Search results for "grain"
showing 10 items of 752 documents
Influence of Ag, Cu dopants on the second and third harmonic response of ZnO films
2009
International audience; Silver- and copper-doped ZnO films were prepared by radio-frequency (RF)-magnetron sputtering on glass and quartz substrates. The influence of dopants content on the microstructural evolution and optical as well as nonlinear optical (NLO) properties were investigated. It has been found that the grain sizes were enlarged with increasing of Ag, Cu dopants amount in ZnO films. The Ag or Cu doping leads to the optical band gap narrowing. Besides, the second-order NLO response of Ag- and Cu-doped ZnO films is lower than that of undoped ZnO film. The second harmonic generation (SHG) efficiency of the ZnO:Ag film was found to be higher than that of the ZnO:Cu film at the si…
Grain size dependent bandgap shift of SnO2 nanofibers
2013
SnO2 nanofibers with various grain sizes ranging from 18.5 to 31.6 nm in diameter were fabricated by electrospinning a polymeric solution and subsequent controlled calcination of the as-spun fibers. The calcined fibers were polycrystalline and composed of densely packed nano-sized SnO2 grains. The effect of the nanograin size on the optical bandgap of SnO2 nanofibers was examined by ultraviolet-visible spectroscopy. The bandgap showed a strong dependence on the nanograin size. The bandgap decreased with increasing nanograin size. Some calculations were performed to understand the relationship between the experimentally obtained bandgaps of the SnO2 nanofibers and the theoretical ones. Quant…
Development and evaluation of occlusive systems employing polyvinyl alcohol for transdermal delivery of sumatriptan succinate
2010
The aim of the present study was to develop a sumatriptan succinate transdermal system for applying migraine treatments efficiently and easily. For this system polyvinyl alcohol was employed as a matrix and Azone((R)) was added as a permeability enhancer. The physical characteristics, mechanical properties, and in vivo bioadhesion of the systems were evaluated, as was in vitro permeation across porcine skin. A uniform distribution of the drug in the matrix was observed, and moisture uptake values were constant. With regard to mechanical parameters, occlusive layer inclusion made the system more resistant, and no significant differences were detected with respect to other systems. Although A…
A three-dimensional cohesive-frictional grain-boundary micromechanical model for intergranular degradation and failure in polycrystalline materials
2013
Abstract In this study, a novel three-dimensional micro-mechanical crystal-level model for the analysis of intergranular degradation and failure in polycrystalline materials is presented. The polycrystalline microstructures are generated as Voronoi tessellations, that are able to retain the main statistical features of polycrystalline aggregates. The formulation is based on a grain-boundary integral representation of the elastic problem for the aggregate crystals, that are modeled as three-dimensional anisotropic elastic domains with random orientation in the three-dimensional space. The boundary integral representation involves only intergranular variables, namely interface displacement di…
An enhanced grain-boundary framework for computational homogenization and micro-cracking simulations of polycrystalline materials
2015
An enhanced three-dimensional (3D) framework for computational homogenization and intergranular cracking of polycrystalline materials is presented. The framework is aimed at reducing the computational cost of polycrystalline micro simulations, with an aim towards effective multiscale modelling. The scheme is based on a recently developed Voronoi cohesive-frictional grain-boundary formulation. A regularization scheme is used to avoid excessive mesh refinements often induced by the presence of small edges and surfaces in mathematically exact 3D Voronoi morphologies. For homogenization purposes, periodic boundary conditions are enforced on non-prismatic periodic micro representative volume ele…
Finite element simulation of diffusion into polycrystalline materials
2008
Diffusion in polycrystalline materials is investigated by means of numerical finite element simulations for constant source conditions. The grain boundaries are assumed to provide fast diffusion paths. Main emphasis is put on situations that typically occur for nanocrystals, viz. on situations in which (i) the diffusion length is significant compared with grain size, (ii) the influence of boundaries that are parallel to the surface become important in addition to the perpendicular ones. Furthermore, we treat the influence of blocking space charge layers sandwiching the core pathways and thus channeling grain boundary diffusion.
Impedance spectroscopy studies of SrMnO3, BaMnO3and Ba0.5Sr0.5MnO3ceramics
2014
The impedance spectrum of hexagonal SrMnO3, rhombohedral BaMnO3 and orthorhombic Ba0.5Sr0.5MnO3 ceramics, synthesized by conventional high-temperature method, was studied in a wide temperature and frequency range. The complex impedance plots of Z″ versus Z′ pointed to two contributions originating from grains and grain boundaries. The parameters of electric equivalent circuit were calculated. The semicircles related to the grain boundary are located at the lower frequency side due to higher resistivity and capacity of the grain boundaries. The Ba0.5Sr0.5MnO3 ceramics is characterized by the lowest activation energy related to the grains. The conductivities σac of all the investigated sample…
Phase separation in the quaternary Heusler compound CoTi(1−x)MnxSb – A reduction in the thermal conductivity for thermoelectric applications
2010
We investigate the phase separation of the solid solution CoTi(1−x)MnxSb into the two Heusler compounds CoTiSb and CoMnSb. Energy-dispersive X-ray spectroscopy measurements on the two-phase material reveal the presence of size- and shape-tunable CoTiSb regions in a CoMnSb matrix. We demonstrate that the formed phase and grain boundaries have a considerable influence on the phonon scattering processes, which leads to a reduction in the thermal conductivity by a factor of three compared to single-phase CoTiSb.
Neutron scattering study of structural and magnetic size effects in NiO
2013
Nickel oxide powders with the grain size of 13–1500 nm have been studied by neutron scattering, scanning electron microscopy and vibrating sample magnetometry. We have found that the atomic structure and the antiferromagnetic ordering are nearly independent of the average size of grains. The existence of the uncompensated spins in nanoparticles with the grain size below 100 nm has been detected.
Spark plasma sintering of cobalt ferrite nanopowders prepared by coprecipitation and hydrothermal synthesis.
2007
International audience; Cobalt ferrite exhibits a high coercivity at room temperature and a strong magnetic anisotropy compared to the other spinel ferrites and, consequently appears as an interesting material for permanent magnets and high-density recording. The magnetic properties depend also on the crystallite size. In order to keep the powder properties in a bulk material, dense nanostructured cobalt ferrite has to be sintered. A field activated sintering process like spark plasma sintering (SPS) may be promising for such challenge. The present paper deals with: (i) the preparation of cobalt ferrite by two methods: coprecipitation and hydrothermal synthesis in supercritical water; (ii) …