Solid-phase diffusion mechanism for GaAs nanowire growth

Controllable production of nanometre-sized structures is an important field of research, and synthesis of one-dimensional objects, such as nanowires, is a rapidly expanding area with numerous applications, for example, in electronics, photonics, biology and medicine. Nanoscale electronic devices created inside nanowires, such as p-n junctions, were reported ten years ago. More recently, hetero-structure devices with clear quantum-mechanical behaviour have been reported, for example the double-barrier resonant tunnelling diode and the single-electron transistor. The generally accepted theory of semiconductor nanowire growth is the vapour-liquid-solid (VLS) growth mechanism, based on growth from a liquid metal seed particle. In this letter we suggest the existence of a growth regime quite different from VLS. We show that this new growth regime is based on a solid-phase diffusion mechanism of a single component through a gold seed particle, as shown by in situ heating experiments of GaAs nanowires in a transmission electron microscope, and supported by highly resolved chemical analysis and finite element calculations of the mass transport and composition profiles.     

Transformation Textures in As‐hot rolled TRIP Steels

The microstructures of TRIP steels finish‐rolled above and below the recrystallization‐stop temperature (T_nr) are compared. Here, the retained austenite grains are equiaxed or elongated, respectively, according to whether final rolling was carried out above or below the T_nr. The recrystallized austenite did not contain a sharp texture, the best defined component of which was the cube. The bainite that formed in this case was characterized by weak concentrations of the Goss and rotated Goss and a fairly strong concentration of the rotated cube. It also displayed the transformation products of a retained rolling fibre in the prior austenite. The deformed austenite contained the typical fcc rolling texture, where the copper is considerably more intense than the brass under these conditions. After transformation to bainite, the presence of a strong transformed copper component is evident, together with somewhat less intense contributions from the three transformed brass components. The data indicate that strong variant selection took place in the deformed austenite and that it was also present in the recrystallized material, but to a lesser extent. The latter displayed evidence of incomplete recrystallization in that the transformation texture included components obtained from both “recrystallized” and “deformed” austenite.     

The influence of training on the accuracy of roentgenographic cephalometric tracings (author's transl)

The accuracy in landmark identification and in the determination of angular and linear measurements used in cephalometric analysis on lateral head film were statistically evaluated comparing operators with different levels of competence. The observed magnitude of methodological error was depending on - prior experience of the investigator - individual radiographic anatomical knowledge - rigour of landmark definition and - accuracy of the tracing procedure. Considering these sources of error some of the variability in the present investigation might be avoided by systematic instructions. In order to decrease the variation in landmark location the reproducibility of some points must be further improved by redefining them in their verbal assessments. From an educational point of view the correlation between individual radiographic anatomical knowledge and the magnitude of error in cephalometric analysis should be considered with particular emphasis.     

Dependence of gas adsorption rates on carbon granule size and linear flow velocity

A study was made of the dependence of the adsorption rate constant of an activated carbon for dimethyl methylphosphonate vapor on carbon granule size and superficial linear velocity using the adsorption kinetics equation to calculate the rate constant from critical bed weight values. Over a 30-fold range of velocities and a 7-fold range of granule diameters it was found, in accord with adsorption kinetics theory, that although the adsorption capacity for the vapor was invariant, the time for vapor breakthrough of the bed varied because of the effects of linear flow velocity and carbon granule size on the adsorption rate constant. In general, the rate constant increased nonlinearly with increase in velocity and decrease in carbon granule size. The slowest adsorption kinetics existed for the largest granule size at the lowest linear flow velocity, becoming increasingly faster as the velocity was increased and/or the granule size was decreased. For the smallest granule size the rate constant reached a limiting value of 2600 sec^?1 becoming essentially independent of linear velocity due to a change in the rate controlling step.     

Users as a Hidden Resource for Creativity: Findings from an Experimental Study on User Involvement

The main objective of this article is to report the empirical findings from a study on user involvement in service innovation. In doing this, we seek to answer the question of how user involvement affects the originality of new service ideas. An experimental investigation was carried out which included 54 participants arranged into three groups of creative users, ordinary users and professional service developers. The empirical data revealed that the users produced more original ideas than the company’s professional service developers. It is thus suggested that business organizations attempt to innovate original products would benefit from involving their customers.     

Solvent effects on hydrolysis stage of the sol-gel process

High-resulution ²⁹Si NMR has been employed to investigate the role of chemical additives in the dynamics of the sol-gel hydrolysis process. The chemical additives selected for the study are formamide, dimethyl formamide, dioxane and acetonitrile. The kinetics of the hydrolysis reaction have been shown to be strongly dependent upon the viscosity of the sol-gel system, and consequently upon the viscosity of the chemical additive considered.     

Residual adsorption capacity of carbon beds

The retention time of a small (0.3 cm³) ethane pulse, to which various dry weights (1–3 g) of activated carbon were exposed, was studied as a nondestructive method of determining the residual adsorption capacity of the bed. Carbon beds, partially saturated with CCl₄ or water, adsorbed the ethane from and then desorbed the ethane into the nitrogen carrier gas stream. At a fixed flowrate and fractional carbon saturation the retention time in the bed varied linearly with carbon weight. A critical bed weight existed, below which the retention time of ethane in the bed was zero. The logarithm of a dimensionless time parameter, normalized with respect to the effective bed weight (total weight minus critical weight), was a linear function of the percentage carbon saturation (or the percentage residual adsorption capacity) of the bed. This approach constitutes a nondestructive, in situ method for determining the residual adsorption capacity of an activated carbon bed that is independent of bed weight.     

Acoustoelastic determination of the higher order orientation distribution function coefficients up tol = 12 and their use for the on-line prediction ofr-value

The higher order orientation distribution function (ODF) coefficients up tol = 12 in cold-rolled and annealed sheet steels were evaluated and calculatednondestructively from the anisotropy of the ultrasonic velocities of the lowest order symmetrical Lamb (S₀) and shear horizontal (SH ₀) waves propagating in the rolling plane. The elastic energy method was employed, together with a decomposition of the texture into the principal preferred orientations, following a procedure originally developed for Young’s modulus data obtained destructively. Plastic strain ratios are estimated using the series expansion method, in conjunction with a relaxed constraint (pancake) grain interaction model. Ther- value predictions based on the ODF coefficients obtained in this study are compared with tensile data and with empirical predictions made using the Modul-r approach. Formerly with the Department of Metallurgical Engineering, McGill University     

Effect of texture and heat treatment on high temperature deformation of Zircaloy-2

Cylindrical specimens machined from Zircaloy-2 plate have been tested under compression at 295, 675 and 1075 K. The plate was in three conditions (a) as hot rolled, (b) oil quenched from 1130 K. and (c) oil quenched from 1340 K. These conditions simulate the structures in fuel cladding in regions near brazed spacers. The structures and crystallographic textures of the material in these conditions were characterized. Specimens with axes along the longitudinal, transverse and thickness directions were tested to determine the effect of temperature, texture and structure on the mechanical properties, particularly with respect to postulated LOCA conditions. The tests showed that (i) factors for mechanical anisotropy derived from room temperature results should be valid for LOCA analysis, (ii) anisotropy factors from the results are consistent with about 67% prism and 33% basal slip in the early stages of deformation and (iii) oil quenching from 1130 K changes the texture and strength in a minor way, while quenching from 1340 K (β phase) gives an almost random texture, little mechanical anisotropy and a small change in the average strength.