4,448 research outputs found

    An Improved Method using RBF Neural Networks to Speed up Optimization Algorithms

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    The paper presents a method using Radial Basis Function (RBF) neural networks to speed up deterministic search algorithms used for the optimization of superconducting magnets for the LHC accelerator project at CERN. The optimization of the iron yoke of the main LHC dipoles requires a number of numerical field computations per trial solution as the field quality depends on the excitation and local iron saturation in the yoke. This results in computation times of about 30 minutes for each objective function evaluation (on DEC-Alpha 600/333). In this paper we present a method for constructing an RBF neural network for a local approximation of the objective function. The computational time required for such a construction is negligible compared to the deterministic function evaluation, and thus yields a speed-up of the overall search process. The effectiveness of this method is demonstrated by means of two- and three-parametric optimization examples. The achieved speed-up of the search routine is up to 30 %

    Influence of synthesis parameters and thermal treatment on functional properties of Fe3O4-BaTiO3 multiferroics obtained by mechanical activation

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    Mechanical activation of a mixture of polycrystalline powders Fe3O4 (50% wt.) and BaTiO3 (50% wt.) was performed in a planetary ball-mill, with different milling times (3 h, 6 h and 12 h). Average crystallite size determined by XRD analysis ranges from 12 to 30 nm, depending on the milling time. The activated powders were pressed into disc-shaped samples, 8 mm in diameter and 1.5 mm thick, applying the pressure of 500 MPa,. Thermoelectric measurements conducted in the temperature range from room temperature to 350 °C revealed that the electrical resistivity of the sample depends on temperature and activation time. At room temperature, the maximum value of specific electrical resistivity (ρ0 = 1 MΩm) was observed for the sample obtained by pressing the powder activated for 6 h. Magnetic properties of pressed powder samples were studied using a modified Faraday method. At room temperature, the pressed powder activated for 3 h exhibited the maximum value of magnetization, M0= 0.86 Am2/kg. Multiple heating of the pressed samples, for 10 min, was performed in a magnetic field of 20 KA/m. After cooling, the highest magnetization values were observed for the samples previously heated at 380 °C, while the maximum one (M’= 1.04 Am2/kg) corresponds to the sample activated for 3 h

    Influence of mechanical activation on synthesis and properties of the MgO-TiO2 system

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    Materials applied in electronics such as multilayer capacitors are an important field of ceramic materials. Magnesium titanate based dielectric materials are used for producing type-I capacitors. A common way of obtaining this material is a solid-state reaction during reaction sintering. The process of sintering can be enhanced if mechanical activation precedes. In this work starting powders of magnesium carbonate (MgCO3) and titanium dioxide (TiO2) with a rutile crystal modification were weighed to attain a 1:1 molar MgCO3:TiO2 ratio. Mechanical activation of the starting mixture was performed by high energy ball milling using ZrO balls and vessels with a ball to powder mass ratio of 40:1. The observed grinding times were 15, 30, 60 and 120 minutes. Powder characterization was conducted using X ray powder diffraction, DTA analysis up to 1000 o C and particle morphology changes were observed with Scanning Electron Microscopy. Isothermal sintering of compacted powders was conducted at 1100ºC during 30, 60 and 180 minutes. For specimens synthesized in such a manner, microwave dielectric properties were measured, quality factor Q, specific electrical resistivity (ρ) and the dielectric constant (Єr). In this work we explain the influence of mechanical activation on the MgCO3-TiO2 system leading to titanate formation during sintering, as well as induced changes in microwave dielectric properties

    Effect of electrodeposition current density on the microstructure and magnetic properties of nickel-cobalt-molybdenum alloy powders

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    Nanostructured nickel-cobalt-molybdenum alloy powders were electrodeposited from an ammonium sulfate bath. The powders mostly consist of an amorphous phase and a very small amount of nanocrystals with an mean size of less than 3 nm. An increase in deposition current density increases the amorphous phase percentage, the density of chaotically distributed dislocations and internal microstrains in the powders, while decreasing the mean nanocrystal size. The temperature range over which the structural relaxation of the powders deposited at higher current densities occurs is shifted towards lower temperatures. A change in relative magnetic permeability during structural relaxation is higher in powders deposited at higher current densities. Powder crystallization takes place at temperatures above 700oC. The formation of the stable crystal structure causes a decrease in relative magnetic permeability

    Development of new material for the environmental-friendly energy generation processes

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    The aim of this study was to develop a new material which has high hydrogen storage capacity and hydrogen absorption/desorption rate. This material, which is based on cobalt powder onto which palladium was chemically deposited can be used in environmentally friendly energy generation processes, thus ensuring preventive protection of human health. Hydrogen absorption was investigated using differential scanning calorimetry (DSC) and by measuring hydrogen pressure under isothermal and non-isothermal conditions in a chamber containing either pure cobalt powder either cobalt powder onto which palladium was chemically deposited (Co-0.003 Pd). The mechanism of hydrogen absorption has been described. It was noticed that palladium catalyses hydrogen absorption by dissociating the adsorbed H2 molecules into H atoms more rapidly on its own atoms than on cobalt ones. The catalysis of dissociation of H2 molecules enabled hydrogen absorption into Co-0.003 Pd powder to occur at lower temperatures. The results of this study enable integration of technological and public health investigation in order to ensure preventive protection of human health through enviromental protection

    Effect of milling and annealing on microstructural, electrical and magnetic properties of electrodeposited Ni-11.3fe-1.4W alloy

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    A nanostructured Ni-11.3Fe-1.4W alloy deposit was obtained from an ammonium citrate bath at a current density of 600 mAcm -2. XRD analysis shows that the deposit contains an amorphous matrix having embedded nanocrystals of the FCC phase of the solid solution of Fe and W in Ni with the average crystal grain size of 8.8 nm. The deposit has a high internal microstrain value and a high minimum density of chaotically distributed dislocations. The effect of milling and annealing of the Ni-11.3Fe-1.4W alloy on electrical and magnetic properties was studied. Structural changes in the alloy take place during both annealing and milling. Upon deposition, the alloy was heated to 420 °C. Heating resulted in structural relaxation which induced a decrease in electrical resistivity and an increase in magnetic permeability of the alloy. Further heating of the alloy at temperatures higher than 420 °C led to crystallization which caused a reduction in both electrical resistivity and magnetic permeability. The milling of the alloy for up to 12 hours caused a certain degree of structural relaxation and crystallization of the alloy. The increase in crystal grain size up to 11 nm and the partial structural relaxation induced a decrease in electrical resistivity and an increase in magnetic permeability of the alloy. Heating the powders obtained by milling at 420 °C led to complete structural relaxation, reduced electrical resistivity, and increased magnetic permeability. During heating of the powders obtained by milling at temperatures above 420 °C, crystallization and a significant increase in crystal grain size occurred, leading to a reduction in both electrical resistivity and magnetic permeability. The best magnetic properties were exhibited by the alloys milled for 12 hours and annealed thereafter at 420 °C. In these alloys, crystal grains were found to have an optimum size, and complete relaxation took place, resulting in a maximum increase in magnetic permeability

    Integrated Design of Superconducting Magnets with the CERN Field Computation Program ROXIE

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    The program package ROXIE has been developed at CERN for the field computation of superconducting accelerator magnets and is used as an approach towards the integrated design of such magnets. It is also an example of fruitful international collaborations in software development.The integrated design of magnets includes feature based geometry generation, conceptual design using genetic optimization algorithms, optimization of the iron yoke (both in 2d and 3d) using deterministic methods, end-spacer design and inverse field calculation.The paper describes the version 8.0 of ROXIE which comprises an automatic mesh generator, an hysteresis model for the magnetization in superconducting filaments, the BEM-FEM coupling method for the 3d field calculation, a routine for the calculation of the peak temperature during a quench and neural network approximations of the objective function for the speed-up of optimization algorithms, amongst others.New results of the magnet design work for the LHC are given as examples

    Synthesis of BaFe12O19-BaTiO3 multiferroics by mechanical activation

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    A mixture of polycrystalline powders of Fe (70 % wt.) and BaTiO3 (30 % wt.) was ball-milled in a planetary mill under air atmosphere, for different time intervals: 60, 120, 180, 240, 300 and 360 min. During the mechanical activation, the powder was exposed to oxygen from the air, resulting in formation of iron oxides: FeO and then Fe2O3 and Fe3O4. XRD and SEM analyses of the activated powders revealed that the weight fraction of the iron oxides in the mixture and microcrystal size depend on the activation time. For the powders activated for different time intervals, average crystallite size (Dhkl), dislocation density (ρn) and average microcrystal size of BaTiO3 and Fe were determined. In order to investigate the influence of thermally induced structural changes on magnetic properties, the change of magnetic properties of the pressed activated powders during multiple heating in a magnetic field of 10KA/m was measured. Maximum magnetization of the samples was reached after heating at 620 K. Pressed powder samples were sintered at temperatures of 1100 oC and 1200 °C for 2h giving the different phase diagrams. The samples sintered at 1100 oC include BaTiO3, BaFe12O19 and BaFeO2,67 as the dominant components. The samples sintered at 1200 °C containing only two components, BaTiO3 and BaFe12O19, exhibited pronounced ferromagnetic and ferroelectric propertie

    Management of tomato bacterial spot in the field by foliar applications of bacteriophages and SAR inducers

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    Various combinations of the harpin protein, acibenzolar-S-methyl, and bacteriophages were compared for controlling tomato bacterial spot in field experiments. Harpin protein and acibenzolar-S-methyl were applied every 14 days beginning twice before transplanting and then an additional four applications throughout the season. Formulated bacteriophages were applied prior to inoculation followed by twice a week at dusk. A standard bactericide treatment, consisting of copper hydroxide plus mancozeb, was applied once prior to inoculation and then every 7 days, while untreated plants served as an untreated control. Experiments were conducted in north and central Florida fields during fall 2001, spring 2002, and fall 2002. In three consecutive seasons, acibenzolar-S-methyl applied in combination with bacteriophage or bacteriophage and harpin significantly reduced bacterial spot compared with the other treatments. However, it did not significantly affect the total yield compared with the standard or untreated control. Application of host-specific bacteriophages was effective against the bacterial spot pathogen in all three experiments, providing better disease control than copper-mancozeb or untreated control. When results of the disease severity assessments or harvested yield from the bacteriophage-treated plots were grouped and compared with the results of the corresponding nonbacteriophage group, the former provided significantly better disease control and yield of total marketable fruit
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