Primary crystal growth

Primary crystallization occurs when chain segments from a molten polymer that is below its equilibrium melting temperature deposit themselves on the growing face of a crystallite or a nucleus. Primary crystal growth takes place in the "a and b directions, relative to the unit cell, as shown schematically in Fig. 7.8. Inevitably, either the a or b direction of growth is thermodynamically favored and lamellae tend to grow faster in one direction than the other. The crystallite thickness, i.e., the c dimension of the crystallite, remains constant for a given crystallization temperature. Crystallite thickness is proportional to the crystallization temperature. 

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... 

The primary application for floating-zone melting is crystal growth rather than purification. Semiconductor-grade siUcon is not purified by zone refining siUcon chlorides are distilled and then reduced with hydrogen. 

When a precipitate separates from a solution, it is not always perfectly pure it may contain varying amounts of impurities dependent upon the nature of the precipitate and the conditions of precipitation. The contamination of the precipitate by substances which are normally soluble in the mother liquor is termed co-precipitation. We must distinguish between two important types of co-precipitation. The first is concerned with adsorption at the surface of the particles exposed to the solution, and the second relates to the occlusion of foreign substances during the process of crystal growth from the primary particles. 

The hypothesis was extended to nucleation of hydrates from liquid water. An alternative hypothesis was proposed by Rodger [1516]. The main difference between these two sets of theories is that Rodger s hypothesis relates the initial formation process to the surface of the water, whereas the theory of Sloan and coworkers considers clusters related to soluted hydrate formers in liquid water as the primary start for joining, agglomeration, and crystal growth. The theories of Sloan and coworkers have been discussed and related to elements of the hypothesis proposed by Rodger [1043]. 

Some of the reports are as follows. Mizukoshi et al. [31] reported ultrasound assisted reduction processes of Pt(IV) ions in the presence of anionic, cationic and non-ionic surfactant. They found that radicals formed from the reaction of the surfactants with primary radicals sonolysis of water and direct thermal decomposition of surfactants during collapsing of cavities contribute to reduction of metal ions. Fujimoto et al. [32] reported metal and alloy nanoparticles of Au, Pd and ft, and Mn02 prepared by reduction method in presence of surfactant and sonication environment. They found that surfactant shows stabilization of metal particles and has impact on narrow particle size distribution during sonication process. Abbas et al. [33] carried out the effects of different operational parameters in sodium chloride sonocrystallisation, namely temperature, ultrasonic power and concentration sodium. They found that the sonocrystallization is effective method for preparation of small NaCl crystals for pharmaceutical aerosol preparation. The crystal growth then occurs in supersaturated solution. Mersmann et al. (2001) [21] and Guo et al. [34] reported that the relative supersaturation in reactive crystallization is decisive for the crystal size and depends on the following factors. 

Polymer crystallization is usually divided into two separate processes primary nucleation and crystal growth [1]. The primary nucleation typically occurs in three-dimensional (3D) homogeneous disordered phases such as the melt or solution. The elementary process involved is a molecular transformation from a random-coil to a compact chain-folded crystallite induced by the changes in ambient temperature, pH, etc. Many uncertainties (the presence of various contaminations) and experimental difficulties have long hindered quantitative investigation of the primary nucleation. However, there are many works in the literature on the early events of crystallization by var-... 

The very fast initial density increase due to nucleation and rapid spherulite growth as shown by the dotted lines, referred to as primary crystallization. 

The spouting bed temperature is generally in the range of 150-170 °C, which is close to the maximum spherulite growth rate, and therefore ensures quick completion of the primary crystallization. The material temperature at the outlet of the pulsed fluid bed is usually <180°C. 

It Is unknown whether suspended a-form crystal or 3-form crystal In the saturated solution displays a solid-solid transformation or not. The reason for above experimental results may be assumed as follows In the standpoint of Industrial crystallization. Since 3-form crystal Is less soluble than a-form crystal at high temperature above 284K. The state that a-form crystal Is suspended In the saturated solution Is considered to be supersaturated for 3-form crystal. So the state has the potential to take place primary nucleatlon and crystal growth for the 3-form. In this way, 3-form crystal may be produced In the suspension of a form crystal. Rewarding to the formation of 3-form crystal, a part of a-form crystal suspended may be dissolved. Opposite phenomena may take place at low temperature below 284K. 

Conceptually similar to those of Lahav are the experiments performed by Kon-depudi as well as by other groups working with saturated solutions of compounds, which exist in the crystal state in two enantiomorphous forms. In particular, the primary homogeneous nucleation process of the chiral crystals of sodium chlorate and sodium bromate is very slow compared to the sequential step of crystal growth. Kondepudi et al. (1990) and McBride et al. (1991) have demonstrated that when... 

Dunkle s Syllabus (1960-1961), pp 13c 17d (Spontaneous detonation of LA and other primary expls during crystal growth)... 

Crystallization of TiO occurs during solvothermal treatment at high pressure, and the crystals grow to primary particle size through homo-coagulation. At this point, excess solvent partially suppresses further crystal growth as a result, the particle size becomes smaller than that in the sol-gel method. However, hydrolysis and condensation reactions occur very rapidly in sol-gel synthesis of transition metal oxides, therefore uniform and ultrafme products are difficult to obtain. 

---