Thermoelectric Properties of Bi-Doped Magnesium Silicide Stannides

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Thermoelectric Properties of Bi-Doped Magnesium Silicide Stannides Leilane Roberta Macario, Xiaoyu Cheng, Daniel Ramirez, Takao Mori, and Holger Kleinke ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b15111 • Publication Date (Web): 02 Nov 2018 Downloaded from http://pubs.acs.org on November 3, 2018

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ACS Applied Materials & Interfaces

Revised version

Thermoelectric Properties of Bi-Doped Magnesium Silicide Stannides Leilane R. Macario†, Xiaoyu Cheng†, Daniel Ramirez†, Takao Mori‡, Holger Kleinke*,† † Department

of Chemistry and Waterloo Institute for Nanotechnology, University of Waterloo, Waterloo, ON N2L 3G1, Canada

‡ Center

for Functional Sensor & Actuator (CFSN) and International Center for

Materials Nanoarchitectonics (WPI-MANA), National Institute for Materials Science (NIMS), Namiki 1-1, Tsukuba 305-0044, Japan

ABSTRACT:

Mg2(Si,Sn)-based

compounds

have

shown

great

promise

for

thermoelectric applications, as they are non-toxic and comprised of abundantly available constituent elements. In this work, the crystal structures and thermoelectric properties of polycrystalline materials with nominal compositions Mg2Si0.35Sn0.65-xBix (x = 0, 0.015, 0.030, and 0.045) and Mg2SiySn0.97-yBi0.03 (y = 0.30, 0.325, and 0.35) have

ACS Paragon Plus Environment

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been investigated. The electrical conductivity, Seebeck coefficient, and thermal conductivity are strongly affected by the presence of Bi. Undoped samples showed higher values of Seebeck coefficients (below 600 K), lower electrical conductivity, and lower thermal conductivity (above 600 K) in comparison to the Bi-doped samples. Furthermore, the signs of Seebeck coefficients are all negative, confirming that n-type conduction is dominant in these materials. Electrical conductivity was enhanced by increasing of the Bi content up to 3% on the Si/Sn site due to the increasing amount of electron donors, and the absolute value of Seebeck coefficient decreased. When the Bi content is greater than 3%, lower zT values were obtained at 773 K. Thermal conductivity values might decrease with increasing Sn alloying for Mg2SiySn0.97-yBi0.03, since mass and strain fluctuation caused by alloying can effectively scatter phonons. However, a different behaviour was observed in higher Sn content material, possibly due to the absence of Mg atoms at the interstitial site (Mgi, on (½, ½, ½)) and vacancies of Mg atoms at the (¼, ¼, ¼) site, as confirmed by Rietveld refinements. Outstanding figure of merit values in excess of unity were achieved with all samples, culminating in

zTmax = 1.35.


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KEYWORDS: Energy, Thermoelectric Materials, Solid Solutions, magnesium, silicon, tin, bismuth


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1. INTRODUCTION Thermoelectric materials (TE) can be deployed to complete direct conversion between heat and electricity in an unobtrusive way, hence arousing wide attention all over the world.1,2 The heat to electric power conversion efficiency of such devices is characterized by the dimensionless figure of merit zT of the constituent thermoelectric materials. zT is calculated as zT = S2σT/к, where S is the Seebeck coefficient, σ is the electrical conductivity, T is the absolute temperature, and к the thermal conductivity. To improve zT, a variety of approaches have been explored to enhance the power factor,

S2σ, and reduce к. An ideal TE material should not only have a high zT, but also high chemical and thermal stability.3 That is why Mg2(Si,Sn) based TE materials have attracted increasing attention since 2006,4 particularly since a zT of above unity was achieved in Sb-doped Mg2(Si,Sn) solid solutions,5–7 comparable with the state-of-the-art intermediate temperature TE materials such as PbTe and filled skutterudites.8,9 Magnesium based compounds have been identified for promising advanced thermoelectric materials in the temperature range from 500 K to 800 K, because they are abundant raw materials, very cheap, benignity to environment and stable at


4

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relatively high temperatures.10–12 Mg2Si based materials crystallize in the antifluoritetype structure where the 8c (¼, ¼, ¼) and 4a (0, 0, 0) sites are occupied by Mg and Si, respectively.13 Thereby, the phase purity and microstructure of the product Mg2Si1-xSnx are difficult to control by conventional technique mainly because of the easy volatilization and oxidation of Mg which causes structural defects. It was shown that Mg2Si crystals may contain a small amount of Mg at the 4b (½, ½, ½) interstitial site, and that vacancy and interstitial defects are more likely to occur in Mg2Si compared with other point defects.

10,1415

The increasing number of studies focused on optimization

through alloying and doping of Mg2(Si,Sn)-based materials is due to their excellent thermoelectric

performance.14,16,17

Bi-doped

Mg2(Si,Sn)

based

materials

might

principally have lower thermal conductivity since Bi has a heavier mass and a larger radius causing local distortions (MBi = 208.9 g mole–1, rBi = 1.60 Å) in comparison to the host atoms (MSi = 28.1 g mole–1, rSi = 1.10 Å; MSn = 118.7 g mole–1, rSn = 1.45 Å). Meanwhile, the Bi-doped Mg2Si samples showed the best thermoelectric properties compared to Cu-, Al-, P-, and Sb-doped samples.18–20 To date, the best materials in this system

include

Mg2.15Si0.28Sn0.71Sb0.006,21

Mg2.16(Si0.4Sn0.6)0.97Bi0.03,22


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Mg2Si0.53Ge0.05Sn0.4Bi0.02,23 and Mg2Si0.3Sn0.665Bi0.035,24 all with outstanding figures of merit values between 1.2 and 1.4 between 750 K and 850 K.25 Many synthesis methods have been applied to obtain Mg2(Si,Sn) based compounds to improve the thermoelectric properties, such as solid state reaction,18 mechanical alloying,26 hot pressing24 and spark-plasma-sintering (SPS),10,27 which are relatively time consuming, complex, and in most of cases use expensive crucibles to avoid Mg volatilization and reaction with the container.24 In this study, we prepared polycrystalline Bi-doped Mg2(Si,Sn)-based materials with various nominal compositions using graphite foil as crucible, which is stable under high temperature, cheap, and remains inactive with Mg. As determined recently, the best performance in this system persists around 35% Si and 3% Bi.20 Therefore, we studied two series, one with varying Bi and fixed Si at 35%, and the second with varying Si and fixed Bi at 3%. In addition, we investigated the effect of Mg interstitial (Mgi) and vacancies of Mg atoms on the thermoelectric properties of the obtained materials.

2. EXPERIMENTAL SECTION


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2.1. Materials preparation. Mg2Si0.35Sn0.65-xBix (x = 0, 0.015, 0.030, and 0.045) and Mg2SiySn0.97-yBi0.03 (y = 0.30, 0.325, and 0.35) solid solutions were synthesized by mixing stoichiometric amounts of Mg chips (99.98%, Sigma-Aldrich), Si powder (99.9%, Alfa Aesar), Sn granules (99.9%, Alfa Aesar), and Bi granules (99.99%, Sigma-Aldrich), with an extra 15 mole-% of Mg. The mixtures were put in graphite foils, shaped to fit into the silica tubes. The tubes were heated in a tube furnace at 1223 K for three hours and thereafter at 923 K for three days. The heat-treated samples were ground into powders and then hot pressed using Oxy-Gon in a 95% Ar - 5% H2 atmosphere at a maximum temperature of 923 K under 30 MPa. The final products showed densities over 98% of the theoretical value as determined via the Archimedes method. 2.2. Characterizations and measurements. The phase purity and the crystal structure of the obtained materials were characterized by X-ray diffraction (XRD) on an Inel powder X-ray diffractometer with Cu Kα1 radiation. Data were collected at room temperature over a period of 15 hours. The phase composition of the solid solution and the lattice parameters were calculated by the Rietveld method using the GSAS28 suite of programs with the EXPGUI interface.29 Background, scale factor, phase fractions,


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displacement, lattice parameters, atomic positions, atomic vibrational parameters, and coefficients for the peak profile were refined in steps until convergence was achieved. The thermal displacement parameters of Si, Sn, and Bi atoms and of the Mg and Mgi atoms were constrained to the same value. The fractional occupancies of Si and Sn were also constrained to sum 0.97, while the Bi occupancy was fixed at 0.03. The obtained pellets were cut into 10 x 2.5 x 2.5 mm rectangle bars for Seebeck coefficient and electrical conductivity measurements under a helium atmosphere between 300 and 800 K by using ULVAC-RIKO ZEM-3 apparatus. Estimated experimental errors are 3% for the Seebeck coefficient, and 5% both for the electrical and thermal conductivity, which results in an error of about 10% for the figure of merit.24 The pellets were also cut into 6 x 6 x 2 mm square pellets for thermal diffusivity measurements under an argon atmosphere between 300 and 800 K by using the NETZSCH LF467 thermal properties analyzer. Thermal conductivity was calculated from the equation к = α ρ Cp, where α = thermal diffusivity, ρ = density of pellets, and Cp = specific heat capacity. The Dulong–Petit approximation was applied for determining

Cp values, which was shown to work well for these materials.30


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3. RESULTS AND DISCUSSION The XRD patterns of the as-synthesized Mg2Si0.35Sn0.65-xBix (x = 0, 0.015, 0.030, and 0.045) and Mg2SiySn0.97-yBi0.03 (y = 0.30, 0.325, and 0.35) materials can be indexed to the face-centered cubic (fcc) antifluorite structure with the space group Fm3¯ m (ICSD #44934) except for the small MgO peak at 2θ = 43° (~1 wt.-%) as shown in Figures 1 and 2(a). MgO is almost unavoidable because of the use of silica tubes (which release traces of oxygen at high temperatures and Mg powder with potentially MgO on its surface.


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Figure 1. XRD pattern of Mg2Si0.35Sn0.65-xBix (x = 0, 0.015, 0.030, and 0.045) and Mg2SiySn0.97-yBi0.03 (y = 0.30, 0.325, and 0.35) and zoom in the 2θ range of 37-44°.

Small amounts of Mg2Si and Mg2Si1-xSnx of < 4% were additionally found; therefore, the yield was about 95 weight-% (Supporting information, Table S-I). Since the impurities contents present in all six samples are nearly identical and in low quantity, the


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systematic changes in the TE properties are primarily caused by the differences in Sn and Bi amounts. As presented in Figure 2(b), the lattice parameters increase with the Sn and Bi content, which indicates that Sn and Bi atoms successfully substitute for Si.

Figure 2. (a) XRD pattern of Mg2Si0.35Sn0.62Bi0.03 together with results of the Rietveld refinement; (b) variation of lattice parameters with Sn and Bi substitution on Mg2Si based materials.

The temperature dependences of the electrical conductivity and Seebeck coefficient for Mg2Si0.35Sn0.65-xBix (x = 0, 0.015, 0.030, and 0.045) and Mg2SiySn0.97-yBi0.03 (y = 0.30,


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0.325, and 0.35) are shown in Figure 3. The undoped sample exhibits a typical semiconducting

behaviour

(increasing

electrical

conductivity

with

increasing

temperature), while the Bi-doped samples display characteristics of heavily doped semiconductors. The latter is evident from a high electrical conductivity caused by the dopant, which decreases with temperature because of decreasing mobility. The mobility reduction is caused by increasing acoustic phonon scattering of the charge carriers. In case of Mg2.08Si0.39Sn0.6Bi0.01, the mobility is around 50 cm2V−1s−1 at 300 K and around 18 cm2V−1s−1 at 700 K.31 The same trend was observed in other Mg(Si, Sn)-doped materials14,20,24,32,33 Bi is known to substitute for the Si/Sn atoms in the Mg2Si1−xSnx crystal structure, and to act as electron donor. A small amount of Bi-doping causes therefore a dramatic change in the temperature-dependent physical properties. For the Mg2Si0.35Sn0.65-xBix samples at 300 K, the electrical conductivity values range from σ = 1440 Ω–1cm–1 (1.5% Bi) to 2180 Ω–1cm–1 (3% Bi), and 1990 Ω–1cm–1 (4.5% Bi). It is observed that σ enhances by increasing of the Bi content, and the 3% Bi sample exhibits the highest σ at 300 K. Among the 3% Bi-doped samples, Mg2Si0.325Sn0.645Bi0.03 has the highest electrical conductivity (2490 Ω−1cm−1) compared to 2140 Ω−1cm−1 for


12

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Mg2Si0.30Sn0.67Bi0.03 at 300 K. The results exhibit comparable σ to Mg2Si0.3Sn0.665Bi0.035 with σ = 2400 Ω−1cm−1 at 320 K, which was prepared via two stage ball milling, followed by annealing using tantalum crucibles sealed by arc melting method.24 The former sample exhibits higher σ than most known high-performance materials in this family, including Mg2Si0.57Sn0.4Bi0.03 with 1780 Ω−1 cm−1 at 320 K, which was synthesized by a two step solid-state reaction combined with hot-pressing, and exhibited a zTmax (850 K) = 1.2.34


13


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Figure 3. Temperature dependences of (a) the electrical conductivity and (b) the Seebeck coefficient for Mg2Si0.35Sn0.65-xBix (x = 0, 0.015, 0.030, and 0.045) and Mg2SiySn0.97-yBi0.03 (y = 0.30, 0.325, and 0.35).

Since the lattice parameter continued to increase from 3% to 4.5% Bi, at least part of the additional Bi atoms is present in the crystals of the 4.5% Bi sample and should provide additional electrons. Thus, we postulate that the experimentally observed lower electrical conductivity of the 4.5% Bi sample results either from the ionized impurity scattering between carriers as found in other n-type Mg2Si doped materials,35–37, and/or from the presence of small additional side products as Mg3Bi2, as found recently.14 In line with this, we notice that the Seebeck coefficient of the 4.5% Bi sample is actually the smallest, indicating that that sample does have a relatively high carrier concentration. The signs of the Seebeck coefficients are all negative for the doped samples, confirming that the n-type conduction is dominant in these materials. The absolute values of the undoped sample initially increases with temperature followed by


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a decrease at T > 350 K due to the presence of thermally excited minority carriers, which increase the electronic concentration.17,36 For Bi-doped samples, the absolute value of Seebeck coefficient increases with temperature and decreases with increasing Bi amount. At higher dopant concentrations, the additional electrons from the Bi atoms populate the parabolic conduction bands, and therefore the Seebeck coefficient is reduced.38 The Seebeck coefficient values range from -145 μV K–1 (1.5% Bi) to -119 μV K–1 (3% Bi), and -113 μV K–1 (4.5% Bi) at 300 K. The compounds with 0.03 Bi and different Sn content exhibit no significant difference in their S values, all increasing from about -115 μV K–1 at 300 K to -215 μV K–1 at 773 K in agreement to the reported values for Mg2SiySn0.97-yBi0.03.24,31,34 The total thermal conductivity is composed of an electrical (кe) and a lattice contribution (кL). кe can be calculated via the Wiedemann–Franz relation, кe = L σ T. Since the single parabolic band model was proved to be as efficient as the three band model for calculating the Lorenz (L) number,12,33 we used the former for the determination of L. Figure 4(a) illustrates the temperature dependence of кL. In the undoped sample, кL decreases with temperature until 500 K and increases afterwards,


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likely due to the bipolar effect. Among the Bi-doped samples, кL decreases as temperature and as Bi content increase. It should be noted that some of these values are equal within experimental error, however.

Figure 4. Temperature dependences of (a) lattice thermal conductivity; and (b) total thermal conductivity.

к of the undoped sample is 2.39 W m−1K−1 at room temperature and decreases to a local minimum value of 1.93 W m−1K−1 at 500 K. Upon Bi doping, the local minimum disappears and the к continues to decrease beyond 500 K as well.


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The dimensionless figure of merit, zT, which is determined based on the Seebeck coefficient, electrical conductivity and total thermal conductivity, is summarized in Figure 5. In the Bi-doped materials, the absolute value of zT rises with increasing temperature and gradually increases up to 3% Bi content at 773 K. When the Bi content is greater than 1.5%, zTmax is larger than 1. The zTmax achieved at 773 K with a value of 1.35 is higher than zT = 0.66 at x = 0.030 and y = 0.4 obtained using microwave reaction,39 zT = 0.66 at x = 0.030 and y = 0.8 obtained after SPS,20 and zT = 0.98 at x = 0.015 and y = 0.985 obtained using high pressure.27 Furthermore, the zT = 1.35 is comparable to the results (zT = 1.4 at x = 0.035 and y = 0.3) obtained using expensive tantalum crucibles,24 to zT = 1.45 at 750 K for Mg2.16(Si0.3Ge0.05Sn0.6.95)0.98Sb0.02 obtained using pyrolytic boron nitride crucibles and SPS,40 zT = 1.4 for Mg2Si0.53Ge0.05Sn0.4Bi0.02 obtained via a two step solid state synthesis in graphite crucibles and hot-pressing,41 and zT = 1.4 at 800 K for Mg2.16(Si0.4Sn0.6)0.97Bi0.03 after SPS.22 These outstanding zT values are also comparable with that of the state-of-the-art PbTe and filled skutterudites based TE materials operating in the intermediate temperature range,8,9 while the Mg2(Si,Sn)-based TE materials are ecologically more favourable.


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Figure 5. Temperature dependences of the dimensionless figure of merit.

It was observed that Mg2Si0.3Sn0.67Bi0.03 exhibits non-expected results in electrical conductivity and thermal conductivity measurements. The introduction of defects can potentially lead to a certain change of the TE performance. Considering the cubic crystal structure of Mg2Si consisting of a face-centered lattice of Si with an embedded simple cubic lattice of Mg (Figure 6a), the central part of the unit cell is unfilled, thereby providing enough space for stable interstitial defects. Theoretical investigations of structural defects in Mg2Si lattice proposed that the central position (4b site in Wyckoff notation) is the only stable position for interstitial atoms42–45. These results are


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consistent with previous studies reporting that vacancy and interstitial defects are more likely to occur in Mg2Si compared with other point defects10,42–44 (Figure 6b).

Figure 6. Crystal structure of Mg2(Si,Sn,Bi) at ambient conditions. (a) A unit cell of the original cubic antifluorite (CaF2)-type structure; (b) an illustrative model of the unit cell with one Mg ion shifted from its regular position toward the interstitial site (4b; ½, ½, ½) and one Mg vacancy (8c; ¼, ¼, ¼).

The phase information obtained from the Rietveld refinements is listed in Table I. Four structures models were assumed: (a) stoichiometric Mg2SiySn0.97-yBi0.03 (y = 0.30, 0.325, and 0.35), (b) Mg-deficient with Mgi, (c) presence of Mgi, and (d) Mg-deficient. The


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significance level of 0.10, deduced from the Hamilton test,46 is sufficient to justify the conclusion that the structure of Mg2Si0.3Sn0.67Bi0.03 is Mg-deficient at the 8c site, and Mg2Si0.325Sn0.645Bi0.03 and Mg2Si0.35Sn0.62Bi0.03 possess instead Mgi. The stoichiometry variation between the nominal compositions and the refined formulae is not significant for Mg2Si0.325Sn0.645Bi0.03 and Mg2Si0.35Sn0.62Bi0.03; however, the refined formula of Mg2Si0.3Sn0.67Bi0.03 revealed significantly less Mg, namely 1.93 vs. 2.02 and 2.04 per formula unit, respectively.

Table I. The crystal structure and phase information of Mg2SiySn0.97-yBi0.03 (y = 0.30, 0.325, and 0.35) obtained from the Rietveld refinement of the XRD patterns.

Mg2Si0.3Sn0.67Bi0.03 Rb (%)/Rwp (%)

3.8/10.6

Occ. Mg (%)

Mg2Si0.325Sn0.645Bi0.03

Mg2Si0.35Sn0.62Bi0.03

2.7/10.9

4.7/9.3

96.6(7)

100

100

Occ. Mgi (%)

0

1.7(8)

4.0(8)

Occ. Si (%)

26.7(8)

34.2(7)

36.0(7)

Occ. Sn (%)

70.3(8)

62.8(7)

61.0(7)

Occ. Bi (%)

3

3

3


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Refined

Mg1.93(1)Si0.27(1)Sn0.70Bi0

Mg2.02(1)Si0.34Sn0.63Bi0. Mg2.04(1)Si0.36(1)Sn0.61Bi0

formula

.03

03

.03

In this context, recent theoretical work predicted that point defects in Mg2Si based materials like the presence of Mgi should be able to improve the thermoelectric performance because it enhances electrical conductivity and decreases thermal conductivity.47 In addition, vacancies on the major Mg site decrease the electron carrier density which slightly lowers the Fermi level, as observed before.48,15 That is in agreement

with

our

observation

that

the

sample

without

interstitial

Mg,

Mg2Si0.3Sn0.67Bi0.03, exhibits lower electrical conductivity and higher thermal conductivity than the one with interstitial Mg, namely Mg2Si0.35Sn0.62Bi0.03.

4. CONCLUSIONS In conclusion, polycrystalline materials with nominal compositions Mg2Si0.35Sn0.65-xBix (x = 0, 0.015, 0.030, and 0.045) and Mg2SiySn0.97-yBi0.03 (y = 0.30, 0.325, and 0.35) were synthesized by solid state reaction using a customized container that reduced the cost


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significantly comparing to any other crucible, followed by hot pressing instead of the more involved spark-plasma-sintering method. Ultimately, various Mg2SiySn0.97-yBi0.03 samples (y = 0.30, 0.325, and 0.35) showed consistently figure of merit values around 1.3 at 773 K using our affordable container, which makes it a prominent candidate for advanced waste heat recovery applications. Independent of the presence of Mg vacancy and lack of Mgi atoms, all 3% Bi-doped samples are promising for large scale industrial synthesis; a certain fluctuation of the Si/Sn ratio and the Bi content is possible while still producing high-performance materials. Compared to other high performing Mg2(Si, Sn) based materials, additional advantages lie in the avoidance of toxic antimony, which would be difficult to use for large production as required by industry.

ASSOCIATED CONTENT Supporting Information. The following files are available free of charge.


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Phase fraction and real composition to each sample obtained by Rietveld refinement. Power factor of the Mg2Si0.35Sn0.65-xBix (x = 0, 0.015, 0.030, and 0.045) and Mg2SiySn0.97-yBi0.03 (y = 0.30, 0.325, and 0.35) solid solutions. Calculated Lorenz numbers between 300 K and 800 K of the Mg2Si0.35Sn0.65-xBix (x = 0, 0.015, 0.030, and 0.045) and Mg2SiySn0.97-yBi0.03 (y = 0.30, 0.325, and 0.35) solid solutions. Calculated electronic thermal conductivity of the Mg2Si0.35Sn0.65-xBix (x = 0, 0.015, 0.030, and 0.045) and Mg2SiySn0.97-yBi0.03 (y = 0.30, 0.325, and 0.35) solid solutions. Temperature dependence of electrical conductivity and Seebeck coefficient of Mg2Si0.30Sn0.67Bi0.03 in a temperature cycling test. Area scan results (percentages) of Mg2Si0.35Sn0.65-xBix (x = 0, 0.015, 0.030, and 0.045) and Mg2SiySn0.97-yBi0.03 (y = 0.30, 0.325, and 0.35). (PDF) AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. ORCID Holger Kleinke: 0000-0002-6777-6140 Leilane R. Macario: 0000-0003-1204-0723


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Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS The authors acknowledge the support provided by Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), Vale Brazil, and Mitacs Globalink Early Career Fellowship (process number 201483/2016-5). Financial support from the Natural Sciences and Engineering Research Council of Canada (NSERC) is appreciated.

TM

acknowledges

CREST

JPMJCR15Q6

and

JSPS

KAKENHI

JP17H02749,JP16H06441 for support.

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TOC Graphic 199x160mm (300 x 300 DPI)


Thermoelectric Properties of Bi-Doped Magnesium Silicide Stannides

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