Sharif Digital Repository

Industrial dehydrogenation of ethylbenzene to styrene is performed using potassium-promoted iron oxide catalyst. Many attempts have been made to understand the deactivation mechanism of the catalyst based on the chemical differences between the fresh and used catalysts. In the present work, in addition to the effect of chemical changes, the effect of structural changes of the internal areas on the catalyst activity was investigated. A fresh and used commercial catalyst from an industrial reactor which had been continuously used for two years under severe conditions (LHSV = 1 h-1, T = 610 °C, mass ratio of steam to ethylbenzene = 1.2, P = 1.2 atm) was studied. Nitrogen adsorption, Hg... 

Polycrystalline (Pr1-x Bax)Ba2Cu 3O7-δ samples (0.0 ≤ x ≤ 0.6) is prepared by the standard solid state reaction technique. X-ray measurements confirm the formation of the 123 phase for x ≤ 0.3, but for 0.4 ≤ x ≤ 0.6, due to the solubility limit, the 123 phase is not a dominant phase. Resistivity measurements show the increasing of resistivity with the increase of Ba-doping up to about x=0.08, and then decreases in the 123 phase up to x = 0.3. There is no observation of superconducting state in our samples, but we observe the sharp decrease of resistivity for x>0.1, which is related to possible superconductivity in Pr123 system by Ba-doping. It is concluded that the Pr/Ba disorder has... 

We have studied the electrical and magnetic properties of single phase polycrystalline Gd(Ba2-x Lax)Cu3O 7+δ and Gd(Ba2-xNdx)Cu3O 7+δ (0≤ x ≤ 0.5) samples. The samples were prepared by the standard solid-state reaction technique. The electrical measurements show the decrease of Tc and increase of ΔTc with the increase of Nd and La-doping. The magnetoresistance of samples has been studied within the thermally activated flux creep (TAFC) model. The pinning energy decreases with the increase of magnetic field and doping for both systems in a similar way. The decrease of pinning energy with applied magnetic field can be scaled with two power law relations for the magnetic field: smaller and... 

An unusual hump on the resistivity vs. temperature curve of Gd(Ba 2-xPrx)Cu3O7+δ samples have been observed for particular values of Pr doping. Investigation of the origin of this effect has led to an important result. Based on the Rietveld refinement of the XRD patterns, we have found that the Ba atom substitution at the rare earth site could lead to superconductivity in some parts of the grains at T p ∼ 80-90 K, which appears as a hump on the p(T) curve. Our result is in line with the previously proposed possibility of existence of superconductivity in Pr-123 due to the Ba atom substituted at Pr site. © 2004 WILEY-VCH Verlag GmbH & Co. KOaA, Weinheim  

Tetragonal Gd1-xPrxBaSrCu3O 7-δ polycrystalline samples (0 ≤ x ≤ 0.9) have been prepared by the standard solid-state reaction and characterized by XRD. A hump on the ρ(T) curve have been observed at about 80 K for large values of x. The normal state resistivity have been investigated with two and three dimensional variable range hopping (2&3D-VRH) and Coulomb gap (CG). For low concentration of Pr, 2D-VRH is the dominant mechanism, but with the increase of x, 3D-VRH is dominant. The magnetoresistance measurements of the samples have been analysed by the Ambegakor and Halperin (AH) phase slip model. The field dependences of the pinning energy and critical current density have been studied for... 

Investigation of BSCCO 2223 phase formation in superconducting thin films of Bi-based cuprates deposited on MgO substrate is reported. A series of films were made by in situ dc-magnetron sputtering method and another series were made by ex situ rf-magnetron sputtering technique. Same target have been used in both technique. In the case of the ex situ method, the films were annealed for different period of time at 800°C, and in the case of in situ method, the films were deposited at different temperature substrate. The influence of annealing time and the temperature of the substrate on the quality and the phase formation for ex situ and in situ method have studied, respectively. The results... 

The photocatalytic degradiation of phenol by nanoTiO2 particles coated on perlite as a new composite nano-catalyst was investigated. Titanium dioxide (Degussa P-25) was immobilized on three different supports (perlite granules, glass plates and steel fiber) by a very simple and inexpensive method. Perlite granules have a porosity of more than 95%, which allows them to stay afloat on water surface. This gives the medium a unique characteristic from the processing point of view, which enables it to get wetted with the polluted solution without requiring any pumping and simultaneously be exposed to the radiation source when coated with the photocatalyst. The photocatalytic activity of prepared... 

Electrochemical co-deposition approach was expanded to prepare composite bio-ceramic coating of hydroxyapatite (HA)/polyvinyl acetate on the surface of titanium. The role is to improve the bioactive and crystallization properties. The results of XRD, XPS, SEM and TEM characterization showed that by increasing amount of vinyl acetate in the composite bio-ceramic coating before and after immersing in the simulated body fluid (SBF), an oriented growth of HA planes on the (0 0 2) direction had been observed on titanium substrate. Also significant surface morphology changes were obtained  

The magnetoresistance of single-phase polycrystalline Gd(Ba2-xPrx)Cu3O7+δ samples has been studied within the thermally activated flux creep model. The decrease of pinning energy with applied magnetic field can be scaled to two power-law relations for magnetic fields, smaller and also larger than about 1 kOe. The derived Hc2(T), Hc2(O), and superconducting coherence length ξ show that the Pr doping, similar to weak links, decreases the vortex flux pinning energy. It is also concluded that the substitution of Pr at the Ba site has a more destructive effect on the flux dynamics than Pr at the rare-earth site  

The compound, lead hydroxide Pb6O4(OH)4, was prepared by a method used for the first time in the authors laboratory. The compound was identified by XRD and it's purity was determined by analytical methods (100%). The thermal characteristics of this compound were investigated by using a thermogravimetry (TGA). This compound was found to be stable at room temperature and at 160°C will decompose completely to produce lead (II) oxide. © Sharif University of Technology, April 2007  

Zinc oxide is used in functional devices, catalysts, pigments, optical materials and many other important applications. ZnO nanopowders can be produced mechanochemically or solochemically. The synthesis of ZnO nanopowder has been carried out via solochemical processing from an aqueous solution of a zinc containing complex in this research. This is the newest economic method for synthesis of ZnO nanopowder. The results obtained from XRD and TEM show that the nanoparticles are single crystals and the mean particle size is 45.3 nm. TEM micrographs of ZnO nanopowder reveal that the particles have elongated particulate shape with a narrow size distribution. Solochemical processing can thus be an... 

We have studied the effect of sintering temperature on the weak link behavior of polycrystalline Bi2223 superconductors with different intergranular coupling using the XRD, electrical resistivity and AC susceptibility techniques. The XRD results show that by increasing sintering temperature up to 865°C the Bi2212 phase fraction decrease. It was found that the Bi2212 phase on the grain boundaries is likely to play the role of weak links and consequently reduces the intergranular critical current densities. © 2006 WILEY-VCH Verlag GmbH & Co. KGaA  

We have studied the phase formation of a CoSi2 layer by solid-state reaction of ternary Co/WxTa(1-x)/Si(1 0 0) systems. The effect of cosputtered WxTa(1-x) nanometric interlayers, with different values of x (0, 0.25, 0.5, 0.75 and 1), on the degree of texturing of a CoSi2 layer and disilicide formation of the refractory metals has been investigated. The annealed samples, in a temperature range of 400-1000 °C, were analysed by x-ray diffraction, sheet resistance measurement, scanning electron microscopy, and energy-dispersive x-ray techniques. Using W0.25Ta0.75 and W interlayers, the best (1 0 0) texture of the CoSi2 layer with a thermal stability in the range of 900-1000 °C was produced. In... 

In this investigation, the crystalline structure of a nanometric CoSi 2 layer, formed in heat treated Co/W x Ta (1-x) /Si(1 0 0) systems, has been studied by XRD analysis. Careful measurements of the diffraction intensities revealed that temporary formation of a metastable diamond cubic structure of CoSi 2 phase, rather than its usual CaF 2 structure, was occurred. It has been shown that formation of this metastable structure depends on the kind of the applied interlayer in addition to the annealing temperature. Among the studied systems with x = 0, 0.25, 0.5, 0.75 and 1, the second and the last systems resulted in growing a (1 0 0) single-texture CoSi 2 layer with the preferred usual CaF 2... 

In FeCo fine particles, induced anisotropy has a considerable contribution in increasing the coercivity. We have prepared Fe0.7Co0.3 fine particles by the borohydride method in the presence of various magnetic fields in the range 0 ≤ H0 ≤ 2 kOe. We have studied and analysed the factors affecting the induced anisotropy and magnetic structure of these particles by the use of SEM, XRD and vibrating sample magnetometer. It was found that heat treatment, and also the presence of a magnetic field of about 1 kOe during the grain growth, result in the maximum coercivity of Fe 0.7Co0.3 fine particles. The value of magnetic field intensity should be such that it causes formation of oriented atomic... 

Denim jeans have consistently been fashionable around the world. This fabric inspired strong opinions from historians, teenagers, and movie stars, and has acquired different styles throughout the years. Most jeans today are stonewashed; this technique first become popular in 1970. Now, in addition to pumice stones, enzymes are used on the cotton fabrics to create the stonewashed look. Nowadays, plasma treatment is a cost-effective and environment-friendly process which is able to modify the properties of the fabric surface in order to achieve this effect. Plasma surface treatment changes the properties of the fabric up to a limited depth (several molecular layers), leaving the bulk... 

In this investigation, fine Fe85Si10Ni5 (at.%) powders with a nanocrystalline structure were prepared by high energy ball milling. The effects of milling time on microstructure and magnetic properties of Fe85Si10Ni5 powders were studied by X-ray diffraction (XRD), scanning electron microscope (SEM) and vibrating sample magnetometer (VSM). The optimum soft magnetic properties (maximum saturation magnetization and minimum coercivity) were obtained in alloy powders with an average grain size of 8 nm after 70 h milling. © 2005 Elsevier B.V. All rights reserved  

In this work, we propose a combined photochemical-chemical method that enables us to grow CdS nanoparticles with ångström precision. CdS nanoparticles were formed through a photo-induced reaction between CdSO 4 and Na2S2O3. Thioglycerol (C 3H8O2S) was used as the capping agent. Fine tuning of the size was subsequently carried out by controlling the pH of the solution. Optical absorption spectroscopy was mainly used to monitor the process of particle formation, and to measure the bandgap and size. X-ray diffraction spectra confirmed the optical sizes and demonstrated crystalline nanoparticles. In the presence of the capping agent there was a dark growth that was completely dependent on the pH... 

Solid-state-reacted polycrystalline Nd(Ba2-xPr x)Cu3O7+8 materials have been prepared and investigated using X-ray diffraction and resistivity measurements. Our results show that Nd(Ba2-xPrx)Cu3O7+8 is a superconductor when x < 0.3. The Tc depression in superconductivity of Pr at Ba sites is due to both hole filling/localization and depairing effects. The normal state conductivity undergoes a dimensional crossover from two-dimensional to three-dimensional in the frame of the variable range hopping regime. The localization length of carriers shows that Pr doping localizes carriers in the normal state and causes the Tc depression. The magnetoresistance of the samples has been investigated... 

Tetragonal Gd1-xPrxBaCaCu3O 7-δ (0≤x≤1) polycrystalline samples have been prepared by the standard solid-state reaction, and characterized by XRD and SEM. Rietveld analysis on X-ray diffraction pattern shows site mixing between rare earth (R) and Ca. Contrary to Gd1-xPrxBa2Cu 3O7-δ, a hump on the ρ(T) curve is observed at about 80 K. The normal state resistivity has been analyzed by the two and three dimensional variable range hopping (2&3D-VRH) and Coulomb gap. For low concentration of Pr (x<0.5-0.6, corresponding to the metal-insulator transition), 2D-VRH is the dominant mechanism, but with the increase of x, the 3D-VRH is dominant. Substitution of Ba by Ca highly increases the...