Structure and phase formation in the Ti–Al–Nb system in the thermal explosion mode

This paper presents the results of the structure and phase formation in the Ti/Nb/2Al, Ti/Nb/2.5Al and Ti/Nb/3Al systems in the thermal explosion mode of self-propagating hightemperature synthesis. The morphology, phase composition, microstructure, and physical properties have been studied. It has been found that compounds with the highest content of aluminum have the most homogeneous composition and the lowest porosity. The main phase of the synthesis product is a phase based a solid solution of Nb in γ-TiAl.     

Defect structure of multiwalled carbon nanotubes studied by Raman spectroscopy

The defect structure of multiwalled carbon nanotubes has been studied by transmission electron microscopy and Raman spectroscopy, with particular attention to the shape and intensity of the defect band D and its overtone D*. Electron-microscopic results demonstrate that multiwalled nanotubes typically have multiple bends. The associated short- and long-range disorder influences the Raman spectrum of the nanotubes. The presence of several defect species, differing in scattering probability, results in a stochastic relationship between the intensities of the D*- and D-bands. This relationship is qualitatively interpreted in terms of general mechanisms of elastic/inelastic interactions of π-electrons with phonons and defects.     

Numerical Evaluation of Toughening by Crack-Face Grain Interlocking in Self-Reinforced Ceramics

Crack bridging associated with the pull-out process of interlocking grains in self-reinforced ceramic materials is studied through a micromechanical simulation. The pullout of a single inclined grain is modeled via the numerical solution of a general contact problem. The bridging-force versus crack-opening-distance curve indicates a nonlinear, springlike response for the pullout of interlocking grains. The sliding friction along the debonded interface, induced by highly localized contact stresses, dominates the total bridging force. The bridging force increases with grain inclination until eventual bridge failure. The pullout of misaligned grains mainly affects short-crack toughening, with a rising R -curve, whereas aligned grains contribute to long-crack toughening. The residual stresses of the thermal expansion anisotropy play a minor role in the pull-out process of grain interlocking and the resultant toughening. The proposed mechanism is operative in both single-phase and composite ceramics in which pullout of elongated grains/reinforcements occurs.     

Cast silicides of molybdenum,tungsten, and niobium by combustion synthesis

Cast silicides of molybdenum, tungsten, and niobium were prepared by combustion synthesis under nitrogen pressure from powder mixtures of respective metal oxides with Al and Si. Upon variation in green composition, cast MoSi₂, WSi₂, NbSi₂, and their solid solutions with variable phase composition have been synthesized. NbSi₂ was obtained by combustion in the presence of added heat-generating CaO₂-Al powder mixture.     

Selective Hydrolysis of Arabinogalactan into Arabinose and Galactose Over Heterogeneous Catalysts

Abstract  

The application of solid acid catalysts for the production of monomers from hemicelluloses can be one of the key steps in developing the concept of an integrated forest biorefinery. Arabinogalactans (AG) are hemicelluloses which can be extracted on an industrial scale from larch wood species and has a great potential as a sustainable feedstock for bio-based products. Hydrolysis of AG to monomers over acidic heterogeneous catalysts (Smopex-101 and Amberlyst 15) was successfully demonstrated for the first time to selectively produce arabinose as the primary product, followed by the release of galactose without further degradation of the monomers.