Papers by Author: Ali Shokuhfar

Abstract: Microwave irradiation is a novel method to accelerate the preparation of inorganic nanoparticles as well as various nanostructures such as nanotubes, nanorods, and nanowires. In this research, silver (Ag) nanostructures with various morphologies were prepared via a rapid microwave-assisted technique. The spherical and polygonal Ag nanoparticles were synthesized in the presence of Polyvinilpyroliden (PVP) as stabilizing agent. Ethylene glycol (EG) serves as a solvent. The silver nanoparticles were produced by Continuous and Discontinuous Wave irradiation. The obtained materials were characterized by UV-visible and transmission electron microscopy (TEM). The results present that the size of nanoparticles increase in continuous irradiation. In this process, the morphology of product is polygonal. In the discontinuous process the colloidal particle size is smaller than that obtained from the continuous process. Furthermore, the Ag nanoparticles have spherical shapes. The final data show that microwave processing has unique advantages over conventional heating processes.

Abstract: Nowadays, different production methods of nanoparticles have been developed. Among novel wet-chemical processes, the Chemical Bath Deposition (CBD) method is used to synthesize nanoparticles more easily than the other method. In this investigation, titanium dioxide (TiO2) nanoparticles were synthesized by the Chemical Bath Deposition (CBD) method. Tetraisopropyl titanate (C12H28O4Ti), sodium hydroxide (NaOH) and ethanol were used as initial materials. Appropriate solvents were used for preparation of samples. CBD process was carried out at 50°C for 90 min and the obtained materials washed and then dried at room temperature for 48 hrs. For determining particle size and evaluation of morphological characteristics, X-ray diffraction (XRD) and transmission electron microscopy (TEM) were used. The TEM observation indicated that the average particles size of powder is about 10-30 nm and the shape of product is semi-spherical shape. The final results present that the CBD method is more suitable than the other process because of it showing a low processing cost and fine powders.

Abstract: Fabrication of alloys in the solid state via mechanical alloying (MA) process has been studied by earlier researchers. The effects of milling time and impact force, defined as the ball-to-powder weight ratio (BPR), on the elemental diffusion during synthesis of nanostructured Fe-50at.%Cu alloy via MA process were evaluated in the current work. X-ray diffraction patterns revealed that increasing the milling time and impact force give rise to increasing the micro-strain, lattice parameter and decreasing the crystallite size during the MA process. Furthermore, scanning electron microscopy (SEM) was utilized not only for evaluating the microstructure of the milled powder particles but also for proving this claim that during MA process, the mutual diffusion of Cu and Fe has occurred. The interpretation of data resulted have been discussed in details.

Abstract: Boron Nitride nanotubes (BNNTs) together with carbon nanotubes (CNTs) have attracted the wide attention of the scientific community and have been considered as promising materials due to their unique structural and physical properties. In this paper, the behavior of BNNTs of different diameters under compressive loading has been studied through molecular dynamic (MD) simulations. We have used a Lennard-Jones pair potential to characterize the interactions between non-bonded atoms and harmonic potentials for bond stretching and bond angle vibrations. Results of the MD simulations determine the critical buckling loads of the BNNTs of various diameters under uniaxial compression, and indicate that for the simulated BNNTs of length L = 6 nm, the critical buckling loads increase by increasing the nanotube diameters.

Abstract: Hard coatings show various tribological behaviors against different contact materials (counter-faces) during dry sliding depended on their microstructure, surface morphology and encountered tribological systems and condition. In this work, the tribological and wear mechanisms of vanadium carbide (VC) and vanadium nitrocarbide (V(N,C)) layers were examined against WC/Co cemented tungsten carbide pin during pin-on-disk sliding wear testing. The V(N,C) layer was produced by a duplex surface treatment involving the gas pre-nitrocarburising followed by thermo-reactive diffusion (TRD) vanadizing technique. The coating layers were characterized by a cross sectional and morphological examination methods and X-ray diffraction analysis to identify damages of the coating’s surface. Wear mechanisms were determined by SEM microscope in BSI and SE mode accompanied by EDS analysis. Results revealed that the surface morphology of the V(N,C) coating consist of dense and smooth layer in comparison with the VC coating surface which reveals a non-uniform structure with chasms. It was determined that the activation of tribo-chemical system and oxidizing of the coating layer together with minor plastic deformation are the dominant wear mechanism in the V(N,C) coated steel. In the case of the VC coating, combination of abrasive wear and adhesion of pin material to coating and vice versa are the major impairing mechanisms.

Abstract: Plasma nitriding is a powerful process for surface modification of different materials. In this study, plasma nitriding is applied on a Nickel-Aluminum composite, coated on ST37 steel. Ni+Al composites were fabricated by electrodeposition process in watts bath containing Al particles. For prediction of electrodeposited Al% during the electroplating and microhardness of coatings after plasma nitriding process artificial neural network (ANN) was used. The numerical results obtained via a neural network model were compared with the experimental results. Agreement between the experimental and numerical results was reasonably good.

Abstract: In this research work, characterization of a broad area zone with ultra fine grain size of 7075-T6 grade aluminum sheets with the thickness of 8 mm was carried out using friction stir processing (FSP) and quenching by liquid nitrogen on both, i.e. upper and lower, surfaces. In the first step, ability and competence of two tools with different designs in creating a stir zone without any defects were examined; selected samples were exposed to FSP by efficient tools and in a highly extensive quenching media. Friction stir processing was carried out on the samples in three different rates (fast, medium and slow), the mechanism of ultra fine grain size as well as the creation of dislocation in thermo-mechanical affected zone was evaluated. Micro structural evaluation using transmission electron microscopy showed that the samples exposed to FSP in slow manner, had the finest grain size (20 nm) in the stir zone, and the lowest amount of dislocation in the thermo-mechanical affected zone. More details are discussed in the presented results.

Abstract: In this paper, sodium bentonite (BT) and organically modified sodium bentonite (OBT) are used to reinforce and modify asphalt binder. The microstructures of the reinforced asphalt binders are investigated using transmission electron microscopy (TEM) and wide angle X-ray diffraction (WAXD). Two models for the microstructure of the BT and OBT modified asphalts are suggested considering the colloidal structure of the base asphalt binder. The physical properties of the modified asphalt binders were studied.

Abstract: Over the last decades, numerous investigations have been conducted on Nitinol properties. However, the effects of alloying elements on Ni-rich NiTi alloys have been considered less. In this research, different effects of homogenization time and cooling rate on the behaviors of Ni-42.5wt%Ti-3wt%Cu alloy were evaluated. The mentioned alloy was fabricated by vacuum arc melting method. Three different homogenization times (half, one and two hours) and three cooling media (water, air and furnace) were selected. The microstructure and hardness were examined by means of optical and scanning electron microscope (SEM) equipped with an energy dispersive X-ray (EDX) analysis and hardness test, respectively. According to the microscopic investigations, no significant changes were observed after half an hour. However, results indicate that increasing time of homogenization leads to finer precipitations and a uniform distribution of them. The various cooling environments result in the formation of two types of precipitation phases. It was seen that in the high cooling rate, the majority of precipitations consisted of Ti2(Ni,Cu) while by decreasing cooling rate NiTiCu precipitates appeared too, which affect the hardness.

Abstract: A great deal of attention in manufacturing engineering has been focused on finishing operations of hard and brittle materials in recent years. This paper reports an experimental work on the analysis of surface roughness and material removal using design of experiment (DOE) method in magnetic abrasive finishing, (MAF) of flat surfaces. Change in surface roughness and material removal were found to increase with an increase in weight percentage of abrasive particles in magnetic abrasive brush, lubricant volume and decrease in working gap. Also, any decrease in the relative size of the abrasive particles vis-à-vis the iron particles would result into an increase of the surface roughness and decrease in material removal. It was observed that the work piece hardness had no considerable effect on the process results. The optimum parameter levels which lead into the best surface finish and highest material removal were also derived from these experimentations. Optimum levels included weight percentage of abrasive particles of 40%, Lubricant volume of 1 ml, working gap of 3 mm, relative size of abrasive particles vis-à-vis the iron particles of 0.22, and work piece hardness of 82-87 HBN. Disk type test pieces were selected from Al 7075 and their two side surfaces were under experiments. Experiments were made using a milling machine spindle as magnetic pole holder, and its table as fixture holder for work pieces.