Dendrite precursors

Nonaallylation of Mesitylene for the Synthesis of Dendritic Precursors of Large Metallodendrimers... 

Fig. 5.7 A 243-ferrocenyl dendrimer prepared from a dendritic precursor, having 81 C02H groups at the periphery, and the tertiary amine NKCH CsI FeiCsHsIb in THF at room temperature. The tiny balls at the periphery of the giant dendrimer represent (C5H4)Fe(C5F[5) units (from ref. 87 reproduced with permission of Wiley-VCH)...

Sisson A, Papp I, Landfester K, Haag R (2009) Functional nanoparticles from dendritic precursors hierarchical assembly in miniemulsion. Macromolecules 42 556-559... 

Deming, W. Edwards See Japanese workmanship, demulsification See emulsion/demulsification. demurrage See cost, demurrage, dendritic plastic A highly branched (dendritic) precursor of synthetic plastics that lead to three-dimensional gels and networks. These systems are broadly recognized as thermoset plastics. See gel. 

The tip of the twined cadmium dendrite precursor from Fig. 2.8b represents the physical equivalent of the scheme of the growth site from Fig. 2.8a. As shown in Fig. 2.8a, a layer of atoms advance in the direction determined by twining laws, an edge is constantly renewed, in which the new layers can be started by one-dimensional nucleation. Further growth and branching of precursor like that from Fig. 2.8b produces the dendrites shown in Fig. 2.8c, d. The deposition on the lateral flat dendrite surfaces takes place by repeated two-dimensional nucleation, as in deposition on dislocation free surface [33]. This makes the deposition rate in the direction of tip motion many times larger, which results in dendrite shape like that from Fig. 2.8d. 

The rate of growth of the protrusions under the conditions of spherical and cylindrical diffusion can be compared as follows. As shown earlier, the limiting diffusion current density at the tips of protmsions growing under the conditions of spherical diffusion (the needle-like dendrite or dendrite precursor), (L.tip inside the diffusion layer of the macroelectrode is given by ... 

Deposits obtained at 300 mV are compact, while those obtained at 600 mV are dendritic ones. Since both overpotentials correspond to the plateau of the limiting diffusion current density [14], it is clear that dendrites are formed at overpotentials larger than certain critical value, as required by the Eq. (2.47). It is seen that the current density to the tips of dendrites depends on the hp/5 ratio (see the Eq. (2.43)), so that larger dendrites are produced at more elevated points of the electrode surface. This is because the effective height of the dendrite precursor in the modeled diffusion layer is equal to the sum of the height of the precursor and the height of the point at which nucleation took places relative to the flat part of the electrode surface. In the same way, for nuclei formed on the tip of a protrusion (Fig. 2.12b), rja (see Eq. (2.46)) is lower than that formed on the flat surface, and a dendrite is formed at the tip of the protrusion while dendrites are not formed on the flat part of the electrode at the same overpotential. 

There is an induction period before the initiatirHi of dendritic growth [5]. During this induction period, dendrite precursors are formed and become sufficiently high to satisfy the Eq. (2.46) at a given overpotential, as illustrated in Fig. 2.13. The cross-like grains seen in Fig. 2.13a, b further develop into dendrite precursors (Fig. 2.13a, c). 

Popov KI, Cekerevac MI (1989) Dendritic electrocrystallization of cadmium from acid sulphate solution II the effect of the geometry of dendrite precursors on the shape of dendrites. Surf Coat Technol 37 435-440... 

It is obvious that the electrochemical conditions, as well as the crystallographic ones, under which dendritic deposits are formed can be precisely determined. One problem that still seems to remain unsolved is the question what causes the dendrite precursors to appear at regularly spaced locations along the dendrite stem. Further investigations in this direction are necessary. 

Precursors of cadmium dendrites [47] obtained by the processes of electrochemical deposition from 0.1 M CdS04 in 0.50 M H2SO4 onto cadmium wire electrodes at different overpotentials are shown in Fig. 2.22. It is obvious that further growth of the dendrite precursors shown in Fig. 2.22 leads to the formation of 2D dendrites (Fig. 2.23). Around the tips of dendrite precursors, as well as around the tips of dendrites, spherical or cylindrical diffusion control can occur, which is in good agreement with the requirements of the mathematical model. 

There is an induction period before initiation of dendritic growth [25, 33,49,50]. During this induction period, dendrite precursors are formed by the growth of suitable nuclei. According to Pangarov and Vitkova [51, 52] the orientation of nuclei is related to the over-potential used. The effect of overpotential of electrodeposition on the shape of cadmium dendrites is illustrated in Fig. 2.23. 

Fig. 6 Crystal structure of the dendritic precursor molecule 28a A central benzene ring, C branching points of central dendrimer structure...

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