Papers by Author: Qing Sen Meng

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.

Abstract: The growth mechanism of Onion-like fullerenes (OLFs) synthesized by arc discharge in liquid benzene was discussed. HRTEM was employed to characterize the morphologies and microstructures of the products. Results showed that the typical OLFs were highly crystallized with uniform diameter of 10-30nm. The growth of OLFs depended on the appropriate temperature gradient and quenching zone provided by the bubbles. The aromatic debris could act as graphite fragments composed of hexagonal carbon rings. The reducing of dangling bonds on the edges of graphite fragments would lower the energy of the system, resulting in formation of a close-caged structure at suitable temperature gradient by the auto-curling of graphite fragments. Based on the impact of the innermost core shape, such as C60 shape, quasi-spherical or polyhedral hollow concentric OLFs was formed. Adding ferrocene, ferrocene directly influenced the morphology and yield of OLFs.

Abstract: Iron disilicide (-FeSi2, and -FeSi2+Cu0.1wt%) were prepared by a field-activated pressure assisted synthesis(FAPAS) method from elemental powders and the thermoelectric properties were investigated. The average grain size of these products is about 0.3m. The thermal conductivity of these materials is 3-4wm-1K-1in the temperature range 300-725K. These products’ figure of merit is 28.50×10-4 in the temperature range 330-450K. The additions of Cu promote the phase transformation of -Fe2Si5 + -FeSi → β-FeSi2 and shorten the annealing time. It is proved that FAPAS is a benign and rapid process for sintering of -FeSi2 thermoelectric materials.

Abstract: Iron disilicide（-FeSi2）compounds were synthesized by field-activated pressure assisted synthesis (FAPAS) process, and their thermoelectric properties were measured. Fine microstructure with small pores was obtained in the fabricated samples. The average grain size was approximately 0.3 m. The thermal conductivity in the temperature range of 300-725 K was 3-4 Wm-1K-1, which was considerably lower than that of the same materials synthesized by other methods including SPS process. Lower thermal conductivity provided a higher figure of merit, ZT of 28.50×10-4 in the temperature range of 330-450 K.

Abstract: Silicon was electrodeposited by electrochemical reduction of silicon tetrachloride in propylene carbonate solvent on an alloy of nickel-titanium-niobium and nickel, which were used as working electrodes respectively. The effect of two substrates as working electrodes on the electrodeposition ability of the silicon was investigated by scanning electron microscopy (SEM) and energy dispersion spectroscopy (EDS). Results show that using an alloy of titanium, nickel and niobium as working electrode is better than nickel in the morphology and deposition yield of silicon.

Abstract: Reactive sintering of elemental powders was used to form Y-doped Mg2Si (Y: 1000, 2000, 3000ppm) using a field-activated pressure assisted synthesis (FAPAS) method. XRD analysis, and the calculation of lattice constant (a) indicates that 1000ppm is the solid solubility of Y in Mg2Si. Sample doped with 2000ppm Y owns better performances, the absolute value of Seebeck Coefficient increases in the temperature of 288-580K and is higher than that of non-doped Mg2Si, and it got higher electric conductivity and higher power factor, which reaches up to 1.67 times of non-doped Mg2Si at 438K and 2.03 times of that of non-doped Mg2Si at about 408K. Meanwhile, the introduction of Y can decrease thermo-conductivity obviously, proving that the introduction of Y is favorable for both electrical and thermal properties.

Abstract: In this paper it was investigated that silicon be electrodeposited in the room temperature from propylene carbonate solvent. Cyclic voltammograms indicated that it was possible to electrochemical reduction of silicon chloride at -2.3984V versuHg/Hg2Cl2/ KCl (saturation) quasi-reference electrode. Potentiostatic electrolysis yielded deposits, accompanied by the change in electrolyte color from transparent to brown. Scanning electron microscopy (SEM) and energy dispersion spectroscopy (EDS) were used to characterize the physical properties of the silicon. The EDS analysis performed simultaneously with SEM observation demonstrates that the deposit consists of Si, O and Cl.

Abstract: In this paper, the synthesis of four-layered functionally graded materials (FGMs) samples of (TiB2)pNi/(TiB2)p(Ni+Ni3Al)/ Ni3Al with different metallic substrate (Ni, Cu and Cr-Ni steel) by field-activated and pressure-assisted synthesis process (FAPAS) was investigated. It was shown that the imposition of the electric field and the combustion reaction of Ni3Al is the key for the successful synthesis of the FGMs during process. The exothermal reaction of the powder of nickel and aluminum during combustion enhanced the diffusion bonding in the interfaces subsequently. The microstructure, phase composition of the interfaces and mechanical properties of the graded materials were characterized, which showed indicative formation of firm bonds by the combustion reactive process. The samples have an excellent thermal shock resistance ability and gradient distribution of micro-hardness across the substrate layers and the surface layer.

Abstract: Using the processes of field-activated and pressure-assisted combustion synthesis (FAPACS), FGM materials (FGMs) were prepared under the conditions of field-assisted and the hot-press. The microstructure and the phase composition of the interface of the graded materials were investigated and the results showed that the metallurgical joining layer was formed in the interfaces of the (TiB2)pNi/Ni3Al/405 steel. The mechanical characterization of the gradient materials showed that the composition and the micro-hardness of the gradient material were gradient distributed, and its surface Rockwell hardness and wear resistance are better than that of hardened 20Cr steel.