Mg2 Si Research Articles

Low Electron Scattering Potentials in High Performance Mg2Si0.45Sn0.55 Based Thermoelectric Solid Solutions with Band Convergence

AbstractUnderstanding the electron and phonon transport characteristics is crucial for designing and developing high performance thermoelectric materials. Weak scattering effects on charge carriers, characterized by deformation potential and alloy scattering potential, are favorable for thermoelectric solid solutions to enable high carrier mobility and thereby promising thermoelectric performance. Mg2(Si,Sn) solid solutions have attracted much attention due to their low cost and environmental compatibility. Usually, their high thermoelectric performance with ZT ∼ 1 is ascribed to the band convergence and reduced lattice thermal conductivity caused by alloying. In this work, both a low deformation potential Ξ = 13 eV and a low alloy scattering potential U = 0.7 eV are found for the thermoelectric alloys by characterizing and modeling of thermoelectric transport properties. The band convergence is also verified by the increased density‐of‐states effective mass. It is proposed that, in addition to band convergence and reduced lattice thermal conductivity, the low deformation potential and alloy scattering potential are additional intrinsic features that contribute to the high thermoelectric performance of the solid solutions.

Study on microstructure and properties of Mg2(Si,Sn) compound phase in Mg-Sn-Si magnesium alloy

Microstructure, characteristic and metamorphism of Mg2(Si,Sn) compound phase are studied in as-cast Mg-Sn-Si alloy. The results indicate that Sn atom can replace the partial Si of Mg2Si then form Mg2(Si,Sn) compound phase, and structure of the ternary compound is the same as Mg2Si and Mg2Sn, all belong to face-centered cubic. The element content of Mg2(Si,Sn) phase is not invariable, the Mg2(Si,Sn) phase with high Si content is in Si enrichment region, while it with low Si content is in Si poverty region. The property of Mg2(Si,Sn) phase with high Si content is close to Mg2Si phase, the property of the phase with high Sn content is close to Mg2Sn phase. In experiments, the nano-hardness, elastic modulus, vickers hardness and other physical properties of Mg2(Si,Sn) phase are between Mg2Si phase and Mg2Sn phase, in the metamorphism process of Chinese-script-like Mg2Si phase, Mg2(Si,Sn) compound phase plays a role as a bridge.

Enhanced hole concentration through Ga doping and excess of Mg and thermoelectric properties of p-type Mg2(1+z)(Si0.3Sn0.7)1−yGay

The influence of the doping amount of Ga as well as the excess of Mg on the phase composition and thermoelectric properties of Mg2(1+z)(Si0.3Sn0.7)1−yGay compounds are analyzed in detail. Regarding the content of Mg, a second phase of Mg is detected at grain boundaries whenever its over-stoichiometry exceeds 7%. On the other hand, XRD and EPMA analysis indicate that phase separation occurs when Mg is more than 3.5% deficient with respect to its stoichiometric amount. Thermoelectric property measurements reveal that doping with Ga, along with some over-stoichiometry of Mg, enhances the concentration of holes and electrical conductivity of Mg2(1+z)(Si0.3Sn0.7)1−yGay while it simultaneously reduces the Seebeck coefficient. However, there is little effect on the lattice thermal conductivity. The results also show that, in p-type Mg2Si0.3Sn0.7 based compounds, antisite point defects MgSi form when the content of Mg is over-stoichiometric. This leads to an enhanced concentration of holes. Mg2.10(Si0.3Sn0.7)0.95Ga0.05, having the optimized content of Ga and Mg, possesses the highest ZT value of 0.35 that is achieved at 650 K. This research reveals that both the doping of Ga and the excess of Mg do not have significant influence on the band structure of Mg2Si0.3Sn0.7, and the transport properties of p-type Mg2Si0.3Sn0.7 in the high hole concentration range can be well described by a simple, parabolic band model. The research work also establishes an important basis for further optimization of the figure of merit of p-type Mg2Si1−xSnx solid solutions by making use of over-stoichiometric amounts of Mg.

Preparation of rare-earth element doped Mg2Si by FAPAS

Rare-earth elements (Re) Sc and Y doped Mg2Si thermoelectric materials were made via a field-activated and pressure-assisted synthesis (FAPAS) method at 1023–1073 K, 50 MPa for 15 min. The samples created using this method have uniform and compact structures. The average grain size was about 1.5–2 μm, the micro-content of Re did not change the matrix morphology. The sample with 2500 ppm Sc obtained the best See-beck coefficient absolute value, about 1.93 times of that belonging to non-doped Mg2 Si at about 408 K. The electric conductivity of the sample doped with 2000 ppm Y becomes 1.69 times of that of pure Mg2Si at 468 K, while the former had a better comprehensive electrical performance. Their thermal conductivity was reduced to 70% and 84% of that of non-doped Mg2Si. Thus, the figure of merit and ZT of these two samples were enhanced visibly, which were 3.3 and 2.4 times of the non-doped samples at 408 K and 468 K, respectively. The maximal ZT belonging to samples doped with 2500 ppm Sc went up to 0.42 at about 498 K, higher than 0.40 of sample doped with 2000 ppm Y at 528 K and 0.25 of non-doped Mg2Si at 678 K, and the samples doped with Sc seemed to get the best thermoelectric performances at lower temperature.

Improving p-type thermoelectric performance of Mg2(Ge,Sn) compounds via solid solution and Ag doping

Mg2X (X = Si, Ge, and Sn) compounds have attracted increasing attention owing to their promising thermoelectric properties and “green” constituent elements. While most studies to date have been on n-type Mg2(Si,Sn) solid solutions, we herein report Ag-doping study in Mg2(Ge,Sn) solid solutions to pursue higher p-type performance. Two series of samples, Mg2Ge1−xSnx and Mg2−yAgyGe0.4Sn0.6, were prepared by melting in evacuated Ta tubes, followed by hot pressing, and the thermoelectric properties of the solid solutions were investigated from room temperature to 723 K. An n- to p-type crossover in the Seebeck coefficient was observed for Mg2Ge0.4Sn0.6 and Mg2Ge0.2Sn0.8 at elevated temperatures. To enhance p-type performance, we doped Ag in Mg2Ge0.4Sn0.6 that exhibited lowest thermal conductivity among the Mg2(Ge,Sn) solid solutions. It was found that (1) p-type behavior has been established in Mg2−yAgyGe0.4Sn0.6 (y = 0.005, 0.01, 0.02, 0.04) samples in the entire temperature range studied; and (2) the electrical conductivity increased with increasing Ag content until reaching the solution limit ∼0.02. As a result, a dimensionless figure of merit ZT ∼0.38 is attained for Mg1.98Ge0.4Sn0.6Ag0.02 at 675 K, which is one of the highest reported values in p-type Mg2X compounds.

Impurity doping into Mg2Sn: A first-principles study

We studied the formation energy and atomic structure of impurities in Mg2Sn using first-principles plane-wave total energy calculations. Twenty elements, namely H, Li, Na, K, Rb, Sc, Y, La, Cu, Ag, Au, B, Al, Ga, In, N, P, As, Sb, and Bi, were selected as the impurity species. We considered structural relaxation of the atoms within the second nearest neighbors of the impurity atom in the 48-atom supercell. The results of the formation energy calculations suggested that Sc, Y, La, P, As, Sb, and Bi are good n-type dopants whereas Li and Na are good p-type dopants. The electrical properties of Li-, Na-, and Ga-doped Mg2Sn and La-doped Mg2(Si, Sn) composites reported previously can be explained by the low formation energies of Li, Na, Ga, and La in Mg2Sn.

Roles of interstitial Mg in improving thermoelectric properties of Sb-doped Mg2Si0.4Sn0.6 solid solutions

While adding extra Mg is intended to compensate for the Mg loss during the synthesis, it often leads to Mg interstitials in Mg2(Si,Sn) materials and profoundly affects their thermoelectric properties. Herein we studied the electrical conductivity, Seebeck coefficient, and thermal conductivity of Mg2(1 + x)Si0.38Sn0.6Sb0.02 (0.05 ≤ x ≤ 0.12) as a function of Mg excess between 300 K and 730 K. The presence of interstitial Mg was corroborated by X-ray powder diffraction, X-ray photoelectron spectroscopy, Hall coefficient measurement, and compositional analysis. The electrical properties have been analyzed in the framework of a single parabolic band model to gain more insight on the roles of Mg interstitials. We found that increasing Mg excess content increased the carrier concentration, electronic effective mass, and electrical conductivity, while it decreased the Seebeck coefficient and led to a non-monotonic change in the lattice thermal conductivity. As a result, a maximum ZT ∼ 0.85 was attained at 700 K for Mg excess x = 0.1, a ∼60% enhancement compared to that of the sample x = 0.05. The Mg interstitials thus provide an extra tuning parameter in optimizing the thermoelectric properties of Mg2(Si,Sn)-based materials.

Effect of Sb Doping on the Thermoelectric Properties of Mg2Si0.7Sn0.3 Solid Solutions

This study focuses on Sb-doped Mg2(Si,Sn) thermoelectric material. Samples were successfully fabricated using a hybrid synthesis method consisting of three different processes: induction melting, solid-state reaction, and a hot-press sintering technique. We found that the carrier concentration increased with Sb content, while the Seebeck coefficient exhibited a decreasing trend. Sb doping was shown to improve the power factor and thermoelectric figure of merit compared with the undoped material, yielding a peak figure of merit (ZT) of ~0.55 at 620 K, while leaving the band gap of Mg2Si0.7Sn0.3 almost unchanged.

First principles study of structural, electronic and elastic properties of Mg2 Si polymorphs

The structural and the elastic properties of the Mg2Si polymorphs are calculated. The calculations are performed by using the plane-wave pseudo-potential method within the framework of first principles. The anti-fluorite structure, the anti-cotunnite structure and the Ni2In-type structure of Mg2Si can retain their mechanical stability in the pressure intervals 07 GPa,7.520.2 GPa and 21.940 GPa, separately. The relationships between pressure and the elastic moduli (elastic constant, bulk modulus, shear modulus, Youngs modulus, Poisson ratio and anisotropy factor) are discussed. The electron density distribution, the density of states, the bond length and the Mulliken population of these polymorphs are systemically investigated. Our results show that the anti-fluorite Mg2Si is a semiconductor and the other two polymorphs are metallic materials. The interaction between Mg 2p, 3s and Si 3p plays a dominant role in the stability of the Mg2Si polymorphs. The strongest interactions in the anti-fluorite Mg2Si and the Ni2In-type Mg2Si are Mg-Mg and Mg-Si interactions, respectively. Our results are concordant with the experimental data and the previous results.

Bulk Nanostructured Thermoelectric Materials: Preparation, Structure and Properties

Bulk nanostructured materials have recently emerged as a new paradigm for improving the performance of existing thermoelectric materials. Here, we fabricated two kinds of bulk nanostructured thermoelectric materials by a bottom-up strategy and an in situ precipitation method, respectively. Binary PbTe was fabricated by a combination of chemical synthesis and hot pressing. The grain sizes of the hot pressed bulk samples varied from 200 nm to 400 nm, which significantly contributed to the reduction of thermal conductivity due to the enhanced boundary phonon scattering. The highest figure of merit ZT of the binary PbTe sample reached 0.8 at 580 K. Mg2(Si,Sn) solid solutions have shown great promise for thermoelectric application, due to good thermoelectric properties, non-toxicity, and abundantly available constituent elements. The nanoscale microstructure observation of the compounds showed the existence of nanophases formed in situ, which is believed to be related to the relatively low lattice thermal conductivity in this material system. The highest ZT of Sb-doped Mg2(Si,Sn) samples reached 1.1 at 770 K.

Plasma characterization for nitridation of aluminium alloy using 50 Hz ac discharge

A 50 Hz ac abnormal glow discharge is characterized as a function of filling pressure (1–4 mbar) using a Langmuir probe. It is found that electron temperature, electron density and electron energy distribution function decrease with the gas filling pressure. Nitriding of aluminum alloy (Al–Mg2–Si) is performed for various time durations (3, 6, 9 and 12 h) and pressures (1 and 2 mbar) by keeping the input power (100 W) and substrate temperature (250 °C) constant. To explore the surface properties induced by plasma, X-ray diffractometer, optical micrography, atomic force microscopy and Vickers micro-hardness tester diagnostic techniques are used. Results exhibit the formation of cubic aluminum nitride. A significant increase in surface hardness is observed when samples are exposed at 1 mbar pressure for 12 h of processing time. A rapid decrease in surface hardness values with the imposed load suggests the formation of a thin modified surface layer.

High figures of merit and natural nanostructures in Mg2Si0.4Sn0.6 based thermoelectric materials

Mg 2 ( Si , Sn ) compounds have shown great promise for thermoelectric applications due to good thermoelectric properties, nontoxicity, and abundantly available constituent elements. Herein we report on the thermoelectric properties and microstructure of high performance Mg2Si0.4−xSn0.6Sbx (0⩽x⩽0.015) alloys. The state-of-the-art ZT value of ∼1.1 has been attained in the samples with x=0.0075 due to the relatively low thermal conductivity. In light of the simple cubic structure and mostly light constituent elements, the reduction in lattice thermal conductivity has been discussed in connection with a fairly large amount of in situ formed nanostructures in these samples.

In situ synthesis and thermoelectric properties of La-doped Mg2(Si, Sn) composites

Utilizing the peritectic reaction and the miscibility gap featured in the pseudo-binary phase diagram of Mg2Si and Mg2Sn, we fabricated environmentally friendly Mg2(Si, Sn) thermoelectric (TE) composites in which the Mg2Si-rich bulk grains were in situ coated by Mg2Sn-rich thin layers. The Mg2Sn-rich grain boundary phase was selectively doped with La. It was found that the La doping dramatically increased the electrical conductivity to thermal conductivity ratio in the composites. As a result, a dimensionless figure of merit ZT ∼ 0.81 has been attained at 810 K for the Mg2−xLax(Si, Sn) in situ composite with x = 0.005, significantly higher than the ZT ∼0.18 at 540 K for the undoped composite and comparable to ZT ∼ 0.8 of state-of-the-art PbTe intermediate temperature TE alloys.

Development of short alumina fibre and in situ Mg2Si particle reinforced magnesium alloy hybrid composite with high temperature strength

Short alumina fibre and in situ Mg2Si particle reinforced hybrid composites were fabricated by infiltration with the molten magnesium alloy into the preforms consisting of the fibres having Si particles attached to their surfaces. P or CaF2 particles were used as the refiners of the Mg2Si particles. All of Si particles reacted with the Mg alloy and melt to form Mg2Si particles. As the melting temperature decreased or the cooling rate after the infiltration increased, the Mg2Si particles became finer. The introduction of P or CaF2 further promoted the refinement of the Mg2Si particles. When the composite was fabricated by squeeze casting in the permanent mould, fine Mg2 Si particles with a grain size of ∼5 μm were formed regardless of introducing the refiners due to the rapid solidification. The strength of the hybrid composites was higher than that of the conventional fibre reinforced composite at both room temperature and high temperature.

DSC and HRTEM investigation of the precipitates in Al–1.0%Mg2 Si–0.5%Ag alloy

The understanding and control of nanoscale precipitation in an Al–1.0 wt-%Mg2 Si–0.5 wt-% Ag alloy during artificial aging is critical for achieving optimum mechanical properties. To achieve this objective, both differential scanning calorimetry (DSC) and high resolution transmission electron microscopy (HRTEM) have been utilised. The non-isothermal DSC thermograms exhibited eight reaction peaks; six are exothermic (precipitation) and two are endothermic (dissolution) reactions. The activation energies associated with the individual precipitates are determined. With the aid of HRTEM, the evolved precipitates have been characterised.

Solid state reaction synthesis and thermoelectric properties of Mg2Si doped with Sb and Te

Doped with Sb and Te, Mg2 Si based compounds were prepared respectively by solid state reaction at 823K for 8h. Effects of dopants of Sb and Te on the structure and thermoelectric properties of the compounds were investigated. By calculating the values of the electrical conductivity for Sb-doped sample, the mechanism of electric conduction at 625 K is different. The figure of merit for sample doped with 0.4wt% Te at 500K is 2.4×10−3W/mK2, and it reaches 3.3×10−3 W/mK2 at 650K for the sample doped with 0.5wt% Sb. The values are more than 1.4 times and 2.3 times of the pure Mg2Si sample.

Contribution of the silicon carbide particles to the process of forming the structure of magnesium cast composites

The work presents results of investigations and microstructural analyses for Mg- SiCp cast composites. The metallographic examination shows that the distribution of SiC particles is uniform. Quantitative image analysis of the microstructure shows the magnesium grain size to be smaller and more uniform than in the base metal. The studies at the level of the interfaces suggest that the bonding between the matrix and the reinforcement is of the adhesive type. The presence of the Mg2 Si compound in the composites has also been revealed. Additionally, some compounds seem to be present at the SiC/matrix interface. The performed analyses have allowed to propose a model description of the process of the formation of the Mg-SiC composite structure. Mechanical testing has shown that composites have increased YoungOs modulus and proof stress (R) and decreased impact strength.

Microstructure and tensile properties of graded Al–Mg2 Si in situ composites fabricated by centrifugal casting

Al–Mg2 Si in situ composite tubes were fabricated by a centrifugal casting process. The microstructure and tensile strength of the composite tubes were examined revealing an inhomogeneous distribution of Mg2 Si particles along the radial direction. Adjacent to the rapidly cooled area, near the outer periphery, in situ particles were forced inwards leaving behind a particle free region in the middle part of the tubes. Increasing Mg2 Si weight percentage in the aluminium matrix induced dendrite formation in the rapidly cooled area and the width of the particle free region in the middle part of the cylinder wall decreased. The particle distribution gradient in the inner region also decreased with increasing Mg2 Si content. The width of the particle free region and the particle distribution gradient were both influenced by the gradually varying viscosity of the liquid alloy as it solidified. The tensile strength of the centrifugally cast tubes increased from inner to outer periphery. High volume fractions of Mg2 Si particles of between 50 and 70 vol.-% reduced the strength of the composites.

Effect of Si content on surface quality of extruded AI–Mg2 Si system alloys

The effect of Si content on surface quality of extruded Al-Mg2Si system alloys has been investigated in terms of surface roughness, specular glossiness, and lightness difference of the as extruded and surface treated products. The mechanism forming surface roughness has been discussed on the basis of results obtained from a new extrusion technique using a split die. The bearing surfaces were polished to a mirrorlike finish so that the effect of chemical composition of the billet on the behaviour of the bearing surface coating might be clearly observed. A mixture of bright and non-bright surfaces was observed in the lower Si content alloys containing 0-23-0-43 wt-%Si. Only the non-bright surface was observed in higher Si content alloys containing 0-53-0-73 wt-%Si. An acceptable uniform surface appearance and roughness across the extrusion could be obtained by Si additions greater than about 0-5 wt-%. A high quality surface with uniform surface roughness could be obtained on extrusion when the continuous coating film was formed uniformly on the bearing surface during the extrusion of a billet with the higher excess Si content and the β-AlFeSi compound.

Effect of Si content on surface quality of extruded AI–Mg2 Si system alloys

Effect of Si content on surface quality of extruded AI–Mg2 Si system alloys