materialsholds great promise for reducing vs of material, consequentlyleading to a reduction in kL. Organic compounds, which arenormally softer than inorganic compounds, are ideal as possi-ble additives for the softening of overall materials. Zinc stearate(C36H70ZnO4), magnesium stearate (C36H70MgO4) and stearicacid (C18H36O2), known for their universe usage in powdermetallurgy,62,63 have been chosen to be added to MgAgSb.As shown in Fig. 1b, the vs of MgAgSb was successfully reducedfrom its original 1826 m s�1 to 1760 m s�1, 1742 m s�1, and1731 m s�1 with C36H70ZnO4, C36H70MgO4 and C18H36O2 addi-tion, respectively. As a result, the reduced vs leads to an obviousdrop in kL of MgAgSb in the whole temperature range, as shownin Fig. 1c, where kL reaches 0.48 W m�1 K�1 at room tempera-ture and 0.21 W m�1 K�1 at 573 K by C18H36O2 addition, whichfurther brings about the enhancement of zT in MgAgSb.As shown in Fig. 1d, MgAgSb by C18H36O2 addition can achievea high zT value of B0.88 at 300 K and a peak zT value of B1.30,which is excellent in the temperature range from 300 K to 573 Kcompared to the previous reports.33,36,47–49 Moreover, this highzT is very competitive among state-of-art p-type TE materials(Fig. 1e).50–58 With this high-performance MgAgSb, its single TEleg can achieve 3.6% and 8.6% maximum conversion efficiencyunder the temperature difference DT of about 80 K and 280 K,respectively, standing out as one of the best p-type single TElegs compared to PbSe, AgSbTe2 and GeTe.50,52,53,59–61Manipulating sound velocity by adjusting C18H36O2 contentTo delve deeper into the global softening strategy aimed atreducing vs, varying amounts (0–1 wt%) of C18H36O2 have beenintroduced into the matrix of MgAgSb. As shown in the X-raydiffraction (XRD) patterns in Fig. 2a, the addition of C18H36O2(0.25–0.75 wt%) does not alter the phase of MgAgSb, whilewhen 1 wt% C18H36O2 is added, a noticeable Sb secondaryphase appears, which has been excluded from further discus-sion. Typically, C18H36O2 is stable below 573 K but decomposesat around 633 K. The presence of C18H36O2 in the MgAgSb canbe evidenced by the increase in the vacuum of SPS chamberwhen the sintering temperature exceeds 700 K (Fig. S1, ESI†).Additionally, as shown in Fig. S2 (ESI†), the gradually increas-ing endothermic peak around 650 K in differential scanningFig. 1 Global softening for high-performance MgAgSb. (a) vs versus kL at 300 K for various compounds;28,45,46 (b) vs of MgAgSb in literature28,36 and thiswork; (c) T dependence of kL of MgAgSb without addition and with organic addition, and their comparison to literature;33,36,44 (d) T dependence of zT ofMgAgSb without addition and with C18H36O2 addition, and their comparison to literature;33,36,47–49 (e) T dependence of zT MgAgSb with C18H36O2addition and its comparison to other state-of-art p-type TE materials;50–58 (f) applied DT dependence of maximum conversion efficiency Zmax of MgAgSbsingle-leg and its comparison to other state-of-art p-type single-leg.50,52,53,59–61Energy & Environmental Science PaperThis journal is © The Royal Society of Chemistry 2024 Energy Environ. Sci., 2024, 17, 8810–8819 |  8813calorimetry (DSC) curves of these C18H36O2-added MgAgSbfurther confirms the successful addition of C18H36O2 in MgAgSb.Due to the addition of C18H36O2, an expected softeningeffect on MgAgSb is observed, as indicated by the decreasedVickers hardness of MgAgSb shown in Fig. 2b. It should benoted that Vickers hardness is also influenced by changes inmicrostructure. Scanning electron microscopy (SEM) was usedto observe the fracture morphology of pure MgAgSb andMgAgSb with 0.75 wt% C18H36O2. As shown in Fig. S3 (ESI†),the grain size decreases in the sample with C18H36O2 addition.However, despite the smaller grain size, the Vickers hardness ofMgAgSb still decreases with C18H36O2 addition. This is surpris-ing, as according to the Hall–Petch relationship, smaller grainsizes usually result in increased strength or hardness. Thedecreased Vickers hardness suggests a more pronounced soft-ening effect with C18H36O2 addition.The gradual softening of MgAgSb further leads to a corres-ponding gradual decrease in vs. As displayed in Fig. 2c, both thelongitudinal sound velocity vl and transverse sound velocity vtdecrease with the increasing C18H36O2 content. Consequently,the derived vs of MgAgSb also gradually decreases. This sug-gests that vs can be easily manipulated and tailored whenadjusting the material’s overall softness by different C18H36O2contents. Due to the deceased vs, it correspondingly resultsin the decreased k of MgAgSb. As shown in Fig. 2d, the kof MgAgSb with 0.75 wt% C18H36O2 addition decreases to0.65 W m�1 K�1 at room temperature, nearly a 30% reductioncompared to 0.95 W m�1 K�1 of MgAgSb without addition.Further, when subtracting ke, calculated based on the simpli-fied single parabolic band (SPB) model (ESI† Note),64 thedecreased kL is also evident (Fig. 2e), where kL is decreasedfrom 0.58 W m�1 K�1 to 0.48 W m�1 K�1 at room temperature.It is important to note that the decreased vs will alsoinfluence the scattering process of phonons. For example, bothU scattering and nanoprecipitates (NP) scattering are inverselycorrelated with phonon group velocity, indicating that lower vswill lead to more extensive phonon scattering. As shown inFig. 2f, employing the Debye–Callaway model65 and includingthe U scattering, boundary (B) scattering, point defect (PD)scattering and NP scattering36,44,66 (Note and Table S1, ESI†),the simulated kL of MgAgSb without addition closely matchesthe experimental results. When incorporating the variation invs with 0.75 wt% C18H36O2 addition, the simulated kL alsoaligns with the experiments around room temperature, con-firming the significant role of vs reduction in decreasing kL.Furthermore, it is worth noting that the minimum thermalconductivity kmin is also related to vs. The kmin models pro-posed by Cahill et al.67 Clarke et al.68 or Snyder et al.69 allexhibit a positive relationship with vs. This suggests thatmaterials with low vs will also have a lower limit of thermalconductivity, offering more rooms and opportunities forfurther decreasing k and achieving high TE performance.Enhanced TE quality factor B and high average figure of meritGenerally, although kL is independent of other TE parameters(S, s and ke), manipulating kL often affects electrical properties.Fig. 2 C18H36O2 addition to manipulate vs and kL. (a) XRD patterns of MgAgSb with different C18H36O2 addition (x = 0–1 wt%); (b) Vickers hardnesses,(c) vl and vt, T dependence of (d) k and (e) kL of MgAgSb with x wt% C18H36O2 addition (x = 0, 0.25, 0.5, 0.75); (f) experimental and simulated kL of MgAgSbwith 0 wt% and 0.75 wt% C18H36O2 addition.Paper Energy & Environmental Science8814 |  Energy Environ. Sci., 2024, 17, 8810–8819 This journal is © The Royal Society of Chemistry 2024While C18H36O2 addition could effectively lower kL of MgAgSb,it is also important that this addition has less detrimentalimpact on electrical performances to achieve high TE perfor-mance. As shown in Fig. 3a and b, it can be noticed that sgradually decreases, whereas the S increases, which indicates adecrease in carrier concentration with increasing C18H36O2content. The overall PF decreases with higher C18H36O2 content(Fig. 3c), indicating the deterioration of overall electrical trans-port performance. Intrinsically, based on the intrinsic electricalconductivity s0 (ESI† Note),64 which evaluates the electricaltransport performance potential, as well as weighted mobilitymW70 and electronic thermoelectric quality factor BE,71 it can beseen in Fig. S4 (ESI†) that s0, mW and BE decrease with increas-ing C18H36O2 content, confirm the intrinsic deterioration ofelectrical transport performance of MgAgSb with C18H36O2addition. Moreover, as shown in Fig. S5 (ESI†), the T�1.5relationship of mW in both pure MgAgSb and MgAgSb withC18H36O2 indicates that carrier scattering is dominated byacoustic phonons. This suggests that the variation in S and sis due to the addition of C18H36O2 rather than the grain sizereduction. Additionally, the calculated effective mass m* forboth pure MgAgSb and MgAgSb with C18H36O2 addition isabout 1.76 me, which matches well with the reported value,44suggesting that the band structure of MgAgSb remains unchangedwith the addition of C18H36O2.However, the overall TE performance is not solely deter-mined by electrical transport properties. Thermal transportproperties also play a significant role. TE quality factor B =S02s0T/kL is used to reflect the overall TE performance potentialof the material, where S0 is a constant and 2S0 E 173 mV K�1.As shown in Fig. 3d, despite the decrease in s0, the decreased kLleads to an unchanged B factor at room temperature and evenimproved at high temperatures, suggesting the better TEpotential for MgAgSb with C18H36O2 addition. Due to theenhanced B factor at high temperature, it results in a superiorzT of MgAgSb with 0.75 wt% C18H36O2 addition, where zT valueof 1.3 can be achieved at 523 K (Fig. 3e). Furthermore, despitethe unchanged B factor at room temperature, the optimizedS in MgAgSb with C18H36O2 addition enables it to exhibit zTB0.88 at 300 K. As a result, the zTavg value reaches 1.17 in thetemperature of 300–548 K, making it one of the best valuescompared to state-of-art MgAgSb, as shown in Fig. 3f.33,36,47–49,72,73Highly reliable MgAgSb by repeatable synthesis and testBesides the high TE performance, facile synthesis and excellentrepeatability are additional benefits facilitated by the C18H36O2addition. Due to the complex phase transition of MgAgSb,32ball milling is usually preferred for synthesizing MgAgSb.However, powder adhesion to the jar walls during ball millingcan pose a serious problem.49 This not only makes it difficult toretrieve the powder but also leads to an inhomogeneouscomposition. The TE performance of MgAgSb is reported tobe highly sensitive to its composition,35,49 and powder adhe-sion is particularly problematic in its ball milling process.49Typically, a two-step ball milling method has been adopted tosynthesize MgAgSb.33,44 However, issues with repeatability ofperformance persist due to the uncontrollable powder adhe-sion during the synthesis of both the MgAg precursor and thefinal MgAgSb compounds.C18H36O2 is an effective process-control agent for addressingthe problem of powder adhesion.63 In this work, the addition ofFig. 3 Electrical properties of MgAgSb with C18H36O2 addition. Temperature dependence of (a) s, (b) S, (c) PF, (d) B factor, and (e) zT of MgAgSb withx wt% C18H36O2 addition (x = 0, 0.25, 0.5, 0.75); (f) the average zT of MgAgSb in literature33,36,47–49,72,73 and MgAgSb with no and 0.75 wt% C18H36O2addition.Energy & Environmental Science PaperThis journal is © The Royal Society of Chemistry 2024 Energy Environ. Sci., 2024, 17, 8810–8819 |  8815C18H36O2 not only softens MgAgSb globally but also resolves thepowder adhesion issue. As shown in Fig. S6 (ESI†), compared toMgAgSb without C18H36O2 addition, no power adhesives tothe jar wall in MgAgSb with 0.75% C18H36O2 addition, makingit easy to retrieve the alloyed MgAgSb powder. It is worth notingthat MgAgSb tends to contain various second phases evenwhen undetected by XRD.49 Energy-dispersive X-ray spectro-scopy (EDS) mapping on both pure MgAgSb and MgAgSb with0.75% C18H36O2 addition has been used to identify distributionof constituent elements. As shown in Fig. S7 (ESI†), bothsamples display uniform element distributions, though someSb-rich second phases are observable in pure MgAgSb. By addres-sing the powder adhesion issue with C18H36O2, the second-phaseproblem can be alleviated. Another advantage of adding C18H36O2is that it enables a one-step ball milling process (5 hours) forsynthesizing the material, making it much more time- and energy-efficient compared to the two-step process, which usually requires5 hours followed by an additional 10 hours.33,44 Importantly, dueto the absence of powder adhesion, excellent repeatability ofMgAgSb can be achieved. As shown in Fig. S8 (ESI†), threesynthesized samples of MgAgSb from three separate ball millingjars exhibit very good phase purity. Furthermore, the TE transportproperties of MgAgSb with C18H36O2 addition are also repeatable.As shown in Fig. 4a–c, the data for S, s and k of three separatesamples match well and are within 5% measurement error ofS and s, and 3% measurement error of k, respectively.In addition to the repeatable synthesis of MgAgSb con-ducted three times, the stability of TE properties in MgAgSbwith C18H36O2 addition warrants investigation, as C18H36O2may enter a liquid phase around its melting point of 343 K.Fig. S9 (ESI†) shows an endothermic peak in MgAgSb with0.75 wt% C18H36O2, likely due to its melting, but the peak isbroad and weak because of its small content. Therefore, itsimpact on TE properties might be minimal. Further measure-ments of s around 343 K show a smooth variation, indicatinglittle influence from the potential melting of C18H36O2 (Fig. S9,ESI†). Additionally, the stability of the sample under prolongedhigh-temperature exposure should also be explored. As shownin Fig. 4d–f, the electrical transport properties (S, s, and PF) at523 K and 573 K exhibit minimal changes over 30 measure-ments. The ratios of Seebeck coefficient (S/S0), electrical con-ductivity (s/s0), and power factor (PF/PF0) for MgAgSb with0.75 wt% C18H36O2 remain at 1, indicating excellent stabilityeven at elevated temperatures. Here, S0, s0 and PF0 representsthe initial values of Seebeck coefficient, electrical conductivityand power factor, respectively. In all, as revealed above, despitebeing an organic compound, C18H36O2 has quite good stabilitybelow 573 K, highlighting its significant role in achievingreliable high TE performance in MgAgSb.High conversion efficiency of MgAgSb single-leg and two-pairmodulesDue to the enhanced average TE performance over a widetemperature range, MgAgSb with 0.75 wt% C18H36O2 additionis highly suitable for the low-grade heat harvesting. To accessthe materials’ heat conversion ability, a single TE leg has beenfabricated, which is sandwiched by two layers of MgCuSb due toits low contact resistance and stability.74 As shown in Fig. 5a,it can be found that the contact resistance is very small,approximately 1 mO cm2. For measuring Z, various DT areFig. 4 Repeatable TE properties of MgAgSb with C18H36O2 addition. T dependence of (a) s, (b) S and (c) k of MgAgSb with 0.75 wt% C18H36O2 addition bythree separate syntheses; (d) S/S0, (e) s/s0 and (f) PF/PF0 of MgAgSb with 0.75 wt% C18H36O2 acid addition under 523 K and 573 K for repeatedmeasurements.Paper Energy & Environmental Science8816 |  Energy Environ. Sci., 2024, 17, 8810–8819 This journal is © The Royal Society of Chemistry 2024applied along the single TE leg.75 The largest hot-side tempera-ture Th is limited to be 573 K considering the a-phase tob-phase transition of MgAgSb occurrs in 573–583 K.32 Themeasured output voltage V and output power P are displayedin Fig. 5b. The good linearity of the current-dependent V can beused to determine the internal resistance and open-circuitvoltage of the single TE leg. A reduced slope indicates increaseds, which is consistent with the performance of our materials,and the increased open-circuit voltage is induced by the largeDT. When Th is 573 K (DT is B276 K), the open-circuit voltage is55 mV and the maximum output power Pmax is 60 mW. Thedecreased k of MgAgSb helps to efficiently utilize the heatenergy by suppressing heat flow to the cold side Q (Fig. 5c),which consequently results in a high maximum conversionefficiency Zmax of 3.6% under 82 K and 8.6% under 276 K(Fig. 5d). Furthermore, given the inherent stability of MgAgSbwith C18H36O2 as revealed above, it is anticipated that legperformance will remain stable when aging, if the interfaceperformance does not degrade.In addition to the single TE leg, a two-pair TE modulehas also been demonstrated, with its optical image shown inthe inset of Fig. 5e. The n-type TE legs are based on Sb-richMg3Sb1.5Bi0.5 due to its high chemical stability, as suggested bythe recent study.80 The TE performance of n-type Mg3Sb1.5Bi0.5-based material is shown in Fig. S10 (ESI†), while Fig. S11 (ESI†)displays the measured V, P and Q under different appliedDT and different applied current I. The Th is also limited to573 K, considering the phase transition of MgAgSb. As shownin Fig. 5e, Zmax of this module can reach approximately 7%under DT of 278 K, comparable to recently reported Mg-basedTE modules under DT of 300 K. It is noteworthy that this 7%conversion efficiency is based on n-type Sb-rich Mg3(Bi,Sb)2-based TE leg. Although its TE performance falls short of theBi-rich Mg3(Bi,Sb)2-based material at room temperature,38,59,74,76,81it could exhibit much higher chemical stability, making it muchmore promising for practical applications.Furthermore, the achieved Zmax B 7% in MgAgSb/Mg3Sb1.5-Bi0.5-based module is quite impressive compared to fullMg3Sb2-based, full Bi2Te3-based and p-type (Bi,Sb)2Te3/n-typeMg3(Bi,Sb)2-based two-pair modules under the same DT.77–79This highlights the superior potential of p-type MgAgSb for low-grade waste heat harvesting. Moreover, repeatable Zmax ofboth the single TE and the two-pair module have been achievedwith two rounds of tests (Fig. S12, ESI†), demonstrating theirgood stability. If the TE performance of MgAgSb and Sb-richMg3Sb1.5Bi0.5 is further improved, greater enhancement of Zcan be expected in the future.ConclusionsIn this work, we introduce a global softening strategy to decreasevs in TE materials, thereby enhancing their TE performance.Unlike traditional methods by softening chemical bonds at theatomic scale to reduce vs, our strategy stems from a macroscopicview, by introducing inherently soft organic compounds into theFig. 5 Conversion efficiency of MgAgSb single TE leg and two-pair module. (a) Contact resistance between MgAgSb and MgCuSb in the single TE leg;(b) I dependence of V and P, and (c) Q of MgAgSb single TE leg under different DT; (d) I dependence of Z of MgAgSb single TE leg, and (e) MgAgSb/Mg3Sb1.5Bi0.5 two-pair module under different DT. The inset is the optical image of the two-pair module; (f) applied DT dependence of Zmax of MgAgSb/Mg3Sb1.5Bi0.5 and full Mg3Sb2-based, full Bi2Te3-based, (Bi,Sb)2Te3/Mg3(Bi,Sb)2-based and MgAgSb/Mg3Bi1.5Sb0.5-based two-pair modules inliterature.76–79Energy & Environmental Science PaperThis journal is © The Royal Society of Chemistry 2024 Energy Environ. Sci., 2024, 17, 8810–8819 |  8817material matrix to soften the overall material. Using MgAgSb,a promising p-type TE material, we demonstrate that addingvarious soft organics, such as zinc stearate (C36H70ZnO4), magne-sium stearate (C36H70MgO4) and stearic acid (C18H36O2), effec-tively reduce vs, which brings about the reduction of kL andenhancement of TE performance.Specifically, it is found the vs of MgAgSb can be graduallytuned when adjusting C18H36O2 contents, and an ultralow kL B0.48 W m�1 K�1 can be achieved at room temperature inMgAgSb with 0.75 wt% C18H36O2 due to the reduced vsand increased phonon scattering, greatly lower than the0.58 W m�1 K�1 in MgAgSb without addition. Despite asimultaneous decrease in PF by C18H36O2 addition, morereduction in kL results in an overall enhancement of the Bfactor. Consequently, a high zT value of B0.88 at 300 K and apeak zT value of B1.30 are achieved, which gives rise to anaverage zT of B1.17 in the temperature range of 300 K to 548 K,surpassing most state-of-art p-type TE materials. Moreover, theMgAgSb with C18H36O2 shows good repeatability and highstability, indicating high reliability. Using this high-performance MgAgSb compound, we achieve a high Z ofB8.6% and B7% in a single TE leg and a two-pair moduleunder DT of B276 K, respectively, demonstrating greatpotential for harvesting low-grade heat. This work not onlyadvances high-performance MgAgSb for low-grade power gen-eration but also proposes a global softening strategy to manip-ulate the vs for performance enhancement in thermoelectrics.MethodsMaterials synthesisMgAg0.97Sb0.99 (denoted as MgAgSb in the main text and below)with x wt% C18H36O2 (x = 0, 0.25, 0.5, 0.75, 1) were synthesizedby using Mg turnings (99.95%), Ag powers (99.99%) Sb shots(99.999%), C36H70ZnO2 power (99.9%), C36H70MgO2 power(99.9%), and C18H36O2 power (99.9%). The raw materials wereweighted stoichiometrically and then put into the ball millingjar with the inside of argon. Then, the jaw was mechanicallyalloyed for continuously 5 h (SPEX-8000D). The alloyed sampleswere scratched from the jaw and then compressed into the bulkby vacuum spark plasma sintering (SPS-322Lx, Dr Sintering)in the carbon die of 10 mm diameter under 573 K and 60 MPafor 5 min. The relative density of all samples reaches B97%.Mg3.2In0.005Sb1.5Bi0.49Te0.01 (denoted as Mg3Sb1.5Bi0.5 in themain text and below) was prepared by using Mg turnings(99.95%), Te shots (99.999%), Bi shots (99.999%), Sb shot(99.999%), and In powder (99.99%), which was weighted stoi-chiometrically and loaded into the ball milling jar with insideof argon, and then ball milled for 5 h (SPEX-8000D). Theobtained powder was consolidated by SPS (SPS-1080 System,SPS SYNTEX INC) under 973 K and 60 MPa for 20 min. MgCuSbwas prepared by using Mg powder (99.95%), Cu powder(99.999%) and Sb powder (99.999%), which was weightedstoichiometrically and loaded into the ball milling jar withthe inside of argon, and then ball milled for 20 h (5 cycles, eachcycle contains 4 hours running and 30 minutes rest) (SPEX-8000D).Characterization and measurementsThe phases of obtained MgAgSb samples were characterized byusing the X-ray diffractometer (SmartLab3, Rigaku) with Cu Karadiation under 40 kV and 15 mA. The thermal analysis DSCwas carried out by STA 449 (Netzsch) to check the existence ofC18H36O2, the samples were loaded into an Al crucibleand heated to 773 K with a heating rate of 10 K min�1. Thefracture morphology of the sample were investigated by usingscanning electron microscopy (FESEM, Hitachi SU8000),which is equipped with an energy dispersive spectrometer(EDS, XFlash FlatQUAD 5060 F) to study the samples’ com-position. The longitudinal (vl) and transverse (vt) sound velocityof obtained MgAgSb were measured by using a sing-aroundultrasonic velocity measuring instrument (UVM-2, UltrasonicEngineering Co., Ltd) with their sound velocity vs calculatedaccording to vs�3 = (vl�3 + 2vt�3)/3. The Vickers hardness ofMgAgSb was measured by a micro-Vickers hardness tester(HMV-G, Shimadzu), where 10 different spots in one samplewere measured. The zT of MgAgSb samples was calculated bythe formula: zT = S2sT/k, in which the S and the s weremeasured by ZEM-3 (Advance Riko, � 5% uncertainty) underhelium atmosphere, and the k was calculated by formula: k =DrCp, where the thermal diffusivity D was measured by LFA467(Netzsch, � 3% uncertainty), the sample density r was esti-mated by the Archimedes method, and heat capacity Cp wasestimated by Dulong–Petit law. The Hall carrier concentrationwas measured by using a physical properties measuring sys-tems, with an AC resistance option (PPMS, Quantum Design).The contact resistance of the MgAgSb/MgCuSb single TE legwas measured by a 2-axis resistance distribution measurementinstrument (S1331, Mottainai energy).Module fabrication and measurementMgAgSb single TE leg was fabricated by sandwiching two layersof MgCuSb as the interface material and then sintered by SPSunder 573 K and 60 MPa for 5 min. The obtained MgCuSb/MgAgSb/MgCuSb joints were cut into dice with dimensions ofB3.8 � 3.8 � 6 mm3. The two-pair module was fabricatedbased on p-type MgCuSb/MgAgSb/MgCuSb TE legs and n-type304 stainless steel/Mg3Sb1.5Bi0.5/304 stainless steel TE legs. Theoutput power and conversion efficiency of the single TE leg andtwo-pair module were measured by Mini-PEM, (ADVANCERIKO, Japan) with cold-side temperature maintained at 293 Kand hot-side temperatures ranging from 373 K to 573 K in avacuum condition.Author contributionsA. L., T. M. designed the project. A. L. prepared the samples andcarried out the transport measurements and conversion effi-ciency measurement with the help of L. W. L. W. and J. L.provided the samples of Mg3(Sb,Bi)2 and MgCuSb, respectively.Paper Energy & Environmental Science8818 |  Energy Environ. Sci., 2024, 17, 8810–8819 This journal is © The Royal Society of Chemistry 2024A. L. analyzed the data and wrote the original manuscript. T. M.supervised the whole project. All the authors reviewed andedited the manuscript.Data availabilityAll data generated or analyzed during this study are included inthe published article and its ESI.† The data that support thefindings of this study are available from the correspondingauthor upon request.Conflicts of interestT. M. and A. L. have filed one Japanese patent application(2024-111372) on the work described here. The remainingauthors declare no competing interests.AcknowledgementsThis work was supported by JST Mirai Program (JPMJMI19A1).References1 J. He and T. M. Tritt, Science, 2017, 357, eaak9997.2 G. J. Snyder and E. S. Toberer, Nat. Mater., 2008, 7, 105–114.3 I. Petsagkourakis, K. Tybrandt, X. Crispin, I. Ohkubo,N. Satoh and T. Mori, Sci. Technol. Adv. Mater., 2018, 19,836–862.4 T. Zhu, Y. Liu, C. Fu, J. P. Heremans, J. G. Snyder andX. Zhao, Adv. Mater., 2017, 29, 1605884.5 Y. Pei, H. Wang and G. J. Snyder, Adv. Mater., 2012, 24,6125–6135.6 A. Li, C. Hu, B. He, M. Yao, C. Fu, Y. Wang, X. Zhao, C. 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