快速凝固富钛Ti-V合金的微观组织和储氢性能

研究目的在于优化Ti0.8-0.9V0.2-0.1二元合金的相结构成分、微观组织和储氢性能。该合金主要用于从含有大量一氧化碳的高温气态混合物中吸收氢气。Ti0.8-0.9V0.2-0.1合金中的α-（HCP）和β-（BCC）相在纯氢气中基于氢化作用,形成单相FCC结构的氢化物,此过程与合金的化学成分无关。同步辐射X射线衍射的原位分析表明,在含有氢气和10%一氧化碳的混合气体中,只有β相转变成相应的氢化物。快速凝固（RS）处理细化了Ti0.8V0.2和Ti0.9V0.1合金的晶粒组织,而且,快速凝固处理增加了Ti0.9V0.1合金中的β相,其所占比例是普通熔铸条件下的两倍。扫描电子显微镜（SEM）分析表明,Ti0.9V0.1合金含有片状组织,层片的厚度约为300nm。热脱附谱（TDS）显示,微观组织的细化可以加快氢脱附的动力学过程。     

Microstructure evolution and mechanical properties of Ti-B-N coatings deposited by plasma-enhanced chemical vapor deposition

Ternary Ti-B-N coatings were synthesized on AISI 304 and Si wafer by plasma-enhanced chemical vapor deposition (PECVD) technique using a gaseous mixture of TiCl4,BCl3,H2,N2,and Ar.By virtue of X-ray diffraction analysis,X-ray photoelectron spectroscopy,scanning electron microscope,and high-resolution transmission electron microscope,the influences of B content on the microstructure and properties of Ti B N coatings were investigated systematically.The results indicated that the microstructure and mechanical properties of Ti-B-N coatings largely depend on the transformation from FCC-TiN phase to HCP-TiB2 phase.With increasing B content and decreasing N content in the coatings,the coating microstructure evolves gradually from FCC-TiN/a-BN to HCP-TiB2 /a-BN via FCC-TiN+HCP-TiB2/a-BN.The highest microhardness of about 34 GPa is achieved,which corresponds to the nanocomposite Ti-63%B-N (mole fraction) coating consisting of the HCP-TiB2 nano-crystallites and amorphous BN phase.The lowest friction-coefficient was observed for the nanocomposite Ti-41%B-N (mole fraction) coating consisting of the FCC-TiN nanocrystallites and amorphous BN phase     

Bifurcations and negative propagation speeds of methane/air premixed flames with repetitive extinction and ignition in a heated microchannel

Detailed behaviors of ignition kernel(s) in a uniform stoichiometric methane/air mixture under the temperature gradient were investigated numerically by using a fundamental system of microcombustion. Bifurcation of the heat release rate peak in an ignition phase at the high wall temperature side, which has been observed in previous experimental and theoretical studies, was successfully reproduced by the present computation. The bifurcated heat release rate peak exhibited negative propagation speed relative to the local flow velocity by consuming a separated methane/air mixture in the downstream side of the boundary zone between an incoming fresh mixture and burned gas. CH₄ was completely consumed at the main peak, whereas CO remained unreacted in the wide region behind the main peak. In a weak reaction phase at the low wall temperature side, two bifurcations of heat release rate peak were newly captured. By the two bifurcations, three heat release rate peaks, namely, a main and two bifurcated peaks appeared. The two bifurcations were caused by remaining intermediates such as CH₃, CO, H, and OH in the downstream side of the boundary zone. The main and one bifurcated peak disappeared, whereas the other bifurcated peak remained and flowed downstream. The main and two bifurcated peaks exhibited negative propagation speeds relative to local flow velocity by consuming the remaining intermediates. CO which formed in the middle of the boundary zone in the weak reaction phase remained unreacted and kept on flowing downstream, but did not flow out since the next cycle of the ignition phase was initiated there.     

A variational void coalescence model for ductile metals

We present a variational void coalescence model that includes all the essential ingredients of failure in ductile porous metals. The model is an extension of the variational void growth model by Weinberg et al. (Comput Mech 37:142–152, 2006). The extended model contains all the deformation phases in ductile porous materials, i.e. elastic deformation, plastic deformation including deviatoric and volumetric (void growth) plasticity followed by damage initiation and evolution due to void coalescence. Parametric studies have been performed to assess the model’s dependence on the different input parameters. The model is then validated against uniaxial loading experiments for different materials. We finally show the model’s ability to predict the damage mechanisms and fracture surface profile of a notched round bar under tension as observed in experiments.     

Tunable generation of correlated vortices in open superconductor tubes

As shown theoretically, geometry determines the dynamics of vortices in the presence of transport currents in open superconductor micro- and nanotubes subject to a magnetic field orthogonal to the axis. In low magnetic fields, vortices nucleate periodically at one edge of the tube, subsequently move along the tube under the action of the Lorentz force and denucleate at the opposite edge of the tube. In high magnetic fields, vortices pass along rows closest to the slit. Intervortex correlations lead to an attraction between vortices moving at opposite sides of a tube. Open superconductor nanotubes provide a tunable generator of superconducting vortices for fluxon-based quantum computing.     

Cascading Wafer-Scale Integrated Graphene Complementary Inverters under Ambient Conditions

The fundamental building blocks of digital electronics are logic gates which must be capable of cascading such that more complex logic functions can be realized. Here we demonstrate integrated graphene complementary inverters which operate with the same input and output voltage logic levels, thus allowing cascading. We obtain signal matching under ambient conditions with inverters fabricated from wafer-scale graphene grown by chemical vapor deposition (CVD). Monolayer graphene was incorporated in self-aligned field-effect transistors in which the top gate overlaps with the source and drain contacts. This results in full-channel gating and leads to the highest low-frequency voltage gain reported so far in top-gated CVD graphene devices operating in air ambient, A(v) ～ -5. Such gain enabled logic inverters with the same voltage swing of 0.56 V at their input and output. Graphene inverters could find their way in realistic applications where high-speed operation is desired but power dissipation is not a concern, similar to emitter-coupled logic.     

Co-precipitation of yttrium and barium fluorides from aqueous solutions

Co-precipitation of barium and yttrium fluorides from aqueous solutions at room temperature produced non-equilibrium Ba_1?xY_xF_2+x nanofluoride phase with face-centered cubic crystal lattice of fluorite-type with the composition interval of the homogeneity for x = 0.35–0.75. Lattice parameter a of this solid solution nanophase varied as a function of the sample chemical composition in a complex manner with two areas of linear dependency, from x = 0.35 to 0.45 and from x = 0.50 to 0.75. A plausible explanation of this phenomenon included a change of the type of crystal lattice defects and the manner of their population with the corresponding ions. An increase of the relative amount of yttrium in the HF reaction system led to the formation of hydroxonium salt of decafluorotriyttrium acid, (H₃O⁺)Y₃F₁₀^?·nH₂O, instead of expected YF₃ hydrate. No formation of oxyfluoride phases under acidic conditions has been observed.     

Influence of small dozes ultra-violet radiation on motion of dislocation in alkali-halide crystals

The purpose of this work was research into influence of ultra-violet radiation on size of run of regional and screw dislocations in beams of dislocation sockets, formed at indentation surface of alkali-halide crystals. In experiments it was used crystals NaCl, with the quantitative maintenance of impurity 10-2 -10-3weight%, the wave length of UV-radiation λ=250 nanometers, the sizes of samples 10mm× 20mm× 2mm,temperature of samples was constant T=290 K.It is established that indentation and the simultaneous irradiation of samples a ultraviolet is increases size of run of head dispositions in dislocation sockets..It is marked, that influence UV-radiation nonequivalence for various times of an exposition. At small times (till 5 minutes) the size of run grows. The length of beams increases on ～ 50 %. At the further increase in time of influence of a ultraviolet the length of beams is reduced till the sizes corresponding stressing without an irradiation (Figs. 1, 2, 3). The effect is observed on dislocation beams of regional and screw orientations and most expressed at small loadings (in our experiments-10 grams) (Fig. 3).Observable effects are explained from positions dislocation-exciton interactions. At UV-radiation exciton cooperates with the charged step on a disposition, causing movement of a step along a disposition on one internuclear distance. Due to this interaction overcoming by a disposition of a grid of stoppers is facilitated.Big times of endurance cause a relaxation of pressure directly in a print that provides convertible movement of dispositions in area of a print and as consequence, reduction of length of beams of dislocation sockets.     

Lightning striking distance of complex structures

Traditionally, the location of lightning strike points has been determined by using the rolling sphere method, but recently the collection volume method (CVM) has also been proposed for the placement of air terminals on complex structures. Both these methods are empirical in nature and a more advanced model based on physics of discharges is needed to improve the state of affairs. This model is used to evaluate the striking distance from corners and air terminals on actual buildings and the results are qualitatively compared with the predictions of the rolling sphere method and the CVM. The results show that the striking distance not only depends upon the prospective return stroke current and the geometry of the building, but also on the lateral position of the downward leader with respect to the strike point. A further analysis is performed to qualitatively compare the lightning attraction zones obtained with the CVM and the leader inception zones obtained for a building with and without air terminals. The obtained results suggest that the collection volume concept overestimates the protection areas of air terminals placed on complex structures, bringing serious doubts on the validity of this method.     

Nanoporous materials: Porous Graphene as an Atmospheric Nanofilter (Small 20/2010)

The fabrication of nanoscale membranes exhibiting high selectivity is an emerging field of research. The possibility to use bottom‐up approaches to fabricate a filter with porous graphene and analyze its functionality with first principle calculations is investigated. Here, the porous network is produced by self‐assembly of the hexaiodo‐substituted macrocycle cyclohexa‐m‐phenylene (CHP). The resulting porous network exhibits an extremely high selectivity in favor of H₂ and He among other atmospheric gases, such as Ne, O₂, N₂, CO, CO₂, NH₃, and Ar. The presented membrane is superior to traditional filters using polymers or silica and could have great potential for further technological applications such as gas sensors or fuel cells.