Effect of Bi-doping and Mg-excess on the thermoelectric properties of Mg2Si materials

Journal of Physics and Chemistry of Solids 75 (2014) 984–991

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Journal of Physics and Chemistry of Solids journal homepage: www.elsevier.com/locate/jpcs

Effect of Bi-doping and Mg-excess on the thermoelectric properties of Mg2Si materials M. Ioannou a, G.S. Polymeris b, E. Hatzikraniotis b, K.M. Paraskevopoulos b, Th. Kyratsi a,n a b

Department of Mechanical and Manufacturing Engineering, University of Cyprus, 1678 Nicosia, Cyprus Department of Physics, Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece

art ic l e i nf o

a b s t r a c t

Article history: Received 23 December 2013 Received in revised form 21 March 2014 Accepted 10 April 2014 Available online 24 April 2014

In this work, Bi-doped magnesium silicide compounds were prepared by applying a combination of both, short-time ball milling and heating treatment. The effect of Mg excess was also studied, aiming towards further improvement in thermoelectric properties. The structural modifications of all materials were followed by Powder X-ray diffraction and Scanning Electron Microscopy. Highly dense pellets of Mg2Si1
xBix (0 rx r 0.035) and Mg2 þ δSi0.975Bi0.025 (δ ¼ 0.04, 0.06 and 0.12) were fabricated via hot pressing and studied in terms of Seebeck coefficient, electrical and thermal conductivities and free carrier concentration. Their thermoelectric performance, at high temperature range, is presented and the maximum value of the dimensionless-figure-of-merit (ZT) is found to be 0.68 at 810 K, for Mg2Si0.97Bi0.03. & 2014 Elsevier Ltd. All rights reserved.

Keywords: A. Intermetallic compounds D. Transport properties D. Thermal conductivity

1. Introduction Thermoelectric (TE) materials have attracted considerable attention in recent years due to their wide use in various applications, as thermoelectric generation technology is expected to play an important role in energy generation [1,2], in the coming years. Their efficiency is related to a dimensionless quantity—called the figure of merit (ZT), which comprises three key transport parameters such as the thermoelectric power, S i.e. the Seebeck coefficient, expressing the ability of the material to generate a potential difference due to applied temperature difference, the electrical conductivity s and the thermal conductivity κ along with the absolute temperature T given by the following expression: ZT ¼

S2 s

κ

T

ð1Þ

In order to achieve high ZT values, thermoelectric materials must possess a special combination of electrical and thermal transport properties: a high metal-like electrical conductivity, a high insulatorlike Seebeck coefficient, and a low glass-like thermal conductivity. Magnesium compounds (Mg2X, where X¼Si, Ge or Sn) and their solid solutions, having the antifluorite structure, have attracted increasing attention as promising high-performance thermoelectric materials which are functional in the temperature range of 400–900 K [3–10]. Pure Mg2Si is an indirect-bandgap semiconductor n

Corresponding author: Tel.: þ 357 22892267. E-mail address: [email protected] (Th. Kyratsi).

http://dx.doi.org/10.1016/j.jpcs.2014.04.008 0022-3697/& 2014 Elsevier Ltd. All rights reserved.

with less interesting thermoelectric properties [3,11]. It has low density (1.99 g/cm3), high compression strength (1640 MPa) and high melting point (1358 K); however, the poor room temperature ductility and low toughness stand as its major shortcomings. Impurity doping drastically affects the thermoelectric properties of Mg2Si compounds and their solid solutions. Therefore, a plethora of dopants has been proposed for binary Mg2Si in order to optimize its thermoelectric properties, while for practical applications thermodynamically stable dopants are required to ensure long-lifetime operation at elevated temperatures. Over the voluminous literature already published, very few p-type dopants have been reported, including Ag and Cu as efficient electron acceptors [7], indicating that p-type doping seems more difficult to be achieved. For the case of n-type Mg2Si, Tani and Kido [12] suggested that elements from groups Ib, IIIb, and Vb are expected to be primarily located at Si sites in Mg2Si and can therefore be used as donors. Therefore, various successful attempts have been reported, for dopants such as Te [13], but mostly using elements from group Vb such as Sb and Bi [14–23]. It is noteworthy that elements Bi and Sb from this group are found to be much more effective to enhance the thermoelectric properties of n-type Mg2Si than other group elements. Finally, calculations from first principles show that Bi is the most stable element in Mg2Si compared with the other dopants. Bi has a low melting point of 545 K [24] while it is nearly twice as heavy and has a larger radius than other n-type dopants such as Al and Sb; therefore, for Bi-doped Mg2Si, a further decrease in thermal conductivity is anticipated and expected. The effect of magnesium deficiency has been also discussed for promising materials doped with Sb [25].

M. Ioannou et al. / Journal of Physics and Chemistry of Solids 75 (2014) 984–991

While in the majority of cases, Bi-doped magnesium silicide was fabricated by Spark Plasma Sintering (SPS), in this work, we deal with Bi-doped Mg2Si materials prepared by the combination of short-time ball milling process and heating treatment. Mg2Si1
xBix series is studied with x values in the range of 0 rx r0.035. The materials were hot pressed and the pellets were studied in terms of structural and morphological characteristics. Thermoelectric properties such as Seebeck coefficient, electrical conductivity, thermal conductivity and ZT are also discussed. Special emphasis was addressed to the impact of Mg content on electron concentration and thermoelectric properties. Finally, an optimum composition giving the largest ZT value in the present system is determined.

2. Materials and methods 2.1. Reagents Chemicals in this work were used as obtained: i) magnesium powder (
20 þ100 mesh [ ¼841–149 μm], 99.8% purity, Alfa Aesar); ii) silicon powder (crystalline, þ100mesh [¼ 149 μm], 99.9% purity, Alfa Aesar); iii) bismuth polycrystalline lump, (Puratronic, 99.999%). All manipulations were carried out under inert gas argon in a dry glove box. 2.2. Mixing and milling The starting elements (Mg, Si and Bi) were mixed in appropriate ratios (Mg2Si1
xBix for 0 rxr 0.035 and Mg2 þ δSi0.975Bi0.025 for δ ¼0.04, 0.06 and 0.12) in a stainless steel bowl and subsequently ball-milled using balls of 10 mm diameter using a Pulverisette 6, Fritsch Planetary Mono Mill. The bowl was sealed by O-rings in the glove box to ensure milling process under argon atmosphere. The materials were milled for 60 min and the process was interrupted every 15 min/for 5 min to avoid heating. The speed was set at 300 rpm and the ball-to-material ratio was 23:1. 2.3. Solid state reaction and densification The powders, after milling, were cold-pressed to pellets at about 0.5 GPa. The pellets were placed inside the Mo foil to prevent them from attacking the hot quartz wall, sealed in silica tubes under vacuum and heated at temperatures 400 1C and 600 1C for 1 h. Solid state reaction causes disintegration of pellets into fine powder. The materials in powder form were sintered in order to perform thermoelectric measurements. Sintering was carried out using a uni-axial hot-press system (HP20) from Thermal Technologies Inc. Hot-pressing was carried out under argon flow using graphite die of 10 mm diameter. The pellets were heated to actual temperature of 860 1C and kept at this temperature for 60 min under a pressure of 80 MPa. The hot-pressed pellets were highly dense with densities corresponding to 498% of the theoretical value for Mg2Si. The density of pellets was estimated from mass (m) and volume (V) measurements using the equation ρMEAS ¼m/V. 2.4. Structural characterization Powder XRD patterns were obtained for all materials (ball milled, after heating treatment and after hot pressing), using a Rigaku Miniflex Powder X-ray Diffractometer with Ni-filtered Cu Kα radiations (30 kV, 15 mA), in order to identify the phases and evaluate the purity of products. Powder XRD data (PXRD) data were refined using a FullProf Suite ToolBar software. For the determination of lattice parameters, Si powder was used as an

985

internal standard. The morphological characterization of powders was carried out by a Scanning Electron Microscope (SEM) (Tescan Vega LSU as well as Jeol 840A). 2.5. Thermoelectric properties 2.5.1. Seebeck coefficient and electrical conductivity Measurements of Seebeck coefficient and electrical resistivity were carried out simultaneously on bar-shaped samples, cut from hot-pressed pellets, using a commercial ZEM-3 system from ULVAC-RIKO. Data were recorded in the temperature range between room temperature and 823 K. The measurements were performed under a residual pressure of helium gas to facilitate good thermal contact. 2.5.2. Thermal conductivity A Netzsch LFA-457 system was used to measure thermal diffusivity and heat capacity of the hot-pressed pellets with 10 mm diameter. The thermal conductivity was calculated from the experimental thermal diffusivity (α), as well as density values (ρ) and a previously reported specific heat capacity data (Cp) of Mg2Si [26], based on the equation κ ¼ α  ρ  Cp. Carrier concentration for each doping level was estimated by applying Fourier Transformed Infrared (FTIR) spectroscopy measurements, in the reflectivity configuration, in conjunction with minimization techniques in fitting experimental data. For each sample, a number of FTIR spectra were recorded on highly polished surfaces, with 100 μm diameter iris. All measurements were performed at room temperature, with near normal incidence light in mid infrared ranges (700–4000 cm
1) using an i-Series FTIR microscope, PerkinElmer connected with an FTIR spectrometer, Spectrum 1000. The reflection coefficient was determined by a typical sample in/sample out method with a gold mirror as reference.

3. Results and discussion Magnesium, sequentially:

silicon

and

bismuth

coarse

powders

were

 ball milled, for the formation of fine and well-mixed Mg/Si/Bi powders, then

 cold-pressed (to achieve good contact between the grains and



enhance the diffusion mechanism) and heated, in order to get the pure single phase magnesium silicide through solid state reaction and finally hot-pressed in order to obtain highly dense pellets and evaluate their thermoelectric properties.

3.1. Synthesis, powder X-ray diffraction and morphology Magnesium, silicon and bismuth powders were ball-milled under argon atmosphere, in order to get homogeneous finegrained powder, to be further used as pristine material for heating treatments. Ball-milling was limited to a duration of 60 min, in order to avoid the formation of Mg2Si at this step as discussed elsewhere [27,28]. Fig. 1a shows the PXRD pattern of the material milled for 60 min, where it becomes evident that only peaks of Mg, Si and Bi2Mg3 are identified. Mg preferential orientation in the ball milled materials is suggested by the intensities of (101) and (100) diffraction peaks, which is discussed elsewhere [27,28]. After solid state reaction the material was found to consist of single Mg2Si phase, see Fig. 1b and c (after heating at 400 1C and 600 1C for 1 h). The addition of a second heating step, as suggested in

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M. Ioannou et al. / Journal of Physics and Chemistry of Solids 75 (2014) 984–991

Fig. 1. PXRD patterns of 60 min (a) ball milled material and (b) solid state reaction at 400 1C for 1 h and (c) at 600 1C for 1 h. The reference materials are shown for comparison.

Table 1 The quantitative analysis of Mg2Si1
xBix (0 r xr 0.035) in terms of lattice parameters and volume fraction of Mg2Si and MgO phases. Nominal composition

aMg2Si

Mg2Si (vol%)

MgO (vol%)

Mg2Si Mg2Si0.990Bi0.010 Mg2Si0.980Bi0.020 Mg2Si0.975Bi0.025 Mg2Si0.970Bi0.030 Mg2Si0.965Bi0.035

6.3467 6.3478 6.3505 6.3526 6.3520 6.3512

91.0 92.5 90.9 92.0 88.8 86.5

9.0 7.5 9.1 8.0 11.2 13.5

Ref. [28], was necessary to avoid the appearance of Bi2Mg3 phase in our product. After hot pressing, PXRD patterns also show the presence of MgO. The existence of MgO is a common impurity for this material [28,29], and, practically, it is difficult to be ignored in this work. The PXRD analysis data showed that the volume fraction of MgO in the products is in the range of 10%, as can be seen in Table 1. As discussed elsewhere [23,28], the heating process affects the morphology of materials due to the observed disintegration of pellets to very fine powder, and that is also evident in the case of Bi-doped materials. Specifically, disintegration was observed after the heating process in all experiments that led to a unique, Mg2Siphase product. The reaction of Mg2Si formation has been recently reported to follow a self-propagating mechanism [30] and the disintegration of the pellets was attributed [30] to the increasing porosity due to both volume reduction and Mg evaporation. In our case, the first argument seems to be more important, since the phenomenon is also observed at temperatures as low as 400 1C where, apparently, there is no evaporation of Mg. However, some Mg evaporation is expected to take place during the second heating step at 600 1C and the hot-pressing process. The PXRD data of Mg2Si1
xBix were refined using the FullProf Suite ToolBar software being a well-known powder profile fitting method for the structural refinement. The cell parameters and other structural parameters were refined using this method by Pauling File Browser. Mg2Si has antifluorite structure, belongs to Fm-3m space group, while the fitted profile and the positions of Bragg peaks for Mg2Si have been shown in Fig. 2a. The quantitative analysis by FullProf Suite ToolBar of Mg2Si1
xBix proves that the received material consists of two phases (Mg2Si and MgO). The refined lattice parameters and the percentage of MgO in the material are presented

Fig. 2. Refined X-ray powder profile of Mg2Si1
xBix. Red line represents the experimental data and the black line represents the calculated powder profile. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

in Table 1. The obtained value of undoped Mg2Si is 6.3467 Å and is lower than the reported values [7,30–35], which can be attributed to some Mg loss during heating/hot pressing. As shown in Table 1, the lattice parameter of Mg2Si1
xBix series increases with Bi incorporation in the lattice. In order to study the grains and morphological changes of the Mg2Si1
xBix samples under various compositions, fractured surfaces of the hot pressed samples are examined by SEM, see typical image in Fig. 3. In general, SEM images show that the products consist mainly of large aggregates ranging from 2 to 4 μm sizes. SEM images were used to measure the average particle size of several individual particles (n 4200). The grains display slightly decreasing average dimensions from 3.7 μm to 2.0 μm for x ¼ 0.010–0.035, respectively.

3.2. Bi doping and carrier concentration Room temperature reflectivity spectra measured for selected samples with different Bi contents are shown in Fig. 4. Experimental IR spectra are dominated by free carrier effects, showing high reflectivity values at low wavenumbers accompanied by a deep reflectivity minimum in the range of 1200–1750 cm
1 (plasma minimum). Plasma minimum is shifted to higher wavenumbers on increasing the Bi content, indicative for the increase of free carriers. The experimentally obtained reflectivity spectrum R(ω), i.e. the response of the material to the incident electromagnetic radiation, was fitted by applying the conventional Drude model, which describes the interaction between free carriers and the infrared radiation. The reflectivity R(ω) is expressed as [36] pffiffiffiffiffiffiffiffiffiffi   εðωÞ
12   RðωÞ ¼ pffiffiffiffiffiffiffiffiffiffi   ε ð ωÞ þ 1 

ð2Þ

while the complex dielectric function describing such interaction is

εðωÞ ¼ ε1


ε1 ω2P ω2 þ iγ P ω

ð3Þ

where ε1 is the high frequency dielectric constant and ωP, γP are the plasmon frequency and the free carrier damping factor, respectively. The plasmon frequency is related to the free carrier

M. Ioannou et al. / Journal of Physics and Chemistry of Solids 75 (2014) 984–991

987

Fig. 3. Typical SEM micrographs of fracture surfaces of Mg2Si1
xBix for (a) x ¼0.01 and (b) x¼ 0.035.

concentration, according to the following equation:

1.0

3% Bi

ω2P ¼

0.8

Reflectivity

2.5% Bi 0.6

0.4

0.2

1.5% Bi 1% Bi

3.5% Bi

0.0 1000

1500

2000

2500

3000

wavenumbers (cm -1) 1.0

Reflectivity

0.8

no excess Mg

0.6

2% excess Mg 4% excess Mg

0.2

6% excess Mg 0.0 1500

2000

ð4Þ

where ε0 is the dielectric function of vacuum, m* the carrier effective mass and n the free carrier concentration. For Mg2Si, the electron effective mass was taken m* ¼ 0.53m0 [25,37], where m0 is the mass of a free electron. The main outcome of this fitting procedure results in three values for the parameters, i.e. ωP, γP and ε1. Conventional IR reflectivity measurements are usually carried out with an iris of about 2–4 mm in diameter. Reducing the iris to 100 μm enables the evaluation of local carrier concentration. The spatially resolved spectral information was obtained by scanning the sample spot by spot at a spatial resolution of 500 μm and collecting at each spot a complete infrared spectrum. For each doping level, mean values for carrier concentration along with the respective standard deviations (in parenthesis) are indicated in Tables 2 and 3. The average values for carrier concentration stand in good agreement with the corresponding values yielded by Hall measurements. The carrier concentration of undoped Mg2Si is 1.5  1018 cm
3, while that of Bi-doped Mg2Si is higher, starting from 9.37  1019 cm
3 for x¼0.01 to 16.60  1019 cm
3 for x¼ 0.035. This increasing trend is in agreement with the expected based on the Si4
/Bi5
substitution as well as with the literature [12]. 3.3. Thermoelectric properties of Mg2Si1
xBix

0.4

1000

ne2 mn ε 0 ε 1

2500

3000

wavenumbers (cm -1) Fig. 4. Typical room-temperature reflectivity spectra for Mg2Si samples with various Bi doping levels. (a) Spectra corresponding to various levels of Mg excess are also plotted for comparison reasons. (b) The plasma frequency, reflecting the reflectivity minimum, is blue-shifted either for ongoing from 1% to 3.5% Bi or for increasing level of Mg excess.

Table 2 presents the results of various measurements for Mg2Si1
xBix (0 rx r0.035), including Seebeck coefficient, electrical and thermal conductivities, all measured at 300 K. The sign of the Seebeck coefficient of the undoped and Bi-doped Mg2Si is negative, indicating that the conductivity is mainly due to electrons, as expected. The electrical conductivity of all Bi-doped Mg2Si is higher than the value of 38 S/cm of the un-doped Mg2Si and increases with Bi concentration reaching the highest value of 1929 S/cm at x ¼0.03. For higher Bi concentration (i.e. x¼ 0.035) the electrical conductivity drops to 1748 S/cm and this is attributed to the lower mobility since the carrier concentration is similar to the member of x ¼0.030, see Table 2. Fig. 5 shows the temperature dependence of (a) electrical conductivity and (b) the Seebeck coefficient of Mg2Si1
xBix (0 rx r0.035) hot-pressed pellets. The electrical conductivity of

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M. Ioannou et al. / Journal of Physics and Chemistry of Solids 75 (2014) 984–991

Table 2 Thermoelectric properties of Bi-doped Mg2Si (Mg2Si1
xBix, 0r xr 0.035) at 300 K. Nominal composition

Conduction type

Carrier concentration N (1019cm
3)

Electrical conductivity (S/cm)

Seebeck coefficient (μV/K)

Thermal conductivity (W/m K)

Mg2Si Mg2Si0.990Bi0.010 Mg2Si0.985Bi0.015 Mg2Si0.980Bi0.020 Mg2Si0.975Bi0.025 Mg2Si0.970Bi0.030 Mg2Si0.965Bi0.035

n n n n n n n

0.22 9.37 (0.95) 11.10 (1.31) 11.25 (0.26) 11.80 (0.47) 16.90 (0.55) 16.60 (1.14)

38 638 1324 1361 1694 1929 1748


315
119
112
109
98
89
82

11.10 9.44 9.91 9.40 8.43 6.61 6.69

Table 3 Thermoelectric properties of Mg2 þ δSi0.975Bi0.025 (0 r δr 0.12) at 300 K. Conduction type

Mg2Si0.975Bi0.025 Mg2.04Si0.975Bi0.025 Mg2.08Si0.975Bi0.025 Mg2.12Si0.975Bi0.025

n n n n

σ (S/cm)

Nominal composition

Carrier concentration N (1019cm
3) 11.80 (0.47) 14.40 (0.48) 15.30 (0.53) 20.7(1.19)

x=0.000 x=0.010 x=0.015 x=0.020 x=0.025 x=0.030 x=0.035

10

300

400

500

600

700

800

S (μV/K)

T (K) -60 -80 -100 -120 -140 -160 -180 -200 -220 -240 -260 -280 -300 -320 -340 -360 -380

x=0.000 x=0.010 x=0.015 x=0.020 x=0.025 x=0.030 x=0.035

Electrical conductivity (S/cm)

Seebeck coefficient (μV/K)

Thermal conductivity (W/m K)

1694 1761 2042 2306


98
90
86
85

8.43 7.66 7.30 6.99

of the un-doped Mg2Si corresponds to semiconducting behavior and is in agreement with the intrinsic semiconducting behavior of Mg2Si [5]. The intrinsic conduction occurs at temperatures higher than 600 K due to the band gap of 0.77 eV [38]. Fig. 5b shows the temperature dependence of Seebeck coefficient for both un-doped and Bi-doped Mg2Si; the negative sign for all samples indicates n-type semiconductor and is in agreement with the literature [34,39]. The different temperature dependence, compared to the un-doped, is due to their significantly higher carrier concentration following typical behavior of heavily doped semiconductors. Fig. 6a shows the thermal conductivity of Mg2Si1
xBix (0 rx r 0.035) as a function of temperature. Room temperature thermal conductivity depends strongly on x and decreases with increasing x reaching its lowest value of 6.61 W/m K for x ¼0.030. The decrease of thermal conductivity with Bi concentration is mainly due to the observed variation in the grain size, as well as the mass fluctuation that is introduced by Bi incorporation in the Mg2Si lattice. This effect is better observed on the lattice thermal conductivity that is calculated by subtracting the electronic contribution (κe) from the total thermal conductivity (κ). κe was calculated via the Wiedemann–Franz law κe ¼LsT, using our measured values for electrical conductivity (s). Lorentz number (L) is estimated based on the Seebeck coefficient measurements as well as assuming scattering from acoustic phonons, from the following equations using Fermi–Dirac statistics: h i 1 ðηÞ S ¼ 7 keB 2F F 0 ðηÞ
η L¼

#  2 " kB 3F 0 ðηÞF 2 ðηÞ
4F 21 ðηÞ e F 20 ðηÞ

F i ðηÞ ¼ 300

400

500

600

700

800

T (K) Fig. 5. Electrical conductivity (a) and Seebeck coefficient (b) of Mg2Si1
xBix (0 r xr 0.035) as a function of temperature.

Mg2Si1
xBix (for x 40) decreases with temperature and this is typical for materials with relatively high carrier concentration, see Fig. 5a. This feature is mainly attributed to the decrease of mobility since scattering by acoustical phonons is the predominant mechanism [4,8]. On the other hand, the electrical conductivity

Z1 0

xi dx 1 þexpðx
ηÞ

ð5Þ

where η is the reduced Fermi energy (EF/kBT), and F i ðηÞ is the Fermi–Dirac integrals, kB the Boltzmann constant. The residual part, κ
κe, corresponds to the lattice contribution in the thermal conductivity since the bipolar contribution in the doped materials is eliminated. The lattice thermal conductivity (Fig. 6b) decreases with Bi concentration reaching the minimum values that correspond to members x¼ 0.03 and x¼ 0.035, as expected. The temperature dependence of both power factor (sS2) and ZT for Mg2Si1
xBix (0 rx r0.035) is shown in Fig. 7. ZT improved

M. Ioannou et al. / Journal of Physics and Chemistry of Solids 75 (2014) 984–991

0.003

11 x=0.000 x=0.010 x=0.015 x=0.020 x=0.025 x=0.030 x=0.035

10

0.002

PF (W/mK2)

9

κ (W/mK)

989

8 7 6

x=0.000 x=0.010 x=0.015 x=0.020 x=0.025 x=0.030 x=0.035

0.001

5 4 3

0.000 300

400

500

600

700

800

900

300

400

500

T (K) 12

0.7 x=0.000 x=0.010 x=0.015 x=0.020 x=0.025 x=0.030 x=0.035

11 10

800

x=0.000 x=0.010 x=0.015 x=0.020 x=0.025 x=0.030 x=0.035

0.6 0.5 0.4

8

0.3

7 6

0.2

5

0.1

4

0.0

3 2

700

ΖΤ

κlattice (W/mK)

9

600

T (K)

300 300

400

500

600

700

800

900

T (K) Fig. 6. Temperature dependence of the total thermal conductivity (a) and the lattice thermal conductivity (b) for Mg2Si1
xBix (0 r xr 0.035).

significantly by Bi doping due to the increase of power factor and the maximum ZT value of 0.68 is obtained for Mg2Si0.970Bi0.030 at 810 K. The power factor (sS2) of Mg2Si0.970Bi0.030 was 2.9 mW/ m K2 at 810 K, which is about 8 times larger than that of intrinsic Mg2Si (0.371 mW/m K2).

3.4. Mg excess on Bi-doped Mg2Si materials The loss of Mg during synthesis is a common problem. Mg vacancies act as acceptors and hence reduce the thermoelectric properties as addressed for Sb-doped materials [25]. In order to explore the potential improvement of thermoelectric properties by the compensation of possible Mg evaporation loss in the Bi-doped materials prepared with the synthesis technique presented here, Mg2 þ δSi0.975Bi0.025 members (δ ¼0.04, 0.08 and 0.12) were studied. The member Mg2Si0.975Bi0.025 was selected for this study, although Mg2Si0.970Bi0.030 exhibited higher ZT, aiming to reach the optimum carrier concentration by adding Mg excess (both, Bidoping and Mg-excess increase the carrier concentration). The room temperature values of thermoelectric properties of Mg2 þ δSi0.975Bi0.025 (δ ¼ 0.04, 0.08 and 0.12) series are shown in the Table 3. n-type carrier concentration seems to increase with the excess of Mg and this can be attributed to the elimination of Mg vacancies, see Table 3, exceeding the carrier concentration of Mg2Si0.97Bi0.03, discussed above as being the best in terms of ZT performance. The electrical conductivity also increases in agreement with the carrier concentration trend, see Table 3.

400

500

600

700

800

T (K) Fig. 7. Temperature dependence of the (a) power factor and (b) ZT for Mg2Si1
xBix (0 rx r 0.035).

The temperature dependence of electrical conductivity and Seebeck coefficient, see Fig. 8a and b, presents typical behavior of materials with relatively high carrier concentration. The power factor (sS2) is 2.95 mW/m K2 at 810 K when δ ¼0.12 and this is attributed to the modification of the carrier concentration. Fig. 8c shows the thermal conductivity of aforementioned samples as a function of temperature that is lower compared to that of the members prepared without Mg excess. Overall, the ZT value is higher for 4% and 6% Mg excess and equal to 0.65 and 0.67 at 810 K, respectively. These ZT values are similar to the best values obtained for materials prepared without Mg excess (i.e. Mg2Si0.97Bi0.03 exhibit ZT of 0.68). Interestingly, the best materials with Mg excess (4% and 6%) have higher electrical conductivity but also higher thermal conductivity compared to the member of x ¼0.03. The member of x¼ 0.03 has lower thermal conductivity due to the mass fluctuation in the lattice that is caused by Bi. Overall, both materials exhibit similar ZT values and they can be both discussed as the best in this work. They are also similar to those reported in the literature for materials prepared with different methods [21,27] although lower than the recently reported Bi-doped Mg2Si1
xSnx series [40]. This difference is due to the much higher thermal conductivity that the binary Mg2Si exhibits compared to that of the alloyed Mg2Si1
xSnx.

4. Conclusion In a nut shell, Bi-doped Mg2Si (Mg2Si1
xBix, 0 r xr 0.035) compounds were prepared by solid-state synthesis; both the

990

M. Ioannou et al. / Journal of Physics and Chemistry of Solids 75 (2014) 984–991

-80

δ=0.00 δ=0.04 δ=0.08

S (μV/K)

σ (S/cm)

δ=0.12

δ=0.00

-100

δ=0.04

-120

δ=0.12

δ=0.08

-140 -160 -180 -200

100

300

400

500

600

700

800

-220

900

300

400

500

T (K)

600

700

800

900

T (K)

9 0.7 8

δ=0.00

0.6

δ=0.04 δ=0.08

0.5

δ=0.12

6

ΖΤ

κ (W/mK)

7

0.4 0.3

5

δ=0.00

0.2 4 3

δ=0.04 δ=0.08

0.1 300

400

500

600

700

800

900

0.0

δ=0.12

300

T (K)

400

500

600

700

800

900

T (K)

Fig. 8. Electrical conductivity (a), Seebeck coefficient (b), thermal conductivity (c) and ZT (d) of Mg2 þ δSi0.975Bi0.025 (0 r δr 0.12) as a function of temperature.

thermoelectric and transport properties were also examined. This synthesis route consists of two steps; ball-milling aiming at good mixing and the creation of very fine powders to be used as starting materials and heating process in order to trigger a solid state reaction. The free carrier concentration, electrical conductivity, Seebeck coefficient and thermal conductivity were strongly affected by the Bi content that acted as donor in the material. Therefore, the absolute value of the Seebeck coefficient decreased and the electrical conductivity increased with Bi concentration. The thermal conductivity decreases with increasing amount of Bi and this feature is attributed to the mass fluctuation introduced by the Si/Bi substitution as well as the decreased grain size. The maximum value of the dimensionless-figure-of-merit (ZT) for Mg2Si0.97Bi0.03 excess was 0.68 at 810 K. Materials prepared with Mg excess, and doped with lower Bi content, present similar ZT values due to the similar carrier concentration, suggesting that Mg excess actually compensates the lower Bi concentration in the lattice.

Acknowledgments The authors wish to acknowledge financial support from the Si-THERM Project supported by the Cyprus Research Promotion Foundation's Framework Programme for Research, Technological Development and Innovation 2009-2010 (DESMI 2009-2010), cofunded by the Republic of Cyprus and the European Regional Development Fund and specially under Grant PENEK/0311/07 and ThermoMag Project, which is co-funded by the European Commission in the 7th Framework Programme (Contract NMP4-SL-2011-263207), by the European Space Agency and by the individual partner organizations.

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