Homogeneous induction time

Figure 9.11. Dioxygen uptake plots for the autoxidation of 1-decanethiol in the presence of [CoPcTs]4- as catalyst (a) unsupported and (b) supported on a MgAl-LDH. A shorter induction time and a larger turnover frequency were obtained for the LDFI assembly compared with the homogeneous case. After Perez Bernal et al. [124],...

Fig. 8. Schematic of shock initiation of energetic materials, based on refs. [65] and [66]. In a homogeneous material such as liquid NM (a)-(d), ignition begins first at the entry face which has been hot for the longest time. The delay between shock entry and ignition is termed the induction time. A fast reaction in the compressed material (superdetonation) runs up to the shock front, eventually relaxing to a stable detonation state. In an inhomogeneous material...

The induction time in homogeneous nucleation of a or p polymorphs of quizalofop-ethyl, i.e. the time the crystals first appeared in a supersaturated solution of quizalofop-e yl-ethanol mixtures was measured at temperatures between 293 and 31 IK in a batch stirred crystallizer. Experimental equations on the induction time have been derived semiquantitatively on the basis of an equation of classical nucleation rate. In addition, the concentration of supersaturated... 

A well-documented advantage of membrane crystallization techniques over conventional methods is represented by the accelerated rate of the crystallization process, as demonstrated by induction time analysis and nucleation/growth rate mea-snrements (Di Prolio et al. 2003). In fact, the membrane surface acts as a promoter of heterogeneous nucleation. As will be detailed in the next paragraphs, this increases the probability of forming stable crystalline nuclei even in supersaturation conditions that would not be adequate for homogeneous nucleation. This aspect is particularly... 

Whether it is homogeneous or heterogeneous growth, the nucleation rate J is difficult to measure, as the critical clusters formed correspond to a small number of molecules and hence are very small. In practice, the induction time (t) is determined, which is the time between supersaturation and the first appearance of visible crystals. Assuming that the first appearance of the crystals is primarily controlled by the nucleation step, then t is inversely proportional to the rate of nucleation ... 

Obviously spherulites cannot be assumed to be homogeneous spherical particles, and the fractional n values observed in their crystallization kinetics is a result of their complex structure. With such crystallizations, however, the rate parameter, Zj, must at the moment be of little mechanistic significance as are parameters such as induction times and half-lives derived from it. Determination of surface free energies from their temperature dependence must also be suspect until the exact significance of the Zi rate parameter be known. 

There is some confusion regarding the induction time of crystallization. While it is definitely present in the case of homogeneous nucleation, its existence is not that obvious in heterogeneous nucleation. While here it is often regarded as 0, there are some experimental indications for its presence even for heterogeneous nucleation (Menczel and Varga 1983). 

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