Microstructure and mechanical stability of Bi doped Mg2Si0.4Sn0.6 thermoelectric material

Journal Pre-proof Microstructure and mechanical stability of Bi doped Mg2Si0.4Sn0.6 thermoelectric material Smita Howlader, R. Vasudevan, B. Jarwal, S. Gupta, K.H. Chen, K. Sachdev, M.K. Banerjee PII:

S0925-8388(19)34134-9

DOI:

https://doi.org/10.1016/j.jallcom.2019.152888

Reference:

JALCOM 152888

To appear in:

Journal of Alloys and Compounds

Received Date: 28 July 2019 Revised Date:

30 October 2019

Accepted Date: 1 November 2019

Please cite this article as: S. Howlader, R. Vasudevan, B. Jarwal, S. Gupta, K.H. Chen, K. Sachdev, M.K. Banerjee, Microstructure and mechanical stability of Bi doped Mg2Si0.4Sn0.6 thermoelectric material, Journal of Alloys and Compounds (2019), doi: https://doi.org/10.1016/j.jallcom.2019.152888. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.

Microstructure and mechanical stability of Bi doped Mg2Si0.4Sn0.6 thermoelectric material Smita Howlader1*, R. Vasudevan2,3, B. Jarwal2,3, S. Gupta4, K.H. Chen2,3, K. Sachdev1, 4,M.K.Banerjee1 1.Materials Research Centre, Malaviya National Institute of Technology, Jaipur 302017 (India) 2. Center for Condensed Matter Sciences, National Taiwan University, Taipei (Taiwan) 3. Institute of Atomic and Molecular Sciences, Academia Sinica (Taiwan) 4. Department of Physics, Malaviya National Institute of Technology, Jaipur 302017 (India) *Corresponding author email id: [email protected]

Abstract Bi doped Mg2Si0.4Sn0.6 had been synthesised in a high energy ball mill followed by compaction using a sintering hot press. The structural and compositional characterization of sintered mass indicated the formation of a highly densified single-phase product. The microstructure of the hot-pressed samples had been critically assessed. Thermoelectric properties were measured between room temperature and 723 K. A decrease in electrical conductivity was found with the increase in temperature but the Seebeck coefficient showed a reverse trend justifying the attainment of degenerate semiconducting behaviour. Meanwhile, the lattice thermal conductivity was subdued to 1.5 W/mK at 623 K. However, the highest zT value of 0.8 was achieved at 723 K. Moreover, the detailed X-ray photoelectron spectroscopic analysis was carried for the determination of binding energy of the constituent elements in the experimental alloy; it also provided the correct estimation of atomic percentage of the concerned elements. The Raman spectrum revealed a shift in F2g peak with respect to that of Mg2Sn and Mg2Si in correspondence with the composition of the synthesised alloy. The synthesised alloy showed micro and nano hardness of 3.7 and 4.03 GPa respectively, which implies that good mechanical strength could be achieved in the synthesised alloy. KEYWORDS- Magnesium silicide stannide; n-type Mg2Si0.4Sn0.6; high energy ball milling; thermoelectric properties; mechanical properties 1. Introduction Thermoelectric (TE) technologies are valuable to a great extent, as the solid-state device implementing the Seebeck effect can be utilised to convert waste heat into electric power whereas the reverse effect of TE cooling can be witnessed via the Peltier effect [1,2].The TE performance is determined by a dimensionless figure of merit, zT, defined by zT= (S2σT)/κ, where, σ is the electrical conductivity, S is Seebeck coefficient and κ is

1

thermal conductivity at a given absolute temperature, T. Because of the interdependence of the transport properties, it is challenging to achieve zT > 2.0 for mid-temperature applications [3–12]. Magnesium silicide stannide are not only promising candidates for mid-temperature TE power generation application but also do have low density, high melting point, abundance and non-toxicity of constituent elements, all leading to green and sustainable development [3,4]. Given commercial potential Mg2Si1-xSnx alloys as a mid-temperature TE material, considerable research efforts are directed towards the development of high zT alloys [3–12]. Taking stock of results of such investigations, it is noticed that various values of x have yielded different zT values in Mg2Si1-xSnx alloys. The attainable maximum zT values of mostly studied Mg2Si1-xSnx alloys are seen to lie between 1.2 – 1.55 where x lies within 0.6-0.7. It has been the subject of the investigation to find out the exact value of x that provides the best TE performance in this alloy system. Theoretical studies conducted for the purpose have demonstrated that convergence of light and heavy conduction bands takes place at x ~0.625; this enhances Seebeck coefficient and hence favours attainment of high zT value [9]. However, increasing Sn content gives rise to mass disorder scattering of phonons reducing lattice thermal conductivity. This, in turn, improves the figure of merit. Along with an enhanced Seebeck coefficient, it is also necessary to heavily dope the system with an n-type dopant to get a considerable electrical conductivity. Different dopants influence the charge carrier concentration in different ways and hence the zT values [3–5,7,10,11]. The pentavalent bismuth provides the extra electron and increases the electron carrier concentration leading to the desired high electrical conductivity. The solubility limit of Bi is also high concluding the absence of Mg-Bi compound in the Mg2(Si,Sn) system which is beneficial as the second -phase particles may exert deteriorating influence on electrical conductivity as well Seebeck coefficient [10–13]. Also, among the group V elements, Bi has the largest ionic radius and density and so result is in increased phonon scattering thereby reducing the thermal conductivity. These comprehend the use of Bi as a promising dopant in the Mg-Si-Sn system. It is understood that microstructural features of different scale of sizes are more effective in an overall reduction in lattice thermal conductivity, thereby aiding in the enhancement of figure of merit [1,3,12]. At the same time, it is apparent that the degree of first and second near neighbour disordering in Mg2Si1-xSnx system influence the interatomic interaction and hence Seebeck coefficient. However, reports on detailed studies on bond quality and fine-scale microstructure are not abundant in the literature. Hence attempts are made to understand the structure of Bi-doped Mg2Si0.4Sn0.6 at micron and sub-micron level.

2

Along with the desirably good thermoelectric properties, this material is also required to possess appreciable mechanical strength for better durability for long term use. The mechanical properties of Mg2Sn are weaker than that of Mg2Si [14,15]; thus, substituting some of the Sn with Si will not only generate TE beneficial conduction band convergence but also will result in better mechanical properties. When the mechanical properties of Mg2Si0.250Sn0.750werecalculated via first principles and compared with the pristine Mg2Sn, an increase of ~ 18.20%, ~8.42% and ~ 25.94% is observed in bulk, shear and Young’s moduli respectively [14]. Also, with the increase in Si content, the yield strength increases by ~3.77% [14]. Because limited information on the mechanical behaviour of Mg-Si-Sn ternary alloys is available in the literature, this entices us to put effort into gaining insight into the mechanical stability of the experimental alloy via hardness tests. As a token of novelty of the present work, elaborate study of X-ray photoelectron spectroscopy (XPS), Raman spectroscopy (RS) and Scanning Electron Microscope (SEM) enabled Electron Backscatter Diffraction (EBSD) are carried out for gaining a better understanding of the causative factors of the thermoelectric behaviour of Mg2Si0.4Sn0.6 alloy. The parameters other than TE properties and XRD measurements are not presented in the literature. Thus, the Mg2Si0.37Sn0.6Bi0.03 nanocomposite was first prepared by mechanical alloying and subsequently hot pressed. Mechanical alloying is advantageous from the conventional high-temperature synthesis routes as the volatilization and oxidation of Mg can be controlled along with the added benefit of refined grains [10]. Thereafter elaborate study concerning the phase stability, microstructure, TE properties and mechanical stability was made. 2. Experimental procedure 2.1 Synthesis of Mg2Si0.37Sn0.6Bi0.03 The magnesium silicide stannide solid solution was prepared using commercially available elemental powders of Mg, Si, Sn and Bi (Alfa Aesar) of99.99 purity%. The elemental powders were weighed in a glove box according to the desired composition of Mg2Si0.37Sn0.6Bi0.03with an excess of 10 mole % of stoichiometric Mg to compensate for the possible Mg loss. A high energy planetary ball mill, Retsch PM 100, was employed along with hardened tungsten carbide vial and balls with a ball to powder ratio of 30:1. A cycle of 30 minutes milling at 300 rpm and 15 minutes break was employed to subdue the heat generation during the dry mill. After the XRD confirmation of the attainment of single-phase alloy, which in this case took 16 hrs, the ball-milled (BMed) powder was loaded to a graphite mold and compacted in a sintering hot press (NANO TEC Sintering

3

hot press) at 990 K for 20 minutes at5 K/min heating rate under nitrogen atmosphere. The density of the hotpressed pellet was calculated applying the Archimedes principle in kerosene medium. The density of the synthesised alloy had been measured to be 3.11 g/cm3 which is 100.97 % of the theoretical density of Mg2Si0.4Sn0.6. Further, observations and measurements were carried after the appropriately cut samples were polished. 2.2Phase and microstructure characterization Bruker D2 diffractometer was employed for the X-ray diffraction of the synthesised BMed powder. Cu-Kα radiation (1.5406 Å) in the 2θ range (20° – 80°) along with a step size of 0.02° was used. A diamond paste of 1 µm diameter particle is used for polishing the sample, for the microstructure view in a SEM (Nova Nano FE-SEM 450 FEI).X Flash 6130 Bruker was employed for the EDX study. The sample was further polished by ion milling using Illion+ II 697 Gatan for EBSD analysis using an e-Flash HR+ EBSD detector. The ARGUSTM imaging system was used to assess the surface quality and to find a convenient area of interest for analysis. The sample was given a tilt of 9.620, an accelerating voltage of 20kV, an exposure time of 35 ms andvan area of 156.4 x 117.3 µm2was scanned. The ESPIRIT software was used for further analysis. For further microstructure study by transmission electron microscopy (T20 ST FEI—Technai G2), 3mm disc sample with a thickness of~100nm was prepared; finally, ion milling (PIPS II 695 Gatan) was done till perforation at the centre takes place which ensures the formation of an electron transparent area around the hole. X-ray Photoelectron Spectroscopy (Omicron NanoTechnology Oxford Instruments) using A1 Kα X-ray source (1486.7 eV) was used. The depth profiling included four times etching of the sample at a rate of 5 minutes/etch via Ar sputtering under an accelerating voltage of 2 keV and 2.50µA current till the oxygen content got stabilized. Casa XPS software was used for XPS spectra analysis.

In the Raman Spectroscopic analysis, 532 nm Ar+ laser line was used for excitation at a power level of 30 mW in TR 500 Confocal Micro Raman Spectrometer (Airix Corp Raman Spectrometer).

2.3 Measurement of thermoelectric properties The standard four probe method (ZEM-3; ULVAC) was used to simultaneously measure Seebeck coefficient and electrical conductivity of the pellet under He atmosphere. The typical dimensions of the sample employed to study the Seebeck coefficient measurement were 14 mm x 2 mm x 2 mm. Netzsch Laser Flash Analyzer (LFA)

4

457 was used to measure thermal diffusivity (α) of the pellet which thereafter was used to calculate the thermal conductivity (κ) by using the relation: κ = α ρ cP, where ρ is density and cP is specific heat capacity. The measurements were performed in He atmosphere within a temperature range 323K – 723 K. The thermal conductivity was measured on a 1 mm thick sample of area 5mm x 5mm. 2.4 Measurement of mechanical properties For Vicker’s hardness test (NEXUS 4000 Innovatest), a load of 2.942 N was applied on a polished surface for a dwell time of 10 s. Nanoindentation measurement was carried out with an AFM (Multimode 8, Bruker). The indenter was a Bercovich tip (stainless steel cantilever with a diamond tip) which is a sharp three-sided pyramid with an equilateral triangle base of 300 half angle. The tip had a deflection sensitivity of 133 nm/V, a spring constant of 225 N/m and a resonant frequency of 70 kHz. A load of 2.9 mN load was applied for the measurements. The same tip was used to obtain the AFM image. NanoScope Analysis was used to interpret the AFM images. 3. Results 3.1 X-ray diffraction analysis The powder X-ray diffractogram of 16 hours ball milled sample is shown in Fig 1a. The characteristic diffraction peaks of Mg2Si0.4Sn0.6 alloys are located between those of the standard Mg2Sn and Mg2Si. All the considerable peaks show similarity with the anti-fluorite Mg2Si0.4Sn0.6 phase (JCPDS card no.01-089-4254); this verifies that a single-phase Mg2Si0.4Sn0.6 alloy with no elemental peak has been successfully achieved. The absence of the Mg-Bi compound in the XRD spectrum authenticates that the added dopant, that is bismuth, is present in elemental form into the lattice of Mg2Si0.4Sn0.6. This corroborates the earlier observation that bismuth added in excess amount (4.5 wt %) tends to form a Mg3Bi2 phase in the microstructure of Mg2X [7]. Although an excess of 10 mole % of Mg had been used initially during the synthesis, the XRD of the synthesized alloy does not show peaks of elemental Mg which implies that there was a loss of Mg during the synthesis process, viz. dry ball milling. The calculated lattice parameter comes out to be 6.65 Å which is 7% greater than that observed in an ideal Mg2Si0.4Sn0.6 alloy. This is attributed to the addition of Bi as dopant. 3.2 Spectroscopic studies

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The SEM assisted EDX and the XPS (Fig.1b and Table 1) too confirm that the sintered alloy has stoichiometric Mg content rather than the excess as was used initially during the ball milling process. The elemental percentage goes well enough with the overall stoichiometry of the desired alloy composition. However, The EDX reveals the

Fig. 1. (a) X-ray Diffractogram (b) EDX spectrum (c) XPS spectrum and (d)Raman spectrum of Mg2Si0.37Sn0.6Bi0.03 presence of oxygen in the sample which is further justified by the XPS. The presence of ~10 atomic percent of oxygen indicates the formation of MgO as impurity; this is discussed while explaining the results of XPS study. Table 1. Atomic percentage as per EDX and XPS Element Mg2.2Si0.37Sn0.6Bi0.03

Mg Si Sn

68.75 11.56 18.75

Mg2Si0.37Sn0.6Bi0.03

66.6 12.3 20.0

Atomic percentage EDX Excluding Including oxygen oxygen 66.18 66.33 13.33 13.67 19.32 17.97

Standard deviation 3.01 0.93 1.54

XPS Excluding Including oxygen oxygen 66.05 59.64 12.39 10.88 20.83 18.28

6

0.93

Bi O

01.0

01.17

0.82 4.19

0.02 0.75

0.73

0.60 10.57

Gaussian–Lorentzian envelopes and Shirley type background were used to fit the XPS spectra. All the highresolution spectra were deconvoluted and quantified to identify the contribution of chemical species comprising each spectrum (Fig.2). The unprocessed surface gives a dominant peak of C 1s and O 1s (Fig.1a). But Sn, Mg, Si and Bi peaks get resurfaced as the sample is etched. Also, C 1s peak and O 1s peak get reduced to finally no carbon content and constant oxygen content from the 3rd etch. When quantification is done on the 4thetch surface spectrum (Fig. 1c), the atomic percentage obtained on excluding oxygen is similar to that obtained in EDX (Table 1). But, considering oxygen, it is seen that there is 10.57 atomic percent oxygen which appears to be that of metal oxides after the broad Mg 2p and O 1s peaks are deconvoluted.

(c)

(b)

3rd Etch

2500

2000

2nd Etch

1500

1st Etch

1000

Si 2p

Position Area Plasmon loss 101.38 1477.77 Mg2Si 98.94 1818.77 Fitted

2700

Intensity (a.u.)

M g 1s

4th Etch

3000

Position Area %Area MgOx 51.66 492.94 20.36 Mg2Si 50.05 645.94 26.68 Mg2Sn 50.36 1281.94 52.96 Fitted

Mg 2p

3000

Intensity (a.u)

C 1s

O 1s

Bi 4 p Sn 3p

Sn M N N

Sn 3 d Sn 3d 3 /2 5 /2

3500

In ten sity (a.u.)

Sn 4d M g 2p M g 2s Sn 4p Si 2p Si 2s B i 4f

(a)

2400

2100

1800

1500

1200

0

200

400

600

800

1000

49

1200

50

51

Binding energy (eV)

54

Bi 4f 2600

15000

10000

5000 482

484

486

488

490

Binding energy (eV)

492

494

496

102

104

106

O 1s

Bi 4f 7/2, Elemental Bi 156.45

9000

Bi 4f 5/2, Elemental Bi 161.78

8500

Position Area % Area Metal oxide 530.35 2960.83 47.51 Some O compound 531.83 3271.91 52.49 Fitted

2400

Intensity (a.u)

20000

100

(f)

2800

Sn 3d 5/2, Mg2Sn 484.62

Sn 3d 3/2 Mg2Sn 493.04

480

98

Binding energy (eV)

8000

2200

Intensity (a.u.)

Sn 3d

96

55

(e)

25000

Intensity (a.u)

53

Binding energy (eV)

(d) 30000

52

2000 1800

7500 7000

1600

6500

1400

6000

154

156

158

160

162

164

Binding energy (eV)

526

528

530

532

534

536

Binding energy (eV)

Fig. 2. (a) XPS Depth profile spectra, Short scan spectrum of (b) Mg 2p, (c) Si 2p, (d) Sn 3d, (e) Bi 4f and (f) O 1s of the 4th etch sample The Mg 2p is deconvoluted to three peaks positioned at 51.66 eV, 50.05 eV and 50.36 eV. Reports suggest that Mg 2p due to MgO occurs at 51.5 eV [17]. Thus, the peak at 51.66 eV may be attributed to MgO whereas the

7

peaks at 50.05 eV and 50.36 eV could be due to Mg getting bonded with Si as in Mg2Si and Mg getting bonded with Sn as in Mg2Sn respectively [18]. It is so because, the binding energy of elemental Mg 2p is 49.8 eV [17], but with the formation of MgO, Mg2Si and Mg2Sn, the binding energy increases as the Mg becomes more and more electropositive. If we exclude the MgO contribution, it is seen that the area of the peak denoting Mg2Si is ~ 40% and that of Mg2Sn is ~60% which is in sync with the targeted alloy composition. Broad O 1s peak is deconvoluted into two, with a peak at 530.35 denoting MgO. The binding energy of the elemental Si 2p is 99.0 eV, but on deconvoluting the broad Si 2p, the peak at 98.94 eV is dedicated to Si on getting bonded with Mg as Si is electronegative in Mg2Si [18].A broad peak at 101.38 eV in the Si 2pspectrum can be attributed to a plasmon loss associated with the Mg 1s electrons [19].TheSn 3d spectrum includes a doublet structure generated by the splitting of multiplet (i.e., Sn 3d 5/2 and Sn 3d 3/2). The Sn 3d 5/2 peak is positioned at 484.62 eV which is due to the decrease in the binding energy of Sn 3d 5/2 on getting bonded with Mg resulting in Mg2Sn [18]. Thus, even the high-resolution spectra justify the formation of Mg2Si and Mg2Sn bonds. The doublet structure generated by the splitting of Bi 4f 7/2 and Bi 4f 5/2 stems from the Bi 4f spectrum. The Bi 4f 7/2 peak is positioned at 156.46 eV which is dedicated to elemental Bi, implying that Bi is in elemental form and not in compound form with Mg [20]. In the Raman spectrum, the fundamental F2g modes of Mg2Sn and Mg2Si of the (111) planes have been observed at 220 cm-1 and 258 cm-1[21–22]. But the fundamental F2g peak in our sample is obtained at 232.97 cm-1 (Fig. 1 d) which can be stated to be a shifted Mg2Sn F2g mode since the alloy composition bears 60% of Mg2Sn. The substitution of Si at Sn site in the alloy lattice leads to a decrease in the overall effective mass, increasing the Raman shift. Also, Raman overtone is observed in Mg2Sn as a broad peak between 400- 550 cm-1, which in our sample has been observed between 450 -550 cm-1.The Raman overtone is likely to be second-order, two phonon scattering phenomenon as the obtained broad peak is found to be positioned around twice of F2g line [21].Further, the spectrum doesn’t depict any peak representing oxide or any other contaminants. 3.3 Microstructure study The secondary scattered electron image (Fig. 3a) of the polished sample does not discern any porosity verifying the formation of a dense sample. The backscatter electron image (Fig. 3b) gives distribution of the major Mg2Si0.4Sn0.6 phase (grey) and minor phases (white) of MgO and/or Sn rich Mg-Si-Sn phase, showing overall homogeneity of the sample.

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EBSD has been carried out with a high hit rate of 92.6%. By feeding three elements (Mg, Si & Sn), the software has successfully identified two candidate phases upon fitting 5 Kikuchi bands. While the major phase could be identified as Mg2Si0.4Sn0.6, the other minor phase seems to be a non-stoichiometric Mg-Si-Sn phase. In the instant case, a maximum band mismatch value of 1.5 is obtained which implies a good fit. The phase distribution map extracted from EBSD (Fig. 4a), depicts 87.9% of Mg2Si0.4Sn0.6 and 4.7 % of an unidentified non-stoichiometric Mg-Si-Sn phase. Thus, it has been highly established by now that the composition obtained is very near to the single-phase Mg2Si0.4Sn0.6 alloy. From the grain boundary map (Fig. 4b, c), it is clear that the average grain size is ~ 1µm. The misorientation angle profile (Fig. 4d) shows a polycrystalline material, around 80 % of which bears the angle of misorientation in excess 300.

(a)

(b)

(c)

Fig.3 (a) Secondary scattered electron image and (b) Backscattered electron obtained from FESEM and (c) HRTEM image

The lattice fringes obtained from the HRTEM (Fig. 3c) corresponds to (111) plane of Mg2Si0.4Sn0.6 which can be said so because of the similarity of observed d-spacing, 0.38 nm. Hence the results of XRD, EBSD and TEM are compatible, which justifies the reliability of the obtained data. 3.4 Thermoelectric transport properties

9

The thermoelectric transport properties were studied in temperature range from 323K to 723K (Fig. 5). The electrical conductivity shows a monotonous decrease from 1.6 x 105 S/m to 9.6 x 104 S/m whereas the Seebeck coefficient increases from 118 µV/K to 197 µV/K with temperature (Fig. 5a). The thermal conductivity shows a mean value of 3.15 W/mK (Fig. 5b) with a standard deviation of 0.17. The highest zT obtained is 0.8 at 723 K (Fig. 5c).

Fig.4(a) phase distribution map, (b) grain boundary map, (c) histogram of grain sizes and (d) misorientation angle distribution obtained from EBSD of our sample. 3.4 Mechanical properties

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Both the diagonals of the impression formed from the Vicker’s tip (Fig. 6a) is found to be 0.0380 mm. Accordingly, Vicker’s hardness of 3.7 GPa is recorded which is ~ 3 times superior to of Mg2Sn [30]. Alloying with Mg2Si (hardness: 3.5–7.0 GPa) [30] has enhanced the hardness of the material. Impressions formed from the Berkovich tip and the depth profile (Fig. 6b,c) is fairly good enough. Any pile-up or sink-in effects were not visible. The hardness value, H is given by,

H=

(

…………………………………………………………………………………………………………

)

where hmax is depth of the nanoindentation imaged with AFM and A(h ) is the cross-section area of the conical indenter specified at the maximum indentation depth. On the analysis of the AFM images of the nanoindentation, a hardness of 4.03 GPa is obtained on applying a load of 5.9 mN.

4.0 -120

-140 3.5

14 -160

3.0 12 -180 2.5 10

-200 2.0 300

400

500

600

Thermal conductivity Electronic thermal conductivity Lattice thermal conductivity

(b)

4.0

Thermal conductivity (W/mK)

Electrical conductivity (S/m) * 104 Seebeck coefficient (µ µV/K) Power factor (W/mK2) * 10-3

(a)

16

3.5

3.0

2.5

2.0

1.5

700

Temperature (K)

300

400

500

600

700

Temperature (K) 0.9 0.8

conductivity and power factor vs temperature, (b) plot

0.7

of total thermal conductivity, electronic thermal

0.6

zT

Fig. 5 (a) Plot of Seebeck coefficient, electrical

conductivity and lattice thermal conductivity vs temperature and (c) plot of zT vs temperature

(c)

0.5 0.4 0.3 0.2 300

400

500

600

700

Temperature (K)

4 Discussion The observed density is 0.97 % higher than the theoretical value due to the addition of 1% of Bi moles into the parent alloy system intended to substitute some of the Si atoms(which happens to be 7.5 %) to synthesise the

11

composition Mg2Si0.37Sn0.6Bi0.03.Also, the increase in density is attributed to the presence of MgO, as verified by XPS and EBSD [16].The XRD depicts an increment in lattice parameter by 7% which can be accounted by incorporation of Bi into the lattice because the atomic radius of Bi is higher than Si. The EBSD phase map (Fig. 3b) shows 87.9 % of Mg2Si0.4Sn0.6 and 8.7% of unidentified non-stochiometric MgSi-Sn phase. Mg2Si0.4Sn0.6 is the desired phase which is intended to be synthesised. But the presence of a nonstochiometric Mg-Si-Sn phase is explained by the fact that although mechano-chemical synthesis during high energy ball milling produces a single phase Mg2Si0.4Sn0.6 , post spark plasma sintering cooling may not avoid the precipitation of minor phase owing to decreasing solubility of Mg2Si in the Mg2Sn with decreasing temperature; however, the minor second phase precipitates tend to disappear at high temperature. Moreover, in accordance with the pseudo-binary Mg2Si-Mg2Sn diagram, the non-stoichiometric phase could be a Sn-rich phase.

4.1 Thermoelectric transport properties The electrical conductivity shows a monotonous decrease from 1.6 x 105 S/m to 9.6 x 104 S/m with temperature indicating that the doping concentration is well enough to generate degeneracy in the semiconductor (Fig 4a). This arises due to the rise in acoustic phonon scattering of charge carriers leading to a reduction in mobility [7]. From the sign of Seebeck coefficient, it is verified that the Bi dopant has provided the n-type conductivity by

12

providing the 5th electron upon substituting some of the X (X- Si/Sn) sites in the Mg2X lattice, thereby increasing the carrier concentration and shifting the Fermi level towards the conduction band. The Seebeck coefficient, as expected, shows a reverse trend with temperature (Fig. 4a) when compared to the electrical conductivity. The Seebeck coefficient is inversely proportional to the charge carrier concentration, but the temperature has a negligible effect on charge carrier concentration until the bipolar conduction, and thus increase in Seebeck coefficient from 118 µV/K to 197 µV/K is because of Seebeck coefficient’s direct proportionality to temperature. Since, Mg2X is a double-valley cubic semiconductor [3,23], band degeneracy accompanied by convergence is in favour of a high-power factor (=S2σ). Thus, a good Seebeck coefficient (118 µV/K to 197 µV/K), as well as electrical conductivity (1.6 x 105 S/m to 9.6 x 104 S/m) are attributed to the attainment of conduction bands’ convergence at Mg2Si0.4Sn0.6 alloy composition. Such band convergence in Mg2Si1-xSnx at x = 0.6 has also been predicted by theoretical calculation [5,9]. Band degeneracy allows to obtain a high density of states (DOS) effective mass (m∗ ) which helps in attaining appreciable Seebeck coefficient without much negatively impacting charge carrier mobility and thus the electrical conductivity [1]. The thermal conductivity κ involves a lattice part (κlat) and an electronic part (κe). According to the Wiedemann–Franz law, κe is equal to LσT, where the Lorenz number L is 2.45 × 10-8 V2K-2, as the sample in consideration is a case of degenerate semiconductor [2]. Electronic thermal conductivity demonstrates an exponential behaviour with temperature (Fig. 4b). Alloying and grain size refinement cause a short-range order disturbance in the lattice of Mg2Si0.4Sn0.6, due to which enhanced phonon scattering leads to a 66% decrease in lattice thermal conductivity as compared to that of Mg2Sn at room temperature. Since, the electron mean free path is two order of magnitude higher than phonon mean free path [2,27], the extent of incoherent scattering of electrons remains low and hence the electrical conductivity is not adversely affected. The κlat first decreases till 423K and then shows short interval non dependency till 673K beyond which there is a significant increase with the increase in temperature (Fig. 4b). The increase is presumably due to the onset of bipolar conduction. Although the expected decrease in S is not observed beyond 673 K, a decrease the rate of change of S with respect to temperature is noticed. The power factor too shows an increasing trend with temperature but till 673K (Fig. 4a). So, the change in behaviour beyond 673K needs to be further analysed. The onset of intrinsic conduction in Mg2Si0.4Sn0.6 is expected to be at a lower temperature than that of Mg2Si (band

13

gap ~ 0.71 eV) as the band gap of Mg2Si0.4Sn0.6is reported to be 0.61 eV [23]. Thus, zT increases with temperature, with an obtainable highest value of 0.8 at 723K. The TE properties obtained in the present investigation are compared with those reported by Gao et al. [28] and Bahk et al. [29] in Table 2. It is apparent that the values of electrical conductivity and Seebeck coefficient achieved in the present experiments are very much similar to the experimental results of Gao et al. and theoretical predictions by Bahk et al. However, the zT values obtained in this investigation is found to be less than those due to the above investigators. The measured thermal conductivity of the presently investigated alloy is found to be 1.5 times higher than that reported by Gao et al.; at the same time, the predicted value by Bahk et al. is considerably lower than the value of the present study. It is evident that the higher thermal conductivity of the investigated alloy has led to a lower zT value in comparison to those of previous investigations. The zT values of this study could be comparable with the others if its thermal conductivity would be similar to those of others. The higher thermal conductivity is presumably due to the non-availability of nanoscale features in the microstructure. High resolution of microscopy and EBSD results have delineated that the grain size of this single-phase alloy has been around 1µm and that the majority of grain boundaries exhibit misorientation greater than 300. Moreover, no nanosized second phase particles could be detected in the high-resolution microscopy. In general, the wavelength of phonon is spread over a wide range say, from the angstrom level to the level of microns. This means that in terms of size scale of structural features, the investigated alloy is only capable of scattering of phonons only of wavelength in the level of microns. Table 2. A Comparative study of TE properties of present and previous investigations TE Property (Room Temperature) Electrical conductivity, S/m Seebeck Coefficient, µV/K Thermal conductivity, W/mK zT

Gao et al. Experimental 1.1 x 105 -200 2.25 1.4

Bahk et al. Theoretical 7 x 104 -240 1.875 1.2

Present 9.6 x 104 -197 3.39 0.8

In the absence of submicron scattering centres, the present alloy is deprived of the ability to effectively scatter a large fraction of phonons of wavelength below 1µ. Moreover, XPS has recorded the presence of 10 atom % MgO which contributes to enhancement of thermal conductivity. All these factors are supposedly responsible for higher thermal conductivity. Since no such events were given ample consideration in the previous investigations [29–28], it is difficult to deduce the reason for the lower

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thermal conductivity reported by those researchers. However, it may be inferred from structural (SEM, TEM, EBSD, XRD) and spectrographic (XPS and Raman Spectroscopy) studies that the creation of small to ultrasmall microstructural features viz. atom clusters of excess phase, nanoscale second particles, etc. are the mandatory requirement to harness the inherent TE properties of Mg2Si0.4Sn0.6and hence, realise the maximum achievable zT value in the alloy. 4.2 Mechanical properties The microhardness of the alloy turns out to be ~3 times more than that of the Mg2Sn. Thus, on the presumption of one to one co-relation between hardness and tensile strength, the hardness result is indicative of enhanced tensile strength. Also, at the average grain size level of ~ 1um, there is a good amount of grain boundary strengthening because the grain boundaries are known to obstruct dislocation propagation thereby enhancing the strength and fracture toughness. Since the dimensions of the ball milled and sintered samples were quite small, it has not been possible to undertake detailed tensile testing. However, one can anticipate that a fine grained material should have sufficiently high strength and ductility unless there is any microvoids or cracks inside the sample; the presence of such defects is reasoned out on ground of the high density (higher than theoretical density) of the synthesised material. The hardness obtained from nanoindentation (4.03 GPa) is higher than that obtained from the Vicker’s indentation (3.7 GPa). This is so because the size of the nanoindentation (~82.5 nm) is very small when compared to the average grain size (984 nm); so as per nanohardness is concerned, the response comes from nanoscale structural features. Moreover, the XPS results do not rule out the presence of small quantity of oxides; at the same time EBSD results hint upon the presence of minor phases by a very small quantity. All these phases have lattice mismatch with the surrounding matrix and there are short range strain fields around these particles. This strain field enhances resistance to nanoindentation and hardness appears to be higher. The larger dimension of Vicker’s indentation averages out these ultra small events and hence, records a lower hardness. Nevertheless, need for tensile testing of the alloy cannot be underestimated to achieve the exact idea of the real strength and ductility of the present alloy. 4. Conclusions The authors wish to conclude that a nearlysingle-phaseMg2Si0.4Sn0.6alloy, free from contaminations, can be produced by controlled mechanical alloying and hot press sintering. During synthesis, a small quantity of unidentified non-stoichiometric Mg-Si-Sn compound was formed. It is established by SEM and XPS studies that

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the stoichiometry,Mg2Si0.4Sn0.6, is achievable by the practiced process of synthesis. Moreover, the mechanical stability of the synthesised alloy is quite satisfactory. The average grain size of the alloy is 1µm and there is no abundance in the presence of nanoscale features in microstructures. The electrical conductivity and Seebeck coefficient of the synthesised alloy is reasonably good and comparable with the values documented so far in the literature. Although in minute quantity, the presence of MgO and the absence of nanostructures have led to the enhanced thermal conductivity. This is why the achievable highest zT value (~0.8) of the experimental alloy is found to be less than the maximum value reported in the literature. The authors finally conclude that X-ray photoelectron spectroscopic, Raman spectroscopic and high-resolution microscopic studies are the effective tools to gain a better insight into the TE behaviour exhibited by Mg2Si0.4Sn0.6alloy.

Acknowledgements This study was supported by a grant from the Department of Science and Technology [GITA/DST/TWN/P68/2015], Government of India. We are thankful to National Taiwan University for the Ball milling, XRD, ZEM & LFA measurements and Materials Research Centre, MNIT Jaipur for FESEM-EDX-EBSD, HRTEM, XPS, Raman spectroscopy, Vicker’s indentation & nanoindentation. References [1]

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Highlights • • • • •

Dense single-phase Mg2Si0.4Sn0.6 is produced by ball milling and hot press Electrical conductivity and Seebeck coefficient are good Average grain size is 1µm and there is no abundance of nanoscale features Micro and nano hardness of 3.7 and 4.03 GPa respectively, is achieved XPS and Raman spectroscopy are effective in providing better insight

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: