Mg2Si Powder Research Articles

Effects of Mg2Si Addition on the Pitting Corrosion Resistance of AA7075 Prepared by Spark Plasma Sintering

AA7075 containing Mg2Si was fabricated by Spark plasma sintering (SPS). Gas-atomized AA7075 powders and Mg2Si powders were mixed and sintering was performed at 773 K for 15 min. After sintering, hot-forging was conducted at 773 K, and heat-treatment was conducted at 688 K for 2 h. The specimens were polished with a diamond paste down to 0.25 µm.The surface of specimens was observed using an optical microscope and a field emission scanning electron microscope (FE-SEM) equipped with energy dispersive X-ray spectroscopy system (EDS). Electrochemical measurements were performed in 0.1 M NaCl at pH 6.0 adjusted with NaOH. With the exception of the electrode area, the surfaces of specimens were coated with an epoxy resin. The size of the exposed electrode area was ca. 5 mm × 5 mm. Dip and dry tests were conducted to evaluate the corrosion resistance of the specimens. The specimens were dipped in 0.1 M NaCl (pH 6.0) for 20 min and dried at 298 K and 50%RH for 60 min. In addition, the effect of Mg2Si on pH of 0.1 M NaCl was investigated. The variation of pH due to addition of Mg2Si to 1 L of 0.1 M NaCl at pH 6.0 was measured.The open circuit potential of Mg2Si-containing AA7075 decreased significantly after immersion compared to AA7075. In 20 cycles of dip and dry, mass loss was reduced by Mg2Si addition. Oxide films containing Al were formed on the Mg2Si-containing AA7075; however, Mg was not detected from the films. When Mg2Si was added to 1 L of 0.1 M NaCl, the pH increased significantly. The increase in pH on the specimen surface is thought to contribute to improvement of the pitting corrosion resistance.

Formation of porous Mg2(SiSn) by nanoparticle alloying and its thermoelectric properties

Thermoelectric materials and devices are regarded as promising alternatives for addressing global warming challenges. This study demonstrates the fabrication and thermoelectric properties of porous Mg2(Si0.4Sn0.6). The composition inhomogeneity enhances the Seebeck coefficient, and the porous structure results in low thermal conductivity. Mg2Si and Mg2Sn powders are synthesized and subsequently used to create the Mg2(Si0.4Sn0.6) alloy. The experimental findings reveal that via nano-engineering of earth-abundant elements, an increase in the Seebeck coefficient and a decrease in thermal conductivity can be achieved. This approach illustrates a feasible alternative to the traditional hot-pressing and spark plasma sintering processes for synthesizing porous Mg2(Si0.4Sn0.6). Furthermore, this study demonstrates that porous Mg2(Si0.4Sn0.6) has high performance/cost value and is a promising candidate for environmentally friendly waste-heat energy conversion.

Studying the process of modification of lithium aluminum alloys

The effect of modification with dispersed compositions on the grain structure and mechanical properties of industrial aluminum alloys has been studied. Aluminum alloys of the Al-Si, Al-Mg-Sc, Al-Cu-Mn systems were modified with dispersed Mg2Si powder with a particle size of up to 200 nm. The amount of modifier to be added to the melt is calculated. The physicochemical properties of dispersed Mg2Si have been studied. Melting of the AMg6, 1570, 2219, AK9ch alloys in the initial state and with the treatment of Mg2Si melts have been carried out. The action of insoluble applications, isomorphic to aluminum, the similarity of the influence of soluble elements holds only when the amount of insoluble addition exceeds the number of crystals formed arbitrarily under the same conditions. Thus, with an increase in the amount of insoluble addition, in particular silicon carbide particles, the grain size first decreases and then remains constant. The mechanism of the influence of dispersed particles of magnesium silicide on the formation of the structure of hypoeutectic aluminum alloys during crystallization is that their bulk is pushed out by the crystallization front into the liquid phase and participates in the refinement of the structural components of the alloy. To determine the optimal amount of silicon carbide modifier, industrial melting and testing were performed on specimens that underwent heat treatment according to the T6 mode (quenching and artificial aging). The quality of cast aluminum alloys during modification depends on many factors: the nature of the dispersed phase, the temperature of the melt, and the modes of its mixing with the introduction of particles. Dependences of the particle size and the amount of the modifier on the mechanical properties of the alloys have been established. The mechanism of interaction of the modifier with aluminum melt during crystallization has been established. In industrial experiments, the most effective size of SiC particles for increasing the σm of the AK9ch alloy from 115 to 260 MPa in the as-cast state has been established. The optimal content of Mg2Si (0.10 %) for increasing the σm of aluminum alloys has been determined.

Modeling the selective laser melting-based additive manufacturing of thermoelectric powders

Thermoelectric generators directly convert thermal energy to electricity when exposed to the proper temperature difference. The efficiency of the thermoelectric module has not been fully explored due to the drawbacks in conventional design and fabrication methods. Selective laser melting (SLM) additive manufacturing offers a unique potential scalable approach for the fabrication of flexible and functionally graded thermoelectric materials with high energy conversion efficiency. In this paper, we developed a physical model to simulate the SLM manufacturing process of thermoelectric materials (Mg2Si powders) with additive material (Si) mixed for better thermoelectric performance. A comprehensive thermal and fluid study of the SLM manufacturing was conducted to understand the phenomena associated with the melting and solidification processes in the melting pool. This physical model was established based on the conservation equations and provided a basis to study the fluid flow driven by the buoyancy force and surface tension in the melting pool. Using this model, the influences of the process parameters, such as laser scanning speed and power energy density, on the temperature distribution, powder bed shrinkage, pool size, and particle aggregation in the powder bed, were studied, which provided critical information for understanding SLM for thermoelectric device fabrication.

Optimization of Degreasing-Sintering Process for Mg2Si/PLA Mixture and Influences of Additive Amount of Al on Sintered Density and Thermoelectric Performance of Mg2Si Fabricated by the Optimized Process

ABSTRACTFused deposition modelling (FDM) type of 3D printing is widely used for manufacturing complex shaped polymer products. Recently, the metal/polymer composite products can be made by 3D printer using metal/polymer composite filament. Now, we are planning to develop a new manufacturing process of the thermoelectric (TE) elements or modules by combining the FDM-type 3D printing and the degreasing-sintering process. In this work, we focused on the degreasing-sintering process of the mixture of Mg2Si and polylactic acid (PLA) powders. Mg2Si compound powder was synthesized by a liquid-solid phase reaction (LSPR) method. The powder mixtures of Mg2Si, Al and PLA were pressed and heated in a pulse discharge sintering (PDS) chamber under a vacuum in various degreasing conditions. Following the degreasing, the sintering of Mg2Si was carried out in the same PDS chamber at various starting sintering temperatures. Sintered density, Seebeck coefficient and electrical resistivity of the consolidated Mg2Si were measured and the power factor as a TE performance was estimated from the TE properties. The optimum conditions of degreasing-sintering process maximizing the sintered density and the TE performance of Al-doped Mg2Si were investigated. Furthermore, the influences of the additive amount of Al on the sintered density and the TE performance of Mg2Si fabricated via the optimized degreasing-sintering process were investigated.

Synthesis of thermoelectric magnesium-silicide pastes for 3D printing, electrospinning and low-pressure spray

In this work, eco-friendly magnesium-silicide (Mg2Si) semiconducting (n-type) thermoelectric pastes for building components concerning energy-harvesting devices through 3D printing, spray and electrospinning were synthetized and tested for the first time. The Mg2Si fine powders were obtained through the combination of ball milling and thermal annealing under Ar atmosphere. While the latter process was crucial for obtaining the desired Mg2Si phase, the ball milling was indispensable for homogenizing and reducing the grain size of the powders. The synthetized Mg2Si powders exhibited a large Seebeck coefficient of ~ 487 µV/K and were blended with a polymeric solution in different mass ratios to adjust the paste viscosity to the different requirements of 3D printing, electrospinning and low-pressure spray. The materials produced in every single stage of the paste synthesis were characterized by a variety of techniques that unequivocally prove their viability for producing thermoelectric parts and components. These can certainly trigger further research and development in green thermoelectric generators (TEGs) capable of adopting any form or shape with enhanced thermoelectric properties. These green TEGs are meant to compete with common toxic materials such as Bi2Te3, PbTe and CoSb that have Seebeck coefficients in the range of ~ 290–700 μV/K, similar to that of the produced Mg2Si powders and lower than that of 3D printed bulk Mg2Si pieces, measured to be ~ 4866 μV/K. Also, their measured thermal conductivities proved to be significantly lower (~ 0.2 W/mK) than that reported for Mg2Si (≥ 4 W/mK). However, it is herein demonstrated that such thermoelectric properties are not stable over time. Pressureless sintering proved to be indispensable, but difficultly achievable by long thermal annealing (even above 32 h) in inert atmosphere at 400 °C, at least for bulk Mg2Si pieces constituted by a mean grain size of 2–3 μm. Hence, for overcoming this sintering challenge and become the silicide’s extrusion viable in the production of bulk thermoelectric parts, alternative pressureless sintering methods will have to be further explored.

Understanding the effect of the reinforcement addition on corrosion behavior of Fe/Mg2Si composites for biodegradable implant applications

Abstract Iron-based materials showed a high potential for degradable biomaterial applications. However, their slow corrosion rate limits their use as a biodegradable implant material. One approach to control and modify their mechanical properties is the use of reinforcement particles, to create metal matrix composites in which the second phase is aimed at tuning not only the mechanical properties but also the corrosion mode and rate in a physiological environment. Here, Fe/Mg2Si composites were produced via powder metallurgy from pure Fe and Mg2Si powders. The different conditions were fabricated by various combinations of milling or mixing processes followed by hot rolling consolidation. The effect of the Mg2Si addition on corrosion behavior of Fe/Mg2Si composites was studied by performing cyclic polarization and static corrosion tests for an immersion time of 24 and 240 h. The presence of the reinforcement particles played a crucial role in the susceptibility of Fe-based composites to localized corrosion attack. The corrosion initiation and its development were systematically monitored. Scanning electron microscopy, X-ray diffraction, and atomic emission spectroscopy were employed to investigate the corrosion mechanism. The importance of Mg2Si particles in the triggering of corrosion processes was explained. Electrochemical measurements and static immersion tests implied that the introduction of Mg2Si particles could accelerate the corrosion rate of Fe. It was confirmed that the size and distribution of the reinforcement influenced considerably the uniformity of the corrosion attack.

Morphological and Structural Modifications of Si-Based Nanostructures Synthesized from Metal Silicide Templates in IP6, Acid and Metal Chloride Solutions

Powders consisting of Si-based nanostructures were synthesized by the extraction of Ca atoms from CaSi2 powders using an inositol hexakisphosphate (IP6) aqueous solution. The raw CaSi2 powders were simply immersed in a diluted IP6 solution, then dried. It is noted that the Si-based nanostructures were easily exfoliated from the powders to expose the surfaces corresponding to the Si {111} planes of the nanostructures. In addition, the Si-based nanostructures were also synthesized by metal atom extraction from SrSi2, BaSi2 and Mg2Si using the IP6 aqueous solution. It was found that the nanostructures mainly including the amorphous Si-oxide phase were obtained for the IP6-treated SrSi2, BaSi2 and Mg2Si powders. Moreover, the amorphous Si oxide-based nanostructures were synthesized from CaSi2 using citric acid, malic acid, FeCl3 and FeCl2 aqueous solutions. It was demonstrated that the morphological and structural properties of the synthesized Si-based nanostructures depend on the silicide templates and the solutions.

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.

In-Situ Synthesis of Carbon-Coated Porous Silicon for Li-Ion Batteries Via Inorganic CO2 Reduction Route with Magnesium Silicide

Carbon coating step in silicon (Si) anode research has been considered as a necessity in order to enhance cycling performance by increasing electronic conductivity and bolstering stable Solid-Electrolyte Interphase (SEI) layer between Si host and liquid electrolytes1-5. Many strategies have been employed to deposit carbon layer on the surface of Si, including chemical vapor deposition (CVD), sol-gel/carbonization method, etc. Although those processes can offer uniform coating on diverse types of Si nanostructures (nanowires, nanotubes, and nanoparticles) with improved battery performance, they require costly procedures; sophisticated equipment and, oftentimes, toxic precursors, that make the overall fabrication process unsuitable to scale up and advance towards commercialization. In this work, we demonstrate a novel strategy that can abridge the additional carbon coating step and yet simultaneously obtain carbon coating and porous structure on Si host via simple/one-pot thermal treatment where CO2 is reduced down to carbon by inorganic magnesium silicide (Mg2Si). Based on the Ellingham diagram which describes oxidative nature of pure metallic elements, Mg has a strong tendency of reducing other oxygen-containing molecules, such as CO2 or silica (SiO2). The Gibbs free energy of the CO2 reduction reaction with Mg2Si is estimated to be -519 kJ/mol at 700 oC, indicating a thermodynamically feasible process. During the thermal treatment, CO2 gas and Mg2Si powder undergo spontaneous redox reaction and result in composite of MgO, Si, and C, confirmed by in-situ high temperature X-ray diffraction analysis. In this redox reaction, element Mg act as a reductant that takes O atoms away from CO2 molecules, leaving elemental carbon layer between MgO and Si. After the MgO removal (sacrificing template for porous structure) via acid leaching, microstructural characterization reveals porous structures in Si host and uniform carbon layers (thickness: 20 ~ 40 nm) on the Si surface. Most importantly, obtaining micron-sized Si with macro-porous structures, the tap density of the material is 3~4 times higher than commercially available nano Si powder, which is greatly beneficial to develop Li-ion batteries with high volumetric densities. Cycling performance tests with CR2032-type coin cell (vs. Li/Li+) also show high initial coulombic efficiency (85~88%), good rate capability (>1000 mAh/g at 1.0 C-rate), and excellent long-term cycling performance (~1300 mAh/g after 300 cycles at 0.5 C-rate & ~1000 mAh/g after 500 cycles at 1.0 C-rate). 1 Lu, Z. et al. Nonfilling Carbon Coating of Porous Silicon Micrometer-Sized Particles for High-Performance Lithium Battery Anodes. ACS Nano 9, 2540-2547, doi:10.1021/nn505410q (2015). 2 de Guzman, R. C., Yang, J., Cheng, M. M.-C., Salley, S. O. & Simon Ng, K. Y. Effects of graphene and carbon coating modifications on electrochemical performance of silicon nanoparticle/graphene composite anode. Journal of Power Sources 246, 335-345, doi:http://dx.doi.org/10.1016/j.jpowsour.2013.07.100 (2014). 3 Shen, L., Wang, Z. & Chen, L. Carbon-coated hierarchically porous silicon as anode material for lithium ion batteries. RSC Advances 4, 15314-15318, doi:10.1039/C4RA01255K (2014). 4 Yi, R., Dai, F., Gordin, M. L., Sohn, H. & Wang, D. Influence of Silicon Nanoscale Building Blocks Size and Carbon Coating on the Performance of Micro-Sized Si–C Composite Li-Ion Anodes. Advanced Energy Materials 3, 1507-1515, doi:10.1002/aenm.201300496 (2013). 5 Gao, P., Tang, H., Xing, A. & Bao, Z. Porous silicon from the magnesiothermic reaction as a high-performance anode material for lithium ion battery applications. Electrochimica Acta 228, 545-552, doi:http://dx.doi.org/10.1016/j.electacta.2017.01.119 (2017).

Examination of oxidation resistance of Mg2Si thermoelectric modules at practical operating temperature

Magnesium silicide (Mg2Si) is as known a thermoelectric material with low environmental load and high power factor around 400 °C. It should be performed to solve the problems. i.e., its low oxidation resistance and high chemical activity. In the current decades, oxidation resistance of Mg2Si has been researched because it is essential to thermoelectric power generation. The oxidation factor of Mg2Si still remains unexplained. In this paper, forming process of oxidation resistant layer of Mg2Si powder is examined by thermal analysis and X-ray diffraction. It was revealed that Mg2Si induced to compound with elements of the atmosphere. The oxidation process of Mg2Si was related to humidity without oxide, since oxidation layer absorbs humidity and forms a magnesium hydroxide (Mg(OH)2) layer. The Mg(OH)2 layer was decomposed at 400 °C, which is the temperature required for practical use of thermoelectric generation. It is found that a humidity resistant layer should be formed on the surface of Mg2Si.

Energetic consideration of compounds at Mg2Si–Ni electrode interlayer produced by spark-plasma sintering

In order to understand the presence of the MgNi2-type phase, the η-phase, and the ω-phase at the boundary layer produced by the simultaneous spark plasma sintering of mixed Ni and Mg2Si powders, we have calculated the electronic energies of these phases. The phase change of MgNi2XSi2(1−X) from the Cu2Mg-type to the MgNi2-type through the Fe2Tb-type structures with increases in X, as observed by Song and Varin [Metall. Mater. Trans. A 32, 5 (2001)], is well-reproduced by the present calculations. The stability of the η-phase (Mg6Ni16Si7) is also explained, but that of the ω-phase is not explained by the energetic calculations, even if the entropy effect of mixing is assumed.

Mg2si Anode with Greatly Enhanced Cycle Life

Lithium-ion batteries (LIBs) with silicon anode have a potential application in electric vehicles and energy storage systems owing to the high capacity of silicon, however, the volume change and thus-resulted fast capacity decay or electrode failure greatly restricts its practical use. Using Si-M composite instead of pure Si has been proved to be an efficient strategy to minimize the capacity fade. Among these Si composites, magnesium silicide (Mg2Si) attracts much attention due to the natural abundance of Mg and cost concern. More importantly, Mg and Si can accommodate Li respectively which brings a higher specific capacity than other Si-M composite. Despite the above advantages, similar to pure Si anode, Mg2Si anode also suffers from the large volume change during the cycling. In this work, we propose using fluoroethylene carbonate (FEC) or vinylene carbonate (VC) additive to enhance the cycling life of Mg2Si anode and greatly improved cycling stability has been achieved on Mg2Si anode. Experiment Mg2Si (Adamas Beta, 98%) and acetylene black (ACB) powder were vacuum dried at 80°C for 12h and then ball milled for 3h to get Mg2Si/ACB mixture (mass ratio of 5:3). The electrode contained 80% above mixture and 20% PVDF binder. 1M LiPF6 in EC and DMC mixture solvent (volume ratio of 1:1) was used as the electrolyte. The cyclic performance was evaluated on LAND CT2001A system between 0.01 and 2.0V with a current of 100mA/g. The impedance spectra were collected on Autolab electrochemical working station. The surface information of the cycled electrode was obtained through SEM observation and XPS measurement. Results and discussions Figure 1 shows the cycling performances of Mg2Si in bare electrolyte or the electrolyte with VC or FEC addition. Rapid capacity fading was found on Mg2Si in bare electrolyte, such as lower than 300 mAh/g after several cycles. In comparison, in FEC-added electrolyte, Mg2Si still released 400 mAh/g capacity after 100 cycles. In VC-added electrolyte, similar improvement was also observed, but not as notable as in FEC-added electrolyte. SEM observation supplied the SEI information of the Mg2Si electrodes cycled in electrolyte with or without additives. In bare electrolyte, the thinnest and most porous SEI layer was found on Mg2Si. While in FEC or VC-added electrolyte, the SEI layer was much thicker and denser, and VC addition led to the most compact SEI layer on Mg2Si anode. EIS impedance further confirmed this difference by the biggest diameter shown by Mg2Si anode in VC-added electrolyte. To understand the reason of the different cycling behavior of Mg2Si, XPS analysis was conducted. Given the fixed amount of LiPF6 in all tested electrolytes, the F-P bond could be used as a reference, and the amount of Li-F could be determined in a semi-quantitative model. The results revealed the highest content of LiF in the SEI formed on Mg2Si in FEC-added electrolyte and the lowest content of LiF in the SEI formed in bare electrolyte. Combined with previous research and some additional experiments, we think FEC could promote the LiF accumulation in the outer layer rather than the inner layer of SEI and reduce the LiF content in the bulk Mg2Si anode, and this eventually brings some protection effect on Mg2Si. Conclusions FEC addition in electrolyte greatly enhanced the cycling stability of Mg2Si, and around 400mAh/g capacity could be obtained after 100cycles. Detailed analysis revealed the protection effect of FEC on Mg2Si anode. Figure 1

Thermoelectric Hexagonal A-Mg-Si with A = Sr and Ba Zintl Phases

Hexagonal A-Mg-Si with A = Sr and Ba Zintl phases are promising candidates for p-type magnesium silicides usable with n-Mg2Si under 900 K. We synthesized p-type A-Mg-Si Zintl phases by the spark plasma synthesis procedure. Mg2Si and Mg2A powders were mixed at the ratio of 1−x/x with x = 0.3–0.4. Two-step synthesis was performed at 850 K for 20 min and 1100 K for 20 min under uniaxial pressure at 30 MPa. Sintered pellets exhibited a stable p-type thermoelectric property. These pellets consisted of several unknown phases. We found two semiconductor phases, namely A2Mg4Si3 and A2Mg12Si7. The crystal structures of the 2/4/3- and 2/12/7-phases were \( P\bar{6}2m\) (No. 189)- and \( P6_{3} /m\) (No. 176)-types, respectively. Sr0.70Mg2Si, Ba3Mg10Si7 and Sr3Mg10Si7 phases are newly found and characterized by a single crystal diffraction study. Previously found Sr2Mg4Si3 single phase polycrystalline 30φ × 10 mm pellets were successfully synthesized. The thermoelectric performance of the Sr2Mg4Si3 single phase sample was tested. The pellets exhibit p-type behavior from room temperature to 700 K. The thermal conductivity, κ, was almost constant at 1.1 W/mK from 350 K to 700 K.

Thermoelectric Properties of Al-doped Mg2Si Compounds Prepared via Three Kinds of Process for Grain Refinement

ABSTRACTDimagnesium silicide is an eco-friendly thermoelectric compound whose constituent elements of both Mg and Si are non-toxic and exist in abundance on the earth. In this study, we attempted to control the thermal conductivity of Al-doped Mg2Si by grain refinement. Three types of Si powders, i.e., commercial coarse powder, its pulverized powder and commercial fine powder were prepared for synthesizing the Al-doped Mg2Si. Mg powder and one of the Si powders were weighed with Mg/Si mole ratio of 67/33, and mixed with Al powder with amount of 0.1 at.%. The Al-doped Mg2Si compounds were synthesized using three different Si powders via a liquid-solid phase reaction process under unified synthesizing conditions. A part of the synthesized Mg2Si powder using the coarse Si powder was pulverized. Four kinds of Mg2Si powders were sintered by pulse discharge sintering method under unified sintering conditions. The sintered samples of the synthesized Mg2Si powders made from the fine and the milled Si powders and of the milled Mg2Si powder had the grain-refined microstructure. Especially, the sintered sample of the milled Mg2Si powder was effective for grain refinement and for reduction of thermal conductivity, and had the best thermoelectric performance of ZT = 1.15 at 873 K.

Si-particle-size dependence of Mg2Si powders directly produced by liquid–solid synthesis method

The synthesis and powder processing of compounds in an Mg2Si system are difficult to achieve. These processes require a multi-step procedure that involves melting, crushing, grinding, and sieving in order to obtain Mg2Si powders from the Mg and Si starting materials. This may lead to the oxidation of Mg and contamination of the obtained powders with impurities. In this study, we developed a direct technique for synthesizing Mg2Si particles using a liquid–solid reaction. Si powders of various particle sizes (20–38, 38–53, 53–75, 75–106, 106–150, and 150–250 μm) were employed in the synthesis. The resulting powders were identified as single phase of Mg2Si by both XRD and EDX analyses. The optimal heat treatment temperature was between the two eutectic temperatures, 911 K and 1218 K. After heat treatment at 1143 K for 4 h, Si powders with particle sizes of less than 53 μm did not yield Mg2Si powders. In contrast, after heat treatment at 973 K for 10 h, all the Si powders produced Mg2Si powders. The Mg2Si powder particles were larger than the starting Si particles; for example Si particles with sizes of 53–75 μm produced Mg2Si powders with sizes ranging from 75 to 106 μm. The yields of Mg2Si powder reached approximately 90%.

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.

Enhancement of thermoelectric properties of Mg2Si compounds with Bi doping through carrier concentration tuning

The Bi-doped Mg2Si powder was fabricated with solid state reaction method and consolidated with hot pressing method and then its thermoelectric properties were investigated. The n-type transport properties were measured in all samples and temperature dependence of the electrical properties shows a behavior of degenerate semiconductors for Bi-doped samples. The electrical resistivity and the Seebeck coefficient were greatly reduced with Bi, which was mainly due to the increment of the carrier concentration. The samples have maximum carrier concentration of 8.2 × 1018 cm−3. The largest ZT value of 0.61 was achieve at 873 K for Mg2.04SiBi0.02. The Bi-doping was found to be an effective n-type dopant to adjust carrier concentration. Open image in new window

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.

Analysis of the Microstructure of Mg2Si Thermoelectric Devices

High-performance Mg2Si thermoelectric devices have been obtained by spark plasma sintering of high-purity, pre-synthesized, all-molten Mg2Si powder. We studied the effects of source powder particle size on thermoelectric performance. To improve the performance, further investigation of the microstructure of the devices is needed. In this work we studied the microstructure of grain boundaries and interfaces between electrodes and Mg2Si sintered bodies to increase understanding of Mg2Si thermoelectric devices.