Sharif Digital Repository

A new and effective hot milling system was designed and applied to the synthesis of PbZr0.95Ti0.05O3 (PZT) powder. PbO, ZrO2 and TiO2 powders were used as starting materials and hot milled at various temperatures ranging from 10 ̊°C to 800 ̊°C. The obtained powders were characterized by X-ray diffraction analysis (XRD) and scanning electron microscopy (SEM). Results indicated that pure, crystalline, and fine PZT powders with the average particle size of 170 nm can be successfully synthesized directly after a relatively short synthesis time of 12 h at lower temperature of 800 ̊°C by the hot milling technique. In contrast to this method, this study demonstrates that conventional solid-state... 

In this paper, dynamic modeling and control problem for transfer of a sloshing liquid container suspended through rigid massless links from a team of quadrotors are investigated. By the proposed solution, pose of the slung container and fluid sloshing modes are stabilized appropriately. Dynamics of the container-liquid-quadrotors system is modeled by Euler-Lagrange method. Fluid slosh dynamics is included using multi-mass-spring model. According to derived model, a proper control law is designed for a system with three or more quadrotors. Implementing the proposed control law, quadrotors can control pose of the container, directions of the links and liquid sloshing modes simultaneously.... 

In this paper, we propose a new method for automatic news event detection. An event is a specific happening in a particular time and place. We propose a new model in this paper to detect news events using a label based clustering approach. The model takes advantage of the fact that news events are news clusters with high internal similarity whose articles are about an event in a specific time and place. Since news articles about a particular event may appear in several consecutive days, we developed this model to be able to distinguish such events and merge the corresponding news articles. Although event detection is propounded as a stand alone news mining task, it has also applications in... 

Up to 8wt. % of Nano-iron oxide was added to CaO refractory matrix. The crystalline phases and microstructure characteristics of specimens sintered at 1650°C for 5h in an electric furnace were studied by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. The physical properties are reported in terms of bulk density, apparent porosity and hydration resistance. The mechanical behavior was studied by a cold crushing strength (CCS) and flexural strength at 1200°C test. As a result, it was found that the presence of Nano-iron oxide in the CaO refractory matrix induced 2CaO.Fe2O3 (C2F), CaO.Fe2O3 (CF) and 3CaO.Al2O3 (C3A) phase's formation, which improved the sintering... 

Magnesia-dolomite refractories have advantages such as stability in the basic environments, clean steel production, high corrosion resistance, proper covering in the cement furnace, and low production costs. However, their application is restricted due to low hydration resistance. In this research, the effects of nano and micro alumina particles on the microstructure, sintering behavior, and properties of magnesia-dolomite refractories were studied. For this purpose, MgO-CaO refractories were formulated using dolomite and magnetite and 0, 2, 4, 6 and 8 wt percentage nano and micro alumina were added to the formulation. Phase and microstructure analysis of the samples were performed using... 

In this research, for the first time, the effect of air quenching on the microstructural and electrical properties of (In, Nb) co-doped TiO2 was investigated. The FE-SEM images showed that the air quenching has no effect on microstructure of co-doped TiO2. However, air quenching affected the electrical properties so that the dielectric constant in the frequency of 1 kHz at room temperature sharply enhanced from 21*103 to 26*104 and the dielectric loss surprisingly decreased from 0.6 to 0.1. This incredible improvement in the dielectric properties is attributed to the electron-pined defect dipoles which has been activated through air quenching. © 2020 Elsevier B.V  

Up to now, attempts for developing coarse-grained SnO2-based varistors which exhibit high nonlinearity property at lower voltage have become a challenge without any prominent result because of its unknown grain growth mechanism. In this study, the effect of CuO addition to SnO2-based varistors as a grain growth enhancer additive on microstructural development, grain growth kinetics, and electrical properties was investigated. The characterization of grain growth kinetics showed that CuO addition encouraged grain growth and enhanced the grains size as it could be seen in the activation energy which decreased from 594 kJ/mol to 364 kJ/mol. In the samples with a low amount of CuO, the solute... 

In this study, SnO 2-based varistors were prepared from mechanically activated nanocrystalline powders. Nanocrystalline powders were derived by subjecting the initial powders to intensive high-energy activation with different times and ball to powder ratio. The effect of activation parameters on the powder properties and sintering temperature, as well as microstructural, micro-electrical and macro-electrical properties of the final specimens was evaluated. Varistors derived from high-energy mechanical activation exhibit a higher density (98.3% relative density) and more refined microstructure upon sintering at 1,300 °C in comparison varistors prepared from conventional powders. Breakdown... 

Up to now, multifunctional varistor-capacitor materials were based on TiO2, even though these materials failed to have practical applications. For the first time, in this research, multifunctional varistor-capacitor properties are observed in the CuO-doped SnO2-CoO-Sb2O3 system. The XRD analysis showed that this system has a single phase microstructure. SEM images indicated that the CuO addition resulted in a better densification and larger grain size. The very low electric breakdown field of the CuO-doped SnO2-CoO-Sb2O3 system (12 V/mm, similar to TiO2), is a consequence of coarse-grained microstructure. This varistor had a suitable surge withstand capability as well. The colossal... 

In this research, effects of two-step sintering parameters, such as first and second step sintering temperatures (T1 and T2, respectively) and soaking time in the second step, on microstructure and electrical properties of ZnO varistors have been studied. In addition, slow rate of grain growth in the second step of sintering has also been investigated. Near fully dense (higher than 99%) and sub-micron grain size (1-0.75 μm) ZnO varistors have been fabricated by different two-step sintering cycles. Decreasing T1 and T2 result in longer necessary time in the second step to reach near full density with smaller final grain size. The superior electrical properties of varistors (VB: 2050-1022 V/mm... 

This paper describes a solution-based technique for fabrication of varistor grade composite nanopowders. The method consists of coating major varistor dopants on the surface of the ZnO nanoparticles. As a result, a homogenous mixture of dopants and ZnO nanoparticles will be achieved. TEM results indicated that a composite layer of dopants with the average particle size of 9 nm on the surface of ZnO nanoparticles has been successfully prepared. Sintering of the coated powders was performed in temperatures as low as 850 °C and final specimens with average particle size of 900 nm and density of 98.5% were achieved. In comparison to conventional mixing, varistors prepared from coated nanopowders... 

The development of noble-metal-free electrocatalysts with enhanced active sites is of great significance for the production of renewable energy on large scale. Ultrathin transition metal compounds (e.g., phosphides and sulfides) have unique structural features but their stability and electroactivity must still be improved for practical applications. We propose a new strategy to synthesis ultrathin sheets of Ni-Cu-Co transition metals on porous nickel and self-assemble them into yarn-shaped microspheres. Enhance catalytic activity and stability are then attained by thermochemical phosphorization without morphological modifications. We employed various analytical techniques including XRD, SEM,...