Gel growth and dielectric studies of K. Rb and Cs perchlorates

Single crystals of K, Rb and Cs perchlorates have been grown by the counter diffusion of the respective ions and CIO4 through the gel medium. Studies on nucleation, growth kinetics, morphological aspects and purity are discussed in this paper. The dielectric constant, ~b, as well as loss measured along the longest axis, exhibits an anomaly at the transition temperature, Tt, in all the three crystals. It is found that the peak values of Tt are approximately 800, 100 and 53 in K, Rb and Cs perchlorates, respectively. The dielectric anomaly and the large value of c b in the cubic phase are discussed in terms of the degree of disorder of the CIO~ group and the possible contribution from defects.

Non-equilibrium trapping in Al---In system

The comparative trapping efficiency of indium in aluminium solid solution during vapour deposition and rapid solidification at room temperature was evaluated by studying the nature of the decomposition of the metastable solid solution. A spinodal decomposition was observed in the case of rapid solidification while vapour deposited foils decompose by a nucleation and growth type of process. From this we conclude that rapid solidification effects a more efficient non-equilibrium trapping of indium in aluminium solid solution compared to vapour deposition at room temperature.

Relaminarization in highly accelerated turbulent boundary layers

The mean flow development in an initially turbulent boundary layer subjected to a large favourable pressure gradient beginning at a point x0 is examined through analyses expected a priori to be valid on either side of relaminarization. The ‘quasi-laminar’ flow in the later stages of reversion, where the Reynolds stresses have by definition no significant effect on the mean flow, is described by an asymptotic theory constructed for large values of a pressure-gradient parameter Λ, scaled on a characteristic Reynolds stress gradient. The limiting flow consists of an inner laminar boundary layer and a matching inviscid (but rotational) outer layer. There is consequently no entrainment to lowest order in Λ−1, and the boundary layer thins down to conserve outer vorticity. In fact, the predictions of the theory for the common measures of boundary-layer thickness are in excellent agreement with experimental results, almost all the way from x0. On the other hand the development of wall parameters like the skin friction suggests the presence of a short bubble-shaped reverse-transitional region on the wall, where neither turbulent nor quasi-laminar calculations are valid. The random velocity fluctuations inherited from the original turbulence decay with distance, in the inner layer, according to inverse-power laws characteristic of quasi-steady perturbations on a laminar flow. In the outer layer, there is evidence that the dominant physical mechanism is a rapid distortion of the turbulence, with viscous and inertia forces playing a secondary role. All the observations available suggest that final retransition to turbulence quickly follows the onset of instability in the inner layer.It is concluded that reversion in highly accelerated flows is essentially due to domination of pressure forces over the slowly responding Reynolds stresses in an originally turbulent flow, accompanied by the generation of a new laminar boundary layer stabilized by the favourable pressure gradient.

Effect of magnetic and electrical fields on dendritic freezing of aqueous solutions of sodium chloride

Aqueous solutions of sodium chloride were solidified under the influence of magnetic and electrical fields using two different freezing systems. In the droplet system, small droplets of the solution are introduced in an organic liquid column at −20°C which acts as the heat sink. In the unidirectional freezing system the solutions are poured into a tygon tube mounted on a copper chill, maintained at −70°C, from which the freezing initiates. Application of magnetic fields caused an increase in the spacing and promoted side branching of primary ice dendrites in the droplet freezing system, but had no measurable effect on the dendrites formed in the unidirectional freezing system. The range of electric fields applied in this investigation had no measurable effect on the dendritic structure. Possible interactions between external magnetic and electrical fields have been reviewed and it is suggested that the selective effect of magnetic fields on dendrite spacings in a droplet system could be due to a change in the nucleation behaviour of the solution in the presence of a magnetic field.

Eutectic crystallization of poly(l-lactic acid) and pentaerythrityl tetrabromide

Diluents (either low molecular weight compounds orother polymers) are known to modify the morphology, the rates of nucleation and growth of polymers 1- 4. Recentlybinary systems in which both the components crystallize simultaneously to give a eutectic solid have been studied with great interest. Carbonnei et al.

Line tension of a two dimensional gas-liquid interface

In two dimensional (2D) gas-liquid systems, the reported simulation values of line tension are known to disagree with the existing theoretical estimates. We find that while the simulation erred in truncating the range of the interaction potential, and as a result grossly underestimated the actual value, the earlier theoretical calculation was also limited by several approximations. When both the simulation and the theory are improved, we find that the estimate of line tension is in better agreement with each other. The small value of surface tension suggests increased influence of noncircular clusters in 2D gas-liquid nucleation, as indeed observed in a recent simulation.

Polarization switching in radiation damaged ferroelectric crystals

The polarization switching processes in radiation damaged ferroelectrics were studied by the Merz method. In irradiated triglycine sulphate and sodium nitrite, the switching time depends exponentially on the applied electric field. Irradiation increases the importance of nucleation and sideways motion of the domain walls in polarization switching.

Kinetics and mechanism of the transformation in antimony trioxide from orthorhombic valentinite to cubic senarmontite

The kinetics of the polymorphic transformation in antimony trioxide from metastable orthorhombic valentinite to cubic senarmontite has been studied in polycrystalline material between 490 and 530°C. Quantitative analysis of the mixtures was done using infrared spectrophotometry. The kinetic data was analyzed and the activation energy for the process was obtained: (i) On the basis of Avrami's equation, which is derived on the basis of a nucleation and growth mechanism; and (ii) from the time required for a constant fraction of the transformation to take place. The values obtained were 50.8 and 46.0 kcal/mole. Observations have also been made on partly transformed single crystals of valentinite using a polarizing microscope. The latter studies and the value of the activation energy suggest that a better understanding of the transformation could be obtained on the basis of a vapor phase mechanism.

A note on the movement of single large liquid bubbles in vertical tubes

The flow of single large liquid bubbles under gravity in closed tubes is studied here for the case when the liquid bubble exhibits micropolar behaviour. The film thickness, velocity profile in the bubble and film, and nonNewtonian effects are studied and compared with those for the correspondingNewtonian fluid. The investigation is restricted to the case where the bubble length is far greater than the tube radius.

Rate Model and Mechanism of Liquid-Phase Oxidation of Propionaldehyde

A rate equation is developed for the liquid-phase oxidation of propionaldehyde with oxygen in the presence of manganese propionate catalyst in a sparged reactor. The equation takes into account diffusional limitations based on Brian's solution for mass transfer accompanied by a pseudo m-. nth-order reaction. Sauter-mean bubble diameter, gas holdup, interfacial area, and bubble rise velocity are measured, and rates of mass transfer within the gas phase and across the gas-liquid interface are computed. Statistically designed experiments show the adequacy of the equation. The oxidation reaction is zero order with respect to oxygen concentration, 3/2 order with respect to aldehyde concentration, and order with respect to catalyst concentration. The activation energy is 12.1 kcal/g mole.

An incipient 310 helix in Piv-Pro-Pro-Ala-NHMe as a model for peptide folding

The molecular mechanism of helix nucleation in peptides and proteins is not yet understood and the question of whether sharp turns in the polypeptide backbone serve as nuclei for protein folding has evoked controversy1,2. A recent study of the conformation of a tetrapeptide containing the stereochemically constrained residue alpha-aminoisobutyric acid, both in solution and the solid state, yielded a structure consisting of two consecutive beta-turns, leading to an incipient 310 helical conformation3,4. This led us to speculate that specific tri- and tetra-peptide sequences may indeed provide a helical twist to the amino-terminal segment of helical regions in proteins and provide a nucleation site for further propagation. The transformation from a 310 helical structure to an alpha-helix should be facile and requires only small changes in the phi and psi conformational angles and a rearrangement of the hydrogen bonding pattern5. If such a mechanism is involved then it should be possible to isolate an incipient 310 helical conformation in a tripeptide amide or tetrapeptide sequence, based purely on the driving force derived from short-range interactions. We have synthesised and studied the model peptide pivaloyl-Pro-Pro-Ala-NHMe (compound I) and provide here spectroscopic evidence for a 310 helical conformation in compound I.

Nano-meter scale plasticity in KBr studied by nanoindenter and force microscopy

The early stages of plasticity in KBr single crystals have been studied by means of nano-meter-scale indentation in complementary experiments using both a nanoindenter and an atomic force microscope. Nanoindentafion experiments precisely correlate indentation depth and forces, while force microscopy provides high-resolution force measurements and images of the surface revealing dislocation activity. The two methods provide very similar results for the onset of plasticity in KBr. Upon loading we observe yield of the surface in atomic layer units which we attribute to the nucleation of single dislocations. Unloading is accompanied by plastic recovery as evident from a non-linear force distance unloading curve and delayed discrete plasticity events.

Development of novel grain morphology during hot extrusion of magnesium AZ21 alloy

Microstructure and microtexture evolution during static annealing of a hot-extruded AZ21 magnesium alloy was studied. Apart from fine recrystallized equiaxed grains and large elongated deformed grains, a new third kind of abnormal grains that are stacked one after the other in a row parallel to the extrusion direction were observed. The crystallographic misorientation inside these grains was similar to that of the fine recrystallized grains. The large elongated grains exhibited significant in-grain misorientation. A self-consistent mechanistic model was developed to describe the formation of these grain morphologies during dynamic recrystallization (DRX). The texture of pre-extruded material, although lost in DRX, leaves a unique signature which manifests itself in the form of these grain morphologies. The origin of abnormal stacked grains was associated with slow nucleation in pre-extruded grains of a certain orientation. Further annealing resulted in large secondary recrystallized grains with occasional extension twins. (c) 2009 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Nanoscale Heterostructures with Molecular-Scale Single-Crystal Metal Wires

Creating nanoscale heterostructures with molecular-scale (<2 nm) metal wires is critical for many applications and remains a challenge. Here, we report the first time synthesis of nanoscale heterostructures with single-crystal molecular-scale Au nanowires attached to different nanostructure substrates. Our method involves the formation of Au nanoparticle seeds by the reduction of rocksalt AuCl nanocubes heterogeneously nucleated on the Substrates and subsequent nanowire growth by oriented attachment of Au nanoparticles from the Solution phase. Nanoscale heterostructures fabricated by such site-specific nucleation and growth are attractive for many applications including nanoelectronic device wiring, catalysis, and sensing.

Formation of ω-Al7Cu2Fe phase during laser processing of quasicrystal-forming Al-Cu-Fe alloy

The formation of an ω-Al7Cu2Fe phase during laser cladding of quasicrystal-forming Al65Cu23.3Fe11.7 alloy on a pure aluminium substrate is reported. This phase is found to nucleate at the periphery of primary icosahedral-phase particles. A large number of ω-phase particles form an envelope around the icosahedral phase. On the outer side, they form an interface with an agr-Al solid solution. Detailed transmission electron microscopic observations show that the ω phase exhibits an orientation relationship with the icosahedral phase. Analysis of experimental results suggests that the ω phase forms by precipitation on an icosahedral phase by heterogeneous nucleation and grows into the aluminium-rich melt until supersaturation is exhausted. The microstructural observations are explained in terms of available models of phase transformations.