Papers by Author: Jian Guo Yu

Abstract: Thermal simulation compression tests were performed on semi-solid billet in order to observe and investigate the behavior of 6061 aluminum alloys while varying the processing parameters such as apparent viscosity, the shear rate and the temperature. Specimens of 6061 aluminum alloys were characterized with their semi-solid behavior during partial melting and holding in the semi-solid state. Furthermore, the constitutive equation of semi-solid 6061 aluminum alloys was investigated. Moreover, the tests allow the apparent viscosity and shear rate of the alloys to be determined as a function of the solid volume fraction and strain rate together with the geometry behavior of the specimen. Utilizing these parameters, semi-solid die casting process of 6061 aluminum alloys could be simulated by the change of the solid volume fractions.

Abstract: The corrosion behaviors and mechanisms of 310S and AL-6XN stainless steels in MgCl2 molten salts were investigated at 800°C for 8-72h. The weight change kinetics, surface and cross-sectional morphologies as well as corrosion products and compositions were analyzed. The results showed that corrosion velocities of both steels were very high in the preliminary period, then diminished quickly and trended to zero afterward. The AL-6XN steels exhibited better corrosion resistance in MgCl2 molten salts than 310S. The predominant peaks of MgO were detected from both steels after corrosion in molten MgCl2 at 800°C, which could serve as the protective barriers between the alloy and the MgCl2 molten salts. The depletion of Cr was very serious throughout the corrosion layer for both steels due to their preferential oxidation for the formation of an outer corrosion layer. The other elements in alloy were corroded in succession through the outward diffusion with the progress of the corrosion. The outward migrations of the metal ions led to the coalescence of vacancies to form the voids. The two steels possessed the uniform corrosion behavior.

Abstract: Mesoporous titania nanoribbons are synthesized via an optimized soft hydrothermal process and the derived titania ion-sieves with lithium selective adsorption property are accordingly prepared via a simple solid-phase reaction between Li2CO3 and TiO2 nanomaterials followed by the acid treatment process to extract lithium from the Li2TiO3 ternary oxide precursors. The physical chemistry structure are characterized by XRD, TEM/HRTEM, SAED and N2 adsorption-desorption analysis; and the lithium selective adsorption properties are tested by the adsorption isotherm measurement and demonstrated with the distribution coefficient of a series of alkaline and alkaline-earth metal ions. Though the high temperature calcinations and lithium insertion-extraction process resulted in the agglomeration of nanostructure to large bulky particles, it implied that that the low-dimensional titania nanoribbons might be functionalized to lithium ion-sieves with remarkable adsorption capacity and selectivity, promising in the utilization of lithium extraction from aqueous resources including brine or seawater.

Abstract: The cathodic reduction of SiO2 in KCl-CaCl2-NaCl-MgCl2 melt was studied using tungsten wires as working electrodes. The results of cyclic voltammetry
square wave voltammetry and chronopotentiometry showed that the cathodic deposition of silicon is a quasi-reversible diffusion-controlled reaction, followed by a four electrons transfer step. The results of current reversal chronopotentiometry and thermodynamic data showed that both the silicon deposition and the side reaction between SiO2 and magnesium result in the loss of magnesium and low current efficiency. A 35.2% current efficiency was obtained with the content of SiO2 0.2% at 700 .

Abstract: Magnesium chloride hex-ammoniate (MgCl2·6NH3) is an intermediate to produce anhydrous magnesium chloride (MgCl2) by method of reaction crystallization. MgCl2·6NH3 is decomposed at 670K to produce anhydrous magnesium chloride. The process of thermal decomposition and its non-isothermal kinetics of MgCl2·6NH3 is studied. Results show that the thermal decomposition process is made up of three stages, the thermal decomposition functions and the thermal decomposition kinetics parameters, such as activation energy (E), pro-exponential factor (A) of MgCl2·6NH3 for each step are obtained by means of the Acher differential, the Coats-Redfern integral and multi-accelerated heating rate method. This study provides a valuable theoretical basis for MgCl2·6NH3 decomposition process on industrialization.

Abstract: Magnesium chloride hexammoniate (MgCl2·6NH3) is an intermediate product for preparation anhydrous magnesium chloride by reaction crystallization method. An experiment study of a semi-batch reaction crystallization is presented. In a single feed operation, magnesium chloride solution is fed to a stirred methanol solution mixed with ammonia to crystallize magnesium chloride hexammoniate. The median crystal size of product increases with increasing stirring rate, reaches a maximum, and then decreases again. Decreasing feed rate or decreasing stirring time increases the crystal size significantly. The reaction temperature and concentration of magnesium ion can also influence the crystal size distribution (CSD). A double feed operation can create larger crystal size than that of single feed operation. The relationship between crystal size and the content of water of the product is discussed.

Abstract: Anhydrous magnesium chloride (MgCl2), the dehydration product from bischofite (MgCl2•6H2O) and as industrial raw material for preparation of electrolytic magnesium, is now the most advanced and perfect technological process. For long, the detailed dehydration process was not known due to its dehydration complexity and lack of appropriate experimental conditions. In this paper, quantum chemistry method based on density functional theory (DFT) was used to study the whole dehydration processes. The molecular geometries of MgCl2•6H2O, MgCl2•4H2O, MgCl2 •2H2O, MgCl2•H2O and MgCl2 were all optimized at level of B3LYP/6-31G*, the optimized geometrical parameters and correspondent energies corrected by the second order Møller-Plesset perturbation theory (MP2) were thus obtained. Results show that the energy variations corresponding to the whole dehydration steps from MgCl2•6H2O via intermediates MgCl2•4H2O, MgCl2•2H2O and MgCl2•H2O, to anhydrous product MgCl2 are 35.55, 41.30, 28.55, 31.08kcal/mol, respectively. For steps of 2H2O removal, the energy variation from MgCl2•2H2O to MgCl2 is 59.63kcal/mol, bigger than the steps from MgCl2•6H2O to MgCl2•4H2O (35.55kcal/mol) and from MgCl2•4H2O to MgCl2•2H2O (41.30kcal/mol), which means the last two water molecules are the most difficult to be removed. All these results are significant for mechanism study of bischofite dehydration and are helpful for industrial production of anhydrous magnesium chloride.