Nucleation step process

Fig. 4. Diagram of the two-step process to manufacture nucleation track membranes, (a) Polycarbonate film is exposed to charged particles in a nuclear reactor, (b) Tracks left by particles are preferentially etched into uniform cylindrical pores (8).

Models used to describe the growth of crystals by layers call for a two-step process (/) formation of a two-dimensional nucleus on the surface and (2) spreading of the solute from the two-dimensional nucleus across the surface. The relative rates at which these two steps occur give rise to the mononuclear two-dimensional nucleation theory and the polynuclear two-dimensional nucleation theory. In the mononuclear two-dimensional nucleation theory, the surface nucleation step occurs at a finite rate, whereas the spreading across the surface is assumed to occur at an infinite rate. The reverse is tme for the polynuclear two-dimensional nucleation theory. Erom the mononuclear two-dimensional nucleation theory, growth is related to supersaturation by the equation. 

Initially, crystallization is a two-step process viz. nucleation and crystal growth requiring a change of free energy (Gibbs, 1928), as shown schematically in Figure 5.1. 

Since the interposition of a barrier layer diminishes the effective contact between reactants, the nucleation step in solid + solid reactions is Usually completed very rapidly at temperatures conveniently used in studies of the bulk reaction and, accordingly, the rate processes are often deceleratory throughout. In addition to the progressive diminution in rate... 

Sections 2.1—2.3 give accounts of kinetic and mechanistic features of the three rate-limiting processes (i) diffusion at a surface or in a gas (including the nucleation step), (ii) reaction at an interface, and (iii) diffusion across a barrier phase, [(ii) and (iii) are growth processes.] In any particular reaction, the slowest of these processes will, at any particular instant, control the rate of product formation. (A kinetic analysis of rate measurements must also incorporate an allowance for the geometric factors.)... 

The seeding-growth procedure is a popular technique that has been used for a century to synthesize metal particles in solution. Recent studies have successfully led to control the dimensionality of the particles where the sizes can be manipulated by varying the ratio of seed to metal salt [23-25]. The step-by-step particle enlargement is more effective than a one-step seeding method to avoid secondary nucleation [26,27]. This mechanism involves a two-step process, i.e. nucleation and then successive growth of the particles as illustrated in Scheme 1. 

Knowledge concerning the mechanism of hydrates formation is important in designing inhibitor systems for hydrates. The process of formation is believed to occur in two steps. The first step is a nucleation step and the second step is a growth reaction of the nucleus. Experimental results of nucleation are difficult to reproduce. Therefore, it is assumed that stochastic models would be useful in the mechanism of formation. Hydrate nucleation is an intrinsically stochastic process that involves the formation and growth of gas-water clusters to critical-sized, stable hydrate nuclei. The hydrate growth process involves the growth of stable hydrate nuclei as solid hydrates [129]. 

Polymers crystallize from the molten state by the two-step process of nucleation and crystal growth. Nucleation initiates crystallization, followed by the addition of linear chain segments to the crystal nucleus. 

The principles of collagen folding differ markedly from other known proteins since single monomers cannot fold. Triple helix folding is a multi-step process involving chain association, registration, nucleation and... 

The electrode reaction can involve the formation of a new phase ( e.g. electro-deposition processes used in galvanizing metals). The formation of a new phase is a multi-stage process since it requires a first nucleation step followed by crystal growth (in which atoms must diffuse through the solid phase to then become located in the appropriate site of the crystal lattice). 

The second step in the two-step process is nucleation. A typical nucleating solution used in industry is... 

A step by step process of nucleation allows the good selectivities obtained in the controlled reduction of silica-supported [Ru(CO)3Cl2]2 or [Ru(CO)3Cl2(HOSi ] in the presence of alkali carbonates (Scheme 16.4) [11]. 

Note 2 In the unstable region bounded by the spinodal curve, phase domains separation is spontaneous, i.e., no nucleation step is required to initiate the separation process. 

As indicated in Sec. IIB, ordinary nucleate boiling is a two-step process. First, nuclei must appear. Second, the nuclei must grow into bubbles large enough to move away from the nucleation sites. The rate of heat absorption by the liquid may be controlled by either one or both of these two processes. The growth of a nucleus (tiny bubble) into ordinary bubbles has received attention recently. The theoretical attack of Forster and Zuber was discussed in Sec. IIB2. Inasmuch as the theory of Zwick and Plesset (P3, P4, Zl, Z2) represents another attempt to obtain exact expressions for bubble growth, and since the theory fits well with the few data for steam bubbles in superheated water, their theoretical method is summarized below. 

Crystallization in general is a two-step process involving (1) nucleation and (2) growth of the nucleus to a macro size. Nucleation involves the activation of small, unstable particles with sufficient excess surface energy to form a new stable phase. This may occur in supersaturated solutions as a result of mechanical shock, the introduction of small crystals of the desired type, or the presence of certain impurities that can act as centers for growth. 

Schoen, H. M. 1961. Crystallization is a two-step process-nucleation and growth. Ind. Eng. Chem. 53, 607-611. 

Kinetic analysis indicates that renaturation is a two-step process. In the slow step effective contact is made between two complementary regions of DNA originated from separate strands. This rate-limiting step called nucleation is a function of the concentration of complementary strands. Nucleation is followed by a relatively rapid zippering up of adjoining base residues into a duplex structure. The steps involved in denaturation and renaturation are depicted in figure 25.14. 

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