Crystallization, secondary

Secondary nucleation is an important particle formation process in industrial crystallizers. Secondary nucleation occurs because of the presence of existing crystals. In industrial crystallizers, existing crystals in suspension induce the formation of attrition-like smaller particles and effectively enhance the nucleation rate. This process has some similarity with attrition but differs in one important respect it occurs in the presence of a supersaturated solution. 

After the bath attained its equilibrium temperature, the crystallizer was charged with about 400 ml of liquid and was inserted into the bath. After about 30 minutes the system attained a constant temperature and a subcooling (difference of equilibrium temperature and constant temperature before initiation of crystaUization) was established. Introduction of the seed crystals ( ter being allowed to warm for a period of a few seconds) on the specially prepared stirrer initiated crystallization (secondary nucleation) and resulted in a change in the temperature of the crystallizer (figure 2). The temperature of the crystallizer attained an uilibrium value of a few minutes after nucleation occurred. The concentration of the sucrose solutions was measured using a refractometer (.1% accuracy). 

Gouty arthritis is an inflammatory response to the deposition of monosodium urate monohydrate crystals secondary to hyperuricemia. It is called monosodium urate crystal deposition disease. Hyperuricemia is a serum urate concentration > 7 mg% in males and >6 mg% in females. Hyperuricemia results from overproduction (10-15% of individuals) or a renal excretion of urate lower than 400 mg uric acid/24 hours (85-90% of individuals). The urate under-excretors have a urate clearance of <6 ml/min or a urate to creatinine clearance ratio of <6%. The combination of a relative excess of dietary purine consumption together with urate under-excretion is often the basis for hyperuricemia. 

Other minor constituents are authigenic pyrite which appears as framboides or separate crystals secondary gypsum, mainly as vein filling detrital quartz grains and authigenic feldspar (microcline). 

All of the organic matter and part of the aluminosilicates, hydroxyoxides, and silica in soil exist in structures too small or too poorly crystalline to be detectable by x-ray diffraction, These amorphous materials are not well understood, but they should logically be among the most reactive of soil components, because their structure is so open and their surface area so great, They represent a transition state between unweathered parent materials and well-crystallized secondary soil minerals. 

A considerable secondary crystallization, typical for PEEK, is seen in Fig. 6.99. Only about 50% of the total crystallinity can be assigned to primary crystallization. Secondary crystallization starts at about 28 h ( 10 s). At that time, the reversing amplitude of the storage modulus is almost constant. At later times, it decreases slowly, indicating very different processes for primary and secondary crystallization. 

Boron nitride (0.5 wt%) was used as nucleating agent in melt-spun fiber from poly-3-hydroxybutyrate. The nucleating agent controlled and stabilized crystallization. Secondary crystallization was suppressed, which otherwise woirld have lead to brittleness and poor mechanical performance. ... 

Keywords entanglement, disentanglement, cross-hatching, lamellae, crystallization, nucleation, reptation, nucleation (crystallization) regimes, nucleation agents, nucleation rate, spherulitic growth rate, Avrami-equation, Ozawa-equation, isothermal crystallization, nonisothermal crystallization, secondary nucleation, supercooling. 

Polymer crystallization theory is a mature area, and there are several review articles available that present and discuss the different theories in great detail (eg, 103,104). Having said that, over the last 5 years or so there has been a flurry of new interest becanse of the increase in computational power, which has the potential to decisively enter the debate in some areas. In the following the underlying themes of the two principle theories of polymer crystallization, secondary nucleation theory and rough-surface or entropic barrier theory, are outlined. The results of more recent simulations are then briefly discussed, in which the constraints of the above theories, introduced to provide analytical solutions, have been relaxed. Finally, some of the more fundamentally different ideas that have recently appeared are discussed. 

Electron diffraction is a collective elastic scattering phenomenon in which the electrons are scattered by atoms in a regular array or a crystal. Secondary waves are generated when an incoming plane electron wave interact with the atoms. Analogous to Huygens principle for diffraction of light waves, the secondary waves interfere with each other, and diffraction patterns thus can be obtained. 

When a folded layer on the surface of a crystal has finished growing, a new nucleus needs to form on the surface for continued growth of the crystal. Secondary nucleation requires a high undercooling because it has a low temperature dependence. The secondary crystallization layers are completed by an attachment-detachment mechanism [1]. 

The process of crystallization of a polymer from the melt can be divided in three stages (1) primary nucleation (2) growth of the crystals (secondary nucleation) (3) secondary crystallization. 

Stereoscopic Observation Compact paste with micropores, cracks with re-crystallized secondary material (photo) with concentration of calcite (photo). High content in aggregates, max size up to 8-10 mm, of mixed-type siliceous and calcific origin ... 

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