Sb-doped Mg2Si Research Articles

Investigation of local structures and electronic states of Sb-doped Mg2Si by fluorescence XAFS and HAXPES

To clarify the origin of the figure of merit in the thermoelectric material Mg2Si, we performed extended x-ray absorption fine structure (EXAFS) analysis and hard x-ray photoelectron spectroscopy (HAXPES). We elucidate the local structures and electronic states of Mg2Si, especially focusing on the contribution of doped Sb. EXAFS analysis revealed that Sb substituted Si and that the nearest neighbor distance expanded around Sb. Furthermore, HAXPES confirmed the formation of density of states near the Fermi level by co-doping with 0.5% Zn and 0.5% Sb. The comparison of the experimental and theoretical results suggested that the density of states at the Fermi level was due to the tail of the conduction band, and it was filled by the electrons provided by doped Sb. From a microscopic viewpoint, the results obtained in this study suitably explain why Sb doping increases electrical conductivity and decreases thermal conductivity in Mg2Si, as reported in a previous study.

Microstructure and Thermoelectric Properties of Hot Extruded Sb-Doped Mg2Si Using MoS2 Nano-particles as Lubricant

Magnesium silicide is a very promising thermoelectric material for applications in the temperature range of 500–800 K, and is of particular interest for large-scale applications because its constituents are non-toxic, inexpensive and very abundant in the Earth’s crust. Although the hot extrusion (HE) method to compact powders has long been considered for thermoelectric applications because it lends itself easily to large-scale industrial applications, advances to obtain Mg2Si by HE are still difficult to implement. We present the transformations undergone by Mg2Si powders during the nascent HE as well as the modifications of the structural, thermal and electronic properties of the compacted solid. MoS2 particles (2 at.%) are added to the starting Mg2Si:Sb (0.5 at.%) powders which play the role of solid lubricant during this process at 873 K. Samples are extracted from different areas of the die along the extrusion direction and separately characterized, describing the transformations of the material through different stages of the nascent extrusion. X-ray diffraction reveals the expected structure for all samples without any significant texturing. The increase in grain size along the HE direction towards the exit has been determined from analysis of scanning electron microscopy observations. The thermoelectric properties have been characterized using the Harman method between 300 K and 700 K, giving Seebeck coefficients which vary between − 200 μV K−1and − 215 μV K−1 at 700 K. The thermal (λ) and electrical (σ) conductivity decrease as the sample progresses in the extrusion process, and in the case of λ can be accounted for by the increase of sample porosity. The highest figure-of-merit $$ \left( {\hbox{ZT}} \right) $$ is to be found for the sample extracted from the exit of the die. It increases with temperature reaching a maximum value of 0.32 at 700 K, the highest temperature we could attain experimentally.

Preparation of Mg2Si-based thermoelectric materials using waste silicon sludge as a source material

Sb-doped Mg2Si was prepared using waste silicon sludge via a liquid-solid phase reaction and pulse-current sintering. Two types of Si sludge were used as a silicon source. One is an unwashed Si sludge, and the other is Si sludge washed with a water-based agent. In the samples prepared from unwashed Si sludge, Mg2Si, Mg, and MgO phases existed. In the case of samples prepared from the washed Si sludge, the X-ray diffraction patterns showed almost a single phase, although a small MgO phase remained.The thermoelectric properties of Mg2Si prepared from the washed Si sludge were improved by control of weighed composition and carrier concentration. The obtained maximum figure of merit of this study was 0.66 in the samples prepared from the washed Si sludge, which is equivalent to materials prepared from reagent Si with identical carrier concentration.

Control of Mg content and carrier concentration via post annealing under different Mg partial pressures for Sb-doped Mg2Si thermoelectric material

An approach to control the Mg content in Sb-doped Mg2Si via post annealing under different Mg partial pressures is developed. Annealing under low and high Mg partial pressures (≤ 1 × 10–1Pa and 1 × 101Pa) result in low and high carrier concentrations of Mg2Si0.99Sb0.01 (1.2 × 1020cm–3 and 1.7 × 1020cm–3) that correspond to a hypo-stoichiometric and a hyper-stoichiometric compositions, respectively. Mg2Si0.99Sb0.01 with a hypo-stoichiometric composition shows a low figure of merit below 573K mainly due to a reduction of the carrier mobility probably by the carrier scattering at Mg-deficient grain boundaries. The carrier mobility and the figure of merit are recovered by increasing the Mg content via annealing under high Mg partial pressure, which indicates that the annealing process is an effective way to recover the thermoelectric performance of Mg-deficient Mg2Si-based materials.

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.

Influence of Humidity, Volume Density, and MgO Impurity on Mg2Si Thermoelectric-Leg

We have studied the influence of humidity on the production yield of Mg2Si thermoelectric (TE)-legs synthesized by spark plasma sintering (SPS) and also the influence of sintered density and MgO impurity on the oxidation resistance of the Mg2Si sintered compacts. We observed a strong correlation between the humidity in air atmosphere and the yield rate of Mg2Si TE-legs. The Mg2Si TE-legs sintered from the raw material powder that was exposed to an atmosphere with humidity >60% contained relatively high density of voids and cracks due to the reaction of adsorbed moisture and Mg2Si during SPS. We found that the Mg2Si sintered density strongly affected the oxidation resistance, whereas a small amount of MgO concentration in the initial sintered compacts had no significant effect on the oxidation resistance. Sb-doped Mg2Si with a high sintered density showed an excellent oxidation resistance in air atmosphere when subjected to an oxidation-resistance test at 600°C for 800 h, which is presumed to be due to the formation of a dense MgO layer on the surface.

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 texture on the structural and transport properties of Sb-doped Mg2Si thin films

With the ever increasing energy demand, the need for miniaturized thermoelectric (TE) devices that minimize energy waste is becoming urgent. In this context, the use of thin film-based devices is a promising strategy provided that suitable materials with appropriate properties are developed. Here, for applications at temperatures ranging from 500 K to 850 K, the TE properties of the Mg2Si compound are investigated after its deposition as thin film by microwave plasma-assisted co-sputtering method. The effects of doping the Mg2Si films with different Sb contents and of the deposition temperature are assessed. Structural characterization shows that the films grown at room temperature exhibit a larger degree of texture and strains compared to those deposited at 463 K. Furthermore, the TE properties (electrical conductivity, Seebeck coefficient, thermal conductivity) are affected in a different way by the deposition temperature, resulting eventually in a higher power factor (based on in-plane characteristics) and higher thermal conductivity (measured out-of-plane) for RT depositions.

Thermoelectric Properties of Mg2Si1-x-yGexSby Prepared by Spark Plasma Sintering

ABSTRACTThe magnesium compound Mg2Si and its solid solutions are expected asn-type thermoelectric (TE) material because they are non-toxic, have a large Clarke number, and are light weight. In this study, we improved TE performance by doping Ge into Sb-doped Mg2Si to cause phonon scattering and increase carrier concentration. A bulk of Sb-doped Si-Ge alloy as the raw material was fabricated using an arc-melting method. A high-purity Mg2Si was synthesized from metal Mg and Sb-doped Si-Ge alloy using spark plasma sintering equipment. For the samples with the same Sb concentration, the electrical conductivity was equivalent. On the other hand, the Seebeck coefficient was dependent on Ge concentration. Due to phonon scattering, thermal conductivity decreased by a small amount of Ge doping andκphdominated for thermal conduction. The minimum thermal conductivity of Mg2Si0.90Ge0.10was 2.25 W/mK (κph: 2.06 W/mK,κel: 0.19 W/mK). The dimensionless figure of merit (ZT) for the Mg2Si0.945Ge0.05Sb0.005sample was higher than that of the others due to reducing thermal conductivity and increasing carrier concentration. The maximumZTwas 0.47 at 713 K.

Phase Segregation and Superior Thermoelectric Properties of Mg2Si(1-x)Sb(x) (0 ≤ x ≤ 0.025) Prepared by Ultrafast Self-Propagating High-Temperature Synthesis.

A series of Sb-doped Mg2Si(1-x)Sb(x) compounds with the Sb content x within 0 ≤ x ≤ 0.025 were prepared by self-propagating high-temperature synthesis (SHS) combined with plasma activated sintering (PAS) method in less than 20 min. Thermodynamic parameters of the SHS process, such as adiabatic temperature, ignition temperature, combustion temperature, and propagation speed of the combustion wave, were determined for the first time. Nanoprecipitates were observed for the samples doped with Sb. Thermoelectric properties were characterized in the temperature range of 300-875 K. With the increasing content of Sb, the electrical conductivity σ rises markedly while the Seebeck coefficient α decreases, which is attributed to the increase in carrier concentration. The carrier mobility μ(H) decreases slightly with the increasing carrier concentration but remains larger than the Sb-doped samples prepared by other methods, which is ascribed to the self-purification process associated with the SHS synthesis. In spite of the increasing electrical conductivity with the increasing Sb content x, the overall thermal conductivity κ decreases on account of a significantly falled lattice thermal conductivity κ(L) due to the strong point defect scattering on Sb impurities and possibly enhanced interface scattering on nanoprecipitates. As a result, the sample with x = 0.02 achieves the thermoelectric figure of merit ZT ∼ 0.65 at 873 K, one of the highest values for the Sb-doped binary Mg2Si compounds investigated so far. A subsequent annealing treatment on the sample with x = 0.02 at 773 K for 7 days has resulted in no noticeble changes in the thermoelectric transport properties, indicating an excellent thermal stability of the compounds prepared by the SHS method. Therefore, SHS method can serve as an effective alternative fabrication route to synthesize Mg-Si based themoelectrics and some other functional materials due to the resulting high performance, perfect thermal stability, and feasible production in large scale for commercial application.

Structural, compositional and functional properties of Sb-doped Mg2Si synthesized in Al2O3-crucibles

Dense Sb-doped Mg2Si materials were obtained with a cheap and fast process. An extensive characterization was carried out to study the effects of Sb doping and other secondary phases on the stability and functional properties of n-type Mg2Si1−xSbx.

First-Principles Study on Structural and Thermoelectric Properties of Al- and Sb-Doped Mg2Si

We theoretically investigate the structural and thermoelectric properties of magnesium silicide (Mg2Si) incorporating Al or Sb atoms as impurities using first-principles calculations. We optimized the structural properties through variable-cell relaxation using a pseudopotential method based on density functional theory. The result indicates that the lattice constant can be affected by the insertion of impurity atoms into the system, mainly because the ionic radii of these impurities differ from those of the matrix constituents Mg and Si. We then estimate, on the basis of the optimized structures, the site preferences of the impurity atoms using a formation energy calculation. The result shows a nontrivial concentration-dependence of the site occupation, such that Al tends to go into the Si, Mg, and interstitial sites with comparable formation energies at low doping levels (<2 at.%); it can start to substitute for the Mg sites preferentially at higher doping levels (<4 at.%). Sb, on the other hand, shows a strong preference for the Si sites at all impurity concentrations. Furthermore, we obtain the temperature-dependence of the thermoelectromotive force (Seebeck coefficient) of the Al- and Sb-doped Mg2Si using the full-potential linearized augmented-plane-wave method and the Boltzmann transport equation.

Microstructural effects on thermoelectric efficiency: A case study on magnesium silicide

Carrier transport and conversion efficiency of thermoelectric materials is not only governed by the intrinsic bulk material properties, but often heavily influenced by microstructural effects. Using Mg2Si—a very attractive, non-toxic and abundant material—as an example, the authors investigate the microstructural effects on thermoelectric transport in the material. Combining microstructural analysis (scanning electron microscopy, X-ray diffraction) with high-temperature transport measurements (Seebeck coefficient, electrical and thermal conductivity, charge carrier mobility), it is verified that small amounts of impurity phases can heavily impair electrical transport. Further electron microscopy studies confirm MgO functioning as a scattering center at the grain boundaries of the material. The effect of the impurity phases on the transport properties can be understood in a simple transport model based on a single parabolic band including grain boundary scattering. The model can be employed to assess material parameters such as the phonon deformation potential, the barrier height of the grain boundaries and the average size of the grains. Furthermore, the detrimental effect of MgO on thermoelectric efficiency is quantified. Compared with Sb-doped Mg2Si with optimized properties, a few per cent MgO decreases the average thermoelectric figure of merit by 30%.

Convenient Melt-Growth Method for Thermoelectric Mg2Si

We have succeeded in growing single-crystalline-like n-type Mg2Si bulk crystals by a convenient melt-growth method that requires no vacuum or inert gas. The Sb-doped, n-type Mg2Si crystals had a density equivalent to the theoretical ideal of 1.99 g cm3 to 2.00 g cm−3 and well-developed crystalline grains. Powder x-ray diffraction measurements and scanning electron microscopy observations confirmed the single-phase Mg2Si nature of the grown crystals, with no MgO or unreacted Si and Mg observed. The crystals had high Hall mobility and power factor compared with Sb-doped sintered Mg2Si crystals. The achieved ZT values were 0.10 at 300 K and 0.36 at 600 K for 0.317 at.%Sb-doped Mg2Si.

Fabrication parameters for optimized thermoelectric Mg2Si

Here, we report on the effect of sintering temperature, particle size, and sintering pressure on the thermoelectric properties of Sb-doped Mg2Si samples. We find a strong dependence of thermoelectric performance on sintering temperature with the best properties achieved at 900 °C. We furthermore show a strong influence of the particle size of the employed powder, with coarser powder giving better properties. The difference stems mainly from the electrical conductivity; Seebeck coefficient and thermal conductivity are affected to a lesser extent. With respect to sintering pressure, a small positive influence of increasing pressure is found. We obtain a peak thermoelectric figure of merit of 0.75 ± 0.18 at 820 K employing optimized fabrication parameters.

Improvement in the Durability and Heat Conduction of uni-leg Thermoelectric Modules Using n-type Mg2Si Legs

We have fabricated several kinds of uni-leg thermoelectric (TE) modules using Sb-doped n-type Mg2Si. In order to evaluate the influence of the structure of the modules on their durability with respect to heat-cycling, modules of two different types were evaluated. One was a conventional-structured module, in which the upper and lower surfaces of the legs were each fixed to a ceramic substrate. The other was a ‘half skeleton’ module, in which the ‘cold-side’ substrate was removed and a thermal-conductive sheet was used instead of a ceramic plate for the cold-side insulator. From the result of this evaluation, it was confirmed that, although some variation in the output power was observed for the ‘half-skeleton’ module, the power variation was markedly less than for the conventional-structured module. Additionally, to improve the output power of the module, we replaced the Al2O3 substrate with Si3N4, which has a higher thermal conductivity than the Al2O3 substrate. The observed output power of a module (25 mm × 24 mm × 8.3 mm) fabricated using the Si3N4 substrate was 1,293 mW at ΔT = 500 K. The output value of the module using the Si3N4 plate was improved by 29 % compared with the output value of the module using the Al2O3 substrate. Moreover, based on the structures of these modules, a 36 mm × 41 mm × 8.3 mm module was fabricated. The expected value of the output power of the module was 1.9 W at ΔT = 500 K.

Development of an Mg2Si Unileg Thermoelectric Module Using Durable Sb-Doped Mg2Si Legs

Mg2Si unileg structure thermoelectric (TE) modules, which are composed only of n-type Mg2Si legs, were fabricated using Sb-doped Mg2Si. The Mg2Si TE legs used in our module were fabricated by a plasma-activated sintering method using material produced from molten commercial doped polycrystalline Mg2Si, and, at the same time, nickel electrodes were formed on the Mg2Si using a monobloc plasma-activated sintering technique. The source material used for our legs has a ZT value of 0.77 at 862 K. The TE modules, which have dimensions of 21 mm × 30 mm × 16 mm, were composed of ten legs that were connected in series electrically using nickel terminals, and the dimensions of a single leg were 4.0 mm × 4.0 mm × 10 mm. From evaluations of the measured output characteristics of the modules, it appeared that the electrical resistance of the wiring that is used to connect each leg considerably affects the power output of the unileg module. Thus, we attempted to reduce the wiring resistance of the module and fabricated a module using copper terminals. The observed values of the open-circuit voltage and output power of the Sb-doped Mg2Si unileg module were 496 mV and 1211 mW at ΔT = 531 K (hot side: 873 K; cool side: 342 K).

The Influence of Grain Boundary Scattering on Thermoelectric Properties of Mg2Si and Mg2Si0.8Sn0.2

The temperature dependences of the Seebeck coefficient, and electrical and thermal conductivities of bulk hot-pressed Sb-doped n-type Mg2Si and Mg2Si0.8Sn0.2 samples were measured in the temperature range from 300 K to 850 K together with the Hall coefficients at room temperature. The features of the complex band structure and scattering mechanisms were analyzed based on experimental data within the relaxation-time approximation. Based on the obtained model parameters, the possibility of improvement of the thermoelectric figure of merit due to nanostructuring and grain boundary scattering was theoretically analyzed for both Mg2Si and the solid solution.

Fabrication and thermoelectric properties of Mg2Si-based composites using reduction reaction with additives

Mg2Si-based composites were successfully synthesized by a reduction reaction of Mg, Si, and a small amount of metal oxide, carbonate, or hydroxide additive (Al2O3, Bi2O3, Sb2O3, La(OH)3, Li2CO3, Ag2O, CuO, Ga2O3, In2O3, Na2CO3, or Y2O3). The reaction was carried out under a flowing Ar–5%H2 atmosphere at 1073 K and the resulting composites were densified under vacuum at 993−1053 K using spark plasma sintering (SPS). The thermoelectric properties of the dense composites were characterized using Hall effect measurements at room temperature, and by measurements of electrical resistivity (ρ), the Seebeck coefficient (S), and thermal conductivity (κ) at 300–900 K. XRD measurements of the composites revealed that the major phase was Mg2Si, and the minor phases were MgO, Si, intermetallics, and metal, obtained by the reduction of additives, and unreacted oxides. Doping of Al, Bi, Sb, La, Li, Ag, In, and Y into Mg2Si was successfully achieved. The microstructure and thermoelectric properties of the composites were drastically influenced by the additive used. The maximum values of dimensionless thermoelectric figure of merit (ZT) for the Mg2Si composites using Al2O3, Bi2O3, and Sb2O3 were found to be 0.58, 0.68, and 0.63 at ∼865 K, respectively, which are comparable to the previously reported values of Al-, Bi-, and Sb-doped Mg2Si.

Fabrication of large sintered pellets of Sb-doped n-type Mg2Si using a plasma activated sintering method

Sb is known to be the most stable n-type dopant in Mg2Si, whereas the reproducibility and sintering scalability of Sb-doped Mg2Si has proved to be difficult. We have developed a metallic binder for the Sb-doped Mg2Si sintering process. A remarkable benefit of the binder is that it enables us to reproducibly fabricate large sintered Sb-doped Mg2Si pellets up to 30mm in diameter with no internal cracks. For binder contents of 3–7wt%, the observed power factor and thermal conductivity were identical to that of conventional Sb-doped samples, while excess binder degraded the TE properties. The binder mixture also plays a part in increasing the yield in the production of TE legs, up to 82% for 5wt% Ni incorporation, as well as improving the hardness of the sintered sample. Incorporation of 5wt% Ni binder affected an increase in the ZT value up to 0.97.