Formation Of New Nuclei Research Articles

Crystallization of poly(aryl‐ether‐ether‐ketone): Effect of thermal history of the melt on crystallization kinetics

AbstractDifferential Scanning Calorimetry (DSC) and optical microscopy were used to investigate the effect of the thermal history of the melt on the crystallization of a commercial sample of poly(aryl‐ether‐ether‐ketone) (PEEK). Heating a film of PEEK at a temperature above the melt temperature for various periods of time changes the nucleation and crystal growth rate upon cooling the sample. Destruction of existing nuclei, formation of new nuclei, chain branching, cross linking, and chemical degradation of the macromolecular chains are all believed to take place at different times and to different extents during the thermal melt processing of the polymer. This study suggests that the thermal history of the melt plays an important role in the crystallization of PEEK samples.

Domain nucleation and wall movement in TbFeCo

The magnetic after-effect in Tbx(Fe85Co15)1−x with 0.19<x<0.33 measured at different applied fields Ha and domain observations using the polar Kerr effect show that the distinct differences of the after-effect between the samples investigated correspond to differences in domain behaviour. Samples exhibiting growth of non-equilibrium domains and simultaneous formation of new nuclei with time are distinguished from samples showing instantaneous formation of all nuclei running out as stripes into an equilibrium maze-like pattern with time. The evaluation of the after-effect measurement as a function of time for one sample yields a thermally activated wall motion with a potential barrier of ≈ 100 eV and an activation volume of ≈ 1.4 × 10−4 μm3.

The evolution of microcrystalline structures in supercooled metal powders

A numerical model has been developed to study the relative effects of nucleation and growth kinetics on the evolution of ultrafine grain structures observed in some supercooled metal powders. The thermal history during solidification is analyzed using a Newtonian heat transfer formulation coupled to classical models for homogeneous nucleation and continuous growth with a diffuse interface. The results indicate that decreasing particle size increases both the supercooling prior to solidification and the thermal excursion beyond the nucleation temperature. After the first nucleus appears, a compctition is established between the formation of new nuclei and the growth of the existing one(s). There is a range of particle sizes in which the achievable supercoolings are high enough to produce massive nucleation before any significant growth—and the ensuing recalescence—can take place. The probability of multiple nucleation may be evaluated from a dimensionless parameter combining the characteristic frequencies of the nucleation, growth, and heat transfer processes at the moment of nucleation. Calculations for Al and Ni model systems confirm the experimental observation that the latter has a stronger tendency to supercool and develop microcrystalline structures.

Direct microscopy of alloy nucleation, solidification and ageing (coarsening) during stir casting

In order to explain the slurry thixotropy and spherical particle morphology observed during the stir casting of metal alloys, we performed real time microscopic observations during the stir casting process using as a model substance the transparent organic alloy neopentyl alcohol with water. An optical set-up was developed consisting of two pulsed lasers, an optically adopted model of a stir casting apparatus, a microscope system and three different recording system to take (simultaneously) holograms, microphotographs and video recordings of the fast moving (up to 10 m s −1) small solidifying particles (10-10 3 μm). Our preliminary observations indicate that several mechanisms occur (nearly) simultaneously. Initially these are: both primary and secondary nucleation, rapid growth of the first nuclei into equiaxed dendrites, fragmentation of dendrite arms, clustering and coarsening. The development of the distribution of particle size and dendrite tip diameter and of the distribution of particle flow velocity and direction is shown quantitatively. We found that: (i) The increase of the fraction solid was mainly due to the formation of new nuclei (even after partial remelting), instead of outgrowth of existing nuclei. (ii) Not the fragmentation or clustering mechanisms but coarsening of the (branches of) equiaxed dendrites leads to the typical spherical particle shapes after stir casting. (iii) The rate of coarsening increases with stirring rate. (iv) Complex flow patterns at higher stirring rates can be attributed to the presence of Taylor vortices. Thixotropy is discussed in terms of coarsening and (local) variations of process parameters. The observation techniques used to determine the development of size and shape distributions, interactions and flow patterns of alloy particles during solidification seem also suitable to study industrial bulk crystallization.

Nucleation and interface formation in thin films

In this paper we present the results of an experimental investigation into the nucleation and growth of ion-plated films. Transmission electron microscopy was used to study the nucleation of ultrathin silver films (10–400 Å thick) deposited on carbon films about 50 Å thick. Test results reveal that the formation of the ion-plated films is dominated either by direct deposition onto previously implanted nuclei or by the formation of new nuclei. The effect of ion implantation is to minimize the now well-known liquid-like behaviour of the deposited films. Furthermore, continuous ion-plated films can be produced with lower thicknesses than continuous films conventionally deposited in a vacuum.

Zur elektrokristallisation von Legierungen und zu ihrer phasenstruktur

In the electrolytic codeposition of metals a deposit is sometimes formed, which is in contradiction to the phase diagram. In certain cases one obtains a single phase of mixed crystals, in other cases two phase alloys, which include one pure component. The reasons for this behaviour were studied in the present work. Experimental results obtained with the systems copper—lead, copper—bismuth, copper—thallium, copper—nickel and silver—copper show that the phase structure of an alloy can be predicted from a comparison of the electrochemical parameters of the alloy deposition (overvoltage) with the quantities characterizing the kinetics of the formation of a new phase (work of formation of new nuclei).In the present investigation the composition of the oversaturated mixed crystals was found to depend on the cathodic overvoltage. This suggests that the cause of the formation of metastable deposits in electrocrystallization is the increased energy of the atoms discharged at a high overvoltage.

Strain Induced Grain Boundary Migration in High Purity Aluminum

A method has been developed for the study of grain boundary migration in high purity aluminum, which allows positive determination of the direction of grain boundary movements. By means of this method it was confirmed that the migration of grain boundaries resulting from surface energy takes place in the direction toward the centers of curvature of the boundaries. On the other hand, strain induced grain boundary migration, such as occurs in recrystallization, proceeds in a direction away from the centers of curvature of the moving boundaries. It was also found that certain strained grains serve as nuclei for strain-free grains of the same orientation which grow at the expense of neighboring strained grains. Most of the strain-free grains formed during the annealing of high purity aluminum cold rolled up to about 40 percent are produced by this mechanism of strain-induced boundary migration, without the formation of new nuclei.

Radioactivity and the Completion of the Periodic System

IN connexion with the article by Prof. F. A. Paneth1 in NATURE of May 23, 1942, I would like to direct attention to the fact that the systematics of isotopes acquires a more simple and logical form if, besides the usual formation of nuclei by addition of a neutron (1on) and a proton (11p), we assume the possibility of the formation of new nuclei by addition of a hypothetical particle (1-1a) of unit mass and charge equal to — e, which may be called the aproton-type formation of nuclei.