Temperature and growth rate

As the temperature drops from T, the melt viscosity, which is a fimction of the molar mass, increases and the diffusion rate decreases, thereby giving the chains greater opportunity to rearrange themselves to form annclens. This means that there will exist an optimum temperamre of crystallization, which not only depends largely on the interval between and Tg, but also on the molar mass of the sample. 

FIGURE 11.1 Radial growth rate r of spherulites of isotactic polystyrene as a function of the crystallization temperature. 

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A study of formation and modification of Ge quantum dots (QD) in Si is the actual problem due to perspectives to apply Ge/Si nanostructures in optoelectronic devices [1], To obtain nanoclusters with specified properties it is important to control sizes and density of Ge QDs. The modification of Ge nanocluster sizes is reached [2,3] by variation of temperature and growth rate, change of interfacial mechanical stresses, creation of buffer layers, insertion of impurities as nucleation centres, and stimulation of island growth by ion beams. In this paper, modification of Ge QDs by pulsed laser radiation was studied by Raman spectroscopy. 

TABLE 28.3 PECVD Reactants, Deposition Temperatures, and Growth Rates ... 

Figure 1. Correlation of typical gas temperatures and growth rates r (in pmh ) of diamond CVD processes (after [3]). See text for the explanation of abbreviations.

The effectiveness of an admixture Is closely bound to the given system and cannot be simply generalized. For the activity of additives on the crystal shape, Buckley (32] defined the measure of the effectiveness of the additive as the number of weight units of the crystallizing substance per one unit of the additive that causes a certain shape modification. Another way of measurement and evaluation of the effectiveness of admixtures has also been described in the literature (219. The influence of admixtures drops with Increasing temperature and growth rate (32). 

I Introduction to molecular motion in polymers 6.6 Temperature and growth rate... 

This difference can be attributed qualitatively to the increase in the equilibrium melting temperature with strain. At the lower temperatures, <20 °C, the growth rates are comparable to one another. At temperatures greater than the maximum there is a precipitous drop in the growth rates in both cases. This effect is more marked with the sample vmder 50% strain, where the growth rate becomes severely retarded. A quantitative analysis of this data is difficult without firm values of the equilibrium melting temperatures and growth rate data for the undeformed, but cross-linked sample. 

Submerged culture oxidizers can also be operated on a continuous basis. Continuous monitoring of ethanol and acetic acid concentrations, temperature, and aeration rates permit control of feed and withdrawal streams. Optimum production, however, is achieved by semicontinuous operation because the composition of vinegar desired in the withdrawal stream is so low in ethanol that vigorous bacterial growth is impeded. Bacterial... 

Crystal Morphology. Size, shape, color, and impurities are dependent on the conditions of synthesis (14—17). Lower temperatures favor dark colored, less pure crystals higher temperatures promote paler, purer crystals. Low pressures (5 GPa) and temperatures favor the development of cube faces, whereas higher pressures and temperatures produce octahedral faces. Nucleation and growth rates increase rapidly as the process pressure is raised above the diamond—graphite equiUbrium pressure. 

Model calculations of the growth rate R are shown in Fig. 3. These are plotted as a function of the driving force for crystallization, A/x ln(p/pg), where p and Pg are the actual and equilibrium vapor pressures, respectively. At very low temperatures, the growth rate is essentially... 

Conclusively, the calculated Avrami exponents reveal a three-dimensional growth of the crystalline regions for each blend. The rate of crystallization of each blend increased with the decrease in crystallization temperature, and the rate of crystallization of the (PHB80-PET20)/PEN blend was faster than that of the (PHB 80-PET20)/PET blend. 

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