Wavefront sensor based on the Talbot effect with the precorrected holographic grating

A holographic wavefront sensor based on the Talbot effect is proposed. Optical wavefronts are measured by sampling the light amplitude distribution with a two-dimensional (2D) precorrected holographic grating. The factors that allow changing an angular measurement range and a spatial resolution of the sensor are discussed. A comparative analysis with the Shack-Hartmann sensor is illustrated with some experimental results.     

Densification kinetics of nanocrystalline zirconia powder using microwave and spark plasma sintering--a comparative study

Two-stage densification process of nanosized 3 mol% yttria-stabilized zirconia (3Y-SZ) polycrystalline compacts during consolidation via microwave and spark-plasma sintering have been observed. The values of activation energies obtained for microwave and spark-plasma sintering 260-275 kJ x mol(-1) are quite similar to that of conventional sintering of zirconia, suggesting that densification during initial stage is controlled by the grain-boundary diffusion mechanism. The sintering behavior during microwave sintering was significantly affected by preliminary pressing conditions, as the surface diffusion mechanism (230 kJ x mol(-1)) is active in case of cold-isostatic pressing procedure was applied.     

Design and Synthesis of Polycyclic Imidazole‐Containing N‐ Heterocycles based on C?H Activation/Cyclization Reactions

A new strategy for the synthesis of polycyclic imidazole‐containing N‐heterocycles, based on the two general synthetic ways, namely the Pd(II)‐catalyzed intramolecular arylation via CH/C Hal and CH/CH coupling reactions, was developed. The method proposed here enables the synthesis of many fused N‐heterocycles containing purine, 1‐deazapurines and benzimidazole structural units.     

Activation effects during hydrogen release and uptake of MgH2

Scandium(II)hydride, ScH₂, and scandium(III)chloride, ScCl₃, are explored as additives to facilitate hydrogen release and uptake for magnesium hydride. These additives are expected to form more homogeneous composites with Mg/MgH₂ as compared to metallic scandium. However, scandium(III)chloride, reacts with MgH₂ during mechano-chemical treatment and form ScH₂ and MgCl₂ (that later crystallise during heat treatment). The composite MgH₂−ScH₂ was investigated using in-situ synchrotron radiation powder X-ray diffraction during up to five cycles of continuous release and uptake of hydrogen at isothermal conditions at 320, 400 and 450 °C and p(H₂) = 100–150 or 10⁻² bar. The data were analysed by Rietveld refinement and no reaction is observed between either MgH₂/ScH₂ or Mg/ScH₂ during cycling. The extracted sigmoidal shaped curves for formation or decomposition of Mg/MgH₂ suggest that a nucleation process is preceding the crystal growth. The reaction rate increases with increasing number of cycles of hydrogen release and uptake at isothermal conditions possibly due to activation effects. This kinetic enhancement is strongest between the first cycles and may be denoted an activation effect.     

Measured kinetics of the acid-catalysed hydrolysis of sugar cane bagasse to produce xylose

Experimental trials of the water hydrolysis of bagasse to produce xylose, arabinose and glucose were conducted using a temperature-controlled microwave digester. The experimental variables were temperature, ratio of water mass to bagasse mass, type of bagasse material and reaction time. The pH of the liquid and concentration of dissolved xylose, arabinose and glucose were measured at the completion of each trial. Kinetic modelling of the global rates of formation of monosaccharide products was performed using schemes based on earlier researchers’ models of acid hydrolysis using mineral acids. For the most plentiful product, xylose, the most accurate kinetic model of the global reactions was determined to be two parallel pathways for hydrolysis of xylan to xylose followed by a single pathway for xylose decomposition. The calculated activation energies of the reactions were within the range reported by other researchers for the hydrolysis of a range of lignocellulosic materials using mineral acids.     

Segregation Development in Multiple Melt Vacuum Arc Remelting

A numerical model of the vacuum arc remelting (VAR) process was used to study multistage VAR processes. The studies of low and high power 3XVAR confirmed the results of the single stage process studies for Ti-10-2-3: (1) high arc power results in strong electromagnetically driven flow and undesirably high macrosegregation; (2) low arc power does not generate significant Lorentz forces and the flow is dominated by weaker buoyancy forces, which cause less segregation; and (3) even short-lived changes in process conditions during the run may result in a switch of the flow regime in low power cases from buoyancy driven to electromagnetically driven. The switch of flow regime results in an increase in macrosegregation levels and a change in the pattern of solute redistribution. The most significant finding in the studies of 3XVAR processing of Ti-10-2-3 is the small effect of the electrode composition distribution on ingot segregation development. In both low and high power VAR cases, macrosegregation levels and patterns in the final ingots were similar to those demonstrated assuming a uniform electrode for that final case. However, for low power cases, nonuniformities in the electrode composition may strongly affect the final ingot macrosegregation. The nonuniformity in the composition of the electrode results in the formation of additional buoyancy forces within the liquid pool, which can cause a switch from buoyancy driven flow to the undesirable electromagnetically driven flow regime and a drastic change in segregation development. This article is based on a presentation given at the International Symposium on Liquid Metal Processing and Casting (LMPC 2007), which occurred in September 2007 in Nancy, France.     

Toward a definition of self-care children: A commentary on Steinberg (1986).

This comment responds to an article by Steinberg (1986) in which he stressed the desirability of investigating a wide variety of after-school experiences when examining consequences for latchkey (self-care) children. In presenting his results, he inadvertently confounded latchkey children with adolescents who spent time at a friend's house or who "hang out" with friends without adult supervision. Steinberg reported that these adolescents showed greater susceptibility to peer pressure to engage in antisocial activities. We point out that these adolescents do not fit the usual conceptualization of latchkey children and that Steinberg's study may lend support to practitioners and policy makers who assume negative latchkey consequences in the absence of research evidence. To avoid future confusion we indicate the need for a definition of latchkey children and offer a tentative definition. (PsycINFO Database Record (c) 2010 APA, all rights reserved)