Yongnian Dai

Preparation of TiC Powders by Carbothermal Reduction Method in Vacuum

The preparation of fine TiC powders by carbothermal reduction of TiO2 in vacuum was investigated by XRD, SEM, XRF and laser particle sizer. Thermodynamic analysis indicates that it is easy to prepare TiC in vacuum and the formation sequence of products are Ti4O7 (Magneli phase), Ti3O5, Ti2O3, TiC(subscript x)O(subscript 1-x) and TiC with the increase of reaction temperature. Experimental results demonstrate that TiC powders with single phase are obtained with molar ratio of TiO2 to C ranging from 1:3.2 to 1:6 at 1550℃ for 4 h when the system pressure is 50 Pa, and TiC1.0 is gained when the molar ratio of TiO2 to C is 1:4 and 1:5. In addition, fine TiC1.0 powders (D50 equals 3.04 μm) with single phase and low impurities are obtained when the molar ratio of TiO2 to C is 1:4. SEM observation shows that uniform shape, low agglomeration, and loose structure are observed on the surface of block product.

Synthesis and Photoluminescence Property of Silicon Carbide Nanowires Via Carbothermic Reduction of Silica

Silicon carbide nanowires have been synthesized at 1400 °C by carbothermic reduction of silica with bamboo carbon under normal atmosphere pressure without metallic catalyst. X-ray diffraction, scanning electron microscopy, energy-dispersive spectroscopy, transmission electron microscopy and Fourier transformed infrared spectroscopy were used to characterize the silicon carbide nanowires. The results show that the silicon carbide nanowires have a core–shell structure and grow along <111> direction. The diameter of silicon carbide nanowires is about 50–200 nm and the length from tens to hundreds of micrometers. The vapor–solid mechanism is proposed to elucidate the growth process. The photoluminescence of the synthesized silicon carbide nanowires shows significant blueshifts, which is resulted from the existence of oxygen defects in amorphous layer and the special rough core–shell interface.

Boron removal in purifying metallurgical grade silicon by CaO−SiO2 slag refining

Abstract Boron removal from metallurgical grade silicon (MG-Si) using a calcium silicate slag was studied. The results show that it is impossible basically to remove boron using a pure SiO 2 refining. The oxidizing ability of CaO–SiO 2 slag for boron removal was characterized by establishing the thermodynamic relationship between the distribution coefficient of boron ( L B ) and the activities of SiO 2 and CaO. The experimental results show that the distribution coefficient and the removal efficiency of boron are greatly improved with the increase of CaO proportion in the slag. The maximal value of L B reaches 1.57 with a slag composition of 60%CaO-40%SiO 2 (mass fraction). The boron content in the refined silicon is reduced from 18×10 −6 to 1.8×10 −6 using slag refining at 1600 °C for 3 h with a CaO–SiO 2 /MG-Si ratio of 2.5, and the removal efficiency of boron reaches 90%.

Impurities Removal From Metallurgical Grade Silicon Using Gas Blowing Refining Techniques

The removal of impurities from metallurgical grade silicon using the O2 and H2O-O2 gas blowing techniques was firstly studied by thermodynamics. The relationships between the boron content in refined silicon and the equilibrium partial pressures of gaseous boride species were established, which shows a theoretical limitation for boron removal from metallurgical grade silicon using the H2O-O2 gas blowing technique. The data also showed that the impurity boron in silicon was mainly volatilized in the form of B3H3O6, BHO2 and BO and the volatilization of boric hydrate species was much more than that of the oxide species. The impurities removal from metallurgical grade silicon including Fe, Al, Ca, Ti, B, P and C was studied using an O2 gas blowing in a ladle and in succession a mixed Ar-H2O-O2 gas blowing was operated in a DC arc furnace for boron removal. It showed a removal efficiency higher than 90 % for Al, Ca and 50 % for B using the O2 gas blowing technique in the ladle. Impurity boron was reduced from 35 ppmw to 18 ppmw in the ladle and it was once again reduced to 0.6 ppmw using an Ar-H2O-O2 gas blowing technique in the DC arc furnace for a systematic pressure of 5 Pa when the ratio of H2O to O2 and the refining times are 2:1 and 12 min, respectively.

Silica nanoparticles functionalized via click chemistry and ATRP for enrichment of Pb(II) ion

Silica nanoparticles have been functionalized by click chemistry and atom transfer radical polymerization (ATRP) simultaneously. First, the silanized silica nanoparticles were modified with bromine end group, and then the azide group was grafted onto the surface via covalent coupling. 3-Bromopropyl propiolate was synthesized, and then the synthesized materials were used to react with azide-modified silica nanoparticles via copper-mediated click chemistry and bromine surface-initiated ATRP. Transmission electron microscopy, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and thermogravimetric analysis were performed to characterize the functionalized silica nanoparticles. We investigated the enrichment efficiency of bare silica and poly(ethylene glycol) methacrylate (PEGMA)-functionalized silica nanoparticles in Pb(II) aqueous solution. The results demonstrated that PEGMA-functionalized silica nanoparticles can enrich Pb(II) more quickly than pristine silica nanoparticles within 1 h.

Si purification by enrichment of primary Si in Al-Si melt

Abstract The primary silicon crystals and Al-Si alloy in hypereutectic Al-Si melt were separated by electromagnetic stirring and directional solidification. Additionally, the distribution feature of impurities in Al-Si system was verified. The results show that the impurities are mainly located in Al-Si alloy and the grain boundaries between the Al-Si alloy and primary silicon. Furthermore, the morphology of primary silicon changes from fish-bone like to plate like and spheroid due to the different Si contents. The amount of impurities decreases with the increasing of Si content in different positions of the sample. The amount of impurities in the bottom of the sample is approximately 10×10 −6 , which is obviously improved compared with the 1248.47×10 −6 in metallurgical Si.

Thermodynamic behavior and morphology of impurities in metallurgical grade silicon in process of O2 blowing

Abstract Gas blowing is a valid method to remove the impurities from metallurgical grade silicon (MG-Si) melt. The thermodynamic behavior of impurities Fe, Al, Ca, Ti, Cu, C, B and P in MG-Si was studied in the process of O 2 blowing. The removal efficiencies of impurities in MG-Si were investigated using O 2 blowing in ladle. It is found that the removal efficiencies are higher than 90% for Ca and Al and nearly 50% for B and Ti. The morphology of inclusions was analyzed and the phases Al 3 Ni, NiSi 2 and Al 3 Ni were confirmed in MG-Si by X-ray diffraction. It was found that SiB 4 exists in Si–B binary system. The chemical composition of inclusions in MG-Si before and after refining was analyzed by SEM-EDS. It is found that the amount of white inclusion reduces for the removal of most Al and Ca in the forms of molten slag inclusion and the contents of Fe, Ni and Mn in inclusion increase for their inertia in silicon melt with O 2 blowing.

Recycling of metals from waste Sn-based alloys by vacuum separation

Abstract In order to recycle waste Sn-based alloys, the vapor–liquid phase equilibrium composition diagrams of Sn–Pb, Sn–Sb and Sn–Zn binary systems were calculated. The calculated results indicate that Pb, Sb and Zn can be separated from Sn effectively. Based on the above calculation, the industrial experiments of vacuum distillation of Sn–Pb alloy, Sn–Pb–Sb alloy, Sn–Pb–Sb–As alloy, crude Sn and Sn–Zn alloy with different contents were carried out. The experimental results show that Pb (>99% Pb) and Sn (≤0.003% Pb) were obtained simultaneously while Sn–Pb alloy was subjected to vacuum distillation; the crude Sn (>90% Sn, ≤ 2% Pb, ≤6% Sb) and crude Pb (≤2% Sn) were obtained simultaneously while a single vacuum distillation was carried out for Sn–Pb–Sb alloy; the Pb and Bi contents in the Sn ingot (99.99% Sn) achieve the grade A of GB/T 728—2010 standard, more than 50% of As and Sb was removed after vacuum distillation of crude Sn; Zn (

Calculation and Characterization of Silicon-Boron Phases in Metallurgical Grade Silicon

IntroductionIt is a challenge that boron is removed in upgrading metallurgical grade silicon (MG-Si) to solar grade silicon (SoG-Si) with a metallurgical process. In this research, the Si-B binary system is thermodynamically assessed and characterized by using the CompuTherm Pandat software, which is helpful to the boron removal in refining MG-Si to SoG-Si.MethodsThe Si-B binary system was thermodynamically assessed and characterized by using the CompuTherm Pandat software. The SEM-EDS and XRD technologies were used.ResultsThe solution phases, including Liquid, diamond-Si and β-B were treated as substitutional solution phases, of which the Gibbs energies were expressed with Redlich–Kister polynomial functions. Meanwhile, the compounds, SiB3, SiB6, SiBn, were modeled as stoichiometric compounds. The thermodynamic parameters formulating the Gibbs energies of various phases were obtained and the equilibria and transition of the phases were discussed. According to XRD technology, the intermediate phase SiB4 was obtained and observed. It was tested that the phase SiB4 existed objectively although it is not found in the assessed Si-B binary phase diagram.DiscussionThe existent forms for Si-B phases in the MG-Si melt were forecast and tested to be SiB6 and Liquid.

Self-Organized Ni Nanocrystal Embedded in BaTiO3 Epitaxial Film

Ni nanocrystals (NCs) were embedded in BaTiO3 epitaxial films using the laser molecular beam epitaxy. The processes involving the self-organization of Ni NCs and the epitaxial growth of BaTiO3 were discussed. With the in situ monitoring of reflection high-energy electron diffraction, the nanocomposite films were engineered controllably by the fine alternation of the self-organization of Ni NCs and the epitaxial growth of BaTiO3. The transmission electron microscopy and the X-ray diffraction characterization confirmed that the composite film consists of the Ni NCs layers alternating with the (001)/(100)-oriented epitaxial BaTiO3 separation layers.