Crystallization increased rate

At HOY speeds, the rate of increase in orientation levels off but the rate of crystallization increases dramatically. Air drag and inertial contributions to the threadline stress become large. Under these conditions, crystallization occurs very rapidly over a small filament length and a phenomenon called neck-draw occurs (68,75,76). The molecular stmcture is stable, fiber tensde strength is adequate for many uses, thermal shrinkage is low, and dye rates are higher than traditional slow speed spun, drawn, and heat-set products (77). 

Increased pressures can lower the temperature at which crystallisation occurs. Experiments performed using Spectrosil (Thermal Syndicate Ltd.) and G.E. Type 204 (General Electric Company) fused siUcas (see Eig. 2) show that at pressures above 2.5 GPa (<25, 000 atm), devitrification occurs at temperatures as low as 500°C and that at 4 GPa (<40, 000 atm), it occurs at as low as 450°C (107). Although the temperatures and pressures were in the coesite-phase field, both coesite and quarts were observed. Both the devitrification rate and the formation of the stable phase (coesite) were enhanced by the presence of water. In the 1000—1700°C region at 500—4000 MPa (<5, 000-40,000 atm), a- and p-quarts were the primary phases. Crystal growth rates... 

Polymers are a little more complicated. The drop in modulus (like the increase in creep rate) is caused by the increased ease with which molecules can slip past each other. In metals, which have a crystal structure, this reflects the increasing number of vacancies and the increased rate at which atoms jump into them. In polymers, which are amorphous, it reflects the increase in free volume which gives an increase in the rate of reptation. Then the shift factor is given, not by eqn. (23.11) but by... 

The size of crystal increases with time gradually approaching an asymptotic value. The higher the stirring rate, the larger the primary crystal sizes. 

The data plotted in the figure clearly support the predicted positive dependence of crystal size on agitation rate. Precipitation in the crystal film both enhances mass transfer and depletes bulk solute concentration. Thus, in the clear film model plotted by broken lines, bulk crystal sizes are initially slightly smaller than those predicted by the crystal film model but quickly become much larger due to increased yield. Taken together, these data imply that while the initial mean crystal growth rate and mixing rate dependence of size are... 

Among the molecules, however, business is going on as usual. Iodine dissolves by the detachment of surface layer molecules from the iodine crystals. The rate at which this process occurs is fixed by the stability of the crystal (tending to hold the molecules in the surface layer) and the temperature (the thermal agitation tending to dislodge the molecules from their lattice positions). As the dissolving continues, the concentration of iodine molecules in the solution increases. 

Besides, without addictive AICI3 as a crystal conversion agent, phase composition of most neogenic Ti02 particles was anatase in our experiment. Conversions active energy finm anatase to rutile was 460 kJ/mol [5], with temperature arose, crystal conversion rate as well as mass fraction of rutile would increase [6,7]. Hence, after a lot of heat accumulated, phase composition of particle-sintered layer was rutile. 

The overall rate of crystallization is determined by both the rate of nuclei formation and by the crystal growth rate. The maximum crystal growth rate lies at temperatures of between 170 and 190 °C [71, 72], as does the overall crystallization rate [51, 61, 75], The former is measured using hot stage optical microscopy while the latter is quantified by the half-time of crystallization. Both are influenced by the rate of nucleation on the crystal surface and the rate of diffusion of polymer chains to this surface. It has been shown that the spherulite growth rate decreases with increasing molecular weight due to the decrease in the rate of diffusion of molecules to this surface [46, 50, 55, 71, 74],... 

Both the rate of nuclei formation and the crystal growth rate can also be expected to influence the spherulite size. It has been reported (hat, in the temperature range 130-180 °C, the spherulite size increases with increasing temperature [74], This trend can be expected to extend to higher temperatures as the nucleation rate decreases. On the other hand, the presence of nucleating... 

As a result of the selective removal of the largest crystals, the specific surface area tends to increase, which imposes a decrease in the crystal growth rate and eventually causes a decrease in the average crystal size. Therefore a product clcussification step should preferably be combined with fines removal. 

Figure 7 shows the relation between the crystal nucleation rate and 0. decreases linearly with increase of 0 in... 

Figure 4-2 Calculated nucleation rate for Vc = 46 x 10 m /mol, E = 250 kj/mol, A5m-c = 50 J-K -moP, Al = 5 x 10 m, the equilibrium temperature of 1500K for (a) and (b), and the equilibrium pressure of 3 GPa for (c). (a) The dependence of crystal nucleation rate on the interface energy. Note that for a small change in interface energy from 0.300 to 0.295 J/m, the peak nucleation rate increases by more than one order of magnitude. If the interface energy changes from 0.3 to 0.2 J/m, the peak nucleation rate would increase by 17 orders of magnitude, (b) The nucleation rate of crystal and melt as a function of temperature, (c) The nucleation rate of crystal and melt as a function of pressure.

On a nucleation rate versus pressure diagram (Figure 4-2c), melt nucleation rate below the crystal-melt equilibrium pressure and crystal nucleation above the pressure are roughly S3mimetric. In Equation 4-9, only AG would vary with pressure or concentration. Hence, both melt nucleation rate and crystal nucleation rate increase monotonically with departure from equilibrium. There is no peak nucleation rate. 

Te increases monotonically and rapidly with increasing temperature. That is, the curve for crystal growth rate below Te and that for melt growth rate above Te are not symmetric. However, when plotted as a function of pressure or degree of saturation (w), the curve for crystal growth and that for melt growth are roughly symmetric. 

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