Defect and Diffusion Forum Vol. 353

Abstract: Discrete Element Method (DEM) computer simulation is used to examine the influence of contact force between two-dimensional aggregates of polystyrene microsphere formed on the air-liquid interface. Colloidal aggregates have been treated as the granular material or discontinuum materials. The interaction force models are related to experiment which had done by digital video microscopy. The interaction mechanisms of the contact forces between particles in the colloidal system can be considered as a combination of spring and dashpot force and van der Waals force. According to the DEM, the interaction forces are evaluated to introduce relations between particles and the result comparison between the computer simulation and the experimental work. This study indicates that the behavior of the colloidal aggregates depends on the long-ranged (spring and dashpot) and the short-ranged interaction force (van der Waals). Besides, the behaviors shown in both computer simulation and the experiment are in good agreement. Thus, this computer simulation method can mimic the behavior of colloidal aggregates forming as a monolayer at the air-liquid interface.

Abstract: The thermodynamic simulation of the oxidation processes of silver selenide by reagents such as NaNO₂, NaNO₃ and NaOH during their heat treatment has been conducted. Gibbs energy of silver selenide oxidation reactions has been estimated. It is shown as a reagent–oxidant of silver selenide it is rationally to use a mixture of sodium nitrite and sodium hydroxide. Oxidation of silver selenide during heat treatment together with sodium nitrite and sodium hydroxide is accompanied by the formation of soluble compounds such as Na₂SeO₃ and Na₂SeO₄, silver being passed in the metallic state. Separation coefficients characterizing oxidation efficiency and the level of selenium transfer in soluble compounds were estimated. Using the data the separation method of materials containing selenides by means of their sintering with sodium-containing reagents is proposed. The method is based on the transfer of selenium in the water-soluble compounds followed by leaching, precious metals being the elemental state. The products obtained are suitable for the production of selenium and precious metals by traditional technologies.

Abstract: The purpose of this paper is to present an experimental study of clay brick drying. For the drying experiments, industrial holed bricks were dried in an oven under controlled conditions of velocity, temperature (constant and variable) and relative humidity of air. The continuous drying experiments ended when the mass reached constant weight. Experimental tests were performed under atmospheric pressure. Results of the drying and heating kinetics and volume variations during the process are shown and analyzed. It was verified that the drying process happens in the falling drying rate period, and air temperature has large influence in the drying rate during process. It was verified that the largest temperature, moisture content and stress gradients are located in the vertexes of the brick.

Abstract: This paper presents a frost numerical model for predicting the frosting behavior in terms of the correlation of the heat transfer coefficient, Nusselt number. In this work the correlations of the Nusselt number from various references is examined and compared with experimental data. In this study, a mathematical model that can predict the behavior of the frost growth on a bank of triangularly arranged tubes is developed. Because of the complex heat transfer during frost formation on a tube bank, this subject is receiving much attention. Heat transfer in flow across a bank of tubes has particular importance in the design of heat exchangers. For this study various empirical relations are analyzed to determine the Nusselt number and compared with experimental data.

Abstract: In the present study the modeling of the HEBM process is presented. The impact velocity, impact angle, rotation speed, mass of balls, ball-to-powder weight ratio and time of milling have been taken into account in order to calculate the energy transferred from the balls to the powder. Two different systems, namely, TiN-AlN and polysalicylic acid were experimentally investigated in order to confirm the validity of the model. The calculation results are in a reasonable agreement with the results of experimental research.

Abstract: An own-designed pressure vessel with glass windows has been employed to perform an in-situ characterization of the temporal evolution of the crystallization process of an amorphous polylactic acid (PLA) under different controlled CO₂ pressures and temperatures. It has been proven that crystallinity can be related to optical parameters such as transmissivity, obtaining information about the whole process by optical measurements. The method has the advantage of measuring in-situ over bulk samples with a non-destructive tool. The obtained results have shown some unexpected trends that have been explained taking into account the complex phenomena occurring during the crystallization process of PLA in the presence of CO₂ at high pressure.

Abstract: The thermal and microstructural properties of five copper-gallium-tin alloys (Cu-Ga-Sn) and two important intermetallic compounds Cu₃Sn and CuGa₂ have been investigated by dilatometry, scanning electron microscopy (SEM) equipped with energy dispersive X-ray spectroscopy (EDX) and X-ray diffraction. The temperature dependence of thermal expansion and coefficient of linear thermal expansion (CLTE) were obtained.

Abstract: In order to enhance the hydrogen permeation, Silicon carbide (SiC) was prepared with CeO₂-NiO mixed oxides via sol-gel process and the SiC membranes with various contents of χ wt% mixed oxides (χ = 20, 40, 60 and 80) were fabricated by hot-press sintering (HPS) at 1173 K under vacuum condition. The effect of the CeO₂-NiO mixed oxides was studied in terms of properties on hydrogen permeation. All the membranes were characterized by XRD, FE-SEM and BET instrument. The hydrogen permeation test was carried out at various temperatures under 0.1 MPa. Hydrogen permeation flux of the 80 wt% CeO₂-NiO/SiC membrane, which has the highest value, was obtained at 2.92×10^-6 mol/m²sPa, 323 K. The reaction enthalpy (ΔH°) was calculated as - 4.95 J/mol by Arrhenius’s plot.

Abstract: The formation of porosity in thin layers of coating under its processing using the flux of high energy Xe⁺⁺ is happen under the physical processes of nucleation of gaseous defects. The process is described by kinetic equations of Kolmogorov-Feller and Einstein-Smolukhovskii. The initial stage of the phase transition is considered as the superposition of two random processes: clustering of defects and their brownian motion. In the paper, we showthat a damaging phenomenon in layers can be predicted by means of computer experiments. The kinetic distribution function (DF) of defects depends on sizes and depth their penetration into materials under irradiation. Computer simulation of DF allows analysing the porosity and elastic stresses in bilayer caused by Xe⁺⁺ blistering. The structures of defects are accumulated in SiC layer due to phase transition as well as a result the action perturbation of elastic forces in thin layers.

Abstract: This work investigates the possibility of improving the kinetics of the diffusion controlled cementation of copper on a fixed bed of zinc Raschig rings in a batch reactor by using flow pulses induced through the bed by a pulsating perforated disc. The kinetics of the cementation reaction was investigated under different conditions of initial concentration of copper ions, frequency and amplitude of oscillation (vibration velocity), zinc ring diameter, bed height and temperature. The effect of temperature was found to fit the Arrhenius equation with an activation energy of 5.5 kcal/mole which confirms the diffusion controlled nature of the reaction. A dimensionless correlation in the form that: Sh = 6.9 Re^0.55 Sc^0.33 (P_d/B_h)^0.49 was deduced. Industrial application of the results obtained was discussed.