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Articles by H. Arabshahi
Total Records ( 30 ) for H. Arabshahi
  H. Arabshahi and G.R. Ebrahimi
  A new simulation method has been developed and used to model mechanical properties of materials at many different length scales, from the nanoscale where an atomic description is appropriate, through a mesoscale where dislocation based descriptions may be useful to macroscopic length scales. In some materials, such as nanocrystalline metals, the range of length scales is compressed and a polycrystalline material may be simulated at the atomic scale. It is observed how the grain boundaries contribute actively to the deformation. At grain sizes below 5-10 nm deformation in the grain boundaries dominate over the traditional dislocation-based deformation mechanisms. This results in a reversal of the normal grain size dependence of the yield stress. It is shown that the material becomes softer when the grain size is reduced.
  M. Gholizadeh and H. Arabshahi
  The effect of magnetism on the internal scale formation responsible for pitting corrosion inside the engine's cooling jackets have been studied. It is found that the presence of permanent magnetic field inhibits scale formation and protects the cooling systems in petrol and diesel engines from internal corrosion.
  A. Binesh and H. Arabshahi
  Radon makes up approximately half of the total dose of radiation were receive naturally. The majority of it comes from the inhalation of progeny of 222Rn and is prominent in a closed atmosphere. The continuous measurement of the levels of 222Rn concentration in different geographical areas is of great importance, particularly in living places. In this study, the concentration of radium and radon in 120 samples of drinking, springs and rivers water sources of north west regions of Mashhad city have been measured. Solid state nuclear track detectors were used for measuring the concentration. The average value of radon and radium concentrations in the studied area is found to be 30.2±5.1 and 18.4±2.2 Bq m-3, respectively. The dose rate due to radon, radium and their progenies received by the population in the studied location between 0.1-0.5 mSv year-1. The arithmetic and geometric mean concentrations are 0.2±0.05 and 0.2 mSv year-1, respectively. The results show no significant radiological risk for the inhabitants of the studied regions.
  F. Abbasi , M. Asmari and H. Arabshahi
  Because of low special resolution of General Circulation models, they can not predict weather and climate accurately. In this regards weather generator technique have been developed by climatologists to downscale GCM outputs into station scale. In this research, girded meteorological outputs of ECHO-G model including precipitation, maximum temperature, minimum temperature and radiation have been downscaled over 18 synoptic stations of Zagros during 2010-2039 with A1 scenario. Results show that the mean annual precipitation will be decreased by 2%, increasing of mean annual temperature by 0.4°C during period of 2010-2039. Maximum increase predicted to occur over Fars, Esfahan provinces. Also thresholds of heavy and extreme rainfall will be increased by 3 and 19%, respectively. In this regards in the future period, the rainfalls will be heavy and flash-flooded and there is a significant decrease in the amount of snow falls.
  Z. Pourhabib , A. Binesh and H. Arabshahi
  Radon is an odorless, radioactive gas formed from the breakdown of uranium and thorium. Chronic exposure to elevated radon (222Rn) decay product concentrations is recognized as health risk. The water used for drinking and other household uses can increase the indoor radon level because dissolved radon escapes from the water and gets into household air in the course of dishwashing showering and other water-using activities. Also, ingesting water with elevated levels of radon can present a risk for developing internal organ cancers. In this study radon concentrations of the 43 water samples have been measured by PRASSI system. The 4 samples have radon concentration higher than 11 Bq L-1 as normal level. Similarly, the annual effective dose in stomach and lung per person has been evaluated in this research. According to the advised of WHO and the EU council none of the samples induced the total annual effective dose >0.1 mSv year-1.
  H. Arabshahi and F. Sarlak
  Temperature and doping dependencies of electron mobility in both wurtzite and zincblende 6H-SiC structures have been calculated using an Iteravive technique. The following scattering mechanisims, i.e., impurity, polar optical phonon, acoustic phonon, piezoelectric and electron plasmon are inculded in the calculation. Ionized imurity scattering has been treated beyound the born approximation using the phase-shift analysis. It is found that the electron mobility decreases monotonically as the temperature increases from 100-600 K. The low temperature value of electron mobilty increases significantly with increasing doping concentration. The iterative results are in fair agreement with other recent calculations obtained using the relaxation-time approximation and experimental methods.
  H. Arabshahi and M. Rezaee Rokn-Abadi
  A Monte Carlo simulation has been used to model steady state and transient electron transport in 6H-SiC field effect transistor. The simulated device geometries and doping are matched to the nominal parameters described for the experimental structures as closely as possible and the predicted 1-5 and transfer characteristics for the intrinsic devices show fair agreement with the available experimental data. Simulations of the effect of modulating, the gate bias have also been carried out to test the device response and derived the frequency bandwidth. Value of 90±10 GHz has been derived for the intrinsic current gain cut-off frequency of the 6H-SiC MESFETs.
  H. Arabshahi and M. Rezaee Rokn Abadi
  The Monte Carlo method is used to simulate electron transport in bulk wurtzite phases of 6H-SiC and 4H-SiC using a three valley analytical band structure. Spherical, non-parabolic conduction band valleys at the Γ, K and U symmetry points of the Brillouin zone are fitted to the first-principles band structure. The electron drift velocity is calculated as a function of temperature and ionized donor concentration in the ranges of 300-600 K and 1016-1020 cm-3, respectively. Due to the freezout of deep donor levels the role of ionized impurity scattering in 6H-SiC is suppressed and the role of phonon scattering is enhanced, compared to 4H-SiC. For two materials, it is found that electron velocity overshoot only occurs when the electric field is increased to a value above a certain field unique to each material. This critical field is strongly dependent on the material parameters.
  H. Arabshahi and M. Rezaee Rokn-Abadi
  An ensemble Monte Carlo simulation has been used to model electron transport in n+-i (n)-n+ ZnO diode at 300 K in comparison with hydrodynamic approach. Electronic states within the conduction band are represented by non-parabolic ellipsoidal valleys centered on important symmetry points of the Brillouin zone. An original decomposition of velocity and energy profiles along the structure in terms of field, convective and diffusion contributions are presented. The anode voltage ranges from 0-3.75 V. The distributions of electron energies and electron velocities and the profiles of the electron density, electric field and average electron velocity are computed. Based on these data, the excellent agreement of the hydrodynamic approach with Monte Carlo simulations is discussed.
  S. Homayooni , F. Karimzadeh and H. Arabshahi
  The relationship of DRC (Domestic Resource Cost) can indicate the relative benefits among activities. If the product having DRC is equal to one this means for gaining a dollar in the international level must be consumed the 1 dollar equivalent of Domestic Resources. The ratio of DRC <1 indicates activities in which the country has an international advantage since the activities which their DRC is >1 is interpreted as lack of the relative advantage. Since, Iran's statistics of foreign commerce indicating the apple export in figure of export products pertaining to the garden surplus to domestic requirement and its import in figure of both the changeable and unchangeable products. Therefore in this study, researchers proceed the analysis of the product's shade prices within the framework of the different scenarios. For calculating apple's shade price from universal prices of this product is used as the import CIF price consider every tone of the product and with adding charging cost and transportation as well as from border to consumption centers, the shade price gains in consumption centers. In sum, resaearchers can proceed to appoint competitive advantage chosen countries in exporting apple in this figure with considering the shade price of exporting apple's exchange product and with considering direct export shade price, researchers can attain Iran's competitive position and specially Razavi Khorasan's chosen countries in relation to direct export of apple. So, the shade price of farm is determinant of chosen countries' competitive position in allocation and without allocation resources. The shade price of apple direct import is explanatory of its replacing power with domestic production and then importing exchange products.
  S. Homayooni , H. Daliri , F. Karimzadeh and H. Arabshahi
  Investment is one of the key factors to create a steady growth in economy of the developing countries. In today’s world, only the countries are considered important that have a production line and technological support to back it up. For this, a great deal of capital is needed and to accumulate it using a foreign source to supplement the domestic resources sounds rational. This also could be a way to import the modern technology in the country. However, it must be born in mind that in order to attract foreign investment, one should find and modify the influencing factors on direct foreign investment. In this study using the VAR model, we reviewed the interaction between foreign direct investment and domestic investment and the economic growth during 1990-2004 in Iran.
  H. Arabshahi
  A new simulation method has been developed and used to model mechanical properties of materials at many different length scales from the nanoscale where an atomic description is appropriate, through a mesoscale where dislocation based descriptions may be useful to macroscopic length scales. In some materials such as nanocrystalline metals, the range of length scales is compressed and a polycrystalline material may be simulated at the atomic scale. It is observed how the grain boundaries contribute actively to the deformation. At grain sizes below 5-10 nm deformation in the grain boundaries dominate over the traditional dislocation based deformation mechanisms. This results in a reversal of the normal grain size dependence of the yield stress. It is shown that the material becomes softer when the grain size is reduced.
  G.R. Ebrahimi , H. Keshmiri and H. Arabshahi
  In making tubes of corrosion resistant and hardly deformed steels and alloys, the pilger rolling method is used for hot rolling of final thick-walled tubes or mother tubes of large diameters (above 300 mm) and small quantities of other size tubes when no other, more efficient tube rolling or extrusion equipment is available. To clarify individual parameters of the production process and make choice of the deformation and temperature parameters, mechanical properties of the alloy type 30Cr25Ni32Mo3 superaustenitic stainless steel at various temperatures were studied. The tests have been performed using samples taken from the forged 400 mm diameter billet to determine strength and plastic properties of the billet metal at various temperatures and its macro and microstructure. The test results will be used in the choice of optimum conditions of preheating of the billets and hot rolling of tubes. On the whole, it should be stated that as forged alloy 30Cr25Ni32Mo3 features a favorable combination of strength and plastic properties in the hot working temperature range of 1075-1200°C.
  H. Arabshahi , G.R. Ebrahimi and S. Gholafroz
  The dipolar electric field arising from the opposite displacement of the negatively and positively charged atoms provides a coupling between the electrons and the lattice which results in electron scattering. In this study, a new model of inelastic electron-phonon scattering has been carried out in which the scattering probability does not depend on macroscopic parameter. The reviewed model gives the good agreement between the theory and experiment.
  H. Arabshahi , M. Rezaee Rokn-Abadi and D. Ghodsi Nahri
  In this research, we have solved the rate equations for GaAs/AlGaAs self-assembled quantum dot laser with considering the homogeneous and inhomogeneous broadening of the optical gain using 4th order Runge-Kutta method. With increasing the Full Width at Half Maximum (FWHM) of homogeneous broadening, the threshold current, turn-on delay and steady-state photons increase because of increasing the density of states in the central group. The calculation results show also that the simulated self-assembled dot laser reaches the steady-state faster and the lasing emission is not single mode due to the gain saturation.
  H. Arabshahi
  A new simulation method has been developed and used to model mechanical properties of materials at many different length scales from the nanoscale where an atomic description is appropriate, through a mesoscale where dislocation based descriptions may be useful to macroscopic length scales. In some materials, such as nanocrystalline metals, the range of length scales is compressed and a polycrystalline material may be simulated at the atomic scale. It is observed how the grain boundaries contribute actively to the deformation. At grain sizes below 5-10 nm deformation in the grain boundaries dominate over the traditional dislocation based deformation mechanisms. This results in a reversal of the normal grain size dependence of the yield stress. It is shown that the material becomes softer when the grain size is reduced.
  H. Arabshahi
  A new numerical method has been carried out to solve the differential equation using Fast Fourier Transform (FFT). The new algorithm has been accompanied by a numerical example. Firstly, we solve a Cauchy problem for an elastic vibrating system using the finite difference method. Then with the values of the approximate solution obtained in the equidistant points from the interval (0, 1), we shall find an interpolation polynomial using FFT. Also we study, the approximation of the numerical solution and stability of the difference scheme which correspond of a second-order differential equation.
  H. Arabshahi , M. Rezaee Rokn-Abadi and F. Badieyan Bagh-Siyahi
  Temperature and doping dependencies of electron drift velocity in wurtzite ZnO structure have been calculated using ensemble Monte Carlo method and compared with electron drift velocity in GaN and SiC in steady-state and transient situation. The following scattering mechanisims i.e., impurity, polar optical phonon and acoustic phonon are inculded in the calculation. The maximum electron drift velocity that is obtained in room temperature for 1023 m-3 donor concentration are 2.2x107, 3x107 cm sec-1 for ZnO and GaN, respectively. SiC shows the negative differential mobility just in low temperature. For high applied electric field transient electron drift velocity shows a significant overshoot in ZnO and GaN.
  A. Binesh , S. Mohammadi , A.A. Mowlavi , P. Parvaresh and H. Arabshahi
  Radon and its radioactive progenies in indoor places are recognized as the main sources of public exposure from the natural radioactive sources. The tap water used for drinking and other household uses can increase the indoor radon level. In the present research drinking water samples were collected from various places and supplies of public water used in Mashhad city which has about 4 millions population. Then radon concentration has been measured by PRASSI system three times for each sample. Results show that about 75% of water samples have radon concentration >10 Bq L-1 which advised EPA as a normal level. According to measurements data, the arithmetic mean of radon concentration for all samples was 16.238±9.322 Bq L-1. As well as the annual effective dose in stomach and long per person has been evaluated in this research. According to the advised of WHO and the EU Council, just 2 samples induced the total annual effective dose greater than 0.1 mSv year-1.
  A.A. Mowlavi , A. Binesh and H. Arabshahi
  In this study, the radial Schrodinger equation for central coulomb potential using numerical Runge-Kutta has been solved. Energy eigenvalues for hydrogen and positronium bound systems is derived -13.6056 and -6.803 eV, respectively. Numerical results of ground state modes of wave functions for hydrogen and positronium R (r) and the presence probability function |rR(r)|2has been presented. These results are in good agreement with analytical calculations of the hydrogen atom in modern physics and quantum mechanics. Therefore, numerical methods can be very useful and effective in solving physical problems.
  H. Arabshahi , M. Hakimi and R. Rezaee
  Solvomagnetic method is a famous method to get pure crystals of organic or inorganic compounds. In this method with applying a certain strong magnetic field new pure crystal structures can be prepared from original samples. The result will be useful for further complete analysis. In this research some new transition metals salts crystals have been produced by using solvomagneticthis method and their crystal structures have been determined using X-ray diffraction pattern and melting point has also been measured.
  A. Binesh , A.A. Mowlavi and H. Arabshahi
  We have solved numerically the Schrodinger wave equation for one-dimension or spherically symmetric potential by writing a code in FORTRAN. The method which we have used in the calculation is based on Runge-Kutta method. The Schrodinger equation has been solved for different V (x) and has been checked for the Hydrogen atom. The solutions are consistent with resent data.
  H. Arabshahi and D. Ghodsi Nahri
  In this study researchers have studied the effects of nonlinear gain and thermal carrier escape on dynamic characteristics of GaAs/InGaAs self-assembled quantum dot laser with considering the homogeneous and inhomogeneous broadening of the optical gain using fourth order Runge-Kutta method. The calculations show that the thermal carrier escape leads to shift the dominant lasing mode at the low injection currents. The number of lasing modes increases for the larger injection currents. With exceeding the FWHM of homogeneous broadening from the full width at half maximum FWHM of inhomogeneous broadening, the dynamic and static characteristics degrade and SAQD-LD reaches the steady-state slower. The threshold current, the steady-state photons and the dynamic-characteristics degrade and SAQD-LD reaches the steady-state slower as the FWHM of inhomogeneous broadening and carrier relaxation life time increase.
  M. Gholizadeh and H. Arabshahi
  A machine for magnetic water treatment is designed and its application on the ions contained in water has been investigated to examine the effect of magnetic water on concrete parameters. Strength parameters of concrete have been studied for >104 concrete samples including the nonmagnetic samples (made by ordinary water) and magnetic samples (made by magnetic water) with slump and compressive strength experiments. Based on slump experiments, magnetic samples were 7 cm more than nonmagnetic group and the average compressive strength of samples made by magnetic water was 23% more than that of samples made by ordinary water. The experimental results show the advantages of magnetic samples in concrete industry because of increase in plasticity, the efficiency and quality of concrete boosts in comparison with nonmagnetic samples.
  H. Arabshahi and G.R. Ebrahimi
  A Monte Carlo model that simulates the primary electron production inside the photoconductors mentioned, for a number of monoenergetic and polyenergetic x-ray spectra that cover the mammographic energies has been developed. The model simulates the primary photon interactions (photoelectric absorption, coherent and incoherent scattering) as well as the atomic deexcitations (fluorescent photon production, Auger and Coster-Kronig electron emission). In addition, a mathematical formulation has been developed for the drifting of primary electrons of a-Ge in vacuum under the influence of a capacitor’s electric field and the electron characteristics on the collecting electrode are being studied. The formulation is based on the Newton’s equations of motion and the theorem for kinetic energy change. Furthermore, a code has been developed that calculates the distribution of the electric potential inside a-Ge using an existing analytical solution, the boundary values of the case and certain numerical calculation methods. Finally, the structure and the mathematical formulation of a model that would simulate the electron interactions inside a-Ge have been developed. An existing model has been reexamined and enriched with certain theoretical considerations and simulation formalisms.
  H. Arabshahi and D. Ghodsi Nahri
  An ensemble Monte Carlo simulation has been developed to simulate the motion of electrons in a submicron GaN diode with a AlxGa1-xN heterojunction cathode. It is shown that the hot electron injection through the heterojunction cathode is effective to increase the mean electron velocity of carriers. The analysis has also shown that the mean drift velocity for electrons in the channel is about 2x105 m sec-1 at bias 4 V. Mean drift velocity in channel decrease with temperature and reach to saturated value about 1.5x105 m sec-1.
  H. Arabshahi and F. Taghavi
  Electron transport properties in GaSb and GaAs are calculated for different temperature, doping dependencies at high electric field applications. The calculations are performed using a three valleys ensemble Monte Carlo model that includes numerical formulations of the phonon scattering rates and ionized impurity scattering rates. For two materials, it is found that electron velocity overshoot only occurs when the electric field in increased to a value above a certain critical field. This critical field is strongly dependent on the material parameters. Results from the two materials are finally compared. The agreement with the available experimental data is found to be satisfactory.
  H. Arabshahi and D. Ghodsi Nahri
  Application of the Genetic Algorithm to the GaAs/AlGaAs quantum wells are presented. We followed a method that is produced by using the Genetic algorithm, Variation method and Monte Carlo integration Scheme (GMV method). We have investigated the effect of the well width on the diamagnetic shift. The effect of the Al doping is also investigated.
  H. Arabshahi and F. Sarlak
  We introduce a new calculation method to compute the electron transport properties in semiconductor devices. Using the relaxation-time approximation, the Boltzmann transport equation for electrons has been solved to calculate the thermal energy flux, electrical conductivity, seebeck coefficient and thermal conductivity.
  H. ARABSHAHI
  The results of thermoelectric power and electron drift mobility in AlxGa1-xN lattice-matched to GaN are calculated for different temperatures, free-electron concentrations and compositions. The two-mode nature of the polar optic phonons is considered jointly with deformation potential acoustic, piezoelectric, alloy and ionized-impurity scattering. Band non-parabolicity, admixture of p functions, arbitrary degeneracy of the electron distribution and the screening effects of free carriers on the scattering probabilities are incorporated. The Boltzmann equation is solved by an iterative technique using the currently established values of the material parameters. The iterative results are in fair agreement with other recent calculations obtained using the relaxation-time approximation and experimental methods.
 
 
 
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