Defect and Diffusion Forum Vols. 297-301

Abstract: A two dimensional mathematical model has been developed to simulate the coupled heat and mass transfer in a porous medium undergoing a strong exothermic reaction. The problem has received a lot of interest due to its relevance in a wide variety of engineering applications such heat pipes, nuclear reactors, drying technologies, catalytic reactors and others. The fluid flow is modelled via the Darcy-Brinkman-Forchheimer equation. This model is solved numerically by the finite volume method, and the code is validated by comparing with previously published works. The influence of the exothermic chemical reaction on the heat and mass transfer in the porous medium is discussed. The effects of pertinent parameters such as the Biot number, the Reynolds number and the Frank-Kamenetskii number were analyzed. Quantitative and qualitative results are presented. Comparisons with other works in the literature are performed and excellent agreement between the results is obtained.

Abstract: During the solidification and the homogenization of single-crystal nickel-base superalloys, pores form which lead to the degradation of the mechanical properties, especially the fatigue lifetime. Hot Isostatic Pressing (HIP) is used for the pores dissolution. In this paper, analytical models for the formation of the homogenization pores and their dissolution during the HIP are developed. The results are compared with experimental data.

Abstract: The molecular dynamics (MD) method for analysis of the electric field intensity affecting iron impurities removal from tantalum in the presence of hydrogen is used. The radial distribution functions and diffusivities of hydrogen and iron atoms in a tantalum melt at 3400K under an electric field and without are obtained. An electric field imposed on liquid tantalum with an iron impurity increases the tantalum diffusivity more considerably than for iron atoms. Hydrogen introduction into the MD - cell without an electric field gives a much lower increase of the tantalum diffusivity, but a more considerable increase for iron. Simultaneous imposing an electric field and hydrogen introduction into the MD – cell keeps the tantalum and iron diffusivities at the level of the effect of an electric field. Thus the imposition of an electric field is a main parameter of the increase of the iron diffusivity.

Abstract: The present study analyzes numerically the mixed thermosolutal convection in a vertical partly porous annular cavity. The fluid motion in the porous matrix is described by the generalized Darcy-Brinkman-Forchheimer model. The results show that the composite porous layer tends to improve the heat transfer and moderately reduce the transfer of aqueous solution compared to the case of a homogeneous porous partition. This conclusion is valid for all characteristic values (Da, ) or the flow modes set up in the porous matrix.

Abstract: A new triangular-shape designed tool as a cathode in microelectromechanical etching process is a precision nanoscale production of a reclamation system of Indium tin oxide (ITO) thin-films defects removal from optoelectronic flat panel displays’ color filter surface is demonstrated in the current study. Through the ultra-precise removal of the thin-film nanostructure, the optoelectronic semiconductor industry can effectively reclaim defective products, reducing production costs. In the current experiment, a large size triangular shape cathode is accompanied by a small gap-width between the cathode and the workpiece takes less time for the same amount of ITO removal. A higher feed rate of displays’ color filter or a small end radius of the cathode combined with enough electric power produces fast machining. Pulsed direct current can improve the effect of dregs discharge and is advantageous in association with a fast workpiece feed rate. However, it raises the current rating. A large flow rate of the electrolyte corresponds to a higher removal rate for the ITO nanostructure. The electrochemical etching just needs a short time to make the ITO remove removal easy and clean.

Abstract: This paper describes the processes of water adsorption and desorption in PE membranes for fuel cells. A simple equation is inferred assuming that the surface of the membrane is uniformly covered by adsorbed molecules to an average depth of some monolayers. The adsorption depth is only controlled by diffusion of adsorbate from the surface towards the bulk through a two-layer or multi-layer mechanisms; so the empty sites formed at the surface can accept further molecules of water. If the diffusion rate is fast enough, cumulative water uptake occurs. The uptake kinetics is described considering the average penetration depth, i.e. neglecting the local concentration spikes below a random number and position of empty sites statistically formed at the surface of the membrane. The model also describes the desorption process, assumed to start at a prefixed time.

Abstract: Friction stir processing (FSP) is a solid state process to modify microstructure and mechanical properties of sheet metals and as-cast materials. In this process stirring action of the tool causes the material to intense plastic deformation that yields a dynamical recrystalyzation. In this study the effect of FSP and process parameters on hardness, and microstructure of stainless steel 316L has been investigated. Also by using of FSP, a composite layer of 316L/SiC has been produced. Results show that FSP leads to a finer and homogenized grain structure, as well as increased hardness, strength, toughness, and elongation at failure of the material. The composites produced by FSP have a uniform distribution of SiC particles between the grains of the base metal.

Abstract: Compared to the many fusion welding processes that are routinely used for joining stainless steel 316L, the friction stir welding (FSW) process is an emerging solid state joining process in which the material that is being welded does not melt and is being recast. The welding parameters play a major role in deciding the weld quality. In this investigation an attempt has been made to understand the influences of rotational speed and traverse speed of the tool on the microstructure of the friction stir processing zone in stainless steel 316L. Five different tool rotational speeds have been used to fabricate the joints at four different traverse speeds from this investigation which is the optimum for the tool speed and higher or lower amounts of these parameters are not useful for the process.

Abstract: Superplastic carburizing (SPC) is a carburizing process that combines carburizing with superplastic deformation. Since SPC involves direct interaction between the superplastically deformed surface and the solid carbon medium, the effect of surface roughness on the process cannot be disregarded. This paper presents the study of surface roughness and initial pressure effects on superplastic carburizing of duplex stainless steel (DSS). SPC was conducted under four different surface roughness (Ra) conditions of 0.9, 0.3, 0.1 and 0.03 μm. The microstructure, surface hardness, and carburized layer thickness were studied. Comparisons were also done on non-superplastic material which has a coarse microstructure. The results showed that the surface roughness strongly affected the properties of the superplastically carburized duplex stainless steel while its effects on the non-superplastic material were not that obvious.

Abstract: In this study, different mechanisms of diffusion such as Knudsen and bulk were investigated for diethylbenzene diffusion into a catalyst and it was concluded that the pore sizes should be in the range that permit transitional diffusion (both Knudsen and bulk diffusion). The catalyst grain size can be controlled and varied by different parameters such as speed and time of mixing, type of alkali, temperature and pH. Particle size distribution experiments were conducted for different types of alkali and speed of mixing to characterize the catalyst. The effects of grain size formed during coprecipitation on pore size distribution of the catalyst pellet which affect the effective diffusivity were discussed. Pore size distribution of the model catalyst was obtained and the effective diffusivities were calculated by numerical integration of Johanson-Stewart equation.