فهرست مطالب mohammadjavad eshraghi

In the current research, Nd2(Fe,Co)14B hard magnetic nanoparticles were synthesized through the reduction diffusion process. For this purpose, Nd(Fe1-xCox)B oxide powders for x = 0.05, x = 0.3, and x = 0.5 were heat-treated once in hydrogen atmosphere (H2) and once in the reduction diffusion process using calcium hydride (CaH2). The phase analysis and chemical composition of the resulting Nd-Fe-Co-B powders were identified by X-ray diffraction and X-ray energy dispersive spectroscopy. The morphology and magnetic properties of the synthesized powders were investigated using a field emission scanning electron microscope, transmission electron microscope, and vibrating sample magnetometer. The results demonstrated that oxide powders reduced with hydrogen gas were characterized by a soft magnetic character due to the formation of the bcc-FeCo magnetic phase. However, oxide powders reduced via reduction diffusion exhibited hard magnetic characteristics due to the direct diffusion of NdH2, Fe, Co, and B phases as well as the production of Nd2(Fe,Co)14B hard magnetic phase. The Nd2(Fe,Co)14B particles were rinsed with water and dilute acetic acid to eliminate the byproducts (CaO) formed during the reduction diffusion process. Followed by washing, coercivity dropped due to the formation of the Nd2Fe14BHx soft magnetic phase; however, saturation magnetization rose due to the elimination of the non-magnetic CaO phase from the final production.

Large area fabrication of graphene, as a leading two-dimensional material as well as an allotrope of carbon, is a challenging requirement prior to its preparation for applications. Chemical Vapor Deposition (CVD) is one of the most effective and promising methods for high-scale and high-quality synthesis of graphene. In this study, graphene layers were grown on copper (Cu) sheets using low-pressure CVD technique at 930 °C, 870 °C, and 760 °C. Raman spectroscopy, Field Emission Scanning Electron Microscopy (FESEM), Optical Microscopy (OM) and Atomic Force Microscopy (AFM) were employed in this study to investigate the effect of the process temperature on the structural properties, morphology, grain boundaries, continuity, purity, and number of layers. The results from analyses revealed that at higher temperatures, the continuity and quality of the layers and number of grain boundaries were higher and lower, respectively. In contrast, at lower temperatures, the nucleation and discontinuity of the deposited layers were relatively high. The surface roughness of the graphene sheets increased with a decrease in temperature.

Abstract     Nowadays, magnesium alloys such as a new generation of biodegradable materials have been noticed by researchers. In this study, to the sanitation of biodegradability and biocompatibility of magnesium alloys, TiO2/MgO dual layer coating was formed by magnetron sputtering on AZ91 and the microstructure, corrosion behavior and biocompatibility properties of coating were investigated. The coating microstructure was studied via field emission scanning electron microscopy (FESEM) X-ray diffraction (XRD). The corrosion resistance of the substrate and coated sample was evaluated by electrochemical polarization test in simulated body solution (SBF). To evaluate biocompatibility MTT and cell viability tests were used. The results showed that the corrosion potential after the surface coating with TiO2/ MgO was more positive, changing from 1.532 V for the uncoated sample to 1.436 V for the coated sample.. The coating was observed in quasi-spherical morphology with MgTi2O5 and MgTiO3 phases in interface of coating and substrate and two layers of coating too according to XRD analysis results. The results of the biocompatibility tests also showed that the viability of the osteoblastic cells on the coated sample increased compared to substrate.

In this study, the effect of phosphorous concentration has been investigated on performance of porous silicon solar cells. For this purpose, silicon substrates were etched into aqueous etching solution containing hydrofluoric acid / hydrogen peroxide with molar ratios 0.82, 0.87 and 0.89 via metal-assisted chemical etching. Surface characteristics were studied by optical microscopy, field-emission electron microscopy and antireflection analysis. In the following, solar cells were fabricated by phosphorous diffusion into optimized porous silicon substrate which phosphorous concentrations had values of 3%, 9%, and 15%. The result presented that in 0.82 molar ratio, the etched silicon had uniform porosities whose sizes were nearly to Ag nano-particles. The reflectance of this substrate was decreased to 11% and was the best substrate for absorbing incident light. Survey of fabricated solar cell performance showed 98% increasing of short circuit current in the solar cell which was made by P2O5 3%. This concentration due to is near the solubility of phosphorous in silicon, caused the increasing efficiency to 10.6%.

ßíÏå In this study the effect of sintering parameters on the microstructure and electrical properties of the barium strontium titanate sputtering target is investigated. For optimizing the sintering temperature, BST compacts sintered at various temperatures ranging from 1200 to 1400 ºC for 2 hours. The sintered Barium Strontium Titanate sputtering target comprising with a high density, purity and a fine-grained microstructure has been also fabricated. Optimized calcining and sintering condition was obtained by controlling sintering temperature and time to fabricate single phase, high relative density sintered body. A relative density of up to 95% and mean grain size of 8 µm were reached at the sintering temperature of 1250C. The relative permittivity of thin films at 10 KHz was around 4400. Also the thin film bandgap was around 3.2 eV. The densities of the sintered samples were measured using the Archimedes method. Characteristics of the body were analyzed by X-ray diffraction (XRD), optical microscopy. Dielectric permittivity (εr) were measured with an HP 4192A impedance analyzer and thin film bandgap was analyzed by UV-Vis method.

In this research diamond-like carbon (DLC) films were deposited on AISI316L stainless steel by plasma enhanced chemical vapor deposition (PECVD) method. In order to improve adhesion of the coating, plasma nitrocarbuizing method was performed on substrates before DLC deposition. Raman spectra of the coating showed that sp3 fraction of the film was 39%. Biological properties of the uncoated, plasma nitrocarburized and DLC coated samples were analyzed. MTT results showed that 316L stainless steel didn?t have appropriate biocompatibility and nitrocarburizing process and DLC deposition improved the biocompatibility of the samples due to preventing the release of toxic elements such as nickel. Cell viability of the uncoated, plasma nitrocarburized and DLC coated samples was about 29%, 40% and 71% respectively.

In this report, we explained the formation and characterization of high-porosity, uniform, and stable porous silicon (PS) layers. An innovative combination of electroless and electrochemical etching of silicon was introduced using a novel parameter of delay time (Td). The electroless chemical etching was optimized by varying the delay time (Td) prior to the electrochemical process to control the uniformity of the pores. The porosity and uniformity of PS samples were enhanced by applying the optimized value of delay time. The PL spectra of porous samples were measured in room temperature. The optimized sample showed the most intense PL peak compare to the other porous samples due to its highest porosity and elevated specific surface area which enhanced the absorption coefficient of this porous surface. The optical characteristics of the optimized sample indicated the highest porosity of this sample due to the application of sufficient duration of electroless chemical etching prior to applying the pulsed current. It was found that the performance of the photodetector on optimized PS sample is much better than the other porous samples.

The effect of evaporation rate on structural, morphological and optical properties of electron beam evaporated CdS thin films have been investigated. CdS thin film deposited by electron beam evaporation method in 12nm/min and 60nm/min evaporation rates on glass substrates. X-ray diffraction, scanning electron microscopy, UV-Vis-NIR spectroscopy and Atomic Force Microscopy were used to characterize thin films. The x-ray diffraction analysis confirms that films have polycrystalline hexagonal phase and exhibited preferred orientation along the (002) plane. The crystallite size were calculated and found to be increased from 23 nm to 30 nm by increasing deposition rate. Results of Atomic Force Microscopy revealed that the RMS roughness values of the CdS films decreased as deposition rate increased. The relation between deposition rates and optical properties of deposited films was investigated. It was found that stoichiometric properties and band gap values of the deposited films are correlated to deposition rates. These dependencies are associated to the Cd/S ratio variation by deposition rate. The optical band gap values of CdS films increased slightly in a range of 2.32–2.34 eV for deposition rate varied from 12nm/min and 60nm/min .