270 resultados para SUPPORTED COBALT CATALYSTS


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For the design of radio frequency micro-electro-mechanical systems (RF MEMS) switches, the reliability issue becomes increasingly important. This paper represents some failure phenomena of doubly supported capacitive RF MEMS switches that include observable destruction failure and directly measurable parameter degradation obtained from the actuating-voltage testing and scanning electron microscope (SEM) observation. The relevant failure modes as well as their failure mechanisms are identified.

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The deformation microstructure of face-centered cubic cobalt subjected to surface mechanical attrition treatment was studied as a function of strain levels. Strain-induced gamma --> epsilon transformation and twinning deformation were evidenced by transmission electron microscopy and were found to progress continuously in ultrafine and nanocrystalline grains as the strain increased.

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The microstructural evolution during surface mechanical attrition treatment of cobalt (a mixture of hexagonal close packed (hep) and face-centered cubic (fcc) phases) was investigated. In order to reveal the mechanism of grain refinement and strain accommodation. The microstructure was systematically characterized by both cross-sectional and planar-view transmission electron microscopy. In the hcp phase, the process of grain refinement. Accompanied by an increase in strain imposed in the surface layer. Involved: (1) the onset of 110 111 deformation twinning, (2) the operation of (1 120) 110 1 0} prismatic and (1 120) (000 1) basal slip, leading to the formation of low-angle dislocation boundaries, and (3) the successive subdivision of grains to a finer and finer scale. Ressulting in the formation of highly misoriented nanocrystalline grains. Moreover. The formation of nanocrystalliies at the grain boundary and triple junction was also observed to occur concurrently with straining. By contrast. The fec phase accommodated strain in a sequence as follows: (1) slip of dislocations by forming intersecting planar arrays of dislocations, (2) {1 1 1} deformation twinning, and (3) the gamma(fcc) --> epsilon(hcp) martensitic phase transformation. The mechanism of grain refinement was interpreted in terms of the structural subdivision of grains together with dynamic recrystallization occurring in the hep phase and the gamma --> E: martensitic transformation in the fcc phase as well.

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The nanocrystalline (nc) formation was studied in cobalt (a mixture of c (hexagonal close packed) and gamma (face-centered cubic) phases) subjected to surface mechanical attrition treatment. Electron microscopy revealed the operation of {10(1) over bar 0}< 11(2) over bar 0 > prismatic and {0001}< 11(2) over bar 0 > basal slip in the E phase, leading to the successive subdivision of grains to nanoscale. In particular, the dislocation splitting into the stacking faults was observed to occur in ultrafine and nc grains. By contrast, the planar dislocation arrays, twins and martensites were evidenced in the gamma phase. The strain-induced gamma ->epsilon martensitic transformation was found to progress continuously in ultrafine and nc grains as the strain increased. The nc formation in the gamma phase was interpreted in terms of the martensitic transformation and twinning.

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This paper addresses the explosive consolidation of amorphous cobalt-based alloys. Using the experimental setup introduced in the present paper, specimens with high compact density, excellent magnetic properties and great wearability have been made. In comparison with permalloy and ferrite, the present specimens exhibit superior magnetic properties. Therefore, the compact is deemed as being a promising material for magnetic recording heads.

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Nanocrystalline intermetallic Co3Fe7 was produced on the surface of cobalt via surface mechanical attrition (SMA). Deformationinduced diffusion entailed the formation of a series of solid solutions. Phase transitions occurred depending on the atomic fraction of Fe in the surface solid solutions: from hexagonal close-packed (<4% Fe) to face-centered cubic (fcc) (4-11% Fe), and from fcc to body-centered cubic (>11% Fe). Nanoscale compositional probing suggested significantly higher Fe contents at grain boundaries and triple junctions than grain interiors. Short-circuit diffusion along grain boundaries and triple junctions dominate in the nanocrystalline intermetallic compound. Stacking faults contribute significantly to diffusion. Diffusion enhancement due to high-rate deformation in SMA was analyzed by regarding dislocations as solute-pumping channels, and the creation of excess vacancies. Non-equilibrium, atomic level alloying can then be ascribed to deformation-induced intermixing of constituent species. The formation mechanism of nanocrystalline intermetallic grains on the SMA surface can be thought of as a consequence of numerous nucleation events and limited growth. (C) 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

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The nanocrystalline (nc) formation was studied in cobalt (a mixture of c (hexagonal close packed) and gamma (face-centered cubic) phases) subjected to surface mechanical attrition treatment. Electron microscopy revealed the operation of {10(1) over bar 0}< 11(2) over bar 0 > prismatic and {0001}< 11(2) over bar 0 > basal slip in the E phase, leading to the successive subdivision of grains to nanoscale. In particular, the dislocation splitting into the stacking faults was observed to occur in ultrafine and nc grains. By contrast, the planar dislocation arrays, twins and martensites were evidenced in the gamma phase. The strain-induced gamma ->epsilon martensitic transformation was found to progress continuously in ultrafine and nc grains as the strain increased. The nc formation in the gamma phase was interpreted in terms of the martensitic transformation and twinning.

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What the real trade-off is among fig-supported wasps and the viable seeds of figs is heatedly debated in the studies of fig/fig wasp mutualism. In the present study, we collected wasp offspring (galls) and the viable seeds of premature fruits, and determined the foundress number in receptive fruits and all the types of wasps supported by Ficus racemosa L. during both the rainy and dry seasons in Xishuangbanna, China. The data show that the galls were positively correlated with viable seeds (n=32;r=0.74; P < 0.001) when the proportion of vacant female flowers (PVFF) was high, in April (68.0%), and were negatively correlated with viable seeds (n=48;r=-0.59; P < 0.05) when PVFF were limited (PVFF 42.6%) during a colder month (January). The mean foundress number per fruit during the colder months is significantly lower than during the warmer months (F-5,F-603 = 27.9; P < 0.001) and pollinator wasps can live longer during the colder months, During the colder months, the proportions of non-pollinators and wasp offspring are higher than those found during other months, whereas the proportion of viable seeds is not different compared with that of other months. Non-pollinator wasps tend to oviposit the female flowers that have been oviposited by pollinator wasps. The non-pollinators only negatively affect pollinator wasps and there is no obvious negative effect of non-pollinator wasps on viable seeds, so ovipositing by non-pollinator wasps will not result in the extinction of the figs during the process of evolution. The results of the present study indicate that figs can allow less foundresses to be in fruit cavities when PVFF are limited, which provides supporting evidence for the previous assumption that the plants have developed a mechanism to maintain a stable system because of the conflicts between the parties involved.

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Well-aligned Zn1-xMgxO nanorods and film with Mg-content x from 0 to 0.051 have been successfully synthesized by metal organic chemical vapor deposition (MOCVD) without any catalysts. The characterization results showed that the diameters and lengths of the nanorods were in the range of 20-80 nm and 330-360 nm, which possessed wurtzite structure with a c-axis growth direction. As the increase of Mg precursor flows into the growth chamber, the morphology of Zn1-xMgxO evolves from nanorods to a film with scale-like surface and the height of the nanorods and the film was almost identical, it is suggested that the growth rate along the c-axis was hardly changed while the growth of six equivalent facets of the type {1 0 (1) over bar 0} of the Zn1-xMgxO has been improved. Photoluminescence and Raman spectra show that the products have a good crystal quality with few oxygen vacancies. With the Mg incorporation, multiple-phonon scattering become weak and broad, and the intensities of all observed vibrational modes decrease. And the ultraviolet near-band-edge emission shows a clear blueshift (x=0.051, as much as 90 meV) and slightly broadening compared with that of pure ZnO nanorods. (C) 2008 Elsevier B.V. All rights reserved.

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The cobalt ferrites with chemical composition Co1+xZnxFe2-2xO4 (r=0.0, 0.1, 0.2, 0.4) were obtained with conventional solid reaction. The ZnO-doped samples have lower lattice constant than CoFe2O4 by adjusting Co ions to the octahedral sites. The results show that doping ZnO could extremely improve the magnetic properties. In comparison with pure CoFe2O4, the little ZnO-doped sample has higher permeability and much lower coercivity at the condition of a little decrease of magnetization saturation. Sample with x=0.1 shows evident magnetostrictive effect at the magnetic field of 30-60 mT while pure cobalt ferrite sample does not, though the saturation magnetostriction decreases. These indicate that ZnO-doping improves the magnetostrictive sensitivity of the cobalt ferrites and have potential applications in magnetoelectric devices and magnetic detector.

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We report on high-frequency (300-700 GHz) ferromagnetic resonance (HF-FMR) measurements on cobalt superparamagnetic particles with strong uniaxial effective anisotropy. We derive the dynamical susceptibility of the system on the basis of an independent-grain model by using a rectangular approach. Numerical simulations give typical line shapes depending on the anisotropy, the gyromagnetic ratio, and the damping constant. HF-FMR experiments have been performed on two systems of ultrafine cobalt particles of different sizes with a mean number of atoms per particles of 150 +/- 20 and 310 +/- 20. In both systems, the magnetic anisotropy is found to be enhanced compared to the bulk value, and increases as the particle size decreases, in accordance with previous determinations from magnetization measurements. Although no size effect has been observed on the gyromagnetic ratio, the transverse relaxation time is two orders of magnitude smaller than the bulk value indicating strong damping effects, possibly originating from surface spin disorders.

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The cobalt ferrites with chemical composition Co1+xZnxFe2-2xO4 (r=0.0, 0.1, 0.2, 0.4) were obtained with conventional solid reaction. The ZnO-doped samples have lower lattice constant than CoFe2O4 by adjusting Co ions to the octahedral sites. The results show that doping ZnO could extremely improve the magnetic properties. In comparison with pure CoFe2O4, the little ZnO-doped sample has higher permeability and much lower coercivity at the condition of a little decrease of magnetization saturation. Sample with x=0.1 shows evident magnetostrictive effect at the magnetic field of 30-60 mT while pure cobalt ferrite sample does not, though the saturation magnetostriction decreases. These indicate that ZnO-doping improves the magnetostrictive sensitivity of the cobalt ferrites and have potential applications in magnetoelectric devices and magnetic detector.