Determination Of Nucleation Rates Research Articles

A study on the crystallization behaviour of polypropylene, polyethylene and their blends by dynamic mechanical and thermal methods

Studies on the crystallization of polymers are usually carried out by dilatometry, differential scanning calorimetry (d.s.c.) and optical microscopy. These methods have been widely used in the analysis of crystallization kinetics using the Avrami equation. Direct determination of nucleation rate and density is only possible by the optical microscope technique. We have employed a new technique for studying the crystallization behaviour of polymers by using a dynamic mechanical method. This method is capable of detecting the onset of nucleation. Semiquantitative data on the nucleation density and the initial crystallization rate may also be deduced by this method. Results from the dynamic mechanical method are cross-compared with d.s.c. and optical microscope data for polypropylene, polyethylene and their blends, and are shown to be mutually consistent.

Determination of nucleation rate in polymers using isothermal crystallization experiments and computer simulation

Within the frame of overall kinetics of crystallization, polymer crystallization is characterized by only three parameters: the initial density of potential nucleiN0, their activation frequencyq and the growth rateG. The growth rateG is rather easy to measure, whereas the nucleation parameters are generally unknown. Our purpose is to determine bothN0 andq using experimental isothermal two-dimensional crystallizations and computer simulation. Both these parameters are deduced from the rate of appearance of spherulites expressed as a function of the transformed surface fraction. The activation times of the spherulites are deduced from the shape of the boundaries between spherulites at the end of the transformation. When the growth rateG is known, the evolution of the transformed surface fraction is rebuilt using a computer simulation, so that only one observation of the final stage of the crystallization is needed.

Surface and bulk crystallization of amorphous alloys Ni-Si-B probed by electrical resistivity

Electrical resistivity measurements are tested as a sensitive probe of the crystallization processes in amorphous metallic alloys of Ni78Si7B15, rendering the determination of nucleation rates at the surface and in bulk. It is shown that the increase in the electrical resistivity just below the crystallization temperature is mainly due to nucleation phenomena. Moreover, the Avrami coefficient, calculated from resistivity data, provides information about types of crystallization mechanisms, dimensionality of the crystallite growth and the sequence of crystallization stages.

Gas-flow diffusion chamber for vapour nucleation studies. Relations between nucleation rate, critical nucleus size and entropy of transition from a metastable into a stable state

Modification of the method of mixing has been considered which makes it possible to carry out nucleation experiments in coaxial laminar vapour-gas streams having different temperatures. The vapour flow diffusion chamber realizes this method and is described together with new experiments on the nucleation rate determination of supersaturated vapour in conditions similar to isobaric or isothermal. These experiments made it possible to determine critical sizes of nuclei of the new phase in supercooled vapour. The possibility of experimental determination of the entropy of phase transition from metastable to stable state is theoretically shown.

Glass Formation, a Contemporary View

The process of glass formation is discussed from several perspectives. Particular attention is directed to kinetic treatments of glass formation and to the question of how fast a given liquid must be cooled in order to form a glass. Specific consideration is paid to the calculation of critical cooling rates for glass formation, to the effects of nucleating heterogeneities and transients in nucleation on the critical cooling rates, to crystallization on reheating a glass, to the experimental determination of nucleation rates and barriers to crystal nucleation, and to the characteristics of materials which are most conducive to glass formation.

Crystallization of sodium perborate from aqueous solutions: I. Nucleation rates in pure solution and in presence of a surfactant

We describe a new experimental method for the determination of nucleation rates in solution, consisting of the measure of the total mass precipitated in a solution where the supersaturation is kept artificially constant. Another experiment is necessary, to determine the growth rate of the crystals. In pure solutions, the nucleation rate is not constant, but is a staircase-form function of time. The addition of the sodium salt of butyl ester oleic acid reduces the nucleation rates and makes the nucleation still more irregular. Sodium perborate NaBO 2, H 2O 2, 3 H 2O is an important constituent of washing powders, and is manufactured in large quantities. By the action of a sodic solution on the boron ore, a sodium metaborate solution is prepared; the crystallization of sodium perborate is obtained by a mixing of this solution with hydrogen peroxyde. As a rule, this is done in presence of surfactants, which confer to the powders a higher apparent density, and a more homogeneous size distribution, which prevents caking. Finding the right surfactant, and its optimum concentration, is mainly empirical; moreover, the exact role played by the surfactant is not fully understood. So it seemed interesting to get a better understanding, or at least an exact quantitative description of the influence of the surfactant. In this first paper we describe the nucleation rate of sodium perborate, first in pure aqueous solution, and then in presence of the industrial surfactant.

Observations Upon Calcium Oxalate Crystallization Kinetics in Simulated Urine

Two major etiologic theories of urinary stones are excessive saturation of urine with crystallizable substances or defects in inhibitors that allow relative supersaturation to occur. To date, it has been difficult to confirm the supersaturation theory in experiments using diffusion-limited crystallization systems because direct measurements of the nucleation process of crystallization could not be performed. We used well developed, continuous crystallizer techniques and adapted them from industrial use to the study of stone disease. Data derived from the experiments allow the absolute measurement of crystal growth rate and determination of nucleation rate. These methods were applied to study the calcium oxalate dihydrate (weddellite) system in artificial urine that lacked only proteinaceous components. Based on these experiments it was not possible to grow crystals large enough within 5 to 20 minutes to obstruct the collecting ducts of the kidney. Therefore, it appears that other processes, such as aggregation or stasis within tissues, may well be related to initiation of stone disease. Under the experimental conditions of this study nucleation rate exceeded growth rate. Therefore, multiple small particles are created at the expense of allowing larger particles to grow. Inhibitors can be tested rapidly in this system by adding them in concentrations compatible with those found in urine.

Crystallization statistics, thermal history and glass formation

The formal theory of transformation kinetics describes the volume fraction of a phase transformed in a given time at a given temperature. The basic concepts are extended for isotropic crystal growth in a material having a known thermal history T( r, t). A crystal distribution function ψ( r, t, R) is defined such that the number of crystallites in a volume d υ at r having radii between R and R + d R at time t is ψ( r, t, R) d υ d R. The function ψ contains essentially complete statistical information about the state of crystallinity of a material. Formal expressions for ψ are obtained. Applications are discussed, including predictions of crystallinity when T( r, t) is known; predictions of glass-forming tendencies; experimental determination of nucleation rates; and the determination of the thermal history of a sample from post mortem crystallinity measurements. As an example, ψ( r, t, R) is calculated for a lunar glass composition subjected to a typical laboratory heat treatment.