Fabrication of Mg（OH）2 Powders by Decomposition of Mg3N2 Powders

Mg（OH）2 powders were formed by the decomposition of Mg3N2 powders synthesized by a simple reaction of Mg with N Ha. X-ray diffraction（XRD）, Fourier transform infrared spectroscopy （FTIR）, and scanning electron microscopy（SEM） were used to study the structure, composition and morphology of the products. Mg （OH）2 nanowires with an average diameter about 300 nm-500 nm were found in these Mg（OH）2 powders.     

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

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

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

The temperature dependences of the Seebeck coefficient, and electrical and thermal conductivities of bulk hot-pressed Sb-doped n-type Mg₂Si and 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.     

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.     

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.     

Mg膜厚度对Mg2Si半导体薄膜结构及方块电阻的影响

采用射频磁控溅射系统和热处理系统制备了Mg2Si半导体薄膜.首先在Si衬底上溅射不同厚度的Mg膜,然后在真空退火炉中进行低真空热处理4h制备一系列Mg2Si半导体薄膜.采用台阶仪、X射线衍射仪(XRD)、扫描电子显微镜(SEM)对Mg2Si薄膜样品的结构、表面形貌、剖面进行表征,研究了Mg膜厚度对Mg2Si半导体薄膜生成的影响.结果表明,在Si衬底上制备出以Mg2Si (220)为主的单一相Mg2Si薄膜,且Mg2Si (220)的衍射峰强度随着Mg膜厚度的增加先增大后减小,Mg膜为2.52 μm时,制备的Mg2Si薄膜表现出了良好的结晶度和平整度.最后,研究了Mg膜厚度对Mg2Si薄膜方块电阻的影响.     

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.     

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.     

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.     

Al掺杂半导体Mg2Si薄膜的制备及光学带隙研究

用磁控溅射方法在Si衬底上制备了Al掺杂Mg2Si薄膜,通过X射线衍射仪(XRD)、扫描电镜(SEM)、原子力显微镜(AFM)和分光光度计研究了掺杂含量对Mg2Si薄膜组分、表面形貌、粗糙度及光学带隙值的影响,XRD结果表明随着Al掺杂量的增加, Mg2Si衍射峰先增强后减弱. SEM及AFM的结果表明随掺杂量的增加,结晶度先增加后降低,晶粒尺寸减小,粗糙度先增加后降低.得到掺杂后薄膜间接跃迁带隙范围为0.423～0.495 eV,直接跃迁带隙范围为0.72～0.748 eV,掺杂前薄膜间接跃迁带隙和直接跃迁带隙分别为0.53 eV、0.833 eV.     

Thermoelectric Performance of Sb- and La-Doped Mg2Si0.5Ge0.5

La _x Mg_2?x Si_0.49Ge_0.5Sb_0.01 compounds (x?=?0, 0.005, 0.01, 0.02) were synthesized by solid-state reaction followed by spark plasma sintering. The thermoelectric properties, such as the Seebeck coefficient, the electrical and thermal conductivities, and ZT, of these compounds have been studied in the temperature range of 300?K to 823?K. The figure of merit of this n-type compound has been raised above unity at 823?K for the sample with x?=?0.01, a value 60% higher than that of Mg₂Si_0.49Ge_0.5Sb_0.01. The reduction of the thermal conductivity via increasing phonon scattering is considered as the main reason for the enhanced ZT. These observations demonstrate an opportunity to improve the thermoelectric performance of Mg₂Si_1?x Ge _x solid solutions.     

OMVPE growth of P-type GaN using solution Cp2Mg

Bis(cyclopentadienyl)magnesium (Cp₂Mg) is a common source for p-type doping in GaN and AlInGaP materials. It is a white crystalline solid with very low vapor pressure, leading to transport problems similar to solid trimethyindium (TMI). Some of these problems can be alleviated by a newly developed source-solution magnesocene, Cp₂Mg, dissolved in a solvent that is essentially nonvolatile. In this paper, we report the growth and comparative results of Mg-doped GaN grown by OMVPE using solid and solution Cp₂Mg. Using both sources, we optimized parameters to obtain high-quality GaN growth with hole concentrations up to 1 10¹⁸/cm³.     

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.     

Power Generation Characteristics of Mg2Si Uni-Leg Thermoelectric Generator

Mg₂Si thermoelectric (TE) elements were fabricated by a plasma-activated sintering method using a commercial polycrystalline n-type Mg₂Si source produced by the Union Material Co., Ltd. This material typically has a ZT value of ??0.6. A monobloc plasma-activated sintering technique was used to form Ni electrodes on the TE elements. The dimensions of a single element were 4.0?mm?×?4.0?mm?×?10?mm, and these were used to construct a TE module comprising nine elements connected in series. To reduce the electrical and thermal contact resistance of the module, each part of the module, i.e., the elements, terminals, and insulating plates, was joined using a Ag-based brazing alloy. In addition, to maintain the temperature difference between the top and bottom of the module, a thermal insulation board was installed in it. The observed values of open-circuit voltage (V _OC) and output power (P) of a uni-leg structure module were 594?mV and 543?mW, respectively, at a maximum ??T?=?500?K.     

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.