Papers by Keyword: Nucleation

Abstract: Sn-0.7Cu-0.05Ni is a widely used Pb-free solder that solidifies into a near-eutectic microstructure and a small fraction of primary Cu₆Sn₅. This paper overviews in-situ time-resolved imaging experiments on the solidification of Sn-0.7Cu-0.05Ni solder under three conditions: (i) directional solidification, (ii) continuous cooling in a near-uniform thermal field, and (iii) solder joint solidification on a Cu substrate. Primary Cu₆Sn₅ grow as rods along [0001] in each case but can also grow as X-shaped crystals in (iii). There are significant differences in eutectic growth due to nucleation difficulties for tin in conditions (ii) and (iii).

Abstract: The initial stages of copper electrodeposition in acid copper sulphate/sodium citrate bath were investigated with varying copper and sodium citrate concentrations. Different electrochemical measurements, including linear sweep voltammetry, cyclic voltammetry, and chronoamperometry were introduced to the study. The Scharifker-Hills model was introduced to identify the nucleation model with analysing current transients. It was observed that the increase of copper ions inhibited the cathodic polarization behaviour for the reduction of ions. On the contrary, sodium citrate promoted the cathodic polarization behaviour. The chronoamperometry results indicated that without the sodium citrate, the nucleation process corresponded to instantaneous nucleation and three-dimensional diffusion limited growth, although obvious deviations were observed. While the addition of sodium citrate changed the deviations and caused that the initial deposition kinetics followed well with the mechanism of instantaneous nucleation.

Abstract: A reaction-diffusion framework (RDF) is used to synthesize and control the size and morphology of single crystals of metal-organic framework-199 (MOF-199). The framework consists of diffusing copper ions (Cu²⁺, outer electrolyte) into a hydrogel medium containing the organic linker, 1,3,5-benzenetricarboxylic acid (BTC, inner electrolyte). The resulting supersaturation gradient, and its nonlinear coupling with nucleation and growth kinetics, provides means to control the crystal size, distrubution and morphology along the diffusion flux. This method is rapid, efficient, scalable, and environmentally friendly. By using this method we demonstrate how assorted experimental parameters, such as temperature, concentrations, and nature of the gel matrix can be easily tuned to produce different particle size distributions and various morphologies.

Abstract: The crystallization of planar solid oxide fuel cells (SOFCs) sealant glasses in the systems BaO-Al₂O₃-SiO₂-B₂O₃ (BaBS) and BaO-Al₂O₃-SiO₂-B₂O₃-ZnO (BaBS-Zn) was investigated via both X-ray diffractometer and scanning electron microscopy with energy dispersive spectroscopy. The effect of nucleation heat-treatment of the BaBS glass at different temperature for 5 hours, i.e. 550 and 590 °C, on the crystallization behavior was also studied. Thermal expansion profiles of the glasses indicate that both glasses have a low sealing temperature. XRD patterns of all BaBS glass-ceramics, devitrified at 800 °C for 30 hours, show that Ba₂Si₃O₈, BaAl₂Si₂O₈, Ba₃B₂O₆ and some unknown crystalline phases were found. It was also found that crystalline size of unknown barium aluminosilicate with low silicon content depends on the nucleation heat-treatment temperature. For the ZnO-containing glass, ZnO reduces the coefficient of thermal expansion value of glass and causes the devitrification of large needle-like barium zinc silicate phases. The crack at the YSZ/BaBS-Zn glass-ceramic interface was also observed. Two barium silicate phases, which are BaZnSiO₄ and BaZn₂Si₂O₇ were devitrified in ZnO-containing glass-ceramic.

Abstract: The effect of TiB on the nucleation and growth of α phase both in as-casted and as-annealed Ti-6Al-4V-0.1B alloy by optical microscopy and EBSD technique. The results show that during the solidification α phases nucleate and grow up in the β phase matrix and the interface between beta matrix and TiB phase. TiB phase acts as a nucleation site for α phase and promotes the nucleation and growth, resulting in refined α colonies in as-cast microstructure. During the furnace cooling process from above β transus temperature, α precipitates is a consequence of its orientation relationship with TiB, which restraints the lath growth. Equiaxed α phase precipitates during the furnace cooling from the above β transus temperature.

Abstract: The saturation of primary tensile twins in heavily textured Mg-alloy AZ31 is investigated, and their strain accommodation limit is evaluated. EBSD results suggest that the mean number of twins per grain saturate rapidly, followed by the stop of twin growth. Twinning saturation is included in a physical model of twin evolution.

Abstract: In the structures of all metastable precipitates in Al-Mg-Cu and Al-Mg-Si alloys, we find that column surrounding of an element column in the needle/lath direction order according to simple principles. Advanced transmission electron microscopy and DFT calculations support the principles originate with a line defect, which is a segment of a <100>Al column shifted to interstitial positions. We propose the defect aids solute decomposition by partitioning the FCC matrix locally into columns of fewer and higher number of nearest neighbours, which suit smaller and larger size solute atoms, respectively. The defect explains how <100> directionality of the precipitates can arise in a cluster. Ordering of a few defects leads naturally to GPB zones in Al-Mg-Cu and to β'' in Al-Mg-Si.

Abstract: The microcellular injection molding technology, commercially offered by Trexel Inc. and other manufacturers, is primarily a close cell foaming technique. This process is capable of offering light weight non-porous thermoplastics moldings. The foaming of thermoplastics with open cellular morphology has got various high end applications among others like tissue engineering and membrane separation. Some of the researchers were successful in synthesis of open cellular thermoplastics at laboratory scale via solid state batch process. The growing demand for microporous thermoplastics, especially the biodegradable plastics (e.g. Polylactic acid), motivated the researchers develop a specialized microcellular injection molding process for processing of open cell thermoplastics using physical blowing agents such as supercritical nitrogen or carbon dioxide gas. A brief of theoretical and conceptual treatment of microcellular injection molding is presented.

Abstract: Nano size (2 - 10 nm) metal particles were formed and accumulated on powder substrate by conventional physical vapor deposition (PVD) process, in which the powder were a non-volatile in vacuum, such as Al₂O₃ powder. The neutral nuclei which were formed on the substrates from vaporized or sputtered metal atoms at an initial thin film growth were not grown up to coalescence and island stage with arrival atoms and ad-atom migration in the continuous deposition process, when the powder in vessel were continuously stirred during the deposition. Nano sized particles on the polymer chips (diameter: 1-2mm) easily dispersed into the polymer matrix by heating the chips, and on non-soluble powder, g-Al₂O₃, were stuck on the supporters stably in air. The nanoparticles on sucrose directly formed colloid with water solvents without dispersive agents. Most of the nano sized particles appear their own characteristic colors due to plasmon effect. Concentration and size of the nanoparticles are controlled by physical parameters in the PVD and the stirring speed of the powder. Surface phenomena on the substrate have been discussed with TEM, SEM, EDX, UV spectroscopy, etc. comparing with the conventional thin film growth.

Abstract: The etching of silicon nitride using phosphoric acid with silicon dioxide as a mask is an important process step used in the production of 3D NAND devices. This paper examines the theory of formation of a silica film onto the silicon dioxide surface during this etching step by performing a shell balance analysis of silica species in the etched out liquid volume of the 3D NAND structures. The method of moments is used to solve for the moments of the distribution of particle sizes, and this is used to solve for the potential energy barrier for silica particles to adhere to the silicon dioxide surface.