Sharif Digital Repository

The magnetoresistance measurement of single phase polycrystalline Tm(Ba2-xPrx)Cu3O7+δ samples have been analyzed by the Ambegaokar and Halperine (AH) phase slip model. The magnetic field dependence of pinning energy power factors increase with the increase of Pr doping. The derived critical current density from the AH theory decreases with increasing magnetic field and Pr-doping for all superconducting samples. It shows that the Pr-doping plays the role of weak link. We have also investigated the normal state resistivity with the hopping model. We have found a CG-VRH cross-over with the increase of Pr content near metal insulator transition. The deacrease of the localization length of the... 

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  

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... 

Gd(Ba2-xPrx)Cu3O7+δ single phase polycrystalline samples with 0.0 ≤ x ≤ 1.0 were investigated for structural, electronic and flux dynamics properties. Two-dimensional variable range hopping (VRH) is the dominant conduction mechanism in the normal state of the system. Pr doping strongly localizes the carriers in normal state, and finally causes the suppression of superconductivity. The effect of Pr substitution in 123 structure of HTSC at R or Ba sites is to increase the pseudogap temperature Ts, although, Pr at Ba sites has a stronger effect on the increase of Ts and suppression of superconductivity. The magnetoresistance of the samples have been studied within thermally activated flux creep... 

This paper describes results of an advanced suction controlled triaxial test device on a loessial soil in an attempt to define the constitutive relationship between suction stress and matric suction (suction stress characteristic curve) for highly collapsible soils. Due to the presence of void spaces with different degrees of collapse potential within the soil matrix, the collapse phenomenon in loess is believed to be a continuous-stepwise reduction in volume rather than a sudden drop during wetting. Due to this unique volume change behavior, the definition of the suction stress characteristic curve for loess may require a soil-specific experimental testing approach capable of making precise... 

A recently developed two-level undercut-profile substrate (2LUPS), containing two levels of plateaus connected by a curved wall with an undercut profile, enables self-forming filaments in a coated conductor during physical line-of-sight deposition of buffer and superconducting layers. In the present study, the 2LUPS concept is applied to a commercial cube-textured Ni-5at.%W tape, and the surface of the 2LUPS coated with two Gd2Zr2O7 buffer layers using chemical solution deposition is examined. Except for narrow regions near the edge of upper plateaus, the plateaus are found to be covered by strongly textured Gd2Zr2O7 buffer layers after dip coating and sintering  

During recent years, extensive studies have been conducted around the world documenting liquefaction induced lateral spreading and its effects on deep foundations. This study is aimed to numerically model a shaking table experiment, to investigate the effect of lateral spreading on piles and also to assess the capability of an advanced critical state two-surface plasticity model in predicting soil and pile responses to lateral spreading. Changes in permeability of the soil layers during the shaking are also accounted for using the software's built-in programming language, FISH. Numerical results showed that the onset of liquefaction occurs after just a few cycles from the beginning of the... 

A critical state model is described within the framework of infinite surface bounding surface/kinematic hardening plasticity. The model can account for both initial anisotropy developing during sample preparation and subsequent evolution of anisotropy in samples along anisotropic loading paths. This model is then used to simulate the anisotropic behaviour of Hostun sand observed in series of stress probing tests conducted in a true triaxial apparatus within a single deviatoric stress plane in stress space. The selection of constitutive parameters for the model is discussed and the model evaluated. Deviatoric stress response envelopes are used to show the deviatoric strains generated in... 

Based on transport and magnetic measurements on Gd(Ba2-xPrx)Cu3O7+δ, and some other properties of high temperature superconductors (HTSC), we have extracted similarities between superconductors, two-dimensional electron gas (2D-EG) i.e. MOSFETs and ultrathin films of conventional superconductors. These are based on properties such as superconductor-insulator transition in superconductors and metal-insulator transition (MIT) in 2D-EG with doping and magnetic field, localization in transport conduction, quantum unit of resistance at MIT, larger change in resistance from critical doping to the insulating side in comparison with change from critical doping to the metallic side and strong... 

A conventional triaxial test device was modified to characterize the hydro-mechanical behavior of a loessial soil during isotropic and shear loadings. This device is capable of precise and continuous measurements of water outflow during the application of loading. The tests were performed on "undisturbed" cylindrical specimens, which were taken from loessial deposits in Gorgan, a city in the northeast of Iran. Experimental measurements indicate that the hydro-mechanical behavior of loess is highly affected by the extent of applied mean net stress and the level of suction. During both isotropic and shearing stages of loading, the tested specimens may exhibit collapse, abrupt decrease in... 

Abstract Lateral Spreading, which usually occurs as a consequence of liquefaction in gently sloped loose saturated sand layers, is known to be a major source of earthquake-induced damages to structures such as quay walls, bridge piers, pipelines, and highway/railways. Therefore evaluation of the liquefaction potential and using appropriate methods for prediction of the adverse consequences of lateral spreading is of great importance. In this study, numerical modeling has been used to study lateral spreading phenomenon behind rigid waterfront structures. Coupled dynamic field equations of the extended Biot's theory with u-P formulation are used for simulating the phenomenon. A fully coupled... 

The lateral movement of a liquefiable soil layer on gentle slopes is the most visible and devastating type of liquefaction-induced ground failure. Recent earthquakes have shown that this phenomenon causes severe damages to coastal structures, pier of the bridges and life-lines by exerting large lateral forces on the structures. In this paper coupled dynamic field equations of extended Biot's theory with u-p formulation are used for simulating the phenomenon and the soil behavior is modeled by a critical state two-surface plasticity model for sands. Furthermore, in this study variation of permeability coefficient during liquefaction is taken into account. The permeability coefficient is... 

To characterize the role of specimen disturbance and structure in the hydromechanical behavior of collapsible soils, two sets of wetting-induced collapse and suction-controlled triaxial tests were conducted on intact and reconstituted specimens of a loessial soil taken from a loess deposit in Gorgan, a city in the northeastern Golestan province of Iran. The testing approach used an advanced triaxial testing device that was specifically modified to control pressures applied to a soil specimen and to monitor and measure the amount of changes in volume and water content of the soil specimens during testing using highly sensitive digital sensors with an accuracy of 60.01 cm3. Results of the... 

The sand–bentonite mixture is used as the buffer layer in nuclear waste disposals. The buffer layer, as a non-permeable protective layer, is generally exposed to temperature gradients and the long-term subjection to temperature results in creep and more intact structure for the layer. In the present study, thermally induced structure and its effects on the isotropic compression behavior and volume change behavior of buffer layer are evaluated. Thermal consolidation tests were conducted using a modified triaxial cell capable of handling temperatures up to 90 °C. In order to investigate the effects of thermal gradients on the behavior of the mixture, saturated specimens were cured at constant... 

Although so many advances have been proposed in the field of artificial intelligence and superconductivity, there are few reports on their combination. On the other hand, because of the nonlinear and multivariable characteristics of the superconductive elements and capabilities of neural networks in this field, it seems useful to apply the neural networks to model and control the superconductive phenomena or devices. In this paper, a new constructive neural network (CNN) trained by two different optimization algorithms; back-propagation and genetic algorithm, is proposed which models the behavior of the superconductive fault current limiters (SFCLs). Simulation results show that the proposed... 

A modified two-surface critical state plasticity model for saturated and unsaturated soil is presented in this study. The key modification in new model is inclusion of an alternative yield surface used to simulate the behavior of unsaturated soils in addition to corresponding saturated conditions. Moreover, a numerical technique is used to obtain an incremental stress–strain response from loading curves. Modification is applied continuously in each incremental step to return the final stress states and hardening parameters to the yield surface. Results revealed that the adopted modeling approach can predict two independent sets of laboratory unsaturated experiments under various conditions... 

The soil permeability coefficient plays a key role in the process of numerical simulation of the liquefaction phenomenon. Liquefaction causes a considerable increase in soil permeability, due to the creation of easier paths for water flow. The work presented in this paper tries to investigate the effects of permeability coefficient on the results of numerical modeling of the liquefaction phenomenon. To do this, a fully coupled (u-P) formulation is employed to analyze soil displacements and pore water pressures. Two different versions of a well-calibrated critical state bounding surface plasticity model, which possesses the capability to utilize a single set of material parameters for a wide... 

In this paper a procedure for two-dimensional unsteady thermo-mechanical analysis of layered structures is presented, allowing the determination of the temperature and stress field at each step the construction period. The finite element method is employed in the methodology. Numerical simulation are focused on concrete structures, particularly roller compacted concrete (RCC) dams. A time varying elasticity modulus is introduced in the model. One case study is presented and analysis under different design approaches. Thermal effects must be considered in the process of designing of certain types of concrete structures in order to prevent the damage during either the construction phase (early... 

The liquefaction phenomenon is usually accompanied by a large amount of settlement. Based on the observations made in past earthquakes, ground improvement by densification is one of the most useful approaches to reduce the liquefaction-induced settlement. Currently, there is no analytical solution for evaluation of the amount of settlement and tilting of footings that are constructed on densified ground surrounded by liquefiable soil. A number of factors, such as underlying soil properties, dimensions of the footing and earthquake loading characteristics, cause the problem to become complicated. In this paper, the dynamic response of shallow foundations on both liquefiable and... 

Liquefaction phenomenon is usually accompanied by large amounts of settlement owing to disruption of soil structure. In addition to that, large settlement also occurs by a significant increase in soil permeability during seismic excitation. To properly simulate the post-liquefaction settlement, it is important to take the compressibility properties of the liquefied sand as well as the permeability increase into account. Using initial permeability coefficient in the course of simulation of liquefaction leads to underestimation of settlement. In addition to that, using unrealistic values for permeability may cause erroneous predictions of other aspects of soil behavior. Therefore, an accurate...