Vacancy and Temperature Effects on Mechanical Properties of Single-Crystal Bulk Mg2Si: A Molecular Dynamics Study

The mechanical properties of single-crystal bulk Mg₂Si have been investigated using the molecular dynamics simulation method, in which a many-body potential energy function including bond and angle interactions is adopted to predict the mechanical properties. Virtual tension tests of specimens under different conditions, including Mg vacancy and temperature effects, were carried out by controlling the strain along the principal crystallographic direction. The simulation results show that single-crystal bulk Mg₂Si exhibits a nonlinear elastic stress–strain response and the mechanical properties degrade significantly with increasing vacancy content. Moreover, the effect of temperature on the mechanical properties of single-crystal bulk Mg₂Si is also discussed in detail.     

Molecular Dynamics Simulation on Mechanics of Mg2Si Nanofilm

Molecular dynamics simulation has been carried out to study the mechanical properties of Mg₂Si nanofilm. For the binary thermoelectric material Mg₂Si with antifluorite crystal structure, a modified Morse potential energy function in which the bond-angle deformation has been taken into account is developed and employed to describe the atomic interactions to shed light on its mechanical properties. In the simulation, the radial distribution function of Mg₂Si nanofilm is computed to validate its crystal structure, and the stress–strain responses of the nanofilm are examined at room temperature. It is found that the mechanical properties of Mg₂Si nanofilm are quite different from those of bulk Mg₂Si due to the impact of surface atoms of the nanostructures. The size effect and the temperature effect on the mechanical properties of Mg₂Si nanofilm are discussed in detail.     

Effect of Biaxial Strain on Electronic and Thermoelectric Properties of Mg2Si

The electronic and thermoelectric properties of biaxially strained magnesium silicide Mg₂Si are analyzed by means of first-principle calculations and semiclassical Boltzmann theory. Electron and hole doping are examined for different doping concentrations and temperatures. Under strain the degeneracy of the electronic orbitals near the band edges is removed, the orbital bands are warped, and the energy gap closes up. These characteristics are rationalized in the light of the electron density transfers upon strain. The electrical conductivity increases with the biaxial strain, whereas neither the Seebeck coefficient nor the power factor (PF) follow this trend. Detailed analysis of the evolution of these thermoelectric properties is given in terms of the in-plane and cross-plane components. Interestingly, the maximum value of the PF is shifted towards lower temperatures when increasingly intensive strain is applied.     

Effect of Synthesis and Sintering Conditions on the Thermoelectric Properties of n-Doped Mg2Si

Magnesium silicide (Mg₂Si)-based alloys are promising candidates for thermoelectric (TE) energy conversion in the middle–high temperature range. The detrimental effect of the presence of MgO on the TE properties of Mg₂Si based materials is widely known. For this reason, the conditions used for synthesis and sintering were optimized to limit oxygen contamination. The effect of Bi doping on the TE performance of dense Mg₂Si materials was also investigated. Synthesis was performed by ball milling in an inert atmosphere starting from commercial Mg₂Si powder and Bi powder. The samples were consolidated, by spark plasma sintering, to a density >95%. The morphology, and the composition and crystal structure of samples were characterized by field-emission scanning electronic microscopy and x-ray diffraction, respectively. Moreover, determination of Seebeck coefficients and measurement of electrical and thermal conductivity were performed for all the samples. Mg₂Si with 0.1 mol% Bi doping had a ZT value of 0.81, indicative of the potential of this method for fabrication of n-type bulk material with good TE performance.     

Synthesis and Characterization of Al-Doped Mg2Si Thermoelectric Materials

Magnesium silicide (Mg₂Si)-based alloys are promising candidates for thermoelectric (TE) energy conversion for the middle to high range of temperature. These materials are very attractive for TE research because of the abundance of their constituent elements in the Earth’s crust. Mg₂Si could replace lead-based TE materials, due to its low cost, nontoxicity, and low density. In this work, the role of aluminum doping (Mg₂Si:Al = 1:x for x = 0.005, 0.01, 0.02, and 0.04 molar ratio) in dense Mg₂Si materials was investigated. The synthesis process was performed by planetary milling under inert atmosphere starting from commercial Mg₂Si pieces and Al powder. After ball milling, the samples were sintered by means of spark plasma sintering to density >95%. The morphology, composition, and crystal structure of the samples were characterized by field-emission scanning electron microscopy, energy-dispersive spectroscopy, and x-ray diffraction analyses. Moreover, Seebeck coefficient analyses, as well as electrical and thermal conductivity measurements were performed for all samples up to 600°C. The resultant estimated ZT values are comparable to those reported in the literature for these materials. In particular, the maximum ZT achieved was 0.50 for the x = 0.01 Al-doped sample at 600°C.     

Modeling of Thermoelectric Properties of Magnesium Silicide (Mg2Si)

Thermoelectric (TE) materials based on alloys of magnesium (Mg) and silicon (Si) possess favorable properties such as high electrical conductivity and low thermal conductivity. Additionally, their abundance in nature and lack of toxicity make them even more attractive. To better understand the electronic transport and thermal characteristics of bulk magnesium silicide (Mg₂Si), we solve the multiband Boltzmann transport equation within the relaxation-time approximation to calculate the TE properties of n-type and p-type Mg₂Si. The dominant scattering mechanisms due to acoustic phonons and ionized impurities were accounted for in the calculations. The Debye model was used to calculate the lattice thermal conductivity. A unique set of semiempirical material parameters was obtained for both n-type and p-type materials through simulation testing. The model was optimized to fit different sets of experimental data from recently reported literature. The model shows consistent agreement with experimental characteristics for both n-type and p-type Mg₂Si versus temperature and doping concentration. A systematic study of the effect of dopant concentration on the electrical and thermal conductivity of Mg₂Si was also performed. The model predicts a maximum dimensionless figure of merit of about 0.8 when the doping concentration is increased to approximately 10²⁰?cm^?C3 for both n-type and p-type devices.     

First-Principles and Experimental Studies of Y-Doped Mg<Subscript>2</Subscript>Si Prepared Using Field-Activated Pressure-Assisted Synthesis

The thermoelectric properties of Y-doped (1000 ppm, 2000 ppm, 3000 ppm) Mg₂Si fabricated using field-activated pressure-assisted synthesis (FAPAS) have been characterized using measurements of electrical resistivity (ρ), Seebeck coefficient (S), and thermal conductivity (κ) at temperatures ranging from 285 K to 810 K. The Y-doped Mg₂Si samples were n-type in the measured temperature range. A first-principles calculation revealed that the Y atoms were expected to be primarily located at Mg sites. In sample doped with 2000 ppm Y, which exhibited the best electrical and thermal conductivity, the absolute value of the Seebeck coefficient increased in the temperature range of 320 K to 680 K, being higher than that of undoped Mg₂Si. Moreover, this sample exhibited a higher level of electrical conductivity and a higher power factor. In addition, introduction of Y decreased the thermal conductivity appreciably, indicating that Y dopants are favorable for improving the properties of Mg₂Si.     

Synchrotron Study of Ag-Doped Mg2Si: Correlation Between Properties and Structure

The crystal structure of Ag-doped Mg₂Si was investigated using synchrotron and neutron powder diffraction analysis, including in situ synchrotron x-ray powder diffraction patterns, recorded during a thermal cycle from room temperature up to 600°C. Rietveld refinement of diffraction patterns indicated that Ag doping results in partial substitution at Si sites. During heating, the Mg₂Si lattice parameters exhibited a shift in the temperature dependence at 300°C to 350°C, which was attributed to Ag precipitation out of Mg₂Si_1?x Ag _x solid solution. In turn, an increase of the Ag present in the Mg₂Si lattice after 350°C could be linked to thermally activated diffusion of Ag from β-AgMg phase. The Ag-dopant migration may explain previously outlined instabilities in the thermopower of Ag-doped Mg₂Si, e.g., the drop of the Seebeck coefficient value after heating to 150°C to 200°C and its subsequent increase after 350°C to 450°C.     

Transport and Mechanical Properties of High-ZT Mg2.08Si0.4−x Sn0.6Sb x Thermoelectric Materials

Mg₂(Si,Sn) compounds are promising candidate low-cost, lightweight, nontoxic thermoelectric materials made from abundant elements and are suited for power generation applications in the intermediate temperature range of 600 K to 800 K. Knowledge on the transport and mechanical properties of Mg₂(Si,Sn) compounds is essential to the design of Mg₂(Si,Sn)-based thermoelectric devices. In this work, such materials were synthesized using the molten-salt sealing method and were powder processed, followed by pulsed electric sintering densification. A set of Mg_2.08Si_0.4?x Sn_0.6Sb _x (0 ≤ x ≤ 0.072) compounds were investigated, and a peak ZT of 1.50 was obtained at 716 K in Mg_2.08Si_0.364Sn_0.6Sb_0.036. The high ZT is attributed to a high electrical conductivity in these samples, possibly caused by a magnesium deficiency in the final product. The mechanical response of the material to stresses is a function of the elastic moduli. The temperature-dependent Young’s modulus, shear modulus, bulk modulus, Poisson’s ratio, acoustic wave speeds, and acoustic Debye temperature of the undoped Mg₂(Si,Sn) compounds were measured using resonant ultrasound spectroscopy from 295 K to 603 K. In addition, the hardness and fracture toughness were measured at room temperature.     

The Use of Transition-Metal Silicides to Reduce the Contact Resistance Between the Electrode and Sintered n-Type Mg2Si

The electrical and thermoelectric characteristics of n-type Mg₂Si equipped with electrodes of Ni and the transition-metal silicides CoSi₂, CrSi₂, TiSi₂, and NiSi were examined. To form the electrodes on the Mg₂Si matrix, a monobloc sintering method, i.e., simultaneous sintering of the electrode material during Mg₂Si sintering, was used. To obtain dense electrodes and to keep an appropriately low sintering temperature for the Mg₂Si matrix, a Ni binder was used for the CoSi₂, CrSi₂, and TiSi₂ monobloc sintering. The mixture ratio between the transition-metal silicide and the Ni was 50:50 in wt.%. The room-temperature I?CV characteristics of the fabricated CoSi₂, CrSi₂, and TiSi₂ electrodes with the Ni binder and NiSi electrodes were considered to be adequate for practical applications in as much as ohmic contacts were obtained. The contact resistance at the Mg₂Si/electrode interface decreased by 35% and 28%, respectively, for the CoSi₂ and CrSi₂ electrodes compared with our standard Ni electrode. The thermoelectric power output was measured at the practical operating temperature of 600?K, with ??T?=?500?K. The observed output powers for 3.0?mm?×?3.0?mm?×?7.5?mm samples equipped with CoSi₂, CrSi₂, and NiSi electrodes were 153?mW, 149?mW, and 125?mW, respectively, representing increases of 27%, 24%, and 4%, respectively, compared with the 120?mW measured for the sample with Ni electrodes.     

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 Mg₂Si_0.8Sn_0.2 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.     

Solid-State Synthesis and Thermoelectric Properties of Al-Doped Mg2Si

The electronic transport and thermoelectric properties of Al-doped Mg₂Si (Mg₂Si:Al _m , m?=?0, 0.005, 0.01, 0.02, 0.03) compounds prepared by solid-state synthesis were examined. Mg₂Si was synthesized by solid-state reaction (SSR) at 773?K for 6?h, and Al-doped Mg₂Si powders were obtained by mechanical alloying (MA) for 24?h. Mg₂Si:Al _m were fully consolidated by hot pressing (HP) at 1073?K for 1?h, and all samples showed n-type conduction, indicating that the electrical conduction is due mainly to electrons. The electrical conductivity increased significantly with increasing Al doping content, and the absolute value of the Seebeck coefficient decreased due to the significant increase in electron concentration from 10¹⁶ cm^?3 to 10¹⁹ cm^?3 by Al doping. The thermal conductivity was increased slightly by Al doping, but was not changed significantly by the Al doping content due to the much larger contribution of lattice thermal conductivity over electronic thermal conductivity. Mg₂Si:Al_0.02 showed a maximum thermoelectric figure of merit of 0.47 at 823?K.     

Thermal evolution of the morphology, structure, and optical properties of multilayer nanoperiodic systems produced by the vacuum evaporation of SiO and SiO2

The alternate vacuum evaporation of SiO and SiO₂ from separate sources is used to produce amorphous a-SiO _x /SiO₂ multilayer nanoperiodic structures with periods of 5–10 nm and a number of layers of up to 64. The effect of annealing at temperatures T _a = 500–1100°C on the structural and optical properties of the nanostructures is studied. The results of transmission electron microscopy of the samples annealed at 1100°C indicate the annealing-induced formation of vertically ordered quasiperiodic arrays of Si nanocrystals, whose dimensions are comparable to the a-SiO _x -layer thickness in the initial nanostructures. The nanostructures annealed at 1100°C exhibit size-dependent photoluminescence in the wavelength range 750–830 nm corresponding to Si nanocrystals. The data on infrared absorption and Raman scattering show that the thermal evolution of structural and phase state of the SiO _x layers with increasing annealing temperature proceeds through the formation of amorphous Si nanoinclusions with the subsequent formation and growth of Si nanocrystals.     

Electrical Properties of (110)-Oriented Nondoped Mg2Si Films with p-Type Conduction Prepared by RF Magnetron Sputtering Method

Magnesium silicide (Mg₂Si) thick films with (110) orientation were fabricated on (001) sapphire substrate using radiofrequency magnetron sputtering. Stoichiometric Mg₂Si films with composition Si/(Mg + Si) = 0.33 were achieved over a range of vacuum from 10 mTorr to 140 mTorr and 300°C. On postannealing the film at 500°C, the out-of-plane lattice parameter shifted to lower values and the electrical conductivity increased by two orders of magnitude. A room-temperature Seebeck coefficient of 517 μV K^?1 was observed and found to decrease with increasing temperature; the Seebeck coefficient remained at a constant positive value of 212 μV K^?1 at 500°C. This can be related to the possibility of p-type conduction in this temperature range.     

Rational Manipulation of Epitaxial Strains Enabled Valence Band Convergence and High Thermoelectric Performances in Mg3Sb2 Films

Strain engineering is demonstrated to effectively regulate the functionality of materials, such as thermoelectric, ferroelectric, and photovoltaic properties. As the straightforward approach of strain engineering, epitaxial strain is usually proposed for rationally manipulating the electronic structure and performances of thermoelectric materials, but has rarely been verified experimentally. In this study, tunable and large epitaxial strains are demonstrated, as well as the resulting valence band convergence can be achieved in the Mg₃Sb₂ epi-films with the choice of substrates. The large epitaxial strains up to 8% in Mg₃Sb₂ films represent one of the most striking results in strain engineering. The angle-resolved photoemission spectroscopy measurements and the theoretical calculations reveal the vital role of epitaxial strain in tuning the crystal field splitting and the band structure of Mg₃Sb₂. Benefiting from the appropriate manipulation of the crystal field effect via in-plane compressive strain, the valence band convergence is unambiguously discovered in the strained Mg₃Sb₂ film grown on InP(111) substrate. As a result, a state-of-the-art thermoelectric power factor of 0.94 mWm⁻¹K⁻² is achieved in the strain-engineered Mg₃Sb₂ film, well exceeding that of the strain-relaxed Mg₃Sb₂. The work paves the way for effectively manipulating epitaxial strain and band convergence for Mg₃Sb₂ and other thermoelectric films.     

Improved thermal stability of Ni-silicides on Si:C epitaxial layers

The thermal stability of Ni-silicides on tensily strained in situ P doped Si:C epitaxial layers was evaluated. The baseline Ni silicidation process was shown to be compatible with Si:C Recessed Source-Drain (RSD) stressors for NMOS strain engineering while the thermal stability of NiSi:C contacts was significantly improved compared to NiSi ones. Dominant degradation mechanism was shown to be the transition to the NiSi₂:C phase. It was demonstrated that the Si:C strain level affects the silicide formation but has no significant effect on the NiSi:C thermal stability. A mechanism responsible for the improved thermal stability of NiSi:C silicides is discussed.     

Thermoelectric Properties of Magnesium Silicide Deposited by Use of an Atmospheric Plasma Thermal Spray

The thermoelectric properties of magnesium silicide (Mg₂Si) samples prepared by use of an atmospheric plasma spray (APS) were compared with those of samples prepared from the same feedstock powder by use of the conventional hot-pressing method. The characterization performed included measurement of thermal conductivity, electrical conductivity, Seebeck coefficient, and figure of merit, ZT. X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive x-ray spectroscopy (EDX) were used to assess how phase and microstructure affected the thermoelectric properties of the samples. Hall effect measurements furnished carrier concentration, and measurement of Hall mobility provided further insight into electrical conductivity and Seebeck coefficient. Low temperature and high velocity APS using an internal-powder distribution system achieved a phase of composition similar to that of the feedstock powder. Thermal spraying was demonstrated in this work to be an effective means of reducing the thermal conductivity of Mg₂Si; this may be because of pores and cracks in the sprayed sample. Vacuum-annealed APS samples were found to have very high Seebeck coefficients. To further improve the figure of merit, carrier concentration must be adjusted and carrier mobility must be enhanced.     

Stress Analysis and Output Power Measurement of an n-Mg2Si Thermoelectric Power Generator with an Unconventional Structure

We examine the mechanical stability of an unconventional Mg₂Si thermoelectric generator (TEG) structure. In this structure, the angle θ between the thermoelectric (TE) chips and the heat sink is less than 90°. We examined the tolerance to an external force of various Mg₂Si TEG structures using a finite-element method (FEM) with the ANSYS code. The output power of the TEGs was also measured. First, for the FEM analysis, the mechanical properties of sintered Mg₂Si TE chips, such as the bending strength and Young’s modulus, were measured. Then, two-dimensional (2D) TEG models with various values of θ (90°, 75°, 60°, 45°, 30°, 15°, and 0°) were constructed in ANSYS. The x and y axes were defined as being in the horizontal and vertical directions of the substrate, respectively. In the analysis, the maximum tensile stress in the chip when a constant load was applied to the TEG model in the x direction was determined. Based on the analytical results, an appropriate structure was selected and a module fabricated. For the TEG fabrication, eight TE chips, each with dimensions of 3 mm × 3 mm × 10 mm and consisting of Sb-doped n-Mg₂Si prepared by a plasma-activated sintering process, were assembled such that two chips were connected in parallel, and four pairs of these were connected in series on a footprint of 46 mm × 12 mm. The measured power generation characteristics and temperature distribution with temperature differences between 873 K and 373 K are discussed.     

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.