فصلنامه مواد پیشرفته در مهندسی
سال سی و پنجم شماره 4 (زمستان 1395)

In this research, the nanopowders of lanthanum and neodymium co-doped yttria were synthesized by sol-gel combustion method. Citric acid and glycine were used as the gel maker and fuel respectively. The effect of molar ratio of citric acid to glycine on the grain size and morphology was evaluated. The optimized products were characterized by X-ray diffraction (XRD), field-emission scanning electron microscope (FESEM), transmission electron microscope (TEM), UV–visible (UV–Vis), thermal gravimetric-differential thermal analysis (TG/DTA), and Fourier transform infrared spectrometer (FTIR). The optimized products which are synthesized with a molar ratio citric acid to glycine 1.06:1.06 have an average grain size of 30-40nm with spherical morphology, and without agglomeration. Also, their band gap is 3.29eV.

In this study, the parameters affecting the synthesis of silver nanoparticles were optimized by green chemical reduction method to make a conductive pattern. The raw materials used in this study, include silver nitrate as a source of silver ions, polyvinylpyrrolidone as surface stabilizer, and glucose as the reducing agent. Effective parameters were investigated by Taguchi statistical design, to determine the optimum conditions and achieve the smallest average particle size. Silver nanoparticles were characterized by X-ray diffraction and field emission scanning electron microscopy. The smallest particle size can be applied by solution adding rate of 0.1 ml/min, temperature 90 °C, weight ratio of glucose to silver nitrate 3 g/g and weight ratio of Polyvinylpyrrolidone to silver nitrate 3.2 g/g. According to.our expectation 20 nm silver nanoparticles were obtained in this condition. FE-SEM confirmed the above results and showed nanoparticles with a size of 25 nm. Finally, A conductive pattern was printed on a glass substrate with synthesized powder. The electrical resistance of the printed pattern was 0.088× 10 -4 Ω.cm.

The electrochemical exfoliation of graphite via intercalation is one of the attractive methods to obtain graphene. However, presence of course graphite particles in product is the drawback of this method. In this research, the effect of adding cetyl trimethyl ammonium chloride (CTAC), on the amount of exfoliated graphene and functional groups was studied. Transmission electron microscopy, weighing, UV-vis spectroscopy and electrical conductivity were used for characterization of the products. According to results, presence of this surfactant decreases the erosion, while it increases the exfoliation of graphene flakes. However, after critical micelle concentration (CMC) of the surfactant, exfoliated weight decreased.

In this study, a Cr-modified NiAl coating was fabricated by weld cladding technique using Gas- Tungsten Arc Welding (GTAW) process on 310 steel. Chemical composition and microstructure of the coating was studied by X-Ray Diffraction (XRD), optical microscopy and scanning electron microscopy equipped with an Energy Dispersive Spectroscopy (EDS). The wear behavior of the coated steel was examined through pin-on-disc tests at ambient temperature and 400 °C. The results showed that the hardness of coated steel increased remarkably due to the formation of Cr-modified NiAl on the surface. Furthermore, the wear experiments showed that the presence of Cr-modified NiAl coating caused significant improvement in wear resistance of cladding 310 steel at both ambient temperature and 400 °C. These results were discussed based on the wear mechanism obtained from examination of the worn surfaces using SEM.

In this study, aluminium composite foams reinforced by different volume fractions of SiC particles as reinforcement and stabilizing agent were fabricated with the direct foaming route of melt using different contents of CaCO3 as foaming agent. The density of produced foams were measured to be from 0.38 to 0.68 g/cm3. The microstructural features and compressive properties of the AA356/SiCp composite foams were investigated. The relation between plateau stress, density and, weight percentage of CaCO3 and SiCp volume fraction with a given particle size was also investigated. The results showed that compressive stress-strain curves of the products were not smooth and exhibit some serrations. Also, it was shown that in the same density of composite foams, the plateau stress of the composite foams increases with increasing volume fraction of SiC particles and decreasing weight percentage of CaCO3.

In this study, fabrication of an in-situ composite through aluminothermic combustion synthesis in An Al–V2O5-NiO system was investigated. Therefore, Al, V2O5 and NiO powders with stoichiometric ratio of 11:1:1, respectively, were milled for an hour and finally the mixtures were compressed. In order to investigate the temperatures of phase transformations, Differential Thermal Analysis (DTA) was utilized. Heat treatment was applied on the raw samples according to their peak temperatures treated in DTA. X Ray Diffraction (XRD) analysis for the samples shows formation of phases such as Al3V and Al3Ni2 at different sintering temperatures. Microstructure and phase analysis showed that during sintering of this sample, Al3V phase was not formed below 700 °C, at 880 °C Al3Ni2 it was formed and after 950 °C, it was transformed to Al4Ni3 phase. In addition, after 950°C, Al3V transformed into Al23V4 phase. Analysis of samples density and hardness showed that, due to increase of volume fraction percentages of reinforcing phase, these two parameters increase as well.

In this work, cobalt ferrite nanocrystallites were synthesized by air annealing of milled Co-Fe compound. Effect of annealing temperature on phase formation of cobalt ferrite and structural and magnetic properties of the product was studied. Analysis of annealed sample in 450 oC showed that around 46 weight percent of the specimen was changed to Co2FeO4. This value increased to 95 and 90% for 800 oC and 900 oC annealed samples respectively. Reduction of saturation magnetization under annealing was related to transformation of Co-Fe to cobalt ferrite. Increasing the value of saturation magnetization in 900 oC annealed sample compared to 800 oC one was attributed to decreased surface to volume ratio and crystallite size. The main reason of occurrence of maximum coercivity in 800 oC annealed sample was its low crystallite size.

In this research titanium nitride (TiN) films were prepared by plasma assisted chemical vapor deposition using TiCl4, H2, N2 and Ar on the AISI H13 tool steel. Coatings were deposited during different substrate temperatures (460°C, 480 ° C and 510 °C). Wear tests were performed in order to study the acting wear mechanisms in the high(400 °C) and low (25 °C) temperatures by ball on disc method. Coating structure and chemical composition were characterized using scanning electron microscopy, microhardness and X-ray diffraction. Wear test result was described in ambient temprature according to wear rate. It was evidenced that the TiN coating deposited at 460 °C has the least weight loss with the highest hardness value. The best wear resistance was related to the coating with the highest hardness (1800 Vickers). Wear mechanisms were observed to change by changing wear temperatures. The result of wear track indicated that low-temprature wear has surface fatigue but high-temperature wear showed adhesive mechanism.

Presence of nanoscale bainitic ferrites and high carbon retained austenites that are stable at ambient temperature within the microstructures of super strong bainitic steels makes it possible to achieve exceptional strengths and ductility properties in these groups of nanostructured steels. This article aims to study the effect of the dislocation density variations during tensile testing in ambient temperature on deformation behavior of nanostructured low temperature bainitic steels. Results indicate that dislocation absorption from bainitic ferrite subunits by surrounding retained austenite reduces the work hardening and therefore increases the formability of bainitic ferrite during deformation, which in turn results in a suitable combination of strength and ductility.

In this study, creation of a silicon aluminide coating on IN738LC nickel-based superalloy has been investigated, using co-deposition process. Thermochemical calculations indicated the possibility of obtaining a silicon aluminide with NH4Cl activated pack powder at 900°C, in order to achieve coating with desirable structures. Two powder mixtures with nominal compositions of 7Si-14Al-(1-3) NH4Cl-Al2O3 (wt. %) and 16Si-4Al-(1-3) NH4Cl-Al2O3 (4 and 0.5 Si/Al ratios, respectively) were used. According to the results, both coatings showed multi-layered structures containing AlNi2Si as dominant phase. In coating created by pack powder with Si/Al ratio of 0.5, a porous and brittle layer of NiSi was formed on the surface which deteriorated the mechanical properties of coating to some extent. It was found that inward diffusion of Al was dominant at the first stage, while afterward, inward diffusion of Si led to conversion of NiAl phase to AlNi2Si and, finally, to NiSi phase. Eventually, the sample coated by Si/Al=4, showed superior microstructural characteristics, containing desirable AlNi2Si phase without undesirable brittle NiSi phase.