Papers by Keyword: Mg₂Si

Abstract: Mg2Si reinforced hypereutectic Al-Si composite was prepared and the effects of electro- magnetic stirring on Mg2Si particle in the composite were researched. The results indicated that there were the important effects of electromagnetic stirring on size and morphology of the particle in the composite. The morphology and size of the particle in the composite were obviously improved by electromagnetic stirring. In comparison with the sample without isothermal holding, the size of the particle isothermally held slightly increased under the same stirring condition, but the roundness of the particle was improved. After isothermal holding, the mechanical properties of the composite prepared by electromagnetic stirring were improved to some extent.

Abstract: Metal matrix composites comprising a magnesium matrix and Mg₂Si/MgO dispersoids obtained by hot pressing silica nanoparticle agglomerates and metal powder in a Degussa press were characterized. Two powder mixtures having weight proportions of Mg:SiO₂ of 10:0.3 and 10:1 were identically sintered. Their microstructures were characterized by optical microscopy and X-ray diffraction. The size and distribution of the Mg₂Si and MgO dispersoids formed in situ were assessed as a function of the original nanosilica content. The behaviour of the composites under compression testing was assessed in 3D by X-ray microtomography using 225kV Nikon X-tek and 150kV Xradia MicroXCT scanners. This provided insights into composite strengthening mechanisms and matrix particle decohesion.

Abstract: Group B^I(Cu, Ag)-, B^II(Zn)- and B^III(Al, In)-doped Mg₂Si compounds were synthesized by solid state reaction and mechanical alloying. Electronic transport properties (Hall coefficient, carrier concentration and mobility) and thermoelectric properties (Seebeck coefficient, electrical conductivity, power factor, thermal conductivity and figure of merit) were examined. Mg₂Si powder was synthesized successfully by solid state reaction at 773 K for 6 h and doped by mechanical alloying for 24 h. It was fully consolidated by hot pressing at 1073 K for 1 h. The electrical conductivity increased by doping due to an increase in the carrier concentration. However, the thermal conductivity did not changed significantly by doping, which was due to much larger contribution of the lattice thermal conductivity over the electronic thermal conductivity. Group B^III(Al, In) elements were more effective to enhance the thermoelectric properties of Mg₂Si.

Abstract: In order to reduce the oxidizing and volatilizing caused by Mg element in the traditional methods for synthesizing Mg₂Si compounds, solid state phase reaction at low temperature was introduced by microwave field. XRD was used to characterize the powders. At the same time, the influences of parameters during the synthesis processing were discussed. The results suggest that the heating profile is also dependent on the initial green density and higher green density provides lower heating rate while power setting are fixed and the oxidation of Mg can be rest rained by changing microwave heating programs. It was found that high purity Mg₂Si intermetallic compound can be obtained with excessive content of 8at% Mg from the stoichiometric Mg₂Si, 853K and 30min

Abstract: Mg₂Si reinforced hypereutectic Al-Si in-situ composite was prepared and the effects of the isothermal holding temperature and time on Mg₂Si in the composite were researched. The results showed that there were the important effects of holding temperature and time in the solid-liquid phase region of the Al-Si alloy on the size and morphology of Mg₂Si. The size and morphology of Mg₂Si became smaller and round with the rising of the holding temperature and prolonging of the holding time. The suitable technology in the test was obtained, in which when being held at 615°C for 300s, the size of Mg₂Si decreased to 20.54μm and the shape factor of Mg₂Si reached about 0.67.

Abstract: Epitaxial films of magnesium silicide Mg₂Si are prepared by magnetron sputtering system on Si (111) substrates. The crystal structures and the surface morphology of the Mg₂Si films are characterized by X-ray diffraction and Field emission scanning electron microscope. The microstructure of Mg₂Si films is obtained. The results show that the Mg₂Si films have a strong Mg₂Si (220) preferential orientation with sputtering power increases, the Mg₂Si (220) peak intensity increases with increasing sputtering power before 100W and then decreases with increasing sputtering power.

Abstract: Mg₂Si particle reinforced hypereutectic Al-Si composite was prepared by direct melt reaction, and the effects of Mg₂Si content on the microstructure of the composite were researched. The results indicated that the morphology of Mg₂Si obviously changed with Mg₂Si contents increasing, in which the morphology of Mg₂Si in the composite had changed from polygon block to characters like and finally became coarse dendrite. Formation of α-Al grains in the composites with the high Si and Mg content was discussed.

Abstract: NaCo₂O₄ and 0.5at%-Sb doped Mg₂Si have excellent thermoelectric properties. We tried to fabricate a thermoelectric module composed of these materials and using Ni plates as electrodes. The fine powder of NaCo₂O₄ was prepared by metal-citric acid complex decomposition. 0.5at%-Sb doped Mg₂Si bulk was ground to powder and sieved to a powder particle size of 75 micrometers or less. These powders were sintered using spark plasma sintering (SPS) to obtain a body of NaCo₂O₄ and 0.5at%-Sb doped Mg₂Si. These thermoelectric materials were connected to the Ni plates by using the SPS method. The whole process took a very short time (less than 2 min) and could be done at low temperature (below 873 K). The open-circuit voltage values were 82.7 mV, and the maxima, maximum output current and maximum output power, for the single module were 212.4 mA and 6.65 mW at ΔT = 470 K.

Abstract: Metallization techniques based on electroless coating are used to coat SiC particles reinforced to make Al-Si/SiC metal matrix composites. Nitric Acid (HNO₃), aluminum powder and different percentage of magnesium addition were used in solutions as electroless coating medium to coat the surface of SiC particles. Five different percentage of Mg was used from 0.1 to 0.5 wt% in electrolyte solution. Metal oxide coating was characterised by scanning electron microscope (SEM), energy dispersion spectroscopy (EDS), X-ray diffraction (XRD) as well as Transmission Electron Microscope (TEM). It is obtained that metal oxide layer formed on the SiC surface was MgAl₂O₄ or spinel which was analysed by XRD and confirmed by EDS. Spinel (MgAl₂O₄) layer was found at all composition of Mg, and such layer improved wettability between SiC and Al-Si. It also is found that the solution with 0.2%wt Mg content generated more homogeneous metal oxide layer on SiC particles therefore the solution with 0.2wt% Mg was selected as electroless coating medium to coat SiC particles reinforced to produce Al-Si/SiC metal matrix composites by stir casting route. The phases present in Al-Si/SiC composites was spinel (MgAl₂O₄), Si eutectic and Mg₂Si which was analysed by XRD.

Abstract: The formation energy, structure relaxation and electronic structure of Mg₂Si and Y-doped Mg₂Si are investigated using first-principle calculations based the density functional theory. The general gradient approximation was used to treat the exchange and correlation potential. The calculated electronic structure shows that Mg₂Si is a semiconductor with a direct gap of 0.27eV at G point. The preferential substitution site of Y inside Mg₂Si is determined to be Mg. Y-doping makes the Si atoms around the impurity outward relaxation and increases the Seebeck coefficient, electrical conductivity and thermal conductivity of Mg₂Si crystals simultaneously.