On the CVD of MoSi2: an experimental study from the MoCl4–SiCl4–H2–Ar precursor with a view to the preparation of C/MoSi2/SiC and SiC/MoSi2/SiC microcomposites

The chemical vapour deposition of MoSi2 on plane substrates (graphite or sintered-SiC) and ceramic fibres has been studied from MoCl4–SiCl4–H2–Ar gas mixtures at 900相似文献   

Hydrogen sorption properties of MgH2–NiCo2O4 composites activated mechanically under argon and hydrogen atmospheres

The effect of the mechanical activation medium on the hydrogen absorption–desorption properties of MgH₂ with NiCo₂O₄ additives is investigated. The composite 90 wt.% MgH₂–10 wt.% NiCo₂O₄ mechanically activated for 180 min under hydrogen reaches a higher absorption capacity as compared to the composite ground for the same time in an argon medium. At T = 573 K and P = 1 MPa the composite activated mechanically in a reactive medium shows a value of 5.67 wt.% while for the composite ground under argon the value is 4.36 wt.% only, both samples preserving a high absorption capacity at temperatures below 573 K. Addition of nickel cobaltite is found to have a favorable effect on the hydriding kinetics of magnesium. In order to elucidate this effect, a composite containing a large amount of NiCo₂O₄ (50 wt.%) is also investigated.     

Catalytic effect of monoclinic WO3, hexagonal WO3 and H0.23WO3 on the hydrogen sorption properties of Mg

The H sorption properties of mixtures Mg + WO₃ (having various structures) and Mg + H_0.23WO₃ are reported. First, the higher conversion of Mg into MgH₂ during reactive mechanical grinding (under 1.1 MPa of H₂) for higher WO₃ content is due to the improvement of the milling efficiency. Then, it is shown that the hydrogen absorption properties are almost independent of the crystal structure of the catalyst and that only the particles' size and the specific surface play a major role. Finally, for the desorption process, it appears that the chemical composition and structure of the catalyst, together with the particle size and specific surface have an effect.     

Enhancement of hydrogen-storage properties of Mg by reactive mechanical grinding with oxide, metallic element(s), and hydride-forming element

In order to improve the hydrogen-storage properties of magnesium, oxides, metallic element(s) and a hydride-forming element were added to Mg by grinding under a hydrogen atmosphere (reactive mechanical grinding). As the oxides, Fe₂O₃ purchased, Fe₂O₃ prepared by spray conversion, MnO purchased, SiO₂ prepared by spray conversion, and Cr₂O₃ prepared by spray conversion were chosen. As the metallic elements, Ni, Fe, and Mo were selected. In addition, as the hydride-forming element, Ti was selected. Samples with the compositions of Mg-10 wt%oxide, 76.5 wt%Mg-23.5 wt%Ni, 71.5 wt%Mg-23.5 wt%Ni-5wt% Fe₂O₃, 71.5 wt%Mg-23.5 wt%Ni-5 wt%Fe, and Mg-14 wt%Ni-2 wt%Fe-2wt%Ti-2 wt%Mo were prepared. The hydrogen-storage properties and changes in phase and microstructure after the hydriding-dehydriding cycling of the prepared samples were then investigated.     

A comparison of models of force production during stimulated isometric ankle dorsiflexion in humans.

In this paper, we compare seven models on their ability to fit isometric muscle force. We stimulated the ankle dorsiflexors of eight subjects at seven ankle angles (85 degrees-120 degrees). Three different stimulation patterns (twitch, triangular, and random) were applied at all ankle angles. Four additional patterns (doublets, steady rates, "catch property," and walking-like) were applied at 95 degrees. Parameter values were optimized for each model at each angle. Parameters for the general linear model were calculated using a novel least-squares algorithm. A linear, second-order critically damped model gave the poorest fits (average root mean square (rms) error: 15 N). The models of Ding et al. (2002) and Bobet and Stein (1998) gave the best fits (average rms errors: 9.2 and 9.4 N). The other models (general linear second-order model, Wiener model, Zhou et al. (1995) model, general linear model) gave intermediate results. Results were similar at all ankle angles. We conclude that the Ding and Bobet-Stein models are the best overall for isometric contractions, that no linear model of any kind will give an error less than 9% of maximum force, and that the models tested are consistent across lengths.     

Improvement of hydrogen storage characteristics of Mg by planetary ball milling under H2 with metallic element(s) and/or Fe2O3

Among samples of Mg-Ni, Mg-Ni-5Fe₂O₃, and Mg-Ni-5Fe, Mg-Ni-5Fe had the highest hydriding and dehydriding rates. For the as-milled Mg-Ni-5Fe alloy and the hydrided Mg-Ni-5Fe alloy after activation, the weight percentages of the constituent phases were calculated using the FullProf program. The creation of defects and the diminution of Mg particle size through reactive mechanical grinding and hydriding-dehydriding cycling, and the formation of the Mg₂Ni phase are considered to increase the hydriding and dehydriding rates. Mg-14Ni-2Fe-2Ti-2Mo had higher hydriding and dehydriding rates than did any of the other samples (Mg-Ni, Mg-Ni-5Fe₂O₃, Mg-Ni-5Fe, and Mg-14Ni-6Fe₂O₃) prepared in this work.     

Analytical Solutions for Shallow Tunnels in Saturated Ground

Estimates of ground deformations and liner stresses in a tunnel are usually obtained from empirical correlations or from past experience on similar tunnels. These correlations account for only a few of the significant factors, and extrapolation to other cases is questionable because similitude conditions are not generally fulfilled. In this paper, complete analytical solutions for a shallow tunnel in saturated ground are obtained. Two different drainage conditions have been considered: full drainage at the ground-liner interface, and no drainage. The solutions cover different construction processes and soil conditions: (1) dry ground; (2) saturated ground with and without air pressure; (3) with and without a gap between the ground and the liner; and (4) applicability for short term analysis (i.e., undrained excavation and liner installation) and for long term analysis. Since the ground and the liner are assumed to behave elastically, the solutions obtained are restricted to cases where ground deformations are small, such as stiff clays and rocks, or when the excavation method prevents large deformations of the ground.     

Structure of a new ternary compound with high magnesium content,so-called Gd13Ni9Mg78

The magnesium-rich composition Gd₁₃Ni₉Mg₇₈ was synthesized from its constituent elements in sealed tantalum tubes in an induction furnace. X-ray diffraction, electron probe microanalysis and dark-field transmission electron microscopy (TEM) images revealed a new compound with a composition ranging from Gd_10–15Ni_8–12Mg_72–78 and low crystallinity. In order to increase the crystallinity, different experimental conditions were investigated for numerous compounds with the initial composition Gd₁₃Ni₉Mg₇₈. In addition, several heat treatments (from 573 to 823 K) and cooling rates (from room temperature quenched down to 2 K h^?1) have been tested. The best crystallinity was obtained for the slower cooling rates ranging from 2 to 6 K h^?1. From the more crystallized compounds, the structure was partially deduced using TEM and an average cubic structure with lattice parameter a = 4.55 Å could be assumed. A modulation along both a¹ and b¹ axis with vectors of modulation q1 = 0.42a¹ and q2 = 0.42b¹ was observed. This compound, so-called Gd₁₃Ni₉Mg₇₈, absorbs around 3 wt.% of hydrogen at 603 K, 30 bars and a reasonable degree of reversibility is possible, because after the first hydrogenation, irreversible decomposition into MgH₂, GdH₂ and NiMg₂H₄ has been shown. The pathway of the reaction is described herein. The powder mixture after decomposition shows an interesting kinetics for magnesium without ball milling.     

Slip propagation along frictional discontinuities

Slip initiation and propagation along non-homogeneous frictional surfaces are investigated by loading specimens of gypsum in biaxial compression. The specimens used in the tests are composed of two individual blocks with perfectly mated contact surfaces. The contact surfaces have on their upper half a frictional strength smaller than on their lower half. This creates a “weak” surface on the upper half and a “strong” surface on the lower half. Four test series are performed using specimens with different surface characteristics along the contact surface. The experiments are conducted by applying first a normal stress across the frictional surface and then increasing the shear stress until final debonding and slip along the strong surface occur. The magnitude of the normal stress used in the experiments ranges from 0.7 to 15 MPa (about 50% of the unconfined compression strength of the material). Slip starts on the weak area and, as the shear stress is increased, propagates towards the strong area. Full slip along the weak area introduces a sharp transition between the area of the discontinuity that has slipped and the area that has not. This transition creates a large concentration of stresses which can be treated within the framework of fracture mechanics as a mode II frictional crack. With further loading, rupture occurs through the strong area as an unstable process that coincides with failure. The results show that the critical energy release rate G_IIC is a good indicator of the rupture. However, G_IIC is not a material property. It depends on the normal stress applied, on the frictional characteristics of the interface, and on the critical slip required for the transition from peak to residual. A slip initiation model is proposed based on experimental observations and on fracture mechanics theory and is incorporated into a finite element method code. Predictions with the model compare very well with experiments.