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.     

Effect of geometric dimensions on thermoelectric and mechanical performance for Mg2Si-based thermoelectric unicouple

The generating efficiency of thermoelectric generation (TEG) depends not only on the thermoelectric (TE) performance of TE device, but also on its mechanical performance. And choosing suitable TE materials and geometric dimension can improve the working performance of TE device. Mg₂Si is one of the most promising TE materials in the medium temperature range, and Mg₂Si-based TE devices have broad application prospects. In this paper, a three-dimensional finite model of the Mg₂Si-based TE unicouple used for recovering vehicle exhaust waste heat is constructed for the performance analysis. The TE performance and mechanical performance of the Mg₂Si-based TE unicouple under the influence of different geometric dimensions are investigated, respectively. The curves of the output power, the power conversion efficiency and the thermal stress distribution varying with different geometric dimensions are discussed in detail. The calculated result would be helpful for further understanding of the TE and mechanical properties of the Mg₂Si-based TE unicouple, and it can also provide guidance for further strength check and optimum geometric design of TE unicouples in general.     

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.     

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.     

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.     

Room-Temperature Mechanical Properties and Slow Crack Growth Behavior of Mg2Si Thermoelectric Materials

Mg₂Si is of interest as a thermoelectric (TE) material in part due to its low materials cost, lack of toxic components, and low mass density. However, harvesting of waste heat subjects TE materials to a range of mechanical and thermal stresses. To understand and model the material??s response to such stresses, the mechanical properties of the TE material must be known. The Mg₂Si specimens included in this study were powder processed and then sintered via pulsed electrical current sintering. The elastic moduli (Young??s modulus, shear modulus, and Poisson??s ratio) were measured using resonant ultrasound spectroscopy, while the hardness and fracture toughness were examined using Vickers indentation. Also, the Vickers indentation crack lengths were measured as a function of time in room air to determine the susceptibility of Mg₂Si to slow crack growth.     

Thermoelectric Behavior of Sb- and Al-Doped <Emphasis Type="Italic">n</Emphasis>-Type Mg<Subscript>2</Subscript>Si Device Under Large Temperature Differences

The thermoelectric (TE) characteristics of Sb- and Al-doped n-type Mg₂Si elemental devices fabricated using material produced from molten commercial doped polycrystalline Mg₂Si were examined. The TE devices were prepared using a plasma-activated sintering (PAS) technique. To complete the devices, Ni electrodes were fabricated on each end of them during the sintering process. To realize durable devices for large temperature differences, thermodynamically stable Sb-doped Mg₂Si (Sb-Mg₂Si) was exposed to the higher temperature and Al-doped Mg₂Si (Al-Mg₂Si) was exposed to the cooler temperature. The devices consisted of segments of Sb-Mg₂Si and Al-Mg₂Si with sizes in the following ratios: Sb-Mg₂Si:Al-Mg₂Si = 4:1, 1:1, and 1:4. A device specimen composed solely of Sb-Mg₂Si showed no notable deterioration even after aging for 1000 h, while some segmented specimens, such as those with Sb-Mg₂Si:Al-Mg₂Si = 1:1 and 1:4, suffered from a considerable drop in output current over the large ΔT range. The observed power generated by specimens with Sb-Mg₂Si:Al-Mg₂Si = 1:1 and 1:4 and sizes of 2 mm × 2 mm × 10 mm were 50.7 mW and 49.5 mW, respectively, with higher and lower temperatures of 873 K and 373 K, respectively. For the sample composed solely of Sb-Mg₂Si, a power of 55 mW was demonstrated. An aging test for up to 1000 h for the same ΔT range indicated drops in output power of between ∼3% and 20%.     

Modeling a Thermoelectric Generator Applied to Diesel Automotive Heat Recovery

Thermoelectric generators (TEGs) are outstanding devices for automotive waste heat recovery. Their packaging, lack of moving parts, and direct heat to electrical conversion are the main benefits. Usually, TEGs are modeled with a constant hot-source temperature. However, energy in exhaust gases is limited, thus leading to a temperature decrease as heat is recovered. Therefore thermoelectric properties change along the TEG, affecting performance. A thermoelectric generator composed of Mg₂Si/Zn₄Sb₃ for high temperatures followed by Bi₂Te₃ for low temperatures has been modeled using engineering equation solver (EES) software. The model uses the finite-difference method with a strip-fins convective heat transfer coefficient. It has been validated on a commercial module with well-known properties. The thermoelectric connection and the number of thermoelements have been addressed as well as the optimum proportion of high-temperature material for a given thermoelectric heat exchanger. TEG output power has been estimated for a typical commercial vehicle at 90°C coolant temperature.     

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.     

Enhanced Thermoelectric Performance of p-Type Mg3Sb2 for Reliable and Low-Cost all-Mg3Sb2-Based Thermoelectric Low-Grade Heat Recovery

Bi₂Te₃-based devices have long dominated the commercial market for thermoelectric cooling applications, but their narrow operating temperature range and high cost have limited their possible applications for conversion of low-grade heat into electric power. The recently developed n-type Mg₃Sb₂-based compounds exhibit excellent transport properties across a wide temperature range, have low material costs, and are nontoxic, so it would be possible to substitute the conventional Bi₂Te₃ module with a reliable and low-cost all-Mg₃Sb₂-based thermoelectric device if a good p-type Mg₃Sb₂ material can be obtained to match its n-type counterpart. In this study, by comprehensively regulating the carrier concentration, carrier mobility, and lattice thermal conductivity, the thermoelectric performance of p-type Mg₃Sb₂ is significantly improved through Na and Yb doping in Mg_1.8Zn_1.2Sb₂. Moreover, p- and n-type Mg₃Sb₂ are similar in terms of their coefficients of thermal expansion and their good performance stability, thus allowing the construction of a reliable all-Mg₃Sb₂-based unicouple. The decent conversion efficiency (≈5.5% at the hot-side temperature of 573 K), good performance stability, and low cost of this unicouple effectively promote the practical application of Mg₃Sb₂-based thermoelectric generators for low-grade heat recovery.     

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.     

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.     

Thermoelectric Properties of Hot-Pressed Materials Based on Mg2Si n Sn1−n

Mg₂Si _n Sn_1?n solid solutions consist of nontoxic widespread elements. In this work a number of samples of Mg₂Si _n Sn_1?n solid solutions, where 1 ≥ n ≥ 0.7 with various carrier concentrations, were obtained using microcrystalline powder by hot pressing in vacuum. The Seebeck coefficient and the thermal and electrical conductivity were measured in the temperature range from 300 K to 800 K. It is shown that the specific thermoelectric figure of merit (the ratio of the thermoelectric figure of merit to the material density) of these samples weakly depends on the composition of the solid solution. Hence, whether a solid solution or pure Mg₂Si is used depends on the application temperature of the material.     

Solid-State Synthesis of Te-Doped Mg<Subscript>2</Subscript>Si

Te-doped Mg₂Si (Mg₂Si:Te _m , m = 0, 0.01, 0.02, 0.03, 0.05) alloys were synthesized by a solid-state reaction and mechanical alloying. The electronic transport properties (Hall coefficient, carrier concentration, and mobility) and thermoelectric properties (Seebeck coefficient, electrical conductivity, thermal conductivity, and figure of merit) were examined. Mg₂Si was synthesized successfully by a solid-state reaction at 673 K for 6 h, and Te-doped Mg₂Si powders were obtained by mechanical alloying for 24 h. The alloys were fully consolidated by hot-pressing at 1073 K for 1 h. All the Mg₂Si:Te _m samples showed n-type conduction, indicating that the electrical conduction is due mainly to electrons. The electrical conductivity increased and the absolute value of the Seebeck coefficient decreased with increasing Te content, because Te doping increased the electron concentration considerably from 10¹⁶ cm⁻³ to 10¹⁸ cm⁻³. The thermal conductivity did not change significantly on Te doping, due to the much larger contribution of lattice thermal conductivity over the electronic thermal conductivity. Thermal conduction in Te-doped Mg₂Si was due primarily to lattice vibrations (phonons). The thermoelectric figure of merit of intrinsic Mg₂Si was improved by Te doping.     

Double-Doping Approach to Enhancing the Thermoelectric Figure-of-Merit of n-Type Mg2Si Synthesized by Use of Spark Plasma Sintering

We report significant enhancement of the thermoelectric figure-of-merit of Mg₂Si by double-doping with a combination of Bi, Pb, and Sb as doping elements. Addition of any two of these three elements to Mg₂Si increases the electrical conductivity by more than three orders of magnitude at 323 K, irrespective of the doping elements used. However, a corresponding decrease in the Seebeck coefficient is observed in comparison with undoped Mg₂Si. Irrespective of the combination of the three elements used for double doping, a figure-of-merit of approximately 0.7 at 873 K is obtained for Mg₂Si; this is primarily because of enhancement of the electrical conductivity.     

High-Performance p-Type Magnesium Silicon Thermoelectrics

Prospective p-type magnesium-silicon compounds have been produced from mixed and sintered Mg₂Si, Mg₂Sr or Mg₂Ba powders. The synthesis was carried out through the spark plasma sintering (SPS) process at 1123 K for 10 min at 50 MPa. p-Type thermoelectric performance was observed for samples prepared from sintered powder at nominal compositions (Mg₂Si)_0.7(Mg₂Sr)_0.3 and (Mg₂Si)_0.7(Mg₂Ba)_0.3. The maximum dimensionless figure of merit, ZT, of the former sample reached 0.24 at 700 K. The crystal structures and chemical contents were carefully elucidated by powder and single-crystal x-ray diffraction (XRD) measurements, scanning electron microscopy (SEM), and transmission electron microscopy (TEM) with the use of energy-dispersive x-ray spectroscopy (EDS). The SPS samples consisted of two new unknown phases. Based on the single-crystal XRD data, the crystal structures of two different A(Sr or Ba)-Mg-Si crystals are successfully determined. The two structures are hexagonal P6₃/m-type A₂Mg₁₂Si₇ and $ {{P}}\bar{6}2{{m}} $ P 6 ¯ 2 m (no. 189)-type A₂Mg₄Si₃. The two structures consist of three-dimensionally interconnecting MgSi₄ tetrahedron networks and alkaline-earth ion columns aligned parallel to the 4.3-Å axes. Both phases have identical [(A²⁺, Mg²⁺)]₂[Si^4?]-type chemical formulae, being identified as a kind of Zintl phase.     

Solid-State Synthesis and Thermoelectric Properties of Sb-Doped Mg2Si Materials

Sb-doped magnesium silicide compounds have been prepared through ball milling and solid-state reaction. Materials produced were near-stoichiometric. The structural modifications have been studied with powder x-ray diffraction. Highly dense pellets of Mg₂Si_1?x Sb _x (0 ≤ x ≤ 0.04) were fabricated via hot pressing and studied in terms of Seebeck coefficient, electrical and thermal conductivity, and free carrier concentration as a function of Sb concentration. Their thermoelectric performance in the high temperature range is presented, and the maximum value of the dimensionless figure of merit was found to be 0.46 at 810 K, for the Mg₂Si_0.915Sb_0.015 member.     

Eutectic Microstructure and Thermoelectric Properties of Mg2Sn

Ingots of undoped and Ag-doped Mg₂Sn were prepared from the melt using a rocking Bridgman furnace at different cooling rates: slow cooling (0.1 K/min), moderate cooling (1 K/min), and rapid quenching. The ingots show very different microstructure and thermoelectric properties. Slow-cooled ingots consist of large Mg₂Sn crystals with minor inclusions. Moderate-cooled ingots show significant variation in composition and microstructure, with Mg-rich material at the topmost section of the ingot and Sn-rich material at the bottom surface of the ingot. Rapid quenching results in ingots with finely dispersed Mg + Mg₂Sn eutectic microstructure in the form of lamellae 200 nm to 500 nm in thickness. Measurements of the Seebeck coefficient and electrical conductivity in the temperature range of T = 80 K to 700 K were carried out to establish correlations between the microstructure and the thermoelectric properties.     

Doping Effects on Thermoelectric Properties in the Mg2Sn System

A weak point of Mg₂X thermoelectrics is the absence of a p-type composition, which motivates research into the Mg₂Sn system. Mg₂Sn thermoelectrics were fabricated by a vacuum melting method and a spark plasma sintering process. As a result, Mg₂Sn single phases were acquired in a wide range of Mg-to-Sn atomic ratios (67:33 to 71:29), showing slightly different thermoelectric characteristics. However, the thermoelectric properties of the undoped system were not sufficient for application in commercial production. To maximize the p-type characteristics, many atoms [Ni (VIIIA), Cu (IB), Ag (IB), Zn (IIB), and In (IIIB)] were doped into the Mg₂Sn phase. Among them, the power factor values increased only in the Ag-doped case. Ag-doping resulted in a power factor that was more than 10 times larger than the value in the undoped case. This result could be important for developing p-type polycrystalline thermoelectrics in the Mg₂X (X?=?Si, Sn) system. However, other atoms [Ni (VIIIA), Cu (IB), Zn (IIB), and In (IIIB)] were not determined to act as acceptor atoms. The maximum ZT value for the Ag-doped Mg₂Sn thermoelectric was more than 0.18, which is comparable to the value for the n-type Mg₂Si system.     

Improvement of Electrical Contact Between TE Material and Ni Electrode Interfaces by Application of a Buffer Layer

A single ??-structure thermoelectric (TE) module based on p-type NaCo₂O₄, n-type Mg₂Si, and Ni electrode was fabricated by the spark plasma sintering (SPS) method. The NaCo₂O₄ powder was synthesized by using a metal?Ccitric acid complex decomposition method. Bulk Mg₂Si prepared by melt quenching was ground into a powder and sieved to particle size of 75???m or less. To obtain a sintered body of NaCo₂O₄ or Mg₂Si, the powder was sintered using SPS. Pressed Ni powder or mixed powder consisting of Ni and SrRuO₃ powder was inserted between these materials and the Ni electrode in order to connect them, and electrical power was passed through the electrodes from the SPS equipment. The open-circuit voltage (V _OC) values of a single module in which TE materials were connected to the Ni electrodes by using pressed Ni powder was 82.7?mV, and the maximum output current (I _max) and maximum output power (P _max) were 212.4?mA and 6.65?mW at ??T?=?470?K, respectively. On the other hand, V _OC of a single module in which TE materials and an Ni electrode were connected with a mixed powder (Ni:SrRuO₃?=?6:4 volume fraction) was 109?mV, and I _max and P _max were 4034?mA and 109?mW at ??T?=?500?K, respectively. These results indicate that the resistance at the interface between the TE materials and the Ni electrode can be decreased and the output power can be increased by application of a buffer layer consisting of Ni and SrRuO₃.