Thermoelectric Properties Of Mg2Si Research Articles

Effect of Bi doping on the thermoelectric properties of Mg2Si0.3Ge0.04Sn0.66 compound

N-type Bi-doped Mg2(Si0.3Ge0.04Sn0.66)1-xBix (0<x≤0.03) compounds were successfully fabricated by solid state reaction-spark plasma sintering (SPS). The XRD results indicate that all prepared samples are nearly phase pure Bi-doped solid solutions. The lattice constant and the electrical conductivity of the samples increases while the Seebeck coefficient decreases with the increase of the doped Bi content. The figure of merit (ZT) decreases at a low temperature but increases at a high temperature while increasing the Bi content. At the temperature of 800K, ZT value at x = 0.03 is about 1.3, comparing to 1.1 for x = 0.01.

Enhancement of thermoelectric performance of Mg2Si via co-doping Sb and C by simultaneous tuning of electronic and thermal transport properties

Thermoelectric power generation using distributed waste heat energy has received attention as a long-life, environmentally friendly power supply. The intermetallic compound Mg2Si is a lightweight, mid-temperature thermoelectric material that contains no toxic elements, and its thermoelectric performance has been enhanced by various methods such as impurity doping, nanostructuring, and alloying. In this study, we examined the influence of the influence of co-doping with Sb and dilute amounts of the isoelectronic impurity C on the thermoelectric properties of Mg2Si. We fabricated dense polycrystalline specimens of Mg2CxSbySi using the melting process and subsequent plasma-activated sintering. Doping Mg2Si with Sb increased the electrical conductivity ~102 times. Further co-doping with isoelectronic C did not significantly change the electrical conductivity; however, it did reduce the thermal conductivity independently of the electrical properties. Consequently, the specimens co-doped with Sb and C achieved higher thermoelectric performance than specimens of Mg2Si single-doped with Sb. The dimensionless figure of merit ZT of the co-doped specimens reached 0.79 at 873 K over the temperature range 323–873 K.

Lifting the Optical and Thermoelectric Properties of Mg2Si as a Function of Sn Incorporation—Potential Thermoelectric Materials

By means of Density Functional Theory (DFT) study, we have performed the structural, electronic, optical and thermoelectric properties calculations of tin doped Mg2Si (Mg2Si1−xSnx, x = 0, 0.125, 0.25, 0.5, 0.75, 0.875, 1) using Full Potential Linearized Augmented Plane Wave (FP-LAPW) Method. The DFT study yields satisfactory results for electronic and thermoelectric properties of Sn doped Mg2Si compared with experimental values. With semiclassical Boltzmann transport theory, the transport properties of Mg2Si and Sn doped Mg2Si alloys has been investigated systematically. According to the calculated band structure, density of states and electron density, the parent Mg2Si/Sn materials having indirect energy gap (Γ−x) with ionic bonding; Sn doped ternary combinations Mg2Si1−xSnx, x = 0.125, 0.25, 0.5, 0.75, 0.875 having direct band gap (Γ−Γ) with a mixed covalent and ionic bonding nature. Band gap decreases linearly with the increase of Sn-concentration in each alloy system except for Mg2Si0.75Sn0.25 combination. The optical properties calculations have been performed for the energy range between 0–13.5 eV. The thermoelectric properties have been calculated for the temperature range 100 K to 800 K. Out of the five studied materials, Mg2Si0.75Sn0.25 found to be a better thermoelectric material with increased Power factor, Seebeck coefficient, electrical conductivity and corresponding thermal conductivity at high temperature range.

Experimental investigation of the predicted band structure modification of Mg2X (X: Si, Sn) thermoelectric materials due to scandium addition

Modification of the electronic band structure via doping is an effective way to improve the thermoelectric properties of a material. Theoretical calculations from a previous study have predicted that Sc substitution on the Mg site in Mg2X materials drastically increase their Seebeck coefficient. Herein, we experimentally studied the influence of scandium substitution on the thermoelectric properties of Mg2Si0.4Sn0.6 and Mg2Sn. We found that the thermoelectric properties of these materials are unaffected by Sc addition, and we did not find hints for a modification of the electronic band structure. The SEM-energy dispersive X-ray analysis revealed that the scandium does not substitute Mg but forms a secondary phase (Sc-Si) in Mg2Si0.4Sn0.6 and remains inert in Mg2Sn, respectively. Thus, this study proves that scandium is an ineffective dopant for Mg2X materials.

Strain engineering on thermoelectric performance of Mg2Si

In this work, we propose to boost the thermoelectric performance of bulk magnesium silicide using isotropic strain. The effect of strain on the electronic and thermoelectric properties of Mg2Si is analyzed using first principles calculations combined with semi-classical Boltzmann theory. The Seebeck coefficient, power factor and electrical conductivity are strongly modified with strain. The lattice thermal conductivity is also tuned with strain. However, the strain effect on the lattice thermal conductivity of Mg2Si has not yet been systematically studied. The effect of strain on lattice thermal conductivity, specific heat, phonon dispersion curves and phonon density of states of Mg2Si are studied for the first time in this paper. The lattice thermal conductivity of bulk Mg2Si is shown to decrease continuously when applied strain changes from compressive to tensile. The results obtained in this paper have great importance in the thermoelectric field.

Scrutinize the effect of Ge and Sn doping on electronic and thermoelectric properties of Mg2Si as thermoelectric material

We have analyzed the effect of Ge and Sn doping on the electronic and thermoelectric properties of Mg2Si using density functional theory and Boltzmann equations. The calculated results show that Mg2Si1−xAx (A = Ge, Sn, 0.125 ≤ x ≤ 0.5) systems exhibit semiconductor nature and doping with Ge slightly widen the energy gap at x = 0.125. In all doped systems, the seebeck coefficient has a negative sign, which indicates that conduction is due to electrons. With an increase in doping concentrations, the seebeck coefficient decreases, while electrical conductivity, electronic thermal conductivity increases. A classical kinetic theory has been employed to calculate the contribution of lattice thermal conductivity. We have elucidated that both doped systems attained minimum lattice thermal conductivity. Mg2Si1−xGex and Mg2Si1−xSnx has maximum figure of merit 0.077 and 0.15 at x = 0.125 respectively.

Unraveling the effect of uniaxial strain on thermoelectric properties of Mg2Si: A density functional theory study**Kulwinder Kaur thanks Council of Scientific & Industrial Research (CSIR), India for providing fellowship.

In this work, the effect of uniaxial strain on electronic and thermoelectric properties of magnesium silicide using density functional theory (DFT) and Boltzmann transport equations has been studied. We have found that the value of band gap increases with tensile strain and decreases with compressive strain. The variations of electrical conductivity, Seebeck coefficient, electronic thermal conductivity, and power factor with temperatures have been calculated. The Seebeck coefficient and power factor are observed to be modified strongly with strain. The value of power factor is found to be higher in comparison with the unstrained structure at 2% tensile strain. We have also calculated phonon dispersion, phonon density of states, specific heat at constant volume, and lattice thermal conductivity of material under uniaxial strain. The phonon properties and lattice thermal conductivity of Mg2Si under uniaxial strain have been explored first time in this report.

Sb Substitution Effect on Thermoelectric Properties of Mg2Si

The effects of Sb dopant on electronic and thermoelectric properties of Mg2Si (Mg2Si1−x Sb x , 0.125 ≤ x ≤ 0.5) have been studied using density functional theory and semiclassical Boltzmann transport theory. Classical kinetic theory was employed to calculate the lattice thermal conductivity. The calculations show that the Sb-doped Mg2Si system exhibits metallic nature and Sb enhances the thermoelectric efficiency of Mg2Si. Increase in the concentration of Sb leads to decrease in the Seebeck coefficient and increase in the electrical and electronic thermal conductivity as well as reduction in the lattice thermal conductivity. We found that the lattice thermal conductivity decreases with increase in temperature. There is little effect on the lattice thermal conductivity as the doping concentration is increased, but the overall thermal conductivity increases with increasing concentration due to increased electronic thermal conductivity. High ZT value of 0.47 is obtained at 1200 K for x = 0.125.

Thermoelectric Properties of Mg2Si0.995Sb0.005 Prepared by the High-Pressure High-Temperature Method

Mg2Si0.995Sb0.005 compound was prepared by the high-pressure high-temperature (HPHT) method. The simultaneous synthesis and consolidation in one step could be completed in <15 min. The effects of pressure and temperature on the thermoelectric properties of Mg2Si0.995Sb0.005 were analyzed in this work. With the pressure and temperature increasing, the electrical conductivity rises markedly, while the Seebeck coefficient changes slightly, which results in significant enhancement of the power factor. The Mg2Si0.995Sb0.005 sample prepared under the condition of 1073 K and 2 GPa achieves the highest power factor of ∼2.12 × 10−3 W m−1 K−2 at 575 K. As the sample prepared at 973 K and 2 GPa retains a lower thermal conductivity, it obtains the highest thermoelectric figure-of-merit ZT ∼0.62 at 800 K. In conclusion, the HPHT method can serve as a route to prepare Sb-doped Mg2Si thermoelectric materials efficiently.

Effect of Silicon Carbide Nanoparticles on the Grain Boundary Segregation and Thermoelectric Properties of Bismuth Doped Mg2Si0.7Ge0.3

The effect of silicon carbide (SiC) nanoparticles on the thermoelectric properties of Mg2Si0.676Ge0.3Bi0.024 was investigated. Increasing the concentration of SiC nanoparticles systematically reduces the electrical conductivity from 431 Ω−1 cm−1 for the pristine sample to 370 Ω−1 cm−1 for the sample with 1.5 wt.% SiC at 773 K, while enhancing the Seebeck coefficient from −202 μV K−1 to −215 μV K−1 at 773 K. In spite of the high thermal conductivity of SiC, its additions could successfully decrease the lattice thermal conductivity from 3.2 W m−1 K−1 to 2.7 W m−1 K−1 at 323 K, presumably by adding more interfaces. The Z contrast transmission electron microscopy imaging (Z = atomic number) and energy dispersive x-ray spectroscopy revealed bismuth segregation at the grain boundary. In summary, the figure of merit reached its maximum value of 0.75 at 773 K for the sample containing 0.5 wt.% SiC.

Effect of pressure on electronic and thermoelectric properties of magnesium silicide: A density functional theory study**Project supported by the Council of Scientific & Industrial Research (CSIR), India.

We study the effect of pressure on electronic and thermoelectric properties of Mg2Si using the density functional theory and Boltzmann transport equations. The variation of lattice constant, band gap, bulk modulus with pressure is also analyzed. Further, the thermoelectric properties (Seebeck coefficient, electrical conductivity, electronic thermal conductivity) have been studied as a function of temperature and pressure up to 1200 K. The results show that Mg2Si is an n-type semiconductor with a band gap of 0.21 eV. The negative value of the Seebeck coefficient at all pressures indicates that the conduction is due to electrons. With the increase in pressure, the Seebeck coefficient decreases and electrical conductivity increases. It is also seen that, there is practically no effect of pressure on the electronic contribution of thermal conductivity. The paper describes the calculation of the lattice thermal conductivity and figure of merit of Mg2Si at zero pressure. The maximum value of figure of merit is attained 1.83×10−3 at 1000 K. The obtained results are in good agreement with the available experimental and theoretical results.

Thermal stability of Mg2Si0.4Sn0.6 in inert gases and atomic-layer-deposited Al2O3 thin film as a protective coating

The structure and thermoelectric properties of Mg2Si0.4Sn0.6 are unstable above 700 K. We demonstrated atomic-layer-deposited Al2O3 as a protective coating to address this problem.

First-principles investigation of structural, electronic, and thermoelectric properties of n- and p-type Mg2Si

Abstract

Local structure and thermoelectric properties of Mg2Si0.977−xGexBi0.023 (0.1 ⩽ x ⩽ 0.4)

We investigated the effect of germanium substitution for silicon in bismuth doped Mg2Si. This alloying reduces the thermal conductivity from above 7Wm−1K−1 to 2.7Wm−1K−1 at around 300K in part due to the added mass contrast. High resolution transmission electron microscopy (HRTEM) revealed the presence of Ge-rich domains within the Mg2(Si,Ge,Bi) particles, contributing to decreasing thermal conductivity with increasing Ge content up to 0.3 Ge per formula unit. The electrical conductivity also decreases with Ge alloying because of the increasing amount of scattering centers, while the Seebeck coefficient increased only very slightly. In total, the positive effect of Ge substitution on the thermoelectric properties of Bi doped Mg2Si resulted in a figure of merit of 0.7 at 773K for Mg2Si0.677Ge0.3Bi0.023 sample. The optimum amount of Bi seems to be 0.023 per formula unit (0.77at%), since lower Bi content resulted in electrical conductivity that is too low, and higher Bi content generated the Mg3Bi2 intermetallic phase.

Electron transport properties of Mg2Si under hydrostatic pressures

The electronic and thermoelectric properties of Mg2Si under hydrostatic pressures have been investigated using the first principles calculations with general potential linearized augmented plane-wave method and the semiclassical Boltzmann theory with the rigid band approach and the constant scattering time relaxation approximation. In this work, the hydrostatic pressure is simulated by applying equiaxial strain method for the cubic anti-fluorite structure of Mg2Si in space group Fm3m. The strain values ranging from -0.03 to 0.03 describe the compressive and tensile Processes under pressure. The band structure, electrical conductivity, Seebeck coefficient and power factor have been calculated and analyzed in detail.#br#From the band structure in Mg2Si one can see that the bottom of the conduction band shows significant changes under strains. Especially, when the strain is up to 0.02, there are two twofold-degeneracy states occurring at the center of the Brillouin zone. The top of the valence band shows a slight change due to the strain effect. For the unstrained structure, our calculated thermoelectric data are in accordance with other reports. Moreover, the results indicate that when the value of strain is up to 0.02, the transport properties get an optimal functioning of Mg2Si due to electron doping. At 300 K, the Seebeck coefficient improves obviously and comes up to 126%. And the power factor is up to 47% (45%) at T=300 K (700 K). Consequently, the thermoelectric properties can be improved through applying negative pressures to the Mg2Si crystal. For the case of hole doping, the transport parameters change obviously at a small strain value, and change gently at a high strain values. When the strain is up to 0.01, the Seebeck coefficient reaches the maximum value 439 μV/K-1. But, the power factor only increases 0.9%–2%. Hence, we can conclude that the hydrostatic pressures have a slight influence on the thermoelectric properties of hole-doped materials.

Effect of GaSb Addition and Sb Doping on the Thermoelectric Properties of Mg2Si0.5Sn0.5 Solid Solutions

Abstract Mg 2 Si 0.487-2 x Sn 0.5 (GaSb) x Sb 0.013 (0.04 ≤ x ≤ 0.10) solid solutions were synthesized by a B 2 O 3 flux method followed by hot pressing. X-ray power diffraction analysis confirms that single-phased samples are obtained. It is found that the Sb-doping effectively enhances the electrical conductivity. The Seebeck coefficients increase while the electrical conductivity decreases for Mg 2 Si 0.487-2 x Sn 0.5 (GaSb) x Sb 0.013 with the increase of temperature. With increasing of GaSb content the electrical conductivity first increases and then decreases. Among all the samples, Mg 2 Si 0.287 Sn 0.5 (GaSb) 0.1 Sb 0.013 sample has the lowest lattice thermal conductivity which is about 15% lower than that of Mg 2 Si 0.5 Sn 0.5 [11] at room temperature. A maximum dimensionless figure of merit of 0.61 at 720 K has been obtained for Mg 2 Si 0.327 Sn 0.5 (GaSb) 0.08 Sb 0.013 mainly due to its high electrical conductivity, which is obviously higher than that (0.019 at 540 K) of Mg 2 Si 0.5 Sn 0.5 [11] .

Transmission Electron Microscopy Study of Mg2Si0.5Sn0.5 Solid Solution for High-Performance Thermoelectrics

A Mg2Si0.5Sn0.5 solid solution was prepared by mixing Mg2Si and Mg2Sn powders and hot-pressing the mixture. The Mg2Si0.5Sn0.5 samples exhibited a much lower thermal conductivity (1.92 W m−1 K−1 at 300 K) than the parent Mg2Si (8.75 W m−1 K−1) and Mg2Sn compounds (6.28 W m−1 K−1). X-ray diffraction measurements confirmed the successful synthesis of the Mg2Si0.5Sn0.5 solid solution. Electron microscopy observations revealed that the grains were mainly 10–20 μm in size and had clean grain boundaries without obvious inclusions and precipitates. The major phase was cubic Mg2Si0.5Sn0.5. MgO nanoparticles 10–20 nm in diameter were evenly dispersed in the Mg2Si0.5Sn0.5 matrix, which probably reduced its thermal conductivity; moreover, uneven structures containing pure Si and Sn particles were found in the Mg2Si0.5Sn0.5 grains. The origin and the formation mechanisms of the MgO and other impurity particles, and their effect on thermoelectric properties of Mg2Si0.5Sn0.5, are discussed. The low thermal conductivity of Mg2Si0.5Sn0.5 resulted in a relatively high dimensionless figure of merit ZT = 0.0132 at 300 K, which may be further increased by optimizing the synthesis procedure, alloy composition, and doping level. This work provides information on the structure and chemistry and their relationship with the thermoelectric properties of the Mg2Si0.5Sn0.5 solid solution; it may help in developing other Mg2Si1−xSnx compounds with superior thermoelectric properties.

Thermoelectric properties of Mg2Si0.7Ge0.3Bi m prepared using a solid-state reaction

Mg2Si0.7Ge0.3Bi m (m = 0 − 0.03) solid solutions were synthesized using a solid-state reaction and consolidated by hot pressing. All specimens showed n-type conduction, and the carrier concentration increased from 4.0 × 1017 to 2.1 × 1020 cm−3 with increasing Bi-doping content; the electrical conductivity thereby increased from 7.3 × 102 to 7.0 × 104 Sm−1 at room temperature. The electrical conductivity of the undoped specimen increased with increasing temperature, and the specimen behaved similar to a non-degenerate semiconductor. The absolute value of the Seebeck coefficient of the undoped specimen was very high at low temperature, but it decreased with increasing temperature. Bi-doped Mg2Si0.7Ge0.3 showed a Seebeck coefficient ranging from −235 to −197 μVK−1 at 823 K. The power factor significantly increased with Bi doping and increased with increasing temperature; it were also improved by around 10 times at 823 K. The lowest thermal conductivity was 2.2 Wm−1K−1 for Mg2Si0.7Ge0.3Bi0.02 at 823 K, and the maximum ZT value of 0.79 was obtained for Mg2Si0.7Ge0.3Bi0.02 at 823 K.

Thermoelectric Properties of Mg2Si Produced by New Chemical Route and SPS

This paper reports about a new synthesis method for preparing Mg2Si in an efficient way. The intermetallic Mg2Si-phase forms gradually from a mixture of Mg and Si fine powder during exposure to hydrogen atmosphere, which reacts in a vacuum vessel at 350 °C. The resulting powder has the same particle size (100 µm) compared with commercial Mg2Si powder, but higher reactivity due to large surface area from particulate morphology. Both types of powders were compacted by spark plasma sintering (SPS) experiments at 627, 602, 597, and 400 °C for 600 s with a compaction pressure of 80 MPa. The thermoelectric characterization was performed with low and high temperature gradients of ΔT = 10 K up to 600 K. The results confirmed a Seebeck coefficient of −0.14 mV/K for specimens sintered from both powders. The small difference in total performance between purchased and produced power is considered to be due to the effect of impurities. The best values were obtained for n-type Mg2Si doped with 3% Bi yielding a Seebeck coefficient of −0.2 mV/K, ZT = 0.45) and electric output power of more than 6 µW.

Enhanced Thermoelectric Properties of Mg&lt;sub&gt;2&lt;/sub&gt;Si&lt;sub&gt;0.3&lt;/sub&gt;Sn&lt;sub&gt;0.7 &lt;/sub&gt;Compounds by Bi Doping

The single phase of Bi-doped Mg2Si0.3Sn0.7compounds have been successfully fabricated by solid state reaction and spark plasma sintering (SPS). The effect of Bi doping concentration on the thermoelectric properties of Mg2Si0.3Sn0.7is investigated. The doping of Bi atom results in the increase of carrier concentrations and ensures the increase of electrical conductivity. Although the thermal conductivity and Seebeck coefficient shows a slight increase, the figure of merit of Mg2Si0.3Sn0.7compounds still increases with the increasing contents of Bi-doping. When Bi-doping content is 1.5at%, the Mg2Si0.3Sn0.7compound obtained the maximum value,ZT, is 1.03 at 640 K.