مجله مواد مهندسی
پیاپی 3 (2009)

Spinel refractory has received much interest in industry because of combined ultra good chemical resistance to alkaline salts، high refractoriness and high mechanical strength. In this study samples which were containing Alumina and Magnesia (mole ratio 1-1)، prepared in order to formation of spinel phase. In the next Step، the first group of samples was sintered in a 2. 45GHz Kitchen Microwave for 30 minutes and the others (group 2، 3 and 4) were sintered in an electrical furnace، where second and third groups were heated for 1 hour، at 1350˚C and 1400˚C، respectively and the last ones was sintered at 1400˚C for 2 hours. Due to low loss factor of Alumina and Magnesia at low temperatures، a silicon carbide substrate was used to improve heating during Microwave sintering. Also، the effect of adding silicon carbide on the sintering of samples was investigated. Results showed that the density of sintered samples in electrical furnace were enhanced by increasing silicon carbide content، in contrast with، density of the Microwave sintered samples were decreased. It was seen، the first group reached to the same amount of spinel in a shorter time by consuming lower energy in comparison with other groups.

In this investigation، the effect of various parameters of aging treatment on the superelastic behavior of Ni50. 9Ti49. 1 wires was studied. The samples were aged at 300، 350، 400، 450، 500، 550 and 600˚C for 10، 30 and 120 minutes. Tensile tests were performed for all samples. Results of the tensile tests for as-received sample show a low superelastic effect. It means that there are remained strains in as receive sample after loading and unloading cycle. The aging treatments with short and intermediate aging times result in increasing the superelastic property، while there are considerably remained strains in samples، aged at 450، 500، 550 and 600˚C for long aging time (120 minutes). Moreover the results indicate that، both upper and lower plateau stress are decreased with increasing the time and temperature of aging. The results of tensile experiments up to fracture for aged samples at different temperatures for 30 minutes show that with increasing the aging temperature up to 400˚C، the magnitude of fracture stress increases slowly. With increasing the aging temperature more than 400˚C، the fracture stress decreases. As the aging temperature increase، the strain content of plateau increase. Therefore، we can conclude that combined use of work-hardening and age-hardening is more effective to increase the critical stress for slip and consequently to improve superelasticity

Segmentation is one of the most complicated procedures in the image processing that has important role in the image analysis. In this paper، using the pixon concept، a segmentation algorithm is proposed based on Markov Random Field (MRF). In the proposed algorithm images are initially modeled using pixonal images، and then the MRF-based segmentation process operates on the pixons to produce the segmented image. The results indicate that the proposed algorithm is superior to the existing image segmentation techniques in terms of segmentation accuracy and speed.

In this paper، synthesis of HAp particles with Dandelion-like morphology، a unique morphology rarely reported previously has been studied via a micelle-directed hydrothermal method. The phase composition، morphology and particle size of synthesized powder were investigated using X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR) techniques and Scanning electron microscopy (SEM). Results revealed that cationic surfactant cetyltrimethylammonium bromide (CTAB) and non-ionic co-surfactant polyethylene glycol 600 (PEG 600) can act as template and co-template، respectively for synthesis of dandelion-like HAp. SEM observations showed HAp crystals have a uniform dandelion-like morphology with an average diameter of about 100-150 nm and aspect ratio of about 20 for each tooth. Fourier transform infrared spectroscopy study revealed that a certain level of carbonate substitution has taken place in the product. The template action of CTAB and co-template action of PEG 600 used to regulate the nucleation and crystal growth are discussed.

Today''s mixed metal oxide anodes are widely used in the chloralkali media. Stability is one of the main properties of these anodes because of their high cost. In this article، the effect of heating rate، environment and oxygen on the morphology and electrochemical stability of Ti/ (10Ti–20Ru–70Sn) O2 anode has been investigated. The anodes were prepared by brushing and thermal decomposition method. Mud cracked morphology appeared on the sample by this process. Air is the best environment and putting samples in the furnace without heating rate are the best result for obtaining the highest stability and the best morphology.

A MoSi2-SiC nano-composite from Mo، Si، C powders has been prepared by employing a combination of high-energy milling and SHS in Oven Chemical mode. Nano powders of (Mo+Si+C) mixtures and co-combustion powders (Mo-Si-Al) were prepared by milling process and mixing، respectively and separately then they was subjected to SHS synthesis. Final Products were subsequently characterized by XRD، SEM and TEM. After optimization of milling-times and use of oven chemical mode، MoSi2-SiC with crystallite sizes in the order of 100nm were successfully synthesized.

In the present work multi step martensitic transformations of Ni51Ti49 single crystal has been investigated by differential scanning calorimetric (DSC). The sample with the mass of 77 mg after solution annealing treatment was aged at 400˚C for different aging times between 10 minutes and 50 hours. DSC experiments have been performed on the sample after each aging treatment. Results show that for low aging times (10 to 45 minutes) martensitic transformations occur in two steps، for middle aging times (75 minutes to 4 hours) in three steps and higher aging times (more than 8 hours) martensitic transformations occur in two steps again. Indeed، the multi step martensitic transformations in single crystals of NiTi alloys indicate 2-3-2 behavior in DSC curves with aging times like polycrystals. In the present investigation proposed mechanism related to the multi step martensitic transformation in single crystals and polycrystals such as heterogeneity of composition in both microscopic and macroscopic scale and existence of stress fields around the Ni4Ti3 particles have been discussed.

Vortex shedding phenomena from circular cylinders imposes oscillatory flow induced forces on these structures. Splitter-plates in the downstream/upstream of the circular cross section cylinder could be embedded in order to reduce flow induced forces and suppress vortex shedding. In this Article، flow over a circular cylinder with detached short splitter-plate is numerically simulated by solving two dimensional، unsteady، and viscous flow fields. Implemented Method is based on finite-volume approach on Cartesian-staggered grid. In addition، the ghost-cell method in conjunction with Great Source Term technique is employed to enforce directly no-slip condition on the cylinder boundary. The accuracy of the solver is validated by simulation of the flow over a single circular cylinder. The results are in excellent agreement with the experiments reported in the literature. Eventually، the flow over a circular cylinder with splitter-plate in its downstream in Re=40، 100 and 150،Your browser may not support display of this image. and Your browser may not support display of this image. (where D is the cylinder diameter، G and Z the horizontal and vertical distances between leading edge of the splitter-plate and the cylinder base) are simulated and analyzed. Critical values of G and Z where vortex shedding is fully suppressed is calculated. The results show that the critical values are significantly dependent on the G and Z.

Titanium aluminide-alumina composite is a new category of advanced materials with unique properties titanium and promising performance for high-temperature applications. In this research، the physical properties of these composites are investigated. The composites were prepared by hot pressing of a powder mixture produced by combustion synthesis. According to the results، this composite exhibits electrical conductivity; however، no thermal conductivity is observed.

Silicon carbide (SiC) particles coated with nano-boron nitride (BN) were synthesized. The SiC/BN ceramic nano-composites and microcomposites were sintered by plasma-activated sintering. For the nano-composites، due to the homogeneous dispersion of the nano-BN crystals around the SiC grains of the matrix، the grain size of nano-composites was smaller than the monolithic SiC and micro-composites. As a result، the bending strength of the nano-composites decreased slowly (for SiC/20 % h-BN nano-composite، about 18% decrease)، while their hardness decreased sharply and the machinability properties were improved noticeably.