Controlled Nucleation

Different methods can be implemented for triggering the nucleation process and reducing the effect of the rather broad distribution of spontaneous nucleation temperature whitdi is one of the main causes of vial-to-vial variation of ice morphology and, consequently, of the distribution of drying times and of distributed morphology parameters in the freeze-dried product. 

Nevertheless, the main drawback of this method is that the distribution of the small ice crystals can be non-uniform along each plate of the freeze-dryer due to a complex and uncontrolled gas flow regime in the interior of the sublimation chamber. 

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The growth of a well ordered fullerene monolayer, by means of molecular beam epitaxy, has been used for the controlled nucleation of single crystalline thin films. The quality and stability of molecular thin films has been shown... 

Stookey, S.D. (1961) Controlled nucleation and crystallization leads to versatile new glass-ceramics, Chem. Eng. News 39/25, 116. 

The use of ordered supramolecular assemblies, such as micelles, monolayers, vesicles, inverted micelles, and lyotropic liquid crystalline systems, allows for the controlled nucleation of inorganic materials on molecular templates with well-defined structure and surface chemistry. Poly(propyleneimine) dendrimers modified with long aliphatic chains are a new class of amphiphiles which display a variety of aggregation states due to their conformational flexibility [38]. In the presence of octadecylamine, poly(propyleneimine) dendrimers modified with long alkyl chains self-assemble to form remarkably rigid and well-defined aggregates. When the aggregate dispersion was injected into a supersaturated... 

In Table 4-2, we show both phase-boundaiy controlled and diffusion-controlled nucleation reactions as a function of both constant and zero rate of nucleation. 

Frens, G. (1973) Controlled nucleation for the regulationof the particles size in monodisperse gold suspensions. Nature Physical Science, 241, 20-22. 

Furthermore, other advantages of controlled nucleation include ... 

In the system copper on Au(lll) [101, 102], the substrate structure controls nucleation despite the presence of a UPD layer. STM images obtained by Nichols et al. 

Frens G. Controlled nucleation for the regulation of particle size in monodispersed gold suspensions. NatPhys Sci 1973 241 20-22. 

Wirges Christian T, Timper J, Fischler M, Sologubenko Alla S, Mayer J, Simon U, Carell T (2009) Controlled nucleation of DNA metallization. Angew Chem Int Ed 48 219-223... 

Pal A (2004) Photochemical synthesis of gold nanoparticles via controlled nucleation using a bioactive molecule. Mater Lett 58 529-534... 

In interfacial electrochemical reaction rates given by the Butler—Volmcr equation (7.24), the current density, or rate of reaction per unit area, is zero at zero oveipotential (equilibrium), but significant net currents are observed if the potential of the working electrode is displaced from the reversible potential by only 1 mV. In the case of rate-controlling nucleation, however, there is no detectable current until the oveipotential exceeds a few millivolts, after which (at, say, 7 mV), the reaction rate suddenly undergoes an explosive increase. 

Applying supercritical fluids in CSS allows one to obtain supersaturation and to control nucleation- and growth rates by temperature-induced variation of the concentration of the solute in systems where no liquid solvents (e.g., VOC = volatile organic compounds) are present. 

McCauley J.W. and Roy R. (1974) Controlled nucleation and crystal growth of various CaCC>3 phases by the silica gel technique. Amer. Mineral. 59, 947-963. 

A pyroceramic material is a special type of glass that has been devitrified by controlled nucleation of crystals. It has certain properties that are like glass and other properties that are like ceramics. Standard round tubing, not square, flat, or distorted walls. 

Molders utilizing this system require equipment to measure and control the amount of entrained gas in the liquid at the desired level. They can include mass flow meters with density devices, nuclear density monitoring devices, as well as a variety of other densities measuring devices to control nucleation level. All these systems work within very defined pressure and temperature limits however, outside these limits, readings become erratic. There are systems that remove the dependence on system pressure and temperature. This system provides more consistent data. 

It is this variability, often uncontrolled, that leads to problems in controlling nucleation in food systems. Even in relatively pure systems, like carefully controlled supersaturated sugar systems, nucleation may occur over a very wide time span. For example, van Hook and Frulla (1952) found that identical supersaturated sucrose samples held at the same temperature nucleated over a very wide time span, with some crystallizing as quickly as 3 hours while other samples took over a day to nucleate. 

In the case of lipid fractionation, however, a different crystal size distribution is desired. As the fat crystals are to be separated from the liquid phase, uniform crystals of distinct size and shape are needed for the most efficient separation. For the most efficient separation by filtration, reasonably large (200 to 300 pm) crystals of fairly uniform size (narrow distribution of sizes) are needed. Fractionation technologies carefully control nucleation and growth to produce this uniform distribution of crystals to enhance filtration and separation of the high-melting stearin phase from the low-melting olein phase. 

This type of method was used by Brus and co-workers in studies on CdS and ZnS [12, 13], which have led to the explosion of interest in solid samples of semiconductors which show quantum confinement effects. They [12] prepared CdS nanoparticles by a process involving the controlled nucleation of CdS in dilute aqueous solutions of cadmium sulfate and ammonium sulfide. The dynamic equilibrium between solvated ions and solid CdS in acetonitrile, as a solvent in the presence of a styrene or maleic anhydride copolymer, allowed the preparation of... 

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