Nuclei heterogeneous

Certain types of contaminants within a molten polymer act as heterogeneous nuclei. Polymer chains that solidify against the pre-existing surface of a contaminant create less new solid/liquid... 

It has been reported that the overall rate of crystallization of pure PHB is relatively low compared to that of common synthetic polymers, showing a maximum in the temperature range of 55-60°C [23]. The spherulite growth rate kinetics have been evaluated [59] in terms of the theory by Hoffmann et al. [63], At about 90 °C, the spherulite growth rate displayed a maximum, which is not excessively low compared to that of common synthetic polymers. Therefore it was stated that the low overall crystallization rate of PHB centers on the nuclea-tion process rather than the subsequent crystal growth. Indeed, it has been shown that PHB has an exceptionally low level of heterogeneous nuclei [18]. 

If particles (or ions) are already present in a supersaturated vapor, nucleation will take place preferentially on these particles at supersaturations far smaller than for the homogeneous vapor. In this case, nucleation takes place heterogeneously on the existing nuclei at a rate dependent on the free energy of a condensate cap forming on or around the nucleus. Heterogeneous nuclei always occur in the earth s atmosphere. They are crucial to the formation of water clouds and to the formation of ice particles in supercooled clouds. 

Alexander McPherson and Paul J. Shlichta have suggested using insoluble minerals as heterogeneous nuclei for the crystallization of macromolecules. They obtained excellent protein crystals, which could be cleaved from the mineral nucleus and used for X-ray diffraction studies. The mineral is introduced into a supersaturated solution of the material to be crystallized. As supersaturation increases, nucleation occurs on a specific face of the mineral nucleus, and a crystal begins to grow. The orientation and periodicity of the molecules on the nucleus surface promote an oriented overgrowth that has a similar periodicity. 

Nucleation by heterogeneous nuclei noble metals, Ti02, Zr02. sulfides, phosphates... 

Figure 19 empirically demonstrates the divergence of nanoparticle properties from bulk properties. At sufficiently large size (i.e., extrapolation beyond the micron), every surface contains a number of active sites and the hysteresis gap between deliquescence and efflorescence approaches a limiting value of zero. However, when the particles move into the nanoregime, the distribution statistics shown by the Poisson distribution in Equation (21) indicate that there are perceptible differences from one particle to the next in terms of how many active sites are on an individual particle. For the smallest particles (e.g., 10 nm), over half of the particles have no active sites at all and are very ineffective heterogeneous nuclei. 

Vapour bubbles almost always develop at particularly favourable positions on solid surfaces or on suspended particles. Therefore, it is generally heterogeneous nuclei formation that prevails. The homogeneous nuclei formation, with bubbles formed by themselves as a result of the natural fluctuations of the molecules, plays a very minor role. 

Impacts upon perfect crystals of high explosives have to be pretty violent to get them to detonate introduce defects into them, and the job is much, much easier. Porosity in the form of voids has been assumed the principal source of hot spots, or heterogeneous nuclei for chemical reaction, as a shock wave passes by [39, 40]. We have investigated the interaction of shock waves with voids at the atomistic level, to see just how the hot spots are formed [41]. 

The conditions of heterogeneous nucleation, as well as the work of formation and the curvature radius of the critical nucleus, may be significantly influenced by the linear tension of the perimeter of wetting, x if x<0, the formation of heterogeneous nuclei becomes easier... 

Case 1 The sample of A contains inclusions in the amount of 10 cm . These inclusions act as heterogeneous nuclei in the phase transformation. The sample is quenched to 980 K and held at that temperature while the transformation to the a-phase occurs. 

A completely different behavior is reported for blends in which the crystallizable phase is dispersed. Fractionated crystallization of the dispersed droplets, associated with different degrees of undercooling and types of nuclei is the rule. The most important reason is a lack of primary heterogeneous nuclei within each crystallizable droplet. An important consequence of fractionated crystallization may be a drastic reduction in the degree of crystallinity. 

C lower, caused by the smaller PA-6 droplets having a lack in heterogeneous nuclei. The ratio of the o/y crystalline form was not altered by the fractionated crystallization, indicating that the lower crystallizing droplets do not crystallize in another crystalline form as in the bulk. 

In the case where disversed droplets crystallize in an already solidified matrix, the same phenomena as in the previously described case can influence the thermal behavior of the dispersed phase. Additionally, nucleation from the already solidified matrix will play a distinguished role. An induction of heterogeneous nuclei often can reduce the fractionated crystallization or even bring the T back at its bulk temperature. 

Glass formation can also be influenced by external factors. Since growth of crystals on heterogeneous nuclei does not require formation... 

At low temperatures, the aetion is attributed to an improvement of the molecular motion of PEN. Namely, the glass transition temperature deereases by the addition of the additives. At high temperatures, it is supposed that a phase separation oeeurs between additive and PEN. The separated droplets of the additives eould serve as heterogeneous nuclei to initiate primary nucleation. 

The Avrami equation is obtained as follows. For spherical growth from heterogeneous nuclei, for which all are assumed to start growing simultaneously, the number of nuclei per unit volume (v) is assumed constant. The growth rate (G) is given by... 

If all nuclei start growing simultaneously at time = 0 (the case of heterogeneous nuclei),... 

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