فهرست مطالب خسرو رحمانی

In recent scientific studies, the use of graphene has been considered due to its positive effect on the properties of composites, including mechanical, electrical and thermal properties. In this research, the microstructural and mechanical properties of pure aluminum nanocomposite cast reinforced with 0.5 wt% graphene with mechanical-electromagnetic stirrer and then hot extrusion and finally annealing, in two methods of using ball milling and without using ball milling process, was investigated. By applying both mechanical and electromagnetic stirring techniques, combining both shear and body forces can cause more turbulence in the molten metal that lead to as well as distribute reinforcing particles, decreasing the dendritic structures and refining the grains during solidification. The microstructural studies showed that in the ball mill method, the distribution of graphene nanoparticles and reduce grain size in the matrix phase is significantly better than the non-ball mill method. Also, in the method of ball mill, hardness, tensile strength and compressive strength increased by 19.7, 142.8 and 11.7%, respectively, and in the method without mill, 9, 85.2 and 13.5%, respectively, compared to pure aluminum. The elongation decreased by 6.8% in the ball mill method and 35.4% in the non-ball mill method.

In this paper, a new model for estimation of the electrical conductivity of polymer carbon nanotube (CNT) nanocomposites based on the conventional power-law model and Halpin-Tsai formulation has been proposed. Halpin-Tsai model was originally presented to calculate the tensile modulus of composites, which can be modified for estimation of the electrical conductivity by replacing the electrical parameters. The nature of “b” exponent in power-law model is defined according to CNT dimensions, CNT electrical conductivity and the interphase thickness and also the impacts of these parameters on the “b” and the electrical conductivity of nanocomposite are taken into consideration. The developed model interprets that the electrical conductivity of polymer-CNT nanocomposite increases as the concentration, length and electrical conductivity of CNT and the interphase thickness increase. Furthermore, reduction in CNT diameter and waviness results in growth of nanocomposite electrical conductivity. In order to validate the developed model, nanocomposite samples with different volume fractions were produced by solid-state technique of the melt-blending method. The results of calculations and experimental procedure show good agreement.

As moisture condenses on the surface of the contaminated insulators, the electrical conductivity of the insulator surface increases, resulting in a leakage current on the surface and causing the insulators to fail. Condensation of moisture on the insulator surface occurs due to the mechanism of radiation cooling, mainly in the early mornings of winters. In this case, the temperature of the insulator surface is lower than the dew point. In this paper, a sample of real fine dust collected from the dust center and chemical analysis and measurement of electrical conductivity in the wet state were performed. By simulating a 230-kV contaminated composite insulator in COMSOL Multiphysics software, the magnitude of leakage current was obtained and verified by an experimental test. Also, using heat transfer governing relations, the temperature of the insulator surface during the leakage current flow, was determined and compared with the images captured by the thermo-vision camera during the test. The results show that the leakage current and the surface temperature of the insulator increase with increasing the conduction and thickness of the contamination layer and consequently the probability of insulator failure also increases. Experimental testing shows that simulation of insulators with finite elements-based commercial software can be a reliable method for pre-analysis of insulators.

In this study, which was conducted by the field and inductive methods and through interviews with semi-structured questions, 38 researchers and employers in the electricity industry were purposefully selected and interviewed in depth. In these interviews, some questions were asked about the research ethics norms of efficient researchers in the industrial field, and the answers were collected based on their research experiences. Then, using open coding, axial coding and selective coding on the data obtained from the interviews, the final analysis was performed by interpretive analysis. The results of this study indicate that the most important characteristics of research ethics about efficient researchers are: Honesty, specialty, patience, good communication, hard work, professional ethics, beyond the commitment to act, known in the industry, being responsive to objections and criticisms, understand the problems and the issue, national patriotism & flag, faithfulness, satisfaction / understanding the needs and position of the employer, being young, problem solving concern, curiosity, persistence, innovation and patent, research articles, applicability of research in one or more companies.

In this paper, the creep strength of hybrid composites used in the new generation of power transmission line conductors is predicted. The hybrid composite rods consist of carbon fiber/epoxy composite core surrounded by a glass fiber/epoxy composite shell. The hybrid rods were fabricated by using the pultrusion process. Dynamic mechanical analysis was carried out at various frequencies on specimens cut from the carbon/epoxy fiber composite core. In addition, the hybrid composite rods were subjected to three-point bending experiments at constant loading rate and different temperatures. The master curve of the storage modulus corresponding to carbon/epoxy composite core was derived at the desired reference temperature based on the time-temperature superposition principle. Consequently, the master curve of the constant strain rate flexural strength was constructed using the time-temperature shift factors and the monotonic flexural strengths of hybrid composites at different temperatures. Based on these data, the creep strength master curve was developed at the operating temperature. The prediction of creep life based on the constructed master curve shows a proper response of these conductors at service condition.

Among various manufacturing processes, the use of cold press- sintering has been considered with low cost, high efficiency, and the ability to produce components with the appropriate dimensional accuracy. In this method, at least two components of the alcoholic solution are used in granulation and milling steps. Removing the solutions is accompanied with energy consumption, as well as, an increment in probability of crack initiation, which may limited the widespread utilization of press-sintering process. Therefore, in the present study, the soluble component in the milling step was removed. In following just using PVA as a soluble component in the granulation step, the ceramic / ceramic composite was made. Doing this, samples based on NiO and YSZ were made with different weight percentages of the reinforcement component (25, 30 and 35% wt. zirconia-stabilized yttrium), and then their microstructure, density and micro-hardness were investigated. Microstructural studies indicate that the amount and distribution of porosity, surface quality and depth of cracks in the samples are depended on reinforcement weight. In detailed, at the surface of green tables, no deep crack was observed. In sintered specimens, the best distribution of the gas phase was observed in NiO- 30% wt. YSZ, which results in maximum micro-hardness. Also, the radiography results provided by the above samples indicated that deep crack in the discs is not visible. Therefore, it seems that the starch as a pore former displayed the beset role via porosity distribution and lessening crack nucleation in sample having 30% reinforcement.

Investigation of Persian Gulf seawater corrosion effect on the carbon steels and welded pieces used in construction of metallic storage tanks is important from this point of view that this investigation may result in using this seawater to do hydro test of these tanks. If the results of this evaluation be positive, it will have lots of economic and environmental advantages. In order to achieve this target in this study, at first a 15 mm thickness plate of ASTM A283 Grade C low carbon steel was welded by E7018 electrode then quality controlled by radiography test and its microstructure evaluated by macro and micro etch tests and also hardness test was done for it. Furthermore chemical composition, pH, conductivity and SRB amount within the water has been measured too. Then corrosion rates for each one of these mentioned steel and electrode in Persian Gulf seawater were measured by electrochemical method of linear polarization resistance and also immersion test. Then corrosion product and corroded surface were analyzed by EDS and SEM methods. Results of polarization and immersion tests proved that E7018 electrode is anode compared to ASTM A283 Grade C steel and this result was confirmed by SEM images.