Dendritic deposits

Two phenomena seem to distinguish dendritic from carrot-like growth [7, 11, 13]  

A certain well-defined critical overpotential value appears to exist below which 

Dendrites exhibit a highly ordered structure and grow and branch in weU-defined directions. According to Wranglen [26], a dendrite is a skeleton of a monociystal and consists of a stalk and branches, thereby resembling a tree. 

It is known that dendritic growth occurs selectively at three types of growth sites [11]  

In all the above cases, the adatoms are incorporated into the lattice by repeated one-dimensional nucleation. On the other hand, deposition to the tip of screw 

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A number of attempts to produce tire refractory metals, such as titanium and zirconium, by molten chloride electrolysis have not met widr success with two exceptions. The electrolysis of caesium salts such as Cs2ZrCl6 and CsTaCle, and of the fluorides Na2ZrF6 and NaTaFg have produced satisfactoty products on the laboratory scale (Flengas and Pint, 1969) but other systems have produced merely metallic dusts aird dendritic deposits. These observations suggest tlrat, as in tire case of metal deposition from aqueous electrolytes, e.g. Ag from Ag(CN)/ instead of from AgNOj, tire formation of stable metal complexes in tire liquid electrolyte is the key to success. 

Matsuda and co-workers [39-41] proposed the addition of some inorganic ions, such as Mg2+, Zn2+, In3+, Ga3+, Al3+,and Sn2+, to PC-based electrolytes in order to improve cycle life. They observed the formation of thin layers of Li/M alloys on the electrode surface during the cathodic deposition of lithium on charge-discharge cycling. The resulting films suppress the dendritic deposition of lithium [40, 41]. The Li/Al layer exhibited low and stable resistance in the electrolyte, but the... 

The potential has to be controlled very precisely. If it is insufficiently negative, the Cd/Te ratio of 1 1 is not obtained. If it is too negative, dendritic deposits of Cd appear. The more complex compounds (see above) are grown anodically (e.g., Cu2S co-deposited with In2 S3). 

It was possible to improve the interfacial properties of Li metal anodes in liquid electrolyte solutions using additives that modify the Li-surface chemistry, such as C02 [23-27] and HF [28,29], Using PEO-based gel electrolyte systems effectively suppressed dendritic deposition of lithium [30], In Section C we report on a very good charge-discharge performance of lithium metal anodes in PVdF-HFP gel electrolyte systems. Furthermore, addition of C02 to the PVdF-HFP gel electrolyte system considerably improves the charge/discharge characteristics [31]. 

Dendritic deposits grow under mass transport-controlled electrodeposition conditions. These conditions involve low concentration of electrolyte and high current density. A dendrite is a skeleton of a monocrystal consisting of stem and branches. The shapes of the dendrites are mainly determined by the directions of preferred growth in the lattice. The simplest dendrites consist of the stem and primary branches. The primary branches may develop secondary and tertiary branches. The angles between the stem and the branches, or between different branches, assume certain definite values in accordance with the space lattice. Thus, dendrites can be two dimensional (2D) or three dimensional (3D). 

Activation-limited growth tends to favor compact columnar or equiaxed deposits, while mass transport-limited growth favors formation of loose dendritic deposits. The deposit morphology is modified by using additives. Additives act as grain refiners and levelers because of their effects on electrode kinetics and the structure of the electrical double layer at the cathode surface. Additives that reduce primarily the nucleation overpotential can be considered to be grain-refining additives because of increased secondary nucleation events. 

The copper electrowinning process requires concentrated solutions to improve mass transport and increase the solution conductivity. The pregnant leach solutions from leaching are too dilute and too impure for the direct production of high-purity cathodes. Electrowinning from these solutions would give impure, dendritic deposits. Solvent extraction provides the means for producing pure,... 

In the case of copper, electrodeposition at low overpotentials produces large grains with relatively well-defined crystal shapes. Further increasing the overpotential leads to the formation of cauliflower-like and carrot-like protrusions, and finally, dendritic deposits are formed in the absence of strong hydrogen co-deposition.13... 

At overpotentials larger than 175 my the current density is considerably larger than the one expected from the linear dependence of current on overpotential. The formation of dendritic deposits (Fig. 16d-f) confirms that the deposition was dominantly under activation control. Thus, the elimination of mass transport limitations in the Ohmic-controlled electrodeposition of metals is due to the initiation of dendritic growth at overpotentials close to that at which complete diffusion control of the process on the flat part of the electrode surface occurs. 

It is necessary to note that the silver deposits shown in Fig. 16d-f are not similar to ideal silver dendrites,49 but they behave as dendritic ones in regard to their electrochemical properties. Hence, they can be considered as degenerate dendritic deposits. 

Aerosol Filtration, Ph.D. dissertation, California Institute of Technology, Pasadena, 1966), forming chainlike agglomerates termed dendrites. The growth of dendritic deposits on fibers has been studied experimentally [Billings, op. cit. Bhutra and Payatakes,/. Aerosol Sci., 10, 445 (1979)], and Payatakes and coworkers [Payatakes and Tien, J. Aerosol Sci., 7, 85 (1976) Payatakes, Am. Inst. Chem. Eng. J., 23,192 (1977) and Payatakes and Gradon, Chem. Eng. Sci., 35,1083 (1980)] have attempted to model the growth of dendrites and its influence on filter efficiency and pressure drop. 

Figure 7 A schematic view of dendritic deposition of active metals.

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