Defect and Diffusion Forum Vols. 334-335

Abstract: Commercial purity titanium samples were gas nitrided for 3, 5, 7 and 12 h at 1250°C. TiN layers with nitrogen diffusion zones were formed on the surfaces, which caused an increase in the surface hardness with respect to the processing time. In addition, nitriding provided a reduction in friction coefficient and an enhancement in wear resistance against an alumina ball under dry sliding conditions.

Abstract: The mechanical properties of open-cell metal foam structures are investigated using the finite element method. The foam structure is modelled by a regular network of anisotropic Weaire-Phelan cells in which the strands are modelled as 3D finite element beams. We consider four types of strand cross sections: (i) circular, (ii) square, (iii) triangular and (iv) Plateau border shape. The numerical results obtained with our proposed mathematical model are checked against the experimental results obtained on real Nickel metallic foam and an excellent agreement is found. In addition, we conducted a parametric analysis to study the effect of some geometrical characteristics on the elasticity of the metal foam. Among these geometrical parameters, the shape, the dimensions of strand cross section, the inertia, the alignment of strands and the structural network irregularities are investigated, discussed and documented.

Abstract: For over a decade, metal foams have attracted a growing research interest because of the development of certain production processes. The advantages of metal foams most cited in the scientific literature are their low weight. However, one of the obstacles to their use on an industrial scale is the dispersion of their mechanical properties, in part because of their microstructural heterogeneity.

Abstract: Drying can be defined as a simultaneous heat and mass transfer process between product and air drying. These phenomena have occurred due to thermal and moisture gradients and they are affected by different parameters. The inadequate management of this process can cause severe damage to the product such as cracking, loss of nutritional properties and even loss of the product quality. Thus, this paper proposes a numerical study of heat and mass transfer that occurs during the rough rice drying process, using the commercial software ANSYS CFX^®. Herein the grain was considered as ellipsoid of revolution.

Abstract: The gel polymer electrolytes (GPEs) composed of polymethylmethacrylate (PMMA) with lithium trifluoromethanesulfonate (LiCF₃SO₃) salt dissolved in a binary mixture of ethylene carbonate (EC) and propylene carbonate (PC) organic solvents have been prepared by the solution casting technique. The samples are prepared by varying the salt concentrations from 5 wt.% to 30 wt.%. Impedance spectroscopy measurement has been carried out to determine the ionic conductivity of the samples. The sample containing 25 wt.% of LiCF₃SO₃ salt exhibits the highest room temperature ionic conductivity of 2.56 x 10^-3 S cm^-1. The conductivity of the GPEs has been found to depend on the salt concentration added to the sample, while at higher salt concentration reveals a decrease in the ionic conductivity due to ions association. The temperature dependence of conductivity from 303 K to 373 K is found to obey the Arrhenius law. The ionic transference number, t_i of GPEs has been estimated by the DC polarization method and the value is found to be 0.98, 0.93, and 0.97 for the sample containing 25 wt.%, 5 wt.% and 30 wt.% respectively. This result is consistent with the conductivity studies.

Abstract: Drying is a method of preservation widely used to prolong the post-harvest life of several agricultural products. In this work, experiments were accomplished involving drying of whole bananas, using hot air at temperature of 40.0 ^ºC and constant velocity of 1.5 m s^-1. The mass loss in regular time intervals was measured using the gravimetric method. In order to describe the process, the liquid diffusion model was used, assuming variable volume and effective mass diffusivity [. Thus, the diffusion equation was numerically solved through the finite volume method, with a fully implicit formulation. Due to the geometry of the product, the diffusion equation was written in generalized coordinates, and then discretized, assuming boundary condition of the third kind [. To take advantage of the symmetry, bananas were considered as revolution solids, obtained by the rotation of an area in the plane (x,y) about the axis y. The area was obtained directly of the photography of a banana, which served to create a non-orthogonal structured grid with 32 x 40 control volumes. The thermo-physical parameters were obtained through an optimization algorithm, based on the inverse method. Once the thermo-physical parameters were known, the drying kinetics as well as the water distribution within the bananas in stipulated times were presented and analyzed. The statistical indicators enable to conclude the methodology proposed to describe whole banana drying presents good results.

Abstract: In this work, a numerical solution for the diffusion equation applied to solids with arbitrary shape considering convective boundary condition is presented. To this end, the diffusion equation, written in generalized coordinates, was discretized by the finite-volume method with a fully implicit formulation. The transport parameters and the dimensions of the solids are considered constant during all process. For each time step, the system of equations obtained for a given non-orthogonal structured mesh was solved by the Gauss-Seidel method. One computational code was developed in FORTRAN, using the CFV 6.6.0 Studio, in a Windows platform. The proposed solution was validated using analytical and numerical solutions of the diffusion equation for different geometries (parallelepiped and finite cylinder). The analysis and comparison of the results showed that the proposed solution provides correct results for the cases investigated. In order to verify the potential of the proposed numerical solution, we used experimental data of the drying of ceramic roof tiles for the following temperature: T = 55.6 °C. The analysis of the results and the statistical indicators enables to affirm that the developed numerical solution satisfactorily describes the drying processes in this temperature for the convective boundary condition.

Abstract: Successive precipitation processes in solution treated Mg3Y3Nd alloy were studied by electrical resistometry, by differential scanning calorimetry and by microhardness. The results were compared to those in the commercial WE43 alloy. Five various phases known from the Mg-Gd-and Mg-Nd-type decomposition sequences form, dissolute or transform in the Mg3Y3Nd alloy. The main difference in the WE43 precipitation sequence is the absence of the β₁ phase particles. Electron microscopy confirmed that not the phase type of hardening particles but their morphology, size and orientation to the matrix determine the hardening effectiveness. Differential scanning calorimetry revealed exothermic effects connected to precipitation processes. Apparent activation energies of individual precipitation processes correspond to those in the MgTbNd and in MgNd alloys.

Abstract: The effect of hot extrusion at 350°C on microstructure, thermal, electrical and mechanical properties of the AlMnScZr alloy was studied. The samples of the cast and of the hot-extruded alloys were annealed from 20°C up to 600°C. Transmission and scanning electron microscopy and electron backscatter diffraction examinations of specimens quenched from temperatures of significant resistivity changes were used to identify microstructural processes responsible for these changes. The cast as well as hot-extruded alloy is characterized by a dispersion of fine coherent Al₃Sc and/or Al₃(Sc,Zr) particles, and furthermore the fine (sub) grain structure was observed in the hot-extruded alloy. Microhardness HV1 and resistivity values reflect different microstructure of the alloys accordingly. The distinct resistivity changes of the alloys are mainly caused by precipitation of Mn-containing particles. The apparent activation energy for the Al₆Mn-phase precipitation in the hot-extruded alloy was also determined. The obtained results agree with those observed in the alloys prepared by powder metallurgy studied in our previous work.

Abstract: Absorption phenomenon of ammonia vapor into ammonia water solution has been investigated experimentally, by allowing superheated ammonia vapor to flow into a test cell containing a stagnant pool of ammonia water solution. Before commencing the experiment, the pressure in the test cell P_1i, corresponds to the equilibrium vapor of the ammonia-water system at room temperature and initial mass fraction C_i. When the valve is opened, mechanical equilibrium is established quickly and the pressure in the test cell becomes equal to that of the ammonia vapor cylinder. The difference between the initial pressure in the vapor cylinder and the initial pressure in the test cell ΔP_i is found to have a major influence on the absorption rate [1]. The interface temperature can be estimated for a transient case, by help of an inverse solution proposed by Monde [2]. The interface concentration C_int obtained by measured ammonia vapour pressure and the estimated interface temperature. The main objective of this study is to investigate the effect of the initial pressure difference and the initial concentration on the interface concentration. A correlation which gives the interface concentration as a function of the initial concentration, the initial pressure difference and time is derived. In addition, the absorbed mass at no pressure difference could be estimated from the absorbed mass at initial pressure difference.